MC4R — Melanocortin 4 Receptor

title: MC4R — Melanocortin 4 Receptor

MC4R — Melanocortin 4 Receptor

Introduction

The MC4R (Melanocortin 4 Receptor) gene encodes a G protein-coupled receptor (GPCR) that plays a critical role in energy homeostasis, appetite regulation, and metabolic function. As one of five melanocortin receptors (MC1R-MC5R), MC4R is expressed primarily in the central nervous system and regulates food intake, energy expenditure, and body weight.[@he2019] Located on chromosome 18q21.3, MC4R is one of the most common monogenic causes of obesity in humans.[@mcr]

<div class="infobox infobox-gene">

title: MC4R — Melanocortin 4 Receptor

MC4R — Melanocortin 4 Receptor

Introduction

The MC4R (Melanocortin 4 Receptor) gene encodes a G protein-coupled receptor (GPCR) that plays a critical role in energy homeostasis, appetite regulation, and metabolic function. As one of five melanocortin receptors (MC1R-MC5R), MC4R is expressed primarily in the central nervous system and regulates food intake, energy expenditure, and body weight.[@he2019] Located on chromosome 18q21.3, MC4R is one of the most common monogenic causes of obesity in humans.[@mcr]

<div class="infobox infobox-gene">

| Property | Value |
|----------|-------|
| Gene Symbol | MC4R |
| Full Name | Melanocortin 4 Receptor |
| Chromosomal Location | 18q21.3 |
| NCBI Gene ID | 4162 |
| OMIM ID | 601665 |
| Ensembl ID | ENSG00000137273 |
| UniProt ID | P32246 |
| Encoded Protein | Melanocortin 4 receptor |
| Protein Length | 332 amino acids |
| Molecular Weight | ~37 kDa |

</div>

Gene Structure and Organization

The MC4R gene consists of a single exon, typical of the melanocortin receptor family. This single-exon structure simplifies transcription and avoids alternative splicing complications. The gene encodes a classic seven-transmembrane GPCR with characteristic features:

• N-terminal extracellular domain: Contains glycosylation sites
• Seven transmembrane helices: Classic GPCR architecture
• Three extracellular loops: Ligand binding interface
• Three intracellular loops: G protein coupling
• C-terminal intracellular tail: Phosphorylation sites and desensitization motifs

Promoter and Regulation

The MC4R promoter contains elements for tissue-specific expression:

• Neuronal enhancers: Drive expression in specific brain regions
• Hormone response elements: Allow modulation by metabolic signals
• Circadian elements: Link to daily feeding rhythms

Protein Structure and Function

Melanocortin Receptor Family

MC4R belongs to the melanocortin receptor subfamily, which includes:

| Receptor | Primary Ligands | Main Function |
|----------|-----------------|----------------|
| MC1R | α-MSH, ACTH | Pigmentation, inflammation |
| MC2R | ACTH | Steroidogenesis |
| MC3R | α-MSH, γ-MSH | Energy homeostasis |
| MC4R | α-MSH, β-MSH | Appetite, energy expenditure |
| MC5R | α-MSH | Exocrine function |

Ligand Binding

MC4R binds several melanocortin peptides:

• α-MSH (α-Melanocyte Stimulating Hormone): Primary agonist
• β-MSH: Agonist with lower potency
• γ-MSH: Agonist at certain splice variants
• ACTH (Adrenocorticotropic Hormone): Agonist

Signaling Mechanisms

MC4R activates multiple signaling pathways:

Primary (Gs-mediated) Pathway:

Alternative Pathways:

• Gi/o coupling: Inhibition of cAMP in some contexts
• ERK/MAPK activation: Through β-arrestin
• Calcium signaling: Through IP3 pathways

Receptor Regulation

MC4R undergoes multiple regulatory processes:

• Phosphorylation: By GRKs, promoting β-arrestin binding
• Internalization: Via clathrin-mediated endocytosis
• Desensitization: Homologous and heterologous
• Recycling: Return to membrane or degradation

Expression Patterns

Central Nervous System

MC4R shows distinctive patterns in the brain:

Hypothalamic Expression:

• Paraventricular nucleus (PVN): Highest expression; projects to autonomic centers
• Arcuate nucleus (ARC): On POMC and NPY neurons; integration of metabolic signals
• Lateral hypothalamus: Feeding behavior control
• Dorsomedial hypothalamus: Energy expenditure regulation

• Spinal cord: Pain modulation
• Dorsal raphe nucleus: Mood and emotional regulation
• Hippocampus: Learning and memory
• Cortex: Cognitive functions

Peripheral Expression

Lower levels of MC4R expression are found in:

• Adrenal gland: Modulation of steroidogenesis
• Peripheral nerves: Pain transduction
• Immune cells: Anti-inflammatory effects
• Gastrointestinal tract: Gut-brain signaling

Role in Energy Homeostasis

Appetite Regulation

MC4R is a central integrator of energy balance signals:

Anorexigenic ( appetite-suppressing) Signaling:

• α-MSH binding activates MC4R
• Reduces food intake
• Increases energy expenditure
• Promotes weight loss

• AGRP acts as inverse agonist
• Blocks α-MSH action
• Increases food intake
• Promotes weight gain

POMC and NPY Integration

The arcuate nucleus houses key MC4R-expressing neurons:

• POMC (Pro-opiomelanocortin) neurons: Produce α-MSH, project to MC4R neurons
• NPY/AgRP neurons: Produce AGRP, antagonize MC4R

• POMC activation → MC4R activation → reduced feeding
• NPY/AGRP activation → MC4R inhibition → increased feeding

Energy Expenditure

Beyond food intake, MC4R regulates:

• Thermogenesis: Brown adipose tissue activation
• Physical activity: Motor behavior and exploration
• Basal metabolic rate: Through autonomic regulation

Disease Associations

Obesity

MC4R mutations are the most common monogenic cause of obesity:

• Prevalence: ~5-6% of severe early-onset obesity
• Inheritance: Autosomal dominant with incomplete penetrance
• Mechanism: Loss-of-function mutations reduce signaling
• Phenotype: Hyperphagia, early-onset obesity, increased linear growth

• Frameshift mutations
• Missense mutations (particularly in transmembrane domains)
• Deletion mutations

• Complete loss-of-function: Severe phenotype
• Partial function: Variable severity
• Heterozygotes: Often manifest obesity

Type 2 Diabetes

MC4R dysfunction contributes to metabolic syndrome:

• Insulin resistance
• Glucose intolerance
• Dyslipidemia
• Increased cardiovascular risk

Alzheimer's Disease

The melanocortin system has been implicated in AD pathogenesis:

Neuroinflammation:

• MC4R signaling has anti-inflammatory effects
• Reduced signaling may contribute to neuroinflammation
• Therapeutic potential for MC4R agonists

• AD patients often have metabolic syndrome
• MC4R variants may modify AD risk
• Energy homeostasis dysregulation in AD

• MC4R in hippocampus may affect memory
• Signaling modulates synaptic plasticity
• Potential for cognitive enhancement

Parkinson's Disease

Emerging evidence links MC4R to PD:

Nigrostriatal System:

• MC4R expressed in substantia nigra
• May modulate dopaminergic function
• Potential role in PD pathogenesis

• PD patients often have weight loss
• MC4R dysfunction may contribute
• Energy expenditure alterations

• MC4R agonists may have neuroprotective effects
• Anti-inflammatory actions relevant to PD
• Clinical trials ongoing

Other Neurological Conditions

Huntington's Disease:

• Metabolic dysfunction is common
• MC4R variants may modify progression

• MC4R signaling has immunomodulatory effects
• Potential therapeutic target

Molecular Genetics

Mutation Spectrum

Over 200 MC4R mutations have been identified:

• Missense mutations (~60%): Protein misfolding or trafficking issues
• Frameshift mutations: Truncated proteins
• Nonsense mutations: Premature stop codons
• Deletions: Loss of functional protein
• Splice variants: Alternative splicing effects

Functional Categories

Class I (Trafficking defects):

• Mutations in extracellular/transmembrane domains
• Protein retained in ER
• Can sometimes be rescued by pharmacological chaperones

• Mutations in ligand-binding pocket
• Reduced agonist affinity

• Mutations in intracellular domains
• Impaired G protein coupling

Population Genetics

• Variant frequency: ~2-3% of population carry rare variants
• Founder mutations: Some populations have specific variants
• Loss-of-function carriers: Higher BMI, increased obesity risk

Therapeutic Implications

MC4R Agonists

Several MC4R agonists have been developed:

Setmelanotide (Imcivree):

• FDA approved for rare genetic obesity (POMC, LEPR deficiency)
• Highly selective MC4R agonist
• Marked weight loss in clinical trials
• Also being studied for Bardet-Biedl syndrome

• Peptide agonists in development
• Small molecule agonists
• Brain-penetrant vs. peripheral-selective options

Inverse Agonists

AGRP-based approaches:

• Block excessive MC4R activation
• May have different side effect profiles

Combination Approaches

• MC4R agonists + other anti-obesity agents
• GIP/GLP-1 receptor co-agonists with MC4R activity
• Lifestyle intervention with pharmacotherapy

Clinical Considerations

Efficacy:

• Weight loss of 5-10% in trials
• Improvements in metabolic parameters
• Long-term maintenance challenging

• Most common: skin darkening (melanin), nausea
• Potential for increased blood pressure
• Psychiatric effects (mood changes)

Research Models

Animal Models

Mc4r Knockout Mice:

• Develop obesity with hyperphagia
• Reduced energy expenditure
• Insulin resistance
• Useful for therapeutic testing

• Express human MC4R
• Rescue knockout phenotype
• Allow testing of human variants

• Conditional knockouts
• Cell-type specific deletion
• Reporter lines

Cellular Models

• Heterologous expression: HEK293, CHO cells
• Neuronal cell lines: For CNS signaling
• iPSC-derived neurons: Patient-specific models

Interactions and Pathways

Protein Interactions

MC4R interacts with:

• G proteins: Gs, Gi/o
• β-arrestins: For internalization
• GRKs: For phosphorylation
• Chaperones: For folding (e.g., RABEP1)

Neural Circuits

MC4R participates in:

• POMC-MC4R circuit: Primary feeding regulation
• NPY-MC4R circuit: Integration of orexigenic signals
• Autonomic circuits: Energy expenditure control

Signaling Networks

• cAMP/PKA pathway: Primary signaling
• MAPK/ERK pathway: Non-canonical
• PI3K/Akt pathway: Metabolic effects

Clinical Summary

| Aspect | Details |
|--------|---------|
| Primary disease | Monogenic obesity (MC4R deficiency) |
| Inheritance | Autosomal dominant |
| Key feature | Hyperphagia, early-onset obesity |
| Treatment | Setmelanotide (for POMC/LEPR), supportive |
| Neurodegeneration | Implicated in AD, PD |

See Also

• [Genes Index](/genes-index)
• [POMC Gene](/genes/pomc)
• [AGRP Gene](/genes/agrp)
• [Obesity and Metabolic Syndrome](/entities/obesity)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Melanocortin System](/entities/melanocortin-system)

External Links

• [NCBI Gene - MC4R](https://www.ncbi.nlm.nih.gov/gene/4162)
• [OMIM - MC4R](https://www.omim.org/entry/601665)
• [UniProt - MC4R](https://www.uniprot.org/uniprot/P32246)
• [GeneReviews - MC4R](https://www.ncbi.nlm.nih.gov/books/NBK1365/)

References

Pathway Diagram

The following diagram shows the key molecular relationships involving MC4R — Melanocortin 4 Receptor discovered through SciDEX knowledge graph analysis: