Acidosis causes endoplasmic reticulum stress and caspase-12-mediated astrocyte death

Endoplasmic reticulum (ER) stress leads to activation of caspase-12, which in turn can lead to activation of caspase-3 and cell death. Here we report that transient acidosis induces ER stress and caspase-12-mediated cell death in mouse astrocytes. After a 3-hour incubation at pH 6.0, astrocytes exhibited delayed cell death associated with nuclear condensation and fragmentation. Cell death was reduced by the protein synthesis inhibitor cycloheximide, further suggesting an active cell death program. Acidosis increased the expression of the ER chaperone protein GRP-78, indicative of ER stress. Acidosis also increased caspase-12 mRNA expression, caspase-12 protein expression, cleavage of caspase-12 to its active form, and activation of caspase-3. Each of these effects was suppressed in astrocytes pretreated with caspase-12 antisense phosphorodiamidate morpholino oligodeoxynucleotides (PMOs). Caspase-12 antisense PMOs also reduced the cell death induced by acidosis. Immunoprecipitation studies showed dissociation of both caspase-12 and Ire1-alpha from GRP-78, thereby suggesting a mechanism by which acidosis can initiate the ER stress response. To evaluate caspase-12 activation in vivo, rats were subjected to middle cerebral artery ischemia-reperfusion. Immunostaining of brain sections harvested 24 hours later showed increased caspase-12 expression and nuclear condensation in astrocytes of the perfinfarct region exposed to acidosis during ischemia. These findings suggest that acidosis induces ER stress and caspase-12 activation, and that these changes may contribute to delayed cell death after ischemia.