Association of the α2δ1 Subunit with Cav3.2 Enhances Membrane Expression and Regulates Mechanically Induced ATP Release in MLO-Y4 Osteocytes

摘要

Voltage sensitive calcium channels (VSCCs) mediate signaling events in bone cells in response to mechanical loading. Osteoblasts predominantly express L-type VSCCs composed of the α1 pore-forming subunit and several auxiliary subunits. Osteocytes, in contrast, express T-type VSCCs, but a relatively small amount of L-type α1 subunits. Auxiliary VSCC subunits have several functions including modulating gating kinetics, trafficking of the channel and phosphorylation events. The influence of the α2δ auxiliary subunit on T-type VSCCs and the physiological consequences of that association are incompletely understood and have yet to be investigated in bone. In this study, we postulated that the auxiliary α2δ subunit of the VSCC complex modulates mechanically-regulated ATP release in osteocytes via its association with the T-type, Cav3.2 (α1H) subunit. We demonstrated by RT-PCR, Western blotting, and immunostaining that MLO-Y4 osteocyte-like cells express the T-type, Cav3.2 (α1H) subunit more abundantly than the L-type, Cav1.2 (α1C). We also demonstrated that the α2δ1 subunit, previously described as an L-type auxiliary subunit, complexes with the T-type Cav3.2 (α1H) subunit in MLO-Y4 cells. Interestingly, siRNA mediated knockdown of α2δ1 completely abrogated ATP release in response to membrane stretch in MLO-Y4 cells. Additionally, knockdown of the α₂δ₁ subunit and resulted in reduced ERK1/2 activation. Together these data demonstrate a functional VSCC complex. Immunocytochemistry following α₂δ₁ knockdown showed decreased membrane localization of Ca_v3.2 (α_1H) at the plasma membrane, suggesting that the diminished ATP release and ERK1/2 activation in response to membrane stretch resulted from a lack of Ca_v3.2 (α_1H) at the cell membrane.