br Disclosures br Acknowledgements This work was supported i
Acknowledgements This work was supported in part by grants from the Ministerio de Ciencia e Innovación, Spain (SAF2010-22051) and Xunta de Galicia, Spain (INCITE08PXIB203092PR).
Introduction Ca is a key element in cardiac excitation–contraction (EC) coupling. In each heartbeat, membrane depolarization during an BMH-21 activates L-type Ca channels located in the sarcolemma. Ca entry through these channels activates intracellular Ca release channels, named ryanodine receptors (RyRs), which are located in the membrane of the sarcoplasmic reticulum (SR). RyRs amplify the initial Ca signal via Ca-induced Ca release (CICR), providing enough Ca to activate contractile myofibrils. Relaxation then occurs when intracellular Ca concentration ([Ca]i) returns to diastolic values, due mainly to Ca pumped back into the SR by the Ca-ATPase (SERCA) and extrusion from the cell via Na+/Ca exchange (NCX) . While Ca in EC coupling is physiologically and pathophysiologically relevant, new roles for cardiac myocyte Ca are being elucidated. For instance, prohypertrophic signaling seems to be activated by perinuclear activation of Ca/calmodulin dependent protein kinase II (CaMKII) promoted by local elevation of nuclear [Ca] ([Ca]n) . By analogy to EC coupling, this process has been named excitation–transcription (ET) coupling. However, it is still not fully understood how [Ca]n variations may be dissociated from bulk [Ca]i oscillations during contraction–relaxation cycles. We recently showed that Epac (exchange protein directly activated by cAMP) could activate SR Ca release in ventricular myocytes, via a CaMKII-dependent phosphorylation of the RyR , and we hypothesized that Epac-dependent Ca signaling may also be implicated in cardiac hypertrophy , . Epac induces cardiomyocyte hypertrophy, both in cultured neonatal ventricular myocytes  and in adult myocytes . Epac is a guanylyl exchange protein (GEF) ,  widely distributed in the organism, including the heart, whose functional roles are just beginning to be defined , . The hypertrophic effects of Epac are independent of the classical effector of cAMP, protein kinase A (PKA), but rather involve the CaMKII . In addition, Epac expression is increased in experimental animal models of cardiac hypertrophy  and contributes to the hypertrophic effect of β-adrenergic receptor . CaMKII is also known to activate nuclear export of class II histone deacetylases (e.g. HDAC4 and 5) , , an effect which derepresses myocyte enhancer factor 2 (MEF2) driven transcription, and contributes to hypertrophic remodeling. Here we dissected the signaling pathway linking Epac activation to Ca mobilization, paying a special attention to [Ca]n, and activation of HDAC5 nuclear export. By simultaneously analyzing cytoplasmic and [Ca]n during selective Epac activation with 8-pCPT-2′-O-Me-cAMP (8-pCPT), we found that Epac preferentially and differently increases [Ca]n (more than bulk [Ca]i). This action involves phospholipase C (PLC)/Inositol 1,4,5-trisphosphate (IP3) signaling and CaMKII activation. In addition, we found that Epac induced HDAC5 export via activation of IP3R and CaMKII, and results in activation of the hypertrophic transcription factor MEF2.
Discussion The present work demonstrates a novel role for endogenous Epac in cardiac Ca and hypertrophic signaling. Moreover, the activation of Epac by cAMP in the physiological context, likely coexists with and parallels the classical cAMP-dependent signaling via PKA, but may have very divergent downstream targets. We find that acute activation of endogenous Epac elevates [Ca]n and Ca sparks in a manner that depends on the activation of PLC, IP3R and CaMKII. Epac is preferentially located near the nucleus, and acute Epac activation also drives HDAC5 nuclear export (and MEF2 activation) in a manner that is IP3R and CaMKII-dependent, paralleling the [Ca]n elevation. We propose a working hypothesis for diastolic [Ca] schematized in Fig. 6 whereby Epac activation can activate PLC, causing DAG and IP3 production, that the IP3 activates Ca release via IP3R which result in local activation of CaMKII. This CaMKII activation may then be responsible for the phosphorylation of both RyR (enhancing Ca sparks and unloading the SR) and HDAC5 (resulting in nuclear HDAC5 export and derepression of MEF2-dependent hypertrophic transcription). Not all of the details of these two Epac-dependent pathways are fully worked out yet (see below), but this represents a novel paradigm in cAMP-dependent Ca signaling in cardiac myocytes that influences both EC coupling and ET coupling.