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

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.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

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.

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.