The twisted link between a dual function glutamate transporter and episodic ataxia

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS) [1].Excitatory Amino Acid Transporters (EAATs) regulate extracellular glutamate by transporting it into cells, mostly glia, to terminate neurotransmission and to avoid neurotoxicity.EAATs also conduct chloride ions via a channel-like process that is thermodynamically uncoupled from transport.However, the molecular mechanisms that allow these dual-function transporters to carry out two seemingly contradictory roles, and the physiological role of Cl-conductance through EAATs, are unknown.I will describe the cryo-electron microscopy structure of a glutamate transporter homologue in an open-channel state, revealing an aqueous cavity that is formed during the transport cycle [2].Using functional studies and molecular dynamics simulations, we show that this cavity is an aqueous-accessible chloride permeation pathway gated by two hydrophobic regions and is conserved across mammalian and archaeal glutamate transporters [3].Mutations of human EAAT1 (hEAAT1) have been identified in patients with episodic ataxia type 6 (EA6).One mutation showed increased Cl-channel activity and decreased glutamate transport, but the relative contributions of each function of hEAAT1 to mechanisms underlying the pathology of EA6 remain unclear.I will describe the characterisation of five additional EA6-related mutations on hEAAT1 function in Xenopus laevis oocytes, and on CNS function in a Drosophila melanogaster model of locomotor behaviour.Our results indicate that mutations with decreased hEAAT1 Cl-channel activity and functional glutamate transport can also contribute to the pathology of EA6, highlighting the importance of Cl-homeostasis in glial cells for proper CNS function [4].Together, these results strongly support the idea that EA6 is primarily an ion channelopathy of CNS glia and provides a framework for the rational development of therapeutics that can differentially modulate substrate transport or channel properties for the treatment of neurological disorders caused by EAAT dysfunction, such as Episodic Ataxia.Figure 1.Chloride ions (blue balls) permeating the archaeal glutamate transporter, GltPh.