PDF(Pubmed)
Abstract：
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 (Atp6ap2Ocn-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 impaired β-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 of β-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 active β-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 regulating β-catenin levels, demonstrating an un-recognized function of osteoblastic ATP6AP2 in promoting Wnt/LRP6/β-catenin signaling and trabecular bone formation.
摘要：
Wnt/β-catenin信号传导对于多种细胞类型的各种细胞过程至关重要。包括成骨细胞（OB）的分化和功能。在OB中Wnt/β-连环蛋白信号传导到底如何被调节仍然难以捉摸。ATP6AP2，V-ATPase的辅助亚基，在多种细胞类型/器官和多种信号通路中起重要作用。然而,OBs中的ATP6AP2是否以及如何调节Wnt/β-catenin信号传导和骨形成鲜为人知。在这里，我们提供了ATP6AP2在OB谱系细胞中选择性地促进OB介导的骨形成和骨小梁区域的骨稳态的证据。在OB谱系细胞（Atp6ap2Ocn-Cre）中条件性敲除（CKO）ATP6AP2减少小梁，但不是皮质，骨形成和骨量。蛋白质组学和细胞生化研究显示，在ATP6AP2-KOBMSCs和OBs中LRP6和N-cadherin减少，但不是骨细胞.另外的体外和体内研究显示ATP6AP2-KOBMSCs和OBs中β-catenin信号传导受损,但不是骨细胞，在基础和Wnt刺激条件下，虽然LRP5在ATP6AP2-KO骨细胞中减少，但不是BMSCs。进一步的细胞生物学研究发现，成骨细胞ATP6AP2不是Wnt3a抑制β-连环蛋白磷酸化所必需的，但对于LRP6/β-catenin和N-cadherin/β-catenin蛋白复合物在细胞膜上的分布是必需的，从而防止其退化。活性β-catenin的表达减少ATP6AP2-KOBMSCs的OB分化缺陷。一起来看,这些结果支持ATP6AP2作为LRP6和N-cadherin蛋白运输和稳定性的关键调节因子的观点，从而调节β-连环蛋白水平，证明了成骨细胞ATP6AP2在促进Wnt/LRP6/β-catenin信号传导和骨小梁形成方面的未识别功能。
