Chemogenetic approaches to identify metabolically important GPCR signaling pathways: Therapeutic implications

Abstract

DREADDs (Designer Receptors Exclusively Activated by a Designer Drug) are designer G protein-coupled receptors (GPCRs) that are widely used in the neuroscience field to modulate neuronal activity. In this review, we will focus on DREADD studies carried out with genetically engineered mice aimed at elucidating signaling pathways important for maintaining proper glucose and energy homeostasis. The availability of muscarinic receptor-based DREADDs endowed with selectivity for one of the four major classes of heterotrimeric G proteins (G_s , G_i , G_q , and G₁₂ ) has been instrumental in dissecting the physiological and pathophysiological roles of distinct G protein signaling pathways in metabolically important cell types. The novel insights gained from this work should inform the development of novel classes of drugs useful for the treatment of several metabolic disorders including type 2 diabetes and obesity.

Keywords: DREADD technology; G protein-coupled receptors; G proteins; mutant mouse models; obesity; type 2 diabetes.

FIGURE 1.

Chemical structures of CNO, clozapine, C21, and DCZ. All four agents can activate muscarinic receptor-based DREADDs with high potency and efficacy. However, clozapine acts on many other high-affinity targets in the CNS. Several studies have shown that CNO can be metabolized to clozapine. C21 and DCZ are CNO-derived, novel DREADD agonists that are not converted to clozapine in vivo (Chen et al., 2015; Nagai et al., 2020; Thompson et al., 2018). See text for details.

FIGURE 2

Coupling properties of muscarinic receptor-based DREADDs. The structural features of the various DREADDs are described in the main body of the text. The four DREADDs shown here harbor the same two point mutations in the transmembrane core (Y->C^3.33 and A->G^5.46; Ballesteros-Weinstein GPCR nomenclature) (Armbruster et al., 2007; Guettier et al., 2009; Inoue et al., 2019). While acetylcholine, the endogenous muscarinic receptor agonist, shows little or no activity at these DREADDs, they can be activated by CNO with high potency and efficacy. The different G proteins are named after the α-subunit contained within the G protein heterotrimer. Activated Gα subunits regulate the activity of distinct intracellular signaling pathways, but free βγ complexes, which are released after G protein activation, can also modulate cellular functions. Gα proteins are grouped into four major subfamilies (G_s: α_s, α_olf; G_i: α_i1, α_i2, α_i3, α_o, etc.; G_q: α_q, α₁₁; G₁₂: α₁₂, α₁₃). Abbreviations: Epac, exchange protein activated by cAMP; GIRK, G-protein-regulated inward-rectifier potassium channel; LARG, Leukemia-Associated RhoGEF; PLC, phospholipase C; PKA, protein kinase A; PKC, protein kinase C; RhoGEF, Rho guanine nucleotide exchange factor; ROCK; Rho-associated coiled-coil-containing protein kinase; VDCC, voltage-dependent Ca^2+-channel.

FIGURE 3.

Summary of in vivo ‘DREADD studies’ investigating signaling pathways regulating glucose and energy homeostasis. This cartoon summarizes data obtained with mutant mice expressing DREADDs in pancreatic β- and α-cells (a), adipocytes (b), hepatocytes (c), skeletal muscle cells (d), and AgRP (e) and POMC neurons (f) of the arcuate nucleus of the hypothalamus. For simplicity, only some key signaling factors/pathways and metabolic phenotypes are listed. See text for details.