Investigation of structure-activity relationships of Oxyntomodulin (Oxm) using Oxm analogs

Chronic treatment with glucagon-like peptide-1 and glucagon receptor co-agonist causes weight loss-independent improvements in hepatic steatosis in mice with diet-induced obesity

OBJECTIVES

Co-agonists at the glucagon-like peptide-1 and glucagon receptors (GLP1R/GCGR) show promise as treatments for metabolic dysfunction-associated steatotic liver disease (MASLD). Although most co-agonists to date have been heavily GLP1R-biased, glucagon directly acts on the liver to reduce fat content. The aims of this study were to investigate a GCGR-biased co-agonist as treatment for hepatic steatosis in mice.

METHODS

Mice with diet-induced obesity (DIO) were treated with Dicretin, a GLP1/GCGR co-agonist with high potency at the GCGR, Semaglutide (GLP1R monoagonist) or food restriction over 24 days, such that their weight loss was matched. Hepatic steatosis, glucose tolerance, hepatic transcriptomics, metabolomics and lipidomics at the end of the study were compared with Vehicle-treated mice.

RESULTS

Dicretin lead to superior reduction of hepatic lipid content when compared to Semaglutide or equivalent weight loss by calorie restriction. Markers of glucose tolerance and insulin resistance improved in all treatment groups. Hepatic transcriptomic and metabolomic profiling demonstrated many changes that were unique to Dicretin-treated mice. These include some known targets of glucagon signaling and others with as yet unclear physiological significance.

CONCLUSIONS

Our study supports the development of GCGR-biased GLP1/GCGR co-agonists for treatment of MASLD and related conditions.

Striking the Balance: GLP-1/Glucagon Co-Agonism as a Treatment Strategy for Obesity

Obesity and Type 2 diabetes represent global health challenges, and there is an unmet need for long-lasting and effective pharmacotherapies. Although long-acting glucagon-like peptide-1 (GLP-1) analogues are now in routine use for diabetes and are now being utilised for obesity per se, the need for ever better treatments has driven the development of co-agonists, with the theoretical advantages of improved efficacy by targeting multiple pathways and reduced adverse effects. In this review, we highlight the past and present progress in our understanding and development of treatments based on GLP-1/glucagon co-agonism. We also reflect on the divergent effects of varying the GLP-1:glucagon activity and ratio in the context of pre-clinical and human clinical trial findings. In particular, the multiple metabolic actions of glucagon highlight the importance of understanding the contributions of individual hormone action to inform the safe, effective and tailored use of GLP-1/glucagon co-agonists to target weight loss and metabolic disease in the future.

Thermo-Responsive self-assembly of a dual glucagon-like peptide and glucagon receptor agonist

The human peptide hormone Oxyntomodulin (Oxm) is known to induce satiety, increase energy expenditure, and control blood glucose in humans, making it a promising candidate for treatment of obesity and/or type 2 diabetes mellitus. However, a pharmaceutical exploitation has thus far been impeded by fast in vivo clearance and the molecule's sensitivity to half-life extending structural modifications. We recently showed that Oxm self-assembles into amyloid-like nanofibrils that continuously release active, soluble Oxm in a peptide-deprived environment. S.c. injected Oxm nanofibrils extended plasma exposure from a few hours to five days in rodents, compared to s.c. applied soluble Oxm. Here we show that Oxm fibril elongation kinetics and thermodynamics display a uniquely low temperature optimum compared to previously reported amyloid-like peptide and protein assemblies. Elongation rate is optimal at room temperature, with association rates 2-3 times higher at 25 °C than at ≥37 °C or ≤20 °C. We deduce from a combination of Cryo electron microscopy and spectroscopic methods that Oxm fibrils have a double-layered, triangular cross-section composed of arch-shaped monomers. We suggest a thermodynamic model that links the necessary molecular rearrangements during fibrillation and peptide release to the unique temperature effects in Oxm self-assembly and disassembly.

Proglucagon-Derived Peptides as Therapeutics

Initially discovered as an impurity in insulin preparations, our understanding of the hyperglycaemic hormone glucagon has evolved markedly over subsequent decades. With description of the precursor proglucagon, we now appreciate that glucagon was just the first proglucagon-derived peptide (PGDP) to be characterised. Other bioactive members of the PGDP family include glucagon-like peptides -1 and -2 (GLP-1 and GLP-2), oxyntomodulin (OXM), glicentin and glicentin-related pancreatic peptide (GRPP), with these being produced via tissue-specific processing of proglucagon by the prohormone convertase (PC) enzymes, PC1/3 and PC2. PGDP peptides exert unique physiological effects that influence metabolism and energy regulation, which has witnessed several of them exploited in the form of long-acting, enzymatically resistant analogues for treatment of various pathologies. As such, intramuscular glucagon is well established in rescue of hypoglycaemia, while GLP-2 analogues are indicated in the management of short bowel syndrome. Furthermore, since approval of the first GLP-1 mimetic for the management of Type 2 diabetes mellitus (T2DM) in 2005, GLP-1 therapeutics have become a mainstay of T2DM management due to multifaceted and sustainable improvements in glycaemia, appetite control and weight loss. More recently, longer-acting PGDP therapeutics have been developed, while newfound benefits on cardioprotection, bone health, renal and liver function and cognition have been uncovered. In the present article, we discuss the physiology of PGDP peptides and their therapeutic applications, with a focus on successful design of analogues including dual and triple PGDP receptor agonists currently in clinical development.

A novel thrombin-based triagonist with diabetes-protective and weight-lowering potential

AIMS

To investigate the diabetes-protective effect and weight-lowering potential of a novel long-acting triagonist at three metabolically related hormone receptors including glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), glucagon receptors.

MAIN METHODS

Triagonist were designed in an iterative manner from native GLP-1, GIP and Glucogan. Main peptide chain (termed TG peptides) and subsequently modified LTG peptides were synthesized via solid phase synthesis. In vitro receptor activity assay was performed to screen the TG peptide with most balanced potency on all three receptors. The in vitro biological activities of modified TG peptides were further investigated by albumin-binding measurement and proteolytic cleavage test. Subsequently, oral glucose tolerance test (OGTT), pharmacokinetic test and chronic study were subjected to the acute and long-term efficacy evaluation of selected fusion peptide, LTG-6.

KEY FINDINGS

TG-8 exhibited equally aligned constituent efficacy and supraphysiological potency on corresponding receptor without cross-reactivity. Modified TG-8, termed LTG-6, exerted the great binding affinity for human serum albumin and the enhanced rational controlled-release of TG-8 in vitro. Further OGTT in different gene knockout mice and diabetic mice demonstrated the promising hypoglycemic and insulinotropic abilities of LTG-6. After long-term treatment for 8 weeks, LTG-6 was proved superior to co-agonists to decrease the body weight and %HbA1c, improve reverse dyslipidemia and glycemic control in the DIO models.

SIGNIFICANCE

LTG-6, as a newly designed long-acting triagonist, holds potential to correct the obesity related metabolic disorders.

Design and characterization of novel oxyntomodulin derivatives with potent dual GLP-1/glucagon receptor activation and prolonged antidiabetic effects

AIMS

To investigate the combination of dimerization and PEGylation to enhance the receptor activation and in vivo stability of Oxyntomodulin (OXM).

MAIN METHODS

All LDM peptides were produced by using standard method of solid phase synthesis. The in vitro effects of LDM peptides were assessed by glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GcgR) binding test and Proteolytic stability test. Subsequently, saline, Liraglutide and three doses of LDM-3 treated groups were subjected to the evaluation of aute and long-term efficacy.

KEY FINDINGS

Five long-acting OXM conjugates, termed LDM-1 to LDM-5, were designed using cysteine (Cys)-specific modification reaction including the activated PEG, bisMal-NH₂, and OXM-Cys, and all prepared with high purity. LDM-3 exhibited greater GLP-1R and GcgR activation and ameliorative serum stability. In addition, LDM-3 was identified with enhanced insulinotropic and glycemic abilities in the gene knockout mice. The prolonged glucose-lowering effects of the LDM-3 were proved by hypoglycemic duration test and multiple oral glucose tolerance tests (OGTTs) in the diet-induced obesity (DIO) mice. Furthermore, the pharmacokinetic tests in Sprague Dawley (SD) rat and cynomolgus monkey exhibited the lifespans of LDM-3 at 90 nmol·kg^-1 were 101.5 h and 119.4 h, respectively. Nevertheless, consecutive 8-week administration of LDM-3 improved the cumulative body weight gain, food intake, % HbA1c, glucose tolerance and the pancreatic of the obese mice.

SIGNIFICANCE

LDM-3, as a dual GLP-1R and GcgR agonist, holds potential to be developed as a promising therapeutic candidate for both diabetes and obesity.

High fat diet induces microbiota-dependent silencing of enteroendocrine cells

Enteroendocrine cells (EECs) are specialized sensory cells in the intestinal epithelium that sense and transduce nutrient information. Consumption of dietary fat contributes to metabolic disorders, but EEC adaptations to high fat feeding were unknown. Here, we established a new experimental system to directly investigate EEC activity in vivo using a zebrafish reporter of EEC calcium signaling. Our results reveal that high fat feeding alters EEC morphology and converts them into a nutrient insensitive state that is coupled to endoplasmic reticulum (ER) stress. We called this novel adaptation 'EEC silencing'. Gnotobiotic studies revealed that germ-free zebrafish are resistant to high fat diet induced EEC silencing. High fat feeding altered gut microbiota composition including enrichment of Acinetobacter bacteria, and we identified an Acinetobacter strain sufficient to induce EEC silencing. These results establish a new mechanism by which dietary fat and gut microbiota modulate EEC nutrient sensing and signaling.

Will medications that mimic gut hormones or target their receptors eventually replace bariatric surgery?

Bariatric surgery is currently the most effective therapeutic modality through which sustained beneficial effects on weight loss and metabolic improvement are achieved. During recent years, indications for bariatric surgery have been expanded to include cases of poorly controlled type 2 (T2DM) diabetes mellitus in lesser extremes of body weight. A spectrum of the beneficial effects of surgery is attributed to robust changes of postprandial gut peptide responses that are observed post operatively. Consolidated knowledge regarding gut peptide physiology as well as emerging new evidence shedding light on the mode of action of previously overlooked gut hormones provide appealing potential obesity and T2DM therapeutic perspectives. The accumulation of evidence from the effect of exogenous administration of native gut peptides alone or in combinations to humans as well as the development of mimetic agents exerting agonistic effects on combinations of gut hormone receptors pave the way for future integrated gut peptide-based treatments, which may mimic the effects of bariatric surgery.

Design, screening and biological evaluation of novel fatty acid chain-modified oxyntomodulin-based derivatives with prolonged glucose-lowering ability and potent anti-obesity effects

Schematic diagram of the discovery, design, screening and evaluation of a fully novel OXM derivative.

Physiology and Pharmacology of DPP-4 in Glucose Homeostasis and the Treatment of Type 2 Diabetes

Dipeptidyl peptidase-4 (DPP-4), also known as the T-cell antigen CD26, is a multi-functional protein which, besides its catalytic activity, also functions as a binding protein and a ligand for a variety of extracellular molecules. It is an integral membrane protein expressed on cells throughout the body, but is also shed from the membrane and circulates as a soluble protein in the plasma. A large number of bioactive molecules can be cleaved by DPP-4 in vitro, but only a few of these have been demonstrated to be physiological substrates. One of these is the incretin hormone, glucagon-like peptide-1 (GLP-1), which plays an important role in the maintenance of normal glucose homeostasis, and DPP-4 has been shown to be the key enzyme regulating its biological activity. This pathway has been targeted pharmacologically through the development of DPP-4 inhibitors, and these are now a successful class of anti-hyperglycaemic agents used to treat type 2 diabetes (T2DM). DPP-4 may additionally influence metabolic control via its proteolytic effect on other regulatory peptides, but it has also been reported to affect insulin sensitivity, potentially mediated through its non-enzymatic interactions with other membrane proteins. Given that altered expression and activity of DPP-4 are associated with increasing body mass index and hyperglycaemia, DPP-4 has been proposed to play a role in linking obesity and the pathogenesis of T2DM by functioning as a local mediator of inflammation and insulin resistance in adipose and hepatic tissue. As well as these broader systemic effects, it has also been suggested that DPP-4 may be able to modulate β-cell function as part of a paracrine system involving GLP-1 produced locally within the pancreatic islets. However, while it is evident that DPP-4 has the potential to influence glycaemic control, its overall significance for the normal physiological regulation of glucose homeostasis in humans and its role in the pathogenesis of metabolic disease remain to be established.

Glucagon-related peptides from phylogenetically ancient fish reveal new approaches to the development of dual GCGR and GLP1R agonists for type 2 diabetes therapy

The insulinotropic and antihyperglycaemic properties of glucagons from the sea lamprey (Petromyzontiformes), paddlefish (Acipenseriformes) and trout (Teleostei) and oxyntomodulin from dogfish (Elasmobranchii) and ratfish (Holocephali) were compared with those of human glucagon and GLP-1 in mammalian test systems. All fish peptides produced concentration-dependent stimulation of insulin release from BRIN-BD11 rat and 1.1 B4 human clonal β-cells and isolated mouse islets. Paddlefish glucagon was the most potent and effective peptide. The insulinotropic activity of paddlefish glucagon was significantly (P < 0.01) decreased after incubating BRIN-BD11 cells with the GLP1R antagonist, exendin-4(9-39) and the GCGR antagonist [des-His¹,Pro⁴, Glu⁹] glucagon amide but GIPR antagonist, GIP(6-30)Cex-K⁴⁰[palmitate] was without effect. Paddlefish and lamprey glucagons and dogfish oxyntomodulin (10 nmol L^-1) produced significant (P < 0.01) increases in cAMP concentration in Chinese hamster lung (CHL) cells transfected with GLP1R and human embryonic kidney (HEK293) cells transfected with GCGR. The insulinotropic activity of paddlefish glucagon was attenuated in CRISPR/Cas9-engineered GLP1R knock-out INS-1 cells but not in GIPR knock-out cells. Intraperitoneal administration of all fish peptides, except ratfish oxyntomodulin, to mice together with a glucose load produced significant (P < 0.05) decreases in plasma glucose concentrations and paddlefish glucagon produced a greater release of insulin compared with GLP-1. Paddlefish glucagon shares the sequences Glu¹⁵-Glu¹⁶ and Glu²⁴-Trp²⁵-Leu²⁶-Lys²⁷-Asn²⁸-Gly²⁹ with the potent GLP1R agonist, exendin-4 so may be regarded as a naturally occurring, dual-agonist hybrid peptide that may serve as a template design of new drugs for type 2 diabetes therapy.

No Guts, No Loss: Toward the Ideal Treatment for Obesity in the Twenty-First Century

Over the last century, our knowledge of the processes which control appetite and weight regulation has developed significantly. The understanding of where gut hormones fit into the control of energy homeostasis in addition to the rapid advancement of pharmacotherapeutics has paved the way for the development of novel gut hormone analogs to target weight loss. Currently, bariatric surgery remains the most efficacious treatment for obesity. The emergence of gut hormone analogs may provide a useful non-surgical addition to the armamentarium in treating obesity. Simply targeting single gut hormone pathways may be insufficiently efficacious, and combination/multiple-agonist approaches may be necessary to obtain the results required for clear clinical impact.

Team Players or Opponents: Coadministration of Selective Glucagon and GLP-1 Receptor Agonists in Obese Diabetic Monkeys

We assessed the therapeutic contribution of the individual components of glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR) agonists alone and in combination upon energy homeostasis and glycemic control in diet-induced obese, diabetic nonhuman primates. The pharmacological active dose ranges of selective agonists were established through a dose-finding study, followed by a 6-week chronic study. Repeated subcutaneous administration of a selective GCGR agonist (30 µg/kg once daily) did not affect food intake or body weight, whereas the selective GLP-1R agonist (3 µg/kg once daily) alone decreased energy intake by 18% and body weight by 3.8% ± 0.9%. Combination of both agonists reduced significantly cumulative food intake by 27% and body weight by 6.6% ± 0.9%. Fasting plasma glucose (FPG) was improved by GLP-1R agonist (baseline vs end of study, 176.7 ± 34.0 vs 115.9 ± 16.1 mg/dL). In contrast, groups exposed to GCGR agonist experienced nonsignificant elevations of FPG. More accurate assessment of therapeutic interventions on glucose homeostasis was tested by an IV glucose tolerance test. Glucose excursion was significantly elevated by chronic GCGR agonist administration, whereas it was significantly decreased in GLP-1R agonist-treated monkeys. In the combination group, a nonsignificant increase of glucose excursion was seen, concomitantly with significantly increased insulin secretion. We conclude that chronic glucagon agonism does not affect energy homeostasis in nonhuman primates. In combination with GLP-1R agonism, glucagon agonism synergistically enhances negative energy balance with resulting larger body weight loss. However, adding GCGR to GLP-1R agonism diminishes glycemic control in diabetic monkeys. Therefore, long-term therapeutic implications of using GLP-1R/GCGR coagonists for weight management in diabetes warrants further scrutiny.

The Gut-Brain Axis, the Human Gut Microbiota and Their Integration in the Development of Obesity

Obesity is a global epidemic, placing socioeconomic strain on public healthcare systems, especially within the so-called Western countries, such as Australia, United States, United Kingdom, and Canada. Obesity results from an imbalance between energy intake and energy expenditure, where energy intake exceeds expenditure. Current non-invasive treatments lack efficacy in combating obesity, suggesting that obesity is a multi-faceted and more complex disease than previously thought. This has led to an increase in research exploring energy homeostasis and the discovery of a complex bidirectional communication axis referred to as the gut-brain axis. The gut-brain axis is comprised of various neurohumoral components that allow the gut and brain to communicate with each other. Communication occurs within the axis via local, paracrine and/or endocrine mechanisms involving a variety of gut-derived peptides produced from enteroendocrine cells (EECs), including glucagon-like peptide 1 (GLP1), cholecystokinin (CCK), peptide YY3-36 (PYY), pancreatic polypeptide (PP), and oxyntomodulin. Neural networks, such as the enteric nervous system (ENS) and vagus nerve also convey information within the gut-brain axis. Emerging evidence suggests the human gut microbiota, a complex ecosystem residing in the gastrointestinal tract (GIT), may influence weight-gain through several inter-dependent pathways including energy harvesting, short-chain fatty-acids (SCFA) signalling, behaviour modifications, controlling satiety and modulating inflammatory responses within the host. Hence, the gut-brain axis, the microbiota and the link between these elements and the role each plays in either promoting or regulating energy and thereby contributing to obesity will be explored in this review.

Oxyntomodulin analogue increases energy expenditure via the glucagon receptor

The gut hormone oxyntomodulin (OXM) causes weight loss by reducing appetite and increasing energy expenditure. Several analogues are being developed to treat obesity. Exactly how oxyntomodulin works, however, remains controversial. OXM can activate both glucagon and GLP-1 receptors but no specific receptor has been identified. It is thought that the anorectic effect occurs predominantly through GLP-1 receptor activation but, to date, it has not been formally confirmed which receptor is responsible for the increased energy expenditure. We developed OX-SR, a sustained-release OXM analogue. It produces a significant and sustained increase in energy expenditure in rats as measured by indirect calorimetry. We now show that this increase in energy expenditure occurs via activation of the glucagon receptor. Blockade of the GLP-1 receptor with Exendin 9-39 does not block the increase in oxygen consumption caused by OX-SR. However, when activity at the glucagon receptor is lost, there is no increase in energy expenditure. Glucagon receptor activity therefore appears to be essential for OX-SR's effects on energy expenditure. The development of future 'dual agonist' analogues will require careful balancing of GLP-1 and glucagon receptor activities to obtain optimal effects.

Mutational Analysis of Neuropeptide Y Reveals Unusual Thermal Stability Linked to Higher-Order Self-Association

Neuropeptide Y (NPY) is a 36-residue peptide, abundant in the central and peripheral nervous system. The peptide interacts with membrane-bound receptors to control processes such as food intake, vasoconstriction, and memory retention. The N-terminal polyproline sequence of NPY folds back onto a C-terminal α-helix to form a hairpin structure. The hairpin undergoes transient unfolding to allow the monomer to interact with its target membranes and receptors and to form reversible dimers in solution. Using computational, functional, and biophysical approaches, we characterized the role of two conserved tyrosines (Y20 and Y27) located within the hydrophobic core of the hairpin fold. Successive mutation of the tyrosines to more hydrophobic phenylalanines increased the thermal stability of NPY and reduced functional activity, consistent with computational studies predicting a more stable hairpin structure. However, mutant stability was high relative to wild-type: melting temperatures increased by approximately 20 °C for the single mutants (Y20F and Y27F) and by 30 °C for the double mutant (Y20F + Y27F). These findings suggested that the mutations were not just simply enhancing hairpin structure stability, but might also be driving self-association to dimer. Using analytical ultracentrifugation, we determined that the mutations indeed increased self-association, but shifted the equilibrium toward hexamer-like species. Notably, these latter species were not unique to the NPY mutants, but were found to preexist at low levels in the wild-type population. Collectively, the findings indicate that NPY self-association is more complex than previously recognized and that the ensemble of NPY quaternary states is tunable by modulating hairpin hydrophobicity.

Peptide degradation and the role of DPP-4 inhibitors in the treatment of type 2 diabetes

Dipeptidyl peptidase-4 (DPP-4) inhibitors are now a widely used, safe and efficacious class of antidiabetic drugs, which were developed prospectively using a rational drug design approach based on a thorough understanding of the endocrinology and degradation of glucagon-like peptide-1 (GLP-1). GLP-1 is an intestinal hormone with potent insulinotropic and glucagonostatic effects and can normalise blood glucose levels in patients with type 2 diabetes, but the native peptide is not therapeutically useful because of its inherent metabolic instability. Using the GLP-1/DPP-4 system and type 2 diabetes as an example, this review summarises how knowledge of a peptide's biological effects coupled with an understanding of the pathways involved in its metabolic clearance can be exploited in a rational, step-by-step manner to develop a therapeutic agent, which is effective and well tolerated, and any side effects are minor and largely predictable. Other peptides with metabolic effects which can also be degraded by DPP-4 will be reviewed, and their potential role as additional mediators of the effects of DPP-4 inhibitors will be assessed.

Controlling the bioactivity of a peptide hormone in vivo by reversible self-assembly

The use of peptides as therapeutic agents is undergoing a renaissance with the expectation of new drugs with enhanced levels of efficacy and safety. Their clinical potential will be only fully realised once their physicochemical and pharmacokinetic properties have been precisely controlled. Here we demonstrate a reversible peptide self-assembly strategy to control and prolong the bioactivity of a native peptide hormone in vivo. We show that oxyntomodulin, a peptide with potential to treat obesity and diabetes, self-assembles into a stable nanofibril formulation which subsequently dissociates to release active peptide and produces a pharmacological effect in vivo. The subcutaneous administration of the nanofibrils in rats results in greatly prolonged exposure, with a constant oxyntomodulin bioactivity detectable in serum for at least 5 days as compared to free oxyntomodulin which is undetectable after only 4 h. Such an approach is simple, cost-efficient and generic in addressing the limitations of peptide therapeutics.

The clinical potential of peptide therapeutic agents can only be fully realised once their physicochemical and pharmacokinetic properties are precisely controlled. Here the authors show a reversible peptide self-assembly strategy to control and prolong the bioactivity of a native peptide hormone in vivo.

GLP-1/glucagon receptor co-agonism for treatment of obesity

Over a relatively short period, obesity and type 2 diabetes have come to represent a large medical and economic burden to global societies. The epidemic rise in the prevalence of obesity has metabolic consequences and is paralleled by an increased occurrence of other diseases, such as diabetes, cancer and cardiovascular complications. Together, obesity and type 2 diabetes constitute one of the more preventable causes of premature death and the identification of novel, safe and effective anti-obesity drugs is of utmost importance. Pharmacological attempts to treat obesity have had limited success, with notable adverse effects, rendering bariatric surgery as the only current therapy for substantially improving body weight. Novel unimolecular, multifunctional peptides have emerged as one of the most promising medicinal approaches to enhance metabolic efficacy and restore normal body weight. In this review, we will mainly focus on the discovery and translational relevance of dual agonists that pharmacologically function at the receptors for glucagon and glucagon-like peptide-1. Such peptides have advanced to clinical evaluation and inspired the pursuit of multiple related approaches to achieving polypharmacy within single molecules.

Degradation Paradigm of the Gut Hormone, Pancreatic Polypeptide, by Hepatic and Renal Peptidases

Pancreatic polypeptide (PP) is a gut hormone that acts on Y4 receptors to reduce appetite. Obese humans display a reduced postprandial increase in PP and remain fully sensitive to the anorectic effects of exogenous PP. The utility of PP as an anti-obesity treatment is limited by its short circulating half-life. Insight into the mechanisms by which PP is degraded could aid in the design of long-acting PP analogs. We investigated the role of peptidases in PP degradation to determine whether inhibition of these enzymes enhanced PP plasma levels and bioactivity in vivo. Dipeptidyl peptidase IV (DPPIV) and neprilysin (NEP) were two peptidase found to cleave PP. Limiting the effect of both peptidases improved the in vivo anorectic effect of PP and PP-based analogs. These findings suggest that inhibiting the degradation of PP using specific inhibitors and/or the design of analogs resistant to cleavage by DPPIV and NEP might be useful in the development of PP as an anti-obesity pharmacotherapy.

Design of Potent and Proteolytically Stable Oxyntomodulin Analogs

Incretin-based peptides are effective therapeutics for treating type 2 diabetes mellitus (T2DM). Oxyntomodulin (OXM), a dual agonist of GLP-1R and GCGR, has shown superior weight loss and glucose lowering effects, compared to single GLP-1R agonists. To overcome the short half-life and rapid renal clearance of OXM, which limit its therapeutic potential, both lipid and PEG modified OXM analogs have been reported. However, these approaches often result in reduced potency or PEG-associated toxicity. Herein, we report a new class of cross-linked OXM analogs that show increased plasma stability and higher potency in activating both GLP-1R and GCGR. Moreover, the extended in vivo half-life results in superior antihyperglycemic activity in mice compared to the wild-type OXM.

Effect of Oxyntomodulin, Glucagon, GLP-1, and Combined Glucagon +GLP-1 Infusion on Food Intake, Appetite, and Resting Energy Expenditure

CONTEXT

The gut hormone, oxyntomodulin, is a proglucagon product with body weight-lowering potential. It binds to both the glucagon-like peptide-1 (GLP-1) receptor and the glucagon receptor; however, the mechanism behind the body weight-lowering effect remains elusive.

OBJECTIVE

We wanted to delineate the contributions of separate and combined GLP-1 receptor and glucagon receptor activation to the body weight-reducing mechanisms of oxyntomodulin.

DESIGN

This was a double-blinded, randomized, crossover study.

SETTING

The study was conducted at a specialized research unit.

PARTICIPANTS

Fifteen young healthy male volunteers (aged 22 [range 18-32] y; body mass index 23 [21-26] kg/m(2); fasting plasma glucose 5.1 [4.4-5.4] mmol/L; and glycated hemoglobin A1c 40 (37-42) mmol/mol).

INTERVENTIONS

Five 4-hour liquid meal tests during the infusion of saline, GLP-1 (1 pmol × kg(-1) × min(-1)), glucagon (0.86 pmol × kg(-1) × min(-1)), oxyntomodulin (3 pmol × kg(-1) × min(-1)), or glucagon+GLP-1 (same doses).

MAIN OUTCOME MEASURES

We evaluated resting energy expenditure (measured as oxygen uptake, gastric emptying (GE), composite appetite scores (CAS), and food intake.

RESULTS

Oxyntomodulin, GLP-1, and GLP-1+glucagon slowed GE and reduced CAS, whereas glucagon did not affect GE and CAS. All infusions caused a similar decrease in food intake compared with saline (total intake (g [95% confidence interval]), saline 811 [729, 892], GLP-1 669 [586, 750], glucagon 686 [604, 768], oxyntomodulin 689 [608, 771], and glucagon+GLP-1 688 [606, 769]). Oxygen uptake did not change significantly from baseline in response to any peptide infusion compared with saline.

CONCLUSIONS

Oxyntomodulin, GLP-1, and glucagon decreased food intake but with no additional effect of combining GLP-1 and glucagon.

New perspectives on exploitation of incretin peptides for the treatment of diabetes and related disorders

The applicability of stable gut hormones for the treatment of obesity-related diabetes is now undisputable. This is based predominantly on prominent and sustained glucose-lowering actions, plus evidence that these peptides can augment insulin secretion and pancreatic islet function over time. This review highlights the therapeutic potential of glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), oxyntomodulin (OXM) and cholecystokinin (CCK) for obesity-related diabetes. Stable GLP-1 mimetics have already been successfully adopted into the diabetic clinic, whereas GIP, CCK and OXM molecules offer promise as potential new classes of antidiabetic drugs. Moreover, recent studies have shown improved therapeutic effects following simultaneous modulation of multiple receptor signalling pathways by combination therapy or use of dual/triple agonist peptides. However, timing and composition of injections may be important to permit interludes of beta-cell rest. The review also addresses the possible perils of incretin based drugs for treatment of prediabetes. Finally, the unanticipated utility of stable gut peptides as effective treatments for complications of diabetes, bone disorders, cognitive impairment and cardiovascular dysfunction is considered.

Increased food intake with oxyntomodulin analogues

Oxyntomodulin analogues offer a novel treatment for obesity. However during analogue screening in a rat model increased food intake was consistently observed. To further investigate this finding, a series of representative analogues (OXM14 and OXM15) and their Glu-3 equivalents (OXM14E3 and OXM15E3) were administered to rats for 7 days and food intake and bodyweight measurements taken. To investigate the role of glucagon receptor activation glutamate (Glu/E) was substituted at amino acid position 3. GLP-1 and glucagon receptor efficacy of the oxyntomodulin analogues and their Glu-3 counterparts were measured at the rat receptors in vitro. Doses of 25 nmol/kg of OXM14 and OXM15 increased food intake by up to 20%. Bodyweight was not significantly increased. Food intake was not increased with the Glu-3 peptides, indicating that a glucagon receptor mechanism may be responsible for the increase in food intake.

Stable oxyntomodulin analogues exert positive effects on hippocampal neurogenesis and gene expression as well as improving glucose homeostasis in high fat fed mice

The weight-lowering and gluco-regulatory actions of oxyntomodulin (Oxm) have been well-documented however potential actions of this peptide in brain regions associated with learning and memory have not yet been evaluated. The present study examined the long-term actions of a stable acylated analogue of Oxm, (dS(2))Oxm(K-γ-glu-Pal), together with parent (dS(2))Oxm peptide, on hippocampal neurogenesis, gene expression and metabolic control in high fat (HF) mice. Groups of HF mice (n = 12) received twice-daily injections of Oxm analogues (both at 25 nmol/kg body weight) or saline vehicle (0.9% wt/vol) over 28 days. Hippocampal gene expression and histology were assessed together with evaluation of energy intake, body weight, non-fasting glucose and insulin, glucose tolerance, insulin sensitivity and lipids. Oxm analogues significantly reduced body weight, improved glucose tolerance, glucose-mediated insulin secretion, insulin sensitivity, islet architecture and lipid profile. Analysis of brain histology revealed significant reduction in hippocampal oxidative damage (8-oxoguanine), enhanced hippocampal neurogenesis (doublecortin) and improved hippocampal and cortical synaptogenesis (synaptophysin) following treatment. Furthermore, Oxm analogues up-regulated hippocampal mRNA expression of MASH1, Synaptophysin, SIRT1, GLUT4 and IRS1, and down-regulated expression of LDL-R and GSK3β. These data demonstrate potential of stable Oxm analogues, and particularly (dS(2))Oxm(K-γ-glu-Pal) to improve metabolic function and enhance neurogenesis, synaptic plasticity, insulin signalling and exert protective effects against oxidative damage in hippocampus and cortex brain regions in HF mice.

Sustained treatment with a stable long-acting oxyntomodulin analogue improves metabolic control and islet morphology in an experimental model of type 1 diabetes

AIM

To assess the therapeutic benefits of regulatory peptides other than insulin, which have to date received limited consideration in the context of type 1 diabetes.

METHODS

We assessed the effects of subchronic administration of the stable, oxyntomodulin (Oxm) analogue, (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], for 28 days in streptozotocin (STZ)-induced insulin-deficient diabetic mice.

RESULTS

Twice-daily injection with (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] significantly countered the excessive food and fluid intake in STZ-induced diabetic mice, and maintained normal body weight. Lean body mass was normalized, whilst fat mass was significantly increased compared with control STZ-induced diabetic mice. In addition, circulating glucose was significantly reduced by the Oxm analogue, whilst plasma and pancreatic insulin concentrations were increased and glucagon decreased by day 28. Plasma lipid profile was normalized by (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL] administration and circulating amylase was not significantly altered by induction of diabetes or Oxm analogue therapy. This was associated with significantly improved glucose tolerance and insulin secretion. Peripheral insulin sensitivity was also significantly improved by Oxm analogue treatment. Histological examination of pancreata showed beneficial elevations of total islet and β-cell area, associated with an increase in the number of smaller-sized islets. Further analysis revealed enhanced islet cell proliferation relative to apoptosis in Oxm analogue-treated mice.

CONCLUSION

These studies emphasize the potential of stable Oxm-based peptides, such as (d-Ser(2) )Oxm[Lys(38) -γ-glu-PAL], as therapeutic agents for insulin-deficient type 1 diabetes.

Pulmonary delivery of anorectic oxyntomodulin in rats: food intake suppression, reduced body weight gain and pharmacokinetics

Background: Oxyntomodulin (OXM1–37) is an anorectic gut-secreting peptide with a promise to treat obesity, but its needle-free delivery has yet to be successful. Results: Pulmonary delivery of OXM1–37, but not its C-terminal octapeptides, caused dose-related, transient 4–6 h food intake suppression in rats. At 0.5 mg/kg, its 30–38% food intake suppression led to 46% reduction in body weight gain by day 8. Its lung absorption was fast, elevating the systemic level rapidly, yet the bioavailability was low at 13%. In the brain, twofold neuronal c-fos activation was seen in the hypothalamus arcuate nucleus and brainstem area postrema. Conclusion: Pulmonary delivery is a promising needle-free systemic delivery option for OXM1–37 to treat obesity, as enabling effective lung absorption and brain interaction.

The role of gut hormones in appetite regulation (review)

Eating process is an aggregate of complex and different forms of behavior. Its regulation is based on energy homeostasis and appetite control which includes two components: the homeostatic and the hedonistic control. Important signals in appetite regulation are gut-derived hormones. They are produced by enteroendocrine cells in response to nutrient and energy intake, and achieve their effects by influencing brain structures involved in food intake regulation. The key brain structure involved in this process is the hypothalamus. Gut hormones reach the hypothalamus from the circulation or by the vagal nerve via the nucleus of the solitary tract. Among gut peptides, ghrelin is the only orexigenic hormone, leading to an increase in food intake and body weight. All others, such as cholecystokinin, glucagon like peptide-1, oxyntomodulin, peptide tyrosine tyrosine or pancreatic polypeptide, are anorexigenic, leading to decrease in food intake. Also, gut-derived endocannabinoids exert orexigenic effect on appetite. Keeping in mind the growing problem of obesity, the crucial issue when considering gut derived peptides is to understand their mechanisms of acting because of potential role in clinical therapy, and discovering long-lasting gut peptides or their analogues, with no or minimal side effects.

Comparison of stability, cellular, glucose-lowering and appetite supressing effects of oxyntomodulin analogues modified at the N-terminus

Oxyntomodulin (Oxm) possesses beneficial biological actions for the potential treatment of obesity-diabetes. However, rapid inactivation by dipeptidyl peptidase-4 (DPP-4) results in a short half-life, hindering therapeutic applicability. In the present study, six Oxm analogues namely, (Thr(2))Oxm, (Asp(3))Oxm, (Aib(2))Oxm, (d-Ser(2))Oxm, (N-acetyl)Oxm and (d-Ser(2))Oxm-Lys-γ-glutamyl-PAL were synthesised and tested for DPP-4 stability and biological activity. Native Oxm, (Thr(2))Oxm and (Asp(3))Oxm were rapidly degraded by DPP-4, while (Aib(2))Oxm, (d-Ser(2))Oxm, (N-acetyl)Oxm and (d-Ser(2))Oxm-Lys-γ-glutamyl-PAL were resistant to degradation. All peptides stimulated cAMP production (P<0.01 to P<0.001) in GLP-1-R, but not in GIP-R, transfected cells. In glucagon-R transfected cells, all peptides except (N-acetyl)Oxm and (Thr(2))Oxm evoked significant cAMP generation. Similarly, all analogues, except (N-acetyl)Oxm, exhibited prominent (P<0.05 to P<0.001) insulinotropic activity in BRIN BD11 cells. When administered in conjunction with glucose to normal mice only native Oxm, (Aib(2))Oxm and (d-Ser(2))Oxm significantly (P<0.05 to P<0.01) increased overall plasma insulin levels. The corresponding glycaemic excursion was significantly (P<0.05 to P<0.001) lowered by all Oxm peptides, barring (N-acetyl)Oxm. Further investigations revealed persistent glucose-lowering and insulin-releasing actions of (d-Ser(2))Oxm-Lys-γ-glutamyl-PAL. Studies in GIP- and GLP-1-receptor KO mice with (Aib(2))Oxm, (d-Ser(2))Oxm, and (d-Ser(2))Oxm-Lys-γ-glutamyl-PAL highlighted the importance of GLP-1 receptor signalling for the beneficial glucose homoeostatic actions of these analogues. All peptides, except (N-acetyl)Oxm, possessed significant appetite suppressive effects in mice. These data highlight the significant therapeutic promise of enzymatically stable Oxm-based peptides, particularly with position 2 modifications, for the treatment of obesity-diabetes.

A novel DPP IV-resistant C-terminally extended glucagon analogue exhibits weight-lowering and diabetes-protective effects in high-fat-fed mice mediated through glucagon and GLP-1 receptor activation

AIMS/HYPOTHESIS

Modification of the structure of glucagon could provide useful compounds for the potential treatment of obesity-related diabetes.

METHODS

This study evaluated N-acetyl-glucagon, (D-Ser(2))glucagon and an analogue of (D-Ser(2))glucagon with the addition of nine amino acids from the C-terminal of exendin(1-39), namely (D-Ser(2))glucagon-exe.

RESULTS

All analogues were resistant to dipeptidyl peptidase IV degradation. N-Acetyl-glucagon lacked acute insulinotropic effects in BRIN BD11 cells, whereas (D-Ser(2))glucagon and (D-Ser(2))glucagon-exe evoked significant (p < 0.001) insulin release. (D-Ser(2))glucagon-exe stimulated cAMP production (p < 0.001) in glucagon- and GLP-1-receptor (GLP-1R)-transfected cells but not in glucose-dependent insulinotropic polypeptide-receptor-transfected cells. In normal mice, N-acetyl-glucagon and (D-Ser(2))glucagon retained glucagon-like effects of increasing (p < 0.001) plasma glucose and insulin levels. (D-Ser(2))glucagon-exe was devoid of hyperglycaemic actions but substantially (p < 0.001) increased plasma insulin levels. (D-Ser(2))glucagon-exe reduced the glycaemic excursion (p < 0.01) and increased the insulin secretory (p < 0.01) response following a glucose challenge 12 h after administration. Studies in GLP-1R knockout mice confirmed involvement of the GLP-1R pathway in the biological actions of (D-Ser(2))glucagon-exe. Twice-daily administration of (D-Ser(2))glucagon-exe to high-fat-fed mice for 28 days significantly (p < 0.05 to p < 0.001) reduced body weight, energy intake and non-fasting glucose levels, as well as increasing insulin concentrations. Glucose tolerance and insulin sensitivity were significantly (p < 0.01) improved and energy expenditure, O2 consumption and locomotor activity were (p < 0.05 to p < 0.001) augmented. The metabolic benefits were accompanied by increases in pancreatic islet number (p < 0.001) and area (p < 0.05), as well as beta cell area (p < 0.05). Beneficial effects were largely retained for 14 days following cessation of treatment.

CONCLUSIONS/INTERPRETATION

This study emphasises the potential of (D-Ser(2))glucagon-exe for the treatment of obesity-related diabetes.

The gut-brain axis in obesity

Currently the only effective treatment for morbid obesity with a proven mortality benefit is surgical intervention. The underlying mechanisms of these surgical techniques are unclear, but alterations in circulating gut hormone levels have been demonstrated to be at least one contributing factor. Gut hormones seem to communicate information from the gastrointestinal tract to the regulatory appetite centres within the central nervous system (CNS) via the so-called 'Gut-Brain-Axis'. Such information may be transferred to the CNS either via vagal or non-vagal afferent nerve signalling or directly via blood circulation. Complex neural networks, distributed throughout the forebrain and brainstem, are in control of feeding and energy homoeostasis. This article aims to review how appetite is potentially regulated by these gastrointestinal hormones. Identification of the underlying mechanisms of appetite and weight control may pave the way to develop better surgical techniques and new therapies in the future.

The role of gastrointestinal hormones in the pathogenesis of obesity and type 2 diabetes

Obesity, influencing the increase of incidence of type 2 diabetes, cardiovascular complications and cancer is a growing medical problem worldwide. The feelings of hunger and satiety are stimulated by the “gut-brain axis”, where a crucial role is played by gastrointestinal hormones: glucagon-like peptide 1, glucose-dependent insulinotropic polypeptide, pancreatic polypeptide, peptide YY, oxyntomodulin, cholecystokinin and ghrelin. These hormones affect not only the functioning of the digestive tract, but also might have effects on insulin secretion and are mediators which affect brain areas involved in the regulation of food intake. The effect of their actions can be antagonistic as well as an additive or synergistic, and their secretion is dependent on many factors, such as dietary nutrients or the energy state of the body. Changes in circulating gut hormones concentrations result in activation of various pathways primarily within the hypothalamus and brain stem areas, which modulate feeding behaviour and a number of metabolic processes.

Systemic delivery of biotherapeutics through the lung: opportunities and challenges for improved lung absorption

The development of Exubera(®) (inhaled insulin) has paved the way for consideration of future inhaled biotherapeutic products for systemic delivery. This route of drug delivery favors highly potent small peptides without self-association and large proteins resistant to enzymatic degradation for high bioavailability, while likely resulting in transient therapeutic effects. Improved therapeutic benefits with a needle-free delivery, such as inhaled insulin, are also rational pursuits. Molecules and their formulations must be carefully chosen and designed to optimize the rates of lung absorption and nonabsorptive loss. Novel molecular or formulation approaches, for example, Technosphere(®), Fc-/scFv-fusion protein, PEGylation, polymeric or lipid-based micro/nanoparticles and liposomes, offer opportunities to improve lung absorption and therapeutic duration of some biotherapeutics. Critical assessments are now essential as to their therapeutic benefits, safety, patient acceptance and market competition, as carried out for Exubera.

Gut hormones and obesity: physiology and therapies

Over the past 30 years, it has been established that hormones produced by the gut, pancreas, and adipose tissue are key players in the control of body weight. These hormones act through a complex neuroendocrine system, including the hypothalamus, to regulate metabolism and energy homeostasis. In obesity, this homeostatic balance is disrupted, either through alterations in the levels of these hormones or through resistance to their actions. Alterations in gut hormone secretion following gastric bypass surgery are likely to underlie the dramatic and persistent loss of weight following this procedure, as well as the observed amelioration in type 2 diabetes mellitus. Medications based on the gut hormone GLP-1 are currently in clinical use to treat type 2 diabetes mellitus and have been shown to produce weight loss. Further therapies for obesity based on other gut hormones are currently in development.

Gut hormones as therapeutic agents in treatment of diabetes and obesity

Obesity and Type 2 Diabetes Mellitus (T2DM) present an ever-increasing threat to global health. Although bariatric surgery is an effective treatment, it cannot be applied to the vast majority of patients. The beneficial effects of bariatric surgery are related to complex alterations in the secretion of gut hormones. By recapitulation of the changes of gut hormone secretion after bariatric surgery, drugs based on gut hormones represent an exciting possibility for the treatment of T2DM and obesity. We review the rapidly emerging role of GLP-1 based treatments as well as the future for new drugs based on other gut hormones such as GIP, ghrelin, oxyntomodulin and peptide YY.

A PEGylated analog of the gut hormone oxyntomodulin with long-lasting antihyperglycemic, insulinotropic and anorexigenic activity

Peptide agonists of the glucagon-like peptide 1 (GLP-1) receptor (GLP1R) are rapidly gaining favor as antidiabetic agents, since in addition to increasing glucose-dependent insulin secretion, they also cause weight loss. Oxyntomodulin (OXM), a natural peptide with sequence homology to both glucagon and GLP-1, has glucose-lowering activity in rodents and anorectic activity in rodents and humans, but its clinical utility is limited by a short circulatory half-life due to rapid renal clearance and degradation by dipeptidyl peptidase IV (DPP-IV). Here, we describe the development of a novel DPP-IV-resistant, long-acting GLP1R agonist, based on derivatization of a suitably chosen OXM analog with high molecular weight polyethylene glycol (PEG) ('PEGylation'). PEG-OXM exerts an anti-hyperglycemic effect in diet-induced obese (DIO) mice in a glucose-dependent manner, with a maximally efficacious dose of 0.1mg/kg, and reduces food intake and body weight with a minimally efficacious dose of 1mg/kg. If this pharmacology is recapitulated in patients with type 2 diabetes, these results indicate PEG-OXM as a potential novel once-weekly GLP-1 mimetic with both glucose-lowering activity and weight loss efficacy.

A DPP-IV-resistant triple-acting agonist of GIP, GLP-1 and glucagon receptors with potent glucose-lowering and insulinotropic actions in high-fat-fed mice

AIMS/HYPOTHESIS

We designed a chemically modified, enzyme-resistant peptide with triple-acting properties based on human glucagon with amino acid substitutions aligned to strategic positions in the sequence of glucose-dependent insulinotropic polypeptide (GIP).

METHODS

Y(1)-dA(2)-I(12)-N(17)-V(18)-I(27)-G(28,29)-glucagon (termed YAG-glucagon) was incubated with dipeptidylpeptidase IV (DPP-IV) to assess stability, BRIN-BD11 cells to evaluate insulin secretion, and receptor-transfected cells to examine cAMP production. Acute glucose-lowering and insulinotropic properties of YAG-glucagon were assessed in National Institutes of Health (NIH) Swiss mice, while longer-term actions on glucose homeostasis, insulin secretion, food intake and body weight were examined in high-fat-fed mice.

RESULTS

YAG-glucagon was resistant to DPP-IV, increased in vitro insulin secretion (1.5-3-fold; p < 0.001) and stimulated cAMP production in GIP receptor-, glucagon-like peptide-1 (GLP-1) receptor- and glucagon receptor-transfected cells. Plasma glucose levels were significantly reduced (by 51%; p < 0.01) and insulin concentrations increased (1.2-fold; p < 0.01) after acute injection of YAG-glucagon in NIH Swiss mice. Acute actions were countered by established GIP, GLP-1 and glucagon antagonists. In high-fat-fed mice, twice-daily administration of YAG-glucagon for 14 days reduced plasma glucose (40% reduction; p < 0.01) and increased plasma insulin concentrations (1.8-fold; p < 0.05). Glycaemic responses were markedly improved (19-48% reduction; p < 0.05) and insulin secretion enhanced (1.5-fold; p < 0.05) after a glucose load, which were independent of changes in insulin sensitivity, food intake and body weight.

CONCLUSIONS/INTERPRETATION

YAG-glucagon is a DPP-IV-resistant triple agonist of GIP, GLP-1 and glucagon receptors and exhibits beneficial biological properties suggesting that it may hold promise for treatment of type 2 diabetes.

Gut hormones: the future of obesity treatment?

Obesity is a major worldwide health problem. The treatment options are severely limited. The development of novel anti-obesity drugs is fraught with efficacy and safety issues. Consequently, several investigational anti-obesity drugs have failed to gain marketing approval in recent years. Anorectic gut hormones offer a potentially safe and viable option for the treatment of obesity. The prospective utility of gut hormones has improved drastically in recent years with the development of longer acting analogues. Additionally, specific combinations of gut hormones have been demonstrated to have additive anorectic effects. This article reviews the current stage of anti-obesity drugs in development, focusing on gut hormone-based therapies.

Analogs of pancreatic polypeptide and peptide YY with a locked PP-fold structure are biologically active

Pancreatic polypeptide (PP), peptide YY (PYY) and neuropeptide Y (NPY), members of the PP-fold family share a high degree of sequence homology. Nuclear magnetic resonance (NMR) and X-ray crystallography studies have shown these peptides can adopt a tightly organized tertiary structure called the PP-fold, which has long been assumed to be the active structure of this family of peptides. To date, however, no studies have been completed with PYY and PP which confirm if the PP-fold structure is important for their physiological actions. The aim of the study was to test if PYY and PP locked into the PP-fold maintained biological activity. Therefore, we designed and produced analogs of PP and PYY in a cyclic conformation with two cysteine amino acid substitutions at the N-terminus and at position 27. These were oxidized to form a cysteine disulfide bond locking the peptides into the PP-fold structure. Studies demonstrate that the cyclic analogs have both similar in vivo activity to their parent molecules, and affinity for the Y2 and Y4 receptors. Results suggest that the proposed PP and PYY-fold is likely to be their biologically active conformation.

Novel therapeutics and targets for the treatment of diabetes

The microvascular complications of insufficiently controlled diabetes (neuropathy, retinopathy and nephropathy) and the marked increased risk of macrovascular events (e.g., stroke and myocardial infarction) have a dire impact on society in both human and economic terms. In Type 1 diabetes total β-cell loss occurs. In Type 2 diabetes, partial β-cell loss occurs before diagnosis, and the progressive β-cell loss during the life of the patient increases the severity of the disease. In patients with diabetes, increased insulin resistance in the muscle and liver are key pathophysiologic defects. In addition, defects in metabolic processes in the fat, GI tract, brain, pancreatic α-cells and kidney are detrimental to the overall health of the patient. This review addresses novel therapies for these deficiencies in clinical and preclinical evaluation, emphasizing their potential to address glucose homeostasis, β-cell mass and function, and the comorbidities of cardiovascular disease and obesity.

Obesity and appetite control

Obesity is one of the major challenges to human health worldwide; however, there are currently no effective pharmacological interventions for obesity. Recent studies have improved our understanding of energy homeostasis by identifying sophisticated neurohumoral networks which convey signals between the brain and gut in order to control food intake. The hypothalamus is a key region which possesses reciprocal connections between the higher cortical centres such as reward-related limbic pathways, and the brainstem. Furthermore, the hypothalamus integrates a number of peripheral signals which modulate food intake and energy expenditure. Gut hormones, such as peptide YY, pancreatic polypeptide, glucagon-like peptide-1, oxyntomodulin, and ghrelin, are modulated by acute food ingestion. In contrast, adiposity signals such as leptin and insulin are implicated in both short- and long-term energy homeostasis. In this paper, we focus on the role of gut hormones and their related neuronal networks (the gut-brain axis) in appetite control, and their potentials as novel therapies for obesity.