Calpain induces proteolysis of neuronal cytoskeleton in ischemic gerbil forebrain

We investigated the relationship between the activity of calcium-dependent protease (calpain) and the ischemic neuronal damage. We also investigated the mechanism of ischemic resistance in astrocytes. In gerbil, a 10-min forebrain ischemia was induced by occlusion of both common carotid arteries. The calpain-induced proteolysis of cytoskeleton (fodrin) was examined by immunohistochemistry. Immunolocalization of micro and m-calpain was also examined. Intact fodrin was observed both in neurons and astrocytes, but proteolyzed fodrin was not observed in normal brain. Fifteen minutes after ischemia, proteolysis of fodrin took place in putamen, parietal cortex and hippocampal CA1. The proteolysis extended to thalamus 4 h after ischemia after which the immunoreactivity faded down in all areas except hippocampus. On day 7, the proteolysis was still observed only in hippocampus. Neurons with the proteolysis of soma resulted in neuronal death. Throughout the experiment, the proteolysis was not observed in astrocytes. micro -Calpain was observed only in neurons but m-calpain was observed both in neurons and astrocytes. The ischemia induced only micro -calpain activation, which resulted in fodrin proteolysis of neurons with differential spatial distribution and temporal course. The proteolysis was developed rapidly and was completed within 24 h in all vulnerable regions except hippocampal CA1. The proteolysis preceded the neuronal death. The mechanism of the proteolysis seemed to be involved by Ca(2+) influx via glutamate receptor and rapid neuronal death seemed reasonable. The reason why neuronal death in CA1 evolved slowly was not clarified. In astrocytes, fodrin was not proteolyzed by m-calpain. The low Ca(2+)-sensitivity of m-calpain may be the reason of ischemic resistance in astrocytes.