Regulation of G protein-coupled receptor trafficking

G protein-coupled receptors (GPCRs) mediate cellular responses to diverse extracellular stimuli to play a vital role in the control of physiology and behaviour. GPCR trafficking is of fundamental importance for the regulation of GPCRs signaling. In this mini review, we will discuss some of the recent findings on the mechanisms that regulate GPCR trafficking, which include (i) large dense-core vesicle (LDCV)-associated GPCR delivery which could be a general cell biological mechanism for rapid modulation of membrane receptors in response to certain stimuli; (ii) lateral diffusion of GPCRs in the plasma membrane for rapid change of the number of neurotransmitter receptors during synaptic plasticity and (iii) constitutive internalization of GPCRs, that contributes to receptor resensitization and distribution, including axonal polarization.     

Role and modulation of G protein-coupled receptor signaling in inflammatory processes

Many extracellular stimuli, such as neurotransmitters, hormones, chemokines, proteinases, inflammatory mediators, odorants, and light, are recognized by the superfamily of G protein-coupled receptors (GPCRs). Immune cells express GPCRs for classical chemoattractants, chemokines, neuropeptides, and neurotransmitters. GPCRs transmit information by interacting with heterotrimeric G proteins, resulting in rapid and transient signaling. The signal given by GPCRs is terminated rapidly by the activity of regulators of G protein signaling (RGS). In addition, GPCR responsiveness diminishes after repeated or prolonged exposure to the agonist. This process of homologous desensitization of GPCRs is dependent on receptor phosphorylation by G protein-coupled receptor kinases (GRKs). In this review, we describe the role of RGS and GRKs in the regulation of GPCR signaling in the immune system, with special emphasis on the role of changes in GRKs and RGS expression during (auto) immune processes. Since altered regulation of GPCR signaling can influence disease states, the molecules involved in this process can also represent attractive therapeutic targets.     

Concepts of GPCR‐controlled navigation in the immune system

G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.     

G-protein-coupled receptor signaling, RGS proteins, and lymphocyte function

The positioning of lymphocytes in immune organs and the migration of lymphocytes that occurs during normal immune surveillance and following immune activation depends on appropriate signaling through receptors that couple to heterotrimeric G-proteins. In addition other mediators that affect lymphocyte function, such as histamine, purine nucleosides, C5A, prostaglandins, leukotrienes, serotonin, epinephrine, opioids, and certain phospholipids, also signal through G-protein-coupled receptors (GPCRs). Downstream of heterotrimeric G-proteins are a limited number of downstream effectors, which, in turn, activate a large number of other signaling molecules, many of which are shared with other signaling pathways, such as those activated by antigen receptors, coreceptors, and adhesion receptors. Crucial to signaling through GPCRs are finely developed regulatory systems, which control the activation of heterotrimeric G-proteins and their interactions with their immediate downstream effectors. This review will focus on the overall importance of GPCR signaling in lymphocyte function and an upstream regulatory system present in lymphocytes, which fine tunes heterotrimeric G-protein signaling.     

G蛋白偶联受体激酶在心血管疾病中的作用

G-protein-coupled receptors (GPCRs) are hitherto the largest family of membrane receptors. They activate associated heterotrimeric G-proteins and participate in the regulation of intracellular signaling pathways, which are accomplished via ligand binding to them. G-protein-coupled receptor kinases (GRKs) are key modulators of GPCR signaling. GRKs, in concert with β-arrestins, classically lead to desensitization and internalization of GPCR, thus preventing hyperactivation of GPCR second messenger cascades. Changes in the GRKs expression and regulation have featured prominently in many cardiovascular diseases, including heart failure, myocardial infarction, cardiac hypertrophy, and hypertension. Intensively studying the role of GRKs in cardiovascular diseases contributes to expounding the mechanism of correlated diseases. Herein, we review the role of GRKs in cardiovascular pathophysiology.     

A review of neurohormone GPCRs present in the fruitfly Drosophila melanogaster and the honey bee Apis mellifera

G protein-coupled receptor (GPCR) genes are large gene families in every animal, sometimes making up to 1-2% of the animal's genome. Of all insect GPCRs, the neurohormone (neuropeptide, protein hormone, biogenic amine) GPCRs are especially important, because they, together with their ligands, occupy a high hierarchic position in the physiology of insects and steer crucial processes such as development, reproduction, and behavior. In this paper, we give a review of our current knowledge on Drosophila melanogaster GPCRs and use this information to annotate the neurohormone GPCR genes present in the recently sequenced genome from the honey bee Apis mellifera. We found 35 neuropeptide receptor genes in the honey bee (44 in Drosophila) and two genes, coding for leucine-rich repeats-containing protein hormone GPCRs (4 in Drosophila). In addition, the honey bee has 19 biogenic amine receptor genes (21 in Drosophila). The larger numbers of neurohormone receptors in Drosophila are probably due to gene duplications that occurred during recent evolution of the fly. Our analyses also yielded the likely ligands for 40 of the 56 honey bee neurohormone GPCRs identified in this study. In addition, we made some interesting observations on neurohormone GPCR evolution and the evolution and co-evolution of their ligands. For neuropeptide and protein hormone GPCRs, there appears to be a general co-evolution between receptors and their ligands. This is in contrast to biogenic amine GPCRs, where evolutionarily unrelated GPCRs often bind to the same biogenic amine, suggesting frequent ligand exchanges ("ligand hops") during GPCR evolution.     

G protein coupled pattern recognition receptors expressed in neutrophils: Recognition,activation/modulation,signaling and receptor regulated functions

Neutrophils, the most abundant white blood cell in human blood, express receptors that recognize damage/microbial associated pattern molecules of importance for cell recruitment to sites of inflammation. Many of these receptors belong to the family of G protein coupled receptors (GPCRs). These receptor-proteins span the plasma membrane in expressing cells seven times and the down-stream signaling rely in most cases on an activation of heterotrimeric G proteins. The GPCRs expressed in neutrophils recognize a number of structurally diverse ligands (activating agonists, allosteric modulators, and inhibiting antagonists) and share significant sequence homologies. Studies of receptor structure and function have during the last 40 years generated important information on GPCR biology in general; this knowledge aids in the overall understanding of general pharmacological principles, governing regulation of neutrophil function and inflammatory processes, including novel leukocyte receptor activities related to ligand recognition, biased/functional selective signaling, allosteric modulation, desensitization, and reactivation mechanisms as well as communication (receptor transactivation/cross-talk) between GPCRs. This review summarizes the recent discoveries and pharmacological hallmarks with focus on some of the neutrophil expressed pattern recognition GPCRs. In addition, unmet challenges, including recognition by the receptors of diverse ligands and how biased signaling mediate different biological effects are described/discussed.     

Regulation of G-protein-coupled signaling pathways in allergic inflammation

Allergic diseases such as asthma are elicited by maladaptive activation of immune cells such as mast cells and lymphocytes by otherwise innocuous allergens. The numerous mediators secreted by such cells promote both acute inflammation and, in many instances, chronic tissue remodeling. Most of these compounds exert their effects on end-organ targets such as epithelial and endothelial cells and airway smooth muscle by activating G-protein-coupled receptors (GPCRs), which are by far the most abundant type of cell surface receptor. Since GPCRs are also the most common target of allergy therapeutics, a better understanding of their intracellular signaling mechanisms is vital to improve the efficacy of such drugs or to develop new targets. In this review, we focus on some of the new regulatory elements that control the duration and amplitude of GPCR signal transduction pathways in immune effector cells and end-organ structural cells affected by allergic inflammation.     

Heterotrimeric G protein signaling: role in asthma and allergic inflammation

Asthma and rhinitis are pathophysiologic conditions associated with a prototypical allergic response to inhaled allergens consisting of both neuromechanical and inflammatory components. Heptahelical receptors that bind guanosine triphosphate-binding proteins (G proteins), referred to as G protein-coupled receptors (GPCRs), have been intimately linked with asthma and allergic inflammation for many years. G protein signaling mediates responses throughout the immune, nervous, and muscular systems that might contribute to the pathogenesis of allergic processes and asthma. For example, GPCR agonists or antagonists are used as therapies for asthma either by promoting airway smooth muscle relaxation (beta2 adrenergic receptor agonists) or by inhibiting inflammation in the nasal mucosa and airways (cysteinyl leukotriene receptor antagonists). The focus of this review is to explore how downstream signaling cascades elicited by GPCR activation contribute to the allergic phenotype and the mechanism by which pharmaceuticals alter signaling to generate a therapeutic effect. We also discuss physiologic modulators of G protein signaling, such as regulator of G protein signaling proteins and G protein receptor kinases, inasmuch as they represent potential new therapeutic targets in the treatment of atopy and other inflammatory conditions.     

Role of RGS proteins in regulating the migration of B lymphocytes

The migration of B lymphocytes into distinct microenvironments in secondary lymphoid tissues and maintenance of cells in these micro-domains is strictly structured and likely supports the proper regulation of immune responses to both foreign and self-antigens. Chemokines' and other chemoattactants' signals serve as signposts to direct cell migration. They signal cells through heptahelical receptors, which couple to heterotrimeric G proteins (G protein-coupled receptors or GPCRs). The regulation of the signals transduced through these receptors ultimately determines the positioning of cells in lymphoid tissues. A variety of mechanisms regulate GPCR signaling including a family of approximately 25 proteins termed regulators of G protein signaling (RGS). These proteins act as GTPase activating proteins for G alpha subunits and can also function as effector antagonists of specific G alpha subunits, thereby attenuating signaling through GPCRs such as chemokine receptors. RGS proteins possess some degree of receptor and G alpha subunit specificity. Thus, the particular spectrum of RGS proteins and their expression levels within a cell will determine the duration and magnitude of G protein signaling initiated by chemokines. In this review we illustrate the role RGS proteins have in regulating B cell signaling responses to chemoattractant stimuli during homeostasis as well as during an immune response.     

GRK5 deficiency exaggerates inflammatory changes in TgAPPsw mice

Background  

Deficiency of membrane G-protein coupled receptor (GPCR) kinase-5 (GRK5) recently has been linked to early AD pathogenesis, and has been suggested to contribute to augmented microglial activation in vitro by sensitizing relevant GPCRs. However, GRK5 deficient mice did not show any signs of microgliosis, except for their moderate increase in axonal defects and synaptic degenerative changes during aging. We have speculated that one possible reason for the absence of microgliosis in these animals might be due to lack of an active inflammatory process involving activated GPCR signaling, since GRKs only act on activated GPCRs. The objective of this study was to determine whether the microgliosis is exaggerated in TgAPPsw (Tg2576) mice also deficient in GRK5, in which fibrillar β-amyloid (Aβ) and an active inflammatory process involving activated GPCR signaling are present.     

Sphingosine 1-phosphate and its type 1 G protein-coupled receptor: trophic support and functional regulation of T lymphocytes

The lysophospholipid (LPL) growth factors sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are generated by macrophages, dendritic cells, mast cells, and platelets, which leads to lymph and plasma concentrations of 0.1-1 microM. Distinctive profiles of G protein-coupled receptors (GPCRs) for S1P and LPA are expressed by each type of immune cell and are regulated by cellular activation. At 1-100 nM, S1P signals T cells through their principal S1P(1) GPCRs with consequent protection from apoptosis, enhancement of chemotaxis, and facilitation of optimal regulatory activity of CD4(+)25(+) T cells. At 0.3-3 microM, S1P inhibits T cell chemotaxis and to a lesser extent other functions. These S1P-S1P(1) GPCR signals suppress homing of blood and spleen T cells to secondary lymphoid tissues. S1P(1) GPCR antagonists evoke lymphopenia by permitting blood T cells to enter lymph nodes and blocking S1P(1) GPCR-dependent T cell efflux from lymph nodes. Inversely, there is a decrease in lymphoid tissue traffic of T cells in transgenic mice, which overexpress lymphocyte S1P(1) GPCRs. The immunotherapeutic activity of S1P(1) GPCR antagonists, which limits T cell access to organ grafts and autoimmune antigens, does not reduce other functional capabilities of T cells. LPLs and their GPCRs thus constitute an immunoregulatory system of sufficient prominence for pharmacological targeting in transplantation, autoimmunity, and immunodeficiency.     

Mining the gene repertoire and ESTs for G protein-coupled receptors with evolutionary perspective

The purpose of this article was to review recent progress in mining the gene repertoire and expressed sequence tags (ESTs) for the super-family of G protein-coupled receptors (GPCRs) in the form of a proceeding from the Nordic GPCR meeting held at the Nobel Forum, Karolinska Institute in August 2006. We update and give an overview of the expansion of the main families of GPCRs; Glutamate, Rhodopsin, Adhesion, Frizzled and Secretin (GRAFS) in perspective of fully sequenced genomes. We look into the most recent findings including the work that has been carried out on the spotted green puffer fish (Tetraodon nigroviridis), mouse (Mus musculus), chicken (Gallus gallus), slime mold (Dictyostelium discoideum) and the plant pathogenic fungus Magnaporthe grisea. We use examples from our recent work on chicken GPCRs to highlight the importance of detailed assembly and curation of sequences and how that can affect percentage similarity and phylogeny. ESTs can give valuable information about expression patterns. GPCRs have comparatively low numbers of EST suggesting that GPCRs are in generally expressed in lower amount than other genes. We discuss similarities in the evolution of the trace amine associated receptors with other sensory receptors.     

The essential role of G protein-coupled receptor (GPCR) signaling in regulating T cell immunity

Aim: The aim of this paper is to clarify the critical role of GPCR signaling in T cell immunity.

Methods: The G protein-coupled receptors (GPCRs) are the most common targets in current pharmaceutical industry, and represent the largest and most versatile family of cell surface communicating molecules. GPCRs can be activated by a diverse array of ligands including neurotransmitters, chemokines as well as sensory stimuli. Therefore, GPCRs are involved in many key cellular and physiological processes, such as sense of light, taste and smell, neurotransmission, metabolism, endocrine and exocrine secretion. In recent years, GPCRs have been found to play an important role in immune system. T cell is an important type of immune cell, which plays a central role in cell-mediated immunity. A variety of GPCRs and their signaling mediators (RGS proteins, GRKs and β-arrestin) have been found to express in T cells and involved T cell-mediated immunity. We will summarize the role of GPCR signaling and their regulatory molecules in T cell activation, homeostasis and function in this article.

Results: GPCR signaling plays an important role in T cell activation, homeostasis and function.

Conclusion: GPCR signaling is critical in regulating T cell immunity.     

Neuropeptides and neuropeptide receptors in the Drosophila melanogaster genome

Recent genetic analyses in worms, flies, and mammals illustrate the importance of bioactive peptides in controlling numerous complex behaviors, such as feeding and circadian locomotion. To pursue a comprehensive genetic analysis of bioactive peptide signaling, we have scanned the recently completed Drosophila genome sequence for G protein-coupled receptors sensitive to bioactive peptides (peptide GPCRs). Here we describe 44 genes that represent the vast majority, and perhaps all, of the peptide GPCRs encoded in the fly genome. We also scanned for genes encoding potential ligands and describe 22 bioactive peptide precursors. At least 32 Drosophila peptide receptors appear to have evolved from common ancestors of 15 monophyletic vertebrate GPCR subgroups (e.g., the ancestral gastrin/cholecystokinin receptor). Six pairs of receptors are paralogs, representing recent gene duplications. Together, these findings shed light on the evolutionary history of peptide GPCRs, and they provide a template for physiological and genetic analyses of peptide signaling in Drosophila.     

Involvement of metabotropic glutamate receptor 1, a G protein coupled receptor,in melanoma development

Melanoma is the aberrant proliferation of melanocytes, the cells in the skin responsible for pigment production. In the United States the current lifetime risk of melanoma development is 1 in 57 in males and 1 in 81 in females [1]. In its early stages melanoma can be surgically removed with great success; however, advanced stages of melanoma have a high mortality rate due to the lack of responsiveness to currently available therapies. The development of animal models to be used in the studies of melanoma will provide the means for developing improved and targeted treatments for this disease. This review focuses on the recent report of a mouse melanoma model, TG-3, which has implicated the ectopic expression of the metabotropic glutamate receptor 1 (Grm1), a G protein coupled receptor (GPCR), in melanomagenesis and metastasis [2]. The involvement of other GPCRs in cellular transformation, particularly GPCRs in melanoma biology, and signaling of Grm1 are also discussed.     

Pepducins: an effective means to inhibit GPCR signaling by neutrophils

G-protein-coupled receptors (GPCRs) have a central role not only in the competent development of an innate myeloid response to foreign pathogens but, if dysregulated, might contribute to phagocyte-mediated organ injury. Here, recent findings from a study in which neutrophil trafficking is inhibited by using a novel family of GPCR signaling inhibitors, known as pepducins, are discussed.     

An mRNA atlas of G protein-coupled receptor expression during primary human monocyte/macrophage differentiation and lipopolysaccharide-mediated activation identifies targetable candidate regulators of inflammation

G protein-coupled receptors (GPCRs) are among the most important targets in drug discovery. In this study, we used TaqMan Low Density Arrays to profile the full GPCR repertoire of primary human macrophages differentiated from monocytes using either colony stimulating factor-1 (CSF-1/M-CSF) (CSF-1 M?) or granulocyte macrophage colony stimulating factor (GM-CSF) (GM-CSF M?). The overall trend was a downregulation of GPCRs during monocyte to macrophage differentiation, but a core set of 10 genes (e.g. LGR4, MRGPRF and GPR143) encoding seven transmembrane proteins were upregulated, irrespective of the differentiating agent used. Several of these upregulated GPCRs have not previously been studied in the context of macrophage biology and/or inflammation. As expected, CSF-1 M? and GM-CSF M? exhibited differential inflammatory cytokine profiles in response to the Toll-like Receptor (TLR)4 agonist lipopolysaccharide (LPS). Moreover, 15 GPCRs were differentially expressed between these cell populations in the basal state. For example, EDG1 was expressed at elevated levels in CSF-1 M? versus GM-CSF M?, whereas the reverse was true for EDG6. 101 GPCRs showed differential regulation over an LPS time course, with 65 of these profiles being impacted by the basal differentiation state (e.g. GPRC5A, GPRC5B). Only 14 LPS-regulated GPCRs showed asynchronous behavior (divergent LPS regulation) with respect to differentiation status. Thus, the differentiation state primarily affects the magnitude of LPS-regulated expression, rather than causing major reprogramming of GPCR gene expression profiles. Several GPCRs showing differential profiles between CSF-1 M? and GM-CSF M? (e.g. P2RY8, GPR92, EMR3) have not been widely investigated in macrophage biology and inflammation. Strikingly, several closely related GPCRs displayed completely opposing patterns of regulation during differentiation and/or activation (e.g. EDG1 versus EDG6, LGR4 versus LGR7, GPRC5A versus GPRC5B). We propose that selective regulation of GPCR5A and GPCR5B in CSF-1 M? contributes to skewing toward the M2 macrophage phenotype. Our analysis of the GPCR repertoire expressed during primary human monocyte to macrophage differentiation and TLR4-mediated activation provides a valuable new platform for conducting future functional analyses of individual GPCRs in human macrophage inflammatory pathways.