CALPAIN INHIBITORS, BUT NOT CASPASE INHIBITORS, PREVENT ACTIN PROTEOLYSIS AND DNA FRAGMENTATION DURING APOPTOSIS

Apoptosis, or programmed cell death, involves a cascade of regulatory events leading to the activation of specific proteases. However, the k ey substrates for these proteases remain to be identified. We previous ly demonstrated that levels of five unidentified polypeptides were spe cifically increased in neurons from embryonic chicken ciliary ganglia undergoing apoptosis by trophic deprivation. Here me show by microsequ encing of two of these polypeptides that they are fragments of actin. One of them represents cleavage of actin at the site of interaction wi th DNase I. The same actin fragments are also found at early stages of apoptosis in chicken and rat dorsal root ganglion neurons, chicken sp inal motoneurons and rat thymocytes. Actin fragmentation may play a ro le in the apoptotic process, since calpain inhibitors I and II both in hibit neuronal death and suppress actin fragmentation. In contrast, ca spase (ICE family) inhibitors, though effective in delaying neuronal d eath, do not prevent actin cleavage or DNA fragmentation. These result s indicate a keg role for calpain-like proteases in neuronal programme d cell death and suggest that actin fragmentation in the cell is corre lated with subsequent DNA fragmentation.