A truncated fragment of Src protein kinase generated by calpain-mediated cleavage is a mediator of neuronal death in excitotoxicity

Excitotoxicity resulting from overstimulation of glutamate receptors is a major cause of neuronal death in cerebral ischemic stroke. The overstimulated ionotropic glutamate receptors exert their neurotoxic effects in part by overactivation 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 ?52 kDa, which we localized predominantly to the cytosol. A cell membrane-permeable fusion peptide derived from the unique domain of Src prevents calpain 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 myristoylation motif and unique domain, was sufficient to induce neuronal death. Furthermore, inactivation of the prosurvival kinase Akt is a key step in its neurotoxic signaling pathway. Because Src maintains neuronal survival, 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. Besides unveiling a new pathological action of Src, our discovery of the neurotoxic action of the truncated Src fragment suggests new therapeutic strategies with the potential to minimize brain damage in ischemic stroke.

Effect of training on activation of extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase pathways in rat soleus muscle

1. The effect of a chronic programme of either low- or moderate-to-high-intensity treadmill running on the activation of the extracellular-signal regulated protein kinase (ERK1/2) and the p38 mitogen-activated protein kinase (MAPK) pathways was determined in rat muscle. 2. Sprague-Dawley rats were assigned to one of three groups: (i) sedentary (NT; n = 8); (ii) low-intensity training (8 m/min; LIT; n = 16); and (iii) moderate-to-high-intensity training (28 m/min; HIT;n = 16). The training regimens were planned so that animals covered the same distance and had similar glycogen utilization for both LIT and HIT exercise sessions. 3. A single bout of LIT or HIT following 8 weeks of training led to a twofold increase in the phosphorylation of ERK1/2 (P = 0.048) and a two- to threefold increase in p38 MAPK (P = 0.005). Extracellular signal-regulated kinase 1/2 phosphorylation in muscle sampled 48 h after the last exercise bout was similar to sedentary values, while p38 MAPK phosphorylation was 70-80% lower than sedentary. One bout of LIT or HIT increased total ERK1/2 and p38 MAPK expression, with the magnitude of this increase being independent of prior exercise intensity or duration. Extracellular signal-regulated kinase 1/2 expression was increased three- to fourfold in muscle sampled 48 h after the last exercise bout irrespective of the prior training programme (P = 0.027), but p38 MAPK expression was approximately 90% lower than sedentary values. 4. In conclusion, exercise-training of different intensities/durations results in selective postexercise activation of intracellular signalling pathways, which may be one mechanism regulating specific adaptations induced by diverse training programmes.