Model Organisms in G Protein-Coupled Receptor Research

Abstract

The study of G protein-coupled receptors (GPCRs) has benefited greatly from experimental approaches that interrogate their functions in controlled, artificial environments. Working in vitro, GPCR receptorologists discovered the basic biologic mechanisms by which GPCRs operate, including their eponymous capacity to couple to G proteins; their molecular makeup, including the famed serpentine transmembrane unit; and ultimately, their three-dimensional structure. Although the insights gained from working outside the native environments of GPCRs have allowed for the collection of low-noise data, such approaches cannot directly address a receptor's native (in vivo) functions. An in vivo approach can complement the rigor of in vitro approaches: as studied in model organisms, it imposes physiologic constraints on receptor action and thus allows investigators to deduce the most salient features of receptor function. Here, we briefly discuss specific examples in which model organisms have successfully contributed to the elucidation of signals controlled through GPCRs and other surface receptor systems. We list recent examples that have served either in the initial discovery of GPCR signaling concepts or in their fuller definition. Furthermore, we selectively highlight experimental advantages, shortcomings, and tools of each model organism.

Graphical abstract

Fig. 1.

Adhesion GPCRs as mechanically activated receptors. (A) Early in Schwann cell development, Laminin-211 interacts with the N terminus of Gpr126 in a conformation that prevents cAMP accumulation through suppression of basal signaling, potentially allowing the Schwann cell to remain immature. After Laminin-211 polymerization in the basal lamina (purple shadow), Laminin-211 facilitates an active conformation of the receptor, perhaps by physical removal of the N terminus, which exposes a cryptic ligand (noted by “S”) (Petersen et al., 2015). (B) The aGPCR Latrophilin/Cirl is present in mechanosensory neurons of Drosophila, where it regulates their sensitivity toward mechanical stimulation and maintains a physiological signal-to-noise ratio. Loss of Latrophilin/Cirl results in numbness, amblyacousia, and proprioception deficits (Monk et al., 2015; Scholz et al., 2015).

Fig. 2.

Cellular context dictates the nature of GPCR signaling. The neuropeptide PDF activates a GPCR in target neurons of the circadian neural circuits in the Drosophila brain. M and E pacemakers refer to neurons that are biased to involvement in either a morning or evening bout of locomotor activity. In vivo observations indicate that PDFR signalosome components differ in the different target neurons: PDFR associates with AC3 in M cells, but in the E cell subgroup, PDF signaling relies on AC78C (AC8) and at least one other (currently unidentified) AC. This figure was originally published by Duvall and Taghert (2013) and is reproduced here with permission from the Journal of Biological Rhythms.

Fig. 3.

Modern tools for investigating GPCR signaling in vivo. (A) Representation of intact (right) and dissected (left) Drosophila larva expressing the FRET-based sensor for monitoring cAMP changes, Epac1-camps. (B) YFP and time-resolved pseudocolor FRET images of Drosophila motoneurons expressing Epac1-camps. The application of agonist mediates the activation of the endogenous receptors, which results in production of cAMP. This cAMP increase is revealed by a loss in FRET caused by a conformational rearrangement of the sensor, induced by cAMP binding. (C) Cartoon depicting the chimeric “Opto-XR” approach, whereby rhodopsin cDNA is fused with wild-type GPCR cDNA intracellular loops and tail to generate a photosensitive receptor system capable of spatiotemporal engagement of canonical GPCR signaling pathways such as G_q, G_s, and G_i or arrestin recruitment in selected cell types when combined with viral and genetic approaches in vivo. (D) Cartoon representing chemogenetic GPCRs termed DREADDs, which selectively respond to the ligand CNO to engage canonical GPCR signaling pathways in selected cell types when also combined with viral or mouse genetic approaches. CFP, cyan fluorescent protein; CNO, cloxapine–nitrous oxide; ERK, extracellular signal-regulated kinase; IP3, inositol trisphosphate; MAPK, mitogen-activated protein kinase; PKC, protein kinase C; YFP, yellow fluorescent protein. Bar, 10 μm.