Minireview: Novel aspects of M3 muscarinic receptor signaling in pancreatic β-cells

Abstract

The release of insulin from pancreatic β-cells is regulated by a considerable number of G protein-coupled receptors. During the past several years, we have focused on the physiological importance of β-cell M3 muscarinic acetylcholine receptors (M3Rs). At the molecular level, the M3R selectively activates G proteins of the G(q) family. Phenotypic analysis of several M3R mutant mouse models, including a mouse strain that lacks M3Rs only in pancreatic β-cells, indicated that β-cell M3Rs play a key role in maintaining blood glucose levels within a normal range. Additional studies with transgenic M3R mouse models strongly suggest that strategies aimed to enhance signaling through β-cell M3Rs may prove useful in the treatment of type 2 diabetes. More recently, we analyzed transgenic mice that expressed an M3R-based designer receptor in a β-cell-specific fashion, which enabled us to chronically activate a β-cell G(q)-coupled receptor by a drug that is otherwise pharmacologically inert. Drug-dependent activation of this designer receptor stimulated the sequential activation of G(q), phospholipase C, ERK1/2, and insulin receptor substrate 2 signaling, thus triggering a series of events that greatly improved β-cell function. Most importantly, chronic stimulation of this pathway protected mice against experimentally induced diabetes and glucose intolerance, induced either by streptozotocin or by the consumption of an energy-rich, high-fat diet. Because β-cells are endowed with numerous receptors that mediate their cellular effects via activation of G(q)-type G proteins, these findings provide a rational basis for the development of novel antidiabetic drugs targeting this class of receptors.

Figure 1.

M3R signaling in pancreatic β-cells. We recently demonstrated that RGS4 (16) and SPL (19) act as potent negative regulators of M3R signaling in pancreatic β-cells. BRET studies (19) suggest that SPL acts as an adaptor protein capable of recruiting RGS4 to the ligand-activated M3R/G_q protein complex, thus limiting the lifetime of M3R-activated G protein α_q/11 subunits. Such a model is consistent with the observation that the lack of SPL or RGS4 affects M3R signaling to a similar degree (19). The figure was modified based on a scheme shown in Ref. . DAG, diacylglycerol; ER, endoplasmic reticulum; K-ATP, ATP-sensitive K⁺ channel; VDCC, voltage-dependent Ca²⁺ channel.

Figure 2.

Structure of M3R-based DREADDs used to explore GPCR regulation of β-cell function. All 3 designer GPCRs shown in this figure contain the Y^3.33C and A^5.46G point mutations in transmembrane domains TM3 and TM5, respectively (red X marks). They lack the ability to bind ACh but can be activated by CNO with high potency and efficacy. A, Structure of the Rq and Rs DREADDs (alternative names: rM3Dq and rM3Ds, respectively) used in the study by Guettier et al (24). B, Structure of the Rq(R165L) DREADD (alternative name: rM3Darr). This designer receptor is unable to activate heterotrimeric G proteins but promotes arrestin-dependent signaling (47). It contains the R165L^3.50 point mutation at the bottom of TM3 within the highly conserved DRY motif. In the presence of CNO, the Rq(R165L) DREADD is able to recruit arrestin-2 and -3 and to mediate arrestin-dependent downstream signaling (47).

Figure 3.

A GPCR/G_q-dependent signaling cascade predicted to be operative in pancreatic β-cells. The scheme shown here is based primarily on data reported by Jain et al (25). It depicts some of the key signaling molecules involved in G_q-mediated activation of IRS2 expression and function in β-cells. Enhanced IRS2 activity is predicted to stimulate the expression of Pdx1 and other key β-cell transcription factors, activating the promoters of many genes important for β-cell function (25).