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.