Over-expression of x-linked inhibitor of apoptosis protein modulates multiple aspects of neuronal Ca²⁺ signaling

X-linked inhibitor of apoptosis (XIAP) protects and preserves the function of neurons in both in vitro and in vivo models of excitotoxicity. Since calcium (Ca²⁺) overload is a pivotal event in excitotoxic neuronal cell death, we have determined whether XIAP over-expression influences Ca ²⁺-signaling in primary cultures of mouse cortical neurons. Using cortical neuron cultures derived from wild-type (Wt) mice transiently transfected with XIAP or from transgenic mice that over-express XIAP, we show that XIAP opposes the rise in intracellular Ca²⁺ concentration by a variety of triggers. Relative to control neurons, XIAP over-expression produced a slight, but significant, elevation of resting Ca²⁺ concentrations. By contrast, the rise in intracellular Ca²⁺ concentrations produced by N-methyl-d-aspartate receptor stimulation and voltage gated Ca²⁺ channel activation were markedly attenuated by XIAP over-expression. The release of Ca²⁺ from intracellular stores induced by the sarco/endoplasmic reticulum Ca²⁺ ATPase inhibitor thapsigargin was also inhibited in neurons transiently transfected with XIAP. The pan-caspase inhibitor zVAD did not, however, diminish the rise in intracellular Ca²⁺ concentrations elicited by l-glutamate suggesting that XIAP influences Ca²⁺ signaling in a caspase-independent manner. Taken together, these findings demonstrate that the ability of XIAP to block excessive rises in intracellular Ca²⁺ by a variety of triggers may contribute to the neuroprotective effects of this anti-apoptotic protein.