Paola Cattaneo

Genome-wide analysis of histone marks identifying an epigenetic signature of promoters and enhancers underlying cardiac hypertrophy

Significance Cardiac failure is a leading cause of mortality worldwide and a major financial burden for healthcare systems. New tools for understanding cardiovascular disease and developing better therapeutic approaches are therefore needed. To this end, transcriptional regulation has been extensively studied in cardiac hypertrophy and failure, but there is still a lack of understanding of the epigenetic framework in which transcription factors act. Our report adds significant knowledge to the field because we demonstrate, in vivo, that a complex and specific epigenetic signature regulates gene expression by modulating promoters and enhancers, a large number of which have been described here. These findings advance our understanding of the mechanisms underlying this pathology. Cardiac hypertrophy, initially an adaptive response of the myocardium to stress, can progress to heart failure. The epigenetic signature underlying this phenomenon is poorly understood. Here, we report on the genome-wide distribution of seven histone modifications in adult mouse cardiomyocytes subjected to a prohypertrophy stimulus in vivo. We found a set of promoters with an epigenetic pattern that distinguishes specific functional classes of genes regulated in hypertrophy and identified 9,207 candidate active enhancers whose activity was modulated. We also analyzed the transcriptional network within which these genetic elements act to orchestrate hypertrophy gene expression, finding a role for myocyte enhancer factor (MEF)2C and MEF2A in regulating enhancers. We propose that the epigenetic landscape is a key determinant of gene expression reprogramming in cardiac hypertrophy and provide a basis for understanding the role of chromatin in regulating this phenomenon.

Origins of cardiac fibroblasts

Cardiac fibroblasts produce the extracellular matrix (ECM) scaffold within which the various cellular components of the heart are organized. As well as providing structural support, it is becoming evident that the quality and quantity of ECM is a key factor for determining cardiac cell behavior during development and in pathological contexts such as heart failure involving fibrosis. Cardiac fibroblasts have long remained a poorly characterized cardiac lineage. Well characterized markers are now paving the way for a better understanding of the roles of these cells in various developmental and disease contexts. Notably, the relevance of processes including endothelial-tomesenchymal transition and the recruitment of circulating fibroblast progenitors in heart failure has been challenged. This review describes the latest findings on cardiac fibroblast markers and developmental origins, and discusses their importance in myocardial remodeling. Effective modulation of cardiac fibroblast activity would likely contribute to successful treatment of various cardiac disorders.

Transcription factor ISL1 is essential for pacemaker development and function

The sinoatrial node (SAN) maintains a rhythmic heartbeat; therefore, a better understanding of factors that drive SAN development and function is crucial to generation of potential therapies, such as biological pacemakers, for sinus arrhythmias. Here, we determined that the LIM homeodomain transcription factor ISL1 plays a key role in survival, proliferation, and function of pacemaker cells throughout development. Analysis of several Isl1 mutant mouse lines, including animals harboring an SAN-specific Isl1 deletion, revealed that ISL1 within SAN is a requirement for early embryonic viability. RNA-sequencing (RNA-seq) analyses of FACS-purified cells from ISL1-deficient SANs revealed that a number of genes critical for SAN function, including those encoding transcription factors and ion channels, were downstream of ISL1. Chromatin immunoprecipitation assays performed with anti-ISL1 antibodies and chromatin extracts from FACS-purified SAN cells demonstrated that ISL1 directly binds genomic regions within several genes required for normal pacemaker function, including subunits of the L-type calcium channel, Ank2, and Tbx3. Other genes implicated in abnormal heart rhythm in humans were also direct ISL1 targets. Together, our results demonstrate that ISL1 regulates approximately one-third of SAN-specific genes, indicate that a combination of ISL1 and other SAN transcription factors could be utilized to generate pacemaker cells, and suggest ISL1 mutations may underlie sick sinus syndrome.

HIF1α Represses Cell Stress Pathways to Allow Proliferation of Hypoxic Fetal Cardiomyocytes.

Transcriptional mediators of cell stress pathways, including HIF1α, ATF4, and p53, are key to normal development and play critical roles in disease, including ischemia and cancer. Despite their importance, mechanisms by which pathways mediated by these transcription factors interact with one another are not fully understood. In addressing the controversial role of HIF1α in cardiomyocytes (CMs) during heart development, we discovered a mid-gestational requirement for HIF1α for proliferation of hypoxic CMs, involving metabolic switching and a complex interplay among HIF1α, ATF4, and p53. Loss of HIF1α resulted in activation of ATF4 and p53, the latter inhibiting CM proliferation. Bioinformatic and biochemical analyses revealed unexpected mechanisms by which HIF1α intersects with ATF4 and p53 pathways. Our results highlight previously undescribed roles of HIF1α and interactions among major cell stress pathways that could be targeted to enhance proliferation of CMs in ischemia and may have relevance to other diseases, including cancer.

Infarct Fibroblasts Do Not Derive From Bone Marrow Lineages

Rationale: Myocardial infarction is a major cause of adult mortality worldwide. The origin(s) of cardiac fibroblasts that constitute the postinfarct scar remain controversial, in particular the potential contribution of bone marrow lineages to activated fibroblasts within the scar. Objective: The aim of this study was to establish the origin(s) of infarct fibroblasts using lineage tracing and bone marrow transplants and a robust marker for cardiac fibroblasts, the Collagen1a1-green fluorescent protein reporter. Methods and Results: Using genetic lineage tracing or bone marrow transplant, we found no evidence for collagen-producing fibroblasts derived from hematopoietic or bone marrow lineages in hearts subjected to permanent left anterior descending coronary artery ligation. In fact, fibroblasts within the infarcted area were largely of epicardial origin. Intriguingly, collagen-producing fibrocytes from hematopoietic lineages were observed attached to the epicardial surface of infarcted and sham-operated hearts in which a suture was placed around the left anterior descending coronary artery. Conclusions: In this controversial field, our study demonstrated that the vast majority of infarct fibroblasts were of epicardial origin and not derived from bone marrow lineages, endothelial-to-mesenchymal transition, or blood. We also noted the presence of collagen-producing fibrocytes on the epicardial surface that resulted at least in part from the surgical procedure.

Revisiting Preadolescent Cardiomyocyte Proliferation in Mice

No Evidence for Cardiomyocyte Number Expansion in Preadolescent Mice Alkass et al Cell . 2015;163:1026–1036. Understanding cardiomyocyte cell cycle regulation after birth is key to optimizing regenerative strategies for the heart post injury, yet poses multiple technical challenges, as evidenced by recent studies that have arrived at divergent conclusions. In a recent publication in Cell , Alkass et al undertook multiple approaches to examine cardiomyocyte cell cycle regulation in the first 3 weeks after birth. Here, we summarize results of Alkass et al and 3 other groups in examining preadolescent cardiomyocyte cell cycle regulation, highlighting the distinct approaches and incumbent caveats . Understanding cardiomyocyte cell cycle activity during the perinatal and preadolescent period is extremely challenging and is a subject of intense debate. From a classical viewpoint, throughout embryonic development, cardiomyocytes progressively lose their ability to divide and proliferate. After the period between postnatal day 5 (P5) and 10 (P10), the second and final wave of nonreplicative DNA synthesis ends with binucleation of existing cardiomyocytes.1 Recently, Naqvi et al proposed that cardiomyocytes undergo an additional burst of synchronized proliferation on postnatal day 15 (P15) that results in a 40% increase in the number of cardiomyocytes.2 However, in a recent study, Alkass et al observed that an increase in cardiomyocyte number after birth is largely restricted to the first postnatal week, with no significant increase in number after postnatal day 11.3 Consistent with Alkass et al, 2 other groups were not able to substantiate a proliferative burst of cardiomyocytes during preadolescence between the second and third postnatal weeks.4,5 Interestingly, Alkass et al observed a peak of polyploidization of binucleated cardiomyocytes between the second and third postnatal weeks, introducing further complexity to the model of cardiomyocyte cell …

Epigenetics in Cardiovascular Biology

Correct gene expression is a conditio sine qua for proper development of the heart; development of cardiovascular disease is often due to deregulation of gene expression. Epigenetics is a major mechanism controlling gene regulation in eukaryotic cells and consists in modulation of gene expression patterns without modifying the actual DNA sequence. The alteration of epigenetic mechanisms has been found to be involved in the etiology of several diseases, such as cancer, diabetes, and neuronal disorders (e.g. Rett syndrome and Parkinson’s disease); epigenetics has also been linked to heart development and disease. In particular, histone acetylation and chromatin remodeling are two important mechanisms in both activation of cardiac-specific genes and repression of non-cardiac ones during heart development, and in the re-programming that underlies cardiac hypertrophy. This chapter gives an overview of the role of epigenetics in heart development and in two important cardiovascular pathologies – heart failure and atherosclerosis.