Calpain activation in apoptosis

Programmed cell death is an active process wherein the cell initiates a sequence of events culminating in the fragmentation of its DNA, nuclear collapse, and disintegration of the cell into small, membrane‐bound apoptotic bodies. Examination of the death program in various models has shown common themes, including a rise in cytoplasmic calcium, cytoskeletal changes, and redistribution of membrane lipids. The calcium‐dependent neutral protease calpain has putative roles in cytoskeletal and membrane changes in other cellular processes; this fact led us to test the role of calpain in a well‐known model of apoptotic cell death, that of thymocytes after treatment with dexamethasone. Assays for calcium‐dependent proteolysis in thymocyte extracts reveal a rise in activity with a peak at about 1 hr of incubation with dexamethasone, falling to background at approximately 2 hr. Western blots indicate autolytic cleavage of the proenzyme precursor to the calpain I isozyme, providing additional evidence for calpain activation. We have also found that apoptosis in thymocytes, whether induced by dexamethasone or by low‐level irradiation, is blocked by specific inhibitors of calpain. Apoptosis of metamyelocytes incubated with cycloheximide is also blocked by calpain inhibitors. These studies suggest a required role for calpain in both “induction” and “release” models of apoptotic cell death. © 1994 wiley‐Liss, Inc.