Quantitative mass spectrometry for human melanocortin peptides in vitro and in vivo suggests prominent roles for β-MSH and desacetyl α-MSH in energy homeostasis

Alpha-melanocyte-stimulating hormone contributes to an anti-inflammatory response to lipopolysaccharide

OBJECTIVE

During infection, metabolism and immunity react dynamically to promote survival through mechanisms that remain unclear. Pro-opiomelanocortin (POMC) cleavage products are produced and released in the brain and in the pituitary gland. One POMC cleavage product, alpha-melanocyte-stimulating hormone (α-MSH), is known to regulate food intake and energy expenditure and has anti-inflammatory effects. However, it is not known whether α-MSH is required to regulate physiological anti-inflammatory responses. We recently developed a novel mouse model with a targeted mutation in Pomc (Pomctm1/tm1 mice) to block production of all α-MSH forms which are required to regulate metabolism. To test whether endogenous α-MSH is required to regulate immune responses, we compared acute bacterial lipopolysaccharide (LPS)-induced inflammation between Pomctm1/tm1 and wild-type Pomcwt/wt mice.

METHODS

We challenged 10- to 14-week-old male Pomctm1/tm1 and Pomcwt/wt mice with single i.p. injections of either saline or low-dose LPS (100 μg/kg) and monitored immune and metabolic responses. We used telemetry to measure core body temperature (Tb), ELISA to measure circulating cytokines, corticosterone and α-MSH, and metabolic chambers to measure body weight, food intake, activity, and respiration. We also developed a mass spectrometry method to measure three forms of α-MSH produced in the mouse hypothalamus and pituitary gland.

RESULTS

LPS induced an exaggerated immune response in Pomctm1/tm1 compared to Pomcwt/wt mice. Both groups of mice were hypoactive and hypothermic following LPS administration, but Pomctm1/tm1 mice were significantly more hypothermic compared to control mice injected with LPS. Pomctm1/tm1 mice also had reduced oxygen consumption and impaired metabolic responses to LPS compared to controls. Pomctm1/tm1 mice had increased levels of key proinflammatory cytokines at 2 h and 4 h post LPS injection compared to Pomcwt/wt mice. Lastly, Pomcwt/wt mice injected with LPS compared to saline had increased total α-MSH in circulation 2 h post injection.

CONCLUSIONS

Our data indicate endogenous α-MSH contributes to the inflammatory immune responses triggered by low-dose LPS administration and suggest that targeting the melanocortin system could be a potential therapeutic for the treatment of sepsis or inflammatory disease.

Variant-to-function analysis of the childhood obesity chr12q13 locus implicates rs7132908 as a causal variant within the 3' UTR of FAIM2

The ch12q13 locus is among the most significant childhood obesity loci identified in genome-wide association studies. This locus resides in a non-coding region within FAIM2; thus, the underlying causal variant(s) presumably influence disease susceptibility via cis-regulation. We implicated rs7132908 as a putative causal variant by leveraging our in-house 3D genomic data and public domain datasets. Using a luciferase reporter assay, we observed allele-specific cis-regulatory activity of the immediate region harboring rs7132908. We generated isogenic human embryonic stem cell lines homozygous for either rs7132908 allele to assess changes in gene expression and chromatin accessibility throughout a differentiation to hypothalamic neurons, a key cell type known to regulate feeding behavior. The rs7132908 obesity risk allele influenced expression of FAIM2 and other genes and decreased the proportion of neurons produced by differentiation. We have functionally validated rs7132908 as a causal obesity variant that temporally regulates nearby effector genes and influences neurodevelopment and survival.

Low resting metabolic rate and increased hunger due to β-MSH and β-endorphin deletion in a canine model

Mutations that perturb leptin-melanocortin signaling are known to cause hyperphagia and obesity, but energy expenditure has not been well studied outside rodents. We report on a common canine mutation in pro-opiomelanocortin (POMC), which prevents production of β-melanocyte-stimulating hormone (β-MSH) and β-endorphin but not α-MSH; humans, similar to dogs, produce α-MSH and β-MSH from the POMC propeptide, but rodents produce only α-MSH. We show that energy expenditure is markedly lower in affected dogs, which also have increased motivational salience in response to a food cue, indicating increased wanting or hunger. There was no difference in satiety at a modified ad libitum meal or in their hedonic response to food, nor disruption of adrenocorticotropic hormone (ACTH) or thyroid axes. In vitro, we show that β-MSH signals comparably to α-MSH at melanocortin receptors. These data implicate β-MSH and β-endorphin as important in determining hunger and moderating energy expenditure and suggest that this role is independent of the presence of α-MSH.

Integrating a Multi-label Deep Learning Approach with Protein Information to Compare Bioactive Peptides in Brain and Plasma

Peptide therapeutics is gaining momentum. Advances in the field of peptidomics have enabled researchers to harvest vital information from various organisms and tissue types concerning peptide existence, expression and function. The development of mass spectrometry techniques for high-throughput peptide quantitation has paved the way for the identification and discovery of numerous known and novel peptides. Though much has been achieved, scientists are still facing difficulties when it comes to reducing the search space of the large mass spectrometry-generated peptidomics datasets and focusing on the subset of functionally relevant peptides. Moreover, there is currently no straightforward way to analytically compare the distributions of bioactive peptides in distinct biological samples, which may reveal much useful information when seeking to characterize tissue- or fluid-specific peptidomes. In this chapter, we demonstrate how to identify, rank, and compare predicted bioactive peptides and bioactivity distributions from extensive peptidomics datasets. To aid this task, we utilize MultiPep, a multi-label deep learning approach designed for classifying peptide bioactivities, to identify bioactive peptides. The predicted bioactivities are synergistically combined with protein information from the UniProt database, which assist in navigating through the jungle of putative therapeutic peptides and relevant peptide leads.

Targeting the central melanocortin system for the treatment of metabolic disorders

A large body of preclinical and clinical data shows that the central melanocortin system is a promising therapeutic target for treating various metabolic disorders such as obesity and cachexia, as well as anorexia nervosa. Setmelanotide, which functions by engaging the central melanocortin circuitry, was approved by the FDA in 2020 for use in certain forms of syndromic obesity. Furthermore, the FDA approvals in 2019 of two peptide drugs targeting melanocortin receptors for the treatment of generalized hypoactive sexual desire disorder (bremelanotide) and erythropoietic protoporphyria-associated phototoxicity (afamelanotide) demonstrate the safety of this class of peptides. These approvals have also renewed excitement in the development of therapeutics targeting the melanocortin system. Here, we review the anatomy and function of the melanocortin system, discuss progress and challenges in developing melanocortin receptor-based therapeutics, and outline potential metabolic and behavioural disorders that could be addressed using pharmacological agents targeting these receptors.

Variant-to-function analysis of the childhood obesity chr12q13 locus implicates rs7132908 as a causal variant within the 3' UTR of FAIM2

The ch12q13 obesity locus is among the most significant childhood obesity loci identified in genome-wide association studies. This locus resides in a non-coding region within FAIM2; thus, the underlying causal variant(s) presumably influence disease susceptibility via an influence on cis-regulation within the genomic region. We implicated rs7132908 as a putative causal variant at this locus leveraging a combination of our inhouse 3D genomic data, public domain datasets, and several computational approaches. Using a luciferase reporter assay in human primary astrocytes, we observed allele-specific cis-regulatory activity of the immediate region harboring rs7132908. Motivated by this finding, we went on to generate isogenic human embryonic stem cell lines homozygous for either rs7132908 allele with CRISPR-Cas9 homology-directed repair to assess changes in gene expression due to genotype and chromatin accessibility throughout a differentiation to hypothalamic neurons, a key cell type known to regulate feeding behavior. We observed that the rs7132908 obesity risk allele influenced the expression of FAIM2 along with other genes, decreased the proportion of neurons produced during differentiation, up-regulated cell death gene sets, and conversely down-regulated neuron differentiation gene sets. We have therefore functionally validated rs7132908 as a causal obesity variant which temporally regulates nearby effector genes at the ch12q13 locus and influences neurodevelopment and survival.

Early-set POMC methylation variability is accompanied by increased risk for obesity and is addressable by MC4R agonist treatment

Increasing evidence points toward epigenetic variants as a risk factor for developing obesity. We analyzed DNA methylation of the POMC (pro-opiomelanocortin) gene, which is pivotal for satiety regulation. We identified sex-specific and nongenetically determined POMC hypermethylation associated with a 1.4-fold (confidence interval, 1.03 to 2.04) increased individual risk of developing obesity. To investigate the early embryonic establishment of POMC methylation states, we established a human embryonic stem cell (hESC) model. Here, hESCs (WA01) were transferred into a naïve state, which was associated with a reduction of DNA methylation. Naïve hESCs were differentiated via a formative state into POMC-expressing hypothalamic neurons, which was accompanied by re-establishment of DNA methylation patterning. We observed that reduced POMC gene expression was associated with increased POMC methylation in POMC-expressing neurons. On the basis of these findings, we treated POMC-hypermethylated obese individuals (n = 5) with an MC4R agonist and observed a body weight reduction of 4.66 ± 2.16% (means ± SD) over a mean treatment duration of 38.4 ± 26.0 weeks. In summary, we identified an epigenetic obesity risk variant at the POMC gene fulfilling the criteria for a metastable epiallele established in early embryonic development that may be addressable by MC4R agonist treatment to reduce body weight.

Recent advances in mass spectrometry analysis of neuropeptides

Due to their involvement in numerous biochemical pathways, neuropeptides have been the focus of many recent research studies. Unfortunately, classic analytical methods, such as western blots and enzyme-linked immunosorbent assays, are extremely limited in terms of global investigations, leading researchers to search for more advanced techniques capable of probing the entire neuropeptidome of an organism. With recent technological advances, mass spectrometry (MS) has provided methodology to gain global knowledge of a neuropeptidome on a spatial, temporal, and quantitative level. This review will cover key considerations for the analysis of neuropeptides by MS, including sample preparation strategies, instrumental advances for identification, structural characterization, and imaging; insightful functional studies; and newly developed absolute and relative quantitation strategies. While many discoveries have been made with MS, the methodology is still in its infancy. Many of the current challenges and areas that need development will also be highlighted in this review.

Obesity and Hyperphagia With Increased Defective ACTH: A Novel POMC Variant

OBJECTIVE

Patients with pro-opiomelanocortin (POMC) defects generally present with early-onset obesity, hyperphagia, hypopigmentation and adrenocorticotropin (ACTH) deficiency. Rodent models suggest that adequate cleavage of ACTH to α-melanocortin-stimulating hormone (α-MSH) and desacetyl-α-melanocortin-stimulating hormone (d-α-MSH) by prohormone convertase 2 at the KKRR region is required for regulating food intake and energy balance.

METHODS

We present 2 sisters with a novel POMC gene variant, leading to an ACTH defect at the prohormone convertase 2 cleavage site, and performed functional studies of this variant.

RESULTS

The patients had obesity, hyperphagia and hypocortisolism, with markerly raised levels of ACTH but unaffected pigmentation. Their ACTH has reduced potency to stimulate the melanocortin (MC) 2 receptor, explaining their hypocortisolism.

CONCLUSION

The hyperphagia and obesity support evidence that adequate cleavage of ACTH to α-MSH and d-α-MSH is also required in humans for feeding control.

Mass Spectrometry Approaches Empowering Neuropeptide Discovery and Therapeutics

The discovery of insulin in the early 1900s ushered in the era of research related to peptides acting as hormones and neuromodulators, among other regulatory roles. These essential gene products are found in all organisms, from the most primitive to the most evolved, and carry important biologic information that coordinates complex physiology and behavior; their misregulation has been implicated in a variety of diseases. The evolutionary origins of at least 30 neuropeptide signaling systems have been traced to the common ancestor of protostomes and deuterostomes. With the use of relevant animal models and modern technologies, we can gain mechanistic insight into orthologous and paralogous endogenous peptides and translate that knowledge into medically relevant insights and new treatments. Groundbreaking advances in medicine and basic science influence how signaling peptides are defined today. The precise mechanistic pathways for over 100 endogenous peptides in mammals are now known and have laid the foundation for multiple drug development pipelines. Peptide biologics have become valuable drugs due to their unique specificity and biologic activity, lack of toxic metabolites, and minimal undesirable interactions. This review outlines modern technologies that enable neuropeptide discovery and characterization, and highlights lessons from nature made possible by neuropeptide research in relevant animal models that is being adopted by the pharmaceutical industry. We conclude with a brief overview of approaches/strategies for effective development of peptides as drugs. SIGNIFICANCE STATEMENT: Neuropeptides, an important class of cell-cell signaling molecules, are involved in maintaining a range of physiological functions. Since the discovery of insulin's activity, over 100 bioactive peptides and peptide analogs have been used as therapeutics. Because these are complex molecules not easily predicted from a genome and their activity can change with subtle chemical modifications, mass spectrometry (MS) has significantly empowered peptide discovery and characterization. This review highlights contributions of MS-based research towards the development of therapeutic peptides.

The G209R mutant mouse as a model for human PCSK1 polyendocrinopathy

PCSK1 encodes an enzyme required for prohormone maturation into bioactive peptides. A striking number of SNPs and rare mutations in PCSK1 are associated with a range of clinical phenotypes. Infants bearing two copies of a catalytically inactivating mutation, such as G209R, exhibit life-threatening chronic diarrhea and subsequently develop systemic endocrinopathies. Using CRISPR/Cas9 technology, we have engineered a mouse model bearing a G209R missense mutation in exon 6 of the murine Pcsk1 locus. Most pups homozygous for the G209R mutation succumbed by day 2, and surviving pups were severely dwarfed. In homozygous (but not heterozygous) pups, blood glucose levels were significantly lower, accompanied by elevated plasma insulin-like immunoreactivity and accumulation of large quantities of unprocessed proinsulin in the pancreas. Peptide hormone processing was also aberrant in G209R mouse pituitary, with mature ACTH levels markedly reduced in homozygotes, accompanied by a significant accumulation of POMC. We also observed a significant reduction in PC1/3 protein in the brains of G209R homozygous mice by Western blotting, while PC2 levels remained unaffected. Most likely due to the continued presence of PC2, pituitary and brain levels of α-MSH were not impaired. Analysis of intestinal cell types indicated a modest reduction of enteroendocrine cells in G209R homozygotes. We suggest that the G209R Pcsk1 mouse model recapitulates many of the dramatic neonatal deficiencies of human patients with this homozygous mutation.

Obesity, POMC, and POMC-processing Enzymes: Surprising Results From Animal Models

Peptides derived from proopiomelanocortin (POMC) are well-established neuropeptides and peptide hormones that perform multiple functions, including regulation of body weight. In humans and some animals, these peptides include α- and β-melanocyte-stimulating hormone (MSH). In certain rodent species, no β-MSH is produced from POMC because of a change in the cleavage site. Enzymes that convert POMC into MSH include prohormone convertases (PCs), carboxypeptidases (CPs), and peptidyl-α-amidating monooxygenase (PAM). Humans and mice with inactivating mutations in either PC1/3 or carboxypeptidase E (CPE) are obese, which was assumed to result from defective processing of POMC into MSH. However, recent studies have shown that selective loss of either PC1/3 or CPE in POMC-expressing cells does not cause obesity. These findings suggest that defects in POMC processing cannot alone account for the obesity observed in global PC1/3 or CPE mutants. We propose that obesity in animals lacking PC1/3 or CPE activity depends, at least in part, on deficient processing of peptides in non-POMC-expressing cells either in the brain and/or the periphery. Genetic background may also contribute to the manifestation of obesity.

Depletion of Alpha-Melanocyte-Stimulating Hormone Induces Insatiable Appetite and Gains in Energy Reserves and Body Weight in Zebrafish

The functions of anorexigenic neurons secreting proopiomelanocortin (POMC)/alpha-melanocyte-stimulating hormone (α-MSH) of the melanocortin system in the hypothalamus in vertebrates are energy homeostasis, food intake, and body weight regulation. However, the mechanisms remain elusive. This article reports on zebrafish that have been genetically engineered to produce α-MSH mutants, α-MSH-7aa and α-MSH-8aa, selectively lacking 7 and 8 amino acids within the α-MSH region, but retaining most of the other normal melanocortin-signaling (Pomc-derived) peptides. The α-MSH mutants exhibited hyperphagic phenotypes leading to body weight gain, as observed in human patients and mammalian models. The actions of several genes regulating appetite in zebrafish are similar to those in mammals when analyzed using gene expression analysis. These include four selected orexigenic genes: Promelanin-concentrating hormone (pmch), agouti-related protein 2 (agrp2), neuropeptide Y (npy), and hypothalamic hypocretin/orexin (hcrt). We also study five selected anorexigenic genes: Brain-derived neurotrophic factor (bdnf), single-minded homolog 1-a (sim1a), corticotropin-releasing hormone b (crhb), thyrotropin-releasing hormone (trh), and prohormone convertase 2 (pcsk2). The orexigenic actions of α-MSH mutants are rescued completely after hindbrain ventricle injection with a synthetic analog of α-MSH and a melanocortin receptor agonist, Melanotan II. We evaluate the adverse effects of MSH depletion on energy balance using the Alamar Blue metabolic rate assay. Our results show that α-MSH is a key regulator of POMC signaling in appetite regulation and energy expenditure, suggesting that it might be a potential therapeutic target for treating human obesity.

Melanocortin System in Kidney Homeostasis and Disease: Novel Therapeutic Opportunities

Melanocortin peptides, melanocortin receptors, melanocortin receptor accessory proteins, and endogenous antagonists of melanocortin receptors are the key components constituting the melanocortin hormone system, one of the most complex and important hormonal systems in our body. A plethora of evidence suggests that melanocortins possess a protective activity in a variety of kidney diseases in both rodent models and human patients. In particular, the steroidogenic melanocortin peptide adrenocorticotropic hormone (ACTH), has been shown to exert a beneficial effect in a number of kidney diseases, possibly via a mechanism independent of its steroidogenic activity. In patients with steroid-resistant nephrotic glomerulopathy, ACTH monotherapy is still effective in inducing proteinuria remission. This has inspired research on potential implications of the melanocortin system in glomerular diseases. However, our understanding of the role of the melanocortinergic pathway in kidney disease is very limited, and there are still huge unknowns to be explored. The most controversial among these is the identification of effector cells in the kidney as well as the melanocortin receptors responsible for conveying the renoprotective action. This review article introduces the melanocortin hormone system, summarizes the existing evidence for the expression of melanocortin receptors in the kidney, and evaluates the potential strategy of melanocortin therapy for kidney disease.

L-Cell Expression of Melanocortin-4-Receptor Is Marginal in Most of the Small Intestine in Mice and Humans and Direct Stimulation of Small Intestinal Melanocortin-4-Receptors in Mice and Rats Does Not Affect GLP-1 Secretion

The molecular sensors underlying nutrient-stimulated GLP-1 secretion are currently being investigated. Peripheral administration of melanocortin-4 receptor (MC4R) agonists have been reported to increase GLP-1 plasma concentrations in mice and humans but it is unknown whether this effect results from a direct effect on the GLP-1 secreting L-cells in the intestine, from other effects in the intestine or from extra-intestinal effects. We investigated L-cell expression of MC4R in mouse and human L-cells by reanalyzing publicly available RNA sequencing databases (mouse and human) and by RT-qPCR (mouse), and assessed whether administration of MC4R agonists to a physiologically relevant gut model, isolated perfused mouse and rat small intestine, would stimulate GLP-1 secretion or potentiate glucose-stimulated secretion. L-cell MC4R expression was low in mouse duodenum and hardly detectable in the ileum and MC4R expression was hardly detectable in human L-cells. In isolated perfused mouse and rat intestine, neither intra-luminal nor intra-arterial administration of NDP-alpha-MSH, a potent MC4R agonist, had any effect on GLP-1 secretion (P ≥0.98, n = 5-6) from the upper or lower-half of the small intestine in mice or in the lower half in rats. Furthermore, HS014-an often used MC4R antagonist, which we found to be a partial agonist-did not affect the glucose-induced GLP-1 response in the rat, P = 0.62, n = 6). Studies on transfected COS7-cells confirmed bioactivity of the used compounds and that concentrations employed were well within in the effective range. Our combined data therefore suggest that MC4R-activated GLP-1 secretion in rodents either exclusively occurs in the colon or involves extra-intestinal signaling.

Mass spectrometric characterisation of the circulating peptidome following oral glucose ingestion in control and gastrectomised patients

RATIONALE

Meal ingestion triggers secretion of a variety of gut and endocrine peptides important in diabetes research which are routinely measured by immunoassays. However, similarities between some peptides (glucagon, oxyntomodulin and glicentin) can cause specificity issues with immunoassays. We used a liquid chromatography/tandem mass spectrometry (LC/MS/MS) methodology to unambiguously monitor multiple gut peptides in human plasma.

METHODS

A simple acetonitrile-based protein precipitation step, followed by evaporation and solid-phase extraction, removed high-abundance proteins from samples prior to nano-LC/MS/MS analysis on an Orbitrap Q-Exactive Plus mass spectrometer using a data-dependent methodology. Database searching using PEAKS identified multiple gut-derived peptides, including peptides in the mid-pg/mL range. The relative levels of these and previously characterised peptides were assessed in plasma samples from gastrectomised and control subjects during an oral glucose tolerance test.

RESULTS

Analysis of plasma extracts revealed significantly elevated levels of a number of peptides following glucose ingestion in subjects who had undergone gastrectomy compared with controls. These included GLP-1(7-36), GLP-1(9-36), glicentin, oxyntomodulin, GIP(1-42), GIP(3-42), PYY(1-36), PYY(3-36), neurotensin, insulin and C-peptide. Motilin levels decreased following glucose ingestion. Results showed good correlation with immunoassay-derived concentrations of some peptides in the same samples. The gastrectomy group also had higher, but non-glucose-dependent, circulating levels of peptides from PIGR and DMBT1.

CONCLUSIONS

Overall, the approach showed that a fast, generic and reproducible LC/MS/MS methodology requiring only a small volume of plasma was capable of the multiplexed detection of a variety of diabetes-related peptides.

Melanocortin pathways: suppressed and stimulated melanocortin-4 receptor (MC4R)

Leptin-melanocortin pathway plays an essential role in the body weight regulation. Enhanced melanocortin signaling in the hypothalamus results in both decreased food intake and increased energy expenditure. The discovery of monogenic obesities with dysfunction of melanocortin-4 receptor (MC4R) greatly contributed to understanding of energy balance regulation. This review presents phenotypical characterization and prevalence of the MC4R gene mutations. Genome-wide association studies revealed that MC4R gene is significantly related not only to monogenic obesities but also to common obesity. An interaction of variants in the MC4R gene with fat mass and obesity associated (FTO) gene significantly increases the risk for obesity, particularly in adolescence. On the other hand, about 15 % of the MC4R gene variants result in a gain of function that protects against obesity and is associated with favorable metabolic profile. Long-term attempts to activate the MC4R have recently been finalized by a discovery of setmelanotide, a novel specific MC4R agonist that is devoid of untoward cardiovascular side-effects. The employment of specific MC4R agonists may open new horizons not only in the treatment of rare monogenic obesities but also in some common obesities where stimulation of MC4R could be achieved.

Proopiomelanocortin Processing in the Hypothalamus Is Directly Regulated by Saturated Fat: Implications for the Development of Obesity

In outbred mice, susceptibility or resistance to diet-induced obesity is associated with rapid changes in hypothalamic proopiomelanocortin (POMC) levels. Here, we evaluated 3 hypotheses that potentially explain the development of the different obesity phenotypes in outbred Swiss mice. First, rapid and differential changes in the gut microbiota in obesity-prone (OP) and obesity-resistant (OR) mice fed on a high-fat diet (HFD) might cause differential efficiencies in fatty acid harvesting leading to changes in systemic fatty acid concentrations that in turn affect POMC expression and processing. Second, independently of the gut microbiota, OP mice might have increased blood fatty acid levels after the introduction of a HFD, which could affect POMC expression and processing. Third, fatty acids might act directly in the hypothalamus to differentially regulate POMC expression and/or processing in OP and OR mice. We evaluated OP and OR male Swiss mice using 16S rRNA sequencing for the determination of gut microbiota; gas chromatography for blood lipid determination; and immunoblot and real-time polymerase chain reaction for protein and transcript determination and indirect calorimetry. Some experiments were performed with human pluripotent stem cells differentiated into hypothalamic neurons. We did not find evidence supporting the first 2 hypotheses. However, we found that in OP but not in OR mice, palmitate induces a rapid increase in hypothalamic POMC, which is followed by increased expression of proprotein convertase subtilisin/kexin type 1 PC1/3. Lentiviral inhibition of hypothalamic PC1/3 increased caloric intake and body mass in both OP and OR mice. In human stem cell-derived hypothalamic cells, we found that palmitate potently suppressed the production of POMC-derived peptides. Palmitate directly regulates PC1/3 in OP mice and likely has a functional impact on POMC processing.

Reduced Stability and pH-Dependent Activity of a Common Obesity-Linked PCSK1 Polymorphism, N221D

Common mutations in the human prohormone convertase (PC)1/3 gene (PCKSI) are linked to increased risk of obesity. Previous work has shown that the rs6232 single-nucleotide polymorphism (N221D) results in slightly decreased activity, although whether this decrease underlies obesity risk is not clear. We observed significantly decreased activity of the N221D PC1/3 enzyme at the pH of the trans-Golgi network; at this pH, the mutant enzyme was less stable than wild-type enzyme. Recombinant N221D PC1/3 also showed enhanced susceptibility to heat stress. Enhanced susceptibility to tunicamycin-induced endoplasmic reticulum stress was observed in AtT-20/PC2 cell clones in which murine PC1/3 was replaced by human N221D PC1/3, as compared with wild-type human PC1/3. However, N221D PC1/3-expressing AtT-20/PC2 clones processed proopiomelanocortin to α-MSH similarly to wild-type PC1/3. We also generated a CRISPR-edited mouse line expressing the N221D mutation in the PCKSI gene. When homozygous N221D mice were fed either a standard or a high-fat diet, we found no increase in body weight compared with their wild-type sibling controls. Sexual dimorphism was observed in pituitary ACTH for both genotypes, with females exhibiting lower levels of pituitary ACTH. In contrast, hypothalamic α-MSH content for both genotypes was higher in females compared with males. Hypothalamic corticotropin-like intermediate peptide content was higher in wild-type females compared with wild-type, but not N221D, males. Taken together, these data suggest that the increased obesity risk linked to the N221D allele in humans may be due in part to PC1/3-induced loss of resilience to stressors rather than strictly to decreased enzymatic activity on peptide precursors.

Obesity-associated mutant melanocortin-4 receptors with normal Gαs coupling frequently exhibit other discoverable pharmacological and biochemical defects

Mutations in the melanocortin-4 receptor (MC4R) are the most common cause of early syndromic obesity known. Most of these mutations result in a loss of protein expression, α-melanocyte-stimulating hormone binding, receptor trafficking or coupling to the stimulatory G-protein, Gα_s . However, approximately 26% of the obesity-associated mutations characterised to date exhibit none of these pharmacological defects. In the present study, we investigated seven of these apparently normal mutant MC4R in more detail and found that the majority (five of the seven) exhibit marked defects including defective binding of another endogenous melanocortin ligand, defective glycosylation, and defective recruitment of β-arrestin. These data provide support for two hypotheses: (i) that the majority of these rare, obesity-associated mutations are likely defective and causative of obesity and (ii) that β-arrestin recruitment is a valuable marker of normal MC4R function. Recent work has demonstrated a statistical correlation between the efficacy of β-arrestin recruitment to the MC4R and body mass index; however, the data reported here demonstrate both decreased and increased β-arrestin signalling in obesity-associated MC4R mutations.

Chronic High-Fat Diet Exacerbates Sexually Dimorphic Pomctm1/tm1 Mouse Obesity

Mice with a targeted mutation in the pro-opiomelanocortin (Pomc) gene (Pomctm1/tm1 mice) are unable to synthesize desacetyl-α-MSH and α-MSH and they develop obesity when fed chow diet. In this study, we hypothesized that a chronic high-fat (HF) diet exacerbates Pomctm1/tm1 mouse obesity. Male and female Pomcwt/wt and Pomctm1/tm1 mice were fed low-fat (LF) (10 kcal percent fat) or HF (45 kcal percent fat) diets from weaning for 23 weeks. We show that Pomctm1/tm1 mouse obesity is sexually dimorphic and exacerbated by an HF diet. Male Pomctm1/tm1 mice develop obesity because they are hyperphagic compared with Pomcwt/wt mice when fed an LF or HF diet. Female Pomctm1/tm1 mice develop obesity when feeding on an LF or HF diet because they exhibit signs of reduced energy expenditure (no change in feed efficiency; body weight gained exceeding energy intake) compared with Pomcwt/wt mice. A chronic HF diet exacerbates male Pomctm1/tm1 and Pomcwt/wt mouse obesity, and the increased energy intake fully accounts for increased weight gain. In contrast, female Pomcwt/wt mice are protected from chronic HF diet-induced obesity because they reduce the amount of HF diet eaten, and they appear to increase their energy expenditure (no change in feed efficiency but energy intake exceeding body weight gained). A chronic HF diet exacerbates female Pomctm1/tm1 mouse obesity due to impaired ability to reduce the amount of HF diet eaten and apparent impaired HF diet-induced adaptive thermogenesis. Our data show that desacetyl-α-MSH and α-MSH are required for sexually dimorphic HF diet-induced C57BL/6J obesity. In conclusion, desacetyl-α-MSH and α-MSH play salutary roles in sexually dimorphic melanocortin obesity and sexually dimorphic HF diet-induced C57BL/6J obesity.