The identification and spatial distribution of different nutrient receptors along the gastrointestinal tract

The gastrointestinal tract generates many signals that play a role in the
regulation of eating behavior, most importantly satiety signals. The gut is
therefore an appealing target for food products to induce satiety and reduce
food intake. Gut peptides, like cholecystokinin (CCK) and glucagon-like
peptide-1 (GLP-1) are important mediators of the satiety signalling. CCK and
GLP-1 have been demonstrated to reduce food intake and hunger after intravenous
administration [1-3]. Long-term use of a GLP-1 agonist has been shown to reduce
body weight in obese individuals [4]. Together these results illustrate the
potential of targeting the gastrointestinal tract in weight management and
weight loss strategies.

In order for different nutrients to influence satiety, the presence of these
nutrients in the small intestine has to be sensed. There appear to be two major
principles of nutrient sensing in the gastrointestinal tract [5]. Firstly,
nutrients or their direct breakdown products in the lumen of the gut can
interact with receptors on the microvilli of enteroendocrine cells. These
enteroendocrine cells respond by basal side secretion of mediators including
cholecystokinin (CCK), Peptide YY (PYY) and Glucagon Like Peptide-1 (GLP-1)
which are either transported through the blood stream or activate their
receptors on vagal nerve endings. In parallel, a second mechanism is operating.
Nutrients (lipids, amino acids) are taken up by enterocytes, where they can
subsequently be converted to nutrient-derived mediators. These mediators also
interact with receptors on vagal nerve endings.

Many different receptors have been suggested to be involved in the sensing of
carbohydrates, proteins and fats. We would like to investigate those receptors
potentially involved in satiety signalling. For carbohydrates the sweet
receptor and glucose transporter have been suggested to induce GLP-1 secretion.
The sweet receptor, a heterodimer of T1R2 and T1R3, has been observed to be
located on the enteroendocrine cells of the gastrointestinal tract [6-8].
However, this might not be the sole carbohydrate sensing receptor. The glucose
transporter, SGLT-1, is also involved in GLP-1 secretion in response to glucose
[9].
G-protein coupled receptor 120 has been described as an important fatty acid
sensing receptor in the gastrointestinal tract. Activation of this receptor
results in GLP-1 and CCK secretion [10, 11].
Proteins and amino acids are more potent in inducing a satiety signal then
carbohydrates and fat, therefore we would like to investigate the distribution
of several potential protein sensing receptors [12]. The oligopeptide
transporter, Pept1, is highly expressed in the small intestine of humans and
has been implicated to be essential for the stimulation of the vagal nerve in
response to peptides [13]. Also the umami receptor, a heterodimer of T1R1 and
T1R3, is expressed in the intestine. Other potential peptide sensing receptors,
which have not been investigated in the human intestine, are G-protein coupled
receptor 93, the calcium sensing receptor and the G-protein coupled receptor
C6A. These are found in the gastrointestinal tract of rodents and observed to
induce GLP-1 or CCK secretion in enteroendocrine cells lines [14].
As mentioned before, not only nutrients but also their derivates are able to
induce satiety. The receptors implicated to be involved in sensing these
derivates are the cannabinoid receptor 1 and the G-protein coupled receptor 119
[15, 16].
Apart from the nutrient sensing receptors we would like to investigate the
receptors for CCK and GLP-1, these receptors are important in the further
signalling of the satiety signal via the vagal nerve towards the brain [17].

Investigating which receptors are involved in the nutient sensing of
enteroendocrine cells and on nerve endings, gives further knowledge on the kind
of nutrients responsible for the secretion of gut hormones involved in satiety
signalling. So far only little is know about the distribution of potential
nutient sensing receptors along the gastrointestinal tract. We are interested
in the intestinal parts which highly secrete CCK and GLP-1 [18, 19]. By
comparing these tissues, it is expected to further elucidate which receptors
are important for satiety signalling in the intestine.
Having this information gives rise to further reseach on how these receptors
stimulate the secretion of gut hormones. Moreover, knowing which receptors are
involved in satiety signalling, gives oppertunity for targeting these receptors
to increase satiety and influencing food intake.
We aim to investigate the distribution of the above mentioned receptors in the
following referred to as *nutrient sensing receptors*, which have been
suggested to be involved in satiety signalling. For these receptors the gene
expression, localisation and protein expression will be determined at different
locations. This will be measured in intestinal biopsies of two locations in the
duodenum, in the terminal ileum, ascending colon, transverse colon and
descending colon.

Hypotheses:
- The nutrient sensing receptors have a different spatial distribution along
the intestinal tract.
- The receptors: T1R2/T1R3, SGLT-1, G-protein coupled receptor 120, T1R1/T1R3,
Pept1, G-protein coupled receptor 93, calcium sensing receptor, G-protein
coupled receptor C6A, G-protein coupled receptor 119, cannabinoid receptor 1,
CCK-1 receptor and GLP-1 receptor are expected to be located along the
intestine.
- The receptors: T1R2/T1R3, SGLT-1, G-protein coupled receptor 120, T1R1/T1R3,
Pept1, G-protein coupled receptor 93, calcium sensing receptor, G-protein
coupled receptor C6A and G-protein coupled receptor 119 are expected to be
located on the enteroendocrine cells.
- Cannabinoid receptor 1, CCK-1 receptor and GLP-1 receptor are expected to be
located on the vagal nerve endings.

This study is designed as an observational study with invasive measurements

All patients undergo gastroduodenoscopy or colonoscopy for a medical reason.
Only difference with regular procedure is taking 6 or 12 extra biopsies at the
end of the procedure. This will extend the duration of the endoscopies with 2-3
minutes.

Gastroduodenoscopy
The gastroduodenoscopy, performed by a gastroenterologist, is a standard
procedure that takes 10 to 20 minutes. These patients have a medical indication
to undergo a gastroduodenoscopy. The only difference, with the standard
procedure, is that 6 extra biopsies will be taken (with a standard biopsy
forceps). Diagnostic upper GI endoscopy is a remarkably safe procedure. One
large US study estimated an overall complication rate (including mucosal
biopsy) of 0.13% and an associated mortality of 0.004%. Taking the additional
biopsies will be the only extra risk for the patient. We would like to include
patients who already need to undergo a gastroduodenoscopy for a medical reason.
By this mean we can diminish the risk of a gastroduodenoscopy for healthy
volunteers.

Colonoscopy
In patients undergoing colonoscopies for a medical reason (which is the case in
the patients used in this study), there is a very small risk (ranging from
0.016-%0.2%) of bowel perforation. Shiffman et al conducted a study on the risk
of bleeding after endoscopic biopsy or polypectomy. They found that 4.6% of all
patients (32 of 694) reported bleeding, 28 had a minor and self-limited,
clinically insignificant bleeding and 4 (0.58%) had a major bleeding which
required hospitalization or treatment. All 4 of these patients had undergone
colonic polypectomy. Since the colonoscopy in these patients is performed
because of medical reasons (not for research reasons), the patients will be
informed about these risks by the gastroenterologist. We are taking a few (12)
extra small biopsies (with standard forceps), and therefore we expect that the
risk of adverse events because of these extra biopsies would be much lower than
0.58%