The data show that nicotinamide riboside, the most energy-efficient among NAD precursors, could be useful for treatment of heart failure, notably in the context of DCM, a disease with few therapeutic options.

Rationale: Mutations in the MYBPC3 gene encoding cardiac myosin-binding protein (cMyBP)-C are frequent causes of hypertrophic cardiomyopathy, but the mechanisms leading from mutations to disease remain elusive. Objective: The goal of the present study was therefore to gain insights into the mechanisms controlling the expression of MYBPC3 mutations. Methods and Results: We developed a cMyBP-C knock-in mouse carrying a point mutation. The level of total cMyBP-C mRNAs was 50% and 80% lower in heterozygotes and homozygotes, respectively. Surprisingly, the single G>A transition on the last nucleotide of exon 6 resulted in 3 different mutant mRNAs: missense (exchange of G for A), nonsense (exon skipping, frameshift, and premature stop codon) and deletion/insertion (as nonsense but with additional partial retention of downstream intron, restoring of the reading frame, and almost full-length protein). Inhibition of nonsense-mediated mRNA decay in cultured cardiac myocytes or in vivo with emetine or cycloheximide increased the level of nonsense mRNAs severalfold but not of the other mRNAs. By using sequential protein fractionation and a new antibody directed against novel amino acids produced by the frameshift, we showed that inhibition of the proteasome with epoxomicin via osmotic minipumps increased the level of (near) full-length mutants but not of truncated proteins. Homozygotes exhibited myocyte and left ventricular hypertrophy, reduced fractional shortening, and interstitial fibrosis; heterozygotes had no major phenotype. Conclusions: These data reveal (1) an unanticipated complexity of the expression of a single point mutation in the whole animal and (2) the involvement of both nonsense-mediated mRNA decay and the ubiquitin-proteasome system in lowering the level of mutant proteins. (Circ Res. 2009;105:239-248.)Key Words: cardiomyopathy Ⅲ hypertrophic cardiomyopathy Ⅲ mRNA stability Ⅲ transgenic mice Ⅲ ubiquitin C ardiac myosin-binding protein (cMyBP)-C is a major component of the A-band of the sarcomere, where it interacts with myosin, actin and titin (see elsewhere 1,2 and reviewed previously 3 ). It is exclusively expressed in the heart in humans and mice. 4,5 Its role has been enigmatic for long, but accumulating recent evidence suggests that cMyBP-C is essential for normal diastolic relaxation by inhibiting actin-myosin interactions at low intracellular Ca 2ϩ concentrations. 6 -10 Mutations in MYBPC3 encoding cMyBP-C cause hypertrophic cardiomyopathy (HCM) (reviewed previously 3,11 ).HCM is an autosomal-dominant disease characterized by left ventricular (LV) hypertrophy, which predominantly involves the interventricular septum and is associated with myocardial disarray and interstitial fibrosis. 12 HCM involves more than 450 mutations in at least 13 genes encoding sarcomeric proteins. 11,13 Out of them, mutations in MYBPC3 are frequent. 14 In contrast to other disease genes, in which the majority of the mutations are missense, Ϸ70% of MYBPC3 mutations result in a frameshift creating a premature termination...

The second messengers cAMP and cGMP can be degraded by specific members of the phosphodiesterase superfamily or by active efflux transporters, namely the multidrug resistance-associated proteins (MRPs) MRP4 and MRP5. To determine the role of MRP4 and MRP5 in cell signaling, we studied arterial SMCs, in which the effects of cyclic nucleotide levels on SMC proliferation have been well established. We found that MRP4, but not MRP5, was upregulated during proliferation of isolated human coronary artery SMCs and following injury of rat carotid arteries in vivo. MRP4 inhibition significantly increased intracellular cAMP and cGMP levels and was sufficient to block proliferation and to prevent neointimal growth in injured rat carotid arteries. The antiproliferative effect of MRP4 inhibition was related to PKA/CREB pathway activation. Here we provide what we believe to be the first evidence that MRP4 acts as an independent endogenous regulator of intracellular cyclic nucleotide levels and as a mediator of cAMP-dependent signal transduction to the nucleus. We also identify MRP4 inhibition as a potentially new way of preventing abnormal VSMC proliferation. Introduction cAMP and cGMP are second messengers that relay external signals to downstream effector proteins. The most common targets are cAMP-dependent PKA and cGMP-dependent PKG, which regulate a large number of processes by phosphorylating target proteins. In addition to PKA, recent evidence has highlighted a major role for guanine-nucleotide exchange factors for Rap proteins (namely EPAC1 and EPAC2) (1) in mediating cAMP signaling. cAMP and cGMP also act by binding certain ion channels (2). Signaling events triggered by extracellular stimuli arise from an ingenious system of regulation that involves the production and elimination of intracellular cyclic nucleotides. Classically, cyclic nucleotide elimination has been attributed to hydrolysis mediated by cyclic nucleotide phosphodiesterases (PDEs). PDEs constitute a large superfamily of enzymes encoded by several genes with tissue-specific expression of a large number of splice variants (3). In several models, including VSMCs, PDEs have been shown to regulate the amplitude and duration of intracellular cyclic nucleotide signaling (4, 5). For instance, sildenafil, a selective PDE5 inhibi-