Rab GTPases in Osteoclastic Endomembrane Systems

Figure 1

Bone resorption and transcytosis. On adhesion to the bone, OCs become polarized and reorganize their cytoskeleton. The sealing zone (or organelle-free clear zone) is formed by a peripheral belt of adhesive structures that delimit the ruffled border, which is a highly specialized area consisting of membrane expansions directed toward the target bone surface. The ruffled border is formed by polarized vesicular trafficking and plays a critical role in the degradation of bone matrix through acidification by V-ATPases and proteases released by the fusion of secretory lysosomal vesicles. The central ruffled membrane represents the uptake zone where degraded bone matrix products are internalized and transported by transcytosis toward the functional secretory domain. Several Rab proteins identified in OCs have been added (in red). Rab7 plays a direct role in the ruffled border formation. Rab3D and Rab27A have been involved in the trafficking of secretory vesicles essential for bone resorption. E: endosome and RE: recycling endosomes.