Heterozygous loss of Rbm24 in the adult mouse heart increases sarcomere slack length but does not affect function.

RNA-binding proteins are key regulators of post-transcriptional processes such as alternative splicing and mRNA stabilization. Rbm24 acts as a regulator of alternative splicing in heart and skeletal muscle, and is essential for sarcomere assembly. Homozygous inactivation of Rbm24 in mice disrupts cardiac development and results in embryonic lethality around E12.5. In the present study, we generated somatic Rbm24 knockout (KO) mice and investigated the effects of reduced levels of Rbm24 in the adult heart. Due to the embryonic lethality of Rbm24 KO mice, we examined cardiac structure and function in adult Rbm24 heterozygotes (HETs). Rbm24 protein expression was 40% downregulated in HET hearts compared to WT hearts. Force measurements on isolated membrane-permeabilized myocytes showed increased sarcomere slack length and lower myofilament passive stiffness in adult Rbm24 HET compared to wildtype cardiomyocytes. As a result of the differences in sarcomere slack length, the relations between force development and sarcomere length differed between WT and Rbm24 HET hearts. No differences in sarcomere structure and titin isoform composition were observed. Likewise, in vivo cardiac function and myocardial structure was unaltered in Rbm24 HET mice compared to WT, at baseline and upon pressure overload after transverse aortic constriction. In conclusion, we generated a somatic Rbm24 KO model and recapitulated the previously reported embryonic phenotype. In adult Rbm24 HET cardiomyocytes we observed increased sarcomere slack length, but no difference in sarcomere structure and cardiac function.

Small sample sizes in high-throughput miRNA screens: A common pitfall for the identification of miRNA biomarkers.

Since the discovery of microRNAs (miRNAs), circulating miRNAs have been proposed as biomarkers for disease. Consequently, many groups have tried to identify circulating miRNA biomarkers for various types of diseases including cardiovascular disease and cancer. However, the replicability of these experiments has been disappointingly low. In order to identify circulating miRNA candidate biomarkers, in general, first an unbiased high-throughput screen is performed in which a large number of miRNAs is detected and quantified in the circulation. Because these are costly experiments, many of such studies have been performed using a low number of study subjects (small sample size). Due to lack of power in small sample size experiments, true effects are often missed and many of the detected effects are wrong. Therefore, it is important to have a good estimate of the appropriate sample size for a miRNA high-throughput screen. In this review, we discuss the effects of small sample sizes in high-throughput screens for circulating miRNAs. Using data from a miRNA high-throughput experiment on isolated monocytes, we illustrate that the implementation of power calculations in a high-throughput miRNA discovery experiment will avoid unnecessarily large and expensive experiments, while still having enough power to be able to detect clinically important differences.

Matrix metalloproteinase inhibition after myocardial infarction: a new approach to prevent heart failure?

Increased activity of matrix metalloproteinases (MMPs) has been implicated in numerous disease processes, including tumor growth and metastasis, arthritis, and periodontal disease. It is now becoming increasingly clear that extracellular matrix degradation by MMPs is also involved in the pathogenesis of cardiovascular disease, including atherosclerosis, restenosis, dilated cardiomyopathy, and myocardial infarction. Administration of synthetic MMP inhibitors in experimental animal models of these cardiovascular diseases significantly inhibits the progression of, respectively, atherosclerotic lesion formation, neointima formation, left ventricular remodeling, pump dysfunction, and infarct healing. This review focuses on the role of MMPs in cardiovascular disease, in particular myocardial infarction and the subsequent progression to heart failure. MMPs, which are present in the myocardium and capable of degrading all the matrix components of the heart, are the driving force behind myocardial matrix remodeling. The recent finding that acute pharmacological inhibition of MMPs or deficiency in MMP-9 attenuates left ventricular dilatation in the infarcted mouse heart led to the proposal that MMP inhibitors could be used as a potential therapy for patients at risk for the development of heart failure after myocardial infarction. Although these promising results encourage the design of clinical trials with MMP inhibitors, there are still several unresolved issues. This review describes the biology of MMPs and discusses new insights into the role of MMPs in several cardiovascular diseases. Attention will be paid to the central role of the plasminogen system as an important activator of MMPs in the remodeling process after myocardial infarction. Finally, we speculate on the use of MMP inhibitors as potential therapy for heart failure.