Alteration of RANKL-induced osteoclastogenesis in primary cultured osteoclasts from SERCA2^+/- mice

RANKL is essential for the terminal differentiation of monocytes/ marcrophages into osteoclasts. RANKL induces long-lasting oscillations in the intracellular concentration of Ca²⁺ ([Ca²⁺]_i) only after 24 h of stimulation. These Ca²⁺ oscillations play a switch-on role in NFATc1 expression and osteoclast differentiation. Which Ca²⁺ transporting pathway is induced by RANKL to evoke the Ca ²⁺ oscillations and its specific role in RANKL-mediated osteoclast differentiation is not known. This study examined the effect of a partial loss of sarco/endoplasmic reticulum Ca²⁺ ATPase type2 (SERCA2) on osteoclast differentiation in SERCA2 heterozygote mice (SERCA2^+/-). The BMD in the tibias of SERCA2^+/- mice increased >1.5-fold compared with wildtype mice (WT). RANKL-induced [Ca²⁺]_i oscillations were generated 48 h after RANKL treatment in the WT mice but not in the SERCA2^+/- bone marrow-derived macrophages (BMMs). Forty-eight hours after RANKL treatment, there was a lower level of NFATc1 protein expression and markedly reduced translocation of NFATc1 into the nucleus during osteoclastogenesis of the SERCA2^+/- BMMs. In addition, RANKL treatment of SERCA2^+/- BMMs incompletely induced formation of multinucleated cells, leading to reduced bone resorption activity. These results suggest that RANKL-mediated induction of SERCA2 plays a critical role in the RANKL-induced [Ca²⁺]_i oscillations that are essential for osteoclastogenesis.