Urea transporters in kidney: molecular analysis and contribution to the urinary concentrating process1

Facilitated urea transporters (UTs) are responsible for urea accumulation in the renal inner medulla of the mammalian kidney and therefore play a central role in the urinary concentrating process. Recently, the cDNAs encoding three members of the UT family, UT1, UT2, and UT3 have been cloned. These transporters are expressed in different structures of the mammalian kidney. In rat, UT1 resides in the apical membrane of terminal inner medullary collecting ducts, where it mediates vasopressin-regulated urea reabsorption. UT2 and UT3 are located in descending thin limbs of Henle's loop and descending vasa recta, respectively, and participate in urinary recycling processes, which minimize urea escape from the inner medulla. UT1 and UT2 are regulated independently and respond differently to changes in dietary protein content and hydration state. Identification and characterization of these urea transporters advances our understanding of the molecular basis and regulation of the urinary concentrating mechanism.

Characterization of a rat Na+-dicarboxylate cotransporter

The metabolism of Krebs cycle intermediates is of fundamental importance for eukaryotic cells. In the kidney, these intermediates are transported actively into epithelial cells. Because citrate is a potent inhibitor for calcium stone formation, excessive uptake results in nephrolithiasis due to hypocitraturia. We report the cloning and characterization of a rat kidney dicarboxylate transporter (SDCT1). In situ hybridization revealed that SDCT1 mRNA is localized in S3 segments of kidney proximal tubules and in enterocytes lining the intestinal villi. Signals were also detected in lung bronchioli, the epididymis, and liver. When expressed in Xenopus oocytes, SDCT1 mediated electrogenic, sodium-dependent transport of most Krebs cycle intermediates (Km = 20-60 microM), including citrate, succinate, alpha-ketoglutarate, and oxaloacetate. Of note, the acidic amino acids L- and D-glutamate and aspartate were also transported, although with lower affinity (Km = 2-18 mM). Transport of citrate was pH-sensitive. At pH 7.5, the Km for citrate was high (0.64 mM), whereas at pH 5.5, the Km was low (57 microM). This is consistent with the concept that the -2 form of citrate is the transported species. In addition, maximal currents at pH 5.5 were 70% higher than those at pH 7.5, and our data show that the -3 form acts as a competitive inhibitor. Simultaneous measurements of substrate-evoked currents and tracer uptakes under voltage-clamp condition, as well as a thermodynamic approach, gave a Na+ to citrate or a Na+ to succinate stoichiometry of 3 to 1. SDCT1-mediated currents were inhibited by phloretin. This plant glycoside also inhibited the SDCT1-specific sodium leak in the absence of substrate, indicating that at least one Na+ binds to the transporter before the substrate. The data presented provide new insights into the biophysical characteristics and physiological implications of a cloned dicarboxylate transporter.

Tubular localization and tissue distribution of peptide transporters in rat kidney

PURPOSE

To define the tubular localization and tissue distribution of PEPT1 (low-affinity, high-capacity transporter) and PEPT2 (high-affinity, low-capacity transporter) in rat kidney.

METHODS

mRNA expression of PEPT1 and PEPT2 was assessed with reverse transcription-polymerase chain reaction (RT-PCR) methods using cDNA prepared from microdissected nephron segments in rat. Tissue localization of rat renal PEPT1 and PEPT2 mRNA was further assessed by in situ hybridization with radiolabeled probes.

RESULTS

RT-PCR analysis of microdissected segments from rat nephron showed that both PEPT1 and PEPT2 are confined to a proximal tubule. While PEPT1 is specific for early regions of the proximal tubule (pars convoluta), PEPT2 is overwhelmingly but not exclusively expressed in latter regions of the proximal tubule (pars recta). All other segments along the nephron were negative for PEPT1 or PEPT2 mRNA transcripts. These finding were supported by in situ hybridization results in which PEPT1 was selectively expressed in kidney cortex and PEPT2 in the outer stripe of outer medulla.

CONCLUSIONS

Contrary to current opinion, the data suggest that peptides are handles in a sequential manner in proximal regions of the nephron, first by the low-affinity, high-capacity transport system and second by the high-affinity, low-capacity transport system.

Comparative analysis of glutamate transporter expression in rat brain using differential double in situ hybridization

This study compares the mRNA expression pattern for the three glutamate transporters EAAC1, GLT1 and GLAST in rat brain, using a sensitive non-radioactive in situ hybridization technique. The results confirm the predominantly neuronal localization of EAAC1 mRNA, the astroglial and ependymal localization of GLAST mRNA and the astroglial and neuronal localization of GLT1 mRNA. Further, we demonstrate, using a novel differential double hybridization protocol, that the presence of GLT1 mRNA in neurons is more widespread than previously thought, and that it encompasses the majority of neurons in the neocortex, neurons in the external plexiform layer in the olfactory bulb, neurons in dorsal and ventral parts of the anterior olfactory nucleus, the majority of neurons in the anteromedial thalamic nuclei, the CA3 pyramidal neurons in the hippocampus and neurons in the inferior olive. In addition, we demonstrate marked variations in the expression levels of GLT1 and GLAST mRNAs in different brain areas, suggesting that their mRNA levels are regulated by different mechanisms. Finally, for EAAC1 we demonstrate also a widespread distribution and a marked heterogeneity in the expression levels. EAAC1 is strongly expressed by a heretofore unrecognized group of cells in white matter tracts such as the corpus callosum, fimbria-fornix or anterior commissure. Also, strong EAAC1 expression is present in groups of scattered cells in grey matter areas of much of the forebrain and the cerebellum. These results provide more detailed information about the precise cellular localization of these three glutamate transporters and their regulation at the mRNA level.

Distribution of mRNA for the facilitated urea transporter UT3 in the rat nervous system

Recently, the cDNA encoding the rat urea transporter UT3 has been cloned from rat kidney. Here we describe the cellular localization of this transporter in the brain as detected by non-radioactive in situ hybridization. UT3 is expressed in astrocytes throughout the central nervous system as well as in Bergmann glia in the cerebellum. The expression in astrocytes was verified by double staining using the astrocytic marker GFAP. UT3 mRNA is also strongly expressed by the ependymal cells lining the cerebral ventricles and by Müller cells in the retina. Furthermore, UT3 expression was detected in subgroups of neurons in the inferior colliculus and dorsal root ganglia, as well as in cells in the anterior pituitary gland. Other types of brain cells, including oligodendrocytes, microglia, tanycytes, endothelial cells of blood vessels, and epithelial cells in the choroid plexus were devoid of UT3 mRNA. Northern blot analysis confirmed that the mRNA species in the brain and in dorsal root ganglia are identical, and that cultured astrocytes and C6 cells also express the UT3 mRNA. UT3 mRNA expression by astrocytes is markedly upregulated in quinolinic acid-induced gliosis, possibly as a result of increased urea levels during gliosis induced polyamine formation. We propose that UT3 in astrocytes represents a mechanism to control urea formed in the brain by equilibrating it throughout the astrocyte network and guiding it to blood vessels and the CSF for disposal.

Cloning and characterization of a potassium-coupled amino acid transporter

Active solute uptake in bacteria, fungi, plants, and animals is known to be mediated by cotransporters that are driven by Na+ or H+ gradients. The present work extends the Na+ and H+ dogma by including the H+ and K+ paradigm. Lepidopteran insect larvae have a high K+ and a low Na+ content, and their midgut cells lack Na+/K+ ATPase. Instead, an H+ translocating, vacuolar-type ATPase generates a voltage of approximately -240 mV across the apical plasma membrane of so-called goblet cells, which drives H+ back into the cells in exchange for K+, resulting in net K+ secretion into the lumen. The resulting inwardly directed K+ electrochemical gradient serves as a driving force for active amino acid uptake into adjacent columnar cells. By using expression cloning with Xenopus laevis oocytes, we have isolated a cDNA that encodes a K+-coupled amino acid transporter (KAAT1). We have cloned this protein from a larval lepidopteran midgut (Manduca sexta) cDNA library. KAAT1 is expressed in absorptive columnar cells of the midgut and in labial glands. When expressed in Xenopus oocytes, KAAT1 induced electrogenic transport of neutral amino acids but excludes alpha-(methylamino)isobutyric acid and charged amino acids resembling the mammalian system B. K+, Na+, and to a lesser extent Li+ were accepted as cotransported ions, but K+ is the principal cation, by far, in living caterpillars. Moreover, uptake was Cl(-)-dependent, and the K+/Na+ selectivity increased with hyperpolarization of oocytes, reflecting the increased K+/Na+ selectivity with hyperpolarization observed in midgut tissue. KAAT1 has 634 amino acid residues with 12 putative membrane spanning domains and shows a low level of identity with members of the Na+ and Cl(-)-coupled neurotransmitter transporter family.

Cloning and functional expression of rNBC, an electrogenic Na(+)-HCO3- cotransporter from rat kidney

We have recently cloned the renal electrogenic Na(+)-bicarbonate contransporter of the salamander Ambystoma tigrinum (aNBC) (M. F. Romero, M. A. Hediger, E. L. Boulpaep, and W. F. Boron. FASEB J. 10: 89, 1996; and Nature 387: 409-413, 1997). Here we report the cloning of a mammalian homolog of aNBC, named rNBC for rat Na(+)-bicarbonate cotransporter. NBC constitutes the major route for HCO3- reabsorption and assists in Na+ reabsorption across the basolateral membrane of the renal proximal tubule (PT). We used aNBC as a probe to screen a rat kidney cortex cDNA library in lambda gt10 and identified several clones. Each has an initiator Met and a large open-reading frame followed by a 3'-untranslated region of approximately 500 bp. The 7.5-kb mRNA for rNBC is present in kidney, liver, lung, brain, and heart. In situ hybridization with the rNBC probe in the rat kidney revealed staining in the S2 segment of PT. rNBC encodes a protein of 1,035 amino acids, with a predicted molecular mass of 116 kDa. Its deduced amino acid sequence is 86% identical to that of aNBC. Comparison of both the aNBC and rNBC sequences to the GenBank database reveals a low level of amino acid identity (approximately 30%) to the AE family of Cl-/HCO3- exchangers. Injection of rNBC cRNA into Xenopus oocytes leads to expression of an electrogenic Na(+)-HCO3- contransporter that is qualitatively similar to that of aNBC but at a much lower level. Placement of the rNBC cDNA into the context of a Xenopus expression vector produces a substantial increase in rNBC expression. Addition of 1.5% CO2/10 mM HCO3- elicits a hyperpolarization of > 50 mV and a rapid decrease of intracellular pH (pHi), followed by an increase in pHi. Subsequent removal of Na+ in the presence of CO2/HCO3- causes a depolarization of > 50 mV and a concomitant decrease of pHi. Thus rNBC is in the same newly identified family of Na(+)-linked HCO3- transporters as is aNBC.

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

Most amino acids filtered by the glomerulus are reabsorbed in the kidney via specialized transport systems. Recently, the cDNA encoding a high-affinity glutamate transporter, EAAC1, has been isolated and shown to be expressed at high levels in the kidney. To determine the potential role of EAAC1 in renal acidic amino acid reabsorption, the distribution of EAAC1 mRNA and protein in rat kidney was examined. In situ hybridization revealed that EAAC1 mRNA is expressed predominantly in S2 and S3 segments of the proximal tubules and at low levels in the inner stripe of outer medulla and inner medulla. Polyclonal antibodies raised against the carboxy terminus of EAAC1 recognized a single band of approximately 70 kDa on Western blots of membrane protein from kidney cortex and medulla. Immunofluorescence microscopy revealed intense signals in the luminal membrane of S2 and S3 segments and weaker signals in S1 segments, descending thin limbs of long-loop nephrons, medullary thick ascending limbs, and distal convoluted tubules. These results are consistent with EAAC1 encoding the previously described apical high-affinity glutamate transporter in the kidney that mediates reabsorption of acidic amino acids in tubules beyond early proximal tubule S1 segments. Potential additional roles of EAAC1 in acid/base balance, cell volume regulation, and amino acid metabolism are discussed.

Differential modulation of the uptake currents by redox interconversion of cysteine residues in the human neuronal glutamate transporter EAAC1

Control of extrasynaptic glutamate concentration in the central nervous system is an important determinant of neurotransmission and excitotoxicity. Mechanisms that modulate glutamate transporter function are therefore critical factors in these processes. The redox modulation of glutamate uptake was examined by measuring transporter-mediated electrical currents and radiolabelled amino acid influx in voltage-clamped Xenopus oocytes expressing the human neuronal glutamate transporter EAAC1. Up and down changes of the glutamate uptake currents in response to treatment with dithiothreitol and 5,5'-dithio-bis-(2-nitrobenzoic) acid (DTNB) were observed in oocytes clamped at -60 mV. The redox interconversion of cysteines induced by dithiothreitol/DTNB influenced the Vmax (Imax) of transport, while the apparent affinity for glutamate was not affected. Formation or breakdown of disulphide groups did not affect the pre-steady-state currents, suggesting that these manipulations do not interfere with the Na+ binding/unbinding and/or the charge distribution on the transporter molecule. The glutamate-evoked net uptake current of EAAC1 was composed of the inward current from electrogenic glutamate transport and the current arising from the glutamate-activated Cl- conductance. The structural rearrangement produced by the formation or breakdown of disulphide groups only affected the current from electrogenic glutamate transport. The electrogenic currents of EAAC1 were significantly reduced by peroxynitrite, an endogenously occurring oxidant formed in certain pathological brain processes, and the mechanism of inhibition partially depended on the formation of disulphide groups.

Cloning and characterization of a mammalian proton-coupled metal-ion transporter

Metal ions are essential cofactors for a wealth of biological processes, including oxidative phosphorylation, gene regulation and free-radical homeostasis. Failure to maintain appropriate levels of metal ions in humans is a feature of hereditary haemochromatosis, disorders of metal-ion deficiency, and certain neurodegenerative diseases. Despite their pivotal physiological roles, however, there is no molecular information on how metal ions are actively absorbed by mammalian cells. We have now identified a new metal-ion transporter in the rat, DCT1, which has an unusually broad substrate range that includes Fe2+, Zn2+, Mn2+, Co2+, Cd2+, Cu2+, Ni2+ and Pb2+. DCT1 mediates active transport that is proton-coupled and depends on the cell membrane potential. It is a 561-amino-acid protein with 12 putative membrane-spanning domains and is ubiquitously expressed, most notably in the proximal duodenum. DCT1 is upregulated by dietary iron deficiency, and may represent a key mediator of intestinal iron absorption. DCT1 is a member of the 'natural-resistance-associated macrophage protein' (Nramp) family and thus its properties provide insight into how these proteins confer resistance to pathogens.

The electrogenic Na/HCO3 cotransporter

The electrogenic Na/HCO3 cotransporter (symporter) is the major HCO3- transporter of the renal proximal tubule (PiT), located at the basolateral membrane (BLM), and also plays a noteworthy role in Na+ reabsorption. HCO3 transporters are important for regulation of intracellular pH (pHi) in most cells and also thereby regulate blood pH. This electrogenic Na/HCO3 cotransporter was first discovered using perfused Ambystoma tigrinum (salamander) renal, proximal tubules. This novel cotransporter mediates the movement of one Na+ ion with several HCO3- ions, making it electrogenic, is blocked by stilbene compounds, but does not depend on intra- or extracellular Cl-. This and similar cotransporters have been found in a number of tissues and cell types. Recently, we used Xenopus-laevis oocytes to expression clone the salamander renal electrogenic Na Bicarbonate Cotransporter (NBC). Using microelectrodes to monitor membrane potential (Vm) and intracellular pH (pHi), we followed oocyte expression after injecting poly (A)+, fractioned poly (A)+, or cRNA. All experimental solutions contained 100 microM ouabain to block the Na+/K+ pump. Our expression assay was to apply 1.5% CO2/10 mM HCO3- (pH 7.5), allow pHi to stabilize from the CO2-induced acidification, and then remove bath Na+. Removing bath Na+ from native oocytes and water-injected controls, hyperpolarized the oocytes by approximately 5 mV and had no effect on pHi. However, for oocytes injected with poly (A)+ RNA, removing Na+ transiently depolarized the cell by approximately 10 mV and caused pHi to decrease; both effects were blocked by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and required HCO3-. Electrophoretic fractionation of the poly (A)+ RNA, enriched the expression signal. From the optimal expression-fraction, we constructed a size-selected cDNA library in pSPORT1. Screening our Ambystoma library yielded a single clone (aNBC). We could detect expression 3 days after injection of NBC cRNA. In aNBC-expressing oocytes, adding CO2/HCO3-elicited a large (> 50mV) and rapid hyperpolarization, followed by a partial relaxation as pHi stabilized. Na+ removal in CO2/HCO3-depolarized the cell by > 40mV and decreased pHi, aNBC encodes a protein of 1035 amino acids with several putative membrane-spanning domains, and has a low level of amino-acid homology (approximately 30% to the AE family of Cl-HCO3 exchangers. aNBC is the first member of a new family of Na(+)-linked HCO3- transporters and, together with the AE family, defines a new superfamily of HCO3- transporters. Using aNBC to screen a rat-kidney cDNA library, we identified a full-length cDNA clone (rNBC), rNBC encodes a protein of 1035 amino acids, is 86% identical to aNBC, and can be functionally expressed in oocytes.

Expression cloning and characterization of a renal electrogenic Na+/HCO3- cotransporter

Bicarbonate transporters are the principal regulators of pH in animal cells, and play a vital role in acid-base movement in the stomach, pancreas, intestine, kidney, reproductive system and central nervous system. The functional family of HCO3- transporters includes Cl- -HCO3- exchangers, three Na+/HCO3- cotransporters, a K+/HCO3- cotransporter, and a Na+-driven Cl- -HCO3- exchanger. Molecular information is sparse on HCO3- transporters, apart from Cl- -HCO3- exchangers ('anion exchangers'), whose complementary DNAs were cloned several years ago. Attempts to clone other HCO3- transporters, based on binding of inhibitors, protein purification or homology with anion exchangers, have so far been unsuccessful. Here we monitor the intracellular pH and membrane voltage in Xenopus oocytes to follow the expression of the most electrogenic transporter known: the renal 1:3 electrogenic Na+/HCO3- cotransporter from the salamander Ambystoma tigrinum. We now report the successful cloning and characterization of a cDNA encoding a cation-coupled HCO3- transporter. The encoded protein is 1,035 amino acids long with several potential membrane-spanning domains. We show that when it is expressed in Xenopus oocytes, this protein is electrogenic, Na+ and HCO3- dependent, and blocked by the anion-transport inhibitor DIDS, and conclude that it is the renal electrogenic sodium bicarbonate cotransporter (NBC).

Segmental localization of urea transporter mRNAs in rat kidney

Renal epithelia express at least two distinct urea transporter mRNAs, termed UT1 and UT2, that are derived from a single UT gene by alternative splicing. Previous immunolocalization studies using a polyclonal antibody that does not distinguish between the protein products of these two transcripts revealed that expression of urea transporter protein is restricted to inner medullary collecting ducts and descending thin limbs of Henle's loop. To identify which transcripts account for protein expression in these two structures, we carried out reverse transcription-polymerase chain reaction studies in microdissected structures using UT1- and UT2-specific primers. UT1 mRNA was detected only in the inner medullary collecting duct, consistent with its identification as the vasopressin-regulated urea transporter. In contrast, UT2-mRNA was detected in the late part of descending thin limbs of short loops of Henle and in the inner medullary part of descending thin limbs of long loops of Henle. This localization is consistent with the predicted role of UT2 in medullary urea recycling. Thus, in conjunction with foregoing physiological studies, our data indicate that these transporters play central roles in the urinary concentrating mechanism.

Cloning and characterization of the urea transporter UT3: localization in rat kidney and testis

Urea transport in the kidney plays an important role in urinary concentration and nitrogen balance. Recently, three types of urea transporters have been cloned, UT1 and UT2 from rat and rabbit kidney and HUT11 from human bone marrow. To elucidate the physiological role of the latter urea transporter, we have isolated the rat homologue (UT3) of HUT11 and studied its distribution of expression and functional characteristics. UT3 cDNA encodes a 384 amino acid residue protein, which has 80% identity to the human HUT11 and 62% identity to rat UT2. Functional expression in Xenopus oocytes induced a large (approximately 50-fold) increase in the uptake of urea compared with water-injected oocytes. The uptake was inhibited by phloretin (0.75 mM) and pCMBS (0.5 mM) (55 and 32% inhibition, respectively). Northern analysis gave a single band of 3.8 kb in kidney inner and outer medulla, testis, brain, bone marrow, spleen, thymus, and lung. In situ hybridization of rat kidney revealed that UT3 mRNA is expressed in the inner stripe of the outer medulla, inner medulla, the papillary surface epithelium, and the transitional urinary epithelium of urinary tracts. Co-staining experiments using antibody against von Willebrand factor showed that UT3 mRNA in the inner stripe of the outer medulla is expressed in descending vasa recta. These data suggest that UT3 in kidney is involved in counter current exchange between ascending and descending vasa recta, to enhance the cortico-papillary osmolality gradient. In situ hybridization of testis revealed that UT3 is located in Sertoli cells of seminiferous tubules. The signal was only detected in Sertoli cells associated with the early stages of spermatocyte development, suggesting that urea may play a role in spermatogenesis.

Symmetry of H+ binding to the intra- and extracellular side of the H+-coupled oligopeptide cotransporter PepT1

Ion-coupled solute transporters exhibit pre-steady-tate currents that resemble those of voltage-dependent ion channels. These currents were assumed to be mostly due to binding and dissociation of the coupling ion near the extracellular transporter surface. Little attention was given to analogous events that may occur at the intracellular surface. To address this issue, we performed voltage clamp studies of Xenopus oocytes expressing the intestinal H+-coupled peptide cotransporter PepT1 and recorded the dependence of transient charge movements in the absence of peptide substrate on changing intra- (pHi) and extracellular pH (pHo). Rapid steps in membrane potential induced transient charge movements that showed a marked dependence on pHi and pHo. At a pHo of 7.0 and a holding potential (Vh) of -50 mV, the charge movements were mostly inwardly directed, whereas reduction of pHo to below 7.0 resulted in outwardly directed charge movements. When pHi was reduced, inwardly directed charge movements were observed. The data on the voltage dependence of the transient charge movements were fitted by the Boltzmann equation, yielding an apparent valence of 0.65 +/- 0.03 (n = 7). The midpoint voltage (V0.5) of the charge distribution shifted linearly as a function of pHi and pHo. Our results indicate that, as a first approximation, the magnitude and polarity of the transient charge movements depend upon the prevailing H+ electrochemical gradient. We propose that PepT1 has a single proton binding site that is symmetrically accessible from both sides of the membrane and that decreasing the H+ chemical potential (DeltamuH) or increasing the membrane potential (Vm) shifts this binding site from an outwardly to an inwardly facing occluded state. This concept constitutes an important extension of previous kinetic models of ion-coupled solute transporters by including a more detailed description of intracellular events.

Stoichiometry and pH dependence of the rabbit proton-dependent oligopeptide transporter PepT1

1. The intestinal H(+)-coupled peptide transporter PepT1, displays a broad substrate specificity and accepts most charged and neutral di- and tripeptides. To study the proton-to-peptide stoichiometry and the dependence of the kinetic parameters on extracellular pH (pHo), rabbit PepT1 was expressed in Xenopus laevis oocytes and used for uptake studies of radiolabelled neutral and charged dipeptides, voltage-clamp analysis and intracellular pH measurements. 2. PepT1 did not display the substrate-gated anion conductances that have been found to be characteristic of members of the Na(+)- and H(+)-coupled high-affinity glutamate transporter family. In conjunction with previous data on the ion dependence of PepT1, it can therefore be concluded that peptide-evoked charge fluxes of PepT1 are entirely due to H+ movement. 3. Neutral, acidic and basic dipeptides induced intracellular acidification. The rate of acidification, the initial rates of the uptake of radiolabelled peptides and the associated charge fluxes gave proton-substrate coupling ratios of 1:1, 2:1 and 1:1 for neutral, acidic and basic dipeptides, respectively. 4. Maximal transport of the neutral and charged dipeptides Gly-Leu, Gly-Glu, Gly-Lys and Ala-Lys occurred at pHo 5.5, 5.2, 6.2 and 5.8, respectively. The Imax values were relatively pHo independent but the apparent affinity (Km(app) values for these peptides were shown to be highly pHo dependent. 5. Our data show that at physiological pH (pHo 5.5-6.0) PepT1 prefers neutral and acidic peptides. The shift in transport maximum for the acidic peptide Gly-Glu to a lower pH value suggests that acidic dipeptides are transported in the protonated form. The shift in the transport maxima of the basic dipeptides to higher pH values may involve titration of a side-chain on the transporter molecule (e.g. protonation of a histidine group). These considerations have led us to propose a model for coupled transport of neutral, acidic and basic dipeptides.

The renal electrogenic Na+:HCO-3 cotransporter

The electrogenic Na+:HCO3- cotransporter (symporter) is the major transporter for HCO3- reabsorption across the basolateral membrane of the renal proximal tubule and also contributes significantly to Na+ reabsorption. We expression-cloned the salamander renal electrogenic Na+:Bicarbonate Cotransporter (NBC) in Xenopus laevis oocytes. After injecting poly(A)+ RNA, fractionated poly(A)+ RNA or cRNA, we used microelectrodes to monitor membrane potential (Vm) and intracellular pH (pHi) All solutions contained ouabain to block the Na+/K+ pump (P-ATPase). After applying 1.5% CO2/10 mmol l-1 HCO3- (pH 7.5) and allowing pHi to stabilize from the CO2-induced acidification, we removed Na+. In native oocytes or water-injected controls, removing Na+ hyperpolarized the cell by -5 mV and had no effect on pHi. In oocytes injected with poly(A)+ RNA, removing Na+ transiently depolarized the cell by -10 mV and caused pHi to decrease; both effects were blocked by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) and required HCO3-. We enriched the signal by electrophoretic fractionation of the poly(A)+ RNA, and constructed a size-selected cDNA library in pSPORT1 using the optimal fraction. Screening the Ambystoma library yielded a single clone (aNBC). Expression was first obvious 3 days after injection of NBC cRNA. Adding CO2/HCO3- induced a large (> 50 mV) and rapid hyperpolarization, followed by a partial relaxation as pHi stabilized. Subsequent Na+ removal depolarized the cell by more than 40 mV and decreased pHi. aNBC is a full-length clone with a start Met and a poly(A)+ tail; it encodes a protein with 1025 amino acids and several putative membrane-spanning domains. aNBC is the first member of a new family of Na(+)-linked HCO3- transporters. We used aNBC to screen a rat kidney cDNA library, and identified a full-length cDNA clone (rNBC) that encodes a protein of 1035 amino acids. rNBC is 86% identical to aNBC and can be functionally expressed in oocytes.

Molecular characteristics of mammalian and insect amino acid transporters: implications for amino acid homeostasis

In mammalian cells, the uptake of amino acids is mediated by specialized, energy-dependent and passive transporters with overlapping substrate specificities. Most energy-dependent transporters are coupled either to the cotransport of Na+ or Cl- or to the countertransport of K+. Passive transporters are either facilitated transporters or channels. As a prelude to the molecular characterization of the different classes of transporters, we have isolated transporter cDNAs by expression-cloning with Xenopus laevis oocytes and we have characterized the cloned transporters functionally by uptake studies into oocytes using radiolabelled substrates and by electrophysiology to determine substrate-evoked currents. Mammalian transporters investigated include the dibasic and neutral amino acid transport protein D2/NBAT (system b0+) and the Na(+)- and K(+)-dependent neuronal and epithelial high-affinity glutamate transporter EAAC1 (system XAG-). A detailed characterization of these proteins has provided new information on transport characteristics and mechanisms for coupling to different inorganic ions. This work has furthermore advanced our understanding of the roles these transporters play in amino acid homeostasis and in various pathologies. For example, in the central nervous system, glutamate transporters are critically important in maintaining the extracellular glutamate concentration below neurotoxic levels, and defects of the human D2 gene have been shown to account for the formation of kidney stones in patients with cystinuria. Using similar approaches, we are investigating the molecular characteristics of K(+)-coupled amino acid transporters in the larval lepidopteran insect midgut. In the larval midgut, K+ is actively secreted into the lumen through the concerted action of an apical H+ V-ATPase and an apical K+/2H+ antiporter, thereby providing the driving force for absorption of amino acids. In vivo, the uptake occurs at extremely high pH (pH 10) and is driven by a large potential difference (approximately -200 mV). Studies with brush-border membrane vesicles have shown that there are several transport systems in the larval intestine with distinct amino acid and cation specificities. In addition to K+, Na+ can also be coupled to amino acid uptake at lower pH, but the Na+/K+ ratio of the hemolymph is so low that K+ is probably the major coupling ion in vivo. The neutral amino acid transport system of larval midgut has been studied most extensively. Apart from its cation selectivity, it appears to be related to the amino acid transport system B previously characterized in vertebrate epithelial cells. Both systems have a broad substrate range which excludes 2-(methylamino)-isobutyric acid, an amino acid analog accepted by the mammalian Na(+)-coupled system A. In order to gain insights into the K(+)-coupling mechanism and into amino acid and K+ homeostasis in insects, current studies are designed to delineate the molecular characteristics of these insect transporters. Recent data showed that injection of mRNA prepared from the midgut of Manduca sexta into Xenopus laevis oocytes induced a 1.5- to 2.5-fold stimulation of the Na(+)-dependent uptake of both leucine and phenylalanine (0.2 mmoll-1, pH 8). The molecular cloning of these transporters is now in progress. Knowledge of their unique molecular properties could be exploited in the future to control disease vectors and insect pests.

Molecular cloning and characterization of the vasopressin-regulated urea transporter of rat kidney collecting ducts

Absorption of urea in the renal inner medullary collecting duct (IMCD) contributes to hypertonicity in the medullary interstitium which, in turn, provides the osmotic driving force for water reabsorption. This mechanism is regulated by vasopressin via a cAMP-dependent pathway and activation of a specialized urea transporter located in the apical membrane. We report here the cloning of a novel urea transporter, designated UT1, from the rat inner medulla which is functionally and structurally distinct from the previously reported kidney urea transporter UT2. UT1 expressed in Xenopus oocytes mediated passive transport of urea that was inhibited by phloretin and urea analogs but, in contrast to UT2, was strongly stimulated by cAMP agonists. Sequence comparison revealed that the coding region of UT1 cDNA contains the entire 397 amino acid residue coding region of UT2 and an additional 1,596 basepair-stretch at the 5' end. This stretch encodes a novel 532 amino acid residue NH2-terminal domain that has 67% sequence identity with UT2. Thus, UT1 consists of two internally homologous portions that have most likely arisen by gene duplication. Studies of the rat genomic DNA further indicated that UT1 and UT2 are derived from a single gene by alternative splicing. Based on Northern analysis and in situ hybridization, UT1 is expressed exclusively in the IMCD, particularly in its terminal portion. Taken together, our data show that UT1 corresponds to the previously characterized vasopressin-regulated urea transporter in the apical membrane of the terminal IMCD which plays a critical role in renal water conservation.

Functional differentiation of the human red blood cell and kidney urea transporters

The recent cloning of two urea transporters will allow to better understand their role in the urinary concentrating mechanism. This physiological approach needs to be sustained by a knowledge of their functional characteristics. We compared the pharmacological properties of the human red blood cell and kidney urea transporters (HUT11 and HUT2) in the Xenopus oocyte expression system. Both proteins allow the rapid transfer of urea but not of water. Both are inhibited by phloretin, although with different half-maximal inhibitory concentrations (IC50; 75 microM, for HUT11 and 230 microM for HUT2). Whereas para-chloromercuribenzene sulfonate inhibits HUT11 with an IC50 of 150 microM, it does not inhibit HUT2, whatever the concentration used. We demonstrate that thiourea diffuses through HUT11 with a Michaelis constant (Km) of 40 mM, but not through HUT2. In contrast, it inhibits urea transport through both proteins. This identification of a substrate binding site independent from the transport activity is the first step in the understanding of the molecular events underlying urea transport.

Mammalian urea transporters

Urea transporters are membrane proteins that mediate rapid, passive movement of urea across cell membranes. Physiological studies have revealed their significant roles in urea accumulation in the kidney inner medulla, and consequently in the urinary concentrating mechanism. Three mammalian urea transporters have been identified and their expression in the kidney was found to occur in a tissue-specific manner. This review discusses our current knowledge with emphasis on the localization and regulation of expression of urea transporters in different physiological conditions.

Structure, regulation and physiological roles of urea transporters

Urea is the major constituent of the urine and the principal means for disposal of nitrogen derived from amino acid metabolism. Specialized phloretin-inhibitable urea transporters are expressed in kidney medulla and play a central role in urea excretion and water balance. These transporters allow accumulation of urea in the medulla and enable the kidney to concentrate urine to an osmolality greater than systemic plasma. Recently, expression cloning with Xenopus oocytes has led to the isolation of a novel phloretin-inhibitable urea transporter (UT2) from rabbit, and subsequently from rat kidney. UT2 from both species has the characteristics of the phloretin-sensitive urea transporter previously defined in kidney by in vitro perfused tubule studies. Based on these advances, Ripoche and colleagues cloned a homologous urea transporter (HUT11) from erythrocytes. UT2 and HUT11 predict 43 kDa polypeptides and exhibit 64% amino acid sequence identity. Since regulation of urea transport in the kidney plays an important role in the orchestration of the antidiuretic response, we have studied the regulation of urea transporter in rat kidney at the mRNA level. On Northern blots probed at high stringency, rat UT2 hybridized to two transcripts of 2.9 kb and 4.0 kb, which have spatially distinct distributions within the kidney. Northern analysis and in situ hybridization of kidneys from rats maintained at different physiologic states revealed that the 2.9 and 4.0 kb transcripts are regulated by separate mechanisms. The 4 kb transcript was primarily responsive to changes in the dietary protein content, whereas the 2.9 kb transcript was highly responsive to changes in the hydration state of the animal. We propose that the two UT2 transcripts are regulated by distinct mechanisms to allow optimal fluid balance and urea excretion.

Cellular and subcellular localization of the vasopressin- regulated urea transporter in rat kidney

The renal urea transporter (RUT) is responsible for urea accumulation in the renal medulla, and consequently plays a central role in the urinary concentrating mechanism. To study its cellular and subcellular localization, we prepared affinity-purified, peptide-derived polyclonal antibodies against rat RUT based on the cloned cDNA sequence. Immunoblots using membrane fractions from rat renal inner medulla revealed a solitary 97-kDa band. Immunocytochemistry demonstrated RUT labeling of the apical and subapical regions of inner medullary collecting duct (IMCD) cells, with no labeling of outer medullary or cortical collecting ducts. Immunoelectron microscopy directly demonstrated labeling of the apical plasma membrane and of subapical intracellular vesicles of IMCD cells, but no labeling of the basolateral plasma membrane. Immunoblots demonstrated RUT labeling in both plasma membrane and intracellular vesicle-enriched membrane fractions from inner medulla, a subcellular distribution similar to that of the vasopressin-regulated water channel, aquaporin-2. In the outer medulla, RUT labeling was seen in terminal portions of short-loop descending thin limbs. Aside from IMCD and descending thin limbs, no other structures were labeled in the kidney. These results suggest that: (i) the RUT provides the apical pathway for rapid, vasopressin-regulated urea transport in the IMCD, (ii) collecting duct urea transport may be increased by vasopressin by stimulation of trafficking of RUT-containing vesicles to the apical plasma membrane, and (iii) the rat urea transporter may provide a pathway for urea entry into the descending limbs of short-loop nephrons.

Knockout of glutamate transporters reveals a major role for astroglial transport in excitotoxicity and clearance of glutamate

Three glutamate transporters have been identified in rat, including astroglial transporters GLAST and GLT-1 and a neuronal transporter EAAC1. Here we demonstrate that inhibition of the synthesis of each glutamate transporter subtype using chronic antisense oligonucleotide administration, in vitro and in vivo, selectively and specifically reduced the protein expression and function of glutamate transporters. The loss of glial glutamate transporters GLAST or GLT-1 produced elevated extracellular glutamate levels, neurodegeneration characteristic of excitotoxicity, and a progressive paralysis. The loss of the neuronal glutamate transporter EAAC1 did not elevate extracellular glutamate in the striatum but did produce mild neurotoxicity and resulted in epilepsy. These studies suggest that glial glutamate transporters provide the majority of functional glutamate transport and are essential for maintaining low extracellular glutamate and for preventing chronic glutamate neurotoxicity.

Nonradioactive monitoring of organic and inorganic solute transport into single Xenopus oocytes by capillary zone electrophoresis

Transport of organic and inorganic solutes into and out of cells requires specialized transport proteins. Given a sufficiently sensitive analytical method for measuring cellular solute concentrations, it should be possible to monitor solute transport across the plasma membrane at the level of single cells. We report a capillary zone electrophoresis approach that is generally applicable to monitor solute transport into Xenopus laevis oocytes, requires only nanoliters of sample, and involves no radioactive materials. The sensitivity of capillary electrophoresis with UV detection is typically on the order of 10(-5)-10(-6) M, resulting in the mass detection limits in the low femtomole range. We show that capillary zone electrophoresis serves as a simple technique to measure solute transport into oocytes. Studies of the mammalian oligopeptide transporter PepT1 and the Na(+)- and K(+)-coupled epithelial and neuronal glutamate transporter EAAC1 expressed in oocytes demonstrate that transport of the dipeptide Trp-Gly via PepT1 and transport of Na+ and K+ via EAAC1 across the oocyte plasma membrane can be monitored by measuring intracellular tryptophan absorption and by indirect UV detection of inorganic ions, respectively. The CZE method allowed the simultaneous detection of changes of intracellular Na+ and K+ concentrations in response to EAAC1-mediated Na+ cotransport and K+ countertransport. This is the first report of a capillary zone electrophoresis-based quantitative analysis of intracellular components of a single cell in response to transport activity.

Molecular characteristics of Na(+)-coupled glucose transporters in adult and embryonic rat kidney

Two distinct Na(+)-coupled glucose transporters (SGLTs) with either a high or a low affinity for glucose were shown to provide reabsorption of filtered glucose in the kidney. We have previously reported the characteristics of the high affinity Na+/glucose cotransporter SGLT1 from rabbit, rat, and human kidney and the low affinity Na+/glucose cotransporter SGLT2 from human kidney. Because the molecular identity of SGLT2 as the kidney cortical low affinity Na+/glucose cotransporter has been recently challenged based on studies of the porcine low affinity Na+/glucoe cotransporter SAAT-pSGLT2 (Mackenzie, B., Panayotova-Heiermann, M., Loo, D. D. F., Lever, J.E., and Wright, E. M. (1994) J. Biol. Chem. 269, 22488-22491), we have reevaluated the properties of SGLT2 in greater detail. We furthermore report new data on the regulation of SGLT1 and SGLT2 during kidney development. To analyze and compare SGLT1 and SGLT2 in adult and embryonic kidney, we have cloned and characterized SGLT2 from rat kidney and determined its tissue distribution based on Northern analysis and in situ hybridization. When expressed in Xenopus oocytes, rat SGLT2 stimulated transport of alpha-methyl-D-glucopyranoside (2 mM) in oocytes up to 4.5-fold over controls with an apparent Km of 3.0 mM. The transport properties (i.e. a Na+ to glucose coupling of 1:1 and lack of galactose transport) generally matched those of the kidney cortical low affinity system. We show that expression of rat SGLT2 mRNA is kidney specific and that it is strongly and exclusively expressed in proximal tubule S1 segments. Hybrid-depletion studies were performed to conclusively determine whether SGLT2 corresponds to the kidney cortical low affinity system. Injection of rat kidney superficial cortex mRNA into oocytes stimulated the uptake of alpha-methyl-D-glucopyranoside (2 mM) 2-3-fold. We show that hybrid depletion of this kidney RNA using an SGLT2 antisense oligonucleotide completely suppresses the uptake. These data strongly indicate that SGLT2 is the major kidney cortical low affinity glucose transporter. We therefore propose that SAAT-pSGLT2 be renamed SGLT3. Experiments addressing the expression of SGLT1 and SGLT2 mRNAs in embryonic rat kidneys reveal that the two Na+/glucose cotransporters are developmentally regulated and that there may be a different splice variant for SGLT2 in embryonic kidney compared to the adult.

Neuronal high-affinity glutamate transport in the rat central nervous system

EAAC1 is a neuronal and epithelial high affinity glutamate transporter previously cloned from rabbit intestine. Here we report the isolation of EAAC 1 from rat brain* and its expression in the central nervous system based on in situ hybridization. Strong signals were detected in brain, spinal cord and retina. Expression of EAAC1 was particularly strong in pyramidal cells of the cerebral cortex, pyramidal cells of the hippocampus, mitral cells of the olfactory bulb, various thalamic nuclei and cells of certain retinal layers. EAAC1 was also expressed in non-glutamatergic neurons such as GABAergic cerebellar Purkinje cells and alpha-motor neurons of the spinal cord. We propose that EAAC1 is not only involved in the sequestration of glutamate at glutamatergic synapses and in protecting neurons from glutamate excitotoxicity, but also in the cellular metabolism involving glutamate.

Cloning and regulation of expression of the rat kidney urea transporter (rUT2)

In mammals, urea is the predominant end-product of nitrogen metabolism and plays a central role in the urinary-concentrating mechanism. Urea accumulation in the renal medulla is critical to the ability of the kidney to concentrate urine to an osmolality greater than systemic plasma. Regulation of urea excretion and accumulation in the renal medulla depends on the functional state of specialized phloretin-sensitive urea transporters. To study these transporters and their regulation of expression we isolated a cDNA which encodes the rat homologue (rUT2) of rabbit UT2 (You, G., C.P. Smith, Y. Kanai, W.-S. Lee, M. Stelzner, and M.A. Hediger, et al. Nature (Lond.). 1993. 365:844-847). Rat UT2 has 88% amino acid sequence identity to rabbit UT2 and 64% identity to the recently cloned human erythrocyte urea transporter, HUT11 (Olives, B., P. Neav, P. Bailly, M.A. Hediger, G. Rousselet, J.P. Cartron, and P. Ripoch J. Biol. Chem. 1994. 269:31649-31652). Analysis of rat kidney mRNA revealed two transcripts of size 2.9 and 4.0 kb which had spatially distinct distributions. Northern analysis and in situ hybridization showed that the 4.0-kb transcript was primarily responsive to changes in the protein content of the diet whereas the 2.9-kb transcript was responsive to changes in the hydration state of the animal. These studies reveal that the expression levels of the two rUT2 transcripts are modulated by different pathways to allow fluid and nitrogen balance to be regulated independently. Our data provide important insights into the regulation of the renal urea transporter UT2 and provide a basis on which to refine our understanding of the urinary concentrating mechanism and its regulation.

Electrogenic properties of the epithelial and neuronal high affinity glutamate transporter

Active ion-coupled glutamate transport is of critical importance for excitatory synaptic transmission, normal cellular function, and epithelial amino acid metabolism. We previously reported the cloning of the rabbit intestinal high affinity glutamate transporter EAAC1 (Kanai, Y., and Hediger, M. A. (1992) Nature 360, 467-471), which is expressed in numerous tissues including intestine, kidney, liver, heart, and brain. Here, we report a detailed stoichiometric and kinetic analysis of EAAC1 expressed in Xenopus laevis oocytes. Uptake studies of 22Na+ and [14C]glutamate, in combination with measurements of intracellular pH with pH microelectrodes gave a glutamate to charge ratio of 1:1, a glutamate to Na+ ratio of 1:2, and a OH-/H+ to charge ratio of 1:1. Since transport is K+ dependent it can be concluded that EAAC1-mediated glutamate transport is coupled to the cotransport of 2 Na+ ions, the countertransport of one K+ ion and either the countertransport of one OH- ion or the cotransport of 1 H+ ion. We further demonstrate that under conditions where the electrochemical gradients for these ions are disrupted, EAAC1 runs in reverse, a transport mode which is of pathologic importance. 22Na+ uptake studies revealed that there is a low level of Na+ uptake in the absence of extracellular glutamate which appears to be analogous to the Na+ leak observed for the intestinal Na+/glucose cotransporter SGLT1. In voltage clamp studies, reducing extracellular Na+ from 100 to 10 mM strongly increased K0.5L-glutamate and decreased I(max). The data indicate that Na+ binding at the extracellular transporter surface becomes rate-limiting. Studies addressing the cooperativity of the substrate-binding sites indicate that there are two distinct Na(+)-binding sites with different affinities and that Na+ binding is modulated by extracellular glutamate. A hypothetical ordered kinetic transport model for EAAC1 is discussed.

Molecular cloning of PEPT 2, a new member of the H+/peptide cotransporter family, from human kidney

Mammalian kidney is known to express a transport system specific for small peptides and pharmacologically active aminocephalosporins. This system is energized by a transmembrane electrochemical H+ gradient. Recently, a H(+)-coupled peptide transporter has been cloned from rabbit and human intestine (Fei et al. (1994) Nature 368, 563-566; Liang et al., J. Biol. Chem., in press). Functional studies have established that the renal peptide transport system is similar but not identical to its intestinal counterpart. Therefore, in an attempt to isolate the renal H+/peptide cotransporter cDNA, we screened a human kidney cDNA library with a probe derived from the rabbit intestinal H+/peptide cotransporter cDNA. This has resulted in the isolation of a positive clone with a 2190 bp long open reading frame. The predicted protein consists of 729 amino acids. Hydropathy analysis of the amino acid sequence indicates the presence of twelve putative transmembrane domains. The primary structure of this protein exhibits 50% identity and 70% similarity to the human intestinal H+/peptide cotransporter. Functional expression of the kidney cDNA in HeLa cells results in the induction of a H(+)-coupled transport system specific for small peptides and aminocephalosporins. Reverse transcription-coupled polymerase chain reaction demonstrates that the cloned transporter is expressed in human kidney but not in human intestine. This transporter, henceforth called PEPT 2, represents a new member in the growing family of H(+)-coupled transport systems in the mammalian plasma membrane.

Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization

In mammalian small intestine, a H(+)-coupled peptide transporter is responsible for the absorption of small peptides arising from digestion of dietary proteins. Recently a cDNA clone encoding a H+/peptide cotransporter has been isolated from a rabbit intestinal cDNA library (Fei, Y.J., Kanai, Y., Nussberger, S., Ganapathy, V., Leibach, F.H., Romero, M.F., Singh, S.K., Boron, W. F., and Hediger, M. A. (1994) Nature 368, 563-566). Screening of a human intestinal cDNA library with a probe derived from the rabbit H+/peptide cotransporter cDNA resulted in the identification of a cDNA which when expressed in HeLa cells or in Xenopus laevis oocytes induced H(+)-dependent peptide transport activity. The predicted protein consists of 708 amino acids with 12 membrane-spanning domains and two putative sites for protein kinase C-dependent phosphorylation. The cDNA-induced transport process accepts dipeptides, tripeptides, and amino beta-lactam antibiotics but not free amino acids as substrates. The human H+/peptide cotransporter exhibits a high degree of homology (81% identity and 92% similarity) to the rabbit H+/peptide cotransporter. But surprisingly these transporters show only a weak homology to the H(+)-coupled peptide transport proteins present in bacteria and yeast. Chromosomal assignment studies with somatic cell hybrid analysis and in situ hybridization have located the gene encoding the cloned human H+/peptide cotransporter to chromosome 13 q33-->q34.

Mammalian ion-coupled solute transporters

Active transport of solutes into and out of cells proceeds via specialized transporters that utilize diverse energy-coupling mechanisms. Ion-coupled transporters link uphill solute transport to downhill electrochemical ion gradients. In mammals, these transporters are coupled to the co-transport of H+, Na+, Cl- and/or to the countertransport of K+ or OH-. By contrast, ATP-dependent transporters are directly energized by the hydrolysis of ATP. The development of expression cloning approaches to select cDNA clones solely based on their capacity to induce transport function in Xenopus oocytes has led to the cloning of several ion-coupled transporter cDNAs and revealed new insights into structural designs, energy-coupling mechanisms and physiological relevance of the transporter proteins. Different types of mammalian ion-coupled transporters are illustrated by discussing transporters isolated in our own laboratory such as the Na+/glucose co-transporters SGLT1 and SGLT2, the H(+)-coupled oligopeptide transporters PepT1 and PepT2, and the Na(+)- and K(+)-dependent neuronal and epithelial high affinity glutamate transporter EAAC1. Most mammalian ion-coupled organic solute transporters studied so far can be grouped into the following transporter families: (1) the predominantly Na(+)-coupled transporter family which includes the Na+/glucose co-transporters SGLT1, SGLT2, SGLT3 (SAAT-pSGLT2) and the inositol transporter SMIT, (2) the Na(+)- and Cl(-)-coupled transporter family which includes the neurotransmitter transporters of gamma-amino-butyric acid (GABA), serotonin, dopamine, norepinephrine, glycine and proline as well as transporters of beta-amino acids, (3) the Na(+)- and K(+)-dependent glutamate/neurotransmitter family which includes the high affinity glutamate transporters EAAC1, GLT-1, GLAST, EAAT4 and the neutral amino acid transporters ASCT1 and SATT1 reminiscent of system ASC and (4) the H(+)-coupled oligopeptide transporter family which includes the intestinal H(+)-dependent oligopeptide transporter PepT1.

Cloning and functional expression of a urea transporter from human bone marrow cells

A rapid passive urea transport has been previously described in the mammalian renal inner medullary collecting duct epithelial cells and in mammalian erythrocytes. Recently, a vasopressin-regulated urea transporter (UT2) has been cloned from a rabbit kidney medullary cDNA library (You, G., Smith, C. P., Kanai, Y., Lee, W. S., Stelzner, M., and Hediger, M. A. (1993) Nature 365, 844-847). We now report the cloning and characterization of a complementary DNA (HUT11) encoding an urea transporter isolated from a human bone marrow library. It encodes a 43,000-Da polypeptide of 391 amino acids that exhibited 63% sequence identity with the rabbit urea transporter and a similar membrane topology. HUT11 carries 2 putative glycosylation sites and 10 cysteines, of which only 7 are conserved at an equivalent position in UT2. HUT11 transcripts have been identified in human erythroid and renal tissues. Expression studies in Xenopus oocytes demonstrated that HUT11 mediates a facilitated urea transport that was inhibited, as described in mammalian erythrocytes, by very low concentrations of phloretin, p-chloromercuribenzene sulfonate, and urea analogues. No unidirectional movements of charged molecules, glycerol, or water were associated with HUT11 expression in oocytes. These findings suggest that HUT11 is most likely responsible for the facilitated urea transport in human red blood cells.

Structure, function and evolution of solute transporters in prokaryotes and eukaryotes

In both prokaryotes and eukaryotes, transport systems of organic solutes can be classified as passive transporters, such as channels and facilitated transporters, and active transporters, which utilize diverse energy-coupling mechanisms. In the past decade, our understanding of the biochemistry and molecular biology of transporters from Escherichia coli has progressed significantly, whereas the analysis of mammalian transporters has initially been limited by the ability to purify membrane proteins. The recent development of methods to detect the activity of recombinant proteins in individual cells, however, has led to the cloning of several novel mammalian transporter cDNAs. One of the most useful expression cloning systems is Xenopus oocytes in conjunction with uptake studies and electrophysiological experiments. Overall, the sequence information and the functional data derived from many transporters has revealed unifying designs, similar energy-coupling mechanisms and common evolutionary origins. Here, I will provide a general survey of the known transport systems in bacteria, yeast, plants, insects and vertebrates and illustrate the different types of transport systems in mammals by discussing transporters recently studied in our laboratory.

The neuronal and epithelial human high affinity glutamate transporter. Insights into structure and mechanism of transport

High affinity transport of glutamate across plasma membranes of brain neurons and epithelial is mediated by a Na(+)- and K(+)-coupled electrogenic transporter. Here we report the primary structure and functional characterization of the human high affinity glutamate transporter (HEAAC1). A unique characteristic of HEAAC1-mediated transport is that the affinity for glutamate and the maximal transport rate are strongly dependent on membrane potential. Our data provide new insights into individual steps of high affinity glutamate transport and show that the transport mechanism is distinct from that of the gamma-aminobutyric acid transporter GAT-1 and the Na+/glucose transporter SGLT1. Under voltage clamp condition, HEAAC1 mediated large substrate-evoked inward currents (up to 1 microA). The substrate specificity, stereospecificity, the Km value (30 +/- 3 microM at -60 mV) of the L-glutamate-evoked current, and Northern analysis all agree with previously reported characteristics of high affinity glutamate transport in brain. In contrast to SGLT1 and GAT-1, voltage jump studies of HEAAC1 yielded only minor relaxation currents. Classic inhibitors of brain glutamate uptake such as DL-threo-beta-hydroxyaspartate, L-trans-pyrrolidine 2,4,-dicarboxylic acid (PDC), and dihydrokainate were found to be either transport substrates or to have no significant effect on glutamate transport. We also found that the maximal transport rate for PDC was markedly reduced compared to that for L-glutamate. We propose that PDC most likely reduces the turnover rate of the transporter. A search of the sequence data bases revealed weak homology of HEAAC1 to the H(+)-coupled vesicular monoamine transporter, suggesting an evolutionary link between plasma membrane and vesicular transporters.

Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney

Electrically silent Na(+)-(K+)-Cl- transporter systems are present in a wide variety of cells and serve diverse physiological functions. In chloride secretory and absorbing epithelia, these cotransporters provide the chloride entry mechanism crucial for transcellular chloride transport. We have isolated cDNAs encoding the two major electroneutral sodium-chloride transporters present in the mammalian kidney, the bumetanide-sensitive Na(+)-K(+)-Cl- symporter and thiazide-sensitive Na(+)-Cl- cotransporter, and have characterized their functional activity in Xenopus laevis oocytes. Despite their differing sensitivities to bumetanide and thiazides and their different requirements for potassium, these approximately 115-kDa proteins share significant sequence similarity (approximately 60%) and exhibit a topology featuring 12 potential membrane-spanning helices flanked by long non-hydrophobic domains at the NH2 and COOH termini. Northern blot analysis and in situ hybridization indicate that these transporters are expressed predominantly in kidney with an intrarenal distribution consistent with their recognized functional localization. These proteins establish a new family of Na(+)-(K+)-Cl- cotransporters.

Expression of a rabbit renal ascorbic acid transporter in Xenopus laevis oocytes

We examined the expression of renal ascorbic acid transporter(s) in Xenopus laevis oocytes after microinjection of cells with poly(A)+ RNA extracted from rabbit kidney cortex. Concomitant expression of the Na+-glucose cotransporter served as a control in these studies. Injection of poly(A)+ RNA into oocytes produced over a fivefold increase in the uptake of [14C]ascorbic acid (570 microM) compared with water-injected cells. Size fractionation of the kidney cortex mRNA by sucrose gradient revealed that the mRNA species that induced ascorbic acid transporter expression in oocytes was present in a fraction centered around 2.0 kilobases (kb) and had a size range of 1.8-3.1 kb. Injection of the active fraction into oocytes produced a > 40-fold increase in ascorbic acid uptake compared with water-injected controls. Expression of ascorbic acid transporter(s) was noticeable as early as 2 days after injection and was maximal after 7 days; it was also dependent on the amount of mRNA injected into oocytes. The induced uptake of [14C]ascorbic acid after injection of mRNA into oocytes was 1) Na+ dependent, as indicated by the almost complete lack of transport on removal of Na+ from the incubation medium; 2) significantly inhibited by unlabeled ascorbic acid and its structural analogue isoascorbic acid but not by D-glucose; and 3) saturable as a function of increasing the substrate concentration in the incubation medium (100-1,000 microM), with an apparent Km of 258 +/- 72.5 microM and a maximum velocity of 29.6 +/- 2.8 pmol.oocyte-1.2 h-1. These data demonstrate that X. laevis oocytes are a suitable system to functionally express the mammalian renal ascorbic acid transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

The high affinity Na+/glucose cotransporter. Re-evaluation of function and distribution of expression

We report the primary structure, functional characterization, and tissue distribution of the high affinity Na+/glucose cotransporter SGLT1 from rat kidney. Rat SGLT1 (665 amino acid residues) is 86-87% identical to SGLT1 from rabbit, pig, and human. High stringency Northern analysis demonstrated that SGLT1 is strongly expressed in small intestine and at lower levels in kidney, liver, and lung. In situ hybridization performed on kidney sections revealed that SGLT1 is predominantly present in S3 segments of the proximal tubule. In small intestine, SGLT1 message was located in cells of the lower two-thirds of intestinal villi. Expression of rat SGLT1 in Xenopus oocytes resulted in a large Na(+)-dependent uptake of [14C]-alpha-methyl-D-glucopyranoside (alpha MeGlc). Overall, the transport characteristics were similar to those of rabbit SGLT1. High affinity Na+/glucose cotransport in membrane vesicles was previously shown to be coupled to the cotransport of two Na+ ions (Turner, R. J., and Moran, A. (1982) J. Membr. Biol. 70, 37-45). Previous kinetic analysis of rat and rabbit SGLT1, however, demonstrated between second and first order dependence of sugar uptake on extracellular Na+ concentration, suggesting the existence of Na(+)-binding sites with different affinities. Here, we directly compared the initial rates of the alpha MeGlc uptake with alpha MeGlc-induced inward currents as an indicator of the Na+ flux. This analysis clearly revealed a Na+ to glucose coupling ratio of 2:1. In summary, our data provide important insights into the function and tissue distribution of the high affinity Na+/glucose cotransporter SGLT1 and clarify its role in the reabsorption mechanism of D-glucose in the kidney.

Expression cloning of a mammalian proton-coupled oligopeptide transporter

In mammals, active transport of organic solutes across plasma membranes was thought to be primarily driven by the Na+ gradient. Here we report the cloning and functional characterization of a H(+)-coupled transporter of oligopeptides and peptide-derived antibiotics from rabbit small intestine. This new protein, named PepT1, displays an unusually broad substrate specificity. PepT1-mediated uptake is electrogenic, independent of extracellular Na+, K+ and Cl-, and of membrane potential. PepT1 messenger RNA was found in intestine, kidney and liver and in small amounts in brain. In the intestine, the PepT1 pathway constitutes a major mechanism for absorption of the products of protein digestion. To our knowledge, the PepT1 primary structure is the first reported for a proton-coupled organic solute transporter in vertebrates and represents an interesting evolutionary link between prokaryotic H(+)-coupled and vertebrate Na(+)-coupled transporters of organic solutes.

Small intestine hexose transport in experimental diabetes. Increased transporter mRNA and protein expression in enterocytes

The effect of insulinopenic diabetes on the expression of glucose transporters in the small intestine was investigated. Enterocytes were sequentially isolated from jejunum and ileum of normal fed rats, streptozotocin-diabetic rats, and diabetic rats treated with insulin. Facilitative glucose transporter (GLUT) 2, GLUT5, and sodium-dependent glucose transporter 1 protein content was increased from 1.5- to 6-fold in enterocytes isolated from diabetic animals in both jejunum and ileum. Insulin was able to reverse the increase in transporter protein expression seen after induction of diabetes. There was a four- to eightfold increase in the amount of enterocyte glucose transporter mRNA after diabetes with greater changes in sodium-dependent glucose transporter 1 and GLUT2 than in GLUT5 levels. In situ hybridization showed that after the induction of diabetes there was new hybridization in lower villus and crypt enterocytes that was reversed by insulin treatment. Thus, the increase in total hexose transport caused by diabetes is due to a premature expression of hexose transporters by enterocytes along the crypt-villus axis, causing a cumulative increase in enterocyte transporter protein during maturation. These changes are likely to represent an adaptive response by the organism to increase nutrient absorption in a perceived state of tissue starvation. These adaptive changes may lead to exacerbation of hyperglycemia in uncontrolled diabetes.

The human kidney low affinity Na+/glucose cotransporter SGLT2. Delineation of the major renal reabsorptive mechanism for D-glucose

The major reabsorptive mechanism for D-glucose in the kidney is known to involve a low affinity high capacity Na+/glucose cotransporter, which is located in the early proximal convoluted tubule segment S1, and which has a Na+ to glucose coupling ratio of 1:1. Here we provide the first molecular evidence for this renal D-glucose reabsorptive mechanism. We report the characterization of a previously cloned human kidney cDNA that codes for a protein with 59% identity to the high affinity Na+/glucose cotransporter (SGLT1). Using expression studies with Xenopus laevis oocytes we demonstrate that this protein (termed SGLT2) mediates saturable Na(+)-dependent and phlorizin-sensitive transport of D-glucose and alpha-methyl-D-glucopyranoside (alpha MeGlc) with Km values of 1.6 mM for alpha MeGlc and approximately 250 to 300 mM for Na+, consistent with low affinity Na+/glucose cotransport. In contrast to SGLT1, SGLT2 does not transport D-galactose. By comparing the initial rate of [14C]-alpha MeGlc uptake with the Na(+)-influx calculated from alpha MeGlc-evoked inward currents, we show that the Na+ to glucose coupling ratio of SGLT2 is 1:1. Using combined in situ hybridization and immunocytochemistry with tubule segment specific marker antibodies, we demonstrate an extremely high level of SGLT2 message in proximal tubule S1 segments. This level of expression was also evident on Northern blots and likely confers the high capacity of this glucose transport system. We conclude that SGLT2 has properties characteristic of the renal low affinity high capacity Na+/glucose cotransporter as previously reported for perfused tubule preparations and brush border membrane vesicles. Knowledge of the structural and functional properties of this major renal Na+/glucose reabsorptive mechanism will advance our understanding of the pathophysiology of renal diseases such as familial renal glycosuria and diabetic renal disorders.

Cloning and characterization of an extracellular Ca(2+)-sensing receptor from bovine parathyroid

Maintenance of a stable internal environment within complex organisms requires specialized cells that sense changes in the extracellular concentration of specific ions (such as Ca2+). Although the molecular nature of such ion sensors is unknown, parathyroid cells possess a cell surface Ca(2+)-sensing mechanism that also recognizes trivalent and polyvalent cations (such as neomycin) and couples by changes in phosphoinositide turnover and cytosolic Ca2+ to regulation of parathyroid hormone secretion. The latter restores normocalcaemia by acting on kidney and bone. We now report the cloning of complementary DNA encoding an extracellular Ca(2+)-sensing receptor from bovine parathyroid with pharmacological and functional properties nearly identical to those of the native receptor. The novel approximately 120K receptor shares limited similarity with the metabotropic glutamate receptors and features a large extracellular domain, containing clusters of acidic amino-acid residues possibly involved in calcium binding, coupled to a seven-membrane-spanning domain like those in the G-protein-coupled receptor superfamily.

A new family of neurotransmitter transporters: the high-affinity glutamate transporters

An essential component of the transmission process at glutamatergic synapses is the removal of glutamate from the synaptic cleft. This is achieved by powerful transport systems which have a high affinity for glutamate and exhibit a novel coupling to inorganic ions. Transporters situated on presynaptic termini sequester glutamate directly from the synaptic cleft. In concert, transporters situated on glial cells maintain a low extracellular glutamate concentration, thereby establishing a diffusion gradient favoring movement of glutamate out of the synaptic cleft. Maintenance of a low extracellular glutamate concentration also serves to protect neurons from the excitotoxic action of glutamate. Despite the physiological importance of the glutamate transporters, little information has been available on their molecular structures. This gap, however, has begun to be bridged with the recent cloning of three species of eukaryotic glutamate transporters. The purpose of this review is to summarize the results of these three cloning successes, to compare and contrast the three novel transporters, and to reinterpret, in the light of these recent breakthroughs, information from previous studies.