STIM1 controls calcineurin/Akt/mTOR/NFATC2-mediated osteoclastogenesis induced by RANKL/M-CSF

Store-operated Ca²⁺ entry (SOCE) is the stable calcium channel influx in most cells. It consists of the cytoplasmic ion channel ORAI and endoplasmic reticulum receptor stromal interaction molecule 1 (STIM1). Abolition of SOCE function due to ORAI1 and STIM1 gene defects may cause non-perspiration, ectoderm dysplasia and skeletal malformations with severe combined immunodeficiency (CID). Calcineurin/mammalian target of rapamycin (mTOR)/nuclear factor of activated T cells 2 (NFATC2) is an important signalling cascade for osteoclast development. Calcineurin is activated by Ca²⁺ via SOCE during osteoclastogenesis, which is induced by receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). However, the underlying mechanism has remained to be fully elucidated, which was therefore the aim of the present study. In the current study, flow cytometry was used to examine the effect of a number of STIM1 mutations on proliferation, differentiation, and expression of osteolysis-associated proteins in Bone marrow-derived mononuclear macrophages (BMDM). The calcineurin/AKT/mTOR/NFATC2 signaling cascade activation were also assessed. BMDMs were obtained from three patients with STIM1 mutations (p.E136X, p.R429C and p.R304W). These mutations, which exhibited abolished (p.E136X, p.R429C) or constitutively activated (p.R304W) SOCE, failed to respond to RANKL/M-CSF-mediated induction of normal osteoclastogenesis. In addition, activation of the calcineurin/Akt/mTOR/NFATC2 signalling cascade induced by RANKL/M-CSF was abnormal in the BMDMs with STIM1 mutants compared with that in BMDMs from healthy subjects. In addition, overexpression of wild-type STIM1 restored SOCE in p.R429C- and p.E136X-mutant BMDMs, but not in p.R304W-mutant BMDMs. Of note, calcineurin, cyclosporin A, mTOR inhibitor rapamycin and NFATC2-specific small interfering RNA restored the function of SOCE in p.R304W-mutant BMDMs. The present study suggests a role for SOCE in calcineurin/Akt/mTOR/NFATC2-mediated osteoclast proliferation, differentiation and function.