Chloride-dependent inhibition of vesicular glutamate uptake by alpha-keto acids accumulated in maple syrup urine disease

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.

Function of essential chloride and arginine residue in nucleotide binding to vesicular nucleotide transporter

Vesicular nucleotide transporter (VNUT) plays a key role in purinergic signalling through its ability to transport nucleotides. VNUT belongs to the SLC17 family, which includes vesicular glutamate transporters (VGLUTs) and Type I Na+/phosphate cotransporters. All of these transporters exhibit membrane potential and Cl--dependent organic anion transport activity and have essential arginine in the transmembrane region. Previously, we reported that ketoacids inhibit these transporters through modulation of Cl- activation. Although this regulation is important to control signal transmission, the mechanisms underlying Cl--dependent regulation are unclear. Here, we examined the functional roles of Cl- and essential arginine residue on ATP binding to VNUT using the fluorescent ATP analogue trinitrophenyl-ATP (TNP-ATP). The fluorescence of TNP-ATP was enhanced by VNUT, whereas no enhancement was observed by VGLUT. Concentration-dependence curves showed that TNP-ATP was a high-affinity fluorescent probe for VNUT, with a Kd of 4.8 μM. TNP-ATP binding was competitive to ATP and showed similar specificity to transport activity. Addition of Cl- and ketoacids did not affect the apparent affinity for TNP-ATP. The Arg119 to Ala mutant retained TNP-ATP binding ability with slightly reduced affinity. Overall, these results indicated that Cl- and essential arginine were not important for ATP binding.

Vesicular nucleotide transporter (VNUT): appearance of an actress on the stage of purinergic signaling

Vesicular storage of ATP is one of the processes initiating purinergic chemical transmission. Although an active transport mechanism was postulated to be involved in the processes, a transporter(s) responsible for the vesicular storage of ATP remained unidentified for some time. In 2008, SLC17A9, the last identified member of the solute carrier 17 type I inorganic phosphate transporter family, was found to encode the vesicular nucleotide transporter (VNUT) that is responsible for the vesicular storage of ATP. VNUT transports various nucleotides in a membrane potential-dependent fashion and is expressed in the various ATP-secreting cells. Mice with knockout of the VNUT gene lose vesicular storage and release of ATP from neurons and neuroendocrine cells, resulting in blockage of the initiation of purinergic chemical transmission. Thus, VNUT plays an essential role in the vesicular storage and release of ATP. The VNUT knockout mice exhibit resistance for neuropathic pain and a therapeutic effect against diabetes by way of increased insulin sensitivity. Thus, VNUT inhibitors and suppression of VNUT gene expression may be used for therapeutic purposes through suppression of purinergic chemical transmission. This review summarizes the studies to date on VNUT and discusses what we have learned about the relevance of vesicular ATP release as a potential drug target.

Glutamate Release

Our aim was to review the processes of glutamate release from both biochemical and neurophysiological points of view. A large body of evidence now indicates that glutamate is specifically accumulated into synaptic vesicles, which provides strong support for the concept that glutamate is released from synaptic vesicles and is the major excitatory neurotransmitter. Evidence suggests the notion that synaptic vesicles, in order to sustain the neurotransmitter pool of glutamate, are endowed with an efficient mechanism for vesicular filling of glutamate. Glutamate-loaded vesicles undergo removal of Synapsin I by CaM kinase II-mediated phosphorylation, transforming to the release-ready pool. Vesicle docking to and fusion with the presynaptic plasma membrane are thought to be mediated by the SNARE complex. The Ca(2+)-dependent step in exocytosis is proposed to be mediated by synaptotagmin.

Metabolic control of vesicular glutamate transport and release

Fasting has been used to control epilepsy since antiquity, but the mechanism of coupling between metabolic state and excitatory neurotransmission remains unknown. Previous work has shown that the vesicular glutamate transporters (VGLUTs) required for exocytotic release of glutamate undergo an unusual form of regulation by Cl(-). Using functional reconstitution of the purified VGLUTs into proteoliposomes, we now show that Cl(-) acts as an allosteric activator, and the ketone bodies that increase with fasting inhibit glutamate release by competing with Cl(-) at the site of allosteric regulation. Consistent with these observations, acetoacetate reduced quantal size at hippocampal synapses and suppresses glutamate release and seizures evoked with 4-aminopyridine in the brain. The results indicate an unsuspected link between metabolic state and excitatory neurotransmission through anion-dependent regulation of VGLUT activity.

VGLUTs--potential targets for the treatment of seizures?

Vesicular glutamate transporters (VGLUTs) load glutamate into synaptic vesicles. In this issue of Neuron, Juge et al. report that ketone bodies compete with chloride-dependent activation of VGLUTs, leading to suppression of glutamate release and seizures. These findings provide a surprising explanation for the efficacy of the ketogenic diet in controlling epilepsy.

Galphao2 regulates vesicular glutamate transporter activity by changing its chloride dependence

Classical neurotransmitters, including monoamines, acetylcholine, glutamate, GABA, and glycine, are loaded into synaptic vesicles by means of specific transporters. Vesicular monoamine transporters are under negative regulation by alpha subunits of trimeric G-proteins, including Galpha(o2) and Galpha(q). Furthermore, glutamate uptake, mediated by vesicular glutamate transporters (VGLUTs), is decreased by the nonhydrolysable GTP-analog guanylylimidodiphosphate. Using mutant mice lacking various Galpha subunits, including Galpha(o1), Galpha(o2), Galpha(q), and Galpha11, and a Galpha(o2)-specific monoclonal antibody, we now show that VGLUTs are exclusively regulated by Galpha(o2). G-protein activation does not affect the electrochemical proton gradient serving as driving force for neurotransmitter uptake; rather, Galpha(o2) exerts its action by specifically affecting the chloride dependence of VGLUTs. All VGLUTs show maximal activity at approximately 5 mm chloride. Activated Galpha(o2) shifts this maximum to lower chloride concentrations. In contrast, glutamate uptake by vesicles isolated from Galpha(o2-/-) mice have completely lost chloride activation. Thus, Galpha(o2) acts on a putative regulatory chloride binding domain that appears to modulate transport activity of vesicular glutamate transporters.

Glutaric acid stimulates glutamate binding and astrocytic uptake and inhibits vesicular glutamate uptake in forebrain from young rats

Glutaric acidemia type I (GA I) is an inherited neurometabolic disorder caused by glutaryl-CoA dehydrogenase deficiency, which leads to accumulation in body fluids and in brain of predominantly glutaric acid (GA), and to a lesser extent of 3-hydroxyglutaric and glutaconic acids. Neurological presentation is common in patients with GA I. Although the mechanisms underlying brain damage in this disorder are not yet well established, there is growing evidence that excitotoxicity may play a central role in the neuropathogenesis of this disease. In the present study, preparations of synaptosomes, synaptic plasma membranes and synaptic vesicles, as well as cultured astrocytes from rat forebrain were exposed to various concentrations of GA for the determination of the basal and potassium-induced release of [(3)H]glutamate by synaptosomes, Na(+)-independent glutamate binding to synaptic membranes and vesicular glutamate uptake and Na(+)-dependent glutamate uptake into astrocytes, respectively. GA (1-100 nM) significantly stimulated [(3)H]glutamate binding to brain plasma membranes (40-70%) in the absence of extracellular Na(+) concentrations, reflecting glutamate binding to receptors. Furthermore, this stimulatory effect was totally abolished by the metabotropic glutamate ligands DHPG, DCG-IV and l-AP4, attenuated by the ionotropic non-NMDA glutamate receptor agonist AMPA and had no interference of the NMDA receptor antagonist MK-801. Moreover, [(3)H]glutamate uptake into synaptic vesicles was inhibited by approximately 50% by 10 and 100 nM GA and Na(+)-dependent [(3)H]glutamate uptake by astrocytes was significantly increased (up to 50%) in a dose-dependent manner (maximal stimulation at 100 microM GA). In contrast, synaptosomal glutamate release was not affected by the acid at concentrations as high as 1 mM. These results indicate that the inhibition of glutamate uptake into synaptic vesicles by low concentrations GA may result in elevated concentrations of the excitatory neurotransmitter in the cytosol and the stimulatory effect of this organic acid on glutamate binding may potentially cause excitotoxicity to neural cells. Finally, taken together these results and previous findings showing that GA markedly decreases synaptosomal glutamate uptake, it is possible that the stimulatory effect of GA on astrocyte glutamate uptake might indicate that astrocytes may protect neurons from excitotoxic damage caused by GA by increasing glutamate uptake and therefore reducing the concentration of this excitatory neurotransmitter in the synaptic cleft.

Kinetic studies on the inhibition of creatine kinase activity by branched-chain alpha-amino acids in the brain cortex of rats

Maple syrup urine disease (MSUD) is a metabolic disorder biochemically characterized by the accumulation of branched-chain alpha-amino acids (BCAA) and their branched-chain alpha-keto acids (BCKA) in blood and tissues. Neurological dysfunction is usually present in the patients, but the mechanisms of brain damage in this disease are far from be understood. The main objective of this study was to investigate the mechanisms by which BCAA inhibit creatine kinase activity, a key enzyme of energy homeostasis, in the brain cortex of 21-day-old Wistar rats. For the kinetic studies, Lineweaver-Burk and a modification of the Chevillard et al. plots were used to characterize the mechanisms of enzyme inhibition. The results indicated that BCAA inhibit creatine kinase by competition with the substrates phosphocreatine and ADP at the active site. Considering the crucial role creatine kinase plays in energy homeostasis in brain, if these effects also occur in the brain of MSUD patients, it is possible that inhibition of this enzyme activity may contribute to the brain damage found in this disease. In this case, it is possible that creatine supplementation to the diet might benefit MSUD patients.

Creatine kinase activity from rat brain is inhibited by branched-chain amino acids in vitro

Maple syrup urine disease (MSUD) is an inherited metabolic disorder biochemically characterized by the accumulation of branched-chain amino acids (BCAAs) and their branched-chain keto acids (BCKAs) in blood and other tissues. Neurological dysfunction is usually present in the affected patients, but the mechanisms of brain damage in this disease are not fully understood. Considering that brain energy metabolism seems to be altered in MSUD, the main objective of this study was to investigate the in vitro effect of BCAAs and BCKAs on creatine kinase activity, a key enzyme of energy homeostasis, in brain cortex of young rats. BCAAs, but not their BCKAs, significantly inhibited creatine kinase activity at concentrations similar to those found in the plasma of MSUD patients (0.5-5 mM). Considering the crucial role creatine kinase plays in energy homeostasis in brain, if this effect also occurs in the brain of MSUD patients, it is possible that inhibition of this enzyme activity may contribute to the brain damage found in this disease.

Methylmercury inhibits glutamate uptake by synaptic vesicles from rat brain

Methylmercury (MeHg) is an environmental contaminant that continues to cause risk to human health. The toxic effects of MeHg on the CNS implicate the involvement of glutamatergic system. In this study, we evaluated the effects of MeHg on [3H]glutamate uptake by synaptic vesicles. MeHg inhibited [3H]glutamate uptake in a concentration dependent manner. Since glutamate uptake by synaptic vesicles is driven by an electrochemical gradient, formed across the vesicle membrane by a bafilomycin A(1)-sensitive H+-ATPase, we further investigated the effect of MeHg on activity of this enzyme. MeHg inhibited the H+-ATPase activity and also dissipated the proton gradient (DeltapH), indicating that MeHg decreased [3H]glutamate uptake involving the H+-ATPase activity. Until now, the toxic effects of MeHg on CNS were attributed mainly to an impairment of glial glutamate transporters. These findings contribute for the understanding of the neurotoxicity by MeHg, pointing to the involvement of vesicular glutamate.

Effect of leucine administration on creatine kinase activity in rat brain

Maple syrup urine disease (MSUD) is a metabolic disorder biochemically characterized by the accumulation of branched-chain amino acids (BCAA) and their branched-chain keto acids (BCKA) in blood and tissues. Neurological dysfunction is usually present in the patients, but the pathophysiology of brain damage is still obscure. Considering that brain energy metabolism is possibly altered in MSUD, the main objective of this study was to determine creatine kinase activity in the brain of rats subjected to acute and chronic administration of leucine. Chronic hyperleucinemia was induced by subcutaneous administrations of 4.8 micromol leucine/g body weight, twice a day, from the 6th to the 21st postnatal day. For acute hyperleucinemia, 21-day-old rats received three administrations of the amino acid at 3 h interval. Twelve hours after the chronic treatment or 1 h after the acute one, rats were killed and creatine kinase activity measured. The results indicated that acute or chronic administration of leucine altered creatine kinase activity in the brain of leucine-treated rats. Considering the crucial role creatine kinase plays in energy homeostasis in brain, if these effects also occur in the brain of MSUD patients, it is possible that alteration of this enzyme activity may contribute to the brain damage found in this disease.