Elsevier

Neuroscience Research

Volume 55, Issue 4, August 2006, Pages 343-351
Neuroscience Research

Review article
VGLUTs: ‘Exciting’ times for glutamatergic research?

https://doi.org/10.1016/j.neures.2006.04.016Get rights and content

Abstract

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system (CNS). Glutamate is first synthesized in the cytoplasm of presynaptic terminals before being loaded into synaptic vesicles, which fuse with the plasma membrane, releasing their contents, in response to neuronal activity. The important process of synaptic vesicle loading is mediated by a transport protein, collectively known as vesicular glutamate transporter (VGLUT). Controlling the activity of these transporters could potentially modulate the efficacy of glutamatergic neurotransmission. In recent years, three isoforms of mammalian VGLUTs have been cloned and molecularly characterized in detail. Probing these three VGLUTs has been proven to be the most reliable way of visualizing sites of glutamate release in the mammalian CNS. Immunohistochemical studies on VGLUTs suggest that glutamatergic neurons are categorized into subgroups depending on which VGLUT isoform they contain. Recent studies on VGLUT1-deficient mice have led various models to be postulated concerning the possible roles of VGLUTs in synaptic physiology, such as presynaptic regulation of quantal size and activity-dependent short-term plasticity.

Introduction

Glutamate is an ubiquitous anionic amino acid that is present in all cell types in our body. In the brain, however, it acts as a signaling molecule, being stored and released from a subpopulation of neurons (so-called glutamatergic neurons). In a resting neuron, glutamate is stored in a tiny membrane-bound organelle found within the synaptic terminal – the synaptic vesicle – and it is released from the neuron by the fusion of this vesicle with the plasma membrane. This process, which occurs in response to neuronal activity, is known as exocytosis. This released glutamate is actually capable of eliciting a response in most of the neurons so far examined in the mammalian central nervous system (CNS).

As is the case for other small classical neurotransmitters, such as GABA and biogenic monoamines, glutamate is synthesized in the presynaptic cytoplasm with the aid of a synthesizing enzyme (a phosphate-activated glutaminase (PAG)) and translocated into the vesicle lumen via a glutamate transport protein (VGLUT) located on the vesicle. Glutamate uptake into synaptic vesicles was initially characterized using a reasonably pure vesicle fraction isolated from mammalian brain (Disbrow et al., 1982, Naito and Ueda, 1985). As is the case for other classical neurotransmitters, glutamate uptake is driven by a proton-dependent electrochemical gradient that exists across the vesicle membrane and which is created by a vacuolar-type ATPase. It exhibits stringent substrate specificity (highly specific for l-glutamate) and a relatively low affinity (Km = 1–3 mM). It depends predominately on the existence of a vesicular membrane potential gradient, rather than a pH gradient, unlike other vesicular neurotransmitter transporters that depend on the existence of a pH gradient as the driving force. A low concentration of chloride (2–5 mM) is necessary for its maximum activity. Although the first successful cloning of a vesicular monoamine transporter stimulated the search to identify and characterize other vesicular neurotransmitter transporters (Liu et al., 1992, Gasnier, 2000), molecular cloning of the VGLUT gene(s) had not been accomplished. In 2000, work from the laboratories of Jahn (Takamori et al., 2000) and Edwards (Bellocchio et al., 2000) shed first light on these enigmatic proteins, allowing for their subsequent molecular characterization and manipulation. In this review, I would like to overview the recent progress on VGLUT proteins and their physiological functions. In addition to the functional studies on VGLUTs in the nervous system, on which I will focus in this article, there is accumulating evidence that VGLUTs are also expressed in peripheral tissues, indicating that intercellular glutamate signaling is not just restricted to the CNS, but actually more broadly adopted in mammalian tissues. Readers who are interested in this aspect should refer to other recent reviews (Li et al., 2005, Moriyama and Yamamoto, 2004).

Section snippets

Vesicular glutamate transporters uncovered

Although it had long been conceptually envisaged that the process of loading glutamate into synaptic vesicles might be a key step in glutamatergic neurotransmission, the protein responsible for this process eluded detection for many years. When the protein was eventually identified it came as a surprise because the protein had already been cloned in 1994, but had been disguised as a different transporter protein (Ni et al., 1994). This protein, originally termed BNP1 (brain-specific Na+

Three VGLUTs in mammalian CNS

In addition to the previously known C. elegans orthologue of VGLUT, eat-4, multiple VGLUT isoforms have been identified in many other organisms such as Drosophila (Daniels et al., 2004), zebrafish (Higashijima et al., 2004) and frog (Gleason et al., 2003), in which glutamate is mainly used as a neurotransmitter at the neuromuscular junction. It seems that VGLUT genes are evolutionary conserved, concomitant with the organisms employing some form of glutamatergic signaling. Such a finding raises

What do mice without VGLUT1 tell us?

One of the main interests in the field is to clarify the functional differences between the three VGLUT isoforms. As mentioned above, intensive biochemical investigations on the three identified VGLUTs have yet to yield satisfactory answers. Moreover, because the VGLUTs (VGLUT1 and VGLUT2) were originally isolated as plasma membrane carriers for inorganic phosphate (Aihara et al., 2000, Ni et al., 1994), and VGLUT1 was shown to exhibit an additional chloride conductance associated with the

Do VGLUTs contribute to presynaptic plasticity?

The finding that the residual excitatory neurotransmission in VGLUT1 knockout mice appeared to be mediated by VGLUT2 led two groups to explore more precisely both how VGLUT1 and VGLUT2 are co-expressed in a single neuron and how VGLUT expression could contribute to synaptic plasticity in glutamatergic neurotransmission. Since it is now evident that the release of a quantum of glutamate (from a single vesicle) does not saturate postsynaptic glutamate receptors (Ishikawa et al., 2002, McAllister

VGLUT expression and pathology

Despite the current uncertainty regarding whether changes in VGLUT expression result in the modulation of synaptic efficacy, reports have started to establish a link between alterations in VGLUT isoform expression and pathological processes and also in the response to pharmacological treatments. For instance, VGLUT1 expression in the cerebral cortex and the hippocampus is increased by chronic administration of antidepressant drugs (Moutsimilli et al., 2005, Tordera et al., 2005). In addition,

Concluding remarks

Recent successes in the molecular identification of VGLUTs have promoted rapid progress in the anatomical identification of glutamatergic synapses in the CNS. Amongst the three VGLUT isoforms, VGLUT1 and VGLUT2 are largely segregated in the brain and are expressed in the presynaptic terminals of asymmetric synapses, revealing most, if not all, authentic glutamatergic synapses in the CNS. By contrast, VGLUT3 expression is less abundant compared to VGLUT1 and VGLUT2 and is expressed in

Acknowledgements

This review was written to acknowledge the grateful receipt of the Japan Neuroscience Society Young Investigator Award in 2005. This work was supported, in part, by a grant from the Japanese Ministry of Education, Science, Sports, Culture and Technology (no. 17680032) and by the 21st Century COE Program on “Brain Integration and its Disorder” at Tokyo Medical and Dental University. I am grateful to Reinhard Jahn (Göttingen, Germany) for his continuous support throughout my work on VGLUTs. I am

References (49)

  • Y. Liu et al.

    A cDNA that suppresses MPP+ toxicity encodes a vesicular amine transporter

    Cell

    (1992)
  • L. Moutsimilli et al.

    Selective cortical VGLUT1 increase as a marker for antidepressant activity

    Neuropharmacology

    (2005)
  • C. Rosenmund et al.

    Differential control of vesicle priming and short-term plasticity by Munc13 isoforms

    Neuron

    (2002)
  • T. Sakaba et al.

    Estimation of quantal parameters at the calyx of Held synapse

    Neurosci. Res.

    (2002)
  • M.K. Schafer et al.

    Molecular cloning and functional identification of mouse vesicular glutamate transporter 3 and its expression in subsets of novel excitatory neurons

    J. Biol. Chem.

    (2002)
  • H. Song et al.

    Expression of a putative vesicular acetylcholine transporter facilitates quantal transmitter packaging

    Neuron

    (1997)
  • J. Williams

    How does a vesicle know it is full?

    Neuron

    (1997)
  • Y. Aihara et al.

    Molecular cloning of a novel brain-type Na(+)-dependent inorganic phosphate cotransporter

    J. Neurochem.

    (2000)
  • E.E. Bellocchio et al.

    The localization of the brain-specific inorganic phosphate transporter suggests a specific presynaptic role in glutamatergic transmission

    J. Neurosci.

    (1998)
  • E.E. Bellocchio et al.

    Uptake of glutamate into synaptic vesicles by an inorganic phosphate transporter

    Science

    (2000)
  • J.L. Boulland et al.

    Expression of the vesicular glutamate transporters during development indicates the widespread corelease of multiple neurotransmitters

    J. Comp. Neurol.

    (2004)
  • F. Conti et al.

    Heterogeneity of axon terminals expressing VGLUT1 in the cerebral neocortex

    Arch. Ital. Biol.

    (2005)
  • R.W. Daniels et al.

    Increased expression of the Drosophila vesicular glutamate transporter leads to excess glutamate release and a compensatory decrease in quantal content

    J. Neurosci.

    (2004)
  • R.T. Fremeau et al.

    The identification of vesicular glutamate transporter 3 suggests novel modes of signaling by glutamate

    Proc. Natl. Acad. Sci. U.S.A.

    (2002)
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