The molecular and cellular interplay between the osteopetrosis-associated proteins SNX10, OSTM1, and CLC-7 during osteoclastogenesis
Bone-resorbing osteoclasts (OCLs) are large, multi-nucleated cells that are formed through well-regulated differentiation and cell fusion of monocyte-macrophage precursors. Disruption of OCL-mediated bone resorption perturbs bone formation, remodeling, and homeostasis that, in turn, can lead to severe illnesses, such as autosomal recessive osteopetrosis (ARO). Mutations in the intracellular trafficking-associated protein sorting nexin 10 (SNX10) lead to "OCL-rich" ARO, in which OCLs are produced but are inactive. Furthermore, OCL fusion is deregulated in SNX10-knockout (SKO) mice: mature mutant OCLs fuse continuously to generate gigantic cells, in vitro and in vivo, unlike wild-type OCLs that stop fusing with each other upon maturation. Mutations in CLC-7, the lysosomal voltage-gated Cl-/H+ exchanger, and OSTM1, the beta-chain of the exchanger, also induce ARO in humans and in mouse models, and are associated with the presence of large OCLs. This study explored the molecular interplay between SNX10, CLC-7 and OSTM1 by directly comparing the phenotypes of cultured OCLs lacking one of these proteins. We show that loss of SNX10, OSTM1, or CLC-7 leads to the formation of similarly-gigantic OCLs in culture, due to deregulated fusion between mature OCLs that proceeds with similar kinetics. All three proteins are associated with LAMP1-positive lysosomes, localized in both perinuclear and peripheral regions of mature wild-type OCLs. Co-immunoprecipitation studies indicated that SNX10 physically interacts with CLC-7. Notably, SNX10-KO OCLs exhibited a significant reduction in peripheral lysosomes containing CLC-7 and OSTM1, suggesting that SNX10 is required for their transport to the cell periphery. Taken together, these findings indicate that SNX10 regulates the subcellular distribution of lysosomes containing CLC-7 and OSTM1, thereby controlling both the fusion and functionality of mature OCLs.