Glutamate as a co-agonist for acid-sensing ion channels to aggravate ischemic brain damage

Abstract

Glutamate is traditionally viewed as the first messenger to activate N-methyl-D-aspartate receptors (NMDARs) and downstream cell death pathways in stroke1,2, but unsuccessful clinical trials with NMDAR antagonists implicate the engagement of other NMDAR-independent mechanisms3-7. Here we show that glutamate and its structural analogs, including NMDAR antagonist L-AP5 (or APV), robustly potentiated currents mediated by acid-sensing ion channels (ASICs) which are known for driving acidosis-induced neurotoxicity in stroke4. Glutamate increased the proton affinity and open probability of ASICs, aggravating ischemic neurotoxicity in both in vitro and in vivo models. Site-directed mutagenesis and structure-based in silico molecular docking and simulations uncovered a novel glutamate binding cavity in the extracellular domain of ASIC1a. Computational drug screening of NMDAR competitive antagonist analogs identified a small molecule, LK-2, that binds to this cavity and abolishes glutamate-dependent potentiation of ASIC currents but spares NMDARs, providing strong neuroprotection efficacy comparable to that in ASIC1a or other cation ion channel knockout mouse models4-7. We conclude that glutamate serves as the first messenger for ASICs to exacerbate neurotoxicity, and that selective blockage of glutamate binding sites on ASICs without affecting NMDARs may be of strategic importance for developing effective stroke therapeutics devoid of the psychotic side effects of NMDAR antagonists.