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

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 π -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.

Inhibition of Osteoclast Differentiation and Bone Resorption by N-Methylpyrrolidone

“…After attachment to bone, the ruffled border secretes enzymes and protons enabling the solubilization and digestion of the bone matrix. Rapid cytoskeletal reorganization is essential for osteoclast function and formation of the specialized membranes (21,35). NMP treatment disrupted actin ring formation in osteoclasts.…”

Endocytic trafficking in actively resorbing osteoclasts

“…Depending on the cell type and labelling conditions, a proportion (up to 25%) of the transferrin has a half-life of only 5-7 minutes inside the cell, whereas a larger proportion accumulates in the recycling endosome and it can take more than 60 minutes before these molecules are recycled back to the plasma membrane (Hopkins et al, 1994). In osteoclasts, it has been shown that endocytosed transferrin reaches the ruffled border after 30 minutes of incubation (Mulari et al, 2003;Palokangas et al, 1997), establishing that there is cross-talk between the recycling and late endosome/ ruffled border. However, the amount of transferrin that reaches the ruffled border has not been established.…”

“…This dynamic equilibrium is supported by the finding that basolaterally endocytosed transferrin is found in the area of the ruffled border after 30 minutes of chase (Palokangas et al, 1997). It was also recently suggested that the ruffled border is divided into a peripheral secretory subdomain and a centrally located uptake zone (Mulari et al, 2003). Studies with differentially targeted virus particles, in conjunction with the localization of lysosomeassociated membrane proteins (LAMPs) and the small GTPase Rab7 to this plasma-membrane domain, also indicate a high similarity of the ruffled border to the late-endosome/lysosome compartment (Palokangas et al, 1997;Salo et al, 1996).…”

“…Actin filaments have been linked to endocytosis from the apical plasma membrane in several polarized cell types (Apodaca, 2001) and the actin-disrupting drug cytochalasin has been shown to slow basolateral to apical transcytosis significantly (Maples et al, 1997). In osteoclasts, both microtubules and F-actin are found in the central part of the ruffled border, which is surrounded by the actin ring of the sealing zone (Mulari et al, 2003). The rearrangement of the actin cytoskeleton at the beginning of the resorption cycle is stimulated by engagement of the major osteoclast integrin αvβ3, which activates a heteromultimeric signalling complex that includes actin-binding proteins, the tyrosine kinase pp(60c-src) and the lipid phosphatidylinositol-3-kinase (PI-3-kinase) (Hruska et al, 1995).…”

Modulation of tumor necrosis factor related apoptosis-inducing ligand (TRAIL) receptors in a human osteoclast model in vitro

“…Once the mineral degradation is underway, the degradation of bone organic matrix (mostly type 1 collagen) can take place. This function is performed by many degradative factors including acid phosphatases (TRAP), matrix metalloproteinases (MMPs) and the lysosomal enzyme cathepsin K, which, like the ion channels necessary for acidification, polarize to the ruffled membrane after osteoclast attachment to the bone, to be as close as possible to the site of action (Mulari et al, 2003) ( Figure 5). …”