Erythrocyte formation occurs throughout life in response to cytokine signaling. We show that microRNA-451 (miR-451) regulates erythropoiesis in vivo. Mice lacking miR-451 display a reduction in hematrocrit, an erythroid differentiation defect, and ineffective erythropoiesis in response to oxidative stress. 14-3-3ζ, an intracellular regulator of cytokine signaling that is repressed by miR-451, is up-regulated in miR-451−/− erythroblasts, and inhibition of 14-3-3ζ rescues their differentiation defect. These findings reveal an essential role of 14-3-3ζ as a mediator of the proerythroid differentiation actions of miR-451, and highlight the therapeutic potential of miR-451 inhibitors.

Summary MicroRNAs (miRNA) are small, non-coding RNAs that regulate gene expression post-transcriptionally. We investigated the hypothesis that Bone Morphogenetic Protein (Bmp)-signaling regulates miRNAs in cardiac progenitor cells. Bmp2 and Bmp4 regulate OFT myocardial differentiation via regulation of the miRNA 17-92 cluster. In Bmp mutant embryos, myocardial differentiation was delayed and multiple miRNAs encoded by miRNA 17-92 were reduced. We uncovered functional miRNA17-92 seed sequences within the 3′ UTR of cardiac progenitor genes such as Isl1 and Tbx1. In both Bmp and miRNA 17-92 mutant embryos, Isl1and Tbx1 expression failed to be correctly downregulated. Transfection experiments indicated that miRNA 17 and miRNA 20a directly repressed Isl1and Tbx1. Genetic interaction studies uncovered a synergistic interaction between miRNA 17-92 cluster and Bmp4 providing direct in vivo evidence for the Bmp-miRNA 17-92 regulatory pathway. Our findings indicate that Bmp-signaling directly regulates a miRNA-mediated effector mechanism that downregulates cardiac progenitor genes and enhances myocardial differentiation.

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Background Pitx2 is the homeobox gene located in proximity to the human 4q25 familial atrial fibrillation locus. When deleted in the mouse germline, Pitx2 haploinsufficiency predisposes to pacing induced atrial fibrillation indicating that reduced Pitx2 promotes an arrhythmogenic substrate. Previous work focused on Pitx2 developmental functions that predispose to atrial fibrillation. Although Pitx2 is expressed in postnatal left atrium, it is unknown whether Pitx2 has distinct postnatal and developmental functions. Methods and Results To investigate Pitx2 postnatal function, we conditionally inactivated Pitx2 in the postnatal atrium while leaving its developmental function intact. Unstressed adult Pitx2 homozygous mutant mice display variable R-R interval with diminished P-wave amplitude characteristic of sinus node dysfunction, an atrial fibrillation risk factor in human patients. An integrated genomics approach in the adult heart revealed Pitx2 target genes encoding cell junction proteins, ion channels, and critical transcriptional regulators. Importantly, many Pitx2 target genes have been implicated in human atrial fibrillation by genome wide association studies. Immunofluorescence and transmission electron microscopy studies in adult Pitx2 mutant mice revealed structural remodeling of the intercalated disc characteristic of human atrial fibrillation patients. Conclusions Our findings, revealing that Pitx2 has genetically separable postnatal and developmental functions, unveil direct Pitx2 target genes that include channel and calcium handling genes as well as genes that stabilize the intercalated disc in postnatal atrium.

The molecular mechanisms underlying atrial fibrillation, the most common sustained cardiac arrhythmia, remain poorly understood. Genome-wide association studies uncovered a major atrial fibrillation susceptibility locus on human chromosome 4q25 in close proximity to the paired-like homeodomain transcription factor 2 (Pitx2) homeobox gene. Pitx2, a target of the left-sided Nodal signaling pathway that initiates early in development, represses the sinoatrial node program and pacemaker activity on the left side. To address the mechanisms underlying this repressive activity, we hypothesized that Pitx2 regulates microRNAs (miRs) to repress the sinoatrial node genetic program. MiRs are small noncoding RNAs that regulate gene expression posttranscriptionally. Using an integrated genomic approach, we discovered that Pitx2 positively regulates miR-17-92 and miR-106b-25. Intracardiac electrical stimulation revealed that both miR-17-92 and miR-106b-25 deficient mice exhibit pacing-induced atrial fibrillation. Furthermore electrocardiogram telemetry revealed that mice with miR-17-92 cardiac-specific inactivation develop prolonged PR intervals whereas mice with miR-17-92 cardiac-specific inactivation and miR-106b-25 heterozygosity develop sinoatrial node dysfunction. Both arrhythmias are risk factors for atrial fibrillation in humans. Importantly, miR-17-92 and miR-106b-25 directly repress genes, such as Shox2 and Tbx3, that are required for sinoatrial node development. Together, to our knowledge, these findings provide the first genetic evidence for an miR loss-of-function that increases atrial fibrillation susceptibility. irregular heart rate | single nucleotide variant | mouse genetics A trial fibrillation (AF), the most common arrhythmia in adult patients, increases in prevalence with age to almost 5% of the population over 65. Patients with AF have an increased risk of stroke, dementia, and heart failure (1). Electrical impulses that are critical for a coordinated, physiologic heartbeat originate in the sinoatrial node (SAN). In AF, abnormal fibrillatory atrial impulses override normal SAN function, with resultant irregular conduction to the ventricles. Many cases of ectopic electrical activity originate in the pulmonary vein (2). Other sites of ectopy include the left atrial posterior wall, superior vena cava, interatrial septum, crista terminalis, and coronary sinus myocardium (3, 4).Multiple approaches have been used to uncover genes that may contribute to the AF phenotype in adult patients. A seminal genome-wide association study (GWAS), subsequently replicated by multiple studies, uncovered a single nucleotide variant (SNV) on human 4q25 that was strongly associated with familial AF (5). Patients with the 4q25 variant exhibited early onset AF that was independent from other risk factors such as hypertension and diabetes, suggesting a novel biologic mechanism involving genes located on chromosome 4q25. The presence of the 4q25 SNV also had prognostic value because patients with the SNV are prone to cardioembolic str...