The accessibility in the external part of the TM5 of the glutamate transporter EAAT1 is conformationally sensitive during the transport cycle

Critical amino acids in the TM2 of EAAT2 are essential for membrane-bound localization, substrate binding, transporter function and anion currents

Excitatory amino acid transporter 2 (EAAT2), the gene of which is known as solute carrier family 1 member 2 (SLC1A2), is an important membrane-bound transporter that mediates approximately 90% of the transport and clearance of l-glutamate at synapses in the central nervous system (CNS). Transmembrane domain 2 (TM2) of EAAT2 is close to hairpin loop 2 (HP2) and far away from HP1 in the inward-facing conformation. In the present study, 14 crucial amino acid residues of TM2 were identified via alanine-scanning mutations. Further analysis in EAAT2-transfected HeLa cells in vitro showed that alanine substitutions of these residues resulted in a decrease in the efficiency of trafficking/targeting to the plasma membrane and/or reduced functionality of membrane-bound, which resulted in impaired transporter activity. After additional mutations, the transporter activities of some alanine-substitution mutants recovered. Specifically, the P95A mutant decreased EAAT2-associated anion currents. The Michaelis constant (Km ) values of the mutant proteins L85A, L92A and L101A were increased significantly, whereas R87 and P95A were decreased significantly, indicating that the mutations L85A, L92A and L101A reduced the affinity of the transporter and the substrate, whereas R87A and P95A enhanced this affinity. The maximum velocity (Vmax) values of all 14 alanine mutant proteins were decreased significantly, indicating that all these mutations reduced the substrate transport rate. These results suggest that critical residues in TM2 affect not only the protein expression and membrane-bound localization of EAAT2, but also its interactions with substrates. Additionally, our findings elucidate that the P95A mutant decreased EAAT2-related anion currents. Our results indicate that the TM2 of EAAT2 plays a vital role in the transport process. The key residues in TM2 affect protein expression in the membrane, substrate transport and the anion currents of EAAT2.

Molecular Mechanisms of Glutamate Toxicity in Parkinson's Disease

Parkinson’s disease (PD) is a common neurodegenerative disease, the pathological features of which include the presence of Lewy bodies and the neurodegeneration of dopaminergic neurons in the substantia nigra pars compacta. However, until recently, research on the pathogenesis and treatment of PD have progressed slowly. Glutamate and dopamine are both important central neurotransmitters in mammals. A lack of enzymatic decomposition of extracellular glutamate results in glutamate accumulating at synapses, which is mainly absorbed by excitatory amino acid transporters (EAATs). Glutamate exerts its physiological effects by binding to and activating ligand-gated ion channels [ionotropic glutamate receptors (iGluRs)] and a class of G-protein-coupled receptors [metabotropic glutamate receptors (mGluRs)]. Timely clearance of glutamate from the synaptic cleft is necessary because high levels of extracellular glutamate overactivate glutamate receptors, resulting in excitotoxic effects in the central nervous system. Additionally, increased concentrations of extracellular glutamate inhibit cystine uptake, leading to glutathione depletion and oxidative glutamate toxicity. Studies have shown that oxidative glutamate toxicity in neurons lacking functional N-methyl-D-aspartate (NMDA) receptors may represent a component of the cellular death pathway induced by excitotoxicity. The association between inflammation and excitotoxicity (i.e., immunoexcitotoxicity) has received increased attention in recent years. Glial activation induces neuroinflammation and can stimulate excessive release of glutamate, which can induce excitotoxicity and, additionally, further exacerbate neuroinflammation. Glutamate, as an important central neurotransmitter, is closely related to the occurrence and development of PD. In this review, we discuss recent progress on elucidating glutamate as a relevant neurotransmitter in PD. Additionally, we summarize the relationship and commonality among glutamate excitotoxicity, oxidative toxicity, and immunoexcitotoxicity in order to posit a holistic view and molecular mechanism of glutamate toxicity in PD.

The 4b-4c loop of excitatory amino acid transporter 1 containing four critical residues essential for substrate transport

In the mammalians, the 4b-4c loop of excitatory amino acid transporters (EAATs) spans more than 50 amino-acid residues that are absent in glutamate transporter homologue of Pyrococcus horikoshii (Glt_Ph). This part of insertion is unique for metazoans and indispensable to the localization of EAATs. The excitatory amino acid transporter (EAAT) 1 is one of the two glial glutamate transporters, which are responsible for efficiently clearing glutamate from the synaptic cleft to prevent neurotoxicity and cell death. Although the crystal structure of EAAT1_cryst (a human thermostable EAAT1) was resolved in 2017, the structure-function relationship of the 4b-4c loop has not been elucidated in EAAT1_cryst. To investigate the role of the 4b-4c loop, we performed alanine-scanning mutagenesis in the mutants and observed dramatically decreased transport activities in T192A, Y194A, N242A, and G245A mutants. The surface expression of T192A and Y194A mutants even decreased by more than 80%, and most of them were detained in the cytoplasm. However, when T192 and Y194 were substituted with conservative residues, the transport activities and the surface expressions of T192S and Y194F were largely recovered, and their kinetic parameters (K_m values) were comparable to the wild-type EAAT1 as well. In contrast, N242 and G245 substituted with conservative residues could not rescue the uptake function, suggesting that N242 and G245 may play irreplaceable roles in the glutamate uptake process. These results indicate that the 4b-4c loop of EAAT1 may not only affect the glutamate uptake activity, but also influence the surface localization of EAAT1 by T192 and Y194.Communicated by Ramaswamy H. Sarma.

Structural intermediates in a model of the substrate translocation path of the bacterial glutamate transporter homologue GltPh

Excitatory amino acid transporters (EAATs) are membrane proteins responsible for reuptake of glutamate from the synaptic cleft to terminate neurotransmission and help prevent neurotoxically high, extracellular glutamate concentrations. Important structural information about these proteins emerged from crystal structures of GltPh, a bacterial homologue of EAATs, in conformations facing outward and inward. These remarkably different conformations are considered to be end points of the substrate translocation path (STP), suggesting that the transport mechanism involves major conformational rearrangements that remain uncharted. To investigate possible steps in the structural transitions of the STP between the two end-point conformations, we applied a combination of computational modeling methods (motion planning, molecular dynamics simulations, and mixed elastic network models). We found that the conformational changes in the transition involve mainly the repositioning the "transport domain" and the "trimerization domain" identified previously in the crystal structures. The two domains move in opposite directions along the membrane normal, and the transport domain also tilts by ∼17° with respect to this axis. Moreover, the TM3-4 loop undergoes a flexible, "restraining bar"-like conformational change with respect to the transport domain. As a consequence of these conformational rearrangements along the transition path we calculated a significant decrease of nearly 20% in the area of the transport-to-trimerization domain interface (TTDI). Water penetrates parts of the TTDI in the modeled intermediates but very much less in the end-point conformations. We show that these characteristics of the modeled intermediate states agree with experimental results from residue-accessibility studies in individual monomers and identify specific residues that can be used to test the proposed STP. Moreover, MD simulations of complete GltPh trimers constructed from initially identical monomer intermediates suggest that asymmetry can appear in the trimer, consonant with available experimental data showing independent transport kinetics by individual monomers in the trimers.