Nobel Lecture. G proteins and regulation of adenylyl cyclase

Binding of β4γ5 by adenosine A1 and A2A receptors determined by stable isotope labeling with amino acids in cell culture and mass spectrometry

Characterization of G protein βγ dimer isoform expression in different cellular contexts has been impeded by low levels of protein expression, broad isoform heterogeneity, and antibodies of limited specificity, sensitivity, or availability. As a new approach, we used quantitative mass spectrometry to characterize native βγ dimers associated with adenosine A(1):α(i1) and adenosine A(2A):α(S) receptor fusion proteins expressed in HEK-293 cells. Cells expressing A(1):α(i1) were cultured in media containing [(13)C(6)]Arg and [(13)C(6)]Lys and βγ labeled with heavy isotopes purified. Heavy βγ was combined with either recombinant βγ purified from Sf9 cells, βγ purified from the A(2A):α(S) expressed in HEK-293 cells cultured in standard media, or an enriched βγ fraction from HEK-293 cells. Samples were separated by SDS-PAGE, protein bands containing β and γ were excised, digested with trypsin, and separated by HPLC, and isotope ratios were analyzed by mass spectrometry. Three β isoforms, β(1), β(2), and β(4), and seven γ isoforms, γ(2), γ(4), γ(5), γ(7), γ(10), γ(11), and γ(12), were identified in the analysis. β(1) and γ(5) were most abundant in the enriched βγ fraction, and this βγ profile was generally mirrored in the fusion proteins. However, both A(2A):α(S) and A(1):α(i1) bound more β(4) and γ(5) compared to the enriched βγ fraction; also, more β(4) was associated with A(2A):α(S) than A(1):α(i1). Both fusion proteins also contained less γ(2), γ(10), and γ(12) than the enriched βγ fraction. These results suggest that preferences for particular βγ isoforms may be driven in part by structural motifs common to adenosine receptor family members.

Energy variational analysis of ions in water and channels: Field theory for primitive models of complex ionic fluids

Ionic solutions are mixtures of interacting anions and cations. They hardly resemble dilute gases of uncharged noninteracting point particles described in elementary textbooks. Biological and electrochemical solutions have many components that interact strongly as they flow in concentrated environments near electrodes, ion channels, or active sites of enzymes. Interactions in concentrated environments help determine the characteristic properties of electrodes, enzymes, and ion channels. Flows are driven by a combination of electrical and chemical potentials that depend on the charges, concentrations, and sizes of all ions, not just the same type of ion. We use a variational method EnVarA (energy variational analysis) that combines Hamilton's least action and Rayleigh's dissipation principles to create a variational field theory that includes flow, friction, and complex structure with physical boundary conditions. EnVarA optimizes both the action integral functional of classical mechanics and the dissipation functional. These functionals can include entropy and dissipation as well as potential energy. The stationary point of the action is determined with respect to the trajectory of particles. The stationary point of the dissipation is determined with respect to rate functions (such as velocity). Both variations are written in one Eulerian (laboratory) framework. In variational analysis, an "extra layer" of mathematics is used to derive partial differential equations. Energies and dissipations of different components are combined in EnVarA and Euler-Lagrange equations are then derived. These partial differential equations are the unique consequence of the contributions of individual components. The form and parameters of the partial differential equations are determined by algebra without additional physical content or assumptions. The partial differential equations of mixtures automatically combine physical properties of individual (unmixed) components. If a new component is added to the energy or dissipation, the Euler-Lagrange equations change form and interaction terms appear without additional adjustable parameters. EnVarA has previously been used to compute properties of liquid crystals, polymer fluids, and electrorheological fluids containing solid balls and charged oil droplets that fission and fuse. Here we apply EnVarA to the primitive model of electrolytes in which ions are spheres in a frictional dielectric. The resulting Euler-Lagrange equations include electrostatics and diffusion and friction. They are a time dependent generalization of the Poisson-Nernst-Planck equations of semiconductors, electrochemistry, and molecular biophysics. They include the finite diameter of ions. The EnVarA treatment is applied to ions next to a charged wall, where layering is observed. Applied to an ion channel, EnVarA calculates a quick transient pile-up of electric charge, transient and steady flow through the channel, stationary "binding" in the channel, and the eventual accumulation of salts in "unstirred layers" near channels. EnVarA treats electrolytes in a unified way as complex rather than simple fluids. Ad hoc descriptions of interactions and flow have been used in many areas of science to deal with the nonideal properties of electrolytes. It seems likely that the variational treatment can simplify, unify, and perhaps derive and improve those descriptions.

Intracellular delivery of an antisense oligonucleotide via endocytosis of a G protein-coupled receptor

Gastrin-releasing peptide receptor (GRPR), a member of the G protein-coupled receptor superfamily, has been utilized for receptor-mediated targeting of imaging and therapeutic agents; here we extend its use to oligonucleotide delivery. A splice-shifting antisense oligonucleotide was conjugated to a bombesin (BBN) peptide, and its intracellular delivery was tested in GRPR expressing PC3 cells stably transfected with a luciferase gene interrupted by an abnormally spliced intron. The BBN-conjugate produced significantly higher luciferase expression compared to unmodified oligonucleotide, and this increase was reversed by excess BBN peptide. Kinetic studies revealed a combination of saturable, receptor-mediated endocytosis and non-saturable pinocytosis for uptake of the conjugate. The K_m value for saturable uptake was similar to the EC₅₀ value for the pharmacological response, indicating that receptor-mediated endocytosis was a primary contributor to the response. Use of pharmacological and molecular inhibitors of endocytosis showed that the conjugate utilized a clathrin-, actin- and dynamin-dependent pathway to enter PC3 cells. The BBN-conjugate partially localized in endomembrane vesicles that were associated with Rab7 or Rab9, demonstrating that it was transported to late endosomes and the trans-golgi network. These observations suggest that the BBN-oligonucleotide conjugate enters cells via a process of GRPR mediated endocytosis followed by trafficking to deep endomembrane compartments.

GPCR theme editorial

This themed section of BJP includes 11 reviews on the biology of G-protein coupled receptors (GPCRs) and the drug targets that these present, 21 research papers on the pharmacology of a range of GPCRs and Commentaries on four of the papers. Areas reviewed include molecular interactions, particular in respect of hetero-dimerisation between receptors and other membrane-located proteins and other key signalling molecules including cAMP and G12/13 proteins and recently de-orphanised receptors including the Neuromedins U & S and the Free Fatty Acid receptors FFA2 & FFA3. The research papers cover the pharmacology of a range of agents acting at GPCRs, including adrenoceptors, purinoceptors, 5HT, opioid, cannabinoid & PAR-2 receptors. A group of papers is concerned with the interesting and rapidly developing pharmacology of drugs acting at beta(2)-adrenoceptors. The reach of GPCRs is illustrated by the range of physiological systems and therapeutic applications involved, including pain, cancer, cardiovascular, gastrointestinal, visual and respiratory and central nervous systems.

Neurotransmitters, receptors, and second messengers galore in 40 years

The past four decades have witnessed extraordinary advances in the molecular understanding of neurotransmitters, their receptors, and second messengers. This essay highlights a selected group of particular notable discoveries, emphasizing seminal findings that have transformed thinking in the field.

Inhibition of heterotrimeric G protein signaling by a small molecule acting on Galpha subunit

The simultaneous activation of many distinct G protein-coupled receptors (GPCRs) and heterotrimeric G proteins play a major role in various pathological conditions. Pan-inhibition of GPCR signaling by small molecules thus represents a novel strategy to treat various diseases. To better understand such therapeutic approach, we have characterized the biomolecular target of BIM-46187, a small molecule pan-inhibitor of GPCR signaling. Combining bioluminescence and fluorescence resonance energy transfer techniques in living cells as well as in reconstituted receptor-G protein complexes, we observed that, by direct binding to the Galpha subunit, BIM-46187 prevents the conformational changes of the receptor-G protein complex associated with GPCR activation. Such a binding prevents the proper interaction of receptors with the G protein heterotrimer and inhibits the agonist-promoted GDP/GTP exchange. These observations bring further evidence that inhibiting G protein activation through direct binding to the Galpha subunit is feasible and should constitute a new strategy for therapeutic intervention.

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.

Crosstalk between G-protein and Ca2+ pathways switches intracellular cAMP levels

Cyclic adenosine monophosphate and cyclic guanosine monophosphate are universal intracellular messengers whose concentrations are regulated by molecular networks comprised of different isoforms of the synthases adenylate cyclase or guanylate cyclase and the phosphodiesterases which degrade these compounds. In this paper, we employ a systems biology approach to develop mathematical models of these networks that, for the first time, take into account the different biochemical properties of the isoforms involved. To investigate the mechanisms underlying the joint regulation of cAMP and cGMP, we apply our models to analyse the regulation of cilia beat frequency in Paramecium by Ca(2+). Based on our analysis of these models, we propose that the diversity of isoform combinations that occurs in living cells provides an explanation for the huge variety of intracellular processes that are dependent on these networks. The inclusion of both G-protein receptor and Ca(2+)-dependent regulation of AC in our models allows us to propose a new explanation for the switching properties of G-protein subunits involved in nucleotide regulation. Analysis of the models suggests that, depending on whether the G-protein subunit is bound to AC, Ca(2+) can either activate or inhibit AC in a concentration-dependent manner. The resulting analysis provides an explanation for previous experimental results that showed that alterations in Ca(2+) concentrations can either increase or decrease cilia beat frequency over particular Ca(2+) concentration ranges.

Role of endogenous RGS proteins on endothelial ERK 1/2 activation

Endothelial cells are maintaining atherosclerotic signaling mediated by Extracellular Regulated Kinases 1 and 2 (ERK). Signaling gets activated upon stimulation of G protein-coupled receptors mediated by G(q) and G(i/o) proteins subjected to regulation by RGS proteins. The goal of the study was to delineate the specificity of RGS proteins modulating induced ERK phosphorylation. We used stimulated HUVEC, silenced specifically RGS proteins and compared assessed ERK 1/2 activation with immunohistochemical stainings on atherosclerotic plaques. Increased ERK phosphorylation was detected upon stimulation with Phenylephrine (2.6+/-0.1 times over basal), Endothelin-1 (1.8+/-0.2), Dopamine (5.1+/-0.2), TNF (9.8+/-0.7) or IL-4 (3.1+/-0.3). RGS silencing increased activation of ERK 1/2: Phen (RGS3, 5), ET-1 (RGS3, 4), Dopa (RGS3), TNF (RGS2, 3, 4) or IL-4 (RGS2, 3, 4). Immunohistochemically, increased ERK activation was detected on atherosclerotic plaques. This data supports the role of RGS proteins on ERK activation in human atherosclerosis which identifies RGS proteins as new therapeutical targets.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

The cyclic AMP phenotype of fragile X and autism

Cyclic AMP (cAMP) is a second messenger involved in many processes including mnemonic processing and anxiety. Memory deficits and anxiety are noted in the phenotype of fragile X (FX), the most common heritable cause of mental retardation and autism. Here we review reported observations of altered cAMP cascade function in FX and autism. Cyclic AMP is a potentially useful biochemical marker to distinguish autism comorbid with FX from autism per se and the cAMP cascade may be a viable therapeutic target for both FX and autism.

RGS protein specificity towards Gq- and Gi/o-mediated ERK 1/2 and Akt activation, in vitro

Extracellular Regulated Kinases (ERK) and Protein Kinase B (Akt) are intermediaries in relaying extracellular growth signals to intracellular targets. Each pathway can become activated upon stimulation of G protein-coupled receptors mediated by G(q) and G(i/o) proteins subjected to regulation by RGS proteins. The goal of the study was to delineate the specificity in which cardiac RGS proteins modulate G(q)and G(i/o)-induced ERK and Akt phosphorylation. To isolate G(q)- and G(i/o)-mediated effects, we exclusively expressed muscarinic M(2) or M(3) receptors in COS-7 cells. Western blot analyses demonstrated increase of phosphorylation of ERK 1.7-/3.3-fold and Akt 2.4-/6-fold in M(2)-/M(3)- expressing cells through carbachol stimulation. In co-expressions, M(3)/G(q)-induced activation of Akt was exclusively blunted through RGS3s/RGS3, whereas activation of ERK was inhibited additionally through RGS2/RGS5. M(2)/G(i/o) induced Akt activation was inhibited by all RGS proteins tested. RGS2 had no effect on M(2)/G(i/o)-induced ERK activation. The high degree of specificity in RGS proteins-depending modulation of G(q)- and G(i/o)-mediated ERK and Akt activation in the muscarinic network cannot merely be attributed exclusively to RGS protein selectivity towards G(q) or G(i/o) proteins. Counter-regulatory mechanisms and inter-signaling cross-talk may alter the sensitivity of GPCR-induced ERK and Akt activation to RGS protein regulation.

Crystal structure of the multifunctional Gbeta5-RGS9 complex

Regulators of G-protein signaling (RGS) proteins enhance the intrinsic GTPase activity of G protein alpha (Galpha) subunits and are vital for proper signaling kinetics downstream of G protein-coupled receptors (GPCRs). R7 subfamily RGS proteins specifically and obligately dimerize with the atypical G protein beta5 (Gbeta5) subunit through an internal G protein gamma (Ggamma)-subunit-like (GGL) domain. Here we present the 1.95-A crystal structure of the Gbeta5-RGS9 complex, which is essential for normal visual and neuronal signal transduction. This structure reveals a canonical RGS domain that is functionally integrated within a molecular complex that is poised for integration of multiple steps during G-protein activation and deactivation.

Human G(salpha) mutant causes pseudohypoparathyroidism type Ia/neonatal diarrhea, a potential cell-specific role of the palmitoylation cycle

Pseudohypoparathyroidism type Ia (PHP-Ia) results from the loss of one allele of G(salpha), causing resistance to parathyroid hormone and other hormones that transduce signals via G(s). Most G(salpha)mutations cause the complete loss of protein expression, but some cause loss of function only, and these have provided valuable insights into the normal function of G proteins. Here we have analyzed a mutant G(salpha) (alphas-AVDT) harboring AVDT amino acid repeats within its GDP/GTP binding site, which was identified in unique patients with PHP-Ia accompanied by neonatal diarrhea. Biochemical and intact cell analyses showed that alphas-AVDT is unstable but constitutively active as a result of rapid GDP release and reduced GTP hydrolysis. This instability underlies the PHP-Ia phenotype. alphas-AVDT is predominantly localized in the cytosol, but in rat and mouse small intestine epithelial cells (IEC-6 and DIF-12 cells) alphas-AVDT was found to be localized predominantly in the membrane where adenylyl cyclase is present and constitutive increases in cAMP accumulation occur in parallel. The likely cause of this membrane localization is the inhibition of an activation-dependent decrease in alphas palmitoylation. Upon the overexpression of acyl-protein thioesterase 1, however, alphas-AVDT translocates from the membrane to the cytosol, and the constitutive accumulation of cAMP becomes attenuated. These results suggest that PHP-Ia results from the instability of alphas-AVDT and that the accompanying neonatal diarrhea may result from its enhanced constitutive activity in the intestine. Hence, palmitoylation may control the activity and localization of G(salpha) in a cell-specific manner.

The cyclic AMP cascade is altered in the fragile X nervous system

Fragile X syndrome (FX), the most common heritable cause of mental retardation and autism, is a developmental disorder characterized by physical, cognitive, and behavioral deficits. FX results from a trinucleotide expansion mutation in the fmr1 gene that reduces levels of fragile X mental retardation protein (FMRP). Although research efforts have focused on FMRP's impact on mGluR signaling, how the loss of FMRP leads to the individual symptoms of FX is not known. Previous studies on human FX blood cells revealed alterations in the cyclic adenosine 3', 5'-monophosphate (cAMP) cascade. We tested the hypothesis that cAMP signaling is altered in the FX nervous system using three different model systems. Induced levels of cAMP in platelets and in brains of fmr1 knockout mice are substantially reduced. Cyclic AMP induction is also significantly reduced in human FX neural cells. Furthermore, cAMP production is decreased in the heads of FX Drosophila and this defect can be rescued by reintroduction of the dfmr gene. Our results indicate that a robust defect in cAMP production in FX is conserved across species and suggest that cAMP metabolism may serve as a useful biomarker in the human disease population. Reduced cAMP induction has implications for the underlying causes of FX and autism spectrum disorders. Pharmacological agents known to modulate the cAMP cascade may be therapeutic in FX patients and can be tested in these models, thus supplementing current efforts centered on mGluR signaling.

R7-binding protein targets the G protein beta 5/R7-regulator of G protein signaling complex to lipid rafts in neuronal cells and brain

Background

Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins), composed of Gα, Gβ, and Gγ subunits, are positioned at the inner face of the plasma membrane and relay signals from activated G protein-coupled cell surface receptors to various signaling pathways. Gβ5 is the most structurally divergent Gβ isoform and forms tight heterodimers with regulator of G protein signalling (RGS) proteins of the R7 subfamily (R7-RGS). The subcellular localization of Gβ 5/R7-RGS protein complexes is regulated by the palmitoylation status of the associated R7-binding protein (R7BP), a recently discovered SNARE-like protein. We investigate here whether R7BP controls the targeting of Gβ5/R7-RGS complexes to lipid rafts, cholesterol-rich membrane microdomains where conventional heterotrimeric G proteins and some effector proteins are concentrated in neurons and brain.

Results

We show that endogenous Gβ5/R7-RGS/R7BP protein complexes are present in native neuron-like PC12 cells and that a fraction is targeted to low-density, detergent-resistant membrane lipid rafts. The buoyant density of endogenous raft-associated Gβ5/R7-RGS protein complexes in PC12 cells was similar to that of lipid rafts containing the palmitoylated marker proteins PSD-95 and LAT, but distinct from that of the membrane microdomain where flotillin was localized. Overexpression of wild-type R7BP, but not its palmitoylation-deficient mutant, greatly enriched the fraction of endogenous Gβ5/R7-RGS protein complexes in the lipid rafts. In HEK-293 cells the palmitoylation status of R7BP also regulated the lipid raft targeting of co-expressed Gβ5/R7-RGS/R7BP proteins. A fraction of endogenous Gβ5/R7-RGS/R7BP complexes was also present in lipid rafts in mouse brain.

Conclusion

A fraction of Gβ5/R7-RGS/R7BP protein complexes is targeted to low-density, detergent-resistant membrane lipid rafts in PC12 cells and brain. In cultured cells, the palmitoylation status of R7BP regulated the lipid raft targeting of endogenous or co-expressed Gβ5/R7-RGS proteins. Taken together with recent evidence that the kinetic effects of the Gβ5 complex on GPCR signaling are greatly enhanced by R7BP palmitoylation through a membrane-anchoring mechanism, our data suggest the targeting of the Gβ5/R7-RGS/R7BP complex to lipid rafts in neurons and brain, where G proteins and their effectors are concentrated, may be central to the G protein regulatory function of the complex.

Phosphorylation energy hypothesis: open chemical systems and their biological functions

Biochemical systems and processes in living cells generally operate far from equilibrium. This review presents an overview of a statistical thermodynamic treatment for such systems, with examples from several key components in cellular signal transduction. Open-system nonequilibrium steady-state (NESS) models are introduced. The models account quantitatively for the energetics and thermodynamics in phosphorylation-dephosphorylation switches, GTPase timers, and specificity amplification through kinetic proofreading. The chemical energy derived from ATP and GTP hydrolysis establishes the NESS of a cell and makes the cell--a mesoscopic-biochemical reaction system that consists of a collection of thermally driven fluctuating macromolecules--a genetically programmed chemical machine.

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.

Platelet-activating factor modulates activity of cyclic nucleotides in fetal ovine pulmonary vascular smooth muscle

At birth, release of endogenous vasodilators such as nitric oxide and prostacyclin facilitate pulmonary vasodilation via the cyclic nucleotides, cGMP and cAMP. Interaction of cyclic nucleotides and platelet-activating factor (PAF)-mediated responses in pulmonary vascular smooth muscle is not known. We studied the effects of cGMP and cAMP on PAF-mediated responses in ovine fetal intrapulmonary venous smooth muscle cells. Studies were done in hypoxia or normoxia with buffer with 8-Br-cGMP (BGMP) and 8-Br-cAMP (BAMP), as well as cGMP-dependent protein kinase (PKG) and cAMP-dependent protein kinase (PKA) inhibitors. All groups were treated with 1 nM PAF and incubated for 30 min for the binding assay or 20 min for measurement of inositol 1,4,5-phosphate (IP(3)) production. BGMP and BAMP decreased PAF binding in normoxia by 63 and 14%, respectively. Incubations with the PKG inhibitor Rp-8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphorothioate sodium and the PKA inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate abrogated the inhibitory effects of BGMP and BAMP. PAF-stimulated IP(3) production was 8565 +/- 314 dpm/10(6) cells in hypoxia and 5418 +/- 118 dpm/10(6) cells in normoxia, a 40% decrease. BGMP attenuated PAF-stimulated IP(3) production by 67 and 37% in hypoxia and normoxia, respectively; the value for BAMP was 44% under both conditions. Pretreatment with PKG or PKA inhibitor abrogated BGMP and BAMP inhibition of IP(3) release. PAF receptor (PAFr) protein expression decreased in normoxia, but pretreatment with 10 nM PAF up-regulated PAFr expression. Pretreatment with PAF decreased expression and activities of PKG or PKA proteins in normoxia and hypoxia. Our data demonstrate the existence of cGMP/cAMP-PAF cross-talk in pulmonary vascular smooth muscle cells, which may be one mechanism by which PAFr-mediated vasoconstriction is down-regulated at birth.

The 7 TM G-protein-coupled receptor target family

Chemical biology approaches have a long history in the exploration of the G-protein-coupled receptor (GPCR) family, which represents the largest and most important group of targets for therapeutics. The analysis of the human genome revealed a significant number of new members with unknown physiological function which are today the focus of many reverse pharmacology drug-discovery programs. As the seven hydrophobic transmembrane segments are a defining common structural feature of these receptors, and as signaling through heterotrimeric G proteins is not demonstrated in all cases, these proteins are also referred to as seven transmembrane (7 TM) or serpentine receptors. This review summarizes important historic milestones of GPCR research, from the beginning, when pharmacology was mainly descriptive, to the age of modern molecular biology, with the cloning of the first receptor and now the availability of the entire human GPCR repertoire at the sequence and protein level. It shows how GPCR-directed drug discovery was initially based on the careful testing of a few specifically made chemical compounds and is today pursued with modern drug-discovery approaches, including combinatorial library design, structural biology, molecular informatics, and advanced screening technologies for the identification of new compounds that activate or inhibit GPCRs specifically. Such compounds, in conjunction with other new technologies, allow us to study the role of receptors in physiology and medicine, and will hopefully result in novel therapies. We also outline how basic research on the signaling and regulatory mechanisms of GPCRs is advancing, leading to the discovery of new GPCR-interacting proteins and thus opening new perspectives for drug development. Practical examples from GPCR expression studies, HTS (high-throughput screening), and the design of monoamine-related GPCR-focused combinatorial libraries illustrate ongoing GPCR chemical biology research. Finally, we outline future progress that may relate today's discoveries to the development of new medicines.

G-protein signaling: a new branch in an old pathway

A recent study provides evidence for a new branch of the yeast mating pathway in which a G-protein alpha subunit directly activates phosphatidylinositol 3-kinase at endosomes.

The G protein beta3 subunit 825C allele is associated with sudden infant death due to infection

AIM

To investigate the Gbeta3 subunit C825T polymorphism with regard to sudden unexpected infant death. The reported association between the Gbeta3s protein and increased immune cell function in humans makes this polymorphism highly interesting both with regard to sudden infant death syndrome (SIDS) and deleterious infectious disease.

METHODS

The cases investigated in the present study consist of 250 SIDS cases, 38 cases of sudden unexpected infant death due to infection and 99 living infant controls. Typing of the C825T polymorphism was performed by real-time PCR with allele-specific probes and melting curve analyses.

RESULTS

The cases of infectious death have a higher percentage of both the C allele (p=0.037 compared to the SIDS cases, p=0.022 compared to the controls) and the CC genotype (p=0.05 compared to the SIDS cases, p=0.016 compared to the controls). There were no differences between SIDS cases and controls.

CONCLUSION

The observed association between the 825C allele and infectious death may indicate that the presence of the 825T allele exerts a protective effect towards serious infection, possibly through enhanced G protein signalling. The C allele, on the other hand, appears to represent a disadvantage in this regard.

Cellular and molecular biology of orphan G protein-coupled receptors

The superfamily of G protein-coupled receptors (GPCRs) is the largest and most diverse group of membrane-spanning proteins. It plays a variety of roles in pathophysiological processes by transmitting extracellular signals to cells via heterotrimeric G proteins. Completion of the human genome project revealed the presence of approximately 168 genes encoding established nonsensory GPCRs, as well as 207 genes predicted to encode novel GPCRs for which the natural ligands remained to be identified, the so-called orphan GPCRs. Eighty-six of these orphans have now been paired to novel or previously known molecules, and 121 remain to be deorphaned. A better understanding of the GPCR structures and classification; knowledge of the receptor activation mechanism, either dependent on or independent of an agonist; increased understanding of the control of GPCR-mediated signal transduction; and development of appropriate ligand screening systems may improve the probability of discovering novel ligands for the remaining orphan GPCRs.

Proteomic analysis of bovine brain G protein gamma subunit processing heterogeneity

We characterized the variable processing of the G protein gamma subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. Ggamma subunits were isolated from purified brain G proteins and characterized by Edman sequencing, by MALDI MS, by chemical and/or enzymatic fragmentation assayed by MALDI MS, and by MS/MS fragmentation and sequencing. Multiple forms of six different Ggamma isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All Ggamma isoforms contain a carboxyl-terminal CAAX motif for prenylation, carboxyl-terminal proteolysis, and carboxymethylation. Characterization of these proteins indicates significant variability in the normal processing of all of these steps in the prenylation reaction, including a new variation of prenyl processing resulting from cysteinylation of the carboxyl terminus. These results have multiple implications for intracellular signaling mechanisms by G proteins, for the role of prenyl processing variation in cell signaling, and for the site of action and consequences of drugs that target the prenylation modification.

Functional MT1 and MT2 melatonin receptors in mammals

Melatonin, dubbed the hormone of darkness, is known to regulate a wide variety of physiological processes in mammals. This review describes well-defined functional responses mediated through activation of high-affinity MT1 and MT2 G protein-coupled receptors viewed as potential targets for drug discovery. MT1 melatonin receptors modulate neuronal firing, arterial vasocon-striction, cell proliferation in cancer cells, and reproductive and metabolic functions. Activation of MT2 melatonin receptors phase shift circadian rhythms of neuronal firing in the suprachiasmatic nucleus, inhibit dopamine release in retina, induce vasodilation and inhibition of leukocyte rolling in arterial beds, and enhance immune responses. The melatonin-mediated responses elicited by activation of MT1 and MT2 native melatonin receptors are dependent on circadian time, duration and mode of exposure to endogenous or exogenous melatonin, and functional receptor sensitivity. Together, these studies underscore the importance of carefully linking each melatonin receptor type to specific functional responses in target tissues to facilitate the design and development of novel therapeutic agent.

Regulation of cyclic nucleotides in the urinary tract

Cyclic nucleotide levels are controlled through their synthesis from nucleotide triphosphates by cyclases and their degradation to 5'-monophosphates by phosphodiesterases (PDEs). Components controlling cyclic AMP-induced relaxation in the urinary tract include receptors, inhibitory and stimulatory G-proteins, isoforms of adenylyl cyclase and PDEs. The responsiveness of PDEs to a variety of physiological challenges is related to the presence of multiple families of isoenzymes with specific localization within tissues and within cells. At least 11 families of PDEs encode more than 50 PDE proteins produced in mammalian cells. In the urinary tract, characterization of PDE isoforms has lagged behind other systems and much of the literature was published prior to identification of PDE7, 8, 9, 10, 11. Specific PDE inhibitors regulate smooth muscle function in the bladder, urethra, prostate and ureter. The pharmacological potential of these inhibitors may include treatment of urge incontinence and the low compliance bladder, and treatment of prostate cancer. G-proteins also regulate cyclic AMP production. Changes in specific G- protein isoforms with aging, most prominently Gialpha2, cause decreased relaxation response in the aging bladder. As we have seen here with aging and certainly in other disease processes, levels of the components of adenylyl cyclase/phosphodiesterase/protein kinase can change and thus affect the relaxation response. By exploitation of differences in PDE expression in disease, such as the overexpression of PDEs in cancer, treatment options may present themselves.

Genomic analysis of G protein gamma subunits in human and mouse - the relationship between conserved gene structure and G protein betagamma dimer formation

Analysis of the genomic sequences, cDNAs and expressed sequence tags (ESTs) in human and mouse for the 12 genes of the gamma subunits of the heterotrimeric G proteins has allowed us to identify the common versus unique elements of the organization and expression of the members of this important gene family. All of the G protein gamma subunit genes are organized around two coding exons, each containing about 100 nucleotides coding for 30-40 amino acids. These two exons each correspond to a functional domain of the protein, which interestingly appears to impose constraints on both the structure of the protein and the structure of the gene. There is large variation in the intron size between these two coding exons, the number and size of 5' and 3' UTRs, and the overall size of the genes. There is general but not absolute conservation in the size and structure of these genes between humans and mice. Alternative splicing and potential differential promoter usage were detected for several Ggamma subunits, indicating possible differential regulation in expression. Only for Ggamma10, however, did we find an alternative coding transcript. This alternative transcript appears to code for a hybrid protein containing a DnaJ domain in place of its Ggamma exon 1 domain, joined to the Ggamma10 second exon domain. The predicted mRNA is expressed in humans, and the protein coded by it is readily translated in vitro. This protein does not form a functional G protein betagamma dimer, but it could generate a chaperone-like protein related to its DNA-J domain. These studies suggest that alternative splicing is not a prominent mechanism for generating G protein subunit diversity from within the human or mouse genomes. Instead, each of the known 12 gamma subunit genes generate transcripts with one prevalent protein.

The role of G proteins in the activity and ethanol modulation of glycine-induced currents in rat neurons freshly isolated from the ventral tegmental area

In freshly isolated neurons of the ventral tegmental area of young rats, we first examined the role of G proteins in the functional modulation of the glycine receptor (GlyR). GTP-gamma-S [guanosine-5'-0-(2-thiotriphosphate)] (2 mM) or GDP-beta-S [guanosine 5'-0-(2-thiodiphosphate)] (2 mM) was added to the pipette solution of whole-cell recordings to regulate G protein activities. GTP-gamma-S enhanced the amplitude of glycine-induced current (I(Gly)), suggesting modulation of GlyRs via a G protein-coupled pathway. GDP-beta-S suppressed I(Gly), suggesting that basal G protein activity positively modulates the GlyRs. We next examined effects of G proteins in ethanol potentiation of GlyR function. Activation of G proteins with 2 mM GTP-gamma-S attenuated, but did not eliminate, ethanol-induced potentiation of I(Gly). These results suggest that GTP-gamma-S and ethanol share the same pathway of activating GlyRs. When G proteins are maximally activated by GTP-gamma-S, the action of ethanol was partially occluded. When 2 mM GDP-beta-S was added in pipette solution, ethanol-induced potentiation of I(Gly) was significantly attenuated, suggesting that GDP-beta-S partially blocked the action of ethanol. However, the inability of GTP-gamma-S (or GDP-beta-S) to eliminate completely the potentiating effect of ethanol indicates that some other factors, in addition to G proteins, may also contribute to the action of ethanol on GlyRs.

Complex haplotype structure of the human GNAS gene identifies a recombination hotspot centred on a single nucleotide polymorphism widely used in association studies

The alpha subunit of the heterotrimeric G protein Gs (Gsalpha) is involved in numerous physiological processes and is a primary determinant of cellular responses to extracellular signals. Genetic variations in the Gsalpha gene may play an important role in complex diseases and drug responses. To characterize the genetic diversity in this locus, we resequenced exons and flanking introns of the gene in 44 genomic samples and analysed the haplotype structure of the gene in an additional 50 African-Americans and 50 Caucasians. Significant differences in allele frequency for nearly all the genotyped single nucleotide polymorphism (SNPs) were detected between the two ethnic groups. Linkage disequilibrium (LD) analysis of this locus revealed two haplotype blocks characterized by strong LD and reduced haplotype diversity, especially in Caucasians. Between the two blocks is a narrow (approximately 3 kb) recombination hotspot centred on exons 4 and 5, and a widely used genetic marker in association studies in this region (rs7121) was in linkage equilibrium with the rest of the gene. The haplotype structure of the GNAS locus warrants reevaluation of previous association studies that used marker rs7121 and affects choice of SNP markers to be used in future studies of this locus.

Competition between lithium and magnesium ions for the G-protein transducin in the guanosine 5'-diphosphate bound conformation

Li(+) is the most effective drug used to treat bipolar disorder; however, its exact mechanism of action has yet to be elucidated. One hypothesis is that Li(+) competes with Mg2+ for the Mg2+ binding sites on guanine-nucleotide binding proteins (G-proteins). Using 7Li T1 relaxation measurements and fluorescence spectroscopy with the Mg2+ fluorophore furaptra, we detected Li(+)/Mg(2+) competition in three preparations: the purified G-protein transducin (Gt), stripped rod outer segment membranes (SROS), and SROS with purified Gt reattached (ROS-T). When purified ROS-T, SROS or transducin were titrated with Li+ in the presence of fixed amounts of Mg(2+), the apparent Li(+) binding constant decreased due to Li(+)/Mg(2+) competition. Whereas for SROS the competition mechanism was monophasic, for G(t), the competition was biphasic, suggesting that in G(t), Li(+)/Mg(2+) competition occurred with different affinities for Mg(2+) in two types of Mg(2+) binding sites. Moreover, as [Li(+)] increased, the fluorescence excitation spectra of both ROS-T and G(t) were blue shifted, indicating an increase in free [Mg(2+)] compatible with Li(+) displacement of Mg(2+) from two low affinity Mg(2+) binding sites of G(t). G(t) release from ROS-T membrane was also inhibited by Li(+) addition. In summary, we found evidence of Li(+)/Mg(2+) competition in G(t)-containing preparations.

Effect of linoleic acid on proliferation and gene expression in the breast cancer cell line T47D

Human and animal studies have linked n-6 polyunsaturated fatty acids with mammary carcinogenesis. We investigated the cellular and molecular effects of linoleic acid on the human breast cancer cell line T47D. Linoleic acid had a stimulatory effect on the growth of T47D cells, associated with an increase in the proportion of cells in the S phase of the cell cycle. Microarray, functional group and quantitative PCR analyses indicate that linoleic acid may affect T47D cell growth by modulation of the estrogen receptor (ERalpha), the G13alpha G protein, and p38 MAP kinase gene expression as well genes involved in RNA transcription and cell cycle regulation.

Differential Contribution of GTPase Activation and Effector Antagonism to the Inhibitory Effect of RGS Proteins on Gq-mediated Signaling in Vivo

RGS proteins act as negative regulators of G protein signaling by serving as GTPase-activating proteins (GAP) for alpha subunits of heterotrimeric G proteins (Galpha), thereby accelerating G protein inactivation. RGS proteins can also block Galpha-mediated signal production by competing with downstream effectors for Galpha binding. Little is known about the relative contribution of GAP and effector antagonism to the inhibitory effect of RGS proteins on G protein-mediated signaling. By comparing the inhibitory effect of RGS2, RGS3, RGS5, and RGS16 on Galpha(q)-mediated phospholipase Cbeta (PLCbeta) activation under conditions where GTPase activation is possible versus nonexistent, we demonstrate that members of the R4 RGS subfamily differ significantly in their dependence on GTPase acceleration. COS-7 cells were transiently transfected with either muscarinic M3 receptors, which couple to endogenous Gq protein and mediate a stimulatory effect of carbachol on PLCbeta, or constitutively active Galphaq*, which is inert to GTP hydrolysis and activates PLCbeta independent of receptor activation. In M3-expressing cells, all of the RGS proteins significantly blunted the efficacy and potency of carbachol. In contrast, Galphaq* -induced PLCbeta activation was inhibited by RGS2 and RGS3 but not RGS5 and RGS16. The observed differential effects were not due to changes in M3, Galphaq/Galphaq*, PLCbeta, or RGS expression, as shown by receptor binding assays and Western blots. We conclude that closely related R4 RGS family members differ in their mechanism of action. RGS5 and RGS16 appear to depend on G protein inactivation, whereas GAP-independent mechanisms (such as effector antagonism) are sufficient to mediate the inhibitory effect of RGS2 and RGS3.

Ligand-receptor-G-protein molecular assemblies on beads for mechanistic studies and screening by flow cytometry

G protein-coupled receptors form a ternary complex of ligand, receptor, and G protein heterotrimer (LRG) during signal transduction from the outside to the inside of a cell. Our goal was to develop a homogeneous, small-volume, bead-based approach compatible with high-throughput flow cytometry that would allow evaluation of G protein coupled receptor molecular assemblies. Dextran beads were derivatized to carry chelated nickel to bind hexahistidine-tagged green fluorescent protein (GFP) and hexahistidine-tagged G proteins. Ternary complexes were assembled on these beads using fluorescent ligand with wild-type receptor or a receptor-Gialpha2 fusion protein, and with a nonfluorescent ligand and receptor-GFP fusion protein. Streptavidin-coated polystyrene beads used biotinylated anti-FLAG antibodies to bind FLAG-tagged G proteins for ternary complex assembly. Validation was achieved by showing time and concentration dependence of ternary complex formation. Affinity measurements of ligand for receptor on particles, of the ligand-receptor complex for G protein on the particles, and receptor-Gialpha2 fusion protein for Gbetagamma, were consistent with comparable assemblies in detergent suspension. Performance was assessed in applications representing the potential of these assemblies for ternary complex mechanisms. We showed the relationship for a family of ligands between LR and LRG affinity and characterized the affinity of both the wild-type and GFP fusion receptors with G protein. We also showed the potential of kinetic measurements to allow observation of individual steps of GTP-induced ternary complex disassembly and discriminated a fast step caused by RG disassembly compared with the slower step of Galphabetagamma disassembly.

Direct modulation of synaptic vesicle priming by GABA(B) receptor activation at a glutamatergic synapse

Second messenger cascades involving G proteins and calcium are known to modulate neurotransmitter release. A prominent effect of such a cascade is the downmodulation of presynaptic calcium influx, which markedly reduces evoked neurotransmitter release. Here we show that G-protein-mediated signalling, such as through GABA (gamma-amino butyric acid) subtype B (GABA(B)) receptors, retards the recruitment of synaptic vesicles during sustained activity and after short-term depression. This retardation occurs through a lowering of cyclic AMP, which blocks the stimulatory effect of increased calcium concentration on vesicle recruitment. In this signalling pathway, cAMP (functioning through the cAMP-dependent guanine nucleotide exchange factor) and calcium/calmodulin cooperate to enhance vesicle priming. The differential modulation of the two forms of synaptic plasticity, presynaptic inhibition and calcium-dependent recovery from synaptic depression, is expected to have interesting consequences for the dynamic behaviour of neural networks.

Modulation of glycine-activated ion channel function by G-protein betagamma subunits

Glycine receptors (GlyRs), together with GABA(A) and nicotinic acetylcholine (ACh) receptors, form part of the ligand-activated ion channel superfamily and regulate the excitability of the mammalian brain stem and spinal cord. Here we report that the ability of the neurotransmitter glycine to gate recombinant and native ionotropic GlyRs is modulated by the G protein betagamma dimer (Gbetagamma). We found that the amplitude of the glycine-activated Cl- current was enhanced after application of purified Gbetagamma or after activation of a G protein-coupled receptor. Overexpression of three distinct G protein alpha subunits (Galpha), as well as the Gbetagamma scavenger peptide ct-GRK2, significantly blunted the effect of G protein activation. Single-channel recordings from isolated membrane patches showed that Gbetagamma increased the GlyR open probability (nP(o)). Our results indicate that this interaction of Gbetagamma with GlyRs regulates both motor and sensory functions in the central nervous system.

Immunolocalization of multiple Galpha subunits in mammalian spermatozoa and additional evidence for Galphas

Like somatic cells, mammalian spermatozoa appear to contain several different heterotrimeric G protein alpha-subunits that could mediate specialized cell responses. However, the precise Galpha subunits present, their subcellular location and their possible roles are still incompletely defined. In this study, using commercially available specific antibodies, we have shown by immunoblotting that Galpha(s) is present in human and mouse sperm lysates. Immunolocalization using intact spermatozoa from both species revealed this protein to be in the acrosomal cap region and the flagellum, particularly the principal piece. Treatment of permeabilized mouse spermatozoa with cholera toxin led to enhanced ADP-ribosylation of a protein the same size as Galpha(s), as well as an increase in cAMP, providing further proof for Galpha(s). Evidence for the presence and distinct localizations of Galpha(i2), Galpha(i3), Galpha(o), Galpha(q/11), and Galpha(olf) was also obtained. Of particular interest was Galpha(i2) which, like Galpha(s), was present in the acrosomal cap region and flagellum, the same regions where stimulatory and inhibitory adenosine receptors are localized. These observations are consistent with our hypothesis that G proteins mediate adenosine receptor modulation of adenylyl cyclase, with consequent alterations in cAMP production, apparently crucial for the spermatozoon's acquisition and maintenance of fertilizing ability.

Estrogen modulation of endothelial nitric oxide synthase

Over the past decade, clinical and basic research has demonstrated that estrogen has a dramatic impact on the response to vascular injury and the development of atherosclerosis. Further work has indicated that this is at least partially mediated by an enhancement in nitric oxide (NO) production by the endothelial isoform of NO synthase (eNOS) due to increases in both eNOS expression and level of activation. The effects on eNOS abundance are primarily mediated at the level of gene transcription, and they are dependent on estrogen receptors (ERs), which classically serve as transcription factors, but they are independent of estrogen response element action. Estrogen also has potent nongenomic effects on eNOS activity mediated by a subpopulation of ERalpha localized to caveolae in endothelial cells, where they are coupled to eNOS in a functional signaling module. These observations, which emphasize dependence on cell surface-associated receptors, provide evidence for the existence of a steroid receptor fast-action complex, or SRFC, in caveolae. Estrogen binding to ERalpha on the SRFC in caveolae leads to G(alphai) activation, which mediates downstream events. The downstream signaling includes activation of tyrosine kinase-MAPK and Akt/protein kinase B signaling, stimulation of heat shock protein 90 binding to eNOS, and perturbation of the local calcium environment, leading to eNOS phosphorylation and calmodulin-mediated eNOS stimulation. These unique genomic and nongenomic processes are critical to the vasoprotective and atheroprotective characteristics of estrogen. In addition, they serve as excellent paradigms for further elucidation of novel mechanisms of steroid hormone action.

Cell condition-dependent regulation of ERK5 by cAMP

ERK5 activity is increased by agents known to activate receptor tyrosine kinases, G-protein coupled receptors, and stress response pathways. We now find a role for cAMP in the regulation of ERK5. ERK5 is activated by forskolin, isoproterenol, and epinephrine in NIH3T3 cells and C2C12 myoblasts. ERK1/2 are also activated by cAMP in NIH3T3 cells, but not in C2C12 myoblasts, demonstrating differential regulation of ERK5 and ERK1/2 by cAMP. We examined the effect of cell context on activation of ERK5 and discovered ERK5 activity is inhibited, rather than activated, by cAMP in confluent, serum-deprived NIH3T3 cells and C2C12 myoblasts. Our results suggest that regulation of MAP kinase pathways by cAMP is not only dictated by cell type, but also by cell context.

Modulation of cardiac PIP2 by cardioactive hormones and other physiologically relevant interventions

Phosphatidylinositol 4,5-bisphosphate (PIP2) affects profoundly several cardiac ion channels and transporters, and studies of PIP2-sensitive currents in excised patches suggest that PIP2 can be synthesized and broken down within 30 s. To test when, and if, total phosphatidylinositol 4-phosphate (PIP) and PIP(2) levels actually change in intact heart, we used a new, nonradioactive HPLC method to quantify anionic phospholipids. Total PIP and PIP2 levels (10-30 micromol/kg wet weight) do not change, or even increase, with activation of Galpha(q)/phospholipase C (PLC)-dependent pathways by carbachol (50 microM), phenylephrine (50 microM), and endothelin-1 (0.3 microM). Adenosine (0.2 mM) and phorbol 12-myristate 13-acetate (1microM) both cause 30% reduction of PIP2 in ventricles, suggesting that diacylglycerol (DAG)-dependent mechanisms negatively regulate cardiac PIP2. PIP2, but not PIP, increases reversibly by 30% during electrical stimulation (2 Hz for 5 min) in guinea pig left atria; the increase is blocked by nickel (2 mM). Both PIP and PIP2 increase within 3 min in hypertonic solutions, roughly in proportion to osmolarity, and similar effects occur in multiple cell lines. Inhibitors of several volume-sensitive signaling mechanisms do not affect these responses, suggesting that PIP2 metabolism might be sensitive to membrane tension, per se.

Rapid activation of endothelial NO synthase by estrogen: evidence for a steroid receptor fast-action complex (SRFC) in caveolae

Estrogen has important atheroprotective and vasoactive properties related to its capacity to stimulate nitric oxide (NO) production by endothelial NO synthase. Previous work has shown that these effects are mediated by estrogen receptor (ER) alpha functioning in a nongenomic manner via calcium-dependent, MAP kinase-dependent mechanisms. Recent studies have demonstrated that estradiol (E(2)) activates eNOS in isolated endothelial plasma membranes in the absence of added calcium, calmodulin or eNOS cofactors. Studies of blockade by ICI 182,780 and by ER alpha antibody, and also immunoidentification experiments indicate that the process is mediated by a subpopulation of plasma membrane-associated ER alpha. Fractionation of endothelial cell plasma membranes has further revealed that ER alpha protein is localized to caveolae, and that E(2) causes stimulation of eNOS in isolated caveolae which is ER-dependent and calcium-dependent, whereas noncaveolae membranes are insensitive. Furthermore, in intact endothelial cells the activation of eNOS by E(2) is prevented by pertussis toxin, and exogenous GDP beta S inhibits the response in isolated plasma membranes. Coimmunoprecipitation studies have shown that E(2) exposure causes interaction between ER alpha and G(alpha i) on the plasma membrane, and eNOS activation by E(2) is enhanced by overexpression of G(alpha i) and attenuated by expression of a protein regulator of G protein signaling (RGS), RGS4. Thus, a subpopulation of ER alpha is localized to caveolae in endothelial cells, where they are coupled via G(alpha i) to eNOS in a functional signaling module. Emphasizing the dependence on cell surface-associated receptors, these observations provide evidence for the existence of a steroid receptor fast-action complex, or SRFC, in caveolae.

Regulation of G protein-initiated signal transduction in yeast: paradigms and principles

All cells have the capacity to evoke appropriate and measured responses to signal molecules (such as peptide hormones), environmental changes, and other external stimuli. Tremendous progress has been made in identifying the proteins that mediate cellular response to such signals and in elucidating how events at the cell surface are linked to subsequent biochemical changes in the cytoplasm and nucleus. An emerging area of investigation concerns how signaling components are assembled and regulated (both spatially and temporally), so as to control properly the specificity and intensity of a given signaling pathway. A related question under intensive study is how the action of an individual signaling pathway is integrated with (or insulated from) other pathways to constitute larger networks that control overall cell behavior appropriately. This review describes the signal transduction pathway used by budding yeast (Saccharomyces cerevisiae) to respond to its peptide mating pheromones. This pathway is comprised by receptors, a heterotrimeric G protein, and a protein kinase cascade all remarkably similar to counterparts in multicellular organisms. The primary focus of this review, however, is recent advances that have been made, using primarily genetic methods, in identifying molecules responsible for regulation of the action of the components of this signaling pathway. Just as many of the constituent proteins of this pathway and their interrelationships were first identified in yeast, the functions of some of these regulators have clearly been conserved in metazoans, and others will likely serve as additional models for molecules that carry out analogous roles in higher organisms.

Galpha(i2) enhances insulin signaling via suppression of protein-tyrosine phosphatase 1B

Suppression of the expression of the heterotrimeric G-protein Galpha(i2) in vivo has been shown to provoke insulin resistance, whereas enhanced insulin signaling is observed when Galpha(i2) is overexpressed in vivo. The basis for Galpha(i2) regulation of insulin signaling was explored in transgenic mice with targeted expression of the GTPase-deficient, constitutively active Q205L Galpha(i2) in fat and skeletal muscle. Phosphorylation of insulin receptor and IRS-1 in response to insulin challenge in vivo was markedly amplified in fat and skeletal muscle expressing Q205L Galpha(i2). The expression and activity of the protein-tyrosine phosphatase 1B (PTP1B), but not protein-tyrosine phosphatases SHP-1, SHP-2, and LAR, were constitutively decreased in tissues expressing the Q205L Galpha(i2), providing a direct linkage between insulin signaling and Galpha(i2). The loss of PTP1B expression may explain, in part, the loss of PTP1B activity in the iQ205L transgenic mice. Activation of Galpha(i2) in mouse adipocytes with lysophosphatidic acid was shown to decrease PTP1B activity, whereas pertussis toxin inactivates Galpha(i2), blocks lysophosphatidic acid-stimulated inhibition of PTP1B activity, and blocks tonic suppression of PTP1B activity by Galpha(i2). Elevation of intracellular cAMP in fat cells is shown to increase PTP1B activity, whereas either depression of cAMP levels or direct activation of Galpha(i2) suppresses PTP1B. These data provide the first molecular basis for the interplay between Galpha(i2) and insulin signaling, i.e. activation of Galpha(i2) can suppress both the expression and activity of PTP1B in insulin-sensitive tissues.