Context-Dependent GluN2B-Selective Inhibitors of NMDA Receptor Function Are Neuroprotective with Minimal Side Effects

Abstract

SUMMARY Stroke remains a significant problem despite decades of work on neuroprotective strategies. NMDA receptor (NMDAR) antagonists are neuroprotective in preclinical models, but have been clinically unsuccessful, in part due to side effects. Here we describe a prototypical GluN2B-selective antagonist with an IC50 value that is 10-fold more potent at acidic pH 6.9 associated with ischemic tissue compared to pH 7.6, a value close to the pH in healthy brain tissue. This should maximize neuroprotection in ischemic tissue while minimizing on-target side-effects associated with NMDAR blockade in non-injured brain regions. We have determined the mechanism underlying pH-dependent inhibition and demonstrate the utility of this approach in vivo. We also identify dicarboxylate dimers as a novel proton sensor in proteins. These results provide insight into the molecular basis of pH-dependent neuroprotective NMDAR block, which could be beneficial in a wide range of neurological insults associated with tissue acidification.

Mechanisms for Zinc and Proton Inhibition of the GluN1/GluN2A NMDA Receptor

“…This “shift” of the structural populations away from the 2-knuckle-asym conformation is consistent with electrophysiological studies showing protons sensitize the receptor to zinc-inhibition, as well as zinc and proton inhibitions being strongly coupled to each other (Erreger and Traynelis, 2005; Low et al, 2000; Zheng et al, 2001). While there is evidence that the proton sensors on the receptor are distributed through various domains, Yuan et al have proposed Glu106 in the R1 lobe and Glu235 in the R2 lobe as the proton sensors in the ATD of the GluN2B, forming carboxylic acid “dimers” at low pH values when one or both sidechains are protonated (Yuan et al, 2015), leading to the stabilization of the closed ATD clamshell conformation. The corresponding residues in the GluN2A subunit are Glu107 and Asp234.…”

“…By contrast, elements of the receptor implicated in proton modulation are distributed throughout the receptor, with both the LBD-TMD linkers and the ATD playing a role in pH dependent gating (Gielen et al, 2009; Yuan et al, 2015). …”

Mechanism of NMDA receptor channel block by MK-801 and memantine

“…pH-dependent GluN2B-selective inhibitors show neuroprotection in traumatic brain injury, creating a strategy for exploring therapy for brain injury with minimal side effects 15 . Moreover, small molecules that target the TMD, including dizolcipine (MK-801) and memantine, show promise in the treatment of excitotoxicity-related disorders 16 .…”

Network-Based Approach to Identify Potential Targets and Drugs that Promote Neuroprotection and Neurorepair in Acute Ischemic Stroke

“…Although the ultimate validation of these novel results will require systematic experimental and clinical studies, we searched the recent literature and found isolated studies that partially validated our results, supporting the reliability of these data. For example, both NMDA receptors and GPCR- related pathways have been identified in M64 disease modules, and related studies have shown that NMDA receptor blockers exert a neuroprotective effect while GPCR agonists are associated with cell apoptosis454647. Otherwise, the activation of NMDA receptors and GPCRs contributes to Ca 2+ overload1042, which occurs within minutes of stroke onset33.…”