Biomedical subjects

E M Wright

Publications and source records attributed to E M Wright.

At least 19 recordsLinked to original sources

PubMed1992

Baculovirus-mediated expression of the Na+/glucose cotransporter in Sf9 cells.

We have used baculovirus (AcNPV) to express the Na+/glucose cotransporter protein in cultured Sf9 cells. We constructed a baculovirus transfer vector containing the cDNA for the rabbit intestinal Na+/glucose cotransporter (SGLT1) under the control of the polyhedrin gene promoter. Recombinant baculovirus was obtained by cotransfection of SF9 cells with wild-type AcNPV DNA and the transfer vector. Recombinant virus was identified by Southern blotting and then purified. Recombinant infected Sf9 cells expressed a protein which was recognized by anti-peptide antibodies raised to sequences of the cloned Na+/glucose cotransporter. This protein migrated with a molecular mass of 55 kD by SDS-PAGE, similar to the in vitro translation product of SGLT1. An identical protein was metabolically labeled with [35S]methionine. Cells which synthesized the transport protein showed Na(+)-dependent alpha MeGlc transport. Micromolar phlorizin inhibited transport. Uninfected and wild-type virus infected Sf9 cells did not have Na(+)-dependent glucose transport. All transport protein migrated at 45% sucrose (w/w) by density gradient sedimentation, suggesting that the expressed transporter is membrane associated. We conclude that we have functionally expressed the rabbit intestinal Na+/glucose cotransporter in Sf9 cells. The transporter is not heavily glycosylated, and this is consistent with previous work showing that glycosylation is not necessary for function. We are poised to purify and characterize this protein from a structure-function perspective.

Amino Acid Sequence↗

PubMed1992

Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies.

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus laevis oocytes. Presteady-state and steady-state currents associated with cotransporter activity were measured with the two-electrode voltage-clamp technique. Steady-state sugar-dependent currents were measured between -150 and +90 mV as a function of external Na+ ([Na]o) and alpha-methyl-D-glucopyranoside concentrations ([alpha MDG]o). K alpha MDG0.5 was found to be dependent upon [Na]o and the membrane potential. At Vm = -50 mV, increasing [Na]o from 10 to 100 mM decreased K alpha MDG0.5 from 1.5 mM to 180 microM. Increasing membrane potential toward negative values decreased K alpha MDG0.5 at nonsaturating [Na]o. For instance, at 10 mM [Na]o, K alpha MDG0.5 decreased from 1.5 mM to 360 microM on increasing the membrane potential from -50 to -150 mV. The i alpha MDGmax was relatively insensitive to [Na]o between 10 and 100 mM and weakly voltage dependent (e-fold increase per 140 mV). KNa0.5 and iNamax were found to be dependent upon membrane potential and [sugar]o. In the presence of 1 mM [alpha MDG]o, KNa0.5 decreased from 50 to 5 mM between 0 and -150 mV and iNamax increased twofold between -30 and -200 mV. The voltage dependence of KNa0.5 is consistent with an effect of potential on Na+ binding (Na(+)-well effect), whereas the voltage dependence of iNamax is compatible with the translocation step being voltage dependent. It is concluded that voltage influences both Na+ binding and translocation. Presteady-state currents were observed for depolarization pulses in the presence of 100 mM [Na]o. The transient current relaxed with a half time of approximately 10 msec, and both the half time and magnitude of the transient varied with the holding potential and the size of depolarization pulse. Presteady-state currents were not observed after the addition of phlorizin or alpha MDG to the external Na+ solution and were not observed for water-injected control oocytes. We conclude that presteady-state currents are due to the activity of the carrier and that they may give a novel insight to the transport mechanism of the Na+/glucose cotransporter.

Animals↗

PubMed1992

Electrogenic properties of the cloned Na+/glucose cotransporter: II. A transport model under nonrapid equilibrium conditions.

The results of the accompanying electrophysiological study of the cloned Na+/glucose cotransporter from small intestine (Parent, L., Supplisson, S., Loo, D.D.F., Wright, E.M. (1992) J. Mémbrane Biol. 125:49-62) were evaluated in terms of a kinetic model. The steady-state and presteady-state cotransporter properties are described by a 6-state ordered kinetic model ("mirror" symmetry) with a Na+:alpha MDG stoichiometry of 2. Carrier translocation in the membrane as well as Na+ and sugar binding and dissociation are treated as a function of their individual rate constants. Empty carrier translocation and Na+ binding/dissociation are the only steps considered to be voltage dependent. Currents were associated with the translocation of the negatively charged carrier in the membrane. Negative membrane potential facilitates sugar transport. One numerical solution was found for the 14 rate constants that account quantitatively for our experiment observations: i.e., (i) sigmoidal shape of the sugar-specific current-voltage curves (absence of outward currents and inward current saturation at high negative potentials), (ii) Na+ and voltage dependence of Ksugar0.5 and isugarmax, (iii) sugar and voltage dependence of KNa0.5 and iNamax, (iv) presteady-state currents and their dependence on external Na+, alpha MDG and membrane potential, and (v) and carrier Na+ leak current. We conclude that the main voltage effect is on carrier translocation. Na+ ions that migrate from the extracellular medium to their binding sites sense 25 to 35% of the transmembrane voltage, whereas charges associated with the carrier translocation experiences 60 to 75% of the membrane electrical field. Internal Na+ ion binding is not voltage dependent. In our nonrapid equilibrium model, the rate-limiting step for sugar transport is a function of the membrane potential, [Na]o and [alpha MDG]o. At 0 mV and at saturating [Na]o and [alpha MDG]o, the rate-limiting step for sugar transport is the empty carrier translocation (5 sec-1). As the membrane potential is made more negative, the empty carrier translocation gets faster and the internal Na+ dissociation becomes increasingly rate limiting. However, as [Na]o is decreased to less than 10 mM, the rate-limiting step is the external Na+ ions binding in the 0 to -150 mV potential range. At 0 mV, the external Na+ dissociation constant KNa' is 80 mM and decreases to 24 mM at -150 mV. The external sugar dissociation constant KNaS' is estimated to be 200 microM and voltage independent. Finally, the internal leak pathway (CNa2 translocation) is insignificant.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding Sites↗

PubMed1992

Neurotrophic factors promote the maturation of developing sensory neurons before they become dependent on these factors for survival.

We have studied the early development of chicken embryo sensory neurons in culture before they become dependent on neurotrophic factors for survival. During this period, they undergo a distinct change in morphology:initially they have small, spindle-shaped, phase-dark cell bodies, which become spherical and phase bright and extend long neurites. Although this maturational change occurs in isolated cells grown in chemically defined medium, it is accelerated by brain-derived neurotrophic factor (BDNF) or neurotrophin-3 and is retarded by antisense oligonucleotides that inhibit expression of the common, low affinity neurotrophic factor receptor (gp75NGFR) and by antisense BDNF oligonucleotides. We conclude that neurotrophic factors play a role in the earliest stages of sensory neuron development and suggest that they operate by an autocrine mechanism at this time.

Animals↗

PubMed1992

Sodium cotransport proteins.

Significant advances have been made in elucidating the structure of Na+ cotransport proteins. Some fifteen of these low-abundance proteins have been cloned, sequenced and functionally expressed. They are members of the 12 membrane-spanning superfamily and they segregate into two groups, the Na+/glucose (SGLT1) and Na+/Cl-/GABA (GAT-1) families. SGLT1 transporters are expressed in bacteria and animal cells, while GAT-1 transporters are mostly expressed in the brain. None have yet been found in plants.

Amino Acid Sequence↗

PubMed1992

Molecular biology approaches to comparative study of Na(+)-glucose cotransport.

The rabbit intestinal Na(+)-glucose cotransporter has been cloned and sequenced. The cDNA encoding the cotransporter has been used in two general lines of research related to comparative aspects of Na(+)-glucose cotransport that are reviewed here. First, defined regions of the predicted amino acid sequence were used to raise antibodies, and the species distribution of epitopes recognized by those antibodies was investigated. Intestinal brush-border membranes from mammals, birds, an amphibian, and a reptile were all found to contain protein that were recognized by the antibodies in Western analysis. The cDNA encoding the rabbit intestinal Na(+)-glucose cotransporter was also used directly to examine the species distribution of related mRNA in Northern studies and to isolate new cDNAs encoding other Na(+)-glucose cotransporters. Northern blots revealed the presence of related mRNAs in intestines of mammals and a fish, as well as rabbit kidney and gills of marine mussels. The cDNAs encoding mammalian Na(+)-glucose cotransporters and bacterial Na(+)-dependent cotransporters for proline and pantothenate share sequence homology. There has been evolutionary conservation of the structure and function of the Na(+)-glucose cotransporter, and there appears to be a gene family that codes for the Na(+)-coupled cotransporters.

Amino Acid Sequence↗

PubMed1992

Cloning of a human kidney cDNA with similarity to the sodium-glucose cotransporter.

We have used low-stringency screening with the human intestinal Na(+)-glucose cotransporter SGLT1 to isolate a 2,271-nucleotide cDNA (Hu14) from human kidney. This clone, which encodes a 672-residue protein, is 59% identical at the amino acid level to SGLT1 and has a similar number and arrangement of predicted membrane-spanning regions. It also shares significant sequence identity with other Na(+)-coupled transporters. Northern blot analysis suggests strong expression of Hu14 in kidney, but, unlike SGLT1, no significant expression in intestine. We have been unable to demonstrate definitive transport of any of a number of substrates (including amino acids, sugars, nucleosides, and vitamins) into Hu14 cRNA-injected Xenopus oocytes, although sequence conservation makes it likely that Hu14 represents another member of the Na+ cotransporter family, possibly a second Na(+)-glucose cotransporter.

Amino Acid Sequence↗

PubMed1992

Colocalization of GLUT2 glucose transporter, sodium/glucose cotransporter, and gamma-glutamyl transpeptidase in rat kidney with double-peroxidase immunocytochemistry.

Glucose is reabsorbed from the glomerular filtrate in the proximal segment of the renal tubule in two stages. The first stage is uphill transport across the brush border membrane by Na(+)-glucose cotransport and the second stage is downhill transport across the basolateral membrane by facilitated diffusion. Genes for both a renal Na(+)-glucose cotransporter (SGLT1) and a renal facilitated glucose transporter (GLUT2) have been cloned and sequenced. To examine whether SGLT1 and GLUT2 colocalize to the same tubular epithelial cells in rat kidney, double-immunoperoxidase studies with dual chromogens and paraformaldehyde perfusion-fixed frozen sections of rat kidney were performed. Antipeptide antisera were prepared against rat GLUT2 (amino acids 510-522) and rabbit SGLT1 (amino acids 402-420). Proximal tubules were identified immunocytochemically with an antiserum raised against a synthetic peptide corresponding to the 21 amino acids at the COOH-terminal of the heavy chain of rat gamma-glutamyl transpeptidase, which is a proximal tubule-specific enzyme. The anti-GLUT2 antiserum strongly stained the basolateral membrane of 46% of cortical tubules, whereas the SGLT1 antiserum stained the brush border of 56% of the cortical tubules. The gamma-glutamyl transpeptidase antiserum also stained the brush border of 51% of the cortical tubules. GLUT2 and SGLT1 colocalized to 40% of cortical epithelium, but 16% of cortical epithelial cells were immunopositive for brush border SGLT1 and immunonegative for basolateral GLUT2. These gamma-glutamyl transpeptidase staining results suggest that at least 50% of the tubules in the cortex are proximal tubules and that SGLT1 and GLUT2 colocalize to most proximal tubules. The fact that SGLT1 antiserum immunoreacted with tubules unreactive to the GLUT2 antiserum suggests that either the SGLT1 epitope is conserved on a related brush border protein or that there is another GLUT transporter responsible for the exit of sugar from these proximal tubule cells.

Animals↗

PubMed1992

The Na+/glucose cotransporter (SGLT1).

An important class of Na+ transport proteins is the cotransporters. They exist in bacteria and animal cells and are responsible for the "active" accumulation of sugars, amino acids, carboxylic acids and some ions, e.g., I-, Cl-, and PO-4, in cells. In the small intestine and renal proximal tubule the cotransporters play an important role in the transport of salt and water across the epithelia. The most well known and best characterized Na+ cotransporter is the intestinal brush border Na+/glucose cotransporter. We have cloned, sequenced, and expressed both the rabbit and human Na+/glucose cotransporters. The cDNAs code for 73kDa proteins with 662-664 residues (86% identity). Secondary structure analysis suggests a 12 membrane-spanning helical model with the N- and C-termini in the cytoplasm. A single N-linked glycosylation site is utilized at Asn248. These sugars are not required for function. Two essential residues for functional expression in oocytes have been identified, Asp28 and Arg300. In two sisters with glucose-galactose malabsorption the transport defect is caused by a missense mutation changing Asp28 to Asn28, and we have found that changing Arg300 to Cys300 eliminated transport. Current research is directed to finding residues and domains essential for ligand binding and transport, and we are using electrophysiological techniques to correlate structure and function.

Amino Acid Sequence↗

PubMed1979

Highly purified basal lateral plasma membranes from rat duodenum. Physical criteria for purity.

Preparations of intestinal epithelial cell basal lateral plasma membranes were analyzed with free flow electrophoresis and density perturbation with digitonin. The initial basal lateral membrane preparations were obtained by equilibrium density gradient centrifugation after two different schemes of homogenization and differential sedimentation (A.K. Mircheff, C.H. van Os, and E.M. Wright. 1978. Membr. Biochem. 1:177, and A.K. Mircheff, S.D. Hanna, M.W. Walling, and E.M. Wright. 1979. Prep. Biochem. 9:33. In these preparations, Na,K-ATPase, a marker for the basal lateral mambrane, was purified 16- to 18-fold over the initial homogenate. The preparations were also enriched in NADPH-cytochrome c reductase, alkaline phosphatase, acid phosphatase, and galactosyltransferase. Both free-flow electrophoresis, which separates on the basis of surface charge, and density perturbation with digitonin, which depends on a specific interaction of digitonin with cholesterol-rich membranes, resolved the preparation into three populations of particles. The major population, which represented basal lateral membranes purified 20- to 32-fold with respect to the initial homogenate, contained Na,K-ATPase, alkaline phosphatase, adenylate cyclase, and acid phosphatase. A second population was defined by its content of NADPH-cytochrome c reductase, and the third was defined by its content of galactosyltransferase. Guanylate cyclase appeared to be partitioned between the Na,K-ATPase-rich and NADPH-cytochrome c reductase-rich populations. Galactosyltransferase is also present in fractions which contain the Na,K-ATPase-rich membranes, but the present data cannot exclude the possibility of spillover by the adjacent, galactosyltransferase-rich population. This work emphasizes the importance of multiple, physical criteria for purity in the isolation of subcellular components.

Animals↗

PubMed1979

Effects of dibutyryl cyclic AMP on the transport of alpha-methyl-D-glucoside and alpha-aminoisobutyric acid in separated tubules and brush border membranes from rabbit kidney.

The effect of dibutyryl cyclic AMP on the transport of alpha-methyl-D-glucoside and alpha-aminoisobutyric acid in separated tubules and purified brush border membranes from rabbit kidney was investigated using a rapid filtration procedure. Dibutyryl cyclic AMP stimulated the uptake of alpha-methyl-D-glucoside and alpha-aminoisobutyric acid by separated renal tubules in agreement iwth prior studies utilizing renal slices (Rea, C. and Segal, S. (1973) Biochim. Biophys. Acta 311, 615--624; Weiss, I.W., Morgan, K. and Phang, J.M. (1972) J. Biol. Chem. 247, 760--764). However, in contrast to previous reports, no preincubation of the tissue with dibutyryl cyclic AMP was required for stimulation of transport to be manifest. Dibutyryl cyclic AMP stimulated oxygen consumption by separated tubules suggesting that stimulation of transport may occur by a linkage with renal oxidative metabolism. Dibutyryl cyclic AMP increased the uptake of alpha-aminoisobutyric acid into purified renal brush border membranes. However the uptakes of alpha-methyl-D-glucoside, proline, leucine and phosphate into brush border membranes were significantly inhibited.

Aminoisobutyric Acids↗

PubMed1979

Transport of p-aminohippuric acid, uric acid and glucose in highly purified rabbit renal brush border membranes.

A procedure for preparing highly purified brush border membranes from rabbit kidney cortex using differential and density gradient centrifugation is described. Brush border membranes prepared by this procedure were substantially free of basal-lateral membranes, mitochondria, endoplasmic reticulum and nuclear material as evidenced by an enrichment factor of less than 0.3 for (Na+ + K+)-ATPase, succinate dehydrogenase, NADPH-cytochrome c reductase and DNA. Alkaline phosphatase was enriched ten fold indicating that the membranes were enriched at least 30 fold with respect to other cellular organelles. The yield of brush border membranes was 20%. Transport of D-glucose by the membranes was identical to that previously reported except that the Arrhenius plot for temperature dependence of transport was curvilinear (EA = 11.3--37.6 kcal/mol) rather than biphasic. Transport of p-aminohippuric acid and uric acid were increased by the presence of NaCl, either gradient or preequilibrated. However, no overshoot was obtained in the presence of a NaCl gradient, and KCl and LiCl also produced equivalent stimulation of transport suggesting a nonspecific ionic strength effect. Uptakes of p-aminohippuric acid and uric acid were not saturable, and were increased markedly by reducing the pH from 7.5 to 5.6. Probenecid (1 mM) reduced p-aminohippuric acid and uric acid (50 muM) uptake by 49% and 21%, respectively. We conclude that the uptake of uric acid and p-aminohippuric acid by renal brush border membranes of the rabbit occurs primarily by a simple solubility-diffusion mechanism.

Aminohippuric Acids↗

PubMed1979

Alkaline phosphatase of basal lateral and brush border plasma membranes from intestinal epithelium.

The alkaline phosphatase present on isolated brush border and basal lateral membranes of rat duodenal epithelium were examined by means of a variety of biochemical assays and physical methods. The two alkaline phosphatases have similar pH optima of 9.6--9.8, similar substrate km's for p-nitrophenyl phosphate (PNPP) of 71 micromolar, similar responses to the inhibitors 2-mercaptoethanol, theophylline, phenylalanine, and ethylenediaminetetraacetic acid (EDTA), similar sensitivities to calcium, magnesium, zinc, sodium, and potassium, and similar insensitivities to digestion with trypsin of papain. The two enzymes also exhibit similar molecular weights on SDS-polyacrylamide gels in the range 124,000--150,000, and both enzymes show an Rf value of 0.092 on Triton X-100 polyacrylamide gels, indicating similar intrinsic charges. The Vmax of the brush border enzyme is ten times greater than that of the basal lateral enzyme, 140 mumoles/mg-h as opposed to 14 mumoles/mg-h. The differences in Vmax are a reflection of the known distribution of alkaline phosphatase in rat duodenum, there being more alkaline phosphatase activity present on the brush border than on the basal lateral surface. One other major difference was observed between the two enzymes, the stimulation of the basal lateral and not the brush border alkaline phosphatase by SDS, Triton X-100, or cholate. We conclude that the enzymes are very similar to one another and probably perform similar membrane functions.

Alkaline Phosphatase↗

PubMed1979

Volume flows across gallbladder epithelium induced by small hydrostatic and osmotic gradients.

The hydraulic conductivity of rabbit gallbladder epithelium has been studied using a continuous volumetric method based on capacitance measurements. The time resolution for measuring osmotic flows is in the range of seconds. Volume flows have been induced by osmotic gradients between 0 and 100 mosmol. In this range the flow-force relation is linear and the Pf value is 9.3 X 10(-3) cm/sec. After correction for solute polarization effects, the Pf value amounts to 0.05 cm/sec. The observed flow is constant between 5 sec up to 20 min after a sudden increase in the osmolarity of the mucosal solution. The wet weight of the gallbladder tissue decreases by 22% and increases by 30% during osmotic flows from mucosa to serosa, respectively. Volume flows induced by hydrostatic pressure gradients on the mucosal surface are linearly related to the driving forces between 0 and 40 mbar. The Pf value is 0.15 cm/sec. The volume flows are constant between 2 sec and 15 min after pressure application. The flow-force relation for pressure gradients on the serosal surface is markedly nonlinear for gradients greater than 5 mbar. Below 5 mbar the Pf value is 4.5 cm/sec. From electrical measurements, e.g., resistance and streaming potentials, and from flux studies with inulin and polyethylene glycol 4000, it is concluded that hydrostatic and osmotic gradients are not comparable when they are applied to gallbladder epithelium. They induce volume flows across different pathways, e.g., osmosis predominantly across the cellular route and pressure filtration predominantly across paracellular routes.

Animals↗

PubMed1979

Transport of tricarboxylic acid cycle intermediates by membrane vesicles from renal brush border.

The uptake of citrate and alpha-ketoglutarate by membrane vesicles from rabbit renal brush border was studied by a rapid filtration technique. Both compounds exhibited transport characteristics similar to those seen for the sodium-dependent cotransport systems previously described for sugars and amino acids in brush border membranes. The estimated sodium-dependent Vmax and Km were 17 nmol per mg of protein per min and 0.18 mM for citrate and 17 nmol per mg of protein per min and 1.0 mM for alpha-ketoglutarate. The initial rate of citrate transport was 5 times that of sugars and amino acids under comparable conditions. Uptake rates of 0.1 mM citrate and alpha-ketoglutarate were inhibited by greater than 90% by 10 mM succinate, malate, fumarate, or oxaloacetate, indicating the presence in the brush border membrane of a transport system highly specialized for the renal conservation of intermediates of the tricarboxylic acid cycle.

Animals↗

PubMed1979

Large scale, analytical method for isolating basal lateral plasma membranes from rat duodenum.

A procedure is described for obtaining large amounts of basal lateral plasma membranes from the rat duodenal epithelium. The yield is approximately 50%, and the purification factor is 18; preparations from 25 rats routinely contain 100 mg of protein. The procedure depends on mild homogenization with a nitrogen cavitation bomb, followed by removal of brush borders by sedimentation in a weak centrifugal field. Basal lateral membranes in the resulting supernatant are partially purified by differential centrifugation in a medium which approximates their equilibrium density, and then further purified by equilibrium density gradient centrifugation in a high capacity zonal rotor. Brush border membranes may be isolated from the 450 x g pellet. Since both brush border and basal lateral membranes may be isolated from the same homogenate, this preparative procedure is suitable for such analytical purposes as determinations of distribution of enzyme activities between the two surfaces of the epithelium. The large scale of the isolation procedure makes it an appropriate starting point for purification of specific basal lateral membrane components.

Animals↗

PubMed1978

An electrogenic NA+/K+ pump in the choroid plexus.

Intracellular electrical potential and potassium activity was measured by means of microelectrodes in the epithelial cells of choroid plexus from bullfrogs (Rana catesbeiana). Ouabain applied from the ventricular side caused an abrupt depolarisation of 10 mV but only a gradual loss of potassium from the cells. Readministration of potassium to the ventricular solution of plexuses which were previously depleted of potassium, caused a hyperpolarisation of about 4 mV. These two experiments are consistent with the notion of an electrogenic Na+/K+ pump situated at the ventricular membrane and which pumps potassium into the cell and sodium into the ventricle. The numerical values obtained suggest that 3 sodium ions are pumped for 2 potassium ions. The permeability coefficient for potassium exit from the cell is calculated to be 1.24 . 10(-5) cm-1 . s-1 expressed per cm2 of flat epithelium.

Animals↗

PubMed1978

Subcellular distribution of nucleotide cyclases in rat intestinal epithelium.

The subcellular distributions of adenylate cyclase and guanylate cyclase were determined for the mature enterocyte from the rat duodenum. Brush-border and basolateral membranes were prepared from isolated cells by an analytical isolation procedure, and multiple linear regression analysis was used to obtain a quantitative estimate of the distribution of recovered cyclase activities between the brush borders and basolateral membranes. Adenylate cyclase was largely confined to the basolateral surface of the epithelium, whereas guanylate cyclase was found on the brush-border and basolateral membrane fractions in the ratio 2.4:1. There was no evidence for the presence of nucleotide cyclases in the cytosol. Guanylate cyclase in both the brush-border and basolateral membranes was stimulated by epinephrine, insulin, and Triton X-100, but not by carbachol. Adenylate cyclase was not influenced by epinephrine, but was markedly stimulated by NaF and vasoactive intestinal peptide. These results are discussed in relation to the effects of hormones on transport across the small intestine.

Adenylyl Cyclases↗