No abstract

Alterations in cardiac G protein-mediated signaling, most prominently G q/11 signaling, are centrally involved in hypertrophy and heart failure development. Several RGS proteins that can act as negative regulators of G protein signaling are expressed in the heart, but their functional roles are still poorly understood. RGS expression changes have been described in hypertrophic and failing hearts. In this study, we report a marked decrease in RGS2 (but not other major cardiac RGS proteins (RGS3-RGS5)) that occurs prior to hypertrophy development in different models with enhanced G q/11 signaling (transgenic expression of activated G␣ q * and pressure overload due to aortic constriction). To assess functional consequences of selective down-regulation of endogenous RGS2, we identified targeting sequences for effective RGS2 RNA interference and used lipid-based transfection to achieve uptake of fluorescently labeled RGS2 small interfering RNA in >90% of neonatal and adult ventricular myocytes. Endogenous RGS2 expression was dose-dependently suppressed (up to 90%) with no major change in RGS3-RGS5. RGS2 knockdown increased phenylephrine-and endothelin-1-induced phospholipase C␤ stimulation in both cell types and exacerbated the hypertrophic effect (increase in cell size and radiolabeled protein) in neonatal myocytes, with no major change in G q/11 -mediated ERK1/2, p38, or JNK activation. Taken together, this study demonstrates that endogenous RGS2 exerts functionally important inhibitory restraint on G q/11 -mediated phospholipase C␤ activation and hypertrophy in ventricular myocytes. Our findings point toward a potential pathophysiological role of loss of fine tuning due to selective RGS2 down-regulation in G q/11 -mediated remodeling. Furthermore, this study shows the feasibility of effective RNA interference in cardiomyocytes using lipid-based small interfering RNA transfection.

RGS proteins act as negative regulators of G protein signaling by serving as GTPase-activating proteins (GAP) for ␣ subunits of heterotrimeric G proteins (G␣), thereby accelerating G protein inactivation. RGS proteins can also block G␣-mediated signal production by competing with downstream effectors for G␣ binding. Little is known about the relative contribution of GAP and effector antagonism to the inhibitory effect of RGS proteins on G protein-mediated signaling. By comparing the inhibitory effect of RGS2, RGS3, RGS5, and RGS16 on G␣ q -mediated phospholipase C␤ (PLC␤) activation under conditions where GTPase activation is possible versus nonexistent, we demonstrate that members of the R4 RGS subfamily differ significantly in their dependence on GTPase acceleration. COS-7 cells were transiently transfected with either muscarinic M 3 receptors, which couple to endogenous G q protein and mediate a stimulatory effect of carbachol on PLC␤, or constitutively active G␣ q * , which is inert to GTP hydrolysis and activates PLC␤ independent of receptor activation. In M 3 -expressing cells, all of the RGS proteins significantly blunted the efficacy and potency of carbachol. In contrast, G␣ q * -induced PLC␤ activation was inhibited by RGS2 and RGS3 but not RGS5 and RGS16. The observed differential effects were not due to changes in M 3 , G␣ q / G␣ q * , PLC␤, or RGS expression, as shown by receptor binding assays and Western blots. We conclude that closely related R4 RGS family members differ in their mechanism of action. RGS5 and RGS16 appear to depend on G protein inactivation, whereas GAP-independent mechanisms (such as effector antagonism) are sufficient to mediate the inhibitory effect of RGS2 and RGS3.Many extracellular stimuli elicit intracellular responses by activating seven-transmembrane receptors that are coupled to heterotrimeric G proteins comprising ␣ and ␤␥ subunits (1, 2). The duration of the response of a cell to external signals is largely determined by the activation/inactivation cycle of G proteins. Activated receptors trigger GTP-for-GDP exchange on G␣ subunits, thus dissociating G␣ from G␤ ␥ and subsequent activation of downstream effectors (such as enzymes and ion channels). The duration of G protein activation is limited by GTPase activity intrinsic to G␣ subunits that catalyzes the conversion of active GTP-bound G␣ into inactive GDP-bound G␣, which in turn can reassociate with G␤␥ and receptors.RGS proteins belong to a family of more than 20 proteins with a conserved RGS core domain of ϳ120 amino acids that is necessary and sufficient for binding to G␣ subunits (3). They are divided into several subfamilies based on their structural similarities, gene organization, and function. RGS proteins act as regulators of G protein signaling by limiting the signals generated by G protein-coupled receptors. They markedly increase the rate at which G␣ subunits hydrolyze GTP to GDP, a property that defines them as GTPase-activating proteins (or GAPs) 1 (4). Hastening of G␣ inactivation facilitates the reass...

In patients with stable and low heart rates, the prospectively ECG-triggered axial scan protocol maintained image quality but reduced radiation exposure by 69% compared with helical scanning. Axial computed tomography data acquisition should be strongly recommended in suitable patients to avoid unnecessarily high radiation exposure.