Nitric oxide synthase, immunophilins and poly(ADP-ribose) synthetase: novel targets for the development of neuroprotective drugs.

During ischemic stroke, massive neural damage occurs due to excess release of glutamate which acts mainly through N-methyl-D-aspartate (NMDA) receptors. Activation of the NMDA receptor stimulates nitric oxide (NO) production by NO synthase (NOS). NO mediates glutamate neurotoxicity as inhibitors of NOS prevent neuronal death. FK506, an immunosuppressant drug, binds to FK506 binding protein (FKBP). One target of the FK506/FKBP complex is the calcium/calmodulin-dependent protein phosphatase calcineurin, whose activity is inhibited upon interaction with FK506/FKBP. FK506 treatment increases phosphorylation level of calcinurin substrates including NOS. As a potent neuroprotective agent in vitro and in vivo, FK506 increases NOS phosphorylation and decreases NO production. NO activates poly(ADP-ribose) synthetase (PARS), a nuclear enzyme that synthesizes poly(ADP-ribose) from NAD. Prolonged activation of PARS depletes NAD and lowers cellular energy levels. Inhibition of PARS also prevents NO toxicity. NOS inhibitors, immunosuppressants and PARS inhibitors may be useful agents to prevent neuronal damage during stroke.