Muscarinic Acetylcholine Receptor M2 (CHRM2)

Muscarinic Acetylcholine Receptor M2 (CHRM2)

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">CHRM2</th></tr>
<tr><td><strong>Protein Name</strong></td><td>Muscarinic Acetylcholine Receptor M2</td></tr>
<tr><td><strong>Gene</strong></td><td>[CHRM2](/genes/chrm2)</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P08172](https://www.uniprot.org/uniprot/P08172)</td></tr>
<tr><td><strong>Protein Family</strong></td><td>GPCR, Class A, Muscarinic</td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>66.7 kDa</td></tr>
<tr><td><strong>Expression</strong></td><td>Heart, brain, smooth muscle</td></tr>
<tr><td><strong>Signal Transduction</strong></td><td>Gi/o-coupled, ↓cAMP</td></tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>
</div>

Overview

CHRM2 (Muscarinic [Acetylcholine](/entities/acetylcholine) Receptor M2) is a Gi/o-protein-coupled receptor that mediates inhibitory cholinergic signaling in the heart, smooth muscle, and central nervous system. In the brain, M2 receptors play crucial roles in modulating neurotransmitter release, regulating cognitive processes, and maintaining neuronal network stability. CHRM2 has been implicated in [Alzheimer's disease](/diseases/alzheimers-disease), [Parkinson's disease](/diseases/parkinsons-disease), and various neuropsychiatric disorders [1].

Structure and Mechanism

Receptor Architecture

Muscarinic Acetylcholine Receptor M2 (CHRM2)

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">CHRM2</th></tr>
<tr><td><strong>Protein Name</strong></td><td>Muscarinic Acetylcholine Receptor M2</td></tr>
<tr><td><strong>Gene</strong></td><td>[CHRM2](/genes/chrm2)</td></tr>
<tr><td><strong>UniProt ID</strong></td><td>[P08172](https://www.uniprot.org/uniprot/P08172)</td></tr>
<tr><td><strong>Protein Family</strong></td><td>GPCR, Class A, Muscarinic</td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>66.7 kDa</td></tr>
<tr><td><strong>Expression</strong></td><td>Heart, brain, smooth muscle</td></tr>
<tr><td><strong>Signal Transduction</strong></td><td>Gi/o-coupled, ↓cAMP</td></tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>
</div>

Overview

CHRM2 (Muscarinic [Acetylcholine](/entities/acetylcholine) Receptor M2) is a Gi/o-protein-coupled receptor that mediates inhibitory cholinergic signaling in the heart, smooth muscle, and central nervous system. In the brain, M2 receptors play crucial roles in modulating neurotransmitter release, regulating cognitive processes, and maintaining neuronal network stability. CHRM2 has been implicated in [Alzheimer's disease](/diseases/alzheimers-disease), [Parkinson's disease](/diseases/parkinsons-disease), and various neuropsychiatric disorders [1].

Structure and Mechanism

Receptor Architecture

CHRM2 is a seven-transmembrane domain GPCR consisting of 460 amino acids. The receptor contains:

• Extracellular N-terminus: Ligand-binding domain for acetylcholine
• Seven transmembrane helices: Form the ligand-binding pocket
• Intracellular C-terminus: Couples to G proteins and contains phosphorylation sites

G Protein Coupling

CHRM2 preferentially couples to Gi/o proteins, leading to:

This Gi/o coupling distinguishes M2 from M1/M3/M5 receptors, which couple to Gq proteins and activate phospholipase C.

Brain Distribution and Function

Regional Expression

In the central nervous system, CHRM2 is highly expressed in:

• [Hippocampus](/brain-regions/hippocampus): CA1-CA3 regions, dentate gyrus
• [Cortex](/brain-regions/cortex): Layers II-III, V
• Basal forebrain: Cholinergic neurons
• Thalamus: Relay and intralaminar nuclei
• Striatum: Indirect pathway neurons

Physiological Roles

CHRM2 regulates several critical brain functions:

Modulation of Neurotransmitter Release

M2 receptors function as autoreceptors on cholinergic neurons, limiting acetylcholine release during high activity. They also modulate release of:

• Dopamine in striatum and prefrontal cortex
• Norepinephrine in locus coeruleus
• Glutamate and GABA in various regions

Cognitive Processes

CHRM2 is implicated in:

• Working memory: Regulation of hippocampal-cortical circuits
• Attention: Modulation of cortical processing
• Learning and plasticity: Influences [LTP](/mechanisms/long-term-potentiation) and LTD
• Reward processing: Dopaminergic modulation

Autonomic Regulation

In peripheral tissues, CHRM2 mediates:

• Decreased heart rate (negative chronotropy)
• Reduced cardiac contractility
• Smooth muscle relaxation
• Bronchoconstriction

CHRM2 in Neurodegenerative Diseases

Alzheimer's Disease

CHRM2 plays a complex role in [Alzheimer's disease](/diseases/alzheimers-disease):

Cholinergic Hypothesis Connection

The cholinergic hypothesis of AD posits that loss of basal forebrain cholinergic neurons contributes to cognitive decline. CHRM2 autoreceptors are strategically positioned to modulate this system:

• Loss of CHRM2 function may dysregulate acetylcholine release
• M2 receptor density is reduced in AD brains
• Genetic variants of CHRM2 are associated with AD risk

Amyloid and Cholinergic Signaling

Evidence suggests bidirectional interactions between [amyloid-beta](/proteins/amyloid-beta) and muscarinic receptors:

• Aβ can bind to muscarinic receptors and disrupt signaling
• M2 activation may protect against Aβ-induced toxicity
• Cholinergic agonists reduce Aβ production via non-amyloidogenic processing

Parkinson's Disease

In [Parkinson's disease](/diseases/parkinsons-disease), CHRM2 influences:

Genetic Associations

Genome-wide association studies (GWAS) have identified CHRM2 polymorphisms associated with:

• Alzheimer's disease risk
• Parkinson's disease susceptibility
• Schizophrenia
• Major depressive disorder
• Intelligence and cognitive performance

Therapeutic Targeting

Agonists

Muscarinic agonists (e.g., xanomeline, talsaclidine) have been investigated for AD treatment:

• Improve cognitive function in clinical trials
• Reduce amyloid pathology in models
• Activate M2/M4 receptors preferentially

Antagonists

M2 antagonists are being explored for:

• Improving cognitive function
• Reducing dyskinesias in PD
• Treating addiction

Allosteric Modators

Positive allosteric modulators (PAMs) offer potential advantages:

• More selective modulation
• Preserve temporal signaling patterns
• Reduced side effects

Summary

CHRM2 is a critical Gi/o-coupled muscarinic receptor that regulates cholinergic signaling throughout the brain. Its role in modulating neurotransmitter release, cognitive processes, and neuronal survival makes it a key player in neurodegenerative diseases. While direct muscarinic agonists have shown promise, their utility is limited by peripheral side effects. Allosteric modulation and subtype-selective targeting represent promising therapeutic strategies for AD and PD.

See Also

• [Alzheimer's disease](/diseases/alzheimers-disease)
• [Parkinson's disease](/diseases/parkinsons-disease)

External Links

• [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
• [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)

References