MK2 Signaling is Critical for RANKL-induced Osteoclastogenesis

Receptor activator of NF-κB ligand (RANKL) activation of p38 mitogen activated protein kinase (MAPK) has been shown to be critical for osteoclast formation. Although MAPK-activated protein kinase 2 (MK2) is a direct phosphorylation target of p38 MAPK, the role of MK2 in RANKL-induced osteoclastogenesis has yet to be elucidated. Objective: We will test the hypothesis that MK2 expression in osteoclasts is critical for maintaining skeletal homeostasis. Methods: To evaluate the skeletal phenotype, tibia of 3 and 6 month old Mk2 ^-/- and WT mice were analyzed via microcomputed tomography to measure cortical and trabecular bone volume fraction (BV/TV), thickness, and trabecular number. Flow cytometry was performed using bone marrow hematopoietic stem cells (HSCs) from Mk2 ^-/-  and WT mice. Osteoclast progenitor populations (OCP), defined as B220^-Gr1^-/lowCD11b^lowCD115⁺, were isolated using magnetic beads conjugated with CD11b antibody and driven to form osteoclasts (OCs) using macrophage colony stimulation factor and RANKL. Tartrate-resistant acid phosphatase (TRAP) assays were used to measure OC number, size, and TRAP enzyme activity on days 3, 5, and 7 post-RANKL stimulation. Results: Mk2 ^-/- mice had a significant increase in cortical BV/TV (P≤0.01) and thickness (P≤0.05). Three month old Mk2 ^-/-  mice exhibited an increase in trabecular number (P≤0.05), yet no difference was seen in trabecular BV/TV or thickness. Flow cytometry results of HSCs showed no differences in OCPs. Surprisingly when male and female results were dichotomized, male Mk2 ^-/-  mice had a significant increase in OCPs compared to WT mice (P≤0.05). Additionally, Mk2 ^-/-  OCPs from males formed fewer and smaller OCs with less TRAP activity than WT cells on days 3 and 5 of RANKL treatment. Conclusion: These data indicate that MK2 has a role in RANKL induced osteoclastogenesis, which may be critical for regulating skeletal homeostasis in vivo.