Structure-function analysis of muscarinic acetylcholine receptors

Lifetime of muscarinic receptor-G-protein complexes determines coupling efficiency and G-protein subtype selectivity

G-protein-coupled receptors (GPCRs) are essential for the detection of extracellular stimuli by cells and transfer the encoded information via the activation of functionally distinct subsets of heterotrimeric G proteins into intracellular signals. Despite enormous achievements toward understanding GPCR structures, major aspects of the GPCR-G-protein selectivity mechanism remain unresolved. As this can be attributed to the lack of suitable and broadly applicable assays, we set out to develop a quantitative FRET-based assay to study kinetics and affinities of G protein binding to activated GPCRs in membranes of permeabilized cells in the absence of nucleotides. We measured the association and dissociation kinetics of agonist-induced binding of Gi/o, Gq/11, Gs, and G12/13 proteins to muscarinic M1, M2, and M3 receptors in the absence of nucleotides between fluorescently labeled G proteins and receptors expressed in mammalian cells. Our results show a strong quantitative correlation between not the on-rates of G-protein-M3-R interactions but rather the affinities of Gq and Go proteins to M3-Rs, their GPCR-G-protein lifetime and their coupling efficiencies determined in intact cells, suggesting that the G-protein subtype-specific affinity to the activated receptor in the absence of nucleotides is, in fact, a major determinant of the coupling efficiency. Our broadly applicable FRET-based assay represents a fast and reliable method to quantify the intrinsic affinity and relative coupling selectivity of GPCRs toward all G-protein subtypes.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Use of muscarinic agonists in the treatment of Sjögren's syndrome

Two muscarinic agonists (pilocarpine and cevimeline) have recently been approved for the treatment of symptoms of xerostomia in Sjögren's syndrome (SS). These agents stimulate the M1 and M3 receptors present on salivary glands, leading to increased secretory function. The use of these agents emphasizes the importance of neuroendocrine mechanisms in SS, which is considered an autoimmune disorder. We review recent studies on the release of cytokines and metalloproteinases in SS-affected glands and their influence on the release of and response to neurotransmitters. Also, we review the structure and function of muscarinic receptors as they may relate to SS and the potential use of novel muscarinic agonists in SS.