Structure and Function of native kainate-type ionotropic glutamate receptor complexes

点击翻译按钮获取中文摘要

PROJECT SUMMARY/ABSTRACT Neuronal ionotropic glutamate receptors (iGluRs) play key roles in mediating excitatory synaptic transmission in the brain and in a wide range of brain diseases, including Alzheimer’s and Huntington’s disease, schizophrenia, epilepsy, autism spectrum, major depression, and mood disorders. Glutamatergic signaling is pivotal for synaptic plasticity, learning, and memory formation. Kainate-type ionotropic glutamate receptors (KARs) are distributed throughout the brain and regulate the release of neurotransmitters and mediate excitatory synaptic transmission. KARs form homo- or hetero-tetramers composed of five homologous subunits of GluK1–5. Each subunit exhibits unique structural, functional, and pharmacological properties and subcellular localization. Moreover, misguided localization and dysfunction of KARs result in neuropathologies, therefore, KARs are a promising drug target. However, KARs are the least well understood group of iGluRs, and their molecular mechanisms remain elusive. The activities of neuronal KARs are regulated by pH, posttranslational modifications, lipid/cholesterol, and small molecules. Additionally, KAR function and localization are further modified by their auxiliary and accessory proteins. Thus, such brain lipids, modifications, and protein co-factors increase the diversity of KAR functional properties. The neuropilin and tolloid-like (NETO) auxiliary proteins, NETO1 and NETO2, are auxiliary proteins of KARs that are distantly homologous compared with auxiliary proteins associated with other iGluRs. How do such ligands and protein co-factors determine the gating of KARs and regulate synaptic signaling? How are the physiological brain lipid environment and posttranslational modifications contributing to receptor activities? To answer these questions, this proposed research will employ structural and electrophysiological approaches to develop our mechanistic understanding of the regulation of native postsynaptic GluK2/GluK5 KARs isolated from rat brains. The program will move forward in two major directions: In one project, I will determine high-resolution cryo-electron microscopy (cryo-EM) structures of native GluK2/GluK5 KARs in an activated state, but also in complex with ligands to capture multiple functional states. This will elucidate the conformational alternations of GluK2/GluK5 KARs by their ligands, which have not been well-observed in previously determined structures. Comparing our structures with other iGluRs will uncover how physiologically relevant heteromeric KARs are structurally and functionally distinct from other iGluR subfamilies. In a second concurrent project, we will elucidate the regulatory mechanisms of native KARs by NETO1 and NETO2 auxiliary proteins. Overall, our studies will provide fundamental insights into how neuronal KAR complexes are controlled by their ligands and auxiliary proteins, and how they mediate synaptic signaling, and thus neural activities. This information will facilitate the development of new therapeutic strategies for treating brain diseases.