Acidic amino acid transport in animal cells and tissues

Expression of Glutamate Transporters in Mouse Liver, Kidney, and Intestine

Glutamate transport activities have been identified not only in the brain, but also in the liver, kidney, and intestine. Although glutamate transporter distributions in the central nervous system are fairly well known, there are still uncertainties with respect to the distribution of these transporters in peripheral organs. Quantitative information is mostly lacking, and few of the studies have included genetically modified animals as specificity controls. The present study provides validated qualitative and semi-quantitative data on the excitatory amino acid transporter (EAAT)1-3 subtypes in the mouse liver, kidney, and intestine. In agreement with the current view, we found high EAAT3 protein levels in the brush borders of both the distal small intestine and the renal proximal tubules. Neither EAAT1 nor EAAT2 was detected at significant levels in murine kidney or intestine. In contrast, the liver only expressed EAAT2 (but 2 C-terminal splice variants). EAAT2 was detected in the plasma membranes of perivenous hepatocytes. These cells also expressed glutamine synthetase. Conditional deletion of hepatic EAAT2 did neither lead to overt neurological disturbances nor development of fatty liver.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

Nonsynaptic localization of the excitatory amino acid transporter 4 in photoreceptors

Excitatory amino acid transporters (EAATs) are involved in regulating extracellular glutamate levels at synaptic regions in the CNS. EAAT1, 2, 3, and 5 have been found in the mammalian retina, but the presence of EAAT4 has remained controversial. Recently, we found a high level of EAAT4 mRNA in the human retina, and this observation lead us to examine whether EAAT4 was expressed in the mammalian retina. Immunoblotting studies showed the presence of EAAT4-immunoreactive proteins in human and mouse retinas, corresponding to EAAT4 monomers and dimers. Immunohistochemistry revealed that EAAT4 was localized in rod and cone photoreceptor outer segments in the human retina, and in the outer and inner segments of mouse and ground squirrel retinas. In no case was EAAT4 found in the outer plexiform layer or in any other layer in the retina. EAAT4 expression by photoreceptors was confirmed by immunoblotting a purified rod outer segment preparation, which showed the presence of a 50-kDa EAAT4-immunoreactive protein. In addition, the EAAT4-associated protein, GTRAP41, was found in the human, mouse, and squirrel retinas as well as in the rod outer segment preparation. Further immunocytochemical and co-immunoprecipitation experiments demonstrated that GTRAP41 was colocalized and interacted in vivo with EAAT4. Importantly, glutamate uptake and drug inhibition experiments showed that an EAAT4-like glutamate uptake system is present in the rod outer segments. Finally, we examined whether glutamate signaling mediated by EAAT4 can modulate rod outer segment phagocytosis by the retinal pigment epithelium. Results of the present study show that EAAT4 is present in the outer segments, a nonsynaptic region of photoreceptors, where it might provide a feedback mechanism for sensing extracellular glutamate or serve as an outer barrier to prevent glutamate from escaping from the retina.

Functional properties of multispecific amino acid transporters and their implications to transporter-mediated toxicity

The absorption, distribution and excretion of most of xenobiotics, drugs, environmental toxins and their metabolites are mediated by membrane transporters. Recent advances in the transporter molecular biology have made it possible to investigate the mechanisms of transport of those exogenous compounds and their transporter-mediated toxicity at the molecular level. Exogenous compounds including drugs and toxic substances occurring in the environment pass through the transporters with broad substrate selectivity, namely "multispecific" transporters, taking advantage of the multispecific nature to exert their toxic effects. The remarkable examples of such transporter-mediated toxicity are 1-methyl-4-phenyl-2,3-dihydropyridinium (MPP+)-neurotoxicity mediated by dopamine transporters, cephaloridine-nephrotoxicity mediated by organic anion transporters and methylmercury-toxicity mediated by system L amino acid transporters. The molecular identification of system L transporter LAT1 (L-type amino acid transporter 1) has lead to the understanding of the mechanisms of their multispecific substrate recognition and revealed their localization at the blood-brain barrier and placental barrier. LAT1 relies on the hydrophobic interaction between substrate amino acid side chains and the substrate binding site, so that many variations are possible for the substrate amino acid side chains, which is the basis of the broad substrate selectivity. System L transporters, thus, function as a path for the membrane permeation of drugs and toxic compounds occurring in the environment with amino acid-related structures. Beside methylmercury-cysteine conjugate, amino acid-related neurotoxins such as beta-N-methylamino-L-alanine, S-(1,2-dichlorovinyl)-L-cysteine and 3-hydroxykynurenine are proposed to pass through system L transporters to exert their toxicity. Because the presence of such transporters is crucial for the manifestation of the organ toxicity, the inhibition of the transporters would be expected to be beneficial to prevent the disorders caused by the transporter-mediated toxicity.

Localization of glutamate and glutamate transporters in the sensory neurons of Aplysia

The sensorimotor synapse of Aplysia has been used extensively to study the cellular and molecular basis for learning and memory. Recent physiologic studies suggest that glutamate may be the excitatory neurotransmitter used by the sensory neurons (Dale and Kandel [1993] Proc Natl Acad Sci USA. 90:7163-7167; Armitage and Siegelbaum [1998] J Neurosci. 18:8770-8779). We further investigated the hypothesis that glutamate is the excitatory neurotransmitter at this synapse. The somata of sensory neurons in the pleural ganglia showed strong glutamate immunoreactivity. Very intense glutamate immunoreactivity was present in fibers within the neuropil and pleural-pedal connective. Localization of amino acids metabolically related to glutamate was also investigated. Moderate aspartate and glutamine immunoreactivity was present in somata of sensory neurons, but only weak labeling for aspartate and glutamine was present in the neuropil or pleural-pedal connective. In cultured sensory neurons, glutamate immunoreactivity was strong in the somata and processes and was very intense in varicosities; consistent with localization of glutamate in sensory neurons in the intact pleural-pedal ganglion. Cultured sensory neurons showed only weak labeling for aspartate and glutamine. Little or no gamma-aminobutyric acid or glycine immunoreactivity was observed in the pleural-pedal ganglia or in cultured sensory neurons. To further test the hypothesis that the sensory neurons use glutamate as a transmitter, in situ hybridization was performed by using a partial cDNA clone of a putative Aplysia high-affinity glutamate transporter. The sensory neurons, as well as a subset of glia, expressed this mRNA. Known glutamatergic motor neurons B3 and B6 of the buccal ganglion also appeared to express this mRNA. These results, in addition to previous physiological studies (Dale and Kandel [1993] Proc Natl Acad Sci USA. 90:7163-7167; Trudeau and Castellucci [1993] J Neurophysiol. 70:1221-1230; Armitage and Siegelbaum [1998] J Neurosci. 18:8770-8779)) establish glutamate as an excitatory neurotransmitter of the sensorimotor synapse.

The elusive transporters with a high affinity for glutamate

Removal of glutamate from the synaptic cleft is an essential component of the transmission process at glutamatergic synapses. This requirement is fulfilled by transporters that have a high affinity for glutamate and exhibit a unique coupling to Na+, K+ and OH- ions. Independently, three groups have succeeded in cloning cDNAs encoding high-affinity Na(+)-dependent glutamate transporters. These transporters are structurally distinct from previously characterized neurotransmitter transporters and show sequence identity with prokaryotic glutamate and dicarboxylate transporters. In addition, they exhibit significant differences in their structure, function and tissue distribution. This review compares and contrasts these differences, and incorporates into the existing body of knowledge these new breakthroughs.

Anionic amino acid uptake by microvillous membrane vesicles from human placenta

The transport mechanisms for anionic amino acids in trophoblast microvillous (maternal facing) membrane were investigated by characterization of L-[3H]aspartate and L-[3H]glutamate uptake in membrane vesicles. Uptake of the anionic amino acids was by a single high-affinity Na+-dependent K+-stimulated cotransporter that is pH sensitive and electrogenic. A second Na+-dependent transporter could not be discriminated, and there was no observable Na+-independent uptake. An outwardly directed K+ gradient (100 mM KCl inside) resulted in a 5- to 10-fold stimulation in glutamate uptake in the presence of Na+. Intravesicular KCl had no effect on transporter affinity but increased transporter velocity in a concentration-dependent manner. Inhibition of Na+-K+-dependent uptake of L-aspartate and L-glutamate (20 mM, 30 s) by 2 mM unlabeled amino acids demonstrated stereoselectivity for L-glutamate but not for L-aspartate. The neutral amino acids (L-alanine, L-threonine, L-serine, L-cysteine, L-phenylalanine) were not effective inhibitors. These data are consistent with an anionic amino acid transporter in the microvillous membrane of the trophoblast, which has characteristics qualitatively similar to the X-AG system found in other epithelia. This system may mediate the concentrative placental uptake of anionic amino acids from maternal blood in utero.

L-glutamate transport in internally dialyzed barnacle muscle fibers

The transport of L-glutamate (Glu) by single muscle fibers of Balanus nubilus was studied by means of internal dialysis in an effort to obtain transport data under well-defined intracellular conditions. It is shown that the method effectively controls the sarcoplasmic amino acid composition and Glu metabolism. It was found that the classic neutral amino acid transport systems are either absent or inactive, while Glu is strongly transported by a single system. Nonsaturable Glu "leak" is small relative to mediated transport. Glu influx under 0-trans conditions obeys simple Michaelis-Menten kinetics and shows simple competition with aspartate. Influx is strictly dependent on external Na. Trans-Na reduces influx, whereas trans-Glu has a small stimulatory effect. The preparation may serve as a general model for muscle Glu transport and can be used for a thorough study of the kinetic mechanism.