Myh6-driven Cre recombinase activates the DNA damage response and the cell cycle in the myocardium in the absence of loxP sites

Runx1 is sufficient but not required for cardiomyocyte cell-cycle activation

Factors responsible for cardiomyocyte proliferation could serve as potential therapeutics to stimulate endogenous myocardial regeneration following insult, such as ischemic injury. A previously published forward genetics approach on cardiomyocyte cell cycle and ploidy led us to the transcription factor, Runx1. Here, we examine the effect of Runx1 on cardiomyocyte cell cycle during postnatal development and cardiac regeneration using cardiomyocyte-specific gain- and loss-of-function mouse models. RUNX1 is expressed in cardiomyocytes during early postnatal life, decreases to negligible levels by 3 wk of age, and increases upon myocardial injury, all consistent with observed rates of cardiomyocyte cell-cycle activity. Loss of Runx1 transiently stymied cardiomyocyte cell-cycle activity during normal postnatal development, a result that corrected itself and did not extend to the context of neonatal heart regeneration. On the other hand, cardiomyocyte-specific Runx1 overexpression resulted in an expansion of diploid cardiomyocytes in uninjured hearts and expansion of 4 N cardiomyocytes in the context of neonatal cardiac injury, suggesting Runx1 overexpression is sufficient to induce cardiomyocyte cell-cycle responses. Persistent overexpression of Runx1 for >1 mo continued to promote cardiomyocyte cell-cycle activity resulting in substantial hyperpolyploidization (≥8 N DNA content). This persistent cell-cycle activation was accompanied by ventricular dilation and adverse remodeling, raising the concern that continued cardiomyocyte cell cycling can have detrimental effects.NEW & NOTEWORTHY Runx1 is sufficient but not required for cardiomyocyte cell cycle.

MicroRNA205: A Key Regulator of Cardiomyocyte Transition from Proliferative to Hypertrophic Growth in the Neonatal Heart

The mammalian myocardium grows rapidly during early development due to cardiomyocyte proliferation, which later transitions to cell hypertrophy to sustain the heart's postnatal growth. Although this cell transition in the postnatal heart is consistently preserved in mammalian biology, little is known about the regulatory mechanisms that link proliferation suppression with hypertrophy induction. We reasoned that the production of a micro-RNA(s) could serve as a key bridge to permit changes in gene expression that control the changed cell fate of postnatal cardiomyocytes. We used sequential expression analysis to identify miR205 as a micro-RNA that was uniquely expressed at the cessation of cardiomyocyte growth. Cardiomyocyte-specific miR205 deletion animals showed a 35% increase in heart mass by 3 months of age, with commensurate changes in cell cycle and Hippo pathway activity, confirming miR205's potential role in controlling cardiomyocyte proliferation. In contrast, overexpression of miR205 in newborn hearts had little effect on heart size or function, indicating a complex, probably redundant regulatory system. These findings highlight miR205's role in controlling the shift from cardiomyocyte proliferation to hypertrophic development in the postnatal period.

IL-13 promotes functional recovery after myocardial infarction via direct signaling to macrophages

There is great interest in identifying signaling pathways that promote cardiac repair after myocardial infarction (MI). Prior studies suggest a beneficial role for IL-13 signaling in neonatal heart regeneration; however, the cell types mediating cardiac regeneration and the extent of IL-13 signaling in the adult heart after injury are unknown. We identified an abundant source of IL-13 and the related cytokine, IL-4, in neonatal cardiac type 2 innate lymphoid cells, but this phenomenon declined precipitously in adult hearts. Moreover, IL-13 receptor deletion in macrophages impaired cardiac function and resulted in larger scars early after neonatal MI. By using a combination of recombinant IL-13 administration and cell-specific IL-13 receptor genetic deletion models, we found that IL-13 signaling specifically to macrophages mediated cardiac functional recovery after MI in adult mice. Single transcriptomics revealed a subpopulation of cardiac macrophages in response to IL-13 administration. These IL-13-induced macrophages were highly efferocytotic and were identified by high IL-1R2 expression. Collectively, we elucidated a strongly proreparative role for IL-13 signaling directly to macrophages following cardiac injury. While this pathway is active in proregenerative neonatal stages, reactivation of macrophage IL-13 signaling is required to promote cardiac functional recovery in adults.

Evidence of Histone H2A.Z Deacetylation and Cardiomyocyte Dedifferentiation in Infarcted/Tip60-depleted Hearts

Myocardial infarction (MI) in the human heart causes death of billions of cardiomyocytes (CMs), resulting in cardiac dysfunction that is incompatible with life or lifestyle. In order to re-muscularize injured myocardium, replacement CMs must be generated via renewed proliferation of surviving CMs. Approaches designed to induce proliferation of CMs after injury have been insufficient. Toward this end, we are targeting the Tip60 acetyltransferase, based on the rationale that its pleiotropic functions conspire to block the CM cell-cycle at several checkpoints. We previously reported that genetic depletion of Tip60 in a mouse model after MI reduces scarring, retains cardiac function, and activates the CM cell-cycle, although it is unclear whether this culminates in the generation of daughter CMs. For pre-existing CMs in the adult heart to resume proliferation, it is becoming widely accepted that they must first dedifferentiate, a process highlighted by loss of maturity, epithelial to mesenchymal transitioning (EMT), and reversion from fatty acid oxidation to glycolytic metabolism, accompanied by softening of the myocardial extracellular matrix. Findings in hematopoietic stem cells, and more recently in neural progenitor cells, have shown that Tip60 induces and maintains the differentiated state via site-specific acetylation of the histone variant H2A.Z. Here, we report that genetic depletion of Tip60 from naïve or infarcted hearts results in the near-complete absence of acetylated H2A.Z in CM nuclei, and that this is accordingly accompanied by altered gene expressions indicative of EMT induction, ECM softening, decreased fatty acid oxidation, and depressed expression of genes that regulate the TCA cycle. These findings, combined with our previous work, support the notion that because Tip60 has multiple targets that combinatorially maintain the differentiated state and inhibit proliferation, its transient therapeutic targeting to ameliorate the effects of cardiac injury should be considered.

Cardiac-Specific Expression of Cre Recombinase Leads to Age-Related Cardiac Dysfunction Associated with Tumor-like Growth of Atrial Cardiomyocyte and Ventricular Fibrosis and Ferroptosis

Transgenic expression of Cre recombinase driven by a specific promoter is normally used to conditionally knockout a gene in a tissue- or cell-type-specific manner. In αMHC-Cre transgenic mouse model, expression of Cre recombinase is controlled by the myocardial-specific α-myosin heavy chain (αMHC) promoter, which is commonly used to edit myocardial-specific genes. Toxic effects of Cre expression have been reported, including intro-chromosome rearrangements, micronuclei formation and other forms of DNA damage, and cardiomyopathy was observed in cardiac-specific Cre transgenic mice. However, mechanisms associated with Cardiotoxicity of Cre remain poorly understood. In our study, our data unveiled that αMHC-Cre mice developed arrhythmias and died after six months progressively, and none of them survived more than one year. Histopathological examination showed that αMHC-Cre mice had aberrant proliferation of tumor-like tissue in the atrial chamber extended from and vacuolation of ventricular myocytes. Furthermore, the αMHC-Cre mice developed severe cardiac interstitial and perivascular fibrosis, accompanied by significant increase of expression levels of MMP-2 and MMP-9 in the cardiac atrium and ventricular. Moreover, cardiac-specific expression of Cre led to disintegration of the intercalated disc, along with altered proteins expression of the disc and calcium-handling abnormality. Comprehensively, we identified that the ferroptosis signaling pathway is involved in heart failure caused by cardiac-specific expression of Cre, on which oxidative stress results in cytoplasmic vacuole accumulation of lipid peroxidation on the myocardial cell membrane. Taken together, these results revealed that cardiac-specific expression of Cre recombinase can lead to atrial mesenchymal tumor-like growth in the mice, which causes cardiac dysfunction, including cardiac fibrosis, reduction of the intercalated disc and cardiomyocytes ferroptosis at the age older than six months in mice. Our study suggests that αMHC-Cre mouse models are effective in young mice, but not in old mice. Researchers need to be particularly careful when using αMHC-Cre mouse model to interpret those phenotypic impacts of gene responses. As the Cre-associated cardiac pathology matched mostly to that of the patients, the model could also be employed for investigating age-related cardiac dysfunction.

Cre toxicity in mouse models of cardiovascular physiology and disease

The Cre-LoxP system provides a widely used method for studying gene requirements in the mouse as the main mammalian genetic model organism. To define the molecular and cellular mechanisms that underlie cardiovascular development, function and disease, various mouse strains have been engineered that allow Cre-LoxP-mediated gene targeting within specific cell types of the cardiovascular system. Despite the usefulness of this system, evidence is accumulating that Cre activity can have toxic effects in cells, independently of its ability to recombine pairs of engineered LoxP sites in target genes. Here, we have gathered published evidence for Cre toxicity in cells and tissues relevant to cardiovascular biology and provide an overview of mechanisms proposed to underlie Cre toxicity. Based on this knowledge, we propose that each study utilising the Cre-LoxP system to investigate gene function in the cardiovascular system should incorporate appropriate controls to account for Cre toxicity.

Disease Models & Mechanisms helps move heart failure to heart success

Heart failure affects ∼64 million people worldwide, resulting in high morbidity, mortality and societal cost. Current treatment strategies are primarily geared at slowing the progression to an advanced disease state, but do not reverse or cure heart failure. A more comprehensive understanding of the underlying biology and development of preclinical models of this heterogeneous group of disorders will improve diagnosis and treatment. Here, we summarise recent preclinical and translational research in this area published in Disease Models & Mechanisms. We also discuss how our Journal is propelling this field forward by launching a Special Issue and ongoing subject collection, ‘Moving Heart Failure to Heart Success: Mechanisms, Regeneration & Therapy’.

Measuring cardiomyocyte cell-cycle activity and proliferation in the age of heart regeneration

During the past two decades, the field of mammalian myocardial regeneration has grown dramatically, and with this expanded interest comes increasing claims of experimental manipulations that mediate bona fide proliferation of cardiomyocytes. Too often, however, insufficient evidence or improper controls are provided to support claims that cardiomyocytes have definitively proliferated, a process that should be strictly defined as the generation of two de novo functional cardiomyocytes from one original cardiomyocyte. Throughout the literature, one finds inconsistent levels of experimental rigor applied, and frequently the specific data supplied as evidence of cardiomyocyte proliferation simply indicate cell-cycle activation or DNA synthesis, which do not necessarily lead to the generation of new cardiomyocytes. In this review, we highlight potential problems and limitations faced when characterizing cardiomyocyte proliferation in the mammalian heart, and summarize tools and experimental standards, which should be used to support claims of proliferation-based remuscularization. In the end, definitive establishment of de novo cardiomyogenesis can be difficult to prove; therefore, rigorous experimental strategies should be used for such claims.

Conditional depletion of the acetyltransferase Tip60 protects against the damaging effects of myocardial infarction

Injury from myocardial infarction (MI) and consequent post-MI remodeling is accompanied by massive loss of cardiomyocytes (CM), a cell type critical for contractile function that is for all practical purposes non-regenerable due to its profound state of proliferative senescence. Identification of factors that limit CM survival and/or constrain CM renewal provides potential therapeutic targets. Tip60, a pan-acetyltransferase encoded by the Kat5 gene, has been reported to activate apoptosis as well as multiple anti-proliferative pathways in non-cardiac cells; however, its role in CMs, wherein it is abundantly expressed, remains unknown. Here, using mice containing floxed Kat5 alleles and a tamoxifen-activated Myh6-MerCreMer recombinase transgene, we report that conditional depletion of Tip60 in CMs three days after MI induced by permanent coronary artery ligation greatly improves functional recovery for up to 28 days. This is accompanied by diminished scarring, activation of cell-cycle transit markers in CMs within the infarct border and remote zones, reduced expression of cell-cycle inhibitors pAtm and p27, and reduced apoptosis in the remote regions. These findings implicate Tip60 as a novel, multifactorial target for limiting the damaging effects of ischemic heart disease.

Corticosteroid Receptors in Cardiac Health and Disease

Nuclear receptors play a central role in both energy metabolism and cardiomyocyte death and survival in the heart. Recent evidence suggests they may also influence cardiomyocyte endowment. Although several members of the nuclear receptor family play key roles in heart maturation (including thyroid hormone receptors) and cardiac metabolism, here, the focus will be on the corticosteroid receptors, the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR). The heart is an important target for the actions of corticosteroids, yet the homeostatic role of GR and MR in the healthy heart has been elusive. However, MR antagonists are important in the treatment of heart failure, a condition associated with mitochondrial dysfunction and energy failure in cardiomyocytes leading to mitochondria-initiated cardiomyocyte death (Ingwall and Weiss, Circ Res 95:135-145, 2014; Ingwall , Cardiovasc Res 81:412-419, 2009; Zhou and Tian , J Clin Invest 128:3716-3726, 2018). In contrast, animal studies suggest GR activation in cardiomyocytes has a cardioprotective role, including in heart failure.

IL4Rα signaling promotes neonatal cardiac regeneration and cardiomyocyte cell cycle activity

Neonatal heart regeneration depends on proliferation of pre-existing cardiomyocytes, yet the mechanisms driving regeneration and cardiomyocyte proliferation are not comprehensively understood. We recently reported that the anti-inflammatory cytokine, interleukin 13 (IL13), promotes neonatal cardiac regeneration; however, the signaling pathway and cell types mediating this regenerative response remain unknown. Here, we hypothesized that expression of the type II heterodimer receptor for IL13, comprised of IL4Rα and IL13Rα1, expressed directly on cardiomyocytes mediates cardiomyocyte cell cycle and heart regeneration in neonatal mice. Our data demonstrate that indeed global deletion of one critical subunit of the type II receptor, IL4Rα (IL4Rα-/-), decreases cardiomyocyte proliferation during early postnatal development and significantly impairs cardiac regeneration following injury in neonatal mice. While multiple myocardial cell types express IL4Rα, we demonstrate that IL4Rα deletion specifically in cardiomyocytes mediates cell cycle activity and neonatal cardiac regeneration. This demonstrates for the first time a functional role for IL4Rα signaling directly on cardiomyocytes in vivo. Reciprocally, we examined the therapeutic benefit of activating the IL4Rα receptor in non-regenerative hearts via IL13 administration. Following myocardial infarction, administration of IL13 reduced scar size and promoted cardiomyocyte DNA synthesis and karyokinesis, but not complete cytokinesis, in 6-day old non-regenerative mice. Our data demonstrate a novel role for IL4Rα signaling directly on cardiomyocytes during heart regeneration and suggest the potential for type II receptor activation as one potential therapeutic target for promoting myocardial repair.

Evidence that the acetyltransferase Tip60 induces the DNA damage response and cell-cycle arrest in neonatal cardiomyocytes

Tip60, a pan-acetyltransferase encoded by the Kat5 gene, is enriched in the myocardium; however, its function in the heart is unknown. In cancer cells, Tip60 acetylates Atm (Ataxia-telangiectasia mutated), enabling its auto-phosphorylation (pAtm), which activates the DNA damage response (DDR). It was recently reported that activation of pAtm at the time of birth induces the DDR in cardiomyocytes (CMs), resulting in proliferative senescence. We therefore hypothesized that Tip60 initiates this process, and that depletion of Tip60 accordingly diminishes the DDR while extending the duration of CM cell-cycle activation. To test this hypothesis, an experimental model was used wherein a Myh6-driven Cre-recombinase transgene was activated on postnatal day 0 (P0) to recombine floxed Kat5 alleles and induce Tip60 depletion in neonatal CMs, without causing pathogenesis. Depletion of Tip60 resulted in reduced numbers of pAtm-positive CMs during the neonatal period, which correlated with reduced numbers of pH2A.X-positive CMs and decreased expression of genes encoding markers of the DDR as well as inflammation. This was accompanied by decreased expression of the cell-cycle inhibitors Meis1 and p27, activation of the cell-cycle in CMs, reduced CM size, and increased numbers of mononuclear/diploid CMs. Increased expression of fetal markers suggested that Tip60 depletion promotes a fetal-like proliferative state. Finally, infarction of Tip60-depleted hearts at P7 revealed improved cardiac function at P39 accompanied by reduced fibrosis, increased CM cell-cycle activation, and reduced apoptosis in the remote zone. These findings indicate that, among its pleiotropic functions, Tip60 induces the DDR in CMs, contributing to proliferative senescence.

First person – Xinrui Wang

First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping early-career researchers promote themselves alongside their papers. Xinrui Wang is first author on ‘Myh6-driven Cre recombinase activates the DNA damage response and the cell cycle in the myocardium in the absence of loxP sites’, published in DMM. Xinrui is a postdoctoral fellow in the lab of John Auchampach at the Medical College of Wisconsin, Milwaukee, WI, USA, investigating cellular and molecular mechanisms of myocardium re-muscularization after injury.