Excitotoxicity resulting from over-stimulation of ionotropic glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The over-stimulated receptors exert their neurotoxic effects in part by over-activation of calpains which induce neuronal death by catalyzing limited proteolysis of specific cellular proteins. Here, we report that in cultured cortical neurons and in vivo in a rat model of focal ischemic stroke, the tyrosine kinase Src is cleaved by calpains at a site in the N-terminal unique domain. This generates a truncated Src fragment of approximately 52 kDa, which we localised predominantly to the cytosol. A synthetic cell membrane-permeable fusion peptide derived from the Src unique domain prevents calpains from cleaving Src in neurons and protects against excitotoxic neuronal death. To explore the role of the truncated Src fragment in neuronal death, we expressed a recombinant truncated Src fragment in cultured neurons and examined how it affects neuronal survival. Expression of this fragment, which lacks the unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the pro-survival kinase Akt is a key step in its neurotoxic signaling pathway. Using inhibitors of Src kinase activity and shRNA suppressing expression of Src, we demonstrated that intact Src is essential for maintenance of neuronal survival. Thus, our results implicate calpain cleavage as a molecular switch converting Src from a promoter of cell survival to a mediator of neuronal death in excitotoxicity. Additionally, our results suggest blockade of calpain cleavage of Src as a potential therapeutic strategy to minimize brain damage in ischemic stroke.

Ref: Hossain et al. (2013) “A Truncated Fragment of Src Protein Kinase Generated by Calpain-Mediated Cleavage is A Mediator of Neuronal Death in Excitotoxicity” J. Biol. Chem. 288:9696