Ischemic preconditioning targets the respiration of synaptic mitochondria via protein kinase Cε

In the brain, ischemic preconditioning (IPC) diminishes mitochondrial dysfunction after ischemia and confers neuroprotection. Activation of epsilon protein kinase C (epsilon PKC) has been proposed to be a key neuroprotective pathway during IPC. We tested the hypothesis that IPC increases the levels of epsilon PKC in synaptosomes from rat hippocampus, resulting in improved synaptic mitochondrial respiration. Preconditioning significantly increased the level of hippocampal synaptosomal epsilon PKC to 152% of sham-operated animals at 2d of reperfusion, the time of peak neuroprotection. We tested the effect of epsilon PKC activation on hippocampal synaptic mitochondrial respiration 2d after preconditioning. Treatment with the specific epsilon PKC activating peptide, tat-psi epsilon RACK (tat-psi epsilon-receptor for activated C kinase), increased the rate of oxygen consumption in the presence of substrates for complexes I, II, and IV to 157, 153, and 131% of control (tat peptide alone). In parallel, we found that epsilon PKC activation in synaptosomes from preconditioned animals resulted in altered levels of phosphorylated mitochondrial respiratory chain proteins: increased serine and tyrosine phosphorylation of 18 kDa subunit of complex I, decreased serine phosphorylation of FeS protein in complex III, increased threonine phosphorylation of COX IV (cytochrome oxidase IV), increased mitochondrial membrane potential, and decreased H2O2 production. In brief, ischemic preconditioning promoted significant increases in the level of synaptosomal epsilon PKC. Activation of epsilon PKC increased synaptosomal mitochondrial respiration and phosphorylation of mitochondrial respiratory chain proteins. We propose that, at 48 h of reperfusion after ischemic preconditioning, epsilon PKC is poised at synaptic mitochondria to respond to ischemia either by direct phosphorylation or activation of the epsilon PKC signaling pathway.