title: V-ATPase subunit ATP6AP1 (Ac45) regulates osteoclast differentiation, extracellular acidification, lysosomal trafficking, and protease exocytosis in osteoclast-mediated bone resorption.
abstract: Lysosomal trafficking and protease exocytosis in osteoclasts are essential for ruffled border formation and bone resorption. Yet the mechanism underlying lysosomal trafficking and the related process of exocytosis remains largely unknown. We found ATP6ap1 (Ac45), an accessory subunit of vacuolar-type H(+)-ATPases (V-ATPases), to be highly induced by receptor activator for nuclear factor kappa B ligand (RANKL) in osteoclast differentiation. Ac45 knockdown osteoclasts formed normal actin rings, but had severely impaired extracellular acidification and bone resorption. Ac45 knockdown significantly reduced osteoclast formation. The decrease in the number of osteoclasts does not result from abnormal apoptosis; rather, it results from decreased osteoclast precursor cell proliferation and fusion, which may be partially due to the downregulation of extracellular signal-regulated kinase (ERK) phosphorylation and FBJ osteosarcoma oncogene (c-fos), nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), and "transmembrane 7 superfamily member 4" (Tm7sf4) expression. Notably, Ac45 knockdown osteoclasts exhibited impaired lysosomal trafficking and exocytosis, as indicated by the absence of lysosomal trafficking to the ruffled border and a lack of cathepsin K exocytosis into the resorption lacuna. Our data revealed that the impaired exocytosis is specifically due to Ac45 deficiency, and not the general consequence of a defective V-ATPase. Together, our results demonstrate the essential role of Ac45 in osteoclast-mediated extracellular acidification and protease exocytosis, as well as the ability of Ac45 to guide lysosomal intracellular trafficking to the ruffled border, potentially through its interaction with the small guanosine-5'-triphosphatase (GTPase) Rab7. Our work indicates that Ac45 may be a novel therapeutic target for osteolytic disease.
authors.affiliation: Department of Pathology, University of Alabama at Birmingham, Birmingham, AL 35294-2182, USA.
authors.forename: De-Qin
authors.initials: DQ
authors.lastname: Yang
authors.forename: Shengmei
authors.initials: S
authors.lastname: Feng
authors.forename: Wei
authors.initials: W
authors.lastname: Chen
authors.forename: Haibo
authors.initials: H
authors.lastname: Zhao
authors.forename: Christie
authors.initials: C
authors.lastname: Paulson
authors.forename: Yi-Ping
authors.initials: YP
authors.lastname: Li
mesh_terms: D000143:Acids / Q000378:metabolism; D000199:Actins / Q000378:metabolism; D000818:Animals; D017209:Apoptosis; D001862:Bone Resorption / Q000201:enzymology / Q000473:pathology*; D056657:Cathepsin K / Q000378:metabolism*; D002454:Cell Differentiation; D002462:Cell Membrane / Q000201:enzymology; D049109:Cell Proliferation; D005089:Exocytosis; D005110:Extracellular Space / Q000378:metabolism; D055785:Gene Knockdown Techniques; D016086:Lentivirus / Q000378:metabolism; D008247:Lysosomes / Q000378:metabolism*; D051379:Mice; D010010:Osteoclasts / Q000201:enzymology* / Q000473:pathology; D010012:Osteogenesis; D011485:Protein Binding; D021122:Protein Subunits / Q000378:metabolism; D021381:Protein Transport; D034741:RNA, Small Interfering / Q000378:metabolism; D025262:Vacuolar Proton-Translocating ATPases / Q000378:metabolism*; D020691:rab GTP-Binding Proteins / Q000378:metabolism; D000090802:rab7 GTP-Binding Proteins
chemical_list: D000143:Acids; D000199:Actins; C484226:Atp6ap1 protein, mouse; D021122:Protein Subunits; D034741:RNA, Small Interfering; D000090802:rab7 GTP-Binding Proteins; C000716465:rab7 GTP-binding proteins, mouse; D056657:Cathepsin K; D025262:Vacuolar Proton-Translocating ATPases; D020691:rab GTP-Binding Proteins