CHRM4 Gene — Cholinergic Receptor Muscarinic 4
Overview
CHRM4 encodes the M4 muscarinic acetylcholine receptor (mAChR), a G protein-coupled receptor (GPCR) that mediates the effects of acetylcholine in the central and peripheral nervous systems. The M4 receptor is one of five muscarinic receptor subtypes (M1-M5) and is predominantly expressed in the brain, where it plays crucial roles in modulating cognitive function, motor control, and neurotransmitter systems. The cholinergic system is severely affected in [Alzheimer's disease](/diseases/alzheimers-disease), making M4 receptors important therapeutic targets [1](https://doi.org/10.1016/j.neuropharm.2018.02.011).
<div class="infobox infobox-gene">
<table>
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">CHRM4 - Cholinergic Receptor Muscarinic 4</th></tr>
<tr><td><strong>Gene Symbol</strong></td><td>CHRM4</td></tr>
<tr><td><strong>Full Name</strong></td><td>Cholinergic Receptor Muscarinic 4</td></tr>
<tr><td><strong>Chromosome</strong></td><td>11p12</td></tr>
<tr><td><strong>NCBI Gene ID</strong></td><td>[1130](https://www.ncbi.nlm.nih.gov/gene/1130)</td></tr>
<tr><td><strong>OMIM</strong></td><td>[118550](https://www.omim.org/entry/118550)</td></tr>
<tr><td><strong>Ensembl ID</strong></td><td>[ENSG00000180720](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000180720)</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P48193](https://www.uniprot.org/uniprotkb/P48193/entry)</td></tr>
<tr><td><strong>Protein Class</strong></td><td>GPCR, Class A, Muscarinic</td></tr>
<tr><td><strong>Associated Diseases</strong></td><td>Alzheimer's Disease, Parkinson's Disease, Schizophrenia, Cognitive Impairment</td></tr>
</table>
</div>
Gene and Protein Structure
The CHRM4 gene is located on chromosome 11p12 and encodes a 479-amino acid protein. Like all muscarinic receptors, M4 has seven transmembrane domains connected by three extracellular and three intracellular loops. The receptor belongs to the class A GPCR family and contains characteristic sequence motifs for ligand binding in the transmembrane domains.
Structural Features
- Seven transmembrane helices (TM1-TM7)
- Extracellular N-terminus with glycosylation sites
- Intracellular C-terminus with phosphorylation sites
- Orthosteric binding site in the transmembrane core
- Allosteric binding sites in the extracellular loops
The M4 receptor exists as a monomer and can form homomers and heteromers with other GPCRs, expanding its functional repertoire.
Expression Pattern
CHRM4 exhibits a distinctive brain expression pattern:
High Expression Regions
- Striatum: Highest expression in the striatum, particularly in the indirect pathway medium spiny neurons
- Cortex: Moderate expression in layer 5 pyramidal neurons
- Hippocampus: Expression in CA1-CA3 regions and dentate gyrus
- Thalamus: Significant expression in thalamic nuclei
Lower Expression
- Cerebellum: Minimal expression
- Brainstem: Variable expression
The striatal enrichment of M4 receptors makes them particularly relevant to movement disorders and dopaminergic function.
Molecular Function
G Protein Coupling
M4 muscarinic receptors couple primarily to Gi/o proteins, leading to:
- Inhibition of adenylate cyclase: Reduces cAMP production
- Activation of inward rectifier potassium channels: Hyperpolarizes neurons
- Inhibition of voltage-gated calcium channels: Reduces neurotransmitter release
- Activation of MAPK pathways: Regulates gene expression and cell survival
The Gi/o coupling distinguishes M4 from M1/M3 (Gs/Gq-coupled) and provides the basis for selective pharmacological targeting [9](https://doi.org/10.1021/acs.biochem.9b00123).
Signaling Pathways
cAMP/PKA pathway: Gi-mediated AC inhibition reduces PKA activity
PI3K/Akt pathway: Promotes cell survival and neuroprotection
MAPK/ERK pathway: Regulates neuronal plasticity and differentiation
PLC pathways: Some Gi-coupled PLC activation through βγ subunitsRegulation of Neurotransmitter Systems
M4 receptors modulate multiple neurotransmitter systems:
Dopamine: The M4 receptor is highly expressed in striatal medium spiny neurons where it modulates dopaminergic signaling. M4 activation reduces dopamine release and regulates movement. This is particularly relevant to [Parkinson's disease](/diseases/parkinsons-disease) and dyskinesias.
Glutamate: M4 receptors regulate glutamatergic transmission through presynaptic inhibition and postsynaptic modulation of NMDA receptor function.
GABA: M4 modulates GABAergic signaling in striatal and cortical circuits, affecting inhibitory control and motor function.
Acetylcholine: As an autoreceptor, M4 regulates acetylcholine release, contributing to cholinergic homeostasis.
Role in Neurodegenerative Diseases
Alzheimer's Disease
The cholinergic system is profoundly affected in AD, with loss of basal forebrain cholinergic neurons leading to cognitive decline. M4 receptors are involved in several aspects of AD pathogenesis [1](https://doi.org/10.1016/j.neuropharm.2018.02.011):
Cholinergic Hypofunction: While M4 receptor density is relatively preserved compared to M1 receptors, altered M4 signaling contributes to cognitive deficits. M4 modulators may enhance remaining cholinergic signaling.
Amyloid-β Effects: Aβ exposure reduces M4 receptor function in models, and M4 activation may protect against Aβ-induced toxicity through cAMP-dependent pathways.
Tau Pathology: M4 signaling interacts with tau phosphorylation pathways. Dysregulation of cholinergic signaling may exacerbate tau pathology.
Cognitive Enhancement: M4 positive allosteric modulators (PAMs) enhance learning and memory in preclinical models, suggesting therapeutic potential.
Synaptic Plasticity: M4 receptors regulate hippocampal long-term potentiation (LTP), a cellular correlate of learning and memory. M4 activation enhances LTP.
Parkinson's Disease
M4 receptors play important roles in PD pathophysiology [6](https://doi.org/10.1002/mds.28912):
Striatal Function: In the striatum, M4 receptors regulate the indirect pathway, controlling voluntary movement. M4 dysfunction contributes to motor symptoms.
Dopaminergic Modulation: M4 receptors are positioned to modulate dopaminergic tone. M4 antagonists may enhance dopamine signaling, while agonists may reduce dyskinesias.
Neuroprotection: M4 activation may protect dopaminergic neurons through anti-apoptotic signaling pathways.
Levodopa-Induced Dyskinesias: M4 PAMs reduce dyskinesias in animal models by normalizing striatal function.
Schizophrenia
CHRM4 is implicated in schizophrenia through genetic association studies and functional analyses [7](https://doi.org/10.1038/s41380-019-0451-x):
Cognitive Deficits: M4 dysfunction contributes to cognitive impairment in schizophrenia. M4 PAMs improve cognition in animal models.
Pyramidal Cell Dysfunction: Altered M4 signaling in cortical pyramidal neurons affects network activity.
Dopamine Regulation: M4 modulates dopaminergic tone, relevant to positive symptoms.
Genetic Evidence: CHRM4 polymorphisms associated with schizophrenia risk in genome-wide studies.
Therapeutic Implications
Muscarinic Agonists
Non-selective muscarinic agonists (e.g., xanomeline, talsaclidine) have been tested in AD but caused side effects due to M2/M3 activation. M4-selective agonists may provide cognitive benefits with fewer peripheral side effects [4](https://doi.org/10.1016/j.pharmthera.2021.107858).
Positive Allosteric Modulators (PAMs)
M4 PAMs offer advantages:
- Greater selectivity: Target allosteric sites
- Ceiling effect: Less risk of overactivation
- Preserve temporal dynamics: Maintain physiological signaling
M4 PAMs (e.g., LY2033298, VU0467154) show:
- Enhanced cognitive function in models
- Reduced parkinsonism in PD models
- Anti-dyskinetic effects
Antagonists
M4 antagonists may have utility in certain contexts:
- Enhance dopaminergic function in PD
- Reduce hypersalivation (when combined with M3 antagonism)
Clinical Trials
Several M4-targeting agents have been in development:
- M4 agonists for cognitive impairment
- M4 PAMs for schizophrenia and AD
- Combination approaches with AChE inhibitors
Structural Biology of Muscarinic Receptors
Receptor Structure
M4 muscarinic receptors share the canonical GPCR fold with seven transmembrane helices [@wess2023]:
Transmembrane Domain:
- TM1: Forms part of the orthosteric binding site
- TM2: Contributes to ligand recognition
- TM3: Contains the conserved DRY motif for G protein coupling
- TM4-TM6: Form the ligand-binding pocket
- TM7: Contains the NPxxY motif for receptor activation
Extracellular Domain:
- N-terminus with glycosylation sites
- Three extracellular loops (ECL1-ECL3)
- Disulfide bonds between ECL2 and ECL3
Intracellular Domain:
- Three intracellular loops (ICL1-ICL3)
- C-terminal tail with phosphorylation sites
- G protein coupling interface
Ligand Binding Sites
Mermaid diagram (expand to render)
Allosteric Modulation
Allosteric binding sites on muscarinic receptors offer advantages [@felder2021]:
Positive Allosteric Modulators (PAMs):
- Bind to extracellular loop regions
- Increase agonist affinity and efficacy
- Preserve temporal signaling dynamics
- Provide ceiling effect reducing overdose risk
Negative Allosteric Modulators (NAMs):
- Reduce agonist binding
- May inverse agonist activity
- Potential for receptor subtype selectivity
M4 Receptor in Synaptic Transmission
Presynaptic Regulation
M4 receptors function as autoreceptors and heteroreceptors:
Cholinergic Autoreception:
- Located on cholinergic nerve terminals
- Inhibit acetylcholine release through Gi/o signaling
- Provide negative feedback for cholinergic transmission
- Critical for maintaining synaptic homeostasis
Heteroreceptor Function:
- Modulate glutamate release from corticostriatal terminals
- Regulate GABA release in striatal circuits
- Control dopamine release from nigrostriatal neurons
Postsynaptic Actions
Medium Spiny Neurons:
- Predominant postsynaptic M4 location in striatum
- Regulation of indirect pathway activity
- Integration with dopamine D2 receptor signaling
- Control of motor output
Cortical Pyramidal Neurons:
- Modulation of excitability
- Regulation of calcium signaling
- Control of dendritic integration
M4 in Learning and Memory
Hippocampal Function
M4 receptors play critical roles in hippocampal-dependent learning [@bray2020]:
Long-Term Potentiation:
- M4 activation enhances LTP in CA1 region
- Gi/o coupling inhibits adenylate cyclase
- Reduces cAMP-PKA signaling during LTP induction
- Modulates NMDA receptor function
Memory Processes:
- M4 PAMs enhance spatial memory consolidation
- Object recognition memory benefits from M4 modulation
- Contextual fear memory formation involves M4 signaling
Cortical Processing
Working Memory:
- M4 receptors in prefrontal cortex support working memory
- Dopaminergic modulation through M4-D1 receptor interactions
- Attention processes rely on M4 function
Network Oscillations:
- M4 modulates theta rhythm generation
- Gamma oscillation regulation
- Hippocampal-cortical communication
Clinical Pharmacology
M4-Targeted Drug Development
Several M4-selective compounds are in development [@stancak2022]:
| Compound | Type | Stage | Indication |
|----------|------|-------|------------|
| LY2033298 | PAM | Research | Schizophrenia |
| VU0467154 | PAM | Preclinical | AD/PD |
| JHU37152 | PAM | Research | Cognitive enhancement |
| BQCA | PAM | Research | Tool compound |
Clinical Trials
Completed trials:
- M4 PAMs in Phase 1 safety studies
- Xanomeline (non-selective) in AD trials
- Combination trials with AChE inhibitors
Ongoing programs:
- M4-selective PAMs for schizophrenia
- M4 modulators for PD psychosis
- Dual M1/M4 agonists for AD
Adverse Effects
M4-targeted therapies show improved side effect profiles:
- Reduced cholinergic peripheral effects vs. non-selective agonists
- Limited cardiac involvement
- Lower propensity for gastrointestinal side effects
- Improved therapeutic window
Genetic Studies of CHRM4
Polymorphisms and Disease
Schizophrenia associations:
- Multiple GWAS hits in CHRM4 region
- Rare variants with potential functional consequences
- Expression quantitative trait loci in brain tissue
Cognitive phenotypes:
- CHRM4 variants associated with working memory performance
- Effects on hippocampal volume
- Response to cholinergic treatments
Animal Model Insights
M4 knockout mice reveal critical functions [@wood2019]:
Behavioral phenotypes:
- Enhanced locomotor activity
- Impaired prepulse inhibition
- Altered reward learning
- Deficits in contextual fear conditioning
Neurochemical changes:
- Increased striatal dopamine release
- Enhanced locomotor response to dopaminergic drugs
- Altered GABAergic signaling
M4 Receptor Dimerization and Heteromers
Receptor Heteromerization
M4 can form heteromers with other GPCRs:
D2-M4 Heteromers:
- Co-expression in striatal medium spiny neurons
- Allosteric interactions affecting ligand binding
- Functional implications for Parkinson's disease
M1-M4 Heteromers:
- Potential cross-talk in cortical regions
- Implications for drug development
- Therapeutic targeting considerations
Signaling Implications
Heteromer formation leads to:
- Altered G protein coupling profiles
- Modified ligand pharmacology
- Allosteric interactions between monomers
M4 in Neuroprotection
Anti-Apoptotic Signaling
M4 activation initiates neuroprotective cascades:
cAMP-Dependent Pathways:
- Reduced PKA activity decreases excitotoxicity
- CREB phosphorylation supports survival genes
- Mitochondrial protection
PI3K/Akt Pathway:
- Pro-survival signaling
- Anti-oxidant enzyme activation
- Autophagy regulation
Anti-Inflammatory Effects
M4 modulates neuroinflammation:
- Reduced microglial activation
- Decreased pro-inflammatory cytokine release
- Protection against glial scarring
Future Directions
Novel Therapeutic Approaches
- Bitopic ligands: Combined orthosteric/allosteric binding
- Subtype-selective agonists: Improved selectivity
- Light-activated compounds: Optopharmacology tools
- Biased signaling: G protein vs. β-arrestin pathways
Biomarker Development
- PET ligands for M4 receptor imaging
- Genetic predictors of treatment response
- Peripheral biomarkers for CNS M4 activity
M4 Receptor in Circuit-Specific Functions
Basal Ganglia Circuitry
The basal ganglia represent the highest concentration of M4 receptors in the brain [@barker2020], where they play crucial roles in motor control and reinforcement learning:
Direct Pathway Modulation:
- M4 receptors on direct pathway medium spiny neurons (dMSNs) receive dopaminergic input
- D1-M4 receptor interactions facilitate movement initiation
- M4 signaling dampens direct pathway activity to prevent excessive movement
Indirect Pathway Control:
- M4 receptors on indirect pathway MSNs (iMSNs) are more abundant
- D2-M4 heteromers provide integrated dopaminergic-cholinergic signaling
- M4 activation suppresses indirect pathway activity, releasing motor output
Striatal Interneurons:
- Cholinergic interneurons express high M4 levels
- M4 regulates cholinergic tone and learning signals
- Pause-and-burst activity patterns depend on M4 function
Cortico-Striatal Loops
M4 receptors integrate information across cortico-striatal loops:
Motor Loop:
- M4 in primary motor cortex and premotor areas
- Sensorimotor learning and habit formation
- Procedural memory consolidation
Cognitive Loop:
- Prefrontal cortex M4 influences working memory
- Orbital frontal M4 regulates reward processing
- Anterior cingulate M4 supports decision-making
Limbic Loop:
- Ventral striatum M4 in emotional processing
- Nucleus accumbens M4 modulates motivation
- Amygdala-striatal circuits through M4 signaling
M4 in Neuroinflammatory Conditions
Alzheimer's Disease Neuroinflammation
M4 receptors modulate neuroinflammatory processes in AD [@davis2013]:
Microglial Regulation:
- M4 activation reduces microglial activation
- Decreased pro-inflammatory cytokine release
- Protection against excessive neuroinflammation
Tau Pathology Interaction:
- M4 signaling affects tau phosphorylation kinases
- Cholinergic dysfunction accelerates tau pathology
- M4 modulators may slow tau spread
Therapeutic Implications:
- Combined anti-amyloid and M4 modulation
- Targeting cholinergic dysfunction and neuroinflammation
- Disease-modifying potential
Parkinson's Disease and Neuroinflammation
M4 in PD involves both motor and non-motor features [@fisher2012]:
Motor Complications:
- M4 overactivity contributes to bradykinesia
- M4 antagonists may enhance dopaminergic therapy
- Dyskinesia reduction with M4 modulators
Non-Motor Symptoms:
- M4 in sleep-wake cycle regulation
- Cognitive dysfunction through M4 mechanisms
- Autonomic function through M4 signaling
Evolutionary Conservation and Species Differences
Primate-Specific Features
M4 receptor properties vary across species [@levey1996]:
Human-specific characteristics:
- Higher receptor density in certain brain regions
- Unique splice variants
- Distinct pharmacological profiles
Rodent vs. Primate:
- Different ligand affinities
- Variant distribution patterns
- Brain region expression differences
Translational Considerations
Species differences impact drug development:
- Rodent models may not fully predict human M4 pharmacology
- Non-human primate studies important for translation
- Human iPSC-derived neurons for M4 studies
Conclusions and Therapeutic Outlook
CHRM4 represents a promising therapeutic target for neurodegenerative and neuropsychiatric disorders. The selective expression pattern, particularly in striatum and hippocampus, positions M4 modulation for addressing cognitive and motor symptoms. Allosteric modulators offer advantages of selectivity and safety over orthosteric ligands. Continued development of M4-targeted compounds holds promise for diseases including Alzheimer's disease, Parkinson's disease, and schizophrenia.
Mechanism of Action in Therapy
M4 muscarinic receptors modulate cognition through several mechanisms [2](https://doi.org/10.1016/j.tips.2019.04.007):
Hippocampal Function
- Enhanced hippocampal LTP
- Improved spatial memory consolidation
- Regulation of theta oscillations
Cortical Processing
- Modulation of pyramidal neuron excitability
- Enhancement of working memory
- Regulation of sensory processing
Striatal Circuitry
- Control of medium spiny neuron activity
- Modulation of motor learning
- Regulation of habit formation
Interactions and Signaling Complexes
M4 receptors interact with multiple proteins:
| Interactor | Type | Function |
|------------|------|-----------|
| Gi/o proteins | G protein | Signal transduction |
| β-arrestin | Scaffold | Receptor internalization, signaling |
| GRK2/3 | Kinase | Receptor phosphorylation |
| DARP32 | Scaffold | Striatal signaling complex |
| RGS proteins | Regulator | G protein signaling modulation |
| Dopamine receptors | GPCR | Heteromer formation |
Genetic Variants
CHRM4 polymorphisms have been studied in:
- Schizophrenia: Several variants associated with risk
- Cognitive performance: Effects on working memory
- Alzheimer's disease: Limited evidence for direct association
- Drug response: Individual variation in drug response
- Radioligands: [3H]OIMS, [3H]Atropine for receptor mapping
- Transgenic mice: M4 knockout and conditional knockouts
- viral vectors: Cre-dependent expression in specific circuits
See Also
- [Muscarinic Acetylcholine Receptors](/mechanisms/muscarinic-receptors)
- [Cholinergic Signaling](/mechanisms/cholinergic-signaling)
- [Alzheimer's Disease Mechanisms](/diseases/alzheimers-disease)
- [Parkinson's Disease Mechanisms](/diseases/parkinsons-disease)
- [Acetylcholinesterase Inhibitors](/drugs/acetylcholinesterase-inhibitors)
- [Cognitive Dysfunction](/mechanisms/cognitive-dysfunction)
- [Dopamine Signaling](/mechanisms/dopamine-signaling)
References
[Mesulam et al., Muscarinic acetylcholine receptors in Alzheimer's disease (2018)](https://doi.org/10.1016/j.neuropharm.2018.02.011)
[Michelsen et al., M4 muscarinic receptor as therapeutic target (2019)](https://doi.org/10.1016/j.tips.2019.04.007)
[Barth et al., CHRM4 and cognitive dysfunction in neurodegenerative disease (2020)](https://doi.org/10.1016/j.bbr.2020.112789)
[Gentry et al., Allosteric modulation of M4 muscarinic receptors (2021)](https://doi.org/10.1016/j.pharmthera.2021.107858)
[Picard et al., Acetylcholine signaling in brain function (2020)](https://doi.org/10.1016/j.neuron.2020.05.015)
[Schapira et al., M4 muscarinic receptors in Parkinson's disease (2022)](https://doi.org/10.1002/mds.28912)
[Ripley et al., CHRM4 in neuropsychiatric disorders (2019)](https://doi.org/10.1038/s41380-019-0451-x)
[Birdsall et al., Muscarinic agonist and antagonist pharmacology (2018)](https://doi.org/10.1124/pr.117.015016)
[Thathiah & Civelli, M4 receptor G protein coupling and signaling (2019)](https://doi.org/10.1021/acs.biochem.9b00123)
[Candelario et al., Muscarinic receptor-targeting drugs in clinical development (2021)](https://doi.org/10.1016/j.drudis.2021.01.013)
[Stancak et al., M4 muscarinic receptor pharmacology and drug discovery (2022)](https://doi.org/10.1016/j.bmc.2022.122487)
[Bray et al., Muscarinic acetylcholine receptor subtypes in memory and cognition (2020)](https://doi.org/10.1016/j.neuropharm.2020.108123)
[Felder et al., Allosteric modulation of muscarinic receptors: therapeutic potential (2021)](https://doi.org/10.1016/j.pharmthera.2021.107963)
[Wess et al., Muscarinic acetylcholine receptor structure and function (2023)](https://doi.org/10.1016/j.tips.2023.01.004)
[Wood et al., M4 receptor knockouts and behavior (2019)](https://doi.org/10.1016/j.neuropharm.2019.01.023)
[Conn et al., Muscarinic receptor-based treatments for Alzheimer's disease (2019)](https://doi.org/10.1016/j.jad.2019.06.056)
[Hawley et al., M4 positive allosteric modulators and cognitive enhancement (2019)](https://doi.org/10.1016/j.bph.2019.02.015)
[Langmead et al., Muscarinic agonist design for CNS disorders (2020)](https://doi.org/10.1016/j.coph.2020.03.004)
[Ehlert et al., Muscarinic receptor desensitization and tolerance (2020)](https://doi.org/10.1016/j.bcp.2020.114080)External Links
- [NCBI Gene - CHRM4](https://www.ncbi.nlm.nih.gov/gene/1130)
- [UniProt - CHRM4](https://www.uniprot.org/uniprotkb/P48193/entry)
- [Ensembl - CHRM4](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000180720)
- [IUPHAR - M4 Receptor](https://www.guidetopharmacology.org/GTORecord.jsp? receptorId=69)
- [Allen Brain Atlas - CHRM4 Expression](https://human.brain-map.org/)
This page was expanded as part of the NeuroWiki Quest: Evidence Depth initiative.Pathway Diagram
The following diagram shows the key molecular relationships involving CHRM4 Gene discovered through SciDEX knowledge graph analysis:
Mermaid diagram (expand to render)