[Frontiers in Bioscience S1, 92-107, June 1, 2009]
Skeletal morphofunctional considerations and the pituitary-thyroid axis
Jameel Iqbal1, Terry F. Davies1, Li Sun, Etsuko Abe1, Angelo Carpi2, Jeffrey Mechanick1, Mone Zaidi1
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TABLE OF CONTENTS
1. ABSTRACT
The past decade has unraveled novel molecular mechanisms not only of skeletal remodeling, which is the process by which the skeleton is restructured throughout adult life, but also the precision by which the skeleton is put together during embryogenesis and later modeled during growth. It is now possible to delete single genes in individual cells and during specified periods of life. This has allowed us to pin down specific molecular events that underlie individual cellular processes, and also importantly, to identify molecular defects underlying disorders of skeletal morphogenesis and remodeling. Particularly novel has been the demonstration of cross-talk, some of which is humoral, between the skeleton and organs as diverse as the brain, pituitary, and even adipose tissue and pancreas. The current review describes these molecular mechanisms in relation to the way thyroid hormones, and the pituitary hormone thyrotropin (TSH), regulate skeletal morphogenesis and remodeling.
2. INTRODUCTION
Von Recklinghausen (1890) first reported an association between bone loss and hyperthyroidism. Since then, it has become accepted by virtue of rich anecdotal experience and clinical research that thyrotoxicosis is associated with a high-turnover osteoporosis. Accelerated bone resorption is not compensated by a coupled increase in bone formation, resulting in net bone loss, and an increased fracture rate. In particular, excessive resorption of the cortical bone in hyperthyroidism results in an increased risk (~1.8 fold) of hip fractures. Furthermore, recovery of bone loss after correction of the thyroid overactivity, even in younger patients, is never complete, leaving such individuals at an increased fracture risk for the remainder of their lives. In addition, therapeutic suppression of TSH for thyroid cancer is associated with increased osteoporosis, again mainly in the post-menopausal period.
We have shown recently that deficiency of TSH receptors (TSHR) is associated with a high-turnover osteoporosis in mice, even in a euthyroid haploinsufficient state. This suggests that bone loss due to hyperthyroidism may also result from low TSH levels; this sets forth a new clinical paradigm where pituitary hormones play a critical role in bone loss.
Despite the known deleterious effects of thyroid hormones on the adult skeleton, it is clear that skeletogenesis, both during development and growth, is tightly regulated by thyroid hormones. The absence of thyroid hormones, for example in congenital forms of hypothyroidism, as well as genetic mouse models of hypothyroidism, results in stunted growth and development. Evidence to date suggests that thyroid hormones regulate endochondral bone formation directly.
This review focuses on our current and evolving understanding of embryonic skeletal development and bone remodeling, a process by which new bone replaces old bone, in the adult. After detailing the two components of bone remodeling, we explore how mouse genetic studies relate these processes to the thyroid and pituitary axis. Finally, the clinical implications of the new data are discussed.
3. LIMB PATTERNING AND JOINT FORMATION
Limb patterning in the skeleton is a critical developmental processes for all mammals (1). The process of skeletal morphogenesis begins with the migration of lateral mesoderm cells into the nascent limb bud to produce a mesenchymal cell population. The determination and patterning of these cells along the proximal-distal axis creates pre-skeletal mesenchymal condensations. There is solid evidence that the Hox gene family, Hox8 through Hox13, exerts supremacy in determining the overall skeletal pattern. A triple Hox10 mutant with all six paralogous alleles missing lacks the femur, while a triple Hox11 mutant lacks the tibia and fibula (2). Likewise, the conditional ablation of all Hoxa and Hoxd functions causes early patterning arrest and shortened limbs3. This phenotype results from the down-regulation of a hedgehog family member, Sonic hedgehog (Shh) (3), while both Hox and Shh can be negatively regulated to prevent inappropriate patterning by specific microRNAs (4).
Complex interactions between bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), epidermal growth factor (EGF), the wingless-ints (Wnts), and patterning factors including the Pax, forkhead-helix and homeodomain families determine the array of shapes and sizes of skeletal elements (5). Early limb specification appears to require the T-box genes Tbx4 and Tbx5; mutations of which in the latter cause Holt-Oram syndrome typified by forelimb abnormalities (6). Likewise, several BMPs, such as BMP-2, -4 and -7 regulate the dorso-ventral and proximo-distal axes, respectively, by interacting with Wnt7a and FGF-8. In contrast, signaling through several distinct FGFs, namely FGF-4, FGF-9 and FGF-17 that allow continued Shh production by down-regulating the BMP antagonist gremlin determines limb size (7,8). Even with these details known to date, precise mechanisms that integrate this molecular diversity to specify skeletal patterns with such exquisite intricacy remain poorly understood. It has emerged only very recently that the exactness in digit patterning is achieved through precise regulation of spatial and temporal gradients of Shh within a limb bud (9).
Whether there is a predetermined genetic program - a "Hox code" - that specifies the location of joints is also not known (10). It was proposed that segmental demarcations in Hox gene expression predetermine the positioning of future joints. However, the interzone, the very first mesenchymal representation for future joint formation, does not correspond with such Hox boundaries. Thus, what determines the address of interzone cells remains a mystery. The interzone is nonetheless unique in being composed of highly condensed mesenchymal cells interconnected by gap junctions (10). It specifies future joint elements, such as the capsule, synovial lining and articular cartilage, each structure arising from ultrastructurally distinct subsets of cells.
Established molecular signals that dictate the conversion of the interzone cells into joint structures include TGFb superfamily members BMP-2, -4, GDF-5 and GDF-6; Wnt4, Wnt14 and Wnt16; the BMP antagonists noggin and chordin; FGF2, FGF4 and FGF13; connexins; the transcription factors Cux-1 and Erg; and other molecules, such as stanniocalcin (11). The Wnts and noggin represent early anti-chondrogenic signals that maintain the mesenchymal nature of the interzone, a feature that is required for joint formation (12). Interestingly, while noggin-/- mice expectedly do not form joints, chordin-/- mice have normal joints, for unclear reasons. Likewise, mice lacking both GDF-5 and GDF-6 and humans with GDF-5 mutations display multiple joint defects (13). Another provocative hypothesis, based on evidence that mice lacking Indian hedgehog (Ihh) also lack joints, is that like Wnts and noggin, Ihh stimulates the target repressor gene Gli3 in interzone cells to delay chondrogenesis and permit joint formation (14). Conditional ablation of Gli3 or the Ihh receptor, smoothened, in the interzone should prove or disprove whether Ihh has this apparently counterintuitive anti-chondrogenic action.
The next step in which the interzone cavitates to form the joint may initially involve a fine line of apoptotic cells that preempts tissue loosening aided by motion. Hyaluronic acid is then secreted that interacts with its cognate receptor, CD44, resulting in the loss of tissue integrity and joint separation, functions that are assisted by the mucin-rich protein lubricin. Following cavitation, the two sides of a joint are shaped into reciprocal interlocking structures (see Figure 1). What remains arcane, however, is the accuracy underlying the genesis of a perfectly shaped humeral head or a patella that fits exactly over the knee.
4. ENDOCHONDRAL BONE FORMATION
The skeleton enlarges and ossifies mostly through the process of endochondral bone formation, and in flat bones like the skull, through intramembranous ossification. These processes require a set of non-redundant mechanisms that are spatially and temporally integrated. The loss of any one mechanism results in abnormal skeletal development, and in humans, a characteristic chondro-osseous dysplasia.
Endochondral bone formation begins with the condensation of mesenchymal cells and their transformation into prechondrocytes. Critical to lineage commitment is the transcription factor Sox9, which cooperates with the homeobox gene, Barx-2 (15). The transcription regulators Pax1 and Pax9, as well as Nkx3.1 and Nkx3.3 regulate Sox9 under the stewardship of Shh (16). These prechondrocytic cells then commit to form early chondroblasts, which undergo intense proliferation and secrete abundant aggrecan and type II collagen. To regulate this step, Sox9 requires downstream partners Sox5 and Sox6, the trio being both necessary and sufficient. Thus, while Sox9-/- and Sox5-/-/Sox6-/- mice display reduced chondrocyte proliferation, impaired lineage commitment is seen only in Sox9-/- mice (17). A somewhat poorly understood step ensues next, in which centrally located chondrocytes stop proliferating and become post-mitotic prehypertrophic chondrocytes that secrete a collagen type X-rich matrix. The rounded cells closest to the hypertrophic zone flatten out into parallel longitudinal columns that continue to proliferate at rates that are highest away from the center. The proliferating cells also begin to express PTHrP. Both Sox5 and Sox6 are necessary for this conversion, as attested by a loss of cell columns in the Sox5-/-/Sox6-/- mutants (18).
A critical phenotypic switch next marks the conversion of columnar and round chondrocytes into prehypertrophic and hypertrophic chondrocytes, both of which express high levels of collagen type IIb and aggrecan. The Sox trio delays hypertrophy at the time when the Runt family transcription factors Runx2 and Runx3 start being expressed. Runx2 is negatively regulated by HDAC4, a histone deacetylase, but cooperates with the two distal-less-related homeobox transcription activators Dlx5 and Dlx6 (19,20). Runx2 is critical for chondrocyte hypertrophy and activation of collagen X, PTHrP receptor and Ihh gene transcription. Not unexpectedly, Runx2-/- mice and Runx2-/-/Runx3-/- mutants lack prehypertrophic and hypertrophic chondrocytes.
Ihh and PTHrP interact in an elegant feedback in which Ihh secreted by prehypertrophic chondrocytes stimulates PTHrP release from periarticular cells. The released PTHrP maintains chondrocyte proliferation and prevents hypertrophy. Ihh also enhances chondrocyte proliferation directly and converts round chondrocytes to flat columnar cells (21,22). This positive feedback by Ihh ensures a constant flow of chondrocytes in and out of the columnar zone allowing for a timed and exactingly accurate linear expansion at the growth plate. Ablation of PTHrP or Ihh signaling disrupts this endochondral sequence producing severe runting and few or no columnar cells, a phenocopy of fetuses with Blomstrand's chondro-osteodystrophy. Conversely, mice over-expressing PTHrP or the constitutively active PTH/PTHrP receptor accumulate proliferating chondrocytes, an abnormality phenocopied in Jaansen's chondrodysplasia arising from a gain-of-function PTH/PTHrP receptor mutation.
Lateral signals from the surrounding perichondrium, namely BMPs, Wnts and FGF-2, regulate Ihh signaling (23). For example, Ihh enhances the expression of BMPs -2, -4 and -7, which in turn up-regulate Wnt-2a; these molecules work synergistically with Ihh to prevent hypertrophic differentiation. BMP-2 also down regulates Wnt-7a to achieve the same goal. TGFb, cleaved from its precursor by metalloproteinases including MMP-13 also mediates Ihh action in inducing PTHrP expression (24). FGF signaling, in contrast, suppresses Ihh expression, reduces chondrocyte proliferation and enhances hypertrophy.
The differential coupling of the PTH/PTHrP receptor to Gsa or Gq exerts a second level of control. The targeted disruption of the Gsa signal causes chondrocyte hypertrophy, while that of Gq surprisingly permits it (25,26). These opposing actions nonetheless appear consistent with the concentration of PTHrP required for PTH/PTHrP receptor coupling with the respective G proteins (27). That lower PTHrP concentrations are required for Gsa coupling makes biological sense as such a coupling, most distant from the PTHrP source, will keep chondrocytes proliferating (27). Inhibition of p57, a cyclin-dependent kinase inhibitor, constitutes the proliferation signal (28), while Sox9 activation prevents hypertrophy (29). Thus, the abrogation of p57 in PTHrP-/-/p57-/- mice rescues, in part, the PTHrP null phenotype (28).
Closure of the chondrocyte differentiation program is initiated when hypertrophic chondrocytes become terminal chondrocytes and lose type X collagen. The transcriptional regulator c-Maf of the leucine zipper family finally mediates their apoptosis (30). Prior to apoptosis, a typical osteoblast gene sequence comprising MMP-13, alkaline phosphatase and osteopontin is initiated. However, it is not clear why the classic osteoblastic gene collagen 1 remains silent in chondrocytes.
Osteogenesis is initiated when prehypertrophic and hypertrophic chondrocytes instruct perichondrial cells to become mineralizing osteoblasts. For this, Ihh signals through the receptor protein smoothened to induce BMP-2, BMP-4 and BMP-7, which then trigger the formation of a bone collar surrounding a primary ossification center. Thus, both smoothened-/- and compound PTHrP receptor/Ihh mice fail to ossify the perichondrium (31). Runx2 expression in the osteoblast, a consequence of the down-regulation of its two repressors, Twist-1 and Twist-2, coincides with its expression in hypertrophic chondrocytes. Twist deletion rescues certain, but not all, osteogenic defects in Runx2-/- mice (32). Runx2, a pre-requisite for osteogenesis, up-regulates a gene program consisting of collagen 1, osteocalcin, osteopontin, osteonectin and MMP-13.
A necessary late-onset player, the blood vessel, invades the perichondrium and penetrates the hypertrophic zone. This allows the removal of terminal chondrocytes and the procurement of perichondrial osteogenic cells that deposit the first traces of true bone, the primary spongiosa. Osteoclasts also enter through this ready vascular route to assist in the formation of a medullary cavity. They release proteolytic enzymes, such as MMP-9, which permit further vascular ingress. Vascularization is initiated by vascular endothelial growth factor (VEGF) that is secreted from hypertrophic chondrocytes and perichondrial cells in a Runx2-regulated manner. Interestingly, targeted VEGF ablation not only impairs vascularization and ossification allowing hypertrophic chondrocytes to accumulate, but also causes massive apoptosis. This pro-survival function of VEGF has been attributed to its epiphysis-specific isoform 120 (33,34).
Despite vascular penetration, the central portion of the growth plate remains relatively hypoxic. Hypoxic cells up-regulate HIF-1a, a transcription factor of the basic helix-loop-helix family that is normally ubiquitylated by the von Hippel Landau (VHL) tumor suppressor protein. VHL null mice thus show a striking decrease in chondrocyte proliferation that is normally seen when HIF-1a levels are upregulated by hypoxia. Additionally, conditional ablation of HIF-1a reveals abundant apoptotic chondrocytes in the central growth plate core; this suggests that HIF-1a is also required for the survival of hypoxic chondrocytes, likely through its known actions on glycolytic enzymes, such as phosphoglycerokinase (35).
The zinc-dependent proteases MMP-9, MMP-13 and membrane type-1 MMP (MT1-MMP) finally model and remodel the developing cartilage and bone. Gene ablation has defined functions for MMP-9 in chondrocyte removal, growth plate angiogenesis and osteoclast recruitment, and together with MMP-13, in ossification center and bone marrow cavity formation. Compound MMP-13-/-/MMP-9-/- mutants thus display persisting hypertrophic cartilage, reduced trabecular bone and delayed marrow cavity formation (36). Likewise, MT1-MMP-/- mice have severe defects in both endochondral and intramembranous ossification (37). Other MMPs, and their inhibitors, TIMPs, have been localized to distinct bone cell populations, but their true function, if any, remains unknown. The entire endochondral sequence (chondrocyte proliferation ®chondrocyte hypertrophy ®vasculogenesis ®osteogenesis) is recapitulated at the epiphysis to result in secondary ossification centers. A growth plate is thus created between primary and secondary ossification centers, a site at which longitudinal growth occurs up to puberty. In addition, appositional growth to further strengthen bone ensues when new bone is deposited beneath the periosteal membrane. Osteoclasts, derived from marrow cells, resorb bone on the endosteal surface. The two processes, sub-periosteal bone apposition and endosteal bone resorption, together determine the eventual thickness of long bones.
5. BONE FORMATION
Osteoblasts arise from stromal cell precursors in bone marrow to serve several distinct roles in post-natal life. They form new bone, regulate the genesis and resorptive activity of osteoclasts, control the egress of hematopoetic stem cells from bone marrow niches, and, in the guise of osteocytes, transduce mechanical stimuli. To form new bone, osteoblasts secrete type 1 collagen and non-collagenous proteins, including osteocalcin, osteopontin, osteonectin, bone sialoprotein and dentine matrix protein-1 (DMP-1), among others (38). These proteins regulate the content and character of the deposited mineral. Thus, mineral content increases in both osteonectin and osteopontin null mice (39-40), and decreases in DMP-1-/- mice (41), testifying to the negative and positive regulatory functions of the respective proteins.
Evidence that osteoblast differentiation and bone formation are governed by Runx2 rests on the absence of bone formation in Runx2-/- mice; cleidocranial dysplasia in humans even with Runx2 haploinsufficiency; and recent evidence for high bone mass Runx2 polymorphisms (42). As a master switch, Runx prevents the differentiation of pluripotent stromal cells to lineages other than the osteoblast. Binding partners permissive to osteoblast differentiation include AP1 proteins, the Smads (Smad1 and 5), glucocorticoid and androgen receptors, several C/EBPs, Oct-1, Dlx5, Menin and Hes1. Those that inhibit Runx2 activation include C/EBPd, Dlx3, Lef1, Msx2, PPARg, Smad3, Stat1 and Twist. In addition, various co-activators, such as p300, CBP, MOZ and MORF, and co-repressors such as HDACs 3, 4, 6, and a mediator of c-src/yes signaling YAP, modify Runx2 activation.
This molecular assortment allows Runx2 to serve as a platform for various cytokine and hormonal modifiers of osteoblast maturation. For example, BMPs, including TGFb, bind to phosphorylated Smads to activate Runx2 indirectly. PTH and the FGFs directly phosphorylate the Ser residue or the C-terminus, respectively. In contrast, TNF induces Runx2 degradation via the E3 ubiquitin ligases smurf1 and smurf2 (43). What is fascinating, however, is that Runx2 delimits these multiple signal cascades to achieve remarkable temporal homogeny in inducing its osteogenic gene program. It has been suggested that Runx2 binding occurs in discrete nuclear matrix compartments, and that chromatin remodeling modifies these spatial domains to specify gene expression sequences during osteoblast differentiation.
Like Runx-/- mice, mice deficient in another transcription factor, osterix do not form bone. Nonetheless, while Runx2-/- mice lack osterix, the reverse is not true, suggesting that osterix is downstream of Runx2. Like Runx2, osterix expression is regulated by BMP-2, IGF-1 and TNF. Importantly, however, to mediate collagen 1 expression, osterix must form a complex with the NFAT transcription factor family member NFAT2 (44). Bone formation is therefore reduced dramatically in mice deficient in the phosphatase, calcineurin that activates NFAT2. Over-expression of calcineurin enhances osteoblast differentiation and bone formation (45). In contrast to osterix, a CREB-related transcription factor, ATF4, is the substrate for a growth factor-regulated kinase, RSK2, a mutation in which causes the skeletal defects in Coffin-Lowry syndrome (46). Ablation of Rsk2 or ATF4 delays bone formation, indicating an effect on the terminal differentiation of osteoblasts. ATF4 transactivation is inhibited by FIAT, a leucine-zipper nuclear protein (47).
Evidence that Wnts regulate osteoblastogenesis comes from the osteopenia and impaired osteoblast proliferation noted either upon the ablation of Wnt co-receptors, Lrp5 and Lrp6, or the use of the Wnt inhibitors, such as sclerostin, soluble frizzled-related protein (sFRP) 1-3, Wnt inhibitory factor-1 (Wif-1) and the Dickkoff family members Dkk1 and 2 (48-51). Likewise, in humans, loss- and gain-of-function mutations of Lrp5 cause osteoporosis pseudoglioma syndrome and a high bone mass phenotype (52), respectively. Canonical Wnt signaling stabilizes and permits the nuclear ingress of b-catenin to stimulate osteoblast differentiation cooperatively with the Tcf/Lef transcription factors. Thus, Wnt10b induces Runx2, Dlx5 and osterix and suppresses PPARg to shift stromal cell differentiation towards the osteoblastic lineage (53). Wnt 3a and 5a additionally prevent osteoblast apoptosis (54). In conjunction with the early B-cell factor EBF2, Wnts also regulate osteoclast formation by altering the production of osteoprotegerin, the decoy receptor for RANK-L (55,56). Thus, b-catenin or EBF2 null mice have osteopenia, striking increases in osteoclasts, and reduced osteoprotegerin levels (56).
That the osteoblast is a key regulatory component of the hematopoetic stem cell niche has only been established recently (57,58). Osteoblastic over-expression of the constitutively activated PTH/PTHrP receptor causes a focal increase in the expression of a Notch ligand, jagged-1, as well as increased stem cell numbers, but importantly, within these cells, the activation of Notch signaling. The latter result unquestionably confirms proximity between hematopoetic stem cells and osteoblasts (58). The conditional ablation of osteoblasts likewise causes substantial decreases in stem cell numbers (59). Impressive, however, has been evidence that stem cell egress from the niche is regulated by sympathetic nerve signals directed to the osteoblast (60). Demyelination in ceramide galactosyltransferase-null mice and, more specifically, abrogation of norepinephrine release prevents the forced egress of hematopoetic stem cells from the niche in response to GM-CSF (60). This provides the first functional evidence that osteoblasts and stem cells talk within a niche.
Once mineral is deposited, an osteoblast becomes buried within its own matrix to become an osteocyte, which, albeit encased, communicates with neighboring osteocytes through a dense canalicular network. Osteocytes sense and transduce changes in fluid flow arising from stress, strain or pressure. The outcome of skeletal loading is unequivocal: even brief loading periods elicit new bone formation in almost every species. Nonetheless, the molecular identity of the putative mechanosensing receptor has remained a mystery. Proposed candidates include L-type voltage-gated Ca2+ channels, b1 integrin, connexins and lipid rafts, as well as the possibility that altered cell shape is sensed directly (61). Loading also induces canalicular hypoxia that up-regulates Hif-1a (62). While downstream events involve the MEK/Erk and IP3/Ca2+ pathways, there is little information on how these signals are integrated in space and time.
6. BONE RESORPTION
The first evidence for the hematopoetic origin of osteoclasts came from parabiosis experiments and spleen cell transplantation. Since then, osteoclasts, macrophages, dendritic cells and lymphocytes have been shown to share molecules and mechanisms for their differentiation. The myeloid and B cell transcription factor PU.1 first determines the osteoclast lineage. Macrophage colony stimulating factor (M-CSF) next ensures the proliferation and survival of committed precursors. The T-cell factor RANK-L then diverts these precursors away from the macrophage toward the osteoclast lineage. Lymph node development is similar in that, like osteoclast formation, it also involves hematopoetic/ stromal cell, integrin/VCAM and RANK/RANK-L interactions. RANK-L-/- mice thus lack osteoclasts and display impaired lymph node maturation.
The action of RANK-L on the osteoclast precursor involves interacting kinase and transcription factor cascades. A critical event is the recruitment of the docking protein TRAF-6 to the cytoplasmic domain of the RANK receptor. TRAF-6 mediates NF-kB and MAP kinase activation. Cytosolic Ca2+ oscillations arise in parallel through phospholipase Cg activation by Syk kinases that are recruited to the ITAM-harboring adapters DAP12 and FcRg (63). That both Syk-/- and DAP12-/-/FcRg-/- mice have osteopetrosis suggests an obligatory role for ITAM co-stimulation in osteoclastogenesis (64). Upstream in this pathway are TREM2 and OSCAR, recently discovered receptors in search of ligands (64). Downstream, the Ca2+ oscillations trigger the phosphatase calcineurin to activate the transcription factor NFAT2. NFAT2 is not only indispensable, but is sufficient for osteoclast formation. Thus, embryonic precursors from NFAT2-/- mice fail to become osteoclasts and over-expression of NFAT2 yields osteoclasts even without RANK-L (65-67). In addition, NFAT2 stimulates its own expression as well as that of OSCAR (68,69). Negative feedback, in contrast, results from the induction of jun dimerization protein (JDP-2) that blocks AP-1-dependent gene transcription (70).
For the resorption of bone, an osteoclast creates a highly acidic compartment beneath itself, within which its membrane is thrown into complex folds, the ruffled border that harbors all secretory activity. Formation of this sealed specialization, akin to a phagolysosome, requires the precise control of adhesion, motility and polarization. For adhesion, matrix RGD peptides trigger the integrin avb3 to recruit c-src that interacts with another kinase Syk; this interaction is regulated by the guanine nucleotide-binding factor Vav-3 (71). Adhesion additionally triggers membrane ruffling and granule extrusion, processes that require the minute-to-minute assembly and disassembly of actin. For this, avb3 activates c-src and Pyk-2; c-src then recruits the multi-site adapters c-Cbl and Cbl-b, which complex with phosphotidylinositol 3-kinase (PI-3kinase) and a GTPase, dynamin (72). Several mechanistic uncertainties exist. First, while src is essential for polarization, we are not confident that its kinase activity is imperative. Second, we are not clear if complexes between gelsolin or cortactin and the integrin-associated proteins paxillin, talin and vinculin also contribute. Third, we can only speculate how the diverse array of proteins that include scaffolds, actin-associated VASPs, ITAM-harboring proteins and src and syk adapters delimit and integrate higher signals towards specific outcomes.
A multi-subunit V-type H+-ATPase pumps H+ across the ruffled border causing the ambient pH to fall to <4U. This allows acid-optimal enzymes, such as cathepsin K, to cleave the helical and telopeptide regions of collagen and release peptides that are transcytosed to exit at the dorsolateral surface. A Cl- countercurrent through ClC-7 balances proton extrusion and an HCO3-/Cl- exchanger corrects any cellular alkalinization. That H+-ATPase, C1C-7 and cathepsin K are obligatory to resorption is attested by the profound osteopetroses resulting from their deficiency. The low pH also causes hydroxyapatite dissolution that elevates ambient Ca2+ to around 40 mM. This activates a putative Ca2+ sensor allowing the cell to detach and retract. The pro-resorptive cytokine interleukin-6 is then released in a feedback loop to inhibit further Ca2+ sensing and reestablish resorption (73). Despite our own efforts to affirmatively establish its identity, the osteoclast Ca2+ sensor still remains a putative entity (73). Homology cloning has largely failed to identify a member of the Ca2+ sensing receptor (CaSR) family in osteoclasts. Furthermore, CaSR deficiency in mice or humans does not cause osteoclast defects. The cation channel, TrypV5, has been implicated, but its deletion results in inactive rather than hyperactive osteoclasts (74). We find that ryanodine receptor-II (RyR-II) of the endoplasmic reticular Ca2+ channel family is located in the osteoclast plasma membrane (75). In addition to its role as a Ca2+ influx channel, RYR-II likely functions as the Ca2+ sensor with its low affinity Ca2+-binding site facing outwards (76). Attesting to this view is the elevated resorption seen in CD38-/- mice, in which the levels of cyclic ADP-ribose, a physiologic RyR agonist, are reduced (77).
Signals that initiate osteoclast apoptosis following multiple episodes of resorption are unknown. In contrast, survival signals such as avb3 and M-CSF are required to ensure the maintenance of osteoclast precursors. Thus, removing negative regulators of M-CSF, such as SHIP, results in abundant osteoclasts (78). Key to the anti-apoptotic effect of M-CSF is the inactivation of the pro-apoptotic gene Bim through Cbl-mediated ubiquitylation. Although the loss of Bim prevents apoptosis, surviving osteoclasts are surprisingly less active (79). This peculiar phenotype possibly arises from the conflicting role of c-Cbl acting upstream, as both a c-src anchor during resorption and ubiquitin ligase during apoptosis. This may be why c-Cbl-/- mice do not display an overt phenotype.
7. NEUROGENIC CONTROL OF BONE MASS
Karsenty's discovery that central leptin inhibits bone formation, and importantly, that the sympathetic nervous system mediates this effect adds a novel dimension to bone physiology80,81. Intracerebroventricular injection of leptin causes profound bone loss, whereas the disruption of leptin signaling increases bone mass despite accompanying hypogonadism and hypercortisolism (80). The effect of central leptin is lost with the ablation of the adrenergic receptor Adrb2 or dopamine b-hydroxylase (Dbh). b-adrenergic drugs therefore expectedly affect bone mass (81). However, they do not affect body mass, suggesting that bone mass and body mass control by central leptin occurs through distinct mechanisms81. Furthermore, high bone mass in the Ardb2-/- mouse arises not only from enhanced bone formation, but also from low bone resorption. Sympathetic signaling through ATF4 up-regulates RANK-L to increase resorption; this effect is inhibited by CART (cocaine amphetamine-related transcript), a leptin-regulated neuropeptide (82). Additionally, by interacting with the clock genes Per and Cry in the osteoblast, adrenergic stimulation provides circadian rhythmicity. Mice lacking Per and Cry, or Per in osteoblasts, have high bone mass. Finally, in contrast to leptinergic control, evidence for the role of peptidergic neurons in bone mass regulation is limited. While the targeted ablation of NPY2 receptors increases bone mass (83), NPY-/- mice lack bone defects (84). Likewise, mice without CGRPa have low bone formation (85), but it unclear whether the effect is central or peripheral.
8. ENDOCRINE REGULATION OF BONE MASS
The main, if not exclusive, hormone regulating bone formation is IGF-1, secreted in response to growth hormone (86). IGF-1-/- mice and compound IGF-1/IGF-1 receptor null mice show profound runting postnatally (87), while IGF-2-/- mice are retarded only in utero but grow normally after birth. There is strong epidemiological and genetic evidence for correlations between serum IGF-1 and bone mass in humans and mice. Nonetheless, the ablation of liver IGF-1 or the acid labile subunit, which prevents IGF-1 degradation, together provide incontrovertible evidence that circulating rather than tissue IGF-1 regulates bone mass (88). These studies also show that the elevated growth hormone levels do not compensate for circulating IGF-1 deficiency.
Hormones that inhibit bone resorption, namely estrogen, calcitonin and TSH do so through direct osteoclastic actions. In contrast, most pro-resorptive stimuli including PTH and 1,25-dihydroxyvitamin D3 first stimulate the osteoblast, which then secretes RANK-L. The exception is FSH. Unlike calcitonin, estrogen does not inhibit bone resorption by mature osteoclasts. Instead, it attenuates osteoclast formation by reducing JNK/AP-1 activation directly; by decreasing stromal cell cytokine release indirectly; and via T lymphocytes by reducing TNF and increasing interferon-g production (89). Estrogen also prevents osteoblast apoptosis through non-genomic actions of unclear relevance (90). Nonetheless, despite being used widely for osteoporosis therapy, we remain uncertain which of these mechanisms are of physiologic importance. In contrast, the exquisite sensitivity of an osteoclast to calcitonin arises from over a million receptors per cell that couple to either Gs or Gq (91). Despite this mechanistic clarity, we remain uncertain if calcitonin is a true in vivo regulator of bone resorption. This is because deletion of calcitonin and its alternate splice product, CGRP, or the calcitonin receptor (CTR) yields unexpected high bone mass rather than osteopenic phenotypes (91-92). In contrast, mice deficient in the related peptide amylin display osteopenia. However, it is unlikely that amylin interacts with the CTR as compound amylin-/-/CTR-/- mice have a dual phenotype (93).
We have recently reported novel actions of the anterior pituitary hormones TSH and FSH on bone resorption. TSH reduces osteoclast formation, function and survival consistent with the osteopenic phenotype in TSH receptor (TSHR) haploinsufficient mice (94) (this is discussed in greater detail in the next section). In contrast, FSH stimulates osteoclast formation and function congruent with the high bone mass phenotype of FSHb haploinsufficient mice (95). TSH and FSH oppositely affect NF-kB activation, whereas TSH additionally inhibits JNK/c-jun and FSH stimulates both Erk and Akt via Gi2a.
9. BONE LOSS FROM ALTERATIONS IN THE PITUITARY OR THYROID AXIS
The osteoporosis of hyperthyroidism has similarly been attributed to high thyroxine levels. However, TSHR haploinsufficient mice are osteopenic with enhanced osteoclastogenesis, despite normal follicular structure and serum thyroxine (94). Supplemented euthyroid patients with TSH receptor mutations likewise experience significant high-turnover bone loss (96). More impressive, however, is the tight correlation between serum TSH levels and fracture risk in hyperthyroid patients, as well as recent evidence for the direct suppression of bone remodeling by recombinant TSH in post-menopausal women (97,98). Together, the evidence suggests that TSH regulates bone mass, and that its deficiency causes bone loss.
Thyroid dysfunction and thyroid hormone therapy have been associated with derangements in bone metabolism. Hyperthyroidism, a state characterized by elevated thyroid hormone levels and suppressed TSH levels, is associated with elevated bone turnover and ultimately decreased bone mass. In hyperthyroid patients each bone resorption cycle is shortened and excessive (99). Compounding the excessive bone degradation, absorption of intestinal calcium and phosphate is decreased, while dermal, fecal, and urinary calcium losses are increased; these changes produce a state of negative calcium balance that negatively impacts bone mass (100). This increased bone turnover in hyperthyroid states is evident in clinical markers of bone turnover; there are increases in bone-specific alkaline phosphatase, osteocalcin, carboxyterminal propeptide of type 1 collagen (P1NP), and carboxy- and N- terminal cross-linked telopeptides of type 1 collagen (CTx and NTx, respectively) (101-103).
Ten years ago, the prevailing paradigm was that thyroid hormones (T3/T4) directly impacted bone metabolism. This notion was supported by numerous longitudinal studies demonstrating associations between the thyroid state and bone status (Table 1) (104-114). In laboratory studies, thyroid hormones were shown to directly activate bone resorption through the nuclear thyroid hormone receptors (TR) α and β (115). Further studies served to elucidate that thyroid hormone-increased osteoclastic resorptive activity occurred indirectly through osteoblasts and fibroblast growth factor receptor-1 (FGFR1); various inflammatory cytokines and other hormones were implicated in thyroid hormone-induced bone loss, including IL-6, PGE2, PTH, and 1,25-dihydroxyvitamin D (116-117).
Thus, thyroid hormones are intertwined amongst the cytokines and hormones regulating bone metabolism. However, with the analysis of thyroid hormone receptor knockouts, it became apparent that the function of thyroid hormones was not to regulate bone metabolism but rather to regulate bone morphogenesis and development.
In bone, the thyroid hormone receptor TRα is expressed at higher levels than TRβ and has thus been thought to be functionally predominant (118-119). TRα-/- mice, which lack TRα, but are biochemically euthyroid because of compensation through TRβ, have defects in skeletal morphogenesis (120) (see Table 2). On the other hand, TRβ-/- mice, which lack TRβ isoforms and have thyroid hormone resistance, do not have defects in skeletal morphogenesis (121-122). TRα-/-β-/- mice do not develop any bone remodeling defects, but rather, manifest skeletal maturation defects with runting and growth plate abnormalities (123).
Despite evidence linking thyroid hormones to bone morphogenesis and skeletal development, clinically, the adverse effects of thyrotoxicosis is due to excess bone resorption (114,124-131). This incongruence prompted us to challenge the notion that thyroid hormone was the key hormone responsible for altering bone metabolism. In 2003, we discovered that thyroid-stimulating hormone (TSH; thyrotropin) can directly regulate bone remodeling (132). We generated euthyroid mice heterozygous for a deletion of the TSH receptor (TSHR) gene and found that these animals had reduced BMD and increased markers of bone turnover (133). Similarly, homozygous TSHR-/- mice made euthyroid through repletion with levo-thyroxine also had a reduced BMD (132). These two animal models suggested that derangement of the TSH levels could affect bone metabolism despite a normal thyroid axis. On characterizing the in vivo bone phenotype of these animals, we found that they had high-turnover osteoporosis with focal osteosclerosis, which is typically associated with uncoupling between bone formation and resorption (132). Ex vivo studies indicated that TSH affects osteoclast and osteoblast function and formation via multiple mechanisms (see Figure 2). Initially, we found that TSH increased osteoclast apoptosis, and decreased osteoblast differentiation and collagen synthesis (132-134). RANKL-induced osteoclastogenesis is inhibited by TSH at two signal transduction steps involving inhibitor kBα (IkBα) and JNK, which in turn lead to depressed nuclear levels of the pro-osteoclastogenic transcription factors c-jun and p65 (132,135,136). The depressed c-jun levels impair tartrate-resistant acidic phosphatase (TRAP) and cathepsin K gene transcription, which are critical to osteoclastic bone resorption (137).
Further experiments defined TNFα as the critical cytokine mediates the downstream effects of TSH (138). Specifically, TSH inhibits cytokine-induced TNFα production in the bone marrow microenvironment (138). Crossing the TSHR-/- animals to TNFα-/- animals showed that the effect of TSH on bone metabolism could be reversed, thus providing in vivo evidence linking TSH and TNFα (138-139).
10. CLINICAL CORRELATIONS AND RECOMMENDATIONS
In line with the discovery that TSH can directly regulate bone metabolism in a thyroid hormone-independent manner, a 2004 review of population-based studies by Murphy and Williams concluded that endogenous and exogenous TSH suppression, but not thyroid hormone therapy per se, was associated with an increased fracture risk; this adverse effect did not necessarily correlate with decreased bone mineral density (BMD) (140). A high fracture risk in the face of a normal bone density is not surprising and is usually an accompaniment of high dose glucocorticoid therapy, immunosuppression and acute immobilization (141). Consistent with this, two cross-sectional studies, which did not report fracture risk, showed no adverse effect of thyroid hormone suppression therapy on biochemical or densitometric parameters of skeletal integrity (142-143).
Moreover, in subjects with TSH receptor mutations, but euthyroid due to l-thyroxine supplementation, there is decreased BMD also with elevated markers of bone remodeling, such as osteocalcin and N-telopeptide (see Table 1) (128). These physiological correlations are consistent with the clinical observations mentioned above, in which patients with suppressed TSH levels have decreased BMD and increased fracture risk.
Similar evidence also comes from an impressive epidemiological study by Bauer and colleagues, who used serum TSH levels to predict the risk of fracture in hyperthyroid post-menopausal women (144). Most impressive was the increase in vertebral and hip fracture risk by 4.5- and 3.3-fold, respectively, when serum TSH was <0.1 mIU/ml (144). No correlations between thyroid hormone levels and fracture risk were observed. Rather than being a causal relationship, this correlation could arise because TSH suppression is more sensitive index of thyroid hormone excess than thyroid hormone levels per se. However, subjects with TSH receptor mutations rendered euthyroid through thyroid hormone supplementation were found to display a high turnover osteoporosis (see Table 1) (128). Thus, in the light of genetic and pharmacological evidence for direct effects of TSH on bone, it is highly likely that a low TSH contributes to hyperthyroid bone loss.
Because of the strong correlation between low TSH levels and a high fracture risk, which appears to be dissociable from long-term decrements in bone mineral density, we suggest maintaining TSH levels during replacement therapy to above 1 mU/mL, unless there is a clinical rationale for TSH suppression as in thyroid cancer patients. In these patients, admittedly without clinical evidence of efficacy, we propose the empiric use of an oral bisphosphonate to prevent the high turnover osteoporosis and associated fracture risk, which appears to be highly correlated to a TSH level of <0.1 mU/mL.
11. ACKNOWLEDGEMENTS
MZ gratefully acknowledges the support of the National Institutes of Health (AG14907, DK70526, and AG23176) and the Department of Veteran Affairs (Merit Award and GRECC).
12. REFERENCES
1. Boisvert, C.A: The pelvic fin and girdle of Panderichthys and the origin of tetrapod locomotion. Nature 438, 1145-1147 (2005)
doi:10.1038/nature04119
PMid:16372007
2. Wellik, D.M, M.R.Capecchi : Hox10 and Hox11 genes are required to globally pattern the mammalian skeleton. Science 301, 363-367 (2003)
doi:10.1126/science.1085672
PMid:12869760
3. Kmita, M., B. Tarchini, J. Zakany, M. Logan, C.J. Tabin, D.Duboule: Early developmental arrest of mammalian limbs lacking HoxA/HoxD gene function. Nature 435, 1113-1116 (2005)
doi:10.1038/nature03648
PMid:15973411
4. Hornstein, E., J.H.Mansfield, S.Yekta, J.K.Hu, B.D. Harfe, M.T. McManus, S. Baskerville, D.P.Bartel, C.J.Tabin: The microRNA miR-196 acts upstream of Hoxb8 and Shh in limb development. Nature 438, 671-674 (2005)
doi:10.1038/nature04138
PMid:16319892
5. Mundlos, S, B.R. Olsen: Heritable diseases of the skeleton. Part II: Molecular insights into skeletal development-matrix components and their homeostasis. FASEB J. 11, 227-233 (1997)
6. Rallis, C., B.G. Bruneau, J. Del Buono, C.E. Seidman, J.G. Seidman, S. Nissim, C.J. Tabin, M.P. Logan: Tbx5 is required for forelimb bud formation and continued outgrowth. Development 130, 2741-2751 (2003)
doi:10.1242/dev.00473
PMid:12736217
7. Zuniga, A., A.P.Haramis, A.P. McMahon, R. Zeller: Signal relay by BMP antagonism controls the SHH/FGF4 feedback loop in vertebrate limb buds. Nature 401, 598-602 (1999)
doi:10.1038/44157
PMid:10524628
8. Scherz, P.J., B.D.Harfe, A.P.McMahon, C.J. Tabin: The limb bud Shh-Fgf feedback loop is terminated by expansion of former ZPA cells. Science 305, 396-399 (2004)
doi:10.1126/science.1096966
PMid:15256670
9. Harfe, B.D., P.J. Scherz, S. Nissim, H. Tian, A.P. McMahon, C.J.Tabin: Evidence for an expansion-based temporal Shh gradient in specifying vertebrate digit identities. Cell 20, 517-528 (2004)
doi:10.1016/j.cell.2004.07.024
PMid:15315763
10. Pacifici, M., E. Koyama, M. Iwamoto: Mechanisms of synovial joint and articular cartilage formation: recent advances, but many lingering mysteries. Birth Defects Res. C Embryo Today. 75, 237-248 (2005)
doi:10.1002/bdrc.20050
11. Iwamoto, M., Y.Higuchi, E. Koyama, M.Enomoto-Iwamoto, H.Yeh, W.R. Abrams, J.Rosenbloom, M. Pacifici: Transcription factor ERG variants and functional diversification of chondrocytes during long bone development. J. Cell Biol. 150, 27-39 (2000)
doi:10.1083/jcb.150.1.27
PMid:10893254 PMCid:2185572
12. Guo, X., T.F.Day, X. Jiang, L.Garrett-Beal, L.Topol, Y.Yang: Wnt/beta-catenin signaling is sufficient and necessary for synovial joint formation. Genes Dev. 18, 2404-2417 (2004)
doi:10.1101/gad.1230704
13. St-Jacques, B., M.Hammerschmidt, A.P.McMahon,: Indian hedgehog signaling regulates proliferation and differentiation of chondrocytes and is essential for bone formation. Genes Dev. 13, 2072-2086 (1999)
doi:10.1101/gad.13.16.2072
14. Settle, S.H. Jr., R.B. Rountree, A. Sinha, A.Thacker, K.Higgins, D.M. Kingsley: Multiple joint and skeletal patterning defects caused by single and double mutations in the mouse Gdf6 and Gdf5 genes. Dev. Biol. 254, 116-130 (2003)
doi:10.1016/S0012-1606(02)00022-2
PMid:12606286
15. Meech, R., D.B. Edelman, F.S. Jones, H.P. Makarenkova: The homeobox transcription factor Barx2 regulates chondrogenesis during limb development. Development 132, 2135-2146 (2005)
doi:10.1242/dev.01811
PMid:15800003
16. Rodrigo, I., R.E. Hill, R.Balling, A. Munsterberg, K.Imai: Pax1 and Pax9 activate Bapx1 to induce chondrogenic differentiation in the sclerotome. Development 130, 473-482 (2003)
doi:10.1242/dev.00240
PMid:12490554
17. Lefebvre, V., P. Li, B.de Crombrugghe: A new long form of Sox5 (L-Sox5), Sox6 and Sox9 are coexpressed in chondrogenesis and cooperatively activate the type II collagen gene. EMBO J. 17, 5718-5733 (1998)
doi:10.1093/emboj/17.19.5718
PMid:9755172 PMCid:1170900
18. Smits, P., P. Dy, S. Mitra, V. Lefebvre: Sox5 and Sox6 are needed to develop and maintain source, columnar, and hypertrophic chondrocytes in the cartilage growth plate. J Cell Biol. 164, 747-758 (2004)
doi:10.1083/jcb.200312045
PMid:14993235 PMCid:2172159
19. Robledo, R.F., L. Rajan, X. Li, T. Lufkin: The Dlx5 and Dlx6 homeobox genes are essential for craniofacial, axial, and appendicular skeletal development. Genes Dev. 16, 1089-1101 (2002)
doi:10.1101/gad.988402
20. Vega, R.B., K. Matsuda, J. Oh, A.C.Barbosa, X.Yang, E.Meadows, J. McAnally, C. Pomajzl, J.M. Shelton, J.A. Richardson, G. Karsenty, E.N. Olson: Histone deacetylase 4 controls chondrocyte hypertrophy during skeletogenesis. Cell 119, 555-566 (2004)
doi:10.1016/j.cell.2004.10.024
PMid:15537544
21. Shimo, T., C.Gentili, M.Iwamoto, C.Wu, E.Koyama, M. Pacifici: Indian hedgehog and syndecans-3 coregulate chondrocyte proliferation and function during chick limb skeletogenesis. Dev Dyn. 229, 607-617 (2004)
doi:10.1002/dvdy.20009
PMid:14991716
22. Kobayashi, T., D.W. Soegiarto, Y.Yang, B. Lanske, E. Schipani, A.P. McMahon, H.M.Kronenberg: Indian hedgehog stimulates periarticular chondrocyte differentiation to regulate growth plate length independently of PTHrP. J. Clin. Invest. 115, 1734-1742 (2005)
doi:10.1172/JCI24397
PMid:15951842 PMCid:1143590
23. Enomoto-Iwamoto, M., J.Kitagaki, E. Koyama, Y. Tamamura, C.Wu, N. Kanatani, T.Koike, H. Okada, T. Komori, T. Yoneda, V.Church, P.Francis-West, K. Kurisu, T. Nohno, M. Pacifici, M. Iwamoto: The Wnt antagonist Frzb-1 regulates chondrocyte maturation and long bone development during limb skeletogenesis. Dev. Biol. 251, 142-156 (2002)
doi:10.1006/dbio.2002.0802
PMid:12413904
24. Alvarez, J., P.Sohn, X.Zeng, T.Doetschman, D.J.Robbins, R.Serra: TGFbeta2 mediates the effects of hedgehog on hypertrophic differentiation and PTHrP expression. Development 129, 1913-1924 (2002)
25. Bastepe, M., L.S.Weinstein, N.Ogata, H.Kawaguchi, H.Juppner, H.M.Kronenberg, U.I.Chung: Stimulatory G protein directly regulates hypertrophic differentiation of growth plate cartilage in vivo. Proc. Natl. Acad. Sci. USA. 101, 14794-14799 (2004)
doi:10.1073/pnas.0405091101
26. Sakamoto, A., M.Chen, T. Kobayashi, H.M.Kronenberg, L.S.Weinstein: Chondrocyte-specific knockout of the G protein G (s)alpha leads to epiphyseal and growth plate abnormalities and ectopic chondrocyte formation. J. Bone Miner. Res. 20, 663-671 (2005)
doi:10.1359/JBMR.041210
PMid:15765186
27. Kronenberg, H.M. PTHrP and skeletal development. Ann. NY Acad. Sci. In the press.
28. MacLean, H.E., J. Guo, M.C.Knight, P.Zhang, D. Cobrinik, H.M.Kronenberg: The cyclin-dependent kinase inhibitor p57 (Kip2) mediates proliferative actions of PTHrP in chondrocytes. J. Clin. Invest. 113, 1334-1343 (2004)
29. Huang, W., U.I.Chung, H.M.Kronenberg, B.de Crombrugghe: The chondrogenic transcription factor Sox9 is a target of signaling by the parathyroid hormone-related peptide in the growth plate of endochondral bones. Proc. Natl. Acad. Sci. USA. 98, 160-165 (2001)
doi:10.1073/pnas.011393998
30. MacLean, H.E., J.I.Kim, M.J.Glimcher, J.Wang, H.M. Kronenberg, L.H.Glimcher: Absence of transcription factor c-maf causes abnormal terminal differentiation of hypertrophic chondrocytes during endochondral bone development. Dev Biol. 262, 51-63 (2003)
doi:10.1016/S0012-1606(03)00324-5
PMid:14512017
31. Long, F., U.IChung, S.Ohba, J.McMahon, H.M.Kronenberg, A.P.McMahon: Ihh signaling is directly required for the osteoblast lineage in the endochondral skeleton. Development 131, 1309-1318 (2004)
32. Bialek, P., B.Kern, X.Yang, M.Schrock, D.Sosic, N.Hong, H.Wu, K.Yu, D.M.Ornitz, E.N.Olson, M.J.Justice, G.Karsenty: A twist code determines the onset of osteoblast differentiation. Dev. Cell 6, 423-435 (2004)
doi:10.1016/S1534-5807(04)00058-9
PMid:15030764
33. Zelzer, E., R.Mamluk, N.Ferrara, R.S.Johnson, E.Schipani B.Olsen: VEGFA is necessary for chondrocyte survival during bone development. Development 131, 2161-2171 (2004)
doi:10.1242/dev.01053
PMid:15073147
34. Zelzer, E., W.McLean, Y.S.Ng, N.Fukai, A.M. Reginato, S.Lovejoy, P.A. D'Amore, B.R. Olsen: Skeletal defects in VEGF (120/120) mice reveal multiple roles for VEGF in skeletogenesis. Development 129, 1893-1904 (2002)
35. Pfander, D., T. Kobayashi, M.C.Knight, E.Zelzer, D.A.Chan, B.R.Olsen, A.J.Giaccia, R.S.Johnson, V.H.Haase, E.Schipani: Deletion of Vhlh in chondrocytes reduces cell proliferation and increases matrix deposition during growth plate development. Development 131, 2497-2508 (2004)
doi:10.1242/dev.01138
PMid:15128677
36. Stickens, D., D.J. Behonick, N.Ortega, B. Heyer, B. Hartenstein, Y. Yu, A.J. Fosang, M. Schorpp-Kistner, P.Angel, Z.Werb: Altered endochondral bone development in matrix metalloproteinase 13-deficient mice. Development. 131, 5883-5895 (2004)
doi:10.1242/dev.01461
PMid:15539485
37. Zhou, Z., S.S.Apte, R. Soininen, R. Cao, G.Y.Baaklini, R.W. Rauser, J. Wang, Y. Cao, K. Tryggvason: Impaired endochondral ossification and angiogenesis in mice deficient in membrane-type matrix metalloproteinase I. Proc. Natl. Acad. Sci. USA. 97, 4052-4057 (2000)
doi:10.1073/pnas.060037197
38. Fini, M., G. Giavaresi, P. Torricelli, V. Borsari, R. Giardino, A. Nocolini, A. Carpi: Osteoporosis and biomaterial osteointegration. Biomed Pharmacother. 58, 487-439 (2004)
39. Boskey, A.L., L. Spevak, E. Paschalis, S.B. Doty, M.D. McKee: Osteopontin deficiency increases mineral content and mineral crystallinity in mouse bone. Calcif. Tiss. Int. 71, 145-154 (2002)
doi:10.1007/s00223-001-1121-z
PMid:12073157
40. Boskey, A.L., D.J.Moore, M.Amling, E. Canalis, A.M. Delany: Infrared analysis of the mineral and matrix in bones of osteonectin-null mice and their wildtype controls. J. Bone Miner. Res. 18, 1005-1011 (2003)
doi:10.1359/jbmr.2003.18.6.1005
PMid:12817752
41. Ling, Y., H.F. Rios, E.R. Myers, Y. Lu, J.Q. Feng, A.L. Boskey: DMP1 depletion decreases bone mineralization in vivo: an FTIR imaging analysis. J. Bone Miner. Res. 20, 2169-2177 (2005)
doi:10.1359/JBMR.050815
PMid:16294270 PMCid:1456072
42. Doecke, J.D., C.J. Day, A.S. Stephens, S.L. Carter, A. van Daal, M.A. Kotowicz, G.C. Nicholson, N.A. Morrison: Association of functionally different RUNX2 P2 promoter alleles with BMD. J. Bone Miner. Res. 21, 265-273 (2006)
doi:10.1359/JBMR.051013
PMid:16418782
43. Kaneki, H., R. Guo, D. Chen, Z. Yao, E.M. Schwarz, Y.E. Zhang, B.F. Boyce, L. Xing: Tumor necrosis factor promotes RUNX2 degradation through up-regulation of SMURF1 and SMURF2 in osteoblasts. J. Biol. Chem. 281, 4326-4333 (2006) .
doi:10.1074/jbc.M509430200
PMid:16373342 PMCid:2647592
44. Koga, T., Y. Matsui, M. Asagiri, T. Kodama, B. de Crombrugghe, K. Nakashima, H. Takayanagi: NFAT and Osterix cooperatively regulate bone formation. Nat. Med. 11, 880-885 (2005)
doi:10.1038/nm1270
PMid:16041384
45. Sun, L., H.C. Blair, Y. Peng, N. Zaidi, O.A. Adebanjo, X.B. Wu, X.Y. Wu, J. Iqbal, S. Epstein, E. Abe, B.S. Moonga, M. Zaidi: Calcineurin regulates bone formation by the osteoblast. Proc. Natl. Acad. Sci. USA. 102, 17130-17135 (2005)
doi:10.1073/pnas.0508480102
46. Yang, X., K. Matsuda, P. Bialek, S. Jacquot, H.C.Masuoka, T. Schinke, L. Li, S. Brancorsini, P. Sassone-Corsi, T.M.Townes, A. Hanauer, G. Karsenty: ATF4 is a substrate of RSK2 and an essential regulator of osteoblast biology; implication for Coffin-Lowry Syndrome. Cell 117, 387-398 (2004)
doi:10.1016/S0092-8674(04)00344-7
PMid:15109498
47. Yu, V.W., G. Ambartsoumian, L. Verlinden, J.M. Moir, J. Prud'homme, C. Gauthier, P.J. Roughley, R. St-Arnaud: FIAT represses ATF4-mediated transcription to regulate bone mass in transgenic mice. J. Cell Biol. 169, 591-601 (2005)
doi:10.1083/jcb.200412139
PMid:15911876 PMCid:2171686
48. Bodine, P.V., W. Zhao, Y.P. Kharode, F.J.Bex, A.J.Lambert, M.B. Goad, T. Gaur, G.S. Stein, J.B. Lian, B.S.Komm: The Wnt antagonist secreted frizzled-related protein-1 is a negative regulator of trabecular bone formation in adult mice. Mol. Endocrinol. 18, 1222-1237 (2004)
doi:10.1210/me.2003-0498
PMid:14976225
49. Tian, E., F. Zhan, R. Walker, E. Rasmussen, Y. Ma, B. Barlogie, J.D.Jr Shaughnessy: The role of the Wnt-signaling antagonist DKK1 in the development of osteolytic lesions in multiple myeloma. N. Engl. J. Med. 349, 2483-2494 (2003)
doi:10.1056/NEJMoa030847
PMid:14695408
50. Semenov, M., K. Tamai, X. He: SOST is a ligand for LRP5/LRP6 and a Wnt signaling inhibitor. J. Biol. Chem. 280, 26770-26775 (2005)
doi:10.1074/jbc.M504308200
PMid:15908424
$Title
51. Li, X., P. Liu, W. Liu, P. Maye, J. Zhang, Y. Zhang, M. Hurley, C. Guo, A. Boskey, L. Sun, S.E. Harris, D.W. Rowe, H.Z. Ke, D. Wu: Dkk2 has a role in terminal osteoblast differentiation and mineralized matrix formation. Nature Genet. 37, 945-952 (2005)
doi:10.1038/ng1614
PMid:16056226
52. Zhang, Y., Y. Wang, X. Li, J. Zhang, J. Mao, Z. Li, J. Zheng, L. Li, S. Harris, D. Wu: The LRP5 high-bone-mass G171V mutation disrupts LRP5 interaction with Mesd. Mol. Cell Biol. 24, 4677-4684 (2004)
doi:10.1128/MCB.24.11.4677-4684.2004
PMid:15143163 PMCid:416395
53. Bennett, C.N., K.A. Longo, W.S. Wright, L.J.Suva, T.F. Lane, K.D. Hankenson, O.A. MacDougald: Regulation of osteoblastogenesis and bone mass by Wnt10b. Proc. Natl. Acad. Sci. USA. 102, 3324-3329 (2005)
doi:10.1073/pnas.0408742102
54. Almeida, M., L. Han, T. Bellido, S.C. Manolagas, S. Kousteni: Wnt proteins prevent apoptosis of both uncommitted osteoblast progenitors and differentiated osteoblasts by beta-catenin-dependent and -independent signaling cascades involving Src/ERK and phosphatidylinositol 3-kinase/AKT. J. Biol. Chem. 280, 41342-41351 (2005)
doi:10.1074/jbc.M502168200
PMid:16251184
55. Glass, D.A. 2nd, P. Bialek, J.D. Ahn, M. Starbuck, M.S. Patel, H. Clevers, M.M.Taketo, F. Long, A.P. McMahon, R.A. Lang, G. Karsenty: Canonical Wnt signaling in differentiated osteoblasts controls osteoclast differentiation. Dev. Cell 8, 751-764 (2005)
doi:10.1016/j.devcel.2005.02.017
PMid:15866165
56. Kieslinger, M., S. Folberth, G. Dobreva, T. Dorn, L. Croci, R. Erben, G.G. Consalez, R. Grosschedl: EBF2 regulates osteoblast-dependent differentiation of osteoclasts. Dev. Cell 9, 757-767 (2005)
doi:10.1016/j.devcel.2005.10.009
PMid:16326388
57. Zhang, J., C. Niu, L. Ye, H. Huang, X.He, W.G. Tong, J. Ross, J. Haug, T. Johnson, J.Q. Feng, S. Harris, L.M. Wiedemann, Y. Mishina, L. Li: Identification of the haematopoietic stem cell niche and control of the niche size. Nature 425, 836-841 (2003)
doi:10.1038/nature02041
PMid:14574412
58. Calvi, L.M., G.B. Adams, K.W. Weibrecht, J.M. Weber, D.P. Olson, M.C. Knight, R.P. Martin, E. Schipani, P. Divieti, F.R. Bringhurst, L.A. Milner, H.M. Kronenberg, D.T. Scadden: Osteoblastic cells regulate the haematopoietic stem cell niche. Nature 425, 841-846 (2003)
doi:10.1038/nature02040
PMid:14574413
59. Visnjic, D., Z. Kalajzic, D.W. Rowe, V. Katavic, J. Lorenzo, H.L. Aguila: Hematopoiesis is severely altered in mice with an induced osteoblast deficiency. Blood 103, 3258-3264 (2004)
doi:10.1182/blood-2003-11-4011
PMid:14726388
60. Katayama, Y., M. Battista, W. M.Kao, A. Hidalgo, A. Peired, S. Thomas, P. Frenette: Signals from the sympathetic nervous system regulate hematopoetic stem cell egress from bone marrow. Cell 124, 407-421 (2006)
doi:10.1016/j.cell.2005.10.041
PMid:16439213
61. Rubin, J., C. Rubin, C.R. Jacobs: Molecular pathways mediating mechanical signaling in bone. Gene 367, 1-16 (2006)
doi:10.1016/j.gene.2005.10.028
PMid:16361069
62. Gross, T.S., N. Akeno, T.L. Clemens, S. Komarova, S. Srinivasan, D.A. Weimer, S. Mayorov: Selected Contribution: Osteocytes upregulate HIF-1alpha in response to acute disuse and oxygen deprivation. J. Appl. Physiol. 90, 2514-2519 (2001)
63. Koga, T., M. Inui, K. Inoue, S. Kim, A. Suematsu, E. Kobayashi, T. Iwata, H. Ohnishi, T. Matozaki, T. Kodama, T. Taniguchi, H. Takayanagi, T. Takai: Costimulatory signals mediated by the ITAM motif cooperate with RANKL for bone homeostasis. Nature. 428, 758-763 (2004)
doi:10.1038/nature02444
PMid:15085135
64. Mocsai, A., M.B. Humphrey, J.A.Van Ziffle, Y. Hu, A. Burghardt, S.C. Spusta, S. Majumdar, L.L.Lanier, C.A. Lowell, M.C. Nakamura: The immunomodulatory adapter proteins DAP12 and Fc receptor gamma-chain (FcRgamma) regulate development of functional osteoclasts through the Syk tyrosine kinase. Proc. Natl. Acad. Sci. USA. 101, 6158-6163 (2004)
doi:10.1073/pnas.0401602101
65. Takayanagi, H., S. Kim, T. Koga, H. Nishina, M. Isshiki, H. Yoshida, A. Saiura, M. Isobe, T. Yokochi, J. Inoue, E.F. Wagner, T.W. Mak, T. Kodama, T. Taniguchi: Induction and activation of the transcription factor NFATc1 (NFAT2) integrate RANKL signaling in terminal differentiation of osteoclasts. Dev. Cell 3, 889-901 (2002)
doi:10.1016/S1534-5807(02)00369-6
PMid:12479813
66. Gohda, J., T. Akiyama, T. Koga, H. Takayanagi, S. Tanaka, J. Inoue: RANK-mediated amplification of TRAF6 signaling leads to NFATc1 induction during osteoclastogenesis. EMBO J. 24, 790-799 (2005)
doi:10.1038/sj.emboj.7600564
PMid:15678102 PMCid:549623
67. Matsuo, K., D.L. Galson, C. Zhao, L.Peng, C. Laplace, K.Z. Wang, M.A. Bachler, H. Amano, H. Aburatani, H. Ishikawa, E.F. Wagner: Nuclear factor of activated T-cells (NFAT) rescues osteoclastogenesis in precursors lacking c-Fos. J. Biol. Chem. 279, 26475-26480 (2004)
doi:10.1074/jbc.M313973200
PMid:15073183
68. Kim, Y., K. Sato, M. Asagiri, I. Morita, K.Soma, H. Takayanagi: Contribution of nuclear factor of activated T cells c1 to the transcriptional control of immunoreceptor osteoclast-associated receptor but not triggering receptor expressed by myeloid cells-2 during osteoclastogenesis. J. Biol. Chem. 280, 32905-32913 (2005)
doi:10.1074/jbc.M505820200
PMid:16046394
69. Asagiri, M., K. Sato, T. Usami, S. Ochi, H. Nishina, H. Yoshida, I. Morita, E.F. Wagner, T.W. Mak, E. Serfling, H. Takayanagi: Autoamplification of NFATc1 expression determines its essential role in bone homeostasis. J. Exp. Med. 202, 1261-1269 (2005)
doi:10.1084/jem.20051150
PMid:16275763 PMCid:2213228
70. Kawaida, R., T. Ohtsuka, J. Okutsu, T. Takahashi, Y. Kadono, H. Oda, A. Hikita, K. Nakamura, S. Tanaka, H. Furukawa: Jun dimerization protein 2 (JDP2), a member of the AP-1 family of transcription factor, mediates osteoclast differentiation induced by RANKL. J. Exp. Med. 197, 1029-1035 (2003)
doi:10.1084/jem.20021321
PMid:12707301 PMCid:2193879
71. Faccio, R., S.L. Teitelbaum, K. Fujikawa, J. Chappel, A. Zallone, V.L. Tybulewicz, E.P. Ross, W. Swat: Vav3 regulates osteoclast function and bone mass. Nat. Med. 11, 284-290 (2005)
doi:10.1038/nm1194
PMid:15711558
72. Bruzzaniti, A., L. Neff, A. Sanjay, W.C.Horne, P. De Camilli, R. Baron: Dynamin forms a Src kinase-sensitive complex with Cbl and regulates podosomes and osteoclast activity. Mol. Biol. Cell. 16, 3301-3313 (2005)
doi:10.1091/mbc.E04-12-1117
PMid:15872089 PMCid:1165412
73. Zaidi, M., B.S. Moonga, C.L.Huang: Calcium sensing and cell signaling processes in the local regulation of osteoclastic bone resorption. Biol. Rev. Camb. Philos. Soc. 79, 79-100 (2004)
doi:10.1017/S1464793103006262
PMid:15005174
74. van der Eerden, B.C., J.G. Hoenderop, T. J. de Vries, T. Schoenmaker, C.J. Buurman, A.G. Uitterlinden, H.A. Pols, R.J. Bindels, J.P.van Leeuwen: The epithelial Ca2+ channel TRPV5 is essential for proper osteoclastic bone resorption. Proc. Natl. Acad. Sci. USA. 102, 17507-17512 (2005)
doi:10.1073/pnas.0505789102
75. Zaidi, M., V.S. Shankar, R.E. Tunwell, O.A. Adebanjo, M.Pazianas, B. Simon, B. R. Rifkin, A. Venkitaraman, C.L.-H.Huang, F.A.Lai: A ryanodine receptor-like molecule in the osteoclast plasma membrane is a functional component of the osteoclast Ca2+ sensor. J. Clin. Invest. 96, 1582-1590 (1995)
doi:10.1172/JCI118197
PMid:7657829 PMCid:185784
76. Moonga, B.S., L. Sun, J. Iqbal, R. Davidson, V.S. Shankar, P.J.R.Bevis, A. Inzerillo, E. Abe, C.L.-H. Huang, M. Zaidi: Ca2+ influx through the osteoclast plasma membrane ryanodine receptor. Am. J. Physiol. 282, F921-F932 (2002)
77. Sun, L., J. Iqbal, S. Dolgilevich, T.Yuen, X.B.Wu, B.S. Moonga, O.A. Adebanjo, P.J.R. Bevis, F. Lund, C.L-H.Huang, H.C.Blair, E. Abe, M.Zaidi: Disordered osteoclast formation and function in CD38 (ADP-ribosyl cyclase)-deficient mouse establishes an essential role for CD38 in bone resorption. FASEB J. 17, 369-375 (2003)
doi:10.1096/fj.02-0205com
PMid:12631576
78. Takeshita, S., N. Namba, J.J. Zhao, Y. Jiang, H.K. Genant, M.J.Silva, M.D. Brodt, C.D.Helgason, J. Kalesnikoff, M.J.Rauh, R.K. Humphries, G. Krystal, S.L.Teitelbaum, F.P.Ross: SHIP-deficient mice are severely osteoporotic due to increased numbers of hyper-resorptive osteoclasts. Nat. Med. 8, 943-949 (2002)
doi:10.1038/nm752
PMid:12161749
79. Akiyama, T., P.Bouillet, T.Miyazaki, Y.Kadono, H.Chikuda, U.I.Chung, A. Fukuda, A. Hikita, H.Seto, T.Okada, T.Inaba, A.Sanjay, R. Baron, H. Kawaguchi, H. Oda, K.Nakamura, A. Strasser, S.Tanaka: Regulation of osteoclast apoptosis by ubiquitylation of proapoptotic BH3-only Bcl-2 family member Bim. EMBO J. 22, 6653-6664 (2003)
doi:10.1093/emboj/cdg635
PMid:14657036 PMCid:291830
80. Ducy, P., M. Amling, S. Takeda, M. Priemel, A.F. Schilling, F.T. Beil, J. Shen, C. Vinson, J.M. Rueger, G. Karsenty: Leptin inhibits bone formation through a hypothalamic relay: a central control of bone mass. Cell. 100, 197-207 (2000)
doi:10.1016/S0092-8674(00)81558-5
PMid:10660043
81. Takeda, S., F. Elefteriou, R. Levasseur, X. Liu, L.Zhao, K.L.Parker, D. Armstrong, P.Ducy, G. Karsenty: Leptin regulates bone formation via the sympathetic nervous system. Cell 111, 305-317 (2002)
doi:10.1016/S0092-8674(02)01049-8
PMid:12419242
82. Elefteriou, F., J.D. Ahn, S. Takeda, M. Starbuck, X.Yang, X. Liu, H. Kondo, W.G. Richards, T.W. Bannon, M. Noda, K. Clement, C.Vaisse, G. Karsenty: Leptin regulation of bone resorption by the sympathetic nervous system and CART. Nature 434, 514-520 (2005)
doi:10.1038/nature03398
PMid:15724149
83. Baldock, P.A., A. Sainsbury, M.Couzens, R.F.Enriquez, G.P.Thomas, E.M.Gardiner, H. Herzog: Hypothalamic Y2 receptors regulate bone formation. J. Clin. Invest. 109, 915-921 (2002)
84. Elefteriou, F., S.Takeda, X.Liu, D. Armstrong, G. Karsenty: Monosodium glutamate-sensitive hypothalamic neurons contribute to the control of bone mass. Endocrinology 144, 3842-3847 (2003)
doi:10.1210/en.2003-0369
PMid:12933656
85. Schinke, T., S. Liese, M. Priemel, M. Haberland, A.F.Schilling, P. Catala-Lehnen, D. Blicharski, J.M. Rueger, R.F. Gagel, R.B. Emeson, M. Amling: Decreased bone formation and osteopenia in mice lacking alpha-calcitonin gene-related peptide. J. Bone Miner. Res. 19, 2049-2056 (2004)
doi:10.1359/JBMR.040915
PMid:15537449
86. Giardino, R., P.Torricelli, G. Giavaresi, M. Fini, N. Nicoli Aldini, G. Ruggeri, L. Lima, A. Carpi: Histomorphometric bone modification induced by growth hormone treatment in a rabbit model of short bowel syndrome. Biomed Pharmacother. 58, 116-122 (2004)
doi:10.1016/j.biopha.2003.09.011
87. Mohan, S., D.J. Baylink: Impaired skeletal growth in mice with haploinsufficiency of IGF-1: genetic evidence that differences in IGF-1 expression could contribute to peak bone mass differences. J. Endocrinol. 185:415-420 (2005)
doi:10.1677/joe.1.06141
PMid:15930167
88. Yakar, S., C.J. Rosen, W.G. Beamer, C.L. Ackert-Bicknell, Y. Wu, J.L. Liu, G.T. Ooi, J. Setser, J. Frystyk, Y.R. Boisclair, D. LeRoith: Circulating levels of IGF-1 directly regulate bone growth and density. J. Clin. Invest. 110: 771-781 (2002)
89. Cenci, S., G. Toraldo, M.N. Weitzmann, C. Roggia, Y. Gao, W.P. Qian, O. Sierra, R. Pacifici: Estrogen deficiency induces bone loss by increasing T cell proliferation and lifespan through IFN-gamma-induced class II transactivator. Proc. Natl. Acad. Sci. USA. 100, 10405-10410 (2003)
doi:10.1073/pnas.1533207100
90. Kousteni, S., T. Bellido, L.I. Plotkin, C.A O'Brien, D.L. Bodenner, L. Han, K. Han, G.B. DiGregorio, J.A. Katzenellenbogen, B.S. Katzenellenbogen, P.K. Roberson, R.S. Weinstein, R.L. Jilka, S.C. Manolagas: Nongenotropic, sex-nonspecific signaling through the estrogen or androgen receptors: dissociation from transcriptional activity. Cell 104, 719-730 (2001)
91. Zaidi, M., B.S. Moonga, E. Abe: Calcitonin and bone formation: a knockout full of surprises. J. Clin. Invest. 110, 1769-1771 (2002)
92. Hoff, A.O., P. Catala-Lehnen, P.M. Thomas, M. Priemel, J.M. Rueger, I. Nasonkin, A. Bradley, M.R. Hughes, N. Ordonez, G.J. Cote, M. Amling, R.F. Gagel: Increased bone mass is an unexpected phenotype associated with deletion of the calcitonin gene. J. Clin. Invest. 110, 1849-1857 (2002)
93. Dacquin, R., R.A.Davey, C. Laplace, R. Levasseur, H.A. Morris, S.R. Goldring, S. Gebre-Medhin, D.L. Galson, J.D. Zajac, G. Karsenty: Amylin inhibits bone resorption while the calcitonin receptor controls bone formation in vivo. J. Cell Biol. 164, 509-514 (2004)
doi:10.1083/jcb.200312135
PMid:14970190 PMCid:2171986
94. Abe, E., R. Marians, X-B.Wu, J.Iqbal, T.Ando, H.C. Blair, T.F. Davies, M. Zaidi: TSH is a negative regulator of bone remodeling. Cell 175, 151-162 (2003)
doi:10.1016/S0092-8674(03)00771-2
PMid:14567913
95. Sun, L., Y. Peng, A. Sharrow, J.Iqbal, Z. Zhang, D.J. Papachristou, S. Zaudu, L.-L Zhu, B.B. Yaroslavskiy, H. Zhao, A. Zallone, M.R. Sairam, T.R. Kumar, W. Bo, J. Braun, L. Cardoso-Landa, M. Schaffler, B.S.Moonga, H.C. Blair, M. Zaidi: FSH directly regulates bone mass. Cell 125, 247-260 (2006)
doi:10.1016/j.cell.2006.01.051
PMid:16630814
96. Zaidi, M., L. Sun: TSH and bone loss. Ann. NY Acad. Sci. In the press (2006)
97. Bauer, D.C., B. Ettinger, M.C. Nevitt, K.L. Stone: Study of Osteoporotic Fractures Research Group. Risk for fracture in women with low serum levels of thyroid-stimulating hormone. Ann. Intern. Med. 134, 561-568 (2001)
98. Mazziotti, G., F. Sorvillo, M. Piscopo, M. Cioffi, P. Pilla, B. Biondi, S. Iorio, A. Giustina, G. Amato, C. Carella: Recombinant human TSH modulates in vivo C-telopeptides of type-1 collagen and bone alkaline phosphatase, but not osteoprotegerin production in postmenopausal women monitored for differentiated thyroid carcinoma. J. Bone. Miner. Res. 20, 480-486 (2005)
doi:10.1359/JBMR.041126
PMid:15746993
99. Eriksen, E.F., L. Mosekilde, F. Melsen: Trabecular bone remodeling and bone balance in hyperthyroidism. Bone 6, 421-8 (1985)
doi:10.1016/8756-3282(85)90218-2
PMid:4096878
100. Mosekilde, L., E.F. Eriksen, P. Charles: Effects of thyroid hormones on bone and mineral metabolism. Endocrinol Metab Clin North Am 19, 35-63 (1990)
$mAuthor101. Allain, T.J., A.M.McGregor: Thyroid hormones and bone. J Endocrinol 139, 9-18 (1993)
doi:10.1677/joe.0.1390009
PMid:8254298
102. Miyakawa, M., T. Tsushima, H.Demura: Carboxy-terminal propeptide of type 1 procollagen (P1CP) and carboxy-terminal telopeptide of type 1 collagen (1CTP) as sensitive markers of bone metabolism in thyroid disease. Endocr J 43, 701-8 (1996)
doi:10.1507/endocrj.43.701
PMid:9075610
103. Nagasaka, S, H. Sugimoto, T. Nakamura, I. Kusaka, G. Fujisawa, N. Sakuma, Y. Tsuboi, S. Fukuda, K. Hinda, K. Okada, S. Ishikawa, T. Saito: Antithyroid therapy improves bony manifestations and bone metabolic markers in patients with Graves' thyrotoxicosis. Clin Endocrinol (Oxf) 47, 215-21 (1997)
doi:10.1046/j.1365-2265.1997.2401045.x
PMid:9302397
104. Jodar E, L.M. Begona, L. Garcia, D. Rigopoulou, G. Martinez, F. Kawkins: Bone changes in pre- and postmenopausal women with thyroid cancer on levothyroxine therapy: evolution of axial and appendicular bone mass. Osteoporos Internation 8, 311-316 (1998)
doi:10.1007/s001980050069
PMid:10024900
105. Kung AW, S.S.Yeung: Prevention of bone loss induced by thryoxine suppressive therapy in postmenopausal women: the effect of calciumj and calcitonin.J Clin Endocrinol Metab 81, 1232-1236 (1996)
doi:10.1210/jc.81.3.1232
106. Mcdermott MT, J:J.Perloff, G.S.Kidd: A longitudinal assessment of bone loss in women with levothyroxine-suppressed benign thyroid disease and thyroid cancer. Calcif Tissue Internat 56, 521-525 (1995)
doi:10.1007/BF00298581
PMid:7648478
107. Pioli G, M. Pedrazzoni, E. Palummeri, M. Siasesi, R. Del Frate, P.P.Vescovi, M. Prisco, V. Ulietti, D.Costi, M.Passeri: Longitudinal study of bone loss after thyroidectomy and suppressive throxine therapy in premenopausal women. Acta Endocrinol 126, 238-242 (1992)
108. Ribot C, F. Tremollieres, J.M. Pouilles, J.P. Louvet: Bone mineral density and thyroid hormone therapy. Clin Endocrinol 33, 143-153 (1990)
doi:10.1111/j.1365-2265.1990.tb00477.x
PMid:2225474
109. Guo CY, A.P.Weetman, R. Eastell: Longitudinal changes of bone mineral density and bone turnover in postmenopausal women on thyroxine. Clin Endocrinol 46, 301-307 (1997)
doi:10.1046/j.1365-2265.1997.1280950.x
PMid:9156039
110. Ross DS: Bone density is not reduced during the short-term administration of levothyroxine to postmenopausal women with subclinical hypothyroidism: a randomized, prospective study. Am J Med 95, 385-388 (1993)
doi:10.1016/0002-9343(93)90307-B
PMid:8213870
111. Garton M, I. Reid, N. Loveridge, S. Robins, L. Murchison, G. Beckett, G. Reid: Bone mineral density and metabolism in presmenopausal women taking l-thyroxine replacement therapy. Clin Endocrinol 41, 747-755(1994)
doi:10.1111/j.1365-2265.1994.tb02789.x
PMid:7889610
112. Bauer DC, B. Ettinger, M.C. Nevitt, K.L.Stone: Study of Osteoporotic Fractures Research Group. Risk for fracture in women with low serum levels of thyroid-stimulating hormone. Ann Internat Med 134, 561-568 (2001)
113. Seeley DG, J. Kelsey, M. Jergas,M.C. Nevitt: Predictors of ankle and foot fractures in elder women. The Study of Osteoporotic Fractures Research Group. J Bone Min Res 11, 1347-1355 (1996)
114. Cummings SR, M.C. Nevitt, W.S.Browner, K. Stone, K.M.Fox, K.E. Ensrud, J. Cauley, D. Black, T.M.Vogt: Risk factors for hip fracture in white women. Study of Osteoporotic Fractures Research Group. New Engl J Med 332, 767-773 (1995)
115. Mundy, G.R., J.L.Shapiro, J.G. Bandelin, E.M. Canalis, L.G.Raisz: Direct stimulation of bone resorption by thyroid hormones. J Clin Invest 58, 529-34 (1976)
doi:10.1172/JCI108497
PMid:182721 PMCid:333209
116. Bassett, J.H., G.R. Williams: The molecular actions of thyroid hormone in bone. Trends Endocrinol Metab 14, 356-64 (2003)
doi:10.1016/S1043-2760(03)00144-9
PMid:14516933
117. Stevens, D.A, C.B. Harvey, A.J. Scott, P.J. O'Shea, J.C. Bernard, A.J. Williams, G. Brady, J. Samarut, O. Chassande, G.R. Williams: Thyroid hormone activates fibroblast growth factor receptor-1 in bone. Mol Endocrinol 17, 1751-66 (2003)
doi:10.1210/me.2003-0137
PMid:12805413
118. O'Shea, P.J, C.B. Hervey, H. Suzuki, M. Kaheshige, K. Kaneshige, S. Y. Cheng, G. R. Williams: A thyrotoxic skeletal phenotype of advanced bone formation in mice with resistance to thyroid hormone. Mol Endocrinol 17, 1410-24 (2003)
doi:10.1210/me.2002-0296
PMid:12677005
119. Stevens, D.A, R.P. Hasserjian, H. Robson, T. Sielber, S.M. Shalet, G.R. Williams: Thyroid hormones regulate hypertrophic chondrocyte differentiation and expression of parathyroid hormone-related peptide and its receptor during endochondral bone formation. J Bone Miner Res 15, 2431-42 (2000)
doi:10.1359/jbmr.2000.15.12.2431
PMid:11127207
120. Gauthier, K, M. Plateroti, C.B. Harvey, G.R. Williams, R.E. Weiss, S. Refetoff, J.F. Willott, V. Sundin, J.P. Roux, L. Malaval, M. Hara, J. Samarut, O. Chassande: Genetic analysis reveals different functions for the products of the thyroid hormone receptor alpha locus. Mol Cell Biol 21, 4748-60 (2001)
doi:10.1128/MCB.21.14.4748-4760.2001
PMid:11416150 PMCid:87157
121. Forrest, D, E. Hanebuth, R.J. Smeyne, N. Everds, C.L. Stewart, J.M. Wehner, T. Curran: Recessive resistance to thyroid hormone in mice lacking thyroid hormone receptor beta: evidence for tissue-specific modulation of receptor function. Embo J 15, 3006-15 (1996)
122. Gauthier, K, O. Chassande, M. Plateroti, J.P. Roux, C. Legrand, B. Pain, B. Rousset, R. Weiss J. Trouillas, J. Samarut: Different functions for the thyroid hormone receptors TRalpha and TRbeta in the control of thyroid hormone production and post-natal development. Embo J 18, 623-31 (1999)
doi:10.1093/emboj/18.3.623
PMid:9927422 PMCid:1171155
123. Gothe, S, Z. Wang, L. Ng, J.M. Kindblom, A.C. Barros, C. Ohlsson, B. Vennstrom, D. Forrest: Mice devoid of all known thyroid hormone receptors are viable but exhibit disorders of the pituitary-thyroid axis, growth, and bone maturation. Genes Dev 13, 1329-41 (1999)
doi:10.1101/gad.13.10.1329
124. Diamond, T., J. Vine, R. Smart, P. Butler: Thyrotoxic bone disease in women: a potentially reversible disorder. Ann Intern Med 120, 8-11 (1994)
125. Franklyn, J, J. Betteridge, R. Holder, J. Daykin, J. Lilley, M. Sheppard: Bone mineral density in thyroxine treated females with or without a previous history of thyrotoxicosis. Clin Endocrinol (Oxf) 41, 425-32 (1994)
doi:10.1111/j.1365-2265.1994.tb02572.x
PMid:7955453
126. Grant, D.J., M.E. McMurdo, P.A. Mole, C.R. Paterson: Is previous hyperthyroidism still a risk factor for osteoporosis in post-menopausal women? Clin Endocrinol (Oxf) 43, 339-45 (1995)
doi:10.1111/j.1365-2265.1995.tb02041.x
PMid:7586604
127. Jodar, E, M. Munoz-Torres, F. Escobar-Jiménez, M. Quesada, J.D. Luna, N.Olea: Antiresorptive therapy in hyperthyroid patients: longitudinal changes in bone and mineral metabolism. J Clin Endocrinol Metab 82, 1989-94 (1997)
doi:10.1210/jc.82.6.1989
128. Serraclara, A., E. Jodar, F. Sarabia, F. Hawkins: Bone mass after long-term euthyroidism in former hyperthyroid women treated with (131)I influence of menopausal status. J Clin Densitom 4, 249-55 (2001)
doi:10.1385/JCD:4:3:249
PMid:11740067
129. Toh, S.H., B.C. Claunch, P.H. Brown: Effect of hyperthyroidism and its treatment on bone mineral content. Arch Intern Med 145, 883-6 (1985)
doi:10.1001/archinte.145.5.883
PMid:3838875
130. Vestergaard, P., L. Mosekilde: Hyperthyroidism, bone mineral, and fracture risk--a meta-analysis. Thyroid 13, 585-93 (2003)
doi:10.1089/105072503322238854
PMid:12930603
131. Vestergaard, P., L. Rejnmark, J. Weeke, L. Mosekilde: Fracture risk in patients treated for hyperthyroidism. Thyroid 10, 341-8 (2000)
doi:10.1089/thy.2000.10.341
PMid:10807063
132. Abe, E., R.C. Marians, W. Yu, X.B. Wu, T. Ando, Y. Li, J. Iqbal, G. Rajendren, H.C. Blair, T.F. Davies, M. Zaidi: TSH is a negative regulator of skeletal remodeling. Cell 115, 151-62 (2003)
doi:10.1016/S0092-8674(03)00771-2
PMid:14567913
133. Marians, R.C, L. Ng, H.C. Blair, P. Unger, P.N. Graves, P.N. Davies: Defining thyrotropin-dependent and -independent steps of thyroid hormone synthesis by using thyrotropin receptor-null mice. Proc Natl Acad Sci U S A 99, 15776-81 (2002)
doi:10.1073/pnas.242322099
134. Ikeda, F, R. Nishimura, T. Matsubara, S. Tanaka, J. Inoue, S.V. Reddy, K. Hata, K.Yamashita, T. Higara, T. Watanabe, T. Kukita, K. Yoshioka, A. Rao, T. Yoneda: Critical roles of c-Jun signaling in regulation of NFAT family and RANKL-regulated osteoclast differentiation. J Clin Invest 114, 475-84 (2004)
135. Shevde, N.K., A.C. Bendixen, K.M. Dienger, J.W. Pike: Estrogens suppress RANK ligand-induced osteoclast differentiation via a stromal cell independent mechanism involving c-Jun repression. Proc Natl Acad Sci U S A 97, 7829-34 (2000)
doi:10.1073/pnas.130200197
136. Srivastava, S, G. Toraldo, M.N. Weitzmann, S. Cenci, F.P.Ross, R. Pacifici: Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-kappa B ligand (RANKL)-induced JNK activation. J Biol Chem 276, 8836-40 (2001)
doi:10.1074/jbc.M010764200
PMid:11121427
137. Kawaida, R, T. Ohtsuka, J. Okutsu, T. Takahashi, Y. Kadono, H. Oda, A. Hikita, K. Nakamura, S. Tanaka, H. Furukawa: Jun dimerization protein 2 (JDP2), a member of the AP-1 family of transcription factor, mediates osteoclast differentiation induced by RANKL. J Exp Med 197, 1029-35 (2003)
doi:10.1084/jem.20021321
PMid:12707301 PMCid:2193879
138. Hase, H, T. Ando, L. Eldeiry, A. Brebene, Y. Peng, L. Liu, H. Amano, T.F. Davies, L. Sun, M. Zaidi, E. Abe: TNFalpha mediates the skeletal effects of thyroid-stimulating hormone. Proc Natl Acad Sci U S A 103, 12849-54 (2006)
doi:10.1073/pnas.0600427103
139. Simsek, G., Y. Karter, S. Aydin, H. Uzun: Osteoporotic cytokines and bone metabolism on rats with induced hyperthyroidism; changes as a result of reversal to euthyroidism. Chin J Physiol 46, 181-6 (2003)
140. Murphy, E., G.R. Williams: The thyroid and the skeleton. Clin Endocrinol (Oxf) 61, 285-98 (2004)
doi:10.1111/j.1365-2265.2004.02053.x
PMid:15355444
141.Epstein, S., A.M. Inzerillo, J. Caminis, M. Zaidi: Disorders associated with acute rapid and severe bone loss. J Bone Miner Res 18, 2083-94 (2003)
doi:10.1359/jbmr.2003.18.12.2083
PMid:14672343
142. Heijckmann, A.C, M.S. Huijberts, P. Geusens, J. de Vries, P.P. Menheere, B.H. Wolffenbuttel: Hip bone mineral density, bone turnover and risk of fracture in patients on long-term suppressive L-thyroxine therapy for differentiated thyroid carcinoma. Eur J Endocrinol 153, 23-9 (2005)
doi:10.1530/eje.1.01933
PMid:15994742
143. Reverter, J.L, S. Holgado, N. Alonso, I. Salinas, M.L. Granada, A. Sanmartì. Lack of deleterious effect on bone mineral density of long-term thyroxine suppressive therapy for differentiated thyroid carcinoma. Endocr Relat Cancer 12, 973-81 (2005)
doi:10.1677/erc.1.01072
PMid:16322336
144. Bauer, D.C., B. Ettinger, M.C. Nevitt, K.L. Stone: Risk for fracture in women with low serum levels of thyroid-stimulating hormone. Ann Intern Med 134, 561-8 (2001)
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Key Words: Thyroid Disease, Metabolic Bone Disease, Osteoporosis, Pituitary Hormones, Systems Biology, Review
Send correspondence to: Angelo Carpi, Department of Reproduction and Ageing, University Hosptal S. Chiara, Via Roma, 67, 56126 Pisa- Italy. Tel: 0039 050992955, Fax: 0039 050992955, E-mail: a.carpi@med.unipi.it