Calpain-Mediated Truncation of Glutamate Ionotropic Receptors: Methods for Studying the Effects of Calpain Activation in Brain Tissue

Identification of the preferred brain substrates of calpain in vivo is not an easy task. The first difficulty is that, given enough time, almost any protein will be degraded when incubated in the presence of calpain and Ca⁺ in a test tube. A second major problem is the lack of completely specific calpain inhibitors that can effectively block calpain activity in the brain in vivo. Finally, even when the criteria for identification of such substrates are satisfied, one would like to know under which experimental conditions particular substrates are degraded and, if possible, the regional and cellular localization of the degradation. Numerous reports have suggested that spectrin is one of the substrates preferred by calpain, and calpain-mediated spectrin degradation has been widely used as an index of calpain activation in vitro as well as in vivo. The identification of the site in α-spectrin cleaved by calpain led to the preparation of antibodies against the newly exposed C-and N-terminal peptides at the spectrin cleavage site. These antibodies were used to localize calpain-mediated spectrin degradation in brain under a number of experimental conditions, showing that seizure activity, ischemia, and tetanic stimulation produce very selective calpain activation in hippocampus and other brain structures. However, it is now known that this spectrin site is cleaved not only by calpain, but also by other proteases, such as some members of the caspase family.