ISCHEMIC BRAIN INJURY IS MEDIATED BY THE ACTIVATION OF POLY(ADP-RIBOSE)POLYMERASE

Poly(ADP-ribose)polymerase (PARP, EC 2.4.2.30), an abundant nuclear pr otein activated by DNA nicks, mediates cell death in vitro by nicotina mide adenine dinucleotide (NAD) depletion after exposure to nitric oxi de. The authors examined whether genetic deletion of PARP (PARP null m ice) or its pharmacologic inhibition by 3-aminobenzamide (3-AB) attenu ates tissue injury after transient cerebral ischemia. Twenty-two hours after reperfusion following 2 hours of filamentous middle cerebral ar tery occlusion, ischemic injury was decreased in PARP-/- and PARP+/- m ice compared with PARP+/+ litter mates, and also was attenuated in 129 /SV wildtype mice after 3-AB treatment compared with controls. Infarct sparing was accompanied by functional recovery in PARP-/- and S-AB-tr eated mice. Increased poly(ADP-ribose) immunostaining observed in isch emic cell nuclei 5 minutes after reperfusion was reduced by 3-AB treat ment. Levels of NAD-the substrate of PARP-were reduced 2 hours after r eperfusion and were 35% of contralateral levels at 24 hours. The decre ases were attenuated in PARP-/- mice and in 3-AB-treated animals. Poly (ADP-ribose)polymerase cleavage by caspase-3 (CPP-32) has been propose d as an important step in apoptotic cell death. Markers of apoptosis, such as oligonucleosomal DNA damage, total DNA fragmentation, and the density of terminal deoxynucleotidyl transferase dUTP nick-end-labelle d (TUNEL +) cells, however, did not differ in ischemic brain tissue of PARP-/- mice or in 3-AB-treated animals versus controls, although the re were differences in the number of TUNEL-stained cells reflecting th e decrease in infarct size. Thus, ischemic brain injury activates PARP and contributes to cell death most likely by NAD depletion and energy failure, although the authors have not excluded a role for PARP in ap optotic cell death at earlier or later stages in ischemic cell death. Inhibitors of PARP activation could provide a potential therapy in acu te stroke.