Critical Role for the Calcium-Release Activated Calcium Channel Orai1 In RANKL-Stimulated Osteoclast Formation From Monocytic Cells

Abstract 928

Calcium signals are major regulators of human osteoclast formation and function, and the molecular mechanisms underlying calcium effects are of interest as possible targets for pharmacologic regulation of bone resorption. IP3-receptor regulated release of calcium stores is linked to NFATc1 activation, which stimulates expression of key osteoclast genes in precursors, but the roles of other calcium channels in osteoclastogenesis are not clear. In particular, the identity of the channel(s) mediating extracellular calcium influx triggered by release of calcium stores remains uncertain. In lymphoid cells, a major mediator of this extracellular calcium influx is the Calcium-Release Activated Calcium (CRAC) channel consisting of Orai1, a plasma-membrane calcium channel, and the calcium-sensitive regulatory protein, STIM1. Calcium released from intracellular stores binds to a low affinity EF-hand in STIM1 causing a conformational change in STIM1 that permits binding to Orai1, aggregation in microscopically distinct puncta at the cell membrane, and opening of the Orai1 channel with consequent influx of extracellular calcium. Targeted deletion of Orai1 or Stim1 in mice results in severe immunodeficiency and early death; this has limited the assessment of Orai1 effects in other tissues. To evaluate the specific role of Orai1 in human osteoclasts, we used peripheral blood monocytes which form multinucleated osteoclasts capable of bone resorption when treated with CSF1 and RANKL. We confirmed Orai1 expression in human monocytes using Western blot and quantitative PCR assays, and found that the protein was down-regulated in mature osteoclasts. We used fura-2 to measure store-dependent and -independent changes in intracellular calcium during osteoclastic differentiation of monocytes over 10–14 days in RANKL and CSF1. RANKL-associated calcium oscillations were detected throughout differentiation, but calcium-release activated influx of extracellular calcium was markedly lower in the mature osteoclasts compared to precursors, paralleling their expression of Orai1. Human monocytes, transfected with Orai1-specific siRNA producing an 80% reduction in Orai1 protein compared to control siRNA treated cells, showed inhibition of store-regulated calcium influx during osteoclastogenesis. Furthermore, monocytes deficient in Orai1 showed impaired osteoclast formation; in particular, multinucleation resulting from osteoclast precursor cell fusion was markedly reduced, impairing bone resorption. Orai1 deficiency in T-cells inhibits activation of NFATc1, but this did not appear prominent in our cells: we found no significant inhibition of NFATc1 regulated gene expression in Orai1 siRNA-transfected cells compared to control siRNA-transfected cells, despite the marked difference in Orai1 protein. Other calcium channels may mediate calcium dependent NFATc1 activation in osteoclast precursors; alternatively, the low level of Orai1 protein remaining in Orai1 siRNA treated monocytes may be sufficient for NFATc1 activation. To define the effects of complete Orai1 deficiency, we examined osteoclast formation and in vivo skeletal development in mice with targeted deletion of the Orai1 gene (Gwack et al. Mol Cell Biol 28 (2008) 5209-22). Consistent with our in vitro results, multinucleated osteoclasts were nearly absent from Orai1-/- mice, but mononuclear cells expressing osteoclast markers such as TRAcP, were seen. Surprisingly, the knock-out mice did not show the osteopetrotic phenotype typical of osteoclast deficiency. Retention of fetal cartilage was seen, indicating defective osteoclastic function in Orai1-/- mice, but marked inhibition of bone formation was also present. Using micro-computed tomography we found significant reductions in both cortical ossification and trabecular bone formation in Orai1-/- mice. This raised the possibility of a previously unrecognized role for Orai1 in osteoblasts, or the osteoblast defect might simply reflect abnormalities of Orai1-/- osteoclasts and/or lymphocytes, since both cell types have regulatory effects on osteoblast formation and function. In summary, our studies identify a requirement for CRAC channel mediated calcium influx, and specifically the Orai1 channel, for normal formation and activity of human osteoclasts; these results are confirmed in an Orai1 knock-out mouse which also shows defective bone formation.