Mild cerebral ischemia induces loss of cyclin-dependent kinase inhibitors and activation of cell cycle machinery before delayed neuronal cell death

After mild ischemic insults, many neurons undergo delayed neuronal death. A berrant activation of the cell cycle machinery is thought to contribute to apoptosis in various conditions including ischemia. We demonstrate that los s of endogenous cyclin-dependent kinase (Cdk) inhibitor p16(INK4a) is an ea rly and reliable indicator of delayed neuronal death in striatal neurons af ter mild cerebral ischemia in vivo. Loss of p27(Kip1), another Cdk inhibito r, precedes cell death in neocortical neurons subjected to oxygen-glucose d eprivation in vitro. The loss of Cdk inhibitors is followed by upregulation of cyclin D1, activation of Cdk2, and subsequent cytoskeletal disintegrati on. Most neurons undergo cell death before entering S-phase, albeit a small number (similar to1%) do progress to the S-phase before their death. Treat ment with Cdk inhibitors significantly reduces cell death in vitro. These r esults show that alteration of cell cycle regulatory mechanisms is a prelud e to delayed neuronal death in focal cerebral ischemia and that pharmacolog ical interventions aimed at neuroprotection may be usefully directed at cel l cycle regulatory mechanisms.