Abstract
The increase in cytosolic Ca2+ concentration ([Ca2+]cyt) and upregulation of calcium-sensing receptor (CaSR) and stromal interaction molecule 2 (STIM2) along with inhibition of voltage-gated K+ (KV) channels in pulmonary arterial smooth muscle cells (PASMC) have been implicated in the development of pulmonary arterial hypertension; however, the precise upstream mechanisms remain elusive. Activation of CaSR, a G protein-coupled receptor (GPCR), results in Ca2+ release from the endoplasmic/sarcoplasmic reticulum (ER/SR) and Ca2+ influx through receptor-operated and store-operated Ca2+ channels (SOC). Upon Ca2+ depletion from the SR, STIM forms clusters to mediate store-operated Ca2+ entry. Activity of KV channels, like KCNA5/KV1.5 and KCNA2/KV1.2, contributes to regulating membrane potential, and inhibition of KV channels results in membrane depolarization that increases [Ca2+]cyt by opening voltage-dependent Ca2+ channels. In this study, we show that activation of Notch by its ligand Jag-1 promotes the clustering of STIM2, and clustered STIM2 subsequently enhances the CaSR-induced Ca2+ influx through SOC channels. Extracellular Ca2+-mediated activation of CaSR increases [Ca2+]cyt in CASR-transfected HEK293 cells. Treatment of CASR-transfected cells with Jag-1 further enhances CaSR-mediated increase in [Ca2+]cyt. Moreover, CaSR-mediated increase in [Ca2+]cyt was significantly augmented in cells co-transfected with CASR and STIM2. CaSR activation results in STIM2 clustering in CASR/STIM2-cotransfected cells. Notch activation also induces significant clustering of STIM2. Furthermore, activation of Notch attenuates whole cell K+ currents in KCNA5- and KCNA2-transfected cells. Together, these results suggest that Notch activation enhances CaSR-mediated increases in [Ca2+]cyt by enhancing store-operated Ca2+ entry and inhibits KCNA5/KV1.5 and KCNA2/KV1.2, ultimately leading to voltage-activated Ca2+ entry.