Bcl2 mitigates Ca2+ entry and mitochondrial Ca2+ overload through downregulation of L-type Ca2+ channels in PC12 cells

Summary Altered calcium homeostasis and increased cytosolic calcium concentrations ([Ca2+ ]c ) are linked to neuronal apoptosis in epilepsy and in cerebral ischemia, respectively. Apoptotic programmed cell death is regulated by the antiapoptotic Bcl2 family of proteins. Here, we investigated the role of Bcl2 on calcium (Ca2+ ) homeostasis in PC12 cells, focusing on L-type voltage-dependent calcium channels (VDCC). Cytosolic Ca2+ transients ([Ca2+ ]c ) and changes of mitochondrial Ca2+ concentrations ([Ca2+ ]m ) were monitored using cytosolic and mitochondrially targeted aequorins of control PC12 cells and PC12 cells stably overexpressing Bcl2. We found that: (i) the [Ca2+ ]c and [Ca2+ ]m elevations elicited by K+ pulses were markedly depressed in Bcl2 cells, with respect to control cells; (ii) such depression of [Ca2+ ]m was not seen either in digitonin-permeabilized cells or in intact cells treated with ionomycin; (iii) the [Ca2+ ]c transient depression seen in Bcl2 cells was reversed by shRNA transfection, as well as by the Bcl2 inhibitor HA14-1; (iv) the L-type Ca2+ channel agonist Bay K 8644 enhanced K+ -evoked [Ca2+ ]m peak fourfold in Bcl2, and twofold in control cells; (v) in current-clamped cells the depolarization evoked by K+ generated a more hyperpolarized voltage step in Bcl2, as compared to control cells. Taken together, our experiments suggest that the reduction of the [Ca2+ ]c and [Ca2+ ]m transients elicited by K+ , in PC12 cells overexpressing Bcl2, is related to the reduction of Ca2+ entry through L-type Ca2+ channels. This may be due to the fact that Bcl2 mitigates cell depolarization, thus diminishing the recruitment of L-type Ca2+ channels, the subsequent Ca2+ entry, and mitochondrial Ca2+ overload.