Patent Application
20090196878
The present invention relates to a mammalian intestinal epithelial glucose sensor, more specifically to a human intestinal epithelial glucose sensor. The invention relates further to the use of this sensor to modulate or monitor intestinal carbohydrate uptake and metabolism and to the use of ligands and/or inhibitors and/or activators of this sensor to treat or prevent diseases such as diabetes and obesity, and to provide nutritional aids for the elderly, infants and athletes.
Sensing nutrients is a fundamental challenge for all living cells. Different types of nutrient sensing receptors have been identified in eukaryotic cells. Sensing nutrients is particularly important for the absorptive cells of the intestinal epithelium. These cells are exposed to a luminal environment that varies considerably with diet, and not surprisingly therefore, they adapt to these changes by regulating their uptake of nutrients from the intestinal lumen (Karasov and Diamond, 1987; Ferraris and Diamond, 1989). Although it is well established that this adaptation is achieved through the modulation of expression/activity of specialised nutrient transporters resident in the enterocyte plasma membrane, a major challenge that remains is to gain an insight into the identity of the receptors that sense the changes in the luminal contents; i.e. the nutrient sensors.
The best example of adaptive response of intestinal nutrient transport to the changes in the luminal nutrients is that of the intestinal Na+/glucose cotransporter, SGLT1. SGLT1 transports dietary monosaccharides, D-glucose and D-galactose from the lumen of the intestine across the luminal membrane (brush border membrane) into enterocytes. Using both in vivo and in vitro models it has been shown that the activity and the expression of SGLT1 is directly regulated by the luminal (medium) monosaccharides, and that the metabolism of glucose is not required for the glucose induction of SGLT1 (Ferraris and Diamond, 1989; Solberg and Diamond, 1987; Lescale-Matys et al., 1993; Shirazi-Beechey, 1996; Dyer et al., 1997). Furthermore a membrane impermeable glucose analogue, when introduced into the lumen of the intestine, also stimulates SGLT1 expression and abundance, implying that a glucose sensor expressed on the luminal membrane of the intestinal cells is involved in sensing the luminal sugar (Dyer, J and Vayro S (joint first) et al., 2003).
The intestinal epithelium is a dynamic structure, undergoing constant and rapid renewal. The stem cells positioned near the base of the crypt undergo several rounds of cell division and give rise to four cell types, absorptive enterocytes, mucous producing goblet cells, hormone producing enteroendocrine cells, and paneth cells. Enterocytes, constituting 90% of cells, along with goblet and some endocrine cells migrate without subsequent division to the villus tip, where they are extruded into the lumen of the intestine. This process takes 3-4 days.
It is well established that SGLT1 is expressed on the luminal membrane throughout the entire villus enterocytes. It is not known, however, which cell type(s) may express the glucose sensor. Generally it is accepted that enteroendocrine cells possess nutrient sensing properties, and secrete gut hormones in response to luminal nutrients. As such enteroendocrine cells may be the cell type that expresses the glucose receptor.
The only knowledge of sugar sensing in the mammalian gastrointestinal tract is from taste transduction mechanisms. Taste cells in the taste buds of the tongue epithelium have mechanisms that can distinguish chemical compounds, such as sugars, having potential nutritional value. It has been shown that transduction of sweet-tasting compounds involves activation of G-protein coupled receptor (GPCR) on the apical surface of taste receptor cells. Recent studies indicate that the members of the taste T1R receptor family (T1R2/T1R3) and gustducin, a taste-specific transducin-like G-protein α subunit, are involved in transduction of sugars in the tongue.
Surprisingly we found that taste receptors, T1R1-3, which were thought to be limited in expression to the tongue, are expressed in the small intestine. Furthermore we demonstrate that the receptors along with Gαgust are expressed luminally, and mainly in the proximal region of the small intestine. As these GPCRs are involved in sensing dietary glucose their manipulation will result in modulation in the capacity of the gut to absorb dietary sugars. This has both nutritional and clinical significance, in the treatment of, as a non-limiting example, obesity and diabetes.
A first aspect of the invention is the use of T1R2 and/or T1R3 and/or α-gustducin, preferably T1R3 and/or α-gustducin, to modulate sugar uptake in the intestine. Preferably, said modulation of the sugar uptake is realized by a modulation of the activity of the SGLT1 transporter. The modulation of the activity may be cis or trans, i.e. SGLT1 may be situated in the same cell as the T1R2 and/or T1R3 and/or α-gustducin, whereby SGLT1 is activated, at transcriptional level and/or on posttranscriptional and or posttranslational levels, by the signalling pathway of the T1R-receptors, or alternatively SGLT1 transporter is situated in another cell, and activated by a compound that is secreted by the cell harbouring the T1R2 and/or T1R3 and/or α-gustducin upon binding of glucose to the T1-receptor and activation of the signalling pathway.
Another aspect of the invention is the use of an inhibitor of T1R2 and/or T1R3, preferably an inhibitor of T1R3, to modulate sugar uptake in the intestine. Preferably, said modulation of the sugar uptake is realized by a modulation of the activity of the SGLT1 transporter. An inhibitor of T1R2 and/or T1R3 can be any compound that inhibits ligand binding, secretion and/or localization into the plasma membrane, clustering of the receptor, posttranslational modification, dimerization and/or the signalling which is normally induced upon binding of the ligand to the receptor. Preferably, said inhibitor is interfering with the ligand binding. One preferred embodiment of the inhibitor is an antibody binding to the ligand binding domain of the receptor. An antibody, as used here can be any antibody known to the person skilled in that art, including, but not limited to single chain antibodies and camelid antibodies and any derived nanobodies. Another preferred embodiment of the inhibitor is a soluble peptide or a peptido-mimetic comprising the ligand binding domain of the receptor. Indeed, such a compound will bind the ligand and act as a competitive inhibitor of the receptor. An inhibitor of the signalling pathway is, as a non-limiting example a G352P-α-gustducin mutant, which acts as a dominant negative mutant (Ruiz-Avila et al., 2001).
Another aspect of the invention is the use of an activator of T1R2 and/or T1R3, preferably an activator of T1R3, to modulate sugar uptake in the intestine. The use of activators of the sensor to increase intestinal carbohydrate uptake can provide nutritional aids for the elderly, infants and athletes. Preferably, said activator is modulating the activity of the SGLT1 transporter. As a non-limiting example, sucralose can be used as activator.
Another aspect of the invention is an isolated intestinal epithelial cell expressing T1R2 and/or T1R3 and/or α-gustducin, preferably an isolated intestinal epithelial cell expressing T1R3 and/or α-gustducin. Intestinal epithelial cell lines are known to the person skilled in the art and include, but are not limited to the primary human small intestinal epithelial cell line (FHs74 Int), primary rat small intestinal cell line (IEC6), and Caco-2 cells (human colon carcinoma cell line, widely used as a model of small intestinal cell). Still another aspect of the invention is the use of an isolated intestinal epithelial cell according to the invention to screen for activators or inhibitors of the T1R2 and/or T1R3 ligand binding and/or to screen for activators or inhibitors of the signalling pathway of the T1R2 and/or T1R3 receptors. Preferably, said isolated intestinal epithelial cell is STC-1 cell or GLUtag cell. As a non-limiting example, said screening can be performed by placing a reporter gene under control of a T1R2 and/or T1R3 responsive promoter. A Reporter gene as used here means any gene that leads to a detectable signal and can be, as a non-limiting example, an antibiotic resistance gene, a toxin gene resulting in cell death, a gene encoding a fluorescent protein such as GFP, or a gene encoding an enzyme activity such as β-galactosidase. The coding sequence is placed under control of a T1R2 and/or T1R3 responsive promoter, i.e. a promoter that is induced by binding of a ligand to the receptor and consequent induction of the signalling pathway. The induction of the reporter may be direct or indirect. A direct induction means that the reporter gene is induced by the signalling pathway, which is activated upon binding of the ligand to the receptor. An indirect induction means that, upon binding of the ligand to the receptor, an intermediate compound is synthesized by the cell, which is secreted and activates a second receptor situated either on the same cell or on another cell type, whereby the activation of the second receptor will induce the reporter gene.
Preferably, said T R2 and/or T1R3 responsive promoter is the SGLT1 promoter. A preferred embodiment is a screening system, comprising (a) exposing cells expressing T1R family members and/or Gαgust (cell A) to a defined level of glucose, which is activating the receptor; (b) contacting said cells with a possible inhibitor or activator (c) removing samples of media and treat the culture of enterocytes comprising a reporter gene operably linked to the SGLT1 promoter (cell B). The read out is the expression of the reporter gene in the enterocytes (cell B) in response to the presence of the inhibitor or activator, using the normal induction (without addition of an inhibitor or activator) as control.
Sections of mouse proximal small intestine were treated as described in the methods and hybridised with digoxigenin-labelled anti-sense riboprobes to unique sequences of the T1R1, T1R2 and T1R3 taste receptors and α-gustducin protein coding regions. Signals were developed using NBT/BCIP and sections were counter-stained with methyl green. Scale bars represent 20 μm.
Male CD-1 and C57BL/6 mice, six weeks old, from Charles River Laboratories were used. The α-gustducin knock out mouse was described by Wong et al. (1996); the T1R3 knock out mouse was described by Damak et al. 2003).
High and low carbohydrate diets were resp. TestDiet® 5810 and TestDiet® 5787-9. For the sucralose test, the low carbohydrate diet was supplemented with sucralose (1,6-dichloro-1,6-dideoxy-beta-D-fructofuranosyl-4-chloro-4-deoxy-alpha-galactopyranoside) at 2 mM.
Animals were killed by concussion followed by cervical dislocation. The entire small intestine was removed and flushed with ice-cold 0.9% NaCl, opened longitudinally, rinsed in saline and mucous removed by blotting. The small intestine was then divided into proximal, mid and distal sections and the mucosa removed by scraping. Mucosal scrapings were frozen immediately in liquid nitrogen and stored at −80° C. until use.
Sections (1 cm) for immunohistochemistry and in situ hybridisation histochemistry were placed in PBS plus 4% paraformaldehyde.
For investigation of expression along the crypt-villus axis cell populations were removed by the technique of Meddings et al. (1990) adapted for use at 4° C.
As positive controls mouse tongues were removed and the epithelium dissected away from the muscle and frozen immediately in liquid nitrogen, or placed in PBS plus 4% paraformaldehyde.
RNA was isolated from intestinal mucosal scrapings using the Qiagen RNeasy Mini Kit with on-column DNase 1 digestion. Poly (A+) RNA was isolated from total RNA using the Qiagen mRNA isolation kit. RT-PCR was performed on 25 ng of mRNA in a single tube reaction with primers designed to homologous regions of the mouse, rat and human sweet taste GPCRs T1R1, T1R2, T1R3 and the G-protein α-gustducin (Gαgustt). PCR products were cloned into pGEM-T and sequenced. CLUSTALW alignment of the DNA sequences was performed using Vector NTi Suite (Informax).
Using the Primer Express software programme (Applied Biosystems) PCR primers and probes (FAM/TAMRA labelled) for the amplification of T1R1, T1R2, T1R3, Gαgust, and the Na+/glucose co-transporter (SGLT1), along with β-actin (JOE/TAMRA labelled) were designed. Primers and probes were purchased from Eurogentec, along with 18S ribosomal RNA controls.
cDNA was synthesised from either total RNA or mRNA using Supercript III reverse transcriptase (Invitrogen) and either oligo(dT)12-18 or random primers, cleaned up using the Machery-Nagel Nucleospin extract kit and 50 ng of cDNA used per reaction.
For Real-Time PCR reactions the enzyme was activated by heating at 95° C. for 2 min. A two-step PCR procedure was used, 15 s at 95° C. and 60 s at 60° C. for 45 cycles in a PCR mix containing 5 μl of cDNA template, 1×Jumpstart qPCR master mix (Sigma-Aldrich), 900 nM of each primer and 250 nM probe in a total volume of 25 μl. Where multiplex reactions were performed the β-actin primers were primer limiting and used at 600 nM. All reactions were performed in a RotorGene 3000 (Corbett Research).
Brush-border membrane vesicles were isolated from intestinal mucosal scrapings and isolated cells by the cation precipitation, differential centrifugation technique described previously (Shirazi-Beechey et al. 1990). Membrane proteins were denatured in SDS-PAGE sample buffer (20 mM Tris/HCl, pH 6.8, 6% SDS, 4% 2-mercaptoethanol and 10% glycerol) by heating at 95° C. for 4 min and were separated on 8% polyacrylamide gels and electrotransferred to PVDF membranes. Membranes were blocked by incubation in TTBS plus 5% non-fat milk for 60 min. Membranes were incubated for 60 min with antisera to SGLT1, T1R2 (Santa-Cruz), T1R3 (AbCam), Gαgust (Santa-Cruz), villin (The Binding Site), and β-actin (Sigma-Aldrich) in TTBS containing 0.5% non-fat milk. Immunoreactive bands were visualised by using horseradish peroxidase-conjugated secondary antibodies and enhanced chemiluminescence (Amersham Biosciences). Scanning densitometry was performed using Phoretix 1D (Non-Linear Dynamics).
Tissue sections (fixed for 6 hours in 4% (w/v) paraformaldehyde in PBS) were paraffin wax-embedded and sectioned at a thickness of 5-7 μm onto Poly-L-lysine-coated slides.
Slides were then de-waxed as follows: 3×5 minutes in xylene; 2×5 minutes in absolute ethanol, 2×5 minutes in 70% (w/v) ethanol, 2×5 minutes in dd H2O. Washes, 2×5 minute, in PBS were performed before antigen retrieval by autoclaving in 10 mM Tris buffer (pH 10) for 11 minutes. A further 2×5 minute washes were performed and then endogenous H2O2 was blocked by incubation in 3% H2O2/PBS for 15 minutes. Another 2×5 minute washes in PBS were carried out and then a 1 hour incubation at room temperature in a humidity chamber in 5% BSA/PBS to block non-specific protein-binding sites in the tissue sections.
The slides were then incubated in primary antibody diluted in 1% BSA/PBS (1:50 for both α-gustducin and T1R2) at room temperature overnight in a humidity chamber. 3×5 minute washes in PBS were performed prior to incubation in HRP-conjugated swine anti-rabbit secondary antibody (DAKO) for α-gustducin or HRP-conjugated rabbit anti-goat for T1R2, diluted 1:200 in 1% BSA/PBS for 1 hour at room temperature in a humidity chamber. A further 3×5 minute washes were performed and then the slides were developed in 0.05% DAB/0.03% H2O2/0.05M Tris-HCl pH 7.6 for 2-10 minutes at room temperature in a humidity chamber in the dark.
The slides were then counterstained in 1% chloroform-extracted methyl green for 5 minutes. The dye was rinsed off in running tap water, slides allowed to slowly air dry and were then mounted/cover-slipped using DPX (Raymond Lamb).
Tissue sections of varying fixation times (12-48 hours) were paraffin wax-embedded and microtomed at a thickness of 5-7 μm onto APES or Poly-L-lysine-coated slides. Slides were then de-waxed in xylene and then rehydrated through graded ethanol to dd H2O.
The tissue was then permeabilised as follows: 20 min wash in 200 mM HCl; 2×3 min washes in 2×SSC; 3 min equilibration in Proteinase K Buffer (0.05M Tris/HCl pH 7.4); 1 hour incubation at 37° C. in Proteinase K Buffer containing 0-10 μg/ml (determined empirically) Proteinase K (Sigma); 2×3 minute washes in 0.2% Glycine/PBS; rinse in PBS.
Anticipated background was reduced as follows: 3 min equilibration in 0.1M Triethanolamine pH 8.0; 10 min wash in 0.1M Triethanolamine pH 8.0 containing 0.25% (v/v) acetic anhydride (added fresh); rinse in PBS; post-fixation in 4% paraformaldehyde/PBS; 1 min block of endogenous alkaline phosphatase in 20% acetic acid; rinse in PBS.
The slides were then pre-hybridised in a hybridisation buffer (50% de-ionised formamide, 300 mM NaCl, 20 mM Tris/HCl pH 8.0, 5 mM EDTA, 1×Denhardt's, 1×RNA Protect (Sigma), 100 mg/ml dextran sulphate) for 1 hour at 60° C. The slides were then hybridised overnight at 50° C. in hybridisation buffer containing 100 μg/ml tRNA and 50-500 ng/ml probe (determined empirically).
After hybridisation, the following stringency washes were performed: 1 hour wash in 2×SSC; 4 hour wash at 50° C. in Riboprobe Wash Buffer (300 mM NaCl, 200 mM Tris/HCl pH 8.0, 10 mM EDTA, 50% formamide, 1×Denhardt's); overnight wash at 50° C. in Riboprobe Wash Buffer; 30 min wash in 2×SSC; 30 min wash in 0.1×SSC.
The slides were then subjected to the following detection procedure: 5 min equilibration in DIG-AP Buffer (100 mM Tris/HCl pH 7.5, 150 mM NaCl); 30 min block in DIG-AP Buffer containing 0.5% (w/v) DIG Blocking Reagent (Roche); 2 hour incubation in DIG-AP Buffer containing 0.5% (w/v) DIG Blocking Reagent (Roche) and anti-DIG AP-conjugated antibody diluted 1:1000; 5 min wash in DIG-AP Buffer; 5 min equilibration in NBT/BCIP Buffer (100 mM Tris/HCl pH 9.5, 100 mM NaCl); 1-24 hour incubation (determined empirically) in the dark in NBT/BCIP Buffer containing NBT/BCIP Mixture (Roche) diluted 1:50; 5 min wash in 10 mM Tris/HCl pH 8.0, 1 mM EDTA.
Slides were then rinsed in tap water, counter-stained in chloroform-extracted 1% methyl green for 5 minutes, washed in tap water, slowly air dried and then cover-slipped and mounted in Glass Bond (Loctite). Slides were viewed using a Nikon Edipse 400 microscope and images captured with a Nikon DXM1200 digital camera.
To examine T1R expression in the small intestine RT-PCR was performed on mRNA isolated from mucosal scrapings of the proximal small intestine of CD-1 mice using specific primers based on the mouse, rat and human sequences. PCR products of the predicted size, 1127 bp for T1R1, 756 bp for T1R2, 855 bp for T1R3 and 900 bp for G %.t were cloned and sequenced. Sequence analysis confirmed that all were 100% homologous to the reported mouse sequences cloned from taste-buds on the tongue. This indicates that taste receptors are expressed in the proximal part of the small intestine.
To determine the expression of the T1R family members and Gαgust throughout the small intestine, and along the crypt-villus axis, the technique of real-time PCR was used with primers and probes designed specifically to detect mouse T1R1, T1R2, T1R3 and Gαgust. Results indicated that all members of the T1R family and gustducin are expressed along the length of the small intestine. Expression levels were low (equivalent to those seen in the tongue) and suggested that the receptors are expressed in only a sub-population of cells rather than in all intestinal cells along the crypt-villus axis (this conclusion is supported by immunohistochemical data). Expression patterns along the crypt-villus axis indicated that expression of T1R2/T1R3 (receptors known to taste sweets) was higher in the villus cell fractions than in the crypts. T1R1 (a component of the umami taste receptor) appear to have a different pattern of expression (see
Expression of the proteins was investigated using the commercially available antibodies to T1R2, T1R3 and Gαgust. Western blotting indicated that the antibodies to T1R2 and T1R3 each identified a single protein in the purified intestinal brush-border membrane vesicles which was present in the proximal mid and distal small intestinal fractions, with slightly higher levels in the mid small intestine. The Gαgust antibody identified two bands of approximately 55 kDa and 110 kDa in the brush-border membrane vesicles from proximal, mid and distal intestine, again with slightly higher levels in the mid small intestine. All data was normalised to the expression of β-actin. This indicates that Gαgust, T1R2 and T1R3 are expressed on the luminal membrane of gut cells, with higher expression in the jejunum (see
Cells isolated from along the crypt-villus axis were western blotted for the expression of SGLT1 and villin, well characterised markers of enterocyte differentiation. The crypt-villus expression of these markers was as previously reported and indicated that the cell fractions were derived from the expected upper villus, mid-to-lower villus and crypt regions. Gαgust expression along the crypt-villus axis indicated that protein expression increased towards the upper part of the villus being lowest in the crypt and correlated with the Gαgust mRNA expression determined by real-time PCR (see
Using real time quantitative PCR and western blot analysis we have demonstrated the presence of T1R1, T1R2, T1R3 and α-gustducin throughout the human small intestine. T1R5 and α-gustducin proteins are associated with the luminal membrane. Very low levels of T1R5 and α-gustducin are also detected in human colon
Employing techniques of in situ hybridisation and immunohistochemistry, we have shown that T1R5 and α-gustducin are co-expressed at protein and mRNA levels in a sub-population of cells along the crypt-villus axis of mouse small intestine rather than being expressed in the entire enterocyte population. The T1R and α-gustducin proteins are associated with the luminal membrane of the same cells. Typical results of in situ hybridization are shown in
To investigate any direct links between T1R5, α-gustducin, and SGLT1 expression, we performed dietary trials on T1R3−/− and α-gustducin−/− knock-out mice.
Firstly, groups of wild-type and T1R3 and α-gustducin “knock-out” (KO) mice (Damak et al. 2003; Wong et al. 1996) were placed on standard diets with the same carbohydrate composition for two weeks. After this time the mice were killed and the small intestine removed, divided into proximal, mid and distal regions, and SGLT1 expression at the levels of mRNA and protein was measured. The rates of glucose transport were also determined in brush-border membrane vesicles isolated from the tissues.
There were no differences in the levels of SGLT1 mRNA, SGLT1 protein and glucose transport in the intestine of wild-type and KO mice. Therefore all animals had the capacity to absorb dietary sugars. This was evident since neither groups showed any signs of intestinal malabsorption.
Second, groups of wildtype and T1R3 and α-gustducin KO mice were placed on each of three iso-caloric diets a) low carbohydrate, b) high carbohydrate, and c) low carbohydrate+artificial sweetener (sucralose), for two weeks. After this time the mice were killed and the small intestines were removed, divided into proximal, mid and distal regions, and SGLT1 expression, at protein and mRNA levels, were measured in each. The results are shown in
Our data show that SGLT1 protein expression is also increased in response to both high carbohydrate and low carbohydrate+sucralose diets (
| Number | Date | Country | Kind |
|---|---|---|---|
| 04077610.6 | Sep 2004 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/54760 | 9/22/2005 | WO | 00 | 3/22/2007 |
