Osteoclasts resorb the bone matrix through a specific adhesion structure called the sealing zone, which is based on a belt of podosome. Whereas the architecture of individual podosomes is getting well understood, a lot remains to be uncovered regarding the molecular mechanisms driving podosome organization into superstructures such as the osteoclast podosome belt. We showed that Dock5, an exchange factor for the small GTPase Rac is essential for podosome organisation in osteoclasts and then for bone resorption in vitro and in vivo (JBMR, 2011; Dev Biol 2014). We also demonstrated that systemic administration of an inhibitor of Dock5 protects the mice against pathological bone loss while preserving bone formation (Nature Communications in press). To understand how Dock5 signalling pathways control podosome belt formation, we performed proteomic analyses and identified Tensin 3 as a partner of Dock5 in osteoclasts. Confocal and 3D-SIM super-resolution microscopy revealed that Dock5 and Tensin 3 are not associated with individual podosomes. By contrast, they colocalise in the podosome cloud region when these assemble into a belt, standing further from the plasma membrane than vinculin. Suppression of Tensin 3 in osteoclasts destabilises podosome organisation and affects bone resorption. At the molecular level, we found that binding to Tensin 3 to Dock5 strongly increases its exchange activity towards Rac. Our results suggest that binding of Dock5 to Tensin 3 allows efficient activation of Rac to ensure the assembly of podosomes into a belt, the basal architecture of the bone resorbing apparatus of osteoclasts.
Disclosure: This work was supported by the Institut National du Cancer (grant # INCa-4361), the Agence Nationale de la Recherche (ANR grant # ANR-2011-BLAN-006 to AB) and the Société Française de Rhumatologie (SFR grant #111488).