Protons Regulate Vesicular Glutamate Transporters through an Allosteric Mechanism

Codon Usage Bias: A Potential Factor Affecting VGLUT Developmental Expression and Protein Evolution

More and more attention has been paid to the role of synonymous substitution in evolution, in which codon usage preference can affect gene expression distribution and protein structure and function. Vesicular glutamate transporter (VGLUT) consists of three isoforms, among which VGLUT3 is significantly different from other VGLUTs in functional importance, expression level, and distribution range, whose reason is still unclear. This study sought to analyze the role of codon preference in VGLUT differentiation. To conduct an evolutionary analysis of the three VGLUTs, this paper uses bioinformatics research methods to analyze the coding sequences of the three VGLUTs in different species and compare the codon usage patterns. Furthermore, the differences among the three VGLUTs were analyzed by combining functional importance, expression level, distribution range, gene structure, protein relationship network, expression at specific developmental stages, and phylogenetic tree, and the influence of codon usage pattern was explored. The results showed that the VGLUT with greater codon preference had less functional importance, lower expression levels, more peripheral distribution away from the CNS, smaller exon density of gene, less conserved and farther away from the CDS region miRNA regulatory sites, simpler and less tight protein interaction networks, delayed developmental expression, and more distant evolutionary relationships. Codon usage preference is a potential factor affecting VGLUT developmental expression and protein evolution.

Proton-Mediated PIEZO2 Channelopathy: Linking Oxaliplatin Treatment to Impaired Proprioception and Cognitive Deficits

Oxaliplatin induces acute neuropathy within a few hours post-treatment, with symptoms persisting for several days. Delayed onset muscle soreness also causes the delayed onset of mechanical pain sensation starting at about 6-8 h and lasting up to a week after exercise. Both conditions come with impaired proprioception and could be chronic if these bouts are repeated frequently. The involvement of PIEZO2 ion channels, as the principal mechanosensory channels responsible for proprioception, is theorized in both conditions as well. The current opinion manuscript is meant to explain how the minor stretch-related microdamage of PIEZO2 on Type Ia proprioceptive terminals could explain the aforementioned symptoms of impaired proprioception. This includes a platinum-induced proton affinity 'switch' on these proprioceptive endings with PIEZO2 content, resulting in this being the likely initiating cause. Furthermore, it postulates how the proton-based ultrafast long-range oscillatory synchronization to the hippocampus could be impaired due to this microdamage on Type Ia proprioceptive terminals. Finally, the manuscript provides insight into how the impairment of the PIEZO2-initiated ultrafast muscle-brain axis may contribute to chemobrain and its associated cognitive and memory deficits.

Progressive Irreversible Proprioceptive Piezo2 Channelopathy-Induced Lost Forced Peripheral Oscillatory Synchronization to the Hippocampal Oscillator May Explain the Onset of Amyotrophic Lateral Sclerosis Pathomechanism

Amyotrophic lateral sclerosis (ALS) is a mysterious lethal multisystem neurodegenerative disease that gradually leads to the progressive loss of motor neurons. A recent non-contact dying-back injury mechanism theory for ALS proposed that the primary damage is an acquired irreversible intrafusal proprioceptive terminal Piezo2 channelopathy with underlying genetic and environmental risk factors. Underpinning this is the theory that excessively prolonged proprioceptive mechanotransduction under allostasis may induce dysfunctionality in mitochondria, leading to Piezo2 channelopathy. This microinjury is suggested to provide one gateway from physiology to pathophysiology. The chronic, but not irreversible, form of this Piezo2 channelopathy is implicated in many diseases with unknown etiology. Dry eye disease is one of them where replenishing synthetic proteoglycans promote nerve regeneration. Syndecans, especially syndecan-3, are proposed as the first critical link in this hierarchical ordered depletory pathomechanism as proton-collecting/distributing antennas; hence, they may play a role in ALS pathomechanism onset. Even more importantly, the shedding or charge-altering variants of Syndecan-3 may contribute to the Piezo2 channelopathy-induced disruption of the Piezo2-initiated proton-based ultrafast long-range signaling through VGLUT1 and VGLUT2. Thus, these alterations may not only cause disruption to ultrafast signaling to the hippocampus in conscious proprioception, but could disrupt the ultrafast proprioceptive signaling feedback to the motoneurons. Correspondingly, an inert Piezo2-initiated proton-based ultrafast signaled proprioceptive skeletal system is coming to light that is suggested to be progressively lost in ALS. In addition, the lost functional link of the MyoD family of inhibitor proteins, as auxiliary subunits of Piezo2, may not only contribute to the theorized acquired Piezo2 channelopathy, but may explain how these microinjured ion channels evolve to be principal transcription activators.

Vesicular glutamate transporters are H+-anion exchangers that operate at variable stoichiometry

Vesicular glutamate transporters accumulate glutamate in synaptic vesicles, where they also function as a major Cl^- efflux pathway. Here we combine heterologous expression and cellular electrophysiology with mathematical modeling to understand the mechanisms underlying this dual function of rat VGLUT1. When glutamate is the main cytoplasmic anion, VGLUT1 functions as H⁺-glutamate exchanger, with a transport rate of around 600 s^-1 at -160 mV. Transport of other large anions, including aspartate, is not stoichiometrically coupled to H⁺ transport, and Cl^- permeates VGLUT1 through an aqueous anion channel with unitary transport rates of 1.5 × 10⁵s^-1 at -160 mV. Mathematical modeling reveals that H⁺ coupling is sufficient for selective glutamate accumulation in model vesicles and that VGLUT Cl^- channel function increases the transport efficiency by accelerating glutamate accumulation and reducing ATP-driven H⁺ transport. In summary, we provide evidence that VGLUT1 functions as H⁺-glutamate exchanger that is partially or fully uncoupled by other anions.

Substrate recognition and proton coupling by a bacterial member of solute carrier family 17

The solute carrier 17 family transports diverse organic anions using two distinct modes of coupling to a source of energy. Transporters that package glutamate and nucleotide into secretory vesicles for regulated release by exocytosis are driven by membrane potential but subject to allosteric regulation by H+ and Cl-. Other solute carrier 17 members including the lysosomal sialic acid exporter couple the flux of organic anion to cotransport of H+. To begin to understand how similar proteins can perform such different functions, we have studied Escherichia coli DgoT, a H+/galactonate cotransporter. A recent structure of DgoT showed many residues contacting D-galactonate, and we now find that they do not tolerate even conservative substitutions. In contrast, the closely related lysosomal H+/sialic acid cotransporter Sialin tolerates similar mutations, consistent with its recognition of diverse substrates with relatively low affinity. We also find that despite coupling to H+, DgoT transports more rapidly but with lower apparent affinity at high pH. Indeed, membrane potential can drive uptake, indicating electrogenic transport and suggesting a H+:galactonate stoichiometry >1. Located in a polar pocket of the N-terminal helical bundle, Asp46 and Glu133 are each required for net flux by DgoT, but the E133Q mutant exhibits robust exchange activity and rescues exchange by D46N, suggesting that these two residues operate in series to translocate protons. E133Q also shifts the pH sensitivity of exchange by DgoT, supporting a central role for the highly conserved TM4 glutamate in H+ coupling by DgoT.

Nanomolar clodronate induces adenosine accumulation in the perfused rat mesenteric bed and mesentery-derived endothelial cells

The vesicular nucleotide transporter (VNUT) is critical for sympathetic co-transmission and purinergic transmission maintenance. To examine this proposal, we assessed whether the bisphosphonate clodronate, claimed as a potent in vitro VNUT blocker, modified spontaneous and/or the electrically evoked overflow of ATP/metabolites and NA from mesentery sympathetic perivascular nerve terminals. Additionally, in primary endothelial cell cultures derived from this tissue, we also evaluated whether clodronate interfered with ATP/metabolite cell outflow and metabolism of N⁶-etheno adenosine 5'-triphosphate (eATP), N⁶-etheno adenosine (eADO), and adenosine deaminase enzyme activity. Rat mesenteries were perfused in the absence or presence of .01-1,000 nM clodronate, 1-1,000 nM Evans blue (EB), and 1-10 µM DIDS; tissue perfusates were collected to determine ATP/metabolites and NA before, during, and after perivascular electrical nerve terminal depolarization. An amount of 1-1,000 nM clodronate did not modify the time course of ATP or NA overflow elicited by nerve terminal depolarization, and only 10 nM clodronate significantly augmented perfusate adenosine. Electrical nerve terminal stimulation increased tissue perfusion pressure that was significantly reduced only by 10 nM clodronate [90.0 ± 18.6 (n = 8) to 35.0 ± 10.4 (n = 7), p = .0277]. As controls, EB, DIDS, or reserpine treatment reduced the overflow of ATP/metabolites and NA in a concentration-dependent manner elicited by nerve terminal depolarization. Moreover, mechanical stimulation of primary endothelial cell cultures from the rat mesentery added with 10 or 100 nM clodronate increased adenosine in the cell media. eATP was metabolized by endothelial cells to the same extent with and without 1-1,000 nM clodronate, suggesting the bisphosphonate did not interfere with nucleotide ectoenzyme metabolism. In contrast, extracellular eADO remained intact, indicating that this nucleoside is neither metabolized nor transported intracellularly. Furthermore, only 10 nM clodronate inhibited (15.5%) adenosine metabolism to inosine in endothelial cells as well as in a commercial crude adenosine deaminase enzyme preparation (12.7%), and both effects proved the significance (p < .05). Altogether, present data allow inferring that clodronate inhibits adenosine deaminase activity in isolated endothelial cells as in a crude extract preparation, a finding that may account for adenosine accumulation following clodronate mesentery perfusion.

Diversity of function and mechanism in a family of organic anion transporters

Originally identified as transporters for inorganic phosphate, solute carrier 17 (SLC17) family proteins subserve diverse physiological roles. The vesicular glutamate transporters (VGLUTs) package the principal excitatory neurotransmitter glutamate into synaptic vesicles (SVs). In contrast, the closely related sialic acid transporter sialin mediates the flux of sialic acid in the opposite direction, from lysosomes to the cytoplasm. The two proteins couple in different ways to the H⁺ electrochemical gradient driving force, and high-resolution structures of the Escherichia coli homolog d-galactonate transporter (DgoT) and more recently rat VGLUT2 now begin to suggest the mechanisms involved as well as the basis for substrate specificity.

Interactive role of acid sensing ion channels and glutamatergic system in opioid dependence

Dysregulation in glutamatergic receptors and transporters has been found to mediate drugs of abuse, including morphine. Among glutamate receptors, ionotropic glutamate receptors (iGluRs) are altered with exposure to drugs of abuse. Acid-sensing ion channels (ASICs) are ligand (H+)-gated channels, which are expressed at the excitatory synaptic clefts and play a role in drug dependence. Overexpression of a specific ASIC subtype, ASIC1a, attenuated reinstatement of cocaine. ASICs are revealed to be involved in cocaine and morphine seeking behaviors, and these effects are mediated through modulation of glutamatergic receptors. In this review, we discussed the interactive role of ASICs and glutamate receptors, mainly iGluRs, in opioid dependence. ASICs are also expressed in astrocytes and are suggested to be involved on regulating glutamate uptake. However, little is known about the coupling between ASICs and the astroglial glutamate transporters. In addition, this review discussed the role of nitric oxide in the modulation of ASIC function and potentially opioid dependence. We also discussed the role of ASICs in the modulation of the function of both glutamatergic receptors in post-synaptic neurons and glutamatergic transporters in astrocytes in animals exposed to drugs of abuse.

Physiological Perspectives on Molecular Mechanisms and Regulation of Vesicular Glutamate Transport: Lessons From Calyx of Held Synapses

Accumulation of glutamate, the primary excitatory neurotransmitter in the mammalian central nervous system, into presynaptic synaptic vesicles (SVs) depends upon three vesicular glutamate transporters (VGLUTs). Since VGLUTs are driven by a proton electrochemical gradient across the SV membrane generated by vacuolar-type H⁺-ATPases (V-ATPases), the rate of glutamate transport into SVs, as well as the amount of glutamate in SVs at equilibrium, are influenced by activities of both VGLUTs and V-ATPase. Despite emerging evidence that suggests various factors influencing glutamate transport by VGLUTs in vitro, little has been reported in physiological or pathological contexts to date. Historically, this was partially due to a lack of appropriate methods to monitor glutamate loading into SVs in living synapses. Furthermore, whether or not glutamate refilling of SVs can be rate-limiting for synaptic transmission is not well understood, primarily due to a lack of knowledge concerning the time required for vesicle reuse and refilling during repetitive stimulation. In this review, we first introduce a unique electrophysiological method to monitor glutamate refilling by VGLUTs in a giant model synapse from the calyx of Held in rodent brainstem slices, and we discuss the advantages and limitations of the method. We then introduce the current understanding of factors that potentially alter the amount and rate of glutamate refilling of SVs in this synapse, and discuss open questions from physiological viewpoints.

Cellular Physiology and Pathophysiology of EAAT Anion Channels

Excitatory amino acid transporters (EAATs) optimize the temporal resolution and energy demand of mammalian excitatory synapses by quickly removing glutamate from the synaptic cleft into surrounding neuronal and glial cells and ensuring low resting glutamate concentrations. In addition to secondary active glutamate transport, EAATs also function as anion channels. The channel function of these transporters is conserved in all homologs ranging from archaebacteria to mammals; however, its physiological roles are insufficiently understood. There are five human EAATs, which differ in their glutamate transport rates. Until recently the high-capacity transporters EAAT1, EAAT2, and EAAT3 were believed to conduct only negligible anion currents, with no obvious function in cell physiology. In contrast, the low-capacity glutamate transporters EAAT4 and EAAT5 are thought to regulate neuronal signaling as glutamate-gated channels. In recent years, new experimental approaches and novel animal models, together with the discovery of a human genetic disease caused by gain-of-function mutations in EAAT anion channels have enabled identification of the first physiological and pathophysiological roles of EAAT anion channels.

Whole Endosome Recording of Vesicular Neurotransmitter Transporter Currents

The analysis of organellar membrane transporters presents many technical problems. In general, their activity depends on a H⁺ electrochemical driving force (Δμ_H+). However, transport itself influences the expression of Δμ_H+ in standard radiotracer flux assays, making it difficult to disentangle the role of the chemical component ΔpH and the membrane potential Δψ. Whole endosome recording in voltage clamp circumvents many of these problems, controlling ionic conditions as well as membrane potential inside and outside the organelle . This approach has been used primarily to study the properties of endolysosomal channels, which generate substantial currents (Saito et al., J Biol Chem 282(37):27327-27333, 2007; Cang et al., Nat Chem Biol 10(6):463-469, 2014; Cang et al., Cell 152(4):778-790, 2013; Chen et al., Nat Protoc 12(8):1639-1658, 2017; Samie et al., Dev Cell 26(5):511-524, 2013; Wang et al., Cell 151(2):372-383, 2012). Electrogenic transport produces much smaller currents, but we have recently reported the detection of transport currents and an uncoupled Cl^- conductance associated with the vesicular glutamate transporters (VGLUTs) that fill synaptic vesicles with glutamate (Chang et al., eLife 7:e34896, 2018). In this protocol, we will focus on the measurement of transport currents on enlarged endosomes of heterologous mammalian cells.

Synaptic vesicle pools are a major hidden resting metabolic burden of nerve terminals

The brain is a metabolically fragile organ as compromises in fuel availability rapidly degrade cognitive function. Nerve terminals are likely loci of this vulnerability as they do not store sufficient ATP molecules, needing to synthesize them during activity or suffer acute degradation in performance. The ability of on-demand ATP synthesis to satisfy activity-driven ATP hydrolysis will depend additionally on the magnitude of local resting metabolic processes. We show here that synaptic vesicle (SV) pools are a major source of presynaptic basal energy consumption. This basal metabolic processes arises from SV-resident V-ATPases compensating for a hidden resting H+ efflux from the SV lumen. We show that this steady-state H+ efflux (i) is mediated by vesicular neurotransmitter transporters, (ii) is independent of the SV cycle, (iii) accounts for up to 44% of the resting synaptic energy consumption, and (iv) contributes substantially to nerve terminal intolerance of fuel deprivation.

Allosteric Inhibition of a Vesicular Glutamate Transporter by an Isoform-Specific Antibody

The role of glutamate in excitatory neurotransmission depends on its transport into synaptic vesicles by the vesicular glutamate transporters (VGLUTs). The three VGLUT isoforms exhibit a complementary distribution in the nervous system, and the knockout of each produces severe, pleiotropic neurological effects. However, the available pharmacology lacks sensitivity and specificity, limiting the analysis of both transport mechanism and physiological role. To develop new molecular probes for the VGLUTs, we raised six mouse monoclonal antibodies to VGLUT2. All six bind to a structured region of VGLUT2, five to the luminal face, and one to the cytosolic. Two are specific to VGLUT2, whereas the other four bind to both VGLUT1 and 2; none detect VGLUT3. Antibody 8E11 recognizes an epitope spanning the three extracellular loops in the C-domain that explains the recognition of both VGLUT1 and 2 but not VGLUT3. 8E11 also inhibits both glutamate transport and the VGLUT-associated chloride conductance. Since the antibody binds outside the substrate recognition site, it acts allosterically to inhibit function, presumably by restricting conformational changes. The isoform specificity also shows that allosteric inhibition provides a mechanism to distinguish between closely related transporters.

Vesicular Glutamate Transporter Expression Ensures High-Fidelity Synaptic Transmission at the Calyx of Held Synapses

Recycling of synaptic vesicles (SVs) at presynaptic terminals is required for sustained neurotransmitter release. Although SV endocytosis is a rate-limiting step for synaptic transmission, it is unclear whether the rate of the subsequent SV refilling with neurotransmitter also influences synaptic transmission. By analyzing vesicular glutamate transporter 1 (VGLUT1)-deficient calyx of Held synapses, in which both VGLUT1 and VGLUT2 are co-expressed in wild-type situation, we found that VGLUT1 loss causes a drastic reduction in SV refilling rate down to ∼25% of wild-type values, with only subtle changes in basic synaptic parameters. Strikingly, VGLUT1-deficient synapses exhibited abnormal synaptic failures within a few seconds during high-frequency repetitive firing, which was recapitulated by manipulating presynaptic Cl^- concentrations to retard SV refilling. Our data show that the speed of SV refilling can be rate limiting for synaptic transmission under certain conditions that entail reduced VGLUT levels during development as well as various neuropathological processes.

Structures and General Transport Mechanisms by the Major Facilitator Superfamily (MFS)

The major facilitator superfamily (MFS) is the largest known superfamily of secondary active transporters. MFS transporters are responsible for transporting a broad spectrum of substrates, either down their concentration gradient or uphill using the energy stored in the electrochemical gradients. Over the last 10 years, more than a hundred different MFS transporter structures covering close to 40 members have provided an atomic framework for piecing together the molecular basis of their transport cycles. Here, we summarize the remarkable promiscuity of MFS members in terms of substrate recognition and proton coupling as well as the intricate gating mechanisms undergone in achieving substrate translocation. We outline studies that show how residues far from the substrate binding site can be just as important for fine-tuning substrate recognition and specificity as those residues directly coordinating the substrate, and how a number of MFS transporters have evolved to form unique complexes with chaperone and signaling functions. Through a deeper mechanistic description of glucose (GLUT) transporters and multidrug resistance (MDR) antiporters, we outline novel refinements to the rocker-switch alternating-access model, such as a latch mechanism for proton-coupled monosaccharide transport. We emphasize that a full understanding of transport requires an elucidation of MFS transporter dynamics, energy landscapes, and the determination of how rate transitions are modulated by lipids.

Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters

The regulation of luminal ion concentrations is critical for the function of, and transport between intracellular organelles. The importance of the acidic pH in the compartments of the endosomal-lysosomal pathway has been well-known for decades. Besides the V-ATPase, which pumps protons into their lumen, a variety of ion transporters and channels is involved in the regulation of the organelles' complex ion homeostasis. Amongst these are the intracellular members of the CLC family, ClC-3 through ClC-7. They localize to distinct but overlapping compartments of the endosomal-lysosomal pathway, partially with tissue-specific expression. Functioning as 2Cl⁻/H⁺ exchangers, they can support the vesicular acidification and accumulate luminal Cl⁻. Mutations in the encoding genes in patients and mouse models underlie severe phenotypes including kidney stones with CLCN5 and osteopetrosis or hypopigmentation with CLCN7. Dysfunction of those intracellular CLCs that are expressed in neurons lead to neuronal defects. Loss of endosomal ClC-3, which heteromerizes with ClC-4, results in neurodegeneration. Mutations in ClC-4 are associated with epileptic encephalopathy and intellectual disability. Mice lacking the late endosomal ClC-6 develop a lysosomal storage disease with reduced pain sensitivity. Human gene variants have been associated with epilepsy, and a gain-of-function mutation causes early-onset neurodegeneration. Dysfunction of the lysosomal ClC-7 leads to a lysosomal storage disease and neurodegeneration in mice and humans. Reduced luminal chloride, as well as altered calcium regulation, has been associated with lysosomal storage diseases in general. This review discusses the properties of endosomal and lysosomal Cl⁻/H⁺ exchange by CLCs and how various alterations of ion transport by CLCs impact organellar ion homeostasis and function in neurodegenerative disorders.

Vesicular Glutamate Transporters (SLCA17 A6, 7, 8) Control Synaptic Phosphate Levels

Vesicular glutamate transporters (VGLUTs) fill synaptic vesicles with glutamate. VGLUTs were originally identified as sodium-dependent transporters of inorganic phosphate (Pi), but the physiological relevance of this activity remains unclear. Heterologous expression of all three VGLUTs greatly augments intracellular Pi levels. Using neuronal models, we show that translocation of VGLUTs to the plasma membrane during exocytosis results in highly increased Pi uptake. VGLUT-mediated Pi influx is counteracted by Pi efflux. Synaptosomes prepared from perinatal VGLUT2^-/- mice that are virtually free of VGLUTs show drastically reduced cytosolic Pi levels and fail to import Pi. Glutamate partially competes with sodium (Na⁺)/Pi (NaPi)-uptake mediated by VGLUTs but does not appear to be transported. A nanobody that blocks glutamate transport by binding to the cytoplasmic domain of VGLUT1 abolishes Pi transport when co-expressed with VGLUT1. We conclude that VGLUTs have a dual function that is essential for both vesicular glutamate loading and Pi restoration in neurons.

Ion transport and regulation in a synaptic vesicle glutamate transporter

Synaptic vesicles accumulate neurotransmitters, enabling the quantal release by exocytosis that underlies synaptic transmission. Specific neurotransmitter transporters are responsible for this activity and therefore are essential for brain function. The vesicular glutamate transporters (VGLUTs) concentrate the principal excitatory neurotransmitter glutamate into synaptic vesicles, driven by membrane potential. However, the mechanism by which they do so remains poorly understood owing to a lack of structural information. We report the cryo-electron microscopy structure of rat VGLUT2 at 3.8-angstrom resolution and propose structure-based mechanisms for substrate recognition and allosteric activation by low pH and chloride. A potential permeation pathway for chloride intersects with the glutamate binding site. These results demonstrate how the activity of VGLUTs can be coordinated with large shifts in proton and chloride concentrations during the synaptic vesicle cycle to ensure normal synaptic transmission.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

The mechanism and regulation of vesicular glutamate transport: Coordination with the synaptic vesicle cycle

The transport of classical neurotransmitters into synaptic vesicles generally relies on a H⁺ electrochemical gradient (∆μ_H+). Synaptic vesicle uptake of glutamate depends primarily on the electrical component ∆ψ as the driving force, rather than the chemical component ∆pH. However, the vesicular glutamate transporters (VGLUTs) belong to the solute carrier 17 (SLC17) family, which includes closely related members that function as H⁺ cotransporters. Recent work has also shown that the VGLUTs undergo allosteric regulation by H⁺ and Cl^-, and exhibit an associated Cl^- conductance. These properties appear to coordinate VGLUT activity with the large ionic shifts that accompany the rapid recycling of synaptic vesicles driven by neural activity. Recent structural information also suggests common mechanisms that underlie the apparently divergent function of SLC17 family members, and that confer allosteric regulation.

Uncoupling endosomal CLC chloride/proton exchange causes severe neurodegeneration

CLC chloride/proton exchangers may support acidification of endolysosomes and raise their luminal Cl⁻ concentration. Disruption of endosomal ClC‐3 causes severe neurodegeneration. To assess the importance of ClC‐3 Cl⁻/H⁺ exchange, we now generate Clcn3^unc/unc mice in which ClC‐3 is converted into a Cl⁻ channel. Unlike Clcn3^−/− mice, Clcn3^unc/unc mice appear normal owing to compensation by ClC‐4 with which ClC‐3 forms heteromers. ClC‐4 protein levels are strongly reduced in Clcn3^−/−, but not in Clcn3^unc/unc mice because ClC‐3^unc binds and stabilizes ClC‐4 like wild‐type ClC‐3. Although mice lacking ClC‐4 appear healthy, its absence in Clcn3^unc/unc/Clcn4^−/− mice entails even stronger neurodegeneration than observed in Clcn3^−/− mice. A fraction of ClC‐3 is found on synaptic vesicles, but miniature postsynaptic currents and synaptic vesicle acidification are not affected in Clcn3^unc/unc or Clcn3^−/− mice before neurodegeneration sets in. Both, Cl⁻/H⁺‐exchange activity and the stabilizing effect on ClC‐4, are central to the biological function of ClC‐3.

VGLUT substrates and inhibitors: A computational viewpoint

The vesicular glutamate transporters (VGLUTs) bind and move glutamate (Glu) from the cytosol into the lumen of synaptic vesicles using a H⁺-electrochemical gradient (ΔpH and Δψ) generated by the vesicular H⁺-ATPase. VGLUTs show very low Glu binding and to date, no three-dimensional structure has been elucidated. Prior studies have attempted to identify the key residues involved in binding VGLUT substrates and inhibitors using homology models and docking experiments. Recently, the inward and outward oriented crystal structures of d-galactonate transporter (DgoT) emerged as possible structure templates for VGLUT. In this review, a new homology model for VGLUT2 based on DgoT has been developed and used to conduct docking experiments to identify and differentiate residues and binding orientations involved in ligand interactions. This review describes small molecule-ligand interactions including docking using a VGLUT2 homology model derived from DgoT.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

A proline-rich motif on VGLUT1 reduces synaptic vesicle super-pool and spontaneous release frequency

Glutamate secretion at excitatory synapses is tightly regulated to allow for the precise tuning of synaptic strength. Vesicular Glutamate Transporters (VGLUT) accumulate glutamate into synaptic vesicles (SV) and thereby regulate quantal size. Further, the number of release sites and the release probability of SVs maybe regulated by the organization of active-zone proteins and SV clusters. In the present work, we uncover a mechanism mediating an increased SV clustering through the interaction of VGLUT1 second proline-rich domain, endophilinA1 and intersectin1. This strengthening of SV clusters results in a combined reduction of axonal SV super-pool size and miniature excitatory events frequency. Our findings support a model in which clustered vesicles are held together through multiple weak interactions between Src homology three and proline-rich domains of synaptic proteins. In mammals, VGLUT1 gained a proline-rich sequence that recruits endophilinA1 and turns the transporter into a regulator of SV organization and spontaneous release.

Structures suggest a mechanism for energy coupling by a family of organic anion transporters

Members of the solute carrier 17 (SLC17) family use divergent mechanisms to concentrate organic anions. Membrane potential drives uptake of the principal excitatory neurotransmitter glutamate into synaptic vesicles, whereas closely related proteins use proton cotransport to drive efflux from the lysosome. To delineate the divergent features of ionic coupling by the SLC17 family, we determined the structure of Escherichia coli D-galactonate/H⁺ symporter D-galactonate transporter (DgoT) in 2 states: one open to the cytoplasmic side and the other open to the periplasmic side with substrate bound. The structures suggest a mechanism that couples H⁺ flux to substrate recognition. A transition in the role of H⁺ from flux coupling to allostery may confer regulation by trafficking to and from the plasma membrane.

The first structures of a family of organic anion transporters reveal an interaction with protons that is conserved from bacterial transporters to the mammalian proteins that transport glutamate into synaptic vesicles.

pH-Lemon, a Fluorescent Protein-Based pH Reporter for Acidic Compartments

Distinct subcellular pH levels, especially in lysosomes and endosomes, are essential for the degradation, modification, sorting, accumulation, and secretion of macromolecules. Here, we engineered a novel genetically encoded pH probe by fusing the pH-stable cyan fluorescent protein (FP) variant, mTurquoise2, to the highly pH-sensitive enhanced yellow fluorescent protein, EYFP. This approach yielded a ratiometric biosensor-referred to as pH-Lemon-optimized for live imaging of distinct pH conditions within acidic cellular compartments. Protonation of pH-Lemon under acidic conditions significantly decreases the yellow fluorescence while the cyan fluorescence increases due to reduced Förster resonance energy transfer (FRET) efficiency. Because of its freely reversible and ratiometric responses, pH-Lemon represents a fluorescent biosensor for pH dynamics. pH-Lemon also shows a sizable pH-dependent fluorescence lifetime change that can be used in fluorescence lifetime imaging microscopy as an alternative observation method for the study of pH in acidic cellular compartments. Fusion of pH-Lemon to the protein microtubule-associated protein 1A/1B-light chain 3B (LC3B), a specific marker of autophagic membranes, resulted in its targeting within autolysosomes of HeLa cells. Moreover, fusion of pH-Lemon to a glycophosphatidylinositol (GPI) anchor allowed us to monitor the entire luminal space of the secretory pathway and the exoplasmic leaflet of the plasma membrane. Utilizing this new pH probe, we revealed neutral and acidic vesicles and substructures inside cells, highlighting compartments of distinct pH throughout the endomembrane system. These data demonstrate, that this novel pH sensor, pH-Lemon, is very suitable for the study of local pH dynamics of subcellular microstructures in living cells.

Expression of plasma membrane calcium ATPases confers Ca2+/H+ exchange in rodent synaptic vesicles

Ca²⁺ transport into synaptic vesicles (SVs) at the presynaptic terminals has been proposed to be an important process for regulating presynaptic [Ca²⁺] during stimulation as well as at rest. However, the molecular identity of the transport system remains elusive. Previous studies have demonstrated that isolated SVs exhibit two distinct Ca²⁺ transport systems depending on extra-vesicular (cytosolic) pH; one is mediated by a high affinity Ca²⁺ transporter which is active at neutral pH and the other is mediated by a low affinity Ca²⁺/H⁺ antiporter which is maximally active at alkaline pH of 8.5. In addition, synaptic vesicle glycoprotein 2 s (SV2s), a major SV component, have been proposed to contribute to Ca²⁺ clearance from the presynaptic cytoplasm. Here, we show that at physiological pH, the plasma membrane Ca²⁺ ATPases (PMCAs) are responsible for both the Ca²⁺/H⁺ exchange activity and Ca²⁺ uptake into SVs. The Ca²⁺/H⁺ exchange activity monitored by acidification assay exhibited high affinity for Ca²⁺ (K_m ~ 400 nM) and characteristic divalent cation selectivity for the PMCAs. Both activities were remarkably reduced by PMCA blockers, but not by a blocker of the ATPase that transfers Ca²⁺ from the cytosol to the lumen of sarcoplasmic endoplasmic reticulum (SERCA) at physiological pH. Furthermore, we rule out the contribution of SV2s, putative Ca²⁺ transporters on SVs, since both Ca²⁺/H⁺ exchange activity and Ca²⁺ transport were unaffected in isolated vesicles derived from SV2-deficient brains. Finally, using a PMCA1-pHluorin construct that enabled us to monitor cellular distribution and recycling properties in living neurons, we demonstrated that PMCA1-pHluorin localized to intracellular acidic compartments and recycled at presynaptic terminals in an activity-dependent manner. Collectively, our results imply that vesicular PMCAs may play pivotal roles in both presynaptic Ca²⁺ homeostasis and the modulation of H⁺ gradient in SVs.

Melanopsin for precise optogenetic activation of astrocyte-neuron networks

Optogenetics has been widely expanded to enhance or suppress neuronal activity and it has been recently applied to glial cells. Here, we have used a new approach based on selective expression of melanopsin, a G-protein-coupled photopigment, in astrocytes to trigger Ca²⁺ signaling. Using the genetically encoded Ca²⁺ indicator GCaMP6f and two-photon imaging, we show that melanopsin is both competent to stimulate robust IP3-dependent Ca²⁺ signals in astrocyte fine processes, and to evoke an ATP/Adenosine-dependent transient boost of hippocampal excitatory synaptic transmission. Additionally, under low-frequency light stimulation conditions, melanopsin-transfected astrocytes can trigger long-term synaptic changes. In vivo, melanopsin-astrocyte activation enhances episodic-like memory, suggesting melanopsin as an optical tool that could recapitulate the wide range of regulatory actions of astrocytes on neuronal networks in behaving animals. These results describe a novel approach using melanopsin as a precise trigger for astrocytes that mimics their endogenous G-protein signaling pathways, and present melanopsin as a valuable optical tool for neuron-glia studies.

Differential pH Dynamics in Synaptic Vesicles From Intact Glutamatergic and GABAergic Synapses

Synaptic transmission requires the presynaptic release of neurotransmitter from synaptic vesicles (SVs) onto the postsynaptic neuron. Vesicular neurotransmitter transporter proteins, which use a V-ATPase-generated proton gradient, play a crucial role in packaging neurotransmitter into SVs. Recent work has revealed different proton dynamics in SVs expressing the vesicular glutamate transporter (VGLUT) or the vesicular GABA transporter (VGAT) proteins. At the whole synapse level, this results in different steady-state pH and different reacidification dynamics during SV recycling (Egashira et al., 2016). In isolated SVs, the presence of VGAT causes a higher steady state pH, which is correlated with a faster proton efflux rate (Farsi et al., 2016). To address whether proton efflux from GABAergic and glutamatergic SVs in intact synapses differs, we applied a glutamatergic- or GABAergic neuron-specific expression strategy (Chang et al., 2014) to express a genetically encoded pH sensor (synaptophysin pHluorin; SypHy) and/or light-activated proton pump (pHoenix; (Rost et al., 2015). We confirm, with SypHy post-stimulation fluorescence dynamics, that the pH profile of recycling GABAergic SVs differs from that of recycling glutamatergic SVs (Egashira et al., 2016). Using light-activation of pHoenix in pH-neutral vesicles, we investigated the pH dynamics of actively filling vesicles, and could show that proton efflux from GABAergic SVs is indeed initially faster than glutamatergic SVs in intact synapses. Finally, we compared the filling rate of empty glutamatergic and GABAergic vesicles using pHoenix as a proton source, and find a slightly faster filling of glutamatergic vs. GABAergic SVs.

Protons as Messengers of Intercellular Communication in the Nervous System

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

Vesicular glutamate transporter isoforms: The essential players in the somatosensory systems

In nervous system, glutamate transmission is crucial for centripetal conveyance and cortical perception of sensory signals of different modalities, which necessitates vesicular glutamate transporters 1-3 (VGLUT 1-3), the three homologous membrane-bound protein isoforms, to load glutamate into the presysnaptic vesicles. These VGLUTs, especially VGLUT1 and VGLUT2, selectively label and define functionally distinct neuronal subpopulations at each relay level of the neural hierarchies comprising spinal and trigeminal sensory systems. In this review, by scrutinizing each structure of the organism's fundamental hierarchies including dorsal root/trigeminal ganglia, spinal dorsal horn/trigeminal sensory nuclear complex, somatosensory thalamic nuclei and primary somatosensory cortex, we summarize and characterize in detail within each relay the neuronal clusters expressing distinct VGLUT protein/transcript isoforms, with respect to their regional distribution features (complementary distribution in some structures), axonal terminations/peripheral innervations and physiological functions. Equally important, the distribution pattern and characteristics of VGLUT1/VGLUT2 axon terminals within these structures are also epitomized. Finally, the correlation of a particular VGLUT isoform and its physiological role, disclosed thus far largely via studying the peripheral receptors, is generalized by referring to reports on global and conditioned VGLUT-knockout mice. Also, researches on VGLUTs relating to future direction are tentatively proposed, such as unveiling the elusive differences between distinct VGLUTs in mechanism and/or pharmacokinetics at ionic/molecular level, and developing VGLUT-based pain killers.

Amino acid transporters revisited: New views in health and disease

Amino acid transporters (AATs) are membrane-bound transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs have diverse functional roles ranging from neurotransmission to acid-base balance, intracellular energy metabolism, and anabolic and catabolic reactions. In cancer cells and diabetes, dysregulation of AATs leads to metabolic reprogramming, which changes intracellular amino acid levels, contributing to the pathogenesis of cancer, obesity and diabetes. Indeed, the neutral amino acid transporters (NATs) SLC7A5/LAT1 and SLC1A5/ASCT2 are likely involved in several human malignancies. However, a clinical therapy that directly targets AATs has not yet been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, their diverse physiological roles in different tissues and organs, their wide-ranging implications in human diseases and the emerging strategies and tools that will be necessary to target AATs therapeutically.

The dual role of chloride in synaptic vesicle glutamate transport

The transport of glutamate into synaptic vesicles exhibits an unusual form of regulation by Cl- as well as an associated Cl- conductance. To distinguish direct effects of Cl- on the transporter from indirect effects via the driving force Δψ, we used whole endosome recording and report the first currents due to glutamate flux by the vesicular glutamate transporters (VGLUTs). Chloride allosterically activates the VGLUTs from both sides of the membrane, and we find that neutralization of an arginine in transmembrane domain four suffices for the lumenal activation. The dose dependence suggests that Cl- permeates through a channel and glutamate through a transporter. Competition between the anions nonetheless indicates that they use a similar permeation pathway. By controlling both ionic gradients and Δψ, endosome recording isolates different steps in the process of synaptic vesicle filling, suggesting distinct roles for Cl- in both allosteric activation and permeation.

Structure-Function Relationship of Transporters in the Glutamate-Glutamine Cycle of the Central Nervous System

Many kinds of transporters contribute to glutamatergic excitatory synaptic transmission. Glutamate is loaded into synaptic vesicles by vesicular glutamate transporters to be released from presynaptic terminals. After synaptic vesicle release, glutamate is taken up by neurons or astrocytes to terminate the signal and to prepare for the next signal. Glutamate transporters on the plasma membrane are responsible for transporting glutamate from extracellular fluid to cytoplasm. Glutamate taken up by astrocyte is converted to glutamine by glutamine synthetase and transported back to neurons through glutamine transporters on the plasma membranes of the astrocytes and then on neurons. Glutamine is converted back to glutamate by glutaminase in the neuronal cytoplasm and then loaded into synaptic vesicles again. Here, the structures of glutamate transporters and glutamine transporters, their conformational changes, and how they use electrochemical gradients of various ions for substrate transport are summarized. Pharmacological regulations of these transporters are also discussed.

Molecular Signatures Underlying Synaptic Vesicle Cargo Retrieval

Efficient retrieval of the synaptic vesicle (SV) membrane from the presynaptic plasma membrane, a process called endocytosis, is crucial for the fidelity of neurotransmission, particularly during sustained neural activity. Although multiple modes of endocytosis have been identified, it is clear that the efficient retrieval of the major SV cargos into newly formed SVs during any of these modes is fundamental for synaptic transmission. It is currently believed that SVs are eventually reformed via a clathrin-dependent pathway. Various adaptor proteins recognize SV cargos and link them to clathrin, ensuring the efficient retrieval of the cargos into newly formed SVs. Here, we summarize our current knowledge of the molecular signatures within individual SV cargos that underlie efficient retrieval into SV membranes, as well as discuss possible contributions of the mechanisms under physiological conditions.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

VGLUT1 functions as a glutamate/proton exchanger with chloride channel activity in hippocampal glutamatergic synapses

Glutamate is the major excitatory transmitter in the vertebrate nervous system. To maintain synaptic efficacy, recycling synaptic vesicles (SV) are refilled with glutamate by vesicular glutamate transporters (VGLUTs). The dynamics and mechanism of glutamate uptake in intact neurons are still largely unknown. Here, we show by live-cell imaging with pH- and chloride-sensitive fluorescent probes in cultured hippocampal neurons of wild-type and VGLUT1-deficient mice that in SVs VGLUT functions as a glutamate/proton exchanger associated with a channel-like chloride conductance. After endocytosis most internalized Cl⁻ is substituted by glutamate in an electrically, and presumably osmotically, neutral manner, and this process is driven by both the Cl⁻ gradient itself and the proton motive force provided by the vacuolar H⁺-ATPase. Our results shed light on the transport mechanism of VGLUT under physiological conditions and provide a framework for how modulation of glutamate transport via Cl⁻ and pH can change synaptic strength.

During neurotransmission synaptic vesicles are filled with glutamate by vesicular glutamate transporters (VGLUTs). Here, authors image intact neurons and show that in synaptic vesicles VGLUT functions as a glutamate/proton exchanger associated with a channel-like chloride conductance.

Clodronate: A Vesicular ATP Release Blocker

Clodronate is a first-generation bisphosphonate used worldwide for antiresorptive therapy for osteoporosis. Although clodronate is analgesic in nature, its mechanism and efficacy were unknown for some time. Recently, clodronate was identified as a selective and potent inhibitor for vesicular nucleotide transporter (VNUT), a transporter responsible for vesicular storage of ATP. Clodronate inhibits vesicular ATP release from neurons and reduces chronic neuropathic and inflammatory pain following blockade of purinergic chemical transmission. Its effectiveness is stronger, faster acting, and longer lasting than that of existing drugs such as pregabalin. Thus, clodronate might be a promising drug for attenuating chronic neuropathic pain and opens a new field of drug discovery as a presynaptic blocker for purinergic chemical transmission.

Neuronal Depolarization Drives Increased Dopamine Synaptic Vesicle Loading via VGLUT

The ability of presynaptic dopamine terminals to tune neurotransmitter release to meet the demands of neuronal activity is critical to neurotransmission. Although vesicle content has been assumed to be static, in vitro data increasingly suggest that cell activity modulates vesicle content. Here, we use a coordinated genetic, pharmacological, and imaging approach in Drosophila to study the presynaptic machinery responsible for these vesicular processes in vivo. We show that cell depolarization increases synaptic vesicle dopamine content prior to release via vesicular hyperacidification. This depolarization-induced hyperacidification is mediated by the vesicular glutamate transporter (VGLUT). Remarkably, both depolarization-induced dopamine vesicle hyperacidification and its dependence on VGLUT2 are seen in ventral midbrain dopamine neurons in the mouse. Together, these data suggest that in response to depolarization, dopamine vesicles utilize a cascade of vesicular transporters to dynamically increase the vesicular pH gradient, thereby increasing dopamine vesicle content.

Proton electrochemical gradient: Driving and regulating neurotransmitter uptake

Accumulation of neurotransmitters in the lumen of synaptic vesicles (SVs) relies on the activity of the vacuolar-type H⁺ -ATPase. This pump drives protons into the lumen, generating a proton electrochemical gradient (Δμ_H+ ) across the membrane. Recent work has demonstrated that the balance between the chemical (ΔpH) and electrical (ΔΨ) components of Δμ_H+ is regulated differently by some distinct vesicle types. As different neurotransmitter transporters use ΔpH and ΔΨ with different relative efficiencies, regulation of this gradient balance has the potential to influence neurotransmitter uptake. Nevertheless, the underlying mechanisms responsible for this regulation remain poorly understood. In this review, we provide an overview of current neurotransmitter uptake models, with a particular emphasis on the distinct roles of the electrical and chemical gradients and current hypotheses for regulatory mechanisms.

Function and expression of a splicing variant of vesicular glutamate transporter 1

Vesicular glutamate transporter (VGLUT) is an active transporter responsible for vesicular storage of glutamate in synaptic vesicles and plays an essential role in glutamatergic neurotransmission. VGLUT consists of three isoforms, VGLUT1, VGLUT2, and VGLUT3. The VGLUT1 variant, VGLUT1v, with an additional 75-base pair sequence derived from a second intron between exons 2 and 3, which corresponds to 25 amino acid residues in the 1st loop of VGLUT1, is the only splicing variant among VGLUTs, although whether VGLUT1v protein is actually translated at the protein level remains unknown. In the present study, VGLUT1v was expressed in insect cells, solubilized, purified to near homogeneity, and its transport activity was examined. Proteoliposomes containing purified VGLUT1v were shown to accumulate glutamate upon imposition of an inside-positive membrane potential (Δψ). The Δψ-driven glutamate uptake activity requires Cl^- and its pharmacological profile and kinetics are comparable to those of other VGLUTs. The retinal membrane contained two VGLUT1 moieties with apparent molecular masses of 65 and 57kDa. VGLUT1v-specific antibodies against an inserted 25-amino acid residue sequence identified a 65-kDa immunoreactive polypeptide. Immunohistochemical analysis indicated that VGLUT1v immunoreactivity is present in photoreceptor cells and is associated with synaptic vesicles. VGLUT1v immunoreactivity is also present in pinealocytes, but not in other areas, including the brain. These results indicated that VGLUT1v exists in a functional state in rat photosensitive cells and is involved in glutamatergic chemical transmission.

Unique pH dynamics in GABAergic synaptic vesicles illuminates the mechanism and kinetics of GABA loading

GABA acts as the major inhibitory neurotransmitter in the mammalian brain, shaping neuronal and circuit activity. For sustained synaptic transmission, synaptic vesicles (SVs) are required to be recycled and refilled with neurotransmitters using an H(+) electrochemical gradient. However, neither the mechanism underlying vesicular GABA uptake nor the kinetics of GABA loading in living neurons have been fully elucidated. To characterize the process of GABA uptake into SVs in functional synapses, we monitored luminal pH of GABAergic SVs separately from that of excitatory glutamatergic SVs in cultured hippocampal neurons. By using a pH sensor optimal for the SV lumen, we found that GABAergic SVs exhibited an unexpectedly higher resting pH (∼6.4) than glutamatergic SVs (pH ∼5.8). Moreover, unlike glutamatergic SVs, GABAergic SVs displayed unique pH dynamics after endocytosis that involved initial overacidification and subsequent alkalization that restored their resting pH. GABAergic SVs that lacked the vesicular GABA transporter (VGAT) did not show the pH overshoot and acidified further to ∼6.0. Comparison of luminal pH dynamics in the presence or absence of VGAT showed that VGAT operates as a GABA/H(+) exchanger, which is continuously required to offset GABA leakage. Furthermore, the kinetics of GABA transport was slower (τ > 20 s at physiological temperature) than that of glutamate uptake and may exceed the time required for reuse of exocytosed SVs, allowing reuse of incompletely filled vesicles in the presence of high demand for inhibitory transmission.

Quantum Disentanglement: Electrical Analysis of the Complex Roles of Ions in Filling Vesicles with Glutamate

By expressing vesicular glutamate transporters at high levels in plasma membrane and applying voltage clamp methods, Eriksen et al. (2016) have identified a Cl(-) channel in the transporter that is coactivated by protons and Cl(-).