The effect of culture and membrane potential on Go alpha expression in neonatal rat cardiac myocytes

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.

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

Human HEK 293 cells present a simple and tractable system to directly test the hypothesis that the G protein gamma subunits contribute to the specificity of receptor signaling pathways in vivo. To begin to elucidate the functions of the individual gamma subunits in these cells, a ribozyme strategy was used to specifically inactivate the mRNA encoding the gamma7 subunit. A phosphorothioated DNA-RNA chimeric hammerhead ribozyme was constructed and analyzed for specificity toward the targeted gamma7 subunit. In vitro cleavage analysis of this ribozyme revealed a highly efficient cleavage activity directed exclusively toward the gamma7 RNA transcript. In particular, this ribozyme did not result in cleavage of the gamma12 RNA transcript, which is 75% identical to the gamma7 RNA transcript. Using a transient transfection assay, in vivo analysis of this ribozyme showed a specific reduction in both the mRNA and protein expression of the gamma7 subunit in HEK 293 cells. Coincident with this loss in gamma7 subunit, there was a specific reduction in the protein expression of the beta1 subunit, suggesting that the beta1 and gamma7 subunits may functionally interact to form a betagamma dimer in vivo. Functional analysis of the consequences of ribozyme-mediated suppression of the gamma7 subunit expression indicated that it was associated with significant attenuation of isoproterenol-, but not prostaglandin E1-, stimulated adenylylcyclase activity. Suppression of the gamma7 subunit expression had no effect on carbachol- and ATP-mediated stimulation of phosphatidylinositol turnover. Taken together, these results not only indicate the feasibility of using the ribozyme technology to determine the roles of individual gamma subunits in receptor-G protein-effector pathways in vivo, but they point to a specific role of the gamma7 subunit in the regulation of adenylylcyclase activity in response to isoproterenol.

Subunit expression of signal transducing G proteins in cardiac tissue: implications for phospholipase C-beta regulation

In the heart, alpha-adrenergic, angiotensin II and endothelin signaling pathways modulate short-term changes in chronotropy and inotropy, and participate in the long-term control of cardiac growth. A shared feature of these signaling pathways is the stimulation of phosphatidylinositol (PI) turnover, which is thought to occur via G protein-mediated regulation of phospholipase C (PLC) activity. However, G protein subunits capable of regulating PLC activity have not been identified in different regions and cell types of the heart and members of the G protein-regulated PLC-beta isozyme family have not been documented in the heart. Using a battery of antipeptide specific antisera directed against the G protein alpha q, beta and gamma subunit families and against members of the PLC-beta, PLC-gamma and PLC-delta families, we demonstrated that heart tissues express the G protein alpha subunits alpha q and alpha 11, multiple G protein beta and gamma subunits, and PLC-beta 3, a phospholipase C isozyme regulated by either G protein alpha or beta gamma subunits. The degree of expression and distribution of these subunits differed between regions of the heart (atria versus ventricle) and changed with development. These data lay the ground work for future studies to determine the functional coupling of specific subsets of these components involved in receptor activation of PI turnover in the heart.

Characterization of alpha 1-adrenoceptors which mediate chronotropy in neonatal rat cardiac myocytes

1. In the present study, we investigated the effect of culture on alpha 1-adrenoceptors that mediate chronotropy and on alpha 1-adrenergic signal transduction in neonatal rat cardiac myocytes. 2. The spontaneous beating rate of neonatal rat myocytes after 3 or 7 days in culture was 37.4 +/- 4.2 or 102.0 +/- 4.3 beats min-1, respectively. The alpha 1-adrenoceptor-mediated chronotropic effect of norepinephrine was positive at day 3 of culture. In contrast to day 3 of culture, the neonatal myocytes exhibited a negative chronotropic response to norepinephrine on day 7 of culture. Both of these effects of norepinephrine were completely abolished by prazosin. 3. The affinity (Kd) and/or density (Bmax) of alpha 1-adrenoceptors labeled with [3H]prazosin in membranes from cultured myocytes were not significantly different between day 3 and day 7 of culture. 4. The expression of Gs, Gi, Gq and Go alpha-subunits in membranes from cultured myocytes was found to be significantly increased with the passage of culture time by immunoblot analysis. In contrast, no significant differences in G beta-subunit expression were observed between day 3 and day 7 of culture. 5. Norepinephrine-stimulated inositol 1,4,5-trisphosphate production by radio-binding protein in neonatal myocytes after 7 days of culture was significantly higher than that of the day 3 counterpart. 6. No significant changes in phospholipid and cholesterol contents in membranes from neonatal myocytes were observed with longer culture times.(ABSTRACT TRUNCATED AT 250 WORDS)