Cellular calcium deficiency plays a role in neuronal death caused by proteasome inhibitors

Cytosolic Ca²⁺ concentration ([Ca²⁺]_i) is reduced in cultured neurons undergoing neuronal death caused by inhibitors of the ubiquitin proteasome system. Activation of calcium entry via voltage-gated Ca²⁺ channels restores cytosolic Ca²⁺ levels and reduces this neuronal death (Snider et al. 2002). We now show that this reduction in [Ca²⁺]_i is transient and occurs early in the cell death process, before activation of caspase 3. Agents that increase Ca²⁺ influx such as activation of voltage-gated Ca²⁺ channels or stimulation of Ca²⁺ entry via the plasma membrane Na-Ca exchanger attenuate neuronal death only if applied early in the cell death process. Cultures treated with proteasome inhibitors had reduced current density for voltage-gated Ca²⁺ channels and a less robust increase in [Ca ²⁺]_i after depolarization. Levels of endoplasmic reticulum Ca²⁺ were reduced and capacitative Ca²⁺ entry was impaired early in the cell death process. Mitochondrial Ca²⁺ was slightly increased. Preventing the transfer of Ca²⁺ from mitochondria to cytosol increased neuronal vulnerability to this death while blockade of mitochondrial Ca²⁺ uptake via the uniporter had no effect. Programmed cell death induced by proteasome inhibition may be caused in part by an early reduction in cytosolic and endoplasmic reticulum Ca^2+, possibly mediated by dysfunction of voltage-gated Ca²⁺ channels. These findings may have implications for the treatment of disorders associated with protein misfolding in which proteasome impairment and programmed cell death may occur.