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

The impact of solute carrier proteins on disrupting substance regulation in metabolic disorders: insights and clinical applications

Carbohydrates, lipids, bile acids, various inorganic salt ions and organic acids are the main nutrients or indispensable components of the human body. Dysregulation in the processes of absorption, transport, metabolism, and excretion of these metabolites can lead to the onset of severe metabolic disorders, such as type 2 diabetes, non-alcoholic fatty liver disease, gout and hyperbilirubinemia. As the second largest membrane receptor supergroup, several major families in the solute carrier (SLC) supergroup have been found to play key roles in the transport of substances such as carbohydrates, lipids, urate, bile acids, monocarboxylates and zinc ions. Based on common metabolic dysregulation and related metabolic substances, we explored the relationship between several major families of SLC supergroup and metabolic diseases, providing examples of drugs targeting SLC proteins that have been approved or are currently in clinical/preclinical research as well as SLC-related diagnostic techniques that are in clinical use or under investigation. By highlighting these connections, we aim to provide insights that may contribute to the development of improved treatment strategies and targeted therapies for metabolic disorders.

Maternal immune cell gene expression associates with maternal gut microbiome, milk composition and infant gut microbiome

BACKGROUND

Pre-pregnancy overweight and obesity promote deleterious health impacts on both mothers during pregnancy and the offspring. Significant changes in the maternal peripheral blood mononuclear cells (PBMCs) gene expression due to obesity are well-known. However, the impact of pre-pregnancy overweight on immune cell gene expression during pregnancy and its association with maternal and infant outcomes is not well explored.

METHODS

Blood samples were collected from healthy normal weight (NW, pre-pregnancy BMI 18.5-24.9) or overweight (OW, pre-pregnancy BMI 25-29.9) 2nd parity pregnant women at 12, 24 and 36 weeks of pregnancy. PBMCs were isolated from the blood and subjected to mRNA sequencing. Maternal and infant microbiota were analyzed by 16S rRNA gene sequencing. Integrative multi-omics data analysis was performed to evaluate the association of gene expression with maternal diet, gut microbiota, milk composition, and infant gut microbiota.

RESULTS

Gene expression analysis revealed that 453 genes were differentially expressed in the OW women compared to NW women at 12 weeks of pregnancy, out of which 354 were upregulated and 99 were downregulated. Several up-regulated genes in the OW group were enriched in inflammatory, chemokine-mediated signaling and regulation of interleukin-8 production-related pathways. At 36 weeks of pregnancy healthy eating index score was positively associated with several genes that include, DTD1, ELOC, GALNT8, ITGA6-AS1, KRT17P2, NPW, POT1-AS1 and RPL26. In addition, at 36 weeks of pregnancy, genes involved in adipocyte functions, such as NG2 and SMTNL1, were negatively correlated to human milk 2'FL and total fucosylated oligosaccharides content collected at 1 month postnatally. Furthermore, infant Akkermansia was positively associated with maternal PBMC anti-inflammatory genes that include CPS1 and RAB7B, at 12 and 36 weeks of pregnancy.

CONCLUSIONS

These findings suggest that prepregnancy overweight impacts the immune cell gene expression profile, particularly at 12 weeks of pregnancy. Furthermore, deciphering the complex association of PBMC's gene expression levels with maternal gut microbiome and milk composition and infant gut microbiome may aid in developing strategies to mitigate obesity-mediated effects.

Obesity and overweight are linked to increased sodium-glucose cotransporter 1 and glucose transporter 5 levels in duodenum

OBJECTIVE

Prior evidence indicates that individuals with obesity have an accelerated intestinal glucose absorption. This cross-sectional study evaluated whether those with overweight or obesity display higher duodenal protein levels of the glucose carriers sodium-glucose cotransporter 1 (SGLT-1), glucose transporter 2 (GLUT-2), and glucose transporter 5 (GLUT-5).

METHODS

SGLT-1, GLUT-2, and GLUT-5 protein levels were assessed on duodenal mucosa biopsies of 52 individuals without diabetes categorized on the basis of their BMI as lean, with overweight, or with obesity.

RESULTS

Individuals with overweight and obesity exhibited progressively increased duodenal protein levels of SGLT-1 and GLUT-5 as compared with the lean group. Conversely, no differences in duodenal GLUT-2 abundance were found among the three groups. Univariate analysis showed that SGLT-1 and GLUT-5 protein levels were positively correlated with BMI, waist circumference, 1-hour post-load glucose, fasting and post-load insulin, and insulin secretion and resistance levels. Furthermore, a positive relationship was detected between intestinal GLUT-5 levels and serum uric acid concentrations, a product of fructose metabolism known to be involved in the pathogenesis of obesity and its complications.

CONCLUSIONS

Individuals with overweight and obesity display enhanced duodenal SGLT-1 and GLUT-5 abundance, which correlates with increased postprandial glucose concentrations, insulin resistance, and hyperinsulinemia.

Intestinal Farnesoid X Receptor Modulates Duodenal Surface Area but Does Not Control Glucose Absorption in Mice

Bile acids facilitate the intestinal absorption of dietary lipids and act as signalling molecules in the maintenance of metabolic homeostasis. Farnesoid X receptor (FXR) is a bile acid-responsive nuclear receptor involved in bile acid metabolism, as well as lipid and glucose homeostasis. Several studies have suggested a role of FXR in the control of genes regulating intestinal glucose handling. We applied a novel dual-label glucose kinetic approach in intestine-specific FXR^-/- mice (iFXR-KO) to directly assess the role of intestinal FXR in glucose absorption. Although iFXR-KO mice showed decreased duodenal expression of hexokinase 1 (Hk1) under obesogenic conditions, the assessment of glucose fluxes in these mice did not show a role for intestinal FXR in glucose absorption. FXR activation with the specific agonist GS3972 induced Hk1, yet the glucose absorption rate remained unaffected. FXR activation increased the duodenal villus length in mice treated with GS3972, while stem cell proliferation remained unaffected. Accordingly, iFXR-KO mice on either chow, short or long-term HFD feeding displayed a shorter villus length in the duodenum compared to wild-type mice. These findings indicate that delayed glucose absorption reported in whole-body FXR^-/- mice is not due to the absence of intestinal FXR. Yet, intestinal FXR does have a role in the small intestinal surface area.

The β2-adrenergic receptor in the apical membrane of intestinal enterocytes senses sugars to stimulate glucose uptake from the gut

The presence of sugar in the gut causes induction of SGLT1, the sodium/glucose cotransporter in intestinal epithelial cells (enterocytes), and this is accompanied by stimulation of sugar absorption. Sugar sensing was suggested to involve a G-protein coupled receptor and cAMP - protein kinase A signalling, but the sugar receptor has remained unknown. We show strong expression and co-localization with SGLT1 of the β2-adrenergic receptor (β ₂-AR) at the enterocyte apical membrane and reveal its role in stimulating glucose uptake from the gut by the sodium/glucose-linked transporter, SGLT1. Upon heterologous expression in different reporter systems, the β ₂-AR responds to multiple sugars in the mM range, consistent with estimated gut sugar levels after a meal. Most adrenergic receptor antagonists inhibit sugar signaling, while some differentially inhibit epinephrine and sugar responses. However, sugars did not inhibit binding of I¹²⁵-cyanopindolol, a β ₂-AR antagonist, to the ligand-binding site in cell-free membrane preparations. This suggests different but interdependent binding sites. Glucose uptake into everted sacs from rat intestine was stimulated by epinephrine and sugars in a β ₂-AR-dependent manner. STD-NMR confirmed direct physical binding of glucose to the β ₂-AR. Oral administration of glucose with a non-bioavailable β ₂-AR antagonist lowered the subsequent increase in blood glucose levels, confirming a role for enterocyte apical β ₂-ARs in stimulating gut glucose uptake, and suggesting enterocyte β ₂-AR as novel drug target in diabetic and obese patients. Future work will have to reveal how glucose sensing by enterocytes and neuroendocrine cells is connected, and whether β ₂-ARs mediate glucose sensing also in other tissues.

Altered intestinal epithelial nutrient transport: an underappreciated factor in obesity modulated by diet and microbiota

Dietary nutrients absorbed in the proximal small intestine and assimilated in different tissues have a profound effect on overall energy homeostasis, determined by a balance between body's energy intake and expenditure. In obesity, altered intestinal absorption and consequently tissue assimilation of nutrients may disturb the energy balance leading to metabolic abnormalities at the cellular level. The absorption of nutrients such as sugars, amino acids and fatty acids released from food digestion require high-capacity transporter proteins expressed in the intestinal epithelial absorptive cells. Furthermore, nutrient sensing by specific transporters/receptors expressed in the epithelial enteroendocrine cells triggers release of gut hormones involved in regulating energy homeostasis via their effects on appetite and food intake. Therefore, the intestinal epithelial cells play a pivotal role in the pathophysiology of obesity and associated complications. Over the past decade, gut microbiota has emerged as a key factor contributing to obesity via its effects on digestion and absorption of nutrients in the small intestine, and energy harvest from dietary fiber, undigested component of food, in the large intestine. Various mechanisms of microbiota effects on obesity have been implicated. However, the impact of obesity-associated microbiota on the intestinal nutrient transporters needs extensive investigation. This review marshals the limited studies addressing the altered structure and function of the gut epithelium in obesity with special emphasis on nutrient transporters and role of diet and microbiota. The review also discusses the thoughts and controversies and research gaps in this field.

Cardiometabolic Syndrome: An Update on Available Mouse Models

Cardiometabolic syndrome (CMS), a disease entity characterized by abdominal obesity, insulin resistance (IR), hypertension, and hyperlipidemia, is a global epidemic with approximately 25% prevalence in adults globally. CMS is associated with increased risk for cardiovascular disease (CVD) and development of diabetes. Due to its multifactorial etiology, the development of several animal models to simulate CMS has contributed significantly to the elucidation of the disease pathophysiology and the design of therapies. In this review we aimed to present the most common mouse models used in the research of CMS. We found that CMS can be induced either by genetic manipulation, leading to dyslipidemia, lipodystrophy, obesity and IR, or obesity and hypertension, or by administration of specific diets and drugs. In the last decade, the ob/ob and db/db mice were the most common obesity and IR models, whereas Ldlr-/- and Apoe-/- were widely used to induce hyperlipidemia. These mice have been used either as a single transgenic or combined with a different background with or without diet treatment. High-fat diet with modifications is the preferred protocol, generally leading to increased body weight, hyperlipidemia, and IR. A plethora of genetically engineered mouse models, diets, drugs, or synthetic compounds that are available have advanced the understanding of CMS. However, each researcher should carefully select the most appropriate model and validate its consistency. It is important to consider the differences between strains of the same animal species, different animals, and most importantly differences to human when translating results.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Glucose transporters in brain in health and disease

Energy demand of neurons in brain that is covered by glucose supply from the blood is ensured by glucose transporters in capillaries and brain cells. In brain, the facilitative diffusion glucose transporters GLUT1-6 and GLUT8, and the Na⁺-d-glucose cotransporters SGLT1 are expressed. The glucose transporters mediate uptake of d-glucose across the blood-brain barrier and delivery of d-glucose to astrocytes and neurons. They are critically involved in regulatory adaptations to varying energy demands in response to differing neuronal activities and glucose supply. In this review, a comprehensive overview about verified and proposed roles of cerebral glucose transporters during health and diseases is presented. Our current knowledge is mainly based on experiments performed in rodents. First, the functional properties of human glucose transporters expressed in brain and their cerebral locations are described. Thereafter, proposed physiological functions of GLUT1, GLUT2, GLUT3, GLUT4, and SGLT1 for energy supply to neurons, glucose sensing, central regulation of glucohomeostasis, and feeding behavior are compiled, and their roles in learning and memory formation are discussed. In addition, diseases are described in which functional changes of cerebral glucose transporters are relevant. These are GLUT1 deficiency syndrome (GLUT1-SD), diabetes mellitus, Alzheimer’s disease (AD), stroke, and traumatic brain injury (TBI). GLUT1-SD is caused by defect mutations in GLUT1. Diabetes and AD are associated with changed expression of glucose transporters in brain, and transporter-related energy deficiency of neurons may contribute to pathogenesis of AD. Stroke and TBI are associated with changes of glucose transporter expression that influence clinical outcome.

A Modified Tripeptide Motif of RS1 (RSC1A1) Down-Regulates Exocytotic Pathways of Human Na+-d-glucose Cotransporters SGLT1, SGLT2, and Glucose Sensor SGLT3 in the Presence of Glucose

A domain of protein RS1 (RSC1A1) called RS1-Reg down-regulates the plasma membrane abundance of Na+-d-glucose cotransporter SGLT1 by blocking the exocytotic pathway at the trans-Golgi. This effect is blunted by intracellular glucose but prevails when serine in a QSP (Gln-Ser-Pro) motif is replaced by glutamate [RS1-Reg(S20E)]. RS1-Reg binds to ornithine decarboxylase (ODC) and inhibits ODC in a glucose-dependent manner. Because the ODC inhibitor difluoromethylornithine (DFMO) acts like RS1-Reg(S20E), and DFMO and RS1-Reg(S20E) are not cumulative, we raised the hypothesis that RS1-Reg(S20E) down-regulates the exocytotic pathway of SGLT1 at the trans-Golgi by inhibiting ODC. We investigated whether QEP down-regulates human SGLT1 (hSGLT1) like hRS1-Reg(S20E) and whether human Na+-d-glucose cotransporter hSGLT2 and the human glucose sensor hSGLT3 are also addressed. We expressed hSGLT1, hSGLT1 linked to yellow fluorescent protein (hSGLT1-YFP), hSGLT2-YFP and hSGLT3-YFP in oocytes of Xenopus laevis, injected hRS1-Reg(S20E), QEP, DFMO, and/or α-methyl-d-glucopyranoside (AMG), and measured AMG uptake, glucose-induced currents, and plasma membrane-associated fluorescence after 1 hour. We also performed in vitro AMG uptake measurements into small intestinal mucosa of mice and human. The data indicate that QEP down-regulates the exocytotic pathway of SGLT1 similar to hRS1-Reg(S20E). Our results suggests that both peptides also down-regulate hSGLT2 and hSGLT3 via the same pathway. Thirty minutes after application of 5 mM QEP in the presence of 5 mM d-glucose, hSGLT1-mediated AMG uptake into small intestinal mucosa was decreased by 40% to 50%. Thus oral application of QEP in a formulation that optimizes uptake into enterocytes but prevents entry into the blood is proposed as novel antidiabetic therapy.

Increased Rates of Meal Absorption Do Not Explain Elevated 1-Hour Glucose in Subjects With Normal Glucose Tolerance

Context

In subjects with normal fasting glucose (NFG) and normal glucose tolerance (NGT), glucose concentrations >155 mg/dL 1 hour after 75 g of oral glucose predict increased risk of progression to diabetes. Recently, it has been suggested that the mechanism underlying this abnormality is increased gut absorption of glucose.

Objective

We sought to determine the rate of systemic appearance of meal-derived glucose in subjects classified by their 1-hour glucose after a 75-g oral glucose challenge.

Design

This was a cross-sectional study. Participating subjects underwent a 75-g oral glucose challenge and a labeled mixed meal test.

Setting

An inpatient clinical research unit at an academic medical center.

Participants

Thirty-six subjects with NFG/NGT participated in this study.

Interventions

Subjects underwent an oral glucose tolerance test. Subsequently, they underwent a labeled mixed meal to measure fasting and postprandial glucose metabolism.

Main Outcome Measures

We examined β-cell function and the rate of meal appearance (Meal R_a) in NFG/NGT subjects. Subsequently, we examined the relationship of peak postchallenge glucose with Meal R_a and indices of β-cell function.

Results

Peak glucose concentrations correlated inversely with β-cell function. No relationship of Meal R_a with peak postchallenge glucose concentrations was observed.

Conclusion

In subjects with NFG/NGT, elevated 1-hour peak postchallenge glucose concentrations reflect impaired β-cell function rather than increased systemic meal appearance.

Genetic Variants in SGLT1, Glucose Tolerance, and Cardiometabolic Risk

BACKGROUND

Loss-of-function mutations in the SGLT1 (sodium/glucose co-transporter-1) gene result in a rare glucose/galactose malabsorption disorder and neonatal death if untreated. In the general population, variants related to intestinal glucose absorption remain uncharacterized.

OBJECTIVES

The goal of this study was to identify functional SGLT1 gene variants and characterize their clinical consequences.

METHODS

Whole exome sequencing was performed in the ARIC (Atherosclerosis Risk in Communities) study participants enrolled from 4 U.S. communities. The association of functional, nonsynonymous substitutions in SGLT1 with 2-h oral glucose tolerance test results was determined. Variants related to impaired glucose tolerance were studied, and Mendelian randomization analysis of cardiometabolic outcomes was performed.

RESULTS

Among 5,687 European-American subjects (mean age 54 ± 6 years; 47% male), those who carried a haplotype of 3 missense mutations (frequency of 6.7%)-Asn51Ser, Ala411Thr, and His615Gln-had lower 2-h glucose and odds of impaired glucose tolerance than noncarriers (β-coefficient: -8.0; 95% confidence interval [CI]: -12.7 to -3.3; OR: 0.71; 95% CI: 0.59 to 0.86, respectively). The association of the haplotype with oral glucose tolerance test results was consistent in a replication sample of 2,791 African-American subjects (β = -16.3; 95% CI: -36.6 to 4.1; OR: 0.39; 95% CI: 0.17 to 0.91) and an external European-Finnish population sample of 6,784 subjects (β = -3.2; 95% CI: -6.4 to -0.02; OR: 0.81; 95% CI: 0.68 to 0.98). Using a Mendelian randomization approach in the index cohort, the estimated 25-year effect of a reduction of 20 mg/dl in 2-h glucose via SGLT1 inhibition would be reduced prevalent obesity (OR: 0.43; 95% CI: 0.23 to 0.63), incident diabetes (hazard ratio [HR]: 0.58; 95% CI: 0.35 to 0.81), heart failure (HR: 0.53; 95% CI: 0.24 to 0.83), and death (HR: 0.66; 95% CI: 0.42 to 0.90).

CONCLUSIONS

Functionally damaging missense variants in SGLT1 protect from diet-induced hyperglycemia in multiple populations. Reduced intestinal glucose uptake may protect from long-term cardiometabolic outcomes, providing support for therapies that target SGLT1 function to prevent and treat metabolic conditions.

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.

Deregulation of transcription factors controlling intestinal epithelial cell differentiation; a predisposing factor for reduced enteroendocrine cell number in morbidly obese individuals

Morbidly obese patients exhibit impaired secretion of gut hormones that may contribute to the development of obesity. After bariatric surgery there is a dramatic increase in gut hormone release. In this study, gastric and duodenal tissues were endoscopically collected from lean, and morbidly obese subjects before and 3 months after laparoscopic sleeve gastrectomy (LSG). Tissue morphology, abundance of chromogranin A, gut hormones, α-defensin, mucin 2, Na⁺/glucose co-transporter 1 (SGLT1) and transcription factors, Hes1, HATH1, NeuroD1, and Ngn3, were determined. In obese patients, the total number of enteroendocrine cells (EEC) and EECs containing gut hormones were significantly reduced in the stomach and duodenum, compared to lean, and returned to normality post-LSG. No changes in villus height/crypt depth were observed. A significant increase in mucin 2 and SGLT1 expression was detected in the obese duodenum. Expression levels of transcription factors required for differentiation of absorptive and secretory cell lineages were altered. We propose that in obesity, there is deregulation in differentiation of intestinal epithelial cell lineages that may influence the levels of released gut hormones. Post-LSG cellular differentiation profile is restored. An understanding of molecular mechanisms controlling epithelial cell differentiation in the obese intestine assists in the development of non-invasive therapeutic strategies.

Maternal Methyl Donor Supplementation during Gestation Counteracts the Bisphenol A-Induced Impairment of Intestinal Morphology, Disaccharidase Activity, and Nutrient Transporters Gene Expression in Newborn and Weaning Pigs

This study was conducted to explore whether exposure to bisphenol A (BPA) during pregnancy could change intestinal digestion and absorption function in offspring using pigs as a model, and whether methyl donor (MET) could counteract the BPA-induced impacts. Fifty Landrace × Yorkshire sows were divided into four dietary groups throughout gestation: control diet (CON); control diet supplemented with BPA (50 mg/kg); control diet supplemented with MET (3 g/kg betaine, 400 mg/kg choline, 150 μg/kg vitamin B12, and 15 mg/kg folic acid); and control diet with BPA and MET supplementation (BPA + MET). Intestine samples were collected from pigs’ offspring at birth and weaning. Maternal BPA exposure during pregnancy significantly reduced the ratio of jejunum villus height to crypt depth, decreased the jejunum sucrase activity, down-regulated the mRNA expression of jejunum peptide transporter 1 (Pept1) and DNA methyl transferase 3a (DNMT3a), and decreased the DNA methylation level of jejunum Pept1 in offspring (p < 0.05). Maternal MET supplementation significantly raised the ratio of villus height to crypt depth in jejunum and ileum, improved the jejunum lactase activity, up-regulated the mRNA expression of jejunum Pept1, lactase (LCT), DNMT1, DNMT3a, and methylenetetrahydrofolate reductase (MTHFR), and increased the DNA methylation level of jejunum Pept1 in offspring (p < 0.05). However, the ratio of jejunum villus height to crypt depth was higher in BPA + MET treatment compared with CON and BPA treatment (p < 0.05). Meanwhile, there was no difference in the jejunum sucrase activity, the mRNA expression of jejunum Pept1 and DNMT3a, and the DNA methylation level of jejunum Pept1 between CON and BPA + MET treatment. These results indicated that maternal exposure to BPA during gestation might suppress offspring’s intestinal digestion and absorption function, whereas supplementation of MET could counteract these damages, which might be associated with DNA methylation.

Saving the sweetness: renal glucose handling in health and disease

Glucose homeostasis is highly controlled, and the function of the kidney plays an integral role in this process. The exquisite control of blood glucose relies, in part, on renal glucose filtration, renal glucose reabsorption, and renal gluconeogenesis. Particularly critical to maintaining glucose homeostasis is the renal reabsorption of glucose; with ~162 g of glucose filtered by the kidney per day, it is imperative that the kidney have the ability to efficiently reabsorb nearly 100% of this glucose back in the bloodstream. In this review, we focus on this central process, highlighting the renal transporters and regulators involved in both the physiology and pathophysiology of glucose reabsorption.

Protein RS1 (RSC1A1) Downregulates the Exocytotic Pathway of Glucose Transporter SGLT1 at Low Intracellular Glucose via Inhibition of Ornithine Decarboxylase

Na+-d-glucose cotransporter 1 (SGLT1) is rate-limiting for glucose absorption in the small intestine. Shortly after intake of glucose-rich food, SGLT1 abundance in the luminal membrane of the small intestine is increased. This upregulation occurs via glucose-induced acceleration of the release of SGLT1-containing vesicles from the trans-Golgi network (TGN), which is regulated by a domain of protein RS1 (RSC1A1) named RS1-Reg. Dependent on phosphorylation, RS1-Reg blocks release of vesicles containing SGLT1 or concentrative nucleoside transporter 1. The hypothesis has been raised that RS1-Reg binds to different receptor proteins at the TGN, which trigger release of vesicles with different transporters. To identify the presumed receptor proteins, two-hybrid screening was performed. Interaction with ornithine decarboxylase 1 (ODC1), the rate-limiting enzyme of polyamine synthesis, was observed and verified by immunoprecipitation. Binding of RS1-Reg mutants to ODC1 was characterized using surface plasmon resonance. Inhibition of ODC1 activity by RS1-Reg mutants and the ODC1 inhibitor difluoromethylornithine (DFMO) was measured in the absence and presence of glucose. In addition, short-term effects of DFMO, RS1-Reg mutants, the ODC1 product putrescine, and/or glucose on SGLT1 expressed in oocytes of Xenopus laevis were investigated. High-affinity binding of RS1-Reg to ODC1 was demonstrated, and evidence for a glucose binding site in ODC1 was provided. Binding of RS1-Reg to ODC1 inhibits the enzymatic activity at low intracellular glucose, which is blunted at high intracellular glucose. The data suggest that generation of putrescine by ODC1 at the TGN stimulates release of SGLT1-containing vesicles. This indicates a biomedically important role of ODC1 in regulation of glucose homeostasis.

Human DNA-Damage-Inducible 2 Protein Is Structurally and Functionally Distinct from Its Yeast Ortholog

Although Ddi1-like proteins are conserved among eukaryotes, their biological functions remain poorly characterized. Yeast Ddi1 has been implicated in cell cycle regulation, DNA-damage response, and exocytosis. By virtue of its ubiquitin-like (UBL) and ubiquitin-associated (UBA) domains, it has been proposed to serve as a proteasomal shuttle factor. All Ddi1-like family members also contain a highly conserved retroviral protease-like (RVP) domain with unknown substrate specificity. While the structure and biological function of yeast Ddi1 have been investigated, no such analysis is available for the human homologs. To address this, we solved the 3D structures of the human Ddi2 UBL and RVP domains and identified a new helical domain that extends on either side of the RVP dimer. While Ddi1-like proteins from all vertebrates lack a UBA domain, we identify a novel ubiquitin-interacting motif (UIM) located at the C-terminus of the protein. The UIM showed a weak yet specific affinity towards ubiquitin, as did the Ddi2 UBL domain. However, the full-length Ddi2 protein is unable to bind to di-ubiquitin chains. While proteomic analysis revealed no activity, implying that the protease requires other factors for activation, our structural characterization of all domains of human Ddi2 sets the stage for further characterization.

Intestinal SGLT1 in metabolic health and disease

The Na(+)-glucose cotransporter 1 (SGLT1/SLC5A1) is predominantly expressed in the small intestine. It transports glucose and galactose across the apical membrane in a process driven by a Na(+) gradient created by Na(+)-K(+)-ATPase. SGLT2 is the major form found in the kidney, and SGLT2-selective inhibitors are a new class of treatment for type 2 diabetes mellitus (T2DM). Recent data from patients treated with dual SGLT1/2 inhibitors or SGLT2-selective drugs such as canagliflozin (SGLT1 IC50 = 663 nM) warrant evaluation of SGLT1 inhibition for T2DM. SGLT1 activity is highly dynamic, with modulation by multiple mechanisms to ensure maximal uptake of carbohydrates (CHOs). Intestinal SGLT1 inhibition lowers and delays the glucose excursion following CHO ingestion and augments glucagon-like peptide-1 (GLP-1) and peptide YY (PYY) secretion. The latter is likely due to increased glucose exposure of the colonic microbiota and formation of metabolites such as L cell secretagogues. GLP-1 and PYY secretion suppresses food intake, enhances the ileal brake, and has an incretin effect. An increase in colonic microbial production of propionate could contribute to intestinal gluconeogenesis and mediate positive metabolic effects. On the other hand, a threshold of SGLT1 inhibition that could lead to gastrointestinal intolerability is unclear. Altered Na(+) homeostasis and increased colonic CHO may result in diarrhea and adverse gastrointestinal effects. This review considers the potential mechanisms contributing to positive metabolic and negative intestinal effects. Compounds that inhibit SGLT1 must balance the modulation of these mechanisms to achieve therapeutic efficacy for metabolic diseases.

Sodium-glucose cotransport

PURPOSE OF REVIEW

Sodium-glucose cotransporters (SGLTs) are important mediators of glucose uptake across apical cell membranes. SGLT1 mediates almost all sodium-dependent glucose uptake in the small intestine, while in the kidney SGLT2, and to a lesser extent SGLT1, account for more than 90% and nearly 3%, respectively, of glucose reabsorption from the glomerular ultrafiltrate. Although the recent availability of SGLT2 inhibitors for the treatment of diabetes mellitus has increased the number of clinical studies, this review has a focus on mechanisms contributing to the cellular regulation of SGLTs.

RECENT FINDINGS

Studies have focused on the regulation of SGLT expression under different physiological/pathophysiological conditions, for example diet, age or diabetes mellitus. Several studies provide evidence of SGLT regulation via cyclic adenosine monophosphate/protein kinase A, protein kinase C, glucagon-like peptide 2, insulin, leptin, signal transducer and activator of transcription-3 (STAT3), phosphoinositide-3 kinase (PI3K)/Akt, mitogen-activated protein kinases (MAPKs), nuclear factor-kappaB (NF-kappaB), with-no-K[Lys] kinases/STE20/SPS1-related proline/alanine-rich kinase (Wnk/SPAK) and regulatory solute carrier protein 1 (RS1) pathways.

SUMMARY

SGLT inhibitors are important drugs for glycemic control in diabetes mellitus. Although the contribution of SGLT1 for absorption of glucose from the intestine as well as SGLT2/SGLT1 for renal glucose reabsorption has been comprehensively defined, this review provides an up-to-date outline for the mechanistic regulation of SGLT1/SGLT2.

A Maternal High-Energy Diet Promotes Intestinal Development and Intrauterine Growth of Offspring

It has been suggested that maternal nutrition during gestation is involved in an offspring’s intestinal development. The aim of this study was therefore to evaluate the effects of maternal energy on the growth and small intestine development of offspring. After mating, twenty gilts (Large White (LW) breeding, body weight (BW) at 135.54 ± 0.66 kg) were randomly allocated to two dietary treatments: a control diet (CON) group and a high-energy diet (HED) group, respectively. The nutrient levels of the CON were referred to meet the nutrient recommendations by the National Research Council (NRC, 2012), while the HED was designed by adding an amount of soybean oil that was 4.6% of the total diet weight to the CON. The dietary treatments were introduced from day 1 of gestation to farrowing. At day 90 of gestation, day 1 post-birth, and day 28 post-birth, the weights of fetuses and piglets, intestinal morphology, enzyme activities, and gene and protein expressions of intestinal growth factors were determined. The results indicated that the maternal HED markedly increased the BW, small intestinal weight, and villus height of fetuses and piglets. Moreover, the activities of lactase in fetal intestine, sucrase in piglet intestine were markedly increased by the maternal HED. In addition, the maternal HED tended to increase the protein expression of insulin-like growth factor 1 receptor (IGF-1R) in fetal intestine, associated with significantly increased the gene expression of IGF-1R. In conclusion, increasing energy intake could promote fetal growth and birth weight, with greater intestinal morphology and enzyme activities.

Sodium glucose cotransporter SGLT1 as a therapeutic target in diabetes mellitus

INTRODUCTION

Glycemic control is important in diabetes mellitus to minimize the progression of the disease and the risk of potentially devastating complications. Inhibition of the sodium-glucose cotransporter SGLT2 induces glucosuria and has been established as a new anti-hyperglycemic strategy. SGLT1 plays a distinct and complementing role to SGLT2 in glucose homeostasis and, therefore, SGLT1 inhibition may also have therapeutic potential.

AREAS COVERED

This review focuses on the physiology of SGLT1 in the small intestine and kidney and its pathophysiological role in diabetes. The therapeutic potential of SGLT1 inhibition, alone as well as in combination with SGLT2 inhibition, for anti-hyperglycemic therapy are discussed. Additionally, this review considers the effects on other SGLT1-expressing organs like the heart.

EXPERT OPINION

SGLT1 inhibition improves glucose homeostasis by reducing dietary glucose absorption in the intestine and by increasing the release of gastrointestinal incretins like glucagon-like peptide-1. SGLT1 inhibition has a small glucosuric effect in the normal kidney and this effect is increased in diabetes and during inhibition of SGLT2, which deliver more glucose to SGLT1 in late proximal tubule. In short-term studies, inhibition of SGLT1 and combined SGLT1/SGLT2 inhibition appeared to be safe. More data is needed on long-term safety and cardiovascular consequences of SGLT1 inhibition.

Phosphorylation of RS1 (RSC1A1) Steers Inhibition of Different Exocytotic Pathways for Glucose Transporter SGLT1 and Nucleoside Transporter CNT1, and an RS1-Derived Peptide Inhibits Glucose Absorption

Cellular uptake adapts rapidly to physiologic demands by changing transporter abundance in the plasma membrane. The human gene RSC1A1 codes for a 67-kDa protein named RS1 that has been shown to induce downregulation of the sodium-D-glucose cotransporter 1 (SGLT1) and of the concentrative nucleoside transporter 1 (CNT1) in the plasma membrane by blocking exocytosis at the Golgi. Injecting RS1 fragments into Xenopus laevis oocytes expressing SGLT1 or CNT1 and measuring the expressed uptake of α-methylglucoside or uridine 1 hour later, we identified a RS1 domain (RS1-Reg) containing multiple predicted phosphorylation sites that is responsible for this post-translational downregulation of SGLT1 and CNT1. Dependent on phosphorylation, RS1-Reg blocks the release of SGLT1-containing vesicles from the Golgi in a glucose-dependent manner or glucose-independent release of CNT1-containing vesicles. We showed that upregulation of SGLT1 in the small intestine after glucose ingestion is promoted by glucose-dependent disinhibition of the RS1-Reg-blocked exocytotic pathway of SGLT1 between meals. Mimicking phosphorylation of RS1-Reg, we obtained a RS1-Reg variant that downregulates SGLT1 in the brush-border membrane at high luminal glucose concentration. Because RS1 mediates short-term regulation of various transporters, we propose that the RS1-Reg-navigated transporter release from Golgi represents a basic regulatory mechanism of general importance, which implies the existence of receptor proteins that recognize different phosphorylated forms of RS1-Reg and of complex transporter-specific sorting in the trans-Golgi. RS1-Reg-derived peptides that downregulate SGLT1 at high intracellular glucose concentrations may be used for downregulation of glucose absorption in small intestine, which has been proposed as strategy for treatment of type 2 diabetes.

Leptin resistance and diet-induced obesity: central and peripheral actions of leptin

Obesity is a chronic disease that represents one of the most serious global health burdens associated to an excess of body fat resulting from an imbalance between energy intake and expenditure, which is regulated by environmental and genetic interactions. The adipose-derived hormone leptin acts via a specific receptor in the brain to regulate energy balance and body weight, although this protein can also elicit a myriad of actions in peripheral tissues. Obese individuals, rather than be leptin deficient, have in most cases, high levels of circulating leptin. The failure of these high levels to control body weight suggests the presence of a resistance process to the hormone that could be partly responsible of disturbances on body weight regulation. Furthermore, leptin resistance can impair physiological peripheral functions of leptin such as lipid and carbohydrate metabolism and nutrient intestinal utilization. The present document summarizes those findings regarding leptin resistance development and the role of this hormone in the development and maintenance of an obese state. Thus, we focused on the effect of the impaired leptin action on adipose tissue, liver, skeletal muscle and intestinal function and the accompanying relationships with diet-induced obesity. The involvement of some inflammatory mediators implicated in the development of obesity and their roles in leptin resistance development are also discussed.

Modulation of olfactory sensitivity and glucose-sensing by the feeding state in obese Zucker rats

The Zucker fa/fa rat has been widely used as an animal model to study obesity, since it recapitulates most of its behavioral and metabolic dysfunctions, such as hyperphagia, hyperglycemia and insulin resistance. Although it is well established that olfaction is under nutritional and hormonal influences, little is known about the impact of metabolic dysfunctions on olfactory performances and glucose-sensing in the olfactory system of the obese Zucker rat. In the present study, using a behavioral paradigm based on a conditioned olfactory aversion, we have shown that both obese and lean Zucker rats have a better olfactory sensitivity when they are fasted than when they are satiated. Interestingly, the obese Zucker rats displayed a higher olfactory sensitivity than their lean controls. By investigating the molecular mechanisms involved in glucose-sensing in the olfactory system, we demonstrated that sodium-coupled glucose transporters 1 (SGLT1) and insulin dependent glucose transporters 4 (GLUT4) are both expressed in the olfactory bulb (OB). By comparing the expression of GLUT4 and SGLT1 in OB of obese and lean Zucker rats, we found that only SGLT1 is regulated in genotype-dependent manner. Next, we used glucose oxidase biosensors to simultaneously measure in vivo the extracellular fluid glucose concentrations ([Gluc]ECF) in the OB and the cortex. Under metabolic steady state, we have determined that the OB contained twice the amount of glucose found in the cortex. In both regions, the [Gluc]ECF was 2 fold higher in obese rats compared to their lean controls. Under induced dynamic glycemia conditions, insulin injection produced a greater decrease of [Gluc]ECF in the OB than in the cortex. Glucose injection did not affect OB [Gluc]ECF in Zucker fa/fa rats. In conclusion, these results emphasize the importance of glucose for the OB network function and provide strong arguments towards establishing the OB glucose-sensing as a key factor for sensory olfactory processing.

The gut as a sensory organ

The gastrointestinal tract presents the largest and most vulnerable surface to the outside world. Simultaneously, it must be accessible and permeable to nutrients and must defend against pathogens and potentially injurious chemicals. Integrated responses to these challenges require the gut to sense its environment, which it does through a range of detection systems for specific chemical entities, pathogenic organisms and their products (including toxins), as well as physicochemical properties of its contents. Sensory information is then communicated to four major effector systems: the enteroendocrine hormonal signalling system; the innervation of the gut, both intrinsic and extrinsic; the gut immune system; and the local tissue defence system. Extensive endocrine-neuro-immune-organ-defence interactions are demonstrable, but under-investigated. A major challenge is to develop a comprehensive understanding of the integrated responses of the gut to the sensory information it receives. A major therapeutic opportunity exists to develop agents that target the receptors facing the gut lumen.

Role of the apical and basolateral domains of the enterocyte in the regulation of cholesterol transport by a high glucose concentration

We have recently shown that a high glucose (HG) concentration raised intestinal cholesterol (CHOL) transport and metabolism in intestinal epithelial cells. The objective of the present work is to determine whether the stimulus for increased CHOL absorption by glucose originates from the apical site (corresponding to the intestinal lumen) or from the basolateral site (related to blood circulation). We tackled this issue by using differentiated Caco-2/15 cells. Only basolateral medium, supplemented with 25 mmol/L glucose, stimulated [(14)C]-CHOL uptake via the up-regulation of the critical CHOL transporter NPC1L1 protein, as confirmed by its specific ezetimibe inhibitor that abolished the rise in glucose-mediated CHOL capture. No significant changes were noted in SR-BI and CD36. Elevated CHOL uptake was associated with an increase in the transcription factors SREBP-2, LXR-β, and ChREBP, along with a fall in RXR-α. Interestingly, although the HG concentration in the apical medium caused modest changes in CHOL processing, its impact was synergetic with that of the basolateral medium. Our results suggest that HG concentration influences positively intestinal CHOL uptake when present in the basolateral medium. In addition, excessive consumption of diets containing high levels of carbohydrates may strengthen intestinal CHOL uptake in metabolic syndrome, thereby contributing to elevated levels of circulating CHOL and, consequently, the risk of developing type 2 diabetes and cardiovascular disease.

Maternal protein restriction during pregnancy affects gene expression and immunolocalization of intestinal nutrient transporters in rats

Intrauterine dietary restriction may cause changes in the functioning of offspring organs and systems later in life, an effect known as fetal programming. The present study evaluated mRNA abundance and immunolocalization of nutrient transporters as well as enterocytes proliferation in the proximal, median and distal segments of small intestine of rats born to protein-restricted dams. Pregnant rats were fed hypoproteic (6% protein) or control (17% protein) diets, and offspring rats were evaluated at 3 and 16 weeks of age. The presence of SGLT1 (sodium–glucose co-transporter 1), GLUT2 (glucose transporter 2), PEPT1 (peptide transporter 1) and the intestinal proliferation were evaluated by immunohistochemical techniques and the abundance of specific mRNA for SGLT1, GLUT2 and PEPT1 was assessed by the real-time PCR technique. Rats born to protein-restricted dams showed higher cell proliferation in all intestinal segments and higher gene expression of SGLT1 and PEPT1 in the duodenum. Moreover, in adult animals born to protein-restricted dams the immunoreactivity of SGLT1, GLUT2 and PEPT1in the duodenum was more intense than in control rats. Taken together, the results indicate that changes in the small intestine observed in adulthood can be programmed during the gestation. In addition, they show that this response is caused by both up-regulation in transporter gene expression, a specific adaptation mechanism, and intestinal proliferation, an unspecific adaptation mechanism.

Glucose transporter 8 (GLUT8) regulates enterocyte fructose transport and global mammalian fructose utilization

Enterocyte fructose absorption is a tightly regulated process that precedes the deleterious effects of excess dietary fructose in mammals. Glucose transporter (GLUT)8 is a glucose/fructose transporter previously shown to be expressed in murine intestine. The in vivo function of GLUT8, however, remains unclear. Here, we demonstrate enhanced fructose-induced fructose transport in both in vitro and in vivo models of enterocyte GLUT8 deficiency. Fructose exposure stimulated [(14)C]-fructose uptake and decreased GLUT8 protein abundance in Caco2 colonocytes, whereas direct short hairpin RNA-mediated GLUT8 knockdown also stimulated fructose uptake. To assess GLUT8 function in vivo, we generated GLUT8-deficient (GLUT8KO) mice. GLUT8KO mice exhibited significantly greater jejunal fructose uptake at baseline and after high-fructose diet (HFrD) feeding vs. wild-type mice. Strikingly, long-term HFrD feeding in GLUT8KO mice exacerbated fructose-induced increases in blood pressure, serum insulin, low-density lipoprotein and total cholesterol vs. wild-type controls. Enhanced fructose uptake paralleled with increased abundance of the fructose and glucose transporter, GLUT12, in HFrD-fed GLUT8KO mouse enterocytes and in Caco2 cultures exposed to high-fructose medium. We conclude that GLUT8 regulates enterocyte fructose transport by regulating GLUT12, and that disrupted GLUT8 function has deleterious long-term metabolic sequelae. GLUT8 may thus represent a modifiable target in the prevention and treatment of malnutrition or the metabolic syndrome.

Role of the transporter regulator protein (RS1) in the modulation of concentrative nucleoside transporters (CNTs) in epithelia

SLC28 genes encode three plasma membrane transporter proteins, human concentrative nucleoside transporter (CNT)1, CNT2, and CNT3, all of which are implicated in the uptake of natural nucleosides and a variety of nucleoside analogs used in the chemotherapy of cancer and viral and inflammatory diseases. Mechanisms determining their trafficking toward the plasma membrane are not well known, although this might eventually become a target for therapeutic intervention. The transporter regulator RS1, which was initially identified as a short-term, post-transcriptional regulator of the high-affinity, Na(+)-coupled, glucose transporter sodium-dependent glucose cotransporter 1, was evaluated in this study as a candidate for coordinate regulation of membrane insertion of human CNT-type proteins. With a combination of studies with mammalian cells, Xenopus laevis oocytes, and RS1-null mice, evidence that RS1 down-regulates the localization and activity at the plasma membrane of the three members of this protein family (CNT1, CNT2, and CNT3) is provided, which indicates the biochemical basis for coordinate regulation of nucleoside uptake ability in epithelia and probably in other RS1-expressing cell types.

A genome-wide shRNA screen for new OxPhos related genes

The mitochondrial oxidative phosphorylation (OxPhos) system produces most of the ATP required by the cell. The structural proteins of the OxPhos holoenzymes are well known, but important aspects of their biogenesis and regulation remain to be uncovered and a significant fraction of mitochondrial proteins have yet to be identified. We have used a high throughput, genome-wide RNA interference (RNAi) approach to identify new OxPhos-related genes. We transduced a mouse fibroblast cell line with a lentiviral-based shRNA-library, and screened the cell population for growth impairment in galactose-based medium, which requires an intact OxPhos system. Candidate genes were ranked according to their co-expression with known genes encoding OxPhos mitochondria-located proteins. For the top ranking candidates the cellular process in which they are involved was evaluated. Our results show that the use of genome-wide RNAi together with screening for deficient growth in galactose medium is a suitable approach to identifying OxPhos-related and cellular energy metabolism-related genes. Interestingly also ubiquitin-proteasome related genes were selected.

Rapid upregulation of sodium-glucose transporter SGLT1 in response to intestinal sweet taste stimulation

OBJECTIVE

We set out to examine the short-term regulation of the intestinal sodium/glucose cotransporter SGLT1 by its substrate glucose and sweet taste analogs.

SUMMARY BACKGROUND DATA

Intestinal SGLT1 is a putative target for antidiabetic therapy; however, its physiological regulation is incompletely understood, limiting its application as a pharmacological target. While it is clearly regulated by dietary composition over a period of days, its short-term regulation by nutrients is unknown.

METHODS

Sprague-Dawley rats were anesthetized, and the duodenum cannulated. D-glucose, D-fructose, saccharin, D-mannitol, and water were infused for 3 hours, before harvest of proximal jejunum for SGLT1 analysis with Western blotting and quantitative polymerase chain reaction. In further experiments, the receptor region was identified by D-glucose infusion of isolated regions. Lastly, the vagus was de-afferented with capsaicin, and 5HT3-receptor activation of vagal afferents inhibited using ondansetron, before repeating experiments using water or D-glucose infusion.

RESULTS

Infusion of D-glucose led to 2.9-fold up-regulation in SGLT1 compared with water or iso-osmotic D-mannitol; this effect was replicated by D-fructose or saccharin. This response was strongest following isolated infusions of duodenum and proximal jejunum, with a blunted effect distally; topography matched the expression profile of sweet taste receptor T1R2/T1R3. The reflex was abolished by capsaicin pretreatment, and blunted by ondansetron.

CONCLUSIONS

The agonist response implicates the luminal-based sweet-taste receptor T1R2/T1R3, with the reflex apparently involving vagal afferents. The proximal nature of the sensor coincides with the excluded biliopancreatic limb in Roux-en-Y gastric bypass, and this may provide a novel explanation for the antidiabetic effect of this procedure.

Restricted feeding phase shifts clock gene and sodium glucose cotransporter 1 (SGLT1) expression in rats

The intestine exhibits striking diurnal rhythmicity in glucose uptake, mediated by the sodium glucose cotransporter (SGLT1); however, regulatory pathways for these rhythms remain incompletely characterized. We hypothesized that SGLT1 rhythmicity is linked to the circadian clock. To investigate this, we examined rhythmicity of Sglt1 and individual clock genes in rats that consumed food ad libitum (AL). We further compared phase shifts of Sglt1 and clock genes in a second group of rats following restricted feeding to either the dark (DF) or light (LF) phase. Rats fed during the DF were pair-fed to rats fed during the LF. Jejunal mucosa was harvested across the diurnal period to generate expression profiles of Sglt1 and clock genes Clock, Bmal1 (brain-muscle Arnt-like 1), ReverbA/B, Per(Period) 1/2, and Cry (Cryptochrome) 1/2. All clock genes were rhythmic in AL rats (P < 0.05). Sglt1 also exhibited diurnal rhythmicity, with peak expression preceding nutrient arrival (P < 0.05). Light-restricted feeding shifted the expression rhythms of Sglt1 and most clock genes (Bmal1, ReverbA and B, Per1, Per2, and Cry1) compared with dark-restricted feeding (P < 0.05). The Sglt1 rhythm shifted in parallel with rhythms of Per1 and ReverbB. These effects of restricted feeding highlight luminal nutrients as a key Zeitgeber in the intestine, capable of simultaneously shifting the phases of transporter and clock gene expression, and suggest a role for clock genes in regulating Sglt1 and therefore glucose uptake. Understanding the regulatory cues governing rhythms in intestinal function may allow new therapeutic options for conditions of dysregulated absorption such as diabetes and obesity.

Impact of Roux-en-Y gastric bypass surgery on rat intestinal glucose transport

Roux-en-Y gastric bypass (RYGB) has become the gold-standard bariatric procedure, partly because of the rapid resolution of accompanying diabetes. There is increasing evidence this is mediated by duodenal exclusion. We hypothesize that duodenal exclusion suppresses intestinal Na(+)/glucose cotransporter SGLT1-mediated glucose transport, improving glucose handling, and aimed to test this in a rodent RYGB model. Sprague-Dawley rats underwent sham procedure or duodenal exclusion by RYGB (10 cm Roux, 16 cm biliopancreatic limbs). Animals were maintained for 3 wk on a Western diet, before harvest at 10 AM, 4 PM, and 10 PM. Sections were taken from each limb for hematoxylin and eosin staining, and morphological assessment was performed. Functional glucose uptake studies, along with Western blotting and quantitative PCR, were performed on Roux limb. Histology showed morphometric changes in Roux and common limbs, with increase in villus height and crypt depth compared with BP and sham jejunum. Despite this, glucose transport was reduced by up to 68% (P < 0.001) in the Roux limb compared with sham jejunum. Normal diurnal rhythms in glucose uptake were ablated. This occurred at a posttranscriptional level, with little change in message but appearance of different weight species of Sglt1 on Western blotting. We have shown duodenal exclusion significantly influences both intestinal structure and glucose transport function, with glucose absorptive capacity reduced after RYGB. This provides a novel mechanistic explanation for some of the antidiabetic effects of RYGB.

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.

Diurnal expression of the rat intestinal sodium-glucose cotransporter 1 (SGLT1) is independent of local luminal factors

BACKGROUND

The intestinal sodium-glucose cotransporter 1 (SGLT1) is responsible for all secondary active transport of dietary glucose, and it presents a potential therapeutic target for obesity and diabetes. SGLT1 expression varies with a profound diurnal rhythm, matching expression to nutrient intake. The mechanisms entraining this rhythm remain unknown. We investigated the role of local nutrient signals in diurnal SGLT1 entrainment.

METHODS

Male Sprague-Dawley rats, which were acclimatized to a 12:12 light:dark cycle, underwent laparotomy with formation of isolated proximal jejunal loops (Thiry-Vella loops). Animals were recovered for 10 days before harvesting at 4 6-h intervals (Zeitgeber times ZT3, ZT9, ZT15, and ZT21, where ZT0 is lights on; n = 6-8). SGLT1 expression was assessed in protein, and mRNA extracts of mucosa were harvested from both isolated loops (LOOP) and remnant jejunum (JEJ).

RESULTS

Isolated loops were healthy but atrophic with minimal changes to villus architecture. A normal anticipatory rhythm was observed in Sglt1 transcription in both LOOP and JEJ, with the peak signal at ZT9 (2.7-fold, P < .001). Normal diurnal rhythms were also observed in the protein signal, with peak expression in both LOOP and JEJ at ZT9 to 15 (2.1-fold, P < .05). However, an additional more mobile polypeptide band was also observed in all LOOP samples but not in JEJ samples (61 kDa vs 69 kDa). Enzymatic deglycosylation suggested this to be deglycosylated SGLT1.

CONCLUSION

The persistence of SGLT1 rhythmicity in isolated loops indicates that diurnal induction is independent of local luminal nutrient delivery, and it suggests a reliance on systemic entrainment pathways. However, local luminal signals may regulate glycosylation and, therefore, the posttranslational handling of SGLT1.

Defining the transcriptional regulation of the intestinal sodium-glucose cotransporter using RNA-interference mediated gene silencing

BACKGROUND

The sodium glucose cotransporter (SGLT1) is responsible for all active intestinal glucose uptake. Hepatocyte nuclear factors 1 alpha and beta (HNF 1 alpha and HNF 1 beta) activate the SGLT1 promoter, whereas GATA-binding protein 5 (GATA-5) and caudal-type homeobox protein 2 (CDX2) regulate transcription of other intestinal genes. We investigated SGLT1 regulation by these transcription factors using promoter studies and RNA interference.

METHODS

Chinese hamster ovary (CHO) cells were transiently cotransfected with an SGLT1-luciferase promoter construct and combinations of expression vectors for HNF 1 alpha, HNF 1 beta, CDX2, and GATA-5. Caco-2 cells were stably transfected with knockdown vectors for either HNF 1 alpha or HNF 1 beta. mRNA levels of HNF 1 alpha, HNF 1 beta, and SGLT1 were determined using quantitative polymerase chain reaction (qPCR).

RESULTS

HNF 1 alpha, GATA-5, and HNF 1 beta significantly activated the SGLT1 promoter (P < .05). Cotransfection of GATA-5 with HNF 1 alpha had an additive effect, whereas HNF 1 beta and CDX2 antagonized HNF 1 alpha and GATA-5. SGLT1 expression was significantly reduced in HNF 1 alpha or HNF 1 beta knockdowns (P < .001). HNF alpha knockdown significantly reduced HNF 1 beta expression and vice versa (P < .005).

CONCLUSIONS

HNF 1 alpha and HNF 1 beta are important transcription factors for endogenous SGLT1 expression by cultured enterocytes. GATA-5 and CDX2 also regulate SGLT1 promoter activity and show cooperativity with the HNF1s. We, therefore, propose a multifactorial model for SGLT1 regulation, with interactions between HNF1, GATA-5, and CDX2 modulating intestinal glucose absorption.

Capsaicin-sensitive vagal afferents modulate posttranscriptional regulation of the rat Na+/glucose cotransporter SGLT1

INTRODUCTION

the intestinal Na(+)/glucose cotransporter (SGLT1) displays rapid anticipatory diurnal rhythms in mRNA and protein expression. The vagus nerve has been implicated in the entrainment of some transporters. We aimed to clarify the influence of the vagus nerve on the diurnal entrainment pathway for SGLT1 and examine the role of vagal afferent fibers.

METHODS

male Sprague-Dawley rats were randomized to three groups, total subdiaphragmatic vagotomy, selective deafferentation of the vagus with capsaicin, or sham laparotomy. Postoperatively, animals were maintained in a 12-h light-dark cycle with food access limited to night. On the ninth postoperative day, animals were euthanized to harvest jejunal mucosa at 6-h intervals starting at 10 AM. Whole cell SGLT1 protein was measured by semiquantitative densitometry of immunoblots. Sglt1 and regulatory subunit RS1 mRNA was assessed by quantitative PCR. Fluorogold tracer technique was used to confirm adequacy of the vagotomy.

RESULTS

the diurnal rhythm in intestinal SGLT1, with a 5.3-fold increase in Sglt1 mRNA at 4 PM, was preserved in both vagotomy and capsaicin groups. However, the rhythmicity in SGLT1 protein expression (2.3-fold peak at 10 PM; P = 0.041) was abolished following either total vagotomy or deafferentation. Lack of change in RS1 mRNA suggests this is independent of the RS1 regulatory pathway.

CONCLUSION

SGLT1 transcription is independent of the vagus. However, dissociation of the protein rhythm from the underlying mRNA signal by vagotomy suggests the vagus may be involved in posttranscriptional regulation of SGLT1 in an RS1 independent pathway. Disruption following afferent ablation by capsaicin suggests this limb is specifically necessary.

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.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

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.