[Frontiers in Bioscience 14, 2149-2169, January 1, 2009]
Crumbs proteins in epithelial morphogenesis
Elsa Bazellieres, Emeline Assemat, Jean-Pierre Arsanto, Andre Le Bivic, Dominique Massey-Harroche
Institut de Biologie du Developpement de Marseille-Luminy, CNRS UMR 6216 et Universite de la Mediterranee, Parc Scientifique et Technologique de Luminy, Case 907, 13288 Marseille Cedex 09, France
TABLE OF CONTENTS
1. ABSTRACT
Cell polarity is an essential feature of most eukaryotic cells, especially epithelial cells in multicellular animals. Polarity protein complexes that regulate epithelial organization have been identified. In this review, it is proposed to describe how the Crumbs complex acts in the process of cell polarity and epithelial organization. During the last decade, several partners of Crumbs, an apical transmembrane protein, have been identified and their direct or indirect associations with the cytoplasmic domain of Crumbs have been dissected. In addition, mutants of several of the genes encoding proteins belonging to the Crumbs network have been obtained in animals ranging from flies to mouse, which have led to a better understanding of their functions in vivo. These functions include polarity axis formation, stabilization of epithelial apico-lateral junctions, photoreceptor organization and ciliogenesis. Since human CRUMBS1 mutations are associated with retina degeneration, it has become essential to define Crumbs network and to understand exactly how this network acts in polarized cells, with a view to developing suitable therapeutic approaches for treating this severe degenerative disease.
2. INTRODUCTION
Cell polarity is first characterized by the formation of a polarity axis that leads to the vectorial distribution of different proteins within the cell where they exert vectorial functions. The processes involved in establishing cell polarity are under the control of protein complexes and involve the re-organization of actin and microtubule cytoskeletons. This occurs in yeast where the tightly regulated formation of a bud leads to the coordinated accumulation of cell material in the bud to give rise to a new cell (for recent review see (1)). In animal cells, additional mechanisms are used to create transient or more stable cell sub-domains along a polarized axis. Cell to matrix adhesion mechanisms induce the formation of an apico-basal axis, while cell to cell adhesion mechanisms stabilize the lateral plasma membrane domains (for review see (2)). Similar processes occur in migrating cells and neurons, where a front to rear or an axonal to dendritic axis is also created by stabilizing membrane domains while keeping a strong plasticity. In animal cells, these polarity axes are governed by a set of genes which were first identified in Caenorhabditis elegans by genetic screens to identify any mutants that were unable to undergo asymmetric cell division in the early embryo (3). These genes encode Par (partition defective) proteins that are acting as two opposite groups (Par3, Par6 and aPKC in the anterior side and Par1, Par2 and Par4 in the posterior side) to polarize the early embryo. These protein complexes are conserved in the course of animal evolution, and further studies on Drosophila melanogaster have shown the existence of new genes which are essential for epithelial polarity. In addition to the Par system, two other genetic complexes are crucial to the establishment and maintenance of epithelial polarity. First there is the Crumbs complex (DCrumbs (DCrb), Stardust (Sdt) and DPatj), its organization and function will be described below, and the second complex Scribble consists of at least three genes: Scribble, Discs Large and Lethal Giant Larvae (which were recently reviewed in (4)). During the early stages of fly embryogenesis, all these complexes act in a coordinated fashion to localize and stabilize apico-lateral junctions providing a visible border between the apical domain facing the outside of the embryo and the lateral domain (for review, see (5)). While most of the genes controlling epithelial polarity are maternally provided and are therefore probably involved in early polarity determination, it is worth noting that in flies, two members of the Crumbs complex, DCrb and Sdt, start to be expressed at an early stage in gastrulation, which suggests that they may play a crucial role in Zonula adherens (ZA) stabilization and epithelial cohesion (6). Early gastrulation, which coincides with the first cell movements, requires strong cell-cell adhesion processes in the first epithelial layer that is provided by the ZA belt located at the apico-lateral border. In this review, we will focus on the role of a specific family of proteins, the Crumbs family, which form a network of interactors involved in epithelial and neuroepithelial morphogenesis. Crumbs proteins probably originated at an early stage in animal evolution, since they have been found to exist in nematodes, echinoderms, arthropods and vertebrates, but their first appearance in the course of evolution has not yet been determined: for this purpose, genome sequencing will have to be performed on species such as sponges and cnidarians originating from the early branching of the animal tree.
3.CRUMBS PROTEINS: EVOLUTION AND SCAFFOLDING
3.1. Evolution of Crumbs proteins among known animal genomes
Crumbs was first identified in Drosophila melanogaster, where mutations in this gene produced an absent or dispersed cuticle, hence giving the name "crumbs" (DCrb in the case of Drosophila Crumbs) (7, 8). This phenotype indicated a role of DCrb in epithelium organization as the cuticle is secreted by the underlying epithelial epidermis. DCrb is a transmembrane protein with a small cytoplasmic domain consisting of 37 amino acids and a large extracellular domain consisting of 28 EGF-like repeats and three laminin G-like repeats (Figure 1A). A few years later, Wodarz et al. (9, 10) established that the cytoplasmic domain of DCrb is crucial for its function in epithelial morphogenesis. This intracellular domain shows a FERM- (4.1-ezrin-radixin-moesin) binding motif (11), a C-terminal PDZ- (PSD-95, Discs Large, ZO1) binding motif (ERLI aminoacids) and a putative DaPKC phosphorylation motif (12).
The first evidence of a conservation of Crumbs between invertebrates and vertebrates was presented 10 years later by den Hollander et al. (13). The authors discovered that a human gene mutated in Retinitis Pigmentosa group 12 (RP12) disease was very similar to DCrb. This gene was called CRB1 and the largest protein isoform expressed has the same domain architecture as both the extra and intracellular domains of DCrb (Figure 1A). Two additional genes, CRB2 and CRB3, were then discovered (14) and found to encode proteins with a highly conserved cytoplasmic domain (showing 51 and 58% of identity with CRB1) (Figure 1A). CRB3 is the only homolog exhibiting a very short extracellular part with no recognizable protein domain and a proline-rich domain in the intracellular part for CRB3A (two isoforms have been described: CRB3A and B, as shown in Figure 1A). Several splice variants have been described, which encode either proteins lacking the transmembrane and intracellular domains, which are therefore presumably secreted (CRB1s and CRB2s) (15-17) or a protein with a C-terminal cytoplasmic domain lacking the proline rich region and showing a different PDZ-binding motif (CLPI instead of ERLI, in the case of CRB3B) (18, 19) (Figure 1A and B).
Subsequent studies have shown that Crumbs is also conserved in other model species such as mouse (20), worm (11, 21) and zebrafish (22). The short intracellular domain was found to serve as the docking site for a highly conserved protein scaffold. This functional domain is frequently used to study the evolution of CRB proteins. In this review, we have compared the transmembrane and cytoplasmic sequences of CRB orthologs using a multi-alignment software programme (ClustalW) (Figure 1B), and we have generated a phylogenic tree (PHYLIP programs) (Figure 1C). This tree shows that insect CRB form a coherent group. Vertebrate CRB1, CRB2 and CRB3 (A and B isoforms) also form compact groups, which can be subdivided into mammals and other vertebrates (zebrafish, frog, chick, etc). Interestingly, two separate clusters have been observed in this tree: one composed of sea urchin (Strongylocentrotus purpuratus) CRB protein and the second one of worm (Caenorhabditis elegans and C. briggsae) CRB1L and CRB3L proteins; this suggests that these proteins have evolved differently. It has been reported that the extracellular part of CRB1L has only EGF domains and lacks the laminin domains found to exist in all CRB1 and CRB2 proteins. In addition, we have observed that the GTY motif (in the FERM-binding domain ) is replaced by G (H/K)Y in worm CRB1 and CRB1L proteins, although this difference does not seem to have crucial consequences at the functional level (11). Another not previously described CRB protein was found to exist in the worm databases, which was more closely related to CRB3 because of its short extracellular domain. We named this protein CRB3L (CRB3-like) because it differs from the other CRB3 proteins: the ERLI and GTY motifs are replaced in this case by EGLI and GEY, respectively. Worm CRB3L and zebrafish (Danio Rerio) CRB3A and B cytoplasmic domains lack the typical proline-rich region predicted to bind to SH3 (Src homology region 3)-domain proteins or WW-domain proteins, which is conserved in mammalian CRB3A proteins. Conversely, CRB3B splicing isoform, which was found to exist only in mammals, might have been acquired more recently during evolution, giving Crumbs proteins a new functional role. All in all, these findings may help us to build new hypothesis for the functional role played by Crumbs orthologs in variousspecies.
3.2. Scaffolding of Crumbs complex in Drosophila melanogaster and mammals
The high level of homology found to exist between the cytoplasmic domains of Crumbs proteins suggests that the protein-protein interactions are conserved. Most of the protein-protein interactions described in this section were detected in genetic studies or studies using yeast two-hybrid or Far-Western blotting approaches and have been confirmed in vivo using GST pull down and/or co-imunoprecipitation methods. The Crumbs complex is a highly dynamic protein scaffold which is conserved in Drosophila and mammals: the first level of this scaffold, which is composed of identified partners of the backbone Crumbs complex i.e. DCrb/CRB, Stardust/PALS1 and DPatj/PATJ, forms an interactome (as shown in Figures. 2 and 3), which is involved in the establishment and maintenance of epithelial junctions and apico-basal polarity.
3.2.1. Crumbs complex in Drosophila melanogaster
The low redundancy of insect genes makes the fly Crumbs complex a useful prototype for studying vertebrate Crumbs complex in general (as described in 3.2.2). The Crumbs complex, which was discovered at the end of the 90s, consists of 3 proteins: DCrb, Sdt (Stardust) and DPatj (protein associated with tight junction) and this tripartite association was taken to form the backbone of the DCrb complex, hence the name DCrb/Sdt/DPatj complex, which will be described below.
DCrb is a 234kDa transmembrane protein (7, 8), which includes a large extracellular part. Since no ligands have been found so far which bind to this large extracellular part, the exact function of this part still remains to be elucidated . The Crumbs complex is formed around the short cytoplasmic part (37 amino acids) of DCrb, in which two protein-interacting domains have been identified: a FERM binding domain (GTY amino acids) and a PDZ binding domain (the last four ERLI amino acids). This PDZ binding domain binds to Sdt PDZ domain, (6, 23, 24). Sdt encodes at least three protein isoforms, Sdt-A, Sdt-B, and Sdt-GUK1. Two of them, Sdt-A (named Sdt-MAGUK1 in (23) and Sdt-B, are scaffolding proteins belonging to the MAGUK (membrane-associated guanylate kinase) family, which is characterized by the presence of a GUK domain (which is homologous to the previously identified guanylate kinases but without enzymatic activity) (23, 24), a SH3 domain, a hook domain, a single PDZ domain and two adjacent L27 (lin2 lin7 binding) domains (called L27N or C, depending on their position in the sequence). Sdt-A and Sdt-B differ in terms of the presence or absence of a large exon, which encodes a 433-amino acid region in the N-terminal (see Sdt-A in Figure 2.). By contrast, the third known isoform, Sdt-GUK1, lacks most of the PDZ domain, the SH3 and hook domains and part of the GUK domain (23). DPatj contains 4 PDZ domains and a L27 domain (25) that binds to the L27N domain of Sdt (26).
In addition to the core complex formed by DCrb, Sdt and DPatj, an increasing number of DCrb network partners have been identified.
3.2.1.1. Partners of DCrumbs
The DCrb PDZ binding motif can also bind to the single PDZ domain of DPar6 (Drosophila Par6) (27). DPar6 consists of one PB1 (Phox/Bem 1) domain followed by a CRIB (Cdc42/Rac interaction binding) and a PDZ domain. The PB1 domain, which is located at the N-terminal part of DPar6, can bind to other PB1 domain-containing proteins such as aPKC (reviewed in (28)). DPar6 belongs to the conserved Par complex (DPar6/Bazooka (Baz)/DaPKC), which is also involved in establishing and maintaining apico-basal epithelium polarity and in generating the anterior-posterior polarity axis in Caenorhabditis elegans (29, 30). In addition, DCrb (see the following paragraphs) has to be phosphorylated by DaPKC to be functional, and DCrb and DaPKC must therefore interact transiently (12) (these interactions are not shown in Figure 2A). Two FERM proteins, DMoesin (31) and Yurt (32), have been found to bind to the DCrb FERM binding domain. Since DCrb is connected to beta Heavy-spectrin and apical spectrin-based membrane skeleton (SBMS) via the DMoesin protein, the DCrb present in the epithelial cell architecture participates in stabilizing Zonula Adherens (ZA) by recruiting the SBMS into the junctional region (33). By contrast, the interactions between Yurt (a basolateral cytoskeleton protein) and DCrb are part of a negative feedback loop that regulates epithelial polarity and apical membrane growth: any loss of Yurt or overexpression of DCrb result in the expansion of the apical membrane (32). A PDZ binding domain has been found to exist at the C-terminal end of the Yurt protein, but the partner involved in the binding process has not yet been identified.
3.2.1.2. Partners of Stardust
To our knowledge, none of the partners interacting with GUK and SH3 domains have yet been identified; whereas the L27C domain interacts with the L27 domains of DLin-7 (34, 35) and the Sdt N-terminal part (21-141 amino-acids) can bind to the CRIB and PDZ domains of DPar6 (36). It is worth noting that since the Sdt-B isoform lacks the DPar6 interaction motif Sdt-B does not interact with DPar6.
3.2.1.3.Partners of DPatj
Among the four PDZ domains of DPatj, only the partners with which the last two interact have been identified. The third PDZ domain binds to the N-terminal PB1 domain of DPar6 (37). The fourth PDZ domain binds to the Frizzled 1 (Fz1) C-terminal domain (38). This molecular link connects apico-basal determinants (DCrb/Sdt/DPatj complex) to planar cell polarity protein complexes (Fz1 and associated signaling proteins). Fz1 belongs to the Frizzled family of receptors, which have seven transmembrane-spanning domains and are activated by the Wnt family of lipoglycoproteins (for a review see (39)). Djiane et al (38) have suggested that the link between DPatj and Fz1 may recruit DaPKC (possibly via DPar6 linked to DPatj), which in turn phosphorylates and thus inhibits Fz1.
It is worth noting that the three partners involved in the DCrb complex can interact with DPar6, which seems to be an essential interface between DCrb and DPar polarity complexes (40). In addition, Nam and Choi (37) have established that these two complexes depend on each other to be able to achieve their proper location in photoreceptors.
3.2.2. Crumbs complex in mammals
In mammals, three genes denoted CRB1, 2, 3 have been identified and their products have been detected in expressed sequence tag (EST) databases (33). Their patterns of distribution suggest the existence of a tissue-related functional specialization of CRB proteins in mammals despite the conserved cytoplasmic parts they have in common. Since CRB3 is mainly expressed in epithelial tissues, unlike CRB1 and 2 and since the CRB3 complex has been more fully documented in the literature than its counterparts, we will focus in this part of the review on CRB3 (17, 41-44).
The mammalian CRB core complex consists of the mammalian homologues of fly proteins, namely CRB3A/DCrb, PALS1/Sdt, and PATJ/DPatj (Figure 3A). In addition, the interacting domains are highly conserved. CRB3A differs from other CRB in that it has a very short extracellular part and its cytoplasmic part has an additional proline-rich domain (Figure 1A). The link between CRB3 (as well as CRB1 (26)) and PALS1 (proteins associated with Lin seven also called MPP5 for membrane protein palmitoylated 5: for a review, see (40)) is based on the recognition between the four ERLI N-terminal amino acids (the PDZ binding domain) of CRB3A and the PDZ domain of PALS1 (42, 45). PALS1 has the same interacting domains as Sdt: two L27, one PDZ, one SH3, one hook and one GUK domains (23, 24, 46) but it has a shorter N-terminal part than Sdt-A.
As in Drosophila, PALS1 and PATJ interact via their cognate L27 domains (26). The structural bases of L27 domain heteromeric complex assembly have been studied using biochemical, NMR spectroscopic and crystallization approaches (47, 48), and the results of these studies have shown the existence of a heterodimerization mechanism driven by hydrophobic interactions. More interestingly, an association between two heterodimers (PALS/PATJ) may occur in an asymmetric (47) or symmetrical tetramer (48) and it seems likely that the tetrameric structure is based on a general assembly mode involving cognate pairs of L27 domains (49). For the sake of clarity , this process of oligomerization has not been included in figure 3A. The PATJ-L27 domain present at the N-terminal is followed by up to ten PDZ domains: their number depends on the alternative splicing process producing at least 5 isoforms (50, 51) (14). In mammals, there exist two homologues of DPatj: PATJ and MUPP1 (multi-PDZ domain protein); their PDZ domain similarities and their respective partners are indicated in Figure 3B. The N-terminal of MUPP1 also has an L27 domain that can bind to PALS1 (26, 52, 53). However, unlike PATJ, MUPP1 is not generally included among the constitutive members of the CRB3A complex and has therefore not been shown in Figure 3A.
As occurs in Drosophila, the mammalian CRB3A/PALS1/PATJ complex constitutes the core of a cytoplasmic protein scaffold in which an increasingly large number of partners have been identified. However, the partners of CRB3A identified so far are all human homologues of those existing in the fly, whereas new partners for PALS1 and PATJ have been identified in mammals
3.2.2.1. Partners of CRB3A
The FERM binding domains of all mammalian CRB bind to two mammalian Yurt orthologs, Epb4.1l5 (erythrocyte protein band 4.1- like 5) and EHM2 (expressed in high metastatic cells) FERM proteins (32). Interestingly, the PDZ binding domain of Crb3A binds to the PAR6PDZ domain (43). In addition, as in Drosophila, PAR6 (consisting of one PB1 domain followed by a CRIB and a PDZ domains) participates in building the PAR complex along with PAR3 and aPKC. The PAR complex is involved in the establishment and maintenance of epithelial tight junctions and apico-basal polarity (for a review, see (40))
3.2.2.2. Partners of PALS1
The evolutionarily conserved N-terminal region of PALS1 is able to bind to the PAR6 PDZ domain (36). However, there is a requirement for the PALS1 L27N domain in the interaction between PAR6 and PALS1, whereas in Drosophila, the Sdt amino terminus lacking the L27 domain still binds to DPar6 (36). In addition, it seems likely that the relationship between PALS1-PATJ and PALS1-PAR6 interactions may be competitive rather than synergistic (36). The L27C domain of PALS1 binds to the L27 domain of Lin-7 (also known as VELI for Vertebrate lin-7, and MALS for mammalian lin-seven) (46), and this interaction stabilizes PALS1 (54). The PALS1 SH3 and hook domains can bind to another MPP protein, MPP4, via its GUK domain (55). Interestingly, the SH3 and GUK domains of PALS1 (like those of other MPP and MAGUK proteins) may be involved in intramolecular binding processes. These processes keep the SH3-GUK domains in the 'closed' state and may prevent heteromerization of PALS1 with MPP4 as well as homodimerization of PALS1 between the SH3 hook domains of one PALS1 molecule and the GUK domain of another PALS1 molecule (56). The hook domain of PALS1 can interact with Epb4.1l5 in mammals (57), as previously found to occur in zebrafish between NOK (zebrafish PALS1) and MOE (a FERM protein) (57, 58).
3.2.2.3.Partners of PATJ
Among the 10 PDZ domains of PATJ, only four have identified partners. Extensive searches have been carried out for the partners of the 2nd and 3rd PATJ PDZ domains because of their homologies with PDZ 1 and 2 of DPatj, and several partners have been identified. First of all, 2nd and 3rd PDZ domains can bind to a tumor suppressor protein, the tuberous sclerosis 2 protein (TSC2) at its C-terminal part, including its PDZ-binding motif (TEFV) and its GAP (GTPase-activating) domain (59). The TSC2 pathway controls many cellular features such as cell size, cell survival, apoptosis and cell cycle progression (for a review, see (60-64)). In addition, these 2nd and 3rd PDZ domains can also bind to the C-terminal PDZ-binding motives of Amot (angiomotin) and MASCOT (MAGI-1-associated coiled-coil tight junction protein), whereas only the 3rd PDZ is responsible for its interaction with JEAP (junction-enriched and -associated protein) (similar results have been obtained on the 2nd and 3rd MUPP1 PDZ domains) (65). These three proteins have a very similar structure and contain a coiled-coil motif followed by a PDZ-binding motif at the C-terminal end (66-68). Amot contributes to stabilizing tight junction structure and maintaining epithelial polarity (69).
The 6th and 8th PATJ-PDZ domains can interact with the C-terminal PDZ-binding motives of ZO3 (zona occludens3, a tight junction MAGUK protein) and claudin1 (51), respectively. Likewise, MUPP1 interacts with claudin1 via its 10th PDZ domain, which is highly similar to the 8th PDZ domain of PATJ (Figure 3B) (70).
Interestingly, the C-terminal PDZ-binding motif of claudin1 can also bind to the 1st PDZ domain of ZO3 and conversely, the GUK domain of ZO3 (like that of ZO1 and 2) can bind to the C-terminal fragment (consisting of 150 amino acids) of occludin (71, 72). In addition, the targeting of PATJ to tight junctions is abolished when the 6th PDZ domain binding to ZO3 is deleted, which suggests that ZO3 plays an important role in the targeting of PATJ to tight junctions. On the other hand, the deletion of the 8th PDZ domain (binding to claudin1) does not affect the tight junction location of PATJ (51).
4.THE CRUMBS COMPLEX: ITS ROLE IN DEVELOPING ORGANISMS
4.1. Early embryogenesis and primary epithelia
During embryogenesis, the first differentiated structure formed is an epithelium. Studies on the early steps in the development of model organisms (such as the fly and mouse) therefore shed interesting light on the molecular events leading to de novo formation of epithelia.
4.1.1. Drosophila melanogaster
After the fertilized egg has been laid, the process of development begins in Drosophila with the occurrence of synchronous syncytial nuclear divisions not involving cytokinesis, which results in the syncytial blastoderm. The process of cellularization results from the invagination of apical plasma membrane, which leads to the formation of the membrane furrow (reviewed in (73-75)). At this stage, only DPatj protein is present (there are no DCrb and Sdt proteins), and participates in membrane formation by moving in the basal direction along the leading edge of the invaginating membrane furrow and concentrating in the basal domain of the columnar epithelial cells. DPatj null mutation causes early first instar larval lethality, which indicates that DPatj, or more specifically, its N-terminal L27-PDZ1 domains, is crucial in the early stages of development (25, 76). By contrast, DPar6 is also present but does not participate in the process of cellularization, and its inactivation does not perturb this process (77). However, DPar6 recruits DPatj apically at the end of the cellularization process. During this step, spots of adherens junctions (enriched with E-cadherin/beta-catenin complexes) appear on the lateral membrane. Epithelial polarity is thus first established during the process of cellularization and requires the presence of DPatj.
DCrb and Sdt proteins are first detected apically and at the top of the lateral domain in low levels during early gastrulation. A fraction of the DPatj protein is also recruited at that time apically, while most of the remainder is still basal at this stage. The apical localization of DCrb seems to rely on a dynein dependent transport process, since in the dynein mutant, DCrb protein and mRNA were not found to be localized apically, and this deficit leads to apico-basal polarity defects (78). Likewise, the apical localization of Sdt A mRNA depends on its binding to dynein (79).
During gastrulation and shortly thereafter, the spot adherens junctions aggregate, forming the apico-lateral belt of ZA (zonula adherens). In DCrb and Sdt mutants, defects in ZA establishment occur rapidly only a few minutes after DCrb is normally expressed and precede deterioration of the epithelial cell structure (80). In addition, these mutants are characterized by the down-regulation of apical markers, loss of polarity, severe disruption of the cuticle and extensive cell death in the epiderm. Interestingly, two copies of Sdt in an amorphic background of DCrb could rescue the Crb phenotype indicating that Sdt acts downstream of DCrb in the complex (6). Sdt is also required for DCrb to be correctly localized and control epithelial polarity (23). Overexpression of DCrb expands the apical cell surface at the expense of the basolateral domain, which suggests that DCrb is a crucial factor defining the apical surface of epithelial cells. The overexpression of DCrb also results in the disruption of epithelial integrity due to the loss of adherens junctions, establishment without any loss of polarity (10, 80). This phenotype might result from the lack of interactions with the cortical cytoskeleton, since it resembles the phenotype induced by the loss of FERM protein Yurt (32). In wild type Drosophila, Yurt acts as a negative regulator of the DCrb complex, restricting its localization to the subapical region.
The establishment of epithelial polarity during Drosophila embryogenesis therefore results from coordinated interactions between three polarity complexes, the DCrb, DPar and Scrib complexes (see introduction and review (81)). Using genetic approaches it has been shown that DCrb and DPar complexes act in the same way while the Scrib complex antagonizes the function of DCrb and DPar complexes (82) (for a review, see (5, 40, 83)).
4.1.2. Vertebrates
In mammals, the first epithelium is a unique single-layered tissue called the trophectoderm, which forms on the outside of the embryo during the cleavage stage and give rise at a later stage in pregnancy to the chorio-allantoic placenta. In the early amphibian embryo, the outer epithelium becomes the basic tissue involved in gastrulation and comprises progenitors responsible for all three germ layers. In both cases, this initial epithelium engages in the polarized transport processes generating the blastoc�le cavity, thus providing the embryo an opportunity for regulating the composition of the internal tissues it requires for metabolic and developmental purposes and, most importantly, providing a malleable platform on which cell rearrangements and the process of morphogenesis can shape the future anatomy of the developing embryo. A role for CRB proteins in these early mammalian development stages will be likely uncovered by future studies.
In the context of vertebrate embryogenesis, epithelial polarity pathways have been most extensively studied in zebrafish. Genetic screens using this model organism showed the presence of a group of loci serving as determinants of apico-basal polarity (84). Cloning of mutant loci revealed a close relationships between vertebrate neuroepithelial polarity pathways and mechanisms regulating the polarity of fly embryo epithelia. The five CRB genes identified in zebrafish show different patterns of expression, which suggests that these genes play various developmental roles. As in mouse and human studies (13, 53, 85), CRB1 knock-down in zebrafish failed to produce a clear-cut embryonic phenotype (22). By contrast, mutations in CRB2A (the oko meduzy (OME) mutant), and morpholino knockdown of CRB2A and CRB3A decrease or abolish the apical surface features, and are frequently associated with severe morphological defects in the developing embryo: the mechanisms involved here still remain to be elucidated, however (22).
In Xenopus, CRB3 promotes the development of the apical membrane identity and inhibits the basolateral domain expansion, as its overexpression induced the expansion of the apical region of the developing embryo (86). In developing mammalian embryos, TJs biogenesis in trophectoderm cells is usually studied in terms of the distribution of a variety of cellular markers including ZO1 (87), occludin (88), but the exact localization of CRB proteins and their contribution to this process have not yet been established. Recent studies have focused on the partners involved in CRB network interactions in the mouse embryo. The FERM protein Epb4.1l5 is an important component of the epithelial architecture and Epb4.1l5 mutant mouse embryos died at E8.5-9.0; whereas CRB localization was not affected in these mice (89). By contrast, the orthologs of Epb4.1l5 in zebrafish (MOE) and Drosophila (Yurt) are required for the proper localization of CRB2A and DCrb, to occur, respectively, which indicates that Epb4.1l5-Crumbs related function is not conserved among species (32, 58, 90). In addition PALS1 zebrafish ortholog (NOK) is essential not only to the proper localization of the CRB protein, but also to CRB protein stabilization, which suggests that the mechanism involved differs between NOK and MOE (58). This function of PALS1 is not conserved in mammals, since the down-regulation of PALS1 did not induce the down-regulation of CRB3 in MDCK cells (91) (see section 5.2.2). All the data obtained in studies where the pattern of CRB expression was modified in vertebrates show the presence of mild defects in comparison with the Drosophila phenotype (embryonic lethality). Since in vertebrates, there exist at least 3 genes encoding different CRB proteins, we can speculate that during embryogenesis, there may exist some functional redundancy.
4.2. Organogenesis
Crumbs is necessary not only during the earliest stage of development (section 3.1); in this part, we will discuss the role of Crumbs in the formation of differentiated epithelia such as retina and kidney.
In human, CRB1 expression was found to be restricted to the retina and brain (13, 20), whereas in mouse, it was also detected in the kidney, stomach, lung and testis (15). The mammalian paralogs CRB2 and CRB3 are expressed in a wider range of tissues, including the retina (14, 17, 42). CRB2 is mainly expressed in the retina, brain and kidney and in low levels in the lung, placenta and heart (17). CRB3 is expressed not only in the retina (53, 92), but also in the kidney, colon, lung, heart and mammary gland (42, 43). Immunolabeling studies have shown the presence of components of the CRB complex, such as PATJ and CRB3, at the most apical membrane domain of the human colonic epithelium (14, 43), more specifically at the tight junctions. On the other hand, cCRB (chickCrumbs, isoforms not fully described so far) and cPALS1 (chickPALS1), two members of the CRB complex, have been detected at the most apical membrane domain of the chick embryonic neural tube epithelium (93).
In zebrafish, highly conserved CRB orthologs were identified (see section 3.1) and analysis of their mutants showed that some of these genes are required for the normal development of structures including the retina and kidney. Since the CRB complex has been found to be mainly expressed in the retinal and renal systems, it is proposed to focus here on these systems.
4.2.1. Retina
4.2.1.1. Drosophila retina
In the fly, DCrb is a central component of a molecular scaffold controlling the assembly of ZA and regulates the size and shape of the stalk (an apical membrane subdomain connecting the rhabdomeres to the ZA), where the DCrb complex is also localized (37, 94-98) (Reviewed in (99)). Several lines of evidence have shown that loss of function of DCrb in Drosophila leads to impaired ZA integrity, stalk membrane and rhabdomere length (both shortened) and the progressive light-induced degeneration of photoreceptors (PhRs) (94, 95, 97, 99). Interestingly, similar defects to those occurring in DCrb mutant PhRs, including shortened stalk membranes and distorted rhabdomeres, were observed in Drosophila retinas lacking the DCrb binding partners Stardust (Sdt) and DPATJ (76, 97, 98, 100).
In addition to the crucial role of these DCrb-binding partners in retinal morphogenesis, Drosophila Sdt and DPatj mutations have shown that the complete absence of Sdt and DPatj expression in late pupal and adult PhRs also leads to similar progressive light-dependent retinal degeneration processes to those occurring in DCrb mutants (96, 100, 101). This finding provided evidence that the DCrb-Sdt-DPatj complex is indispensable for Drosophila eye integrity (23, 94-96, 98, 100). Recent findings of Berger et al. (100) showed that only a subset of Sdt alleles controls PhR morphogenesis or prevents light-induced retinal degeneration, which suggests that the DCrb complex may vary in its Sdt composition during development.
The proper localization and function of DCrb in Drosophila retina have also been found to depend on the function of DCrb-binding proteins, such as Mbt (mushroom bodies tiny), a protein binding to Cdc42, which is probably involved in actin cytoskeleton reorganization (102), and FERM proteins such as DMoesin (103) and Yurt (32). The dynamic distribution of DMoesin during differentiation and the impact of DMoesin gain- and loss-of-function during PhR morphogenesis suggest that this protein may play a crucial role in the organization of the rhabdomeres (103). However, DMoesin and DCrb define complementary sub-domains in the PhR apical surface, which are located in the rhabdomere base and the stalk membrane, respectively: this finding suggests that DMoesin is not a DCrb-binding partner in PhRs (103) as reported to occur in the case of embryonic epithelia (31). Laprise et al. (32) have reported that Yurt mutant PhRs, without disrupting apico-basal polarity, exhibit longer apical stalks which are similar to those of the PhRs overexpressing DCrb and unlike the PhRs lacking DCrb, which display shorter stalks. The authors concluded that Yurt is a negative regulator of DCrb which is involved in the local control of apical membrane growth.
4.2.1.2. Zebrafish retina
The zebrafish Danio rerio model system is highly suitable for studies on retinal development for at least two reasons: their larvae have large eyes, which can be easily manipulated experimentally, and numerous mutants in which the neuronal pattern of the eye differs from that of normal zebrafish have been identified (104). In particular, mutations induced in the CRB2 ortholog named CRB2A (OME mutants) resulted in impairments affecting the epithelial shape of the retinal neuroepithelium , whereas apico-basal organization was maintained in brain neuroepithelium since both aPKC and PAR3 are correctly localized in OME (22, 105). Consistent with the OME neuroepithelial phenotype, CRB2A is localized in the retina just apically from the adherens junctions (AJs), which supports the idea that this protein is essential to the epithelial integrity of the retinal cells (22). These authors further established that CRB2B, an CRB2A paralog almost exclusively expressed in PhRs (in both the retina and the pineal gland), is a determinant of the apical surface in these cells. The CRB2B knockdown showed a dramatic reduction of the PhR inner segment size (22), a phenotype in zebrafish similar to that involving a decrease in the rhabdomeric membrane stalk observed in DCrb fly mutants (94). However, since PhR maturation was only slightly delayed in CRB2B-deficient animals, CRB2B may not play a general role in PhR differentiation. In addition, Omori and Malicki (22) reported that zebrafish CRB3A, another CRB3 paralog, is also required for the morphogenesis of the kinetocilium in ciliated cells of the ear.
On the other hand, nagie oko (NOK) mutants, defective in a homolog of the Drosophila Stardust and mammalian PALS1/MPP5, resulted in a severe disruption of the retinal architecture, which showed that NOK is essential to zebrafish eye integrity and retina cellular patterning (106, 107). The fact that NOK protein was found to be localized in the vicinity of junctional complexes in the retinal neuroepithelial sheet suggests that nok is required for the establishment and/or the maintenance of the OLM (outer limiting membrane) (107).
Lastly, it was established that three zebrafish CRB proteins (CRB1, CRB2A, CRB2B) can interact directly with zebrafish mosaic eyes (MOE), a FERM protein (fly Yurt ortholog), which co-immunoprecipitates with CRB2A and is colocalized apically with CRB proteins (58). Genetic analyses showed that MOE is required for normal retinal lamination and RPE (retinal pigmented epithelium) integrity, and that the apical domain is expanded in rod PhRs lacking MOE, which suggests that MOE negatively regulates CRB protein function, and especially the size of the rod PhRs (58). Interestingly, the embryonic phenotypes of MOE mutants are similar to both the phenotypes of NOK mutants and those of CRB2A morphants (morpholino knockdown mutants), opening the possibility of genetic interactions between these genes (58, 90).
4.2.1.3.Mammalian retina
In humans, mutations in the CRB1 gene were identified as a major cause of severe inherited retinal diseases, including Retinitis Pigmentosa type 12 (RP12), an early onset form of autosomal recessive RP characterized by a preserved para-arteriolar RPE, autosomal Leber congenital amaurosis (LCA), which is the earliest and most severe form of all inherited retinal dystrophies, RP with coats-like exudative vasculopathy, and Pigmented Paravenous Chorio-retinal Atrophy ( (13, 108-113) recently reviewed in (101)).
No pathological variants have been detected to date in the CRB2 and CRB3 genes of patients with RP12 or LCA (17). However, the CRB2 gene is highly polymorphic, unlike the CRB1 gene (109). It is thus possible that a more complex clinical phenotype may be associated with the loss or altered function of CRB2 or CRB3 in humans (101). The CRB2 sequence variants detected in RP12 and LCA patients (17) may be involved in digenic and polygenic diseases (101).
Human retinal degenerative diseases associated with CRB1 gene mutations can be partly mimicked by the CRB1rd8 and Crb1-/- mouse models (53, 85), since deficiencies in mouse CRB1 also cause retinal abnormalities (13, 41, 101, 108, 109). The rather thick appearance of the retina in the mouse CRB1 knockout correlates with the thick retinas of LCA patients, for instance (112). However, the retinal patterning defects occurring in the mouse are less severe than those observed in humans, although bright light also increases PhR death in CRB1-/- mice as in DCrb mutant retinas (96), without being required to induce retinal degeneration (53). Retinal disorganization, which is always most striking in older animals, has been observed in both mouse mutant models lacking CRB1 (53, 85). In mouse retina, CRB1 protein was found to be confined to the region of the ZA, referred to as the OLM, more specifically at a subapical region (SAR) located just above the AJs connecting PhRs to Müller glia cells (32, 53, 85, 92, 94) (Figure 4). As in Drosophila, the loss of CRB1, that plays a critical role in the maintenance of ZA, leads to defective junctional complexes, and consequently to the loss of retinal integrity in mouse mutant models (53, 85). Confocal and immunoelectron microscopy observations have shown that CRB1 is nearly restricted to the Müller glia cells, since this protein is barely expressed in PhRs, whereas CRB2, CRB3, PATJ and MUPP1 are all present in AJ-associated CRB complexes in both cell types (92). It was recently specified that loss of CRB1 in CRB1 -/- mouse retina results in irregularities in the number and size of the apical microvilli of Müller glia cells, which suggests that CRB1 is required to control the regular development of these microvilli (114).
In addition, the CRB complex localized at the OLM in the mammalian retina contains other proteins, including MAGUK members of the MPP-family, such as MPP5/PALS1, MPP4, MPP3, MPP1/p55 (56, 110-115) and FERM proteins, such as Epb4.1l5 and EHM2 (32, 58). As in Drosophila, Sdt ortholog PALS1/MPP5 is required in mouse retina for the correct localization of the CRB family members (CRB1, CRB2, and to some extent CRB3), MUPP1 and Veli3 (mLin7C/MALS-3) (92) at the SAR of the OLM.
However, PALS1 was found to be exclusively confined to the SAR of the OLM (92, 115) where PALS1 recruits MPP4 and MPP3 into the CRB1 complex (55, 116, 117), while MPP4, which is expressed in high levels in the retina (118, 119), was also detected in the PhR connecting cilia (120) and in the OPL (outer plexiform layer) at the PhR terminals (115, 117, 120, 121). MPP4, which has been found to interact directly with VELI isoforms in vitro (115), was found to be colocalized with both VELI3 and PSD95 at the PhR presynaptic terminals (117, 121), which suggests that MPP4 coordinates the assembly of these synaptic components and plays a role in organizing protein scaffolds at the PhR synapses. However, no retinal degeneration and only a slight PhR displacement not accompanied by any changes in the CRB complex at the OLM were detected in MPP4 knockout mice (117) and mutation analysis of the MPP4 gene in several patients showed the occurrence of no pathologic mutations (119), which suggests that MPP4 is not involved in either the function or the localization of the CRB complex. Regarding FERM proteins found in human and mouse retina, Laprise et al. (32) estblished that the Yurt orthologs Epb4.1l5 (also known as Lulu or YMO1 (89)) and EHM2 can bind to the three human CRB proteins. Hsu et al. (58) reported that Epb4.1l5 is coexpressed with CRB1 and CRB2 in PhRs and cells of the Inner Nuclear Layer, and Laprise et al. (32) estblished that the Epb4.1l5 protein is colocalized with CRB2 in the inner PhR segments of the adult mouse retina. The CRB complex therefore plays an important role in both the morphogenesis and the functional role of photoreceptors and the retina.
4.2.2. Kidney
4.2.2.1. Zebrafish kidney
Studies on zebrafish have confirmed that the CRB complex also regulates ciliogenesis. In CRB2B-deficient animals, apical surface defects are associated with the formation of pronephretic cysts and with short cilia showing defective motility (22). Interestingly, these defects in the cilia can be rescued by injecting CRB2B mRNA, indicating that CRB2B is a determinant of cilial length and motility in the zebrafish nephric epithelia. It is noteworthy that it is CRB3A rather than CRB2B that regulates the length of the hair cell kinocilia. This idea was supported by the shortening of the latter which occurred after knockdown of CRB3A gene and the fact that CRB3A protein accumulated in the basal bodies of these cilia (22).
On the other hand, most interestingly and unexpectedly, Omori and Malicki (22) also reported that the strength of the rescue process was similar with CRB2A, CRB2B, and CRB3A mRNA, which suggested that the CRB genes are at least partly functionally interchangeable in the zebrafish.
4.2.2.2 Mammalian kidney
Confocal analysis of adult rat kidney sections showed that CRB3 is present at the apical membrane domain, including the primary cilia (122). MALS-3, via its L27 domain, mediates stable assembly of the apical CRB complex and the basolateral SCRIB complex. These polarity complexes were disrupted in renal epithelia from MALS-3 knockout mice, especially those derived from metanephric mesenchyme (123). MALS-3 mutant mice have hypomorphic kidneys characterized by cysts and fibrosis and show defective polarization of proximal tubular epithelial cells, where the tight junctions lack the CRB complex. MALS-3, which interacts with the CRB and SCRIB polarity complexes, is therefore essential for correct polarization of renal epithelia and hence, for normal kidney development to occur.
5.CRUMBS COMPLEX: ROLE IN EPITHELIAL CELL CULTURE
In this part of this review, we will focus on CRB3A, as CRB3A is highly expressed in epithelial tissues (42, 43) and this isoform has been extensively studied in in vitro systems. In contrast, the role of the CRB3B splicing variant has been described only during ciliogenesis (18).
5.1. Cell cultures: model systems for studying the function of the CRB3A complex
Functional studies on the role of CRB3A in cell polarization have mainly focused on tissues such as retina or kidney. However, since it is difficult to obtain homogeneous populations of differentiated cells from these tissues, detailed analyzes of the functional contribution of the CRB3A complex in epithelial polarity in vivo are hard to address. This can be achieved, in cultured cell lines, and the cells most commonly used for this purpose are MDCK (Madin-Darby canine kidney) cells that derived from canine kidney and Caco2 cells derived from a human colon adenocarcinoma. Both of these cell lines are still able to polarize, build tight junctions and form a transporting epithelial monolayer (124-127).
Two different methods of growing these cell lines are commonly used to study epithelial cell polarity: in two-dimension with cells grown on a filter with a predetermined free apical surface, and in three-dimension the cells are grown in collagen or Matrigel matrix, with no forced axis of polarization. In the latter case, MDCK cells form cysts, which are polarized monolayers containing an enclosed lumen and encircled by a basement membrane, tight junctions being restricted to the apico-lateral membrane boundary. The ability of epithelial MDCK cells to mimick a tissue-like organization is useful because it helps to understand how interactions between extracellular factors and the intrinsic differentiation program contribute to specifying the architecture of the epithelial tissue (128). This is not the only advantage of growing cysts in vitro, since this system also seems to be more sensitive than the two-dimensional monolayer culture system as a means of detecting cell polarity defects. For instance, Lemmers et al (43) and Roh et al (45) have reported that overexpression of CRB3A in MDCK cells led to a more severe phenotype when these cells were grown in collagen (instead of developing into cysts, they formed multicellular aggregates with a loss of apico-basal polarity), whereas only apical surface expansion (no loss of apico-basal polarity) occurred when these cells were grown on filters.
However, three-dimensional cell culture method has limitations: for example, it is not amenable to forward genetic analyses or for large biochemical experiments, for which two-dimensional methods are much more suitable. In two-dimensional system, MDCK cells and Caco2 cells are grown on filters and form a monolayer with the basal surface in contact with the filters and the apical surface facing the medium.
An elegant study by Saaf et al (129) has shown that the temporal pattern of gene expression observed during Caco2 cell polarization in two-dimensional cell cultures showed similar molecular targets to those involved in the temporal and spatial in vivo gene expression program of enterocyte cell differentiation along the crypt-villus axis. It is worth noting that similar parallels can be drawn between proliferating unpolarized Caco2 cells and tumorogenic colonic cells on the one hand and between fully polarized differentiated Caco2 cells and healthy human colonic cells on the other hand. Several groups have therefore used this in vitro approach to dissect the molecular events linking CRB3 to apical epithelial morphogenesis.
5.2. Genetic approaches
5.2.1. CRB3 knockdown and overexpressing cells
In cell cultures, as in vivo, endogenous CRB3 localizes not only at tight junctions but also to the apical membrane and to the cilium (a specialized apical structure) (42, 43, 122). Fan et al. (18, 122), who studied the role of CRB3A/B in MDCK cells using the knockdown approach, did not observe any polarity defects apart from those involved in cilia morphogenesis. Knockdown of splice variants CRB3A and CRB3B led to the loss of cilia, which indicates that both CRB3 variants are required for ciliogenesis in MDCK cells (122, 130), and that the PDZ binding domain of CRB3A is necessary for this variant to be able to target the cilium. The fact that PAR6, which is known to interact directly with the CRB3A cytoplasmic domain (43), and PAR3 have also been detected in the primary cilia (122) suggests that the PAR complex may also be involved in ciliogenesis (122, 130). Since knockdown of PAR3 results in shorter cilia, the elongation/growth of cilia presumably requires PAR3 (130). The authors proposed a model in which PAR3, via PAR6, acts as a linker between the transmembrane protein CRB3A, which is essential for ciliogenesis, and the ciliary axoneme.
Like endogenous CRB3A, overexpressed CRB3A localized to the apical membrane, where it accumulates at the tight junctions along with PATJ, PALS1, ZO1 and occludin (42, 43). In MDCK cells, since overexpressed CRB3A and apical surface markers (gp135, BC11, BC44) were detected in both the apical and lateral regions, it is likely that Crb3A overexpression may result in apical membrane expansion (43, 45). Interestingly, the overall polarity of these cells was not affected. Several morphological studies using calcium-switch assay have shown that overexpression of CRB3A induces a delay in the establishment of tight junction markers (PATJ, ZO1) in a complete belt of tight junction staining (43, 45). Inulin leakage assay and transepithelial resistance measurements combined with calcium-switch methods showed the occurrence of a much slower but complete process of functional tight junction recovery. Since adherens junctions form normally under these conditions, these results suggest that CRB3A may play a specific but important role in the establishment of tight junctions and the apical membrane domain. This discrepancy between knockdown and overexpression phenotypes might be the consequence of redundant expression of CRB proteins in MDCK cells.
The results of biochemical analyses led to the conclusion that the phenotype of CRB3A overexpression in MDCK cells depends on the last four residues (ERLI), as overexpression of CRB3A missing ERLI motif has no effect on cyst and tight junction formation (43, 45), while correctly targeted to the apical membrane (43). These results strongly suggest that the defects observed may be mediated by potential interactions between PDZ proteins and the ERLI motif of CRB3A. Two partners have been reported to bind to the ERLI motif of CRB3A: PALS1 (26, 42) and PAR6 (43). It has been established that overexpression of PAR6 in MDCK cells delays the assembly of functional tight junctions (131), which show a very similar phenotype to that resulting from CRB3A overexpression. Thus, the two proteins act in the same pathway for the building of tight junctions.
5.2.2. PALS1 knockdown cells
In the case of PALS1, no overexpression experiments have been performed so far. Roh et al disrupted the endogeneous interaction between PALS1 and CRB3A using a dominant negative construct containing the PALS1 PDZ domain, which binds to CRB3A (45). The same group also used PALS1 different specific siRNAs to suppress PALS1 expression in MDCK cells (91, 132). In both cases, tight junction formation was delayed as occurred with the CRB3A overexpression phenotype. Additionally in one study (91) defects in adherens junction formation were observed in PALS1 down-regulated cells. Modification of PALS1 expression does not affect the level of CRB3A expression or CRB3A localization at the apical surface and tight junctions. By contrast, reduction of PALS1 expression levels resulted in a corresponding loss of PATJ, the third member of the CRB3A complex. All in all, these results indicate that the absence of PALS1, which acts as an adaptor between CRB3A and PATJ, is essential to the establishment and maintenance of tight junctions. Since PATJ becomes downregulated in PALS1 knockdown cells, it is difficult to attribute the defects observed to the impairment of PALS1 alone. Wang et al (91) have reported that when exogenous PALS1 was reintroduced, the normal levels of PATJ expression and the formation of tight junctions and adherens junction were restored. Interestingly, PALS1 N-terminal region (L27 and PDZ domains) or mutated in its SH3 domain rescue PATJ expression, but these two proteins were diffusely distributed in the cells, and no recovery of the formation of tight and adherens junctions was observed. One can hypothesize that the SH3 mutation of PALS1 or the deletion of the C-terminal domain may induce changes in the protein conformation, thus preventing its interaction with CRB3A. In this case, PALS1 and PATJ will not be recruited to the apical membrane; which will prevent junctions from being properly formed. The PALS1-L27N mutant, defective in binding PATJ, cannot rescue either PATJ re-expression or the formation of tight and adherens junctions in PALS1 knockdown cells. Interestingly, a previous study by Roh et al (26) showed that this L27N domain of PALS1 is necessary for PALS1 to be targeted to the tight junctions via its interaction with PATJ. Taken as a whole, these results suggest that PALS1, via its L27N domain, may contribute to the stabilization of PATJ (since unbound PATJ might be destabilized and then degraded); and that not only the interaction between PALS1 and PATJ but also their localization to the apical membrane and tight junction may be essential for the tight and adherens junctions to be properly formed.
5.2.3.PATJ knockdown cells
Several studies have focused on the role of PATJ during epithelial polarization. In non-polarized MDCK cells, PATJ was found to be localized in the vacuolar apical compartment and to target to the apical region and tight junctions during the process of polarization (133). Targeting to tight junction occurs via the interaction of the 6th domain of PATJ and ZO3 (51). Contrary to the outcome of PALS1 down-regulation, but in line with that of CRB3A overexpression, PATJ down-regulation affects only tight junction formation: the adherens junctions form normally (133, 134) and the levels of expression of PALS1 and CRB3A were not affected. It is not yet clear whether the functional integrity of tight junctions is affected by the loss of PATJ, since Michel et al (134) observed a normal transepithelial resistance, whereas Shin et al (133) did not. These phenotypic differences might point out tissue-specific defects: Michel et al (134) worked with Caco2 cells, which are derived from human colon while Shin et al (133) worked with MDCK that derived from canine kidney. In both studies, mislocalization of PALS1 was observed, as in previous studies by Roh et al (51) and Hurd et al (135) (using a PATJ dominant negative) in which PATJ was found to be responsible for restricting PALS1 to the tight junctions. Michel et al (134) also observed that loss of PATJ induced occludin and ZO3 leakage along the lateral membrane and an accumulation of CRB3A to the early endosomal network. It is therefore tempting to speculate that one of the functions of PATJ in Caco2 cells might consist in restricting or stabilizing CRB3A, PALS1, occludin and ZO3 to the tight junctions. All the data available on the CRB3A/PALS1/PATJ complex therefore support the idea that this complex is involved in tight junction formation.
5.3.CRB3A induces de novo tight junction formation
Fogg et al (44) used MCF10A cell line, derived from mammary epithelium, which lacks tight junctions and shows very low levels of endogeneous CRB3A expression. In these cells, endogeneous ZO1, occludin, claudin1, PALS1 and PATJ yielded a diffuse cytoplasmic staining pattern. In response to mycCRB3A overexpression, PALS1 and PATJ were recruited to tight junctions and were co-stained with ZO1, occludin and claudin1, and all these proteins formed a continuous belt-like pattern around the cell. The ability of CRB3A to induce tight junction formation was lost when the protein carried ERLI or FERM-binding motif mutations. These data suggest that CRB3A may act as an anchor to recruit the proteins that together are able to build tight junctions.
6.PERSPECTIVES FOR CRB FUNCTION
6.1. CRB3A is linked to actin-binding proteins
As described above, PALS1 and PAR6 can bind to the last amino acid ERLI of CRB3A (43, 45) and are known to play an important role during cell polarization. In a recent study, a FERM protein, Epb4.1l5, was newly identified as a partner of the mammalian CRB3A complex (57). Epb4.1l5 is the first mammalian FERM protein that is known to bind to the FERM domain of CRB3 and to the hook domain of PALS1. Interestingly, overexpression of Epb4.1l5 in MDCK cells leads to defects in the tightness and formation of tight junctions indicating that similarly to CRB3A overexpression, this protein contributes importantly to cell organization. It is worth noting that the proteins belonging to the FERM-domain family serve as adaptors linking transmembrane proteins to the cortical actin cytoskeleton (136). Interestingly, in Epb4.1l5 knockout mice, Lee et al (89) observed an apical redistribution of F-actin with ectopic F-actin foci, and the overexpression of Epb4.1l5 in HelaS cells resulted in actin cytoskeleton reorganization. On the other hand PAR6 and PALS1, which are both partners of CRB3A, seem to be also linked to actin cytoskeleton. PAR6 is known to interact with cytoskeleton-related proteins, such as 14.3.3 protein (137) and Cdc42 (138), and the pattern of F-actin staining obtained was more diffuse in PALS1-downregulated cells (91). All these data strongly suggest that Epb4.1l5, PAR6 and PALS1 may link CRB3A to actin cytoskeleton. In addition, recruitment and reorganization of cytoskeletal elements through these proteins may contribute to tight junction formation. More studies on the role of the Crumbs complex in regulating cortical actin cytoskeleton and junction remodelling should be performed in the future, both in flies and mammals.
6.2. CRB3A and trafficking
Exocytosis and endocytosis are thought regulate the formation of the apical membrane and tight junctions. It has been reported that the exocyst is localized to the tight junctions in polarized MDCK cells (139, 140) and that it mediates the targeting of intracellular vesicles to specific sites on the plasma membrane (141). Wang et al (91) have observed that Sec8, an exocyst marker, was co-localized with PALS1, and that upon PALS1 downregulation, the exocyst was mislocalized, while newly synthesized E-cadherin accumulated in punctate structures in cell periphery. These authors hypothezised that in PALS1 knockdown cells, the E-cadherin exocytic vesicles may be correctly sorted to the cell surface from the recycling endosomes, but cannot be targeted or fused with the plasma membrane. One can speculate that the formation and maintenance of tight junctions regulated by the CRB complex requires exo- and endocytic trafficking. This idea was supported by the finding that in Drosophila mutants such as avalanche, a Syntaxin encoding gene, mislocalization of DCrb and epithelial defects are observed (for review, see (142)). Further studies are therefore now required on the links between intracellular trafficking and Crumbs complex function in order to determine the exact mechanisms underlying their interdependence.
6.3.CRB complex and cell proliferation
The CRB complex is also connected to several signaling pathways whereby it might control cell growth and proliferation. For example, PATJ binds to TSC2, a key component of cell homeostasis (59). TSC2 exhibits a GAP activity domain for the small G protein Rheb (143-145), but this enzymatic activity requires the interaction with TSC1. The TSC1/TSC2 complex has been shown to be a negative regulator of mTORC1 (Target of Rapamycin), which in turn controls cell growth and apoptosis. This data led to the hypothesis that once tight junctions have been established, a signal is sent to the mTOR pathway via PATJ and TSC, to down-regulate epithelial proliferation thus preventing over-proliferation and tumor development. Thus through the intracellular proteins binding to Crumbs cytoplasmic domain, emerges a new picture of how epithelial cell homeostasis is regulated, based on the links between apico-lateral junctions and the cell cycle and survival. While these connections between members of the Crumbs complex and the TOR pathway have still to be uncovered in vivo (TSC2 in flies, for example does not exhibit a typical PDZ-binding motif able to interact with DPatj, to our knowledge this potential interaction has not been tested so far); these new findings have interesting potential applications in the field of cancer (this is already the case for PAR4/LKB1 that both control polarity and cell metabolism (146). It seems likely that increasing numbers of links between polarity complexes and cell proliferation will soon be discovered, leading to a more comprehensive picture of epithelial cell biology.
7. ACKNOWLEGMENTS
EB and EA equally contributed to this article.We would like to thank Giovanna Mottola for stimulating discussions and for reading the manuscript. We also thank Kathy Su and Nicolas Gompel (IBDML) for their help with the phylogenic analyses. This work was supported by the CNRS (UMR 6216), ANR Neuroscience DYOR and ANR n�BLAN07-2_186738. Le Bivic's group is an "Equipe labellisee de la Ligue Nationale contre le Cancer". E. Bazellieres and E. Assemat are recipients of fellowships awarded by the French Ministery of Education and Research and by Fondation de France and Ligue Nationale contre le Cancer, respectively.
8. REFERENCES
1. Brennwald, P. & G. Rossi: Spatial regulation of exocytosis and cell polarity: yeast as a model for animal cells. FEBS Lett, 581, 2119-24(2007)
doi:10.1016/j.febslet.2007.03.043
2. Wang, Q. & B. Margolis: Apical junctional complexes and cell polarity. Kidney Int, 72, 1448-58(2007)
doi:10.1038/sj.ki.5002579
3.Kemphues, K. J., J. R. Priess, D. G. Morton & N. S. Cheng: Identification of genes required for cytoplasmic localization in early C. elegans embryos. Cell, 52, 311-20(1988)
doi:10.1016/S0092-8674(88)80024-2
4.Dow, L. E. & P. O. Humbert: Polarity regulators and the control of epithelial architecture, cell migration, and tumorigenesis. Int Rev Cytol, 262, 253-302(2007)
doi:10.1016/S0074-7696(07)62006-3
5.Muller, H. A. & O. Bossinger: Molecular networks controlling epithelial cell polarity in development. Mech Dev, 120, 1231-56(2003)
doi:10.1016/j.mod.2003.06.001
6.Tepass, U. & E. Knust: Crumbs and stardust act in a genetic pathway that controls the organization of epithelia in Drosophila melanogaster. Dev Biol, 159, 311-26(1993)
doi:10.1006/dbio.1993.1243
7. Tepass, U., C. Theres & E. Knust: crumbs encodes an EGF-like protein expressed on apical membranes of Drosophila epithelial cells and required for organization of epithelia. Cell, 61, 787-99(1990)
doi:10.1016/0092-8674(90)90189-L
8. Jürgens, G., E. Wieschaus, C. Nüsslein-Volhard & H. Kluding: Mutations affecting the pattern of the larval cuticle in Drosophila melanogaster. Development Genes and Evolution 193, 283-295 (1984)
9. Wodarz, A., F. Grawe & E. Knust: CRUMBS is involved in the control of apical protein targeting during Drosophila epithelial development. Mech Dev, 44, 175-87(1993)
doi:10.1016/0925-4773(93)90066-7
10. Wodarz, A., U. Hinz, M. Engelbert & E. Knust: Expression of crumbs confers apical character on plasma membrane domains of ectodermal epithelia of Drosophila. Cell, 82, 67-76(1995)
doi:10.1016/0092-8674(95)90053-5
11. Klebes, A. & E. Knust: A conserved motif in Crumbs is required for E-cadherin localisation and zonula adherens formation in Drosophila. Curr Biol, 10, 76-85(2000)
doi:10.1016/S0960-9822(99)00277-8
12. Sotillos, S., M. T. Diaz-Meco, E. Caminero, J. Moscat & S. Campuzano: DaPKC-dependent phosphorylation of Crumbs is required for epithelial cell polarity in Drosophila. J Cell Biol, 166, 549-57(2004)
doi:10.1083/jcb.200311031
13.den Hollander, A. I., J. B. ten Brink, Y. J. de Kok, S. van Soest, L. I. van den Born, M. A. van Driel, D. J. van de Pol, A. M. Payne, S. S. Bhattacharya, U. Kellner, C. B. Hoyng, A. Westerveld, H. G. Brunner, E. M. Bleeker-Wagemakers, A. F. Deutman, J. R. Heckenlively, F. P. Cremers & A. A. Bergen: Mutations in a human homologue of Drosophila crumbs cause retinitis pigmentosa (RP12). Nat Genet, 23, 217-21(1999)
doi:10.1038/13848
14.Lemmers, C., E. Medina, M. H. Delgrossi, D. Michel, J. P. Arsanto & A. Le Bivic: hINADl/PATJ, a homolog of discs lost, interacts with crumbs and localizes to tight junctions in human epithelial cells. J Biol Chem, 277, 25408-15(2002)
doi:10.1074/jbc.M202196200
15.Watanabe, T., S. Miyatani, I. Katoh, S. Kobayashi & Y. Ikawa: Expression of a novel secretory form (Crb1s) of mouse Crumbs homologue Crb1 in skin development. Biochem Biophys Res Commun, 313, 263-70(2004)
doi:10.1016/j.bbrc.2003.11.122
16.Katoh, M. & M. Katoh: Identification and characterization of Crumbs homolog 2 gene at human chromosome 9q33.3.Int J Oncol, 24, 743-9(2004)
17. van den Hurk, J. A., P. Rashbass, R. Roepman, J. Davis, K. E. Voesenek, M. L. Arends, M. N. Zonneveld, M. H. van Roekel, K. Cameron, K. Rohrschneider, J. R. Heckenlively, R. K. Koenekoop, C. B. Hoyng, F. P. Cremers & A. I. den Hollander: Characterization of the Crumbs homolog 2 (CRB2) gene and analysis of its role in retinitis pigmentosa and Leber congenital amaurosis. Mol Vis, 11, 263-73(2005)
18. Fan, S., V. Fogg, Q. Wang, X. W. Chen, C. J. Liu & B. Margolis: A novel Crumbs3 isoform regulates cell division and ciliogenesis via importin beta interactions. J Cell Biol, 178, 387-98(2007)
doi:10.1083/jcb.200609096
19. Hung, A. Y. & M. Sheng: PDZ domains: structural modules for protein complex assembly. J Biol Chem, 277, 5699-702(2002)
doi:10.1074/jbc.R100065200
20. den Hollander, A. I., M. Ghiani, Y. J. de Kok, J. Wijnholds, A. Ballabio, F. P. Cremers & V. Broccoli: Isolation of Crb1, a mouse homologue of Drosophila crumbs, and analysis of its expression pattern in eye and brain. Mech Dev, 110, 203-7(2002)
doi:10.1016/S0925-4773(01)00568-8
21. Bossinger, O., A. Klebes, C. Segbert, C. Theres & E. Knust: Zonula adherens formation in Caenorhabditis elegans requires dlg-1, the homologue of the Drosophila gene discs large. Dev Biol, 230, 29-42(2001)
doi:10.1006/dbio.2000.0113
22. Omori, Y. & J. Malicki: oko meduzy and related crumbs genes are determinants of apical cell features in the vertebrate embryo. Curr Biol, 16, 945-57(2006)
doi:10.1016/j.cub.2006.03.058
23.Bachmann, A., M. Schneider, E. Theilenberg, F. Grawe & E. Knust: Drosophila Stardust is a partner of Crumbs in the control of epithelial cell polarity. Nature, 414, 638-43(2001)
doi:10.1038/414638a
24.Hong, Y., B. Stronach, N. Perrimon, L. Y. Jan & Y. N. Jan: Drosophila Stardust interacts with Crumbs to control polarity of epithelia but not neuroblasts. Nature, 414, 634-8(2001)
doi:10.1038/414634a
25.Bhat, M. A., S. Izaddoost, Y. Lu, K. O. Cho, K. W. Choi & H. J. Bellen: Discs Lost, a novel multi-PDZ domain protein, establishes and maintains epithelial polarity. Cell, 96, 833-45(1999)
doi:10.1016/S0092-8674(00)80593-0
26.Roh, M. H., O. Makarova, C. J. Liu, K. Shin, S. Lee, S. Laurinec, M. Goyal, R. Wiggins & B. Margolis: The Maguk protein, Pals1, functions as an adapter, linking mammalian homologues of Crumbs and Discs Lost. J Cell Biol, 157, 161-72(2002)
doi:10.1083/jcb.200109010
27. Kempkens, O., E. Medina, G. Fernandez-Ballester, S. Ozuyaman, A. Le Bivic, L. Serrano & E. Knust: Computer modelling in combination with in vitro studies reveals similar binding affinities of Drosophila Crumbs for the PDZ domains of Stardust and DmPar-6.Eur J Cell Biol, 85, 753-67(2006)
doi:10.1016/j.ejcb.2006.03.003
28. Bose, R. & J. L. Wrana: Regulation of Par6 by extracellular signals. Curr Opin Cell Biol, 18, 206-12(2006)
doi:10.1016/j.ceb.2006.02.005
29. Kaltschmidt, J. A. & A. H. Brand: Asymmetric cell division: microtubule dynamics and spindle asymmetry. J Cell Sci, 115, 2257-64(2002)
30. Suzuki, A. & S. Ohno: The PAR-aPKC system: lessons in polarity. J Cell Sci, 119, 979-87(2006)
doi:10.1242/jcs.02898
31. Medina, E., J. Williams, E. Klipfell, D. Zarnescu, G. Thomas & A. Le Bivic: Crumbs interacts with moesin and beta(Heavy)-spectrin in the apical membrane skeleton of Drosophila. J Cell Biol, 158, 941-51(2002)
doi:10.1083/jcb.200203080
32. Laprise, P., S. Beronja, N. F. Silva-Gagliardi, M. Pellikka, A. M. Jensen, C. J. McGlade & U. Tepass: The FERM protein Yurt is a negative regulatory component of the Crumbs complex that controls epithelial polarity and apical membrane size. Dev Cell, 11, 363-74(2006)
doi:10.1016/j.devcel.2006.06.001
33.Medina, E., C. Lemmers, L. Lane-Guermonprez & A. Le Bivic: Role of the Crumbs complex in the regulation of junction formation in Drosophila and mammalian epithelial cells. Biol Cell, 94, 305-13(2002)
doi:10.1016/S0248-4900(02)00004-7
34.Bachmann, A., M. Timmer, J. Sierralta, G. Pietrini, E. D. Gundelfinger, E. Knust & U. Thomas: Cell type-specific recruitment of Drosophila Lin-7 to distinct MAGUK-based protein complexes defines novel roles for Sdt and Dlg-S97. J Cell Sci, 117, 1899-909(2004)
doi:10.1242/jcs.01029
35.Bachmann, A., F. Grawe, K. Johnson & E. Knust: Drosophila Lin-7 is a component of the Crumbs complex in epithelia and photoreceptor cells and prevents light-induced retinal degeneration. Eur J Cell Biol, 87, 123-136(2008)
doi:10.1016/j.ejcb.2007.11.002
36.Wang, Q., T. W. Hurd & B. Margolis: Tight junction protein Par6 interacts with an evolutionarily conserved region in the amino terminus of PALS1/stardust. J Biol Chem, 279, 30715-21(2004)
doi:10.1074/jbc.M401930200
37. Nam, S. C. & K. W. Choi: Interaction of Par-6 and Crumbs complexes is essential for photoreceptor morphogenesis in Drosophila. Development, 130, 4363-72(2003)
doi:10.1242/dev.00648
38. Djiane, A., S. Yogev & M. Mlodzik: The apical determinants aPKC and dPatj regulate Frizzled-dependent planar cell polarity in the Drosophila eye. Cell, 121, 621-31(2005)
doi:10.1016/j.cell.2005.03.014
39. Schulte, G. & V. Bryja: The Frizzled family of unconventional G-protein-coupled receptors. Trends Pharmacol Sci, 28, 518-25(2007)
doi:10.1016/j.tips.2007.09.001
40. Assemat, E., E. Bazellieres, E. Pallesi-Pocachard, A. Le Bivic & D. Massey-Harroche: Polarity complex proteins. Biochim Biophys Acta, 1778, 614-30(2008)
doi:10.1016/j.bbamem.2007.08.029
41. den Hollander, A. I., K. Johnson, Y. J. de Kok, A. Klebes, H. G. Brunner, E. Knust & F. P. Cremers: CRB1 has a cytoplasmic domain that is functionally conserved between human and Drosophila. Hum Mol Genet, 10, 2767-73(2001)
doi:10.1093/hmg/10.24.2767
42. Makarova, O., M. H. Roh, C. J. Liu, S. Laurinec & B. Margolis: Mammalian Crumbs3 is a small transmembrane protein linked to protein associated with Lin-7 (Pals1). Gene, 302, 21-9(2003)
doi:10.1016/S0378111902010843
43.Lemmers, C., D. Michel, L. Lane-Guermonprez, M. H. Delgrossi, E. Medina, J. P. Arsanto & A. Le Bivic: CRB3 binds directly to Par6 and regulates the morphogenesis of the tight junctions in mammalian epithelial cells. Mol Biol Cell, 15, 1324-33(2004)
doi:10.1091/mbc.E03-04-0235
44.Fogg, V. C., C. J. Liu & B. Margolis: Multiple regions of Crumbs3 are required for tight junction formation in MCF10A cells. J Cell Sci, 118, 2859-69(2005)
doi:10.1242/jcs.02412
45.Roh, M. H., S. Fan, C. J. Liu & B. Margolis: The Crumbs3-Pals1 complex participates in the establishment of polarity in mammalian epithelial cells. J Cell Sci, 116, 2895-906(2003)
doi:10.1242/jcs.00500
46.Kamberov, E., O. Makarova, M. Roh, A. Liu, D. Karnak, S. Straight & B. Margolis: Molecular cloning and characterization of Pals, proteins associated with mLin-7. J Biol Chem, 275, 11425-31(2000)
doi:10.1074/jbc.275.15.11425
47. Li, Y., D. Karnak, B. Demeler, B. Margolis & A. Lavie: Structural basis for L27 domain-mediated assembly of signaling and cell polarity complexes. Embo J, 23, 2723-33(2004)
doi:10.1038/sj.emboj.7600294
48. Feng, W., J. F. Long & M. Zhang: A unified assembly mode revealed by the structures of tetrameric L27 domain complexes formed by mLin-2/mLin-7 and Patj/Pals1 scaffold proteins. Proc Natl Acad Sci U S A, 102, 6861-6(2005)
doi:10.1073/pnas.0409346102
49. Feng, W., J. F. Long, J. S. Fan, T. Suetake & M. Zhang: The tetrameric L27 domain complex as an organization platform for supramolecular assemblies. Nat Struct Mol Biol, 11, 475-80(2004)
doi:10.1038/nsmb751
50. Philipp, S. & V. Flockerzi: Molecular characterization of a novel human PDZ domain protein with homology to INAD from Drosophila melanogaster. FEBS Lett, 413, 243-8(1997)
doi:10.1016/S0014-5793(97)00877-6
51. Roh, M. H., C. J. Liu, S. Laurinec & B. Margolis: The carboxyl terminus of zona occludens-3 binds and recruits a mammalian homologue of discs lost to tight junctions. J Biol Chem, 277, 27501-9(2002)
doi:10.1074/jbc.M201177200
52. Ullmer, C., K. Schmuck, A. Figge & H. Lubbert: Cloning and characterization of MUPP1, a novel PDZ domain protein. FEBS Lett, 424, 63-8(1998)
doi:10.1016/S0014-5793(98)00141-0
53.van de Pavert, S. A., A. Kantardzhieva, A. Malysheva, J. Meuleman, I. Versteeg, C. Levelt, J. Klooster, S. Geiger, M. W. Seeliger, P. Rashbass, A. Le Bivic & J. Wijnholds: Crumbs homologue 1 is required for maintenance of photoreceptor cell polarization and adhesion during light exposure. J Cell Sci, 117, 4169-77(2004)
doi:10.1242/jcs.01301
54.Straight, S. W., J. N. Pieczynski, E. L. Whiteman, C. J. Liu & B. Margolis: Mammalian lin-7 stabilizes polarity protein complexes. J Biol Chem, 281, 37738-47(2006)
doi:10.1074/jbc.M607059200
55.Kantardzhieva, A., I. Gosens, S. Alexeeva, I. M. Punte, I. Versteeg, E. Krieger, C. A. Neefjes-Mol, A. I. den Hollander, S. J. Letteboer, J. Klooster, F. P. Cremers, R. Roepman & J. Wijnholds: MPP5 recruits MPP4 to the CRB1 complex in photoreceptors. Invest Ophthalmol Vis Sci, 46, 2192-201(2005)
doi:10.1167/iovs.04-1417
56.McGee, A. W., S. R. Dakoji, O. Olsen, D. S. Bredt, W. A. Lim & K. E. Prehoda: Structure of the SH3-guanylate kinase module from PSD-95 suggests a mechanism for regulated assembly of MAGUK scaffolding proteins. Mol Cell, 8, 1291-301(2001)
doi:10.1016/S1097-2765(01)00411-7
57. Gosens, I., A. Sessa, A. I. den Hollander, S. J. Letteboer, V. Belloni, M. L. Arends, A. Le Bivic, F. P. Cremers, V. Broccoli & R. Roepman: FERM protein EPB41L5 is a novel member of the mammalian CRB-MPP5 polarity complex. Exp Cell Res, 313, 3959-70(2007)
doi:10.1016/j.yexcr.2007.08.025
58. Hsu, Y. C., J. J. Willoughby, A. K. Christensen & A. M. Jensen: Mosaic Eyes is a novel component of the Crumbs complex and negatively regulates photoreceptor apical size. Development, 133, 4849-59(2006)
doi:10.1242/dev.02685
59. Massey-Harroche, D., M. H. Delgrossi, L. Lane-Guermonprez, J. P. Arsanto, J. P. Borg, M. Billaud & A. Le Bivic: Evidence for a molecular link between the tuberous sclerosis complex and the Crumbs complex. Hum Mol Genet, 16, 529-36(2007)
doi:10.1093/hmg/ddl485
60. Yang, Q. & K. L. Guan: Expanding mTOR signaling. Cell Res, 17, 666-81(2007)
doi:10.1038/cr.2007.64
61. Shaw, R. J. & L. C. Cantley: Ras, PI(3)K and mTOR signalling controls tumour cell growth. Nature, 441, 424-30(2006)
doi:10.1038/nature04869
62. De Virgilio, C. & R. Loewith: The TOR signalling network from yeast to man. Int J Biochem Cell Biol, 38, 1476-81(2006)
doi:10.1016/j.biocel.2006.02.013
63.Martin, D. E. & M. N. Hall: The expanding TOR signaling network. Curr Opin Cell Biol, 17, 158-66(2005)
doi:10.1016/j.ceb.2005.02.008
64.Kwiatkowski, D. J. & B. D. Manning: Tuberous sclerosis: a GAP at the crossroads of multiple signaling pathways. Hum Mol Genet, 14, R251-8(2005)
doi:10.1093/hmg/ddi260
65.Sugihara-Mizuno, Y., M. Adachi, Y. Kobayashi, Y. Hamazaki, M. Nishimura, T. Imai, M. Furuse & S. Tsukita: Molecular characterization of angiomotin/JEAP family proteins: interaction with MUPP1/Patj and their endogenous properties. Genes Cells, 12, 473-86(2007)
doi:10.1111/j.1365-2443.2007.01066.x
66.Troyanovsky, B., T. Levchenko, G. Mansson, O. Matvijenko & L. Holmgren: Angiomotin: an angiostatin binding protein that regulates endothelial cell migration and tube formation. J Cell Biol, 152, 1247-54(2001)
doi:10.1083/jcb.152.6.1247
67. Bratt, A., W. J. Wilson, B. Troyanovsky, K. Aase, R. Kessler, E. G. Van Meir & L. Holmgren: Angiomotin belongs to a novel protein family with conserved coiled-coil and PDZ binding domains. Gene, 298, 69-77(2002)
doi:10.1016/S0378-1119(02)00928-9
68. Patrie, K. M.: Identification and characterization of a novel tight junction-associated family of proteins that interacts with a WW domain of MAGI-1. Biochim Biophys Acta, 1745, 131-44(2005)
doi:10.1016/j.bbamcr.2005.05.011
69. Wells, C. D., J. P. Fawcett, A. Traweger, Y. Yamanaka, M. Goudreault, K. Elder, S. Kulkarni, G. Gish, C. Virag, C. Lim, K. Colwill, A. Starostine, P. Metalnikov & T. Pawson: A Rich1/Amot complex regulates the Cdc42 GTPase and apical-polarity proteins in epithelial cells. Cell, 125, 535-48(2006)
doi:10.1016/j.cell.2006.02.045
70. Hamazaki, Y., M. Itoh, H. Sasaki, M. Furuse & S. Tsukita: Multi-PDZ domain protein 1 (MUPP1) is concentrated at tight junctions through its possible interaction with claudin-1 and junctional adhesion molecule. J Biol Chem, 277, 455-61(2002)
doi:10.1074/jbc.M109005200
71. Haskins, J., L. Gu, E. S. Wittchen, J. Hibbard & B. R. Stevenson: ZO-3, a novel member of the MAGUK protein family found at the tight junction, interacts with ZO-1 and occludin. J Cell Biol, 141, 199-208(1998)
doi:10.1083/jcb.141.1.199
72. Peng, B. H., M. A. White, G. A. Campbell, J. J. Robert, J. C. Lee & R. B. Sutton: Crystallization and preliminary X-ray diffraction of the ZO-binding domain of human occludin. Acta Crystallogr Sect F Struct Biol Cryst Commun, 61, 369-71(2005)
doi:10.1107/S1744309105007475
73.Mazumdar, A. & M. Mazumdar: How one becomes many: blastoderm cellularization in Drosophila melanogaster. Bioessays, 24, 1012-22(2002)
doi:10.1002/bies.10184
74.Tepass, U., G. Tanentzapf, R. Ward & R. Fehon: Epithelial cell polarity and cell junctions in Drosophila. Annu Rev Genet, 35, 747-84(2001)
doi:10.1146/annurev.genet.35.102401.091415
75.Lecuit, T.: Junctions and vesicular trafficking during Drosophila cellularization. J Cell Sci, 117, 3427-33(2004)
doi:10.1242/jcs.01312
76.Nam, S. C. & K. W. Choi: Domain-specific early and late function of Dpatj in Drosophila photoreceptor cells. Dev Dyn, 235, 1501-7(2006)
doi:10.1002/dvdy.20726
77. Hutterer, A., J. Betschinger, M. Petronczki & J. A. Knoblich: Sequential roles of Cdc42, Par-6, aPKC, and Lgl in the establishment of epithelial polarity during Drosophila embryogenesis. Dev Cell, 6, 845-54(2004)
doi:10.1016/j.devcel.2004.05.003
78. Li, Z., L. Wang, T. S. Hays & Y. Cai: Dynein-mediated apical localization of crumbs transcripts is required for Crumbs activity in epithelial polarity. J Cell Biol, 180, 31-8(2008)
doi:10.1083/jcb.200707007
79. Horne-Badovinac, S. & D. Bilder: Dynein Regulates Epithelial Polarity and the Apical Localization of stardust A mRNA. PLoS Genet, 4, e8(2008)
doi:10.1371/journal.pgen.0040008
80. Grawe, F., A. Wodarz, B. Lee, E. Knust & H. Skaer: The Drosophila genes crumbs and stardust are involved in the biogenesis of adherens junctions. Development, 122, 951-9(1996)
81. Margolis, B. & J. P. Borg: Apicobasal polarity complexes. J Cell Sci, 118, 5157-9(2005)
doi:10.1242/jcs.02597
82. Bilder, D., M. Schober & N. Perrimon: Integrated activity of PDZ protein complexes regulates epithelial polarity. Nat Cell Biol, 5, 53-8(2003)
doi:10.1038/ncb897
83.Malicki, J.: Harnessing the power of forward genetics--analysis of neuronal diversity and patterning in the zebrafish retina. Trends Neurosci, 23, 531-41(2000)
doi:10.1016/S0166-2236(00)01655-6
84.Mehalow, A. K., S. Kameya, R. S. Smith, N. L. Hawes, J. M. Denegre, J. A. Young, L. Bechtold, N. B. Haider, U. Tepass, J. R. Heckenlively, B. Chang, J. K. Naggert & P. M. Nishina: CRB1 is essential for external limiting membrane integrity and photoreceptor morphogenesis in the mammalian retina. Hum Mol Genet, 12, 2179-89(2003)
doi:10.1093/hmg/ddg232
85.Chalmers, A. D., M. Pambos, J. Mason, S. Lang, C. Wylie & N. Papalopulu: aPKC, Crumbs3 and Lgl2 control apicobasal polarity in early vertebrate development. Development, 132, 977-86(2005)
doi:10.1242/dev.01645
86.Fleming, T. P. & M. J. Hay: Tissue-specific control of expression of the tight junction polypeptide ZO-1 in the mouse early embryo. Development, 113, 295-304(1991)
87. Sheth, B., B. Moran, J. M. Anderson & T. P. Fleming: Post-translational control of occludin membrane assembly in mouse trophectoderm: a mechanism to regulate timing of tight junction biogenesis and blastocyst formation. Development, 127, 831-40(2000)
88. Lee, J. D., N. F. Silva-Gagliardi, U. Tepass, C. J. McGlade & K. V. Anderson: The FERM protein Epb4.1l5 is required for organization of the neural plate and for the epithelial-mesenchymal transition at the primitive streak of the mouse embryo. Development, 134, 2007-16(2007)
doi:10.1242/dev.000885
89. Jensen, A. M. & M. Westerfield: Zebrafish mosaic eyes is a novel FERM protein required for retinal lamination and retinal pigmented epithelial tight junction formation. Curr Biol, 14, 711-7(2004)
doi:10.1016/j.cub.2004.04.006
90. Wang, Q., X. W. Chen & B. Margolis: PALS1 regulates E-cadherin trafficking in mammalian epithelial cells. Mol Biol Cell, 18, 874-85(2007)
doi:10.1091/mbc.E06-07-0651
91. van Rossum, A. G., W. M. Aartsen, J. Meuleman, J. Klooster, A. Malysheva, I. Versteeg, J. P. Arsanto, A. Le Bivic & J. Wijnholds: Pals1/Mpp5 is required for correct localization of Crb1 at the subapical region in polarized Muller glia cells. Hum Mol Genet, 15, 2659-72(2006)
doi:10.1093/hmg/ddl194
92. Afonso, C. & D. Henrique: PAR3 acts as a molecular organizer to define the apical domain of chick neuroepithelial cells. J Cell Sci, 119, 4293-304(2006)
doi:10.1242/jcs.03170
93.Pellikka, M., G. Tanentzapf, M. Pinto, C. Smith, C. J. McGlade, D. F. Ready & U. Tepass: Crumbs, the Drosophila homologue of human CRB1/RP12, is essential for photoreceptor morphogenesis. Nature, 416, 143-9(2002)
doi:10.1038/nature721
94.Izaddoost, S., S. C. Nam, M. A. Bhat, H. J. Bellen & K. W. Choi: Drosophila Crumbs is a positional cue in photoreceptor adherens junctions and rhabdomeres. Nature, 416, 178-83(2002)
doi:10.1038/nature720
95.Johnson, K., F. Grawe, N. Grzeschik & E. Knust: Drosophila crumbs is required to inhibit light-induced photoreceptor degeneration. Curr Biol, 12, 1675-80(2002)
doi:10.1016/S0960-9822(02)01180-6
96.Hong, Y., L. Ackerman, L. Y. Jan & Y. N. Jan: Distinct roles of Bazooka and Stardust in the specification of Drosophila photoreceptor membrane architecture. Proc Natl Acad Sci U S A, 100, 12712-7(2003)
doi:10.1073/pnas.2135347100
97. Richard, M., F. Grawe & E. Knust: DPATJ plays a role in retinal morphogenesis and protects against light-dependent degeneration of photoreceptor cells in the Drosophila eye. Dev Dyn, 235, 895-907(2006)
doi:10.1002/dvdy.20595
98. Tepass, U. & K. P. Harris: Adherens junctions in Drosophila retinal morphogenesis. Trends Cell Biol, 17, 26-35(2007)
doi:10.1016/j.tcb.2006.11.006
99. Berger, S., N. A. Bulgakova, F. Grawe, K. Johnson & E. Knust: Unraveling the genetic complexity of Drosophila stardust during photoreceptor morphogenesis and prevention of light-induced degeneration. Genetics, 176, 2189-200(2007)
doi:10.1534/genetics.107.071449
100. Richard, M., R. Roepman, W. M. Aartsen, A. G. van Rossum, A. I. den Hollander, E. Knust, J. Wijnholds & F. P. Cremers: Towards understanding CRUMBS function in retinal dystrophies. Hum Mol Genet, 15, R235-43(2006)
doi:10.1093/hmg/ddl195
101. Schneeberger, D. & T. Raabe: Mbt, a Drosophila PAK protein, combines with Cdc42 to regulate photoreceptor cell morphogenesis. Development, 130, 427-37(2003)
doi:10.1242/dev.00248
102. Karagiosis, S. A. & D. F. Ready: Moesin contributes an essential structural role in Drosophila photoreceptor morphogenesis. Development, 131, 725-32(2004)
doi:10.1242/dev.00976
103.Malicki, J., S. C. Neuhauss, A. F. Schier, L. Solnica-Krezel, D. L. Stemple, D. Y. Stainier, S. Abdelilah, F. Zwartkruis, Z. Rangini & W. Driever: Mutations affecting development of the zebrafish retina. Development, 123, 263-73(1996)
104.Malicki, J. & W. Driever: oko meduzy mutations affect neuronal patterning in the zebrafish retina and reveal cell-cell interactions of the retinal neuroepithelial sheet. Development, 126, 1235-46(1999)
105.Wei, X. & J. Malicki: nagie oko, encoding a MAGUK-family protein, is essential for cellular patterning of the retina. Nat Genet, 31, 150-7(2002)
doi:10.1038/ng883
106.Wei, X., J. Zou, M. Takechi, S. Kawamura & L. Li: Nok plays an essential role in maintaining the integrity of the outer nuclear layer in the zebrafish retina. Exp Eye Res, 83, 31-44(2006)
doi:10.1016/j.exer.2005.10.030
107. den Hollander, A. I., J. R. Heckenlively, L. I. van den Born, Y. J. de Kok, S. D. van der Velde-Visser, U. Kellner, B. Jurklies, M. J. van Schooneveld, A. Blankenagel, K. Rohrschneider, B. Wissinger, J. R. Cruysberg, A. F. Deutman, H. G. Brunner, E. Apfelstedt-Sylla, C. B. Hoyng & F. P. Cremers: Leber congenital amaurosis and retinitis pigmentosa with Coats-like exudative vasculopathy are associated with mutations in the crumbs homologue 1 (CRB1) gene. Am J Hum Genet, 69, 198-203(2001)
doi:10.1086/321263
108. den Hollander, A. I., J. Davis, S. D. van der Velde-Visser, M. N. Zonneveld, C. O. Pierrottet, R. K. Koenekoop, U. Kellner, L. I. van den Born, J. R. Heckenlively, C. B. Hoyng, P. A. Handford, R. Roepman & F. P. Cremers: CRB1 mutation spectrum in inherited retinal dystrophies. Hum Mutat, 24, 355-69(2004)
doi:10.1002/humu.20093
109. Lotery, A. J., S. G. Jacobson, G. A. Fishman, R. G. Weleber, A. B. Fulton, P. Namperumalsamy, E. Heon, A. V. Levin, S. Grover, J. R. Rosenow, K. K. Kopp, V. C. Sheffield & E. M. Stone: Mutations in the CRB1 gene cause Leber congenital amaurosis. Arch Ophthalmol, 119, 415-20(2001)
110. Lotery, A. J., A. Malik, S. A. Shami, M. Sindhi, B. Chohan, C. Maqbool, P. A. Moore, M. J. Denton & E. M. Stone: CRB1 mutations may result in retinitis pigmentosa without para-arteriolar RPE preservation. Ophthalmic Genet, 22, 163-9(2001)
doi:10.1076/opge.22.3.163.2222
111. Jacobson, S. G., A. V. Cideciyan, T. S. Aleman, M. J. Pianta, A. Sumaroka, S. B. Schwartz, E. E. Smilko, A. H. Milam, V. C. Sheffield & E. M. Stone: Crumbs homolog 1 (CRB1) mutations result in a thick human retina with abnormal lamination. Hum Mol Genet, 12, 1073-8(2003)
doi:10.1093/hmg/ddg117
112. McKay, G. J., S. Clarke, J. A. Davis, D. A. Simpson & G. Silvestri: Pigmented paravenous chorioretinal atrophy is associated with a mutation within the crumbs homolog 1 (CRB1) gene. Invest Ophthalmol Vis Sci, 46, 322-8(2005)
doi:10.1167/iovs.04-0734
113.van de Pavert, S. A., A. S. Sanz, W. M. Aartsen, R. M. Vos, I. Versteeg, S. C. Beck, J. Klooster, M. W. Seeliger & J. Wijnholds: Crb1 is a determinant of retinal apical Muller glia cell features. Glia, 55, 1486-97(2007)
doi:10.1002/glia.20561
114.Stohr, H., L. L. Molday, R. S. Molday, B. H. Weber, B. Biedermann, A. Reichenbach & F. Kramer: Membrane-associated guanylate kinase proteins MPP4 and MPP5 associate with Veli3 at distinct intercellular junctions of the neurosensory retina. J Comp Neurol, 481, 31-41(2005)
doi:10.1002/cne.20367
115.Kantardzhieva, A., S. Alexeeva, I. Versteeg & J. Wijnholds: MPP3 is recruited to the MPP5 protein scaffold at the retinal outer limiting membrane. Febs J, 273, 1152-65(2006)
doi:10.1111/j.1742-4658.2006.05140.x
116.Aartsen, W. M., A. Kantardzhieva, J. Klooster, A. G. van Rossum, S. A. van de Pavert, I. Versteeg, B. N. Cardozo, F. Tonagel, S. C. Beck, N. Tanimoto, M. W. Seeliger & J. Wijnholds: Mpp4 recruits Psd95 and Veli3 towards the photoreceptor synapse. Hum Mol Genet, 15, 1291-302(2006)
doi:10.1093/hmg/ddl047
117. Stohr, H. & B. H. Weber: Cloning and characterization of the human retina-specific gene MPP4, a novel member of the p55 subfamily of MAGUK proteins. Genomics, 74, 377-84(2001)
doi:10.1006/geno.2001.6559
118. Conte, I., M. Lestingi, A. den Hollander, M. G. Miano, G. Alfano, D. Circolo, M. Pugliese, F. Testa, F. Simonelli, E. Rinaldi, M. Baiget, S. Banfi & A. Ciccodicola: Characterization of MPP4, a gene highly expressed in photoreceptor cells, and mutation analysis in retinitis pigmentosa. Gene, 297, 33-8(2002)
doi:10.1016/S0378-1119(02)00872-7
119. Stohr, H., J. Stojic & B. H. Weber: Cellular localization of the MPP4 protein in the mammalian retina. Invest Ophthalmol Vis Sci, 44, 5067-74(2003)
doi:10.1167/iovs.03-0702
120. Yang, J., B. Pawlyk, X. H. Wen, M. Adamian, M. Soloviev, N. Michaud, Y. Zhao, M. A. Sandberg, C. L. Makino & T. Li: Mpp4 is required for proper localization of plasma membrane calcium ATPases and maintenance of calcium homeostasis at the rod photoreceptor synaptic terminals. Hum Mol Genet, 16, 1017-29(2007)
doi:10.1093/hmg/ddm047
121. Fan, S., T. W. Hurd, C. J. Liu, S. W. Straight, T. Weimbs, E. A. Hurd, S. E. Domino & B. Margolis: Polarity proteins control ciliogenesis via kinesin motor interactions. Curr Biol, 14, 1451-61(2004)
doi:10.1016/j.cub.2004.08.025
122. Olsen, O., L. Funke, J. F. Long, M. Fukata, T. Kazuta, J. C. Trinidad, K. A. Moore, H. Misawa, P. A. Welling, A. L. Burlingame, M. Zhang & D. S. Bredt: Renal defects associated with improper polarization of the CRB and DLG polarity complexes in MALS-3 knockout mice. J Cell Biol, 179, 151-64(2007)
doi:10.1083/jcb.200702054
123.Grasset, E., J. Bernabeu & M. Pinto: Epithelial properties of human colonic carcinoma cell line Caco-2: effect of secretagogues. Am J Physiol, 248, C410-8(1985)
124.Wice, B. M., G. Trugnan, M. Pinto, M. Rousset, G. Chevalier, E. Dussaulx, B. Lacroix & A. Zweibaum: The intracellular accumulation of UDP-N-acetylhexosamines is concomitant with the inability of human colon cancer cells to differentiate. J Biol Chem, 260, 139-46(1985)
125.Le Bivic, A., A. Quaroni, B. Nichols & E. Rodriguez-Boulan: Biogenetic pathways of plasma membrane proteins in Caco-2, a human intestinal epithelial cell line. J Cell Biol, 111, 1351-61(1990)
doi:10.1083/jcb.111.4.1351
126.Matter, K., M. Brauchbar, K. Bucher & H. P. Hauri: Sorting of endogenous plasma membrane proteins occurs from two sites in cultured human intestinal epithelial cells (Caco-2). Cell, 60, 429-37(1990)
doi:10.1016/0092-8674(90)90594-5
127. O'Brien, L. E., M. M. Zegers & K. E. Mostov: Opinion: Building epithelial architecture: insights from three-dimensional culture models. Nat Rev Mol Cell Biol, 3, 531-7(2002)
doi:10.1038/nrm859
128. Saaf, A. M., J. M. Halbleib, X. Chen, S. T. Yuen, S. Y. Leung, W. J. Nelson & P. O. Brown: Parallels between global transcriptional programs of polarizing Caco-2 intestinal epithelial cells in vitro and gene expression programs in normal colon and colon cancer. Mol Biol Cell, 18, 4245-60(2007)
doi:10.1091/mbc.E07-04-0309
129. Sfakianos, J., A. Togawa, S. Maday, M. Hull, M. Pypaert, L. Cantley, D. Toomre & I. Mellman: Par3 functions in the biogenesis of the primary cilium in polarized epithelial cells. J Cell Biol, 179, 1133-40(2007)
doi:10.1083/jcb.200709111
130. Gao, L., G. Joberty & I. G. Macara: Assembly of epithelial tight junctions is negatively regulated by Par6.Curr Biol, 12, 221-5(2002)
doi:10.1016/S0960-9822(01)00663-7
131. Straight, S. W., K. Shin, V. C. Fogg, S. Fan, C. J. Liu, M. Roh & B. Margolis: Loss of PALS1 expression leads to tight junction and polarity defects. Mol Biol Cell, 15, 1981-90(2004)
doi:10.1091/mbc.E03-08-0620
132. Shin, K., S. Straight & B. Margolis: PATJ regulates tight junction formation and polarity in mammalian epithelial cells. J Cell Biol, 168, 705-11(2005)
doi:10.1083/jcb.200408064
133.Michel, D., J. P. Arsanto, D. Massey-Harroche, C. Beclin, J. Wijnholds & A. Le Bivic: PATJ connects and stabilizes apical and lateral components of tight junctions in human intestinal cells. J Cell Sci, 118, 4049-57(2005)
doi:10.1242/jcs.02528
134.Hurd, T. W., L. Gao, M. H. Roh, I. G. Macara & B. Margolis: Direct interaction of two polarity complexes implicated in epithelial tight junction assembly. Nat Cell Biol, 5, 137-42(2003)
doi:10.1038/ncb923
135.Chishti, A. H., A. C. Kim, S. M. Marfatia, M. Lutchman, M. Hanspal, H. Jindal, S. C. Liu, P. S. Low, G. A. Rouleau, N. Mohandas, J. A. Chasis, J. G. Conboy, P. Gascard, Y. Takakuwa, S. C. Huang, E. J. Benz, Jr., A. Bretscher, R. G. Fehon, J. F. Gusella, V. Ramesh, F. Solomon, V. T. Marchesi, S. Tsukita, S. Tsukita, K. B. Hoover & et al.: The FERM domain: a unique module involved in the linkage of cytoplasmic proteins to the membrane. Trends Biochem Sci, 23, 281-2(1998)
doi:10.1016/S0968-0004(98)01237-7
136.Hurd, T. W., S. Fan, C. J. Liu, H. K. Kweon, K. Hakansson & B. Margolis: Phosphorylation-dependent binding of 14-3-3 to the polarity protein Par3 regulates cell polarity in mammalian epithelia. Curr Biol, 13, 2082-90(2003)
doi:10.1016/j.cub.2003.11.020
137. Etienne-Manneville, S., J. B. Manneville, S. Nicholls, M. A. Ferenczi & A. Hall: Cdc42 and Par6-PKCzeta regulate the spatially localized association of Dlg1 and APC to control cell polarization. J Cell Biol, 170, 895-901(2005)
doi:10.1083/jcb.200412172
138. Grindstaff, K. K., C. Yeaman, N. Anandasabapathy, S. C. Hsu, E. Rodriguez-Boulan, R. H. Scheller & W. J. Nelson: Sec6/8 complex is recruited to cell-cell contacts and specifies transport vesicle delivery to the basal-lateral membrane in epithelial cells. Cell, 93, 731-40(1998)
doi:10.1016/S0092-8674(00)81435-X
139. Yeaman, C., K. K. Grindstaff & W. J. Nelson: Mechanism of recruiting Sec6/8 (exocyst) complex to the apical junctional complex during polarization of epithelial cells. J Cell Sci, 117, 559-70(2004)
doi:10.1242/jcs.00893
140. Hsu, S. C., C. D. Hazuka, D. L. Foletti & R. H. Scheller: Targeting vesicles to specific sites on the plasma membrane: the role of the sec6/8 complex. Trends Cell Biol, 9, 150-3(1999)
doi:10.1016/S0962-8924(99)01516-0
141. Lu, H. & D. Bilder: Endocytic control of epithelial polarity and proliferation in Drosophila. Nat Cell Biol, 7, 1232-9(2005)
doi:10.1038/ncb1324
142. Garami, A., F. J. Zwartkruis, T. Nobukuni, M. Joaquin, M. Roccio, H. Stocker, S. C. Kozma, E. Hafen, J. L. Bos & G. Thomas: Insulin activation of Rheb, a mediator of mTOR/S6K/4E-BP signaling, is inhibited by TSC1 and 2. Mol Cell, 11, 1457-66(2003)
doi:10.1016/S1097-2765(03)00220-X
143.Inoki, K., Y. Li, T. Xu & K. L. Guan: Rheb GTPase is a direct target of TSC2 GAP activity and regulates mTOR signaling. Genes Dev, 17, 1829-34(2003)
doi:10.1101/gad.1110003
144.Saucedo, L. J., X. Gao, D. A. Chiarelli, L. Li, D. Pan & B. A. Edgar: Rheb promotes cell growth as a component of the insulin/TOR signalling network. Nat Cell Biol, 5, 566-71(2003)
doi:10.1038/ncb996
145.Katajisto, P., T. Vallenius, K. Vaahtomeri, N. Ekman, L. Udd, M. Tiainen & T. P. Makela: The LKB1 tumor suppressor kinase in human disease. Biochim Biophys Acta, 1775, 63-75(2007)
146.Krapivinsky, G., I. Medina, L. Krapivinsky, S. Gapon & D. E. Clapham: SynGAP-MUPP1-CaMKII synaptic complexes regulate p38 MAP kinase activity and NMDA receptor-dependent synaptic AMPA receptor potentiation. Neuron, 43, 563-74(2004)
doi:10.1016/j.neuron.2004.08.003
147. Kimber, W. A., L. Trinkle-Mulcahy, P. C. Cheung, M. Deak, L. J. Marsden, A. Kieloch, S. Watt, R. T. Javier, A. Gray, C. P. Downes, J. M. Lucocq & D. R. Alessi: Evidence that the tandem-pleckstrin-homology-domain-containing protein TAPP1 interacts with Ptd(3,4)P2 and the multi-PDZ-domain-containing protein MUPP1 in vivo. Biochem J, 361, 525-36(2002)
doi:10.1042/0264-6021:3610525
148. Dakoji, S., S. Tomita, S. Karimzadegan, R. A. Nicoll & D. S. Bredt: Interaction of transmembrane AMPA receptor regulatory proteins with multiple membrane associated guanylate kinases. Neuropharmacology, 45, 849-56(2003)
doi:10.1016/S0028-3908(03)00267-3
149. Griffon, N., F. Jeanneteau, F. Prieur, J. Diaz & P. Sokoloff: CLIC6, a member of the intracellular chloride channel family, interacts with dopamine D(2)-like receptors. Brain Res Mol Brain Res, 117, 47-57(2003)
doi:10.1016/S0169-328X(03)00283-3
150. Barritt, D. S., M. T. Pearn, A. H. Zisch, S. S. Lee, R. T. Javier, E. B. Pasquale & W. B. Stallcup: The multi-PDZ domain protein MUPP1 is a cytoplasmic ligand for the membrane-spanning proteoglycan NG2. J Cell Biochem, 79, 213-24(2000)
doi:10.1002/1097-4644(20001101)79:2<213::AID-JCB50>3.0.CO;2-G
151. Jeansonne, B., Q. Lu, D. A. Goodenough & Y. H. Chen: Claudin-8 interacts with multi-PDZ domain protein 1 (MUPP1) and reduces paracellular conductance in epithelial cells. Cell Mol Biol (Noisy-le-grand), 49, 13-21(2003)
152. Mancini, A., A. Koch, M. Stefan, H. Niemann & T. Tamura: The direct association of the multiple PDZ domain containing proteins (MUPP-1) with the human c-Kit C-terminus is regulated by tyrosine kinase activity. FEBS Lett, 482, 54-8(2000)
doi:10.1016/S0014-5793(00)02036-6
153.Coyne, C. B., T. Voelker, S. L. Pichla & J. M. Bergelson: The coxsackievirus and adenovirus receptor interacts with the multi-PDZ domain protein-1 (MUPP-1) within the tight junction. J Biol Chem, 279, 48079-84(2004)
doi:10.1074/jbc.M409061200
Abbreviations: 5-HTR: Serotonin receptor, AJ :adherens junctions, Amot: Angiomotin, aPKC: atypical protein kinase C, CAR: Coxsackievirus and Adenovirus Receptor, C-kit: proto-oncogene (receptor for stem cell factor), CRB: crumbs, CRIB: Cdc42 Rac interaction binding, DLG: discs large, EHM2 : expressed in high metastatic cells, Epb4.1l5: erythrocyte protein band 4.1-like 5, EST: expressed sequence tag, FERM: band 4.1 ezrin radixin moesin, JAM: Junctional Adhesion Molecule, L27: Lin2 lin7, LCA : Leber congenital amaurosis, MAGUK: membrane associated guanylate kinase, MALS:mammalian lin seven, MOE: mosaic eyes protein, MPP: membrane protein palmitoylated, MUPP1: multi PDZ domain protein, NG2: membrane-spanning proteoglycan, NOK: nagie oko protein, OLM : outer limiting membrane, OME: oko meduzy mutant, OPL : outer plexiform layer, PALS: protein associated with Lin seven, PAR: partition defective, PATJ: protein associated with tight junction, PDZ: PSD-95, Discs large, ZO1, PhR : photoreceptors, PSD95: post-synaptic density 95, RP : Retinitis Pigmentosa , RPE : Retinal Pigmented Epithelium, SAR : subapical region, Scrib: scribble, Sdt: stardust, SH3: Src homology domain 3, TAPP: tandem-PH (pleckstrin-homology) domain-containing protein, TARPs: transmembrane AMPA receptor regulated proteins, TSC: tuberous sclerosis complex, Veli : vertebrate lin seven, ZA : zonula adherens, ZO: zonula occludens
Note: Drosophila melanogaster and Caenohabditis elegans gene and Protein names are in small print, whereas vertebrate GENE and PROTEIN names are in capital letters. All gene names are given in italics.
Key Words: Epithelium, Polarity, PATJ,PALS1, MUPP1,PAR6, Stardust, Review
Send correspondence to: Dominique Massey-Harroche, IBDML, CNRS UMR 6216, Campus de Luminy, Case 907, 13288 Marseille Cedex 9, France, Tel: 33-0-4 91 26 97 44, Fax: 33-0-4 91 26 97 48, E-mail:massey@ibdml.univ-mrs.fr