Disruption of BMP Signaling in Osteoclasts Regulates their Differentiation

Objective: Twisted gastrulation (TWSG1) regulates bone morphogenetic protein (BMP) mediated signaling by binding to BMPs and preventing them from binding to their receptors.  We previously showed that TWSG1-deficient mice are osteopenic due to enhanced osteoclast formation and increased bone resorption mediated by increased BMP-signaling.  In the current study we tested the hypothesis that genetic disruption of BMP signaling specifically in osteoclasts by targeted deletion of either Bmpr2 or Twsg1 using the Cre-loxP approach will affect osteoclast formation.

Methods: Using Cre-recombinase expressing adenovirus (Ad-Cre), we examined M-CSF and RANKL mediated osteoclast formation and gene expression in primary bone marrow derived precursors from mice carrying floxed alleles of Bmpr2 orTwsg1. Efficient deletion of Bmpr2 andTwsg1 were verified by real time RT-PCR and western blot analysis. Regulation of BMP signaling was confirmed by examining pSmad1/5/8 levels. Effect of BMPRII and TWSG1-deletion on osteoclast formation was examined by quantitation of multinucleated TRAP-positive cells.  Expression of osteoclast differentiation and fusion genes was examined by real-time RT-PCR.             

Results: We demonstrate that Ad-Cre mediated targeted deletion of BMPRII decreased size and number of multinuclear TRAP-positive osteoclasts along with a decrease in expression of osteoclast markers.  On the contrary, Ad-Cre mediated targeted deletion of TWSG1 increased the size and number of multinuclear TRAP-positive osteoclasts along with an increase in expression of osteoclast genes.

Conclusion: We report that down regulation of BMP pathway inhibits RANKL-mediated osteoclast formation, while activation of BMP pathway by deleting BMP inhibitor TWSG1 enhances osteoclast formation.  Therefore, BMP pathway is necessary for RANKL-mediated osteoclastogenesis. Investigating the mechanism(s) by which BMPs and TWSG1 control osteoclast function will improve our understanding of physiological regulation of bone remodeling and pathogenesis of diseases that result from altered bone remodeling, as well as the understanding of the therapeutic efficacy of BMP-induced bone regenerative protocols.