RANKL-mediated reactive oxygen species pathway that induces long lasting Ca²⁺ oscillations essential for osteoclastogenesis

RANKL (receptor activator of NF-κB ligand) induces osteoclastogenesis by activating multiple signaling pathways in osteoclast precursor cells, chief among which is induction of long lasting oscillations in the intracellular concentration of Ca²⁺ ([Ca²⁺]_i). The [Ca ²⁺]_i oscillations activate calcineurin, which activates the transcription factor NFATc1. The pathway by which RANKL induces [Ca ²⁺]_i oscillations and osteoclastogenesis is poorly understood. Here we report the discovery of a novel pathway induced by RANKL to cause a long lasting increase in reactive oxygen species (ROS) and [Ca ²⁺]_i oscillations that is essential for differentiation of bone marrow-derived monocytes into osteoclasts. The pathway includes RANKL-mediated stimulation of Rac1 to generate ROS, which stimulate phospholipase Cγ1 to evoke [Ca²⁺]_i oscillations by stimulating Ca²⁺ release from the inositol 1,4,5-trisphosphate pool and STIM1-regulated Ca²⁺ influx. Induction and activation of the pathway is observed only after 24-h stimulation with RANKL and lasts for at least 3 days. The physiological role of the pathway is demonstrated in mice with deletion of the Peroxiredoxin II gene and results in a mark increase is ROS and, consequently, a decrease in bone density. Moreover, bone marrow-derived monocytes in PrxII^-/- primary culture show increased ROS and spontaneous [Ca²⁺]_i oscillations. These findings identify the primary RANKL-stimulated pathway to trigger the late stages of osteoclastogenesis and regulate bone resorption.