Targeted deletion of microRNA-22 promotes stress-induced cardiac dilation and contractile dysfunction.

BACKGROUND: Delineating the role of microRNAs (miRNAs) in the posttranscriptional gene regulation offers new insights into how the heart adapts to pathological stress. We developed a knockout of miR-22 in mice and investigated its function in the heart. METHODS AND RESULTS: Here, we show that miR-22-deficient mice are impaired in inotropic and lusitropic response to acute stress by dobutamine. Furthermore, the absence of miR-22 sensitized mice to cardiac decompensation and left ventricular dilation after long-term stimulation by pressure overload. Calcium transient analysis revealed reduced sarcoplasmic reticulum Ca(2+) load in association with repressed sarcoplasmic reticulum Ca(2+) ATPase activity in mutant myocytes. Genetic ablation of miR-22 also led to a decrease in cardiac expression levels for Serca2a and muscle-restricted genes encoding proteins in the vicinity of the cardiac Z disk/titin cytoskeleton. These phenotypes were attributed in part to inappropriate repression of serum response factor activity in stressed hearts. Global analysis revealed increased expression of the transcriptional/translational repressor purine-rich element binding protein B, a highly conserved miR-22 target implicated in the negative control of muscle expression. CONCLUSION: These data indicate that miR-22 functions as an integrator of Ca(2+) homeostasis and myofibrillar protein content during stress in the heart and shed light on the mechanisms that enhance propensity toward heart failure.

"Triplet" polycistronic vectors encoding Gata4, Mef2c, and Tbx5 enhances postinfarct ventricular functional improvement compared with singlet vectors.

OBJECTIVE: The in situ reprogramming of cardiac fibroblasts into induced cardiomyocytes by the administration of gene transfer vectors encoding Gata4 (G), Mef2c (M), and Tbx5 (T) has been shown to improve ventricular function in myocardial infarction models. The efficacy of this strategy could, however, be limited by the need for fibroblast targets to be infected 3 times--once by each of the 3 transgene vectors. We hypothesized that a polycistronic "triplet" vector encoding all 3 transgenes would enhance postinfarct ventricular function compared with use of "singlet" vectors. METHODS: After validation of the polycistronic vector expression in vitro, adult male Fischer 344 rats (n=6) underwent coronary ligation with or without intramyocardial administration of an adenovirus encoding all 3 major vascular endothelial growth factor (VEGF) isoforms (AdVEGF-All6A positive), followed 3 weeks later by the administration to AdVEGF-All6A-positive treated rats of singlet lentivirus encoding G, M, or T (1×10(5) transducing units each) or the same total dose of a GMT "triplet" lentivirus vector. RESULTS: Western blots demonstrated that triplet and singlet vectors yielded equivalent GMT transgene expression, and fluorescence activated cell sorting demonstrated that triplet vectors were nearly twice as potent as singlet vectors in generating induced cardiomyocytes from cardiac fibroblasts. Echocardiography demonstrated that GMT triplet vectors were more effective than the 3 combined singlet vectors in enhancing ventricular function from postinfarct baselines (triplet, 37%±10%; singlet, 13%±7%; negative control, 9%±5%; P<.05). CONCLUSIONS: These data have confirmed that the in situ administration of G, M, and T induces postinfarct ventricular functional improvement and that GMT polycistronic vectors enhance the efficacy of this strategy.

microRNA-22 promotes heart failure through coordinate suppression of PPAR/ERR-nuclear hormone receptor transcription.

Increasing evidence suggests that microRNAs are intimately involved in the pathophysiology of heart failure. MicroRNA-22 (miR-22) is a muscle-enriched miRNA required for optimum cardiac gene transcription and adaptation to hemodynamic stress by pressure overload in mice. Recent evidence also suggests that miR-22 induces hypertrophic growth and it is oftentimes upregulated in end stage heart failure. However the scope of mRNA targets and networks of miR-22 in the heart failure remained unclear. We analyzed transgenic mice with enhanced levels of miR-22 expression in adult cardiomyocytes to identify important pathophysiologic targets of miR-22. Our data shows that forced expression of miR-22 induces a pro-hypertrophic gene expression program, and it elicits contractile dysfunction leading to cardiac dilation and heart failure. Increased expression of miR-22 impairs the Ca(2+) transient, Ca(2+) loading into the sarcoplasmic reticulum plus it interferes with transcription of estrogen related receptor (ERR) and PPAR downstream genes. Mechanistically, miR-22 postranscriptionally inhibits peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), PPARα and sirtuin 1 (SIRT1) expression via a synergistic circuit, which may account for deleterious actions of unchecked miR-22 expression on the heart.