A novel missense mutation in SLC5A2 encoding SGLT2 underlies autosomal-recessive renal glucosuria and aminoaciduria

The Sweet Pee model for Sglt2 mutation

Inhibiting renal glucose transport is a potential pharmacologic approach to treat diabetes. The renal tubular sodium-glucose transporter 2 (SGLT2) reabsorbs approximately 90% of the filtered glucose load. An animal model with sglt2 dysfunction could provide information regarding the potential long-term safety and efficacy of SGLT2 inhibitors, which are currently under clinical investigation. Here, we describe Sweet Pee, a mouse model that carries a nonsense mutation in the Slc5a2 gene, which results in the loss of sglt2 protein function. The phenotype of Sweet Pee mutants was remarkably similar to patients with mutations in the Scl5a2 gene. The Sweet Pee mutants had improved glucose tolerance, higher urinary excretion of calcium and magnesium, and growth retardation. Renal physiologic studies demonstrated a prominent distal osmotic diuresis without enhanced natriuresis. Sweet Pee mutants did not exhibit increased KIM-1 or NGAL, markers of acute tubular injury. After induction of diabetes, Sweet Pee mice had better overall glycemic control than wild-type control mice, but had a higher risk for infection and an increased mortality rate (70% in homozygous mutants versus 10% in controls at 20 weeks). In summary, the Sweet Pee model allows study of the long-term benefits and risks associated with inhibition of SGLT2 for the management of diabetes. Our model suggests that inhibiting SGLT2 may improve glucose control but may confer increased risks for infection, malnutrition, volume contraction, and mortality.

Abnormal expression and dysfunction of novel SGLT2 mutations identified in familial renal glucosuria patients

Familial renal glucosuria (FRG) is characterized by persistent glucosuria despite normal serum glucose and in the absence of overt tubular dysfunction. Mutation of sodium/glucose co-transporter 2 (SGLT2) has been identified and was recently reported to be involved in FRG. However, the functional and pathological consequences of such mutations remain unknown. In the current study, we collected four families with FRG. Sequencing of the SGLT2 coding region, intronic segments and cDNA revealed three missense mutations (294C>A: F98L; 1388T>G: L463R; 1435C>G: R479G) and two splice mutations (IVS 1-16 C>A: Del exon3; IVS 11+1 G>C: Del exon11). The probands were either heterozygous or compound heterozygous for SGLT2 mutations, and had glucosuria quantified at 6-27 g/day. Human 293 cells were transfected with the plasmid constructs to study the expression and function of SGLT2 mutants in vitro. Confocal microscopy using green fluorescent protein (GFP) revealed that the mutation results in a loss of punctate membrane pattern typical of the wild-type SGLT2 except in the 294C>A mutant. All mutants had significantly lower transport capacity in comparison to the wild-type control (26.49-71.48%). Renal biopsy in one consenting proband revealed significantly lower SGLT2 expression in the apical side of the proximal convoluted tubule in comparison to both healthy and disease controls (minimal change disease and diabetic nephropathy). The current study provides functional clues regarding the SGLT2 molecule from genotype to phenotype in FRG families.

Discovery of novel N-β-D-xylosylindole derivatives as sodium-dependent glucose cotransporter 2 (SGLT2) inhibitors for the management of hyperglycemia in diabetes

A novel series of N-linked β-D-xylosides were synthesized and evaluated for inhibitory activity against sodium-dependent glucose cotransporter 2 (SGLT2) in a cell-based assay. Of these, the 4-chloro-3-(4-cyclopropylbenzyl)-1-(β-D-xylopyranosyl)-1H-indole 19m was found to be the most potent inhibitor, with an EC(50) value similar to that of the natural SGLT2 inhibitor phlorizin. Further studies in Sprague-Dawley (SD) rats indicated that 19m significantly increased urine glucose excretion in a dose-dependent manner with oral administration. The antihyperglycemic effect of 19m was also observed in streptozotocin (STZ) induced diabetic SD rats. These results described here are a good starting point for further investigations into N-glycoside SGLT2 inhibitors.

Sodium-glucose transport: role in diabetes mellitus and potential clinical implications

PURPOSE OF REVIEW

Current options for glycemic control are less than optimal in terms of efficacy and to reduce complications in the diabetic population. Selective inhibition of SGLT2 in the proximal tubule increases urinary glucose excretion thereby reducing plasma glucose levels, which may present a novel therapeutic approach.

RECENT FINDINGS

SGLT2 inhibitors enhance glucose excretion and improve glycemic control in patients with type 2 diabetes in the absence of clinically relevant hypoglycemia or sustained changes in volume status or glomerular filtration rate. This is associated with lowering of body weight and may reduce systolic blood pressure. The increased glucosuria appears to increase the risk of genital infections but may not increase the risk of urinary tract infections.

SUMMARY

The ability of SGLT2 inhibitors to reduce plasma glucose without inducing increased insulin secretion, clinically relevant hypoglycemia, or weight gain constitutes a major advance. The ability to increase glucose excretion provides a powerful means to treat caloric excess conditions. Important questions remain to be resolved and more clinical research is needed on the long-term effects of SGLT2 inhibition. Potential extrarenal effects need to be explored in order to determine the safety of these compounds. It also remains to be determined whether these drugs lower the toxicity of glucose directly on renal cells, independent of hyperglycemia, which may slow or prevent the progressive nature of diabetic nephropathy.

SGLT2 mediates glucose reabsorption in the early proximal tubule

Mutations in the gene encoding for the Na(+)-glucose co-transporter SGLT2 (SLC5A2) associate with familial renal glucosuria, but the role of SGLT2 in the kidney is incompletely understood. Here, we determined the localization of SGLT2 in the mouse kidney and generated and characterized SGLT2-deficient mice. In wild-type (WT) mice, immunohistochemistry localized SGLT2 to the brush border membrane of the early proximal tubule. Sglt2(-/-) mice had glucosuria, polyuria, and increased food and fluid intake without differences in plasma glucose concentrations, GFR, or urinary excretion of other proximal tubular substrates (including amino acids) compared with WT mice. SGLT2 deficiency did not associate with volume depletion, suggested by similar body weight, BP, and hematocrit; however, plasma renin concentrations were modestly higher and plasma aldosterone levels were lower in Sglt2(-/-) mice. Whole-kidney clearance studies showed that fractional glucose reabsorption was significantly lower in Sglt2(-/-) mice compared with WT mice and varied in Sglt2(-/-) mice between 10 and 60%, inversely with the amount of filtered glucose. Free-flow micropuncture revealed that for early proximal collections, 78 ± 6% of the filtered glucose was reabsorbed in WT mice compared with no reabsorption in Sglt2(-/-) mice. For late proximal collections, fractional glucose reabsorption was 93 ± 1% in WT and 21 ± 6% in Sglt2(-/-) mice, respectively. These results demonstrate that SGLT2 mediates glucose reabsorption in the early proximal tubule and most of the glucose reabsorption by the kidney, overall. This mouse model mimics and explains the glucosuric phenotype of individuals carrying SLC5A2 mutations.

Familial renal glucosuria and SGLT2: from a mendelian trait to a therapeutic target

Four members of two glucose transporter families, SGLT1, SGLT2, GLUT1, and GLUT2, are differentially expressed in the kidney, and three of them have been shown to be necessary for normal glucose resorption from the glomerular filtrate. Mutations in SGLT1 are associated with glucose-galactose malabsorption, SGLT2 with familial renal glucosuria (FRG), and GLUT2 with Fanconi-Bickel syndrome. Patients with FRG have decreased renal tubular resorption of glucose from the urine in the absence of hyperglycemia and any other signs of tubular dysfunction. Glucosuria in these patients can range from <1 to >150 g/1.73 m(2) per d. The majority of patients do not seem to develop significant clinical problems over time, and further description of specific disease sequelae in these individuals is reviewed. SGLT2, a critical transporter in tubular glucose resorption, is located in the S1 segment of the proximal tubule, and, as such, recent attention has been given to SGLT2 inhibitors and their utility in patients with type 2 diabetes, who might benefit from the glucose-lowering effect of such compounds. A natural analogy is made of SGLT2 inhibition to observations with inactivating mutations of SGLT2 in patients with FRG, the hereditary condition that results in benign glucosuria. This review provides an overview of renal glucose transport physiology, FRG and its clinical course, and the potential of SGLT2 inhibition as a therapeutic target in type 2 diabetes.

Glucose control by the kidney: an emerging target in diabetes

The full significance of the kidney's role in glucose homeostasis is now well recognized. For example, it is now known that renal gluconeogenesis contributes substantially to total-body glucose release in the postabsorptive state. The kidney contributes to glucose homeostasis by filtering and reabsorbing glucose. Under normal circumstances, glucose filtered by glomeruli is completely reabsorbed, but glucosuria may occur under conditions of hyperglycemia or reduced reabsorptive capacity. The sodium-glucose cotransporter SGLT2 (encoded by the SLC5A2 gene), which is expressed almost exclusively in proximal tubules, mediates approximately 90% of active renal glucose reabsorption. This transporter can be blocked by SGLT2 inhibitors, a class of compound that may prove effective in managing type 2 diabetes. The glucosuria induced by these compounds has a naturally occurring parallel in familial renal glucosuria (FRG), a condition in which SGLT2 mutations reduce renal reabsorptive capacity. Interestingly, the chronic glucosuria of patients with FRG does not appear to be associated with other pathological changes, and patients with FRG are mostly asymptomatic. This suggests that glucosuria is not intrinsically detrimental. Selective SGLT2 inhibitors are currently in clinical trials.

Sodium glucose co-transporter 2 inhibitors: blocking renal tubular reabsorption of glucose to improve glycaemic control in patients with diabetes

BACKGROUND

The kidney plays a central role in the regulation of plasma glucose levels, although until recently this has not been widely appreciated or considered a target for therapeutic intervention. The sodium glucose co-transporter type 2 (SGLT2) located in the plasma membrane of cells lining the proximal tubule mediates the majority of renal glucose reabsorption from the tubular fluid, which normally prevents the loss of glucose in the urine. Competitive inhibitors of SGLT2 that provoke the renal excretion of glucose have been discovered, thereby providing a unique mechanism to potentially lower the elevated blood glucose levels in patients with diabetes.

OBJECTIVE

To explore the physiology of SGLT2 action and discuss several SGLT2 inhibitors that have entered early clinical development.

METHODS

All publicly available data were identified by searching the internet for 'SGLT2' and 'SGLT2 inhibitor' through 1 November 2007. Published articles, press releases and abstracts presented at national and international meetings were considered.

RESULTS/CONCLUSION

Sodium glucose co-transporter type 2 inhibition is a novel treatment option for diabetes, which has been studied in preclinical models and a few potent and selective SGLT2 inhibitors have been reported and are currently in clinical development. These agents appear to be safe and generally well tolerated, and will potentially be a beneficial addition to the growing battery of oral antihyperglycaemic agents.

Twenty-one additional cases of familial renal glucosuria: absence of genetic heterogeneity, high prevalence of private mutations and further evidence of volume depletion

INTRODUCTION

Familial renal glucosuria (FRG) is a rare renal tubular disorder caused by mutations within the SLC5A2 gene. It is characterized by persistent glucosuria in the absence of hyperglycaemia and any other signs of generalized tubular dysfunction. In small series of patients previously reported, the molecular and phenotypic findings in FRG families, including first hints of extracellular volume depletion and activation of the renin-angiotensin-aldosterone system induced by natriuresis, have been described. We have now extended this analysis to another 21 consecutive cases from 17 pedigrees, including 11 cases with severe glucose excretion.

METHODS

Mutation analysis was performed by direct sequencing of the genomic coding segments of the SLC5A2 gene. In two cases with severe glucosuria, basal plasma renin activity and serum aldosterone concentrations were determined.

RESULTS

Within the 17 pedigrees, we have identified a total of 20 different SLC5A2 mutations. Fifteen have not been previously reported. In all glucosuric individuals tested, at least one SLC5A2 mutation could be identified. Heterozygous individuals were found to have only mild glucose excretion whereas homozygous or compound heterozygous patients had severe glucosuria, ranging from 10 to 86.5 g/1.73 m(2)/24 h. In two patients of the latter group, basal plasma renin activity and serum aldosterone concentration were determined and found to be raised to an average of 4.6-fold and 3.1-fold of the upper limit of the normal range, respectively. Discussion. The identification of at least one mutated allele in every affected individual in this cohort of 17 consecutively investigated families strongly suggests that genetic heterogeneity is not prevalent in FRG. Although 5 of the detected alleles have been described previously, 15 are novel, confirming that most mutations in FRG are private. Our finding of an activation of compensatory mechanisms for salt loss may warrant more detailed studies of long-term hormonal and metabolic imbalances in patients with FRG.

Regulatory mechanisms of Na(+)/glucose cotransporters in renal proximal tubule cells

Glucose is a key fuel and an important metabolic substrate in mammals. Renal proximal tubular cells (PTCs) not only reabsorb filtered glucose but are also believed to play a role in the glucotoxicity associated with renal pathogenesis, such as in diabetes. The proximal tubule environment is where 90% of the filtered glucose is reabsorbed by the low-affinity/high-capacity Na(+)/glucose cotransporter 2 (SGLT2) and facilitated diffusion glucose transporter 2 (GLUT2). Both active and facilitative glucose transporters have distinct distribution profiles along the proximal tubule related to their particular kinetic characteristics. A number of mechanisms contribute to the changes in the cellular functions, which occur in response to exposure to various endogenous factors. Hyperglycemia was reported to regulate the renal SGLT activities through the reactive oxygen species-nuclear factor-kappaB pathways, which suggests that the transcellular glucose uptake within the PTCs contribute to the development of diabetic-like nephropathy. Angiotensin II (ANG II) plays an important role in its development through epidermal growth factor receptor (EGFR) transactivation. Therefore, a combination of high glucose, ANG II, and EGF are involved in diabetic-like nephropathy by regulating the SGLT activity. In addition, endogenously enhanced SGLTs have a cytoprotective function. The renal proximal tubules play a major role in regulating the plasma glucose levels, and there is increasing interest in the renal glucose transporters on account of their potential implications in the treatment of various conditions including diabetes mellitus.

Absence of ClC5 in knockout mice leads to glycosuria, impaired renal glucose handling and low proximal tubule GLUT2 protein expression

Glycosuria is one of the well-documented characteristics in ClC-5 knockout (KO) mice and patients with Dent's disease. However, the underlying pathophysiology of its occurrence is unknown. In this study, we have compared ClC-5 KO mice with age and gender matched wild-type (WT) control mice to investigate if the underlying cause of manifested glycosuria is an impairment of glucose homeostasis and/or an alteration in expression levels of proximal tubule (PT) glucose transporters. We observed that, the blood glucose concentration (n=12, p<0.01) and the fractional excretion of glucose and insulin (n=6, p<0.05) were higher in KO mice. In contrast, the fasting blood glucose levels (n=7) were not significantly different in the two groups. Plasma glucose increased to a greater extent in KO mice (n=7, p<0.05) when challenged by an intraperitoneal injection of glucose. However, no peripheral tissue insulin resistance was observed following an intraperitoneal injection of insulin (n=9) in the KO mice. ELISA analysis demonstrated low plasma insulin concentrations after a 12 hour fasting period and also following glucose injection in KO mice. The total insulin released during a 2 hour period following glucose challenge was significantly lower in KO mice (n=6, p<0.05). By western blot, we observed a significant decrease in GLUT2 protein expression levels in isolated PT ((n=10, p<0.01)) of KO mice. This decrease in protein levels was corroborated by a significant decrease in GLUT2 mRNA levels estimated semi quantitatively by RT-PCR in isolated PT (n=10, p<0.01). No significant changes in mRNA expression levels of SGLT2, SGLT1 and GLUT1, as analyzed by RT-PCR, could be detected in the isolated PT (n=10). Also, we have shown by western blot analysis that expression of megalin is lower in the renal cortex of KO mice when compared to WT mice (n=3, p<0.05). Our results suggest that low plasma insulin concentration together with renal function changes observed in KO mice significantly contribute towards the glucose intolerance and documented glycosuria observed in this animal.

Active sugar transport in health and disease

Secondary active glucose transport occurs by at least four members of the SLC5 gene family. This review considers the structure and function of two premier members, SGLT1 and SGLT2, and their role in intestinal glucose absorption and renal glucose reabsorption. Genetics disorders of SGLTs include Glucose-Galactose Malabsorption, and Familial Renal Glucosuria. SGLT1 plays a central role in Oral Rehydration Therapy used so effectively to treat secretory diarrhoea such as cholera. Increasing attention is being focused on SGLTs as drug targets for the therapy of diabetes.

Diagnostic Approach in Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common Mendelian disorder of the kidney and affects all racial groups worldwide. It is characterized by focal development of renal and extrarenal cysts in an age-dependent manner. Typically, only a few renal cysts are detected in most affected individuals before 30 yr of age. However, by the fifth decade of life, hundreds to thousands of renal cysts will be found in the majority of patients. ADPKD is genetically heterogeneous. Mutations of two genes, PKD1 and PKD2, account for approximately 85 and 15% of cases, respectively. Although the clinical manifestations of these two genotypes overlap completely, patients with PKD1 have much more severe renal disease compared with those with PKD2, as evidenced by their ESRD occurring approximately 15 yr earlier. Renal ultrasonography commonly is used for the assessment of ADPKD, and age-dependent ultrasound diagnostic criteria with high sensitivity and specificity have been established for individuals who are born with 50% risk for PKD1. Although these diagnostic criteria are used widely for genetic counseling and for the evaluation of at-risk individuals as living-related kidney donors to their affected relatives, their application to individuals who are at risk for PKD2 or have undefined genotype needs to be refined further. Molecular genetic testing is available for ADPKD and may be useful for evaluation of at-risk individuals with equivocal imaging results, younger at-risk individuals as a living-related kidney donor, and individuals with atypical or de novo renal cystic disease.

Familial renal glucosuria: SLC5A2 mutation analysis and evidence of salt-wasting

Familial renal glucosuria (FRG) is an inherited renal tubular disorder characterized by persistent isolated glucosuria in the absence of hyperglycemia. Mutations in the sodium/glucose co-transporter SGLT2 coding gene, SLC5A2, were recently found to be responsible for the disorder. Here, we report the molecular and phenotype study of five unrelated FRG families. Five patients were identified and their family members screened for glucosuria. SLC5A2 coding region of index cases was polymerase chain reaction amplified and sequenced. Five different mutations are reported, including four novel alleles. The IVS12+1G>A and p.A102V alleles were identified in homozygosity in index patients of two unrelated families. A proband from another family was compound heterozygous for the p.R132H and p.A219T mutations, and the heterozygous p.Q167fsX186 frameshift allele was the only mutation detected in the affected individual from an additional pedigree. For the remaining family no mutations were detected. The patient homozygous for the p.A102V mutation had glucosuria of 65.6 g/1.73 m(2)/24 h, evidence of renal sodium wasting, mild volume depletion, and raised basal plasma renin and serum aldosterone levels. Our findings confirm previous observations that in FRG, transmitted as a codominant trait with incomplete penetrance, most mutations are private. In the only patient with massive glucosuria in our cohort there was evidence evocative of renin-angiotensin aldosterone system activation by extracellular volume depletion induced by natriuresis. Definite proof of renin-angiotensin aldosterone system activation in FGR should rely on evaluation of additional patients with massive glucosuria.