ATP6AP2,a regulator of LRP6/β-catenin protein trafficking,promotes Wnt/β-catenin signaling and bone formation in a cell type dependent mannerOAMEDLINE

Wnt/β-catenin signaling is critical for various cellular processes in multiple cell types,including osteoblast(OB)differentiation and function.Exactly how Wnt/β-catenin signaling is regulated in OBs remain elusive.ATP6AP2,an accessory subunit of V-ATPase,plays important roles in multiple cell types/organs and multiple signaling pathways.However,little is known whether and how ATP6AP2 in OBs regulates Wnt/β-catenin signaling and bone formation.Here we provide evidence for ATP6AP2 in the OB-lineage cells to promote OB-mediated bone formation and bone homeostasis selectively in the trabecular bone regions.Conditionally knocking out(CKO)ATP6AP2 in the OB-lineage cells(Atp6ap2^(Ocn-Cre))reduced trabecular,but not cortical,bone formation and bone mass.Proteomic and cellular biochemical studies revealed that LRP6 and N-cadherin were reduced in ATP6AP2-KO BMSCs and OBs,but not osteocytes.Additional in vitro and in vivo studies revealed impairedβ-catenin signaling in ATP6AP2-KO BMSCs and OBs,but not osteocytes,under both basal and Wnt stimulated conditions,although LRP5 was decreased in ATP6AP2-KO osteocytes,but not BMSCs.Further cell biological studies uncovered that osteoblastic ATP6AP2 is not required for Wnt3a suppression ofβ-catenin phosphorylation,but necessary for LRP6/β-catenin and N-cadherin/β-catenin protein complex distribution at the cell membrane,thus preventing their degradation.Expression of activeβ-catenin diminished the OB differentiation deficit in ATP6AP2-KO BMSCs.Taken together,these results support the view for ATP6AP2 as a critical regulator of both LRP6 and N-cadherin protein trafficking and stability,and thus regulatingβ-catenin levels,demonstrating an un-recognized function of osteoblastic ATP6AP2 in promoting Wnt/LRP6/β-catenin signaling and trabecular bone formation.