A Novel phospholipase Ce-dependent mechanism forB-adrenergic receptor signaling in the heart

Agonist regulation of intracellular Ca2+ release and protein kinase C activation through stimulation of phospholipase C (PLC) enzymes modulates a variety of physiological responses in multiple cell types. PLCe is a recently identified enzyme regulated by multiple agonists through a wide range of signaling molecules including Ras-family small GTPases, RhoA, Gα12/13, and Gβγ. In addition to phospholipase activity, PLCe uniquely acts as a guanine nucleotide exchange factor (GEF) for Rap1. The physiological consequences of PLCe signaling remain largely undefined. Our analysis of a PLCe knock-out mouse model revealed a surprising role for PLCe in β- adrenergic receptor (βAR) modulation of cardiac contractile function. This was the first implication of a physiological role for PLC activity downstream of βAR stimulation, but the mechanisms remained unknown. Subsequently, a novel upstream regulatory pathway for PLCe requiring the cAMP-responsive Rap GEF, Epac and Rap-GTP was found to be critical for maximal βAR-stimulated enhancement of SRCa 2+ release in isolated cardiac myocytes. This pathway appears to operate in parallel to the canonical PKA pathway. Having identified Epac as the crucial connection between βAR stimulation and PLCe, the downstream mechanism for PLCe modulation of SR-Ca2+ release remained to be identified. Our investigations revealed a requirement for both phospholipase C and Rap GEF activities of PLCe suggesting a possible feed forward potentiation of PLCe activity. Downstream of PLCe, PKCe- dependent activation of CamKII was found to be the critical pathway modulating SRviii Ca2+ release following βAR stimulation in ventricular cardiac myocytes. This study provides a novel and potentially important mechanism for regulation of CamKII activity in the heart. Knowledge gained from this work identifies an important and previously unappreciated physiological role for PLCe in cardiac signaling, thus providing a more complete picture of βAR regulation of cardiac myocyte Ca2+- handling. Sympathetic regulation of cardiac function is critical for the normal response of the heart to stress as perturbation of the βAR signaling cascade is one of the major predictors of progressive heart failure. A comprehensive understanding of all the components of βAR signaling in the heart may provide new therapeutic targets for the treatment of heart failure.