Neuropeptide Y, Y1, Y2 and Y4 receptors mediate Y agonist responses in isolated human colon mucosa

Illuminating the neuropeptide Y4 receptor and its ligand pancreatic polypeptide from a structural, functional, and therapeutic perspective

The neuropeptide Y4 receptor (Y4R), a rhodopsin-like G protein-coupled receptor (GPCR) and the hormone pancreatic polypeptide (PP) are members of the neuropeptide Y family consisting of four receptors (Y1R, Y2R, Y4R, Y5R) and three highly homologous peptide ligands (neuropeptide Y, peptide YY, PP). In this family, the Y4R is of particular interest as it is the only subtype with high affinity to PP over NPY. The Y4R, as a mediator of PP signaling, has a pivotal role in appetite regulation and energy homeostasis, offering potential avenues for the treatment of metabolic disorders such as obesity. PP as anorexigenic peptide is released postprandial from the pancreas in response to food intake, induces satiety signals and contributes to hamper excessive food intake. Moreover, this system was also described to be associated with different types of cancer: overexpression of Y4R have been found in human adenocarcinoma cells, while elevated levels of PP are related to the development of pancreatic endocrine tumors. The pharmacological relevance of the Y4R advanced the search for potent and selective ligands for this receptor subtype, which will be significantly progressed through the elucidation of the active state PP-Y4R cryo-EM structure. This review summarizes the development of novel PP-derived ligands, like Obinepitide as dual Y2R/Y4R agonist in clinical trials or UR-AK86c as small hexapeptide agonist with picomolar affinity, as well as the first allosteric modulators that selectively target the Y4R, e.g. VU0506013 as potent Y4R positive allosteric modulator or (S)-VU0637120 as allosteric antagonist. Here, we provide valuable insights into the complex physiological functions of the Y4R and PP and the pharmacological relevance of the system in appetite regulation to open up new avenues for the development of tool compounds for targeted therapies with potential applications in metabolic disorders.

Appetite- and Weight-Regulating Neuroendocrine Circuitry in Hypothalamic Obesity

Since hypothalamic obesity (HyOb) was first described over 120 years ago by Joseph Babinski and Alfred Fröhlich, advances in molecular genetic laboratory techniques have allowed us to elucidate various components of the intricate neurocircuitry governing appetite and weight regulation connecting the hypothalamus, pituitary gland, brainstem, adipose tissue, pancreas, and gastrointestinal tract. On a background of an increasing prevalence of population-level common obesity, the number of survivors of congenital (eg, septo-optic dysplasia, Prader-Willi syndrome) and acquired (eg, central nervous system tumors) hypothalamic disorders is increasing, thanks to earlier diagnosis and management as well as better oncological therapies. Although to date the discovery of several appetite-regulating peptides has led to the development of a range of targeted molecular therapies for monogenic obesity syndromes, outside of these disorders these discoveries have not translated into the development of efficacious treatments for other forms of HyOb. This review aims to summarize our current understanding of the neuroendocrine physiology of appetite and weight regulation, and explore our current understanding of the pathophysiology of HyOb.

Peptide Tyrosine-Tyrosine Triggers GLP-2-Mediated Intestinal Hypertrophy After Roux-en-Y Gastric Bypass

Purpose 

Intestinal remodeling and adaptation of the alimentary limb after Roux-en-Y gastric bypass (RYGB) play an important role in the pathophysiological events that lead to type 2 diabetes mellitus (T2DM) improvement. Intestinal absorptive loop hypertrophy and growth following surgery have been related to GLP-2 secretion by ileal L-cells. The secretion of peptide tyrosine-tyrosine (PYY) enterohormone after a meal has been proposed as a trigger for ileal secretion of GLP-1. Our aim is to determine the role of PYY as a GLP-2 secretion modulator as an adaptation result in the alimentary limb after RYGB.

Method

We used a non-obese euglycemic rodent model. Circulating glucose, insulin, PYY, and GLP-2 were measured in the experimental and control groups. We used four groups: fasting control, Sham-operated, RYGB-operated (RYGB), and RYGB-operated and treated with BIIE0246 (RYGB + BII). BIIE0246 is a NPY2 receptor antagonist in L-cells. Intestinal glucose transporters and GLP-1 and PYY gut expression and hypertrophy were analyzed after 12 weeks of surgery.

Results

RYGB increased PYY3-36 plasma levels in rats with or without BII treatment. A high-insulin response was observed in the RYGB group but not in the control or RYGB + BII groups. BIIE0246 treatment limited plasma GLP-2 levels. In the alimentary intestinal limb, hypertrophy and SGLT1 and GLUT1 expression appeared to be reduced after RYGB compared to controls.

Conclusion

The postprandial ileal PYY secretion is enhanced after RYGB. This increase mediates GLP-2 release through its binding to the Y2 receptor on L-cells. This mechanism plays a role in alimentary limb hypertrophy after surgery.

Graphical abstract

Intestinal secretory mechanisms and diarrhea

One of the primary functions of the intestinal epithelium is to transport fluid and electrolytes to and from the luminal contents. Under normal circumstances, absorptive and secretory processes are tightly regulated such that absorption predominates, thereby enabling conservation of the large volumes of water that pass through the intestine each day. However, in conditions of secretory diarrhea, this balance becomes dysregulated, so that fluid secretion, driven primarily by Cl- secretion, overwhelms absorptive capacity, leading to increased loss of water in the stool. Secretory diarrheas are common and include those induced by pathogenic bacteria and viruses, allergens, and disruptions to bile acid homeostasis, or as a side effect of many drugs. Here, we review the cellular and molecular mechanisms by which Cl- and fluid secretion in the intestine are regulated, how these mechanisms become dysregulated in conditions of secretory diarrhea, currently available and emerging therapeutic approaches, and how new strategies to exploit intestinal secretory mechanisms are successfully being used in the treatment of constipation.

Fecal microbiota transplantation for irritable bowel syndrome: An intervention for the 21st century

Irritable bowel syndrome (IBS) affects about 12% of the global population. Although IBS does not develop into a serious disease or increase mortality, it results in a considerable reduction in the quality of life. The etiology of IBS is not known, but the intestinal microbiota appears to play a pivotal role in its pathophysiology. There is no effective treatment for IBS, and so the applied treatments clinically focus on symptom relief. Fecal microbiota transplantation (FMT), an old Chinese treatment, has been applied to IBS patients in seven randomized controlled trials (RCTs). Positive effects on IBS symptoms in various degrees were obtained in four of these RCTs, while there was no effect in the remaining three. Across the seven RCTs there were marked differences in the selection processes for the donor and treated patients, the transplant dose, the route of administration, and the methods used to measure how the patients responded to FMT. The present frontier discusses these differences and proposes: (1) criteria for selecting an effective donor (superdonor); (2) selection criteria for patients that are suitable for FMT; (3) the optimal FMT dose; and (4) the route of transplant administration. FMT appears to be safe, with only mild, self-limiting side effects of abdominal pain, cramping, tenderness, diarrhea, and constipation. Although it is early to speculate about the mechanisms underlying the effects of FMT, the available data suggest that changes in the intestinal bacteria accompanied by changes in fermentation patterns and fermentation products (specifically short-chain fatty acids) play an important role in improving the IBS symptoms seen after FMT. FMT appears to be a promising treatment for IBS, but further studies are needed before it can be applied in everyday clinical practice.

Dysregulation of epithelial ion transport and neurochemical changes in the colon of a parkinsonian primate

The pathological changes underlying gastrointestinal (GI) dysfunction in Parkinson’s disease (PD) are poorly understood and the symptoms remain inadequately treated. In this study we compared the functional and neurochemical changes in the enteric nervous system in the colon of adult, L-DOPA-responsive, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated common marmoset, with naïve controls. Measurement of mucosal vectorial ion transport, spontaneous longitudinal smooth muscle activity and immunohistochemical assessment of intrinsic innervation were each performed in discrete colonic regions of naïve and MPTP-treated marmosets. The basal short circuit current (I_sc) was lower in MPTP-treated colonic mucosa while mucosal resistance was unchanged. There was no difference in basal cholinergic tone, however, there was an increased excitatory cholinergic response in MPTP-treated tissues when NOS was blocked with L-Nω-nitroarginine. The amplitude and frequency of spontaneous contractions in longitudinal smooth muscle as well as carbachol-evoked post-junctional contractile responses were unaltered, despite a decrease in choline acetyltransferase and an increase in the vasoactive intestinal polypeptide neuron numbers per ganglion in the proximal colon. There was a low-level inflammation in the proximal but not the distal colon accompanied by a change in α-synuclein immunoreactivity. This study suggests that MPTP treatment produces long-term alterations in colonic mucosal function associated with amplified muscarinic mucosal activity but decreased cholinergic innervation in myenteric plexi and increased nitrergic enteric neurotransmission. This suggests that long-term changes in either central or peripheral dopaminergic neurotransmission may lead to adaptive changes in colonic function resulting in alterations in ion transport across mucosal epithelia that may result in GI dysfunction in PD.

Possible role of peptide YY (PYY) in the pathophysiology of irritable bowel syndrome (IBS)

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder of unknown aetiology for which there is no effective treatment. Although IBS does not increase mortality, it reduces the quality of life and is an economic burden to both the patients themselves and society as a whole. Peptide YY (PYY) is localized in endocrine cells located in the ileum, colon and rectum. The concentration of PYY and the density of PYY cells are decreased in both the colon and rectum but unchanged in the ileum of patients with IBS. The low density of PYY cells in the large intestine may be caused by a decreased number of stem cells and their progeny toward endocrine cells. PYY regulates the intestinal motility, secretion and absorption as well as visceral sensitivity via modulating serotonin release. An abnormality in PYY may therefore contribute to the intestinal dysmotility and visceral hypersensitivity seen in IBS patients. Diet management involving consuming a low-FODMAP diet restores the density of PYY cells in the large intestine and improves abdominal symptoms in patients with IBS. This review shows that diet management appears to be a valuable tool for correcting the PYY abnormalities in the large intestine of IBS patients in the clinic.

Discovery of small molecule agonists targeting neuropeptide Y4 receptor using homology modeling and virtual screening

Neuropeptide Y4 receptor has the most significant effect on body weight and fat mass in its physiological functions, and the activation of Y4 receptor has explicit role on losing weight. The Y4 receptor has been successfully applied in the development of anti-obesity agent, thus representing a potential therapeutic target for obesity treatment. Here, we reported the first discovery of small molecule agonists targeting Y4 receptor: three Y4 receptor models with active and inactive conformations were built, each model was submitted following structure-based virtual screening, and finally six hits were identified as Y4 receptor agonists. These results confirm the reliability of the constructed Y4 receptor models and the proposed computational strategy for investigating novel Y4 receptor agonists. These new small molecule Y4 receptor agonists will contribute to the further development of Y4 agonists as potential therapeutics and functional probes.

Signaling of free fatty acid receptors 2 and 3 differs in colonic mucosa following selective agonism or coagonism by luminal propionate

BACKGROUND

Propionate exhibits affinity for free fatty acid receptor 2 (FFA2, formerly GPR43) and FFA3 (GPR41). These two G protein-coupled receptors (GPCRs) are expressed by enteroendocrine L cells that contain anorectic peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), while FFA3 is also expressed by enteric neurons. Few studies have investigated the individual roles of FFA2 and FFA3 in propionate's gastrointestinal (GI) effects. Here, we compared FFA2, FFA3, and propionate mucosal responses utilizing selective ligands including an FFA3 antagonist, in mouse and human colonic mucosa.

METHODS

Vectorial ion transport was measured in native colonic preparations from normal mouse and human colon with intact submucosal innervation. Endogenous fecal pellet propulsion was monitored in colons isolated from wild-type (WT) and PYY-/- mice.

KEY RESULTS

FFA2 and FFA3 signaling differed significantly. FFA2 agonism involved endogenous L cell-derived PYY and was glucose dependent, while FFA3 agonism was independent of PYY and glucose, but required submucosal enteric neurons for activity. Tonic FFA3 activity was observed in mouse and human colon mucosa. Apical propionate responses were a combination of FFA2-PYY mediation and FFA3 neuronal GLP-1- and CGRP-dependent signaling in mouse ascending colon mucosa. Propionate also slowed WT and PYY-/- colonic transit, and this effect was blocked by a GLP-1 receptor antagonist.

CONCLUSIONS & INFERENCES

We conclude that luminal propionate costimulates FFA2 and FFA3 pathways, reducing anion secretion and slowing colonic motility; FFA2 via PYY mediation and FFA3 signaling by activation of enteric sensory neurons.

Bidirectional GPR119 Agonism Requires Peptide YY and Glucose for Activity in Mouse and Human Colon Mucosa

The lipid sensor G protein-coupled receptor 119 (GPR119) is highly expressed by enteroendocrine L-cells and pancreatic β-cells that release the hormones, peptide YY (PYY) and glucagonlike peptide 1, and insulin, respectively. Endogenous oleoylethanolamide (OEA) and the dietary metabolite, 2-monoacylglycerol (2-OG), can each activate GPR119. Here, we compared mucosal responses with selective, synthetic GPR119 agonists (AR440006 and AR231453) and the lipids, OEA, 2-OG, and N-oleoyldopamine (OLDA), monitoring epithelial ion transport as a readout for L-cell activity in native mouse and human gastrointestinal (GI) mucosae. We also assessed GPR119 modulation of colonic motility in wild-type (WT), GPR119-deficient (GPR119-/-), and PYY-deficient (PYY-/-) mice. The water-soluble GPR119 agonist, AR440006 (that cannot traverse epithelial tight junctions), elicited responses, when added apically or basolaterally in mouse and human colonic mucosae. In both species, GPR119 responses were PYY, Y1 receptor mediated, and glucose dependent. AR440006 efficacy matched the GI distribution of L-cells in WT tissues but was absent from GPR119-/- tissue. OEA and 2-OG responses were significantly reduced in the GPR119-/- colon, but OLDA responses were unchanged. Alternative L-cell activation via free fatty acid receptors 1, 3, and 4 and the G protein-coupled bile acid receptor TGR5 or by the melanocortin 4 receptor, was unchanged in GPR119-/- tissues. The GPR119 agonist slowed transit in WT but not the PYY-/- colon in vitro. AR440006 (intraperitoneally) slowed WT colonic and upper-GI transit significantly in vivo. These data indicate that luminal or blood-borne GPR119 agonism can stimulate L-cell PYY release with paracrine consequences and slower motility. We suggest that this glucose-dependent L-cell response to a gut-restricted GPR119 stimulus has potential therapeutic advantage in modulating insulinotropic signaling with reduced risk of hypoglycemia.

Agonism of free fatty acid receptors 1 and 4 generates peptide YY-mediated inhibitory responses in mouse colon

BACKGROUND AND PURPOSE

Free fatty acid receptors FFA1 and FFA4 are located on enteroendocrine L cells with the highest gastrointestinal (GI) expression in descending colon. Their activation causes the release of glucagon-like peptide 1 and peptide YY (PYY) from L cells. Additionally, FFA1 agonism releases insulin from pancreatic β cells. As these receptors are modulators of nutrient-stimulated glucose regulation, the aim of this study was to compare the pharmacology of commercially available agonists (TUG424, TUG891, GW9508) with proven selective agonists (JTT, TAK-875, AZ423, Metabolex-36) in mice.

EXPERIMENTAL APPROACH

Mouse mucosa was mounted in Ussing chambers, voltage-clamped and the resultant short-circuit current (I_sc ) was recorded continuously. Pretreatments included antagonists of FFA1, Y₁  or Y₂ receptors. Glucose sensitivity was investigated by mannitol replacement apically, and colonic and upper GI transit was assessed in vitro and in vivo.

KEY RESULTS

FFA1 and FFA4 agonism required glucose and reduced I_sc in a PYY-Y₁ receptor-dependent manner. The novel compounds were more potent than GW9508. The FFA1 antagonists (GW1100 and ANT825) blocked FFA1 activity only and revealed FFA1 tonic activity. The FFA4 agonist, Metabolex-36, slowed colonic transit in vitro but increased small intestinal transit in vivo.

CONCLUSIONS AND IMPLICATIONS

The selective FFA1 and FFA4 agonists were more potent at reducing I_sc than GW9508, a dual FFA1 and FFA4 agonist. A paracrine epithelial mechanism involving PYY-stimulated Y₁ receptors mediated their responses, which were glucose sensitive, potentially limiting hypoglycaemia. ANT825 revealed tonic activity and the possibility of endogenous FFA1 ligands causing PYY release. Finally, FFA4 agonism induced regional differences in transit.

Gastrointestinal neuroendocrine peptides/amines in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic recurrent condition whose etiology is unknown, and it includes ulcerative colitis, Crohn’s disease, and microscopic colitis. These three diseases differ in clinical manifestations, courses, and prognoses. IBD reduces the patients’ quality of life and is an economic burden to both the patients and society. Interactions between the gastrointestinal (GI) neuroendocrine peptides/amines (NEPA) and the immune system are believed to play an important role in the pathophysiology of IBD. Moreover, the interaction between GI NEPA and intestinal microbiota appears to play also a pivotal role in the pathophysiology of IBD. This review summarizes the available data on GI NEPA in IBD, and speculates on their possible role in the pathophysiology and the potential use of this information when developing treatments. GI NEPA serotonin, the neuropeptide Y family, and substance P are proinflammatory, while the chromogranin/secretogranin family, vasoactive intestinal peptide, somatostatin, and ghrelin are anti-inflammatory. Several innate and adaptive immune cells express these NEPA and/or have receptors to them. The GI NEPA are affected in patients with IBD and in animal models of human IBD. The GI NEPA are potentially useful for the diagnosis and follow-up of the activity of IBD, and are candidate targets for treatments of this disease.

Distribution and characterisation of CCK containing enteroendocrine cells of the mouse small and large intestine

There is general consensus that enteroendocrine cells, EEC, containing the enteric hormone cholecystokinin (CCK) are confined to the small intestine and predominate in the duodenum and jejunum. Contrary to this, EEC that express the gene for CCK have been isolated from the large intestine of the mouse and there is evidence for EEC that contain CCK-like immunoreactivity in the mouse colon. However, the human and rat colons do not contain CCK cells. In the current study, we use immunohistochemistry to investigate CCK peptide presence in endocrine cells, PCR to identify cck transcripts and chromatography to identify CCK peptide forms in the mouse small and large intestine. The colocalisation of CCK and 5-HT, hormones that have been hypothesised to derive from cells of different lineages, was also investigated. CCK immunoreactivity was found in EEC throughout the mouse small and large intestine but positive cells were rare in the rectum. Immunoreactive EEC were as common in the caecum and proximal colon as they were in the duodenum and jejunum. CCK gene transcripts were found in the mucosa throughout the intestine but mRNA for gastrin, a hormone that can bind some anti-CCK antibodies, was only found in the stomach and duodenum. Characterisation of CCK peptides of the colon by extraction, chromatographic separation and radioimmunoassay revealed bioactive amidated and sulphated forms, including CCK-8 and CCK-33. Moreover, CCK-containing EEC in the large intestine bound antibodies that target the biologically active sulfated form. Colocalisation of CCK and 5-HT occurred in a proportion of EEC throughout the small intestine and in the caecum but these hormones were not colocalised in the colon, where there was CCK and PYY colocalisation. It is concluded that authentic, biologically active, CCK occurs in EEC of the mouse large intestine.

High molecular weight PEGylation of human pancreatic polypeptide at position 22 improves stability and reduces food intake in mice

BACKGROUND AND PURPOSE

Human pancreatic polypeptide (hPP) is known to suppress appetite and food intake, thereby representing a potential therapeutic approach against obesity and associated metabolic disorders. The aim of this study was to improve hPP stability by covalent PEGylation with diverse molecular weight polyethylene glycols (PEGs) at two positions using promising lead structures while maintaining target activity.

EXPERIMENTAL APPROACH

Modified peptides were synthesized by combined solid-phase and solution-phase peptide synthesis. Their potency was investigated in constitutively expressing human epithelial cells and isolated human colonic mucosa as well as receptor-transfected artificial cell lines. Human blood plasma and porcine liver homogenates were used to examine the in vitro stability of the analogues. The most promising variants were injected s.c. in C57BL/6JRj mice to monitor fasting-induced food intake and bioavailability.

KEY RESULTS

In human epithelia and colonic mucosal preparations, activity of the modified hPP peptides depended on the core sequence and latency of the peptides was related to PEG size. Peptides modified with a 22 kDa PEG (PEG22) remained intact in blood plasma and on incubation with liver homogenates for more than 96 h. Finally, hPP_2-36 , [K²² (PEG22)]hPP_2-36 and [K²² (PEG22),Q³⁴ ]hPP significantly reduced cumulative food intake in mice over 16 h after s.c. administration.

CONCLUSIONS AND IMPLICATIONS

Modification with PEG22 at position 22 stabilizes hPP significantly while extending its biological activities and could be used in drug development prospectively.

Angiotensin II Type 1 Receptor-Dependent GLP-1 and PYY Secretion in Mice and Humans

Angiotensin II (Ang II) is the key hormone mediator of the renin angiotensin system, which regulates blood pressure and fluid and electrolyte balance in the body. Here we report that in the colonic epithelium, the Ang II type 1 receptor is highly and exclusively expressed in enteroendocrine L cells, which produce the gut hormones glucagon-like peptide-1 and peptide YY (PYY). Ang II stimulated glucagon-like peptide-1 and PYY release from primary cultures of mouse and human colon, which was antagonized by the specific Ang II type 1 receptor blocker candesartan. Ang II raised intracellular calcium levels in L cells in primary cultures, recorded by live-cell imaging of L cells specifically expressing the fluorescent calcium sensor GCaMP3. In Ussing chamber recordings, Ang II reduced short circuit currents in mouse distal colon preparations, which was antagonized by candesartan or a specific neuropeptide Y1 receptor inhibitor but insensitive to amiloride. We conclude that Ang II stimulates PYY secretion, in turn inhibiting epithelial anion fluxes, thereby reducing net fluid secretion into the colonic lumen. Our findings highlight an important role of colonic L cells in whole-body fluid homeostasis by controlling water loss through the intestine.

Neuroendocrine peptide mechanisms controlling intestinal epithelial function

Enteroendocrine cells (EECs) contain different combinations of hormones, which are released following stimulation of nutrient receptors that are selectively expressed by these cells. This chemosensation varies according to the intestinal area and species of interest, and responses to meals are rapidly modified following bariatric surgery. Such surgically-induced gastrointestinal (GI) changes highlight considerable enteroplasticity, however our understanding of even the acute physiological control and consequences of neuroendocrine peptide release is still under-developed. This review focuses on recent advances in nutrient G protein-coupled receptor (GPCR)-chemosensation in L cells, the patterns of peptide release and consequent changes in GI function. A clearer resolution of these mucosal mechanisms will shed light on potential receptor-target combinations that could provide less-invasive anti-diabesity strategies in future.

The role of the neuropeptide Y (NPY) family in the pathophysiology of inflammatory bowel disease (IBD)

Inflammatory bowel disease (IBD) includes three main disorders: ulcerative colitis, Crohn's disease, and microscopic colitis. The etiology of IBD is unknown and the current treatments are not completely satisfactory. Interactions between the gut neurohormones and the immune system are thought to play a pivot role in inflammation, especially in IBD. These neurohormones are believed to include members of the neuropeptide YY (NPY) family, which comprises NPY, peptide YY (PYY), and pancreatic polypeptide (PP). Understanding the role of these peptides may shed light on the pathophysiology of IBD and potentially yield an effective treatment tool. Intestinal NPY, PYY, and PP are abnormal in both patients with IBD and animal models of human IBD. The abnormality in NPY appears to be primarily caused by an interaction between immune cells and the NPY neurons in the enteric nervous system; the abnormalities in PYY and PP appear to be secondary to the changes caused by the abnormalities in other gut neurohormonal peptides/amines that occur during inflammation. NPY is the member of the NPY family that can be targeted in order to decrease the inflammation present in IBD.

The melanocortin-4 receptor is expressed in enteroendocrine L cells and regulates the release of peptide YY and glucagon-like peptide 1 in vivo

The melanocortin-4 receptor (MC4R) is expressed in the brainstem and vagal afferent nerves and regulates a number of aspects of gastrointestinal function. Here we show that the receptor is also diffusely expressed in cells of the gastrointestinal system, from stomach to descending colon. Furthermore, MC4R is the second most highly enriched GPCR in peptide YY (PYY) and glucagon-like peptide 1 (GLP-1) expressing enteroendocrine L cells. When vectorial ion transport is measured across mouse or human intestinal mucosa, administration of α-MSH induces a MC4R-specific PYY-dependent antisecretory response consistent with a role for the MC4R in paracrine inhibition of electrolyte secretion. Finally, MC4R-dependent acute PYY and GLP-1 release from L cells can be stimulated in vivo by intraperitoneal (i.p.) administration of melanocortin peptides to mice. This suggests physiological significance for MC4R in L cells and indicates a previously unrecognized peripheral role for the MC4R, complementing vagal and central receptor functions.

Endogenous PYY and GLP-1 mediate l-glutamine responses in intestinal mucosa

BACKGROUND AND PURPOSE

l-glutamine (Gln) is an energy source for gastrointestinal (GI) epithelia and can stimulate glucagon-like peptide 1 (GLP-1) release from isolated enteroendocrine L-cells. GLP-1 and peptide YY (PYY) are co-secreted postprandially and both peptides have functional roles in glucose homeostasis and energy balance. The primary aim of this project was to establish the endogenous mechanisms underpinning Gln responses within intact GI mucosae using selective receptor antagonists.

EXPERIMENTAL APPROACH

Mouse mucosae from different GI regions were voltage-clamped and short-circuit current (Isc) was recorded to Gln added to either surface in the absence or presence of antagonists, using wild-type (WT) or PYY-/- tissues. The glucose sensitivity of Gln responses was also investigated by replacement with mannitol.

KEY RESULTS

Colonic apical and basolateral Gln responses (at 0.1 and 1 mM) were biphasic; initial increases in Isc were predominantly GLP-1 mediated. GLP-1 receptor antagonism significantly reduced the initial Gln response in the PYY-/- colon. The slower reductions in Isc to Gln were PYY-Y1 mediated as they were absent from the PYY-/- colon and were blocked selectively in WT tissue by a Y1 receptor antagonist. In jejunum mucosa, Gln stimulated monophasic Isc reductions that were PYY-Y1 receptor mediated. Gln effects were partially glucose sensitive, and Calhex 231 inhibition indicated that the calcium-sensing receptor (CaSR) was involved.

CONCLUSION AND IMPLICATIONS

Gln stimulates the co-release of endogenous GLP-1 and PYY from mucosal L-cells resulting in paracrine GLP-1 and Y1 receptor-mediated electrogenic epithelial responses. This glucose-sensitive mechanism appears to be CaSR mediated and could provide a significant therapeutic strategy releasing two endogenous peptides better known for their glucose-lowering and satiating effects.

Peptide modifications differentially alter G protein-coupled receptor internalization and signaling bias

Although G protein-coupled receptors (GPCRs) are targeted by more clinically used drugs than any other type of protein, their ligand development is particularly challenging. Humans have four neuropeptide Y receptors: hY1R and hY5R are orexigenic, while hY2R and hY4R are anorexigenic, and represent important anti-obesity drug targets. We show for the first time that PEGylation and lipidation, chemical modifications that prolong the plasma half-lives of peptides, confer additional benefits. Both modifications enhance pancreatic polypeptide preference for hY2R/hY4R over hY1R/hY5R. Lipidation biases the ligand towards arrestin recruitment and internalization, whereas PEGylation confers the opposite bias. These effects were independent of the cell system and modified residue. We thus provide novel insights into the mode of action of peptide modifications and open innovative venues for generating peptide agonists with extended therapeutic potential.

Evidence for tachykinin NK3 receptors-triggered peptide YY release from isolated guinea-pig distal colon

The anorectic gut hormone, peptide YY (PYY), is released from colonic mucosal endocrine cells, but little is known about the role for tachykinin NK3 receptor in the control of PYY release from the colonic mucosa. We investigated the functional role for NK3 receptors in the control of PYY release from isolated guinea-pig distal colon, and the role for NK3 receptors-triggered PYY release in the control of colonic motility. Isolated colonic preparations were mounted in organ baths for measurement of PYY release and mechanical activity. The release of PYY from these preparations was determined by enzyme immunoassays. The NK3 receptor agonist senktide produced a tetrodotoxin/atropine-sensitive sustained increase in the release of PYY from the colonic preparations. Basal PYY release was transiently inhibited by the NK3 receptor antagonist SB222200. The neuropeptide Y1 receptor antagonist BIBO3304 produced a leftward shift of the concentration-response curves for senktide-evoked neurogenic contraction, but neither the neuropeptide Y2 receptor antagonist BIIE0246 nor the neuropeptide Y5 receptor antagonist CGP71683 affected the senktide concentration-response curves. NK3 receptors appear to play an important role in the control of PYY release from colonic mucosa, and NK3 receptor-triggered PYY release can exert Y1 receptor-mediated inhibition of tachykinergic neuromuscular transmission. This indicates a pathophysiological role for the NK3 receptor-triggered PYY release in the control of colonic motility.

A role for endogenous peptide YY in tachykinin NK(2) receptor-triggered 5-HT release from guinea pig isolated colonic mucosa

BACKGROUND AND PURPOSE

The colon-derived peptide hormone, peptide YY (PYY), regulates colonic motility, secretion and postprandial satiety; but little is known about the influence of endogenous PYY on 5-HT release from colonic mucosa. Tachykinin NK(2) receptor-selective agonist, βAla-NKA-(4-10) induces 5-HT release from guinea pig colonic mucosa via NK(2) receptors on the mucosal layer. The present study was designed to determine the influence of endogenous PYY on 5-HT release from guinea pig colonic mucosa, evoked by the NK(2) receptor agonist, βAla-NKA-(4-10).

EXPERIMENTAL APPROACH

Muscle layer-free mucosal preparations of guinea pig colon were incubated in vitro and the outflow of PYY or 5-HT and its metabolite, 5-HIAA, from these preparations were determined by enzyme immunoassays or HPLC with electrochemical detection respectively.

KEY RESULTS

βAla-NKA-(4-10) produced a tetrodotoxin-resistant sustained increase in the outflow of PYY and 5-HT from the mucosal preparations. The βAla-NKA-(4-10)-evoked 5-HT outflow was partially inhibited by Y(1) receptor antagonist, BIBO3304, and Y(2) receptor antagonist, BIIE0246, but with less potency. Exogenously-applied PYY also produced a sustained increase in the outflow of 5-HT that was inhibited by Y(1) blockade but not Y(2) blockade.

CONCLUSION AND IMPLICATIONS

Our findings support the view that the NK(2) receptor-selective agonist, βAla-NKA-(4-10) produces a long-lasting PYY release from guinea pig colonic mucosa via NK(2) receptors on L cells and βAla-NKA-(4-10)-evoked 5-HT release is in part mediated by endogenously released PYY, acting mainly on Y(1) receptors on EC cells. The PYY-containing L cells appear to play a role in controlling the release of 5-HT from colonic EC cells.

Neuropeptide Y, peptide YY and pancreatic polypeptide in the gut-brain axis

The gut-brain axis refers to the bidirectional communication between the gut and the brain. Four information carriers (vagal and spinal afferent neurons, immune mediators such as cytokines, gut hormones and gut microbiota-derived signalling molecules) transmit information from the gut to the brain, while autonomic neurons and neuroendocrine factors carry outputs from the brain to the gut. The members of the neuropeptide Y (NPY) family of biologically active peptides, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are expressed by cell systems at distinct levels of the gut-brain axis. PYY and PP are exclusively expressed by endocrine cells of the digestive system, whereas NPY is found at all levels of the gut-brain and brain-gut axis. The major systems expressing NPY comprise enteric neurons, primary afferent neurons, several neuronal pathways throughout the brain and sympathetic neurons. In the digestive tract, NPY and PYY inhibit gastrointestinal motility and electrolyte secretion and in this way modify the input to the brain. PYY is also influenced by the intestinal microbiota, and NPY exerts, via stimulation of Y1 receptors, a proinflammatory action. Furthermore, the NPY system protects against distinct behavioural disturbances caused by peripheral immune challenge, ameliorating the acute sickness response and preventing long-term depression. At the level of the afferent system, NPY inhibits nociceptive input from the periphery to the spinal cord and brainstem. In the brain, NPY and its receptors (Y1, Y2, Y4, Y5) play important roles in regulating food intake, energy homeostasis, anxiety, mood and stress resilience. In addition, PP and PYY signal to the brain to attenuate food intake, anxiety and depression-related behaviour. These findings underscore the important role of the NPY-Y receptor system at several levels of the gut-brain axis in which NPY, PYY and PP operate both as neural and endocrine messengers.

Modulatory role of PYY in transport and metabolism of cholesterol in intestinal epithelial cells

Background

Gastrointestinal peptides are involved in modulating appetite. Other biological functions attributed to them include the regulation of lipid homeostasis. However, data concerning PYY remain fragmentary. The objectives of the study were: (i) To determine the effect of PYY on intestinal transport and synthesis of cholesterol, the biogenesis of apolipoproteins (apos) and assembly of lipoproteins and (ii) To analyze whether the effects of PYY are similar according to whether cells are exposed to PYY on apical or basolateral surface.

Methodology/Principal Findings

Caco-2/15 cells were incubated with PYY (1–36) administered either to the apical or basolateral medium, at concentrations of 50 or 200 nM for 24 hours. De novo synthesis of cholesterol, cholesterol uptake, and assembly of lipoproteins were evaluated through the incorporation of [¹⁴C]-acetate, [¹⁴C]-cholesterol, and [¹⁴C]-oleate, respectively. Biogenesis of apos (A-I, A-IV, E, B-48 and B-100) was examined by the incorporation of [³⁵S]-methionine. The influence of PYY on protein and mRNA levels of many key mediators of lipid metabolism was analyzed by Western blot and PCR, respectively. Our results show that PYY influenced cholesterol metabolism in Caco-2/15 cells depending on the site of PYY delivery. Apical addition of PYY significantly lowered the incorporation of [¹⁴C]-cholesterol likely via the reduction of NPC1L1, stimulated intracellular cholesterol synthesis probably through an increase in SREBP-2 expression, whereas it concomitantly increased apo A-I synthesis and decreased LDL secretion. In contrast, basolateral PYY reduced the production of chylomicrons (CM) as well as the biogenesis of apos B-48 and B-100, while lowering the expression of the transcription factors RXRα and PPAR(α,β).

Conclusions/Significance

PYY is capable of influencing cholesterol homeostasis in intestinal Caco-2/15 cells depending on the site delivery. Apical PYY was able to decrease cholesterol uptake via NPC1L1 downregulation, whereas basolateral PYY diminished CM output through the biogenesis decline of apos B-48 and B-100.

Endogenous peptide YY and neuropeptide Y inhibit colonic ion transport, contractility and transit differentially via Y₁ and Y₂ receptors

BACKGROUND AND PURPOSE

Peptide YY (PYY) and neuropeptide Y (NPY) activate Y receptors, targets under consideration as treatments for diarrhoea and other intestinal disorders. We investigated the gastrointestinal consequences of selective PYY or NPY ablation on mucosal ion transport, smooth muscle activity and transit using wild-type, single and double peptide knockout mice, comparing mucosal responses with those from human colon.

EXPERIMENTAL APPROACH

Mucosae were pretreated with a Y₁ (BIBO3304) or Y₂ (BIIE0246) receptor antagonist and changes in short-circuit current recorded. Colonic transit and colonic migrating motor complexes (CMMCs) were assessed in vitro and upper gastrointestinal and colonic transit measured in vivo.

KEY RESULTS

Y receptor antagonists revealed tonic Y₁ and Y₂ receptor-mediated antisecretory effects in human and wild-type mouse colon mucosae. In both, Y₁ tone was epithelial while Y₂ tone was neuronal. Y₁ tone was reduced 90% in PYY⁻/⁻ mucosa but unchanged in NPY⁻/⁻ tissue. Y₂ tone was partially reduced in NPY⁻/⁻ or PYY⁻/⁻ mucosae and abolished in tetrodotoxin-pretreated PYY⁻/⁻ tissue. Y₁ and Y₂ tone were absent in NPYPYY⁻/⁻ tissue. Colonic transit was inhibited by Y₁ blockade and increased by Y₂ antagonism indicating tonic Y₁ excitation and Y₂ inhibition respectively. Upper GI transit was increased in PYY⁻/⁻ mice only. Y₂ blockade reduced CMMC frequency in isolated mouse colon.

CONCLUSIONS AND IMPLICATIONS

Endogenous PYY and NPY induced significant mucosal antisecretory tone mediated by Y₁ and Y₂ receptors, via similar mechanisms in human and mouse colon mucosa. Both peptides contributed to tonic Y₂-receptor-mediated inhibition of colonic transit in vitro but only PYY attenuated upper GI transit.

Therapeutic potential of neuropeptide Y (NPY) receptor ligands

Neuropeptide Y (NPY) is widely distributed in the human body and contributes to a vast number of physiological processes. Since its discovery, NPY has been implicated in metabolic regulation and, although interest in its role in central mechanisms related to food intake and obesity has somewhat diminished, the topic remains a strong focus of research concerning NPY signalling. In addition, a number of other uses for modulators of NPY receptors have been implied in a range of diseases, although the development of NPY receptor ligands has been slow, with no clinically approved receptor therapeutics currently available. Nevertheless, several interesting small molecule compounds, notably Y2 receptor antagonists, have been published recently, fueling optimism in the field. Herein we review the role of NPY in the pathophysiology of a number of diseases and highlight instances where NPY receptor signalling systems are attractive therapeutic targets.

Neuropeptide Y and peptide YY: important regulators of energy metabolism

PURPOSE OF REVIEW

An overview of recent developments documenting the neuropeptide Y (NPY) family's role in energy metabolism. Specifically focusing on site-specific functions of NPY and increasing evidence of peptide YY (PYY) as a weight loss therapeutic.

RECENT FINDINGS

Studying the NPY family in hypothalamic nuclei, other than the arcuate and paraventricular nuclei, is a recent shift in metabolic research. NPY overexpression in the dorsomedial hypothalamus increases food intake whereas its ablation in this area reduces hyperphagia and obesity. Similarly, NPY exerts orexigenic effects in the ventromedial nucleus. However, specific arcuate Y2 receptor ablation leads to positive energy balance, suggesting the NPY family demonstrates location-specific functions. Peripherally, dual blockade of cannabinoid and NPY pathways has synergistic effects on weight loss, as does combined administration of PYY3-36 and oxyntomodulin in reducing food intake, perhaps due to the recently discovered role of PYY in mediating intestinal Gpr119 activity and controlling glucose tolerance.

SUMMARY

Conditional Y receptor knockout models have provided deeper insights on NPY's functions according to location. Further study of PYY appears vital, due to recent evidence of its role in intestinal motility, with exercise positively influencing PYY levels.

Peptide YY is critical for acylethanolamine receptor Gpr119-induced activation of gastrointestinal mucosal responses

Peptide YY (PYY) is released following food intake and regulates intestinal function and glucose homeostasis, but the mechanisms underpinning these processes are unclear. Enteroendocrine L cells contain PYY and express the acylethanolamine receptor, Gpr119. Here, we show that Gpr119 activation inhibited epithelial electrolyte secretion in human and mouse colon in a glucose-sensitive manner. Endogenous PYY selectively mediated these effects, since PYY(-/-) mice showed no Gpr119 response, but responses were observed in NPY(-/-) mice. Importantly, Gpr119 responses in wild-type (WT) mouse tissue and human colon were abolished by Y(1) receptor antagonism, but were not enhanced by dipeptidylpeptidase IV blockade, indicating that PYY processing to PYY(3-36) was not important. In addition, Gpr119 agonism reduced glycemic excursions after oral glucose delivery to WT mice but not PYY(-/-) mice. Taken together, these data demonstrate a previously unrecognized role of PYY in mediating intestinal Gpr119 activity and an associated function in controlling glucose tolerance.

Antisecretory effects of neuropeptide Y in the mouse colon are region-specific and are lost in DSS-induced colitis

Regulation of water movement in the gut is an important homeostatic event that is critical to normal intestinal function. We assessed the effect of neuropeptide Y (NPY) on epithelial ion transport in the normal and inflamed mouse colons. Colitis was induced by dextran sodium sulfate (DSS, 4% wt./vol.) administered in the drinking water for 5 days followed by 3 days of regular water. Segments of proximal and distal colons were excised and short-circuit current (I(SC)) was measured in Ussing chambers to assess net electrogenic active ion transport. NPY Y(1) receptor (Y(1)R) expression was measured by quantitative real-time PCR and immunohistochemistry. Challenge of distal colon from normal mice with NPY (10(-7)M) evoked a drop in I(SC) (51.4±9.1 μA/cm(2)), which was dependent on Cl(-) flux, was insensitive to neural blockade with tetrodotoxin and was mediated primarily through the Y(1)R. In contrast, the proximal colon was largely unresponsive to NPY, expressing ~ten-fold less Y(1)R mRNA compared to the distal colon. These findings confirm that specific regional regulation of ion transport occurs in the colon. Segments of proximal and distal colons from mice with DDS-induced colitis were virtually unresponsive to NPY, expressed less Y(1)R mRNA than tissues from control mice and displayed loss of Y(1)R protein expression in the colonic epithelium. This hypo-responsiveness to an antisecretory stimulus adds to the well-documented loss of responsiveness to pro-secretory agents during inflammation, attesting to a profound loss of control of active ion transport during enteric inflammatory disease.

NPY Y2 receptor agonist PYY(3-36) inhibits diarrhea by reducing intestinal fluid secretion and slowing colonic transit in mice

Peptide YY (PYY)(3-36), a neuropeptide Y (NPY) Y2 receptor agonist, is a powerful inhibitor of intestinal secretion. Based on this anti-secretory effect, NPY Y2 receptor agonists may be useful as novel anti-diarrheal agents, but anti-diarrheal efficacy has yet to be determined. We therefore examined the anti-diarrheal efficacy of PYY(3-36) and a selective Y2 receptor agonist, N-acetyl-[Leu28, Leu31]-NPY(24-36), in experimental mouse models of diarrhea. Intraperitoneal administration of PYY(3-36) (0.01-1mg/kg) and N-acetyl-[Leu28, Leu31]-NPY(24-36) (10mg/kg) significantly inhibited diarrhea (increase in wet fecal weight and diarrhea score) induced by dimethyl-prostaglandin E2, 5-hydroxytryptamine, and castor oil. Anti-diarrheal activities of PYY(3-36) and N-acetyl-[Leu28, Leu31]-NPY(24-36) were comparable to the effects of loperamide (1mg/kg), a widely used anti-diarrheal drug. To clarify the anti-diarrheal mechanisms of NPY Y2 receptor agonists, we investigated the effects of PYY(3-36) and N-acetyl-[Leu28, Leu31]-NPY(24-36) on intestinal fluid secretion and colonic transit. PYY(3-36) (1mg/kg) and N-acetyl-[Leu28, Leu31]-NPY(24-36) (10mg/kg) significantly reduced dimethyl-prostaglandin E2-induced intestinal fluid accumulation in conscious mice, suggesting that NPY Y2 receptor agonists inhibit diarrhea, at least in part, by reducing intestinal secretion. In addition, PYY(3-36) (0.01-1mg/kg) and N-acetyl-[Leu28, Leu31]-NPY(24-36) (10mg/kg) potently inhibited normal fecal output, suggesting that NPY Y2 receptor activation inhibits colonic motor function and NPY Y2 receptor agonists inhibit diarrhea partly by slowing colonic transit. These results indicate that NPY Y2 receptor agonists inhibit diarrhea in mice by not only reducing intestinal fluid secretion, but also slowing colonic transit, and illustrate the therapeutic potential of NPY Y2 receptor agonists as effective treatments for diarrhea.

Endogenous PYY and NPY mediate tonic Y1- and Y2-mediated absorption in human and mouse colon

Objective

To establish the functional significance of endogenous peptide YY (PYY) and neuropeptide Y (NPY) as mediators of Y₁ and Y₂ absorptive tone in colonic mucosa.

Methods

Functional studies utilized descending colon from adult mice (wild type [WT] and peptide nulls) and ex vivo human colonic tissue (from patients undergoing bowel resections) measuring changes in basal ion transport. Peak increases in ion transport to Y₁ or Y₂ antagonists (BIBO3304 300 nM; BIIE0246 1 μM) were pooled (mean ± SEM) and compared using Student's unpaired t test (P ≤ 0.05); some tissues received tetrodotoxin (TTX; 100 nM). PYY-positive L-cell numbers and NPY innervation were also compared.

Results

Y₁ and Y₂ tones were present in human and WT mouse colon mucosa and only the latter was TTX sensitive. Y₁ tone was unchanged in NPY^−/− but was ∼90% inhibited in PYY^−/− and abolished in PYYNPY^−/− colon mucosa. Y₂ tone was reduced ∼50% in NPY^−/− and PYY^−/− tissues and was absent from PYYNPY^−/− colon. Residual Y₂ and Y₁ tones present in PYY^−/− mucosa were abolished by TTX. PYY ablation had no apparent effect on NPY innervation and PYY-positive cells were observed at the same frequency in NPY^−/− (56.7 ± 6.8 cells/section) and WT (55.0 ± 4.6 cells/section) colons. Double knockouts lacked PYY and NPY expression, but endocrine cells and enteric nerves were present with similar frequencies to those of WT mice.

Conclusion

Endogenous PYY mediates Y₁ absorptive tone that is epithelial in origin, whereas Y₂ tone is a combination of PYY and NPY mediation.

Peptide YY containing enteroendocrine cells and peripheral tissue sensitivity to PYY and PYY(3-36) are maintained in diet-induced obese and diet-resistant rats

Peptide YY (PYY) is a gastrointestinal hormone, localized in enteroendocrine L-cells. Its hydrolyzed form PYY(3-36) is a satiety factor. The aim of this study was to identify if intestinal PYY enteroendocrine cells or content correlate with the diet-induced obese (DIO) or diet-resistant (DR) phenotypes. We also examined intestinal sensitivity to PYY and PYY(3-36) in DIO and DR rats. Animals were maintained on a medium-high fat diet and split into DIO and DR groups based on weight gain. PYY immunoreactive cells were unaltered in DIO intestine and stomach compared to DR rats. PYY content and circulating levels were also unchanged in DIO rats. Intestinal PYY and PYY(3-36) responses were enhanced in fasted rats, and equipotent in both DIO and DR jejunum. We conclude that PYY cell number, tissue content and peripheral sensitivity are maintained in DIO rats. Our data suggests that neither PYY nor PYY(3-36) contribute to the maintenance of either the DIO or DR phenotype, and that peripheral resistance to PYY and PYY(3-36) does not accompany DIO.

Peptide YY: a neuroendocrine neighbor of note

Endocrine cells, enteric neurons and enterocytes provide an integrated functional defense against luminal factors, including nutrients, microbes and toxins. Prominent among intrinsic mediators is peptide YY (PYY) which is present in approximately 50% of colorectal endocrine cells and neuropeptide Y (NPY), a neurotransmitter expressed in submucous and myenteric nerves. Both peptides and their long fragments (PYY(3-36) and NPY(3-36)) are potent, long-lasting anti-secretory agents in vitro and in vivo and, they provide significant Y receptor-mediated absorptive tone in human and mouse colon mucosa. The main function of the colon is to absorb 90% of approximately 2l of daily ileal effluent (in adult humans) and Y-absorptive tone can contribute significantly to this electrolyte absorption. Blockade or loss of this mucosal Y-absorptive tone (i.e. with Y(1) or Y(2) antagonists) leads to hypersecretion and potentially to diarrhea, so Y agonists are predicted to rescue absorption by mimicking endogenous neuroendocrine PYY or neuronal NPY.

Neuropeptide Y receptors; antisecretory control of intestinal epithelial function

This paper reviews the cellular localisation, mechanisms of release and intestinal absorptive actions of neuropeptide Y and its related peptides, peptide YY, pancreatic polypeptide and major fragments NPY(3-36) and PYY(3-36). While NPY is commonly found in inhibitory enteric neurons that can be interneurons, motor neurons or secretomotor-nonvasodilator in nature, its analogue, peptide YY in contrast, is located in neuroendocrine L-cells that predominate in the colorectal mucosa. Peptide YY is released from these cells when nutrients arrive in the small or large bowel, exerting paracrine as well as hormonal actions. Pancreatic polypeptide is found in relatively few, scattered intestinal endocrine cells, the majority of this peptide being produced by, and released from pancreatic islet F-cells in response to food intake. An introduction to the current pharmacology of this family of peptides is provided and the different types of neuropeptide Y (termed Y) receptors, their agonist preferences, antagonism, and preferred signalling pathways, are described. Our current understanding of specific Y receptor localisation within the intestine as determined by immunohistochemistry, is presented as a prelude to an assessment of functional studies that have monitored ion transport across isolated mucosal preparations. It is becoming clear that three Y receptor types are significant functionally in human colon, as well as particular rodent models (e.g. mouse) and these, namely the Y(1), Y(2) and Y(4) receptors, are discussed in detail. Their presence within the basolateral aspect of the epithelial layer (Y(1) and Y(4) receptors) or on enteric neurons (Y(1) and Y(2) receptors) and their activation by endogenous neuropeptide Y, peptide YY (Y(1) and Y(2) receptors) or pancreatic polypeptide (which prefers Y(4) receptors) results consistently in antisecretory/absorptive responses. The recent use of novel mouse knockouts has helped establish loss of specific intestinal functions including Y(1) and Y(2) receptor-mediated absorptive tone in colon mucosa. Progress in this field has been rapid recently, aided by the availability of selective antagonists and mutant mice lacking either one (e.g. Y(4)-/-, for which no antagonists exist at present) or more Y receptor types. It is therefore timely to review this work and present a rational basis for developing stable synthetic Y receptor agonists as novel anti-diarrhoeals.

Y(4) receptors mediate the inhibitory responses of pancreatic polypeptide in human and mouse colon mucosa

The antisecretory effects of several Y agonists, including pancreatic polypeptide (PP), indicate the presence of Y(1), Y(2), and Y(4) receptors in mouse and human (h) colon mucosae. Here, we used preparations from human and from wild-type (WT), Y(4), and Y(1) receptor knockout ((-/-)) mice, alongside Y(4) receptor-transfected cells to define the relative functional contribution of the Y(4) receptor. First, rat (r) PP antisecretory responses were lost in murine Y(4)(-/-) preparations, but hPP and Pro(34) peptide YY (PYY) costimulated Y(4) and Y(1) receptors in WT mucosa. The Y(1) antagonist/Y(4) agonist GR231118 [(Ile,Glu,Pro,Dpr,Tyr,Arg,Leu,Arg,Try-NH(2))-2-cyclic(2,4'),(2',4)-diamide] elicited small Y(4)-mediated antisecretory responses in human tissues pretreated with the Y(1) antagonist, BIBO3304 [(R)-N-[[4-(aminocarbonylaminomethyl)-phenyl]methyl]-N(2)-(diphenylacetyl)-argininamide trifluoroacetate)], and attenuated Y(4)-mediated hPP responses in mouse and human mucosa. GR231118 and rPP were also antisecretory in hY(4)-transfected epithelial monolayers but were partial agonists compared with hPP at this receptor. In Y(4)-transfected human embryonic kidney (HEK) 293 cells, Y(4) ligands displaced [(125)I]hPP binding with orders of affinity (pK(i)) at human (hPP = rPP > GR231118 > Pro(34)PYY = PYY) and mouse (rPP = hPP > GR231118 > Pro(34)PYY > PYY) Y(4) receptors. GR231118- and rPP-stimulated guanosine 5'-3-O-(thio)triphosphate binding through hY(4) receptors with significantly lower efficacy than hPP. GR231118 marginally increased basal but abolished further PP-induced hY(4) internalization to recycling (transferrin-labeled) pathways in HEK293 cells. Taken together, these findings show that Y(4) receptors play a definitive role in attenuating colonic anion transport and may be useful targets for novel antidiarrheal agents due to their limited peripheral expression.

Enzyme-based visualization of receptor-ligand binding in tissues

New methods of elucidating the ligand-binding activity of receptors could improve our understanding of receptor function, key events they control, and their presence in normal and pathological states. We describe a method for visualizing receptor-ligand binding in cells and tissues that substitutes fluorescein for radioactive labels, and detects receptor bound, fluoresceinated ligand with an antifluorescein/horseradish peroxidase amplification system. Receptor-bound ligand is then visualized by light microscopy against a standard hemotoxylin-stained background of cell structure. Quantitative versions of the assay provide an apparent dissociation constant and number of receptors per cell at saturation in cell or tissue specimens. Receptors examined include the folate receptor, bombesin peptide-binding receptors, the epidermal growth factor receptor, the neuropeptide Y receptor, the asialoglycoprotein receptor, and RGD peptide-binding integrins. Using fluoresceinated versions of molecules, we show the method can visualize and quantitate receptor-bound ligands in cell culture monolayers and animal tissue specimens. Ligand binding to receptors present in tissues was visualized in normal and pathological samples of human tissue microarrays. The enzyme-amplified detection of receptor-bound fluoresceinated ligand is a simple and nonradioactive-based method that provides information on the receptor activity in tissue specimens.

Adaptation of estrogen-regulated genes in long-term estradiol deprived MCF-7 breast cancer cells

First line treatment of hormone dependent breast cancer initially causes tumor regression but later results in adaptive changes and tumor re-growth. Responses to second line treatments occur but tumors again begin to progress after a period of 12???18??months. In depth understanding of the adaptive process would allow the identification of targets to abrogate the development of hormonal resistance and prolong the efficacy of endocrine therapy. We have developed a model system to examine adaptive changes in human MCF-7 breast cancer cells. Upon deprivation of estradiol for a prolonged period of time, a maneuver analogous to surgical oophorectomy in pre-menopausal women and use of aromatase inhibitors in post-menopausal patients, tumor cells adapt and become hypersensitive to estradiol. We reasoned that the expression pattern of multiple genes would change in response to estradiol deprivation and that cDNA microarrays would provide an efficient means of assessing these changes. Accordingly, we examined the transcriptional responses to estradiol in long-term estradiol deprived (LTED) MCF-7 cells with a cDNA microarray containing 1901 known genes and ESTs. To assess the changes induced by long-term estradiol deprivation, we compared the effects of estradiol administration in LTED cells with those in MCF-7 cells, which we had previously reported, and confirmed with real time PCR using the parental and LTED cells. Seven genes and one EST were induced by estradiol in LTED but not in wild type MCF-7 cells, whereas ten genes were down-regulated by estradiol only in LTED cells. The expression of seven genes increased concurrently and five decreased in response to estradiol in both cell types. From these observations, we generated testable hypotheses regarding several genes including DKFZP, RAP-1, ribosomal protein S6, and TM4SF1. Based upon the known functions of these genes and the patterns of observed changes, we postulate that divergent regulation of these genes may contribute to the different biologic responses to estrogen in these cell lines. These results provide targets for further mechanistic studies in our experimental system. Our findings indicate that long-term estradiol deprivation causes expression changes in multiple genes and emphasizes the complexity of the process of cellular adaptation.

The regulation of veratridine-stimulated electrogenic ion transport in mouse colon by neuropeptide Y (NPY), Y1 and Y2 receptors

1 Neuropeptide Y (NPY) is a prominent enteric neuropeptide with prolonged antisecretory effects in mammalian intestine. Veratridine depolarises neurons consequently causing epithelial anion secretion across mouse colon mucosa. Our aim was to characterise functionally, veratridine-stimulated mucosal responses and to determine the roles for NPY, Y(1), and Y(2) receptors in modulating these neurogenic effects. 2 Colon mucosae (with intact submucous innervation) from wild-type mice (+/+) and knockouts lacking either NPY (NPY-/-), Y(1)-/- or Y(2)-/- were placed in Ussing chambers and voltage clamped at 0 mV. Veratridine-stimulated short-circuit current (I(sc)) responses in +/+, Y(1) or Y(2) antagonist pretreated +/+ colon, Y(1)-/- and NPY-/- colon were insensitive to cholinergic blockade by atropine (At; 1 microM) and hexamethonium (Hex; 10 microM). Tetrodotoxin (TTX, 100 nM) abolished veratridine responses, but had no effect upon carbachol (CCh) or vasoactive intestinal polypeptide (VIP)-induced secretory responses. 3 To establish the functional roles for Y(1) and Y(2) receptors, +/+ tissues were pretreated with either the Y(1) or Y(2) receptor antagonist (BIBO3304 (300 nM) or BIIE0246 (1 microM), respectively) and veratridine responses were compared with those from Y(1)-/- or Y(2)-/- colon. Neither BIBO3304 nor Y(1)-/- altered veratridine-induced secretion, but Y(1) agonist responses were abolished in both preparations. In contrast, the Y(2) antagonist BIIE0246 significantly amplified veratridine responses in +/+ mucosa. Unexpectedly, NPY-/- colon exhibited significantly attenuated veratridine responses (between 1 and 5 min). 4 We demonstrate that electrogenic veratridine responses in mouse colon are noncholinergic and that NPY can act directly upon epithelia, a Y(1) receptor effect. The enhanced veratridine response observed in +/+ tissue following BIIE0246, indicates that Y(2) receptors are located on submucosal neurons and that their activation by NPY will inhibit enteric noncholinergic secretory neurotransmission. 5 We also demonstrate Y(1) and Y(2) receptor-mediated antisecretory tone in +/+ colon and show selective loss of each in Y(1) and Y(2) null colon respectively. In NPY-/- tissue, only Y(1)-mediated tone was present, this presumably being mediated by endogenous endocrine peptide YY. Y(2) tone was absent from NPY-/- (and Y(2)-/-) colon and we conclude that NPY activation of neuronal Y(2) receptors attenuates secretory neurotransmission thereby providing an absorptive electrolyte tone in isolated colon.

Selective increase of dark phase water intake in neuropeptide-Y Y2 and Y4 receptor knockout mice

Neuropeptide-Y (NPY) is involved in the regulation of ingestive behaviour and energy homeostasis. Since deletion of the NPY Y2 and Y4 receptor gene increases and decreases food intake, respectively, we examined whether water intake during the light and dark phases is altered in Y2 and Y4 receptor knockout mice. The water consumption of mice staying in their home cages was measured by weighing the water bottles at the beginning and end of the light phase during 4 consecutive days. Control, Y2 and Y4 receptor knockout mice did not differ in their water intake during the light phase. However, during the dark phase Y2 and Y4 receptor knockout mice drank significantly more (46-63%, P<0.05) water than the control mice. The total daily water intake over 24 h was also enhanced. The enhanced water intake during the dark phase was not altered by the beta-adrenoceptor antagonist propranolol or the angiotensin AT1 receptor antagonist telmisartan (each injected intraperitoneally at 10 mg/kg). These data indicate that NPY acting via Y2 and Y4 receptors plays a distinctive role in the regulation of nocturnal water consumption. While beta-adrenoceptors and angiotensin AT1 receptors do not seem to be involved, water intake in Y2 and Y4 receptor knockout mice may be enhanced because presynaptic autoinhibition of NPY release and inhibition of orexin neurons in the central nervous system are prevented.

Role of the C terminus in neuropeptide Y Y1 receptor desensitization and internalization

We have studied truncation mutants of the rat neuropeptide Y (NPY) Y1 receptor lacking four (Thr361stop, Y1T361*) or eight (Ser352stop, Y1S352*) potential serine/threonine C-terminal phosphorylation sites. NPY-stimulated hemagglutinin-tagged Y1, Y1T361*, and Y1S352* receptors all efficiently activated G proteins in Chinese hamster ovary (CHO) cell membranes, but desensitization after NPY pretreatment was only prevented in the HAY1S352* clone. In transfected colonic carcinoma epithelial layers, functional Y1 and Y1T361* peptide YY responses became more transient as the agonist concentration increased, whereas those mediated by the Y1S352* receptor remained sustained. NPY-stimulated HAY1 receptor phosphorylation was increased by transient overexpression of G protein-coupled receptor kinase 2, and only Ser352stop truncation abolished this response in CHO or human embryonic kidney (HEK) 293 cells. Rapid internalization of cell-surface HAY1 receptors in HEK293 cells was observed in response to agonist, resulting in partial colocalization with transferrin, a marker for clathrin-mediated endocytosis and recycling. It is surprising that both truncated receptors were constitutively internalized, predominantly in transferrin-positive compartments. NPY increased cell-surface localization of HAY1S352* receptors, whereas the distribution of both mutants was unaltered by BIBO3304. Recruitment of green fluorescent protein-tagged beta-arrestin2 to punctate endosomes was observed only for HAY1 and HAY1T361* receptors and solely under NPY-stimulated conditions. Thus, the key C-terminal sequence between Ser352 and Lys360 is a major site for Y1 receptor phosphorylation, is critical for its desensitization, and contributes to the association between the receptor and beta-arrestin proteins. However, additional beta-arrestin-independent mechanisms control Y1 receptor trafficking under basal conditions.

The ability of neuropeptide Y to mediate responses in the murine cutaneous microvasculature: an analysis of the contribution of Y1 and Y2 receptors

1. The ability of neuropeptide Y (NPY) to modulate skin blood flow, oedema formation and neutrophil accumulation was investigated. Experiments were designed to examine the possible contribution of the Y2 receptor, in addition to the Y1 receptor, through use of Y2 receptor knockout mice (Y2-/-) and selective receptor antagonists. 2. The development of a 99mTc clearance technique for the measurement of microvascular blood flow changes in mouse dorsal skin revealed a dose-dependent ability of picomole amounts of NPY, and also of the Y1-preferred agonist Pro34NPY and the Y2-preferred agonist PYY(3-36) to decrease blood flow. 3. The Y1 receptor antagonist BIBO3304 blocked responses to the Y1 agonist at the lower doses, but only partially inhibited at the higher doses tested in Y2+/+. In Y2-/- receptor mice, the responses to the Y2 agonist were abolished at the lower doses and partially reduced at the highest dose tested, while those to the Y1 agonist were similar in both Y2+/+ and Y2-/-receptor mice. 4. In Y2+/+ receptor mice, the simultaneous injection of the Y2 antagonist BIIE0246 with BIBO3304 abolished Y2 agonist-induced decreases in blood flow over the dose range used (10-100 pmol). When the Y2 receptor antagonist BIIE0246 was given alone, it was not able to significantly affect the PYY(3-36)-induced response, whereas the Y1 receptor antagonist BIBO3304 partially (P<0.001) inhibited the decrease in blood flow evoked by PYY(3-36) at the highest dose. 5. NPY did not mediate either oedema formation, even when investigated in the presence of the vasodilator calcitonin gene-related peptide (CGRP), or neutrophil accumulation in murine skin. 6. We conclude that the major vasoactive activity of NPY in the cutaneous microvasculature is to act in a potent manner to decrease blood flow via Y1 receptors, with evidence for the additional involvement of postjunctional Y2 receptors. Our results do not provide evidence for a potent proinflammatory activity of NPY in the cutaneous microvasculature.

Control of signalling efficacy by palmitoylation of the rat Y1 receptor

(1) We have investigated the properties of native and haemagglutinin (HA)-tagged neuropeptide Y (NPY) Y(1) receptors after mutation of the palmitoylation site Cys(337) to Ser or Ala. (2) In Chinese hamster ovary cells expressing similar receptor levels, the C337A mutation abolished incorporation of [(3)H]palmitic acid into the HA-Y(1) receptor. (3) Cys(337) substitution did not alter the affinities of Y(1) receptor agonists or antagonists, but it eliminated the ability of guanosine-5'-O-(3-thio)triphosphate (GTPgammaS) to displace [(125)I]PYY-specific binding (compared to approximately 50% inhibition in Y(1) or HA-Y(1) clones). (4) Stimulation of GTPgamma[(35)S] binding by native and HA-Y(1) receptors in standard incubation buffer (100 mM NaCl, 10 micro M GDP) was prevented by Cys(337) mutation. In this assay, the function of Y(1)(C337S) receptors could be partially rescued by reducing the Na(+) concentration, and when overexpressed (B(max): approximately 10 pmol mg(-1)), both HA-Y(1) and HA-Y(1)(C337A) receptors displayed similar responses to NPY and peptide YY (PYY). (5) In stably transfected adenocarcinoma cells expressing Y(1) or Y(1)(C337S) receptors, PYY inhibited anion secretion stimulated by vasoactive intestinal peptide (VIP; measured as short-circuit current, I(SC)) with similar potency (EC(50): 26-53 nM). In contrast to the transient Y(1) receptor-mediated responses observed at maximal PYY concentrations, I(SC) reductions in both Y(1)(C337S) clones were sustained. (6) We conclude that nonpalmitoylation of the Y(1) receptor reduces its coupling efficiency to G proteins, and may also indirectly influence desensitisation processes that depend on the formation of an active agonist-receptor conformation.

Functional consequences of neuropeptide Y Y 2 receptor knockout and Y2 antagonism in mouse and human colonic tissues

1 Neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) differentially activate three Y receptors (Y(1), Y(2) and Y(4)) in mouse and human isolated colon. 2 The aim of this study was to characterise Y(2) receptor-mediated responses in colon mucosa and longitudinal smooth muscle preparations from wild type (Y(2)+/+) and knockout (Y(2)-/-) mice and to compare the former with human mucosal Y agonist responses. Inhibition of mucosal short-circuit current and increases in muscle tone were monitored in colonic tissues from Y(2)+/+ and Y(2)-/- mice+/-Y(1) ((R)-N-[[4-(aminocarbonylaminomethyl)phenyl)methyl]-N(2)-(diphenylacetyl)-argininamide-trifluoroacetate (BIBO3304) or Y(2) (S)-N(2)-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6H)-oxodibenz[b,e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl]acetyl]-N-[2-[1,2-dihydro-3,5(4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide (BIIE0246) antagonists. 3 Predictably, Y(2)-/- tissues were insensitive to Y(2)-preferred agonist PYY(3-36) (</=100 nM), but unexpectedly Y(4)-preferred PP responses were right-shifted probably as a consequence of elevated circulating PP levels, particularly in male Y(2)-/- mice (Sainsbury et al., 2002). 4 BIBO3304 and BIIE0246 elevated mucosal ion transport, indicating blockade of inhibitory mucosal tone in Y(2)+/+ tissue. While BIBO3304 effects were unchanged, those to BIIE0246 were absent in Y(2)-/- mucosae. Neither antagonist altered muscle tone; however, BIIE0246 blocked NPY and PYY(3-36) increases in Y(2)+/+ basal tone. BIBO3304 abolished residual Y(1)-mediated NPY responses in Y(2)-/- smooth muscle. 5 Tetrodotoxin significantly reduced BIIE0246 and PYY(3-36) effects in Y(2)+/+ mouse and human mucosae, but had no effect upon Y-agonist contractile responses, indicating that Y(2) receptors are located on submucosal, but not myenteric neurones. 6 Tonic activation of submucosal Y(2) receptors by endogenous NPY, PYY or PYY(3-36) could indirectly reduce mucosal ion transport in murine and human colon, while direct activation of Y(2) receptors on longitudinal muscle results in contraction.

Recent developments in our understanding of the physiological role of PP-fold peptide receptor subtypes

The three peptides pancreatic polypeptide (PP), peptide YY (PYY), and neuropeptide Y (NPY) share a similar structure known as the PP-fold. There are four known human G-protein coupled receptors for the PP-fold peptides, namely Y1, Y2, Y4, and Y5, each of them being able to bind at least two of the three endogenous ligands. All three peptides are found in the circulation acting as hormones. Although NPY is only released from neurons, PYY and PP are primarily found in endocrine cells in the gut, where they exert such effects as inhibition of gall bladder secretion, gut motility, and pancreatic secretion. However, when PYY is administered in an experimental setting to animals, cloned receptors, or tissue preparations, it can mimic the effects of NPY in essentially all studies, making it difficult to study the effects of PP-fold peptides and to delineate what receptor and peptide accounts for a particular effect. Initial studies with transgenic animals confirmed the well-established action of NPY on metabolism, food-intake, vascular systems, memory, mood, neuronal excitability, and reproduction. More recently, using transgenic techniques and novel antagonists for the Y1, Y2, and Y5 receptors, NPY has been found to be a key player in the regulation of ethanol consumption and neuronal development.