Calpains are a highly conserved family of intracellular, non-lysosomal, calcium-dependent cysteine proteases which are found in organisms ranging from mammals to drosophila. Currently, at least 12 different calpains have been identified in mammals with ubiquitous (m- and μ-calpains) and tissue-specific (p94calpain-3 and calpain-4 in skeletal muscle) expression patterns. The m-calpains, μ-calpains and calpastatin are the best-characterized members in the family. They are pivotal proteases participating in limited proteolysis of a number of structural proteins, regulatory proteins and the tumor-suppressing protein Rb. One of the best-documented functions for these enzymes is the regulation of integrin-mediated cell migration. Calpains release the link between the integrin-dependent FA complex and the actin cytoskeleton by proteolysis of talin, which allows proper cell migration.Calpain-mediated proteolysis represents a major pathway of post-translational modification of cellular proteins. Activation of Src induces the synthesis of calpain, which promotes the proteolytic cleavage of FAK. FAK is then released from the cytoskeletal fraction into the cytoplasm, where it is further cleaved into fragments. Calpain-mediated cleavage of FAK results in dissociation of a focal-adhesion-targeting sequence from its amino-terminal and kinase domains, with a loss of FAK catalytic activity at sites of focal adhesion. This contributes to the disassembly of FA structures and results in the loss of integrin, growth factor- and Src-signaling to downstream effectors. During cell migration, μ-calpain (calpain-1) acts at the leading edge in response to integrin signals or calcium fluctuations due to the stress activated calcium channels. It cleaves its target proteins, talin, ezrin, paxillin and the cytoplasmic tail of integrins to release the adhesion and form new adhesions. Calpain-2 (m-calpain) is membrane-bound and functions at the trailing edge of the migrating cell to cleave integrins in response to growth factor signals. Signals derived from EGFR activation enhance calpain activity. Furthermore, inhibitors of calpain suppress EGF-induced substrate de-adhesion and motility.
Enhanced calpain activity is also associated with cell cycle progression. It accelerates the progression of transformed cells through the G1-phase of the cell cycle, increases hyperphosphorylation of Rb protein and increases protein levels of cyclin D, cyclin A and CDK2. Calpain promotes the cleavage of p27 (KIP1), a known inhibitor of CDK2. Calpain provides a common link that regulates the ability of v-Src, and possibly c-Src, to promote both cell migration and proliferation. Increased levels of calpain overcome the inhibitory action of calpastatin and promote proteolytic degradation of calpastatin, which further enhances calpain activity. Members of the calpain family function in various biological processes including integrin-mediated cell migration, cytoskeletal remodeling, cell differentiation and apoptosis. At the pathological level, over-activation of calpain as well as mutations abrogating calpain activity have been implicated in several diseases. Administration of calpain inhibitors reduces or prevents tissue damage, but the toxicity and lack of specificity of current inhibitors limits the effectiveness of such therapies.