[Frontiers in Bioscience 14, 1414-1432, January 1, 2009]

The assembly and maintenance of epithelial junctions in C. elegans

Allison M. Lynch1, Jeff Hardin1,2

1Program in Genetics, University of Wisconsin-Madison, 2Department of Zoology, University of Wisconsin-Madison

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. The C. elegans apical junction
4. The C. elegans cadherin-catenin complex
4.1. HMR-1/cadherin, HMP-2/beta-catenin, and HMP-1/alpha-catenin: core components of the classical cadherin-catenin complex
4.2. Connecting to the cytoskeleton: HMP-1/alpha-catenin
4.3. Regulation of the cadherin complex: JAC-1/p120-catenin
4.4. Regulation of HMP-2/beta-catenin: a role for FRK-1/Fer kinase?
4.5. VAB-9: a divergent claudin family member regulated by the cadherin complex
4.6. The search for adhesion molecules that are functionally redundant with the cadherin complex
4.7. Comparison with vertebrates suggests the C. elegans cadherin complex is a "minimalist" system
5. The DLG-1/AJM-1 complex
5.1. DLG-1 and AJM-1: core components of the basolateral domain
5.2. The role of DLG-1 subdomains in its localization
5.3. The DLG-1/AJM-1 complex: a self-reinforcing complex?
5.4. Do the DLG-1/AJM-1 and cadherin complexes interact?
5.5. UNC-34/Enabled: a potential functional integrator of the cadherin and DLG-1/AJM-1 complexes
5.6. Remaining questions: What links DLG-1 to the membrane?
6. The apical membrane domain
6.1. Organization and function
6.2. The PAR/aPKC complex: regulator of junctional organization
6.3. The Crumbs complex: surprisingly minor roles
6.4. Spectrin-based membrane skeletons: structural stabilizers of the apex
7. Summary and Perspectives
8. Acknowledgements
9. References

1. ABSTRACT

The epithelial tissues of the C. elegans embryo provide a "minimalist" system for examining phylogenetically conserved proteins that function in epithelial polarity and cell-cell adhesion in a multicellular organism. In this review, we provide an overview of three major molecular complexes at the apical surface of epithelial cells in the C. elegans embryo: the cadherin-catenin complex, the more basal DLG-1/AJM-1 complex, and the apical membrane domain, which shares similarities with the subapical complex in Drosophila and the PAR/aPKC complex in vertebrates. We discuss how the assembly of these complexes contributes to epithelial polarity and adhesion, proteins that act as effectors and/or regulators of each subdomain, and how these complexes functionally interact during embryonic morphogenesis. Although much remains to be clarified, significant progress has been made in recent years to clarify the role of these protein complexes in epithelial morphogenesis, and suggests that C. elegans will continue to be a fruitful system in which to elucidate functional roles for these proteins in a living embryo.