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Rationale: RBPs (RNA binding proteins) play critical roles in the cell by regulating mRNA transport, splicing, editing, and stability. The RBP SRSF3 (serine/arginine-rich splicing factor 3) is essential for blastocyst formation and for proper liver development and function. However, its role in the heart has not been explored. Objective: To investigate the role of SRSF3 in cardiac function. Methods and Results: Cardiac SRSF3 expression was high at mid gestation and decreased during late embryonic development. Mice lacking SRSF3 in the embryonic heart showed impaired cardiomyocyte proliferation and died in utero. In the adult heart, SRSF3 expression was reduced after myocardial infarction, suggesting a possible role in cardiac homeostasis. To determine the role of this RBP in the adult heart, we used an inducible, cardiomyocyte-specific SRSF3 knockout mouse model. After SRSF3 depletion in cardiomyocytes, mice developed severe systolic dysfunction that resulted in death within 8 days. RNA-Seq analysis revealed downregulation of mRNAs encoding sarcomeric and calcium handling proteins. Cardiomyocyte-specific SRSF3 knockout mice also showed evidence of alternative splicing of mTOR (mammalian target of rapamycin) mRNA, generating a shorter protein isoform lacking catalytic activity. This was associated with decreased phosphorylation of 4E-BP1 (eIF4E-binding protein 1), a protein that binds to eIF4E (eukaryotic translation initiation factor 4E) and prevents mRNA decapping. Consequently, we found increased decapping of mRNAs encoding proteins involved in cardiac contraction. Decapping was partially reversed by mTOR activation. Conclusions: We show that cardiomyocyte-specific loss of SRSF3 expression results in decapping of critical mRNAs involved in cardiac contraction. The molecular mechanism underlying this effect likely involves the generation of a short mTOR isoform by alternative splicing, resulting in reduced 4E-BP1 phosphorylation. The identification of mRNA decapping as a mechanism of systolic heart failure may open the way to the development of urgently needed therapeutic tools.

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1Aims─ Heart failure (HF) has become an epidemic and constitutes a major medical, 2 social and economic problem worldwide. Despite advances in medical treatment, HF 3 prognosis remains poor. The development of efficient therapies is hampered by the lack 4 of appropriate animal models in which HF can be reliably determined, particularly in 5 mice. The development of HF in mice is often assumed based on the presence of cardiac 6 dysfunction, but HF itself is seldom proved. Lung ultrasound (LUS) has become a 7 helpful tool for lung congestion assessment in patients at all stages of HF. We aimed to 8 apply this non-invasive imaging tool to evaluate HF in mouse models of both systolic 9 and diastolic dysfunction.

Macrophages (Mφs) produce factors that participate in cardiac repair and remodeling after myocardial infarction (MI); however, how these factors crosstalk with other cell types mediating repair is not fully understood. Here, we demonstrated that cardiac Mφs increased expression of Mmp14 (MT1-MMP) 7 days post-MI. We selectively inactivated the Mmp14 gene in Mφs using a genetic strategy (Mmp14f/f:Lyz2-Cre). This conditional KO (MAC-Mmp14 KO) resulted in attenuated post-MI cardiac dysfunction, reduced fibrosis, and preserved cardiac capillary network. Mechanistically, we showed that MT1-MMP activates latent TGFβ1 in Mφs, leading to paracrine SMAD2-mediated signaling in endothelial cells (ECs) and endothelial-to-mesenchymal transition (EndMT). Post-MI MAC-Mmp14 KO hearts contained fewer cells undergoing EndMT than their wild-type counterparts, and Mmp14-deficient Mφs showed a reduced ability to induce EndMT in co-cultures with ECs. Our results indicate the contribution of EndMT to cardiac fibrosis and adverse remodeling post-MI and identify Mφ MT1-MMP as a key regulator of this process.

Increasing evidences advocate for an important function of T cells in controlling immune homeostasis and pathogenesis after myocardial infarction (MI), although the underlying molecular mechanisms remain elusive. In this study, a broad analysis of immune markers in 283 patients revealed a significant CD69 overexpression on Treg cells after MI. Our results in mice showed that CD69 expression on Treg cells increased survival after left-anterior-descending coronary artery (LAD)-ligation. Cd69 -/mice developed strong IL-17 + T cell responses after ischemia that increased myocardial inflammation and, consequently, worsened cardiac function. CD69 + Treg cells, by induction of AhR-dependent CD39 ectonucleotidase activity, induced apoptosis and decreased IL-17A production in T cells. Adoptive transfer of CD69 + Treg cells to Cd69 -/mice after LAD-ligation reduced IL-17 + T cell recruitment, thus increasing survival. Consistently, clinical data from two independent cohorts of patients indicated that increased CD69 expression in peripheral blood cells after acute MI was associated with a lower risk of re-hospitalization for heart failure (HF) after 2.5 years of follow-up. This result remained significant after adjustment for age, sex and traditional cardiac damage biomarkers. Our data highlight CD69 expression on Treg cells as a potential prognostic factor and a therapeutic option to prevent HF after MI.

Rationale: RNA-binding proteins (RBPs) play critical roles in human biology and disease. Aberrant RBP expression affects various steps in RNA processing, altering the function of the target RNAs. The RBP serine/arginine-rich splicing factor 4 (SRSF4) has been linked to neuropathies and cancer. However, its role in the heart is completely unknown. Objective: To investigate the role of SRSF4 in the heart. Methods and Results: Echocardiography of mice specifically lacking SRSF4 in the heart (SRSF4 KO) revealed left ventricular hypertrophy and increased cardiomyocyte area, which led to progressive diastolic dysfunction with age. SRSF4 KO mice showed altered electrophysiological activity under isoproterenol-induced cardiac stress, with a post-QRS depression and a longer QT interval, indicating an elevated risk of sudden cardiac death. RNA-Seq analysis revealed expression changes in several long non-coding RNAs (lncRNAs), including GAS5 (growth arrest specific 5), which we identified as a direct SRSF4 target in cardiomyocytes by individual-nucleotide-resolution cross-linking and immuno-precipitation (iCLIP). GAS5 is a repressor of the glucocorticoid receptor (GR) and was downregulated in SRSF4 KO hearts. This corresponded with elevated GR transcriptional activity in cardiomyocytes, leading to increases in hypertrophy markers and cell size. Furthermore, hypertrophy in SRSF4 KO cardiomyocytes was reduced by overexpressing GAS5. Conclusions: Loss of SRSF4 expression results in cardiac hypertrophy, diastolic dysfunction, and abnormal repolarization. The molecular mechanism underlying this effect involves GAS5 downregulation and consequent elevation of GR transcriptional activity. Our findings may help to develop new therapeutic tools for the treatment of cardiac hypertrophy and myocardial pathology in Cushing's syndrome patients.

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