Desensitization dynamics of the AMPA receptor

Structural Insights into Kainate Receptor Desensitization

Kainate receptors (KARs), along with AMPA and NMDA receptors, belong to the ionotropic glutamate receptor (iGluR) family and play critical roles in mediating excitatory neurotransmission throughout the central nervous system. KARs also regulate neurotransmitter release and modulate neuronal excitability and plasticity. Receptor desensitization plays a critical role in modulating the strength of synaptic transmission and synaptic plasticity. While KARs share overall structural similarity with AMPA receptors, the desensitized state of KARs differs strikingly from that of other iGluRs. Despite extensive studies on KARs, a fundamental question remains unsolved: why do KARs require large conformational changes upon desensitization, unlike other iGluRs? To address this, we present cryo-electron microscopy structures of GluK2 with double cysteine mutations in non-desensitized, shallow-desensitized and deep-desensitized conformations. In the shallow-desensitized conformation, two cysteine crosslinks stabilize the receptors in a conformation that resembles the desensitized state of AMPA receptors. However, unlike the tightly closed pore observed in the deep-desensitized KAR and desensitized AMPAR conformations, the channel pore in the shallow-desensitized state remains incompletely closed. Patch-clamp recordings and fluctuation analysis suggest that this state remains ion-permeable, indicating that the lateral rotational movement of KAR ligand-binding domains (LBDs) is critical for complete channel closure and stabilization of the receptor in desensitization states. Together with the multiple conformations representing different degree of desensitization, our results define the unique mechanism and conformational dynamics of KAR desensitization.

Highlights

We present cryo-EM structures of GluK2 kainate receptors with engineered cysteine crosslinks at the inter-dimer interface, which restrict subunit lateral rotation and attenuate receptor desensitization.The structure of GluK2 double cysteine mutant in complex with the allosteric potentiator BPAM344 and glutamate represents a non-desensitized state, highlighting the critical conformational changes required for ion channel gating.The glutamate-bound GluK2 mutant adopts multiple conformations, representing both shallow- and deep-desensitized states. Electrophysiological recordings indicate that the GluK2 kainate receptor mutant recovers from desensitization more rapidly, resembling AMPA receptors. Our structural and functional data suggest that shallow-desensitized KARs remain conductive, implying that the large lateral LBD rotation during KAR desensitization is essential for complete channel closure, distinguishing KARs from other iGluRs.

Regulating AMPA Receptors with Isoxazole-4-Carboxamide Derivatives: An Electrophysiological Study

Isoxazole carboxamide derivatives are intriguing modulators of ionotropic glutamate receptors; more specifically, their prospective analgesic activities based on non-opioid pathways have sparked widespread research. α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, especially Ca2+-permeable subtypes that are highly expressed in the spinal dorsal horn, play a critical role in nociceptive transmission and inflammatory pain. Herein, the neuromodulatory effects of these derivatives on AMPA receptor activity have been studied, focusing on their potential as modulators of AMPA receptors, a target implicated in pain and neurological disorders. The whole-cell patch clamp technique for electrophysiological recordings was used to investigate the effect of twelve isoxazole-4-carboxamide derivatives (CIC-1-12) on AMPA receptors' whole-cell currents and kinetics, including deactivation and desensitization. The isoxazole-4-carboxamide derivatives tested as inhibitors of AMPA receptor activity were very potent, with an 8-fold inhibition by CIC-1 and a 7.8-fold reduction by CIC-2. Additionally, these compounds profoundly altered the biophysical gating properties of both homomeric and heteromeric receptor subunits. These findings emphasize the therapeutic promise of isoxazole-4-carboxamide derivatives due to their potential as AMPA receptor modulators. Their ability to affect receptor activity and gating properties makes them promising candidates for future treatments for controlling pain.

Structural dynamics in α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor gating

The ionotropic glutamate receptors (iGluRs) are comprised of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), N-methyl-d-aspartate receptor, kainate, and delta subtypes and are pivotal in neuronal plasticity. Recent structural studies on AMPA receptors reveal intricate conformational changes during activation and desensitization elucidating the steps from agonist binding to channel opening and desensitization. Additionally, interactions with auxiliary subunits, including transmembrane AMPA-receptor regulatory proteins, germ-cell-specific gene 1-like protein, and cornichon homologs, intricately modulate AMPA receptors. We discuss the recent high-resolution structures of these complexes that unveil stoichiometry, subunit positioning, and differences in specific side-chain interactions that influence these functional modulations.

Progress on functions of intracellular domain trimeric ligand-gated ion channels

Ligand-gated ion channels are a large category of essential ion channels, modulating their state by binding to specific ligands to allow ions to pass through the cell membrane. Purinergic ligand-gated ion channel receptors (P2XRs) and acid-sensitive ion channels (ASICs) are representative members of trimeric ligand-gated ion channel. Recent studies have shown that structural differences in the intracellular domain of P2XRs may determine the desensitization process. The lateral fenestrations of P2XRs potentially serve as a pathway for ion conductance and play a decisive role in ion selectivity. Phosphorylation of numerous amino acid residues in the P2XRs are involved in regulating the activity of ion channels. Additionally, the P2XRs interact with other ligand-gated ion channels including N-methyl-D-aspartate receptors, γ-aminobutyric acid receptors, 5-hydroxytryptamin receptors and nicotinic acetylcholine receptors, mediating physiological processes such as synaptic plasticity. Conformational changes in the intracellular domain of the ASICs expose binding sites of intracellular signal partners, facilitating metabolic signal transduction. Amino acids such as Val16, Ser17, Ile18, Gln19 and Ala20 in the ASICs participate in channel opening and membrane expression. ASICs can also bind to intracellular proteins, such as CIPP and p11, to regulate channel function. Many phosphorylation sites at the C-terminus and N-terminus of ASICs are involved in the regulation of receptors. Furthermore, ASICs are involved in various physiological and pathophysiological processes, which include pain, ischemic stroke, psychiatric disorders, and neurodegenerative disease. In this article, we review the roles of the intracellular domains of these trimeric ligand-gated ion channels in channel gating as well as their physiological and pathological functions, in order to provide new insights into the discovery of related drugs.