MRAP2 regulates energy homeostasis by promoting primary cilia localization of MC4R

The E3 ubiquitin ligase MGRN1 targets melanocortin receptors MC1R and MC4R via interactions with transmembrane adapters

E3 ubiquitin ligases play a crucial role in modulating receptor stability and signaling at the cell surface, yet the mechanisms governing their substrate specificity remain incompletely understood. Mahogunin Ring Finger 1 (MGRN1) is a membrane-tethered E3 ligase that fine-tunes signaling sensitivity by targeting surface receptors for ubiquitination and degradation. Unlike cytosolic E3 ligases, membrane-tethered E3s require transmembrane adapters to selectively recognize and regulate surface receptors, yet few such ligases have been studied in detail. While MGRN1 is known to regulate the receptor Smoothened (SMO) within the Hedgehog pathway through its interaction with the transmembrane adapter Multiple Epidermal Growth Factor-like 8 (MEGF8), the broader scope of its regulatory network has been speculative. Here, we identify Attractin (ATRN) and Attractin-like 1 (ATRNL1) as additional transmembrane adapters that recruit MGRN1 and regulate cell surface receptor turnover. Through co-immunoprecipitation, we show that ATRN and ATRNL1 likely interact with the RING domain of MGRN1. Functional assays reveal that MGRN1 requires these transmembrane adapters to ubiquitinate and degrade the melanocortin receptors MC1R and MC4R, in a process analogous to its regulation of SMO. Loss of MGRN1 leads to increased surface and ciliary localization of MC4R in fibroblasts and elevated MC1R levels in melanocytes, with the latter resulting in enhanced eumelanin production. These findings expand the repertoire of MGRN1-regulated receptors and provide new insight into a shared mechanism by which membrane-tethered E3 ligases utilize transmembrane adapters to dictate substrate receptor specificity. By elucidating how MGRN1 selectively engages with surface receptors, this work establishes a broader framework for understanding how this unique class of E3 ligases fine-tunes receptor homeostasis and signaling output.

Obesity-associated MRAP2 variants impair multiple MC4R-mediated signaling pathways

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed at hypothalamic neurons that has an important role in appetite suppression and food intake. Mutations in MC4R are the most common cause of monogenic obesity and can affect multiple signaling pathways including Gs-cAMP, Gq, ERK1/2, β-arrestin recruitment, internalization and cell surface expression. The melanocortin-2 receptor accessory protein 2 (MRAP2), is a single-pass transmembrane protein that interacts with and regulates signaling by MC4R. Variants in MRAP2 have also been identified in overweight and obese individuals. However, functional studies that have only measured the effect of MRAP2 variants on MC4R-mediated cAMP signaling have produced inconsistent findings and most do not reduce MC4R function. Here we investigated the effect of twelve of these previously reported MRAP2 variants and showed that all variants that have been identified in overweight or obese individuals impair MC4R function. When expressed at equal concentrations, seven MRAP2 variants impaired MC4R-mediated cAMP signaling, while nine variants impaired IP3 signaling. Four mutations in the MRAP2 C-terminus affected internalization. MRAP2 variants had no effect on total or cell surface expression of either the MRAP2 or MC4R proteins. Structural models predicted that MRAP2 interacts with MC4R transmembrane helices 5 and 6, and mutations in two MRAP2 residues in putative contact sites impaired the ability of MRAP2 to facilitate MC4R signaling. In summary, our studies demonstrate that human MRAP2 variants associated with obesity impair multiple MC4R signaling pathways and that both Gs-cAMP and Gq-IP3 pathways should be assessed to determine variant pathogenicity.

Tonic ubiquitination of the central body weight regulator melanocortin receptor 4 (MC4R) promotes its constitutive exit from cilia

The G protein-coupled receptor (GPCR) melanocortin receptor 4 (MC4R) is an essential regulator of body weight homeostasis. MC4R is unusual among GPCRs in that its activity is regulated by 2 opposing physiological ligands, the agonist ⍺-MSH and the antagonist/inverse agonist AgRP. Paradoxically, while MC4R localizes and functions at the cilium of hypothalamic neurons, the ciliary levels of MC4R are very low under unrestricted feeding conditions. Here, we find that the constitutive activity of MC4R is responsible for the continuous depletion of MC4R from cilia and that inhibition of MC4R's activity via AgRP leads to a robust accumulation of MC4R in cilia. Ciliary targeting of MC4R is mediated by its partner MRAP2 and the constitutive exit of MC4R from cilia relies on the sensor of activation β-arrestin, on ubiquitination, and on the BBSome ciliary trafficking complex. Thus, while MC4R exits cilia via conventional mechanisms, it only accumulates in cilia when its activity is suppressed by AgRP.

Stochastic neuropeptide signals compete to calibrate the rate of satiation

Neuropeptides have important roles in neural plasticity, spiking and behaviour1. Yet, many fundamental questions remain regarding their spatiotemporal transmission, integration and functions in the awake brain. Here we examined how MC4R-expressing neurons in the paraventricular nucleus of the hypothalamus (PVHMC4R) integrate neuropeptide signals to modulate feeding-related fast synaptic transmission and titrate the transition to satiety2-6. We show that hunger-promoting AgRP axons release the neuropeptide NPY to decrease the second messenger cAMP in PVHMC4R neurons, while satiety-promoting POMC axons release the neuropeptide αMSH to increase cAMP. Each release event is all-or-none, stochastic and can impact multiple neurons within an approximately 100-µm-diameter region. After release, NPY and αMSH peptides compete to control cAMP-the amplitude and persistence of NPY signalling is blunted by high αMSH in the fed state, while αMSH signalling is blunted by high NPY in the fasted state. Feeding resolves this competition by simultaneously elevating αMSH release and suppressing NPY release7,8, thereby sustaining elevated cAMP in PVHMC4R neurons throughout a meal. In turn, elevated cAMP facilitates potentiation of feeding-related excitatory inputs with each bite to gradually promote satiation across many minutes. Our findings highlight biochemical modes of peptide signal integration and information accumulation to guide behavioural state transitions.

The expanding landscape of genetic causes of obesity

Obesity and weight regulation disorders are determined by the combined effects of genetics and environment. Polygenic obesity results from the combination of common variants in several genes which predisposes the individual to obesity and its related complications. In contrast, monogenic obesity results from changes in single genes, especially those in leptin-melanocortin pathway, and presents with early onset severe obesity, with or without other syndromic features. Rare variants in melanocortin 4 receptor are the commonest form of monogenic obesity. In addition, structural variation in small or large segments of chromosomes may also present with syndromic forms of obesity. Prader-Willi Syndrome, caused by imprinting errors in chromosome 15q11-13, is the most prevalent genetic cause of severe hyperphagia and obesity. With the advances in technologies, the past decade has witnessed a revolution in the identification of novel genetic causes of obesity, primarily in genes related to the leptin melanocortin pathway. The availability of safe melanocortin analogs holds the potential for targeted therapies for some of these disorders. This review summarizes known and novel rare genetic forms of obesity, along with approaches for the clinical investigation of copy number and sequence variants. The goal is to provide a reference for practicing clinicians to encourage genetic testing in obesity. IMPACT: What does this article add to the existing literature? Genetic obesity is an expanding frontier with potential to change management. Here, we summarize current information on the genetic causes of obesity and provide guidance for genetic testing. Emerging treatments may provide targeted precise treatment and change management practices.

Melanocortin 1 receptor mediates melanin production by interacting with the BBSome in primary cilia

Production of melanin pigments is a protective mechanism of the skin against ultraviolet (UV)-induced damage and carcinogenesis. However, the molecular basis for melanogenesis is still poorly understood. Herein, we demonstrate a critical interplay between the primary cilium and the melanocortin 1 receptor (MC1R) signaling. Our data show that UV and α-melanocyte-stimulating hormone (α-MSH) trigger cilium formation in human melanocytes and melanoma cells. Deficiency of MC1R or the presence of its red hair color (RHC) variations significantly attenuates the UV/α-MSH-induced ciliogenesis. Further investigation reveals that MC1R enters the cilium upon UV/α-MSH stimulation, which is facilitated by the interaction of MC1R with the BBSome and the palmitoylation of MC1R. MC1R interacts with the BBSome through the second and third intercellular loops, which contain the common RHC variant alleles (R151C and R160W). These RHC variants of MC1R exhibit attenuated ciliary localization, and enforced ciliary localization of these variants elevates melanogenesis. Ciliary MC1R triggers a sustained cAMP signaling and selectively stimulates Sox9, which appears to up-regulate melanogenesis-related genes as the transcriptional cofactor for MITF. These findings reveal a previously unrecognized nexus between MC1R and cilia and suggest an important mechanism for RHC variant-related pigmentary defects.

The MRAP2 accessory protein directly interacts with melanocortin-3 receptor to enhance signaling

The central melanocortin system links nutrition to energy expenditure, with melanocortin-4 receptor (MC4R) controlling appetite and food intake, and MC3R regulating timing of sexual maturation, rate of linear growth and lean mass accumulation. Melanocortin-2 receptor accessory protein-2 (MRAP2) is a single transmembrane protein that interacts with MC4R to potentiate it's signalling, and human mutations in MRAP2 cause obesity. Previous studies have been unable to consistently show whether MRAP2 affects MC3R activity. Here we used single-molecule pull-down (SiMPull) to confirm that MC3R and MRAP2 interact in HEK293 cells. Analysis of fluorescent photobleaching steps showed that MC3R and MRAP2 readily form heterodimers most commonly with a 1:1 stoichiometry. Human single-nucleus and spatial transcriptomics show MRAP2 is co-expressed with MC3R in hypothalamic neurons with important roles in energy homeostasis and appetite control. Functional analyses showed MRAP2 enhances MC3R cAMP signalling, impairs β-arrestin recruitment, and reduces internalization in HEK293 cells. Structural homology models revealed putative interactions between the two proteins and alanine mutagenesis of five MRAP2 and three MC3R transmembrane residues significantly reduced MRAP2 effects on MC3R signalling. Finally, we showed genetic variants in MRAP2 that have been identified in individuals that are overweight or obese prevent MRAP2's enhancement of MC3R-driven signalling. Thus, these studies reveal MRAP2 as an important regulator of MC3R function and provide further evidence for the crucial role of MRAP2 in energy homeostasis.

Medical semiology of patients with monogenic obesity: A systematic review

Patients with monogenic obesity display numerous medical features on top of hyperphagic obesity, but no study to date has provided an exhaustive description of their semiology. Two reviewers independently conducted a systematic review of MEDLINE, Embase, and Web of Science Core Collection databases from inception to January 2022 to identify studies that described symptoms of patients carrying pathogenic mutations in at least one of eight monogenic obesity genes (ADCY3, LEP, LEPR, MC3R, MC4R, MRAP2, PCSK1, and POMC). Of 5207 identified references, 269 were deemed eligible after title and abstract screening, full-text reading, and risk of bias and quality assessment. Data extraction included mutation spectrum and mode of inheritance, clinical presentation (e.g., anthropometry, energy intake and eating behaviors, digestive function, puberty and fertility, cognitive features, infectious diseases, morphological characteristics, chronic respiratory disease, and cardiovascular disease), biological characteristics (metabolic profile, endocrinology, hematology), radiological features, and treatments. The review provides an exhaustive description of mandatory, non-mandatory, and unique symptoms in heterozygous and homozygous carriers of mutation in eight monogenic obesity genes. This information is critical to help clinicians to orient genetic testing in subsets of patients with suspected monogenic obesity and provide actionable treatments (e.g., recombinant leptin and MC4R agonist).

Updates on Rare Genetic Variants, Genetic Testing, and Gene Therapy in Individuals With Obesity

PURPOSE OF REVIEW

The goal of this paper is to aggregate information on monogenic contributions to obesity in the past five years and to provide guidance for genetic testing in clinical care.

RECENT FINDINGS

Advances in sequencing technologies, increasing awareness, access to testing, and new treatments have increased the utilization of genetics in clinical care. There is increasing recognition of the prevalence of rare genetic obesity from variants with mean allele frequency < 5% -new variants in known genes as well as identification of novel genes- causing monogenic obesity. While most of these genes are in the leptin melanocortin pathway, those in adipocytes may also contribute. Common variants may contribute either to higher lifetime tendency for weight gain or provide protection from monogenic obesity. While specific genetic mutations are rare, these segregate in individuals with early-onset severe obesity; thus, collectively genetic etiologies are not as rare. Some genetic conditions are amenable to targeted treatment. Research into the discovery of novel genetic causes as well as targeted treatment is growing over time. The utility of therapeutic strategies based on the genetic risk of obesity is an advancing frontier.

Emerging mechanistic understanding of cilia function in cellular signalling

Primary cilia are solitary, immotile sensory organelles present on most cells in the body that participate broadly in human health, physiology and disease. Cilia generate a unique environment for signal transduction with tight control of protein, lipid and second messenger concentrations within a relatively small compartment, enabling reception, transmission and integration of biological information. In this Review, we discuss how cilia function as signalling hubs in cell-cell communication using three signalling pathways as examples: ciliary G-protein-coupled receptors (GPCRs), the Hedgehog (Hh) pathway and polycystin ion channels. We review how defects in these ciliary signalling pathways lead to a heterogeneous group of conditions known as 'ciliopathies', including metabolic syndromes, birth defects and polycystic kidney disease. Emerging understanding of these pathways' transduction mechanisms reveals common themes between these cilia-based signalling pathways that may apply to other pathways as well. These mechanistic insights reveal how cilia orchestrate normal and pathophysiological signalling outputs broadly throughout human biology.

Postnatal Dynamic Ciliary ARL13B and ADCY3 Localization in the Mouse Brain

Primary cilia are hair-like structures found on nearly all mammalian cell types, including cells in the developing and adult brain. A diverse set of receptors and signaling proteins localize within cilia to regulate many physiological and developmental pathways, including the Hedgehog (Hh) pathway. Defects in cilia structure, protein localization, and function lead to genetic disorders called ciliopathies, which present with various clinical features that include several neurodevelopmental phenotypes and hyperphagia-associated obesity. Despite their dysfunction being implicated in several disease states, understanding their roles in central nervous system (CNS) development and signaling has proven challenging. We hypothesize that dynamic changes to ciliary protein composition contribute to this challenge and may reflect unrecognized diversity of CNS cilia. The proteins ARL13B and ADCY3 are established markers of cilia in the brain. ARL13B is a regulatory GTPase important for regulating cilia structure, protein trafficking, and Hh signaling, and ADCY3 is a ciliary adenylyl cyclase. Here, we examine the ciliary localization of ARL13B and ADCY3 in the perinatal and adult mouse brain. We define changes in the proportion of cilia enriched for ARL13B and ADCY3 depending on brain region and age. Furthermore, we identify distinct lengths of cilia within specific brain regions of male and female mice. ARL13B+ cilia become relatively rare with age in many brain regions, including the hypothalamic feeding centers, while ADCY3 becomes a prominent cilia marker in the mature adult brain. It is important to understand the endogenous localization patterns of these proteins throughout development and under different physiological conditions as these common cilia markers may be more dynamic than initially expected. Understanding regional- and developmental-associated cilia protein composition signatures and physiological condition cilia dynamic changes in the CNS may reveal the molecular mechanisms associated with the features commonly observed in ciliopathy models and ciliopathies, like obesity and diabetes.

Competition between stochastic neuropeptide signals calibrates the rate of satiation

We investigated how transmission of hunger- and satiety-promoting neuropeptides, NPY and αMSH, is integrated at the level of intracellular signaling to control feeding. Receptors for these peptides use the second messenger cAMP. How cAMP integrates opposing peptide signals to regulate energy balance, and the in vivo spatiotemporal dynamics of endogenous peptidergic signaling, remain largely unknown. We show that AgRP axon stimulation in the paraventricular hypothalamus evokes probabilistic NPY release that triggers stochastic cAMP decrements in downstream MC4R-expressing neurons (PVHMC4R). Meanwhile, POMC axon stimulation triggers stochastic, αMSH-dependent cAMP increments. Release of either peptide impacts a ~100 μm diameter region, and when these peptide signals overlap, they compete to control cAMP. The competition is reflected by hunger-state-dependent differences in the amplitude and persistence of cAMP transients: hunger peptides are more efficacious in the fasted state, satiety peptides in the fed state. Feeding resolves the competition by simultaneously elevating αMSH release and suppressing NPY release, thereby sustaining elevated cAMP in PVHMC4R neurons. In turn, cAMP potentiates feeding-related excitatory inputs and promotes satiation across minutes. Our findings highlight how biochemical integration of opposing, quantal peptide signals during energy intake orchestrates a gradual transition between stable states of hunger and satiety.

Competition between stochastic neuropeptide signals calibrates the rate of satiation

We investigated how transmission of hunger- and satiety-promoting neuropeptides, NPY and αMSH, is integrated at the level of intracellular signaling to control feeding. Receptors for these peptides use the second messenger cAMP, but the messenger's spatiotemporal dynamics and role in energy balance are controversial. We show that AgRP axon stimulation in the paraventricular hypothalamus evokes probabilistic and spatially restricted NPY release that triggers stochastic cAMP decrements in downstream MC4R-expressing neurons (PVH MC4R ). Meanwhile, POMC axon stimulation triggers stochastic, αMSH-dependent cAMP increments. NPY and αMSH competitively control cAMP, as reflected by hunger-state-dependent differences in the amplitude and persistence of cAMP transients evoked by each peptide. During feeding bouts, elevated αMSH release and suppressed NPY release cooperatively sustain elevated cAMP in PVH MC4R neurons, thereby potentiating feeding-related excitatory inputs and promoting satiation across minutes. Our findings highlight how state-dependent integration of opposing, quantal peptidergic events by a common biochemical target calibrates energy intake.

Current Treatments for Patients with Genetic Obesity

Obesity derives from impaired central control of body weight, implying interaction between environment and an individual genetic predisposition. Genetic obesities, including monogenic and syndromic obesities, are rare and complex neuro-endocrine pathologies where the genetic contribution is predominant. Severe and early-onset obesity with eating disorders associated with frequent comorbidities make these diseases challenging. Their current estimated prevalence of 5-10% in severely obese children is probably underestimated due to the limited access to genetic diagnosis. A central alteration of hypothalamic regulation of weight implies that the leptin-melanocortin pathway is responsible for the symptoms. The management of genetic obesity has so far been only based, above all, on lifestyle intervention, especially regarding nutrition and physical activity. New therapeutic options have emerged in the last years for these patients, raising great hope to manage their complex situation and improve quality of life. Implementation of genetic diagnosis in clinical practice is thus of paramount importance to allow individualized care. This review describes the current clinical management of genetic obesity and the evidence on which it is based. Some insights will also be provided into new therapies under evaluation.