Neurotransmitter transporter family cDNAs in a rat midbrain library: 'orphan transporters' suggest sizable structural variations

Critical transporters of methionine and methionine hydroxyl analogue supplements across the intestine: What we know so far and what can be learned to advance animal nutrition

DL-methionine (DL-Met) and its analogue DL-2-hydroxy-4-(methylthio) butanoic acid (DL-methionine hydroxyl analogue or DL-MHA) have been used as nutritional supplements in the diets of farmed raised animals. Knowledge of the intestinal transport mechanisms involved in these products is important for developing dietary strategies. This review provides updated information of the expression, function, and transport kinetics in the intestine of known Met-linked transporters along with putative MHA-linked transporters. As a neutral amino acid (AA), the transport of DL-Met is facilitated by multiple apical sodium-dependent/-independent high-/low-affinity transporters such as ASCT2, B⁰AT1 and rBAT/b^0,+AT. The basolateral transport largely relies on the rate-limiting uniporter LAT4, while the presence of the basolateral antiporter y⁺LAT1 is probably necessary for exchanging intracellular cationic AAs and Met in the blood. In contrast, the intestinal transport kinetics of DL-MHA have been scarcely studied. DL-MHA transport is generally accepted to be mediated simply by the proton-dependent monocarboxylate transporter MCT1. However, in-depth mechanistic studies have indicated that DL-MHA transport is also achieved through apical sodium monocarboxylate transporters (SMCTs). In any case, reliance on either a proton or sodium gradient would thus require energy input for both Met and MHA transport. This expanding knowledge of the specific transporters involved now allows us to assess the effect of dietary ingredients on the expression and function of these transporters. Potentially, the resulting information could be furthered with selective breeding to reduce overall feed costs.

Loratadine and analogues: discovery and preliminary structure-activity relationship of inhibitors of the amino acid transporter B(0)AT2

B(0)AT2, encoded by the SLC6A15 gene, is a transporter for neutral amino acids that has recently been implicated in mood and metabolic disorders. It is predominantly expressed in the brain, but little is otherwise known about its function. To identify inhibitors for this transporter, we screened a library of 3133 different bioactive compounds. Loratadine, a clinically used histamine H1 receptor antagonist, was identified as a selective inhibitor of B(0)AT2 with an IC50 of 4 μM while being less active or inactive against several other members of the SLC6 family. Reversible inhibition of B(0)AT2 was confirmed by electrophysiology. A series of loratadine analogues were synthesized to gain insight into the structure-activity relationships. Our studies provide the first chemical tool for B(0)AT2.

Involvement of the neutral amino acid transporter SLC6A15 and leucine in obesity-related phenotypes

Brain pathways, including those in hypothalamus and nucleus of the solitary tract, influence food intake, nutrient preferences, metabolism and development of obesity in ways that often differ between males and females. Branched chain amino acids, including leucine, can suppress food intake, alter metabolism and change vulnerability to obesity. The SLC6A15 (v7-3) gene encodes a sodium-dependent transporter of leucine and other branched chain amino acids that is expressed by neurons in hypothalamus and nucleus of the solitary tract. We now report that SLC6A15 knockout attenuates leucine's abilities to reduce both: a) intake of normal chow and b) weight gain produced by access to a high fat diet in gender-selective fashions. We identify SNPs in the human SLC6A15 that are associated with body mass index and insulin resistance in males. These observations in mice and humans support a novel, gender-selective role for brain amino acid compartmentalization mediated by SLC6A15 in diet and obesity-associated phenotypes.

Characterization of the transporterB0AT3 (Slc6a17) in the rodent central nervous system

BACKGROUND

The vesicular B0AT3 transporter (SLC6A17), one of the members of the SLC6 family, is a transporter for neutral amino acids and is exclusively expressed in brain. Here we provide a comprehensive expression profile of B0AT3 in mouse brain using in situ hybridization and immunohistochemistry.

RESULTS

We confirmed previous expression data from rat brain and used a novel custom made antibody to obtain detailed co-labelling with several cell type specific markers. B0AT3 was highly expressed in both inhibitory and excitatory neurons. The B0AT3 expression was highly overlapping with those of vesicular glutamate transporter 2 (VGLUT2) and vesicular glutamate transporter 1 (VGLUT1). We also show here that Slc6a17mRNA is up-regulated in animals subjected to short term food deprivation as well as animals treated with the serotonin reuptake inhibitor fluoxetine and the dopamine/noradrenaline reuptake inhibitor bupropion.

CONCLUSIONS

This suggests that the B0AT3 transporter have a role in regulation of monoaminergic as well as glutamatergic synapses.

B(0)AT2 (SLC6A15) is localized to neurons and astrocytes, and is involved in mediating the effect of leucine in the brain

The B(0)AT2 protein is a product of the SLC6A15 gene belonging to the SLC6 subfamily and has been shown to be a transporter of essential branched-chain amino acids. We aimed to further characterize the B(0)AT2 transporter in CNS, and to use Slc6a15 knock out (KO) mice to investigate whether B(0)AT2 is important for mediating the anorexigenic effect of leucine. We used the Slc6a15 KO mice to investigate the role of B(0)AT2 in brain in response to leucine and in particular the effect on food intake. Slc6a15 KO mice show lower reduction of food intake as well as lower neuronal activation in the ventromedial hypothalamic nucleus (VMH) in response to leucine injections compared to wild type mice. We also used RT-PCR on rat tissues, in situ hybridization and immunohistochemistry on mouse CNS tissues to document in detail the distribution of SLC6A15 on gene and protein levels. We showed that B(0)AT2 immunoreactivity is mainly neuronal, including localization in many GABAergic neurons and spinal cord motor neurons. B(0)AT2 immunoreactivity was also found in astrocytes close to ventricles, and co-localized with cytokeratin and diazepam binding inhibitor (DBI) in epithelial cells of the choroid plexus. The data suggest that B(0)AT2 play a role in leucine homeostasis in the brain.

A variant of the neuronal amino acid transporter SLC6A15 is associated with ACTH and cortisol responses and cognitive performance in unipolar depression

Major depressive disorder (MDD) is accompanied by both cognitive impairments and a hyperactivity of the hypothalamic-pituitary-adrenocortical (HPA) system, resulting in an enhanced glucocorticoid secretion. Cortisol acts via mineralocorticoid and glucocorticoid receptors densely located in the hippocampus, a brain area that is important regarding cognitive functions and especially memory functions. Recently, a variant (rs1545843) affecting transcription of the human SLC6A15 gene has been associated with depression in a genome-wide association study. In an animal model, the neuronal amino acid transporter SLC6A15 was found to be decreased in stress-susceptible mice. Against the background of stress impacting on the activity of the HPA axis, we have investigated alterations of adrenocorticotropic hormone (ACTH) and cortisol secretion in the combined dexamethasone/corticotrophin-releasing hormone (Dex/CRH) test as well as memory and attention performance in a sample of 248 patients with unipolar depression and 172 healthy control subjects genotyped for rs1545843. MDD patients carrying the depression-associated AA genotype showed enhanced maximum and area under the curve ACTH and cortisol answers (p = 0.03) as well as an impaired memory and impaired sustained attention performance (p = 0.04) compared to carriers of at least one G allele. No effects of the SLC6A15 variant were found in the healthy control group. Our findings argue for a role of the SLC6A15 gene in ACTH and cortisol secretion during the Dex/CRH test and furthermore in the occurrence of cognitive impairments in unipolar depression.

The solute carrier 6 family of transporters

The solute carrier 6 (SLC6) family of the human genome comprises transporters for neurotransmitters, amino acids, osmolytes and energy metabolites. Members of this family play critical roles in neurotransmission, cellular and whole body homeostasis. Malfunction or altered expression of these transporters is associated with a variety of diseases. Pharmacological inhibition of the neurotransmitter transporters in this family is an important strategy in the management of neurological and psychiatric disorders. This review provides an overview of the biochemical and pharmacological properties of the SLC6 family transporters.

The role of the neutral amino acid transporter B0AT1 (SLC6A19) in Hartnup disorder and protein nutrition

Hartnup disorder (OMIM 234500) is an autosomal recessive disorder, which was first described in 1956 as an aminoaciduria of neutral amino acids accompanied by a variety of symptoms, such as a photo‐sensitive skin‐rash and cerebellar ataxia. The disorder is caused by mutations in the neutral amino acid transporter B⁰AT1 (SLC6A19)1. To date 21 mutations have been identified in more than twenty families. SLC6A19 requires either collectrin or angiotensin‐converting enzyme 2 for surface expression in the kidney and intestine, respectively. This ties SLC6A19 together with more complex functions such as blood‐pressure control, glomerular structure, and exocytosis. © 2009 IUBMB IUBMB Life, 61(6): 591–599, 2009

The orphan transporter Rxt1/NTT4 (SLC6A17) functions as a synaptic vesicle amino acid transporter selective for proline, glycine, leucine, and alanine

Rxt1/NTT4 (SLC6A17) belongs to a gene family of "orphan transporters" whose substrates and consequently functions remain unidentified. Although Rxt1/NTT4 was previously thought to function as a sodium-dependent plasma membrane transporter, recent studies localized the protein to synaptic vesicles of glutamatergic and GABAergic neurons. Here, we provide evidence indicating that Rxt1/NTT4 functions as a vesicular transporter selective for proline, glycine, leucine, and alanine. Using Western blot, immunoprecipitation, immunocytochemistry, and polymerase chain reaction approaches, we demonstrate that PC12 cells express the Rxt1/NTT4 gene and protein. Small interfering RNA (siRNA)-mediated knockdown of Rxt1/NTT4 in PC12 cells resulted in selective reductions in uptake levels for proline, glycine, leucine, and alanine. Likewise, gas chromatography analysis of amino acid content in an enriched synaptic vesicle fraction from wild-type and siRNA-Rxt1/NTT4 PC12 cells revealed that proline, glycine, leucine, and alanine levels were decreased in siRNA-treated cells compared with wild-type cells. Furthermore, Rxt1/NTT4-transfected Chinese hamster ovary (CHO) cells exhibited significant uptake increases of these amino acids compared with mock-transfected CHO cells. Finally, proline uptake in both PC12 cells and Rxt1/NTT4-transfected CHO cells was dependent on the electrochemical gradient maintained by the vacuolar-type H(+)-ATPase. These data indicate that the orphan Rxt1/NTT4 protein functions as a vesicular transporter for proline, glycine, leucine, and alanine, further suggesting its important role in synaptic transmission.

Apical transporters for neutral amino acids: physiology and pathophysiology

Absorption of amino acids in kidney and intestine involves a variety of transporters for different groups of amino acids. This is illustrated by inherited disorders of amino acid absorption, such as Hartnup disorder, cystinuria, iminoglycinuria, dicarboxylic aminoaciduria, and lysinuric protein intolerance, affecting separate groups of amino acids. Recent advances in the molecular identification of apical neutral amino acid transporters has shed a light on the molecular basis of Hartnup disorder and iminoglycinuria.

Deletion of v7-3 (SLC6A15) transporter allows assessment of its roles in synaptosomal proline uptake, leucine uptake and behaviors

V7-3 (SLC6A15) is the prototype for a gene subfamily whose members have sequence homologies to classical Na+- and Cl(-)-dependent neurotransmitter transporters but display unusual features that include characteristic large fourth extracellular loops. Interest in v7-3 has been increased by the elucidation of its expression in neurons located in cerebral cortex, hippocampus, cerebellum, midbrain and olfactory bulb. To help clarify the role of v7-3 in brain functions, we have created and characterized v7-3 knockout mice. These mice lack functional v7-3 protein but are viable and fertile. While our studies were in progress, v7-3 expression was reported to confer transport of proline and branched-chain amino acids in in vitro expression systems [Takanaga, H., Mackenzie, B., Peng, J.B., Hediger, M.A., 2005b. Characterization of a branched-chain amino-acid transporter SBAT1 (SLC6A15) that is expressed in human brain. Biochem. Biophys. Res. Commun. 337, 892-900; Broer, A., Tietze, N., Kowalczuk, S., Chubb, S., Munzinger, M., Bak, L.K., Broer, S., 2006. The orphan transporter v7-3 (slc6a15) is a Na+-dependent neutral amino acid transporter (B0AT2). Biochem. J. 393, 421-430]. Assessment of amino acid uptake into cortical synaptosomes of v7-3 knockouts identified 15% and 40% reductions in sodium-dependent proline and leucine transport, respectively, compared to wild type controls. Despite these biochemical changes, v7-3 knockout mice demonstrate only modest alterations in rotarod performance with aging and lack reproducible alterations in other motor, memory, anxiety or olfactory tests. Compensation for the lack of v7-3 via other amino acid carriers is likely to leave v7-3 knockouts without gross behavioral manifestations. The current results place v7-3 in the context of other brain transporters that accumulate proline and branched-chain amino acids.

Molecular characterization of the first aromatic nutrient transporter from the sodium neurotransmitter symporter family

Nutrient amino acid transporters (NATs, subfamily of sodium neurotransmitter symporter family SNF, a.k.a. SLC6) represent a set of phylogenetically and functionally related transport proteins, which perform intracellular absorption of neutral, predominantly essential amino acids. Functions of NATs appear to be critical for the development and survival in organisms. However, mechanisms of specific and synergetic action of various NAT members in the amino acid transport network are virtually unexplored. A new transporter, agNAT8, was cloned from the malaria vector mosquito Anopheles gambiae (SS). Upon heterologous expression in Xenopus oocytes it performs high-capacity, sodium-coupled (2:1)uptake of nutrients with a strong preference for aromatic catechol-branched substrates, especially phenylalanine and its derivatives tyrosine and L-DOPA,but not catecholamines. It represents a previously unknown SNF phenotype, and also appears to be the first sodium-dependent B0 type transporter with a narrow selectivity for essential precursors of catecholamine synthesis pathways. It is strongly and specifically transcribed in absorptive and secretory parts of the larval alimentary canal and specific populations of central and peripheral neurons of visual-, chemo- and mechano-sensory afferents. We have identified a new SNF transporter with previously unknown phenotype and showed its important role in the accumulation and redistribution of aromatic substrates. Our results strongly suggest that agNAT8 is an important, if not the major, provider of an essential catechol group in the synthesis of catecholamines for neurochemical signaling as well as ecdysozoan melanization and sclerotization pathways, which may include cuticle hardening/coloring, wound curing, oogenesis, immune responses and melanization of pathogens.

The orphan transporter v7-3 (slc6a15) is a Na+-dependent neutral amino acid transporter (B0AT2)

Transporters of the SLC6 (solute carrier 6) family play an important role in the removal of neurotransmitters in brain tissue and in amino acid transport in epithelial cells. In the present study, we demonstrate that mouse v7-3 (slc6a15) encodes a transporter for neutral amino acids. The transporter is functionally and sequence related to B(0)AT1 (slc6a19) and was hence named B(0)AT2. Leucine, isoleucine, valine, proline and methionine were recognized by the transporter, with values of K(0.5) (half-saturation constant) ranging from 40 to 200 microM. Alanine, glutamine and phenylalanine were low-affinity substrates of the transporter, with K(0.5) values in the millimolar range. Transport of neutral amino acids via B(0)AT2 was Na+-dependent, Cl--independent and electrogenic. Superfusion of mouse B(0)AT2-expressing oocytes with amino acid substrates generated robust inward currents. Na+-activation kinetics of proline transport and uptake under voltage clamp suggested a 1:1 Na+/amino acid co-transport stoichiometry. Susbtrate and co-substrate influenced each other's K(0.5) values, suggesting that they share the same binding site. A mouse B(0)AT2-like transport activity was detected in synaptosomes and cultured neurons. A potential role of B(0)AT2 in transporting neurotransmitter precursors and neuromodulators is proposed.

The repertoire of solute carriers of family 6: Identification of new human and rodent genes

Tremendous amount of primary sequence information has been made available from the genome sequencing projects, although a complete annotation and identification of all genes is still far from being complete. Here, we present the identification of two new human genes from the pharmacologically important family of transporter proteins, solute carriers family 6 (SLC6). These were named SLC6A17 and SLC6A18 by HUGO. The human repertoire of SLC6 proteins now consists of 19 functional members and four pseudogenes. We also identified the corresponding orthologues and additional genes from mouse and rat genomes. Detailed phylogenetic analysis of the entire family of SLC6 proteins in mammals shows that this family can be divided into four subgroups. We used Hidden Markov Models for these subgroups and identified in total 430 unique SLC6 proteins from 10 animal, one plant, two fungi, and 196 bacterial genomes. It is evident that SLC6 proteins are present in both animals and bacteria, and that three of the four subfamilies of mammalian SLC6 proteins are present in Caenorhabditis elegans, showing that these subfamilies are evolutionary very ancient. Moreover, we performed tissue localization studies on the entire family of SLC6 proteins on a panel of 15 rat tissues and further, the expression of three of the new genes was studied using quantitative real-time PCR showing expression in multiple central and peripheral tissues. This paper presents an overall overview of the gene repertoire of the SLC6 gene family and its expression profile in rats.

Characterization of a branched-chain amino-acid transporter SBAT1 (SLC6A15) that is expressed in human brain

The SLC6 gene family comprises membrane proteins that transport neurotransmitters, amino acids, or osmolytes. We report the first functional characterization of the human SLC6A15 gene, which codes for a sodium-coupled branched-chain amino-acid transporter 1 (SBAT1). SBAT1 expression is specific to the brain. When expressed in Xenopus oocytes, SBAT1 mediated Na+-coupled transport of hydrophobic, zwitterionic alpha-amino and imino acids. SBAT1 exhibited a strong preference for branched-chain amino acids (BCAA) and methionine (K0.5 80-160 microM). SBAT1 excluded aromatic or charged amino acids, beta-amino acids, glycine, and GABA. SBAT1-mediated transport of amino or imino acids was extremely temperature-dependent (Q10=9) and was inhibited at acidic pH. PKC activation reduced the plasma-membrane population of SBAT1 protein. SBAT1-mediated transport of BCAA, particularly leucine, may be an important source of amino nitrogen for neurotransmitter synthesis in glutamatergic and GABAergic neurons.

Microarray analysis of thyroid hormone-induced changes in mRNA expression in the adult rat brain

To determine which genes in the adult rat brain are regulated by thyroid hormone (TH), we used microarrays to examine the effect of hyperthyroidism on neuron-specific gene expression. Four-month-old male Fisher 344 rats were rendered hyperthyroid by intraperitoneal injection of 3,5,3'-L-triiodothyronine (T3, 15 microg/100 g body weight) for 10 consecutive days. To minimize interindividual variability, pooled cerebral tissue RNA from four-control and five-hyperthyroid rats was hybridized in duplicates to the Affymetrix (Santa Clara, CA) U34N rat neurobiology microarray, which contains probes for 1224 neural-specific genes. Changes in gene expression were considered significant only if they were observed in both pair-wise comparisons as well as by Northern blot analysis. Hyperthyroidism was associated with modest changes in the expression of only 11 genes. The expression of the phosphodiesterase Enpp2, myelin oligodendrocyte glycoprotein (Mog), microtubule-associated protein 2 (MAP2), growth hormone (GH), Ca(2+)/calmodulin-dependent protein kinase beta-subunit (Camk2b), neuron-specific protein PEP-19 (Pcp4), a sodium-dependent neurotransmitter, and the myelin-associated glycoprotein (S-MAG) was significantly increased. Three genes were suppressed by hyperthyroidism, including the activity and neurotransmitter-induced early genes-1 and -7 (ANIA-1 and ANIA-7) and the guanine nucleotide-binding protein one (Gnb1). The present study underscores the paucity of TH responsive genes in adult cerebral tissue.

Altered mRNA expression of genes related to nerve cell activity in the fracture callus of older rats: A randomized, controlled, microarray study

Background

The time required for radiographic union following femoral fracture increases with age in both humans and rats for unknown reasons. Since abnormalities in fracture innervation will slow skeletal healing, we explored whether abnormal mRNA expression of genes related to nerve cell activity in the older rats was associated with the slowing of skeletal repair.

Methods

Simple, transverse, mid-shaft, femoral fractures with intramedullary rod fixation were induced in anaesthetized female Sprague-Dawley rats at 6, 26, and 52 weeks of age. At 0, 0.4, 1, 2, 4, and 6 weeks after fracture, a bony segment, one-third the length of the femur, centered on the fracture site, including the external callus, cortical bone, and marrow elements, was harvested. cRNA was prepared and hybridized to 54 Affymetrix U34A microarrays (3/age/time point).

Results

The mRNA levels of 62 genes related to neural function were affected by fracture. Of the total, 38 genes were altered by fracture to a similar extent at the three ages. In contrast, eight neural genes showed prolonged down-regulation in the older rats compared to the more rapid return to pre-fracture levels in younger rats. Seven genes were up-regulated by fracture more in the younger rats than in the older rats, while nine genes were up-regulated more in the older rats than in the younger.

Conclusions

mRNA of 24 nerve-related genes responded differently to fracture in older rats compared to young rats. This differential expression may reflect altered cell function at the fracture site that may be causally related to the slowing of fracture healing with age or may be an effect of the delayed healing.

Hypertension and impaired glycine handling in mice lacking the orphan transporter XT2

A family of orphan transporters has been discovered that are structurally related to the Na(+)-Cl(-)-dependent neurotransmitter transporters, including the dopamine transporter. One member of this family, the mouse XT2 gene, is predominantly expressed in the kidney and has 95% homology to rat ROSIT (renal osmotic stress-induced Na(+)-Cl(-) organic solute cotransporter). To study the physiological functions of this transporter, we generated XT2-knockout mice by gene targeting. XT2(-/-) mice develop and survive normally with no apparent abnormalities. To attempt to identify potential substrates for XT2, we screened urine from XT2-knockout mice by high-pressure liquid chromatography and mass spectrometry and found significantly elevated concentrations of glycine. To study glycine handling, XT2(+/+) and XT2(-/-) mice were injected with radiolabeled glycine, and urine samples were collected to monitor glycine excretion. After 2 h, XT2(-/-) mice were found to excrete almost twice as much glycine as the XT2(+/+) controls (P = 0.03). To determine whether the absence of the XT2 transporter affected sodium and fluid homeostasis, we measured systolic blood pressure by computerized tail-cuff manometry. Systolic blood pressure was significantly higher in XT2(-/-) mice (127 +/- 3 mmHg) than in wild-type controls (114 +/- 2 mmHg; P < 0.001). This difference in systolic blood pressure was maintained on high and low salt feeding. To examine whether the alteration in blood pressure and the defect in glycine handling were related, we measured systolic blood pressure in the XT2(-/-) mice during dietary glycine supplementation. Glycine loading caused systolic blood pressure to fall in the XT2(-/-) mice from 127 +/- 3 to 115 +/- 3 mmHg (P < 0.001), a level virtually identical to that of the wild-type controls. These data suggest that the XT2 orphan transporter is involved in glycine reabsorption and that the absence of this transporter is sufficient to cause hypertension.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Characterization of a functional bacterial homologue of sodium-dependent neurotransmitter transporters

The tnaT gene of Symbiobacterium thermophilum encodes a protein homologous to sodium-dependent neurotransmitter transporters. Expression of the tnaT gene product in Escherichia coli conferred the ability to accumulate tryptophan from the medium and the ability to grow on tryptophan as a sole source of carbon. Transport was Na(+)-dependent and highly selective. The K(m) for tryptophan was approximately 145 nm, and tryptophan transport was unchanged in the presence of 100 microM concentrations of other amino acids. Tryptamine and serotonin were weak inhibitors with K(I) values of 200 and 440 microM, respectively. By using a T7 promoter-based system, TnaT with an N-terminal His(6) tag was expressed at high levels in the membrane and was purified to near-homogeneity in high yield.

Cloning and characterization of human NTT5 and v7-3: two orphan transporters of the Na+/Cl- -dependent neurotransmitter transporter gene family

Orphan transporters form a growing subfamily of genes related by sequence similarity to the Na+/Cl- -dependent neurotransmitter superfamily. Using a combination of database similarity searching and cloning methods, we have identified and characterized two novel human orphan transporter genes, v7-3 and NTT5. Similar to other known orphan transporters, v7-3 and NTT5 contain 12 predicted transmembrane domains, intracellular N- and C-terminal domains, and large extracellular loops between transmembrane (TM) domains 3 and 4 and between TM domains 7 and 8. Residues within the extracellular loops are also predicted to contain sites for N-linked glycosylation. Human v7-3, the species orthologue of rat v7-3, contains an open reading frame (ORF) of 730 amino acids. Human NTT5 is a new member of the orphan transporter family and has an ORF of 736 amino acids. The amino acid sequences of human v7-3 and NTT5 are greater than 50% similar to other known orphan neurotransmitter transporters and also show sequence similarity to the human serotonin and dopamine transporters. Radiation hybrid mapping located the human v7-3 and NTT5 genes on chromosomes 12q21.3-q21.4 and 19q13.1-q13.4, respectively. Human mRNA distribution analysis by TaqMan reverse transcription-polymerase chain reaction showed that v7-3 mRNA is predominantly expressed in neuronal tissues, particularly amygdala, putamen, and corpus callosum, with low-level expression in peripheral tissues. In contrast, NTT5 mRNA was highly expressed in peripheral tissues, particularly in testis, pancreas, and prostate. Transient transfection with epitope-tagged transporter constructs demonstrated v7-3 to be expressed at the cell surface, whereas NTT5 was predominantly intracellular, suggestive of a vesicular location. Although the substrates transported by these transporters remain unknown, their specific but widespread distribution suggests that they may mediate distinct and important functions within the brain and the periphery.

Ontogeny of Rxt1, a vesicular "orphan" Na(+)/Cl(-)-dependent transporter, in the rat

The developmental expression of the orphan Na(+)/Cl(-)-dependent transporter, Rxt1, was studied in the rat using a specific [(35)S]complementary RNA probe and affinity purified antibodies. Western blotting experiments allowed the detection of Rxt1 in brain as early as on embryonic day 16. After birth, the brain levels of Rxt1 increased dramatically up to a maximum around postnatal day 30 and then decreased slightly to the adult value. In situ hybridization experiments allowed the earliest detection of Rxt1 messenger RNA in the brain and spinal cord at embryonic day 14. In embryonic day 18 embryos, Rxt1 messenger RNA was present not only in the nervous system but also in the pituitary, the thymus and the heart. Immunoautoradiograms of whole embryo at embryonic days 16 and 18 showed high amounts of the Rxt1 protein in the spinal cord and brain. Moreover, at embryonic day 18, the orphan transporter was expressed in the thymus, heart and liver. At these ages, Rxt1 immunolabeling was localized in neurons of the subplate and in the ventricular zone of the brain. During early postnatal stages, Rxt1 messenger RNA expression demonstrated dynamic and complex changes until postnatal day 13. In particular, this transcript was relatively abundant in the striatum at postnatal days 3 and 5 and then decreased to very low levels after postnatal day 10. At the same period, Rxt1 immunostaining in the hippocampus and the cerebral cortex was observed all over the gray matter, in cell bodies as well as in the neuropil. Finally, the adult pattern was reached around postnatal day 13 for Rxt1 messenger RNA, but only at postnatal day 20 for the Rxt1 protein. The presence of Rxt1 messenger RNA and protein at embryonic stages and the high expression of the protein during synaptogenesis suggest that this vesicular "orphan" transporter is involved in the brain maturation process.

bloated tubules (blot) encodes a Drosophila member of the neurotransmitter transporter family required for organisation of the apical cytocortex

We have identified a novel member of the vertebrate sodium- and chloride-dependent neurotransmitter symporter family from Drosophila melanogaster. This gene, named bloated tubules (blot), shows significant sequence similarity to a subgroup of vertebrate orphan transporters. blot transcripts are maternally supplied and during embryogenesis exhibit a complex and dynamic pattern in a subset of ectodermally derived epithelia, notably in the Malpighian tubules, and in the nervous system. Animals mutant for this gene are larval lethals, in which the Malpighian tubule cells are distended with an enlarged and disorganised apical surface. Embryos lacking the maternal component of blot expression die during early stages of development. They show an inability to form actin filaments in the apical cortex, resulting in impaired syncytial nuclear divisions, severe defects in the organisation of the cortical cytoskeleton, and a failure to cellularise. For the first time, a neurotransmitter transporter-like protein has been implicated in a function outside the nervous system. The isolation of blot thus provides the basis for an analysis of the relationship between the function of this putative transporter and epithelial morphogenesis.

The "orphan" Na+/Cl(-)-dependent transporter, Rxt1, is primarily localized within nerve endings of cortical origin in the rat striatum

Previous studies have shown that the striatum expresses very low levels of Na+/Cl(-)-dependent "orphan" transporter Rxt1 transcripts but contains high levels of protein. This study investigated the origin of Rxt1 expression in rat striatum. Striatal Rxt1 contents assessed by immunocytochemistry or western blotting were found to be significantly reduced after corticostriatal denervation but not after striatal or thalamic lesion with kainic acid or selective 6-hydroxydopamine-induced nigrostriatal deafferentation. Corticostriatal neurons retrogradely labeled by intrastriatal fluorogold injections were shown to express Rxt1 mRNA. Combination of anterograde biotin-dextran amine labeling of the corticostriatal pathway with Rxt1 immunogold detection at the ultrastructural level demonstrated the presence of Rxt1 in about one-third of the corticostriatal synaptic terminals and in numerous unidentified synaptic terminals. All the Rxt1-positive terminals formed asymmetrical contacts on spines. These data provide evidence that striatal Rxt1 immunoreactivity is mainly of extrinsic origin and more specifically associated with the corticostriatal pathway. Rxt1 appears as a selective presynaptic marker of synapses formed by presumably excitatory amino acid afferents, but it segregates a subclass of these synapses, thereby revealing a functional heterogeneity among excitatory amino acid systems.

Immunocytochemical evidence of vesicular localization of the orphan transporter RXT1 in the rat spinal cord

Rxt1, a member of the Na+/Cl- orphan transporter family, exhibits numerous features suggesting a role as plasma membrane transporter. Despite numerous attempts, its substrate has not yet been identified, although immunocytochemical studies have shown that Rxt1 distribution generally matches that of glutamate or GABA. In order to further characterize Rxt1, its detailed immunocytochemical distribution in the rat spinal cord and dorsal root ganglia was studied at both light microscope and ultrastructural levels. The widespread distribution of Rxt1 in spinal cord and ganglia cannot be correlated with any known classical or peptidergic transmitter. Rxt1 is expressed in a subpopulation of glutamatergic primary afferent fibers, in large and medium-sized ganglion cells, while small glutamate cells exhibit generally no Rxt1-like immunoreactivity. In the spinal cord, Rxt1-immunoreactive cell body distribution is quite ubiquitous since Rxt1 is expressed in all laminae in various neuronal types like interneurons, some projection neurons and motoneurons. Some of these neurons are cholinergic. At the electron microscope level, the peroxidase labeling was never localized to the plasma membrane, but rather associated with different organelles including the outer membrane of small synaptic vesicles and large granular vesicles. This localization resembles that of vesicular transporters detected with the same method and suggests that Rxt1, in contrast to other Na+/Cl- transporters, is expressed on vesicles. This was confirmed using a pre-embedding silver-intensified colloidal gold method. Indeed, most gold particles appeared to be localized into the axoplasm on synaptic vesicle accumulations; only few gold particles were observed close to the plasma membrane. These results suggest that Rxt1, despite its molecular characteristics predicting a plasma membrane localization, might be a vesicular transporter.

Characterization and distribution of Hxt1, a Na(+)/Cl(-)-dependent orphan transporter, in the human brain

Rxt1, a transporter-like protein structurally related to the large family of Na(+)/Cl(-)-dependent carriers, was isolated from the rat brain. In the present study, Hxt1, the homologue of Rxt1, was isolated from human cortex cDNA. Comparison of their respective nucleotidic sequences revealed a 96% conservation between Hxt1 and Rxt1. Genetic mapping with human genome radiation hybrids allowed the location of the gene coding for Hxt1 between 323ya5 and 084xb3 AFM markers, on a portion of chromosome 1p which spans over 7 cM or 118 cRay. Northern blot analyses demonstrated that Hxt1 mRNA ( approximately 7.5 Kb) is expressed in the human brain but not in peripheral tissues. The immunodistribution of Hxt1 was determined with antibodies raised against the C-terminus of Rxt1. Hxt1 is concentrated in the cerebral cortex, caudate-putamen, substantia nigra, hippocampus, and cerebellum, appearing as a diffuse or a punctate labeling at the light microscope level. This regional and cellular distribution suggests that Hxt1, as its rat homologue, could be present in axon terminals of glutamatergic neurons. The high pressure of selection exerted upon this protein, its strategic anatomical and subcellular distributions suggest that this orphan transporter could be involved in critical functions in the central nervous system.

Unexpected localization of the Na+/Cl--dependent-like orphan transporter, Rxt1, on synaptic vesicles in the rat central nervous system

Numerous features of its primary structure demonstrate that the orphan transporter Rxt1 belongs to the Na+/Cl--dependent neurotransmitter plasma membrane transporter superfamily, which includes the dopamine, norepinephrine, serotonin and gamma-aminobutyric acid (GABA) transporters. Initial immunocytochemical investigations with affinity-purified antibodies have established that Rxt1 is localized, almost exclusively, in axon terminals of glutamatergic neurons and subsets of GABAergic neurons in the CNS. Further studies were carried out to determine its subcellular distribution. In a first series of experiments, PC-12 cells were transfected with plasmids encoding either the dopamine transporter or Rxt1. Immunofluorescence experiments showed that the dopamine transporter was expressed in these cells, and, as expected, addressed to their plasma membrane. Surprisingly, this was never the case with Rxt1, which was targeted to the same subcellular compartment as synaptophysin, a vesicular protein. In a second set of experiments, subcellular fractionation of rat striatum showed that Rxt1, but not the dopamine transporter, was relatively abundant in the purified synaptic vesicle fraction. Finally, electron microscopic immunocytochemistry with anti-Rxt1 antibodies showed peroxidase as well as pre- and post-embedding immunogold labelling confined to the intracellular compartment in various brain regions. Moreover, quantitative analysis of post-embedding experiments demonstrated that the immunogold particles corresponding to Rxt1 immunoreactivity were mostly localized to small synaptic vesicles. These data indicate that, in contrast with the other members of the Na+/Cl--dependent neurotransmitter transporter superfamily, which are targeted to the plasma membrane, Rxt1 is distributed as a vesicular protein in the CNS.

Cloning of a bovine orphan transporter and its short splicing variant

We have isolated a cDNA (bv7-3) encoding a member of the Na+,Cl(-)-dependent transporter family and its short splicing variant (bv7-3s) by screening a bovine retina cDNA library. Sequence analysis revealed that bv7-3 encodes a protein of 729 amino acids and is a bovine homologue of the rat orphan transporter v7-3-2. bv7-3s contains 265 amino acids, sharing 252 N-terminal amino acids with bv7-3. Both mRNAs for bv7-3 and bv7-3s were detected in nervous system by Northern blot analysis. In immunofluorescence analysis in transfected HEK 293T cells, myc-tagged bv7-3 was mainly detected on the plasma membrane, whereas myc-tagged bv7-3s showed a pattern of intracellular membrane staining.

Developmental expression of the neurotransmitter transporter NTT4

Neurotransmitter transporters are involved in termination of the synaptic neurotransmission and are implicated as the sites of action of antidepressant medicines and illicit drugs. In addition to their function in neurotransmission, neurotransmitter transporters play a key role in neuroregulation and brain development. In this report, the developmental distribution of the "orphan" transporter NTT4, whose substrate has not yet been shown, is described. Immunohistochemical studies have previously shown NTT4 to be specifically and widely localized to the central nervous system. In this report, the distribution of NTT4 in brain areas enriched in glutamatergic and gamma-aminobutyric acid-ergic innervations is further substantiated. NTT4 is detected beginning at E18 in various parts of the rat brain, including the cerebral cortex, fimbria hippocampi, fornix, lateral lemniscus, anterior commissure, and spinal cord. At E18, strong immunoreactivity of NTT4 is observed in the cortical subplate and marginal layers that later develops into the fimbria hippocampi, and at P22, the expression of NTT4 in the hippocampal formation reaches the mature form. The expression of NTT4 in the spinal cord begins at E18 in the ventral white matter. Heavy staining for NTT4 is observed in the substantia nigra since birth and through all time points examined. Transient immunoreactivity is observed in the inferior colliculus, reaching maximal expression at P10, whereas the superior colliculus commences to express NTT4 only after this time point. The globus pallidus is highly stained after birth, and the caudate putamen shows strong staining for NTT4 only at P22. In the adult rat brain, NTT4 is strongly expressed in the olfactory bulb, cerebral cortex, striatum, hippocampus, thalamus, substantia nigra, pontine nucleus, cerebellum, and spinal cord. The developmental distribution of NTT4 suggests involvement in central nervous system maturation.

Molecular biology of mammalian plasma membrane amino acid transporters

Molecular biology entered the field of mammalian amino acid transporters in 1990-1991 with the cloning of the first GABA and cationic amino acid transporters. Since then, cDNA have been isolated for more than 20 mammalian amino acid transporters. All of them belong to four protein families. Here we describe the tissue expression, transport characteristics, structure-function relationship, and the putative physiological roles of these transporters. Wherever possible, the ascription of these transporters to known amino acid transport systems is suggested. Significant contributions have been made to the molecular biology of amino acid transport in mammals in the last 3 years, such as the construction of knockouts for the CAT-1 cationic amino acid transporter and the EAAT2 and EAAT3 glutamate transporters, as well as a growing number of studies aimed to elucidate the structure-function relationship of the amino acid transporter. In addition, the first gene (rBAT) responsible for an inherited disease of amino acid transport (cystinuria) has been identified. Identifying the molecular structure of amino acid transport systems of high physiological relevance (e.g., system A, L, N, and x(c)- and of the genes responsible for other aminoacidurias as well as revealing the key molecular mechanisms of the amino acid transporters are the main challenges of the future in this field.

Studies of the biogenic amine transporters. VII. Characterization of a novel cocaine binding site identified with [125I]RTI-55 in membranes prepared from human, monkey and guinea pig caudate

[125I]RTI-55 is a cocaine analog with high affinity for dopamine (DA) and serotonin (5-HT) transporters. Quantitative ligand binding studies revealed a novel high affinity [125I]RTI-55 binding site assayed under 5-HT transporter (SERT) conditions which has low affinity for almost all classic biogenic amine transporter ligands, including high affinity 5-HT transporter inhibitors such as paroxetine, but which retains high affinity for cocaine analogs. This site, termed SERT(site2) for its detection under 5-HT transporter conditions (not for an association with the SERT) occurs in monkey caudate, human caudate, and guinea pig caudate membranes, but not in rat caudate membranes. SERT(site2) is distinguished from the DA transporter (DAT) and SERT by several criteria, including a distinct ligand-selectivity profile, the inability to detect SERT(site2) in cells stably expressing the cloned human DAT, and insensitivity to irreversible ligands which inhibit [125I]RTI-55 binding to the DAT and SERT. Perhaps the most striking finding about SERT(site2) is that a wide range of representative antidepressant agents have very low affinity for SERT(site2). The affinity of cocaine for this site is not very different from the concentration cocaine achieves in the brain at pharmacological doses. Viewed collectively with the observation that ligands with high affinity for SERT(site2) are mostly cocaine analogs, these data lead us to speculate that actions of cocaine which differ from those of classic biogenic amine uptake inhibitors may be mediated in part via SERT(site2).

Renal osmotic stress-induced cotransporter: expression in the newborn, adult and post-ischemic rat kidney

The renal osmotic stress-induced cotransporter (ROSIT), a new putative member of a family of organic solute transporters, is highly expressed in the kidney. Our in situ hybridization data now reveal that large amounts of ROSIT mRNA can be found in the S3 segment of the proximal tubule. In the developing kidney, ROSIT mRNA is expressed after the S-shaped body stage. Because the S3 segment is the major site of damage in the post-ischemic kidney, we evaluated alterations in ROSIT mRNA expression after ischemic acute tubular necrosis. Renal osmotic stress-induced cotransporter mRNA levels were already decreased eight hours post-ischemia. At seven days post-ischemia, ROSIT mRNA reappeared in a mosaic pattern in the regenerating S3 segment, being fully expressed three weeks after the insult except for focal areas. The exact localization of this putative osmolyte transporter in the kidney, together with that of other known osmolyte transporter will contribute to a better understanding of the mechanism of medullary osmolyte accumulation and its vectorial transport.

Analysis of the transmembrane topology and membrane assembly of the GAT-1 gamma-aminobutyric acid transporter

The transmembrane topology of the Na+- and Cl--dependent gamma-aminobutyric acid transporter GAT-1 has been studied using protein chimeras in Xenopus oocytes. A series of COOH-terminal truncations was generated to which a prolactin epitope was fused. Following expression of transporter-prolactin chimeras in Xenopus oocytes, the transmembrane orientation of each chimera was determined by testing for protease sensitivity in an oocyte membrane preparation. Data from protease protection assays with GAT-1-prolactin chimeras has shown that residues in the loops connecting hydrophobic domain (HD)3 and HD4 and HD7 and HD8 are accessible to protease in the cytoplasm and suggest the presence of pore loop structures which extend into the membrane from the extracellular face. Such pore loop structures may be involved in the formation of the substrate-binding pocket. Studies presented herein confirm that the NH2 and COOH termini are cytosolic and hydrophobic domains span the membrane in a manner consistent with the predicted hydropathy model for Na+- and Cl--dependent transporters. These data also provide insight into GAT-1 transmembrane assembly and suggest that a complex series of topogenic sequences directs this process. A potential pause-transfer sequence has been identified and may be responsible for the translocational pausing observed in this study.

Molecular biology of glycinergic neurotransmission

Glycine is a major inhibitory neurotransmitter in the spinal cord and brainstem of vertebrates. Glycine is accumulated into synaptic vesicles by a proton-coupled transport system and released to the synaptic cleft after depolarization of the presynaptic terminal. The inhibitory action of glycine is mediated by pentameric glycine receptors (GlyR) that belong to the ligand-gated ion channel superfamily. The synaptic action of glycine is terminated by two sodium- and chloride-coupled transporters, GLYT1 and GLYT2, located in the glial plasma membrane and in the presynaptic terminals, respectively. Dysfunction of inhibitory glycinergic neurotransmission is associated with several forms of inherited mammalian myoclonus. In addition, glycine could participate in excitatory neurotransmission by modulating the activity of the NMDA subtype of glutamate receptor. In this article, we discuss recent progress in our understanding of the molecular mechanisms that underlie the physiology and pathology of glycinergic neurotransmission.

Distribution pattern and ultrastructural localization of Rxt1, an orphan Na+/Cl(-)-dependent transporter, in the central nervous system of rats and mice

The cellular and subcellular localization of Rxt1 protein, an orphan Na+/Cl(-)-dependent transporter, was investigated in the central nervous system of rats and mice, with rabbit polyclonal antibodies specifically directed against its C-terminal region. At the light microscope level, the distribution of Rxt1, visualized by the immunoperoxidase method, was found to be similar in rats and mice. Labelled elements were present in numerous gray matter regions of the central nervous system, from the olfactory bulb to the spinal cord. In all labelled regions, immunoreactivity was confined to the neuropil where both a diffuse labelling of low intensity and an intense punctate staining were noted. To further identify the nature of the cellular elements bearing the punctate staining, possible changes in this labelling pattern were investigated: (i) in deep cerebellar nuclei and lateral vestibular nucleus of the Lurcher mutant mouse, in which all cerebellar Purkinje cells are missing and (ii) in the rat cervical spinal cord, 10 days after multiple resections of dorsal roots. The vast majority of the punctate structures, delineating the neuronal perikaryal and stem dendritic contours, had disappeared in the mutant mouse, providing evidence that they belong to Purkinje cell axon terminals. In rhizotomized rats, the intense labelling in laminae I and III had disappeared, demonstrating that it occurred in subclasses of axonal projections of primary afferent fibres. These results strongly suggest that Rxt1 is present in presynaptic axon terminals. The electron microscopic study was carried out in the hippocampus, cerebellum and lateral vestibular nucleus of control mice, where Rxt1-labelled punctate structures were found to be abundant. Immunostaining was confined to axon terminals, particularly in hippocampal and cerebellar mossy fibres and in Purkinje cell axonal terminations of the cerebellar deep nuclei and lateral vestibular nucleus. In the cerebellar cortex, axon terminals belonging to inhibitory local circuit neurons (basket and Golgi cells), were free of labelling. The observations reported in this study have shown that: (1) The Rxt1 transporter is neuron-specific, and is expressed by only some classes or even subclasses of neuronal systems. (2) This transporter can be encountered in excitatory axons using glutamate as neurotransmitter (hippocampal and cerebellar mossy fibres: primary afferent fibres), as well as in inhibitory axons known by their GABAergic nature (Purkinje cell axon terminals) where it might be involved in the re-uptake process of one or several molecules released from corresponding terminals.

Widespread brain distribution of mRNA encoding the orphan neurotransmitter transporter v7-3

Orphan transporter v7-3 is a member of a new subfamily of Na+, Cl- dependent neurotransmitter transporters with two large extracellular loops. Distribution of v7-3 mRNA was investigated in the rat brain. In situ hybridization study revealed that v7-3 mRNA was widely distributed in the rat central nervous system, including the olfactory bulb, the hypothalamus, the cerebral cortex, the hippocampus, and the cerebellum. In addition, intense v7-3 mRNA expression was found in the motor nuclei including the oculomotor nucleus, abducens nucleus, trigeminal motor nucleus, facial nucleus, hypoglossal nucleus and ventral horn of spinal cord. Intense hybridization signals were also observed in the nuclei containing monoaminergic neurons, such as locus coeruleus, the substantia nigra pars compacta, the ventral tegmental area, the dorsal raphe nucleus and the median raphe nucleus. This multifocal and broad nature of the v7-3 distribution suggests widespread roles for this gene product in neurons mediating several important brain function.

Distribution and sites of synthesis of NTT4, an orphan member of the Na+/Cl(-)-dependent neurotransmitter transporter family, in the rat CNS

The distribution and sites of synthesis in rat CNS of NTT4, a novel orphan member of the Na+/Cl(-)-dependent neurotransmitter transporter family, were determined by immunohistochemistry and hybridization histochemistry. Antibodies raised against recombinant fusion proteins, corresponding to residues of NTT4, and 35S-labelled oligodeoxyribonucleotide probes, were used to delineate the cellular distribution of the transporter at the protein and mRNA levels. High levels of immunoreactivity (mainly in the neuropil) were found in the olfactory bulb, cerebral cortex, striatum, hippocampus, thalamus, substantia nigra, pontine nuclei, cerebellum and spinal cord. The lowest levels were associated with the lateral hypothalamic area and deep mesencephalic nuclei. In situ hybridization signals correlated well with the immunoreactivity, and demonstrated a widespread distribution of NTT4 transcripts exclusively in neurons. NTT4 transcripts appeared widely codistributed with the N-methyl-D-aspartate receptor subunit 1 (1-4b), i.e. spliced variants characterized by a common 5' 63 bp insertion. These results indicate that the transporter was associated with neuronal processes in specific glutamate innervated CNS regions. Although the substrate transported by NTT4 remains unknown, our findings suggest a possible role for this carrier protein in glutamate/glycine neurotransmission.

Metabolism and inactivation of neurotransmitters in nematodes

The nematode nervous system employs many of the same neurotransmitters as are found in higher animals. The inactivation of neurotransmitters is absolutely essential for the correct functioning of the nervous system. In this article we discuss the various mechanisms used generally in animal nervous systems for synaptic inactivation of neurotransmitters and review the evidence for similar mechanisms operating in parasitic and free-living nematodes. The sequencing of the entire Caenorhabditis elegans genome means that the sequence of nematode genes can be accessed from the C. elegans database (ACeDB) and this wealth of information together with the increasing knowledge of the genetics of this free-living nematode will have great impact on all aspects of nematode neurobiology. The review will provide an insight into how this information may be exploited to identify and characterize target proteins for the development of novel anti-nematode drugs.

Phylogenetic conservation of 4-aminobutyric acid (GABA) transporter isoforms. Cloning and pharmacological characterization of a GABA/beta-alanine transporter from Torpedo

A family of structurally related, Na+/Cl(-)-dependent plasma-membrane transporters catalyze the uptake of several neurotransmitters, osmolytes and other metabolites into cells. Four different members of this transporter family have been cloned from mammalian sources which all transport 4-aminobutyric acid (GABA) but differ in their pharmacological profiles and in their tissue distribution. We report on the cloning, sequencing and functional expression of a transporter for GABA and beta-alanine from the electric lobe of Torpedo. According to similarity of amino acid sequence (77% identity), pharmacological properties, and tissue distribution (nervous-system-specific), it appears to be the counterpart of the beta-alanine-sensitive GABA transporter, GAT-B/GAT-3/GAT4, previously cloned from rat and mouse. The identification of another GABA transporter isoform from Torpedo (after the recent characterization of a Torpedo GAT-1 transporter) indicates that GABA-transporter isoforms are phylogenetically ancient and arose before the divergence of vertebrates. Sequence comparison between isofunctional transporters from evolutionarily distant species aids in the identification of amino acid residues that are critical for functional specificity. The expression of transporters for GABA and beta-alanine raises questions regarding the unidentified physiological role of these amino acids in Torpedo electric lobe.

Immunolabeling of the Na+/Cl(-)-dependent "orphan" transporter Rxt1 in the rat central nervous system

Previous studies have shown that the mRNA encoding the Na+/Cl(-)-dependent "orphan" transporter Rxt1 is expressed exclusively in the central nervous system (CNS). In the present study, specific antibodies were raised in rabbits for the detailed mapping of this transporter in the rat. The C-terminal part of Rxt1 was fused with glutathione-S-transferase (Rxt1ct-GST) and the resulting fusion protein was used as antigen. The specificity of the antiserum toward Rxt1 was confirmed by immunofluorescent, Western blot, and immunoautoradiographic experiments. In cerebral cortex membranes, Rxt1-like material recognized by the antiserum is a glycosylated protein of 97-116 kDa. This protein was the most abundant in the caudate-putamen, followed, in decreasing order, by the cerebral cortex approximately hippocampus > cerebellum > brainstem > spinal cord. In contrast, no immunoreactive material could be detected in peripheral tissues (tongue, thymus, heart, lung, spleen, kidney, adrenals, liver, skeletal muscle, intestine, testis). Immunoautoradiographic labeling with affinity-purified anti-Rxt1ct-GST antibodies showed high levels of Rxt1-like material in the olfactory bulb, cerebral cortex, striatal complex, hippocampal formation, superior layer of the anterior colliculus, cortex, and deep nuclei in the cerebellum. The regional distribution of Rxt1-like material generally matched that of GABAergic and glutamatergic projections in agreement with previous in situ hybridization data.

Molecular cloning of an orphan transporter. A new member of the neurotransmitter transporter family

A complementary DNA clone predicted to encode a novel transporter was isolated from rat brain and the localization of its mRNA was examined. The cDNA, designated rB21a, predicts a protein with 12 putative transmembrane domains that exhibits significant sequence homology with neurotransmitter transporters. Expression studies have not yet identified the endogenous substrate for this transporter, but the presence of rB21a mRNA within the leptomeninges of the brain suggests the transporter may regulate CSF levels of its substrate. The cloning of rB21a provides the means to determine its physiological functions and the potential to design novel, transporter-based therapeutic agents for neurological and psychiatric disorders.

PC12 variants deficient in norepinephrine transporter mRNA have wild type activities of several other related transporters

Wild type PC12 pheochromocytoma cells express a Na(+)-dependent norepinephrine transporter that operates in the uptake of catecholamines. In addition to the previously described Na(+)-dependent system A for the uptake of alpha-amino-isobutyric acid and system Gly for glycine, we have identified two other Na(+)-dependent transporter systems for amino acid uptake in these cells: 1) system beta for beta-alanine and taurine; and 2) a system for creatine. Uptake of alpha-amino-isobutyric acid, glycine, beta-alanine, and creatine is not affected in some PC12 variants that were previously shown to be deficient in catecholamine uptake and to have decreased levels of norepinephrine transporter mRNA. We have isolated two PC12 cDNA clones that are essentially identical in sequence to recently reported cDNAs for rat brain taurine and creatine transporters, respectively, and a third cDNA that appears to code for a novel transporter. mRNAs for these three transporters are present at wild type levels in those variants that express no or little norepinephrine transporter mRNA. These results support the notion that the expression of catecholamine reuptake transporters may be particularly susceptible to down-regulation.

'Choline/orphan V8-2-1/creatine transporter' mRNA is expressed in nervous, renal and gastrointestinal systems

Several cDNAs with substantial sequence homologies to members of the neurotransmitter transporter gene family currently remain 'orphan' transporters, without clearly-identified substrates. We were concerned that a cDNA 'V8-2-1' isolated from a ventral midbrain cDNA library in this laboratory and a virtually-identical cDNA 'CHOT1' reported by Mayser et al. [J. Neurochem., 20 (1973) 581-593] might represent such an orphan. Despite initial reports that it could mediate some choline uptake; neither CHOT1 nor V8-2-1 was demonstrated to confer pharmacologically appropriate choline uptake not already present in either Xenopus oocytes or COS cells. Determination of the regional and tissue-specific distribution of mRNA hybridizing with V8-2-1 cDNA was undertaken to aid in identifying its function. Examination of the distribution of V8-2-1 expression reveals several novel features of this transporter gene family member's distribution, including several features that add to current evidence suggesting that the clone may not encode the classical pharmacologically-defined, hemicholinium-3 sensitive high affinity transporter of cholinergic neurons. These data fit with and extend recent data that suggest that this cDNA represents creatine transporter, and provide initial documentation of its regional distribution in brain.

Regulation of renal cell organic osmolyte transport by tonicity

Madin-Darby canine kidney cells accumulate several nonperturbing organic osmolytes when cultured in a hypertonic medium. Myo-inositol, betaine, and taurine are accumulated secondary to an increase in uptake, the first coupled to sodium entry, the latter two coupled to sodium and chloride entry. The transport rates increase as the result of an increase in maximum velocity for each cotransporter, with peak activity 24 h after the increase in tonicity. The cDNA for each cotransporter has been cloned. Their sequences indicate that the myo-inositol cotransporter belongs to the gene family that includes the sodium-coupled glucose transporter (SGLT1); the betaine and taurine cotransporters belong to the gene family of sodium- and chloride-coupled transporters that are responsible for neuronal uptake of many neurotransmitters. Assays of mRNA abundance and nuclear run-on assays reveal that shifts in tonicity have a major effect on transcription of the genes for the sodium-myo-inositol (SMIT) and sodium-chloride-betaine (BGT1) cotransporters. The ensuing increase in mRNA abundance for the two cotransporters and presumed increase in synthesis of the cotransporter proteins can explain the increase in transport activity in response to changes in tonicity.