Regulation of cardiac L-type Ca2+ channel by coexpression of G(alpha s) in Xenopus oocytes.
Activation of G(alpha s) via beta-adrenergic receptors enhances the activity of cardiac voltage-dependent Ca2+ channels of the L-type, mainly via protein kinase A (PKA)-dependent phosphorylation. Contribution of a PKA-independent effect of G(alpha s) has been proposed but remains controversial. We demonstrate that, in Xenopus oocytes, antisense knockdown of endogenous G(alpha s) reduced, whereas coexpression of G(alpha s) enhanced, currents via expressed cardiac L-type channels, independently of the presence of the auxiliary subunits alpha2/delta or beta2A. Coexpression of G(alpha s) did not increase the amount of alpha1C protein in whole oocytes or in the plasma membrane (measured immunochemically). Activation of coexpressed beta2 adrenergic receptors did not cause a detectable enhancement of channel activity; rather, a small cAMP-dependent decrease was observed. We conclude that coexpression of G(alpha s), but not its acute activation via beta-adrenergic receptors, enhances the activity of the cardiac L-type Ca2+ channel via a PKA-independent effect on the alpha1C subunit. (+info)
Facilitation of signal onset and termination by adenylyl cyclase.
The alpha subunit (Gsalpha) of the stimulatory heterotrimeric guanosine triphosphate binding protein (G protein) Gs activates all isoforms of mammalian adenylyl cyclase. Adenylyl cyclase (Type V) and its subdomains, which interact with Gsalpha, promoted inactivation of the G protein by increasing its guanosine triphosphatase (GTPase) activity. Adenylyl cyclase and its subdomains also augmented the receptor-mediated activation of heterotrimeric Gs and thereby facilitated the rapid onset of signaling. These findings demonstrate that adenylyl cyclase functions as a GTPase activating protein (GAP) for the monomeric Gsalpha and enhances the GTP/GDP exchange factor (GEF) activity of receptors. (+info)
Distinct roles for Galphai2, Galphai3, and Gbeta gamma in modulation offorskolin- or Gs-mediated cAMP accumulation and calcium mobilization by dopamine D2S receptors.
Previous studies have shown that a single G protein-coupled receptor can regulate different effector systems by signaling through multiple subtypes of heterotrimeric G proteins. In LD2S fibroblast cells, the dopamine D2S receptor couples to pertussis toxin (PTX)-sensitive Gi/Go proteins to inhibit forskolin- or prostaglandin E1-stimulated cAMP production and to stimulate calcium mobilization. To analyze the role of distinct Galphai/o protein subtypes, LD2S cells were stably transfected with a series of PTX-insensitive Galphai/o protein Cys --> Ser point mutants and assayed for D2S receptor signaling after PTX treatment. The level of expression of the transfected Galpha mutant subunits was similar to the endogenous level of the most abundant Galphai/o proteins (Galphao, Galphai3). D2S receptor-mediated inhibition of forskolin-stimulated cAMP production was retained only in clones expressing mutant Galphai2. In contrast, the D2S receptor utilized Galphai3 to inhibit PGE1-induced (Gs-coupled) enhancement of cAMP production. Following stable or transient transfection, no single or pair set of mutant Galphai/o subtypes rescued the D2S-mediated calcium response following PTX pretreatment. On the other hand, in LD2S cells stably transfected with GRK-CT, a receptor kinase fragment that specifically antagonizes Gbeta gamma subunit activity, D2S receptor-mediated calcium mobilization was blocked. The observed specificity of Galphai2 and Galphai3 for different states of adenylyl cyclase activation suggests a higher level of specificity for interaction of Galphai subunits with forskolin- versus Gs-activated states of adenylyl cyclase than has been previously appreciated. (+info)
Both Gs and Gi proteins are critically involved in isoproterenol-induced cardiomyocyte hypertrophy.
Activation of beta-adrenoreceptors induces cardiomyocyte hypertrophy. In the present study, we examined isoproterenol-evoked intracellular signal transduction pathways leading to activation of extracellular signal-regulated kinases (ERKs) and cardiomyocyte hypertrophy. Inhibitors for cAMP and protein kinase A (PKA) abolished isoproterenol-evoked ERK activation, suggesting that Gs protein is involved in the activation. Inhibition of Gi protein by pertussis toxin, however, also suppressed isoproterenol-induced ERK activation. Overexpression of the Gbetagamma subunit binding domain of the beta-adrenoreceptor kinase 1 and of COOH-terminal Src kinase, which inhibit functions of Gbetagamma and the Src family tyrosine kinases, respectively, also inhibited isoproterenol-induced ERK activation. Overexpression of dominant-negative mutants of Ras and Raf-1 kinase and of the beta-adrenoreceptor mutant that lacks phosphorylation sites by PKA abolished isoproterenol-stimulated ERK activation. The isoproterenol-induced increase in protein synthesis was also suppressed by inhibitors for PKA, Gi, tyrosine kinases, or Ras. These results suggest that isoproterenol induces ERK activation and cardiomyocyte hypertrophy through two different G proteins, Gs and Gi. cAMP-dependent PKA activation through Gs may phosphorylate the beta-adrenoreceptor, leading to coupling of the receptor from Gs to Gi. Activation of Gi activates ERKs through Gbetagamma, Src family tyrosine kinases, Ras, and Raf-1 kinase. (+info)
A cluster of oppositely imprinted transcripts at the Gnas locus in the distal imprinting region of mouse chromosome 2.
Imprinted genes tend to occur in clusters. We have identified a cluster in distal mouse chromosome (Chr) 2, known from early genetic studies to contain both maternally and paternally imprinted, but unspecified, genes. Subsequently, one was identified as Gnas, which encodes a G protein alpha subunit, and there is clinical and biochemical evidence that the human homologue GNAS1, mutated in patients with Albright hereditary osteodystrophy, is also imprinted. We have used representational difference analysis, based on parent-of-origin methylation differences, to isolate candidate imprinted genes in distal Chr 2 and found two oppositely imprinted genes, Gnasxl and Nesp. Gnasxl determines a variant G protein alpha subunit associated with the trans-Golgi network and Nesp encodes a secreted protein of neuroendocrine tissues. Gnasxl is maternally methylated in genomic DNA and encodes a paternal-specific transcript, whereas Nesp is paternally methylated with maternal-specific expression. Their reciprocal imprinting may offer insight into the distal Chr 2 imprinting phenotypes. Remarkably, Gnasxl, Nesp, and Gnas are all part of the same transcription unit; transcripts for Gnasxl and Nesp are alternatively spliced onto exon 2 of Gnas. This demonstrates an imprinting mechanism in which two oppositely imprinted genes share the same downstream exons. (+info)
Examining the efficiency of receptor/G-protein coupling with a cleavable beta2-adrenoceptor-gsalpha fusion protein.
Reconstitution of high-affinity agonist binding at the beta2-adrenoceptor (beta2AR) expressed in Sf9 insect cells requires a large excess of the stimulatory G-protein of adenylyl cyclase, Gsalpha, relative to receptor [R. Seifert, T. W. Lee, V. T. Lam & B. K. Kobilka, (1998) Eur. J. Biochem. 255, 369-382]. In a fusion protein of the beta2AR and Gsalpha (beta2AR-Gsalpha), which has only a 1 : 1 stoichiometry of receptor and G-protein, high-affinity agonist binding and agonist-stimulated GTP hydrolysis, guanosine 5'-O-(3-thiotriphosphate) (GTP[S]) binding and adenylyl cyclase (AC) activation are more efficient than in the nonfused coexpression system. In order to analyze the stability of the receptor/G-protein interaction, we constructed a fusion protein with a thrombin-cleavage site between beta2AR and Gsalpha (beta2AR-TS-Gsalpha). beta2AR-TS-Gsalpha efficiently reconstituted high-affinity agonist binding, agonist-stimulated GTP hydrolysis, GTP[S] binding and AC activation. Thrombin cleaves approximately 70% of beta2AR-TS-Gsalpha molecules in Sf9 membranes. Thrombin cleavage did not impair high-affinity agonist binding and GTP[S] binding but strongly reduced ligand-regulated GTPase activity and AC activity. We conclude that fusion of the beta2AR to Gsalpha promotes tight physical association of the two partners and that this association remains stable for a single activation/deactivation cycle even after cleavage of the link between the receptor and G-protein. Dilution of Gsalpha in the membrane and release of activated Gsalpha into the cytosol can both prevent cleaved beta2AR-TS-Gsalpha from undergoing multiple activation/deactivation cycles. (+info)
Differential regulation of inotropy and lusitropy in overexpressed Gsalpha myocytes through cAMP and Ca2+ channel pathways.
We investigated the mechanisms responsible for altered contractile and relaxation function in overexpressed Gsalpha myocytes. Although baseline contractile function (percent contraction) in Gsalpha mice was similar to that of wild-type (WT) mice, left ventricular myocyte contraction, fura-2 Ca2+transients, and Ca2+ channel currents (ICa) were greater in Gsalpha mice in response to 10(-8) M isoproterenol (ISO) compared with WT mice. The late phase of relaxation of the isolated myocytes and fura-2 Ca2+ transients was accelerated at baseline in Gsalpha but did not increase further with ISO. In vivo measurements using echocardiography also demonstrated enhanced relaxation at baseline in Gsalpha mice. Forskolin and CaCl2 increased contraction similarly in WT and Gsalpha mice. Rp-cAMP, an inhibitor of protein kinase, blocked the increases in contractile response and Ca2+ currents to ISO in WT and to forskolin in both WT and Gsalpha. It also blocked the accelerated relaxation in Gsalpha at baseline but not the contractile response to ISO in Gsalpha myocytes. Baseline measurements of cAMP and phospholambation phosphorylation were enhanced in Gsalpha compared with WT. These data indicate that overexpression of Gsalpha accelerates relaxation at end diastolic but does not affect baseline systolic function in isolated myocytes. However, the enhanced responses to sympathetic stimulation partly reflect increased Ca2+ channel activity; i.e the cellular mechanisms mediating these effects appear to involve a cAMP-independent as well as a cAMP-dependent pathway. (+info)
A mutation in the heterotrimeric stimulatory guanine nucleotide binding protein alpha-subunit with impaired receptor-mediated activation because of elevated GTPase activity.
It has been reported that substitution of Arg258, a residue within the GTPase domain of the heterotrimeric guanine nucleotide binding protein (G protein) alpha-subunit (alphas), to alanine (alphas-R258A) results in decreased activation by receptor or aluminum fluoride (AlF4-) and increased basal GDP release. Arg258 interacts with Gln170 in the helical domain, and, presumably, loss of this interaction between the GTPase and helical domain leads to more rapid GDP release, resulting in decreased activation by AlF4- and increased thermolability. In this study, we mutate Gln170 to alanine (alphas-Q170A) and demonstrate that this mutant, like alphas-R258A, has decreased activation by AlF4-, increased thermolability (both reversed in the presence of excess guanine nucleotide), and an increased rate of GDP release. However, unlike alphas-R258A, alphas-Q170A does not have impaired receptor-mediated activation. Therefore, this interdomain interaction is critical to maintain normal guanine nucleotide binding (and hence normal activation by AlF4-) but is not important for receptor-mediated activation. In single turnover GTPase assays, the catalytic rate for GTP hydrolysis of alphas-R258A was 14-fold higher than normal whereas that of alphas-Q170A was unaffected. Examination of the alphas crystal structure suggests that Arg258, through interactions with Glu50, might constrain the position of Arg201, a residue critical for catalyzing the GTPase reaction. This is an example of a mutation in a heterotrimeric G protein that results in an increased intrinsic GTPase activity and provides another mechanism by which G protein mutations can impair signal transduction. (+info)