The E2F/Rb pathway regulates cardiac growth and development and holds great potential as a therapeutic target. The E2F6 repressor is a unique E2F member that acts independently of pocket proteins. Forced expression of E2F6 in mouse myocardium induced heart failure and mortality, with severity of symptoms correlating to E2F6 levels. Echocardiography demonstrated a 37% increase (P<0.05) in left ventricular end-diastolic diameter and reduced ejection fraction (<40%, P<0.05) in young transgenic (Tg) mice. Microarray and qPCR analysis revealed a paradoxical increase in E2F-responsive genes, which regulate the cell cycle, without changes in cardiomyocyte cell number or size in Tg mice. Young adult Tg mice displayed a 75% (P<0.01) decrease in gap junction protein connexin-43, resulting in abnormal electrocardiogram including a 24% (P<0.05) increase in PR interval. Further, mir-206, which targets connexin-43, was up-regulated 10-fold (P<0.05) in Tg myocardium. The mitogen-activated protein kinase pathway, which regulates the levels of miR-206 and connexin-43, was activated in Tg hearts. Thus, deregulated E2F6 levels evoked abnormal gene expression at transcriptional and post-transcriptional levels, leading to cardiac remodeling and dilated cardiomyopathy. The data highlight an unprecedented role for the strict regulation of the E2F pathway in normal postnatal cardiac function.

N-myristoylation on glycine is an irreversible modification that has long been recognized to govern protein localization and function. In contrast, the biological roles of lysine myristoylation remain ill-defined. We demonstrate that the cytoplasmic scaffolding protein, gravin-α/A kinase–anchoring protein 12, is myristoylated on two lysine residues embedded in its carboxyl-terminal protein kinase A (PKA) binding domain. Histone deacetylase 11 (HDAC11) docks to an adjacent region of gravin-α and demyristoylates these sites. In brown and white adipocytes, lysine myristoylation of gravin-α is required for signaling via β2- and β3-adrenergic receptors (β-ARs), which are G protein–coupled receptors (GPCRs). Lysine myristoylation of gravin-α drives β-ARs to lipid raft membrane microdomains, which results in PKA activation and downstream signaling that culminates in protective thermogenic gene expression. These findings define reversible lysine myristoylation as a mechanism for controlling GPCR signaling and highlight the potential of inhibiting HDAC11 to manipulate adipocyte phenotypes for therapeutic purposes.

Passive stiffness of the heart is determined largely by extracellular matrix and titin, which functions as a molecular spring within sarcomeres. Titin stiffening is associated with the development of diastolic dysfunction (DD), while augmented titin compliance appears to impair systolic performance in dilated cardiomyopathy. We found that myofibril stiffness was elevated in mice lacking histone deacetylase 6 (HDAC6). Cultured adult murine ventricular myocytes treated with a selective HDAC6 inhibitor also exhibited increased myofibril stiffness. Conversely, HDAC6 overexpression in cardiomyocytes led to decreased myofibril stiffness, as did ex vivo treatment of mouse, rat, and human myofibrils with recombinant HDAC6. Modulation of myofibril stiffness by HDAC6 was dependent on 282 amino acids encompassing a portion of the PEVK element of titin. HDAC6 colocalized with Z-disks, and proteomics analysis suggested that HDAC6 functions as a sarcomeric protein deacetylase. Finally, increased myofibril stiffness in HDAC6-deficient mice was associated with exacerbated DD in response to hypertension or aging. These findings define a role for a deacetylase in the control of myofibril function and myocardial passive stiffness, suggest that reversible acetylation alters titin compliance, and reveal the potential of targeting HDAC6 to manipulate the elastic properties of the heart to treat cardiac diseases.

The sarcolemmal membrane associated proteins (SLMAPs) belong to the super family of tail anchored membrane proteins which serve diverse roles in biology including cell growth, protein trafficking and ion channel regulation. Mutations in human SLMAP have been linked to Brugada syndrome with putative deficits in trafficking of the sodium channel (Na v 1.5) to the cell membrane resulting in aberrant electrical activity and heart function. Three main SLMAP isoforms (SLMAP1 (35 kDa), SLMAP2 (45 kDa), and SLMAP3 (91 kDa)) are expressed in myocardium but their precise role remains to be defined. Here we generated transgenic (Tg) mice with cardiac-specific expression of the SLMAP3 isoform during postnatal development which present with a significant decrease (20%) in fractional shortening and (11%) in cardiac output at 5 weeks of age. There was a lack of any notable cardiac remodeling (hypertrophy, fibrosis or fetal gene activation) in Tg hearts but the electrocardiogram indicated a significant increase (14%) in the PR interval and a decrease (43%) in the R amplitude. Western blot analysis indicated a selective and significant decrease (55%) in protein levels of Na v 1.5 while 45% drop in its transcript levels were detectable by qRT-PCR. Significant decreases in the protein and transcript levels of the calcium transport system of the sarcoplasmic reticulum (SERCA2a/PLN) were also evident in Tg hearts. These data reveal a novel role for SLMAP3 in the selective regulation of important ion transport proteins at the level of gene expression and suggest that it may be a unique target in cardiovascular function and disease.

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