Synaptic NMDA receptors mediate hypoxic excitotoxic death.

Excessive NMDA receptor activation and excitotoxicity underlies pathology in many neuropsychiatric and neurological disorders, including hypoxia/ischemia. Thus, the development of effective therapeutics for these disorders demands a complete understanding of NMDA receptor (NMDAR) activation during excitotoxic insults. The extrasynaptic NMDAR hypothesis posits that synaptic NMDARs are neurotrophic/neuroprotective and extrasynaptic NMDARs are neurotoxic. The extrasynaptic hypothesis is built in part on observed selectivity for extrasynaptic receptors of a neuroprotective use-dependent NMDAR channel blocker, memantine. In rat hippocampal neurons, we found that a neuroprotective concentration of memantine shows little selectivity for extrasynaptic NMDARs when all receptors are tonically activated by exogenous glutamate. This led us to test the extrasynaptic NMDAR hypothesis using metabolic challenge, where the source of excitotoxic glutamate buildup may be largely synaptic. Three independent approaches suggest strongly that synaptic receptors participate prominently in hypoxic excitotoxicity. First, block of glutamate transporters with a nonsubstrate antagonist exacerbated rather than prevented damage, consistent with a primarily synaptic source of glutamate. Second, selective, preblock of synaptic NMDARs with a slowly reversible, use-dependent antagonist protected nearly fully against prolonged hypoxic insult. Third, glutamate pyruvate transaminase, which degrades ambient but not synaptic glutamate, did not protect against hypoxia but protected against exogenous glutamate damage. Together, these results suggest that synaptic NMDARs can mediate excitotoxicity, particularly when the glutamate source is synaptic and when synaptic receptor contributions are rigorously defined. Moreover, the results suggest that in some situations therapeutically targeting extrasynaptic receptors may be inappropriate.

Nicotinamide mononucleotide adenylyl transferase 1 protects against acute neurodegeneration in developing CNS by inhibiting excitotoxic-necrotic cell death.

Hypoxic-ischemic (H-I) injury to the developing brain is a significant cause of morbidity and mortality in humans. Other than hypothermia, there is no effective treatment to prevent or lessen the consequences of neonatal H-I. Increased expression of the NAD synthesizing enzyme nicotinamide mononucleotide adenylyl transferase 1 (Nmnat1) has been shown to be neuroprotective against axonal injury in the peripheral nervous system. To investigate the neuroprotective role of Nmnat1 against acute neurodegeneration in the developing CNS, we exposed wild-type mice and mice overexpressing Nmnat1 in the cytoplasm (cytNmnat1-Tg mice) to a well-characterized model of neonatal H-I brain injury. As early as 6 h after H-I, cytNmnat1-Tg mice had strikingly less injury detected by MRI. CytNmnat1-Tg mice had markedly less injury in hippocampus, cortex, and striatum than wild-type mice as assessed by loss of tissue volume 7 d days after H-I. The dramatic protection mediated by cytNmnat1 is not mediated through modulating caspase3-dependent cell death in cytNmnat1-Tg brains. CytNmnat1 protected neuronal cell bodies and processes against NMDA-induced excitotoxicity, whereas caspase inhibition or B-cell lymphoma-extra large (Bcl-XL) protein overexpression had no protective effects in cultured cortical neurons. These results suggest that cytNmnat1 protects against neonatal HI-induced CNS injury by inhibiting excitotoxicity-induced, caspase-independent injury to neuronal processes and cell bodies. As such, the Nmnat1 protective pathway could be a useful therapeutic target for acute and chronic neurodegenerative insults mediated by excitotoxicity.