Ischemic Tolerance and Cell Signaling in the Rat Brain

Sammanfattning: A brief period of sublethal ischemia in the brain induces resistance to a subsequent, otherwise lethal ischemic insult. This phenomenon is known as ischemic tolerance or preconditioning. A model of ischemic preconditioning in the rat brain using the two-vessel occlusion model of global cerebral ischemia was established. Using this model we have demonstrated that ischemic preconditioning protects the brain against a subsequent ischemic insult that normally causes neuronal damage. It is hypothesized that normal cell signaling during reperfusion following the insult is disrupted and thus contributes to cell death. From this hypothesis, we examined changes in protein tyrosine phosphorylation (Ptyr), protein kinase C (PKC), calcium calmodulin kinase II (CaMKII), and the extracellular regulated protein kinase (ERK) signal transduction pathways in the rat hippocampus after both the brief sublethal preconditioning ischemia and the subsequent ischemic insult. We found that tyrosine phosphorylation that increased persistently in the reperfusion phase after a lethal ischemic insult eventually normalized to control levels in the preconditioned brains within one day of reperfusion. The CaMKII-a translocated to the cell membranes and become autophosphorylated at threonine 286 during and after the lethal ischemic insult. Theses changes were persistent throughout reperfusion in the nonconditioned brains and returned to control levels in the preconditioned brains within one day of reperfusion. Protein kinase Cg, which translocated to the cell membranes during and after the lethal ischemic insults was rapidly down-regulated in the preconditioned brains during the second ischemic insult. Also, more calpain degradation products were found in preconditioned brains at the end of the second ischemic insult, which indicated that the preconditioning activated the calpain proteolysis system. Furthermore, we found that the ERK and ERK kinase (MEK), two central kinases in the extracellular regulated protein kinase cascade, became phosphorylated after short sublethal preconditioning. We conclude that the neuroprotection achieved by ischemic preconditioning was a combination effect of several mechanisms. An up-regulation of the neuroprotective signaling pathways (ERK) prior to a second ischemic insult, plus a suppression of the detrimental signaling pathways (PKC, CaMKII, Ptyr) after the second ischemic insult. In addition a change in the cell structure or function may contribute to this remarkable neuroprotection.