Family of neutral and acidic amino acid transporters: molecular biology, physiology and medical implications

Genome-wide identification of AAAP gene family and expression analysis in response to saline-alkali stress in foxtail millet (Setaria italica L.)

Amino acid/auxin permease (AAAP) genes encode a large family of protein transporters that play important roles in various aspects of plant growth and development. Here, we performed genome-wide identification of members in the foxtail millet (Setaria italica L.) AAAP family (SiAAAP) and their saline-alkali stress-induced expression patterns, resulting in the identification of 65 SiAAAP genes, which could be divided into eight subfamilies. Except for SiAAAP65, the remaining 64 genes were located on nine chromosomes of foxtail millet. Gene structure and conserved motif analyses indicated that the members in the same subfamily are highly conserved. Gene duplication event analysis suggested that tandem duplication may be the main factor driving the expansion of this gene family, and Ka/Ks analysis indicated that all the duplicated genes have undergone purifying selection. Transcriptome analysis showed differential expression of SiAAAPs in roots, stems, leaves, and tassel inflorescence. Analysis of cis-acting elements in the promoter indicated that SiAAAPs contain stress-responsive cis-acting elements. Under saline-alkali stress, qRT-PCR analysis showed that SiAAP3, SiLHT2, and SiAAP16 were differentially expressed between salt-alkali tolerant millet variety JK3 and salt-alkali sensitive millet variety B175. These results suggest that these genes may be involved in or regulate the response to saline-alkali stress, providing a theoretical basis for further studying the function of SiAAAPs.

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Inhibiting Glutamate Activity during Consolidation Suppresses Age-Related Long-Term Memory Impairment in Drosophila

In Drosophila, long-term memory (LTM) formation requires increases in glial gene expression. Klingon (Klg), a cell adhesion molecule expressed in both neurons and glia, induces expression of the glial transcription factor, Repo. However, glial signaling downstream of Repo has been unclear. Here we demonstrate that Repo increases expression of the glutamate transporter, EAAT1, and EAAT1 is required during consolidation of LTM. The expressions of Klg, Repo, and EAAT1 decrease upon aging, suggesting that age-related impairments in LTM are caused by dysfunction of the Klg-Repo-EAAT1 pathway. Supporting this idea, overexpression of Repo or EAAT1 rescues age-associated impairments in LTM. Pharmacological inhibition of glutamate activity during consolidation improves LTM in klg mutants and aged flies. Altogether, our results indicate that LTM formation requires glial-dependent inhibition of glutamate signaling during memory consolidation, and aging disrupts this process by inhibiting the Klg-Repo-EAAT1 pathway.

Genome-wide identification and expression analysis of the AAAP family in Medicago truncatula

The amino acid/auxin permease (AAAP) gene family plays an important role in the long-distance amino acid transport pathway and takes part in various stages of plant growth and development. However, little is known about the AAAP gene family in Medicago truncatula. Here, we identified 86 putative MtAAAP family members using genome sequence information. Based on phylogenetic analysis, these MtAAAP genes were categorized into eight distinct subfamilies. The MtAAAP genes were mapped on 8 chromosomes and duplication events appeared widely, with 19 and 21 pairs of MtAAAP genes showing segment and tandem duplication events, respectively. Ratio of Ka/Ks indicated that duplicated genes underwent purifying selection. Analysis of RNA-seq data showed that MtAAAP genes exhibited specific expression patterns among different tissues and abiotic stress, indicating that MtAAAP members were involved in plant developmental regulation and stress responses. Expression patterns of 16 MtAAAP genes under abiotic stress were verified by qRT-PCR. The present study provides a foundation for the functional analysis of MtAAAPs in developmental regulation and stress responses.

Intracellular boron accumulation in CHO-K1 cells using amino acid transport control

BPA used in BNCT has a similar structure to some essential amino acids and is transported into tumor cells by amino acid transport systems. Previous study groups have tried various techniques of loading BPA to increase intracellular boron concentration. CHO-K1 cells demonstrate system L (LAT1) activity and are suitable for specifying the transport system of a neutral amino acid. In this study, we examined the intracellular accumulation of boron in CHO-K1 cells by amino acid transport control, which involves co-loading with L-type amino acid esters. Intracellular boron accumulation in CHO-K1 cells showed the greatest increased upon co-loading 1.0mM BPA, with 1.0mM l-Tyr-O-Et and incubating for 60min. This increase is caused by activation of a system L amino acid exchanger between BPA and l-Tyr. The amino acid esters are metabolized to amino acids by intracellular hydrolytic enzymes that increase the concentrations of intracellular amino acids and stimulate exchange transportation. We expect that this amino acid transport control will be useful for enhancing intracellular boron accumulation.

Upregulation of glutamate transporter GLT-1 by mTOR-Akt-NF-кB cascade in astrocytic oxygen-glucose deprivation

Excessive extracellular glutamate leads to neuronal death in central nervous system. Excitatory glutamate transporter subtype 2 (GLT-1) carries bulk of glutamate reuptake in cerebral ischemia. Although GLT-1 expression fluctuates during the period of ischemia, little is known about its regulatory mechanism. Here we show an up-regulation of GLT-1 via mammalian target of rapamycin (mTOR)-Akt-nuclear factor-кB (NF-кB) signaling cascade in oxygen glucose deprivation (OGD). We found that brief rapamycin treatment significantly increased GLT-1 expression in cultured astrocytes. Rapamycin increased phosphorylation of raptor at Ser792 and decreased phosphorylation of rictor at Thr1135, suggesting that both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are involved in GLT-1 expression. This conclusion was further confirmed by raptor and rictor disruption experiments. Akt was activated by mTORC1 inhibition and required for GLT-1 expression because triciribine, a specific inhibitor of Akt, blocked the increase of GLT-1 expression. mTOR-Akt cascade then activated NF-кB and increased кB-motif-binding phosphoprotein (KBBP) expression and GLT-1 transcription. We next demonstrated that mTOR-Akt-NF-кB cascade was activated in OGD and subsequently caused the upregulation of GLT-1. Supporting evidence included: (1) inhibition of Akt or NF-кB occluded OGD-induced GLT-1 upregulation; (2) Raptor knock-down plus OGD did not add to the increase of GLT-1 expression; (3) Intact mTORC2 was required for GLT-1 enhancement. In summary, our data first showed that mTOR-Akt-NF-кB cascade played critical roles to up-regulate GLT-1 in OGD. This signaling cascade may work to promote glutamate uptake in brain ischemia and neurodegenerative diseases.

Intracellular pH regulation in unstimulated Calliphora salivary glands is Na+ dependent and requires V-ATPase activity

Salivary gland cells of the blowfly Calliphora vicina have a vacuolar-type H+-ATPase (V-ATPase) that lies in their apical membrane and energizes the secretion of a KCl-rich primary saliva upon stimulation with serotonin (5-hydroxytryptamine). Whether and to what extent V-ATPase contributes to intracellular pH (pHi) regulation in unstimulated gland cells is unknown. We used the fluorescent dye BCECF to study intracellular pHi regulation microfluorometrically and show that: (1) under resting conditions, the application of Na+-free physiological saline induces an intracellular alkalinization attributable to the inhibition of the activity of a Na+-dependent glutamate transporter; (2) the maintenance of resting pHi is Na+, Cl–, concanamycin A and DIDS sensitive; (3) recovery from an intracellular acid load is Na+ sensitive and requires V-ATPase activity; (4) the Na+/H+ antiporter is not involved in pHi recovery after a NH4Cl prepulse; and (5) at least one Na+-dependent transporter and the V-ATPase maintain recovery from an intracellular acid load. Thus, under resting conditions, the V-ATPase and at least one Na+-dependent transporter maintain normal pHi values of pH 7.5. We have also detected the presence of a Na+-dependent glutamate transporter, which seems to act as an acid loader. Despite this not being a common pHi-regulating transporter, its activity affects steady-state pHi in C. vicina salivary gland cells.

A brief comparison of the pathophysiology of inflammatory versus neuropathic pain

PURPOSE OF REVIEW

The causes of inflammatory pain and neuropathic pain are fundamentally different. There are, however, common mechanisms underlying the generation of each pain state. We will discuss some specific elements observed in both tissue and nerve injury pain states and consider the hypothesis that these two states actually demonstrate a convergence over time.

RECENT FINDINGS

The increased pain sensation following tissue and nerve injury results from several mechanisms, including altered ion channel expression in dorsal root ganglion neurons, enhanced dorsal horn glutamate release from primary afferents, enhanced glutamate receptor function in second-order neurons, disinhibition in the dorsal horn and glia cell activation. The role of specific subtypes of receptors, ion channels and glutamate transporters is revealed at peripheral and central sites. Importantly over time, a number of changes, in the dorsal root ganglion and in dorsal horn observed after tissue injury resemble changes observed after nerve injury.

SUMMARY

Recognition of mechanisms common to both inflammatory pain and neuropathic pain might shed light on the understanding of the transition from acute pain to persistent pain.

Ischemia-reperfusion injury of the cochlea: pharmacological strategies for cochlear protection and implications of glutamate and reactive oxygen species

A large amount of energy produced by active aerobic metabolism is necessary for the cochlea to maintain its function. This makes the cochlea vulnerable to blockade of cochlear blood flow and interruption of the oxygen supply. Although certain forms of human idiopathic sudden sensorineural hearing loss reportedly arise from ischemic injury, the pathological mechanism of cochlear ischemia-reperfusion injury has not been fully elucidated. Recent animal studies have shed light on the mechanisms of cochlear ischemia-reperfusion injury. It will help in the understanding of the pathology of cochlear ischemia-reperfusion injury to classify this injury into ischemic injury and reperfusion injury. Excitotoxicity, mainly observed during the ischemic period, aggravates the injury of primary auditory neurons. On the other hand, oxidative damage induced by hydroxyl radicals and nitric oxide enhances cochlear reperfusion injury. This article briefly summarizes the generation mechanisms of cochlear ischemia-reperfusion injury and potential therapeutic targets that could be developed for the effective management of this injury type.

Transport characteristics of guanidino compounds at the blood-brain barrier and blood-cerebrospinal fluid barrier: relevance to neural disorders

Guanidino compounds (GCs), such as creatine, phosphocreatine, guanidinoacetic acid, creatinine, methylguanidine, guanidinosuccinic acid, γ-guanidinobutyric acid, β-guanidinopropionic acid, guanidinoethane sulfonic acid and α-guanidinoglutaric acid, are present in the mammalian brain. Although creatine and phosphocreatine play important roles in energy homeostasis in the brain, accumulation of GCs may induce epileptic discharges and convulsions. This review focuses on how physiologically important and/or neurotoxic GCs are distributed in the brain under physiological and pathological conditions. Transporters for GCs at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid (CSF) barrier (BCSFB) have emerged as substantial contributors to GCs distribution in the brain. Creatine transporter (CRT/solute carrier (SLC) 6A8) expressed at the BBB regulates creatine concentration in the brain, and represents a major pathway for supply of creatine from the circulating blood to the brain. CRT may be a key factor facilitating blood-to-brain guanidinoacetate transport in patients deficient in S-adenosylmethionine:guanidinoacetate N-methyltransferase, the creatine biosynthetic enzyme, resulting in cerebral accumulation of guanidinoacetate. CRT, taurine transporter (TauT/SLC6A6) and organic cation transporter (OCT3/SLC22A3) expressed at the BCSFB are involved in guanidinoacetic acid or creatinine efflux transport from CSF. Interestingly, BBB efflux transport of GCs, including guanidinoacetate and creatinine, is negligible, though the BBB has a variety of efflux transport systems for synthetic precursors of GCs, such as amino acids and neurotransmitters. Instead, the BCSFB functions as a major cerebral clearance system for GCs. In conclusion, transport of GCs at the BBB and BCSFB appears to be the key determinant of the cerebral levels of GCs, and changes in the transport characteristics may cause the abnormal distribution of GCs in the brain seen in patients with certain neurological disorders.

HIV-associated neurocognitive disorder: pathogenesis and therapeutic opportunities

Human immunodeficiency virus type 1 (HIV) infection presently affects more that 40 million people worldwide, and is associated with central nervous system (CNS) disruption in at least 30% of infected individuals. The use of highly active antiretroviral therapy has lessened the incidence, but not the prevalence of mild impairment of higher cognitive and cortical functions (HIV-associated neurocognitive disorders) as well as substantially reduced a more severe form dementia (HIV-associated dementia). Furthermore, improving neurological outcomes will require novel, adjunctive therapies that are targeted towards mechanisms of HIV-induced neurodegeneration. Identifying such molecular and pharmacological targets requires an understanding of the events preceding irreversible neuronal damage in the CNS, such as actions of neurotoxins (HIV proteins and cellular factors), disruption of ion channel properties, synaptic damage, and loss of adult neurogenesis. By considering the specific mechanisms and consequences of HIV neuropathogenesis, unified approaches for neuroprotection will likely emerge using a tailored, combined, and non-invasive approach.

Activation of ERK/MAPK in the lateral amygdala of the mouse is required for acquisition of a fear-potentiated startle response

There is considerable interest in examining the genes that may contribute to anxiety. We examined the function of ERK/MAPK in the acquisition of conditioned fear, as measured by fear-potentiated startle (FPS) in mice as a model for anticipatory anxiety in humans. We characterized the following for the first time in the mouse: (1) the expression of the ERK/MAPK signaling pathway components at the protein level in the lateral amygdala (LA); (2) the time course of activation of phospho-activated MAPK in the LA after fear conditioning; (3) if pharmacological inhibition of pMAPK could modulate the acquisition of FPS; (4) the cell-type specificity of pMAPK in the LA after fear conditioning. Using western blot and immunohistochemistry techniques and injecting the MEK inhibitor U0126 in the LA, we showed the following: (1) both MEK1/MEK2 and ERK1/ERK2 were co-expressed in the LA of the adult mouse brain; (2) there is a peak of pMAPK at 60 min after fear conditioning; (3) the ERK/MAPK signaling pathway activation is essential for the acquisition of an FPS response; (4) at 60 min, the pMAPK are exclusively neuronal and not glial. These results emphasize the importance of this signaling pathway in the acquisition of conditioned fear in the mouse. Given the widely held view that conditioned fear models the essential aspects of anxiety disorders, the results confirm the ERK/MAPK signaling pathway as a molecular target for the treatment of anxiety disorders in the clinic.

Organization and postembryonic development of glial cells in the adult central brain of Drosophila

Glial cells exist throughout the nervous system, and play essential roles in various aspects of neural development and function. Distinct types of glia may govern diverse glial functions. To determine the roles of glia requires systematic characterization of glia diversity and development. In the adult Drosophila central brain, we identify five different types of glia based on its location, morphology, marker expression, and development. Perineurial and subperineurial glia reside in two separate single-cell layers on the brain surface, cortex glia form a glial mesh in the brain cortex where neuronal cell bodies reside, while ensheathing and astrocyte-like glia enwrap and infiltrate into neuropils, respectively. Clonal analysis reveals that distinct glial types derive from different precursors, and that most adult perineurial, ensheathing, and astrocyte-like glia are produced after embryogenesis. Notably, perineurial glial cells are made locally on the brain surface without the involvement of gcm (glial cell missing). In contrast, the widespread ensheathing and astrocyte-like glia derive from specific brain regions in a gcm-dependent manner. This study documents glia diversity in the adult fly brain and demonstrates involvement of different developmental programs in the derivation of distinct types of glia. It lays an essential foundation for studying glia development and function in the Drosophila brain.

Posttraumatic epilepsy: hemorrhage, free radicals and the molecular regulation of glutamate

Traumatic brain injury causes development of posttraumatic epilepsy. Bleeding within neuropil is followed by hemolysis and deposition of hemoglobin in neocortex. Iron from hemoglobin and transferring is deposited in brains of patients with posttraumatic epilepsy. Iron compounds form reactive free radical oxidants. Microinjection of ferric ions into rodent brain results in chronic recurrent seizures and liberation of glutamate into the neuropil, as is observed in humans with epilepsy. Termination of synaptic effects of glutamate is by removal via transporter proteins. EAAC-1 is within neurons while GLT-1 and GLAST are confined to glia. Persistent down regulation of GLAST production is present in hippocampal regions in chronic seizure models. Down regulation of GLAST may be fundamental to a sequence of free radical reactions initiated by brain injury with hemorrhage. Administration of antioxidants to animals causes interruption of the sequence of brain injury responses induced by hemorrhage, suggesting that such a strategy needs to be evaluated in patients with traumatic brain injury.

Up-regulation of L type amino acid transporter 1 after spinal cord injury in rats

BACKGROUND

L-type amino acid transporter 1 (LAT1) is proposed to be a major nutrient transporter at the blood brain barrier. LAT1 requires the heavy chain of 4F2 cell surface antigen (4F2hc) for functional expression.

METHODS

We investigated the expression of this heterodimeric transporter after traumatic spinal cord injury in rat by using immunohistochemical and western blot analyses.

FINDINGS

LAT1 immunoreactivities were up-regulated in the capillary endothelia in close to the injury epicenter 24 hours after injury. It reached a peak at 48 hours after injury, and thereafter decreased. 4F2hc was abundant and unchanged all through the time course after SCI. Western blot analysis under reductive and non-reductive conditions showed that LAT1 and 4F2hc were conjugated as a heterodimeric transporter and the functional regulation was dependent on the light chain, LAT1.

CONCLUSIONS

We suggest that LAT1 may be transiently upregulated as part of the tissue-repair process after traumatic contusion injury in the spinal cord.

Focal reduction of neuronal glutamate transporters in human neocortical epilepsy

PURPOSE

To study the differential expression of excitatory amino acid transporters (EAATs) at localized epileptic foci compared to nonepileptic regions in human neocortical epilepsy. Decreased expression of EAATs, the predominant mechanism to remove synaptic-released glutamate, may explain mechanisms of heightened excitability at these epileptic foci.

METHODS

The differential expression of EAAT1-4 at the mRNA and protein levels was measured in electrically mapped human neocortical tissues using quantitative real-time PCR and immunoblotting. This required a novel way to prevent aggregation of EAAT proteins through cold solubilization. Layer-specific neuronal densities were measured to control for potential differences in neuronal density.

RESULTS

While focal epileptic brain regions show marked increases in immediate early genes, they have significant reductions in the neuronal glutamate transporter mRNAs (EAAT3 and EAAT4). These changes were not associated with changes in relative neuronal density, suggesting a reduction in EAAT mRNA per neuron. Immunohistochemical staining of epileptic human neocortex confirmed the presence of EAAT1 and EAAT2 proteins in astroglial cells and EAAT3 and EAAT4 proteins in human cortical neurons. At the protein level, western blots of the same epileptic and nonepileptic regions for a subset of these patients showed a similar decrease of EAAT3 and EAAT4. Despite no change in EAAT2 mRNA, EAAT2 protein expression was significantly reduced at epileptic foci.

CONCLUSION

Regional reductions in EAAT expression at human neocortical epileptic foci could produce increased local glutamate levels that in turn may contribute to both hyperexcitability and the spontaneous generation of epileptic discharges that characterize human epileptic foci.

Differential mRNA distribution of components of the ERK/MAPK signalling cascade in the adult mouse brain

The mitogen-activated protein kinases (MAPKs), also called extracellular signal-regulated kinases (ERKs), are a group of serine/threonine terminal protein kinases activated downstream of a pleiotrophy of transmembrane receptors. Main intracellular components of the MAPK signalling pathway are the RAF, MEK, and ERK proteins, which work in a cascade of activator and effector proteins. They regulate many fundamental cellular functions, including cell proliferation, cell survival, and cell differentiation by transducing extracellular signals to cytoplasmic and nuclear effectors. To reveal more details about possible activation cascades in this pathway, the present study gives a complete description of the differential expression of Braf, Mek1, Mek2, Mek5, Erk1, Erk2, Erk3, and Erk5 in the adult murine brain by way of in situ hybridization analysis. In this study, we found that each gene is widely expressed in the whole brain, except for Mek2, but each displays a very distinct expression pattern, leading to distinct interactions of the MAPK components within different regions. Most notably we found that 1) Braf and Erk3 are coexpressed in the hippocampus proper, confirming a possible functional interaction; 2) in most forebrain areas, Mek5 and Erk5 are coexpressed; and 3) in the neurogenic regions of the brain, namely, the olfactory bulb and the dentate gyrus, Braf is absent, indicating that other activator proteins have to take over its function. Despite these differences, our results show widespread coexpression of the pathway components, thereby confirming the hypothesis of redundant functions among several MEK and ERK proteins in some regions of the brain.

Autoantigen specific T cells inhibit glutamate uptake in astrocytes by decreasing expression of astrocytic glutamate transporter GLAST: a mechanism mediated by tumor necrosis factor‐α

Glutamate excitotoxicity is increasingly being recognized as a pathogenic mechanism in autoimmune inflammatory disorders of the central nervous system (CNS). Astrocytes are the predominant players in clearing the extracellular space from glutamate and normally have extensive spare capacities in terms of glutamate uptake. We asked what might be the basis of glutamate accumulation in T cell triggered autoimmune inflammation. In vitro, coculture of primary rat astrocytes with activated myelin basic protein (MBP)-specific T cells resulted in a decrease of astrocytic glutamate uptake rates (Vmax). In parallel, the amount of the Na+-dependent glutamate transporter GLAST was reduced within 48-60 h. Significant decreases of GLAST protein were observed in astrocytes harvested after incubation with T cells activated by MBP during coculture or after incubation with T cell blasts preactivated in the presence of splenocytes beforehand. Since exposure of astrocytes to cell-free supernatant of MBP-activated T cells also resulted in reduced expression of GLAST, a humoral factor appeared to be the driving agent. In blocking experiments using neutralizing antibodies and by incubation of astrocytes with recombinant cytokines, tumor necrosis factor-alpha (TNF-alpha) was identified as being responsible for the down-modulation of GLAST. GLAST was also down-regulated in the CNS of autoimmune encephalomyelitic rats but not in animals suffering from systemic inflammation. Since the loss of GLAST was not confined to inflammatory infiltrates, here too, a humoral factor seemed to be causative. In conclusion, T cell derived TNF-alpha impairs glutamate clearance capacity of astrocytes in vitro and probably also in vivo providing a pathogenic link to glutamate excitotoxicity that may contribute to early axonal dysfunction remote from active autoimmune inflammatory demyelination.

Neutral amino acid transport in epithelial cells and its malfunction in Hartnup disorder

Hartnup disorder is an autosomal recessive abnormality of renal and gastrointestinal neutral amino acid transport. A corresponding transport activity has been characterized in kidney and intestinal cells and named system B(0). The failure to resorb amino acids in this disorder is thought to be compensated by a protein-rich diet. However, in combination with a poor diet and other factors, more severe symptoms can develop in Hartnup patients, including a photosensitive pellagra-like skin rash, cerebellar ataxia and other neurological symptoms. Homozygosity mapping in a Japanese family and linkage analysis on six Australian pedigrees placed the Hartnup disorder gene at a locus on chromosome 5p15. This fine mapping facilitated a candidate gene approach within the interval, which resulted in the cloning and characterization of a novel member of the sodium-dependent neurotransmitter transporter family (B(0)AT1, SLC6A19) from mouse and human kidney, which shows all properties of system B(0). Flux experiments and electrophysiological recording showed that the transporter is Na(+) dependent and Cl(-) independent, electrogenic and actively transports most neutral amino acids. In situ hybridization showed strong expression in intestinal villi and in the proximal tubule of the kidney. Expression of B(0)AT1 was restricted to kidney, intestine and skin. A total of ten mutations have been identified in SLC6A19 that co-segregate with disease in the predicted recessive manner, with the majority of affected individuals being compound heterozygotes. These mutations lead to altered neutral amino acid transport function compared to the wild-type allele in vitro. One of the mutations occurs in members of the original Hartnup family described in 1956, thereby defining SLC6A19 as the 'Hartnup'-gene.

Glyoxal inactivates glutamate transporter-1 in cultured rat astrocytes

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive motor paralysis and selective motor neuron death. There is increasing evidence that motor neuron death in ALS is mediated by glutamate toxicity resulting from reduced activity of astrocytic glutamate transporter-1 (GLT-1). Recent morphological studies have shown that Nepsilon-(carboxymethyl)lysine (CML) accumulates in reactive astrocytes of ALS spinal cords. CML is a product of post-translational protein modification by glyoxal, a reactive aldehydic intermediate. In considering these documents, it is important to determine whether GLT-1 protein modification by glyoxal might cause reduced GLT-1 activity. To address this issue, we investigated the effects of glyoxal on GLT-1 properties in cultured rat astrocytes. High performance liquid chromatography showed reduced glutamate uptake activity in the glyoxal-exposed cells. Immunocytochemical analysis displayed CML accumulation in the cytoplasm of astrocytes by glyoxal exposure. Immunoblots of immunoprecipitated GLT-1 disclosed GLT-1 CML adduct formation in the glyoxal-exposed cells. Our results indicate that glyoxal modifies GLT-1 to form CML and simultaneously deprives its glutamate uptake activity. Thus, these toxic effects of glyoxal on astrocytes might be implicated in motor neuron death in ALS.

Gliotoxicity in hippocampal cultures is induced by transportable, but not by nontransportable, glutamate uptake inhibitors

Extracellular glutamate is kept below a toxic level by glial and neuronal glutamate transporters. Here we show that the transportable glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate (t-PDC) induced cell death in mature, but not in immature, hippocampal neuron-enriched cultures. The cell death produced by a 24-hr treatment with t-PDC was dose-dependent and reached 85% of the cell population at a 250 microM concentration at 23 days in vitro (DIV). Immunocytochemistry experiments showed that, under these experimental conditions, t-PDC killed not only neurons as expected but also glial cells. The N-methyl-D-aspartate (NMDA) antagonist D-2-aminophosphonovalerate (D-APV; 250 microM) only partially reversed this toxicity, completely protecting the neuronal cell population but not the glial population. The antioxidant compounds alpha-tocopherol or Trolox, used at concentrations that reverse the oxidative stress-induced toxicity, did not block the gliotoxicity specifically produced by t-PDC in the presence of D-APV. The nontransportable glutamate uptake inhibitor DL-threo-beta-benzyloxyaspartate (TBOA) elicited cell death only in mature, but not in immature, hippocampal cultures. The TBOA toxic effect was dose dependent and reached a plateau at 100 microM in 23-DIV cultures. About 50% of the cell population died. TBOA affected essentially the neuronal population. D-APV (250 microM) completely reversed this toxicity. It is concluded that nontransportable glutamate uptake inhibitors are neurotoxic via overactivation of NMDA receptors, whereas transportable glutamate uptake inhibitors induce both an NMDA-dependent neurotoxicity and an NMDA- and oxidative stress-independent gliotoxicity, but only in mature hippocampal cultures.

Characterization of receptors for glutamate and GABA in retinal neurons

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.

Evidence of neuronal excitatory amino acid carrier 1 expression in rat dorsal root ganglion neurons and their central terminals

The expression and distribution of the neuronal glutamate transporter, excitatory amino acid carrier-1 (EAAC1), are demonstrated in the dorsal root ganglion neurons and their central terminals. Reverse transcriptase-polymerase chain reaction shows expression of EAAC1 mRNA in the dorsal root ganglion. Immunoblotting analysis further confirms existence of EAAC1 protein in this region. Immunocytochemistry reveals that approximately 46.6% of the dorsal root ganglion neurons are EAAC1-positive. Most EAAC1-positive neurons are small and around 250-750 microm2 in surface area, and some co-label with calcitonin gene-related peptide (CGRP) or isolectin IB4. In the spinal cord, EAAC-1 immunoreactive small dot- or patch-like structures are mainly localized in the superficial dorsal horn, and some are positive for CGRP or labeled by isolectin IB4. Unilateral dorsal rhizotomy experiments further show that EAAC1 immunoreactivity is less intense in superficial dorsal horn on the side ipsilateral to the dorsal rhizotomy than on the contralateral side. The results indicate the presence of EAAC1 in the dorsal root ganglion neurons and their central terminals. Our findings suggest that EAAC1 might play an important role in transmission and modulation of nociceptive information via the regulation of pre-synaptically released glutamate.

Inducible antisense RNA targeting amino acid transporter ATB0/ASCT2 elicits apoptosis in human hepatoma cells

Amino acid transporter B(0)/ASC transporter 2 (ATB(0)/ASCT2) is responsible for most glutamine uptake in human hepatoma cells. Because this transporter is not expressed in normal hepatocytes, we hypothesized that its expression is necessary for growth of human liver cancer cells. To test this hypothesis, Sloan Kettering hepatoma (SK-Hep) cells were stably transfected with an inducible 1.3-kb ATB(0)/ASCT2 antisense RNA expression plasmid under the transcriptional control of mifepristone, a synthetic steroid. Induced antisense RNA expression in monolayer cultures decreased ATB(0)/ASCT2 mRNA levels by 73% and glutamine transport rates by 65% compared with controls after 24 h, leading to a 98% decrease in cell number after 48 h. Cellular death was attributable to apoptosis based on cellular blebbing, caspase-3 activation, vital dye and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling staining, and poly-(ADP-ribose) polymerase (PARP) cleavage. Transporter knockdown also markedly increased activities of caspases-2 and -9, marginally enhanced caspase-8 activity, and dramatically increased ASCT1 mRNA levels, presumably as a futile compensatory response. Apoptosis elicited via transporter silencing was not attributable to the double-stranded RNA-dependent protein kinase R (PKR) pathway. For comparison, glutamine deprivation also caused apoptotic cell death but with slower temporal kinetics, stimulated caspases-2 and -3 but not caspases-8 or -9 activities, and led to considerable PARP cleavage. Thus ASCT2 suppression exerts proapoptotic effects transcending those of glutamine starvation alone. We conclude that ATB(0)/ASCT2 expression is necessary for SK-Hep cell growth and viability and suggest that it be further explored as a selective target for human hepatocellular carcinoma.

Nicergoline Enhances Glutamate Uptake via Glutamate Transporters in Rat Cortical Synaptosomes

To elucidate the mechanisms of neuroprotective action of nicergoline, we examined its effect on glutamate transport in rat cortical synaptosomes and cloned glutamate transporters. In synaptosomes, nicergoline enhanced the glutamate uptake at 1-10 microM in standard medium and suppressed the increase of extracellular glutamate by reversed transport in low Na(+) medium. Apparent increase of extracellular glutamate concentration by dihydrokinate, an inhibitor of glial glutamate transporter GLT-1, was antagonized by nicergoline. In Xenopus oocytes expressing mouse neuronal glutamate transporter (mEAAC1), the glutamate-induced inward current was enhanced by nicergoline. These results suggest that nicergoline reduces the extracellular glutamate concentration through its effect on glutamate transporters.

Early loss of the glutamate transporter splice-variant GLT-1v in rat cerebral cortex following lateral fluid-percussion injury

Glutamate transporter proteins are essential for the control of interstitial glutamate levels, with an impairment of their function or levels being a major potential contributor to excitotoxicity. We have investigated the effects of lateral fluid percussion on the levels of the glutamate transporter proteins GLT-1alpha, its splice variant GLT-1v, GLAST, and EAAC1 in the rat in order to evaluate their pathogenetic role in this model of traumatic brain injury (TBI). Immunoblot analysis revealed neuronal loss in the cerebral cortex was accompanied by a 54% decrease in GLT-1v 6 h following the insult which progressed to an 83% loss of the transporter after 24 h. No changes in GLT-1alpha, GLAST, or EAAC1 were observed in this brain region at either time point. GLT-1v content was also decreased by 55% and 68% in the hippocampus and thalamus, respectively, at 6 h post-injury, but recovered fully after 24 h in both brain regions. In contrast, levels of GLT-1alpha were increased in the hippocampus at 6 h and 24 h post-TBI. These alterations in transporter protein content were also confirmed using immunohistochemical methods. Our results show for the first time a pattern of early, dynamic changes in the levels of GLT-1 transporter splice variants in different brain regions in this trauma model. In addition, correlation of GLT-1v levels with both neuronal cell loss and alpha-internexin content in the injured cortex suggests that loss of this novel glutamate transporter may be a key factor in determining cerebral vulnerability following this type of brain injury.

Measurement of Glutamate Uptake and Reversed Transport by Rat Synaptosome Transporters

To establish an assay system for evaluation of the uptake and reversed transport of glutamate, we examined the effects of Na(+)-concentration and pharmacological agents on the extracellular glutamate concentration ([Glu](o)) in rat cortical synaptosomes in vitro. There was a decrease and increase of the [Glu](o) at high and low Na(+) concentrations, respectively, in a Ca(2+)-free medium. The changes in [Glu](o) in both directions were temperature-sensitive, and reversed at around 30 mM of Na(+). Dihydrokainate (DHK), a non-transportable inhibitor selective for glial glutamate transporter GLT-1, suppressed the decrease in [Glu](o), and the reversal of [Glu](o) change was shifted to about 60 mM Na(+). There was no change in the maximum [Glu](o) at total Na(+) substitution. Further pharmacological analysis revealed that D-aspartate and DL-threo-beta-hydroxy-aspartate (THA), transportable substrates of glutamate transporters, increased the [Glu](o) in standard media. In contrast, beta-phenylglutamic acid, a structural analogue of glutamate, suppressed both the decrease in [Glu](o) in standard medium and the increase in [Glu](o) in low Na(+) medium. It is, thus, concluded that both the direction and the amount of [Glu](o) changes are determined by a balance of the uptake and reversed transport of glutamate, and that this assay system is suitable for evaluation of the effect of this on glutamate transporters.

Synaptosomal glutamate and GABA transport in patients with temporal lobe epilepsy

High-affinity glutamate and GABA transporters found in the plasma membrane of neurons and glial cells terminate neurotransmission by rapidly removing extracellular transmitter. Impairment of transporter function has been implicated in the pathophysiologic mechanisms underlying epileptogenesis. We characterized glutamate and gamma-aminobutyric acid (GABA) transport in synaptosomes, isolated from neocortical and hippocampal biopsies of patients with temporal lobe epilepsy (TLE). We analyzed K(+)-evoked release in the presence and absence of Ca(2+) to determine vesicular and transporter-mediated release, respectively. We also analyzed (3)H-glutamate and (3)H-GABA uptake, the effect of glutamate uptake inhibitors L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) and DL-threo-beta-benzyloxyaspartate (TBOA), and GABA uptake inhibitor N-(4,4-diphenyl-3-butenyl)-3-piperidinecarboxylic acid (SK&F 89976-A). Neocortical synaptosomes from TLE patients did not show vesicular glutamate release, strongly reduced transporter-mediated release, and an increased basal release compared to that in rat synaptosomes. Furthermore, basal release was less sensitive to tPDC, and (3)H-glutamate uptake was reduced compared to that in rat synaptosomes. Vesicular GABA release from neocortical synaptosomes of TLE patients was reduced compared to that in rat synaptosomes, whereas transporter-mediated release was hardly affected. Furthermore, basal GABA release was more than doubled, but neither basal nor stimulated release were increased by SK&F 89976-A, which did significantly increase both types of GABA release in rat synaptosomes. Finally, (3)H-GABA uptake by synaptosomes from TLE patients was reduced significantly in hippocampus (0.19 +/- 0.04%), compared to that in neocortex (0.32 +/- 0.04%). Control experiments with human peritumoral cortical tissue suggest that impaired uptake of glutamate, but not of GABA, was caused in part by the hypoxic state of the biopsy. Our findings provide evidence for impaired function of glutamate and GABA transporters in human TLE.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

Identification of a novel system L amino acid transporter structurally distinct from heterodimeric amino acid transporters

A cDNA that encodes a novel Na+-independent neutral amino acid transporter was isolated from FLC4 human hepatocarcinoma cells by expression cloning. When expressed in Xenopus oocytes, the encoded protein designated LAT3 (L-type amino acid transporter 3) transported neutral amino acids such as l-leucine, l-isoleucine, l-valine, and l-phenylalanine. The LAT3-mediated transport was Na+-independent and inhibited by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, consistent with the properties of system L. Distinct from already known system L transporters LAT1 and LAT2, which form heterodimeric complex with 4F2 heavy chain, LAT3 was functional by itself in Xenopus oocytes. The deduced amino acid sequence of LAT3 was identical to the gene product of POV1 reported as a prostate cancer-up-regulated gene whose function was not determined, whereas it did not exhibit significant similarity to already identified transporters. The Eadie-Hofstee plots of LAT3-mediated transport were curvilinear, whereas the low affinity component is predominant at physiological plasma amino acid concentration. In addition to amino acid substrates, LAT3 recognized amino acid alcohols. The transport of l-leucine was electroneutral and mediated by a facilitated diffusion. In contrast, l-leucinol, l-valinol, and l-phenylalaninol, which have a net positive charge induced inward currents under voltage clamp, suggesting these compounds are transported by LAT3. LAT3-mediated transport was inhibited by the pretreatment with N-ethylmaleimide, consistent with the property of system L2 originally characterized in hepatocyte primary culture. Based on the substrate selectivity, affinity, and N-ethylmaleimide sensitivity, LAT3 is proposed to be a transporter subserving system L2. LAT3 should denote a new family of organic solute transporters.

The glutamate/neutral amino acid transporter family SLC1: molecular, physiological and pharmacological aspects

The solute carrier family 1 (SLC1) includes five high-affinity glutamate transporters, EAAC1, GLT-1, GLAST, EAAT4 and EAAT5 (SLC1A1, SLC1A2, SLC1A3, SLC1A6, and SLC1A7, respectively) as well as the two neutral amino acid transporters, ASCT1 and ASCT2 (SLC1A4 and ALC1A5, respectively). Although each of these transporters have similar predicted structures, they exhibit distinct functional properties which are variations of a common transport mechanism. The high-affinity glutamate transporters mediate transport of l-Glu, l-Asp and d-Asp, accompanied by the cotransport of 3 Na(+) and 1 H(+), and the countertransport of 1 K(+), whereas ASC transporters mediate Na(+)-dependent exchange of small neutral amino acids such as Ala, Ser, Cys and Thr. The unique coupling of the glutamate transporters allows uphill transport of glutamate into cells against a concentration gradient. This feature plays a crucial role in protecting neurons against glutamate excitotoxicity in the central nervous system. During pathological conditions, such as brain ischemia (e.g. after a stroke), however, glutamate exit can occur due to "reversed glutamate transport", which is caused by a reversal of the electrochemical gradients of the coupling ions. Selective inhibition of the neuronal glutamate transporter EAAC1 (SLC1A1) may be of therapeutic interest to block glutamate release from neurons during ischemia. On the other hand, upregulation of the glial glutamate transporter GLT1 (SLC1A2) may help protect motor neurons in patients with amyotrophic lateral sclerosis (ALS), since loss of function of GLT1 has been associated with the pathogenesis of certain forms of ALS.

Neuronal glutamate transporter EAAT4 is expressed in astrocytes

High-affinity excitatory amino acid transporters (EAATs) are essential to terminate glutamatergic neurotransmission and to prevent excitotoxicity. To date, five distinct EAATs have been cloned from animal and human tissues: GLAST (EAAT1), GLT-1 (EAAT2), EAAC1 (EAAT3), EAAT4, and EAAT5. EAAT1 and EAAT2 are commonly known as glial glutamate transporters, whereas EAAT3, EAAT4, and EAAT5 are neuronal. EAAT4 is largely expressed in cerebellar Purkinje cells. In this study, using immunohistochemistry and Western blotting, we found that EAAT4-like immunoreactivity (ir) is enriched in the spinal cord and forebrain. Double-labeled fluorescent immunostaining and confocal image analysis indicated that EAAT4-like ir colocalizes with an astrocytic marker, glial fibrillary acidic protein (GFAP). The astrocytic localization of EAAT4 was further confirmed in astrocyte cultures by double-labeled fluorescent immunocytochemistry and Western blotting. Reverse transcriptase-polymerase chain reaction analysis demonstrated mRNA expression of EAAT4 in astrocyte cultures. Sequencing confirmed the specificity of the amplified fragment. These results demonstrate that EAAT4 is expressed in astrocytes. This astrocytic localization of neuronal EAAT4 may reveal a new function of EAAT4 in the central nervous system.

The glutamate and neutral amino acid transporter family: physiological and pharmacological implications

The solute carrier family 1 (SLC1) is composed of five high affinity glutamate transporters, which exhibit the properties of the previously described system XAG-, as well as two Na+-dependent neutral amino acid transporters with characteristics of the so-called "ASC" (alanine, serine and cysteine). The SLC1 family members are structurally similar, with almost identical hydropathy profiles and predicted membrane topologies. The transporters have eight transmembrane domains and a structure reminiscent of a pore loop between the seventh and eighth domains [Neuron 21 (1998) 623]. However, each of these transporters exhibits distinct functional properties. Glutamate transporters mediate transport of L-Glu, L-Asp and D-Asp, accompanied by the cotransport of 3 Na+ and one 1 H+, and the countertransport of 1 K+, whereas ASC transporters mediate Na+-dependent exchange of small neutral amino acids such as Ala, Ser, Cys and Thr. Given the high concentrating capacity provided by the unique ion coupling pattern of glutamate transporters, they play crucial roles in protecting neurons against glutamate excitotoxicity in the central nervous system (CNS). The regulation and manipulation of their function is a critical issue in the pathogenesis and treatment of CNS disorders involving glutamate excitotoxicity. Loss of function of the glial glutamate transporter GLT1 (SLC1A2) has been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS), resulting in damage of adjacent motor neurons. The importance of glial glutamate transporters in protecting neurons from extracellular glutamate was further demonstrated in studies of the slc1A2 glutamate transporter knockout mouse. The findings suggest that therapeutic upregulation of GLT1 may be beneficial in a variety of pathological conditions. Selective inhibition of the neuronal glutamate transporter EAAC1 (SLC1A1) but not the glial glutamate transporters may be of therapeutic interest, allowing blockage of glutamate exit from neurons due to "reversed glutamate transport" of EAAC1, which will occur during pathological conditions, such as during ischemia after a stroke.

Effect of zonisamide on molecular regulation of glutamate and GABA transporter proteins during epileptogenesis in rats with hippocampal seizures

Epileptiform discharges and behavioral seizures may be the consequences of excess excitation associated with the neurotransmitter glutamate, or from inadequate inhibitory effects associated with gamma-aminobutyric acid (GABA). Synaptic effects of these neurotransmitters are terminated by the action of transporter proteins that remove amino acids from the synaptic cleft. Excitation initiated by the synaptic release of glutamate is attenuated by the action of glial transporters glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1), and the neuronal transporter excitatory amino-acid carrier-1 (EAAC-1). GABA is removed from synaptic regions by the action of the transporters proteins GABA transporter-1 (GAT-1) and GABA transporter-3 (GAT-3). In this experiment, albino rats with chronic, spontaneous recurrent seizures induced by the amygdalar injection of FeCl3 were treated for 14 days with zonisamide (ZNS) (40 mg/kg, i.p.). Control animals underwent saline injection into the same amygdalar regions. Treatment control for both groups of intracerebrally injected animals was i.p. injection of equal volumes of saline. Western blotting was used to measure the quantity of glutamate and GABA transporters in hippocampus and frontal cortex. ZNS caused increase in the quantity of EAAC-1 protein in hippocampus and cortex and down regulation of the GABA transporter GAT-1. These changes occurred in both experimental and ZNS treated control animals. These data show that the molecular effect of ZNS, with up-regulation of EAAC-1 and decreased production of GABA transporters, should result in increased tissue and synaptic concentrations of GABA. Although many antiepileptic drugs have effects on ion channels when measured in vitro our study suggests that additional mechanisms of action may be operant. Molecular effects on regulation of transporter proteins may aid in understanding epileptogenesis and inform investigators about future design and development of drugs to treat epilepsy.

A mRNA molecule encoding truncated excitatory amino acid carrier 1 (EAAC1) protein (EAAC2) is transcribed from an independent promoter but not an alternative splicing event

Glutamate transporter EAAC1 removes excitatory neurotransmitter in central nervous system, and also absorbs glutamate in epithelia of intestine, kidney, liver and heart for normal cell growth. When a mouse cDNA was screened using EAAC1 cDNA fragment as probe in our lab, a transcript (GenBank U75214) encoding an EAAC1 protein with 148 residues truncated at N-terminal was cloned and named as EAAC2. Sequence analysis shows that EAAC2 has it's own start code and unique 5'UTR that is different from that of EAAC1. A mouse genomic library was screened and a positive clone including EAAC1 CDS was sequenced (GenBank AF 322393) and indicates that normal EAAC1 transcript (GenBank U73521) is transcribed from 10 exons in terms of exon I, II, III, IV, V, VI, VII, VIII, IX, X, and EAAC2 transcript is consisted by exons from IV to IX as same as that of EAAC1 and with its unique exon beta upstream to exon IV and exon delta downstream to IX. EAAC2 transcript has a cluster of transcriptional start sites not overlapping with the transcriptional start sites of EAAC1. These results indicate that EAAC2 is transcribed from an independent promoter but not an alternative splicing event.

N-linked glycosylation and sequence changes in a critical negative control region of the ASCT1 and ASCT2 neutral amino acid transporters determine their retroviral receptor functions

A widely dispersed interference group of retroviruses that includes the feline endogenous virus (RD114), baboon endogenous virus (BaEV), human endogenous virus type W (HERV-W), and type D primate retroviruses uses the human Na(+)-dependent neutral amino acid transporter type 2 (hASCT2; gene name, SLC1A5) as a common cell surface receptor. Although hamster cells are fully resistant to these viruses and murine cells are susceptible only to BaEV and HERV-W pseudotype viruses, these rodent cells both become highly susceptible to all of the viruses after treatment with tunicamycin, an inhibitor of protein N-linked glycosylation. A partial explanation for these results was recently provided by findings that the orthologous murine transporter mASCT2 is inactive as a viral receptor, that a related (ca. 55% identity) murine paralog (mASCT1; gene name, SLC1A4) mediates infections specifically of BaEV and HERV-W, and that N-deglycosylation of mASCT1 activates it as a receptor for all viruses of this interference group. Because the only two N-linked oligosaccharides in mASCT1 occur in the carboxyl-terminal region of extracellular loop 2 (ECL2), it was inferred that this region contributes in an inhibitory manner to infections by RD114 and type D primate viruses. To directly and more thoroughly investigate the receptor active sites, we constructed and analyzed a series of hASCT2/mASCT2 chimeras and site-directed mutants. Our results suggest that a hypervariable sequence of 21 amino acids in the carboxyl-terminal portion of ECL2 plays a critical role in determining the receptor properties of ASCT2 proteins for all viruses in this interference group. In addition, we analyzed the tunicamycin-dependent viral susceptibility of hamster cells. In contrast to mASCT1, which contains two N-linked oligosaccharides that partially restrict viral infections, hamster ASCT1 contains an additional N-linked oligosaccharide clustered close to the others in the carboxyl-terminal region of ECL2. Removal of this N-linked oligosaccharide by mutagenesis enabled hamster ASCT1 to function as a receptor for all viruses of this interference group. These results strongly suggest that combinations of amino acid sequence changes and N-linked oligosaccharides in a critical carboxyl-terminal region of ECL2 control retroviral utilization of both the ASCT1 and ASCT2 receptors.

The glutamate transporters EAAT2 and EAAT3 mediate cysteine uptake in cortical neuron cultures

Cysteine availability is normally the rate-limiting factor in glutathione synthesis. How neurons obtain cysteine from extracellular space is not well established. Here we used mouse cortical neuron cultures to examine the role of the excitatory amino acid transporters (EAATs) in neuronal cysteine uptake. The cultured neurons expressed both EAAT2 and EAAT3. Cysteine uptake was predominantly (> 85%) Na+-dependent, with an apparent Km of 37 microm. Cysteine uptake was reduced by the EAAT substrates l-glutamate and l-aspartate and by synthetic EAAT inhibitors. The non-selective EAAT inhibitor threo-beta-hydroxyaspartate had a significantly greater maximal inhibitory effect than did the EAAT2-selective inhibitor, dihydrokainate, indicating uptake by both EAAT2 and EAAT3. Serine, a substrate of ASC uptake system, had negligible effects on cysteine uptake at 10-fold excess concentrations. To assess the functional importance of EAAT-mediated cysteine uptake in neuronal glutathione synthesis, cultures were treated with diethylmaleate to deplete glutathione, then incubated with cysteine in the presence or absence of EAAT inhibitors. Threo-beta-benzyloxyaspartate and the non-transportable inhibitor threo-beta-hydroxyaspartate both inhibited the cysteine-dependent glutathione synthesis. The findings suggest that neuronal EAAT activity can be a rate-limiting step for neuronal glutathione synthesis and that the primary function of EAATs expressed by neurons in vivo may be to transport cysteine.

Adenosine-induced modulation of excitatory amino acid transport across isolated brain arterioles

OBJECT

Excitatory amino acid (EAA) uptake by neurons and glia acts synergistically with stereoselective transport across the blood-brain barrier (BBB) to maintain EAA homeostasis in the brain. The endogenous neuroprotectant adenosine counteracts many aspects of excitotoxicity by increasing cerebral blood flow and by producing pre- and postsynaptic actions on neurons. In the present study, the authors explored the effect of adenosine on EAA transport across the BBB.

METHODS

The effects of adenosine on the permeability of the BBB and transport of aspartate and glutamate across the BBB were studied in a well-characterized isolated penetrating cerebral arteriole preparation suitable for simultaneous investigations of changes in diameter and permeability. At concentrations within the physiological to low pathophysiological range (10(-7)-10(-6) M), the net vectorial transport of [3H]L-glutamate or [3H]L-aspartate from blood to brain was significantly attenuated, whereas there was no effect of adenosine on paracellular BBB permeability to [14C]sucrose or [3H]D-aspartate. With higher concentrations of adenosine (10(-4) M and 10(-3) M) the net vectorial transport of [3H]L-glutamate and [3H]Laspartate returned toward baseline. At 10(-3) M, the permeability to [14C]sucrose was significantly altered, indicating a breakdown in the BBB. The effect of adenosine (10(-6) M) was blocked by theophylline, a blocker of the A1 and A2 receptors of adenosine.

CONCLUSIONS

Adenosine-mediated modulation of glutamate and aspartate transport across the BBB is a novel physiological finding.

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.

Recent advances in the brain-to-blood efflux transport across the blood-brain barrier

Elucidating the details of the blood-brain barrier (BBB) transport mechanism is a very important step towards successful drug targeting to the brain and understanding what happens in the brain. Although several brain uptake methods have been developed to characterize transport at the BBB, these are mainly useful for investigating influx transport across the BBB. In 1992, P-glycoprotein was found to act as an efflux pump for anti-cancer drugs at the BBB using primary cultured bovine brain endothelial cells. In order to determine the direct efflux transport from the brain to the circulating blood of exogenous compounds in vivo, the Brain Efflux Index method was developed to characterize several BBB efflux transport systems. Recently, we have established conditionally immortalized rat (TR-BBB) and mouse (TM-BBB) brain capillary endothelial cell lines from transgenic rats and mice harboring temperature-sensitive simian virus 40 large T-antigen gene to characterize the transport mechanisms at the BBB in vitro. TR-BBB and TM-BBB cells possess certain in vivo transport functions and express mRNAs for the BBB. Using a combination of newly developed in vivo and in vitro methods, we have elucidated the efflux transport mechanism at the BBB for neurosteroids, excitatory neurotransmitters, suppressive neurotransmitters, amino acids, and other organic anions to understand the physiological role played by the BBB as a detoxifying organ for the brain.

Methylmercury increases glutamate extracellular levels in frontal cortex of awake rats

A current hypothesis about methylmercury (MeHg) neurotoxicity proposes that neuronal damage is due to excitotoxicity following glutamate uptake alterations in the astrocyte. By sampling from a microdialysis probe implanted in the frontal cortex of adult Wistar rats, we measured the effects of acute exposure to either 10 or 100 microM MeHg through the microdialysis probe, on glutamate extracellular levels in 15 awake animals. After baseline measurements, the perfusion of MeHg during 90 min induced immediate and significant elevations in extracellular glutamate at 10 microM (9.8-fold, P<.001) and at 100 microM (2.4-fold, P=.001). This in vivo demonstration of increments of extracellular glutamate supports the hypothesis that dysfunction of glutamate neurotransmission plays a key role in MeHg-induced neural damage.

Identification of a novel Na+-independent acidic amino acid transporter with structural similarity to the member of a heterodimeric amino acid transporter family associated with unknown heavy chains

We identified a novel Na(+)-independent acidic amino acid transporter designated AGT1 (aspartate/glutamate transporter 1). AGT1 exhibits the highest sequence similarity (48% identity) to the Na(+)-independent small neutral amino acid transporter Asc (asc-type amino acid transporter)-2 a member of the heterodimeric amino acid transporter family presumed to be associated with unknown heavy chains (Chairoungdua, A., Kanai, Y., Matsuo, H., Inatomi, J., Kim, D. K., and Endou, H. (2001) J. Biol. Chem. 276, 49390-49399). The cysteine residue responsible for the disulfide bond formation between transporters (light chains) and heavy chain subunits of the heterodimeric amino acid transporter family is conserved for AGT1. Because AGT1 solely expressed or coexpressed with already known heavy chain 4F2hc (4F2 heavy chain) or rBAT (related to b(0,+)-amino acid transporter) did not induce functional activity, we generated fusion proteins in which AGT1 was connected with 4F2hc or rBAT. The fusion proteins were sorted to the plasma membrane and expressed the Na(+)-independent transport activity for acidic amino acids. Distinct from the Na(+)-independent cystine/glutamate transporter xCT structurally related to AGT1, AGT1 did not accept cystine, homocysteate, and l-alpha-aminoadipate and exhibited high affinity to aspartate as well as glutamate, suggesting that the negative charge recognition site in the side chain-binding site of AGT1 would be closer to the alpha-carbon binding site compared with that of xCT. The AGT1 message was predominantly expressed in kidney. In mouse kidney, AGT1 protein was present in the basolateral membrane of the proximal straight tubules and distal convoluted tubules. In the Western blot analysis, AGT1 was detected as a high molecular mass band in the nonreducing condition, whereas the band shifted to a 40-kDa band corresponding to the AGT1 monomer in the reducing condition, suggesting the association of AGT1 with other protein via a disulfide bond. The finding of AGT1 and Asc-2 has established a new subgroup of the heterodimeric amino acid transporter family whose members associate not with 4F2hc or rBAT but with other unknown heavy chains.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Metabolic signaling between neurons and glial cells: a short review

There is convincing evidence that astrocytes transform blood-born glucose to lactate, alpha-Keto-glutarate and alanine and supply the neurons. There is a tight regulation of this metabolic coupling by means of chemical signals released by functioning neurons. Previous, pioneer, studies have explored several signals-candidates the major being K(+), Ca(++) and several neuromodulators. However, recent results of numerous studies identify glutamate as the major signal that traffics between excited neurons and astrocytes. The excited neurons also produce and release NH(4)(+) in the extracellular space. Both glutamate and ammonium are taken up preferentially by astrocytes and form glutamine. Ammonia fixation by glutamine synthase controls the amount of lactate, glutamine and alanine produced and released by Muller cells in the extracellular space and then taken up by neurons. Thus, there is a tight coupling between function and metabolism in the central neurons system.

A novel voltage-sensitive Na(+) and Ca(2+) channel blocker, NS-7, prevents suppression of cyclic AMP-dependent protein kinase and reduces infarct area in the acute phase of cerebral ischemia in rat

Binding of cyclic AMP to the regulatory subunit of cyclic AMP-dependent protein kinase (PKA) is an essential step in cyclic AMP-mediated intracellular signal transduction. This binding is, however, rapidly inhibited in the acute phase of cerebral ischemia, indicating that the signal transduction via PKA is very vulnerable to ischemia, although this signal pathway is very important for neuronal survival in the brain. Several lines of evidence suggest that the activation of voltage-sensitive Na+ and Ca(2+) channels is an important mediator of acute ischemic brain damage. In the present study, therefore, we examined the effect of a novel Na+ and Ca(2+) channel blocker, NS-7 (4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride), on changes in the binding activity of PKA to cyclic AMP in permanent focal cerebral ischemia, which was induced by occlusion of the middle cerebral artery by the intraluminal suture method for 5 h in the rat. NS-7 (1 mg/kg) or saline was intravenously infused 5 min after occlusion. The binding activity of PKA to cyclic AMP and local cerebral blood flow were assessed by the in vitro [(3)H]cyclic AMP binding and the [(14)C]iodoantipyrine methods, respectively. NS-7 significantly suppressed inhibition of the binding activity of PKA to cyclic AMP in the ischemic regions such as the frontal and parietal cortices and the medial region of the caudate-putamen without affecting cerebral blood flow or arterial blood pressure. Infarct area measured in the brain slices stained with cresyl violet was significantly smaller in animals treated with NS-7 than in those treated with saline. Blockade of voltage-sensitive Na+ and Ca(2+) channels by NS-7 was expected to reduce ischemia-induced depolarization and thus prevent a massive formation of free radicals, which is known to inhibit the binding activity of PKA to cyclic AMP. These data clearly indicate that NS-7 provides very efficient neuroprotection in the acute phase of cerebral ischemia, and sustains the normal function of PKA.

Genetics of epilepsy

Understanding the molecular biology of epilepsy is a challenge for modern science. Epilepsy results from alternations in fundamental mechanisms of brain and membrane function. Although an understanding of the mode of inheritance and the etiology of genetic epilepsy syndromes forms the basis for genetic counseling, the development of specific therapies will come from knowing the basic mechanisms of epilepsy. Defining the genes causing epilepsy requires an unambiguous definition of seizure phenotype, along with the stability of that trait, an unremitting clinical course, and an abundance of clinical material. This article reviews the task of defining the genetics of epilepsy and discusses genetic methodology, idiopathic generalized and localization-related partial epilepsies, neuronal migration disorders, progressive myoclonus epilepsies, molecular biology of epileptogenesis, and future research.