Background β-adrenergic receptor (β-AR) activation can provoke cardiac arrhythmias mediated by cAMP-dependent alterations of Ca2+ signaling. However cAMP can activate both PKA and an Exchange protein directly activated by cAMP (Epac) but their functional interaction is unclear. In heart selective Epac activation can induce potentially arrhythmogenic sarco­plas­mic reticulum (SR) Ca2+ release that involves CaMKII effects on the ryanodine receptor (RyR). Methods and results We tested whether physiological β-AR activation causes Epac-mediated SR Ca2+ leak and arrhythmias, whether it requires Epac1 vs. Epac2, β1-AR vs. β2-AR and CaMKIIδ-dependent phosphorylation of RyR2-S2814. We used knockout mice for Epac1, Epac2 or both (DKO). All knockouts exhibited unaltered basal cardiac function, Ca2+ handling and hypertrophy in response to pressure overload. However, SR Ca2+ leak induced by the specific Epac activator 8-CPT in wild-type was abolished in Epac2-KO and DKO, but unaltered in Epac1-KO. β-AR-induced arrhythmias were also less inducible in Epac2-KO vs. wild-type. β-AR activation with PKA inhibition, mimicked 8-CPT effects on SR Ca2+ leak, and was prevented by blockade of β1-AR but not β2-AR. CaMKII inhibition (KN93) and genetic ablation of either CaMKIIδ or CaMKII phosphorylation on RyR2-S2814 prevented 8-CPT-induced SR Ca2+ leak. Conclusions β1-AR activates Epac2 to induce SR Ca2+ leak via CaMKIIδ-dependent phosphorylation of RyR2-S2814. This pathway contributes to β-AR-induced arrhythmias and reduced cardiac function.

The prototypic second messenger cyclic AMP (cAMP) is essential for controlling cellular metabolism, including glucose and lipid homeostasis. In mammals, the majority of cAMP functions are mediated by cAMP-dependent protein kinase (PKA) and exchange proteins directly activated by cAMP (Epacs). To explore the physiological functions of Epac1, we generated Epac1 knockout mice. Here we report that Epac1 null mutants have reduced white adipose tissue and reduced plasma leptin levels but display heightened leptin sensitivity. Epac1-deficient mice are more resistant to high-fat diet-induced obesity, hyperleptinemia, and glucose intolerance. Furthermore, pharmacological inhibition of Epac by use of an Epac-specific inhibitor reduces plasma leptin levels in vivo and enhances leptin signaling in organotypic hypothalamic slices. Taken together, our results demonstrate that Epac1 plays an important role in regulating adiposity and energy balance. Obesity is a grave health problem, as it is closely related to the leading causes of morbidity and mortality, such as cardiovascular diseases, type 2 diabetes, hypertension, depression, and cancer (1). Over the last 2 decades, obesity has reached epidemic proportions in the United States: more than 35% of adults in the United States are obese, and more than two-thirds are overweight (2). Furthermore, 500 million people worldwide are obese, representing approximately 12% of the adult population on earth (3). Chronic excessive food/energy intake, mediated by leptin resistance, is a major factor contributing to obesity. To date, few effective treatment options are available for obesity (4). Therefore, a better understanding of the underlying molecular mechanisms of obesity development and effective, safe therapeutic interventions are urgently needed. Cyclic AMP (cAMP)-mediated signaling pathways are important for maintaining metabolic homeostasis and have been implicated in regulating leptin production and secretion (5-7). In mammals, the majority of cAMP functions are mediated by cAMP-dependent protein kinase (PKA) and exchange proteins directly activated by cAMP (Epacs) (8-10). A recent study revealed that activation of Epacs by an Epac-selective cAMP analog, 8-CPT-2=-O-Me-cAMP (11), interferes with leptin signaling in the hypothalamus, suggesting that Epacs may contribute to the pathophysiology of leptin resistance and represent a novel pharmacological target for treatment of obesity (12). To investigate the functional significance of Epac1 in leptin-mediated energy balance in vivo, we generated global loss-of-function mutants for Epac1. Analysis of these animals indicated resistance to high-fat diet (HFD)-induced obesity, heightened leptin signaling in the arcuate nucleus (AN), and improved glucose tolerance. These findings reveal an important role of Epac1 in metabolism and suggest that Epac1 may represent a novel therapeutic target for obesity. MATERIALS AND METHODS Mice.To construct an Epac1 targeting vector, two loxP sites were inserted into introns 2 and 5. A 3.8-kb upstream ...

Background: Talin is an integrin-actin linker essential for integrin activation.Results: Talin1 has distinct developmental and postnatal expression in heart versus Talin2. Cardiac-myocyte specific Talin1 deletion alters physiological and molecular responses of the myocardium to stress.Conclusion: Talin1 has a unique mechanotransductive role in the cardiomyocyte.Significance: Reduction of talin1 in cardiomyocytes may have beneficial effects in the stressed myocardium.