Dietary sugars are transported from the intestinal lumen into absorptive enterocytes by the sodium-dependent glucose transporter isoform 1 (SGLT1). Regulation of this protein is important for the provision of glucose to the body and avoidance of intestinal malabsorption. Although expression of SGLT1 is regulated by luminal monosaccharides, the luminal glucose sensor mediating this process was unknown. Here, we show that the sweet taste receptor subunit T1R3 and the taste G protein gustducin, expressed in enteroendocrine cells, underlie intestinal sugar sensing and regulation of SGLT1 mRNA and protein. Dietary sugar and artificial sweeteners increased SGLT1 mRNA and protein expression, and glucose absorptive capacity in wild-type mice, but not in knockout mice lacking T1R3 or ␣-gustducin. Artificial sweeteners, acting on sweet taste receptors expressed on enteroendocrine GLUTag cells, stimulated secretion of gut hormones implicated in SGLT1 upregulation. Gut-expressed taste signaling elements involved in regulating SGLT1 expression could provide novel therapeutic targets for modulating the gut's capacity to absorb sugars, with implications for the prevention and/or treatment of malabsorption syndromes and diet-related disorders including diabetes and obesity.carbohydrate absorption ͉ gastrointestinal chemosensation ͉ glucose sensor ͉ glucose uptake T o date, the only identified sugar sensors in the mammalian gastrointestinal tract are those involved in taste transduction (1). Although the gut epithelium senses luminal sugars and modulates its glucose absorptive capacity accordingly, the nature of the sugar-sensing molecule(s) and downstream events remain unknown. Several studies have shown that in many species expression of the intestinal sodium-dependent glucose transporter 1 (SGLT1) is directly regulated by monosaccharides in the lumen of the gut independently of metabolism and appears to involve a G protein-linked second messenger pathway (2-6). Furthermore, the addition of membrane-impermeable glucose analogues to the lumen of the intestine stimulates SGLT1 expression, implying that a glucose sensor on luminal membranes is involved (6).In taste cells, the detection of sugars and sweeteners depends on T1R2ϩT1R3, a heterodimer of type 1 taste receptor subunits (T1Rs) (7,8). The taste receptor cells of the anterior tongue that express T1R2ϩT1R3 typically also express gustducin, a transducin-like heterotrimeric G protein (9). Gustducin's ␣-subunit (G␣ gust ) has been detected in brush cells of the rat stomach, duodenum, and pancreatic ducts (10). G␣ gust and bitter-responsive type 2 taste receptors (T2Rs) are expressed in mouse intestinal endocrine cells and in the murine enteroendocrine cell line STC-1 (11).We reported previously that T1R taste receptors and G␣ gust are expressed in the mouse small intestinal epithelium and proposed that they function as luminal sugar sensors to control SGLT1 expression in response to dietary sugar (12). Here, we provide three lines of evidence in favor of T1R taste receptors an...

The composition of the intestinal luminal content varies considerably with diet. It is important therefore that the intestinal epithelium senses and responds to these significant changes and regulates its functions accordingly. Although it is becoming evident that the gut epithelium senses and responds to luminal nutrients, little is known about the nature of the nutrient sensing molecule and the downstream cellular events. A prototype example is the modulation in the capacity of the gut to absorb monosaccharides via the intestinal luminal membrane Na(+)/glucose cotransporter, SGLT1. The experimental evidence suggests that luminal sugar is sensed by a glucose sensor residing on the luminal membrane of the gut epithelium and linked to a G-protein-coupled receptor, cAMP/PKA (protein kinase A) pathway, resulting ultimately in modulation of intestinal monosaccharide absorption. Here we report the expression, at mRNA and protein levels, of members of the T1R sweet taste receptors, and the alpha-subunit of the G-protein gustducin, in the small intestine and the enteroendocrine cell line, STC-1. In the small intestine, there is a highly coordinated expression of sweet taste receptors and gustducin, a G-protein implicated in intracellular taste signal transduction, throughout the gut. The potential involvement of these receptors in sugar sensing in the intestine will facilitate our understanding of intestinal nutrient sensing, with implications for better nutrition and health maintenance.

SummaryD i e t a ry carbohydrates, when digested and absorbed in the small intestine of the horse, provide a substantial fraction of metabolisable energy. However, if levels in diets exceed the capacity of the equine small intestine to digest and absorb them, they reach the hindgut, cause alterations in micro b i a l populations and the metabolite products and predispose the horse to gastrointestinal diseases. We set out to determine, at the m o l e c u l a r level, the mechanisms, pro p e rt i e s and the site of e x p ression of carbohydrate digestive and absorptive functions of the equine small intestinal brush-border membrane. We have demonstrated that the disaccharidases sucrase, lactase and maltase are expressed diversely along the length of the intestine and D-glucose is transported across the equine intestinal brushb o r d e r membrane by a high aff i n i t y, low capacity, Na + / g l u c o s e c o t r a n s p o rt e r type 1 isoform (SGLT1). The highest rate of t r a n s p o rt is in duodenum > jejunum > ileum. We have cloned and sequenced the cDNA encoding equine SGLT1 and alignment with SGLT1 of other species indicates 85-89% homology at the nucleotide and 84-87% identity at the amino acid levels. We have shown that there is a good corre l a t i o n between levels of functional SGLT1 protein and SGLT1 mRNA abundance along the length of the small intestine. This indicates that the major site of glucose absorption in horses maintained on conventional grass-based diets is in the proximal intestine, and the expression of equine intestinal SGLT1 along the p roximal to distal axis of the intestine is regulated at the level of m R N A abundance. The data presented in this paper a re the first to provide information on the capacity of the equine intestine to digest and absorb soluble carbohydrates and has implications for a better feed management, pharmaceutical intervention and for d i e t a ry supplementation in horses following intestinal re s e c t i o n .

Experimental and epidemiological evidence suggests that consumption of hydrolyzable carbohydrate, hCHO (grain), by horses is an important risk factor for colic, a common cause of equine mortality. It is unknown whether the small intestinal capacity to digest hCHO and/or to absorb monosaccharides is limiting, or even if horses can adapt to increased carbohydrate load. We investigated changes in the brush-border membrane carbohydrate digestive enzymes and glucose absorptive capacity of horse small intestine in response to increased hCHO. Expression of the Na(+)/glucose co-transporter, SGLT1, was assessed by Western blotting, immunohistochemistry, Northern blotting, QPCR, and Na(+)-dependent D-glucose transport. Glucose transport rates, SGLT1 protein, and mRNA expression were all 2-fold higher in the jejunum and 3- to 5-fold higher in the ileum of horses maintained on a hCHO-enriched diet compared to pasture forage. Activity of the disaccharidases was unaltered by diet. In a well-controlled study, we determined SGLT1 expression in the duodenal and ileal biopsies of horses switched, gradually over a 2-month period, from low (<1.0 g/kg bwt/day) to high hCHO (6.0 g/kg bwt/day) diets of known composition. We show that SGLT1 expression is enhanced, with time, 2-fold in the duodenum and 3.3-fold in the ileum. The study has important implications for dietary management of the horse.

SGLT1 (Na(+)/glucose co-transporter 1) transports the dietary sugars, D-glucose and D-galactose, from the lumen of the intestine into enterocytes. SGLT1 regulation has important consequences for the provision of glucose to the respiring tissues and is therefore essential for maintaining glucose homoeostasis. SGLT1 expression is directly regulated in response to changes in the sugar content of the diet. To monitor these variations, there is a requirement for a glucose-sensing system located on the luminal membrane of gut cells. This short review focuses on recent findings on intestinal sugar sensing and the downstream mechanisms responsible for enhancement in SGLT1 expression.