Osteoclast Ruffled Border Has Distinct Subdomains for Secretion and Degraded Matrix Uptake

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Abstract
Subosteoclastic bone resorption is a result of HCl and proteinase secretion through a late endosome‐like bone facing membrane domain called ruffled border. As bone matrix is degraded, it enters osteoclasts' transcytotic vesicles for further processing and is then finally exocytosed to the intercellular space. The present study clarifies the spatial relationship between these vesicle fusion and matrix uptake processes at the ruffled border. Our results show the presence of vacuolar H+‐ATPase, small GTPase rab7 as well as dense aggregates of F‐actin at the peripheral ruffled border, where basolaterally endocytosed transferrin and cathepsin K are delivered. On the contrary, rhodamine‐labeled bone matrix enters transcytotic vesicles at the central ruffled border, where the vesicle budding proteins such as clathrin, AP‐2 and dynamin II are also localized. We present a model for the mechanism of ruffled border turnover and suggest that, due to its late endosomal characteristics, the ruffled border serves as a valuable model for studying the dynamic organization of other endosomal compartments as well.