Cloning of a membrane-associated protein which modifies activity and properties of the Na(+)-D-glucose cotransporter

RS1 (Rsc1A1) deficiency limits cerebral SGLT1 expression and delays brain damage after experimental traumatic brain injury

Acute cerebral lesions are associated with dysregulation of brain glucose homeostasis. Previous studies showed that knockdown of Na⁺ -D-glucose cotransporter SGLT1 impaired outcome after middle cerebral artery occlusion and that widely expressed intracellular RS1 (RSC1A1) is involved in transcriptional and post-translational down-regulation of SGLT1. In the present study, we investigated whether SGLT1 is up-regulated during traumatic brain injury (TBI) and whether removal of RS1 in mice (RS1-KO) influences SGLT1 expression and outcome. Unexpectedly, brain SGLT1 mRNA in RS1-KO was similar to wild-type whereas it was increased in small intestine and decreased in kidney. One day after TBI, SGLT1 mRNA in the ipsilateral cortex was increased 160% in wild-type and 40% in RS1-KO. After RS1 removal lesion volume 1 day after TBI was reduced by 12%, brain edema was reduced by 28%, and motoric disability determined by a beam walking test was improved. In contrast, RS1 removal did neither influence glucose and glycogen accumulation 1 day after TBI nor up-regulation of inflammatory cytokines TNF-α, IL-1β and IL-6 or microglia activation 1 or 5 days after TBI. The data provide proof of principle that inhibition or down-regulation of SGLT1 by targeting RS1 in brain could be beneficial for early treatment of TBI.

L-Theanine Administration Modulates the Absorption of Dietary Nutrients and Expression of Transporters and Receptors in the Intestinal Mucosa of Rats

L-theanine has various advantageous functions for human health; whether or not it could mediate the nutrients absorption is unknown yet. The effects of L-theanine on intestinal nutrients absorption were investigated using rats ingesting L-theanine solution (0, 50, 200, and 400 mg/kg body weight) per day for two weeks. The decline of insulin secretion and glucose concentration in the serum was observed by L-theanine. Urea and high-density lipoprotein were also reduced by 50 mg/kg L-theanine. Jejunal and ileac basic amino acids transporters SLC7a1 and SLC7a9, neutral SLC1a5 and SLC16a10, and acidic SLC1a1 expression were upregulated. The expression of intestinal SGLT3 and GLUT5 responsible for carbohydrates uptake and GPR120 and FABP2 associated with fatty acids transport were inhibited. These results indicated that L-theanine could inhibit the glucose uptake by downregulating the related gene expression in the small intestine of rats. Intestinal gene expression of transporters responding to amino acids absorption was stimulated by L-theanine administration.

A substrate binding hinge domain is critical for transport-related structural changes of organic cation transporter 1

Organic cation transporters are membrane potential-dependent facilitative diffusion systems. Functional studies, extensive mutagenesis, and homology modeling indicate the following mechanism. A transporter conformation with a large outward-open cleft binds extracellular substrate, passes a state in which the substrate is occluded, turns to a conformation with an inward-open cleft, releases substrate, and subsequently turns back to the outward-open state. In the rat organic cation transporter (rOct1), voltage- and ligand-dependent movements of fluorescence-labeled cysteines were measured by voltage clamp fluorometry. For fluorescence detection, cysteine residues were introduced in extracellular parts of cleft-forming transmembrane α-helices (TMHs) 5, 8, and 11. Following expression of the mutants in Xenopus laevis oocytes, cysteines were labeled with tetramethylrhodamine-6-maleimide, and voltage-dependent conformational changes were monitored by voltage clamp fluorometry. One cysteine was introduced in the central domain of TMH 11 replacing glycine 478. This domain contains two amino acids that are involved in substrate binding and two glycine residues (Gly-477 and Gly-478) allowing for helix bending. Cys-478 could be modified with the transported substrate analog [2-(trimethylammonium)-ethyl]methanethiosulfonate but was inaccessible to tetramethylrhodamine-6-maleimide. Voltage-dependent movements at the indicator positions of TMHs 5, 8, and 11 were altered by substrate applications indicating large conformational changes during transport. The G478C exchange decreased transporter turnover and blocked voltage-dependent movements of TMHs 5 and 11. [2-(Trimethylammonium)-ethyl]methanethiosulfonate modification of Cys-478 blocked substrate binding, transport activity, and movement of TMH 8. The data suggest that Gly-478 is located within a mechanistically important hinge domain of TMH 11 in which substrate binding induces transport-related structural changes.

MAP17 and the double-edged sword of ROS

Reactive oxygen species, ROS, are beneficially involved in many signaling pathways that control development and maintain cellular homeostasis. In physiological conditions, a tightly regulated redox balance protects cells from injurious ROS activity, but if the balance is altered, it promotes various pathological conditions including cancer. Understanding the duality of ROS as cytotoxic molecules and key mediators in signaling cascades may provide novel opportunities for improved cancer therapy. MAP17 is a small 17-kDa non-glycosylated membrane protein that is overexpressed in many tumors of different origins, including carcinomas. Immunohistochemical analysis of MAP17 during cancer progression demonstrates that overexpression of the protein strongly correlates with the progression of most types of tumor. Tumor cells that overexpress MAP17 show an increased tumoral phenotype associated with an increase in ROS. However, in non-tumor cells MAP17 increases ROS, resulting in senescence or apoptosis. Therefore, in tumor cells, MAP17 could be a marker for increased oxidative stress and could define new therapeutic approaches. Here, we review the role of MAP17 as a putative oncogene, as well as its role in cancer and anticancer therapies.

SLC5 and SLC2 transporters in epithelia-cellular role and molecular mechanisms

Members of the SLC5 and SLC2 family are prominently involved in epithelial sugar transport. SGLT1 (sodium-glucose transporter) and SGLT2, as representatives of the former, mediate sodium-dependent uptake of sugars into intestinal and renal cells. GLUT2 (glucose transporter), as representative of the latter, facilitates the sodium-independent exit of sugars from cells. SGLT has played a major role in the formulation and experimental proof for the existence of sodium cotransport systems. Based on the sequence data and biochemical and biophysical analyses, the role of extramembranous loops in sugar and inhibitor binding can be delineated. Crystal structures and homology modeling of SGLT reveal that the sugar translocation involves operation of two hydrophobic gates and intermediate exofacial and endofacial occluded states of the carrier in an alternating access model. The same basic model is proposed for GLUT1. Studies on GLUT1 have pioneered the isolation of eukaryotic transporters by biochemical methods and the development of transport kinetics and transporter models. For GLUT1, results from extensive mutagenesis, cysteine substitution and accessibility studies can be incorporated into a homology model with a barrel-like structure in which accessibility to the extracellular and intracellular medium is altered by pinching movements of some of the helices. For SGLT1 and GLUT1, the extensive hydrophilic and hydrophobic interactions between sugars and binding sites of the various intramembrane helices occur and lead to different substrate specificities and inhibitor affinities of the two transporters. A complex network of regulatory steps adapts the transport activity to the needs of the body.

The large extracellular loop of organic cation transporter 1 influences substrate affinity and is pivotal for oligomerization

Polyspecific organic anion transporters (OATs) and organic cation transporters (OCTs) of the SLC22 transporter family play a pivotal role in absorption, distribution, and excretion of drugs. Polymorphisms in these transporters influence therapeutic effects. On the basis of functional characterizations, homology modeling, and mutagenesis, hypotheses for how OCTs bind and translocate structurally different cations were raised, assuming functionally competent monomers. However, homo-oligomerization has been described for OATs and OCTs. In the present study, evidence is provided that the large extracellular loops (EL) of rat Oct1 (rOct1) and rat Oat1 (rOat1) mediate homo- but not hetero-oligomerization. Replacement of the cysteine residues in the EL of rOct1 by serine residues (rOct1(6ΔC-l)) or breaking disulfide bonds with dithiothreitol prevented oligomerization. rOct1 chimera containing the EL of rOat1 (rOct1(rOat1-l)) showed oligomerization but reduced transporter amount in the plasma membrane. For rOct1(6ΔC-l) and rOct1(rOat1-l), similar K(m) values for 1-methyl-4-phenylpyridinium(+) (MPP(+)) and tetraethylammonium(+) (TEA(+)) were obtained that were higher compared with rOct1 wild type. The increased K(m) of rOct1(rOat1-l) indicates an allosteric effect of EL on the cation binding region. The similar substrate affinity of the oligomerizing and non-oligomerizing loop mutants suggests that oligomerization does not influence transport function. Independent transport function of rOct1 monomers was also demonstrated by showing that K(m) values for MPP(+) and TEA(+) were not changed after treatment with dithiothreitol and that a tandem protein with two rOct1 monomers showed about 50% activity with unchanged K(m) values for MPP(+) and TEA(+) when one monomer was blocked. The data help to understand how OCTs work and how mutations in patients may affect their functions.

Novel shuttling domain in a regulator (RSC1A1) of transporter SGLT1 steers cell cycle-dependent nuclear location

The gene product of RSC1A1, RS1, participates in the regulation of the Na(+)-D-glucose cotransporter SGLT1. RS1 inhibits release of SGLT1 from the trans Golgi network. In subconfluent LLC-PK(1) cells, RS1 migrates into the nucleus and modulates transcription of SGLT1, whereas most confluent cells do not contain RS1 in the nuclei. We showed that confluence-dependent nuclear location of RS1 is because of different phases of the cell cycle and identified a RS1 nuclear shuttling domain (RNS) with an associated protein kinase C (PKC) phosphorylation site (RNS-PKC) that mediates cell cycle-dependent nuclear location. RNS-PKC contains a novel non-conventional nuclear localization signal interacting with importin beta1, a nuclear export signal mediating export via protein CRM1 and a Ca(2+)-dependent calmodulin binding site. PKC and calmodulin compete for binding to RNS-PKC. Mutagenesis experiments and analyses of the phosphorylation status suggest the following sequences of events. Subconfluent cells without and with synchronization to the G2/M phase contain non-phosphorylated RNS-PKC that mediates nuclear import of RS1 but not its export. During confluence or synchronization of subconfluent cells to the G2/M phase, phosphorylation of RNS-PKC mediates rapid nuclear export of RS1.

SGLT1 is a novel cardiac glucose transporter that is perturbed in disease states

AIMS

Cardiac myocytes depend on a delicate balance of glucose and free fatty acids as energy sources, a balance that is disrupted in pathological states such as diabetic cardiomyopathy and myocardial ischaemia. There are two families of cellular glucose transporters: the facilitated-diffusion glucose transporters (GLUT); and the sodium-dependent glucose transporters (SGLT). It has long been thought that only the GLUT isoforms, GLUT1 and GLUT4, are responsible for cardiac myocyte glucose uptake. However, we discovered that one SGLT isoform, SGLT1, is also an important glucose transporter in heart. In this study, we aimed to determine the human and murine cardiac expression pattern of SGLT1 in health and disease and to determine its regulation.

METHODS AND RESULTS

SGLT1 was largely localized to the cardiac myocyte sarcolemma. Changes in SGLT1 expression were observed in disease states in both humans and mouse models. SGLT1 expression was upregulated two- to three-fold in type 2 diabetes mellitus and myocardial ischaemia (P < 0.05). In humans with severe heart failure, functional improvement following implantation of left ventricular assist devices led to a two-fold increase in SGLT1 mRNA (P < 0.05). Acute administration of leptin to wildtype mice increased cardiac SGLT1 expression approximately seven-fold (P < 0.05). Insulin- and leptin-stimulated cardiac glucose uptake was significantly (P < 0.05) inhibited by phlorizin, a specific SGLT1 inhibitor.

CONCLUSION

Our data suggest that cardiac SGLT1 expression and/or function are regulated by insulin and leptin, and are perturbed in disease. This is the first study to examine the regulation of cardiac SGLT1 expression by insulin and leptin and to determine changes in SGLT1 expression in cardiac disease.

Tripeptides of RS1 (RSC1A1) inhibit a monosaccharide-dependent exocytotic pathway of Na+-D-glucose cotransporter SGLT1 with high affinity

The human gene RSC1A1 codes for a 67-kDa protein named RS1 that mediates transcriptional and post-transcriptional regulation of Na(+)-D-glucose cotransporter SGLT1. The post-transcriptional regulation occurs at the trans-Golgi network (TGN). We identified two tripeptides in human RS1 (Gln-Cys-Pro (QCP) and Gln-Ser-Pro (QSP)) that induce posttranscriptional down-regulation of SGLT1 at the TGN leading to 40-50% reduction of SGLT1 in plasma membrane. For effective intracellular concentrations IC(50) values of 2.0 nM (QCP) and 0.16 nm (QSP) were estimated. Down-regulation of SGLT1 by tripeptides was attenuated by intracellular monosaccharides including non-metabolized methyl-alpha-D-glucopyranoside and 2-deoxyglucose. In small intestine post-transcriptional regulation of SGLT1 may contribute to glucose-dependent regulation of liver metabolism and intestinal mobility. QCP and QSP are transported by the H(+)-peptide cotransporter PepT1 that is colocated with SGLT1 in small intestinal enterocytes. Using coexpression of SGLT1 and PepT1 in Xenopus oocytes or polarized Caco-2 cells that contain both transporters we demonstrated that the tripeptides were effective when applied to the extracellular compartment. After a 1-h perfusion of intact rat small intestine with QSP, glucose absorption was reduced by 30%. The data indicate that orally applied tripeptides can be used to down-regulate small intestinal glucose absorption, e.g. in diabetes mellitus.

RS1 (RSC1A1) regulates the exocytotic pathway of Na+-d-glucose cotransporter SGLT1

The product of gene RSC1A1, named RS1, participates in transcriptional and posttranscriptional regulation of the sodium-d-glucose cotransporter SGLT1. Using coexpression in oocytes of Xenopus laevis, posttranscriptional inhibition of human SGLT1 (hSGLT1) and some other transporters by human RS1 (hRS1) was demonstrated previously. In the present study, histidine-tagged hRS1 was expressed in oocytes or Sf9 cells and purified using nickel(II)-charged nitrilotriacetic acid-agarose. hRS1 protein was injected into oocytes expressing hSGLT1 or the human organic cation transporter hOCT2, and the effect on hSGLT1-mediated uptake of methyl-α-d-[14C]glucopyranoside ([14C]AMG) or hOCT2-mediated uptake of [14C]tetraethylammonium ([14C]TEA) was measured. Within 30 min after the injection of hRS1 protein, hSGLT1-expressed AMG uptake or hOCT2-expressed TEA uptake was inhibited by ∼50%. Inhibition of AMG uptake was decreased when a dominant negative mutant of dynamin I was coexpressed and increased after stimulation of PKC. Inhibition remained unaltered when endocytosis was inhibited by chlorpromazine, imipramine, or filipin but was prevented when exocytosis was inhibited by botulinum toxin B or when the release of vesicles from the TGN and endosomes was inhibited by brefeldin A. Inhibition of hSGLT1-mediated AMG uptake and hOCT2-mediated TEA uptake by hRS1 protein were decreased at an enhanced intracellular AMG concentration. The data suggest that hRS1 protein exhibits glucose-dependent, short-term inhibition of hSGLT1 and hOCT2 by inhibiting the release of vesicles from the trans-Golgi network.

Transporter regulator RS1 (RSC1A1) coats the trans-Golgi network and migrates into the nucleus

The product of gene RSC1A1, named RS1, is involved in transcriptional and posttranscriptional regulation of sodium-d-glucose cotransporter SGLT1, and removal of RS1 in mice led to an increase of SGLT1 expression in small intestine and to obesity (Osswald C, Baumgarten K, Stümpel F, Gorboulev V, Akimjanova M, Knobeloch K-P, Horak I, Kluge R, Joost H-G, and Koepsell H. Mol Cell Biol 25: 78-87, 2005). Previous data showed that RS1 inhibits transcription of SGLT1 in LLC-PK1 cells derived from porcine kidney. A decrease of the intracellular amount of RS1 protein was observed during cell confluence, which was paralleled by transcriptional upregulation of SGLT1. In the present study, the subcellular distributions of endogenously expressed RS1 and SGLT1 were compared in LLC-PK1 cells and human embryonic kidney (HEK)-293 cells using immunofluorescence microscopy. RS1 was located at the plasma membrane, at the entire trans-Golgi network (TGN), and within the nucleus. Treatment of LLC-PK1 cells with brefeldin A induced rapid release of RS1 from the TGN, and confluence of LLC-PK1 cells was accompanied by reduction of nuclear location of RS1; 84-90% of subconfluent cells and 5-34% of confluent cells contained RS1 in the nuclei. This suggests that confluence-dependent transcriptional inhibition by RS1 is partially regulated by nuclear migration. Furthermore, we assigned SGLT1 to microtubule-associated tubulovesicular structures and dynamin-containing parts of the TGN. The data indicate that RS1 inhibits the dynamin-dependent release of SGLT1-containing vesicles from the TGN.

Intestinal sugar transport

Carbohydrates are an important component of the diet. The carbohydrates that we ingest range from simple monosaccharides (glucose, fructose and galactose) to disaccharides (lactose, sucrose) to complex polysaccharides. Most carbohydrates are digested by salivary and pancreatic amylases, and are further broken down into monosaccharides by enzymes in the brush border membrane (BBM) of enterocytes. For example, lactase-phloridzin hydrolase and sucrase-isomaltase are two disaccharidases involved in the hydrolysis of nutritionally important disaccharides. Once monosaccharides are presented to the BBM, mature enterocytes expressing nutrient transporters transport the sugars into the enterocytes. This paper reviews the early studies that contributed to the development of a working model of intestinal sugar transport, and details the recent advances made in understanding the process by which sugars are absorbed in the intestine.

IRIP, a new ischemia/reperfusion-inducible protein that participates in the regulation of transporter activity

We report the identification and characterization of a new ischemia/reperfusion-inducible protein (IRIP), which belongs to the SUA5/YrdC/YciO protein family. IRIP cDNA was isolated in a differential display analysis of an ischemia/reperfusion-treated kidney RNA sample. Mouse IRIP mRNA was expressed in all tissues tested, the highest level being in the testis, secretory, and endocrine organs. Besides ischemia/reperfusion, endotoxemia also activated the expression of IRIP in the liver, lung, and spleen. The transporter regulator RS1 was identified as an IRIP-interacting protein in yeast two-hybrid screening. The interaction between IRIP and RS1 was further confirmed in coimmunoprecipitation assays. A possible role of IRIP in regulating transporter activity was subsequently investigated. IRIP overexpression inhibited endogenous 1-methyl-4-phenylpyridinium (MPP+) uptake activity in HeLa cells. The activities of exogenous organic cation transporters (OCT2 and OCT3), organic anion transporter (OAT1), and monoamine transporters were also inhibited by IRIP. Conversely, inhibition of IRIP expression by small interfering RNA or antisense RNA increased MPP+ uptake. We measured transport kinetics of OCT2-mediated uptake and demonstrated that IRIP overexpression significantly decreased V(max) but did not affect K(m). On the basis of these results, we propose that IRIP regulates the activity of a variety of transporters under normal and pathological conditions.

Amino acids critical for substrate affinity of rat organic cation transporter 1 line the substrate binding region in a model derived from the tertiary structure of lactose permease

To identify functionally relevant amino acids in the rat organic cation transporter 1 (rOCT1), 18 consecutive amino acids in the presumed fourth transmembrane alpha helix (TMH) were mutated and functionally characterized after expression in Xenopus laevis oocytes. After mutation of three amino acids on successive turns of the alpha helix, K(m) values for tetraethylammonium (TEA) and/or 1-methyl-4-phenylpyridinium (MPP) were decreased. After replacement of Trp218 by tyrosine (W218Y) and Tyr222 by leucine (Y222L), the K(m) values for both TEA and MPP were decreased. In mutants Y222F and T226A, only the K(m) values for TEA and MPP were decreased, respectively. The data suggest that amino acids Trp218 and Tyr222 participate in the binding of both TEA and MPP, whereas Thr226 is only involved in the binding of MPP. Using the crystal structure of the lactose permease LacY from Escherichia coli that belongs to the same major facilitator superfamily as rOCT1, we modeled the tertiary structure of the presumed 12 transmembrane alpha helices. The validity of the model was suggested because seven amino acids that have been shown to participate in the binding of cations by mutagenesis experiments [fourth TMH Trp218, Tyr222, and Thr226 (this work); 10th TMH Ala443, Leu447, and Gln448 (companion work in this issue of Molecular Pharmacology); 11th TMH Asp475 (previous report)] are located in one region surrounding a large cleft that opens to the intracellular side. The dimensions of TEA in comparison with the interacting amino acids in the modeled cleft suggest that more than one TEA molecule can bind in parallel to the modeled conformation of the transporter.

Mice without the regulator gene Rsc1A1 exhibit increased Na+-D-glucose cotransport in small intestine and develop obesity

The product of the intronless single copy gene RSC1A1, named RS1, is an intracellular 617-amino-acid protein that is involved in the regulation of the Na(+)-d-glucose cotransporter SGLT1. We generated and characterized RS1 knockout (RS1(-/-) mice. In the small intestines of RS1(-/-) mice, the SGLT1 protein was up-regulated sevenfold compared to that of wild-type mice but was not changed in the kidneys. The up-regulation of SGLT1 was posttranscriptional. Small intestinal d-glucose uptake measured in jointly perfused small bowel and liver was increased twofold compared to that of the wild-type, with increased peak concentrations of d-glucose in the portal vein. At birth, the weights of RS1(-/-) and wild-type mice were similar. At the age of 3 months, male RS1(-/-) mice had 5% higher weights and 15% higher food intakes, whereas their energy expenditures and serum leptin concentrations were similar to those of wild-type mice. At the age of 5 months, male and female RS1(-/-) mice were obese, with 30% increased body weight, 80% increased total fat, and 30% increased serum cholesterol. At this age, serum leptin was increased, whereas food intake was the same as for wild-type mice. The data suggest that the removal of RS1 leads to leptin-independent up-regulation of food intake, which causes obesity.

Oligomer formation by Na+-Cl--coupled neurotransmitter transporters

Na+-Cl--dependent neurotransmitter transporters (or neurotransmitter:Na+ symporters, NSS) share many structural and functional features, e.g. a conserved topology of 12 transmembrane spanning alpha-helices, the capacity to operate in two directions and in an electrogenic manner. Biochemical and biophysical experiments indicate that these transporters interact in oligomeric quaternary structures. Fluorescence resonance energy transfer (FRET) microscopy has provided evidence for a constitutive physical interaction of NSS at the cell surface and throughout the biosynthetic pathway. Two interfaces for protein-protein interaction have been shown to be important in NSS; these comprise a glycophorin-like motif and a leucine heptad repeat. Upon mutational modification of the latter, surface targeting is considerably impaired without concomitant loss in uptake activity. This supports a role of oligomer formation in the passage of the quality control mechanisms of the endoplasmic reticulum and/or Golgi. In contrast, oligomerisation is dispensable for substrate binding and translocation.

Human cardiomyocytes express high level of Na+/glucose cotransporter 1 (SGLT1)

We have quantitatively measured gene expression for the sodium-dependent glucose cotransporters 1 and 2 (SGLT1 and SGLT2) in 23 human tissues using the method of real time PCR. As predicted, our results revealed that the expression of SGLT1 was very high in the small intestine (1.2E + 6 molecules/microg total RNA) relative to that in the kidney (3E + 4 molecules/microg total RNA). Surprisingly, we observed that the expression of SGLT1 in human heart was unexpectedly high (3.4E + 5 molecules/microg total RNA), approximately 10-fold higher than that observed in kidney tissue. DNA sequencing confirmed that the PCR amplified fragment was indeed the human SGLT1 gene. Moreover, in situ hybridization studies using a digoxigenin (DIG)-labeled antisense cRNA probe corresponding to human SGLT1 cDNA confirm that human cardiomyocytes express SGLT1 mRNA. In contrast, the expression of SGLT2 in human tissues appears to be ubiquitous, with levels ranging from 6.7E + 4 molecules/microg total RNA (in skeletal muscle) to 3.2E + 6 molecules/microg total RNA (in kidney), levels 10-100-fold higher than the expression of SGLT1 in the same tissues. Our finding that human cardiomyocytes express high levels of SGLT1 RNA suggests that SGLT1 may have a functional role in cardiac glucose transport. Since several SGLT inhibitors are currently in development as potential anti-diabetic agents, it may be important to assess the functional consequences of inhibition of SGLT1 in the heart.

Rat kidney MAP17 induces cotransport of Na-mannose and Na-glucose in Xenopus laevis oocytes

Renal reabsorption is the main mechanism that controls mannose homeostasis. This takes place through a specific Na-coupled uphill transport system, the molecular identity of which is unknown. We prepared and screened a size-selected rat kidney cortex cDNA library through the expression of mannose transport in Xenopus laevis oocytes. We have identified a membrane protein that induces high-affinity and specific Na-dependent transport of d-mannose and d-glucose in X. laevis oocytes, most likely through stimulation of the capacity of an endogenous transport system of the oocyte. Sequencing has revealed that the cDNA encodes the counterpart of the human membrane-associated protein MAP17, previously known by its overexpression in renal, colon, lung, and breast carcinomas. We show that MAP17 is a 12.2-kDa nonglycosylated membrane protein that locates to the brush-border plasma membrane and the Golgi apparatus of transfected cells and that it is expressed in the proximal tubules of the kidney cortex and in the spermatids of the seminiferous tubules. It spans twice the cell membrane, with both termini inside the cell, and seems to form homodimers through intracellular Cys55, a residue also involved in transport expression. MAP17 is responsible for mannose transport expression in oocytes by rat kidney cortex mRNA. The induced transport has the functional characteristics of a Na-glucose cotransporter (SGLT), because d-glucose and alpha-methyl-d-glucopyranoside are also accepted substrates that are inhibited by phloridzin. The corresponding transporter from the proximal tubule remains to be identified, but it is different from the known mammalian SGLT-1, -2, and -3.

A cocaine insensitive chimeric insect serotonin transporter reveals domains critical for cocaine interaction

Serotonin transporters are key target sites for clinical drugs and psychostimulants, such as fluoxetine and cocaine. Molecular cloning of a serotonin transporter from the central nervous system of the insect Manduca sexta enabled us to define domains that affect antagonist action, particularly cocaine. This insect serotonin transporter transiently expressed in CV-1 monkey kidney cells exhibits saturable, high affinity Na+ and Cl- dependent serotonin uptake, with estimated Km and Vmax values of 436 +/- 19 nm and 3.8 +/- 0.6 x 10-18 mol.cell.min-1, respectively. The Manduca high affinity Na+/Cl- dependent transporter shares 53% and 74% amino acid identity with the human and fruit fly serotonin transporters, respectively. However, in contrast to serotonin transporters from these two latter species, the Manduca transporter is inhibited poorly by fluoxetine (IC50 = 1.23 micro m) and cocaine (IC50 = 12.89 micro m). To delineate domains and residues that could play a role in cocaine interaction, the human serotonin transporter was mutated to incorporate unique amino acid substitutions, detected in the Manduca homologue. We identified a domain in extracellular loop 2 (amino acids 148-152), which, when inserted into the human transporter, results in decreased cocaine sensitivity of the latter (IC50 = 1.54 micro m). We also constructed a number of chimeras between the human and Manduca serotonin transporters (hSERT and MasSERT, respectively). The chimera, hSERT1-146/MasSERT106-587, which involved N-terminal swaps including transmembrane domains (TMDs) 1 and 2, was remarkably insensitive to cocaine (IC50 = 180 micro m) compared to the human (IC50 = 0.431 micro m) and Manduca serotonin transporters. The chimera MasSERT1-67/hSERT109-630, which involved only the TMD1 swap, showed greater sensitivity to cocaine (IC50 = 0.225 micro m) than the human transporter. Both chimeras showed twofold higher serotonin transport affinity compared to human and Manduca serotonin transporters. Our results show TMD1 and TMD2 affect the apparent substrate transport and antagonist sensitivity by possibly providing unique conformations to the transporter. The availability of these chimeras facilitates elucidation of specific amino acids involved in interactions with cocaine.

Characterization of glucose transport by cultured rabbit kidney proximal convoluted and proximal straight tubule cells

Rabbit kidney proximal convoluted tubule (RPCT) and proximal straight tubule (RPST) cells were independently isolated and cultured. The kinetics of the sodium-dependent glucose transport was characterized by determining the uptake of the glucose analog alpha-methylglucopyranoside. Cell culture and assay conditions used in these experiments were based on previous experiments conducted on the renal cell line derived from the whole kidney of the Yorkshire pig (LLC-PK1). Results indicated the presence of two distinct sodium-dependent glucose transporters in rabbit renal cells: a relatively high-capacity, low-affinity transporter (V(max) = 2.28 +/- 0.099 nmoles/mg protein min, Km = 4.1 +/- 0.27 mM) in RPCT cells and a low-capacity, high-affinity transporter (V(max) = 0.45 +/- 0.076 nmoles/mg protein min, K(m) = 1.7 +/- 0.43 mM) in RPST cells. A relatively high-capacity, low-affinity transporter (V(max) = 1.68 +/- 0.215 nmoles/mg protein min, Km = 4.9 +/- 0.23 mM) was characterized in LLC-PK1 cells. Phlorizin inhibited the uptake of alpha-methylglucopyranoside in proximal convoluted, proximal straight, and LLC-PK1 cells by 90, 50, and 90%, respectively. Sodium-dependent glucose transport in all three cell types was specific for hexoses. These data are consistent with the kinetic heterogeneity of sodium-dependent glucose transport in the S1-S2 and S3 segments of the mammalian renal proximal tubule. The RPCT-RPST cultured cell model is novel, and this is the first report of sodium-dependent glucose transport characterization in primary cultures of proximal straight tubule cells. Our results support the use of cultured monolayers of RPCT and RPST cells as a model system to evaluate segment-specific differences in these renal cell types.

Vimentin affects localization and activity of sodium-glucose cotransporter SGLT1 in membrane rafts

It has been reported that vimentin, a cytoskeleton filament that is expressed only in mesenchymal cells after birth, is re-expressed in epithelial cells in vivo under pathological conditions and in vitro in primary culture. Whether vimentin re-expression is only a marker of cellular dedifferentiation or is instrumental in the maintenance of cell structure and/or function is a matter of debate. To address this issue, we used renal proximal tubular cells in primary culture from vimentin-null mice (Vim-/-) and from wild-type littermates (Vim+/+). The absence of vimentin did not affect cell morphology, proliferation and activity of hydrolases, but dramatically decreased Na-glucose cotransport activity. This phenotype was associated with a specific reduction of SGLT1 protein in the detergent-resistant membrane microdomains (DRM). In Vim+/+cells, disruption of these microdomains by methyl-β-cyclodextrin decreased SGLT1 protein abundance in DRM, a change that was paralleled by a decrease of Na-glucose transport activity. Importantly, we showed that vimentin is located to DRM, but it disappeared after methyl-β-cyclodextrin treatment. In Vim-/- cells,supplementation of cholesterol with cholesterol-methyl-β-cyclodextrin complexes completely restored Na-glucose transport activity. Interestingly,neither cholesterol content nor cholesterol metabolism changed in Vim-/- cells. Our results are consistent with the view that re-expression of vimentin in epithelial cells could be instrumental to maintain the physical state of rafts and, thus, the function of DRM-associated proteins.

The apical localization of SGLT1 glucose transporter is determined by the short amino acid sequence in its N-terminal domain

SGLT1, an isoform of Na+-dependent glucose cotransporters, is localized at the apical plasma membrane in the epithelial cells of the small intestine and the kidney, where it plays a pivotal role in the absorption and reabsorption of sugars, respectively. To search the domain responsible for the apical localization of SGLT1, we constructed an N-terminal deletion clone series of rat SGLT1 and analyzed the localization of the respective products in Madin-Darby canine kidney (MDCK) cells. The products of N-terminal deletion clones up to the 19th amino acid were localized at the apical plasma membrane, whereas the products of N-terminal 20- and 23-amino-acid deletion clones were localized along the entire plasma membrane. Since single-amino-acid mutations of either D28N or D28G in the N-terminal domain give rise to glucose/galactose malabsorption disease, we examined the localization of these mutants. The products of D28N and D28G clones were localized in the cytoplasm, showing that the aspartic acid-28 may be essential for the delivery of SGLT1 to the plasma membrane. These results suggest that a short amino acid sequence of the N-terminal domain of SGLT1 plays important roles in plasma membrane targeting and specific apical localization of the protein.

The plasma membrane-associated protein RS1 decreases transcription of the transporter SGLT1 in confluent LLC-PK1 cells

Previously we cloned RS1, a 67-kDa polypeptide that is associated with the intracellular side of the plasma membrane. Upon co-expression in Xenopus laevis oocytes, human RS1 decreased the concentration of the Na(+)-D-glucose co-transporter hSGLT1 in the plasma membrane (Valentin, M., Kühlkamp, T., Wagner, K., Krohne, G., Arndt, P., Baumgarten, K., Weber, W.-M., Segal, A., Veyhl, M., and Koepsell, H. (2000) Biochim. Biophys. Acta 1468, 367-380). Here, the porcine renal epithelial cell line LLC-PK1 was used to investigate whether porcine RS1 (pRS1) plays a role in transcriptional up-regulation of SGLT1 after confluence and in down-regulation of SGLT1 by high extracellular D-glucose concentrations. Western blots indicated a dramatic decrease of endogenous pRS1 protein at the plasma membrane after confluence but no significant effect of D-glucose. In confluent LLC-PK1 cells overexpressing pRS1, SGLT1 mRNA, protein, and methyl-alpha-D-glucopyranoside uptakes were drastically decreased; however, the reduction of methyl-alpha-D-glucopyranoside uptake after cultivation with 25 mm D-glucose remained. In confluent pRS1 antisense cells, the expression of SGLT1 mRNA and protein was strongly increased, whereas the reduction of SGLT1 expression during cultivation with high D-glucose was not influenced. Nuclear run-on assays showed that the transcription of SGLT1 was 10-fold increased in the pRS1 antisense cells. The data suggest that RS1 participates in transcriptional up-regulation of SGLT1 after confluence but not in down-regulation by D-glucose.

Molecular and cellular biology of the human reduced folate carrier

The natural folates are water-soluble members of the B class of vitamins that are essential for cell proliferation and tissue regeneration. Since mammalian cells cannot synthesize folates de novo, tightly regulated and sophisticated cellular uptake processes have evolved to sustain sufficient levels of intracellular tetrahydrofolate cofactors to support the biosynthesis of purines, pyrimidines, serine, and methione. Membrane transport is also a critical determinant of the antitumor activity of antifolate therapeutics (methotrexate, Tomudex) used in cancer chemotherapy, and impaired uptake of antifolates is a frequent mode of drug resistance. The reduced folate carrier is the major transport system for folates and classical antifolates in mammalian cells and tissues. This review summarizes the remarkable advances in the cellular and molecular biology of the human reduced folate carrier over the past decade, relating to its molecular structure and transport function, mechanisms of transcriptional and posttranscriptional regulation, and its critical role in antifolate response and resistance. Many key in vitro findings have now begun to be extended to studies of reduced folate carrier levels and function in patient specimens, paving the way for translating basic laboratory studies in cultured cells to improvements in human health and treatment of disease. The results of research into the human reduced folate carrier should clarify the roles of changes in expression and function of this system that accompany nutritional folate deficiency and human disease, and may lead to improved therapeutic strategies for enhancing drug response and circumventing resistance in cancer patients undergoing chemotherapy with antifolates.

Interaction of cations, anions, and weak base quinine with rat renal cation transporter rOCT2 compared with rOCT1

The rat organic cation transporter (rOCT)-2 was characterized by electrical and tracer flux measurements compared with rOCT1. By applying choline gradients to voltage-clamped Xenopus oocytes expressing rOCT2, potential-dependent currents could be induced in both directions. Tracer flux measurements with seven organic cations revealed similar Michaelis-Menten constant values for both transporters, with the exception of guanidine. In parallel experiments with rOCT2 and rOCT1, inhibition of tetraethylammonium transport by 12 cations, 2 weak bases, corticosterone, and the anions para-amminohippurate, alpha-ketoglutarate, and probenecid was characterized. The IC(50) values of many inhibitors were similar for both transporters, whereas others were significantly different. Mepiperphenidol and O-methylisoprenaline showed an approximately 70-fold lower and corticosterone a 38-fold higher affinity for rOCT2. With the use of these inhibitors together with previous information on cation transporters, experimental protocols are proposed to dissect out the individual contributions of rOCT2 and rOCT1 in intact proximal tubule preparations. Inhibition experiments at different pH levels strongly suggest that the weak base quinine passively permeates the plasma membrane at physiological pH and inhibits rOCT2 from the intracellular side.

The transport modifier RS1 is localized at the inner side of the plasma membrane and changes membrane capacitance

Previously we cloned membrane associated (M(r) 62000-67000) polypeptides from pig (pRS1), rabbit (rbRS1) and man (hRS1) which modified transport activities that were expressed in Xenopus laevis oocytes by the Na(+)-D-glucose cotransporter SGLT1 and/or the organic cation transporter OCT2. These effects were dependent on the species of RS1 and on the target transporters. hRS1 and rbRS1 were shown to be intronless single copy genes which are expressed in various tissues and cell types. Earlier immunohistochemical data with a monoclonal IgM antibody suggested an extracellular membrane association of RS1. In the present paper antibodies against recombinant pRS1 were raised and the distribution and membrane localization of RS1 reevaluated. After subcellular fractionation of renal cortex RS1 was found associated with brush border membranes and an about 1:200 relation between RS1 and SGLT1 protein was estimated. Also after overexpression in X. laevis oocytes RS1 was associated with the plasma membrane, however, at variance to the kidney it was also observed in the cytosol. Labeling experiments with covalently binding lipid-permeable and lipid-impermeable biotin analogues showed that RS1 is localized at the inner side of the plasma membrane. Western blots with plasma membranes from Xenopus oocytes revealed that SGLT1 protein in the plasma membrane was reduced when hRS1 was coexpressed with human SGLT1 which leads to a reduction in V(max) of expressed glucose transport. Measurements of membrane capacitance and electron microscopic inspection showed that the expression of hRS1 leads to a reduction of the oocyte plasma membrane surface. The data suggest that RS1 is an intracellular regulatory protein that associates with the plasma membrane. Overexpression of RS1 may effect the incorporation and/or retrieval of transporters into the plasma membrane.

Transcriptional control of Ca(2+)-activated K(+) channel expression: identification of a second, evolutionarily conserved, neuronal promoter

Neuronal signaling properties are largely determined by the quantity and combination of ion channels expressed. The Drosophila slowpoke gene encodes a Ca(2+)-activated K(+) channel used throughout the nervous system. The slowpoke transcriptional control region is large and complex. To simplify the search for sequences responsible for tissue-specific expression, we relied on evolutionary conservation of functionally important sequences. A number of conserved segments were found between two Drosophila species. One led us to a new 5′ exon and a new transcriptional promoter: Promoter C0. In larvae and adults, Promoter C0 was demonstrated to be neural-specific using flies transformed with reporter genes that either contain or lack the promoter. The transcription start site of Promoter C0 was mapped, and the exon it appends to the 5′ end of the mRNA was sequenced. This is the second neural-specific slowpoke promoter to be identified, the first being Promoter C1. Promoter choice does not alter the encoded polypeptide sequence. RNAase protection assays indicate that Promoter C0 transcripts are approximately 12 times more abundant that Promoter C1 transcripts. Taken together, these facts suggest that promoter choice may be a means for cells to control channel density.

Effects of ginseng radix on sugar absorption in the small intestine

Ginseng radix (GR) is often used in traditional Japanese kampo medicine. We studied the effect of GR on glucose and maltose transport in rat and human duodenal mucosa by Ussing's method, and on smooth muscle movement in rat duodenal muscle by Magnus' method. GR inhibited absorption of glucose or maltose in rat and human duodenal mucosa, but increased duodenal muscle movement. It suggests that the inhibition of sugar absorption by GR is more dominant than enhancement of duodenal muscle movement by GR.

Cloning and characterization of the transport modifier RS1 from rabbit which was previously assumed to be specific for Na+-D-glucose cotransport

Previously we cloned membrane associated polypeptides from pig and man (pRS1, hRS1) which altered rate and glucose dependence of Na+-d-glucose cotransport expressed by SGLT1 from rabbit and man. This paper describes the cloning of a related cDNA sequence from rabbit intestine (rbRS1) which encodes a gene product with about 65% amino acid identity to pRS1 and hRS1. Hybridization of endonuclease-restricted genomic DNA with cDNA fragments of rbRS1 showed that there is only one gene with similarity to rbRS1 in rabbit, and genomic PCR amplifications revealed that the rbRS1 gene is intronless. Comparing the transcription of rbRS1 and rbSGLT1 in various tissues and cell types, different mRNA patterns were obtained for both genes. In Xenopus oocytes the Vmax of expressed Na+-d-glucose cotransport was increased or decreased when rbRS1 was coexpressed with rbSGLT1 or hSGLT1, respectively. After coexpression with hSGLT1 the glucose dependence of the expressed transport was changed. By coexpression of rbRS1 with the human organic cation transporter hOCT2 the expressed cation uptake was not altered; however, the expressed cation uptake was drastically decreased when hRS1 was coexpressed with hOCT2. The data show that RS1 can modulate the function of transporters with non-homologous primary structures.

Membrane localization of the electrogenic cation transporter rOCT1 in rat liver

The polyspecific cation transporter rOCT1 in the rat was the first identified member of a new protein family with 12 presumed membrane-spanning alpha-helices and two large hydrophilic loops. Previous studies showed that rOCT1 is mainly expressed in liver and mediates electrogenic uptake of small organic cations into cells. Antibodies against partial sequences of rOCT1 were raised and their specificity was verified. Immunohistochemistry with rat liver and Western blots with isolated membranes showed that rOCT1 is localized within sinusoidal membranes of hepatocytes. Antibody reactions were also performed with intact and permeabilized human embryonic kidney cells that were stably transfected with rOCT1. They showed that the large hydrophilic loop after the first alpha-helix of rOCT1 is located extracellularly, while the C-terminus is located intracellularly. Translational regulation is suggested since the message of rOCT1 was distributed throughout the liver lobuli, whereas rOCT1 protein was observed only in hepatocytes surrounding the central veins.

Cholecystokinin decreases intestinal hexose absorption by a parallel reduction in SGLT1 abundance in the brush-border membrane

The dual lumenaly and vascularly perfused small intestine was used to determine the mechanism by which cholecystokinin octapeptide (CCK-8) decreases the rate of glucose absorption. With CCK-8 in the vascular perfusate the rate of 3-O-methyl-D-glucose absorption decreased, whereas the rate of D-fructose absorption was unaffected. The substrate pool size within the tissue during steady-state transport, in the presence and absence of CCK-8, was estimated by compartmental analysis of the 3-O-methyl-D-glucose washout into the vascular bed. When CCK-8 was included in the vascular perfusate, the absorptive cell pool size decreased when compared with untreated tissue. Both the steady-state hexose absorption data and the washout studies indicated that the locus of action of CCK-8 was the SGLT1 transporter located in the brush-border membrane. The SGLT1 protein abundance in isolated brush-border membranes, as quantified by Western blotting, showed a decrease that paralleled the decrease in the steady-state transport rate induced by CCK-8. These results indicate that CCK-8 diminishes the rate of intestinal hexose absorption by decreasing SGLT1 protein abundance in the brush-border membrane of the rat jejunum and therefore provides evidence for acute enteric hormonal regulation of the rate of glucose absorption across the small intestine.

Hyperglycemia during normothermic cardiopulmonary bypass: the role of the kidney

BACKGROUND

Hyperglycemia commonly occurs during cardiopulmonary bypass. We studied the quantitative impact of glucose input and its renal excretion on hyperglycemia during cardiopulmonary bypass.

METHODS

The quantity of glucose infused and metabolite and hormone concentrations in plasma, as well as oxygen consumption, carbon dioxide production, and renal glucose excretion, were determined before, during, and after cardiopulmonary bypass in 8 patients.

RESULTS

Hyperglycemia (14 to 29 mmol/L) was accompanied by an increase in plasma insulin levels. The degree of hyperglycemia was directly related to the amount of glucose infused. The rate of oxygen consumption did not decrease and the rate of urea appearance (gluconeogenesis) did not rise. Despite a very high filtered load of glucose, there was very little glucosuria, indicating a markedly enhanced renal absorption of glucose.

CONCLUSIONS

Hormonal and metabolic factors permit the development of hyperglycemia during cardiopulmonary bypass but its severity depends on the quantity of glucose infused and, what appears to be a new finding, a markedly enhanced renal reabsorption of filtered glucose. Thus the kidney plays an important role in the development of severe hyperglycemia during cardiopulmonary bypass.

Transport of the new chemotherapeutic agent beta-D-glucosylisophosphoramide mustard (D-19575) into tumor cells is mediated by the Na+-D-glucose cotransporter SAAT1

For beta-D-glucosylisophosphoramide mustard (beta-D-Glc-IPM), a new alkylating drug in which isophosphoramide mustard is stabilized, a higher selectivity and lower myelotoxicity was observed than for the currently used cytostatic ifosfamide. Because beta-D-Glc-IPM is hydrophilic and does not diffuse passively through the lipid bilayer, we investigated whether a transporter may be involved in the cellular uptake. A variety of cloned Na+-sugar cotransporters were expressed in Xenopus oocytes, and uptake measurements were performed. By tracer uptake and electrical measurements it was found that beta-D-Glc-IPM was transported by the low-affinity Na+-D-glucose cotransporter SAAT1, which had been cloned from pig and is also expressed in humans. At membrane potentials between -50 and -150 mV, a 10-fold higher substrate affinity (Km approximately 0.25 mM) and a 10-fold lower Vmax value were estimated for beta-D-Glc-IPM transport than for the transport of D-glucose or methyl-alpha-D-glucopyranoside (AMG). Transport of beta-D-Glc-IPM and glucose by SAAT1 is apparently performed by the same mechanism because similar sodium dependence, dependence on membrane potential, electrogenicity, and phlorizin inhibition were determined for beta-D-Glc-IPM, D-glucose, and AMG. Transcription of human SAAT1 was demonstrated in various human carcinomas and tumor cell lines. In one of these, the human carcinoma cell line T84, phlorizin inhibitable uptake of beta-D-Glc-IPM was demonstrated with substrate saturation and an apparent Km of 0.4 mM. The data suggest that the Na+-D-glucose cotransporter SAAT1 transports beta-D-Glc-IPM into human tumor cells and may accumulate the drug in the cells. They provide an example for drug targeting by employing a plasma membrane transporter.

Upregulation of SGLT-1 transport activity in rat jejunum induced by GLP-2 infusion in vivo

The effect of in vivo infusion of the peptide hormone glucagon-like peptide 2 (GLP-2) on glucose transport across the rat jejunal brush-border membrane (BBM) was assessed using isolated membrane vesicles. A 2-h infusion of GLP-2 produced a marked acceleration of sodium-dependent glucose uptake into BBM vesicles with a significant overshoot. There was no change in vesicle space or permeability resulting from the hormone infusion. Kinetic analysis showed this stimulation to be the result of a three-fold increase in the maximal rate of transport, with no consistent change in the affinity constant (Km). The time course of this response showed that the effect was observable, but smaller, after only 30 min of hormone infusion and was maximal after 1 h. Sodium-dependent phloridzin binding to the membrane vesicles showed a parallel increase in maximal binding after 1 and 2 h of hormone infusion. Western blotting showed a similar increase in sodium-dependent glucose transporter 1 (SGLT-1) abundance. The effect of GLP-2 could be blocked by luminal brefeldin A or wortmannin. These results indicate that GLP-2 is able to induce trafficking of SGLT-1 from an intracellular pool into the BBM within 60 min and that phosphoinositol 3-kinase may well be involved in the intracellular signaling pathway in this response.

Cloning and characterization of a pH-sensing regulatory factor that modulates transport activity of the human H+/peptide cotransporter, PEPT1

We have isolated a cDNA encoding a pH-sensing regulatory factor protein that modulates transport activity of the human H+/peptide cotransporter, hPEPT1, from the human duodenum cDNA library. The cDNA (1,724 bp) for the regulatory factor (hPEPT1-RF) had an open reading frame encoding a 208-amino acid. The 18-195 amino acid residues of hPEPT1-RF were completely consistent with the 8-185 amino acid residues of hPEPT1, whereas the 1-17 and 196-208 residues were unique sequences. Using a reticulocyte lysate, the in vitro synthesized hPEPT1-RF RNA translated a product of approximately 23 kDa. Northern blot analysis and reverse transcription-coupled PCR revealed that both hPEPT1 and hPEPT1-RF mRNA transcripts are expressed in Caco-2 cells. When expressed in Xenopus oocytes, hPEPT1-RF showed no transport activity of glycylsarcosine, but shifted pH profile of the dipeptide transport mediated by the coexpressed hPEPT1. The pH profile of glycylsarcosine uptake in oocytes coexpressing hPEPT1 and hPEPT1-RF was almost similar to that in the Caco-2 cells. This is the first demonstration of cDNA isolation of a regulatory factor which modulates hPEPT1 activity.

Radiation-inactivation analysis of the oligomeric structure of the renal sodium/D-glucose symporter

The radiation-inactivation size (RIS) of the rat renal brush-border membrane sodium/D-glucose cotransporter was estimated from the loss of transport activity in irradiated membrane vesicles. The RIS depended on the electrochemical conditions present when measuring transport activity. A RIS of 294 +/- 40 kDa was obtained when transport was measured in the presence of a sodium electrochemical gradient. Under sodium equilibrium conditions, the RIS was 84 +/- 25 kDa in the presence of a glucose gradient, and 92 +/- 20 kDa in its absence. In the absence of a sodium gradient, but in the presence of an electrical potential gradient, the RIS increased to 225 +/- 49 kDa. The 294 kDa result supports earlier suggestions that the Na+ gradient-dependent glucose transport activity is mediated by a tetramer. Individual monomers appear, however, to carry out glucose transport under equilibrium exchange conditions or when a glucose gradient serves as the only driving force. The electrical potential gradient-driven glucose transport RIS appears to involve three functional subunits.

Cloning and characterization of two human polyspecific organic cation transporters

Previously we cloned a polyspecific transporter from rat (rOCT1) that is expressed in renal proximal tubules and hepatocytes and mediates electrogenic uptake of organic cations with different molecular structures. Recently a homologous transporter from rat kidney (rOCT2) was cloned but not characterized in detail. We report cloning and characterization of two homologous transporters from man (hOCT1 and hOCT2) displaying approximately 80% amino acid identity to rOCT1 and rOCT2, respectively. Northern blots showed that hOCT1 is mainly transcribed in liver, while hOCT2 is found in kidney. Using in situ hybridization and immunohistochemistry, expression of hOCT2 was mainly detected in the distal tubule where the transporter is localized at the luminal membrane. After expression in Xenopus laevis oocytes, hOCT1 and hOCT2 mediate tracer influx of N-1-methylnicotinamide (NMN), tetraethylammonium (TEA), and 1-methyl-4-phenylpyridinium (MPP). For cation transport by hOCT2 apparent K(m) and K(i) values were determined in tracer flux measurements. In addition, electrical measurements were performed with voltage-clamped oocytes. Similar to rOCT1, cation transport by hOCT2 was pH independent, electrogenic, and polyspecific; however, the cation specificity was different. In voltage-clamped hOCT2-expressing oocytes, inward currents were induced by superfusion with MPP, TEA, choline, quinine, d-tubocurarine, pancuronium, and cyanine863. Cation transport in distal tubules is indicated for the first time. Here hOCT2 mediates the first step in cation reabsorption. hOCT1 may participate in hepatic excretion of organic cations.

Human K562 transfectants expressing high levels of reduced folate carrier but exhibiting low transport activity

A human reduced folate carrier (hRFC) cDNA was transfected into transport-deficient K562 cells to circumvent complications that may result from carrier expression in a heterologous mammalian species. Relative to wild-type cells, hRFC transcript levels were increased 11- and 19-fold, respectively, in the K43-6 and K43-1 transfectants. Although photoaffinity labeling of hRFC protein revealed similar increases of 15- and 19-fold, respectively, only a 2-fold enhancement in methotrexate (Mtx) transport was observed. This suggests that only a small portion of the cDNA-encoded hRFC protein is actively engaged in membrane transport. Kinetic analysis of [3H]Mtx transport indicated that K43-6 cells exhibited a similar affinity (Kt) but an increased Vmax (1.7-fold) when compared with K562 cells. The restored transport was similar to that of wild-type cells in its capacity to be trans-stimulated by intracellular folates and in its sensitivity to competitive transport inhibitors (1843U89, bromosulfophthalein, folic acid, leucovorin, and ZD1694) and to irreversible inhibition by N-hydroxysuccinimide-methotrexate. Further, deglycosylated photoaffinity-labeled hRFC protein in both K562 and K43-6 cells migrated at approximately 65-70 kDa on SDS-gels, consistent with the molecular mass from the predicted amino acid sequence. These data further establish that the expression of hRFC, alone, is sufficient to confer transport properties typical of the "classical" hRFC. However, the discrepancy between the stoichiometry of carrier expression and transport activity implies that membrane translocation of bound substrate may be regulated by additional undefined mechanisms.

Na:K:2Cl cotransporter (NKCC) of intestinal epithelial cells. Surface expression in response to cAMP

During intestinal chloride secretion, epithelial uptake of salts is accomplished largely by a bumetanide-sensitive Na:K:2Cl cotransporter designated here as NKCC. Using monoclonal antibodies directed against NKCC from the human crypt epithelial cell line, T84, we define its surface localization as a function of cotransporter activation. Immunoelectron microscopy, confocal localization, and selective surface biotinylation studies revealed that the 195-kDa NKCC protein is polarized to the basolateral domain. Following immunoprecipitation, several polypeptides coprecipitated with the 195-kDa cotransporter including two prominent proteins of molecular mass 160 and 130 kDa. Immunoblotting with three distinct anti-NKCC monoclonal antibodies in conjunction with deglycosylation experiments suggested that the 160- and 130-kDa bands represented novel proteins unrelated to the cotransporter. Stimulation of T84 monolayers with cAMP agonists, a condition which elicits chloride secretion and leads to microfilament-dependent NKCC activation, did not significantly increase the number of bumetanide-binding sites and only marginally increased surface expression of the 195-kDa cotransporter available for surface biotinylation. In contrast, cAMP agonist stimulation increased the surface expression of the coprecipitating 160- and 130-kDa proteins approximately 6-fold. The increase in surface 160- and 130-kDa proteins was attenuated by phalloidin preloading the cells, a condition which also prevents activation of NKCC without influencing the activity of other membrane transporters participating in chloride secretion. These studies define the polarized distribution of the NKCC protein on intestinal epithelia, indicate that NKCC may be associated with two other previously unidentified membrane proteins and such association is influenced by the F-actin cytoskeleton.

The human gene of a protein that modifies Na(+)-D-glucose co-transport

Recently, a cDNA (pRS1) was cloned from pig kidney cortex that encodes a membrane-associated protein involved in Na(+)-coupled sugar transport. pRS1 alters sugar transport by SGLT1 from rabbit intestine or by SMIT from dog kidney which is homologous to SGLT1. In contrast, pRS1 does not influence transporters from other genetic families. We report the cloning of the intronless human gene hRS1 (6,743 bp), which encodes a 617-amino-acid protein with 74% amino acid identity to pRS1. By fluorescence in situ hybridization, hRS1 was localized to chromosome 1p36.1. The localization to one chromosome and Southern blot analysis of restricted genomic DNA suggest that there is only one RS1-homologous gene in humans. Functionality of hRS1 was demonstrated by co-expression experiments of hRS1 and SGLT1 from human intestine in oocytes from Xenopus laevis. They show that hRS1-protein inhibits Na(+)-D-glucose co-transport expressed by human SGLT1 by decreasing both the Vmax and the apparent Km value of the transporter. The analysis of the 5'-noncoding sequence of hRS1 revealed different enhancer consensus sequences that are absent in the SGLT1 gene, e.g., several consensus sequences for steroid-binding proteins.

The dopamine transporter carboxyl-terminal tail. Truncation/substitution mutants selectively confer high affinity dopamine uptake while attenuating recognition of the ligand binding domain

In order to delineate structural motifs regulating substrate affinity and recognition for the human dopamine transporter (DAT), we assessed [3H]dopamine uptake kinetics and [3H]CFT binding characteristics of COS-7 cells transiently expressing mutant DATs in which the COOH terminus was truncated or substituted. Complete truncation of the carboxyl tail from Ser582 allowed for the expression of biphasic [3H]dopamine uptake kinetics displaying both a low capacity (Vmax approximately 0.4 pmol/10(5) cells/min) high affinity (Km approximately 300 nM) component and one exhibiting low affinity (Km approximately 15 microM] and high capacity (Vmax approximately 5 pmol/10(5)cells/min) with a concomitant 40% decrease in overall apparent Vmax relative to wild type (WT) DAT. Truncation of the last 22 amino acids or substitution of the DAT-COOH tail with sequences encoding the intracellular carboxyl-terminal of either dopamine D1 or D5 receptors produced results that were identical to those with the fully truncated DAT, suggesting that the induction of biphasic dopamine uptake kinetics is likely conferred by removal of DAT-specific sequence motifs distal to Pro597. The attenuation of WT transport activity, either by lowering levels of DAT expression or by pretreatment of cells with phorbol 12-myristate 13-acetate (1 microM), did not affect the kinetics of [3H]dopamine transport. The estimated affinity of dopamine (Ki approximately 180 nM) for all truncated/substituted DAT mutants was 10-fold lower than that of WT DAT (approximately 2000 nM) and appears selective for the endogenous substrate, since the estimated inhibitory constants for numerous putative substrates or uptake inhibitors were virtually identical to those obtained for WT DATs. In marked contrast, DAT truncation/substitution mutants displayed significantly reduced high affinity [3H]CFT binding interactions with estimated Ki values for dopamine and numerous other substrates and inhibitors tested from 10-100-fold lower than that observed for WT DAT. Moreover, co-expression of truncated and/or substituted DATs with WT transporter failed to reconstitute functional or pharmacological activities associated with both transporters. Instead, complete restoration of uniphasic low affinity [3H]dopamine uptake kinetics (Km approximately 2000 nM) and high affinity substrate and inhibitor [3H]CFT binding interactions attributable to WT DATs were evident. These data clearly suggest the functional independence and differential regulation of the dopamine translocation process from the characteristics exhibited by its ligand binding domain. The lack of functional phenotypic expression of mutant DAT activities in cells co-expressing WT transporter is consistent with the contention that native DATs may exist as multisubunit complexes, the formation and maintenance of which is dependent upon sequences encoded within the carboxyl tail.

Cell-specific sorting of biogenic amine transporters expressed in epithelial cells

We have utilized polarized epithelial cells stably expressing neurotransmitter transporters to analyze the sorting behavior of these membrane proteins. The transporters for serotonin (5-HT), dopamine (DA), and norepinephrine (NE) are expected to be present in situ in the most distal extremities of axonal membranes, where they terminate the action of their biogenic amine substrates. Both Madin-Darby canine kidney (MDCK) and LLC-PK1 cells were stably transfected with cDNAs encoding either the rat 5-HT transporter (SERT), the human NE transporter (NET), or the rat or human DA transporter (DAT). These cells were grown on permeable filter supports, and the transporters were localized by three independent techniques. Confocal immunofluorescence microscopy indicated that each of the transporters expressed in LLC-PK1 cells was sorted to the basolateral membrane, co-localizing with the Na+/K+-ATPase. In MDCK cells, however, DAT was located primarily on the apical surface, while SERT and NET were found on the basolateral membranes. Cell surface biotinylation using an impermeant biotinylating reagent confirmed the immunocytochemistry results. Thus, SERT and NET in MDCK cells were labeled more efficiently from the basolateral medium than the apical medium, and DAT in MDCK cells was labeled more efficiently from the apical side than the basolateral side. Transport measurements in transfected MDCK cells agreed with the immunocytochemistry and biotinylation results. These results suggest the existence of cell-specific mechanisms that discriminate between neurotransmitter transporters for surface expression and render unlikely any simple hypothesis that sorting mechanisms in neurons and epithelia are identical.