Xenin, a gastrointestinal peptide, regulates feeding independent of the melanocortin signaling pathway

Central administered xenin induced Fos expression in nesfatin-1 neurons in rats

Xenin is a 25-amino acid peptide identified in human gastric mucosa, which is widely expressed in peripheral and central tissues. It is known that the central or peripheral administration of xenin decreases food intake in rodents. Nesfatin-1/NUCB2 (nesfatin-1) has been identified as an anorexic neuropeptide, it is often found co-localized with many peptides in the central nervous system. After the intracerebroventricular administration of xenin on nesfain-1-like immunoreactivity (LI) neurons, we examined its effects on food intake and water intake in rats. As a result, Fos-LI neurons were observed in the organum vasculosum of the laminae terminalis (OVLT), the median preoptic nucleus (MnPO), the subfornical organ (SFO), the supraoptic nucleus (SON), the paraventricular nucleus (PVN), the arcuate nucleus (Arc), the lateral hypothalamic area (LHA), the central amygdaloid nucleus (CAN), the dorsal raphe nucleus (DR), the locus coeruleus (LC), the area postrema (AP) and the nucleus of the solitary tract (NTS). After the administration, the number of Fos-LI neurons was significantly increased in the LC and the OVLT, the MnPO, the SFO, the SON, the PVN, the Arc, the LHA, the CAN, the DR, the AP and the NTS, compared with the control group. After the administration of xenin, we conducted double immunohistochemistry for Fos and nesfatin-1, and found that the number of nesfatin-1-LI neurons expressing Fos were significantly increased in the SON, the PVN, the Arc, the LHA, the CAN, the DR, the AP and the NTS, compared with the control group. The pretreatment of nesfatin-1 antisense significantly attenuated this xenin-induced feeding suppression, while that of nesfatin-1 missense showed no improvement. These results indicate that central administered xenin may have anorexia effects associated with activated central nesfatin-1 neurons.

CX3CL1 Action on Microglia Protects from Diet-Induced Obesity by Restoring POMC Neuronal Excitability and Melanocortin System Activity Impaired by High-Fat Diet Feeding

Both hypothalamic microglial inflammation and melanocortin pathway dysfunction contribute to diet-induced obesity (DIO) pathogenesis. Previous studies involving models of altered microglial signaling demonstrate altered DIO susceptibility with corresponding POMC neuron cytological changes, suggesting a link between microglia and the melanocortin system. We addressed this hypothesis using the specific microglial silencing molecule, CX3CL1 (fractalkine), to determine whether reducing hypothalamic microglial activation can restore POMC/melanocortin signaling to protect against DIO. We performed metabolic analyses in high fat diet (HFD)-fed mice with targeted viral overexpression of CX3CL1 in the hypothalamus. Electrophysiologic recording in hypothalamic slices from POMC-MAPT-GFP mice was used to determine the effects of HFD feeding and microglial silencing via minocycline or CX3CL1 on GFP-labeled POMC neurons. Finally, mice with hypothalamic overexpression of CX3CL1 received central treatment with the melanocortin receptor antagonist SHU9119 to determine whether melanocortin signaling is required for the metabolic benefits of CX3CL1. Hypothalamic overexpression of CX3CL1 increased leptin sensitivity and POMC gene expression, while reducing weight gain in animals fed an HFD. In electrophysiological recordings from hypothalamic slice preparations, HFD feeding was associated with reduced POMC neuron excitability and increased amplitude of inhibitory postsynaptic currents. Microglial silencing using minocycline or CX3CL1 treatment reversed these HFD-induced changes in POMC neuron electrophysiologic properties. Correspondingly, blockade of melanocortin receptor signaling in vivo prevented both the acute and chronic reduction in food intake and body weight mediated by CX3CL1. Our results show that suppressing microglial activation during HFD feeding reduces DIO susceptibility via a mechanism involving increased POMC neuron excitability and melanocortin signaling.

Minimum biological domain of xenin-25 required to induce anion secretion in the rat ileum

Xenin-25 has a variety of physiological functions in the gastrointestinal tract, including ion transport and motility. Xenin-25 and neurotensin show sequence homology, especially near their C-terminal regions. The sequence similarity between xenin-25 and neurotensin indicates that the effects of xenin-25 is mediated by the neurotensin receptor but some biological actions of xenin-25 are independent. We have previously reported that xenin-25 modulates intestinal ion transport and colonic smooth muscle activity. However, minimal biological domain of xenin-25 to induce ion transport was not clear. To improve the mechanistic understanding of xenin-25 and to gain additional insights into the functions of xenin-25, the present study was designed to determine the minimal biological domain of xenin-25 required for ion transport in the rat ileum using various truncated xenin fragments and analogues in an Ussing chamber system. The present results demonstrate that the minimum biological domain of xenin-25 to induce Cl-/HCO3- secretion in the ileum contains the C-terminal pentapeptide. Furthermore, Arg at position 21 is important to retain the biological activity of xenin-25 and induces Cl-/HCO3- secretion in the rat ileum.

Xenin and Related Peptides: Potential Therapeutic Role in Diabetes and Related Metabolic Disorders

Xenin bioactivity and its role in normal physiology has been investigated by several research groups since its discovery in 1992. The 25 amino acid peptide hormone is secreted from the same enteroendocrine K-cells as the incretin hormone glucose-dependent insulinotropic polypeptide (GIP), with early studies highlighting the biological significance of xenin in the gastrointestinal tract, along with effects on satiety. Recently there has been more focus directed towards the role of xenin in insulin secretion and potential for diabetes therapies, especially through its ability to potentiate the insulinotropic actions of GIP as well as utilisation in dual/triple acting gut hormone therapeutic approaches. Currently, there is a lack of clinically approved therapies aimed at restoring GIP bioactivity in type 2 diabetes mellitus, thus xenin could hold real promise as a diabetes therapy. The biological actions of xenin, including its ability to augment insulin secretion, induce satiety effects, as well as restoring GIP sensitivity, earmark this peptide as an attractive antidiabetic candidate. This minireview will focus on the multiple biological actions of xenin, together with its proposed mechanism of action and potential benefits for the treatment of metabolic diseases such as diabetes.

The Role of Central Neurotensin in Regulating Feeding and Body Weight

The small peptide neurotensin (Nts) is implicated in myriad processes including analgesia, thermoregulation, reward, arousal, blood pressure, and modulation of feeding and body weight. Alterations in Nts have recently been described in individuals with obesity or eating disorders, suggesting that disrupted Nts signaling may contribute to body weight disturbance. Curiously, Nts mediates seemingly opposing regulation of body weight via different tissues. Peripherally acting Nts promotes fat absorption and weight gain, whereas central Nts signaling suppresses feeding and weight gain. Thus, because Nts is pleiotropic, a location-based approach must be used to understand its contributions to disordered body weight and whether the Nts system might be leveraged to improve metabolic health. Here we review the role of Nts signaling in the brain to understand the sites, receptors, and mechanisms by which Nts can promote behaviors that modify body weight. New techniques permitting site-specific modulation of Nts and Nts receptor-expressing cells suggest that, even in the brain, not all Nts circuitry exerts the same function. Intriguingly, there may be dedicated brain regions and circuits via which Nts specifically suppresses feeding behavior and weight gain vs other Nts-attributed physiology. Defining the central mechanisms by which Nts signaling modifies body weight may suggest strategies to correct disrupted energy balance, as needed to address overweight, obesity, and eating disorders.

Neurotensin and Xenin Show Positive Correlations With Perceived Stress, Anxiety, Depressiveness and Eating Disorder Symptoms in Female Obese Patients

Objective

Neurotensin and xenin are two closely related anorexigenic neuropeptides synthesized in the small intestine that exert diverse peripheral and central functions. Both act via the neurotensin-1-receptor. In animal models of obesity reduced central concentrations of these peptides have been found. Dysregulations of the acute and chronic stress response are associated with development and maintenance of obesity. Until now, associations of both peptides with stress, anxiety, depressiveness, and eating disorder symptoms have not been investigated. The aim of the present study was to examine associations of neurotensin and xenin with these psychological characteristics under conditions of obesity.

Materials and Methods

From 2010 to 2016 we consecutively enrolled 160 inpatients (63 men and 97 women), admitted due to obesity and its mental and somatic comorbidities. Blood withdrawal und psychometric tests (PSQ-20, GAD-7, PHQ-9, and EDI-2) occurred within one week after admission. We measured levels of neurotensin and xenin in plasma by ELISA.

Results

Mean body mass index was 47.2 ± 9.5 kg/m². Concentrations of neurotensin and xenin positively correlated with each other (women: r = 0.788, p < 0.001; men: r = 0.731, p < 0.001) and did not significantly differ between sexes (p > 0.05). Women generally displayed higher psychometric values than men (PSQ-20: 58.2 ± 21.7 vs. 47.0 ± 20.8, p = 0.002; GAD-7: 9.7 ± 5.8 vs. 7.1 ± 5.3, p = 0.004; PHQ-9: 11.6 ± 6.6 vs. 8.8 ± 5.9, p = 0.008; EDI-2: 50.5 ± 12.8 vs. 39.7 ± 11.9, p < 0.001). Only women showed positive correlations of both neuropeptides with stress (neurotensin: r = 0.231, p = 0.023; xenin: r = 0.254, p = 0.013), anxiety (neurotensin: r = 0.265, p = 0.009; xenin: r = 0.257, p = 0.012), depressiveness (neurotensin: r = 0.281, p = 0.006; xenin: r = 0.241, p = 0.019) and eating disorder symptoms (neurotensin: r = 0.276, p = 0.007; xenin: r = 0.26, p = 0.011), whereas, men did not (p > 0.05).

Conclusion

Neurotensin and xenin plasma levels of female obese patients are positively correlated with perceived stress, anxiety, depressiveness, and eating disorder symptoms. These associations could be influenced by higher prevalence of mental disorders in women and by sex hormones. In men, no correlations were observed, which points toward a sex-dependent regulation.

Reduction in Nesfatin-1 Levels in the Cerebrospinal Fluid and Increased Nigrostriatal Degeneration Following Ventricular Administration of Anti-nesfatin-1 Antibody in Mice

Nesfatin-1 is one of several brain-gut peptides that have a close relationship with the central dopaminergic system. Our previous studies have shown that nesfatin-1 is capable of protecting nigral dopaminergic neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity. A recent study also revealed a reduced blood level of nesfatin-1 in patients with Parkinson’s disease (PD). The current study was designed to investigate whether reduced nesfatin-1 in cerebrospinal fluid (CSF) induces nigrostriatal system degeneration. An intra-cerebroventricular (ICV) injection technique was used to administer anti-nesfatin-1 antibody directly into the lateral ventricle of the brain. Enzyme-linked immunosorbent assay (ELISA) results showed that ICV injection of anti-nesfatin-1 antibody into the lateral ventricle of the brain once daily for 2 weeks caused a significant reduction in nesfatin-1 levels in the CSF (93.1%). Treatment with anti-nesfatin-1 antibody resulted in a substantial loss (23%) of TH-positive (TH+) dopaminergic neurons in the substantia nigra pars compacta (SNpc), as shown by immunofluorescence staining, a depletion in dopamine and its metabolites in the striatum detected by high-performance liquid chromatography (HPLC), and obvious nuclear shrinkage and mitochondrial lesions in dopaminergic neurons in the SNpc detected by transmission electron microscopy (TEM). Furthermore, the results from our Western blot and ELISA experiments demonstrated that anti-nesfatin-1 antibody injection induced an upregulation of caspase-3 activation, increased the expression of p-ERK, and elevated brain-derived neurotrophic factor (BDNF) levels in the SNpc. Taken together, these observations suggest that reduced nesfatin-1 in the brain may induce nigrostriatal dopaminergic system degeneration; this effect may be mediated via mitochondrial dysfunction-related apoptosis. Our data support a role of nesfatin-1 in maintaining the normal physiological function of the nigrostriatal dopaminergic system.

Antidiabetic effects and sustained metabolic benefits of sub-chronic co-administration of exendin-4/gastrin and xenin-8-Gln in high fat fed mice

The present study has examined the antidiabetic effects of 21 days co-administration of xenin-8-Gln with the dual-acting fusion peptide, exendin-4/gastrin, as well as persistence of beneficial metabolic benefits, in high fat fed (HFF) mice. Xenin-8-Gln, exendin-4 and gastrin represent compounds that activate receptors of the gut-derived hormones, xenin, glucagon-like peptide-1 (GLP-1) and gastrin, respectively. Twice-daily administration of exendin-4/gastrin, xenin-8-Gln or a combination of both peptides significantly reduced circulating glucose, HbA1c and cumulative energy intake. Combination therapy with xenin-8-Gln and exendin-4/gastrin increased circulating insulin. All HFF mice treated with exendin-4/gastrin presented with body weight similar to lean control mice on day 21. Each treatment improved glucose tolerance and the glucose-lowering actions of glucose dependent insulinotropic polypeptide (GIP), as well as augmenting glucose- and GIP-induced insulin secretion, with benefits being most prominent in the combination group. Administration of exendin-4/gastrin alone, and in combination with xenin-8-Gln, increased pancreatic insulin content and improved the insulin sensitivity index. Pancreatic beta-cell area was significantly increased, and alpha cell area decreased, by all treatments, with the combination group also displaying enhanced overall islet area. Notably, metabolic benefits were generally retained in all groups of HFF mice, and especially in the combination group, following discontinuation of the treatment regimens for 21 days. This was associated with maintenance of increased islet and beta-cell areas. Together, these data confirm the antidiabetic effects of co-activation of GLP-1, gastrin and xenin cell signalling pathways, and highlight the sustainable benefits this type of treatment paradigm can offer in T2DM.

Effects of an enzymatically stable C-terminal hexapseudopeptide fragment peptide of xenin-25, ψ-xenin-6, on pancreatic islet function and metabolism

Xenin-25 undergoes rapid enzyme metabolism following secretion. Early studies demonstrated bioactivity of a C-terminal hexapeptide fragment of xenin-25, namely xenin-6, which were enhanced through introduction of a reduced N-terminal peptide bond, to yield Ψ-xenin-6. The present study was undertaken to define the biological actions and potential antidiabetic properties of Ψ-xenin-6. In vitro enzymatic stability, insulin and glucagon secretory activity, as well as effects on beta-cell survival were determined. Studies in mice were used to assess the impact of Ψ-xenin-6 on glucose homeostasis and satiety. Ψ-xenin-6 was resistant to murine plasma degradation. In BRIN-BD11 cells and isolated murine islets, Ψ-xenin-6 significantly stimulated insulin secretion, and prominently enhanced the insulinotropic actions of GIP. Xenin-6 and Ψ-xenin-6 had no impact on glucagon secretion, although xenin-6 partially reversed the glucagonotropic action of GIP. Further in vitro investigations revealed that, similar to GLP-1, Ψ-xenin-6 significantly augmented proliferation of human and rodent clonal beta-cells, whilst also fully protecting against cytokine-induced beta-cell cytotoxicity, with greater potency than xenin-25 and xenin-6. When administered to mice in combination with glucose, Ψ-xenin-6 significantly reduced glucose levels and enhanced glucose-induced insulin release, with a duration of biological action beyond 8 h. Ψ-xenin-6 also significantly enhanced the glucose-lowering action of GIP in vivo. In overnight fasted mice, Ψ-xenin-6 exhibited satiety actions at both 25 and 250 nmol/kg. These data demonstrates that Ψ-xenin-6 is a metabolically stable C-terminal fragment analogue of xenin-25, with a metabolic action profile that merits further study as a potential antidiabetic compound.

Emerging therapeutic potential for xenin and related peptides in obesity and diabetes

Xenin-25 is a 25-amino acid peptide hormone co-secreted from the same enteroendocrine K-cell as the incretin peptide glucose-dependent insulinotropic polypeptide. There is no known specific receptor for xenin-25, but studies suggest that at least some biological actions may be mediated through interaction with the neurotensin receptor. Original investigation into the physiological significance of xenin-25 focussed on effects related to gastrointestinal transit and satiety. However, xenin-25 has been demonstrated in pancreatic islets and recently shown to possess actions in relation to the regulation of insulin and glucagon secretion, as well as promoting beta-cell survival. Accordingly, the beneficial impact of xenin-25, and related analogues, has been assessed in animal models of diabetes-obesity. In addition, studies have demonstrated that metabolically active fragment peptides of xenin-25, particularly xenin-8, possess independent therapeutic promise for diabetes, as well as serving as bioactive components for the generation of multi-acting hybrid peptides with antidiabetic potential. This review focuses on continuing developments with xenin compounds in relation to new therapeutic approaches for diabetes-obesity.

Connexin36 Expression in Primary Afferent Neurons in Relation to the Axon Reflex and Modality Coding of Somatic Sensation

Electrical coupling mediated by connexin36-containing gap junctions that form electrical synapses is known to be prevalent in the central nervous system, but such coupling was long ago reported also to occur between cutaneous sensory fibers. Here, we provide evidence supporting the capability of primary afferent fibers to engage in electrical coupling. In transgenic mice with enhanced green fluorescent protein (eGFP) serving as a reporter for connexin36 expression, immunofluorescence labeling of eGFP was found in subpopulations of neurons in lumbar dorsal root and trigeminal sensory ganglia, and in fibers within peripheral nerves and tissues. Immunolabeling of connexin36 was robust in the sciatic nerve, weaker in sensory ganglia than in peripheral nerve, and absent in these tissues from Cx36 null mice. Connexin36 mRNA was detected in ganglia from wild-type mice, but not in those from Cx36 null mice. Labeling of eGFP was localized within a subpopulation of ganglion cells containing substance P and calcitonin gene-releasing peptide, and in peripheral fibers containing these peptides. Expression of eGFP was also found in various proportions of sensory ganglion neurons containing transient receptor potential (TRP) channels, including TRPV1 and TRPM8. Ganglion cells labeled for isolectin B4 and tyrosine hydroxylase displayed very little co-localization with eGFP. Our results suggest that previously observed electrical coupling between peripheral sensory fibers occurs via electrical synapses formed by Cx36-containing gap junctions, and that some degree of selectivity in the extent of electrical coupling may occur between fibers belonging to subpopulations of sensory neurons identified according to their sensory modality responsiveness.

Xenin is a novel anorexigen in goldfish (Carassius auratus)

Xenin, a highly conserved 25 amino acid peptide cleaved from the N-terminus of the coatomer protein alpha (COPA), is emerging as a food intake regulator in mammals and birds. To date, no research has been conducted on xenin biology in fish. This study aims to identify the copa mRNA encoding xenin in goldfish (Carassius auratus) as a model, to elucidate its regulation by feeding, and to describe the role of xenin on appetite. First, a partial sequence of copa cDNA, a region encoding xenin, was identified from goldfish brain. This sequence is highly conserved among both vertebrates and invertebrates. RT-qPCR revealed that copa mRNAs are widely distributed in goldfish tissues, with the highest levels detected in the brain, gill, pituitary and J-loop. Immunohistochemistry confirmed also the presence of COPA peptide in the hypothalamus and enteroendocrine cells on the J-loop mucosa. In line with its anorexigenic effects, we found important periprandial fluctuations in copa mRNA expression in the hypothalamus, which were mainly characterized by a gradually decrease in copa mRNA levels as the feeding time was approached, and a gradual increase after feeding. Additionally, fasting differently modulated the expression of copa mRNA in a tissue-dependent manner. Peripheral and central injections of xenin reduce food intake in goldfish. This research provides the first report of xenin in fish, and shows that this peptide is a novel anorexigen in goldfish.

Stimulation of white adipose tissue lipolysis by xenin, a neurotensin-related peptide

Xenin is a gastrointestinal hormone that belongs to the neurotensin family. Central administration of xenin to obese mice reduces food intake and body weight gain and causes alterations in the expression of lipid metabolism-related genes and proteins in white adipose tissue (WAT). However, it has not been tested whether or not xenin directly acts on adipose tissue and alters lipid metabolism. The present study was performed to address this possibility by examining the effect of xenin treatment on the levels of glycerol and free fatty acids (FFA) and expression levels of lipolysis marker proteins ex vivo in cultured mouse WAT. Xenin treatment significantly increased concentrations of glycerol and FFA in culture media and increased phosphorylation of hormone sensitive lipase (HSL) in ex vivo cultured WAT. These findings support the hypothesis that xenin directly acts on adipose tissues and stimulates lipolysis. Thus, enhancement of xenin action and its downstream signaling may offer a novel and effective therapy for obese patients by reducing the amount of stored fat in adipose tissue.

Cholinergic signaling mediates the effects of xenin-25 on secretion of pancreatic polypeptide but not insulin or glucagon in humans with impaired glucose tolerance

We previously demonstrated that infusion of an intestinal peptide called xenin-25 (Xen) amplifies the effects of glucose-dependent insulinotropic polypeptide (GIP) on insulin secretion rates (ISRs) and plasma glucagon levels in humans. However, these effects of Xen, but not GIP, were blunted in humans with type 2 diabetes. Thus, Xen rather than GIP signaling to islets fails early during development of type 2 diabetes. The current crossover study determines if cholinergic signaling relays the effects of Xen on insulin and glucagon release in humans as in mice. Fasted subjects with impaired glucose tolerance were studied. On eight separate occasions, each person underwent a single graded glucose infusion- two each with infusion of albumin, Xen, GIP, and GIP plus Xen. Each infusate was administered ± atropine. Heart rate and plasma glucose, insulin, C-peptide, glucagon, and pancreatic polypeptide (PP) levels were measured. ISRs were calculated from C-peptide levels. All peptides profoundly increased PP responses. From 0 to 40 min, peptide(s) infusions had little effect on plasma glucose concentrations. However, GIP, but not Xen, rapidly and transiently increased ISRs and glucagon levels. Both responses were further amplified when Xen was co-administered with GIP. From 40 to 240 min, glucose levels and ISRs continually increased while glucagon concentrations declined, regardless of infusate. Atropine increased resting heart rate and blocked all PP responses but did not affect ISRs or plasma glucagon levels during any of the peptide infusions. Thus, cholinergic signaling mediates the effects of Xen on insulin and glucagon release in mice but not humans.

Anti-diabetic potential of peptides: Future prospects as therapeutic agents

Diabetes mellitus is a metabolic disorder in which the glucose level in blood exceeds beyond the normal level. Persistent hyperglycemia leads to diabetes late complication and obviously account for a large number of morbidity and mortality worldwide. Numerous therapeutic options are available for the treatment of diabetes including insulin for type I and oral tablets for type II, but its effective management is still a dream. To date, several options are under investigation in various research laboratories for efficacious and safer agents. Of them, peptides are currently amongst the most widely investigated potential therapeutic agents whose design and optimal uses are under development. A number of natural and synthetic peptides have so far been found with outstanding antidiabetic effect mediated through diverse mechanisms. The applications of new emerging techniques and drug delivery systems further offer opportunities to achieve the desired target outcomes. Some outstanding peptides in preclinical and clinical studies with better efficacy and safety profile have already been identified. Further detail studies on these peptides may therefore lead to significant clinically useful antidiabetic agents.

Central action of xenin affects the expression of lipid metabolism-related genes and proteins in mouse white adipose tissue

Xenin is a gastrointestinal hormone that reduces food intake when administered centrally and it has been hypothesized that central action of xenin participates in the regulation of whole-body metabolism. The present study was performed to address this hypothesis by investigating the central effect of xenin on the expression of genes and proteins that are involved in the regulation of lipid metabolism in white adipose tissue (WAT). Male obese ob/ob mice received intracerebroventricular (i.c.v.) injections of xenin (5μg) twice 12h apart. Food intake and body weight change during a 24-h period after the first injection were measured. Epididymal WAT was collected at the end of the 24-h treatment period and levels of lipid metabolism-related genes and proteins were measured. Xenin treatment caused significant reductions in food intake and body weight compared to control vehicle treatment. Levels of fatty acid synthase (FASN) protein were significantly reduced by xenin treatment, while levels of adipose triglyceride lipase (Atgl) and beta-3 adrenergic receptor (Adrb3) mRNA and phosphorylated hormone sensitive lipase (Ser⁶⁶⁰-pHSL and Ser⁵⁶³-pHSL) were significantly increased by xenin treatment. These findings suggest that central action of xenin causes alterations in lipid metabolism in adipose tissue toward reduced lipogenesis and increased lipolysis, possibly contributing to xenin-induced body weight reduction. Thus, enhancing central action of xenin and its downstream targets may be possible targets for the treatment of obesity by reducing the amount of stored fat in adipose tissue.

An enzymatically stable GIP/xenin hybrid peptide restores GIP sensitivity, enhances beta cell function and improves glucose homeostasis in high-fat-fed mice

AIMS/HYPOTHESIS

Glucose-dependent insulinotropic polypeptide (GIP) and xenin, regulatory gut hormones secreted from enteroendocrine K cells, exert important effects on metabolism. In addition, xenin potentiates the biological actions of GIP. The present study assessed the actions and therapeutic utility of a (DAla²)GIP/xenin-8-Gln hybrid peptide, in comparison with the parent peptides (DAla²)GIP and xenin-8-Gln.

METHODS

Following confirmation of enzymatic stability, insulin secretory activity of (DAla²)GIP/xenin-8-Gln was assessed in BRIN-BD11 beta cells. Acute and persistent glucose-lowering and insulin-releasing effects were then examined in vivo. Finally, the metabolic benefits of twice daily injection of (DAla²)GIP/xenin-8-Gln was determined in high-fat-fed mice.

RESULTS

All peptides significantly (p < 0.05 to p < 0.001) enhanced in vitro insulin secretion from pancreatic clonal BRIN-BD11 cells, with xenin (and particularly GIP)-related signalling pathways, being important for this action. Administration of (DAla²)GIP or (DAla²)GIP/xenin-8-Gln in combination with glucose significantly (p < 0.05) lowered blood glucose and increased plasma insulin in mice, with a protracted response of up to 4 h. All treatments elicited appetite-suppressive effects (p < 0.05), particularly (DAla²)GIP/xenin-8-Gln and xenin-8-Gln at elevated doses of 250 nmol/kg. Twice-daily administration of (DAla²)GIP/xenin-8-Gln or (DAla²)GIP for 21 days to high-fat-fed mice returned circulating blood glucose to lean control levels. In addition, (DAla²)GIP/xenin-8-Gln treatment significantly (p < 0.05) reduced glycaemic levels during a 24 h glucose profile assessment. Neither of the treatment regimens had an effect on body weight, energy intake or circulating insulin concentrations. However, insulin sensitivity was significantly (p < 0.001) improved by both treatments. Interestingly, GIP-mediated glucose-lowering (p < 0.05) and insulin-releasing (p < 0.05 to p < 0.01) effects were substantially improved by (DAla²)GIP and (DAla²)GIP/xenin-8-Gln treatment. Pancreatic islet and beta cell area (p < 0.001), as well as pancreatic insulin content (p < 0.05), were augmented in (DAla²)GIP/xenin-8-Gln-treated mice, related to enhanced proliferation and decreased apoptosis of beta cells, whereas (DAla²)GIP evoked increases (p < 0.05 to p < 0.01) in islet number.

CONCLUSIONS/INTERPRETATION

These studies highlight the clear potential of GIP/xenin hybrids for the treatment of type 2 diabetes.

Metabolic responses to xenin-25 are altered in humans with Roux-en-Y gastric bypass surgery

Xenin-25 (Xen) is a neurotensin-related peptide secreted by a subset of enteroendocrine cells located in the proximal small intestine. Many effects of Xen are mediated by neurotensin receptor-1 on neurons. In healthy humans with normal glucose tolerance (NGT), Xen administration causes diarrhea and inhibits postprandial glucagon-like peptide-1 (GLP-1) release but not insulin secretion. This study determines (i) if Xen has similar effects in humans with Roux-en-Y gastric bypass (RYGB) and (ii) whether neural pathways potentially mediate effects of Xen on glucose homeostasis. Eight females with RYGB and no history of type 2 diabetes received infusions with 0, 4 or 12pmol Xen/kg/min with liquid meals on separate occasions. Plasma glucose and gastrointestinal hormone levels were measured and insulin secretion rates calculated. Pancreatic polypeptide and neuropeptide Y levels were surrogate markers for parasympathetic input to islets and sympathetic tone, respectively. Responses were compared to those in well-matched non-surgical participants with NGT from our earlier study. Xen similarly increased pancreatic polypeptide and neuropeptide Y responses in patients with and without RYGB. In contrast, the ability of Xen to inhibit GLP-1 release and cause diarrhea was severely blunted in patients with RYGB. With RYGB, Xen had no statistically significant effect on glucose, insulin secretory, GLP-1, glucose-dependent insulinotropic peptide, and glucagon responses. However, insulin and glucose-dependent insulinotropic peptide secretion preceded GLP-1 release suggesting circulating GLP-1 does not mediate exaggerated insulin release after RYGB. Thus, Xen has unmasked neural circuits to the distal gut that inhibit GLP-1 secretion, cause diarrhea, and are altered by RYGB.

Xenin-induced feeding suppression is not mediated through the activation of central extracellular signal-regulated kinase signaling in mice

Xenin is a gut hormone that reduces food intake by partly acting through the hypothalamus via neurotensin receptor 1 (Ntsr1). However, specific signaling pathways that mediate xenin-induced feeding suppression are not fully understood. Activation of Ntsr1 leads to the activation of the extracellular signal-regulated kinase (ERK). Hypothalamic ERK participates in the regulation of food intake by mediating the effect of hormonal signals. Therefore, we hypothesized that the anorectic effect of xenin is mediated by hypothalamic ERK signaling. To address this hypothesis, we compared levels of phosphorylation of ERK1/2 (pERK1/2) in the hypothalamus of both control and xenin-treated mice. The effect of xenin on ERK1/2 phosphorylation was also examined in mouse hypothalamic neuronal cell lines with or without Ntsr1. We also examined the effect of the blockade of central ERK signaling on xenin-induced feeding suppression in mice. The intraperitoneal (i.p.) injection of xenin caused a significant increase in the number of pERK1/2-immunoreactive cells in the hypothalamic arcuate nucleus. The majority of pERK1/2-positive cells expressed neuronal nuclei (NeuN), a marker for neurons. Xenin treatment increased pERK1/2 levels in one cell line expressing Ntsr1 but not another line without Ntsr1 expression. Both i.p. and intracerebroventricular (i.c.v.) injections of xenin reduced food intake in mice. The i.c.v. pre-treatment with U0126, a selective inhibitor of ERK1/2 upstream kinases, did not affect xenin-induced reduction in food intake. These findings suggest that although xenin activates ERK signaling in subpopulations of hypothalamic neurons, xenin does not require the activation of hypothalamic ERK signaling pathway to elicit feeding suppression.

Hormonal Responses to Cholinergic Input Are Different in Humans with and without Type 2 Diabetes Mellitus

Peripheral muscarinic acetylcholine receptors regulate insulin and glucagon release in rodents but their importance for similar roles in humans is unclear. Bethanechol, an acetylcholine analogue that does not cross the blood-brain barrier, was used to examine the role of peripheral muscarinic signaling on glucose homeostasis in humans with normal glucose tolerance (NGT; n = 10), impaired glucose tolerance (IGT; n = 11), and type 2 diabetes mellitus (T2DM; n = 9). Subjects received four liquid meal tolerance tests, each with a different dose of oral bethanechol (0, 50, 100, or 150 mg) given 60 min before a meal containing acetaminophen. Plasma pancreatic polypeptide (PP), glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), glucose, glucagon, C-peptide, and acetaminophen concentrations were measured. Insulin secretion rates (ISRs) were calculated from C-peptide levels. Acetaminophen and PP concentrations were surrogate markers for gastric emptying and cholinergic input to islets. The 150 mg dose of bethanechol increased the PP response 2-fold only in the IGT group, amplified GLP-1 release in the IGT and T2DM groups, and augmented the GIP response only in the NGT group. However, bethanechol did not alter ISRs or plasma glucose, glucagon, or acetaminophen concentrations in any group. Prior studies showed infusion of xenin-25, an intestinal peptide, delays gastric emptying and reduces GLP-1 release but not ISRs when normalized to plasma glucose levels. Analysis of archived plasma samples from this study showed xenin-25 amplified postprandial PP responses ~4-fold in subjects with NGT, IGT, and T2DM. Thus, increasing postprandial cholinergic input to islets augments insulin secretion in mice but not humans.

Trial Registration: ClinicalTrials.gov NCT01434901

Biological Activity and Antidiabetic Potential of C-Terminal Octapeptide Fragments of the Gut-Derived Hormone Xenin

Xenin is a peptide that is co-secreted with the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP), from intestinal K-cells in response to feeding. Studies demonstrate that xenin has appetite suppressive effects and modulates glucose-induced insulin secretion. The present study was undertaken to determine the bioactivity and antidiabetic properties of two C-terminal fragment xenin peptides, namely xenin 18-25 and xenin 18-25 Gln. In BRIN-BD11 cells, both xenin fragment peptides concentration-dependently stimulated insulin secretion, with similar efficacy as the parent peptide. Neither fragment peptide had any effect on acute feeding behaviour at elevated doses of 500 nmol/kg bw. When administered together with glucose to normal mice at 25 nmol/kg bw, the overall insulin secretory effect was significantly enhanced in both xenin 18-25 and xenin 18-25 Gln treated mice, with better moderation of blood glucose levels. Twice daily administration of xenin 18-25 or xenin 18-25 Gln for 21 days in high fat fed mice did not affect energy intake, body weight, circulating blood glucose or body fat stores. However, circulating plasma insulin concentrations had a tendency to be elevated, particularly in xenin 18-25 Gln mice. Both treatment regimens significantly improved insulin sensitivity by the end of the treatment period. In addition, sustained treatment with xenin 18-25 Gln significantly reduced the overall glycaemic excursion and augmented the insulinotropic response to an exogenous glucose challenge on day 21. In harmony with this, GIP-mediated glucose-lowering and insulin-releasing effects were substantially improved by twice daily xenin 18-25 Gln treatment. Overall, these data provide evidence that C-terminal octapeptide fragments of xenin, such as xenin 18-25 Gln, have potential therapeutic utility for type 2 diabetes.

Regulation of appetite, satiation, and body weight by enteroendocrine cells. Part 1: characteristics of enteroendocrine cells and their capability of weight regulation

The gastrointestinal tract is the gateway for food in our body. Food ingestion and the ensuing digestive processes depend on the composition and amount of ingested nutrients. This complex process of nutrient digestion and absorption is effectively regulated by the enteroendocrine system. Enteroendocrine cells (EECs) reside scattered throughout the intestinal epithelium. They express nutrient receptors that face the lumen and secrete peptide hormones in response to food. Besides regulating digestion, gastrointestinal endocrine cells are involved in the regulation of appetite and satiety. The first part of this review describes the anatomical and biological characteristics of EECs and discusses the capability of their hormones to influence appetite, satiety, and body weight. In the second part, we then discuss the therapeutic potential of EECs in the treatment of obesity.

Xenin-25[Lys13PAL]: a novel long-acting acylated analogue of xenin-25 with promising antidiabetic potential

AIMS

Xenin-25 is co-secreted with glucose-dependent insulinotropic polypeptide (GIP) from intestinal K-cells following a meal. Xenin-25 is believed to play a key role in glucose homoeostasis and potentiate the insulinotropic effect of GIP.

METHODS

This study investigated the effects of sub-chronic administration of the stable and longer-acting xenin-25 analogue, xenin-25[Lys(13)PAL] (25 nmol/kg), in diabetic mice fed with a high-fat diet.

RESULTS

Initial studies confirmed the significant persistent glucose-lowering (p < 0.05) and insulin-releasing (p < 0.05) actions of xenin-25[Lys(13)PAL] compared with native xenin-25. Interestingly, xenin-25 retained significant glucose-lowering activity in GIP receptor knockout mice. Twice-daily intraperitoneal (i.p.) injection of xenin-25[Lys(13)PAL] for 14 days had no significant effect on food intake or body weight in high-fat-fed mice. Non-fasting glucose and insulin levels were also unchanged, but overall glucose levels during an i.p. glucose tolerance and oral nutrient challenge were significantly (p < 0.05) lowered by xenin-25[Lys(13)PAL] treatment. These changes were accompanied by significant improvements in i.p. (p < 0.05) and oral (p < 0.001) nutrient-stimulated insulin concentrations. No appreciable changes in insulin sensitivity were observed between xenin-25[Lys(13)PAL] and saline-treated high-fat mice. However, xenin-25[Lys(13)PAL] treatment restored notable sensitivity to the biological actions of exogenous GIP injection. Consumption of O2, production of CO2, respiratory exchange ratio and energy expenditure were not altered by 14-day twice-daily treatment with xenin-25[Lys(13)PAL]. In contrast, ambulatory activity was significantly (p < 0.05 to p < 0.001) increased during the dark phase in xenin-25[Lys(13)PAL] mice compared with high-fat controls.

CONCLUSIONS

These data indicate that sustained administration of a stable analogue of xenin-25 exerts a spectrum of beneficial metabolic effects in high-fat-fed mice.

The gut sensor as regulator of body weight

The gastrointestinal (GI) tract comprises a large endocrine organ that regulates not only nutrient sensing and metabolising but also satiety and energy homeostasis. More than 20 hormones secreted from the stomach, intestine, and pancreas as well as signaling mediators of the gut microbiome are involved in this process. A better understanding of how related pathways affect body weight and food intake will help us to find new strategies and drugs to treat obesity. For example, weight loss secondary to lifestyle intervention is often accompanied by unfavorable changes in multiple GI hormones, which may cause difficulties in maintaining a lower body weight status. Conversely, bariatric surgery favorably changes the hormone profile to support improved satiety and metabolic function. This partially explains stronger sustained body weight reduction resulting in better long-term results of improved metabolic functions. This review focuses on GI hormones and signaling mediators of the microbiome involved in satiety regulation and energy homeostasis and summarizes their changes following weight loss. Furthermore, the potential role of GI hormones as anti-obesity drugs is discussed.

Global biochemical profiling identifies β-hydroxypyruvate as a potential mediator of type 2 diabetes in mice and humans

Glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 are incretins secreted by respective K and L enteroendocrine cells after eating and amplify glucose-stimulated insulin secretion (GSIS). This amplification has been termed the "incretin response." To determine the role(s) of K cells for the incretin response and type 2 diabetes mellitus (T2DM), diphtheria toxin-expressing (DT) mice that specifically lack GIP-producing cells were backcrossed five to eight times onto the diabetogenic NONcNZO10/Ltj background. As in humans with T2DM, DT mice lacked an incretin response, although GLP-1 release was maintained. With high-fat (HF) feeding, DT mice remained lean but developed T2DM, whereas wild-type mice developed obesity but not diabetes. Metabolomics identified biochemicals reflecting impaired glucose handling, insulin resistance, and diabetes complications in prediabetic DT/HF mice. β-Hydroxypyruvate and benzoate levels were increased and decreased, respectively, suggesting β-hydroxypyruvate production from d-serine. In vitro, β-hydroxypyruvate altered excitatory properties of myenteric neurons and reduced islet insulin content but not GSIS. β-Hydroxypyruvate-to-d-serine ratios were lower in humans with impaired glucose tolerance compared with normal glucose tolerance and T2DM. Earlier human studies unmasked a neural relay that amplifies GIP-mediated insulin secretion in a pattern reciprocal to β-hydroxypyruvate-to-d-serine ratios in all groups. Thus, K cells may maintain long-term function of neurons and β-cells by regulating β-hydroxypyruvate levels.

Evaluation of serum xenin and ghrelin levels and their relationship with nonalcoholic fatty liver disease and insulin resistance in obese adolescents

AIM

Xenin is a peptide of the neurotensin/xenopsin/xenin family secreted from gastric cells and other tissues. The first aim of this study was to investigate the serum xenin and ghrelin levels in obese children and compare the patients with healthy controls. The second aim was to compare the xenin levels in patients with nonalcoholic fatty liver disease (NAFLD) and also with insulin resistance with the patients without these complications.

METHODS

62 obese adolescents (27 with NAFLD) and 32 healthy controls were enrolled in the study. Obesity was defined as a body mass index exceeding the 95th percentile for the patients' age and sex. NAFLD was diagnosed via ultrasonographic examination. The insulin resistance was calculated by a homeostasis model assessment (HOMA-IR) index. Serum xenin and ghrelin levels were assessed by enzyme-linked immunosorbent assay.

RESULTS

The mean serum xenin concentration was significantly higher in obese adolescents than the healthy peers (68.15 ± 0.63 vs 16.54 ± 0.07 pg/mL, p = 0.000). Serum xenin levels were not different between the patients with and without NAFLD and also between the patients with and without IR (p > 0.05). There was a positive correlation between xenin levels and relative weight (r = 0.663, p < 0.001) and HOMA-IR (r = 0.612, p < 0.001). Ghrelin was negatively correlated with relative weight (r = -0.283, p < 0.05).

CONCLUSION

In this study, serum xenin levels of both groups of obese patients were found higher than controls. On the other hand, xenin levels were not different in patients with and without NAFLD. High levels of xenin may be in relation with obesity.

Characterisation of the biological activity of xenin-25 degradation fragment peptides

Xenin-25, a peptide co-secreted with the incretin hormone glucose-dependent insulinotropic polypeptide (GIP), possesses promising therapeutic actions for obesity-diabetes. However, native xenin-25 is rapidly degraded by serum enzymes to yield the truncated metabolites: xenin 9-25, xenin 11-25, xenin 14-25 and xenin 18-25. This study has examined the biological activities of these fragment peptides. In vitro studies using BRIN-BD11 cells demonstrated that native xenin-25 and xenin 18-25 possessed significant (P<0.05 to P<0.001) insulin-releasing actions at 5.6 and 16.7 mM glucose, respectively, but not at 1.1  mM glucose. In addition, xenin 18-25 significantly (P<0.05) potentiated the insulin-releasing action of the stable GIP mimetic (D-Ala²)GIP. In contrast, xenin 9-25, xenin 11-25 and xenin 14-25 displayed neither insulinotropic nor GIP-potentiating actions. Moreover, xenin 9-25, xenin 11-25 and xenin 14-25 significantly (P<0.05 to P<0.001) inhibited xenin-25 (10⁻⁶ M)-induced insulin release in vitro. I.p. administration of xenin-based peptides in combination with glucose to high fat-fed mice did not significantly affect the glycaemic excursion or glucose-induced insulin release compared with controls. However, when combined with (D-Ala²)GIP, all xenin peptides significantly (P<0.01 to P<0.001) reduced the overall glycaemic excursion, albeit to a similar extent as (D-Ala²)GIP alone. Xenin-25 and xenin 18-25 also imparted a potential synergistic effect on (D-Ala²)GIP-induced insulin release in high fat-fed mice. All xenin-based peptides lacked significant satiety effects in normal mice. These data demonstrate that the C-terminally derived fragment peptide of xenin-25, xenin 18-25, exhibits significant biological actions that could have therapeutic utility for obesity-diabetes.

Xenin-25 delays gastric emptying and reduces postprandial glucose levels in humans with and without type 2 diabetes

Xenin-25 (Xen) is a neurotensin-related peptide secreted by a subset of glucose-dependent insulinotropic polypeptide (GIP)-producing enteroendocrine cells. In animals, Xen regulates gastrointestinal function and glucose homeostasis, typically by initiating neural relays. However, little is known about Xen action in humans. This study determines whether exogenously administered Xen modulates gastric emptying and/or insulin secretion rates (ISRs) following meal ingestion. Fasted subjects with normal (NGT) or impaired (IGT) glucose tolerance and Type 2 diabetes mellitus (T2DM; n = 10-14 per group) ingested a liquid mixed meal plus acetaminophen (ACM; to assess gastric emptying) at time zero. On separate occasions, a primed-constant intravenous infusion of vehicle or Xen at 4 (Lo-Xen) or 12 (Hi-Xen) pmol · kg(-1) · min(-1) was administered from zero until 300 min. Some subjects with NGT received 30- and 90-min Hi-Xen infusions. Plasma ACM, glucose, insulin, C-peptide, glucagon, Xen, GIP, and glucagon-like peptide-1 (GLP-1) levels were measured and ISRs calculated. Areas under the curves were compared for treatment effects. Infusion with Hi-Xen, but not Lo-Xen, similarly delayed gastric emptying and reduced postprandial glucose levels in all groups. Infusions for 90 or 300 min, but not 30 min, were equally effective. Hi-Xen reduced plasma GLP-1, but not GIP, levels without altering the insulin secretory response to glucose. Intense staining for Xen receptors was detected on PGP9.5-positive nerve fibers in the longitudinal muscle of the human stomach. Thus Xen reduces gastric emptying in humans with and without T2DM, probably via a neural relay. Moreover, endogenous GLP-1 may not be a major enhancer of insulin secretion in healthy humans under physiological conditions.

Xenin-25 increases cytosolic free calcium levels and acetylcholine release from a subset of myenteric neurons

Xenin-25 (Xen) is a 25 amino acid neurotensin-related peptide reportedly produced with glucose-dependent insulinotropic polypeptide (GIP) by a subset of K cells in the proximal gut. We previously showed exogenously administered Xen, with GIP but not alone, increases insulin secretion in humans and mice. In mice, this effect is indirectly mediated via a central nervous system-independent cholinergic relay in the periphery. Xen also delays gastric emptying, reduces food intake, induces gall bladder contractions, and increases gut motility and secretion from the exocrine pancreas, suggesting that some effects of Xen could be mediated by myenteric neurons (MENs). To determine whether Xen activates these neurons, MENs were isolated from guinea pig proximal small intestines. Cells expressed neuronal markers and exhibited typical neuron-like morphology with extensive outgrowths emanating from cell bodies. Cytosolic free Ca(2+) levels ([Ca(2+)](i)) were measured using Fura-2. ATP/UTP, KCl, and forskolin increased [Ca(2+)](i) in 99.6%, 92%, and 23% of the MENs imaged, respectively, indicating that they are functional and activated by nucleotide receptor signaling, direct depolarization, and cAMP. [Ca(2+)](i) increased in only 12.7% of MENs treated with Xen. This rise was blocked by pretreatment with EGTA, diazoxide, SR48692, and neurotensin. Thus the Xen-mediated increase in [Ca(2+)](i) involves influx of extracellular Ca(2+) and activation of neurotensin receptor-1 (NTSR1). Xen also increased acetylcholine release from MENs. Amylin, produced by β-and enteroendocrine cells, delays gastric emptying and increased [Ca(2+)](i) almost exclusively in Xen-responsive MENs. Immunohistochemistry demonstrated NTSR1 expression in human duodenal MENs. Thus myenteric rather than central neurons could mediate some effects of Xen and amylin.

Peripheral neural targets in obesity

Interest in pharmacological treatments for obesity that act in the brain to reduce appetite has increased exponentially over recent years, but failures of clinical trials and withdrawals due to adverse effects have so far precluded any success. Treatments that do not act within the brain are, in contrast, a neglected area of research and development. This is despite the fact that a vast wealth of molecular mechanisms exists within the gut epithelium and vagal afferent system that could be manipulated to increase satiety. Here we discuss mechano- and chemosensory pathways from the gut involved in appetite suppression, and distinguish between gastric and intestinal vagal afferent pathways in terms of their basic physiology and activation by enteroendocrine factors. Gastric bypass surgery makes use of this system by exposing areas of the intestine to greater nutrient loads resulting in greater satiety hormone release and reduced food intake. A non-surgical approach to this system is preferable for many reasons. This review details where the opportunities may lie for such approaches by describing nutrient-sensing mechanisms throughout the gastrointestinal tract.

The migrating motor complex: control mechanisms and its role in health and disease

The migrating motor complex (MMC) is a cyclic, recurring motility pattern that occurs in the stomach and small bowel during fasting; it is interrupted by feeding. The MMC is present in the gastrointestinal tract of many species, including humans. The complex can be subdivided into four phases, of which phase III is the most active, with a burst of contractions originating from the antrum or duodenum and migrating distally. Control of the MMC is complex. Phase III of the MMC with an antral origin can be induced in humans through intravenous administration of motilin, erythromycin or ghrelin, whereas administration of serotonin or somatostatin induces phase III activity with duodenal origin. The role of the vagus nerve in control of the MMC seems to be restricted to the stomach, as vagotomy abolishes the motor activity in the stomach, but leaves the periodic activity in the small bowel intact. The physiological role of the MMC is incompletely understood, but its absence has been associated with gastroparesis, intestinal pseudo-obstruction and small intestinal bacterial overgrowth. Measuring the motility of the gastrointestinal tract can be important for the diagnosis of gastrointestinal disorders. In this Review we summarize current knowledge of the MMC, especially its role in health and disease.

Xenin delays gastric emptying rate and activates the brainstem in mice

Xenin, a 25-amino acid gastrointestinal peptide, inhibits feeding by acting through the central nervous system. Gastrointestinal hormones reduce food intake partly by activating the brainstem and inhibiting gastric emptying. Therefore, we hypothesized that xenin delays gastric emptying through the activation of the brainstem cells. To address this hypothesis, we examined the effect of intraperitoneal (i.p.) injection of xenin on gastric emptying rate and brainstem Fos expression in mice. Gastric emptying rate was reduced by about 93% in xenin-treated mice compared to saline-treated control mice. The i.p. xenin injection significantly increased Fos-immunoreactive cells in the nucleus of the solitary tract (NTS) of the brainstem, but not area postrema (AP) and dorsal motor nucleus of the vagus (DMV). These findings support the hypothesis that xenin-induced anorexia is at least partly due to delayed gastric emptying and the activation of the NTS cells.

Xenin-25 potentiates glucose-dependent insulinotropic polypeptide action via a novel cholinergic relay mechanism

The intestinal peptides GLP-1 and GIP potentiate glucose-mediated insulin release. Agents that increase GLP-1 action are effective therapies in type 2 diabetes mellitus (T2DM). However, GIP action is blunted in T2DM, and GIP-based therapies have not been developed. Thus, it is important to increase our understanding of the mechanisms of GIP action. We developed mice lacking GIP-producing K cells. Like humans with T2DM, "GIP/DT" animals exhibited a normal insulin secretory response to exogenous GLP-1 but a blunted response to GIP. Pharmacologic doses of xenin-25, another peptide produced by K cells, restored the GIP-mediated insulin secretory response and reduced hyperglycemia in GIP/DT mice. Xenin-25 alone had no effect. Studies with islets, insulin-producing cell lines, and perfused pancreata indicated xenin-25 does not enhance GIP-mediated insulin release by acting directly on the beta-cell. The in vivo effects of xenin-25 to potentiate insulin release were inhibited by atropine sulfate and atropine methyl bromide but not by hexamethonium. Consistent with this, carbachol potentiated GIP-mediated insulin release from in situ perfused pancreata of GIP/DT mice. In vivo, xenin-25 did not activate c-fos expression in the hind brain or paraventricular nucleus of the hypothalamus indicating that central nervous system activation is not required. These data suggest that xenin-25 potentiates GIP-mediated insulin release by activating non-ganglionic cholinergic neurons that innervate the islets, presumably part of an enteric-neuronal-pancreatic pathway. Xenin-25, or molecules that increase acetylcholine receptor signaling in beta-cells, may represent a novel approach to overcome GIP resistance and therefore treat humans with T2DM.

Endogenous ACTH, not only alpha-melanocyte-stimulating hormone, reduces food intake mediated by hypothalamic mechanisms

ACTH and alpha-melanocyte-stimulating hormone (alpha-MSH) are both consecutively processed from proopiomelanocortin (POMC), which is synthesized in hypothalamic arcuate neurons innervating the paraventricular nuclei (PVN). POMC secretion/synthesis is regulated by energy availability. ACTH and alpha-MSH bind with equal affinity to melanocortin-4 receptors and elicit similar effects on signal transduction in-vitro. Endogenous alpha-MSH thus far is believed to be the major physiological agonist and to act in an anorexigenic manner. Until now, it was fully unknown whether endogenous ACTH is also involved in the regulation of appetite and food intake. In this study in rats, we now show that icv ACTH as well as alpha-MSH possess anorexigenic effects in the PVN or areas in close proximity in vivo and that the effect of ACTH is direct and not mediated via alpha-MSH. We investigated the roles of endogenous ACTH and alpha-MSH by PVN application of the respective antibodies under different physiological conditions. In satiated rats with high levels of ACTH and alpha-MSH in the PVN, antibody administration increased food intake and body weight gain; hungry animals were unaffected. Finally, repeated injections of ACTH antibodies into PVN resulted in persistently increased food intake during the light period. These data now provide robust evidence that endogenous ACTH without further processing acts in the PVN or areas in close proximity to reduce food intake under conditions of feeding-induced satiety.