The Rac1 exchange factor Dock5 is essential for bone resorption by osteoclasts

Abstract

Osteoporosis, which results from excessive bone resorption by osteoclasts, is the major cause of morbidity for elder people. Identification of clinically relevant regulators is needed to develop novel therapeutic strategies. Rho GTPases have essential functions in osteoclasts by regulating actin dynamics. This is of particular importance because actin cytoskeleton is essential to generate the sealing zone, an osteoclast-specific structure ultimately mediating bone resorption. Here we report that the atypical Rac1 exchange factor Dock5 is necessary for osteoclast function both in vitro and in vivo. We discovered that establishment of the sealing zone and consequently osteoclast resorbing activity in vitro require Dock5. Mechanistically, our results suggest that osteoclasts lacking Dock5 have impaired adhesion that can be explained by perturbed Rac1 and p130Cas activities. Consistent with these functional assays, we identified a novel small-molecule inhibitor of Dock5 capable of hindering osteoclast resorbing activity. To investigate the in vivo relevance of these findings, we studied Dock5 -/-mice and found that they have increased trabecular bone mass with normal osteoclast numbers, confirming that Dock5 is essential for bone resorption but not for osteoclast differentiation. Taken together, our findings characterize Dock5 as a regulator of osteoclast function and as a potential novel target to develop antiosteoporotic treatments. ß

Pharmacological inhibition of Dock5 prevents osteolysis by affecting osteoclast podosome organization while preserving bone formation

“…Genetic suppression of genes required for its formation results in higher bone mass in vivo with normal osteoclast differentiation, as we observed in Dock5 À / À mice 11 . Dock5 expression is mainly restricted to osteoclasts and its knockout does not affect mouse survival and breeding while hindering podosome organization into a sealing zone and then bone degradation by cultured osteoclasts 11 , without affecting longitudinal bone growth 12 . We identified C21, a chemical compound blocking Rac activation by Dock5 in cultured cells that inhibits bone degradation by osteoclasts in vitro 11 .…”

“…We next analysed whether C21 could recapitulate the inhibition of Rac activation and disruption of podosome patterning observed on Dock5 knockdown 11 . Mouse bone marrow-derived osteoclasts were treated with 100 mM C21 and monitored the activation of Rac and the organization of podosomes.…”

“…Dock5 expression is mainly restricted to osteoclasts and its knockout does not affect mouse survival and breeding while hindering podosome organization into a sealing zone and then bone degradation by cultured osteoclasts 11 , without affecting longitudinal bone growth 12 . We identified C21, a chemical compound blocking Rac activation by Dock5 in cultured cells that inhibits bone degradation by osteoclasts in vitro 11 . The aim of the present study was to use this compound as a model to test whether targeting the osteoclast podosome patterning via the pharmacological inhibition of Dock5 could prevent pathological bone loss while preserving bone formation.…”

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

“…40 Interestingly, the osteoclasts derived from mice knockout for these GEFs could not assemble sealing zones on bone and showed an impaired resorptive activity associated with a decreased bone density. [68][69][70] However, even if Dock5 and Vav3 are both required for cytoskeleton organization, they regulate Rac function through distinct signaling pathways. 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.…”

Opening up on ELMO regulation

“…1,3 Recent studies in mice have uncovered important in vivo roles for DOCK180, DOCK5 and DOCK2 in myoblast fusion, cardiovascular development, osteoclast-mediated bone resorption and immune homeostasis. [7][8][9][10][11] Recent human data has also closely linked deficiencies in DOCK8 to hyper IgE syndromes that can be corrected by transplantation of healthy hematopoietic stem cells in afflicted patients. 12,13 Similarly, studies in cancer cells have highlighted that DOCK3, via its Rac GEF activity, promotes a mesenchymal type of movement 14 while DOCK10, via its Cdc42 GEF activity, supports amoeboid migration.…”