Stress Proteins and Tolerance to Focal Cerebral Ischemia

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Abstract
Stress proteins are induced after a variety of neuronal injuries. The inducible 72-kDa heat shock protein (hsp70) is a stress protein that protects neurons from glutamate toxicity in vitro. Hsp70 has also been proposed to underlie the phenomenon of ischemic tolerance whereby brief sublethal intervals of global ischemia protect the hippocampus from subsequent lethal prolonged ischemia. To determine if the phenomenon of tolerance occurs in cortex after focal ischemia, the rat middle cerebral artery (MCA) was occluded by the suture method. Three 10-min intervals of transient ischemia (3 × 10-isc) separated by 45-min periods of reperfusion made up the most effective paradigm of preconditioning ischemia studied, and substantially reduced the volume of infarction 72 h after subsequent 100-min MCA occlusion. This approach induced protection if the interval between the 3 × 10-isc and the 100-min ischemia was 2, 3, or 5 days but not 1 or 7 days. Three 10-min intervals of transient ischemia alone produced minimal histological changes in the cortex at 72 h. Moreover, there were no significant changes in regional cerebral blood flow in the tolerant regions at 72 h after 3 × 10-isc before or during MCA occlusion. To explore the role of stress proteins in the induction of tolerance, expression of hsp70 and the glucose-regulated proteins grp75 and grp78 were studied. Samples from tolerant regions of the brain that had undergone preconditioning ischemia were evaluated at 1, 2, 3, 5, 7, and 14 days after 3 × 10-isc by Western blot analysis. The time course of hsp70 expression most closely correlated with tolerance. Hsp70 protein expression increased during times when tolerance was present (at 2–5 days) but did not increase thereafter (at 7 and 14 days). However, hsp70 was also increased before tolerance was present (at 1 day). Immunocytochemistry showed that hsp70 protein was expressed in neurons in the tolerant regions 24 h after 3 × 10-isc and was expressed in both neurons and glia after 72 h. Although immunocytochemistry suggested that there was increased neuronal expression of grp75 and grp78, no significant differences were found in protein expression as determined by Western blot before (at 1 day), during (at 2–5 days), and after (at 7 days and thereafter) tolerance. Thus, the time course of grp75 and grp78 expression did not correlate with that of tolerance. This model of ischemic tolerance is a useful method by which mechanisms of endogenous neuroprotection may be explored.