Paracrine relationship between incretin hormones and endogenous 5-hydroxytryptamine in the small and large intestine.

BACKGROUND: Enterochromaffin (EC) cell-derived 5-hydroxytryptamine (5-HT) is a mediator of toxin-induced reflexes, initiating emesis via vagal and central 5-HT3 receptors. The amine is also involved in gastrointestinal (GI) reflexes that are prosecretory and promotile, and recently 5-HT's roles in chemosensation in the distal bowel have been described. We set out to establish the efficacy of 5-HT signaling, local 5-HT levels and pharmacology in discrete regions of the mouse small and large intestine. We also investigated the inter-relationships between incretin hormones, glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) and endogenous 5-HT in mucosal and motility assays. METHODS: Adult mouse GI mucosae were mounted in Ussing chambers and area-specific studies were performed to establish the 5-HT3 and 5-HT4 pharmacology, the sidedness of responses, and the inter-relationships between incretins and endogenous 5-HT. Natural fecal pellet transit in vitro and full-length GI transit in vivo were also measured. KEY RESULTS: We observed the greatest level of tonic and exogenous 5-HT-induced ion transport and highest levels of 5-HT in ascending colon mucosa. Here both 5-HT3 and 5-HT4 receptors were involved but elsewhere in the GI tract epithelial basolateral 5-HT4 receptors mediate 5-HT's prosecretory effect. Exendin-4 and GIP induced 5-HT release in the ascending colon, while L cell-derived PYY also contributed to GIP mucosal effects in the descending colon. Both peptides slowed colonic transit. CONCLUSIONS & INFERENCES: We provide functional evidence for paracrine interplay between 5-HT, GLP-1 and GIP, particularly in the colonic mucosal region. Basolateral epithelial 5-HT4 receptors mediated both 5-HT and incretin mucosal responses in healthy colon.

Vertical Sleeve Gastrectomy Lowers SGLT2/Slc5a2 Expression in the Mouse Kidney.

Bariatric surgery improves glucose homeostasis, but the underlying mechanisms are not fully elucidated. Here, we show that the expression of sodium-glucose cotransporter 2 (SGLT2/Slc5a2) is reduced in the kidney of lean and obese mice following vertical sleeve gastrectomy (VSG). Indicating an important contribution of altered cotransporter expression to the impact of surgery, inactivation of the SGLT2/Slc5a2 gene by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 attenuated the effects of VSG, with glucose excursions following intraperitoneal injection lowered by ∼30% in wild-type mice but by ∼20% in SGLT2-null animals. The effects of the SGLT2 inhibitor dapaglifozin were similarly blunted by surgery. Unexpectedly, effects of dapaglifozin were still observed in SGLT2-null mice, consistent with the existence of metabolically beneficial off-target effects of SGLT2 inhibitors. Thus, we describe a new mechanism involved in mediating the glucose-lowering effects of bariatric surgery.

Selective release of gastrointestinal hormones induced by an orally active GPR39 agonist.

OBJECTIVES: Obesity is a complex disease associated with a high risk of comorbidities. Gastric bypass surgery, an invasive procedure with low patient eligibility, is currently the most effective intervention that achieves sustained weight loss. This beneficial effect is attributed to alterations in gut hormone signaling. An attractive alternative is to pharmacologically mimic the effects of bariatric surgery by targeting several gut hormonal axes. The G protein-coupled receptor 39 (GPR39) expressed in the gastrointestinal tract has been shown to mediate ghrelin signaling and control appetite, food intake, and energy homeostasis, but the broader effect on gut hormones is largely unknown. A potent and efficacious GPR39 agonist (Cpd1324) was recently discovered, but the in vivo function was not addressed. Herein we studied the efficacy of the GPR39 agonist, Cpd1324, on metabolism and gut hormone secretion. METHODS: Body weight, food intake, and energy expenditure in GPR39 agonist-treated mice and GPR39 KO mice were studied in calorimetric cages. Plasma ghrelin, glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and peptide YY (PYY) levels were measured. Organoids generated from murine and human small intestine and mouse colon were used to study GLP-1 and PYY release. Upon GPR39 agonist administration, dynamic changes in intracellular GLP-1 content were studied via immunostaining and changes in ion transport across colonic mucosa were monitored in Ussing chambers. The G protein activation underlying GPR39-mediated selective release of gut hormones was studied using bioluminescence resonance energy transfer biosensors. RESULTS: The GPR39 KO mice displayed a significantly increased food intake without corresponding increases in respiratory exchange ratios or energy expenditure. Oral administration of a GPR39 agonist induced an acute decrease in food intake and subsequent weight loss in high-fat diet (HFD)-fed mice without affecting their energy expenditure. The tool compound, Cpd1324, increased GLP-1 secretion in the mice as well as in mouse and human intestinal organoids, but not in GPR39 KO mouse organoids. In contrast, the GPR39 agonist had no effect on PYY or GIP secretion. Transepithelial ion transport was acutely affected by GPR39 agonism in a GLP-1- and calcitonin gene-related peptide (CGRP)-dependent manner. Analysis of Cpd1324 signaling properties showed activation of Gαq and Gαi/o signaling pathways in L cells, but not Gαs signaling. CONCLUSIONS: The GPR39 agonist described in this study can potentially be used by oral administration as a weight-lowering agent due to its stimulatory effect on GLP-1 secretion, which is most likely mediated through a unique activation of Gα subunits. Thus, GPR39 agonism may represent a novel approach to effectively treat obesity through selective modulation of gastrointestinal hormonal axes.

Highly Selective Y4 Receptor Antagonist Binds in an Allosteric Binding Pocket.

Human neuropeptide Y receptors (Y1R, Y2R, Y4R, and Y5R) belong to the superfamily of G protein-coupled receptors and play an important role in the regulation of food intake and energy metabolism. We identified and characterized the first selective Y4R allosteric antagonist (S)-VU0637120, an important step toward validating Y receptors as therapeutic targets for metabolic diseases. To obtain insight into the antagonistic mechanism of (S)-VU0637120, we conducted a variety of in vitro, ex vivo, and in silico studies. These studies revealed that (S)-VU0637120 selectively inhibits native Y4R function and binds in an allosteric site located below the binding pocket of the endogenous ligand pancreatic polypeptide in the core of the Y4R transmembrane domains. Taken together, our studies provide a first-of-its-kind tool for probing Y4R function and improve the general understanding of allosteric modulation, ultimately contributing to the rational development of allosteric modulators for peptide-activated G protein-coupled receptors (GPCRs).

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 (Isc) 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.

Identification and Characterization of the First Selective Y4 Receptor Positive Allosteric Modulator.

The human Y4 receptor (Y4R) and its cognate ligand, pancreatic polypeptide (PP), are involved in the regulation of energy expenditure, satiety, and food intake. This system represents a potential target for the treatment of metabolic diseases and has been extensively investigated and validated in vivo. Here, we present the compound tBPC (tert-butylphenoxycyclohexanol), a novel and selective Y4R positive allosteric modulator that potentiates Y4R activation in G-protein signaling and arrestin3 recruitment experiments. The compound has no effect on the binding of the orthosteric ligands, implying its allosteric mode of action at the Y4R and evidence for a purely efficacy-driven positive allosteric modulation. Finally, the ability of tBPC to selectively potentiate Y4R agonism initiated by PP was confirmed in mouse descending colon mucosa preparations expressing native Y4R, demonstrating Y4R positive allosteric modulation in vitro.

Properties of cholinergic and non-cholinergic submucosal neurons along the mouse colon.

Submucosal neurons are vital regulators of water and electrolyte secretion and local blood flow in the gut. Due to the availability of transgenic models for enteric neuropathies, the mouse has emerged as the research model of choice, but much is still unknown about the murine submucosal plexus. The progeny of choline acetyltransferase (ChAT)-Cre × ROSA26(YFP) reporter mice, ChAT-Cre;R26R-yellow fluorescent protein (YFP) mice, express YFP in every neuron that has ever expressed ChAT. With the aid of the robust YFP staining in these mice, we correlated the neurochemistry, morphology and electrophysiology of submucosal neurons in distal colon. We also examined whether there are differences in neurochemistry along the colon and in neurally mediated vectorial ion transport between the proximal and distal colon. All YFP(+) submucosal neurons also contained ChAT. Two main neurochemical but not electrophysiological groups of neurons were identified: cholinergic (containing ChAT) or non-cholinergic. The vast majority of neurons in the middle and distal colon were non-cholinergic but contained vasoactive intestinal peptide. In the distal colon, non-cholinergic neurons had one or two axons, whereas the cholinergic neurons examined had only one axon. All submucosal neurons exhibited S-type electrophysiology, shown by the lack of long after-hyperpolarizing potentials following their action potentials and fast excitatory postsynaptic potentials (EPSPs). Fast EPSPs were predominantly nicotinic, and somatic action potentials were mediated by tetrodotoxin-resistant voltage-gated channels. The size of submucosal ganglia decreased but the proportion of cholinergic neurons increased distally along the colon. The distal colon had a significantly larger nicotinic ion transport response than the proximal colon. This work shows that the properties of murine submucosal neurons and their control of epithelial ion transport differ between colonic regions. There are several key differences between the murine submucous plexus and that of other animals, including a lack of conventional intrinsic sensory neurons, which suggests there is an incomplete neuronal circuitry within the murine submucous plexus.

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.

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.

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) (

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

1. The aim of this study was to provide a pharmacological characterization of the Y receptor types responsible for neuropeptide Y (NPY), peptide YY (PYY) and pancreatic polypeptide (PP) effects upon electrogenic ion transport in isolated human colonic mucosa. 2. Preparations of descending colon were voltage-clamped at 0 mV in Ussing chambers and changes in short-circuit current (I(sc)) continuously recorded. Basolateral PYY, NPY, human PP (hPP), PYY(3 - 36), [Leu(31), Pro(34)]PYY (Pro(34)PYY) and [Leu(31), Pro(34)]-NPY (Pro(34)NPY) all reduced basal I(sc) in untreated colon. Of all the Y agonists tested PYY(3 - 36) responses were most sensitive to tetrodotoxin (TTX) pretreatment, indicating that Y(2)-receptors are located on intrinsic neurones as well as epithelia in this tissue. 3. The EC(50) values for Pro(34)PYY, PYY(3 - 36) and hPP were 9.7 nM (4.0 - 23.5), 11.4 nM (7.6 - 17.0) and 14.5 nM (10.2 - 20.5) and response curves exhibited similar efficacies. The novel Y(5) agonist [Ala(31), Aib(32)]-NPY had no effect at 100 nM. 4. Y(1) receptor antagonists, BIBP3226 and BIBO3304 both increased basal I(sc) levels per se and inhibited subsequent PYY and Pro(34)PYY but not hPP or PYY(3 - 36) responses. The Y(2) antagonist, BIIE0246 also raised basal I(sc) levels and attenuated subsequent PYY(3 - 36) but not Pro(34)PYY or hPP responses. 5. We conclude that Y(1) and Y(2) receptor-mediated inhibitory tone exists in human colon mucosa. PYY and NPY exert their effects via both Y(1) and Y(2) receptors, but the insensitivity of hPP responses to either Y(1) or Y(2) antagonism, or to TTX, indicates that Y(4) receptors are involved and that they are predominantly post-junctional in human colon.