Caspase 3 activation is essential for neuroprotection in preconditioning

Sublethal insults can induce tolerance to subsequent stressors in neurons. As cell death activators such as ROS generation and decreased ATP can initiate tolerance, we tested whether other cellular elements normally associated with neuronal injury could add to this process. In an in vivo model of ischemic tolerance, we were surprised to observe widespread caspase 3 cleavage, without cell death, in preconditioned tissue. To dissect the preconditioning pathways activating caspases, and the mechanisms by which these proteases are held in check, we developed an in vitro model of excitotoxic tolerance. In this model, antioxidants and caspase inhibitors blocked ischemia-induced protection against N-methyl-d-aspartate toxicity. Moreover, agents that blocked preconditioning also attenuated induction of HSP 70; transient overexpression of a constitutive form of this protein prevented HSP 70 up-regulation and blocked tolerance. We outline a neuroprotective pathway where events normally associated with apoptotic cell death are critical for cell survival.