Effect of deletion of the major brain G-protein alpha subunit (alpha(o)) on coordination of G-protein subunits and on adenylyl cyclase activity

Global Analysis of Predicted G Protein-Coupled Receptor Genes in the Filamentous Fungus, Neurospora crassa

G protein−coupled receptors (GPCRs) regulate facets of growth, development, and environmental sensing in eukaryotes, including filamentous fungi. The largest predicted GPCR class in these organisms is the Pth11-related, with members similar to a protein required for disease in the plant pathogen Magnaporthe oryzae. However, the Pth11-related class has not been functionally studied in any filamentous fungal species. Here, we analyze phenotypes in available mutants for 36 GPCR genes, including 20 Pth11-related, in the model filamentous fungus Neurospora crassa. We also investigate patterns of gene expression for all 43 predicted GPCR genes in available datasets. A total of 17 mutants (47%) possessed at least one growth or developmental phenotype. We identified 18 mutants (56%) with chemical sensitivity or nutritional phenotypes (11 uniquely), bringing the total number of mutants with at least one defect to 28 (78%), including 15 mutants (75%) in the Pth11-related class. Gene expression trends for GPCR genes correlated with the phenotypes observed for many mutants and also suggested overlapping functions for several groups of co-transcribed genes. Several members of the Pth11-related class have phenotypes and/or are differentially expressed on cellulose, suggesting a possible role for this gene family in plant cell wall sensing or utilization.

Heterotrimeric G protein-mediated signaling and its non-canonical regulation in the heart

Heterotrimeric guanine nucleotide-binding proteins (G proteins) regulate a multitude of signaling pathways in mammalian cells by transducing signals from G protein-coupled receptors (GPCRs) to effectors, which in turn regulate cellular function. In the myocardium, G protein signaling occurs in all cardiac cell types and is centrally involved in the regulation of heart rate, pump function, and vascular tone and in the response to hemodynamic stress and injury. Perturbations in G protein-mediated signaling are well known to contribute to cardiac hypertrophy, failure, and arrhythmias. Most of the currently used drugs for cardiac and other diseases target GPCR signaling. In the canonical G protein signaling paradigm, G proteins that are located at the cytoplasmic surface of the plasma membrane become activated after an agonist-induced conformational change of GPCRs, which then allows GTP-bound Gα and free Gβγ subunits to activate or inhibit effector proteins. Research over the past two decades has markedly broadened the original paradigm with a GPCR-G protein-effector at the cell surface at its core by revealing novel binding partners and additional subcellular localizations for heterotrimeric G proteins that facilitate many previously unrecognized functional effects. In this review, we focus on non-canonical and epigenetic-related mechanisms that regulate heterotrimeric G protein expression, activation, and localization and discuss functional consequences using cardiac examples where possible. Mechanisms reviewed involve microRNAs, histone deacetylases, chaperones, alternative modes of G protein activation, and posttranslational modifications. Some of these newly characterized mechanisms may be further developed into novel strategies for the treatment of cardiac disease and beyond.

Enhanced calcium cycling and contractile function in transgenic hearts expressing constitutively active G alpha o* protein

In contrast to the other heterotrimeric GTP-binding proteins (G proteins) Gs and Gi, the functional role of G o is still poorly defined. To investigate the role of G alpha o in the heart, we generated transgenic mice with cardiac-specific expression of a constitutively active form of G alpha o1* (G alpha o*), the predominant G alpha o isoform in the heart. G alpha o expression was increased 3- to 15-fold in mice from 5 independent lines, all of which had a normal life span and no gross cardiac morphological abnormalities. We demonstrate enhanced contractile function in G alpha o* transgenic mice in vivo, along with increased L-type Ca2+ channel current density, calcium transients, and cell shortening in ventricular G alpha o*-expressing myocytes compared with wild-type controls. These changes were evident at baseline and maintained after isoproterenol stimulation. Expression levels of all major Ca2+ handling proteins were largely unchanged, except for a modest reduction in Na+/Ca2+ exchanger in transgenic ventricles. In contrast, phosphorylation of the ryanodine receptor and phospholamban at known PKA sites was increased 1.6- and 1.9-fold, respectively, in G alpha o* ventricles. Density and affinity of beta-adrenoceptors, cAMP levels, and PKA activity were comparable in G alpha o* and wild-type myocytes, but protein phosphatase 1 activity was reduced upon G alpha o* expression, particularly in the vicinity of the ryanodine receptor. We conclude that G alpha o* exerts a positive effect on Ca2+ cycling and contractile function. Alterations in protein phosphatase 1 activity rather than PKA-mediated phosphorylation might be involved in hyperphosphorylation of key Ca2+ handling proteins in hearts with constitutive G alpha o activation.

Gz proteins are functionally coupled to dopamine D2-like receptors in vivo

The receptors that couple to the G protein Gz in vivo are still relatively unknown. In this study, we investigated the effects of various dopamine receptor agonists in a mouse deficient in the alpha subunit of Gz. The dopamine D1-like receptor agonist SKF38393 stimulated comparable locomotor activity in both wildtype mice and mice lacking Galphaz. In contrast, the dopamine D2-like receptor agonist quinpirole suppressed locomotor activity in both groups of mice, but this suppression was significantly smaller in Galphaz knockout mice. Consistent with these behavioural observations, quinpirole inhibition of dopamine release in the forebrain nucleus accumbens evoked by electrical stimulation of dopamine axons was significantly attenuated in mice lacking Galphaz. In addition, hypothermia and adrenocorticotropic hormone release resulting from activation of dopamine D2-like receptors were also significantly reduced in Galphaz knockout mice. However, adrenocorticotropic hormone secretion induced by corticotrophin releasing hormone and the serotonin 1A receptor agonist 8-hydroxy-dipropylamino-tetralin were similar between wildtype and Galphaz knockout mice. Western blot analysis showed that the expression levels of Galphai, Galphao, Galphas, Galphaq and Gbeta were the same in the brains of mice of both genotypes. Overall, our data suggest that Gz proteins are functionally coupled to dopamine D2-like receptors in vivo.

Coordinate expression of the alpha and beta subunits of heterotrimeric G proteins involves regulation of protein degradation in CHO cells

Individual cell types express a characteristic balance between heterotrimeric G protein alpha and betagamma subunits, but little is known about the regulatory mechanism. We systemically examined the regulatory mechanism in CHO cells. We found that expression of Galphas, Galphai2, and Galphaq proteins increased in direct proportion to the increase of Gbeta1gamma2 overexpressed transiently. Expression of Gbeta protein also increased following overexpression of Galphas, Galphai2, and Galphaq. The Gbetagamma overexpression stimulated degradation of Gbeta in contrast to reduction of Galphas degradation. We conclude that coordinate expression of the G protein subunits involves regulation of protein degradation via proteasome in CHO cells.

A G-protein beta subunit required for sexual and vegetative development and maintenance of normal G alpha protein levels in Neurospora crassa

The genome of the filamentous fungus Neurospora crassa contains a single gene encoding a heterotrimeric G-protein beta subunit, gnb-1. The predicted GNB-1 protein sequence is most identical to G beta proteins from the filamentous fungi Cryphonectria parasitica and Aspergillus nidulans. N. crassa GNB-1 is also 65% identical to the human GNB-1 protein but only 38 and 45% identical to G beta proteins from budding and fission yeasts. Previous studies in animal and fungal systems have elucidated phenotypes of G beta null mutants, but little is known about the effects of G beta loss on G alpha levels. In this study, we analyzed a gnb-1 deletion mutant for cellular phenotypes and levels of the three G alpha proteins. Delta gnb-1 strains are female-sterile, with production of aberrant fertilized reproductive structures. Delta gnb-1 strains conidiate more profusely and have altered mass on solid medium. Loss of gnb-1 leads to inappropriate conidiation and expression of a conidiation-specific gene during growth in submerged culture. Intracellular cyclic AMP levels are reduced by 60% in vegetative plate cultures of delta gnb-1 mutants. Loss of gnb-1 leads to lower levels of the three G alpha proteins under a variety of conditions. Analysis of transcript levels for the gna-1 and gna-2 G alpha genes in submerged cultures indicates that regulation of G alpha protein levels by gnb-1 is posttranscriptional. The results suggest that GNB-1 directly regulates apical extension rate and mass accumulation. In contrast, many other delta gnb-1 phenotypes, including female sterility and defective conidiation, can be explained by altered levels of the three N. crassa G alpha proteins.

Impaired parasympathetic heart rate control in mice with a reduction of functional G protein betagamma-subunits

Acetylcholine released on parasympathetic stimulation slows heart rate through activation of muscarinic receptors on the sinus nodal cells and subsequent opening of the atrial muscarinic potassium channel (K(ACh)). K(ACh) is directly activated by G protein betagamma-subunits. To elucidate the physiological role of Gbetagamma for the regulation of heart rate and electrophysiological function in vivo, we created transgenic mice with a reduced amount of membrane-bound Gbeta protein by overexpressing nonprenylated Ggamma(2)-subunits in their hearts using the alpha-myosin heavy chain promoter. At baseline and after muscarinic stimulation with carbachol, heart rate and heart rate variability were determined with electrocardiogram telemetry in conscious mice and in vivo intracardiac electrophysiological studies in anesthetized mice. Reduction of the amount of functional Gbetagamma protein by >50% caused a pronounced blunting of the carbachol-induced bradycardia as well as the increases in time- and frequency-domain indexes of heart rate variability and baroreflex sensitivity that were observed in wild types. In addition, sinus node recovery time and inducibility of atrial arrhythmias were reduced in transgenic mice. Our data demonstrate in vivo that Gbetagamma plays a crucial role for parasympathetic heart rate control, sinus node automaticity, and atrial arrhythmia vulnerability.

RGS proteins: lessons from the RGS9 subfamily

RGS proteins enhance the time resolution of G protein signaling cascades by accelerating GTP hydrolysis of G alpha subunits of heterotrimeric G proteins. RGS9-1, a photoreceptor-specific RGS protein, is the first vertebrate member of this sizeable family whose physiological function in a well-defined G protein pathway has been identified. It is essential for normal subsecond recovery kinetics of the light responses in retinal photoreceptors. Understanding this role allows RGS9-1 to serve as a useful model for understanding how specificity and regulation of RGS function are achieved. In addition to the catalytic RGS domain, shared among all members of this family, RGS9-1 contains several other domains, which are also found in a closely related subset of RGS proteins, the RGS9 subfamily. One of these domains, the G gamma-like (GGL) domain, has been identified as the attachment site for G beta 5 proteins, which act as obligate subunits for this subfamily. Results from RGS9-1 and other subfamily members suggest that specificity is achieved by cell type-specific transcription, RNA processing, and G beta 5-dependent protein stabilization. In addition, membrane localization via specific targeting domains likely plays an important role.

Altered regulation of potassium and calcium channels by GABA(B) and adenosine receptors in hippocampal neurons from mice lacking Galpha(o)

To examine the role of G(o) in modulation of ion channels by neurotransmitter receptors, we characterized modulation of ionic currents in hippocampal CA3 neurons from mice lacking both isoforms of Galpha(o). In CA3 neurons from Galpha(o)(-/-) mice, 2-chloro-adenosine and the GABA(B)-receptor agonist baclofen activated inwardly rectifying K(+) currents and inhibited voltage-dependent Ca(2+) currents just as effectively as in Galpha(o)(+/+) littermates. However, the kinetics of transmitter action were dramatically altered in Galpha(o)(-/-) mice in that recovery on washout of agonist was much slower. For example, recovery from 2-chloro-adenosine inhibition of calcium current was more than fourfold slower in neurons from Galpha(o)(-/-) mice [time constant of 12.0 +/- 0.8 (SE) s] than in neurons from Galpha(o)(+/+) mice (time constant of 2.6 +/- 0.2 s). Recovery from baclofen effects was affected similarly. In neurons from control mice, effects of both baclofen and 2-chloro-adenosine on Ca(2+) currents and K(+) currents were abolished by brief exposure to external N-ethyl-maleimide (NEM). In neurons lacking Galpha(o), some inhibition of Ca(2+) currents by baclofen remained after NEM treatment, whereas baclofen activation of K(+) currents and both effects of 2-chloro-adenosine were abolished. These results show that modulation of Ca(2+) and K(+) currents by G protein-coupled receptors in hippocampal neurons does not have an absolute requirement for Galpha(o). However, modulation is changed in the absence of Galpha(o) in having much slower recovery kinetics. A likely possibility is that the very abundant Galpha(o) is normally used but, when absent, can readily be replaced by G proteins with different properties.

Signal transduction in atria and ventricles of mice with transient cardiac expression of activated G protein alpha(q)

We recently showed that the transient expression of a hemagglutinin (HA) epitope-tagged, constitutively active mutant of the G protein alpha(q) subunit (HAalpha(q)*) in the hearts of transgenic mice is sufficient to induce cardiac hypertrophy and dilatation that continue to progress after HAalpha(q)* protein becomes undetectable. We demonstrated that the activity of phospholipase Cbeta, the immediate downstream target of activated Galpha(q), is increased at 2 weeks, when HAalpha(q)* is expressed, but also at 10 weeks, when HAalpha(q)* is no longer detectable. This observation suggested that the transient HAalpha(q)* expression causes multiple, persistent changes in cellular signaling pathways. We now demonstrate changes in the level, activity, or both of several signaling components, including changes in the amount and hormone responsiveness of phospholipase Cbeta enzymes, in the basal level of diacylglycerol (which predominantly reflects activation of phospholipase D), in the amount or distribution of protein kinase C (PKC) isoforms (PKCalpha, PKCdelta, and PKCepsilon), and in the amount of several endogenous G proteins. These changes vary depending on the isoform of the signaling molecule, the chamber in which it is expressed, and the presence or absence of HAalpha(q)*. Our results suggest that a network of linked signaling functions determines the development of hypertrophy. They also suggest that atria and ventricles represent different signaling domains. It is likely that such changes occur in other model systems in which the activity of a single signaling component is increased, either due to an activating mutation or due to overexpression of the wild type.