Molecular mechanisms of NMDA receptor-mediated excitotoxicity: Implications for neuroprotective therapeutics for stroke

Abstract

Excitotoxicity is a process observed in many disease states by which an excessive synaptic excitation causes neuronal death, and is thought to be triggered by the extracellular accumulation of the excitatory neurotransmitter glutamate, which binds and activates ionotropic N-methyl-D-aspartate glutamatergic receptors (NMDARs) in the brain. Normally, NMDARs mediate calcium entry into the cell to regulate physiological processes such as synaptic plasticity and memory, but excessive stimulation can cause a pathological rise in intracellular calcium, which activates cell death signaling to produce apoptosis. This phenomenon is implicated in numerous conditions such as Alzheimer's disease, traumatic brain injury, and alcohol withdrawal, and is widely studied to better understand disease processes and possible treatment strategies. Particularly in stroke, excitotoxicity has been demonstrated to be the primary mechanism by which neuronal damage occurs and is a popular target for many recent attempts at developing stroke therapeutics. Stroke is an acute brain insult leading to neuronal damage that has virtually no effective neuroprotective treatments in clinical use. Immediately following stroke, brain tissue loses blood perfusion and the core of the infarct deteriorates rapidly. The surrounding penumbra experiences milder ischemia and many neurons within it will undergo delayed death that can take hours or even days. For these cells, studies show that the mechanism of death is primarily NMDA receptor-dependent excitotoxicity. In ischemic regions, extracellular glutamate levels acutely rise several fold via increased release and compromised uptake, while preventing glutamate release, synaptic activity, or NMDAR activation is able to rescue cell death in numerous stroke models (Lai et al., 2014). Therefore, blocking excitotoxicity should prove a viable strategy for mitigating brain damage and improving patient outcome following a stroke, and this has indeed promoted extensive academic and industrial efforts in developing NMDA receptor-based stroke treatments in the last two decades. Unfortunately, these have thus far largely met with rather disappointing results; several large scale clinical trials have failed to find the expected efficacy of NMDAR antagonists in reducing brain injuries (reviewed in Lai et al., 2014). The reasons underlying the apparent contradiction between basic research results and clinical trials remain uncertain, but several reasons have been proposed. These include, but are not limited to, the inability to use the antagonists at the doses required for neuroprotection due to side effects, the inability to administer the drugs within their neuroprotective windows, poor experimental designs, and heterogeneity in the patient population (Lai et al., 2014). However, as briefly summarized below, advancement in our understanding of the mechanisms of physiologic and pathologic NMDAR activation, and in particular, the distinct pathways linked to different NMDAR subtypes, has reignited hope and allowed scientists to develop novel treatments that improve therapeutic windows and increase specificity for death signaling pathways, achieving neuroprotection without indiscriminate disruption of other signaling pathways downstream of the receptor. Developing novel and effective neuroprotectants by differentially targeting NMDAR subtypes Different NMDAR subtypes serve opposing functions in normal physiology and excitotoxicity: The NMDAR is a heteromeric receptor consisting of four subunits. NMDARs generally contain two GluN1 (previously also known as NR1) subunits and two subunits from the GluN2 subfamily (GluN2A-2D, previously also known as NR2A-2D). In the cortex, the major subpopulations of NMDARs are GluN2A-, GluN2B-, or GluN2A and 2B-containing receptors. GluN2A-containing receptors are preferentially localized at synapses, while GluN2B-containing receptors are predominantly found on extrasynaptic membranes. GluN2A- and GluN2B-containing receptors oppose each other as key mediators of plasticity, favoring either long-term potentiation (GluN2A) or depression (GluN2B) by virtue of their different electrophysiological and pharmacological properties and coupled downstream signaling proteins. Additionally, these receptors play an additional role in promoting cell survival (GluN2A) or death (GluN2B) following excitotoxic stimulation (Liu et al., 2007). Because GluN2A-containing receptors are primarily localized to synapses, while GluN2B-containing receptors are on both synaptic and extrasynaptic membranes, when excitotoxic conditions cause excess glutamate to spill beyond synapses, GluN2B-mediated death signaling becomes stronger relative to GluN2A-mediated survival signaling and results in increased death. Therefore, during stroke, indiscriminate targeting of NMDARs is unlikely to tip the balance to favor cell survival and could instead cause detrimental effects by inhibiting important normal physiological functi