Bone Abstracts

Kit ligand/c-Kit receptor tyrosine kinase complex has been implicated as a target for bone remodeling process. Loss of function mutation in c-Kit causes low bone mass in Kit^W/W-v (W/W^v) mice. However, these mice are sterile and it is unclear whether the observed skeletal phenotype is secondary to sex hormone deficiency. To address this question, the skeletal phenotype of Kit^W-sh/W-sh (W^sh/W^sh) mice, which are fertile, was identified. W^sh/W^sh mice, which carried an inversion mutation affecting the transcriptional regulatory elements upstream of the c-Kit promoter region, exhibited osteopenia with elevated bone resorption and bone formation. Histomorphometry indicated an increase in osteoclast number, bone formation rate and mineral apposition rate at 6 and 9 weeks old. c-Kit mutation increased osteoclast differentiation. FACS analysis indicated an increase in the percentage of c-Fms⁺CD11b⁺ cells in spleen of W^sh/W^sh mice compared to controls. In primary osteoblast culture, W^sh/W^sh osteoblasts had increased number of committed osteoblast progenitors, alkaline phosphatase and mineralized bone nodules. These changes were associated with increases in steady-state mRNA levels for osteoblast marker genes, including osteocalcin, Osterix, alkaline phosphatase, type I collagen and Runx2, in femurs. c-Kit was expressed in both osteoclasts and osteoblasts and mutation of c-Kit decreased its expression level in W^sh/W^sh osteoclasts but not osteoblasts, suggesting an indirect effect of c-Kit on bone formation. Osteoclast-derived coupling factor Wnt10b mRNA was increased in W^sh/W^sh osteoclasts. W^sh/W^sh osteoclasts produced elevated Wnt10b protein level. Antagonizing Wnt10b with Wnt inhibitor DKK1 reduced W^sh/W^sh osteoclast conditioned medium-induced alkaline phosphatase activity and mineralization in osteoblast cultures. Our data suggest that c-Kit is a negative regulator of bone turnover, and that increased bone formation following disruption of c-Kit signaling is driven by osteoclast-derived Wnt 10b.