The process of store-operated calcium entry (SOCE) leads to refilling the endoplasmic reticulum (ER) with calcium ions (Ca2+) after their release into the cytoplasm. Interactions between (ER)-located Ca2+ sensors (stromal interaction molecule 1 [STIM1] and STIM2) and plasma membrane-located Ca2+ channel-forming protein (Orai1) underlie SOCE and are well described in non-excitable cells. In neurons, however, SOCE appears to be more complex because of the importance of Ca2+ influx via voltage-gated or ionotropic receptor-operated Ca2+ channels. We found that the SOCE inhibitors ML-9 and SKF96365 reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-induced [Ca2+]i amplitude by 80% and 53%, respectively. To assess the possible involvement of AMPA receptors (AMPARs) in SOCE, we used their specific inhibitors. As estimated by Fura-2 acetoxymethyl (AM) single-cell Ca2+ measurements in the presence of CNQX or NBQX, thapsigargin (TG)-induced Ca2+ influx decreased 2.2 or 3.7 times, respectively. These results suggest that under experimental conditions of SOCE when Ca2+ stores are depleted, Ca2+ can enter neurons also through AMPARs. Using specific antibodies against STIM proteins or GluA1/GluA2 AMPAR subunits, co-immunoprecipitation assays indicated that when Ca2+ levels are low in the neuronal ER, a physical association occurs between endogenous STIM proteins and endogenous AMPAR receptors. Altogether, our data suggest that STIM proteins in neurons can control AMPA-induced Ca2+ entry as a part of the mechanism of SOCE.
Keywords: AMPA receptors; STIM; calcium signaling; neurons; store-operated calcium entry (SOCE).