Temperature Sensitive Glutamate Gating of AMPA-subtype iGluRs.

Abstract

Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that mediate the majority of excitatory neurotransmission1. iGluRs are gated by glutamate, where upon glutamate binding, they open their ion channels to enable cation influx into post-synaptic neurons, initiating signal transduction2. The structural mechanism of iGluR gating by glutamate has been extensively studied in the context of positive allosteric modulators (PAMs)3-15. A fundamental question has remained - are the PAM activated states of iGluRs representative of glutamate gating in the absence of PAMs? Here, using the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype iGluR (AMPAR) we show that glutamate gating is unique from gating in the presence of PAMs. We demonstrate that glutamate gating is temperature sensitive, and through temperature-resolved cryo-electron microscopy (cryo-EM), capture all major glutamate gating states. Physiological temperatures augment channel activation and conductance. Activation by glutamate initiates ion channel opening that involves all ion channel helices hinging away from the pores axis in a motif that is conserved across all iGluRs. Desensitization occurs when the local dimer pairs decouple and enables closure of the ion channel below through restoring the channel hinges and refolding the channel gate. Our findings define how glutamate gates iGluRs, provide foundations for therapeutic design, and point to iGluR gating being temperature sensitive.