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, making M4 receptors important therapeutic targets 1.
| CHRM4 - Cholinergic Receptor Muscarinic 4 | |
|---|---|
| Gene Symbol | CHRM4 |
| Full Name | Cholinergic Receptor Muscarinic 4 |
| Chromosome | 11p12 |
| NCBI Gene ID | [1130](https://www.ncbi.nlm.nih.gov/gene/1130) |
| OMIM | [118550](https://www.omim.org/entry/118550) |
| Ensembl ID | [ENSG00000180720](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000180720) |
| UniProt ID | [P48193](https://www.uniprot.org/uniprotkb/P48193/entry) |
| Protein Class | GPCR, Class A, Muscarinic |
| Associated Diseases | Alzheimer's Disease, Parkinson's Disease, Schizophrenia, Cognitive Impairment |
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.
The M4 receptor exists as a monomer and can form homomers and heteromers with other GPCRs, expanding its functional repertoire.
CHRM4 exhibits a distinctive brain expression pattern:
The striatal enrichment of M4 receptors makes them particularly relevant to movement disorders and dopaminergic function.
M4 muscarinic receptors couple primarily to Gi/o proteins, leading to:
The Gi/o coupling distinguishes M4 from M1/M3 (Gs/Gq-coupled) and provides the basis for selective pharmacological targeting 9.
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 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.
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:
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.
M4 receptors play important roles in PD pathophysiology 6:
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.
CHRM4 is implicated in schizophrenia through genetic association studies and functional analyses 7:
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.
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.
M4 PAMs offer advantages:
M4 PAMs (e.g., LY2033298, VU0467154) show:
M4 antagonists may have utility in certain contexts:
Several M4-targeting agents have been in development:
M4 muscarinic receptors share the canonical GPCR fold with seven transmembrane helices [1]:
Transmembrane Domain:
Extracellular Domain:
Intracellular Domain:
Allosteric binding sites on muscarinic receptors offer advantages [2]:
Positive Allosteric Modulators (PAMs):
Negative Allosteric Modulators (NAMs):
M4 receptors function as autoreceptors and heteroreceptors:
Cholinergic Autoreception:
Heteroreceptor Function:
Medium Spiny Neurons:
Cortical Pyramidal Neurons:
M4 receptors play critical roles in hippocampal-dependent learning [3]:
Long-Term Potentiation:
Memory Processes:
Working Memory:
Network Oscillations:
Several M4-selective compounds are in development [4]:
| Compound | Type | Stage | Indication |
|---|---|---|---|
| LY2033298 | PAM | Research | Schizophrenia |
| VU0467154 | PAM | Preclinical | AD/PD |
| JHU37152 | PAM | Research | Cognitive enhancement |
| BQCA | PAM | Research | Tool compound |
Completed trials:
Ongoing programs:
M4-targeted therapies show improved side effect profiles:
Schizophrenia associations:
Cognitive phenotypes:
M4 knockout mice reveal critical functions [5]:
Behavioral phenotypes:
Neurochemical changes:
M4 can form heteromers with other GPCRs:
D2-M4 Heteromers:
M1-M4 Heteromers:
Heteromer formation leads to:
M4 activation initiates neuroprotective cascades:
cAMP-Dependent Pathways:
PI3K/Akt Pathway:
M4 modulates neuroinflammation:
The basal ganglia represent the highest concentration of M4 receptors in the brain [6], where they play crucial roles in motor control and reinforcement learning:
Direct Pathway Modulation:
Indirect Pathway Control:
Striatal Interneurons:
M4 receptors integrate information across cortico-striatal loops:
Motor Loop:
Cognitive Loop:
Limbic Loop:
M4 receptors modulate neuroinflammatory processes in AD [7]:
Microglial Regulation:
Tau Pathology Interaction:
Therapeutic Implications:
M4 in PD involves both motor and non-motor features [8]:
Motor Complications:
Non-Motor Symptoms:
M4 receptor properties vary across species [9]:
Human-specific characteristics:
Rodent vs. Primate:
Species differences impact drug development:
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.
M4 muscarinic receptors modulate cognition through several mechanisms 2:
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 |
CHRM4 polymorphisms have been studied in:
Muscarinic acetylcholine receptor structure and function. 2023. ↩︎
Muscarinic acetylcholine receptor subtypes in memory and cognition. 2020. ↩︎
M4 muscarinic receptor pharmacology and drug discovery. 2022. ↩︎
M1 and M4 muscarinic receptor agonists for Alzheimer's disease. 2012. ↩︎
Muscarinic acetylcholine receptor expression in the primate brain. 1996. ↩︎