Calpain and caspase: can you tell the difference?

Abstract

Both necrotic and apoptotic neuronal death are observed in various neurological and neurodegenerative disorders. Calpain is activated in various necrotic and apoptotic conditions, while caspase 3 is only activated in neuronal apoptosis. Despite the difference in cleavage-site specificity, an increasing number of cellular proteins are found to be dually susceptible to these cysteine proteases. These include alpha- and beta-fodrin, calmodulin-dependent protein kinases, ADP-ribosyltransferase (ADPRT/PARP) and tau. Intriguingly, calpastatin is susceptible to caspase-mediated fragmentation. Neurotoxic challenges such as hypoxia-hypoglycemia, excitotoxin treatment or metabolic inhibition of cultured neurons result in activation of both proteases. Calpain inhibitors can protect against necrotic neuronal death and, to a lesser extent, apoptotic death. Caspase inhibitors strongly suppress apoptotic neuronal death. Thus, both protease families might contribute to structural derangement and functional loss in neurons under degenerative conditions.