Localization of the high-affinity glutamate transporter EAAC1 in rat kidney

How Many Cell Types Are in the Kidney and What Do They Do?

The kidney maintains electrolyte, water, and acid-base balance, eliminates foreign and waste compounds, regulates blood pressure, and secretes hormones. There are at least 16 different highly specialized epithelial cell types in the mammalian kidney. The number of specialized endothelial cells, immune cells, and interstitial cell types might even be larger. The concerted interplay between different cell types is critical for kidney function. Traditionally, cells were defined by their function or microscopical morphological appearance. With the advent of new single-cell modalities such as transcriptomics, epigenetics, metabolomics, and proteomics we are entering into a new era of cell type definition. This new technological revolution provides new opportunities to classify cells in the kidney and understand their functions.

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

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

Amino acid homeostasis and signalling in mammalian cells and organisms

Cells have a constant turnover of proteins that recycle most amino acids over time. Net loss is mainly due to amino acid oxidation. Homeostasis is achieved through exchange of essential amino acids with non-essential amino acids and the transfer of amino groups from oxidised amino acids to amino acid biosynthesis. This homeostatic condition is maintained through an active mTORC1 complex. Under amino acid depletion, mTORC1 is inactivated. This increases the breakdown of cellular proteins through autophagy and reduces protein biosynthesis. The general control non-derepressable 2/ATF4 pathway may be activated in addition, resulting in transcription of genes involved in amino acid transport and biosynthesis of non-essential amino acids. Metabolism is autoregulated to minimise oxidation of amino acids. Systemic amino acid levels are also tightly regulated. Food intake briefly increases plasma amino acid levels, which stimulates insulin release and mTOR-dependent protein synthesis in muscle. Excess amino acids are oxidised, resulting in increased urea production. Short-term fasting does not result in depletion of plasma amino acids due to reduced protein synthesis and the onset of autophagy. Owing to the fact that half of all amino acids are essential, reduction in protein synthesis and amino acid oxidation are the only two measures to reduce amino acid demand. Long-term malnutrition causes depletion of plasma amino acids. The CNS appears to generate a protein-specific response upon amino acid depletion, resulting in avoidance of an inadequate diet. High protein levels, in contrast, contribute together with other nutrients to a reduction in food intake.

Novel cystine transporter in renal proximal tubule identified as a missing partner of cystinuria-related plasma membrane protein rBAT/SLC3A1

Heterodimeric amino acid transporters play crucial roles in epithelial transport, as well as in cellular nutrition. Among them, the heterodimer of a membrane protein b(0,+)AT/SLC7A9 and its auxiliary subunit rBAT/SLC3A1 is responsible for cystine reabsorption in renal proximal tubules. The mutations in either subunit cause cystinuria, an inherited amino aciduria with impaired renal reabsorption of cystine and dibasic amino acids. However, an unsolved paradox is that rBAT is highly expressed in the S3 segment, the late proximal tubules, whereas b(0,+)AT expression is highest in the S1 segment, the early proximal tubules, so that the presence of an unknown partner of rBAT in the S3 segment has been proposed. In this study, by means of coimmunoprecipitation followed by mass spectrometry, we have found that a membrane protein AGT1/SLC7A13 is the second partner of rBAT. AGT1 is localized in the apical membrane of the S3 segment, where it forms a heterodimer with rBAT. Depletion of rBAT in mice eliminates the expression of AGT1 in the renal apical membrane. We have reconstituted the purified AGT1-rBAT heterodimer into proteoliposomes and showed that AGT1 transports cystine, aspartate, and glutamate. In the apical membrane of the S3 segment, AGT1 is suggested to locate itself in close proximity to sodium-dependent acidic amino acid transporter EAAC1 for efficient functional coupling. EAAC1 is proposed to take up aspartate and glutamate released into luminal fluid by AGT1 due to its countertransport so that preventing the urinary loss of aspartate and glutamate. Taken all together, AGT1 is the long-postulated second cystine transporter in the S3 segment of proximal tubules and a possible candidate to be involved in isolated cystinuria.

The transcription factor Pax6 contributes to the induction of GLT-1 expression in astrocytes through an interaction with a distal enhancer element

The Na(+) -dependent glutamate transporter GLT-1 (EAAT2) shows selective expression in astrocytes, and neurons induce the expression of GLT-1 in astrocytes. In an unpublished analysis of GLT-1 promoter reporter mice, we identified an evolutionarily conserved domain of 467 nucleotides ~ 8 kb upstream of the GLT-1 translation start site that is required for astrocytic expression. Using in silico approaches, we identified Pax6 as a transcription factor that could contribute to the control of GLT-1 expression by binding within this region. We demonstrated the expression of Pax6 protein in astrocytes in vivo. Lentiviral transduction of astrocytes with exogenous Pax6 increased the expression of enhanced green fluorescent protein (eGFP) in astrocytes prepared from transgenic mice that use a bacterial artificial chromosome containing a large genomic region surrounding the GLT-1 gene to control expression of eGFP. It also increased GLT-1 protein and GLT-1-mediated uptake, whereas there was no effect on the levels of the other astroglial glutamate transporter, glutamate aspartate transporter (GLAST). Transduction of astrocytes with an shRNA directed against Pax6 reduced neuron-dependent induction of GLT-1 or eGFP. Finally, we confirmed Pax6 interaction with the predicted DNA-binding site in electrophoretic mobility assays and chromatin immunoprecipitation (ChIP). Together, these studies show that Pax6 contributes to the regulation of GLT-1 through an interaction with these distal elements and identify a novel role of Pax6 in astrocyte biology. The astroglial glutamate transporter GLT-1 shows selective expression in astrocytes and its expression can be induced by neurons. In this study, we demonstrate that Pax6 is expressed in astrocytes and binds to the GLT-1 promoter in vitro and in vivo. Exogenous expression of Pax6 increases GLT-1 and enhanced green fluorescent protein (eGFP) expression in astrocytes from a transgenic mouse line that uses the GLT-1 gene to drive eGFP expression, and an shRNA directed against Pax6 attenuates neuron-dependent induction of GLT-1/eGFP. We therefore conclude that Pax6 contributes to the neuron-dependent induction of GLT-1.

Proximal tubule function and response to acidosis

The human kidneys produce approximately 160-170 L of ultrafiltrate per day. The proximal tubule contributes to fluid, electrolyte, and nutrient homeostasis by reabsorbing approximately 60%-70% of the water and NaCl, a greater proportion of the NaHCO3, and nearly all of the nutrients in the ultrafiltrate. The proximal tubule is also the site of active solute secretion, hormone production, and many of the metabolic functions of the kidney. This review discusses the transport of NaCl, NaHCO3, glucose, amino acids, and two clinically important anions, citrate and phosphate. NaCl and the accompanying water are reabsorbed in an isotonic fashion. The energy that drives this process is generated largely by the basolateral Na(+)/K(+)-ATPase, which creates an inward negative membrane potential and Na(+)-gradient. Various Na(+)-dependent countertransporters and cotransporters use the energy of this gradient to promote the uptake of HCO3 (-) and various solutes, respectively. A Na(+)-dependent cotransporter mediates the movement of HCO3 (-) across the basolateral membrane, whereas various Na(+)-independent passive transporters accomplish the export of various other solutes. To illustrate its homeostatic feat, the proximal tubule alters its metabolism and transport properties in response to metabolic acidosis. The uptake and catabolism of glutamine and citrate are increased during acidosis, whereas the recovery of phosphate from the ultrafiltrate is decreased. The increased catabolism of glutamine results in increased ammoniagenesis and gluconeogenesis. Excretion of the resulting ammonium ions facilitates the excretion of acid, whereas the combined pathways accomplish the net production of HCO3 (-) ions that are added to the plasma to partially restore acid-base balance.

Excitatory amino acid transporter 5 is widely expressed in peripheral tissues

It is routinely stated in the literature that Excitatory Amino Acid Transporter 5 (EAAT5) is a retina-specific glutamate transporter. EAAT5 is expressed by retinal photoreceptors and bipolar cells, where it serves as a slow transporter and as an inhibitory glutamate receptor, the latter role is due to the gating of a large chloride conductance. The dogma of an exclusively retinal distribution has arisen because Northern blot analyses have previously shown only modest hybridisation in non-retinal tissues. Others have re-interpreted this as indicating that EAAT5 was only present in retinal tissues. However, this view appears to be erroneous; recent evidence demonstrating abundant expression of EAAT5 in rat testis prompted us to re-examine this dogma. A new antibody was developed to an intracellular loop region of rat EAAT5. This new tool, in concert with RT-PCR and sequencing, demonstrated that EAAT5 is widely distributed at the mRNA and protein levels in many non-nervous tissues including liver, kidney, intestine, heart, lung, and skeletal muscle. We conclude that EAAT5 is a widely distributed protein. Whether it functions in all locations as a glutamate transporter, or mainly as a glutamate-gated chloride conductance, remains to be determined.

Nuclear factor-κB contributes to neuron-dependent induction of glutamate transporter-1 expression in astrocytes

The glutamate transporter-1 [GLT-1 (excitatory amino acid transporter 2)] subtype of glutamate transporter ensures crisp excitatory signaling and limits excitotoxicity in the CNS. Astrocytic expression of GLT-1 is regulated during development, by neuronal activity, and in neurodegenerative diseases. Although neurons activate astrocytic expression of GLT-1, the mechanisms involved have not been identified. In the present study, astrocytes from transgenic mice that express enhanced green fluorescent protein (eGFP) under the control of a bacterial artificial chromosome (BAC) containing a very large region of DNA surrounding the GLT-1 gene (BAC GLT-1 eGFP mice) were used to assess the role of nuclear factor-κB (NF-κB) in neuron-dependent activation of the GLT-1 promoter. We provide evidence that neurons activate NF-κB signaling in astrocytes. Transduction of astrocytes from the BAC GLT-1 eGFP mice with dominant-negative inhibitors of NF-κB signaling completely blocked neuron-dependent activation of a NF-κB reporter construct and attenuated induction of eGFP. Exogenous expression of p65 and/or p50 NF-κB subunits induced expression of eGFP or GLT-1 and increased GLT-1-mediated transport activity. Using wild-type and mutant GLT-1 promoter reporter constructs, we found that NF-κB sites at -583 or -251 relative to the transcription start site were required for neuron-dependent reporter activation. Electrophoretic mobility shift and supershift assays reveal that p65 and p50 interact with these same sites ex vivo. Finally, chromatin immunoprecipitation showed that p65 and p50 interact with these sites in adult cortex, but not in kidney (a tissue that expresses no detectable GLT-1). Together, these studies strongly suggest that NF-κB contributes to neuron-dependent regulation of astrocytic GLT-1 transcription.

Loss-of-function mutations in the glutamate transporter SLC1A1 cause human dicarboxylic aminoaciduria

Solute carrier family 1, member 1 (SLC1A1; also known as EAAT3 and EAAC1) is the major epithelial transporter of glutamate and aspartate in the kidneys and intestines of rodents. Within the brain, SLC1A1 serves as the predominant neuronal glutamate transporter and buffers the synaptic release of the excitatory neurotransmitter glutamate within the interneuronal synaptic cleft. Recent studies have also revealed that polymorphisms in SLC1A1 are associated with obsessive-compulsive disorder (OCD) in early-onset patient cohorts. Here we report that SLC1A1 mutations leading to substitution of arginine to tryptophan at position 445 (R445W) and deletion of isoleucine at position 395 (I395del) cause human dicarboxylic aminoaciduria, an autosomal recessive disorder of urinary glutamate and aspartate transport that can be associated with mental retardation. These mutations of conserved residues impeded or abrogated glutamate and cysteine transport by SLC1A1 and led to near-absent surface expression in a canine kidney cell line. These findings provide evidence that SLC1A1 is the major renal transporter of glutamate and aspartate in humans and implicate SLC1A1 in the pathogenesis of some neurological disorders.

Kidney amino acid transport

Near complete reabsorption of filtered amino acids is a main specialized transport function of the kidney proximal tubule. This evolutionary conserved task is carried out by a subset of luminal and basolateral transporters that together form the transcellular amino acid transport machinery similar to that of small intestine. A number of other amino acid transporters expressed in the basolateral membrane of proximal kidney tubule cells subserve either specialized metabolic functions, such as the production of ammonium, or are part of the cellular housekeeping equipment. A new finding is that the luminal Na(+)-dependent neutral amino acid transporters of the SLC6 family require an associated protein for their surface expression as shown for the Hartnup transporter B(0)AT1 (SLC6A19) and suggested for the L: -proline transporter SIT1 (IMINO(B), SLC6A20) and for B(0)AT3 (XT2, SLC6A18). This accessory subunit called collectrin (TMEM27) is homologous to the transmembrane anchor region of the renin-angiotensin system enzyme ACE2 that we have shown to function in small intestine as associated subunit of the luminal SLC6 transporters B(0)AT1 and SIT1. Some mutations of B(0)AT1 differentially interact with these accessory subunits, providing an explanation for differential intestinal phenotypes among Hartnup patients. The basolateral efflux of numerous amino acids from kidney tubular cells is mediated by heteromeric amino acid transporters that function as obligatory exchangers. Thus, other transporters within the same membrane need to mediate the net efflux of exchange substrates, controlling thereby the net basolateral amino transport and thus the intracellular amino acid concentration.

Organization of the pronephric kidney revealed by large-scale gene expression mapping

Gene expression mapping reveals 8 functionally distinct domains in the Xenopus pronephros. Interestingly, no structure equivalent to the mammalian collecting duct is identified.

Background

The pronephros, the simplest form of a vertebrate excretory organ, has recently become an important model of vertebrate kidney organogenesis. Here, we elucidated the nephron organization of the Xenopus pronephros and determined the similarities in segmentation with the metanephros, the adult kidney of mammals.

Results

We performed large-scale gene expression mapping of terminal differentiation markers to identify gene expression patterns that define distinct domains of the pronephric kidney. We analyzed the expression of over 240 genes, which included members of the solute carrier, claudin, and aquaporin gene families, as well as selected ion channels. The obtained expression patterns were deposited in the searchable European Renal Genome Project Xenopus Gene Expression Database. We found that 112 genes exhibited highly regionalized expression patterns that were adequate to define the segmental organization of the pronephric nephron. Eight functionally distinct domains were discovered that shared significant analogies in gene expression with the mammalian metanephric nephron. We therefore propose a new nomenclature, which is in line with the mammalian one. The Xenopus pronephric nephron is composed of four basic domains: proximal tubule, intermediate tubule, distal tubule, and connecting tubule. Each tubule may be further subdivided into distinct segments. Finally, we also provide compelling evidence that the expression of key genes underlying inherited renal diseases in humans has been evolutionarily conserved down to the level of the pronephric kidney.

Conclusion

The present study validates the Xenopus pronephros as a genuine model that may be used to elucidate the molecular basis of nephron segmentation and human renal disease.

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.

Regulation of renal amino acid transporters during metabolic acidosis

The kidney plays a major role in acid-base homeostasis by adapting the excretion of acid equivalents to dietary intake and metabolism. Urinary acid excretion is mediated by the secretion of protons and titratable acids, particularly ammonia. NH(3) is synthesized in proximal tubule cells from glutamine taken up via specific amino acid transporters. We tested whether kidney amino acid transporters are regulated in mice in which metabolic acidosis was induced with NH(4)Cl. Blood gas and urine analysis confirmed metabolic acidosis. Real-time RT-PCR was performed to quantify the mRNAs of 16 amino acid transporters. The mRNA of phosphoenolpyruvate carboxykinase (PEPCK) was quantified as positive control for the regulation and that of GAPDH, as internal standard. In acidosis, the mRNA of kidney system N amino acid transporter SNAT3 (SLC38A3/SN1) showed a strong induction similar to that of PEPCK, whereas all other tested mRNAs encoding glutamine or glutamate transporters were unchanged or reduced in abundance. At the protein level, Western blotting and immunohistochemistry demonstrated an increased abundance of SNAT3 and reduced expression of the basolateral cationic amino acid/neutral amino acid exchanger subunit y(+)-LAT1 (SLC7A7). SNAT3 was localized to the basolateral membrane of the late proximal tubule S3 segment in control animals, whereas its expression was extended to the earlier S2 segment of the proximal tubule during acidosis. Our results suggest that the selective regulation of SNAT3 and y(+)LAT1 expression may serve a major role in the renal adaptation to acid secretion and thus for systemic acid-base balance.

Expression of concentrative nucleoside transporters SLC28 (CNT1, CNT2, and CNT3) along the rat nephron: Effect of diabetes

BACKGROUND

The renal reabsorption of natural nucleosides and a variety of nucleoside-derived drugs relies on the function of the apically located, Na(+)-dependent, concentrative nucleoside transporters CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3). The aims of this study were to determine the segmental localization of the three SLC28 family members and to establish whether streptozotocin-induced diabetes alters their expression.

METHODS

SLC28 expression was measured by real-time polymerase chain reaction (PCR) on microdissected sections of rat nephrons. Diabetes was induced by streptozotocin treatment and the biochemical profiles of control, diabetic, and insulin-treated rats were established. The effect of diabetes on SLC28 expression was assessed in those segments that significantly express SLC28 genes.

RESULTS

CNT1-3 mRNAs were expressed in the proximal tubule and glomerulus. In addition, CNT2 and CNT3 mRNAs were expressed in the outer medullary and cortical collecting duct, respectively. Diabetes reduced expression of the three CNTs in almost all nephron segments, and this effect was not prevented by an insulin treatment that normalized all blood and urine parameters. Diabetes increased CNT1 and CNT3 expression in the glomerulus and insulin treatment decreased it.

CONCLUSION

The relative distribution of SLC28 gene expression suggests a role for the proximal tubule in renal nucleoside clearance and an accessory role for CNT2 and CNT3, in adenosine-mediated regulation of collecting duct functions. Diabetes probably may impair nucleoside clearance independently of insulin.

Novel renal amino acid transporters

Reabsorption of amino acids, similar to that of glucose, is a major task of the proximal kidney tubule. Various amino acids are actively transported across the luminal brush border membrane into proximal tubule epithelial cells, most of which by cotransport. An important player is the newly identified cotransporter (symporter) B0AT1 (SLC6A19), which imports a broad range of neutral amino acids together with Na+ across the luminal membrane and which is defective in Hartnup disorder. In contrast, cationic amino acids and cystine are taken up in exchange for recycled neutral amino acids by the heterodimeric cystinuria transporter. The basolateral release of some neutral amino acids into the extracellular space is mediated by unidirectional efflux transporters, analogous to GLUT2, that have not yet been definitively identified. Additionally, cationic amino acids and some other neutral amino acids leave the cell basolaterally via heterodimeric obligatory exchangers.

The Genetics of Heteromeric Amino Acid Transporters

Heteromeric amino acid transporters (HATs) are composed of a heavy ( SLC3 family) and a light ( SLC7 family) subunit. Mutations in system b0,+(rBAT-b0,+AT) and in system y+L (4F2hc-y+LAT1) cause the primary inherited aminoacidurias (PIAs) cystinuria and lysinuric protein intolerance, respectively. Recent developments [including the identification of the first Hartnup disorder gene (B0AT1; SLC6A19)] and knockout mouse models have begun to reveal the basis of renal and intestinal reabsorption of amino acids in mammals.

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.

Translocation of glutamate transporter subtype excitatory amino acid carrier 1 protein in kainic acid-induced rat epilepsy

Glutamate excitotoxicity has been implicated in the pathophysiology of epilepsy. Systemic injection of kainic acid (KA) in the rat produces an animal model of human temporal lobe epilepsy. We examined the temporal expression of the sodium-dependent neuronal glutamate transporter, excitatory amino acid carrier 1 (EAAC1), in KA-induced rat epilepsy. As an early alteration, perinuclear deposits of EAAC1 protein were found mainly in the large pyramidal neurons at the hippocampus, neocortex, piriform cortex, and amygdala with the reduction of neuropil staining 6 hours after KA injection. Immunoelectron microscopic study revealed that the perinuclear EAAC1 immunoreactivity corresponded to the translocation to the Golgi complex. At this time point, EAAC1 mRNA was down-regulated. The intracellular aggregation of EAAC1 primarily disappeared by 24 hours. In vitro studies indicated that internalization of EAAC1 from the plasma membrane to the intracellular compartment by KA treatment was associated with the reduction of electrogenic transporter currents. Our results suggest that the transient EAAC1 internalization participates in the modulation of the transporter function preventing excessive glutamate uptake to pyramidal neurons during the early stage of epilepsy.

A novel sorting motif in the glutamate transporter excitatory amino acid transporter 3 directs its targeting in Madin-Darby canine kidney cells and hippocampal neurons

The glutamate transporter excitatory amino acid transporter 3 (EAAT3) is polarized to the apical surface in epithelial cells and localized to the dendritic compartment in hippocampal neurons, where it is clustered adjacent to postsynaptic sites. In this study, we analyzed the sequences in EAAT3 that are responsible for its polarized localization in Madin-Darby canine kidney (MDCK) cells and neurons. Confocal microscopy and cell surface biotinylation assays demonstrated that deletion of the EAAT3 C terminus or replacement of the C terminus of EAAT3 with the analogous region in EAAT1 eliminated apical localization in MDCK cells. The C terminus of EAAT3 was sufficient to redirect the basolateral-preferring EAAT1 and the nonpolarized EAAT2 to the apical surface. Using alanine substitution mutants, we identified a short peptide motif in the cytoplasmic C-terminal region of EAAT3 that directs its apical localization in MDCK cells. Mutation of this sequence also impairs dendritic targeting of EAAT3 in hippocampal neurons but does not interfere with the clustering of EAAT3 on dendritic spines and filopodia. These data provide the first evidence that an identical cytoplasmic motif can direct apical targeting in epithelia and somatodendritic targeting in neurons. Moreover, our results demonstrate that the two fundamental features of the localization of EAAT3 in neurons, its restriction to the somatodendritic domain and its clustering near postsynaptic sites, are mediated by distinct molecular mechanisms.

Rat erythrocytes express the anionic amino acid transport protein EAAC1

EAAC1 is a widely expressed protein which transports anionic amino acids in a Na(+)-dependent fashion. Rat erythrocytes have generally been thought to be impermeant to anionic amino acids. Utilizing immunoelectron microscopy, we have demonstrated the presence of EAAC1-immunoreactive protein within rat erythrocytes. Immunoblotting revealed the presence of an approximately 60-kDa protein, consistent with EAAC1, in erythrocyte membranes. Specificity was confirmed by peptide competition. In conclusion, EAAC1 is expressed in rat erythrocytes.

Aortic leucine-to-glutamate pathway: metabolic route and regulation of contractile responses

Rat aortic endothelium is differentiated regionally for three signal pathways capable of regulating the cGMP content of the underlying smooth muscle. Formation of nitric oxide (NO) from L-arginine and of glutamate from L-leucine increase cGMP; however, formation of prostaglandin H2 (PGH2) decreases cGMP. All three have peak activity in the windkessel area just distal to the aortic arch and decrease peripherally. We report evidence that the biochemical route of the leucine-to-glutamate (Leu-->Glu) pathway is via metabolism of leucine to acetyl CoA, that the controlling reaction of the pathway is mediated by the branched chain alpha-ketoacid dehydrogenase complex (BCDC), and that glutamate formation via the Leu-->Glu pathway is a major source of aortic segment free glutamate in vitro. Interruption of the pathway by treatment of precontracted rat aortic rings in vitro with each of three classes of inhibitors (leucine analogs, competitors for the BCDC reaction, or inhibitors of L-glutamate transport) enhances contractile responses. The enhancement requires an intact endothelium and is not owing to reductions in NO formation. The results support the hypothesis that the Leu-->Glu pathway functions in the regulation of aortic contractility and compliance.

Na+-alanine uptake activates a Cl- conductance in frog renal proximal tubule cells via nonconventional PKC

Hyposmotically induced swelling of frog renal proximal tubule cells activates a DIDS-sensitive, outwardly rectifying Cl- conductance via a conventional protein kinase C (PKC). This study examines whether Na+-alanine cotransport similarly activates a DIDS-sensitive Cl- conductance in frog renal proximal tubule cells. On stimulation of Na+-alanine cotransport, the DIDS-sensitive current (I(DIDS-Ala)) increased markedly over time. I(DIDS-Ala) exhibited outward rectification, a Na+/Cl- selectivity ratio of 0.19 +/- 0.03, and an anion selectivity sequence Br- = Cl- > I- > gluconate-. Activation of I(DIDS-Ala) was dependent on ATP hydrolysis and PKC-mediated phosphorylation and was inhibited by hyperosmotic conditions. Activation could be not ascribed to a conventional PKC isoform, as I(DIDS-Ala) was not affected by removing Ca2+ or by phorbol ester treatment, suggesting a role for a nonconventional PKC isoform, either novel or atypical. We conclude that Na+-alanine cotransport activates a DIDS-sensitive Cl- conductance via a nonconventional PKC isoform. This contrasts with the hyposmotically activated Cl- conductance, which requires conventional PKC activation.

Modulation of the neuronal glutamate transporter EAAC1 by the interacting protein GTRAP3-18

Excitatory amino-acid carrier 1 (EAAC1) is a high-affinity Na+-dependent L-glutamate/D,L-aspartate cell-membrane transport protein. It is expressed in brain as well as several non-nervous tissues. In brain, EAAC1 is the primary neuronal glutamate transporter. It has a polarized distribution in cells and mainly functions perisynaptically to transport glutamate from the extracellular environment. In the kidney it is involved in renal acidic amino-acid re-absorption and amino-acid metabolism. Here we describe the identification and characterization of an EAAC1-associated protein, GTRAP3-18. Like EAAC1, GTRAP3-18 is expressed in numerous tissues. It localizes to the cell membrane and cytoplasm, and specifically interacts with carboxy-terminal intracellular domain of EAAC1. Increasing the expression of GTRAP3-18 in cells reduces EAAC1-mediated glutamate transport by lowering substrate affinity. The expression of GTRAP3-18 can be upregulated by retinoic acid, which results in a specific reduction of EAAC1-mediated glutamate transport. These studies show that glutamate transport proteins can be regulated potently and that GTRAP can modulate the transport functions ascribed to EAAC1. GTRAP3-18 may be important in regulating the metabolic function of EAAC1.