Compartmentalized cAMP signaling in cardiac ventricular myocytes

The mechanism of 25-hydroxycholesterol-mediated suppression of atrial β1-adrenergic responses

25-Hydroxycholesterol (25HC) is a biologically active oxysterol, whose production greatly increases during inflammation by macrophages and dendritic cells. The inflammatory reactions are frequently accompanied by changes in heart regulation, such as blunting of the cardiac β-adrenergic receptor (AR) signaling. Here, the mechanism of 25HC-dependent modulation of responses to β-AR activation was studied in the atria of mice. 25HC at the submicromolar levels decreased the β-AR-mediated positive inotropic effect and enhancement of the Ca2+ transient amplitude, without changing NO production. Positive inotropic responses to β1-AR (but not β2-AR) activation were markedly attenuated by 25HC. The depressant action of 25HC on the β1-AR-mediated responses was prevented by selective β3-AR antagonists as well as inhibitors of Gi protein, Gβγ, G protein-coupled receptor kinase 2/3, or β-arrestin. Simultaneously, blockers of protein kinase D and C as well as a phosphodiesterase inhibitor did not preclude the negative action of 25HC on the inotropic response to β-AR activation. Thus, 25HC can suppress the β1-AR-dependent effects via engaging β3-AR, Gi protein, Gβγ, G protein-coupled receptor kinase, and β-arrestin. This 25HC-dependent mechanism can contribute to the inflammatory-related alterations in the atrial β-adrenergic signaling.

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Opposite regulation of glycogen metabolism by cAMP produced in the cytosol and at the plasma membrane

Cyclic AMP is produced in cells by two different types of adenylyl cyclases: at the plasma membrane by the transmembrane adenylyl cyclases (tmACs, ADCY1~ADCY9) and in the cytosol by the evolutionarily more conserved soluble adenylyl cyclase (sAC, ADCY10). By employing high-resolution extracellular flux analysis in HepG2 cells to study glycogen breakdown in real time, we showed that cAMP regulates glycogen metabolism in opposite directions depending on its location of synthesis within cells and the downstream cAMP effectors. While the canonical tmAC-cAMP-PKA signaling promotes glycogenolysis, we demonstrate here that the non-canonical sAC-cAMP-Epac1 signaling suppresses glycogenolysis. Mechanistically, suppression of sAC-cAMP-Epac1 leads to Ser-15 phosphorylation and thereby activation of the liver-form glycogen phosphorylase to promote glycogenolysis. Our findings highlight the importance of cAMP microdomain organization for distinct metabolic regulation and establish sAC as a novel regulator of glycogen metabolism.

Cardiac contraction and relaxation are regulated by distinct subcellular cAMP pools

Cells interpret a variety of signals through G-protein-coupled receptors (GPCRs) and stimulate the generation of second messengers such as cyclic adenosine monophosphate (cAMP). A long-standing puzzle is deciphering how GPCRs elicit different physiological responses despite generating similar levels of cAMP. We previously showed that some GPCRs generate cAMP from both the plasma membrane and the Golgi apparatus. Here we demonstrate that cardiomyocytes distinguish between subcellular cAMP inputs to elicit different physiological outputs. We show that generating cAMP from the Golgi leads to the regulation of a specific protein kinase A (PKA) target that increases the rate of cardiomyocyte relaxation. In contrast, cAMP generation from the plasma membrane activates a different PKA target that increases contractile force. We further validated the physiological consequences of these observations in intact zebrafish and mice. Thus, we demonstrate that the same GPCR acting through the same second messenger regulates cardiac contraction and relaxation dependent on its subcellular location.

Beta-blocker treatment of patients with atrial fibrillation attenuates spontaneous calcium release-induced electrical activity

AIMS

Atrial fibrillation (AF) has been associated with excessive spontaneous calcium release, linked to cyclic AMP (cAMP)-dependent phosphorylation of calcium regulatory proteins. Because β-blockers are expected to attenuate cAMP-dependent signaling, we aimed to examine whether the treatment of patients with β-blockers affected the incidence of spontaneous calcium release events or transient inward currents (I_TI).

METHODS

The impact of treatment with commonly used β-blockers was analyzed in human atrial myocytes from 371 patients using patch-clamp technique, confocal calcium imaging or immunofluorescent labeling. Data were analyzed using multivariate regression analysis taking into account potentially confounding effects of relevant clinical factors RESULTS: The L-type calcium current (I_Ca) density was diminished significantly in patients with chronic but not paroxysmal AF and the treatment of patients with β-blockers did not affect I_Ca density in any group. By contrast, the I_TI frequency was elevated in patients with either paroxysmal or chronic AF that did not receive treatment, and β-blocker treatment reduced the frequency to levels observed in patients without AF. Confocal calcium imaging showed that β-blocker treatment also reduced the calcium spark frequency in patients with AF to levels observed in those without AF. Furthermore, phosphorylation of the ryanodine receptor (RyR2) at Ser-2808 and phospholamban at Ser-16 was significantly lower in patients with AF that received β-blockers.

CONCLUSION

Together, our findings demonstrate that β-blocker treatment may be of therapeutic utility to prevent spontaneous calcium release-induced atrial electrical activity; especially in patients with a history of paroxysmal AF displaying preserved I_Ca density.

Impact of R-Carvedilol on β2-Adrenergic Receptor-Mediated Spontaneous Calcium Release in Human Atrial Myocytes

A hallmark of atrial fibrillation is an excess of spontaneous calcium release events, which can be mimicked by β1- or β2-adrenergic stimulation. Because β1-adrenergic receptor blockers (β1-blockers) are primarily used in clinical practice, we here examined the impact of β2-adrenergic stimulation on spontaneous calcium release and assessed whether the R- and S-enantiomers of the non-selective β- blocker carvedilol could reverse these effects. For this purpose, human atrial myocytes were isolated from patients undergoing cardiovascular surgery and subjected to confocal calcium imaging or immunofluorescent labeling of the ryanodine receptor (RyR2). Interestingly, the β2-adrenergic agonist fenoterol increased the incidence of calcium sparks and waves to levels observed with the non-specific β-adrenergic agonist isoproterenol. Moreover, fenoterol increased both the amplitude and duration of the sparks, facilitating their fusion into calcium waves. Subsequent application of the non β-blocking R-Carvedilol enantiomer reversed these effects of fenoterol in a dose-dependent manner. R-Carvedilol also reversed the fenoterol-induced phosphorylation of the RyR2 at Ser-2808 dose-dependently, and 1 µM of either R- or S-Carvedilol fully reversed the effect of fenoterol. Together, these findings demonstrate that β2-adrenergic stimulation alone stimulates RyR2 phosphorylation at Ser-2808 and spontaneous calcium release maximally, and points to carvedilol as a tool to attenuate the pathological activation of β2-receptors.