OA/Gpnmb Mutation Impairs Osteoblast and Osteoclast Differentiation and Function

Osteoactivin/Gpnmb (OA/Gpnmb) is a glycoprotein identified in bone, and later found to be strongly localized to bone-forming osteoblasts. There exists in mice a naturally occurring mutation for OA/Gpnmb, which is attributable to premature stop codon resulting in a truncated 150 amino acid OA/Gpnmb protein. Micro-CT analysis of aged mice show decreased bone volume (BV/TV), trabecular number (Tb.N), and trabecular thickness (Tb. Th.) in OA/Gpnmb mutant mice when compared to wild-type, implying that OA/Gpnmb mutant mice are more osteoporotic. Immunoflourescent analysis has shown OA/Gpnmb protein in mutant osteoblasts (OB) is retained in the peri-nuclear/cis-Golgi area. Electron microscopy analysis of osteoblasts showed no morphological differences in mutant osteoblasts versus control, suggesting OA/Gpnmb retention is not cytotoxic. qPCR mRNA and western blot analyses showed increased RANKL expression in OB from OA/Gpnmb mutant. Co-culture of bone marrow cells showed marked increase in osteoclast (OC) numbers and size in mutant cultures when compared to WT. Treatment of WT and Mutant osteoblasts with recombinant OA/Gpnmb protein resulted in enhanced osteoclast number and size in vitro in WT and but not in mutant OB, suggesting an inherent defect to in mutant compared to WT OC. Collectively, these data suggest OA/Gpnmb acts as a regulator of osteoblast-osteoclast differentiation and function. Grant Funding Source: NIAMS