Potential contribution of proteases to neuronal damage.

Calpain was first discovered 30 years ago. Two major isoforms were subsequently isolated and purified. The presence of an endogenous protein inhibitor, calpastatin, was later discovered. Calpain activity is tightly regulated by Ca(2+). At physiological levels of Ca(2+), the role of calpain remains poorly understood, but is believed to be involved in mitosis and muscle cell differentiation. Calpain has also been implicated in various membrane fusion events through remodeling of the cytoskeletal network. Calpain activation has been shown to be increased during normal aging and in muscular dystrophy, cataract, arthritis and Alzheimer's disease, and in acute traumas such as traumatic brain injury (TBI), spinal cord injury and cerebral and cardiac ischemia. Early work on calpain inhibitors was limited to protein inhibitors and other nonselective enzyme inhibitors. Peptidyl aldehydes such as leupeptin and antipain are also among the earliest reported calpain inactivators. Irreversible inhibitors such as the E64 family have also been studied, and peptidyl halomethanes and diazomethanes have long been used as protease inhibitors. A variety of calpain inhibitors are under development. From a therapeutic perspective, calpain inhibitors may have several advantages over other more conventional targets such as ion channel blockers and glumate antagonists, since calpain proteolysis represents a later component of a pathway mediating cell death initiated by excitotoxicity and elevated Ca(2+) levels. Although the potential clinical utility of calpain inhibitors seems well established, a number of important considerations remain to be addressed. The role of other proteolytic cascades contributing to neuronal cell damage following TBI must also be considered.