Ravi Karra

Inefficient Reprogramming of Fibroblasts into Cardiomyocytes Using Gata4, Mef2c, and Tbx5

Rationale: Direct reprogramming of fibroblasts into cardiomyocytes is a novel strategy for cardiac regeneration. However, the key determinants involved in this process are unknown. Objective: To assess the efficiency of direct fibroblast reprogramming via viral overexpression of GATA4, Mef2c, and Tbx5 (GMT). Methods and Results: We induced GMT overexpression in murine tail tip fibroblasts (TTFs) and cardiac fibroblasts (CFs) from multiple lines of transgenic mice carrying different cardiomyocyte lineage reporters. We found that the induction of GMT overexpression in TTFs and CFs is inefficient at inducing molecular and electrophysiological phenotypes of mature cardiomyocytes. In addition, transplantation of GMT infected CFs into injured mouse hearts resulted in decreased cell survival with minimal induction of cardiomyocyte genes. Conclusions: Significant challenges remain in our ability to convert fibroblasts into cardiomyocyte-like cells and a greater understanding of cardiovascular epigenetics is needed to increase the translational potential of this strategy.

Fibronectin is deposited by injury-activated epicardial cells and is necessary for zebrafish heart regeneration.

Unlike adult mammals, adult zebrafish vigorously regenerate lost heart muscle in response to injury. The epicardium, a mesothelial cell layer enveloping the myocardium, is activated to proliferate after cardiac injury and can contribute vascular support cells or provide mitogens to regenerating muscle. Here, we applied proteomics to identify secreted proteins that are associated with heart regeneration. We found that Fibronectin, a main component of the extracellular matrix, is induced and deposited after cardiac damage. In situ hybridization and transgenic reporter analyses indicated that expression of two fibronectin paralogues, fn1 and fn1b, are induced by injury in epicardial cells, while the itgb3 receptor is induced in cardiomyocytes near the injury site. fn1, the more dynamic of these paralogs, is induced chamber-wide within one day of injury before localizing epicardial Fn1 synthesis to the injury site. fn1 loss-of-function mutations disrupted zebrafish heart regeneration, as did induced expression of a dominant-negative Fibronectin cassette, defects that were not attributable to direct inhibition of cardiomyocyte proliferation. These findings reveal a new role for the epicardium in establishing an extracellular environment that supports heart regeneration.

Nrg1 is an injury-induced cardiomyocyte mitogen for the endogenous heart regeneration program in zebrafish

Heart regeneration is limited in adult mammals but occurs naturally in adult zebrafish through the activation of cardiomyocyte division. Several components of the cardiac injury microenvironment have been identified, yet no factor on its own is known to stimulate overt myocardial hyperplasia in a mature, uninjured animal. In this study, we find evidence that Neuregulin1 (Nrg1), previously shown to have mitogenic effects on mammalian cardiomyocytes, is sharply induced in perivascular cells after injury to the adult zebrafish heart. Inhibition of Erbb2, an Nrg1 co-receptor, disrupts cardiomyocyte proliferation in response to injury, whereas myocardial Nrg1 overexpression enhances this proliferation. In uninjured zebrafish, the reactivation of Nrg1 expression induces cardiomyocyte dedifferentiation, overt muscle hyperplasia, epicardial activation, increased vascularization, and causes cardiomegaly through persistent addition of wall myocardium. Our findings identify Nrg1 as a potent, induced mitogen for the endogenous adult heart regeneration program. DOI: http://dx.doi.org/10.7554/eLife.05871.001

Translational profiling of cardiomyocytes identifies an early Jak1/Stat3 injury response required for zebrafish heart regeneration.

Certain lower vertebrates like zebrafish activate proliferation of spared cardiomyocytes after cardiac injury to regenerate lost heart muscle. Here, we used translating ribosome affinity purification to profile translating RNAs in zebrafish cardiomyocytes during heart regeneration. We identified dynamic induction of several Jak1/Stat3 pathway members following trauma, events accompanied by cytokine production. Transgenic Stat3 inhibition in cardiomyocytes restricted injury-induced proliferation and regeneration, but did not reduce cardiogenesis during animal growth. The secreted protein Rln3a was induced in a Stat3-dependent manner by injury, and exogenous Rln3 delivery during Stat3 inhibition stimulated cardiomyocyte proliferation. Our results identify an injury-specific cardiomyocyte program essential for heart regeneration.

An Injury-Responsive Gata4 Program Shapes the Zebrafish Cardiac Ventricle

A common principle of tissue regeneration is the reactivation of previously employed developmental programs. During zebrafish heart regeneration, cardiomyocytes in the cortical layer of the ventricle induce the transcription factor gene gata4 and proliferate to restore lost muscle. A dynamic cellular mechanism initially creates this cortical muscle in juvenile zebrafish, where a small number of internal cardiomyocytes breach the ventricular wall and expand upon its surface. Here, we find that emergent juvenile cortical cardiomyocytes induce expression of gata4 in a manner similar to during regeneration. Clonal analysis indicates that these cardiomyocytes make biased contributions to build the ventricular wall, whereas gata4(+) cardiomyocytes have little or no proliferation hierarchy during regeneration. Experimental microinjuries or conditions of rapid organismal growth stimulate production of ectopic gata4(+) cortical muscle, implicating biomechanical stress in morphogenesis of this tissue and revealing clonal plasticity. Induced transgenic inhibition defined an essential role for Gata4 activity in morphogenesis of the cortical layer and the preservation of normal cardiac function in growing juveniles, and again in adults during heart regeneration. Our experiments uncover an injury-responsive program that prevents heart failure in juveniles by fortifying the ventricular wall, one that is reiterated in adults to promote regeneration after cardiac damage.

Modulation of tissue repair by regeneration enhancer elements

How tissue regeneration programs are triggered by injury has received limited research attention. Here we investigate the existence of enhancer regulatory elements that are activated in regenerating tissue. Transcriptomic analyses reveal that leptin b (lepb) is highly induced in regenerating hearts and fins of zebrafish. Epigenetic profiling identified a short DNA sequence element upstream and distal to lepb that acquires open chromatin marks during regeneration and enables injury-dependent expression from minimal promoters. This element could activate expression in injured neonatal mouse tissues and was divisible into tissue-specific modules sufficient for expression in regenerating zebrafish fins or hearts. Simple enhancer-effector transgenes employing lepb-linked sequences upstream of pro- or anti-regenerative factors controlled the efficacy of regeneration in zebrafish. Our findings provide evidence for ‘tissue regeneration enhancer elements’ (TREEs) that trigger gene expression in injury sites and can be engineered to modulate the regenerative potential of vertebrate organs.

Single epicardial cell transcriptome sequencing identifies Caveolin 1 as an essential factor in zebrafish heart regeneration

In contrast to mammals, adult zebrafish have a high capacity to regenerate damaged or lost myocardium through proliferation of cardiomyocytes spared from damage. The epicardial sheet covering the heart is activated by injury and aids muscle regeneration through paracrine effects and as a multipotent cell source, and has received recent attention as a target in cardiac repair strategies. Although it is recognized that epicardium is required for muscle regeneration and itself has high regenerative potential, the extent of cellular heterogeneity within epicardial tissue is largely unexplored. Here, we performed transcriptome analysis on dozens of epicardial lineage cells purified from zebrafish harboring a transgenic reporter for the pan-epicardial gene tcf21. Hierarchical clustering analysis suggested the presence of at least three epicardial cell subsets defined by expression signatures. We validated many new pan-epicardial and epicardial markers by alternative expression assays. Additionally, we explored the function of the scaffolding protein and main component of caveolae, caveolin 1 (cav1), which was present in each epicardial subset. In BAC transgenic zebrafish, cav1 regulatory sequences drove strong expression in ostensibly all epicardial cells and in coronary vascular endothelial cells. Moreover, cav1 mutant zebrafish generated by genome editing showed grossly normal heart development and adult cardiac anatomy, but displayed profound defects in injury-induced cardiomyocyte proliferation and heart regeneration. Our study defines a new platform for the discovery of epicardial lineage markers, genetic tools, and mechanisms of heart regeneration. Highlighted article: Gene expression analyses reveal that zebrafish epicardial cells are heterogeneous and identify many new epicardial markers, including Caveolin 1, which is shown to be essential for heart regeneration.

Myocardial NF-κB activation is essential for zebrafish heart regeneration

Significance Heart failure occurs when the heart cannot pump enough blood to meet the body’s demands. The heart most often weakens through the loss of muscle cells, called cardiomyocytes. Heart failure is a devastating disease, and one possible cure is to replace the lost cardiomyocytes through regeneration. Unlike humans, zebrafish can efficiently regenerate their hearts after injury. Interestingly, zebrafish heart cells are similar to human heart cells at the molecular level. Understanding how zebrafish can regenerate cardiac tissue can help identify regenerative therapies for humans. Here, we find that NF-κB signaling is a link between the injury response and the regenerative program in zebrafish. Heart regeneration offers a novel therapeutic strategy for heart failure. Unlike mammals, lower vertebrates such as zebrafish mount a strong regenerative response following cardiac injury. Heart regeneration in zebrafish occurs by cardiomyocyte proliferation and reactivation of a cardiac developmental program, as evidenced by induction of gata4 regulatory sequences in regenerating cardiomyocytes. Although many of the cellular determinants of heart regeneration have been elucidated, how injury triggers a regenerative program through dedifferentiation and epicardial activation is a critical outstanding question. Here, we show that NF-κB signaling is induced in cardiomyocytes following injury. Myocardial inhibition of NF-κB activity blocks heart regeneration with pleiotropic effects, decreasing both cardiomyocyte proliferation and epicardial responses. Activation of gata4 regulatory sequences is also prevented by NF-κB signaling antagonism, suggesting an underlying defect in cardiomyocyte dedifferentiation. Our results implicate NF-κB signaling as a key node between cardiac injury and tissue regeneration.

Essential and Unexpected Role of Yin Yang 1 to Promote Mesodermal Cardiac Differentiation

Rationale: Cardiogenesis is regulated by a complex interplay between transcription factors. However, little is known about how these interactions regulate the transition from mesodermal precursors to cardiac progenitor cells (CPCs). Objective: To identify novel regulators of mesodermal cardiac lineage commitment. Methods and Results: We performed a bioinformatic-based transcription factor binding site analysis on upstream promoter regions of genes that are enriched in embryonic stem cell–derived CPCs. From 32 candidate transcription factors screened, we found that Yin Yang 1 (YY1), a repressor of sarcomeric gene expression, is present in CPCs in vivo. Interestingly, we uncovered the ability of YY1 to transcriptionally activate Nkx2.5, a key marker of early cardiogenic commitment. YY1 regulates Nkx2.5 expression via a 2.1-kb cardiac-specific enhancer as demonstrated by in vitro luciferase-based assays, in vivo chromatin immunoprecipitation, and genome-wide sequencing analysis. Furthermore, the ability of YY1 to activate Nkx2.5 expression depends on its cooperative interaction with Gata4 at a nearby chromatin. Cardiac mesoderm–specific loss-of-function of YY1 resulted in early embryonic lethality. This was corroborated in vitro by embryonic stem cell–based assays in which we showed that the overexpression of YY1 enhanced the cardiogenic differentiation of embryonic stem cells into CPCs. Conclusions: These results demonstrate an essential and unexpected role for YY1 to promote cardiogenesis as a transcriptional activator of Nkx2.5 and other CPC-enriched genes.

Redirecting cardiac growth mechanisms for therapeutic regeneration

Heart failure is a major source of morbidity and mortality. Replacing lost myocardium with new tissue is a major goal of regenerative medicine. Unlike adult mammals, zebrafish and neonatal mice are capable of heart regeneration following cardiac injury. In both contexts, the regenerative program echoes molecular and cellular events that occur during cardiac development and morphogenesis, notably muscle creation through division of cardiomyocytes. Based on studies over the past decade, it is now accepted that the adult mammalian heart undergoes a low grade of cardiomyocyte turnover. Recent data suggest that this cardiomyocyte turnover can be augmented in the adult mammalian heart by redeployment of developmental factors. These findings and others suggest that stimulating endogenous regenerative responses can emerge as a therapeutic strategy for human cardiovascular disease.