In humans, the kidneys filter approximately 180 g of D-glucose from plasma each day, and this is normally reabsorbed in the proximal tubules. Although the mechanism of reabsorption is well understood, Na(+)-glucose cotransport across the brush-border membrane and facilitated diffusion across the basolateral membrane, questions remain about the identity of the genes responsible for cotransport across the brush border. Genetic studies suggest that two different genes regulate Na(+)-glucose cotransport, and there is evidence from animal studies to suggest that the major bulk of sugar is reabsorbed in the convoluted proximal tubule by a low-affinity, high-capacity transporter and that the remainder is absorbed in the straight proximal tubule by a high-affinity, low-capacity transporter. There are at least three different candidates for these human renal Na(+)-glucose cotransporters. This review will focus on the structure-function relationships of these three transporters, SGLT1, SGLT2, and SGLT3. (+info)
Na+-to-sugar stoichiometry of SGLT3.
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Sodium-glucose cotransporters (SGLTs) mediate active transport of sugar across cell membranes coupled to Na+, by using the electrochemical gradient as a driving force. In the kidney, there is evidence for two kinds of cotransporters, a high-affinity, low-capacity system, and a low-affinity, high-capacity system, with differences in substrate specificity and kinetics. Three renal SGLT clones have been identified: SGLT1 corresponding to the high-affinity system, and SGLT2 and SGLT3 with properties reminiscent of the low-affinity system. We have determined the stoichiometry of pig SGLT3 (pSGLT3) by using a direct method, comparing the substrate-induced inward charge to 22Na or [14C]alpha-methyl-D-glucopyranoside uptake in the same oocyte. pSGLT3 stoichiometry is 2 Na+:1 sugar, the same as that for SGLT1, but different from SGLT2 (1:1). The Na+ Hill coefficient for SGLT3 is approximately 1.5, suggesting low cooperativity between Na+ binding sites. Thus SGLT3 has functional characteristics intermediate between SGLT1 and SGLT2, so, whereas SGLT3 stoichiometry is the same as that for SGLT1 (2:1), sugar affinity and specificity are similar to SGLT2. (+info)
HNF1alpha controls renal glucose reabsorption in mouse and man.
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Recently it has been shown that dominant mutations in the human hepatocyte nuclear factor 1alpha (HNF1alpha) gene, encoding for a homeoprotein that is expressed in liver, kidney, pancreas and intestine, result in maturity onset diabetes of the young type 3 (MODY3). HNF1alpha-null mice are diabetic, but at the same time suffer from a renal Fanconi syndrome characterized by urinary glucose loss. Here we show that MODY3 patients are also characterized by a reduced tubular reabsorption of glucose. The renal murine defect is due to reduced expression of the low affinity/high capacity glucose cotransporter (SGLT2). Our results show that HNF1alpha directly controls SGLT2 gene expression. Together these data indicate that HNF1alpha plays a key role in glucose homeostasis in mammals. (+info)
Upregulation of H(+)-peptide cotransporter PEPT2 in rat remnant kidney.
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The progression of renal damage resulting from reduced nephron mass has been extensively studied in the 5/6 nephrectomized rat. However, reabsorption of small peptides and D-glucose across the renal proximal tubule in this model remains poorly understood. In this study, we examined the alterations of H(+)-peptide cotransporters (PEPT1 and PEPT2) and Na(+)-D-glucose cotransporters (SGLT1 and SGLT2) in chronic renal failure. Two weeks after surgery, H(+)-dependent [(14)C]glycylsarcosine uptake by the renal brush-border membrane vesicles isolated from 5/6 nephrectomized rats was significantly increased compared with that from sham-operated controls. Kinetic analysis revealed that the maximum velocity value for [(14)C]glycylsarcosine uptake by the high-affinity-type of peptide transporter was increased threefold by 5/6 nephrectomy, without significant changes in the apparent Michaelis-Menten constant value. Competitive PCR analyses indicated that the expression of PEPT2 mRNA was markedly increased in the remnant kidney, but PEPT1, SGLT1, and SGLT2 mRNA levels showed no significant changes. These findings indicated that the high-affinity-type H(+)-peptide cotransport activity is upregulated by 5/6 nephrectomy, accompanied by the increased expression of PEPT2. The upregulation of PEPT2 expression would result in an increase in reabsorption of small peptides and peptide-like drugs across the brush-border membranes in chronic renal failure. (+info)
Cloning and characterization of a novel Na+-dependent glucose transporter (NaGLT1) in rat kidney.
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To identify novel transporters in the kidney, we have constructed an mRNA data base composed of 1000 overall clones by random sequencing of a male rat kidney cDNA library. After a BLAST search, approximately 40% of the clones were unknown and/or unannotated and were screened by measuring the uptake of various compounds using Xenopus oocytes. One clone stimulated the uptake of alpha-methyl-d-glucopyranoside and therefore was termed rat Na(+)-dependent glucose transporter 1 (rNaGLT1). The rNaGLT1 cDNA (2173 bp) has an open reading frame encoding a 484-amino acid protein, showing <22% homology to known SGLT and GLUT glucose transporters. alpha-Methyl-d-glucopyranoside uptake by rNaGLT1 cRNA-injected oocytes showed saturability, with an apparent K(m) of 3.7 mm and a coupling ratio of 1:1 with Na(+). rNaGLT1 mRNA was expressed predominantly in the kidney upon Northern blot analysis and reverse transcription-PCR. Reverse transcription-PCR in microdissected nephron segments revealed that rNaGLT1 mRNA was primarily localized in the proximal tubules. A clear signal corresponding to rNaGLT1 protein was recognized in the brush-border (but not basolateral) membrane fraction by immunoblot analysis. The rNaGLT1 mRNA level in the kidney was significantly higher than rat SGLT1 and SGLT2 mRNA levels. These findings suggest that rNaGLT1 is a novel Na(+)-dependent glucose transporter with low substrate affinity that mediates tubular reabsorption of glucose. (+info)
Angiotensin II-dependent increased expression of Na+-glucose cotransporter in hypertension.
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Glucose uptake is increased in hypertension. Thus we investigated Na+-glucose cotransporter (SGLT2) activity and expression in proximal tubules from renovascular hypertensive rats. Sham-operated rats, aortic coarctation rats, and aortic coarctation rats treated with either ramipril (2.5 mg.kg-1.day-1 for 21 days) or losartan (10 mg.kg-1.day-1 for 21 days) were used. Na+-dependent glucose uptake was measured in brush-border membrane vesicles (BBMV). Vmax in BBMV from hypertensive rats was greater compared with those from normotensive rats (3 +/- 0.2 vs. 1.5 +/- 0.1 nmol.mg protein-1.min-1) without a change in Km. Renal immunostaining was greater, and Western blot analysis and RT-PCR showed a higher expression of SGLT2 in hypertensive rats than in normotensive rats (1,029 +/- 71 vs. 5,003 +/- 292, 199 +/- 15 vs. 95 +/- 10, and 1.4 +/- 0.2 vs. 0.3 +/- 0.1 arbitrary units, respectively). In rats treated with either ramipril or losartan, Vmax decreased to 2.1 +/- 0.3 and 1.8 +/- 0.4 nmol.mg protein-1.min-1, respectively, as well as did the intensity of immunostaining and levels of protein and mRNA. We suggest that in renovascular hypertension, angiotensin II induced SGLT2 via the AT1 receptor, which was evidenced at both the functional and expression levels, probably contributing to increased absorption of Na+ and thereby to the development or maintenance of hypertension. (+info)
Molecular analysis of the SGLT2 gene in patients with renal glucosuria.
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The role of SGLT2 (the gene for a renal sodium-dependent glucose transporter) in renal glucosuria was evaluated. Therefore, its genomic sequence and its intron-exon organization were determined, and 23 families with index cases were analyzed for mutations. In 21 families, 21 different SGLT2 mutations were detected. Most of them were private; only a splice mutation was found in 5 families of different ethnic backgrounds, and a 12-bp deletion was found in two German families. Fourteen individuals (including the original patient with 'renal glucosuria type 0') were homozygous or compound heterozygous for an SGLT2 mutation resulting in glucosuria in the range of 14.6 to 202 g/1.73 m(2)/d (81 - 1120 mmol/1.73 m(2)/d). Some, but not all, of their heterozygous family members had an increased glucose excretion of up to 4.4 g/1.73 m(2)/d (24 mmol/1.73 m(2)/d). Likewise, in index cases with glucosuria below 10 g/1.73 m(2)/d (55 mmol/1.73 m(2)/d) an SGLT2 mutation, if present, was always detected in the heterozygous state. We conclude that SGLT2 plays an important role in renal tubular glucose reabsorption. Inheritance of renal glucosuria shows characteristics of a codominant trait with variable penetrance. (+info)
Renal transplantation modulates expression and function of receptors and transporters of rat proximal tubules.
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Kidney transplantation often leads to disturbances of solute and volume maintenance in humans. To investigate underlying mechanisms, expression and function of renal transporters and receptors of the proximal tubule (PT) were analyzed in an acute rejection model of rat kidney transplantation. Semiquantitative RT-PCR and Western blot, histology, immunohistochemistry, and microfluorometry were performed on whole kidneys and isolated PT. With acute rejection, Na+/H+-exchanger type-3 (NHE-3) was markedly downregulated. Na+-HCO(3)(-)-cotransporter (NBC-1) and Na+-glucose transporter type-2 (SGLT2) were upregulated after transplantation. Expressions of Na+/H+-exchanger type-1 (NHE-1), Na+/K+-ATPase (NKA), angiotensin II (AngII) receptor (AT-1), or natriuretic peptide receptor (GC-A) were unaltered. Microfluorometric analyses of intracellular pH, Na+, and Ca2+ demonstrated a decrease in NHE-3 function and AngII-mediated stimulation of NHE-3. AngII-mediated inhibition of NHE-1 and function of all other transporters tested remained unaltered. Function of AT-1 and GC-A were unaffected. Reduced expression of NHE-3 was also confirmed by semiquantitative immunohistochemistry. These findings suggest that expression and function of transmembrane proteins involved in Na+-transport after transplantation and rejection is specifically modulated. The local renin-angiotensin-system is apparently not altered. Downregulation of NHE-3 may be a protective mechanism occurring in the graft. (+info)