Professional technology integration to support the entire R&D process

Within the G protein-coupled receptor (GPCR) superfamily, muscarinic acetylcholine receptors (CHRM, or mAChRs) act as the primary conduits for cholinergic signaling. From modulating cognitive function in the central nervous system to maintaining physiological homeostasis in peripheral organs, the five mAChR subtypes (M1–M5) orchestrate a sophisticated regulatory network.

As our understanding of the pathophysiology underlying Alzheimer’s disease, schizophrenia, Chronic Obstructive Pulmonary Disease (COPD), and overactive bladder (OAB) continues to evolve, the distinct therapeutic potential of each CHRM subtype has come into sharp focus. Consequently, they have emerged as high-priority targets in neuropharmacology, respiratory medicine, and cardiovascular research.

Capitalizing on our established GPCR drug discovery platform, ReqBio has successfully engineered a complete suite of stable cell lines covering the M1 through M5 subtypes. By offering functional assays for both the Gq-coupled (IP-One) and Gi-coupled (HTRF cAMP) pathways, we provide global researchers with high-performance, reliable tools to streamline early-stage lead identification and optimization.

I. CHRM1–5: Five Distinct Subtypes with High Homology and Divergent Functions

All five CHRM1–5 subtypes are single-subunit, seven-transmembrane (7TM) glycoproteins belonging to Class A (rhodopsin-like) GPCRs. While their core structures are highly conserved, they are categorized into two distinct functional groups based on their G-protein coupling profiles:

Table 1: G Protein-Coupled Types

Structure Dictates Function: Despite the high degree of homology across their transmembrane domains, the five subtypes exhibit significant sequence divergence within the third intracellular loop (ICL3). This structural variation is the primary determinant of G-protein coupling specificity. Understanding these differences provides the structural framework for developing subtype-selective ligands, making the screening of highly specific agonists and antagonists achievable. 

II. Signal Transduction: Differential Detection of Gq vs. Gi Pathways

 Gq-Coupled Subtypes (M1, M3, M5): Upon activation, these receptors engage PLCβ to hydrolyze PIP2, generating IP3 and DAG. IP3 triggers the release of Ca²⁺ from the endoplasmic reticulum, while DAG activates PKC. The gold standard for measuring the activity of these receptors is the IP-One assay, which detects the accumulation of IP1 (a more stable metabolite than IP3). Notably, M3 can also partially activate the Gs pathway, leading to elevated cAMP levels.

Gi-Coupled Subtypes (M2, M4): Activation leads to the inhibition of adenylyl cyclase (AC) via the Gαi subunit, resulting in decreased cAMP levels. Simultaneously, the βγ subunits activate GIRK potassium channels and inhibit calcium channels. The gold standard for assessing these receptors is the HTRF cAMP assay, where agonists are evaluated by their ability to inhibit forskolin-induced cAMP elevation.

Of particular interest is M4, which can also signal through β-arrestin in a G-protein-independent manner. This characteristic adds a new dimension to drug discovery by enabling the development of biased ligands.

III. Targeting CHRM: From Classic Pharmacology to Frontier

TherapeuticsCurrent drug development targeting CHRM primarily focuses on three major therapeutic areas: neurodegenerative diseases, respiratory disorders, and psychiatric conditions. The core R&D strategies center on small-molecule agonists, antagonists, and allosteric modulators.

Representative Marketed Drugs:

Ipratropium & Tiotropium (Primarily M3, with M1/M2 antagonism): Used for bronchodilation in asthma and COPD.

Atropine (Non-selective M1–M5 antagonist): Used to treat bradycardia.

Tolterodine & Solifenacin (M3-selective antagonists): Standard-of-care treatments for Overactive Bladder (OAB).

Pipeline Highlights (Selected):

Table 2: Drugs in the R&D Pipeline

 As evidenced by the current clinical pipeline, the M4 receptor has emerged as the most prominent subtype, primarily due to its unique regulatory role in schizophrenia and Alzheimer’s disease. By modulating the dopaminergic system without the risk of addiction, it offers a transformative therapeutic approach. Meanwhile, the M1 and M3 subtypes continue to be focal points in addressing cognitive impairment and smooth muscle disorders, respectively. 

IV. Accelerating CHRM Drug Discovery: ReqBio’s Comprehensive Cell Models and Data Advantages

 In GPCR drug discovery, subtype selectivity and signaling pathway specificity are the two critical benchmarks for evaluating lead compounds. To meet diverse screening requirements, ReqBio has developed a robust portfolio of stable cell models covering all five CHRM1–5 subtypes, optimized for both Gq-pathway (IP-One) and Gi-pathway (cAMP) functional assays.

Table 3:ReqBio CHRM Series: Stable Cell Line Portfolio

Validation Data & Advantage Analysis

1.CHO-K1 Human CHRM1 Cell Line（RQP71180）

Figure 4. HTRF IP-One Assay with CHRM1 CHO-K1(C3)

Assay Principle: M1 is a Gq-coupled receptor that triggers IP1 accumulation upon activation. We utilize the HTRF IP-One kit, employing a competitive immunoassay format to quantify IP1 levels.

Data Highlights: Figure 4 illustrates a classic dose-response profile (increasing agonist concentration leads to a corresponding rise in the IP1 signal) with a well-defined EC50and a robust signal window (>5-fold). This demonstrates that the cell line is highly sensitive to reference agonists (such as Carbachol), making it an ideal tool for agonist potency ranking and determining the IC50 of prospective antagonists.

2. CHO-K1 Human CHRM2 Cell Line（RQP71427）

Figure 5: Surface expression or total protein levels of CHRM2 in CHO cells are confirmed via flow cytometry or Western Blot, ensuring high stability and receptor density for model reliability.

Figure 6: The HTRF cAMP assay demonstrates that M2 agonists induce a dose-dependent inhibition of forskolin-stimulated cAMP levels (characteristic sigmoidal dose-response curve). This model effectively distinguishes between full agonists, partial agonists, and inverse agonists, making it highly suitable for high-throughput screening (HTS) of drugs targeting the Gi pathway.

3. CHO-K1 Human CHRM3 Cell Line（RQP71512）

Figure 7: Stable expression of CHRM3 is confirmed, ensuring consistent receptor density across cell generations.

 Figure 8: The IP-One assay exhibits a classic dose-dependent agonist response curve. Given that M3 is a central therapeutic target for COPD and Overactive Bladder (OAB), this model enables highly efficient screening for M3-selective antagonists (such as tiotropium analogs) and allows for the rigorous assessment of subtype cross-reactivity.

4.CHO-K1 Human CHRM4 Gα15 Cell Line（RQP71528）

Unique Design: While wild-type M4 naturally couples with Gi (traditionally measured via cAMP inhibition), this cell line has been engineered to co-express the Gα15 protein. This modification allows M4 activation to be "re-routed" through the Gα15 pathway to activate PLCβ, making the model simultaneously compatible with IP-One assays.

Figure 9 (cAMP): Demonstrates the dose-dependent inhibition of cAMP by M4 agonists, successfully validating the functional integrity of the classic Gi pathway.

Figure 10 (IP-One): Shows that, following engineering, M4 agonists can also induce IP1 accumulation. This provides a powerful alternative for researchers who prefer not to use cAMP assays. This dual-mode validation capability is the standout feature of this specific cell model.

Summary of Core Product Advantages：

 Conclusion

The five subtypes, CHRM1–5, constitute the core network of cholinergic signaling. Their differentiated distribution and functions provide a rich array of targets for precision medical intervention. This field is currently witnessing a new surge in R&D—spanning from M1/M4 agonists for Alzheimer’s disease and M4-selective agonists for schizophrenia, to M3 antagonists for COPD and M1 antagonists for multiple sclerosis.

With a deep understanding of GPCR drug screening, ReqBio has launched a suite of high-quality cell models that offer full subtype coverage and dual-detection platforms. We are committed to empowering the global discovery and translation of innovative medicines.