Comparison of a Na+/D-glucose cotransporter from rat intestine expressed in oocytes of Xenopus laevis with the endogenous cotransporter

Human SLC2A9a and SLC2A9b isoforms mediate electrogenic transport of urate with different characteristics in the presence of hexoses

Human SLC2A9 (GLUT9) is a novel high-capacity urate transporter belonging to the facilitated glucose transporter family. In the present study, heterologous expression in Xenopus oocytes has allowed us to undertake an in-depth radiotracer flux and electrophysiological study of urate transport mediated by both isoforms of SLC2A9 (a and b). Addition of urate to SLC2A9-producing oocytes generated outward currents, indicating electrogenic transport. Urate transport by SLC2A9 was voltage dependent and independent of the Na(+) transmembrane gradient. Urate-induced outward currents were affected by the extracellular concentration of Cl(-), but there was no evidence for exchange of the two anions. [(14)C]urate flux studies under non-voltage-clamped conditions demonstrated symmetry of influx and efflux, suggesting that SLC2A9 functions in urate efflux driven primarily by the electrochemical gradient of the cell. Urate uptake in the presence of intracellular hexoses showed marked differences between the two isoforms, suggesting functional differences between the two splice variants. Finally, the permeant selectivity of SLC2A9 was examined by testing the ability to transport a panel of radiolabeled purine and pyrimidine nucleobases. SLC2A9 mediated the uptake of adenine in addition to urate, but did not function as a generalized nucleobase transporter. The differential expression pattern of the two isoforms of SLC2A9 in the human kidney's proximal convoluted tubule and its electrogenic transport of urate suggest that these transporters play key roles in the regulation of plasma urate levels and are therefore potentially important participants in hyperuricemia and hypouricemia.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Locally and systemically active glucocorticosteroids modify intestinal absorption of sugars in rats

Glucocorticosteroids enhance digestive and absorptive functions of the intestine of weaning and adult rats. This study was undertaken to assess the influence of treatment of weaning male rats with budesonide (Bud), prednisone (Pred), or control vehicle on the in vitro jejunal and ileal uptake of glucose and fructose. Bud and Pred had no effect on the uptake of d-glucose by sodium glucose transporter-1. In contrast, the uptake of d-fructose by GLUT-5 was similarly increased with Bud and with Pred. The increases in the uptake of fructose were not due to variations in the weight of the intestinal mucosa, food intake, or in GLUT-5 protein or mRNA expression. There were no steroid-associated changes in mRNA expression of c-myc, c-jun, c-fos, proglucagon, or selected cytokines. However, the abundance of ileal ornithine decarboxylase mRNA was increased with Pred. Giving postweaning rats 4 wk of Bud or Pred in doses equivalent to those used in clinical practice increases fructose but not glucose uptake. This enhanced uptake of fructose was likely regulated by posttranslational processes.

Dietary lipids alter the effect of steroids on the transport of glucose after intestinal resection: Part I. Phenotypic changes and expression of transporters

BACKGROUND/PURPOSE

Glucocorticosteroids alter the function of the intestine. This study was undertaken to assess the effect on D-glucose uptake of budesonide (Bud), prednisone (Pred), or dexamethasone (Dex) in animals with a 50% intestinal resection and fed chow or a diet enriched with saturated (SFA) or polyunsaturated fatty acids (PUFA).

METHODS

In vitro ring uptake technique, Western blots, and Northern blots were performed.

RESULTS

Bud increased the jejunal D-glucose uptake, and this effect was prevented by feeding PUFA. SGLT1 and Na+/K+ ATPase protein and mRNA abundance did not correlate with the change in the rate of uptake of glucose.

CONCLUSIONS

(1) Bud increased the jejunal glucose uptake, (2) the activity of the sugar transporter does not correlate with the abundance of protein or their respective mRNAs, (3) th Bud effect on glucose uptake is prevented by feeding PUFA. Thus, the desired intestinal adaptive response after intestinal resection may be enhanced further by the administration of the locally acting steroid budesonide and by feeding a saturated compared with a polyunsaturated fatty acid diet.

Cloning and functional expression of an SGLT-1-like protein from the Xenopus laevis intestine

A cDNA encoding an Na+-glucose cotransporter type 1 (SGLT-1)-like protein was cloned from the Xenopus laevis intestine by the 5'- and 3'-rapid amplification of cDNA ends method. The deduced amino acid sequence was 673 residues long, with a predicted mass of 74.1 kDa and 52-53% identity to mammalian SGLT-1s. This gene was expressed in the small intestine and kidney, reflecting a tissue distribution similar to that of SGLT-1. The function of the protein was studied using the two-microelectrode voltage-clamp technique after injection of cRNA into Xenopus laevis oocytes. Perfusion with myo-inositol elicited about twofold larger inward currents than perfusion with D-glucose. The order of the substrate specificity was myo-inositol > D-glucose > D-galactose >/= alpha-methyl-D-glucoside. The current induced by myo-inositol increased with membrane hyperpolarization and depended on external myo-inositol and Na+: the apparent Michaelis-Menten constant was 0.25 +/- 0.07 (SD) mM with myo-inositol, whereas the apparent concentration for half-maximal activation was 12.5 +/- 1.0 mM and the Hill coefficient was 1.6 +/- 0.1 with Na+. In conclusion, the cloned protein shares features with both SGLT-1 and the Na+-myo-inositol cotransporter.

Stretch-independent activation of the mechanosensitive cation channel in oocytes of Xenopus laevis

Oocytes of the South African clawed toad Xenopus laevis possess in their plasma membrane a so-called stretch-activated cation channel (SAC) which is activated by gently applying positive or negative pressure (stretch) to the membrane patch containing the channels. We show here that this mechanosensitive channel acted as a spontaneously opening, stretch-independent non-selective cation channel (NSCC) in more than half of the oocytes that we investigated. In 55% of cell-attached patches (total number of patches, 58) on 30 oocytes from several different donors, we found NSCC opening events. These currents were increased by elevating the membrane voltage or raising the temperature. NSCC and SAC currents shared some properties regarding the relative conductances of Na+>Li+>Ca2+, gating behaviour and amiloride sensitivity. Stretch-independent currents could be clearly distinguished from stretch induced SAC currents by their voltage and temperature dependence. Open events of NSCC increased strongly when temperature was raised from 21 to 27 degrees C. NSCC currents could be partly inhibited by high concentrations of extracellular Gd3+ and amiloride (100 and 500 microM, respectively). We further show exemplarily that NSCC can seriously hamper investigations when oocytes are used for the expression of foreign ion channels. In particular, NSCC complicated investigations on cation channels with small conductance as we demonstrate for a 4 pS epithelial Na+ channel (ENaC) from guinea pig distal colon. Our studies on NSCCs suggest the involvement of these channels in oocyte temperature response and ion transport regulation. From our results we suggest that NSCC and SAC currents are carried by one protein operating in different modes.

cAMP-activation of amiloride-sensitive Na+ channels from guinea-pig colon expressed in Xenopus oocytes

Guinea-pig distal colonic mRNA injection into Xenopus laevis oocytes resulted in expression of functional active epithelial Na+ channels in the oocyte plasma membrane. Poly(A)+ RNA was extracted from distal colonic mucosa of animals fed either a high-salt (HS) or a low-salt (LS) diet. The electrophysiological properties of the expressed amiloride-sensitive Na+ conductances were investigated by conventional two-electrode voltage-clamp and patch-clamp measurements. Injection of poly(A)+ RNA from HS-fed animals [from hereon referred to as HS-poly(A)+ RNA] into oocytes induced the expression of amiloride-sensitive Na+ conductances. On the other hand, oocytes injected with poly(A)+ RNA from LS-fed animals [LS-poly(A)+ RNA] expressed a markedly larger amount of amiloride-blockable Na+ conductances. LS-poly(A)+ RNA-induced conductances were completely inhibitable by amiloride with a Ki of 77 nM, and were also blocked by benzamil with a Ki of 1.8 nM. 5-(N-Ethyl-N-isopropyl)-amiloride (EIPA), even in high doses (25 "mu"M), had no detectable effect on the Na+ conductances. Expressed amiloride-sensitive Na+ channels could be further activated by cAMP leading to nearly doubled clamp currents. When Na+ was replaced by K+, amiloride (1 "mu"M) showed no effect on the clamp current. Single-channel analysis revealed slow gating behaviour, open probabilities (Po) between 0.4 and 0.9, and slope conductances of 3. 8 pS for Na+ and 5.6 pS for Li+. The expressed channels showed to be highly selective for Na+ over K+ with a permeability ratio PNa/PK > 20. Amiloride (500 nM) reduced channel Po to values < 0.05. All these features make the guinea-pig distal colon of LS-fed animals an interesting mRNA source for the expression of highly amiloride-sensitive Na+ channels in Xenopus oocytes, which could provide new insights in the regulatory mechanism of these channels.

Two endogenous methyl-alpha-D-glucopyranoside uptake activities in Xenopus oocytes

Endogenous methyl-alpha-D-glucopyranoside (AMG) uptake in Xenopus oocytes was measured with 14C-labeled AMG. Two AMG uptake activities, one Na(+)-dependent and the other Na(+)-independent, were observed in oocytes incubated with 2 mM AMG. However, only Na(+)-dependent uptake was observed at 0.1 mM AMG. Na(+)-dependent AMG uptake was attributed to the endogenous Na+/glucose co-transporter since it was inhibited by phlorizin. On the other hand, Na(+)-independent AMG uptake was inhibited by cytochalasin B or 2-deoxy-D-glucose. This suggests that AMG can be transported by the endogenous facilitated glucose transporter as well as by the endogenous co-transporter. In addition, a substrate range of the endogenous co-transporter was examined by competition experiments.

Glucose entry into rat mesangial cells is mediated by both Na(+)-coupled and facilitative transporters

Since previous studies from our laboratory have demonstrated that increased glucose consumption by cultured rat mesangial cells is accompanied by an accelerated production of type IV and type VI collagen, we have now examined the manner by which glucose is transported into these cells. A progressive stimulation of glucose uptake by the mesangial cells was observed with increasing concentrations of NaCl so that at 145 mmol/l about twice as much glucose entered the cells as in its absence (substituted by choline chloride). Moreover, since phlorizin inhibited the NaCl-promoted uptake of glucose and this salt was found to increase the accumulation of alpha-methylglucoside in a manner which could not be duplicated by KCl or mannitol, both Na(+)-coupled and facilitative glucose transporters appeared to be present in the cells. Km values of 1.93 mmol/l and 1.36 mmol/l were determined for the co-transport and facilitated transport pathways, respectively, with their Vmax being 29.5 and 18.0 nmol.mg protein-1.h-1. Both uptake activities were found to be down-regulated by exposure of the cells to high glucose and furthermore the Na(+)-dependent transport could no longer be detected after about 12 passages of the cells. Hybridization of mesangial cell mRNA with cDNA probes revealed transcripts for the Na+/glucose co-transporter as well as GLUT1 and to a lesser extent GLUT4.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of osmotically-activated potassium transporters after injection of mRNA from A6 cells in Xenopus oocytes

The different potassium pathways, under iso-osmotic or hypo-osmotic conditions, were examined in Xenopus oocytes that were micro-injected with mRNA from A6 cells. Hypo-osmotically stimulated 86Rb (K+) effluxes could be measured from intact oocytes 1-4 days after injection of 25 ng of poly (A)+ RNA isolated from A6 cells. 86Rb (K+) effluxes were 2.2 times higher from oocytes micro-injected with 25 ng of poly(A)+ RNA, than from water injected control oocytes. Water-injected oocytes themselves, however, were 7-fold more responsive to a hypo-osmotic shock than non-injected Xenopus oocytes. There was no significant effect of the different K+ transport blockers tested (TEA, bumetanide, glybenclamide or quinidine) on the endogenous 86Rb (K+) effluxes from non-injected oocytes in either iso- or hypo-osmotic media. The 86Rb (K+) effluxes from water-and mRNA-injected oocytes in hypo-osmotic media were both inhibited by TEA. In mRNA-injected oocytes the increase in 86Rb (K+) transport following a medium dilution was also inhibited in the presence of glybenclamide or bumetanide. The present study reports that the activation of hypo-osmotically-activated potassium transporters in the oocytes of Xenopus laevis. after injection of mRNA from A6 cells differs quantitatively and in part qualitatively (glybenclamide-sensitivity) from the endogenous K+ pathways of non-injected and of water-injected Xenopus oocytes.

Function and presumed molecular structure of Na(+)-D-glucose cotransport systems

Functional characterization of Na(+)-D-glucose cotransport in intestine and kidney indicates the existence of heterogeneous Na(+)-D-glucose cotransport systems. Target size analysis of the transporting unit and model analysis of substrate binding have been performed and proteins have been cloned which mediate (SGLT1) and modulate (RS1) the expression of Na(+)-D-glucose cotransport. The experiments support the hypothesis that functional Na(+)-D-glucose cotransport systems in mammals are composed of two SGLT1-type subunits and may contain one or two RS1-type proteins. SGLT1 contains up to twelve membrane-spanning alpha-helices, whereas RS1 is a hydrophilic extracellular protein which is anchored in the brush-border membrane by a hydrophobic alpha-helix at the C-terminus. SGLT1 alone is able to translocate glucose together with sodium; however, RS1 increases the Vmax of transport expressed by SGLT1. In addition, the biphasic glucose dependence of transport, which is typical for kidney and has been often observed in intestine, was only obtained after coexpression of SGLT1 and RS1.

Expression of a rat renal sodium-dependent dicarboxylate transporter in Xenopus oocytes

Microinjection of mRNA isolated from rat kidney cortex into Xenopus laevis oocytes resulted in the expression of a Na(+)-dependent dicarboxylate transporter, as detected by uptake measurements with [14C]succinate as substrate. The expressed transporter showed an S-shaped Na(+)-dependence with half-maximal activation at 19-21 mM Na+ and a Hill coefficient between 2 and 3. Endogenous succinate uptake was not Na(+)-dependent. Na(+)-stimulated succinate uptake in mRNA-injected oocytes exhibited a maximum at pH 7.5, whereas endogenous Na(+)-independent transporter was fastest at pH 8.5. The expressed dicarboxylate transporter also differed from the endogenous transporter in its sensitivity to citrate as well as dicarboxylates in trans and cis configurations. The expressed transporter resembled the renal basolateral transporter, especially with respect to affinity for succinate (Km 28 microM), activation by Na+, pH-dependence and substrate specificity. After injection of size-fractionated mRNA, succinate uptake was expressed by mRNA of 2-3 kb. Our results suggest expression of the basolateral Na(+)-dependent dicarboxylate transporter after injection of mRNA from rat kidney into Xenopus oocytes.

Renal amino acid transport: cellular and molecular events from clearance studies to frog eggs

This article reviews recent advances in the mechanisms of renal amino acid transport. Renal amino acid transport is necessary to efficiently reclaim approximately 450 mmol amino acids from the glomerular ultrafiltrate each day in man. In general, individual amino acids are transported across the epithelial membrane of the proximal tubule by a sodium (Na+) dependent mechanism. This cotransport process utilizes the energy of the Na+ gradient to enter the cell. The amino acid then exits the basolateral surface and Na+ is pumped out by the Na(+)-K(+)-ATPase located in the basolateral membrane. In addition to the cellular accumulation of amino acids across the luminal membrane, these compounds may be taken up by the cell from the basolateral surface. Most amino acids are transported both individually and in a series of seven group specific processes. Human disorders of amino acid transport have been described for six of the seven transport systems. The process of ontogeny of amino acid accumulation by the proximal tubule is a complex one and will be further discussed in this review. A number of factors including pH, ion dependency, electrogenicity of transport process, as well as a variety of hormonal factors, may contribute to the regulation of amino acid transport. Gene expression of several amino acid transporters has been successfully performed using the oocyte of the frog Xenopus laevis. Using this system, a number of transporters have been cloned. Such a strategy will permit the cloning of virtually all transporter molecules, and thus we can anticipate the elucidation of the structure of the transporters. However, for a comprehensive understanding of cytoskeletal interactions protein phosphorylation and phospholipid domains and their linkage to the primary structure of the transporter need to be studied. The future for research in this area is indeed a bright one.

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Endogenous L-glutamate transport in oocytes of Xenopus laevis

The existence of an endogenous Na(+)-glutamate cotransporter in the oocytes of Xenopus laevis is demonstrated. The transporter does not accept D-glutamate as substrate. The dependence on substrate displays two saturating components with low (K1/2 = 9 mM) and high (K1/2 = 0.35 microM) affinities for L-glutamate. The dependence on external Na+ exhibits a saturating component with a K1/2 value of about 5 mM and a component that has not saturated up to 110 mM Na+. In voltage-clamped oocytes, it is possible to demonstrate that Na(+)-dependent L-glutamate transport is directly coupled to countertransport of Rb+. The analysis of the voltage dependence of the Na+,K(+)-dependent L-glutamate uptake suggests that positive charges are moved inwardly during the transport cycle.