Pitx2 maintains mitochondrial function during regeneration to prevent myocardial fat deposition

Nrf3-Mediated Mitochondrial Superoxide Promotes Cardiomyocyte Apoptosis and Impairs Cardiac Functions by Suppressing Pitx2

BACKGROUND

Myocardial infarction (MI) elicits mitochondria reactive oxygen species (ROS) production and cardiomyocyte (CM) apoptosis. Nrf3 (nuclear factor erythroid 2-related factor 3) has an established role in regulating redox signaling and tissue homeostasis. Here, we aimed to evaluate the role and mechanism of Nrf3 in injury-induced pathological cardiac remodeling.

METHODS

Global (Nrf3-KO) and CM-specific (Nrf3△CM) Nrf3 knockout mice were subjected to MI or ischemia/reperfusion injury, followed by functional and histopathological analysis. Primary neonatal mouse and rat ventricular myocytes and CMs derived from human induced pluripotent stem cells were used to evaluate the impact of Nrf3 on CM apoptosis and mitochondrial ROS production. Chromatin immunoprecipitation sequencing and immunoprecipitation-mass spectrometry analysis were used to uncover potential targets of Nrf3. MitoParaquat administration and CM-specific adeno-associated virus vectors were used to further confirm the in vivo relevance of the identified signal pathways.

RESULTS

Nrf3 was expressed mainly in CMs in healthy human hearts, and an increased level of Nrf3 was observed in CMs within the border zone of infarcted human hearts and murine cardiac tissues after MI. Both global and CM-specific Nrf3 knockout significantly decreased injury-induced mitochondrial ROS production, CM apoptosis, and pathological cardiac remodeling, consequently improving cardiac functions. In addition, cardiac-specific Nrf3 overexpression reversed the ameliorative cardiac phenotypes observed in Nrf3-KO mice. Functional studies showed that Nrf3 promoted neonatal mouse ventricular myocyte, neonatal rat ventricular myocyte, and CMs derived from human induced pluripotent stem cell apoptosis by increasing mitochondrial ROS production. Critically, augmenting mitochondrial ROS with MitoParaquat blunted the beneficial effects of Nrf3 deletion on cardiac function and remodeling. Mechanistically, a redox regulator Pitx2 (paired-like homeodomain transcription factor 2) was identified as one of the main target genes of Nrf3. Specifically, Nrf3 binds to Pitx2 promoter, where it increases DNA methylation through recruiting heterogeneous nuclear ribonucleoprotein K and DNA-methyltransferase 1 complex, thereby inhibiting Pitx2 expression. CM-specific knockdown of Pitx2 blunted the beneficial effects of Nrf3 deletion on cardiac function and remodeling, and cardiac-specific Pitx2 overexpression attenuated MI-induced mitochondrial ROS production and CM apoptosis, as well as preserved cardiac functions after MI.

CONCLUSIONS

Nrf3 promotes injury-induced CM apoptosis and deteriorates cardiac functions by increasing mitochondrial ROS production through suppressing Pitx2 expression. Targeting the Nrf3-Pitx2-mitochondrial ROS signal axis may therefore represent a novel therapeutic approach for MI treatment.

RNA-binding protein SAMD4A targets FGF2 to regulate cardiomyocyte lineage specification from human embryonic stem cells

BACKGROUND

RNA-binding proteins (RBPs) are essential in cardiac development. However, a large of them have not been characterized during the process.

METHODS

We applied the human embryonic stem cells (hESCs) differentiated into cardiomyocytes model and constructed SAMD4A-knockdown/overexpression hESCs to investigate the role of SAMD4A in cardiomyocyte lineage specification.

RESULTS

SAMD4A, an RBP, exhibits increased expression during early heart development. Suppression of SAMD4A inhibits the proliferation of hESCs, impedes cardiac mesoderm differentiation, and impairs the function of hESC-derived cardiomyocytes. Correspondingly, forced expression of SAMD4A enhances proliferation and promotes cardiomyogenesis. Mechanistically, SAMD4A specifically binds to FGF2 via a specific CNGG/CNGGN motif, stabilizing its mRNA and enhancing translation, thereby upregulating FGF2 expression, which subsequently modulates the AKT signaling pathway and regulates cardiomyocyte lineage differentiation. Additionally, supplementation of FGF2 can rescue the proliferation defect of hESCs in the absence of SAMD4A.

CONCLUSIONS

Our study demonstrates that SAMD4A orchestrates cardiomyocyte lineage commitment through the post-transcriptional regulation of FGF2 and modulation of AKT signaling. These findings not only underscore the essential role of SAMD4A in cardiac organogenesis, but also provide critical insights into the molecular mechanisms underlying heart development, thereby informing potential therapeutic strategies for congenital heart disease.

Oxidative Stress Causes Mitochondrial and Electrophysiologic Dysfunction to Promote Atrial Fibrillation in Pitx2+/- Mice

BACKGROUND

The strongest genetic risk factors for atrial fibrillation (AF) are DNA variants on chromosome 4q25 near the transcription factor gene PITX2 (Pitx2:Paired-like homeodomain transcription factor 2). Mice deficient in Pitx2 (Pitx2+/-) have increased AF susceptibility, although the molecular mechanism(s) remains controversial. Pitx2 encodes a transcription factor that activates an antioxidant response to promote cardiac repair. Increased reactive oxygen species causing oxidation of polyunsaturated fatty acids generates reactive lipid dicarbonyl moieties that adduct to proteins and other macromolecules to promote cellular injury. We tested the hypothesis that oxidative stress, and specifically isolevuglandins, the most reactive lipid dicarbonyls identified, are increased in the setting of Pitx2 deficiency to promote proarrhythmic remodeling and AF.

METHODS

Pitx2+/- and Pitx2+/+ wild-type littermate control mice were treated orally with vehicle, the lipid dicarbonyl scavenger 2-hydroxybenzylamine, or an inactive control compound at weaning, until study at age 16 to 18 weeks.

RESULTS

Pitx2+/- mice demonstrated increased P wave duration indicative of slowed atrial conduction, as well as increased inducible AF burden and sustained AF, compared with wild type, and these abnormalities were prevented by 2-hydroxybenzylamine. Both reactive oxygen species and isolevuglandin protein adducts were elevated in Pitx2+/- atria with reduced expression of reactive oxygen species-protective genes. High-resolution respirometry demonstrated impaired mitochondrial function in Pitx2+/- atria, with disruption of mitochondrial integrity and cell-cell junctions with connexin lateralization, as well as decreased mitochondrial biogenesis gene expression. Proarrhythmic ionic current remodeling in Pitx2+/- atrial myocytes included elevated resting membrane potential, abbreviated action potential duration, and reduced maximum phase 0 upstroke velocity compared with wild type. Most of these abnormalities were ameliorated or prevented by 2-hydroxybenzylamine.

CONCLUSIONS

These results demonstrate a critical role for lipid dicarbonyl mediators of oxidative stress in the proarrhythmic remodeling and AF susceptibility that occurs with Pitx2 deficiency, implying the possibility of genotype-specific therapy to prevent AF.

Cardiac Regeneration in Adult Zebrafish: A Review of Signaling and Metabolic Coordination

PURPOSE OF REVIEW

This review investigates how post-injury cellular signaling and energy metabolism are two pivotal points in zebrafish's cardiomyocyte cell cycle re-entry and proliferation. It seeks to highlight the probable mechanism of action in proliferative cardiomyocytes compared to mammals and identify gaps in the current understanding of metabolic regulation of cardiac regeneration.

RECENT FINDINGS

Metabolic substrate changes after birth correlate with reduced cardiomyocyte proliferation in mammals. Unlike adult mammalian hearts, zebrafish can regenerate cardiomyocytes by re-entering the cell cycle, characterized by a metabolic switch from oxidative metabolism to increased glycolysis. Zebrafish provide a valuable model for studying metabolic regulation during cell cycle re-entry and cardiac regeneration. Proliferative cardiomyocytes have upregulated Notch, hippo, and Wnt signaling and decreased ROS generation, DNA damage in different zebrafish cardiac regeneration models. Understanding the correlation between metabolic switches during cell cycle re-entry of already differentiated zebrafish cardiomyocytes is being increasingly recognized as a critical factor in heart regeneration. Zebrafish studies provide insights into metabolic adaptations during heart regeneration, emphasizing the importance of a metabolic switch. However, there are mechanistic gaps, and extensive studies are required to aid in formulating therapeutic strategies for cardiac regenerative medicine.

PITX2 deficiency leads to atrial mitochondrial dysfunction

AIMS

Reduced left atrial PITX2 is associated with atrial cardiomyopathy and atrial fibrillation (AF). PITX2 is restricted to left atrial cardiomyocytes (aCMs) in the adult heart. The links between PITX2 deficiency, atrial cardiomyopathy, and AF are not fully understood.

METHODS AND RESULTS

To identify mechanisms linking PITX2 deficiency to AF, we generated and characterized PITX2-deficient human aCMs derived from human induced pluripotent stem cells (hiPSC) and their controls. PITX2-deficient hiPSC-derived atrial cardiomyocytes showed shorter and disorganized sarcomeres and increased mononucleation. Electron microscopy found an increased number of smaller mitochondria compared with isogenic controls. Mitochondrial protein expression was altered in PITX2-deficient hiPSC-derived atrial cardiomyocytes. Single-nuclear RNA-sequencing found differences in cellular respiration pathways and differentially expressed mitochondrial and ion channel genes in PITX2-deficient hiPSC-derived atrial cardiomyocytes. PITX2 repression in hiPSC-derived atrial cardiomyocytes replicated dysregulation of cellular respiration. Mitochondrial respiration was shifted to increased glycolysis in PITX2-deficient hiPSC-derived atrial cardiomyocytes. PITX2-deficient human hiPSC-derived atrial cardiomyocytes showed higher spontaneous beating rates. Action potential duration was more variable with an overall prolongation of early repolarization, consistent with metabolic defects. Gene expression analyses confirmed changes in mitochondrial genes in left atria from 42 patients with AF compared with 43 patients with sinus rhythm. Dysregulation of left atrial mitochondrial (COX7C) and metabolic (FOXO1) genes was associated with PITX2 expression in human left atria.

CONCLUSION

PITX2 deficiency causes atrial mitochondrial dysfunction and a metabolic shift to glycolysis in human aCMs. PITX2-dependent metabolic changes can contribute to the structural and functional defects found in PITX2-deficient atria.

Cardiomyocyte-fibroblast interaction regulates ferroptosis and fibrosis after myocardial injury

Neonatal mouse hearts have transient renewal capacity, which is lost in juvenile and adult stages. In neonatal mouse hearts, myocardial infarction (MI) causes an initial loss of cardiomyocytes. However, it is unclear which type of regulated cell death (RCD) occurs in stressed cardiomyocytes. In the current studies, we induced MI in neonatal and juvenile mouse hearts and showed that ischemic cardiomyocytes primarily undergo ferroptosis, a non-apoptotic and iron-dependent form of RCD. We demonstrated that cardiac fibroblasts (CFs) protect cardiomyocytes from ferroptosis through paracrine effects and direct cell-cell interaction. CFs show strong resistance to ferroptosis due to high ferritin expression. The fibrogenic activity of CFs, typically considered detrimental to heart function, is negatively regulated by paired-like homeodomain 2 (Pitx2) signaling from cardiomyocytes. In addition, Pitx2 prevents ferroptosis in cardiomyocytes by regulating ferroptotic genes. Understanding the regulatory mechanisms of cardiomyocyte survival and death can identify potentially translatable therapeutic strategies for MI.

Comparative Analysis of Heart Regeneration: Searching for the Key to Heal the Heart-Part II: Molecular Mechanisms of Cardiac Regeneration

Cardiovascular diseases are the leading cause of death worldwide, among which ischemic heart disease is the most representative. Myocardial infarction results from occlusion of a coronary artery, which leads to an insufficient blood supply to the myocardium. As it is well known, the massive loss of cardiomyocytes cannot be solved due the limited regenerative ability of the adult mammalian hearts. In contrast, some lower vertebrate species can regenerate the heart after an injury; their study has disclosed some of the involved cell types, molecular mechanisms and signaling pathways during the regenerative process. In this 'two parts' review, we discuss the current state-of-the-art of the main response to achieve heart regeneration, where several processes are involved and essential for cardiac regeneration.

Unravelling the Interplay between Cardiac Metabolism and Heart Regeneration

Ischemic heart disease (IHD) is the leading cause of heart failure (HF) and is a significant cause of morbidity and mortality globally. An ischemic event induces cardiomyocyte death, and the ability for the adult heart to repair itself is challenged by the limited proliferative capacity of resident cardiomyocytes. Intriguingly, changes in metabolic substrate utilisation at birth coincide with the terminal differentiation and reduced proliferation of cardiomyocytes, which argues for a role of cardiac metabolism in heart regeneration. As such, strategies aimed at modulating this metabolism-proliferation axis could, in theory, promote heart regeneration in the setting of IHD. However, the lack of mechanistic understanding of these cellular processes has made it challenging to develop therapeutic modalities that can effectively promote regeneration. Here, we review the role of metabolic substrates and mitochondria in heart regeneration, and discuss potential targets aimed at promoting cardiomyocyte cell cycle re-entry. While advances in cardiovascular therapies have reduced IHD-related deaths, this has resulted in a substantial increase in HF cases. A comprehensive understanding of the interplay between cardiac metabolism and heart regeneration could facilitate the discovery of novel therapeutic targets to repair the damaged heart and reduce risk of HF in patients with IHD.

Multi-species meta-analysis identifies transcriptional signatures associated with cardiac endothelial responses in the ischaemic heart

AIM

Myocardial infarction remains the leading cause of heart failure. The adult human heart lacks the capacity to undergo endogenous regeneration. New blood vessel growth is integral to regenerative medicine necessitating a comprehensive understanding of the pathways that regulate vascular regeneration. We sought to define the transcriptomic dynamics of coronary endothelial cells following ischaemic injuries in the developing and adult mouse and human heart and to identify new mechanistic insights and targets for cardiovascular regeneration.

METHODS AND RESULTS

We carried out a comprehensive meta-analysis of integrated single-cell RNA-sequencing data of coronary vascular endothelial cells from the developing and adult mouse and human heart spanning healthy and acute and chronic ischaemic cardiac disease. We identified species-conserved gene regulatory pathways aligned to endogenous neovascularization. We annotated injury-associated temporal shifts of the endothelial transcriptome and validated four genes: VEGF-C, KLF4, EGR1, and ZFP36. Moreover, we showed that ZFP36 regulates human coronary endothelial cell proliferation and defined that VEGF-C administration in vivo enhances clonal expansion of the cardiac vasculature post-myocardial infarction. Finally, we constructed a coronary endothelial cell meta-atlas, CrescENDO, to empower future in-depth research to target pathways associated with coronary neovascularization.

CONCLUSION

We present a high-resolution single-cell meta-atlas of healthy and injured coronary endothelial cells in the mouse and human heart, revealing a suite of novel targets with great potential to promote vascular regeneration, and providing a rich resource for therapeutic development.

Cardiomyocyte-fibroblast interaction regulates ferroptosis and fibrosis after myocardial injury

Neonatal mouse hearts have transient renewal capacity which is lost in juvenile and adult hearts. After myocardial infarction (MI) in neonatal hearts, an initial loss of cardiomyocytes occurs but it is unclear through which type of regulated cell death (RCD). In the current studies, we induced MI in neonatal and juvenile mouse hearts, and show that ischemic cardiomyocytes primarily undergo ferroptosis, a non-apoptotic and iron-dependent form of RCD. We demonstrate that cardiac fibroblasts (CFs) protect cardiomyocytes from ferroptosis through paracrine factors and direct cell-cell interaction. CFs show strong resistance to ferroptosis due to high ferritin expression. Meanwhile, the fibrogenic role of CFs, typically considered detrimental to heart function, is negatively regulated by paired-like homeodomain 2 (Pitx2) signaling from cardiomyocytes. In addition, Pitx2 prevents ferroptosis in cardiomyocytes by regulating ferroptotic genes. Understanding the regulatory mechanisms of cardiomyocyte survival and death can identify potentially translatable therapeutic strategies for MI.

Integrated multi-omic characterization of congenital heart disease

The heart, the first organ to develop in the embryo, undergoes complex morphogenesis that when defective results in congenital heart disease (CHD). With current therapies, more than 90% of patients with CHD survive into adulthood, but many suffer premature death from heart failure and non-cardiac causes1. Here, to gain insight into this disease progression, we performed single-nucleus RNA sequencing on 157,273 nuclei from control hearts and hearts from patients with CHD, including those with hypoplastic left heart syndrome (HLHS) and tetralogy of Fallot, two common forms of cyanotic CHD lesions, as well as dilated and hypertrophic cardiomyopathies. We observed CHD-specific cell states in cardiomyocytes, which showed evidence of insulin resistance and increased expression of genes associated with FOXO signalling and CRIM1. Cardiac fibroblasts in HLHS were enriched in a low-Hippo and high-YAP cell state characteristic of activated cardiac fibroblasts. Imaging mass cytometry uncovered a spatially resolved perivascular microenvironment consistent with an immunodeficient state in CHD. Peripheral immune cell profiling suggested deficient monocytic immunity in CHD, in agreement with the predilection in CHD to infection and cancer2. Our comprehensive phenotyping of CHD provides a roadmap towards future personalized treatments for CHD.

Metabolic Regulation of Cardiac Regeneration

The mortality due to heart diseases remains highest in the world every year, with ischemic cardiomyopathy being the prime cause. The irreversible loss of cardiomyocytes following myocardial injury leads to compromised contractility of the remaining myocardium, adverse cardiac remodeling, and ultimately heart failure. The hearts of adult mammals can hardly regenerate after cardiac injury since adult cardiomyocytes exit the cell cycle. Nonetheless, the hearts of early neonatal mammals possess a stronger capacity for regeneration. To improve the prognosis of patients with heart failure and to find the effective therapeutic strategies for it, it is essential to promote endogenous regeneration of adult mammalian cardiomyocytes. Mitochondrial metabolism maintains normal physiological functions of the heart and compensates for heart failure. In recent decades, the focus is on the changes in myocardial energy metabolism, including glucose, fatty acid, and amino acid metabolism, in cardiac physiological and pathological states. In addition to being a source of energy, metabolites are becoming key regulators of gene expression and epigenetic patterns, which may affect heart regeneration. However, the myocardial energy metabolism during heart regeneration is majorly unknown. This review focuses on the role of energy metabolism in cardiac regeneration, intending to shed light on the strategies for manipulating heart regeneration and promoting heart repair after cardiac injury.

The asymmetric Pitx2 gene regulates gut muscular-lacteal development and protects against fatty liver disease

Intestinal lacteals are essential lymphatic channels for absorption and transport of dietary lipids and drive the pathogenesis of debilitating metabolic diseases. However, organ-specific mechanisms linking lymphatic dysfunction to disease etiology remain largely unknown. In this study, we uncover an intestinal lymphatic program that is linked to the left-right (LR) asymmetric transcription factor Pitx2. We show that deletion of the asymmetric Pitx2 enhancer ASE alters normal lacteal development through the lacteal-associated contractile smooth muscle lineage. ASE deletion leads to abnormal muscle morphogenesis induced by oxidative stress, resulting in impaired lacteal extension and defective lymphatic system-dependent lipid transport. Surprisingly, activation of lymphatic system-independent trafficking directs dietary lipids from the gut directly to the liver, causing diet-induced fatty liver disease. Our study reveals the molecular mechanism linking gut lymphatic function to the earliest symmetry-breaking Pitx2 and highlights the important relationship between intestinal lymphangiogenesis and the gut-liver axis.

Gene Therapy: Targeting Cardiomyocyte Proliferation to Repopulate the Ischemic Heart

ABSTRACT

Adult mammalian cardiomyocytes show scarce division ability, which makes the heart ineffective in replacing lost contractile cells after ischemic cardiomyopathy. In the past decades, there have been increasing efforts in the search for novel strategies to regenerate the injured myocardium. Among them, gene therapy is one of the most promising ones, based on recent and emerging studies that support the fact that functional cardiomyocyte regeneration can be accomplished by the stimulation and enhancement of the endogenous ability of these cells to achieve cell division. This capacity can be targeted by stimulating several molecules, such as cell cycle regulators, noncoding RNAs, transcription, and metabolic factors. Therefore, the proposed target, together with the selection of the vector used, administration route, and the experimental animal model used in the development of the therapy would determine the success in the clinical field.

Pitx genes in development and disease

Homeobox genes encode sequence-specific transcription factors (SSTFs) that recognize specific DNA sequences and regulate organogenesis in all eukaryotes. They are essential in specifying spatial and temporal cell identity and as a result, their mutations often cause severe developmental defects. Pitx genes belong to the PRD class of the highly evolutionary conserved homeobox genes in all animals. Vertebrates possess three Pitx paralogs, Pitx1, Pitx2, and Pitx3 while non-vertebrates have only one Pitx gene. The ancient role of regulating left-right (LR) asymmetry is conserved while new functions emerge to afford more complex body plan and functionalities. In mouse, Pitx1 regulates hindlimb tissue patterning and pituitary development. Pitx2 is essential for the development of the oral cavity and abdominal wall while regulates the formation and symmetry of other organs including pituitary, heart, gut, lung among others by controlling growth control genes upon activation of the Wnt/ß-catenin signaling pathway. Pitx3 is essential for lens development and migration and survival of the dopaminergic neurons of the substantia nigra. Pitx gene mutations are linked to various congenital defects and cancers in humans. Pitx gene family has the potential to offer a new approach in regenerative medicine and aid in identifying new drug targets.

Molecular regulation of myocardial proliferation and regeneration

Heart regeneration is a fascinating and complex biological process. Decades of intensive studies have revealed a sophisticated molecular network regulating cardiac regeneration in the zebrafish and neonatal mouse heart. Here, we review both the classical and recent literature on the molecular and cellular mechanisms underlying heart regeneration, with a particular focus on how injury triggers the cell-cycle re-entry of quiescent cardiomyocytes to replenish their massive loss after myocardial infarction or ventricular resection. We highlight several important signaling pathways for cardiomyocyte proliferation and propose a working model of how these injury-induced signals promote cardiomyocyte proliferation. Thus, this concise review provides up-to-date research progresses on heart regeneration for investigators in the field of regeneration biology.

Ex uno, plures-From One Tissue to Many Cells: A Review of Single-Cell Transcriptomics in Cardiovascular Biology

Recent technological advances have revolutionized the study of tissue biology and garnered a greater appreciation for tissue complexity. In order to understand cardiac development, heart tissue homeostasis, and the effects of stress and injury on the cardiovascular system, it is essential to characterize the heart at high cellular resolution. Single-cell profiling provides a more precise definition of tissue composition, cell differentiation trajectories, and intercellular communication, compared to classical bulk approaches. Here, we aim to review how recent single-cell multi-omic studies have changed our understanding of cell dynamics during cardiac development, and in the healthy and diseased adult myocardium.

Recapturing embryonic potential in the adult epicardium: Prospects for cardiac repair

Research into potential targets for cardiac repair encompasses recognition of tissue‐resident cells with intrinsic regenerative properties. The adult vertebrate heart is covered by mesothelium, named the epicardium, which becomes active in response to injury and contributes to repair, albeit suboptimally. Motivation to manipulate the epicardium for treatment of myocardial infarction is deeply rooted in its central role in cardiac formation and vasculogenesis during development. Moreover, the epicardium is vital to cardiac muscle regeneration in lower vertebrate and neonatal mammalian‐injured hearts. In this review, we discuss our current understanding of the biology of the mammalian epicardium in development and injury. Considering present challenges in the field, we further contemplate prospects for reinstating full embryonic potential in the adult epicardium to facilitate cardiac regeneration.

The Gene-Regulatory Footprint of Aging Highlights Conserved Central Regulators

Many genes and pathways have been linked to aging, yet our understanding of underlying molecular mechanisms is still lacking. Here, we measure changes in the transcriptome, histone modifications, and DNA methylome in three metabolic tissues of adult and aged mice. Transcriptome and methylome changes dominate the liver aging footprint, whereas heart and muscle globally increase chromatin accessibility, especially in aging pathways. In mouse and human data from multiple tissues and regulatory layers, age-related transcription factor expression changes and binding site enrichment converge on putative aging modulators, including ZIC1, CXXC1, HMGA1, MECP2, SREBF1, SREBF2, ETS2, ZBTB7A, and ZNF518B. Using Mendelian randomization, we establish possible epidemiological links between expression of some of these transcription factors or their targets, including CXXC1, ZNF518B, and BBC3, and longevity. We conclude that conserved modulators are at the core of the molecular footprint of aging, and variation in tissue-specific expression of some may affect human longevity.

An Important Role for DNMT3A-Mediated DNA Methylation in Cardiomyocyte Metabolism and Contractility

Supplemental Digital Content is available in the text.

Background:

DNA methylation acts as a mechanism of gene transcription regulation. It has recently gained attention as a possible therapeutic target in cardiac hypertrophy and heart failure. However, its exact role in cardiomyocytes remains controversial. Thus, we knocked out the main de novo DNA methyltransferase in cardiomyocytes, DNMT3A, in human induced pluripotent stem cells. Functional consequences of DNA methylation-deficiency under control and stress conditions were then assessed in human engineered heart tissue from knockout human induced pluripotent stem cell–derived cardiomyocytes.

Methods:

DNMT3A was knocked out in human induced pluripotent stem cells by CRISPR/Cas9gene editing. Fibrin-based engineered heart tissue was generated from knockout and control human induced pluripotent stem cell–derived cardiomyocytes. Development and baseline contractility were analyzed by video-optical recording. Engineered heart tissue was subjected to different stress protocols, including serum starvation, serum variation, and restrictive feeding. Molecular, histological, and ultrastructural analyses were performed afterward.

Results:

Knockout of DNMT3A in human cardiomyocytes had three main consequences for cardiomyocyte morphology and function: (1) Gene expression changes of contractile proteins such as higher atrial gene expression and lower MYH7/MYH6 ratio correlated with different contraction kinetics in knockout versus wild-type; (2) Aberrant activation of the glucose/lipid metabolism regulator peroxisome proliferator-activated receptor gamma was associated with accumulation of lipid vacuoles within knockout cardiomyocytes; (3) Hypoxia-inducible factor 1α protein instability was associated with impaired glucose metabolism and lower glycolytic enzyme expression, rendering knockout-engineered heart tissue sensitive to metabolic stress such as serum withdrawal and restrictive feeding.

Conclusion:

The results suggest an important role of DNA methylation in the normal homeostasis of cardiomyocytes and during cardiac stress, which could make it an interesting target for cardiac therapy.

Reduced left atrial cardiomyocyte PITX2 and elevated circulating BMP10 predict atrial fibrillation after ablation

BACKGROUNDGenomic and experimental studies suggest a role for PITX2 in atrial fibrillation (AF). To assess if this association is relevant for recurrent AF in patients, we tested whether left atrial PITX2 affects recurrent AF after AF ablation.METHODSmRNA concentrations of PITX2 and its cardiac isoform, PITX2c, were quantified in left atrial appendages (LAAs) from patients undergoing thoracoscopic AF ablation, either in whole LAA tissue (n = 83) or in LAA cardiomyocytes (n = 52), and combined with clinical parameters to predict AF recurrence. Literature suggests that BMP10 is a PITX2-repressed, atrial-specific, secreted protein. BMP10 plasma concentrations were combined with 11 cardiovascular biomarkers and clinical parameters to predict recurrent AF after catheter ablation in 359 patients.RESULTSReduced concentrations of cardiomyocyte PITX2, but not whole LAA tissue PITX2, were associated with AF recurrence after thoracoscopic AF ablation (16% decreased recurrence per 2-(ΔΔCt) increase in PITX2). RNA sequencing, quantitative PCR, and Western blotting confirmed that BMP10 is one of the most PITX2-repressed atrial genes. Left atrial size (HR per mm increase [95% CI], 1.055 [1.028, 1.082]); nonparoxysmal AF (HR 1.672 [1.206, 2.318]), and elevated BMP10 (HR 1.339 [CI 1.159, 1.546] per quartile increase) were predictive of recurrent AF. BMP10 outperformed 11 other cardiovascular biomarkers in predicting recurrent AF.CONCLUSIONSReduced left atrial cardiomyocyte PITX2 and elevated plasma concentrations of the PITX2-repressed, secreted atrial protein BMP10 identify patients at risk of recurrent AF after ablation.TRIAL REGISTRATIONClinicalTrials.gov NCT01091389, NL50069.018.14, Dutch National Registry of Clinical Research Projects EK494-16.FUNDINGBritish Heart Foundation, European Union (H2020), Leducq Foundation.

The Hippo Pathway Blocks Mammalian Retinal Müller Glial Cell Reprogramming

In response to retinal damage, the Müller glial cells (MGs) of the zebrafish retina have the ability to undergo a cellular reprogramming event in which they enter the cell cycle and divide asymmetrically, thereby producing multipotent retinal progenitors capable of regenerating lost retinal neurons. However, mammalian MGs do not exhibit such a proliferative and regenerative ability. Here, we identify Hippo pathway-mediated repression of the transcription cofactor YAP as a core regulatory mechanism that normally blocks mammalian MG proliferation and cellular reprogramming. MG-specific deletion of Hippo pathway components Lats1 and Lats2, as well as transgenic expression of a Hippo non-responsive form of YAP (YAP5SA), resulted in dramatic Cyclin D1 upregulation, loss of adult MG identity, and attainment of a highly proliferative, progenitor-like cellular state. Our results reveal that mammalian MGs may have latent regenerative capacity that can be stimulated by repressing Hippo signaling.

Rueda et al. identify the Hippo pathway as an endogenous molecular mechanism normally preventing mammalian Müller glial reprogramming to a proliferative, progenitor-like state.

Epigenetic and Transcriptional Networks Underlying Atrial Fibrillation

Genome-wide association studies have uncovered over a 100 genetic loci associated with atrial fibrillation (AF), the most common arrhythmia. Many of the top AF-associated loci harbor key cardiac transcription factors, including PITX2, TBX5, PRRX1, and ZFHX3. Moreover, the vast majority of the AF-associated variants lie within noncoding regions of the genome where causal variants affect gene expression by altering the activity of transcription factors and the epigenetic state of chromatin. In this review, we discuss a transcriptional regulatory network model for AF defined by effector genes in Genome-wide association studies loci. We describe the current state of the field regarding the identification and function of AF-relevant gene regulatory networks, including variant regulatory elements, dose-sensitive transcription factor functionality, target genes, and epigenetic states. We illustrate how altered transcriptional networks may impact cardiomyocyte function and ionic currents that impact AF risk. Last, we identify the need for improved tools to identify and functionally test transcriptional components to define the links between genetic variation, epigenetic gene regulation, and atrial function.

Cell migration by swimming: Drosophila adipocytes as a new in vivo model of adhesion-independent motility

Several cell lineages migrate through the developing and adult tissues of our bodies utilising a variety of modes of motility to suit the different substrates and environments they encounter en route to their destinations. Here we describe a novel adhesion-independent mode of single cell locomotion utilised by Drosophila fat body cells - the equivalent of vertebrate adipocytes. Like their human counterpart, these large cells were previously presumed to be immotile. However, in the Drosophila pupa fat body cells appear to be motile and migrate in a directed way towards wounds by peristaltic swimming through the hemolymph. The propulsive force is generated from a wave of cortical actomyosin that travels rearwards along the length of the cell. We discuss how this swimming mode of motility overcomes the physical constraints of microscopic objects moving in fluids, how fat body cells switch on other "motility machinery" to plug the wound on arrival, and whether other cell lineages in Drosophila and other organisms may, under certain circumstances, also adopt swimming as an effective mode of migration.

Leading progress in heart regeneration and repair

Ischemic heart disease is one of the leading causes of mortality. Myocardial infarction causes loss of cardiomyocytes in the injury area accompanied by formation of a fibrotic scar. This initiates a cascade of events including further loss of myocyte, increased fibrosis, and pathological cardiac hypertrophy, eventually leading to the heart failure. Cardiomyocytes in mammals have limited regenerative potential due to post mitotic nature of cardiomyocytes. Recently, multiple studies have provided substantial insights in to the molecular pathways governing this block in adult cardiomyocyte proliferation, and successfully employed that understanding to achieve cardiac regeneration. These strategies include directly reprograming the cardiomyocytes or manipulating the cardiac interstitium to repair the injured heart. In this review, we discuss the recent advances made in the field in the past two years.

Early sarcomere and metabolic defects in a zebrafish pitx2c cardiac arrhythmia model

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia. The major AF susceptibility locus 4q25 establishes long-range interactions with the promoter of PITX2, a transcription factor gene with critical functions during cardiac development. While many AF-linked loci have been identified in genome-wide association studies, mechanistic understanding into how genetic variants, including those at the 4q25 locus, increase vulnerability to AF is mostly lacking. Here, we show that loss of pitx2c in zebrafish leads to adult cardiac phenotypes with substantial similarities to pathologies observed in AF patients, including arrhythmia, atrial conduction defects, sarcomere disassembly, and altered cardiac metabolism. These phenotypes are also observed in a subset of pitx2c ^+/- fish, mimicking the situation in humans. Most notably, the onset of these phenotypes occurs at an early developmental stage. Detailed analyses of pitx2c loss- and gain-of-function embryonic hearts reveal changes in sarcomeric and metabolic gene expression and function that precede the onset of cardiac arrhythmia first observed at larval stages. We further find that antioxidant treatment of pitx2c ^-/- larvae significantly reduces the incidence and severity of cardiac arrhythmia, suggesting that metabolic dysfunction is an important driver of conduction defects. We propose that these early sarcomere and metabolic defects alter cardiac function and contribute to the electrical instability and structural remodeling observed in adult fish. Overall, these data provide insight into the mechanisms underlying the development and pathophysiology of some cardiac arrhythmias and importantly, increase our understanding of how developmental perturbations can predispose to functional defects in the adult heart.

Long-range Pitx2c enhancer-promoter interactions prevent predisposition to atrial fibrillation

Genome-wide association studies found that increased risk for atrial fibrillation (AF), the most common human heart arrhythmia, is associated with noncoding sequence variants located in proximity to PITX2 Cardiomyocyte-specific epigenomic and comparative genomics uncovered 2 AF-associated enhancers neighboring PITX2 with varying conservation in mice. Chromosome conformation capture experiments in mice revealed that the Pitx2c promoter directly contacted the AF-associated enhancer regions. CRISPR/Cas9-mediated deletion of a 20-kb topologically engaged enhancer led to reduced Pitx2c transcription and AF predisposition. Allele-specific chromatin immunoprecipitation sequencing on hybrid heterozygous enhancer knockout mice revealed that long-range interaction of an AF-associated region with the Pitx2c promoter was required for maintenance of the Pitx2c promoter chromatin state. Long-range looping was mediated by CCCTC-binding factor (CTCF), since genetic disruption of the intronic CTCF-binding site caused reduced Pitx2c expression, AF predisposition, and diminished active chromatin marks on Pitx2 AF risk variants located at 4q25 reside in genomic regions possessing long-range transcriptional regulatory functions directed at PITX2.

Polycystic ovary syndrome and mitochondrial dysfunction

Polycystic ovary syndrome (PCOS) is a prevalent hormonal disorder of premenopausal women worldwide and is characterized by reproductive, endocrine, and metabolic abnormalities. The clinical manifestations of PCOS include oligomenorrhea or amenorrhea, hyperandrogenism, ovarian polycystic changes, and infertility. Women with PCOS are at an increased risk of suffering from type 2 diabetes; me\tabolic syndrome; cardiovascular events, such as hypertension, dyslipidemia; gynecological diseases, including infertility, endometrial dysplasia, endometrial cancer, and ovarian malignant tumors; pregnancy complications, such as premature birth, low birthweight, and eclampsia; and emotional and mental disorders in the future. Although numerous studies have focused on PCOS, the underlying pathophysiological mechanisms of this disease remain unclear. Mitochondria play a key role in energy production, and mitochondrial dysfunction at the cellular level can affect systemic metabolic balance. The recent wide acceptance of functional mitochondrial disorders as a correlated factor of numerous diseases has led to the presupposition that abnormal mitochondrial metabolic markers are associated with PCOS. Studies conducted in the past few years have confirmed that increased oxidative stress is associated with the progression and related complications of PCOS and have proven the relationship between other mitochondrial dysfunctions and PCOS. Thus, this review aims to summarize and discuss previous and recent findings concerning the relationship between mitochondrial dysfunction and PCOS.

PITX2 and NEURL1 SNP polymorphisms in Hungarian atrial fibrillation patients determined by quantitative real-time PCR and melting curve analysis

Atrial fibrillation (AF) is the most common cardiac arrhythmia affecting 1-2% of the general population. Some common variants located in or next to PITX2 and NEURL1 genes are proved to play role in the occurrence of AF. The aim of our study was to investigate whether rs2595104 in the 4q25 chromosome region and rs6584555 SNP in the NEURL1 gene on chromosome 10 is associated with AF in a Caucasian population. We genotyped DNA samples of 76 AF patients and 77 healthy controls using quantitative real-time PCR followed by melting curve analysis. The minor A allele frequency of rs2595104 in PITX2 was 0.38 and 0.44 in the control group and in AF patients, respectively. There was no significant difference in allele and genotype distribution between the two groups (p = 0.52). The allele frequency based log additive odds ratio is 1.22 (C.I. = 0.76-1.94; p = 0.42). The frequency of minor rs6584555 C allele in NEURL1 was 0.22 in the control group and 0.23 in AF patients. Again there were no significant differences in allele and genotype frequencies between AF patients and controls (p = 0.92). The log additive odds ratio is 1,15 (C.I. = 0.66-2.01; p = 0,63). The heterozygous genotype of rs2595104 had the highest frequency compared to the other genotypes in both groups. In case of the rs6584555 SNP the homozygous genotype of the major allele (TT) had the highest frequency in both groups (0.59). The frequency of homozygous genotype for risk allele had the lowest frequency for both SNPs [rs2595104 (AA): 0.19 in patients, 0.12 in controls; rs6584555 (CC): 0.05 in patients, 0.03 in controls]. We did not find significant association between SNP rs2595104 and rs6584555 andAF. We performed a protein-protein network analysis to assess functional connection among the protein products. The proteins coded by PITX2 and NEURL1 are connected indirectly via CTNNB1 and either JAG1 or DLL4 proteins. These interactive proteins are components of two major channels of cell communication pathways, the Wnt and Notch signaling pathways.

A cellular atlas of Pitx2-dependent cardiac development

The Pitx2 gene encodes a homeobox transcription factor that is required for mammalian development. Disruption of PITX2 expression in humans causes congenital heart diseases and is associated with atrial fibrillation; however, the cellular and molecular processes dictated by Pitx2 during cardiac ontogeny remain unclear. To characterize the role of Pitx2 during murine heart development we sequenced over 75,000 single cardiac cell transcriptomes between two key developmental timepoints in control and Pitx2 null embryos. We found that cardiac cell composition was dramatically altered in mutants at both E10.5 and E13.5. Interestingly, the differentiation dynamics of both anterior and posterior second heart field-derived progenitor cells were disrupted in Pitx2 mutants. We also uncovered evidence for defects in left-right asymmetry within atrial cardiomyocyte populations. Furthermore, we were able to detail defects in cardiac outflow tract and valve development associated with Pitx2 Our findings offer insight into Pitx2 function and provide a compilation of gene expression signatures for further detailing the complexities of heart development that will serve as the foundation for future studies of cardiac morphogenesis, congenital heart disease and arrhythmogenesis.

Cardiomyocyte proliferation, a target for cardiac regeneration

Cardiac diseases, characterized by cardiomyocyte loss, lead to dramatic impairment of cardiac function and ultimately to congestive heart failure. Despite significant advances, conventional treatments do not correct the defects in cardiac muscle cell numbers and the prognosis of congestive heart failure remains poor. The existence, in adult mammalian heart, of low but detectable cardiomyocyte proliferative capacities has shifted the target of regenerative therapy toward new therapeutical strategy. Indeed, the stimulation of terminally differentiated cardiomyocyte proliferation represents the main therapeutic approach for heart regeneration. Increasing evidence demonstrating that the loss of mammalian cardiomyocyte renewal potential shortly after birth causes the loss of regenerative capacities, strongly support the hypothesis that a detailed understanding of the molecular mechanisms controlling fetal and postnatal cardiomyocyte proliferation is essential to identify targets for cardiac regeneration. Here, we will review major developmental mechanisms regulating fetal cardiomyocyte proliferation and will describe the impact of the developmental switch, operating at birth and driving postnatal heart maturation, on the regulation of adult cardiomyocyte proliferation, all these mechanisms representing potential targets for cardiac repair and regeneration.

Requirement of Pitx2 for skeletal muscle homeostasis

Skeletal muscle is generated by the successive incorporation of primary (embryonic), secondary (fetal), and tertiary (adult) fibers into muscle. Conditional excision of Pitx2 function by an MCKCre driver resulted in animals with histological and ultrastructural defects in P30 muscles and fibers, respectively. Mutant muscle showed severe reduction in mitochondria and FoxO3-mediated mitophagy. Both oxidative and glycolytic energy metabolism were reduced. Conditional excision was limited to fetal muscle fibers after the G1-G0 transition and resulted in altered MHC, Rac1, MEF2a, and alpha-tubulin expression within these fibers. The onset of excision, monitored by a nuclear reporter gene, was observed as early as E16. Muscle at this stage was already severely malformed, but appeared to recover by P30 by the expansion of adjoining larger fibers. Our studies demonstrate that the homeodomain transcription factor Pitx2 has a postmitotic role in maintaining skeletal muscle integrity and energy homeostasis in fetal muscle fibers.