p130Cas, Crk-Associated Substrate, Plays Important Roles in Osteoclastic Bone Resorption

Abstract

p130Cas, Crk-associated substrate (Cas), is an adaptor/scaffold protein that plays a central role in actin cytoskeletal reorganization. We previously reported that p130Cas is not tyrosine-phosphorylated in osteoclasts derived from Src-deficient mice, which are congenitally osteopetrotic, suggesting that p130Cas serves as a downstream molecule of c-Src and is involved in osteoclastic bone resorption. However, the physiological role of p130Cas in osteoclasts has not yet been confirmed because the p130Cas-deficient mice displayed embryonic lethality. Osteoclast-specific p130Cas conditional knockout (p130Cas DOCL-) mice exhibit a high bone mass phenotype caused by defect in multinucleation and cytoskeleton organization causing bone resorption deficiency. Bone marrow cells from p130Cas DOCL-mice were able to differentiate into osteoclasts and wild-type cells in vitro. However, osteoclasts from p130Cas DOCL-mice failed to form actin rings and resorb pits on dentine slices. Although the initial events of osteoclast attachment, such as b3-integrin or Src phosphorylation, were intact, the Rac1 activity that organizes the actin cytoskeleton was reduced, and its distribution was disrupted in p130Cas DOCL-osteoclasts. Dedicator of cytokinesis 5 (Dock5), a Rho family guanine nucleotide exchanger, failed to associate with Src or Pyk2 in osteoclasts in the absence of p130Cas. These results strongly indicate that p130Cas plays pivotal roles in osteoclastic bone resorption.

Role of CrkII Signaling in RANKL-Induced Osteoclast Differentiation and Function

“…Several studies have shown that Crk and p130Cas play similar roles in regulating actin dynamics (35,37). Furthermore, recent studies have verified that tyrosine phosphorylation of p130Cas is involved in actin organization in osteoclasts (35), and osteoclast-specific p130Cas conditional knockout mice exhibit a high bone mass phenotype caused by a defect in multinucleation and cytoskeleton organization (17). Our present study showed that p130Cas is tyrosine phosphorylated upon stimulation of osteoclastogenic cytokines and subsequently binds CrkII in preosteoclasts (Fig.…”

“…Recent studies have indicated that p130Cas plays pivotal roles in osteoclasts and engages in complex formation with CrkII in fibroblasts (17,35,36). Therefore, we investigated whether the association between p130Cas and CrkII also regulates osteoclast differentiation.…”

“…Two previous studies have elucidated the essential roles of p130Cas and Dock5 in osteoclastic bone resorption through activation of Rac1, in vitro and in vivo. These have also been shown to be highly indispensable for the function of Rac1 in osteoclasts (16,17).…”

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

“…52 Upon binding, various proteins such as c-Src and Pyk2 are recruited with scaffold proteins to transduce a signal required for actin ring formation and bone degradation. 44,45,54,55,56 Interestingly, Rho and Rac activations in response to M-CSF are deficient in αvβ3 integrins knockout osteoclasts. 57 Different approaches have been used to determine the importance of Rho, Cdc42 and Rac in osteoclasts adhesion structures organization but they have often produced contradictory results.…”

“…We and others showed that Dock5 is part of αvβ3 integrins downstream signaling by forming a Src/Pyk2/p130Cas/ Dock5 complex which ultimately leads to Rac1 activation and localization to the sealing zone. 56,70 On the other hand, Vav3 is activated by M-CSF and adhesion and it is recruited with Rac1 at the plasma membrane of osteoclasts. 71 So Dock5 and Vav3 seem to regulate Rac1 activation at distinct locations in osteoclasts and at different phases of the bone-resorption cycle.…”