Hypoxia-ischemia (H-I) in the developing brain results in brain injury with prominent features of both apoptosis and necrosis. A peptide-based pan-caspase inhibitor is neuroprotective against neonatal H-I brain injury, suggesting a central role of caspases in brain injury. Because previously studied peptide-based caspase inhibitors are not potent and are only partially selective, the exact contribution of specific caspases and other proteases to injury after H-I is not clear. In this study, we explored the neuroprotective effects of a small, reversible caspase-3 inhibitor M826. M826 selectively and potently inhibited both caspase-3 enzymatic activity and apoptosis in cultured cells in vitro. In a rat model of neonatal H-I, M826 blocked caspase-3 activation and cleavage of its substrates, which begins 6 h and peaks 24 h after H-I. Although M826 significantly reduced DNA fragmentation and brain tissue loss, it did not prevent calpain activation in the cortex. This activation, which is associated with excitotoxic/necrotic cell injury, occurred within 30 min to 2 h after H-I even in the presence of M826. Similar to calpain activation, we found evidence of caspase-2 processing within 30 min to 2 h after H-I that was not affected by M826. Caspase-2 processing appeared to be secondary to calpain-mediated cleavage and was not associated with caspase-2 activation. These data suggest that caspase-3 specifically contributes to delayed cell death and brain injury after neonatal H-I and that calpain activation is associated with and likely a marker for the early component of excitotoxic/necrotic brain injury previously demonstrated in this model.
Hypoxic-ischemic (H-I)1 encephalopathy in the prenatal and perinatal period is a major cause of morbidity and mortality and often results in cognitive impairment, seizures, and motor impairment leading to cerebral palsy (1, 2). Many studies of neonatal H-I brain injury have utilized the well characterized Levine model in which unilateral carotid ligation is followed by exposure to hypoxia in postnatal day (P) 7 rats (3-5). This model of H-I results in a reproducible pattern of hemispheric injury ipsilateral, but not contralateral, to the carotid ligation (5-7). There are prominent features of both apoptosis and necrosis when this model is performed in neonatal rats and mice (1, 8 -11). Inhibition of caspases utilizing a pan-caspase inhibitor partially protects against brain injury after neonatal H-I injury in this model (12), and similar inhibitors have been shown to partially protect against ischemic injury in adult models (13-16). Previously utilized peptide-based caspase inhibitors (e.g. Boc-D-fmk, z-VAD-fmk, z-DEVD-fmk) required relatively large doses in vivo for their protective effects, and at high concentrations, their effects are more likely to be less selective. Thus, although these studies suggest a role for caspases, the specific caspases and other proteases, which contribute to brain injury after neonatal H-I, have not been clarified.Caspases are a family of cysteine asp...