Excessive activation of the nuclear enzyme, poly(ADP-ribose) polymerase-1 (PARP-1) plays a prominent role in various of models of cellular injury. Here, we identify poly(ADP-ribose) (PAR) polymer, a product of PARP-1 activity, as a previously uncharacterized cell death signal. PAR polymer is directly toxic to neurons, and degradation of PAR polymer by poly(ADP-ribose) glycohydrolase (PARG) or phosphodiesterase 1 prevents PAR polymer-induced cell death. PARP-1-dependent, NMDA excitotoxicity of cortical neurons is reduced by neutralizing antibodies to PAR and by overexpression of PARG. Neuronal cultures with reduced levels of PARG are more sensitive to NMDA excitotoxicity than WT cultures. Transgenic mice overexpressing PARG have significantly reduced infarct volumes after focal ischemia. Conversely, mice with reduced levels of PARG have significantly increased infarct volumes after focal ischemia compared with WT littermate controls. These results reveal PAR polymer as a signaling molecule that induces cell death and suggests that interference with PAR polymer signaling may offer innovative therapeutic approaches for the treatment of cellular injury.excitotoxicity ͉ poly(ADP-ribose) glycohydrolase ͉ poly(ADP-ribose) polymerase ͉ stroke P oly(ADP-ribose) polymerase-1 (PARP-1) is an abundant nuclear protein that is involved in the DNA base excision repair system, where it is potently activated by DNA strand nicks and breaks (1, 2). Using NAD ϩ as a substrate, PARP-1 builds up homopolymers of ADP ribose units on various nuclear proteins including histones, DNA polymerases, topoisomerases, DNA ligase-2, transcription factors (3, 4), and PARP-1 itself (5, 6). Although the exact physiologic function of PARP-1 is not completely understood, in some tissues it plays an important role in DNA repair and genomic stability (5,7,8). Poly(ADP-ribose) (PAR) catabolism and metabolism is a dynamic process, with PAR glycohydrolase (PARG) playing the major role in the degradation of the polymer (9).Recent studies using pharmacologic inhibition of PARP or genetic KO of PARP-1 indicate that PARP-1 plays a dramatic and significant role in cellular injury after stroke, trauma, ischemiareperfusion of the heart, spleen, skeletal muscle, and retina, arthritis, ␤-islet cytotoxicity͞diabetes mellitus, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of Parkinson's disease, experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, endotoxic shock, multiple-system organ failure, and liver damage (for review, see refs. 1 and 10). PARP-1 activation also plays a prominent role in NMDA excitotoxicity, because PARP-1 KO mice are remarkably resistant both in vitro and in vivo to the excitotoxic effects of glutamate and NMDA (11,12). A cell-suicide hypothesis has been proposed (1,2,13,14) to explain the actions of PARP-1 in mediating cell death. However, studies in mice lacking PARG suggest that PAR polymer formed during the activation of PARP-1 might play a role in PARP-1-dependent cell death. PARG KO mice die at embryoni...

The water channel AQP4 is concentrated in perivascular and subpial membrane domains of brain astrocytes. These membranes form the interface between the neuropil and extracerebral liquid spaces. AQP4 is anchored at these membranes by its carboxyl terminus to ␣-syntrophin, an adapter protein associated with dystrophin. To test functions of the perivascular AQP4 pool, we studied mice homozygous for targeted disruption of the gene encoding ␣-syntrophin (␣-Syn ؊/؊ ). These animals show a marked loss of AQP4 from perivascular and subpial membranes but no decrease in other membrane domains, as judged by quantitative immunogold electron microscopy. In the basal state, perivascular and subpial astroglial end-feet were swollen in brains of ␣-Syn ؊/؊ mice compared to WT mice, suggesting reduced clearance of water generated by brain metabolism. When stressed by transient cerebral ischemia, brain edema was attenuated in ␣-Syn ؊/؊ mice, indicative of reduced water influx. Surprisingly, AQP4 was strongly reduced but ␣-syntrophin was retained in perivascular astroglial end-feet in WT mice examined 23 h after transient cerebral ischemia. Thus ␣-syntrophin-dependent anchoring of AQP4 is sensitive to ischemia, and loss of AQP4 from this site may retard the dissipation of postischemic brain edema. These studies identify a specific, syntrophin-dependent AQP4 pool that is expressed at distinct membrane domains and which mediates bidirectional transport of water across the brain-blood interface. The anchoring of AQP4 to ␣-syntrophin may be a target for treatment of brain edema, but therapeutic manipulations of AQP4 must consider the bidirectional water flux through this molecule. C erebral edema is essentially a loss of water homeostasis entailing a net increase of water flux into the brain. The route of water influx in this life-threatening condition is unknown, and no efficient therapy exists. We have previously shown that the brain expresses a water channel molecule, AQP4, that is strongly enriched in those astrocyte membrane domains forming the interface between brain neuropil and extracerebral spaces filled with blood or cerebrospinal fluid (1-3). To determine whether the pools of AQP4 in these specialized membrane domains are responsible for the fast influx of water that occurs during the development of brain edema, one must specifically eliminate the perivascular and subpial pools of AQP4 while leaving other pools of AQP4 intact. This can be achieved by deletion of ␣-syntrophin (␣-syn), an adapter protein in the dystrophin-associated protein complex that is required for anchoring AQP4 at these specialized membrane domains (4). Mice homozygous for targeted disruption of the gene encoding ␣-syntrophin (␣-Syn Ϫ/Ϫ ) exhibit a marked reduction of AQP4 in perivascular and subpial membranes but not in other locations in brain, because total brain AQP4 protein content is not reduced (4).The first aim of the present study was to use ␣-Syn Ϫ/Ϫ mice to investigate whether a selective depletion of the perivascular AQP4 pool reduces the vol...