Novel insights into the function of β-cell M3 muscarinic acetylcholine receptors: therapeutic implications

Abstract

Impaired function of pancreatic β-cells is one of the hallmarks of type 2 diabetes. β-cell function is regulated by the activity of many hormones and neurotransmitters, which bind to specific cell surface receptors. The M(3) muscarinic acetylcholine receptor (M3R) belongs to the superfamily of G protein-coupled receptors and, following ligand dependent activation, selectively activates G proteins of the G(q/11) family. Recent studies with M3R mutant mice strongly suggest that β-cell M3Rs play a central role in promoting insulin release and maintaining correct glucose homeostasis. In this review, we highlight recent studies indicating that β-cell M3Rs and components of downstream signaling pathways might represent promising new targets for the treatment of type 2 diabetes.

Figure 1

GPCR-dependent signaling pathways involved in regulating insulin release from pancreatic β-cells. Like essentially all other cell types, pancreatic β-cells express many different GPCRs [2,5] which are linked to different functional classes of heteromeric G proteins, primarily (a) G_q/11-, (b) G_s-, and (c) G_i -type G proteins. The different classes of G proteins activate or inhibit specific signaling pathways or networks that play important roles in modulating glucose-dependent insulin release. Please note that only the major effectors known to be critical for G protein-regulated insulin secretion are shown in this figure. In general, activation of G_s- and G_q/11- coupled receptors enhances insulin release, whereas activation of G_i-coupled receptors usually leads to inhibition of insulin secretion [2]. This figure shows only a few representative GPCRs found on the surface of pancreatic β-cells. The M₃ muscarinic receptor (M3R), the main focus of this review, is shown boxed. AC, adenylyl cyclase; ACh, acetylcholine; GLP-1, glucagon-like peptide 1; CCK, cholecystokinin; NPY, neuropeptide Y; CGRP, calcitonin gene-related peptide; GIP, glucose-dependent insulinotropic peptide i; M3R, M₃ muscarinic receptor; PACAP, pituitary adenylate cyclase-activating polypeptide; VIP, vasoactive intestinal polypeptide; VDCC, voltage-dependent Ca²⁺ channel; DAG, diacylglycerol; IP₃, inositol 1,4,5-trisphosphate; PKA, protein kinase A; PKC, protein kinase C; ER, endoplasmic reticulum; Epac, exchange protein activated by cAMP; K-ATP, ATP-sensitive K⁺ channel.

Figure 2

Scheme highlighting several key components involved in M3R-mediated facilitation of insulin release from pancreatic β-cells. Recent studies suggest that RGS4 is a potent negative regulator of M3R signaling in pancreatic β-cells [21] and that ankyrin-B and protein kinase D1 (PKD1) are required for M3R-mediated augmentation of insulin release (note that RGS4, like ankyrin-B and PKD1, is located in the cytoplasm). Whereas ankyrin-B plays a role in stabilizing IP₃ receptors present in the ER (IP₃R; [44]), PKD1 is thought to promote membrane fission events necessary for insulin exocytosis [45].

Figure 3

Central role of β-cell M3Rs in maintaining proper blood glucose homeostasis. Food intake results in the activation of glucoreceptors in the gut, liver, and brain, leading to enhanced central parasympathetic outflow [7]. Acetylcholine (ACh) released from pancreatic parasympathetic nerve terminals promotes insulin release via stimulation of β-cell M3Rs, due to an increase in intracellular calcium levels and the enhanced formation of several other second messengers [7]. Disruption of this pathway in mice lacking β-cell M3Rs leads to reduced insulin release and impaired glucose tolerance, whereas overexpression of M3Rs in β-cells of transgenic mice results in enhanced insulin secretion and greatly improved glucose tolerance [12].

Figure 4

Studies with clozapine-N-oxide (CNO)-responsive mutant M3Rs endowed with distinct G protein-coupling properties. (a) Structure of the M3R-based R-q and R-s designer receptors that can be selectively activated by CNO. The Y148C and A238G point mutations (rat M3R sequence) prevent acetylcholine binding to the M3R [35, 36]. Note that the Y148C and A238G point mutations (rat M3R sequence) correspond to the Y149C and A239G substitutions in the human M3R [35]. Both designer receptors can be efficiently activated by CNO, a pharmacologically inert compound. CNO binding to R-q leads to the selective activation of G proteins of the G_q/11 family [35, 36]. On the other hand, binding of CNO to R-s results in the selective activation of G_s [36]. (b) Phenotypic features of transgenic mice that express the R-q designer receptor selectively in pancreatic β-cells (β-CNO-R-q Tg mice). This short summary lists the key phenotypes that were observed after acute or chronic treatment of β-CNO-R-q Tg mice with CNO [36].