Melanocortin 4 receptor-mediated effects of amylin on thermogenesis and regulation of food intake

Identification and functional analysis of six melanocortin-4-receptor-like (MC4R-like) mutations in goldfish (Carassius auratus)

Melanocortin receptor-4 (MC4R) belongs to the G protein-coupled receptor family, characterized by a classical structure of seven transmembrane domains (7TMD). They play an important role in food intake and weight regulation. In the present study, we identified melanocortin-4-receptor-like (caMC4RL) mutants of goldfish from the Qian River in the Qin Ling region and characterized their functional properties, including the constitutive activities of the mutants, ligand-induced cAMP and ERK1/2 accumulation, and AMPK activation. The results show that six caMC4RL mutants were identified in goldfish from the Qian River in the Qin Ling region, and are located in the conserved position of the Cyprinidae MC4Rs. The mutations (E57K, P296S, and R302T/K) result in the loss of Gs signaling function. The mutations (P296 and R302T/K) exhibited biased signaling in response to ACTH stimulation in the MAPK/ERK pathway. In addition, the E57K mutant may play a role in weight regulation and could serve as molecular markers for molecular breeding. These data will provide fundamental information for functional studies of teleost GPCR mutants and MC4R isoforms.

Central nervous system pathways targeted by amylin in the regulation of food intake

Amylin is a peptide hormone co-released with insulin from pancreatic β-cells during a meal and primarily serves to promote satiation. While the caudal hindbrain was originally implicated as a major site of action in this regard, it is becoming increasingly clear that amylin recruits numerous central nervous system pathways to exert multifaceted effects on food intake. In this Review, we discuss the evidence derived from preclinical studies showing that amylin and the related peptide salmon calcitonin (sCT) directly or indirectly target genetically distinct neurons in the caudal hindbrain (nucleus tractus solitarius and area postrema), rostral hindbrain (lateral parabrachial nucleus), midbrain (lateral dorsal tegmentum and ventral tegmental area) and hypothalamus (arcuate nucleus and parasubthalamic nucleus) via activation of amylin and/or calcitonin receptors. Given that the stable amylin analogue cagrilintide is under clinical development for the treatment of obesity, it is important to determine whether this drug recruits overlapping or distinct central nervous system pathways to that of amylin and sCT with implications for minimising any aversive effects it potentially causes. Such insight will also be important to understand how amylin and sCT analogues synergize with other molecules as part of dual or triple agonist therapies for obesity, especially the glucagon-like peptide 1 receptor (GLP-1R) agonist semaglutide, which has been shown to synergistically lower body weight with cagrilintide (CagriSema) in clinical trials.

Amylin-like immunoreactivity in the extra-islet peptide YY-producing and glucagon-immunoreactive cells in Japanese quail pancreas

In this study, we investigated amylin-like substance distribution in the pancreas of Japanese quail (Coturnix japonica) using a specific anti-rat amylin serum. We detected amylin-immunoreactive cells dispersed in the pancreatic extra-islet region but not in the islet region. The synthetic rat amylin-containing serum pre-absorption abolished the staining profile. Almost all amylin-immunoreactive cells were immuno-positive for peptide YY (PYY). In addition, certain amylin-immunoreactive cells stained immuno-positive for glucagon. Amylin and PYY co-secreted from the extra-islet cells might participate in the insulin and glucagon release regulation in the pancreas and food intake modulation through the central nervous system.

Evidence for a feeding related association between melanocortin in the NTS and Neuropeptide-Y in the PVN

Melanocortin and neuropeptide-Y (NPY) are both involved in feeding and energy regulation, and they have opposite effects in the paraventricular nucleus of the hypothalamus (PVN). The present study examined an interaction between melanocortin in the nucleus of the solitary tract (NTS) and NPY in the PVN. Male Sprague-Dawley rats were implanted with cannulae in the injection sites of interest. In Experiment 1, subjects received either the melanocortin 3/4-receptor (MC3/4) antagonist SHU9119 (0, 10, 50 and 100 pmol/0.5 μl) or the MC3/4 agonist MTII (0, 10, 50, 100 and 200 pmol/0.5 μl) into the NTS. Food intake was measured at 1, 2, 4, 6 and 24-h post-injection. Administration of SHU9119 into the NTS significantly and dose-dependently increased food intake at 1, 2, 4, 6 and 6-24-h, and administration of MTII into the NTS significantly and dose-dependently decreased 24-h free feeding. In Experiment 2, subjects received the MC3/4 agonist MTII (0, 10, 50, 100 and 200 pmol/0.5 μl) into the NTS just prior to NPY (0 and 1μg/0.5 μl) in the PVN. PVN injection of NPY stimulated feeding, and administration of MTII (50, 100 and 200 pmol) into the NTS significantly and dose-dependently decreased NPY-induced feeding at 2, 4, 6 and 6-24-h. These data suggest that there could be a neuronal association between melanocortin in the NTS and NPY in the PVN, and that the melanocortin system in the NTS has an antagonistic effect on NPY-induced feeding in the PVN.

Glutamatergic melanocortin-4 receptor neurons regulate body weight

The locus coeruleus (LC), enriched in vesicular glutamate transporter 2 (VGlut2) neurons, is a potential homeostasis-regulating hub. However, the identity of melanocortin-4 receptor (MC4R) neurons in the paraventricular nucleus (PVN) of the hypothalamus, PVNVGlut2::MC4R and LCVGlut2::MC4R regulation of body weight, and axonal projections of LCVGlut2 neurons remain unclear. Conditional knockout of MC4R in chimeric mice was used to confirm the effects of VGlut2. Interscapular brown adipose tissue was injected with pseudorabies virus to study the central nervous system projections. We mapped the LCVGlut2 circuitry. Based on the Cre-LoxP recombination system, specific knockdown of MC4R in VGlut2 neurons resulted in weight gain in chimeric mice. Adeno-associated virus-mediated knockdown of MC4R expression in the PVN and LC had potential superimposed effects on weight gain, demonstrating the importance of VGlut2 neurons. Unlike these wide-ranging efferent projections, the PVN, hypothalamic arcuate nucleus, supraoptic nucleus of the lateral olfactory tegmental nuclei, and nucleus tractus solitarius send excitatory projections to LCVGlut2 neurons. The PVN → LC glutamatergic MC4R long-term neural circuit positively affected weight management and could help treat obesity.

Amylin regulates testosterone levels via steroidogenesis-related enzymes in the central nervous system of male mice

Amylin is a peripheral satiation signal polypeptide co-secreted with insulin by pancreatic β-cells in response to nutrient ingestion. Amylin participates in the eating-inhibitory effect and regulates energy metabolism by acting on the central nervous system (CNS). However, the role of amylin in regulating the biosynthesis of steroid hormones, such as testosterone, through the hypothalamic-pituitary-gonadal axis (HPG) remains unexplored. However, only limited evidence is available on the involvement of amylin in steroid synthesis, we hypothesize that amylin regulates testosterone levels via steroidogenesis-related enzymes in the CNS. In this study, we elucidated the effect of intraperitoneal injection of amylin on the protein expression of steroidogenesis-related enzymes, including 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 17A1 (CYP17A1), and steroidogenic acute regulatory protein (StAR), and phospho-extracellular signal-regulated kinase (pERK). Additionally, the effect of amylin on testosterone levels in male mice was examined. Our results suggested that 3β-HSD and CYP17A1 neurons were widely expressed in the CNS of male mice, whereas StAR neurons were mainly expressed in the zona incerta (ZI) and locus coeruleus (LC) regions. Intraperitoneal injection of amylin significantly reduced (p < 0.01) the expression of 3β-HSD, CYP17A1, and StAR in ZI and other areas near the third ventricle (3 V) but increased (p < 0.01) pERK expression, brain testosterone levels, serum FSH, serum LH, and decreased (p < 0.01) serum testosterone levels in mice. In conclusion, amylin regulates testosterone levels via steroidogenesis-related enzymes in the central nervous system of male mice.

Identification, tissue distribution, and anorexigenic effect of amylin in Siberian sturgeon (Acipenser baeri)

Amylin is a 37-amino acid polypeptide that has been found to be involved in feeding regulation in some mammals, birds, and goldfish. We cloned amylin of Siberian sturgeon and detected its distribution pattern in 15 tissues. The expression levels in the periprandial period (pre-and post-feeding), the changes in the food intake, and the expression levels of related appetite factors after the intraperitoneal injection of amylin were detected. The expression of amylin was found to be the highest in the hypothalamus. Compared with 1 h pre-feeding, the expression levels of amylin in the hypothalamus and duodenum were increased significantly 1 h post-feeding. Compared with the control group (saline), intraperitoneal injection of 50 ng/g, 100 ng/g, and 200 ng/g of amylin significantly inhibited food intake at 1 h post injection, but not at 3 h and 6 h. The injection of 50 ng/g, 100 ng/g, and 200 ng/g amylin significantly inhibited the cumulative feed. After 1 h of 50 ng/g amylin injection, the levels of MC4R and somatostatin in the hypothalamus increased significantly, while the levels of amylin and NPY decreased significantly. The levels of CCK in the valvular intestine were increased significantly. Insulin in the duodenum was also increased significantly, but there was no significant change in ghrelin in the duodenum. These results show that amylin inhibits feeding in Siberian sturgeon by down-regulating the appetite-stimulating factor NPY and up-regulating the appetite-suppressing factors somatostatin, MC4R, CCK, and insulin. This study provides a theoretical basis for studying the feeding function and action mechanisms of amylin in Siberian sturgeon.

Impact of Gut and Metabolic Hormones on Feeding Reward

Ingestion of food activates a cascade of endocrine responses (thereby reflecting a contemporaneous feeding status) that include the release of hormones from the gastrointestinal (GI) tract, such as cholecystokinin (CCK), glucagonlike peptide YY (PYY), peptide PP, and oleoylethanolamide, as well as suppression of ghrelin secretion. The pancreas and adipose tissue, on the other hand, release hormones that serve as a measure of the current metabolic state or the long-term energy stores, that is, insulin, leptin, and adiponectin. It is well known and intuitively understandable that these hormones target either directly (by crossing the blood-brain barrier) or indirectly (e.g., via vagal input) the "homeostatic" brainstem-hypothalamic pathways involved in the regulation of appetite. The current article focuses on yet another target of the metabolic and GI hormones that is critical in inducing changes in food intake, namely, the reward system. We discuss the physiological basis of this functional interaction, its importance in the control of appetite, and the impact that disruption of this crosstalk has on energy intake in select physiological and pathophysiological states. We conclude that metabolic and GI hormones have a capacity to strengthen or weaken a response of the reward system to a given food, and thus, they are fundamental in ensuring that feeding reward is plastic and dependent on the energy status of the organism. © 2021 American Physiological Society. Compr Physiol 11:1425-1447, 2021.

Inhibitory effect of central ghrelin on steroid synthesis affecting reproductive health in female mice

Ghrelin is a 28-amino acid peptide hormone that regulates ovarian steroid hormone synthesis; however, there is limited evidence regarding the regulation of this pathway by ghrelin in mice ovary. Thus, we aimed to investigate whether central ghrelin action plays a role in murine reproductive health by inhibiting steroid synthesis. Further, we sought to examine the mechanism of central ghrelin action in ovarian steroid hormone synthesis. After the administration of intracerebroventricular ghrelin (1 nmol), we found reduced serum concentrations of oestradiol and progesterone and reduced secretion of follicle-stimulating hormone and luteinising hormone. Although ghrelin reduced 3β-hydroxysteroid dehydrogenase mRNA and protein levels in the hypothalamus, it did not affect the expression of steroidogenic acute regulatory protein and cytochrome P450 17A1. In the ovary, central ghrelin regulation indirectly inhibited the mRNA and protein levels of steroidogenic acute regulatory protein, cytochrome P450 17A1, and 3β-hydroxysteroid dehydrogenase. Moreover, no changes were observed in the expression of proliferating cell nuclear antigen and phosphorylation of extracellular signal-regulated kinase. We hypothesised that central ghrelin regulation suppressed serum oestradiol and progesterone levels by indirectly inhibiting the expression of steroidogenic acute regulatory protein, cytochrome P450 17A1, and 3β-hydroxysteroid dehydrogenase in the ovary. In this regulation, the suppressed secretion of the follicle-stimulating hormone and luteinising hormone in the pituitary by ghrelin could be involved. Furthermore, hypothalamic 3β-hydroxysteroid dehydrogenase expression is reduced by ghrelin injection.

Amylin brain circuitry

Amylin is a peptide hormone that is mainly known to be produced by pancreatic β-cells in response to a meal but amylin is also produced by brain cells in discrete brain areas albeit in a lesser amount. Amylin receptor (AMY) is composed of the calcitonin core-receptor (CTR) and one of the 3 receptor activity modifying protein (RAMP), thus forming AMY1-3; RAMP enhances amylin binding properties to the CTR. However, amylin receptor agonist such as salmon calcitonin is able to bind CTR alone. Peripheral amylin's main binding site is located in the area postrema (AP) which then propagate the signal to the nucleus of the solitary tract and lateral parabrachial nucleus (LPBN) and it is then transmitted to the forebrain areas such as central amygdala and bed nucleus of the stria terminalis. Amylin's activation of these different brain areas mediates eating and other metabolic pathways controlling energy expenditure and glucose homeostasis. Peripheral amylin can also bind in the arcuate nucleus of the hypothalamus where it acts independently of the AP to activate POMC and NPY neurons. Amylin activation of NPY neurons has been shown to be transmitted to LPBN neurons to act on eating while amylin POMC signaling affects energy expenditure and locomotor activity. While a large amount of experiments have already been conducted, future studies will have to further investigate how amylin is taken up by forebrain areas and deepen our understanding of amylin action on peripheral metabolism.

Neuroanatomy of melanocortin-4 receptor pathway in the mouse brain

Objective

Melanocortin-4 receptors (MC4Rs) are key regulators of energy homeostasis and adipose deposition in the central nervous system. Considering that MC4R expression regions and function-related research mainly focus on the paraventricular nucleus (PVN), little is known about their distribution throughout the mouse brain, although its messenger RNA distribution has been analyzed in the rat. Therefore, MC4R protein localization in mouse neurons was the focus of this study.

Methods

MC4R protein distribution was assessed in mice through immunofluorescence and Western blotting.

Results

MC4R was differentially expressed throughout the arcuate nucleus (ARC), nucleus of the solitary tract (NTS), raphe pallidus (RPa), medial cerebellar nucleus, intermediolateral nucleus, and brainstem. The highest MC4R protein levels were found in the ARC and ventromedial hypothalamic nucleus, while they were significantly lower in the parabrachial nucleus and NTS. The lowest MC4R protein levels were found in the PVN; there was no difference in the protein levels between the area postrema and RPa.

Conclusions

These data provide a basic characterization of MC4R-expressing neurons and protein distribution in the mouse brain and may aid further research on its role in energy homeostasis.

Systemic and Central Amylin, Amylin Receptor Signaling, and Their Physiological and Pathophysiological Roles in Metabolism

This article in the Neural and Endocrine Section of Comprehensive Physiology discusses the physiology and pathophysiology of the pancreatic hormone amylin. Shortly after its discovery in 1986, amylin has been shown to reduce food intake as a satiation signal to limit meal size. Amylin also affects food reward, sensitizes the brain to the catabolic actions of leptin, and may also play a prominent role in the development of certain brain areas that are involved in metabolic control. Amylin may act at different sites in the brain in addition to the area postrema (AP) in the caudal hindbrain. In particular, the sensitizing effect of amylin on leptin action may depend on a direct interaction in the hypothalamus. The concept of central pathways mediating amylin action became more complex after the discovery that amylin is also synthesized in certain hypothalamic areas but the interaction between central and peripheral amylin signaling remains currently unexplored. Amylin may also play a dominant pathophysiological role that is associated with the aggregation of monomeric amylin into larger, cytotoxic molecular entities. This aggregation in certain species may contribute to the development of type 2 diabetes mellitus but also cardiovascular disease. Amylin receptor pharmacology is complex because several distinct amylin receptor subtypes have been described, because other neuropeptides [e.g., calcitonin gene-related peptide (CGRP)] can also bind to amylin receptors, and because some components of the functional amylin receptor are also used for other G-protein coupled receptor (GPCR) systems. © 2020 American Physiological Society. Compr Physiol 10:811-837, 2020.

Amylin/Calcitonin Receptor-Mediated Signaling in POMC Neurons Influences Energy Balance and Locomotor Activity in Chow-Fed Male Mice

Amylin, a pancreatic hormone and neuropeptide, acts principally in the hindbrain to decrease food intake and has recently been shown to act as a neurotrophic factor to control the development of area postrema → nucleus of the solitary tract and arcuate hypothalamic nucleus → paraventricular nucleus axonal fiber outgrowth. Amylin is also able to activate ERK signaling specifically in POMC neurons independently of leptin. For investigation of the physiological role of amylin signaling in POMC neurons, the core component of the amylin receptor, calcitonin receptor (CTR), was depleted from POMC neurons using an inducible mouse model. The loss of CTR in POMC neurons leads to increased body weight gain, increased adiposity, and glucose intolerance in male knockout mice, characterized by decreased energy expenditure (EE) and decreased expression of uncoupling protein 1 (UCP1) in brown adipose tissue. Furthermore, a decreased spontaneous locomotor activity and absent thermogenic reaction to the application of the amylin receptor agonist were observed in male and female mice. Together, these results show a significant physiological impact of amylin/calcitonin signaling in CTR-POMC neurons on energy metabolism and demonstrate the need for sex-specific approaches in obesity research and potentially treatment.

Central control of energy balance by amylin and calcitonin receptor agonists and their potential for treatment of metabolic diseases

The prevalence of obesity and associated comorbidities such as type 2 diabetes and cardiovascular disease is increasing globally. Body-weight loss reduces the risk of morbidity and mortality in obese individuals, and thus, pharmacotherapies that induce weight loss can be of great value in improving the health and well-being of people living with obesity. Treatment with amylin and calcitonin receptor agonists reduces food intake and induces weight loss in several animal models, and a number of companies have started clinical testing for peptide analogues in the treatment of obesity and/or type 2 diabetes. Studies predominantly performed in rodent models show that amylin and the dual amylin/calcitonin receptor agonist salmon calcitonin achieve their metabolic effects by engaging areas in the brain associated with regulating homeostatic energy balance. In particular, signalling via neuronal circuits in the caudal hindbrain and the hypothalamus is implicated in mediating effects on food intake and energy expenditure. We review the current literature investigating the interaction of amylin/calcitonin receptor agonists with neurocircuits that induce the observed metabolic effects. Moreover, the status of drug development of amylin and calcitonin receptor agonists for the treatment of metabolic diseases is summarized.

Effects of amylin on food intake and body weight via sympathetic innervation of the interscapular brown adipose tissue

Objective: Amylin acts on the lateral dorsal tegmental nucleus (LDT), resulting in anorexic and weight-loss effects and activates thermogenesis in the interscapular brown adipose tissue (IBAT). In addition, it induces neuronal nitric oxide synthase (nNOS) and choline acetyltransferase (ChAT)-mediated feeding. However, the influence of the intact sympathetic nervous system (SNS) in mediating amylin's effects has not been fully characterised. We investigated whether extracellular signal-regulated kinase (ERK), nNOS, and ChAT activities in the LDT are responsible for amylin's anorexigenic effects and whether this requires an intact SNS.Methods: C57BL/6J mice [wild-type (WT), sham, and sympathetic denervation of IBAT] were used. Food consumption, body weight, and distribution of pERK, nNOS, and ChAT positive neurons in the brain were examined following acute and chronic amylin administration.Results: Food intake was significantly decreased in WT and sham animals following acute amylin injection, but not in the denervated mice. Chronic amylin reduced body weight and serum glucose levels after 6 weeks, but increased insulin levels; no changes were observed in the denervated mice. Acute amylin increased the expression of nNOS, ChAT, and uncoupling protein-1 in the IBAT of WT and sham mice, while no changes were observed in the denervated mice and pERK from the above effect.Conclusions: Intact SNS of IBAT influences amylin-induced suppression of food intake and body weight, thus affecting nNOS and ChAT signalling in the LDT and locus coeruleus.

Response of the expression of oxytocin neurons to ghrelin in female mice

Ghrelin is an orexigenic agonist that acts directly on neurons in the hypothalamus, controlling appetite and energy balance. Although its role in appetite-associated neurons has been described, the relationship between peripheral ghrelin stimulation and oxytocin expression in the paraventricular nucleus is not fully understood. We evaluated the suppressive function of ghrelin in oxytocin-positive paraventricular nucleus neurons in ovariectomized C57BL/6 mice 2 h after ghrelin injection. The results showed that, in intact mice, peripheral ghrelin stimulation activated estrogen receptor alpha-expressing neurons during the estrous cycle and that agouti-related peptide mRNA expression was remarkably increased. Agouti-related peptide neuron axons co-localized with oxytocin neurons in the paraventricular nucleus. Moreover, the response of oxytocin-positive paraventricular nucleus neurons to ghrelin was suppressed in the proestrus period, while ghrelin decreased the serum concentration of estradiol in the proestrus phase. These data suggest that ghrelin may suppress oxytocin-positive neuron expression via the arcuate nucleus agouti-related peptide circuit, with the possible influence of estradiol in the murine estrous cycle. Unraveling the mechanism of ghrelin-induced oxytocin expression in the hypothalamus paraventricular nucleus broadens the horizon for ghrelin-related appetite research.

Amylin and Leptin interaction: Role During Pregnancy, Lactation and Neonatal Development

Amylin is co-secreted with insulin by pancreatic β-cells in response to a meal and produced by neurons in discrete hypothalamic brain areas. Leptin is proportionally secreted by the adipose tissue. Both hormones control food intake and energy homeostasis post-weaning in rodents. While amylin's main site of action is located in the area postrema (AP) and leptin's is located in the mediobasal hypothalamus, both hormones can also influence the other's signaling pathway; amylin has been shown enhance hypothalamic leptin signaling, and amylin signaling in the AP may rely on functional leptin receptors to modulate its effects. These two hormones also play major roles during other life periods. During pregnancy, leptin levels rise as a result of an increase in fat depot resulting in gestational leptin-resistance to prepare the maternal body for the metabolic needs during fetal development. The role of amylin is far less studied during pregnancy and lactation, though amylin levels seem to be elevated during pregnancy relative to insulin. Whether amylin and leptin interact during pregnancy and lactation remains to be assessed. Lastly, during brain development, amylin and leptin are major regulators of cell birth during embryogenesis and act as neurotrophic factors in the neonatal period. This review will highlight the role of amylin and leptin, and their possible interaction, during these dynamic time periods of pregnancy, lactation, and early development.