Different beta-subunits determine G-protein interaction with transmembrane receptors

Non-canonical G protein signaling

The original paradigm of classical - also referred to as canonical - cellular signal transduction of heterotrimeric G proteins (G protein) is defined by a hierarchical, orthograde interaction of three players: the agonist-activated G protein-coupled receptor (GPCR), which activates the transducing G protein, that in turn regulates its intracellular effectors. This receptor-transducer-effector concept was extended by the identification of regulators and adapters such as the regulators of G protein signaling (RGS), receptor kinases like βARK, or GPCR-interacting arrestin adapters that are integrated into this canonical signaling process at different levels to enable fine-tuning. Finally, the identification of atypical signaling mechanisms of classical regulators, together with the discovery of novel modulators, added a new and fascinating dimension to the cellular G protein signal transduction. This heterogeneous group of accessory G protein modulators was coined "activators of G protein signaling" (AGS) proteins and plays distinct roles in canonical and non-canonical G protein signaling pathways. AGS proteins contribute to the control of essential cellular functions such as cell development and division, intracellular transport processes, secretion, autophagy or cell movements. As such, they are involved in numerous biological processes that are crucial for diseases, like diabetes mellitus, cancer, and stroke, which represent major health burdens. Although the identification of a large number of non-canonical G protein signaling pathways has broadened the spectrum of this cellular communication system, their underlying mechanisms, functions, and biological effects are poorly understood. In this review, we highlight and discuss atypical G protein-dependent signaling mechanisms with a focus on inhibitory G proteins (Gi) involved in canonical and non-canonical signal transduction, review recent developments and open questions, address the potential of new approaches for targeted pharmacological interventions.

Oscillatory calcium release and sustained store-operated oscillatory calcium signaling prevents differentiation of human oligodendrocyte progenitor cells

Endogenous remyelination in demyelinating diseases such as multiple sclerosis is contingent upon the successful differentiation of oligodendrocyte progenitor cells (OPCs). Signaling via the Gα_q-coupled muscarinic receptor (M_1/3R) inhibits human OPC differentiation and impairs endogenous remyelination in experimental models. We hypothesized that calcium release following Gα_q-coupled receptor (G_qR) activation directly regulates human OPC (hOPC) cell fate. In this study, we show that specific G_qR agonists activating muscarinic and metabotropic glutamate receptors induce characteristic oscillatory calcium release in hOPCs and that these agonists similarly block hOPC maturation in vitro. Both agonists induce calcium release from endoplasmic reticulum (ER) stores and store operated calcium entry (SOCE) likely via STIM/ORAI-based channels. siRNA mediated knockdown (KD) of obligate calcium sensors STIM1 and STIM2 decreased the magnitude of muscarinic agonist induced oscillatory calcium release and attenuated SOCE in hOPCs. In addition, STIM2 expression was necessary to maintain the frequency of calcium oscillations and STIM2 KD reduced spontaneous OPC differentiation. Furthermore, STIM2 siRNA prevented the effects of muscarinic agonist treatment on OPC differentiation suggesting that SOCE is necessary for the anti-differentiative action of muscarinic receptor-dependent signaling. Finally, using a gain-of-function approach with an optogenetic STIM lentivirus, we demonstrate that independent activation of SOCE was sufficient to significantly block hOPC differentiation and this occurred in a frequency dependent manner while increasing hOPC proliferation. These findings suggest that intracellular calcium oscillations directly regulate hOPC fate and that modulation of calcium oscillation frequency may overcome inhibitory Gα_q-coupled signaling that impairs myelin repair.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Gene Polymorphisms and Susceptibility to Functional Dyspepsia: A Systematic Review and Meta-Analysis

Functional dyspepsia (FD) is a common chronic gastrointestinal disorder with a complex, undefined mechanism. Clustering of patients with FD in families highlights the role of genetic factors in the pathogenesis of FD. We performed a systematic review and meta-analysis to clarify the associations between specific gene polymorphisms and FD susceptibility. PubMed, EMBASE, the Cochrane Library, and HuGE database were searched. An additive model was adopted to determine whether previous studied genes are associated with FD susceptibility. Carriers of minor allele in GNB3 825C>T (OR = 1.15, 95% CI 0.99-1.34, P = 0.07), SCL6A4 5HTTLPR (OR = 0.92, 95% CI 0.75-1.12, P = 0.40), and CCK-1R 779T>C (OR = 0.86, 95% CI 0.72-1.03, P = 0.09) genes failed to demonstrate susceptibility to FD. In a subgroup analysis, only minor allele (T) in GNB3 825C>T was associated with an increased susceptibility to the epigastric pain syndrome subtype (OR = 1.34, 95% CI 1.10-1.63, P = 0.003). Our meta-analysis based on available studies using an additive model failed to show that GNB3, SCL6A4, and CCK-1R polymorphisms are associated with FD susceptibility.

The in vivo specificity of synaptic Gβ and Gγ subunits to the α2a adrenergic receptor at CNS synapses

G proteins are major transducers of signals from G-protein coupled receptors (GPCRs). They are made up of α, β, and γ subunits, with 16 Gα, 5 Gβ and 12 Gγ subunits. Though much is known about the specificity of Gα subunits, the specificity of Gβγs activated by a given GPCR and that activate each effector in vivo is not known. Here, we examined the in vivo Gβγ specificity of presynaptic α_2a-adrenergic receptors (α_2aARs) in both adrenergic (auto-α_2aARs) and non-adrenergic neurons (hetero-α_2aARs) for the first time. With a quantitative MRM proteomic analysis of neuronal Gβ and Gγ subunits, and co-immunoprecipitation of tagged α_2aARs from mouse models including transgenic FLAG-α_2aARs and knock-in HA-α_2aARs, we investigated the in vivo specificity of Gβ and Gγ subunits to auto-α_2aARs and hetero-α_2aARs activated with epinephrine to understand the role of Gβγ specificity in diverse physiological functions such as anesthetic sparing, and working memory enhancement. We detected Gβ₂, Gγ₂, Gγ₃, and Gγ₄ with activated auto α_2aARs, whereas we found Gβ₄ and Gγ₁₂ preferentially interacted with activated hetero-α_2aARs. Further understanding of in vivo Gβγ specificity to various GPCRs offers new insights into the multiplicity of genes for Gβ and Gγ, and the mechanisms underlying GPCR signaling through Gβγ subunits.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

Noncanonical Roles of G Protein-coupled Receptor Kinases in Cardiovascular Signaling

G protein-coupled receptor kinases (GRKs) are classically known for their role in regulating the activity of the largest known class of membrane receptors, which influence diverse biological processes in every cell type in the human body. As researchers have tried to uncover how this family of kinases, containing only 7 members, achieves selective and coordinated control of receptors, they have uncovered a growing number of noncanonical activities for these kinases. These activities include phosphorylation of nonreceptor targets and kinase-independent molecular interactions. In particular, GRK2, GRK3, and GRK5 are the predominant members expressed in the heart. Their canonical and noncanonical actions within cardiac and other tissues have significant implications for cardiovascular function in healthy animals and for the development and progression of disease. This review summarizes what is currently known regarding the activity of these kinases, and particularly the role of GRK2 and GRK5 in the molecular alterations that occur during heart failure. This review further highlights areas of GRK regulation that remain poorly understood and how they may represent novel targets for therapeutic development.

Regulation of G Protein βγ Signaling

Heterotrimeric guanine nucleotide-binding proteins (G proteins) deliver external signals to the cell interior, upon activation by the external signal stimulated G protein-coupled receptors (GPCRs).While the activated GPCRs control several pathways independently, activated G proteins control the vast majority of cellular and physiological functions, ranging from vision to cardiovascular homeostasis. Activated GPCRs dissociate GαGDPβγ heterotrimer into GαGTP and free Gβγ. Earlier, GαGTP was recognized as the primary signal transducer of the pathway and Gβγ as a passive signaling modality that facilitates the activity of Gα. However, Gβγ later found to regulate more number of pathways than GαGTP does. Once liberated from the heterotrimer, free Gβγ interacts and activates a diverse range of signaling regulators including kinases, lipases, GTPases, and ion channels, and it does not require any posttranslation modifications. Gβγ family consists of 48 members, which show cell- and tissue-specific expressions, and recent reports show that cells employ the subtype diversity in Gβγ to achieve desired signaling outcomes. In addition to activated GPCRs, which induce free Gβγ generation and the rate of GTP hydrolysis in Gα, which sequester Gβγ in the heterotrimer, terminating Gβγ signaling, additional regulatory mechanisms exist to regulate Gβγ activity. In this chapter, we discuss structure and function, subtype diversity and its significance in signaling regulation, effector activation, regulatory mechanisms as well as the disease relevance of Gβγ in eukaryotes.

Quantitative Multiple-Reaction Monitoring Proteomic Analysis of Gβ and Gγ Subunits in C57Bl6/J Brain Synaptosomes

Gβγ dimers are one of the essential signaling units of activated G protein-coupled receptors (GPCRs). There are five Gβ and 12 Gγ subunits in humans; numerous studies have demonstrated that different Gβ and Gγ subunits selectively interact to form unique Gβγ dimers, which in turn may target specific receptors and effectors. Perturbation of Gβγ signaling can lead to impaired physiological responses. Moreover, previous targeted multiple-reaction monitoring (MRM) studies of Gβ and Gγ subunits have shown distinct regional and subcellular localization patterns in four brain regions. Nevertheless, no studies have quantified or compared their individual protein levels. In this study, we have developed a quantitative MRM method not only to quantify but also to compare the protein abundance of neuronal Gβ and Gγ subunits. In whole and fractionated crude synaptosomes, we were able to identify the most abundant neuronal Gβ and Gγ subunits and their subcellular localizations. For example, Gβ₁ was mostly localized at the membrane while Gβ₂ was evenly distributed throughout synaptosomal fractions. The protein expression levels and subcellular localizations of Gβ and Gγ subunits may affect the Gβγ dimerization and Gβγ-effector interactions. This study offers not only a new tool for quantifying and comparing Gβ and Gγ subunits but also new insights into the in vivo distribution of Gβ and Gγ subunits, and Gβγ dimer assembly in normal brain function.

Influence of G-protein β-Polypeptide 3 C825T Polymorphism on Antihypertensive Response to Telmisartan and Amlodipine in Chinese Patients

Background:

G-protein β-polypeptide 3 (GNB3) is a β subunit isoform of G-protein that plays important role in signal transduction of membrane G-protein coupled receptors (GPCRs). The GNB3 splice variant C825T (rs5443) is associated with risk for essential hypertension (EH) and efficacy of therapeutic drugs targeting GPCRs. It is unknown whether the polymorphism is associated with blood pressure (BP) response to telmisartan or amlodipine, two widely prescribed antihypertensive drugs.

Methods:

A total of 93 subjects initially diagnosed as EH were recruited and underwent a 4-week treatment with telmisartan (42 patients) or amlodipine (51 patients) monotherapy. Both baseline and after-treatment BP were measured. GNB3 C825T polymorphism was genotyped by polymerase chain reaction-restriction fragment length polymorphism.

Results:

Baseline systolic BP (SBP) and diastolic BP (DBP) were comparable among C825T genotypes in both telmisartan and amlodipine treatment groups. Patients with the CT or TT genotypes showed significantly lower body mass index (BMI) as compared with CC homozygotes in both groups (P < 0.05, respectively). GNB3 825TT homozygotes showed significantly higher after-treatment DBP and mean arterial pressure (MAP) than those carrying at least one 825C allele (P < 0.01) in the telmisartan treatment group. No difference in after-treatment SBP, DBP, and MAP levels among C825T genotypes was observed in the amlodipine treatment group. No significant difference in absolute changes in BP levels was observed among the genotypes in either treatment group.

Conclusion:

The GNB3 C825T splice variant is associated with the DBP-lowering effect of telmisartan but not amlodipine in Chinese EH patients.

Gene polymorphisms associated with functional dyspepsia

Functional dyspepsia (FD) is a constellation of functional upper abdominal complaints with poorly elucidated pathophysiology. However, there is increasing evidence that susceptibility to FD is influenced by hereditary factors. Genetic association studies in FD have examined genotypes related to gastrointestinal motility or sensation, as well as those related to inflammation or immune response. G-protein b3 subunit gene polymorphisms were first reported as being associated with FD. Thereafter, several gene polymorphisms including serotonin transporter promoter, interlukin-17F, migration inhibitory factor, cholecystocynine-1 intron 1, cyclooxygenase-1, catechol-o-methyltransferase, transient receptor potential vanilloid 1 receptor, regulated upon activation normal T cell expressed and secreted, p22PHOX, Toll like receptor 2, SCN10A, CD14 and adrenoreceptors have been investigated in relation to FD; however, the results are contradictory. Several limitations underscore the value of current studies. Among others, inconsistencies in the definitions of FD and controls, subject composition differences regarding FD subtypes, inadequate samples, geographical and ethnical differences, as well as unadjusted environmental factors. Further well-designed studies are necessary to determine how targeted genes polymorphisms, influence the clinical manifestations and potentially the therapeutic response in FD.

Association of genetic variants in GNβ3 with functional dyspepsia: a meta-analysis

BACKGROUND

Functional dyspepsia (FD) is a functional upper gastrointestinal disorder. The etiology and pathogenesis of FD remain unclear, with genetic factors playing an important role. Previous studies investigated the association of C825T in GNβ3 with FD, with conflicting results reported.

AIMS

The aim of this meta-analysis is to assess the association of genetic variants in GNβ3 with FD.

METHODS

We performed a systematic literature search in PubMed, Cochrane Library, Google Scholar, and Web of Knowledge, and conducted a meta-analysis to assess the association of C825T in GNβ3 with FD. For sensitivity analysis, we analyzed the association between C825T and subtypes of FD. We also performed meta-analyses separately for individual ethnic groups/countries of origin.

RESULTS

A total of eight studies met the eligibility criteria and were included in our analyses. Our meta-analysis finds no association between 825CC and FD (OR 1.19, 95% CI 0.84-1.67, p = 0.328). However, the association is significant under an additive model (OR 0.59, 95% CI 0.38-0.92, p = 0.018). Sensitivity analysis indicated a significant association of C825T with FD in participants from Korea but not in those from Japan, Europe, or the United States. We also detected a significant association of this SNP with dysmotility.

CONCLUSIONS

The genetic variant C825T in GNβ3 is significantly associated with FD under an additive model and the association is race-specific. Further studies with larger samples sizes are needed to validate our findings and to explore the potential mechanism underlying the association.

Somatostatin system: molecular mechanisms regulating anterior pituitary hormones

The somatostatin (SRIF) system, which includes the SRIF ligand and receptors, regulates anterior pituitary gland function, mainly inhibiting hormone secretion and to some extent pituitary tumor cell growth. SRIF-14 via its cognate G-protein-coupled receptors (subtypes 1-5) activates multiple cellular signaling pathways including adenylate cyclase/cAMP, MAPK, ion channel-dependent pathways, and others. In addition, recent data have suggested SRIF-independent constitutive SRIF receptor activity responsible for GH and ACTH inhibition in vitro. This review summarizes current knowledge on ligand-dependent and independent SRIF receptor molecular and functional effects on hormone-secreting cells in the anterior pituitary gland.

No association of G-protein beta polypeptide 3 polymorphism with irritable bowel syndrome: Evidence from a meta-analysis

AIM: To clarify the associations between G-protein beta polypeptide 3 (GNB3) C825T polymorphism and risk of the irritable bowel syndrome (IBS) by a meta-analysis.

METHODS: We searched relevant studies in PubMed, EMBASE, CNKI, Google Scholar, Ovid and Cochrane library prior to October 2013. The strengths of the associations between GNB3 C825T polymorphism and IBS risk were estimated by odds ratios (ORs) with 95% confidence interval (CIs).

RESULTS: We identified seven case-control studies with 1085 IBS cases and 1695 controls for the analysis. We found no significantly associations of GNB3 C825T polymorphism with IBS risk in the overall population (CC vs TT, OR = 1.12, 95%CI: 0.86-1.45; CC + CT vs TT, OR = 1.17, 95%CI: 0.92-1.49; TT + CT vs CC, OR = 0.93, 95%CI: 0.80-1.08; C vs T, OR = 1.08, 95%CI: 0.97-1.21). Subgroup analysis did not reveal significant associations either in Asian population or Caucasian population. The pooled results of four studies fail to show associations of GNB3 C825T polymorphism with subtypes of IBS (constipation-dominant type, diarrhea-dominant type and mixed type).

CONCLUSION: The present study suggests no associations of GNB3 C825T polymorphism with IBS risk.

Differential localization of G protein βγ subunits

G protein βγ subunits play essential roles in regulating cellular signaling cascades, yet little is known about their distribution in tissues or their subcellular localization. While previous studies have suggested specific isoforms may exhibit a wide range of distributions throughout the central nervous system, a thorough investigation of the expression patterns of both Gβ and Gγ isoforms within subcellular fractions has not been conducted. To address this, we applied a targeted proteomics approach known as multiple-reaction monitoring to analyze localization patterns of Gβ and Gγ isoforms in pre- and postsynaptic fractions isolated from cortex, cerebellum, hippocampus, and striatum. Particular Gβ and Gγ subunits were found to exhibit distinct regional and subcellular localization patterns throughout the brain. Significant differences in subcellular localization between pre- and postsynaptic fractions were observed within the striatum for most Gβ and Gγ isoforms, while others exhibited completely unique expression patterns in all four brain regions examined. Such differences are a prerequisite for understanding roles of individual subunits in regulating specific signaling pathways throughout the central nervous system.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Fine-tuning of GPCR signals by intracellular G protein modulators

Heterotrimeric G proteins convey receptor signals to intracellular effectors. Superimposed over the basic GPCR-G protein-effector scheme are three types of auxiliary proteins that also modulate Gα. Regulator of G protein signaling proteins and G protein signaling modifier proteins respectively promote GTPase activity and hinder GTP/GDP exchange to limit Gα activation. There are also diverse proteins that, like GPCRs, can promote nucleotide exchange and thus activation. Here we review the impact of these auxiliary proteins on GPCR signaling. Although their precise physiological functions are not yet clear, all of them can produce significant effects in experimental systems. These signaling changes are generally consistent with established effects on isolated Gα; however, the activation state of Gα is seldom verified and many such changes appear also to reflect the physical disruption of or indirect effects on interactions between Gα and its associated GPCR, Gβγ, and/or effector.

The Gβ3 splice variant associated with the C825T gene polymorphism is an unstable and functionally inactive protein

A splice variant of Gβ3, termed Gβ3s, has been associated with the C825T polymorphism in the Gβ3 gene and linked with many human disorders. However, the biochemical properties and functionality of Gβ3s remain controversial. Here, using multidisciplinary approaches including co-immunoprecipitation analysis and bioluminescence resonance energy transfer (BRET) measurements, we showed that unlike Gβ3, Gβ3s failed to form complexes with either Gγ or Gα subunits. Moreover, using a mutant Gγ2 deficient in lipid modification to purify Gβ3s from Sf9 cells without the use of detergents, we further showed that the failure of Gβ3s to form dimers with Gγ was not due to the instability of the dimers in detergents, but rather, reflected the intrinsic properties of Gβ3s. Additional studies indicated that Gβ3s is unstable, and unable to localize properly to the plasma membrane and to activate diverse Gβγ effectors including PLCβ2/3, PI3Kγ, ERKs and the Rho guanine exchange factor (RhoGEF) PLEKHG2. Thus, these data suggest that the pathological effects of Gβ3 C825T polymorphism may result from the downregulation of Gβ3 function. However, we found that the chemokine SDF1α transmits signals primarily through Gβ1 and Gβ2, but not Gβ3, to regulate chemotaxis of several human lymphocytic cell lines, indicating the effects of Gβ3 C825T polymorphism are likely to be tissue and/or stimuli specific and its association with various disorders in different tissues should be interpreted with great caution.

N-palmitoyl-ethanolamine (PEA) induces peripheral antinociceptive effect by ATP-sensitive K+-channel activation

Although the antinociceptive effects of N-palmitoyl-ethanolamine (PEA) were first characterized nearly 50 years ago, the identity of the mechanism that mediates these actions has not been elucidated. The present study investigated the contribution of K(+) channels on peripheral antinociception induced by the CB(2) agonist PEA. Nociceptive thresholds to mechanical paw stimulation of Wistar rats treated with intraplantar prostaglandin E(2) to induce hyperalgesia were measured, and other agents were also given by local injection. PEA (5, 10, and 20 µg/paw) elicited a local peripheral antinociceptive effect. This effect was antagonized by glibenclamide, a selective blocker of ATP-sensitive K(+) channels (20, 40, and 80 µg/paw). In addition, neither the voltage-dependent K(+) channel-specific blocker tetraethylammonium (30 µg/paw) nor the small and large conductance blockers of Ca(2+)-activated K(+) channels, dequalinium (50 µg/paw) and paxilline (20 µg/paw), respectively, were able to block the local antinociceptive effect of PEA. These results indicate that the activation of ATP-sensitive K(+) channels could be the mechanism that induces peripheral antinociception by PEA and that voltage-dependent K(+) channels and small and large conductance Ca(2+)-activated K(+) channels do not appear to be involved in this mechanism.

LTB4 is a signal-relay molecule during neutrophil chemotaxis

Neutrophil recruitment to inflammation sites purportedly depends on sequential waves of chemoattractants. Current models propose that leukotriene B(4) (LTB(4)), a secondary chemoattractant secreted by neutrophils in response to primary chemoattractants such as formyl peptides, is important in initiating the inflammation process. In this study we demonstrate that LTB(4) plays a central role in neutrophil activation and migration to formyl peptides. We show that LTB(4) production dramatically amplifies formyl peptide-mediated neutrophil polarization and chemotaxis by regulating specific signaling pathways acting upstream of actin polymerization and MyoII phosphorylation. Importantly, by analyzing the migration of neutrophils isolated from wild-type mice and mice lacking the formyl peptide receptor 1, we demonstrate that LTB(4) acts as a signal to relay information from cell to cell over long distances. Together, our findings imply that LTB(4) is a signal-relay molecule that exquisitely regulates neutrophil chemotaxis to formyl peptides, which are produced at the core of inflammation sites.

Fluorescence/bioluminescence resonance energy transfer techniques to study G-protein-coupled receptor activation and signaling

Fluorescence and bioluminescence resonance energy transfer (FRET and BRET) techniques allow the sensitive monitoring of distances between two labels at the nanometer scale. Depending on the placement of the labels, this permits the analysis of conformational changes within a single protein (for example of a receptor) or the monitoring of protein-protein interactions (for example, between receptors and G-protein subunits). Over the past decade, numerous such techniques have been developed to monitor the activation and signaling of G-protein-coupled receptors (GPCRs) in both the purified, reconstituted state and in intact cells. These techniques span the entire spectrum from ligand binding to the receptors down to intracellular second messengers. They allow the determination and the visualization of signaling processes with high temporal and spatial resolution. With these techniques, it has been demonstrated that GPCR signals may show spatial and temporal patterning. In particular, evidence has been provided for spatial compartmentalization of GPCRs and their signals in intact cells and for distinct physiological consequences of such spatial patterning. We review here the FRET and BRET technologies that have been developed for G-protein-coupled receptors and their signaling proteins (G-proteins, effectors) and the concepts that result from such experiments.

Receptor signaling and the cell biology of synaptic transmission

This volume describes a series of psychiatric and neuropsychiatric disorders, connects some aspects of somatic and psychiatric medicine, and describes various current and emerging therapies. The purpose of this chapter is to set the stage for the volume by developing the theoretical basis of synaptic transmission and introducing the various neurotransmitters and their receptors involved in the process. The intent is to provide not only a historical context through which to understand neurotransmitters, but a current contextual basis for understanding neuronal signal transduction and applying this knowledge to facilitate treatment of maladies of the brain and mind.

Nuclear GPCRs in cardiomyocytes: an insider's view of β-adrenergic receptor signaling

In recent years, we have come to appreciate the complexity of G protein-coupled receptor signaling in general and β-adrenergic receptor (β-AR) signaling in particular. Starting originally from three β-AR subtypes expressed in cardiomyocytes with relatively simple, linear signaling cascades, it is now clear that there are large receptor-based networks which provide a rich and diverse set of responses depending on their complement of signaling partners and the physiological state. More recently, it has become clear that subcellular localization of these signaling complexes also enriches the diversity of phenotypic outcomes. Here, we review our understanding of the signaling repertoire controlled by nuclear β-AR subtypes as well our understanding of the novel roles for G proteins themselves in the nucleus, with a special focus, where possible, on their effects in cardiomyocytes. Finally, we discuss the potential pathological implications of alterations in nuclear β-AR signaling.

Pituitary somatostatin receptor signaling

Somatotropin-release inhibitory factor (SRIF) is a major regulator of pituitary function, mostly inhibiting hormone secretion and to a lesser extent pituitary cell growth. Five SRIF receptor subtypes (SSTR1-5) are ubiquitously expressed G-protein coupled receptors. In the pituitary, SSTR1, 2, 3 and 5 are expressed, with SSTR2 and SSTR5 predominating. As new SRIF analogs have recently been introduced for treatment of pituitary disease, we evaluate the current knowledge of cell-specific pituitary SRIF receptor signaling and highlight areas of future research for comprehensive understanding of these mechanisms. Elucidating pituitary SRIF receptor signaling enables understanding of pituitary hormone secretion and cell growth, and also encourages future therapeutic development for pituitary disorders.

Structural evidence for a sequential release mechanism for activation of heterotrimeric G proteins

Heptahelical G-protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors couple to heterotrimeric G proteins to relay extracellular signals to intracellular signaling networks, but the molecular mechanism underlying guanosine 5'-diphosphate (GDP) release by the G protein alpha-subunit is not well understood. Amino acid substitutions in the conserved alpha5 helix of G(i), which extends from the C-terminal region to the nucleotide-binding pocket, cause dramatic increases in basal (receptor-independent) GDP release rates. For example, mutant Galpha(i1)-T329A shows an 18-fold increase in basal GDP release rate and, when expressed in culture, it causes a significant decrease in forskolin-stimulated cAMP accumulation. The crystal structure of Galpha(i1)-T329A.GDP shows substantial conformational rearrangement of the switch I region and additional striking alterations of side chains lining the catalytic pocket that disrupt the Mg(+2) coordination sphere and dislodge bound Mg(+2). We propose a "sequential release" mechanism whereby a transient conformational change in the alpha5 helix alters switch I to induce GDP release. Interestingly, this mechanistic model for heterotrimeric G protein activation is similar to that suggested for the activation of the plant small G protein Rop4 by RopGEF8.

The role of Gbetagamma subunits in the organization, assembly, and function of GPCR signaling complexes

The role of Gbetagamma subunits in cellular signaling has become well established in the past 20 years. Not only do they regulate effectors once thought to be the sole targets of Galpha subunits, but it has become clear that they also have a unique set of binding partners and regulate signaling pathways that are not always localized to the plasma membrane. However, this may be only the beginning of the story. Gbetagamma subunits interact with G protein-coupled receptors, Galpha subunits, and several different effector molecules during assembly and trafficking of receptor-based signaling complexes and not simply in response to ligand stimulation at sites of receptor cellular activity. Gbetagamma assembly itself seems to be tightly regulated via the action of molecular chaperones and in turn may serve a similar role in the assembly of specific signaling complexes. We propose that specific Gbetagamma subunits have a broader role in controlling the architecture, assembly, and activity of cellular signaling pathways.

Structural determinants involved in the formation and activation of G protein betagamma dimers

Heterotrimeric G proteins, composed of an alpha, beta and gamma subunit, represent one of the most important and dynamic families of signaling proteins. As a testament to the significance of G protein signaling, the hundreds of seven-transmembrane-spanning receptors that interact with G proteins are estimated to occupy 1-2% of the human genome. This broad diversity of receptors is echoed in the number of potential heterotrimer combinations that can arise from the 23 alpha subunit, 7 beta subunit and 12 gamma subunit isoforms that have been identified. The potential for such vast complexity implies that the receptor G protein interface is the site of much regulation. The historical model for the activation of a G protein holds that activated receptor catalyzes the exchange of GDP for GTP on the alpha subunit, inducing a conformational change that substantially lowers the affinity of alpha for betagamma. This decreased affinity enables dissociation of betagamma from alpha and receptor. The free form of betagamma is thought to activate effectors, until the hydrolysis of GTP by G alpha (aided by RGS proteins) allows the subunits to re-associate, effectively deactivating the G protein until another interaction with activated receptor.

Molecular mechanisms of go signaling

Go is the most abundant G protein in the central nervous system, where it comprises about 1% of membrane protein in mammalian brains. It functions to couple cell surface receptors to intercellular effectors, which is a critical process for cells to receive, interpret and respond to extracellular signals. Go protein belongs to the pertussis toxin-sensitive Gi/Go subfamily of G proteins. A number of G-protein-coupled receptors transmit stimuli to intercellular effectors through Go. Go regulates several cellular effectors, including ion channels, enzymes, and even small GTPases to modulate cellular function. This review summarizes some of the advances in Go research and proposes areas to be further addressed in exploring the functional role of Go.

Coupling specificity of NOP opioid receptors to pertussis-toxin-sensitive Galpha proteins in adult rat stellate ganglion neurons using small interference RNA

The opioid receptor-like 1 (NOP or ORL1) receptor is a G-protein-coupled receptor the endogenous ligand of which is the heptadecapeptide, nociceptin (Noc). NOP receptors are known to modulate pain processing at spinal, supraspinal, and peripheral levels. Previous work has demonstrated that NOP receptors inhibit N-type Ca2+ channel currents in rat sympathetic stellate ganglion (SG) neurons via pertussis toxin (PTX)-sensitive Galphai/o subunits. However, the identification of the specific Galpha subunit that mediates the Ca2+ current modulation is unknown. The purpose of the present study was to examine coupling specificity of Noc-activated NOP receptors to N-type Ca2+ channels in SG neurons. Small interference RNA (siRNA) transfection was employed to block the expression of PTX-sensitive Galpha subunits. RT-PCR results showed that siRNA specifically decreased the expression of the intended Galpha subunit. Evaluation of cell surface protein expression and Ca2+ channel modulation were assessed by immunofluorescence staining and electrophysiological recordings, respectively. Furthermore, the presence of mRNA of the intended siRNA target Galpha protein was examined by RT-PCR experiments. Fluorescence imaging showed that Galphai1, Galphai3, and Galphao were expressed in SG neurons. The transfection of Galphai1-specific siRNA resulted in a significant decrease in Noc-mediated Ca2+ current inhibition, while silencing of either Galphai3 or Galphao was without effect. Taken together, these results suggest that in SG neurons Galphai1 subunits selectively couple NOP receptors to N-type Ca2+ channels.

G protein betagamma subunits as targets for small molecule therapeutic development

G proteins mediate the action of G protein coupled receptors (GPCRs), a major target of current pharmaceuticals and a major target of interest in future drug development. Most pharmaceutical interest has been in the development of selective GPCR agonists and antagonists that activate or inhibit specific GPCRs. Some recent thinking has focused on the idea that some pathologies are the result of the actions of an array of GPCRs suggesting that targeting single receptors may have limited efficacy. Thus, targeting pathways common to multiple GPCRs that control critical pathways involved in disease has potential therapeutic relevance. G protein betagamma subunits released from some GPCRs upon receptor activation regulate a variety of downstream pathways to control various aspects of mammalian physiology. There is evidence from cell- based and animal models that excess Gbetagamma signaling can be detrimental and blocking Gbetagamma signaling has salutary effects in a number of pathological models. Gbetagamma regulates downstream pathways through modulation of enzymes that produce cellular second messengers or through regulation of ion channels by direct protein-protein interactions. Thus, blocking Gbetagamma functions requires development of small molecule agents that disrupt Gbetagamma protein interactions with downstream partners. Here we discuss evidence that small molecule targeting Gbetagamma could be of therapeutic value. The concept of disruption of protein-protein interactions by targeting a "hot spot" on Gbetagamma is delineated and the biochemical and virtual screening strategies for identification of small molecules that selectively target Gbetagamma functions are outlined. Evaluation of the effectiveness of virtual screening indicates that computational screening enhanced identification of true Gbetagamma binding molecules. However, further refinement of the approach could significantly improve the yield of Gbetagamma binding molecules from this screen that could result in multiple candidate leads for future drug development.

An exploratory study of the association of adrenergic and serotonergic genotype and gastrointestinal motor functions

Adrenergic and serotonergic mechanisms alter human gut motor functions. Genotype variation influences phenotype. Our aim was to test the hypothesis that variation in genes that control these functions is associated with gastrointestinal (GI) motor functions in humans with functional GI disorders (FGID). A database of 251 people was assembled by combining genotype data with measurements of gut transit and gastric volumes. Genetic variations evaluated were: alpha(2A) adrenergic (C-1291G), alpha(2C) (Del 332-325), 5-HT transporter (SLC6A4) and GNbeta3 (C825T). We sought associations between motor function or disease groups and genotypes, adjusting for age, gender and body mass index. Among 251 participants, 82 were healthy, 20 with irritable bowel syndrome (IBS) with mixed bowel habit, 49 with constipation-predominant IBS, 67 with diarrhoea-predominant IBS and 33 with functional dyspepsia. For all candidate genes, there was no significant association between motor function and wildtype vs non-wildtype gene status. There were significant interactions between genotype and motility phenotype, specifically GNbeta3 and alpha(2A) and gastric emptying at 4 h. Borderline associations were noted for SCL6A4 and alpha(2A) and postprandial gastric volume, and for alpha(2C) and gastric emptying at 2 h. We conclude that genotype variation may affect gastric motor functions in different FGID phenotypes. However, these candidate genes account for only a limited amount of the variance in gastric function of patients with FGID.

Homozygous 825T allele of the GNB3 protein influences the susceptibility of Japanese to dyspepsia

The role of genetics in the susceptibility to functional dyspepsia (FD) is not well established. Recently, two different associations were reported between FD and G-protein beta3 (GNB3) subunit gene polymorphism. We aim to clarify the association between GNB3 protein C825T polymorphism and dyspepsia in the Japanese population. Eight-nine dyspeptics and 94 nondyspeptic subjects enrolled in this study. All subjects underwent gastroscopy and patients with significant upper gastrointestinal findings were excluded. Other diseases were also excluded by face-to-face history and physical examination. GNB3 protein C825T polymorphisms were determined by polymerase chain reaction-restriction fragment-length polymorphism. H. pylori infection status was examined by histology or antibody against H. pylori. Nonsignificant correlation was found between GNB3 protein homozygous 825T and unexplained dyspepsia (OR = 1.65, 95% CI: 0.87-3.13). However, among H. pylori-negative subjects, homozygous GNB3 protein 825T significantly increased the risk of dyspepsia (16.7% versus 40.5%; CC versus TT; OR = 5.10, 95% CI: 1.21-21.43, CC versus others; OR = 3.40, 95% CI: 1.16-9.93, respectively). This significant association remained after logistic regression analysis with adjustment for sex and age (CC versus TT; OR = 5.73, 95% CI: 1.27-25.82, CC versus others; OR = 3.08, 95% CI: 1.02-9.25). No significant correlation was found between GNB3 polymorphism and any dyspeptic symptoms. Our data suggest that the homozygous 825T allele of GNB3 protein is associated with dyspepsia in the H. pylori-negative Japanese population. The role of genetics in the development of dyspepsia needs further evaluation.

Somatostatin modulates voltage-dependent Ca2+ channels in GH3 cells via a specific G(o) splice variant

In rat pituitary GH3 cells Ca2+ current through L-type channels is reduced by somatostatin. This modulation of channel activity by somatostatin receptors is mediated by a guanine nucleotide-binding regulatory protein (G protein). It is sensitive to pertussis toxin, indicating the involvement of a G(o)- or Gi-type G protein in this pathway. The identity of this G protein was determined by suppressing the expression of endogenous G proteins individually via intranuclear injection of antisense oligonucleotides. This method was applied to GH3 cells to screen several G protein alpha, beta and gamma subunits for their roles in the defined signal transduction pathway. The loss of somatostatin's modulating activity on the voltage-dependent Ca2+ channel after oligonucleotide injection revealed the involvement of G(o) alpha 2 beta 1 gamma 3 to the exclusion of other closely related subtypes.

Interaction of somatostatin receptors with G proteins and cellular effector systems

Somatostatin induces its multiple biological actions by interacting with a family of receptors, referred to as sstr1-sstr5. To determine the molecular mechanisms of action of somatostatin, we have investigated the interaction of the different cloned receptors with G proteins and cellular effector systems. sstr2, sstr3 and sstr5 associate with pertussis toxin-sensitive G proteins and are able to mediate the inhibition of adenylyl cyclase activity by somatostatin. Two forms of sstr2, sstr2A and sstr2B, are generated by alternative splicing and differ in their C-terminal amino acid sequence. sstr2B couples to adenylyl cyclase whereas sstr2A does not. To investigate the basis for the differential coupling to adenylyl cyclase, we truncated sstr2B to the point of amino acid sequence divergence from sstr2A. The truncated sstr2B mediated the inhibition of cAMP formation by somatostatin, indicating that the C-terminus is not needed for coupling sstr2 to adenylyl cyclase. It is likely that the C-terminus of sstr2A hinders coupling to adenylyl cyclase. sstr2A associates with Gi alpha 3 and G(o) alpha but does not effectively interact with Gi alpha 1, a G protein that is necessary for coupling somatostatin receptors to adenylyl cyclase. The differential association of the splice variants with Gi alpha 1 may explain their contrasting effects on adenylyl cyclase activity. sstr3 also couples to adenylyl cyclase. Gi alpha 1 links sstr3 to adenylyl cyclase and mutagenesis studies have shown that the C-terminus of Gi alpha 1 is necessary for this coupling. The C-terminus of the Gi alpha proteins differ by only a few amino acid residues and only Gi alpha 1 couples sstr3 to adenylyl cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificities of g protein-coupled receptor kinase-2 and -3 at cardiac myocyte receptors provide basis for distinct roles in regulation of myocardial function

The closely related G protein-coupled receptor kinases GRK2 and GRK3 are both expressed in cardiac myocytes. Although GRK2 has been extensively investigated in terms of regulation of cardiac beta-adrenergic receptors, the substrate specificities of the two GRK isoforms at G protein-coupled receptors (GPCR) are poorly understood. In this study, the substrate specificities of GRK2 and GRK3 at GPCRs that control cardiac myocyte function were determined in fully differentiated adult cardiac myocytes. Concentration-effect relationships of GRK2, GRK3, and their respective competitive inhibitors, GRK2ct and GRK3ct, at endogenous endothelin, alpha(1)-adrenergic, and beta(1)-adrenergic receptor-generated responses in cardiac myocytes were achieved by adenovirus gene transduction. GRK3 and GRK3ct were highly potent and efficient at the endothelin receptors (IC(50) for GRK3, 5 +/- 0.7 pmol/mg of protein; EC(50) for GRK3ct, 2 +/- 0.2 pmol/mg of protein). The alpha(1)-adrenergic receptor was also a preferred substrate of GRK3 (IC(50),7 +/- 0.4 pmol/mg of protein). GRK2 lacked efficacy at both endothelin and alpha(1)-adrenergic receptors despite massive overexpression. On the contrary, both GRK2ct and GRK3ct enhanced beta(1)-adrenergic receptor-induced cAMP production with comparable potencies. However, the potency of GRK3ct at beta(1)-adrenergic receptors was at least 20-fold lower than that at endothelin receptors. In conclusion, this study demonstrates distinct substrate specificities of GRK2 and GRK3 at different GPCRs in fully differentiated adult cardiac myocytes. As inferred from the above findings, GRK2 may play its primary role in regulation of cardiac contractility and chronotropy by controlling beta(1)-adrenergic receptors, whereas GRK3 may play important roles in regulation of cardiac growth and hypertrophy by selectively controlling endothelin and alpha(1)-adrenergic receptors.

Expansion of signal transduction by G proteins. The second 15 years or so: from 3 to 16 alpha subunits plus betagamma dimers

The first 15 years, or so, brought the realization that there existed a G protein coupled signal transduction mechanism by which hormone receptors regulate adenylyl cyclases and the light receptor rhodopsin activates visual phosphodiesterase. Three G proteins, Gs, Gi and transducin (T) had been characterized as alphabetagamma heterotrimers, and Gsalpha-GTP and Talpha-GTP had been identified as the sigaling arms of Gs and T. These discoveries were made using classical biochemical approaches, and culminated in the purification of these G proteins. The second 15 years, or so, are the subject of the present review. This time coincided with the advent of powerful recombinant DNA techniques. Combined with the classical approaches, the field expanded the repertoire of G proteins from 3 to 16, discovered the superfamily of seven transmembrane G protein coupled receptors (GPCRs) -- which is not addressed in this article -- and uncovered an amazing repertoire of effector functions regulated not only by alphaGTP complexes but also by betagamma dimers. Emphasis is placed in presenting how the field developed with the hope of conveying why many of the new findings were made.

Heterotrimeric G-proteins: a short history

Some 865 genes in man encode G-protein-coupled receptors (GPCRs). The heterotrimeric guanine nucleotide-binding proteins (G-proteins) function to transduce signals from this vast panoply of receptors to effector systems including ion channels and enzymes that alter the rate of production, release or degradation of intracellular second messengers. However, it was not until the 1970s that the existence of such transducing proteins was even seriously suggested. Combinations of bacterial toxins that mediate their effects via covalent modification of the alpha-subunit of certain G-proteins and mutant cell lines that fail to generate cyclic AMP in response to agonists because they either fail to express or express a malfunctional G-protein allowed their identification and purification. Subsequent to initial cloning efforts, cloning by homology has defined the human G-proteins to derive from 35 genes, 16 encoding alpha-subunits, five beta and 14 gamma. All function as guanine nucleotide exchange on-off switches and are mechanistically similar to other proteins that are enzymic GTPases. Although not readily accepted initially, it is now well established that beta/gamma complexes mediate as least as many functions as the alpha-subunits. The generation of chimeras between different alpha-subunits defined the role of different sections of the primary/secondary sequence and crystal structures and cocrystals with interacting proteins have given detailed understanding of their molecular structure and basis of function. Finally, further modifications of such chimeras have generated a range of G-protein alpha-subunits with greater promiscuity to interact across GPCR classes and initiated the use of such modified G-proteins in drug discovery programmes.

Immunohistochemical localization of G protein betagamma subunits in the lateral wall of the rat cochlea

The role of G protein-mediated signal transduction in the production of endolymph, an extracellular fluid of unusual ionic composition, is beginning to be understood. The identity of Galpha subunits in the stria vascularis and the spiral ligament of the lateral wall of the cochlear duct is well established. However, little is known about the presence of betagamma subunits. This study used immunohistochemistry to investigate the distribution of G protein betagamma subunits in the lateral wall of the cochlea. Temporal bones of 6- to 8-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The results show that the pattern of immunoreactivity varies for the G protein beta1-4 and gamma1-3, 5 and 7 subunits in the stria vascularis and spiral ligament. In the stria vascularis, immunoreactivity was detected for beta2, beta3, beta4, gamma1, gamma2 and gamma7 subunits. All five types of fibrocytes in the spiral ligament exhibited positive staining for gamma2 and gamma7. However, immunoreactivity for beta1-4 subunits was variable. Immunoreactivity for gamma3 and gamma5 subunits was not detected in the lateral cochlear wall. The expression pattern of G protein betagamma subunits in lateral wall provides a basis for interpreting the functions of G protein-coupled receptors in cochlear fluid homeostasis.

A study of candidate genotypes associated with dyspepsia in a U.S. community

BACKGROUND

The role of genetic predisposition to the development of dyspepsia is unclear. Recently, a significant association was reported with CC genotype of GNbeta3.

AIM

To explore the association of candidate genotypes altering adrenergic, serotonergic, CCKergic, and G protein functions, and dyspepsia in a sample from a U.S. community.

METHODS

Dyspeptics and healthy controls were identified among community respondents who had been randomly selected to complete validated questionnaires. Other diseases were excluded by face-to-face history and physical examination. Polymorphisms of candidate genes for alpha(2A), alpha(2C), 5-HT(1A), 5-HT(2A), 5-HT(2C), CCK-1 receptors and CCK promoter, GNbeta3 protein, and SERT-promoter (SERT-P) were studied. The association between polymorphisms and meal-related or meal-unrelated dyspepsia, high somatic symptom scores, and somatization were evaluated using Fisher's exact test.

RESULTS

DNA was available from 41 dyspeptics and 47 healthy controls from Olmsted County. Community dyspepsia unrelated to meals was associated with both homozygous GNbeta3 protein 825T and C alleles. There were no significant associations with meal-related dyspepsia. Using Rome II subgroups, the same genotype was associated with dysmotility-like and other dyspepsia. Higher somatization scores were not significantly associated with any of the candidate genes when considered as single factors.

CONCLUSION

Meal-unrelated dyspepsia in a U.S. community study is associated with the homozygous 825T or C alleles of GNbeta3 protein. Candidate genes controlling adrenergic, serotonergic, and CCKergic functions do not appear to be associated with dyspepsia.

Targeted knockdown of G protein subunits selectively prevents receptor-mediated modulation of effectors and reveals complex changes in non-targeted signaling proteins

Heterotrimeric G protein signaling specificity has been attributed to select combinations of Galpha, beta, and gamma subunits, their interactions with other signaling proteins, and their localization in the cell. With few exceptions, the G protein subunit combinations that exist in vivo and the significance of these specific combinations are largely unknown. We have begun to approach these problems in HeLa cells by: 1) determining the concentrations of Galpha and Gbeta subunits; 2) examining receptor-dependent activities of two effector systems (adenylyl cyclase and phospholipase Cbeta); and 3) systematically silencing each of the Galpha and Gbeta subunits by using small interfering RNA while quantifying resultant changes in effector function and the concentrations of other relevant proteins in the network. HeLa cells express equimolar amounts of total Galpha and Gbeta subunits. The most prevalent Galpha proteins were one member of each Galpha subfamily (Galpha(s), Galpha(i3), Galpha(11), and Galpha(13)). We substantially abrogated expression of most of the Galpha and Gbeta proteins expressed in these cells, singly and some in combinations. As expected, agonist-dependent activation of adenylyl cyclase or phospholipase Cbeta was specifically eliminated following the silencing of Galpha(s) or Galpha(q/11), respectively. We also confirmed that Gbeta subunits are necessary for stable accumulation of Galpha proteins in vivo. Gbeta subunits demonstrated little isoform specificity for receptor-dependent modulation of effector activity. We observed compensatory changes in G protein accumulation following silencing of individual genes, as well as an apparent reciprocal relationship between the expression of certain Galpha(q) and Galpha(i) subfamily members. These findings provide a foundation for understanding the mechanisms that regulate the adaptability and remarkable resilience of G protein signaling networks.

Gialpha and Gbeta subunits both define selectivity of G protein activation by alpha2-adrenergic receptors

Previous studies of the specificity of receptor interactions with G protein subunits in living cells have relied on measurements of second messengers or other downstream responses. We have examined the selectivity of interactions between alpha2-adrenergic receptors (alpha2R) and various combinations of Gialpha and Gbeta subunit isoforms by measuring changes in FRET between Gialpha-yellow fluorescent protein and cyan fluorescent protein-Gbeta chimeras in HeLa cells. All combinations of Gialpha1, -2, or -3 with Gbeta1, -2, or -4 were activated to some degree by endogenous alpha2Rs as judged by agonist-dependent decreases in FRET. The degree of G protein activation is determined by the combination of Gialpha and Gbeta subunits rather than by the identity of an individual subunit. RT-PCR analysis and small interfering RNA knockdown of alpha2R subtypes, followed by quantification of radiolabeled antagonist binding, demonstrated that HeLa cells express alpha2a- and alpha2b-adrenergic receptor isoforms in a 2:1 ratio. Increasing receptor number by overexpression of the alpha2aR subtype minimized the differences among coupling preferences for Gialpha and Gbeta isoforms. The molecular properties of each Gialpha, Gbeta, and alpha2-adrenergic receptor subtype influence signaling efficiency for the alpha2-adrenergic receptor-mediated signaling pathway.

Regulation of receptor-coupling to (multiple) G proteins. A challenge for basic research and drug discovery

G protein-coupled receptors induce intracellular signals via interaction of with cytosolic/peripheral membrane proteins, mainly G proteins. There has been much debate about the mode of interaction between the receptors, G proteins and effectors, their mobility and the ways of determining the specificity of interaction. Additional complexity has been added to system upon the discovery of i) coupling of single receptors to several G proteins and ii) active direction of this by different ligands (stimulus trafficking). These data suggest that the most primary unit in the signal transduction is the receptor complexed with a specific G protein, making the investigation of the mechanism of receptor-G protein selection and interaction even more important. In this review, I will summarize the general knowledge of receptor interaction with G proteins and effectors and the ways of investigating this.

TYRA-2 (F01E11.5): a Caenorhabditis elegans tyramine receptor expressed in the MC and NSM pharyngeal neurons

Tyramine appears to regulate key processes in nematodes, such as pharyngeal pumping, and more complex behaviors, such as foraging. Recently, a Caenorhabditis elegans tyramine receptor, SER-2, was identified that is involved in the TA-dependent regulation of these processes. In the present study, we have identified a second C. elegans gene, tyra-2 (F01E11.5) that encodes a tyramine receptor. This is the first identification of multiple tyramine receptor genes in any invertebrate. Membranes from COS-7 cells expressing TYRA-2 bind [(3)H]tyramine with high affinity with a K(d) of 20 +/- 5 nM. Other physiologically relevant biogenic amines, such as octopamine and dopamine, inhibit [(3)H]tyramine binding with much lower affinity (K(i)s of 1.55 +/- 0.5 and 1.78 +/- 0.6 microM, respectively), supporting the identification of TYRA-2 as a tyramine receptor. Indeed, tyramine also dramatically increases GTPgammaS binding to membranes from cells expressing TYRA-2 (EC(50) of 50 +/- 13 nM) and the TA-dependent GTPgammaS binding is PTX-sensitive suggesting that TYRA-2 may couple to Galpha(i/o). Based on fluorescence from tyra::gfp fusion constructs, TYRA-2 expression appears to be exclusively neuronal in the MC and NSM pharyngeal neurons, the AS family of amphid neurons and neurons in the nerve ring, body and tail. Taken together, these results suggest that TYRA-2 encodes a second Galpha(i/o)-coupled tyramine receptor and suggests that TA-dependent neuromodulation may be mediated by multiple receptors and more complex than previously appreciated.

Statin-induced blockade of prenylation alters nucleocytoplasmic shuttling of GTP-binding proteins gamma2 and beta2 and enhances their suppressive effect on glucocorticoid receptor transcriptional activity

BACKGROUND

We previously reported that the guanine tri-phosphate-binding proteins (G) beta and gamma are both localized in the nucleus, in addition to their expected cytoplasmic/plasma membrane localization. These proteins, as a heterodimeric complex, suppress glucocorticoid response element-mediated transcriptional activity of the glucocorticoid receptor through direct physical interactions between Gbeta and the glucocorticoid receptor.

MATERIALS AND METHODS

As Ggamma is prenylated at a cysteine residue in its C-terminal portion, and as this post-translational modification is required for many of the known Gbeta/Ggamma activities, we examined the effect of its absence or diminution on Gbeta/Ggamma-induced suppression of glucocorticoid receptor-induced transcriptional activity.

RESULTS

In a functional reporter assay, Ggamma2C68S, which is defective at the prenylation site, was more potent than the wild-type Ggamma2 at increasing Gbeta2-induced suppression of glucocorticoid receptor transactivation. Interestingly, the enhanced green fluorescent protein fusion of this mutant Ggamma2 was localized preferentially in the nucleus, while it was absent from the plasma membrane. Lovastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor that abrogates the prenylation of Ggamma, shifted the subcellular localization of enhanced green fluorescence protein-fused Ggamma2 and Gbeta2 from the cytoplasm/plasma membrane to the nucleus and further suppressed glucocorticoid receptor-induced transcriptional activity.

CONCLUSIONS

These findings indicate that not only is the natural covalent addition of the prenyl residue to Ggamma unnecessary for the transcriptional suppression induced by Gbeta/Ggamma on the glucocorticoid receptor, but rather helps retain the Gbeta/Ggamma complex away from the nucleus decreasing its antiglucocorticoid actions.