Ribozyme-mediated suppression of the G protein gamma7 subunit suggests a role in hormone regulation of adenylylcyclase activity

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

Identification of GNG7 as a novel biomarker and potential therapeutic target for gastric cancer via bioinformatic analysis and in vitro experiments

Gastric cancer (GC) is one of the most common malignancies with unfavorable prognoses. The present study aimed to identify novel biomarkers or potential therapeutic targets in GC via bioinformatic analysis and in vitro experiments. The Gene Expression Omnibus and The Cancer Genome Atlas databases were used to screen the differentially expressed genes (DEGs). After protein-protein interaction network construction, both module and prognostic analyses were performed to identify prognosis-related genes in GC. The expression patterns and functions of G protein γ subunit 7 (GNG7) in GC were then visualized in multiple databases and further verified using in vitro experiments. A total of 897 overlapping DEGs were detected and 20 hub genes were identified via systematic analysis. After accessing the prognostic value of the hub genes using the online server Kaplan-Meier plotter, a six-gene prognostic signature was identified, which was also significantly correlated with the process of immune infiltration in GC. The results of open-access database analyses suggested that GNG7 is downregulated in GC; this downregulation was associated with tumor progression. Furthermore, the functional enrichment analysis unveiled that the GNG7-coexpressed genes or gene sets were closely correlated with the proliferation and cell cycle processes of GC cells. Finally, in vitro experiments further confirmed that GNG7 overexpression inhibited GC cell proliferation, colony formation, and cell cycle progression and induced apoptosis. As a tumor suppressor gene, GNG7 suppressed the growth of GC cells via cell cycle blockade and apoptosis induction and thus may be used as a potential biomarker and therapeutic target for GC.

Targeting G protein-coupled receptor signalling by blocking G proteins. Nat Rev Drug Discov. 2018;17:789-803. [PMID: 30262890 DOI: 10.1038/nrd.2018.135]

G protein-coupled receptors (GPCRs) are the largest class of drug targets, largely owing to their druggability, diversity and physiological efficacy. Many drugs selectively target specific subtypes of GPCRs, but high specificity for individual GPCRs may not be desirable in complex multifactorial disease states in which multiple receptors may be involved. One approach is to target G protein subunits rather than the GPCRs directly. This approach has the potential to achieve broad efficacy by blocking pathways shared by multiple GPCRs. Additionally, because many GPCRs couple to multiple G protein signalling pathways, blocking specific G protein subunits can 'bias' GPCR signals by inhibiting only a subset of these signals. Molecules that target G protein α or βγ-subunits have been developed and show strong efficacy in multiple preclinical disease models and biased inhibition of G protein signalling. In this Review, we discuss the development and characterization of G protein α and βγ-subunit ligands and the preclinical evidence that this exciting new approach has potential for therapeutic efficacy in a number of indications, such as pain, thrombosis, asthma and heart failure.

Gβ1 is required for neutrophil migration in zebrafish

Signaling mediated by G protein-coupled receptors (GPCRs) is essential for the migration of cells toward chemoattractants. The recruitment of neutrophils to injured tissues in zebrafish larvae is a useful model for studying neutrophil migration and trafficking in vivo. Indeed, the study of this process led to the discovery that PI3Kγ is required for the polarity and motility of neutrophils, features that are necessary for the directed migration of these cells to wounds. However, the mechanism by which PI3Kγ is activated remains to be determined. Here we show that signaling by specifically the heterotrimeric G protein subunit Gβ1 is critical for neutrophil migration in response to wounding. In embryos treated with small-molecule inhibitors of Gβγ signaling, neutrophils failed to migrate to wound sites. Although both the Gβ1 and Gβ4 isoforms are expressed in migrating neutrophils, only deficiency for the former (morpholino-based knockdown) interfered with the directed migration of neutrophils towards wounds. The Gβ1 deficiency also impaired the ability of cells to change cell shape and reduced their general motility, defects that are similar to those in neutrophils deficient for PI3Kγ. Transplantation assays showed that the requirement for Gβ1 in neutrophil migration is cell autonomous. Finally, live imaging revealed that Gβ1 is required for polarized activation of PI3K, and for the actin dynamics that enable neutrophil migration. Collectively, our data indicate that Gβ1 signaling controls proper neutrophil migration by activating PI3K and modulating actin dynamics. Moreover, they illustrate a role for a specific Gβ isoform in chemotaxis in vivo.

Expression profile of G-protein βγ subunit gene transcripts in the mouse olfactory sensory epithelia

Heterotrimeric G-proteins mediate a variety of cellular functions, including signal transduction in sensory neurons of the olfactory system. Whereas the Gα subunits in these neurons are well characterized, the gene transcript expression profile of Gβγ subunits is largely missing. Here we report our comprehensive expression analysis to identify Gβ and Gγ subunit gene transcripts in the mouse main olfactory epithelium (MOE) and the vomeronasal organ (VNO). Our reverse transcriptase PCR (RT-PCR) and realtime qPCR analyses of all known Gβ (β_1,2,3,4,5) and Gγ (γ_{1,2,2t,3,4,5,7,8,10,11,12,13}) subunits indicate presence of multiple Gβ and Gγ subunit gene transcripts in the MOE and the VNO at various expression levels. These results are supported by our RNA in situ hybridization (RISH) experiments, which reveal the expression patterns of two Gβ subunits and four Gγ subunits in the MOE as well as one Gβ and four Gγ subunits in the VNO. Using double-probe fluorescence RISH and line intensity scan analysis of the RISH signals of two dominant Gβγ subunits, we show that Gγ₁₃ is expressed in mature olfactory sensory neurons (OSNs), while Gβ₁ is present in both mature and immature OSNs. Interestingly, we also found Gβ₁ to be the dominant Gβ subunit in the VNO and present throughout the sensory epithelium. In contrast, we found diverse expression of Gγ subunit gene transcripts with Gγ₂, Gγ₃, and Gγ₁₃ in the Gα_i2-expressing neuronal population, while Gγ₈ is expressed in both layers. Further, we determined the expression of these Gβγ gene transcripts in three post-natal developmental stages (p0, 7, and 14) and found their cell-type specific expression remains largely unchanged, except the transient expression of Gγ₂ in a single basal layer of cells in the MOE during P7 and P14. Taken together, our comprehensive expression analyses reveal cell-type specific gene expression of multiple Gβ and Gγ in sensory neurons of the olfactory system.

Age-dependent dystonia in striatal Gγ7 deficient mice is reversed by the dopamine D2 receptor agonist pramipexole

Gγ7 is enriched in striatum and forms a heterotrimeric complex with Gαolf /Gβ, which is coupled to D1 receptor (D1R). Here, we attempted to characterize the pathophysiological, neurochemical, and pharmacological features of mice deficient of Gγ7 gene. Gγ7 knockout mice exhibited age-dependent deficiency in rotarod behavior and increased dystonia-like clasping reflex without loss of striatal neurons. The neurochemical basis for the motor manifestations using immunoblot analysis revealed increased levels of D1R, ChAT and NMDA receptor subunits (NR1 and NR2B) concurrent with decreased levels of D2R and Gαolf , possibly because of the secondary changes of decreased Gαolf /Gγ7-mediated D1R transmission. These behavioral and neurochemical changes are closely related to those observed in Huntington's disease (HD) human subjects and HD model mice. Taking advantage of the finding of D2R down-regulation in Gγ7 knockout mice and the dopamine-mediated synergistic relationship in the control of locomotion between D2R-striatopallidal and D1R-stritonigral neurons, we hypothesized that D2-agonist pramipexole would reverse behavioral dyskinesia caused by defective D1R/Gαolf signaling. Indeed, the rotarod deficiency and clasping reflex were reversed by pramipexole treatment under chronic administration. These findings suggest that Gγ7 knockout mice could be a new type of movement disorders, including HD and useful for the evaluation of therapeutic candidates.

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.

Preferential assembly of G-αβγ complexes directed by the γ subunits

Assembly of the G-αβγ heterotrimer is required for receptor signaling. Although much has been learned about the assembly process itself, the identities of the G-αβγ combinations that actually exist in physiological setting are largely unknown. Moreover, there is uncertainty regarding whether the individual subunits associate by a random process, or combine by a regulated process to form quasi-stable G-αβγ complexes. In this chapter, we will focus on emerging genetic -evidence that supports the latter model. Specifically, we will discuss how use of gene targeted mice has revealed preferential assembly of the striatal-specific Gα(olf)β(2)γ(7) complex occurs by a sequential process that is directed by the γ(7) subunit. The existence of specific G-αβγ complexes responsible for transducing the signals from different receptors may have profound implications by providing a possible explanation for biased agonism.

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.

Multicolor BiFC analysis of G protein βγ complex formation and localization

Cells co-express multiple G protein β and γ subunit isoforms, but the extent to which individual subunits associate to form particular βγ complexes is not known. This issue is important because in vivo knockout experiments suggest that specific βγ complexes may have unique functions despite the fact that most complexes exhibit similar properties when assayed in reconstituted systems. This chapter describes how multicolor bimolecular fluorescence complementation (BiFC) can be used in living cells to study the association preferences of β and γ subunits. Multicolor BiFC determines the association preferences of these subunits by quantifying the two fluorescent complexes formed when β or γ subunits fused to amino terminal fragments of yellow fluorescent protein (YFP-N) and cyan fluorescent protein (CFP-N) compete for interaction with limiting amounts of a common γ or β subunit, respectively, fused to a carboxyl terminal fragment of CFP (CFP-C). One means by which βγ complexes may differ from each other and thereby mediate unique functions in vivo is in the kinetics and patterns of their internalization responses to stimulation of G protein-coupled receptors (GPCRs). Methods are described for imaging and quantifying the internalization of pairs of βγ complexes in response to GPCR stimulation in living cells.

Adenosine A2A receptor signaling and golf assembly show a specific requirement for the gamma7 subtype in the striatum

The adenosine A(2A) receptor (A(2A)R) is increasingly recognized as a novel therapeutic target in Parkinson disease. In striatopallidal neurons, the G-protein α(olf) subtype is required to couple this receptor to adenylyl cyclase activation. It is now well established that the βγ dimer also performs an active role in this signal transduction process. In principal, sixty distinct βγ dimers could arise from combinatorial association of the five known β and 12 γ subunit genes. However, key questions regarding which βγ subunit combinations exist and whether they perform specific signaling roles in the context of the organism remain to be answered. To explore these questions, we used a gene targeting approach to specifically ablate the G-protein γ(7) subtype. Revealing a potentially new signaling paradigm, we show that the level of the γ(7) protein controls the hierarchial assembly of a specific G-protein α(olf)β(2)γ(7) heterotrimer in the striatum. Providing a probable basis for the selectivity of receptor signaling, we further demonstrate that loss of this specific G-protein heterotrimer leads to reduced A(2A)R activation of adenylyl cyclase. Finally, substantiating an important role for this signaling pathway in pyschostimulant responsiveness, we show that mice lacking the G-protein γ(7) subtype exhibit an attenuated behavioral response to caffeine. Collectively, these results further support the A(2A)R G-protein α(olf)β(2)γ(7) interface as a possible therapeutic target for Parkinson disease.

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.

beta-Arrestin1 interacts with the G-protein subunits beta1gamma2 and promotes beta1gamma2-dependent Akt signalling for NF-kappaB activation. Biochem J. 2009;417:287-296. [PMID: 18729826 DOI: 10.1042/BJ20081561]

beta-Arrestins are known to regulate G-protein signalling through interactions with their downstream effectors. In the present study, we report that beta-arrestin1 associates with the G-protein beta1gamma2 subunits in transfected cells, and purified beta-arrestin1 interacts with G(beta1gamma2) derived from in vitro translation. Deletion mutagenesis of beta-arrestin1 led to the identification of a region, comprising amino acids 181-280, as being responsible for its interaction with G(beta1gamma2). Overexpression of beta-arrestin1 facilitates G(beta1gamma2)-mediated Akt phosphorylation, and inhibition of endogenous beta-arrestin1 expression by siRNA (small interfering RNA) diminishes this effect. Through investigation of NF-kappaB (nuclear factor kappaB), a transcription factor regulated by Akt signalling, we have found that overexpression of beta-arrestin1 significantly enhances G(beta1gamma2)-mediated nuclear translocation of NF-kappaB proteins and expression of a NF-kappaB-directed luciferase reporter. Overexpression of beta-arrestin1 also promotes bradykinin-induced, G(betagamma)-mediated NF-kappaB luciferase-reporter expression, which is reverted by silencing the endogenous beta-arrestin1 with a specific siRNA. These results identify novel functions of beta-arrestin1 in binding to the beta1gamma2 subunits of heterotrimeric G-proteins and promoting G(betagamma)-mediated Akt signalling for NF-kappaB activation.

G protein βγ subunits: central mediators of G protein-coupled receptor signaling

G protein betagamma subunits are central participants in G protein-coupled receptor signaling pathways. They interact with receptors, G protein alpha subunits and downstream targets to coordinate multiple, different GPCR functions. Much is known about the biology of Gbetagamma subunits but mysteries remain. Here, we will review what is known about general aspects of structure and function of Gbetagamma as well as discuss emerging mechanisms for regulation of Gbetagamma signaling. Recent data suggest that Gbetagamma is a potential therapeutic drug target. Thus, a thorough understanding of the molecular and physiological functions of Gbetagamma has significant implications.

G protein betagamma dimer expression in cardiomyocytes: developmental acquisition of Gbeta3

Heterotrimeric G proteins are comprised of a guanine nucleotide binding Galpha subunit and the Gbetagamma dimers that link G protein-coupled receptors (GPCRs) to effectors. This study focuses on the expression and localization patterns for certain Gbeta and Ggamma subunits in neonatal and adult cardiomyocytes. We identify developmental downregulation of Gbeta1, Gbeta2, and Ggamma2, and a switch in the molecular form of Ggamma3, in cardiomyocytes. Gbeta1 is highly localized to caveolae membranes, whereas Gbeta2 is identified in caveolae and other membrane fractions. Gbeta3 is not detected in neonatal cardiomyocytes, but rather Gbeta3 is upregulated in adult cardiomyocytes and detected in the caveolae and soluble fractions. The observation that cardiomyocytes co-express multiple Gbeta and Ggamma subunits in a developmentally regulated manner, and that these Gbeta and Ggamma subunits assume distinct subcellular localization patterns, provides for a high level of signaling specificity in the heart.

Live cell analysis of G protein beta5 complex formation, function, and targeting

The G protein beta(5) subunit differs from other beta subunits in having divergent sequence and subcellular localization patterns. Although beta(5)gamma(2) modulates effectors, beta(5) associates with R7 family regulators of G protein signaling (RGS) proteins when purified from tissues. To investigate beta(5) complex formation in vivo, we used multicolor bimolecular fluorescence complementation in human embryonic kidney 293 cells to compare the abilities of 7 gamma subunits and RGS7 to compete for interaction with beta(5). Among the gamma subunits, beta(5) interacted preferentially with gamma(2), followed by gamma(7), and efficacy of phospholipase C-beta2 activation correlated with amount of beta(5)gamma complex formation. beta(5) also slightly preferred gamma(2) over RGS7. In the presence of coexpressed R7 family binding protein (R7BP), beta(5) interacted similarly with gamma(2) and RGS7. Moreover, gamma(2) interacted preferentially with beta(1) rather than beta(5). These results suggest that multiple coexpressed proteins influence beta(5) complex formation. Fluorescent beta(5)gamma(2) labeled discrete intracellular structures including the endoplasmic reticulum and Golgi apparatus, whereas beta(5)RGS7 stained the cytoplasm diffusely. Coexpression of alpha(o) targeted both beta(5) complexes to the plasma membrane, and alpha(q) also targeted beta(5)gamma(2) to the plasma membrane. The constitutively activated alpha(o) mutant, alpha(o)R179C, produced greater targeting of beta(5)RGS7 and less of beta(5)gamma(2) than did alpha(o). These results suggest that alpha(o) may cycle between interactions with beta(5)gamma(2) or other betagamma complexes when inactive, and beta(5)RGS7 when active. Moreover, the ability of beta(5)gamma(2) to be targeted to the plasma membrane by alpha subunits suggests that functional beta(5)gamma(2) complexes can form in intact cells and mediate signaling by G protein-coupled receptors.

Analysis of G protein betagamma dimer formation in live cells using multicolor bimolecular fluorescence complementation demonstrates preferences of beta1 for particular gamma subunits

The specificity of G protein betagamma signaling demonstrated by in vivo knockouts is greater than expected based on in vitro assays of betagamma function. In this study, we investigated the basis for this discrepancy by comparing the abilities of seven beta1gamma complexes containing gamma1, gamma2, gamma5, gamma7, gamma10, gamma11, or gamma12 to interact with alphas and of these gamma subunits to compete for interaction with beta1 in live human embryonic kidney (HEK) 293 cells. betagamma complexes were imaged using bimolecular fluorescence complementation, in which fluorescence is produced by two nonfluorescent fragments (N and C) of cyan fluorescent protein (CFP) or yellow fluorescent protein (YFP) when brought together by proteins fused to each fragment. Plasma membrane targeting of alphas-CFP varied inversely with its expression level, and the abilities of YFP-N-beta1YFP-C-gamma complexes to increase this targeting varied by 2-fold or less. However, there were larger differences in the abilities of the CFP-N-gamma subunits to compete for association with CFP-C-beta1. When the intensities of coexpressed CFP-C-beta1CFP-N-gamma (cyan) and CFP-C-beta1YFP-N-gamma2 (yellow) complexes were compared under conditions in which CFP-C-beta1 was limiting, the CFP-N-gamma subunits exhibited a 4.5-fold range in their abilities to compete with YFP-N-gamma2 for association with CFP-C-beta1. CFP-N-gamma12 and CFP-N-gamma1 were the strongest and weakest competitors, respectively. Taken together with previous demonstrations of a role for betagamma in the specificity of receptor signaling, these results suggest that differences in the association preferences of coexpressed beta and gamma subunits for each other can determine which complexes predominate and participate in signaling pathways in intact cells.

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.

G-protein signaling: back to the future

Heterotrimeric G-proteins are intracellular partners of G-protein-coupled receptors (GPCRs). GPCRs act on inactive Galpha.GDP/Gbetagamma heterotrimers to promote GDP release and GTP binding, resulting in liberation of Galpha from Gbetagamma. Galpha.GTP and Gbetagamma target effectors including adenylyl cyclases, phospholipases and ion channels. Signaling is terminated by intrinsic GTPase activity of Galpha and heterotrimer reformation - a cycle accelerated by 'regulators of G-protein signaling' (RGS proteins). Recent studies have identified several unconventional G-protein signaling pathways that diverge from this standard model. Whereas phospholipase C (PLC) beta is activated by Galpha(q) and Gbetagamma, novel PLC isoforms are regulated by both heterotrimeric and Ras-superfamily G-proteins. An Arabidopsis protein has been discovered containing both GPCR and RGS domains within the same protein. Most surprisingly, a receptor-independent Galpha nucleotide cycle that regulates cell division has been delineated in both Caenorhabditis elegans and Drosophila melanogaster. Here, we revisit classical heterotrimeric G-protein signaling and explore these new, non-canonical G-protein signaling pathways.

Antinociception depends on the presence of G protein gamma2-subunits in brain

We have shown previously [Hosohata, K., Logan, J.K., Varga, E., Burkey, T.H., Vanderah, T.W., Porreca, F., Hruby, V.J., Roeske, W.R., Yamamura, H.I., 2000. The role of the G protein gamma2 subunit in opioid antinociception in mice. Eur. J. Pharmacol. 392, R9-R11] that intracerebroventricular (i.c.v.) treatment of mice with a phosphorothioate oligodeoxynucleotide antisense to the gamma2 subunit (Ggamma2) of the heterotrimeric G proteins (antisense ODN) significantly attenuates antinociception by a delta-opioid receptor agonist. In the present study, we examined the involvement of Ggamma2 in antinociception mediated by other (mu- or kappa-opioid, cannabinoid, alpha2-adrenoreceptor) analgesic agents in a warm (55 degrees C) water tail-flick test in mice. Interestingly, i.c.v. treatment with the antisense ODN attenuated antinociception by each analgesic agent. Missense phosphorothioate oligodeoxynucleotide treatment, on the other hand, had no effect on antinociception mediated by these agonists. The antinociceptive response recovered in 6 days after the last antisense ODN injection, indicating a lack of nonspecific tissue damage in the animals. These results suggest a pervasive role for the G protein gamma2 subunits in supraspinal antinociception.

Live cell imaging of Gs and the beta2-adrenergic receptor demonstrates that both alphas and beta1gamma7 internalize upon stimulation and exhibit similar trafficking patterns that differ from that of the beta2-adrenergic receptor

To visualize and investigate the regulation of the localization patterns of Gs and an associated receptor during cell signaling, we produced functional fluorescent fusion proteins and imaged them in HEK-293 cells. alphas-CFP, with cyan fluorescent protein (CFP) inserted into an internal loop of alphas, localized to the plasma membrane and exhibited similar receptor-mediated activity to that of alphas. Functional fluorescent beta1gamma7 dimers were produced by fusing an amino-terminal yellow fluorescent protein (YFP) fragment to beta1 (YFP-N-beta1) and a carboxyl-terminal YFP fragment to gamma7 (YFP-C-gamma7). When expressed together, YFP-N-beta1 and YFP-C-gamma7 produced fluorescent signals in the plasma membrane that were not seen when the subunits were expressed separately. Isoproterenol stimulation of cells co-expressing alphas-CFP, YFP-N-beta1/YFP-C-gamma7, and the beta2-adrenergic receptor (beta2AR) resulted in internalization of both fluorescent signals from the plasma membrane. Initially, alphas-CFP and YFP-N-beta1/YFP-C-gamma7 stained the cytoplasm diffusely, and subsequently they co-localized on vesicles that exhibited minimal overlap with beta2AR-labeled vesicles. Moreover, internalization of beta2AR-GFP, but not alphas-CFP or YFP-N-beta1/YFP-C-gamma7, was inhibited by a fluorescent dominant negative dynamin 1 mutant, Dyn1(K44A)-mRFP, indicating that the Gs subunits and beta2AR utilize different internalization mechanisms. Subsequent trafficking of the Gs subunits and beta2AR also differed in that vesicles labeled with the Gs subunits exhibited less overlap with RhoB-labeled endosomes and greater overlap with Rab11-labeled endosomes. Because Rab11 regulates traffic through recycling endosomes, co-localization of alphas and beta1gamma7 on these endosomes may indicate a means of recycling specific alphasbetagamma combinations to the plasma membrane.

Visualization of G protein betagamma dimers using bimolecular fluorescence complementation demonstrates roles for both beta and gamma in subcellular targeting

To investigate the role of subcellular localization in regulating the specificity of G protein betagamma signaling, we have applied the strategy of bimolecular fluorescence complementation (BiFC) to visualize betagamma dimers in vivo. We fused an amino-terminal yellow fluorescent protein fragment to beta and a carboxyl-terminal yellow fluorescent protein fragment to gamma. When expressed together, these two proteins produced a fluorescent signal in human embryonic kidney 293 cells that was not obtained with either subunit alone. Fluorescence was dependent on betagamma assembly in that it was not obtained using beta2 and gamma1, which do not form a functional dimer. In addition to assembly, BiFC betagamma complexes were functional as demonstrated by more specific plasma membrane labeling than was obtained with individually tagged fluorescent beta and gamma subunits and by their abilities to potentiate activation of adenylyl cyclase by alpha(s) in COS-7 cells. To investigate isoform-dependent targeting specificity, the localization patterns of dimers formed by pair-wise combinations of three different beta subunits with three different gamma subunits were compared. BiFC betagamma complexes containing either beta1 or beta2 localized to the plasma membrane, whereas those containing beta5 accumulated in the cytosol or on intracellular membranes. These results indicate that the beta subunit can direct trafficking of the gamma subunit. Taken together with previous observations, these results show that the G protein alpha, beta, and gamma subunits all play roles in targeting each other. This method of specifically visualizing betagamma dimers will have many applications in sorting out roles for particular betagamma complexes in a wide variety of cell types.

Translating G protein subunit diversity into functional specificity

Historically, it has been assumed that the functional roles of G proteins in receptor recognition and effector regulation are specified by their diverse alpha subunits. However, the discovery of similarly diverse betagamma subunits that participate in both of these functional processes has called this assumption into question; recent work suggests that G proteins function as heterotrimers whose roles in particular receptor signaling pathways are determined by their specific alphabetagamma subunit combinations. Although much remains to be learned, the assembly of specific alphabetagamma subunit combinations seems to involve both structural and spatial factors.

Ribozyme- and siRNA-Mediated Suppression of RGS-Containing RhoGEF Proteins

Given recent efforts to determine the sequence information on thousands of genes in the human genome, the current challenge is to identify the functions of these genes, including those encoding the regulator of G-protein signaling protein gene superfamily, and to establish their roles in particular signaling pathways in a native system. Increasingly, reverse genetic approaches are being used to address these questions. This article compares two powerful approaches [ribozyme and "short interfering" RNA (siRNA) techniques] under identical conditions for the first report on the suppression of endogenous RGS domain-containing RhoGEFs. The siRNA technique was found to be much more potent than ribozyme targeting at the same mRNA site of RGS-RhoGEFs. Also, the three siRNAs targeting LARG, PDZ-RhoGEF, and p115-RhoGEF are able to discriminate the closely related sequences within this RGS-RhoGEF gene family.

Loss of G protein gamma 7 alters behavior and reduces striatal alpha(olf) level and cAMP production

The G protein beta gamma-dimer is required for receptor interaction and effector regulation. However, previous approaches have not identified the physiologic roles of individual subtypes in these processes. We used a gene knockout approach to demonstrate a unique role for the G protein gamma(7)-subunit in mice. Notably, deletion of Gng7 caused behavioral changes that were associated with reductions in the alpha(olf)-subunit content and adenylyl cyclase activity of the striatum. These data demonstrate that an individual gamma-subunit contributes to the specificity of a given signaling pathway and controls the formation or stability of a particular G protein heterotrimer.

Receptor-selective effects of endogenous RGS3 and RGS5 to regulate mitogen-activated protein kinase activation in rat vascular smooth muscle cells

Regulators of G protein signaling (RGS) proteins compose a highly diverse protein family best known for inhibition of G protein signaling by enhancing GTP hydrolysis by Galpha subunits. Little is known about the function of endogenous RGS proteins. In this study, we used synthetic ribozymes targeted to RGS2, RGS3, RGS5, and RGS7 to assess their function. After demonstrating the specificity of in vitro cleavage by the RGS ribozymes, rat aorta smooth muscle cells were used for transient transfection with the RGS-specific ribozymes. RGS3 and RGS5 ribozymes differentially enhanced carbachol- and angiotensin II-induced MAP kinase activity, respectively, whereas RGS2 and RGS7 ribozymes had no effect. This enhancement was pertussis toxin-insensitive. Thus RGS3 is a negative modulator of muscarinic m3 receptor signaling, and RGS5 is a negative modulator of angiotensin AT1a receptor signaling through G(q/11). Also, RGS5 ribozyme enhanced angiotensin-stimulated inositol phosphate release. These results indicate the feasibility of using the ribozyme technology to determine the functional role of endogenous RGS proteins in signaling pathways and to define novel receptor-selective roles of endogenous RGS3 and RGS5 in modulating MAP kinase responses to either carbachol or angiotensin.

Ribozyme-mediated suppression of G protein gamma subunits

Efforts to determine the sequence of the human genome have resulted in sequence information on thousand of genes. Now, the challenge is to determine the functions of this myriad of genes, including those encoding the G protein subunit families. In this chapter, we describe the successful use of ribozymes to inactivate mRNAs expressed from the G protein gamma subunit genes. Ribozymes are unique in that they can inactivate specific gene expression, and thereby can be used to help identify the function of a protein or the role of a gene in a functional cascade. Compared to other means of identifying the role of a gene (i.e., transgenic or knockout animals), ribozymes are specific and relatively easy to use. Moreover, ribozymes are able to discriminate closely related, or even mutated, sequences within gene families. Thus, in addition to elucidating functions, ribozymes have the potential to be used in treating genetic disorders associated with mutations of G protein subunits.

A highly effective dominant negative alpha s construct containing mutations that affect distinct functions inhibits multiple Gs-coupled receptor signaling pathways

To investigate the subcellular organization of receptor-G protein signaling pathways, a robust dominant negative alpha(s) mutant containing substitutions that alter distinct functions was produced and tested for its effects on G(s)-coupled receptor activity in HEK-293 cells. Mutations in the alpha3beta5 loop region, which increase receptor affinity, decrease receptor-mediated activation, and impair activation of adenylyl cyclase, were combined with G226A, which increases affinity for betagamma, and A366S, which decreases affinity for GDP. This triple alpha(s) mutant can inhibit signaling to G(s) from the luteinizing hormone receptor by 97% and from the calcitonin receptor by 100%. In addition, this alpha(s) mutant blocks all signaling from the calcitonin receptor to G(q). These results lead to two conclusions about receptor-G protein signaling. First, individual receptors have access to multiple types of G proteins in HEK-293 cell membranes. Second, different G protein alpha subunits can compete with each other for binding to the same receptor. This dominant negative alpha(s) construct will be useful for determining interrelationships among distinct receptor-G protein interactions in a wide variety of cells and tissues.

G protein specificity: traffic direction required

This review focuses on the coupling specificity of the Galpha and Gbetagamma subunits of pertussis toxin (PTX)-sensitive G(i/o) proteins that mediate diverse signaling pathways, including regulation of ion channels and other effectors. Several lines of evidence indicate that specific combinations of G protein alpha, beta and gamma subunits are required for different receptors or receptor-effector networks, and that a higher degree of specificity for Galpha and Gbetagamma is observed in intact systems than reported in vitro. The structural determinants of receptor-G protein specificity remain incompletely understood, and involve receptor-G protein interaction domains, and perhaps other scaffolding processes. By identifying G protein specificity for individual receptor signaling pathways, ligands targeted to disrupt individual pathways of a given receptor could be developed.

Hammerhead ribozymes for target validation

Expensive failures in the pharmaceutical industry might be avoided by target validation at an early stage. Often, the full consequences of inhibiting a chosen drug target do not emerge until late in the development process. One option is to use hammerhead ribozymes as highly specific ribonucleases targeted exclusively at the mRNA encoding the target protein. The first part of this review is concerned with the mechanism and design of hammerhead ribozymes. This includes the chemistry of their action, specificity of cleavage and ability to discriminate between different mRNAs and selection of suitable cleavage sites. In considering their use for target validation, hammerhead ribozymes are divided into two categories. Endogenous ribozymes are transcribed inside the cell where they act whilst exogenous are introduced into the cell from outside. Exogenous ribozymes are synthesised chemically and must be protected against cellular nucleases. Information is provided on transfection methods and vectors that have been used with endogenous ribozymes as well as synthesis and chemical modification of exogenous ribozymes. Of proteins inhibited in cells or whole organisms, those in animal experiments are emphasised. Comparisons are made with other approaches, especially the use of antisense oligonucleotides or RNA.

Gbetagamma affinity for bovine rhodopsin is determined by the carboxyl-terminal sequences of the gamma subunit

Two native betagamma dimers, beta(1)gamma(1) and beta(1)gamma(2), display very different affinities for receptors. Since these gamma subunits differ in both primary structure and isoprenoid modification, we examined the relative contributions of each to Gbetagamma interaction with receptors. We constructed baculoviruses encoding gamma(1) and gamma(2) subunits with altered CAAX (where A is an aliphatic amino acid) motifs to direct alternate or no prenylation of the gamma chains and a set of gamma(1) and gamma(2) chimeras with the gamma(2) CAAX motif at the carboxyl terminus. All the gamma constructs coexpressed with beta(1) in Sf9 cells yielded beta(1)gamma dimers, which were purified to near homogeneity, and their affinities for receptors and Galpha were quantitatively determined. Whereas alteration of the isoprenoid of gamma(1) from farnesyl to geranylgeranyl and of gamma(2) from geranylgeranyl to farnesyl had no impact on the affinities of beta(1)gamma dimers for Galpha(t), the non-prenylated beta(1)gamma(2) dimer had significantly diminished affinity. Altered prenylation resulted in a <2-fold decrease in affinity of the beta(1)gamma(2) dimer for rhodopsin and a <3-fold change for the beta(1)gamma(1) dimer. In each case with identical isoprenylation, the beta(1)gamma(2) dimer displayed significantly greater affinity for rhodopsin compared with the beta(1)gamma(1) dimer. Furthermore, dimers containing chimeric Ggamma chains with identical geranylgeranyl modification displayed rhodopsin affinities largely determined by the carboxyl-terminal one-third of the protein. These results indicate that isoprenoid modification of the Ggamma subunit is essential for binding to both Galpha and receptors. The isoprenoid type influences the binding affinity for receptors, but not for Galpha. Finally, the primary structure of the Ggamma subunit provides a major contribution to receptor binding of Gbetagamma, with the carboxyl-terminal sequence conferring receptor selectivity.

Differential dependence of the D1 and D5 dopamine receptors on the G protein gamma 7 subunit for activation of adenylylcyclase

The D(1) dopamine receptor, G protein gamma(7) subunit, and adenylylcyclase are selectively expressed in the striatum, suggesting their potential interaction in a common signaling pathway. To evaluate this possibility, a ribozyme strategy was used to suppress the expression of the G protein gamma(7) subunit in HEK 293 cells stably expressing the human D(1) dopamine receptor. Prior in vitro analysis revealed that the gamma(7) ribozyme possessed cleavage activity directed exclusively toward the gamma(7) RNA transcript (Wang, Q., Mullah, B., Hansen, C., Asundi, J., and Robishaw, J. D. (1997) J. Biol. Chem. 272, 26040-26048). In vivo analysis of cells transfected with the gamma(7) ribozyme showed a specific reduction in the expression of the gamma(7) protein. Coincident with the loss of the gamma(7) protein, there was a noticeable reduction in the expression of the beta(1) protein, confirming their interaction in these cells. Finally, functional analysis of ribozyme-mediated suppression of the beta(1) and gamma(7) proteins revealed a significant attenuation of SKF81297-stimulated adenylylcyclase activity in D(1) dopamine receptor-expressing cells. By contrast, ribozyme-mediated suppression of the beta(1) and gamma(7) proteins showed no reduction of SKF81297-stimulated adenylylcyclase activity in D(5) dopamine receptor-expressing cells. Taken together, these data indicate that the structurally related D(1) and D(5) dopamine receptor subtypes utilize G proteins composed of distinct betagamma subunits to stimulate adenylylcyclase in HEK 293 cells. Underscoring the physiological relevance of these findings, single cell reverse transcriptase-polymerase chain reaction analysis revealed that the D(1) dopamine receptor and the G protein gamma(7) subunit are coordinately expressed in substance P containing neurons in rat striatum, suggesting that the G protein gamma(7) subunit may be a new target for drugs to selectively alter dopaminergic signaling within the brain.

Receptor crosstalk protein, calcyon, regulates affinity state of dopamine D1 receptors

The recently cloned protein, calcyon, potentiates crosstalk between G(s)-coupled dopamine D1 receptors and heterologous G(q/11)-coupled receptors allowing dopamine D1 receptors to stimulate intracellular Ca(2+) release, in addition to cAMP production. This crosstalk also requires the participating G(q/11)-coupled receptors to be primed by their agonists. We examined the ability of calcyon and priming to regulate the affinity of dopamine D1 receptors for its ligands. Receptor binding assays were performed on HEK293 cell membrane preparations expressing dopamine D1 receptors either alone or in combination with calcyon. Co-expression of dopamine D1 receptor and calcyon affected neither the affinity of this receptor for antagonists nor the affinity of agonist binding to this receptor high and low-affinity states. However, the presence of calcyon dramatically decreased the proportion of the high-affinity dopamine D1 receptor agonist binding sites. This decrease was reversed by carbachol, which primes the receptor crosstalk by stimulating endogenous G(q/11)-coupled muscarinic receptors. Our findings suggest that calcyon regulates the ability of dopamine D1 receptors to achieve the high-affinity state for agonists, in a manner that depends on priming of receptor crosstalk.

Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors

G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.

Evidence for the involvement of Gi2 in activation of extracellular signal-regulated kinases in hepatocytes

BACKGROUND

Activation of the extracellular signal-regulated kinases ERK1 and ERK2 in hepatocytes by prostaglandin (PG)F2alpha was recently found to be inhibited by pertussis toxin (PTX) suggesting a role for Gi proteins.

RESULTS

Targeting the Gi2alpha expression by a specific ribozyme inhibited the PGF2alpha -induced ERK1/2 activation in hepatocytes. On the other hand a non-cleaving form of the Gi2alpha ribozyme did not significantly decrease the ERK1/2 activation. In ribozyme-treated cells the Gi2alpha protein level was reduced, while the Gqalpha level was not affected thus confirming the specificity of the ribozyme.

CONCLUSION

The present data suggest an important role of Gi2 in PGF2alpha -induced ERK1/2 signaling in hepatocytes.

Regulation of Xenopus oocyte meiosis arrest by G protein betagamma subunits

BACKGROUND

Progesterone induces the resumption of meiosis (maturation) in Xenopus oocytes through a nongenomic mechanism involving inhibition of an oocyte adenylyl cyclase and reduction of intracellular cAMP. However, progesterone action in Xenopus oocytes is not blocked by pertussis toxin, and this finding indicates that the inhibition of the oocyte adenylyl cyclase is not mediated by the alpha subunits of classical G(i)-type G proteins.

RESULTS

To investigate the possibility that G protein betagamma subunits, rather than alpha subunits, play a key role in regulating oocyte maturation, we have employed two structurally distinct G protein betagamma scavengers (G(t)alpha and betaARK-C(CAAX)) to sequester free Gbetagamma dimers. We demonstrated that the injection of mRNA encoding either of these Gbetagamma scavengers induced oocyte maturation. The Gbetagamma scavengers bound an endogenous, membrane-associated Gbeta subunit, indistinguishable from Xenopus Gbeta1 derived from mRNA injection. The injection of Xenopus Gbeta1 mRNA, together with bovine Ggamma2 mRNA, elevated oocyte cAMP levels and inhibited progesterone-induced oocyte maturation.

CONCLUSION

An endogenous G protein betagamma dimer, likely including Xenopus Gbeta1, is responsible for maintaining oocyte meiosis arrest. Resumption of meiosis is induced by Gbetagamma scavengers in vitro or, naturally, by progesterone via a mechanism that suppresses the release of Gbetagamma.

Secondary structure prediction and in vitro accessibility of mRNA as tools in the selection of target sites for ribozymes

We have investigated the relative merits of two commonly used methods for target site selection for ribozymes: secondary structure prediction (MFold program) and in vitro accessibility assays. A total of eight methylated ribozymes with DNA arms were synthesized and analyzed in a transient co-transfection assay in HeLa cells. Residual expression levels ranging from 23 to 72% were obtained with anti-PSKH1 ribozymes compared to cells transfected with an irrelevant control ribozyme. Ribozyme efficacy depended on both ribozyme concentration and the steady state expression levels of the target mRNA. Allylated ribozymes against a subset of the target sites generally displayed poorer efficacy than their methylated counterparts. This effect appeared to be influenced by in vivo accessibility of the target site. Ribozymes designed on the basis of either selection method displayed a wide range of efficacies with no significant differences in the average activities of the two groups of ribozymes. While in vitro accessibility assays had limited predictive power, there was a significant correlation between certain features of the predicted secondary structure of the target sequence and the efficacy of the corresponding ribozyme. Specifically, ribozyme efficacy appeared to be positively correlated with the presence of short stem regions and helices of low stability within their target sequences. There were no correlations with predicted free energy or loop length.

Gbetagamma subunit combinations differentially modulate receptor and effector coupling in vivo

In vitro, little specificity is seen for modulation of effectors by different combinations of Gbetagamma subunits from heterotrimeric G proteins. Here, we demonstrate that the coupling of specific combinations of Gbetagamma subunits to different receptors leads to a differential ability to modulate effectors in vivo. We have shown that the beta(1)AR and beta(2)AR can activate homomultimers of the human inwardly rectifying potassium channel Kir 3.2 when coexpressed in Xenopus oocytes, and that this requires a functional mammalian Gs heterotrimer. Modulation was independent of cAMP production, suggesting a membrane-delimited mechanism. To analyze further the importance of different Gbetagamma combinations, we have tested the facilitation of Kir 3.2 activation by betaAR mediated by different Gbetagamma subunits. The subunits tested were Gbeta(1,5) and Ggamma(1,2,7,11). These experiments demonstrated significant variation between the ability of the Gbetagamma combinations to activate the channels after receptor stimulation. This was in marked contrast to the situation in vitro where little specificity for binding of a Kir 3.1 C-terminal GST fusion protein by different Gbetagamma combinations was detected. More importantly, neither receptor, although homologous both structurally and functionally, shared the same preference for Gbetagamma subunits. In the presence of beta(1)AR, Gbeta(5)gamma(1) and Gbeta(5)gamma(11) activated Kir 3.2 to the greatest extent, while for the beta(2)AR, Gbeta(1)gamma(7), Gbeta(1)gamma(11,) and Gbeta(5)gamma(2) produced the greatest responses. Interestingly, no preference was seen in the ability of different Gbetagamma subunits to facilitate receptor-stimulated GTPase activity of the Gsalpha. These results suggest that it is not the receptor/G protein alpha subunit interaction or the Gbetagamma/effector interaction that is altered by Gbetagamma, but rather that the ability of the receptor to interact productively with the Gbetagamma subunit directly and/or the G protein/effector complex is dependent on the specific G protein heterotrimer associated with the receptor.

Human G protein gamma(11) and gamma(14) subtypes define a new functional subclass

The mammalian gamma subunit family consists of a minimum of 12 members. Analysis of the amino acid sequence conservation suggests that the gamma subunit family can be divided into three distinct subclasses. The division of the gamma subunit family into these classes is based not only on amino acid homology, but also to some extent on functional similarities. In the present study, two new members of the gamma subunit family, the gamma(11) and gamma(14) subunits, are identified and characterized in terms of their expression and function. The gamma(11) and gamma(14) subunits are most closely related to the gamma(1) subunit and share similar biochemical properties, suggesting their inclusion in class I. However, despite their close phylogenetic relationship and similar biochemical properties, the gamma(1), gamma(11), and gamma(14) subunits exhibit very distinct expression patterns, suggesting that class I should be further subdivided and that the signaling functions of each subgroup are distinct. In this regard, the gamma(11) and gamma(14) subunits represent a new subgroup of farnesylated gamma subunits that are expressed outside the retina and have functions other than phototransduction.

Coupling function of endogenous alpha(1)- and beta-adrenergic receptors in mouse cardiomyocytes

Genetically altered mouse models constitute unique systems to delineate the role of adrenergic receptor (AR) signaling mechanisms as modulators of cardiomyocyte function. The interpretation of results from these models depends on knowledge of the signaling properties of endogenous ARs in mouse cardiomyocytes. In the present study, we identify for the first time several defects in AR signaling in cardiomyocytes cultured from mouse ventricles. beta(1)-ARs induce robust increases in cAMP accumulation and the amplitude of the calcium and cell motion transients in mouse cardiomyocytes. Selective beta(2)-AR stimulation increases the amplitude of calcium and motion transients, with only a trivial rise in cAMP accumulation in comparison. beta(2)-AR responses are not influenced by pertussis toxin in cultured mouse cardiomyocytes. alpha(1)-ARs fail to activate phospholipase C, the extracellular signal-regulated protein kinase, p38-MAPK, or stimulate hypertrophy in mouse cardiomyocytes. Control experiments establish that this is not due to a lesion in distal elements in the signaling machinery, because these responses are induced by protease-activated receptor-1 agonists and phospholipase C is activated by Pasteurella multocida toxin (a G(q) alpha-subunit agonist). Surprisingly, norepinephrine activates p38-MAPK via beta-ARs in mouse cardiomyocytes, but beta-AR activation of p38-MAPK alone is not sufficient to induce cardiomyocyte hypertrophy. Collectively, these results identify a generalized defect in alpha(1)-AR signaling and a defect in beta(2)-AR linkage to cAMP (although not to an inotropic response) in cultured mouse cardiomyocytes. These naturally occurring vagaries in AR signaling in mouse cardiomyocytes provide informative insights into the requirements for hypertrophic signaling and impact on the value of mouse cardiomyocytes as a reconstitution system to investigate AR signaling in the heart.

The role of the G protein gamma(2) subunit in opioid antinociception in mice

We examined the role of the gamma(2) subunit of G proteins (Ggamma(2)) in the antinociception produced by c[D-Pen(2), D-Pen(5)]enkephalin (DPDPE) in mice. DPDPE produced 84.0+/-9.0% antinociception in vehicle-treated mice. After intracerebroventricular (i.c.v.) treatment with an antisense phosphorothioate oligodeoxynucleotide to the Ggamma(2) subunit, DPDPE-mediated antinociception decreased to 24.4+/-7.4%. The mismatch phosphorothioate oligodeoxynucleotide-treated mice showed 65.1+/-10.3% antinociception, while the missense phosphorothioate oligodeoxynucleotide-treated mice showed 76.4+/-23.6% antinociception by DPDPE. The reduction of analgesia in antisense phosphorothioate oligodeoxynucleotide-treated mice was significant in comparison with vehicle-treated (P<0.001), mismatch phosphorothioate oligodeoxynucleotide-treated (P<0.01) and missense phosphorothioate oligodeoxynucleotide-treated (P<0.05) mice. These results suggest that the G protein gamma(2) subunit is involved in the transduction pathway leading to antinociception by DPDPE.

Dual signaling regulated by calcyon, a D1 dopamine receptor interacting protein

The synergistic response of cells to the stimulation of multiple receptors has been ascribed to receptor cross talk; however, the specific molecules that mediate the resultant signal amplification have not been defined. Here a 24-kilodalton single transmembrane protein, designated calcyon, we functionally characterize that interacts with the D1 dopamine receptor. Calcyon localizes to dendritic spines of D1 receptor-expressing pyramidal cells in prefrontal cortex. These studies delineate a mechanism of Gq- and Gs-coupled heterotrimeric GTP-binding protein-coupled receptor cross talk by which D1 receptors can shift effector coupling to stimulate robust intracellular calcium (Ca2+i) release as a result of interaction with calcyon. The role of calcyon in potentiating Ca2+-dependent signaling should provide insight into the D1 receptor-modulated cognitive functions of prefrontal cortex.

Crosstalk between Galpha(i)- and Galpha(q)-coupled receptors is mediated by Gbetagamma exchange

Activation of Galpha(i)-coupled receptors often causes enhancement of the inositol phosphate signal triggered by Galpha(q)-coupled receptors. To investigate the mechanism of this synergistic receptor crosstalk, we studied the Galpha(i)-coupled adenosine A(1) and alpha(2C) adrenergic receptors and the Galpha(q)-coupled bradykinin B(2) and a UTP-preferring P2Y receptor. Stimulation of either Galpha(i)-coupled receptor expressed in COS cells increased the potency and the efficacy of inositol phosphate production by bradykinin or UTP. Likewise, overexpression of Gbeta(1)gamma(2) resulted in a similar increase in potency and efficacy of bradykinin or UTP. In contrast, these stimuli did not affect the potency of direct activators of Galpha(q); a truncated Gbeta(3) mutant had no effect on the receptor-generated signals whereas signals generated at the G-protein level were still enhanced. This suggests that the Gbetagamma-mediated signal enhancement occurs at the receptor level. Almost all possible combinations of Gbeta(1-3) with Ggamma(2-7) were equally effective in enhancing the signals of the B(2) and a UTP-preferring P2Y receptor, indicating a very broad specificity of this synergism. The enhancement of the bradykinin signal by (i) Galpha(i)-activating receptor ligands or (ii) cotransfection of Gbetagamma was suppressed when the B(2) receptor was replaced by a B(2)Gbeta(2) fusion protein. Gbetagamma enhanced the B(2) receptor-stimulated activation of G-proteins as determined by GTPgammaS-induced decrease in high affinity agonist binding and by B(2) receptor-enhanced [(35)S]GTPgammaS binding. These findings support the concept that Gbetagamma exchange between Galpha(i)- and Galpha(q)-coupled receptors mediates this type of receptor crosstalk.

Inhibition of luciferase expression by synthetic hammerhead ribozymes and their cellular uptake

Two synthetic hammerhead ribozymes, one unmodified and the other with 2"-modifications and four phosphorothioate groups, targeting a single GUA site in the luciferase mRNA, were compared for their inhibition of gene expression in cell cultureand their cellular uptake was also analysed. A HeLa X1/5 cell line stably expressing luciferase, under an inducible promoter, was treated with these ribozymes by liposome-mediated transfection to determine their activity. Luciferase expression in cells was inhibited to approximately 50% with little difference between the unmodified and the 2"-modified ribozyme. A similar degree of inhibition was observed with two catalytically inactive ribozymes, indicating that inhibition was mainly due to an antisense effect. A ribozyme carrying a cholesterol moiety, applied to the cells without carrier, showed no inhibition. Northern blotting indicated a similar amount of cellular uptake of all ribozymes. The unmodified ribozyme was essentially evenly distributed between cytoplasm and nucleus, whereas a higher proportion of the phosphorothioate-containing ribozyme was observed in the nucleus. Fluorescence microscopy, including confocal microscopy using 5"-fluorescein-labelled ribozymes, showed that the unmodified and 2"-modified ribozymes were present in the cytoplasm and in the nucleus to a similar extent, whereas the fluorescence of the phosphorothioate-containing ribozyme was much stronger in the nucleus. Both ribozymes inhibited luciferase expression to a comparable degree, suggesting that the ribozyme in the nucleus did not contribute significantly to the inhibition. Ribozymes with a cholesterol moiety were predominantly trapped in the cell membrane, explaining their inability to interfere with gene expression.

Ribozyme approach identifies a functional association between the G protein beta1gamma7 subunits in the beta-adrenergic receptor signaling pathway

The complex role that the heterotrimeric G proteins play in signaling pathways has become increasingly apparent with the cloning of countless numbers of receptors, G proteins, and effectors. However, in most cases, the specific combinations of alpha and betagamma subunits comprising the G proteins that participate in the most common signaling pathways, such as beta-adrenergic regulation of adenylyl cyclase activity, are not known. The extent of this problem is evident in the fact that the identities of the betagamma subunits that combine with the alpha subunit of Gs are only now being elucidated almost 20 years after its initial purification. In a previous study, we described the first use of a ribozyme strategy to suppress specifically the expression of the gamma7 subunit of the G proteins, thereby identifying a specific role of this protein in coupling the beta-adrenergic receptor to stimulation of adenylyl cyclase activity in HEK 293 cells. In the present study, we explored the potential utility of a ribozyme approach directed against the gamma7 subunit to identify functional associations with a particular beta and alphas subunit of the G protein in this signaling pathway. Accordingly, HEK 293 cells were transfected with a ribozyme directed against the gamma7 subunit, and the effects of this manipulation on levels of the beta and alphas subunits were determined by immunoblot analysis. Among the five beta alphas subunits detected in these cells, only the beta1 subunit was coordinately reduced following treatment with the ribozyme directed against the gamma7 subunit, thereby demonstrating a functional association between the beta1 and gamma7 subunits. The mechanism for coordinate suppression of the beta1 subunit was due to a striking change in the half-life of the beta1 monomer versus the beta1 heterodimer complexed with the gamma7 subunit. Neither the 52- nor 45-kDa subunits were suppressed following treatment with the ribozyme directed against the gamma7 subunit, thereby providing insights into the assembly of the Gs heterotrimer. Taken together, these data show the utility of a ribozyme approach to identify the role of not only the gamma subunits but also the beta subunits of the G proteins in signaling pathways.