CHRM5 Gene — Muscarinic Acetylcholine Receptor 5
Introduction
CHRM5 (Cholinergic Receptor Muscarinic 5) encodes the M5 muscarinic acetylcholine receptor, also known as the M5 subtype. This receptor is a member of the muscarinic acetylcholine receptor family, which consists of five subtypes (M1-M5) that belong to the G protein-coupled receptor (GPCR) superfamily. The M5 receptor is the least studied of these subtypes, yet emerging research suggests it plays important roles in the central nervous system (CNS), particularly in regions affected by neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD)[@muscarinic].
The CHRM5 gene is located on chromosome 15q26 and encodes a 532-amino acid protein. Like other muscarinic receptors, M5 contains seven transmembrane domains connected by three extracellular and three intracellular loops, with an extracellular N-terminus and intracellular C-terminus. The receptor couples primarily to Gq/11 proteins, leading to activation of phospholipase C (PLC) and subsequent intracellular signaling cascades[@gqcoupled].
<div class="infobox infobox-gene">
| Property | Value |
|----------|-------|
| Gene Symbol | CHRM5 |
| Full Name | Cholinergic Receptor Muscarinic 5 |
| Chromosomal Location | 15q26 |
| NCBI Gene ID | 1129 |
| OMIM ID | 118491 |
| Ensembl ID | ENSG00000121966 |
| UniProt ID | P08912 |
| Encoded Protein | Muscarinic acetylcholine receptor M5 |
| Gene Type | Protein-coding |
| Protein Family | GPCR, Class A, Muscarinic receptors |
| Associated Diseases | Alzheimer's disease, Parkinson's disease, schizophrenia |
</div>
Receptor Pharmacology and Signaling
G Protein Coupling
The M5 receptor is uniquely characterized by its selective coupling to Gq/11 proteins among the muscarinic receptor family. Unlike M1 and M3 receptors, which also couple to Gq/11, M5 appears to have more restricted G protein coupling. This specificity has important implications for its downstream signaling pathways and therapeutic targeting.
When activated by acetylcholine or muscarinic agonists, M5 triggers the following signaling cascade:
Mermaid diagram (expand to render)
Downstream Pathways
The M5 receptor activates multiple downstream signaling pathways:
Phospholipase C (PLC) pathway: Leads to generation of inositol trisphosphate (IP3) and diacylglycerol (DAG), causing calcium release and protein kinase C activation
MAPK pathway: Activation of ERK1/2 and p38 MAP kinases
PI3K/Akt pathway: Promotes cell survival and neuroprotection
cAMP pathway: Can modulate cAMP levels depending on cellular contextThese pathways influence various neuronal functions including neurotransmitter release, synaptic plasticity, gene expression, and cell survival.
Ligand Pharmacology
The M5 receptor has traditionally been considered an "orphan" receptor due to the lack of highly selective ligands. However, several compounds have been developed:
- Non-selective muscarinic agonists: Oxotremorine, pilocarpine, carbachol (activate all subtypes)
- M5-preferring compounds: Several synthetic compounds show some selectivity for M5
- Allosteric modulators: Positive and negative allosteric modulators (PAMs and NAMs) have been identified
The development of M5-selective ligands remains an active area of research due to the therapeutic potential of subtype-selective targeting.
Brain Distribution and Expression
Regional Localization
The M5 receptor exhibits a distinctive distribution pattern in the brain, with highest expression in:
- Substantia nigra pars compacta (SNc): High density of M5 receptors on dopaminergic neurons
- Ventral tegmental area (VTA): Located in the mesolimbic pathway
- Hippocampus: Particularly in CA1 and CA3 regions
- Cortex: Layer-specific expression in pyramidal neurons
- Striatum: Both in medium spiny neurons and interneurons
- Hypothalamus: Especially in the supraoptic and paraventricular nuclei
- Basal forebrain: Cholinergic neuron bodies
This distribution closely overlaps with dopaminergic pathways and regions involved in learning, memory, and movement control[@receptor].
Cellular Localization
At the cellular level, M5 receptors are found on:
- Dopaminergic neurons in the substantia nigra and VTA
- Pyramidal neurons in cortex and hippocampus
- GABAergic interneurons
- [Astrocytes](/cell-types/astrocytes) Microglia (at lower levels)
The presynaptic localization on dopamine terminals in the striatum and cortex is particularly important for understanding M5's role in modulating dopamine release.
Expression Changes in Disease
Post-mortem studies have revealed altered M5 receptor expression in neurodegenerative diseases:
Alzheimer's Disease:
- Reduced M5 receptor binding in hippocampus and cortex
- Correlation with cholinergic neuron loss
- Correlation with cognitive decline severity
Parkinson's Disease:
- Altered M5 expression in the basal ganglia
- Changes in striatal M5 receptor density
- Potential compensatory changes in response to dopamine loss
Role in Alzheimer's Disease
Cholinergic Hypothesis and M5
The cholinergic hypothesis of AD posits that loss of basal forebrain cholinergic neurons and subsequent deficits in acetylcholine signaling contribute to cognitive impairment. While M1 and M4 receptors have been the primary focus of therapeutic development, M5 receptors also play important roles in cholinergic signaling in brain regions affected by AD[@cholinergic].
Amyloid-Beta Interactions
M5 receptor signaling can influence amyloid precursor protein (APP) processing and amyloid-beta (Aβ) production:
- BACE1 regulation: Muscarinic receptor activation can modulate β-secretase expression
- APP processing: Gq-coupled receptors influence α- and β-secretase activity
- Aβ clearance: May affect mechanisms of Aβ clearance including autophagy
The relationship between M5 and Aβ is complex and context-dependent, with both protective and potentially detrimental effects reported.
Tau Pathology
M5 receptor activation may influence tau phosphorylation through:
- Modulation of tau kinases (GSK3β, CDK5)
- Effects on protein phosphatases
- Influence on cellular stress responses
Synaptic Function and Cognition
M5 receptors contribute to cognitive function through:
- Hippocampal plasticity: Modulation of long-term potentiation (LTP)
- Cortical processing: Influence on working memory and attention
- Learning and memory: Involvement in various memory paradigms
Studies using M5 knockout mice show subtle cognitive deficits, though less severe than M1 receptor knockout, suggesting M5 plays a modulatory rather than essential role in cognition.
Neuroprotection
M5 receptor activation can provide neuroprotective effects through:
- Anti-apoptotic signaling: Akt pathway activation
- Oxidative stress reduction: Decreased ROS production
- Mitochondrial protection: Improved mitochondrial function
- Anti-inflammatory effects: Modulation of glial activation
These neuroprotective properties suggest potential therapeutic applications for M5-targeted compounds in AD[@felder2001].
Role in Parkinson's Disease
Basal Ganglia Circuitry
Parkinson's disease is characterized by degeneration of dopaminergic neurons in the substantia nigra pars compacta, leading to dysfunction of the basal ganglia motor circuit. Muscarinic receptors, including M5, play important roles in modulating this circuitry[@muscarinica].
In the basal ganglia, M5 receptors are located on:
- Dopaminergic nerve terminals (presynaptic)
- Medium spiny neurons (postsynaptic)
- Cholinergic interneurons (striatal aspiny neurons)
The interaction between cholinergic and dopaminergic systems in the striatum is critical for motor control, and M5 receptors represent a key interface between these neurotransmitter systems.
Dopamine Release Modulation
M5 receptors on dopaminergic terminals modulate dopamine release:
- Presynaptic modulation: M5 activation can enhance dopamine release
- Frequency-dependent effects: Different effects at low vs. high firing rates
- Interaction with D1/D2 receptors: Complex interplay in striatal signaling
This modulation has implications for both PD symptoms and levodopa-induced dyskinesias[@berger2002].
Motor Control
Within the basal ganglia, M5 influences motor control:
Mermaid diagram (expand to render)
Therapeutic Implications
The role of M5 in PD has several therapeutic implications:
Anticholinergic therapy: Classic treatments use non-selective muscarinic antagonists
M5-selective targeting: Potential for subtype-specific approaches
Dyskinesia management: M5 modulation may influence levodopa-induced dyskinesiasCurrent PD treatments that target muscarinic receptors include:
- Trihexyphenidyl (Artane)
- Benztropine (Cogentin)
- Procyclidine (Kemadrin)
These non-selective antagonists provide symptomatic relief but have significant side effects due to actions at multiple muscarinic subtypes.
Therapeutic Targeting
M5 as a Drug Target
The M5 receptor represents a potential therapeutic target for:
Neurodegenerative Diseases:
- Alzheimer's disease: Cognitive enhancement, neuroprotection
- Parkinson's disease: Motor symptom modulation, dyskinesia management
Other CNS Disorders:
- Schizophrenia: Modulation of dopaminergic and cholinergic systems
- Addiction: Influence on mesolimbic dopamine pathways
- Depression: Potential mood effects through neurotransmitter modulation
Challenges in M5-Targeted Drug Development
Several challenges have hindered M5-targeted drug development:
Lack of selective ligands: Limited availability of M5-specific compounds
Expression in peripheral organs: Side effects from peripheral actions
Complex pharmacology: Multiple signaling pathways and tissue-specific effects
Blood-brain barrier penetration: CNS delivery challengesCurrent Approaches
Allosteric Modulation:
Allosteric modulators offer subtype selectivity advantages:
- Positive allosteric modulators (PAMs) enhance agonist responses
- Negative allosteric modulators (NAMs) reduce agonist effects
- May provide more selective targeting than orthosteric ligands
M1/M4 Selective Agonists:Some compounds show preferential activity at M1 and M4:
- Xanomeline: M1/M4 preferring, in clinical trials for AD
- Talsaclidine: M1 preferring
Novel M5-Selective Compounds:Several research programs have developed M5-selective compounds in preclinical development[@conn2009].
Genetic Studies
CHRM5 Polymorphisms
Genetic studies have examined CHRM5 polymorphisms in neurodegenerative diseases:
- AD association studies: Some variants show suggestive associations
- PD association studies: Limited evidence for major effects
- Cognitive function: Genetic variation may influence cognitive trajectories
Animal Models
Knockout mice:
M5 knockout mice show:
- Reduced dopamine release in striatum
- Altered locomotor responses
- Subtle cognitive changes
- Normal development and viability
Transgenic models:M5 overexpression studies show:
- Enhanced dopamine release
- Altered behavioral responses
- Modified drug responses
These models have helped elucidate M5 function in vivo.
Research Directions
Unresolved Questions
Why is M5 the least studied muscarinic receptor? - Historical challenges with selective ligands
What are the distinct functions of M5 compared to other subtypes? - Need for comparative studies
Can M5-selective modulation provide therapeutic benefits? - Clinical validation needed
What are the long-term effects of M5-targeted interventions? - Safety and efficacy concernsEmerging Research Areas
- Optogenetics: Using light to control M5-expressing circuits
- Designer receptors: Chemogenetic (DREADD) approaches
- Single-cell RNAseq: Understanding M5 expression at cellular resolution
- Protein structures: Cryo-EM structures of M5 bound to various ligands
See Also
- [Acetylcholine](/entities/acetylcholine)
- [Muscarinic Receptors](/entities/muscarinic-receptors)
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
- [Dopamine](/entities/dopamine)
- [Substantia Nigra](/brain-regions/substantia-nigra)
- [Striatum](/brain-regions/striatum)
- [Hippocampus](/brain-regions/hippocampus)
- [Basal Ganglia](/brain-regions/basal-ganglia)
- [Cholinergic System](/mechanisms/cholinergic-system-neurodegeneration)
- [GPCR signaling in neurodegeneration](/mechanisms/gpcr-signaling-neurodegeneration)
- [Dopamine release mechanisms](/mechanisms/dopamine-release)
External Links
- [NCBI Gene: CHRM5](https://www.ncbi.nlm.nih.gov/gene/1129)
- [UniProt: P08912](https://www.uniprot.org/uniprot/P08912)
- [IUPHAR: CHRM5](https://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=65)
- [PubMed: CHRM5](https://pubmed.ncbi.nlm.nih.gov/?term=CHRM5+muscarinic+receptor)
- [Ensembl: CHRM5](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000121966)
References
[Birdsall & Lazareno, Muscarinic acetylcholine receptors (1999)](https://pubmed.ncbi.nlm.nih.gov/10491354/)
[Rosenblaum et al., Gq-coupled muscarinic receptors (2002)](https://pubmed.ncbi.nlm.nih.gov/11888521/)
[Weiner & Brann, M5 receptor distribution (2003)](https://pubmed.ncbi.nlm.nih.gov/12711704/)
[Cai et al., Cholinergic therapy in Alzheimer's disease (2004)](https://pubmed.ncbi.nlm.nih.gov/14598217/)
[Michell et al., Muscarinic antagonists in Parkinson's disease (2005)](https://pubmed.ncbi.nlm.nih.gov/15654760/)
[Felder et al., Therapeutic opportunities for muscarinic receptors (2001)](https://doi.org/10.1021/jm000279a020)
[Eglen et al., Muscarinic receptor ligands and CNS disorders (2001)](https://doi.org/10.1016/S0165-6147(00)01736-X)
[Hedlund & Gundlach, Muscarinic receptors in basal ganglia (2002)](https://doi.org/10.1007/s007020200067)
[Levey et al., Muscarinic receptor proteins in brain (1993)](https://pubmed.ncbi.nlm.nih.gov/8386254/)
[Volpicelli & Levey, Muscarinic receptor subtypes in cortex and hippocampus (2003)](https://doi.org/10.1016/j.brainres.2003.08.064)
[Wess, Muscarinic receptor knockout mice phenotypes (2004)](https://doi.org/10.1146/annurev.pharmtox.44.101802.121752)
[Berger et al., Acetylcholine and dopamine interactions (2002)](https://pubmed.ncbi.nlm.nih.gov/12097644/)
[Zhang et al., Muscarinic receptor-mediated dopamine release (2002)](https://pubmed.ncbi.nlm.nih.gov/11978834/)
[Toth et al., Muscarinic modulation of NMDA responses (2001)](https://pubmed.ncbi.nlm.nih.gov/11378148/)
[Conn et al., Subtype-selective muscarinic modulators (2009)](https://doi.org/10.1016/j.tips.2008.12.003)
[Langmead et al., Muscarinic receptors as CNS drug targets (2008)](https://doi.org/10.1016/j.pharmthera.2007.10.009)
[Bray et al., M5 muscarinic receptors and neuronal survival (2003)](https://pubmed.ncbi.nlm.nih.gov/14533277/)
[Olsson et al., Muscarinic receptors in neurodegenerative diseases (2006)](https://doi.org/10.1016/j.brainresrev.2005.10.001)
[Caccin et al., Muscarinic receptors in Alzheimer's disease (2011)](https://pubmed.ncbi.nlm.nih.gov/21143141/)
[Fisher, Cholinergic modulation of APP processing (2012)](https://doi.org/10.1007/s00702-011-0713-7)Summary
CHRM5 encodes the M5 muscarinic acetylcholine receptor, a GPCR with important roles in dopaminergic and cholinergic systems of the brain. While less studied than other muscarinic subtypes, M5 receptor signaling influences neurotransmitter release, synaptic plasticity, and neuroprotection in regions affected by Alzheimer's and Parkinson's disease. The development of M5-selective compounds represents a promising avenue for therapeutic intervention in neurodegenerative diseases.