Development and characterisation of novel, enzymatically stable oxytocin analogues with beneficial antidiabetic effects in high fat fed mice

Design, synthesis, conformational analysis, and biological activity of Cα1-to-Cα6 1,4- and 4,1-disubstituted 1H-[1,2,3]triazol-1-yl-bridged oxytocin analogues

Oxytocin (OT) is a neurohypophyseal peptide hormone containing a disulphide-bridged pseudocyclic conformation. The biomedical use of OT peptides is limited amongst others by disadvantageous pharmacokinetic parameters. To increase the stability of OT by replacing the disulphide bridge with the stable and more rigid [1,2,3]triazol-1-yl moiety, we employed the Cu2+-catalysed side chain-to-side chain azide-alkyne 1,3-cycloaddition. Here we report the design, synthesis, conformational analysis, and in vitro pharmacological activity of a homologous series of Cα1-to-Cα6 side chain-to-side chain [1,2,3]triazol-1-yl-containing OT analogues differing in the length of the bridge, location, and orientation of the linking moiety. Exploiting this macrocyclisation approach, it was possible to generate a systematic series of compounds providing interesting insight into the structure-conformation-function relationship of OT. Most analogues were able to adopt similar conformation to endogenous OT in water, namely, a type I β-turn. This approach may in the future generate stabilised pharmacological peptide tools to advance understanding of OT physiology.

Oxytocin as a potential pharmacological tool to combat obesity

The neuropeptide oxytocin (OT) has emerged as an important anorexigen in the regulation of food intake and energy balance. It has been shown that the release of OT and activation of hypothalamic OT neurons coincide with food ingestion. Its effects on feeding have largely been attributed to limiting meal size through interactions in key regulatory brain regions governing the homeostatic control of food intake such as the hypothalamus and hindbrain in addition to key feeding reward areas such as the nucleus accumbens and ventral tegmental area. Furthermore, the magnitude of an anorexigenic response to OT and feeding-related activation of the brain OT circuit are modified by the composition and flavor of a diet, as well as by a social context in which a meal is consumed. OT is particularly effective in reducing consumption of carbohydrates and sweet tastants. Pharmacologic, genetic, and pair-feeding studies indicate that OT-elicited weight loss cannot be fully explained by reductions of food intake and that the overall impact of OT on energy balance is also partly a result of OT-elicited changes in lipolysis, energy expenditure, and glucose regulation. Peripheral administration of OT mimics many of its effects when it is given into the central nervous system, raising the questions of whether and to what extent circulating OT acts through peripheral OT receptors to regulate energy balance. Although OT has been found to elicit weight loss in female mice, recent studies have indicated that sex and estrous cycle may impact oxytocinergic modulation of food intake. Despite the overall promising basic research data, attempts to use OT in the clinical setting to combat obesity and overeating have generated somewhat mixed results. The focus of this mini-review is to briefly summarize the role of OT in feeding and metabolism, address gaps and inconsistencies in our knowledge, and discuss some of the limitations to the potential use of chronic OT that should help guide future research on OT as a tailor-made anti-obesity therapeutic.

Ac3IV, a V1a and V1b receptor selective vasopressin analogue, protects against hydrocortisone-induced changes in pancreatic islet cell lineage

The Avpr1a (V1a) and Avpr1b (V1b) receptor selective, vasopressin (AVP) analogue, Ac3IV has been shown to improve metabolism and pancreatic islet structure in diabetes and insulin resistance. The present study further investigates these actions by assessing the ability of Ac3IV to protect against pancreatic islet architectural disturbances induced by hydrocortisone (HC) treatment in transgenic Ins1^Cre/+;Rosa26-eYFP mice, that possess beta-cell lineage tracing capabilities. HC intervention increased (p < 0.001) energy intake but reduced (p < 0.01) body weight gain, with no impact of Ac3IV. All HC mice had reduced (p < 0.05) circulating glucose, but plasma insulin and glucagon concentrations remained unchanged. However, HC mice presented with increased (p < 0.001) pancreatic insulin content, which was further augmented by Ac3IV. In addition, Ac3IV treatment countered HC-induced increases in islet-, beta- and alpha-cell areas (p < 0.01), as well as promoting islet number towards control levels. This was accompanied by reduced (p < 0.05) beta-cell growth, but enhanced (p < 0.001) alpha-cell proliferation. There were no changes in islet cell apoptotic rates in any of the groups of HC mice, but co-expression of CK19 with insulin in pancreatic ductal cells was reduced by Ac3IV. Assessment of beta-cell lineage revealed that Ac3IV partially protected against HC-mediated de-differentiation of mature beta-cells, whilst also decreasing (p < 0.01) beta- to alpha-cell transdifferentiation. Our data indicate that sustained activation of V1a and V1b receptors exerts positive islet cell transition effects to help retain beta-cell identity in HC mice.

Biological networks in gestational diabetes mellitus: insights into the mechanism of crosstalk between long non-coding RNA and N6-methyladenine modification

Background

Gestational diabetes mellitus (GDM) is one of the most common complications of pregnancy. The mechanism underlying the crosstalk between long non-coding RNAs (lncRNAs) and N⁶-methyladenine (m6A) modification in GDM remain unclear.

Methods

We generated a lncRNA-mediated competitive endogenous RNA (ceRNA) network using comprehensive data from the Gene Expression Omnibus database, published data, and our preliminary findings. m6A-related lncRNAs were identified based on Pearson correlation coefficient (PCC) analysis using our previous profiles. An integrated pipeline was established to constructed a m6A-related subnetwork thereby predicting the potential effects of the m6A-related lncRNAs.

Results

The ceRNA network was composed of 16 lncRNAs, 17 microRNAs, 184 mRNAs, and 338 edges. Analysis with the Kyoto Encyclopedia of Genes and Genomes database demonstrated that genes in the ceRNA network were primarily involved in the development and adverse outcomes of GDM, such as those in the fatty acid-metabolism pathway, the peroxisome proliferator-activated receptor signaling pathway, and thyroid hormone signaling pathway. Four m6A-related lncRNAs were involved in the ceRNA network, including LINC00667, LINC01087, AP000350.6, and CARMN. The m6A-related subnetwork was generated based on these four lncRNAs, their ceRNAs, and their related m6A regulators. Genes in the subnetwork were enriched in certain GDM-associated hormone (thyroid hormone and oxytocin) signaling pathways. LINC00667 was positively correlated with an m6A “reader” (YTHDF3; PCC = 0.95) and exhibited the highest node degree in the ceRNA network. RIP assays showed that YTHDF3 directly bind LINC00667. We further found that MYC possessed the highest node degree in a protein–protein interaction network and competed with LINC00667 for miR-33a-5p. qPCR analysis indicated that LINC00667, YTHDF3 and MYC levels were upregulated in the GDM placentas, while miR-33a-5p was downregulated. In a support-vector machine classifier, an m6A-related module composed of LINC00667, YTHDF3, MYC, and miR-33a-5p showed excellent classifying power for GDM in both the training and the testing dataset, with an accuracy of 76.19 and 71.43%, respectively.

Conclusions

Our results shed insights into the potential role of m6A-related lncRNAs in GDM and have implications in terms of novel therapeutic targets for GDM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12884-022-04716-w.

Peptides in the regulation of glucagon secretion

Glucose homeostasis is maintained by the glucoregulatory hormones, glucagon, insulin and somatostatin, secreted from the islets of Langerhans. Glucagon is the body's most important anti-hypoglycemic hormone, mobilizing glucose from glycogen stores in the liver in response to fasting, thus maintaining plasma glucose levels within healthy limits. Glucagon secretion is regulated by both circulating nutrients, hormones and neuronal inputs. Hormones that may regulate glucagon secretion include locally produced insulin and somatostatin, but also urocortin-3, amylin and pancreatic polypeptide, and from outside the pancreas glucagon-like peptide-1 and 2, peptide tyrosine tyrosine and oxyntomodulin, glucose-dependent insulinotropic polypeptide, neurotensin and ghrelin, as well as the hypothalamic hormones arginine-vasopressin and oxytocin, and calcitonin from the thyroid. Each of these hormones have distinct effects, ranging from regulating blood glucose, to regulating appetite, stomach emptying rate and intestinal motility, which makes them interesting targets for treating metabolic diseases. Awareness regarding the potential effects of the hormones on glucagon secretion is important since secretory abnormalities could manifest as hyperglycemia or even lethal hypoglycemia. Here, we review the effects of each individual hormone on glucagon secretion, their interplay, and how treatments aimed at modulating the plasma levels of these hormones may also influence glucagon secretion and glycemic control.

Potential Therapeutic Targeting Neurotransmitter Receptors in Diabetes

Neurotransmitters are signaling molecules secreted by neurons to coordinate communication and proper function among different sections in the central neural system (CNS) by binding with different receptors. Some neurotransmitters as well as their receptors are found in pancreatic islets and are involved in the regulation of glucose homeostasis. Neurotransmitters can act with their receptors in pancreatic islets to stimulate or inhibit the secretion of insulin (β cell), glucagon (α cell) or somatostatin (δ cell). Neurotransmitter receptors are either G-protein coupled receptors or ligand-gated channels, their effects on blood glucose are mainly decided by the number and location of them in islets. Dysfunction of neurotransmitters receptors in islets is involved in the development of β cell dysfunction and type 2 diabetes (T2D).Therapies targeting different transmitter systems have great potential in the prevention and treatment of T2D and other metabolic diseases.

Beneficial impact of Ac3IV, an AVP analogue acting specifically at V1a and V1b receptors, on diabetes islet morphology and transdifferentiation of alpha- and beta-cells

Ac3IV (Ac-CYIQNCPRG-NH2) is an enzymatically stable vasopressin analogue that selectively activates Avpr1a (V1a) and Avpr1b (V1b) receptors. In the current study we have employed streptozotocin (STZ) diabetic transgenic Ins1Cre/+;Rosa26-eYFP and GluCreERT2;Rosa26-eYFP mice, to evaluate the impact of sustained Ac3IV treatment on pancreatic islet cell morphology and transdifferentiation. Twice-daily administration of Ac3IV (25 nmol/kg bw) to STZ-diabetic Ins1Cre/+;Rosa26-eYFP mice for 12 days increased pancreatic insulin (p<0.01) and significantly reversed the detrimental effects of STZ on pancreatic islet morphology. Such benefits were coupled with increased (p<0.01) beta-cell proliferation and decreased (p<0.05) beta-cell apoptosis. In terms of islet cell lineage tracing, induction of diabetes increased (p<0.001) beta- to alpha-cell differentiation in Ins1Cre/+;Rosa26-eYFP mice, with Ac3IV partially reversing (p<0.05) such transition events. Comparable benefits of Ac3IV on pancreatic islet architecture were observed in STZ-diabetic GluCreERT2;ROSA26-eYFP transgenic mice. In this model, Ac3IV provoked improvements in islet morphology which were linked to increased (p<0.05-p<0.01) transition of alpha- to beta-cells. Ac3IV also increased (p<0.05-p<0.01) CK-19 co-expression with insulin in pancreatic ductal and islet cells. Blood glucose levels were unchanged by Ac3IV in both models, reflecting the severity of diabetes induced. Taken together these data indicate that activation of islet receptors for V1a and V1b positively modulates alpha- and beta-cell turnover and endocrine cell lineage transition events to preserve beta-cell identity and islet architecture.

Weight-reducing, lipid-lowering and antidiabetic activities of a novel arginine vasopressin analogue acting at the V1a and V1b receptors in high-fat-fed mice

AIM

To assess the beneficial metabolic effects of the nonapeptide hormone, arginine vasopressin (AVP), on metabolism.

MATERIALS AND METHODS

We exchanged amino acids at position 3 and 8 of AVP, namely phenylalanine and arginine, with those of oxytocin, to generate novel analogues with altered receptor selectivity. Secondary modification by N-terminal acetylation was used to impart stability to circulating endopeptidases. Analogues were screened for degradation, bioactivity in rodent/human clonal beta cells and primary murine islets, together with evaluation of receptor activation profile.

RESULTS

Analogue Ac3IV, which lacked effects at the V2 receptors responsible for modulation of fluid balance, was selected as the lead compound for assessment of antidiabetic efficacy in high-fat-fed mice. Twice-daily administration of Ac3IV, or the gold standard control exendin-4, for 22 days, reduced energy intake as well as body weight and fat content. Both interventions decreased circulating glucose levels, enhanced insulin sensitivity, and substantially improved glucose tolerance and related insulin secretion in response to an intraperitoneal or oral glucose challenge. The peptides decreased total- and increased HDL-cholesterol, but only Ac3IV decreased LDL-cholesterol, triglyceride and non-fasting glucagon concentrations. Elevations of islet and beta-cell areas were partially reversed, accompanied by suppressed islet cell proliferation, decreased beta-cell apoptosis and, in the case of exendin-4, also decreased alpha-cell apoptosis.

CONCLUSION

AVP-based therapies that exclusively target V1a and V1b receptors may have significant therapeutic potential for the treatment of obesity and related diabetes, and merit further clinical exploration.

Oxytocin and Bone: Review and Perspectives

Recent data demonstrate the anabolic effect of oxytocin on bone. Bone cells express oxytocin receptors. Oxytocin promotes osteoblasts differentiation and function, leading to an increased bone formation with no effect on bone resorption and an improvement of bone microarchitecture. Oxytocin is synthetized by osteoblasts, and this synthesis is stimulated by estrogen. Animal studies demonstrate a direct action of oxytocin on bone, as the systemic administration of oxytocin prevents and reverses the bone loss induced by estrogen deficiency. Although oxytocin is involved in bone formation in both sexes during development, oxytocin treatment has no effect on male osteoporosis, underlining the importance of estrogen that amplifies its local autocrine and paracrine secretion. There are few human data showing a decrease in the oxytocin serum level in anorexia nervosa independently of estrogen and in amenorrheic women associated with impaired bone microarchitecture; in post-menopausal women a higher oxytocin serum level is associated with higher bone density, but not in osteoporotic men. Oxytocin displays many effects that may be beneficial in the management of osteoporosis, cardiovascular diseases, cognitive disorders, breast cancer, diabetes and body fat gain, all age-related diseases affecting elderly women, opening exciting therapeutic perspectives, although the issue is to find a single route, dosage and schedule able to reach all these targets.

The Effects of Oxytocin on Appetite Regulation, Food Intake and Metabolism in Humans

The hypothalamic peptide oxytocin and its receptor are involved in a range of physiological processes, including parturition, lactation, cell growth, wound healing, and social behavior. More recently, increasing evidence has established the effects of oxytocin on food intake, energy expenditure, and peripheral metabolism. In this review, we provide a comprehensive description of the central oxytocinergic system in which oxytocin acts to shape eating behavior and metabolism. Next, we discuss the peripheral beneficial effects oxytocin exerts on key metabolic organs, including suppression of visceral adipose tissue inflammation, skeletal muscle regeneration, and bone tissue mineralization. A brief summary of oxytocin actions learned from animal models is presented, showing that weight loss induced by chronic oxytocin treatment is related not only to its anorexigenic effects, but also to the resulting increase in energy expenditure and lipolysis. Following an in-depth discussion on the technical challenges related to endogenous oxytocin measurements in humans, we synthesize data related to the association between endogenous oxytocin levels, weight status, metabolic syndrome, and bone health. We then review clinical trials showing that in humans, acute oxytocin administration reduces food intake, attenuates fMRI activation of food motivation brain areas, and increases activation of self-control brain regions. Further strengthening the role of oxytocin in appetite regulation, we review conditions of hypothalamic insult and certain genetic pathologies associated with oxytocin depletion that present with hyperphagia, extreme weight gain, and poor metabolic profile. Intranasal oxytocin is currently being evaluated in human clinical trials to learn whether oxytocin-based therapeutics can be used to treat obesity and its associated sequela. At the end of this review, we address the fundamental challenges that remain in translating this line of research to clinical care.