CHRM4 Gene

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).

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

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

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

Signaling Pathways

Regulation 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

• 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

• N-terminus with glycosylation sites
• Three extracellular loops (ECL1-ECL3)
• Disulfide bonds between ECL2 and ECL3

• Three intracellular loops (ICL1-ICL3)
• C-terminal tail with phosphorylation sites
• G protein coupling interface

Ligand Binding Sites

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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• Prefrontal cortex M4 influences working memory
• Orbital frontal M4 regulates reward processing
• Anterior cingulate M4 supports decision-making

• 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

• M4 signaling affects tau phosphorylation kinases
• Cholinergic dysfunction accelerates tau pathology
• M4 modulators may slow tau spread

• 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

• 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

• 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

Research Tools

• 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

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/)

Pathway Diagram

The following diagram shows the key molecular relationships involving CHRM4 Gene discovered through SciDEX knowledge graph analysis: