Background: Cardiac fibroblasts (CF) have a central role in the ventricular remodeling process associated with different types of fibrosis. Recent studies have shown that fibroblasts do not respond homogeneously to heart injury. Due to the limited set of bona fide fibroblast markers, a proper characterization of fibroblast population heterogeneity in response to cardiac damage is still missing. The purpose of this study was to define the CF heterogeneity during ventricular remodeling and the underlying mechanisms that regulate their function. Methods: Collagen1α1-GFP + CF were characterized after myocardial infarction (MI) by single-cell and bulk RNA-seq, ATAC-seq and functional assays. Swine and patient samples were studied using bulk RNA-seq. Results: We identified and characterized a unique CF subpopulation that emerges after MI in mice. These activated fibroblasts exhibit a clear pro-fibrotic signature, express high levels of Collagen Triple Helix Repeat Containing 1 ( Cthrc1 ) and localize into the scar. Non-canonical TGF-β signaling and different transcription factors including SOX9 are important regulators mediating their response to cardiac injury. Moreover, the absence of CTHRC1 results in pronounced lethality due to ventricular rupture. Finally, a population of CF with a similar transcriptome was identified in a swine model of MI and in heart tissue from patients with MI and dilated cardiomyopathy. Conclusions: We report CF heterogeneity, their dynamics during the course of MI and redefine the CF that respond to cardiac injury and participate in myocardial remodeling. Our study identifies Cthrc1 as a novel regulator of the healing scar process, and as a target for future translational studies.

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Adeno-associated viruses (AAV) have become one of the most promising tools for gene transfer in clinics. Among all the serotypes, AAV9 has been described as the most efficient for cardiac transduction. In order to achieve optimal therapeutic delivery in heart disease, we have explored AAV9 transduction efficiency in an infarcted heart using different routes of administration and promoters, including a cardiacspecific one. AAV9 vectors carrying luciferase or green fluorescence protein under the control of the ubiquitous elongation-factor-1-alpha or the cardiac-specific troponin-T (TnT) promoters were administered by intramyocardial or intravenous injection, either in healthy or myocardial-infarcted mice. The transduction efficacy and specificity, the time-course expression, and the safety of each vector were tested. High transgene expression levels were found in the heart, but not in the liver, of mice receiving AAV-TnT, which was significantly higher after intramyocardial injection regardless of ischemia-induction. On the contrary, high hepatic transgene expression levels were detected with the elongation-factor-1-alpha-promoter, independently of the administration route and heart damage. Moreover, tissue-specific green fluorescence protein expression was found in cardiomyocytes with the TnT vector, whereas minimal cardiac expression was detected with the ubiquitous one.Interestingly, we found that myocardial infarction greatly increased the transcriptional activity of AAV genomes. Our findings show that the use of cardiac promoters allows for specific and stable cardiac gene expression, which is optimal and robust when intramyocardially injected. Furthermore, our data indicate that the pathological status of the tissue can alter the transcriptional activity of AAV genomes, an aspect that should be carefully evaluated for clinical applications.

Myelodysplastic syndromes (MDS) are hematopoietic stem cell (HSC) malignancies characterized by ineffective hematopoiesis, with increased incidence in older individuals. Here we analyze the transcriptome of human HSCs purified from young and older healthy adults, as well as MDS patients, identifying transcriptional alterations following different patterns of expression. While aging-associated lesions seem to predispose HSCs to myeloid transformation, disease-specific alterations may trigger MDS development. Among MDS-specific lesions, we detect the upregulation of the transcription factor DNA Damage Inducible Transcript 3 (DDIT3). Overexpression of DDIT3 in human healthy HSCs induces an MDS-like transcriptional state, and dyserythropoiesis, an effect associated with a failure in the activation of transcriptional programs required for normal erythroid differentiation. Moreover, DDIT3 knockdown in CD34+ cells from MDS patients with anemia is able to restore erythropoiesis. These results identify DDIT3 as a driver of dyserythropoiesis, and a potential therapeutic target to restore the inefficient erythroid differentiation characterizing MDS patients.