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)[1].
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[2].
| 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 |
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:
The M5 receptor activates multiple downstream signaling pathways:
These pathways influence various neuronal functions including neurotransmitter release, synaptic plasticity, gene expression, and cell survival.
The M5 receptor has traditionally been considered an "orphan" receptor due to the lack of highly selective ligands. However, several compounds have been developed:
The development of M5-selective ligands remains an active area of research due to the therapeutic potential of subtype-selective targeting.
The M5 receptor exhibits a distinctive distribution pattern in the brain, with highest expression in:
This distribution closely overlaps with dopaminergic pathways and regions involved in learning, memory, and movement control[3].
At the cellular level, M5 receptors are found on:
The presynaptic localization on dopamine terminals in the striatum and cortex is particularly important for understanding M5's role in modulating dopamine release.
Post-mortem studies have revealed altered M5 receptor expression in neurodegenerative diseases:
Alzheimer's Disease:
Parkinson's Disease:
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[4].
M5 receptor signaling can influence amyloid precursor protein (APP) processing and amyloid-beta (Aβ) production:
The relationship between M5 and Aβ is complex and context-dependent, with both protective and potentially detrimental effects reported.
M5 receptor activation may influence tau phosphorylation through:
M5 receptors contribute to cognitive function through:
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.
M5 receptor activation can provide neuroprotective effects through:
These neuroprotective properties suggest potential therapeutic applications for M5-targeted compounds in AD[5].
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[6].
In the basal ganglia, M5 receptors are located on:
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.
M5 receptors on dopaminergic terminals modulate dopamine release:
This modulation has implications for both PD symptoms and levodopa-induced dyskinesias[7].
Within the basal ganglia, M5 influences motor control:
The role of M5 in PD has several therapeutic implications:
Current PD treatments that target muscarinic receptors include:
These non-selective antagonists provide symptomatic relief but have significant side effects due to actions at multiple muscarinic subtypes.
The M5 receptor represents a potential therapeutic target for:
Neurodegenerative Diseases:
Other CNS Disorders:
Several challenges have hindered M5-targeted drug development:
Allosteric Modulation:
Allosteric modulators offer subtype selectivity advantages:
M1/M4 Selective Agonists:
Some compounds show preferential activity at M1 and M4:
Novel M5-Selective Compounds:
Several research programs have developed M5-selective compounds in preclinical development[8].
Genetic studies have examined CHRM5 polymorphisms in neurodegenerative diseases:
Knockout mice:
M5 knockout mice show:
Transgenic models:
M5 overexpression studies show:
These models have helped elucidate M5 function in vivo.
Birdsall NJ, Lazareno A. Muscarinic acetylcholine receptors. Life Sciences. 1999. ↩︎
Rosenblaum K, Young D, Manis P. Gq-coupled muscarinic receptors. Journal of Neurophysiology. 2002. ↩︎
Weiner DM, Brann MR. M5 receptor distribution. Neuroscience. 2003. ↩︎
Cai X, Hodgson S, Wang H. Cholinergic therapy in Alzheimer's disease. Current Alzheimer Research. 2004. ↩︎
Felder CC, Bymaster FP, Ward J, DeLapp N. Therapeutic opportunities for muscarinic receptors in the central nervous system. Journal of Medicinal Chemistry. 2001. ↩︎
Michell B, Ebersole B, Jacobson K. Muscarinic antagonists in Parkinson's disease. Movement Disorders. 2005. ↩︎
Berger M, McIlroy S, Chen Y, et al. Acetylcholine and dopamine interactions in the basal ganglia. Journal of Neurochemistry. 2002. ↩︎
Conn PJ, Jones CK, Lindsley CW. Subtype-selective allosteric modulators of muscarinic receptors for the treatment of CNS disorders. Trends in Pharmacological Sciences. 2009. ↩︎