Transcriptional and translational control over sodium-glucose-linked transporter (SGLT1) gene expression in adult rat small intestine

Immunohistochemical Study of Glucose Transporter GLUT-5 in Duodenal Epithelium in Norm and in T-2 Mycotoxicosis

Although patterns of glucose transporter expression and notes about diseases leading to adaptive changes in intestinal fructose transport have been well-characterized, the connection between infection and fructose transportation has been lightly investigated. Up to now only few studies on GLUT-5 expression and function under pathological conditions in bird intestines have been carried out. The aim of our current research was to immunolocalize GLUT-5 in chicken duodenal epithelium in norm and during T-2 mycotoxicosis. Material from chicken (Gallus gallus domesticus) duodenum was collected from twelve seven-day-old female broilers, divided into control group and broilers with T-2 mycotoxicosis. The material was fixed with 10% formalin and thereafter embedded into paraffin; slices 7 μm in thickness were cut, followed by immunohistochemical staining, according to the manufacturers guidelines (IHC kit, Abcam, UK) using polyclonal primary antibody Rabbit anti-GLUT-5. Our study revealed the strong expression of GLUT-5 in the apical parts of the duodenal epithelial cells in the control group chickens and weak staining for GLUT-5 in the intestinal epithelium in the T-2 mycotoxicosis group. Our results confirmed decreased the expression of GLUT-5 in the duodenal epithelium during T-2 mycotoxicosis.

Comparative expression of hexose transporters (SGLT1, GLUT1, GLUT2 and GLUT5) throughout the mouse gastrointestinal tract

Hexose transporters play a pivotal role in the absorption of food-derived monosaccharides in the gastrointestinal tract. Although a basic knowledge of the hexose transporters has already been gained, their detailed distribution and comparative intensities of expression throughout the gastrointestinal tract have not been fully elucidated. In this study, we quantitatively evaluated the expression of SGLT1, GLUT1, GLUT2, and GLUT5 by in situ hybridization and real-time PCR techniques using a total of 28 segments from the gastrointestinal tract of 9-week-old mice. GLUT2 and GLUT5 mRNA expressions were detected predominantly from the proximal to middle parts of the small intestine, showing identical expression profiles, while SGLT1 mRNA was expressed not only in the small intestine but also in the large intestine. Notably, GLUT1 mRNA was expressed at a considerable level in both the stomach and large intestine but was negligible in the small intestine. Immunohistochemistry demonstrated the polarized localization of hexose transporters in the large intestine: SGLT1 on the luminal surface and GLUT1 on the basal side of epithelial cells. The present study provided more elaborate information concerning the localization of hexose transporters in the small intestine. Furthermore, this study revealed the significant expression of glucose transporters in the large intestine, suggesting the existence of the physiological uptake of glucose in that location in mice.

Revised immunolocalization of the Na+-d-glucose cotransporter SGLT1 in rat organs with an improved antibody

Previously, we characterized localization of Na+-glucose cotransporter SGLT1 ( Slc5a1) in the rat kidney using a polyclonal antibody against the synthetic COOH-terminal peptide of the rat protein (Sabolić I, Škarica M, Gorboulev V, Ljubojević M, Balen D, Herak-Kramberger CM, Koepsell H. Am J Physiol Renal Physiol 290: 913–926, 2006). However, the antibody gave some false-positive reactions in immunochemical studies. Using a shortened peptide for immunization, we have presently generated an improved, more specific anti-rat SGLT1 antibody (rSGLT1-ab), which in immunochemical studies with isolated membranes and tissue cryosections from male (M) and female (F) rats exhibited 1) in kidneys and small intestine, labeling of a major protein band of ∼75 kDa; 2) in kidneys of adult animals, localization of rSGLT1 to the proximal tubule (PT) brush-border membrane (S1 < S2 < S3) and intracellular organelles (S1 > S2 > S3), with zonal (cortex < outer stripe) and sex differences (M < F) in the protein expression, which correlated well with the tissue expression of its mRNA in RT-PCR studies; 3) in kidneys of castrated adult M rats, upregulation of the protein expression; 4) in kidneys of prepubertal rats, weak and sex-independent labeling of the 75-kDa protein band and immunostaining intensity; 5) in small intestine, sex-independent regional differences in protein abundance (jejunum > duodenum = ileum); and 6) thus far unrecognized localization of the transporter in cortical thick ascending limbs of Henle and macula densa in kidney, bile ducts in liver, enteroendocrine cells and myenteric plexus in the small intestine, and initial ducts in the submandibular gland. Our improved rSGLT1-ab may be used to identify novel sites of SGLT1 localization and thus unravel additional physiological functions of this transporter in rat organs.

The effect of massive small bowel resection and oral epidermal growth factor therapy on SGLT-1 distribution in rabbit distal remnant

Small bowel resection decreases brush border membrane (BBM) glucose uptake kinetics. Oral epidermal growth factor (EGF) returns net glucose transport across intact tissue to control levels despite persistence of a defect in BBM glucose uptake. The purpose of this study was to examine the effects of resection and EGF treatment on sodium-dependent glucose cotransporter 1 (SGLT-1) expression in distal remnant tissue. New Zealand White rabbits (1 kg) underwent 70% small bowel resection (R). One group of resected animals (R-EGF) received oral EGF (40 microg/kg, days 3-8). Distal remnant tissue was harvested 10 d after surgery, and compared with controls (C). Mucosal SGLT-1 mRNA was measured by Northern blot, BBM SGLT-1 content by Western blot, and villus distribution of SGLT-1 protein and mRNA by immunofluorescence and in situ hybridization. Western blot indicated BBM from both resected and EGF-treated tissue had decreased SGLT-1 content (C, 0.55 +/- 0.04; R, 0.35 +/- 0.04; R-EGF, 0.35 +/- 0.03 trace OD; n = 5; p < 0.05). Northern blot revealed no alterations in mucosal SGLT-1 mRNA content in any group. SGLT-1 protein and mRNA localization in control tissues was characterized by a gradual increase in stain intensity from the base of the villus to the villus tip. Resection altered SGLT-1 protein and mRNA expression along the villus axis with intensity being strongest in the mid-villus region and little expression at the tip of the villus. Oral EGF normalized SGLT-1 protein and mRNA expression to control patterns. Resection alters SGLT-1 protein and mRNA expression along the villus axis, despite no change in total mucosal SGLT-1 mRNA content. EGF normalized villus SGLT-1 protein and mRNA distribution, without altering overall BBM SGLT-1 content or mucosal mRNA levels.

Modelling molecular mechanisms controlling sequential gene expression in differentiating mammalian enterocytes

The gut epithelium represents a continuous developmental system in which cell proliferation in intestinal crypts is followed by the sequential expression of digestive and absorptive functions as enterocytes migrate out of crypts to the tips of intestinal villi. We have developed a mathematical model in the present work to mimic these sequential aspects of enterocyte differentiation. Using this model allows the characteristics of lactase expression to be ascribed to transcriptional control. In the case of a glucose transporter, however, it became necessary to assume an additional translational control that decreased exponentially as enterocytes migrated along villi. The suggestion that this type of modelling is useful in predicting which set of enterocytes is likely to use translation or transcription to control gene expression is also discussed.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Dietary and developmental regulation of intestinal sugar transport

The Na(+)-dependent glucose transporter SGLT1 and the facilitated fructose transporter GLUT5 absorb sugars from the intestinal lumen across the brush-border membrane into the cells. The activity of these transport systems is known to be regulated primarily by diet and development. The cloning of these transporters has led to a surge of studies on cellular mechanisms regulating intestinal sugar transport. However, the small intestine can be a difficult organ to study, because its cells are continuously differentiating along the villus, and because the function of absorptive cells depends on both their state of maturity and their location along the villus axis. In this review, I describe the typical patterns of regulation of transport activity by dietary carbohydrate, Na(+) and fibre, how these patterns are influenced by circadian rhythms, and how they vary in different species and during development. I then describe the molecular mechanisms underlying these regulatory patterns. The expression of these transporters is tightly linked to the villus architecture; hence, I also review the regulatory processes occurring along the crypt-villus axis. Regulation of glucose transport by diet may involve increased transcription of SGLT1 mainly in crypt cells. As cells migrate to the villus, the mRNA is degraded, and transporter proteins are then inserted into the membrane, leading to increases in glucose transport about a day after an increase in carbohydrate levels. In the SGLT1 model, transport activity in villus cells cannot be modulated by diet. In contrast, GLUT5 regulation by the diet seems to involve de novo synthesis of GLUT5 mRNA synthesis and protein in cells lining the villus, leading to increases in fructose transport a few hours after consumption of diets containing fructose. In the GLUT5 model, transport activity can be reprogrammed in mature enterocytes lining the villus column. Innovative experimental approaches are needed to increase our understanding of sugar transport regulation in the small intestine. I close by suggesting specific areas of research that may yield important information about this interesting, but difficult, topic.

Partial sequence and expression of the gene for and activity of the sodium glucose transporter in the small intestine of fed, starved and refed chickens

A 970 bp cDNA Na(+)/glucose cotransporter (SGLT1) was isolated and sequenced from chicken jejunum by reverse transcriptase polymerase chain reaction (RT-PCR) using primers based on conserved regions. Using the 970 bp PCR product as a specific probe, Northern Blot hybridization indicated a transcript of ca. 4 kb. The isolated chicken intestinal SGLT1 cDNA was used to quantitate mRNA expression. Glucose uptake activity and kinetics were determined in brush border membrane vesicles (BBMV) from jejunum tissue of chickens which were either fed, food-deprived or refed following food deprivation. Net glucose uptake to BBMV was higher (P: < 0.02) in the control and refed chicks (149 +/- 11.9, 139.6 +/- 7.43 pmol x mg protein(-1) x s(-1)) than in food-deprived chicks (107 +/- 4.23 pmol x mg protein(-1) x s(-1)). The k(m) (150 micromol/L) and V:max (1111.1 pmol x mg protein(-1) x s(-1)) were higher in the food-deprived chicks compared to control and refed birds (25, 24 micromol/L and 227,142 pmol x mg protein(-1) x s(-1), respectively). Expression of SGLT1 mRNA was significantly enhanced in the food-deprived and refed birds. In food-deprived chicks the lower affinity and higher activity of the SGLT1 transporter for glucose were accompanied by higher expression of mRNA which might indicate that the transporter was upregulated by low substrate concentration. Quantification of expression of intestinal mRNA of SGLT1 provides important information concerning control of nutrient uptake.

The small intestinal fructose transporters: site of dietary perception and evidence for diurnal and fructose sensitive control elements

To obtain an insight into the mechanisms responsible for GLUT5 diurnality and fructose responsiveness, rats were gavaged at 9:00 AM or 6:00 PM with 1 g of fructose in the presence or absence of cycloheximide. After 4 h of fructose exposure, GLUT5 mRNA and protein levels increased 2-3.5-fold above the natural diurnal levels of expression. In situ hybridization and immunochemical analysis of GLUT5 mRNA and protein demonstrated that both diurnality and fructose responsiveness was confined to mature enterocytes. The protein synthesis inhibitor, cycloheximide, blunted the diurnal and fructose driven increase in GLUT5 mRNA expression in the morning, but had minimal effect on the pattern of expression in the evening. This differential sensitivity of intestinal GLUT5 mRNA to de novo protein synthesis may reflect the increasing presence of diurnal and fructose sensitive control factors during the day. Following vehicle gavage, Cycloheximide was more effective in reducing GLUT5 protein expression levels in the morning when compared to the evening. These data suggest that the turnover of GLUT5 protein may be diurnally influenced.

The molecular basis of intractable diarrhoea of infancy

The intractable diarrhoeas of infancy present very major problems of clinical management. However, the conceptual importance of these conditions lies in the information that they may provide about normal small-intestinal function in humans: among such infants will be found the human equivalents of the 'knock-out' mice, in which targeted gene disruption allows sometimes unexpected insight into the regulation of intestinal function. The challenge posed by the intractable diarrhoeal syndromes, of working backwards from an apparently common phenotype to probably multiple genotypes, is, however, immense. Very few of these conditions have been described at the genetic level, although the molecular basis of pathogenesis has been better explored in recent years. The two major groups of intractable diarrhoea are due to (1) primary epithelial abnormalities (which usually present within the first few days of life) and (2) immunologically mediated (which generally present after the first few weeks). The high prevalence of autoimmune enteropathy among infantile autoimmune disease, in contrast to adult autoimmunity, is intriguing and may reflect constitutive abnormality of extrathymic lymphocyte maturation. The use of potent immunosuppressive drugs and increasing expertise with parenteral nutrition are improving the outlook of these previously fatal conditions. Viewed globally, however, the pressing problem is to treat effectively the millions of infants who die from severe persistent diarrhoea and wasting, which would certainly not be considered intractable in wealthy countries.