Calpains are a family of 14 proteases that are important for cell migration in a variety of tissues. Unlike other proteases, calpains typically cleave substrates once or twice, thereby altering their localization and function, as opposed to simply degrading them. There are two ubiquitously expressed isoforms of calpain, calpain 1 (u) and calpain 2 (m), that are distinguished by their calcium requirements for activity. These isoforms form heterodimers with the small calpain subunit, calpain 4, which stabilizes calpain 1 and 2.
Many of calpain's cleavage substrates are components of focal adhesions and their cleavage can alter focal adhesion dynamics and cell migration. For instance, calpain mediated cleavage of talin, FAK and paxillin is required for focal adhesion turnover. In the absence of calpain 4, there is a loss of both calpain 1 and 2 ex
To date, the function of calpains in T cells has been studied using cell permeable inhibitors. Several reports using calpain inhibitors have shown that calpain is required for T cell adhesion mediated by β1 and β2 integrins. However, a more recent study has suggested calpain is not required for T cell adhesion to ICAM-1 and that some of the initial findings of decreased adhesion may have been due to offtarget effects of inhibitors and rapid induction of apoptosis following stimulation. Despite the presence of known calpain substrates at the immune synapse, including talin, vinculin and paxilhn, no one has investigated the role of calpain in T cell IS formation and activation.
Calpain, a calcium-dependent protease with papain-like activity, is a cytoplasmic cysteine protease that is activated by calcium. It is a highly conserved protease with homologues present in invertebrates, plants, fungi, and mammals, and it consists of more than 15 ubiquitous and tissue specific isoforms in humans. The catalytic subunits found in the CNS include isoforms 1, 2, 3, 5, and 10. The major calpains expressed in all cell types are p-calpain, and m-calpain. Mu-calpain (aka calpain I) is located in the cytosol or near the membrane and is activated by µM concentrations of intracellular calcium. In contrast, m-calpain (aka calpain II) is located at the membrane and requires mM concentrations of calcium for activation. The threshold calcium sensitivity is functionally important, since physiologic concentrations of calcium range from 100-1,000 nM but rise to 5-10 pM during excitatory conditions. Upon calcium activation, the 80-kDa calpain subunit is auto-catalytically processed to a 76-kDa fragment, and the 30-kDa regulatory subunit is processed to 18 kDa. Calpain activity is modulated by the endogenous inhibitor calpastatin. Both calpain and caspase-3 can cleave calpastatin. The calpain cleavage products retain their calpain-inhibitory activity, while caspase cleavage abrogates the inhibitory activity of calpastatin. Thus, a positive feedback loop of calpain activity can be established following upregulation of caspase-3 activity. The prototypical calpain substrate is the cytoskeletal protein α-spectrin. Calpain cleaves this protein into 150 and 145 kDa fragments, which are considered surrogates for the quantification of calpain activity.