Associate editor: J. Wess
Mouse models to study G-protein-mediated signaling

https://doi.org/10.1016/j.pharmthera.2003.10.005Get rights and content

Abstract

The G-protein-mediated signaling system has evolved as one of the most widely used transmembrane signaling mechanisms in eukaryotic organisms. Mammalian cells express many G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. This review focuses on recent data from studies in mutant mice, which have elucidated some of the roles of G-protein-mediated signaling in physiology and pathophysiology.

Introduction

Perception of extracellular stimuli is crucial to allow adaptation of the organism to altered internal and external conditions. Among the cell surface receptors, G-protein coupled receptors (GPCRs) constitute the largest group, and numerous hormones, neurotransmitters, or sensory stimuli-like light or odorants exert their biological effects through them. GPCRs activate heterotrimeric G-proteins, which consist of an α-subunit that binds and hydrolyses guanosine triphosphate (GTP), a β and a γ subunit Hepler & Gilman, 1992, Neer, 1995, Gudermann et al., 1996, Hamm, 1998. G-proteins are divided according to structural and functional similarities of their α subunits into 4 families: Gs, which couples receptors in a stimulatory fashion to adenylyl cyclase (AC); Gi/o, which couples receptors in an inhibitory fashion to AC and mediates the activation of potassium channels and the inhibition of voltage-dependent Ca2+ channels; Gq/11, which couples receptors to the stimulation of β isoforms of phospholipase C (PLC); and G12/13, which mediates the activation of the small GTPase Rho. The β and γ subunits of heterotrimeric G-proteins form an undissociable complex and represent a functional unit. Altogether, genes coding for 16 α, 5 β, and 12 γ subunits have been described (Simon et al., 1991), and some of the functional properties of these proteins are listed in Table 1, Table 2.

To convey a signal from an activated receptor to an effector, the heterotrimeric G-protein undergoes an activation-inactivation cycle, which allows the G-protein to function as a regulatable molecular switch. In the basal state, the guanosine diphosphate (GDP)-bound α subunit and the βγ complex form an inactive heterotrimer. On receptor activation, GDP dissociates from the α subunit and is replaced by GTP, which causes dissociation of the α subunit and the βγ complex. The GTP-bound α subunit and the βγ complex are now able to interact with effector proteins. A GTPase activity inherent to the G-protein α subunit terminates the G-protein activation by hydrolyzing GTP to GDP, leading to the reassociation of the G-protein heterotrimer.

It is not yet clear how the structural and functional complexities of the G-protein signaling system leads to a specific and adequate response to extracellular stimuli on the cellular level. It has, however, become increasingly obvious that heterotrimeric G-proteins are not simple switches that couple receptors to effectors but that they represent a central part of a sophisticated machinery, which is able to receive, process, and integrate information carried by extracellular signals. In addition to the 3 main components of the G-protein-mediated signaling system, the receptor, the heterotrimeric G-protein, and the effector, there are various proteins that are able to modulate the G-protein-mediated signaling process such as the regulators of G-protein signaling (RGSproteins), which are able to adjust the sensitivity of the signal transduction system De Vries et al., 2000, Neubig & Siderovski, 2002, Ross & Wilkie, 2002. While the current knowledge about G-protein-mediated signal transduction is mainly based on studies of cellular and subcellular structures, relatively little is known about its role in the development and function of the whole organism. This review focuses on recent data from studies in mutant mice, which have helped to elucidate some of the roles of G-proteins under physiological and pathological conditions.

Section snippets

Autonomic regulation of heart function

The parasympathetic regulation of the heart is mediated by Gi/o-coupled muscarinic acetylcholine (M2) receptors. One of the major M2/Gi/o-regulated effectors in the atrium are G-protein-regulated inward rectifier IK-Ach potassium channels (GIRK) consisting of Kir3.1 (GIRK1) and Kir3.4 (GIRK4) subunits, which are activated by βγ subunits released from activated Gi/o (see Fig. 1G; Stanfield et al., 2002). In mice lacking GIRK4, IK-Ach is absent (Wickman et al., 1998), and in mice lacking GIRK1,

Endocrine system and metabolism

Regulation of intracellular cAMP levels by Gs and Gi/o family G-proteins plays an important role in both endocrine and metabolic functions. Homozygous inactivation of the Gαs gene in mice leads to embryonic lethality during early postimplantation development (Yu et al., 1998). Heterozygotes, which inherited the intact allele from their fathers (Gαs(m−/p+)) or mothers (Gαs(m+/p−)), have distinct phenotypical manifestations, which lead to early postnatal death in the majority of animals (Yu et

Immune system

Immune cells express a variety of GPCRs, such as receptors for chemokines, lysophospholipids, prostanoids, leukotrienes, catecholamines, nucleotides, or histamine (Lombardi et al., 2002). Direct genetic evidence for a role of G-protein-mediated signaling in the immune system was so far only provided for the Gi family (see below and Fig. 1I), which plays an important role in the signaling of receptors for chemokines and various chemoattractant factors. However, there is reasonable evidence that

Nervous system

Most neurotransmitters of the central nervous system (CNS) act on GPCRs to modulate neuronal activity. The receptors are found presynaptically and postsynaptically and mediate relatively slow responses. Inhibitory modulation is mostly mediated by coupling of receptors to members of the Gi/o family, whereas Gq and Gs family members are primarily involved in excitatory responses.

Conclusions

The G-protein-mediated signaling system serves multiple functions in the mammalian organism. GPCRs and downstream signaling systems have been and will be prime targets for the development of drugs. During the last decade, many mouse models were produced by transgenic expression of wild-type or mutated components of the G-protein signaling system and by genetic targeting of their genes. These genetic models have provided a deep insight into the physiological role of G-protein-mediated signaling

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