Conflicting roles for protein kinase C (PKC) isozymes in cardiac disease have been reported. Here, ␦PKC-selective activator and inhibitor peptides were designed rationally, based on molecular modeling and structural homology analyses. Together with previously identified activator and inhibitor peptides of PKC, ␦PKC peptides were used to identify cardiac functions of these isozymes. In isolated cardiomyocytes, perfused hearts, and transgenic mice, ␦PKC and PKC had opposing actions on protection from ischemiainduced damage. Specifically, activation of PKC caused cardioprotection whereas activation of ␦PKC increased damage induced by ischemia in vitro and in vivo. In contrast, ␦PKC and PKC caused identical nonpathological cardiac hypertrophy; activation of either isozyme caused nonpathological hypertrophy of the heart. These results demonstrate that two related PKC isozymes have both parallel and opposing effects in the heart, indicating the danger in the use of therapeutics with nonselective isozyme inhibitors and activators. Moreover, reduction in cardiac damage caused by ischemia by perfusion of selective regulator peptides of PKC through the coronary arteries constitutes a major step toward developing a therapeutic agent for acute cardiac ischemia.

Abstract-The role of cardiac myosin binding protein-C (cMyBP-C) phosphorylation in cardiac physiology or pathophysiology is unclear. To investigate the status of cMyBP-C phosphorylation in vivo, we determined its phosphorylation state in stressed and unstressed mouse hearts. cMyBP-C phosphorylation is significantly decreased during the development of heart failure or pathologic hypertrophy. We then generated transgenic (TG) mice in which the phosphorylation sites of cMyBP-C were changed to nonphosphorylatable alanines (MyBP-C AllPϪ ). A TG line showing Ϸ40% replacement with MyBP-C AllPϪ showed no changes in morbidity or mortality but displayed depressed cardiac contractility, altered sarcomeric structure and upregulation of transcripts associated with a hypertrophic response. To explore the effect of complete replacement of endogenous cMyBP-C with MyBP-C AllPϪ , the mice were bred into the MyBP-C (t/t) background, in which less than 10% of normal levels of a truncated MyBP-C are present. Although MyBP-C AllPϪ was incorporated into the sarcomere and expressed at normal levels, the mutant protein could not rescue the MyBP-C (t/t) phenotype. The mice developed significant cardiac hypertrophy with myofibrillar disarray and fibrosis, similar to what was observed in the MyBP-C (t/t) animals. In contrast, when the MyBP-C (t/t) mice were bred to a TG line expressing normal MyBP-C (MyBP-C WT ), the MyBP-C (t/t) phenotype was rescued. These data suggest that cMyBP-C phosphorylation is essential for normal cardiac function. Key Words: mouse Ⅲ mouse mutants Ⅲ muscle Ⅲ muscle contraction Ⅲ myocardial contractility U nderstanding the structure/function relations for cardiac myosin binding protein-C (cMyBP-C) is clinically relevant, as cMyBP-C mutations are a widely recognized cause of familial hypertrophic cardiomyopathy. 1 Various cMyBP-C transgenic (TG) and gene-targeted mouse models have demonstrated the importance of the protein for long-term integrity of sarcomeric structure and for maintaining normal cardiac contractility. 2,3 Functional insight can be gained from appreciating the crucial structural differences between cMyBP-C and the skeletal isoform. Only the cardiac isoform contains an extra immunoglobulin domain at the N terminus (C0), an insertion of 28 residues within the C5 domain, and three phosphorylation sites (Ser273, that are substrates for cAMP-dependent protein kinase A (PKA), Ca 2ϩ -calmodulin-activated kinase and protein kinase C.In vivo, PKA-mediated phosphorylation of cMyBP-C is linked to modulation of cardiac contraction. 4 On adrenergic stimulation, PKA phosphorylates Ser273, -282, and -302, whereas protein kinase C phosphorylates only Ser273 and -302. 5 These residues, located near the N terminus of the protein, are of particular interest, as this region binds to the S2 segment of the myosin heavy chain (MHC), 6,7 which is close to the lever arm domain of myosin. It has been hypothesized that cMyBP-C/MHC interactions are dynamically regulated by the phosphorylation/dephosphorylation of cMyBP-C. 8...