Elsevier

Biochemical Pharmacology

Volume 114, 15 August 2016, Pages 40-52
Biochemical Pharmacology

Review
The impact of RGS and other G-protein regulatory proteins on Gαi-mediated signaling in immunity

https://doi.org/10.1016/j.bcp.2016.04.005Get rights and content

Abstract

Leukocyte chemoattractant receptors are members of the G-protein coupled receptor (GPCR) family. Signaling downstream of these receptors directs the localization, positioning and homeostatic trafficking of leukocytes; as well as their recruitment to, and their retention at, inflammatory sites. Ligand induced changes in the molecular conformation of chemoattractant receptors results in the engagement of heterotrimeric G-proteins, which promotes α subunits to undergo GTP/GDP exchange. This results in the functional release of βγ subunits from the heterotrimers, thereby activating downstream effector molecules, which initiate leukocyte polarization, gradient sensing, and directional migration. Pertussis toxin ADP ribosylates Gαi subunits and prevents chemoattractant receptors from triggering Gαi nucleotide exchange. The use of pertussis toxin revealed the essential importance of Gαi subunit nucleotide exchange for chemoattractant receptor signaling. More recent studies have identified a range of regulatory mechanisms that target these receptors and their associated heterotrimeric G-proteins, thereby helping to control the magnitude, kinetics, and duration of signaling. A failure in these regulatory pathways can lead to impaired receptor signaling and immunopathology. The analysis of mice with targeted deletions of Gαi isoforms as well as some of these G-protein regulatory proteins is providing insights into their roles in chemoattractant receptor signaling.

Introduction

Upon ligand binding chemoattractant GPCRs undergo conformational rearrangements that change their interaction with signal transducer proteins [1], [2], [3]. The major proximal signal transducers of an activated GPCR are their cognate heterotrimeric G-proteins, although ligand activated GPCRs engage other proteins including G-protein receptor kinases (GRKs) and β-arrestins [4], [5], [6]. Chemoattractant receptors predominately couple to the Gi family of heterotrimeric G-proteins as their signaling is sensitive to treatment with pertussis toxin, which ADP ribosylates a cysteine residue near the C-termini of αi subunit [7], [8]. This inhibits Gαi from undergoing GPCR-mediated nucleotide exchange. The GTP/GDP binding status of Gαi controls its interaction with the other members of the trimeric G protein, the Gβγ dimer [2], [9]. Composed of one of 5 β and one of 12 γ subunits, the Gβγ dimers, once assembled, are inseparable. Receptor activation and Gαi subunit nucleotide exchange causes the Gαi subunit and Gβγ subunits to functionally dissociate. GTP bound Gαi and the freed Gβγ can then engage downstream effector molecules. However, it is the Gβγ effectors that are the most crucial for gradient sensing and directional cell migration [10]. The activated Gα subunits remain only transiently GTP bound as they possess an intrinsic GTPase activity, which converts their bound GTP to GDP. This encourages the reassembly of the heterotrimeric G-protein terminating signaling, but also allows the G-protein to engage other activated receptors, thereby initiating another round of signaling. The intrinsic GTPase activity of Gα subunits can be greatly enhanced by GTPase activating proteins termed GAPs. Most Gαi GAPs contain a Regulator of G-protein Signaling (RGS) domain [11], [12], [13]. Other G-protein regulatory proteins include those that have a G-protein regulatory (GPR) domain, also called a GoLoco motif, which acts as a guanine nucleotide dissociation inhibitor (GDI) much like Gβγ [14], [15], [16]. Both RGS and GPR domain containing proteins impact Gi signaling. This review will focus predominately on the Gαi proteins and their regulators in the context of chemokine and chemoattractant receptor signaling.

Section snippets

Gi proteins

The heterotrimeric G proteins are divided into four classes; Gi, Gq, G12/13, and Gs, based on the amino acid sequences of their α subunit [2]. The expression levels of the various Gα subunits in murine leukocytes based on mRNA sequencing are shown in Table 1. The “inhibitory class” of heterotrimeric G-proteins were originally named based on the ability of Gi/o proteins to inhibit adenylyl cyclase activity. Based on amino acid sequence homology the Gi family now includes not only Gi/o, but also

Ric-8A

Ric-8 was identified in Caenorhabditis elegansbased on its role in asymmetric cell divisions during early development [58], [59], [60]. A human homolog, Ric-8A, was shown to recruit a signaling complex to the cell cortex that helped orient the mitotic spindle in response to spatial clues [61]. Targeting ric8 in mice caused early embryonic lethality, however, derived ric8−/− embryonic cell lines had pleiotropic G protein signaling defects and major losses of Gαi1/2, Gαq, and Gα13 proteins due to

Conclusions

Many of the GPCRs expressed by leukocytes function as chemoattractant receptors. The signals emanating from these receptors recruit leukocytes to inflammatory sites; organize the positioning of leukocytes in immune organs, help maintain the overall architecture of immune organs; coordinate the movements of leukocytes through tissues; and facilitate the trafficking of leukocytes into and out of lymph nodes, the bone marrow, skin, and the gut associated lymphoid tissues (GALT). The signaling

Conflict of interest

There are no conflict of interests.

Acknowledgements

The author would like to thank the members of his laboratory that contributed to some of the studies described in this review and thank Dr. Anthony Fauci for his long standing support. The intramural program of the National Institutes of Allergy and Infectious Diseases supported some of the research described in this review.

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