STIM1 and TRPV4 regulate fluid flow-induced calcium oscillation at early and late stages of osteoclast differentiation.

Bone resorption is mainly mediated by osteoclasts (OCs), whose formation and function are regulated by intracellular Ca2+ oscillation. Our previous studies demonstrated that fluid shear stress (FSS) lead to Ca2+ oscillation through mechanosensitive cation-selective channels. However, the specific channels responsible for this FSS-induced Ca2+ oscillation remain unknown. In the present study, we examined the expression of several Ca2+ channels in OCs, including STIM1, ORAI1, TRPV1, TRPV4, TRPV5, and TRPV6, by western blotting and reverse transcription-polymerase chain reaction. The results showed that STIM1 was highly expressed in early stage OCs, while TRPV4 was highly expressed in late stage OCs. We observed intracellular Ca2+ responses in OCs that were mechanically stimulated by FSS. When we blocked STIM1-dependent store-operated Ca2+ entry or inhibited TRPV4 using siRNA or drug inhibition, FSS-induced Ca2+ oscillations were almost undetectable in early and late stage OCs, respectively. These results indicate that STIM1 and TRPV4 act as mechanical transduction channels for OCs during the early and late differentiation stages, respectively, suggesting that these calcium channel could serve as markers of osteoclastogenesis or bone resorption.