Sexual Dimorphism in MAPK-Activated Protein Kinase-2 (MK2) Regulation of RANKL-Induced Osteoclastogenesis in Osteoclast Progenitor Subpopulations

Osteoclasts (OCs) are bone-resorptive cells critical for maintaining skeletal integrity through coupled bone turnover. OC differentiation and activation requires receptor activator of NF-kB ligand (RANKL) signaling through the p38 MAPK pathway. However the role of the p38 MAPK substrate, MAPK-activated protein kinase 2 (MK2), is not clearly delineated. Within the bone marrow exists a specific subpopulation of defined osteoclast progenitor cells (dOCPs) with surface expression of B220-Gr1-CD11blo/-CD115+ (dOCPlo/-). In this study, we isolated dOCPs from male and female mice to determine sex-specific effects of MK2 signaling in osteoclastogenesis (OCgen). Male Mk2-/- mice display an increase in the dOCPlo cell population when compared to Mk2+/+ mice, while female Mk2-/- and Mk2+/+ mice exhibit no difference. Defined OCPs from male and female Mk2+/+ and Mk2-/- bone marrow were treated with macrophage colony stimulation factor (M-CSF) and RANKL cytokines to promote OCgen. RANKL treatment of dOCPlo cells stimulated p38 and MK2 phosphorylation. Tartrate-resistant acid phosphatase (TRAP) assays were used to quantify OC number, size, and TRAP enzyme activity post-RANKL stimulation. MK2 signaling was critical for male dOCPlo OCgen, yet MK2 signaling regulated OCgen from female dOCP- and CD11bhi subpopulations as well. The functional gene, Ctsk, was attenuated in both male and female Mk2-/- dOCPlo-derived OCs. Conversely, MK2 signaling was only critical for gene expression of pre-OC fusion genes, Oc-stamp andTm7sf4, in male OCgen. Therefore, these data suggest there is a sexual dimorphism in MK2 signaling of OCP subpopulations.