Complete cardiac regeneration in a mouse model of myocardial infarction

Developmental stage dependent effects of posterior and germline regeneration on sexual maturation in Platynereis dumerilii

Regeneration, regrowing lost and injured body parts, is an ability that generally declines with age or developmental transitions (i.e. metamorphosis, sexual maturation). Regeneration is also an energetically costly process, and trade-offs occur between regeneration and other costly processes such as growth, or sexual reproduction. Here we investigate the interplay of regeneration, reproduction, and developmental stage in the segmented worm Platynereis dumerilii. P. dumerilii can regenerate its whole posterior body axis, along with its reproductive cells, thereby having to carry out the two costly processes (somatic and germ cell regeneration) after injury. We specifically examine how developmental stage affects the success of germ cell regeneration and sexual maturation in developmentally young versus developmentally old organisms. We hypothesized that developmentally younger individuals (i.e. with gametes in early mitotic stages) will have higher regeneration success than the individuals at developmentally older stages (i.e. with gametes undergoing meiosis and maturation). Surprisingly, older amputated worms grew faster and matured earlier than younger amputees. To analyze germ cell regeneration during and after posterior regeneration, we used Hybridization Chain Reaction for the germline marker vasa. We found that regenerated worms start repopulating new segments with germ cell clusters as early as 14 days post amputation. In addition, vasa expression is observed in a wide region of newly-regenerated segments, which appears different from expression patterns during normal growth or regeneration in worms before gonial cluster expansion.

Effects and mechanisms of the myocardial microenvironment on cardiomyocyte proliferation and regeneration

The adult mammalian cardiomyocyte has a limited capacity for self-renewal, which leads to the irreversible heart dysfunction and poses a significant threat to myocardial infarction patients. In the past decades, research efforts have been predominantly concentrated on the cardiomyocyte proliferation and heart regeneration. However, the heart is a complex organ that comprises not only cardiomyocytes but also numerous noncardiomyocyte cells, all playing integral roles in maintaining cardiac function. In addition, cardiomyocytes are exposed to a dynamically changing physical environment that includes oxygen saturation and mechanical forces. Recently, a growing number of studies on myocardial microenvironment in cardiomyocyte proliferation and heart regeneration is ongoing. In this review, we provide an overview of recent advances in myocardial microenvironment, which plays an important role in cardiomyocyte proliferation and heart regeneration.

Targeting cardiomyocyte cell cycle regulation in heart failure

Heart failure continues to be a significant global health concern, causing substantial morbidity and mortality. The limited ability of the adult heart to regenerate has posed challenges in finding effective treatments for cardiac pathologies. While various medications and surgical interventions have been used to improve cardiac function, they are not able to address the extensive loss of functioning cardiomyocytes that occurs during cardiac injury. As a result, there is growing interest in understanding how the cell cycle is regulated and exploring the potential for stimulating cardiomyocyte proliferation as a means of promoting heart regeneration. This review aims to provide an overview of current knowledge on cell cycle regulation and mechanisms underlying cardiomyocyte proliferation in cases of heart failure, while also highlighting established and novel therapeutic strategies targeting this area for treatment purposes.

BMP7 promotes cardiomyocyte regeneration in zebrafish and adult mice

Zebrafish have a lifelong cardiac regenerative ability after damage, whereas mammals lose this capacity during early postnatal development. This study investigated whether the declining expression of growth factors during postnatal mammalian development contributes to the decrease of cardiomyocyte regenerative potential. Besides confirming the proliferative ability of neuregulin 1 (NRG1), interleukin (IL)1b, receptor activator of nuclear factor kappa-Β ligand (RANKL), insulin growth factor (IGF)2, and IL6, we identified other potential pro-regenerative factors, with BMP7 exhibiting the most pronounced efficacy. Bmp7 knockdown in neonatal mouse cardiomyocytes and loss-of-function in adult zebrafish during cardiac regeneration reduced cardiomyocyte proliferation, indicating that Bmp7 is crucial in the regenerative stages of mouse and zebrafish hearts. Conversely, bmp7 overexpression in regenerating zebrafish or administration at post-mitotic juvenile and adult mouse stages, in vitro and in vivo following myocardial infarction, enhanced cardiomyocyte cycling. Mechanistically, BMP7 stimulated proliferation through BMPR1A/ACVR1 and ACVR2A/BMPR2 receptors and downstream SMAD5, ERK, and AKT signaling. Overall, BMP7 administration is a promising strategy for heart regeneration.

Leveraging chromatin state transitions for the identification of regulatory networks orchestrating heart regeneration

The limited regenerative capacity of the human heart contributes to high morbidity and mortality worldwide. In contrast, zebrafish exhibit robust regenerative capacity, providing a powerful model for studying how to overcome intrinsic epigenetic barriers maintaining cardiac homeostasis and initiate regeneration. Here, we present a comprehensive analysis of the histone modifications H3K4me1, H3K4me3, H3K27me3 and H3K27ac during various stages of zebrafish heart regeneration. We found a vast gain of repressive chromatin marks one day after myocardial injury, followed by the acquisition of active chromatin characteristics on day four and a transition to a repressive state on day 14, and identified distinct transcription factor ensembles associated with these events. The rapid transcriptional response involves the engagement of super-enhancers at genes implicated in extracellular matrix reorganization and TOR signaling, while H3K4me3 breadth highly correlates with transcriptional activity and dynamic changes at genes involved in proteolysis, cell cycle activity, and cell differentiation. Using loss- and gain-of-function approaches, we identified transcription factors in cardiomyocytes and endothelial cells influencing cardiomyocyte dedifferentiation or proliferation. Finally, we detected significant evolutionary conservation between regulatory regions that drive zebrafish and neonatal mouse heart regeneration, suggesting that reactivating transcriptional and epigenetic networks converging on these regulatory elements might unlock the regenerative potential of adult human hearts.

Harnessing developmental dynamics of spinal cord extracellular matrix improves regenerative potential of spinal cord organoids

Neonatal spinal cord tissues exhibit remarkable regenerative capabilities as compared to adult spinal cord tissues after injury, but the role of extracellular matrix (ECM) in this process has remained elusive. Here, we found that early developmental spinal cord had higher levels of ECM proteins associated with neural development and axon growth, but fewer inhibitory proteoglycans, compared to those of adult spinal cord. Decellularized spinal cord ECM from neonatal (DNSCM) and adult (DASCM) rabbits preserved these differences. DNSCM promoted proliferation, migration, and neuronal differentiation of neural progenitor cells (NPCs) and facilitated axonal outgrowth and regeneration of spinal cord organoids more effectively than DASCM. Pleiotrophin (PTN) and Tenascin (TNC) in DNSCM were identified as contributors to these abilities. Furthermore, DNSCM demonstrated superior performance as a delivery vehicle for NPCs and organoids in spinal cord injury (SCI) models. This suggests that ECM cues from early development stages might significantly contribute to the prominent regeneration ability in spinal cord.

Structural, angiogenic, and immune responses influencing myocardial regeneration: a glimpse into the crucible

Complete cardiac regeneration remains an elusive therapeutic goal. Although much attention has been focused on cardiomyocyte proliferation, especially in neonatal mammals, recent investigations have unearthed mechanisms by which non-cardiomyocytes, such as endothelial cells, fibroblasts, macrophages, and other immune cells, play critical roles in modulating the regenerative capacity of the injured heart. The degree to which each of these cell types influence cardiac regeneration, however, remains incompletely understood. This review highlights the roles of these non-cardiomyocytes and their respective contributions to cardiac regeneration, with emphasis on natural heart regeneration after cardiac injury during the neonatal period.

Regeneration in Mice of Injured Skin, Heart, and Spinal Cord by α-Gal Nanoparticles Recapitulates Regeneration in Amphibians

The healing of skin wounds, myocardial, and spinal cord injuries in salamander, newt, and axolotl amphibians, and in mouse neonates, results in scar-free regeneration, whereas injuries in adult mice heal by fibrosis and scar formation. Although both types of healing are mediated by macrophages, regeneration in these amphibians and in mouse neonates also involves innate activation of the complement system. These differences suggest that localized complement activation in adult mouse injuries might induce regeneration instead of the default fibrosis and scar formation. Localized complement activation is feasible by antigen/antibody interaction between biodegradable nanoparticles presenting α-gal epitopes (α-gal nanoparticles) and the natural anti-Gal antibody which is abundant in humans. Administration of α-gal nanoparticles into injuries of anti-Gal-producing adult mice results in localized complement activation which induces rapid and extensive macrophage recruitment. These macrophages bind anti-Gal-coated α-gal nanoparticles and polarize into M2 pro-regenerative macrophages that orchestrate accelerated scar-free regeneration of skin wounds and regeneration of myocardium injured by myocardial infarction (MI). Furthermore, injection of α-gal nanoparticles into spinal cord injuries of anti-Gal-producing adult mice induces recruitment of M2 macrophages, that mediate extensive angiogenesis and axonal sprouting, which reconnects between proximal and distal severed axons. Thus, α-gal nanoparticle treatment in adult mice mimics physiologic regeneration in amphibians. These studies further suggest that α-gal nanoparticles may be of significance in the treatment of human injuries.

A Multimodal Omics Framework to Empower Target Discovery for Cardiovascular Regeneration

Ischaemic heart disease is a global healthcare challenge with high morbidity and mortality. Early revascularisation in acute myocardial infarction has improved survival; however, limited regenerative capacity and microvascular dysfunction often lead to impaired function and the development of heart failure. New mechanistic insights are required to identify robust targets for the development of novel strategies to promote regeneration. Single-cell RNA sequencing (scRNA-seq) has enabled profiling and analysis of the transcriptomes of individual cells at high resolution. Applications of scRNA-seq have generated single-cell atlases for multiple species, revealed distinct cellular compositions for different regions of the heart, and defined multiple mechanisms involved in myocardial injury-induced regeneration. In this review, we summarise findings from studies of healthy and injured hearts in multiple species and spanning different developmental stages. Based on this transformative technology, we propose a multi-species, multi-omics, meta-analysis framework to drive the discovery of new targets to promote cardiovascular regeneration.

Characterization of cardiac fibroblast-extracellular matrix crosstalk across developmental ages provides insight into age-related changes in cardiac repair

Heart failure afflicts an estimated 6.5 million people in the United States, driven largely by incidents of coronary heart disease (CHD). CHD leads to heart failure due to the inability of adult myocardial tissue to regenerate after myocardial infarction (MI). Instead, immune cells and resident cardiac fibroblasts (CFs), the cells responsible for the maintenance of the cardiac extracellular matrix (cECM), drive an inflammatory wound healing response, which leads to fibrotic scar tissue. However, fibrosis is reduced in fetal and early (<1-week-old) neonatal mammals, which exhibit a transient capability for regenerative tissue remodeling. Recent work by our laboratory and others suggests this is in part due to compositional differences in the cECM and functional differences in CFs with respect to developmental age. Specifically, fetal cECM and CFs appear to mitigate functional loss in MI models and engineered cardiac tissues, compared to adult CFs and cECM. We conducted 2D studies of CFs on solubilized fetal and adult cECM to investigate whether these age-specific functional differences are synergistic with respect to their impact on CF phenotype and, therefore, cardiac wound healing. We found that the CF migration rate and stiffness vary with respect to cell and cECM developmental age and that CF transition to a fibrotic phenotype can be partially attenuated in the fetal cECM. However, this effect was not observed when cells were treated with cytokine TGF-β1, suggesting that inflammatory signaling factors are the dominant driver of the fibroblast phenotype. This information may be valuable for targeted therapies aimed at modifying the CF wound healing response and is broadly applicable to age-related studies of cardiac remodeling.

Right ventricular cardiomyocyte expansion accompanies cardiac regeneration in newborn mice after large left ventricular infarcts

Cauterization of the root of the left coronary artery (LCA) in the neonatal heart on postnatal day 1 (P1) resulted in large, reproducible lesions of the left ventricle (LV), and an attendant marked adaptive response in the right ventricle (RV). The response of both chambers to LV myocardial infarction involved enhanced cardiomyocyte (CM) division and binucleation, as well as LV revascularization, leading to restored heart function within 7 days post surgery (7 dps). By contrast, infarction of P3 mice resulted in cardiac scarring without a significant regenerative and adaptive response of the LV and the RV, leading to subsequent heart failure and death within 7 dps. The prominent RV myocyte expansion in P1 mice involved an acute increase in pulmonary arterial pressure and a unique gene regulatory response, leading to an increase in RV mass and preserved heart function. Thus, distinct adaptive mechanisms in the RV, such as CM proliferation and RV expansion, enable marked cardiac regeneration of the infarcted LV at P1 and full functional recovery.

Animal models to study cardiac regeneration

Permanent fibrosis and chronic deterioration of heart function in patients after myocardial infarction present a major health-care burden worldwide. In contrast to the restricted potential for cellular and functional regeneration of the adult mammalian heart, a robust capacity for cardiac regeneration is seen during the neonatal period in mammals as well as in the adults of many fish and amphibian species. However, we lack a complete understanding as to why cardiac regeneration takes place more efficiently in some species than in others. The capacity of the heart to regenerate after injury is controlled by a complex network of cellular and molecular mechanisms that form a regulatory landscape, either permitting or restricting regeneration. In this Review, we provide an overview of the diverse array of vertebrates that have been studied for their cardiac regenerative potential and discuss differential heart regeneration outcomes in closely related species. Additionally, we summarize current knowledge about the core mechanisms that regulate cardiac regeneration across vertebrate species.

The cost and payout of age on germline regeneration and sexual maturation in Platynereis dumerilii

Regeneration, regrowing lost and injured body parts, is an ability that generally declines with age or developmental transitions (i.e. metamorphosis, sexual maturation) in many organisms. Regeneration is also energetically a costly process, and trade-offs occur between regeneration and other costly processes such as somatic growth, or sexual reproduction. Here we investigate the interplay of regeneration, reproduction, and age in the segmented worm Platynereis dumerilii. P. dumerilii can regenerate its whole posterior body axis, along with its reproductive cells, thereby having to carry out the two costly processes (somatic and germ cell regeneration) after injury. We specifically examine how age affects the success of germ cell regeneration and sexual maturation in developmentally young versus old organisms. We hypothesized that developmentally younger individuals (i.e. lower investment state, with gametes in early mitotic stages) will have higher regeneration success and reach sexual maturation faster than the individuals at developmentally older stages (i.e. higher investment state, with gametes in the process of maturation). Surprisingly, older amputated worms grew faster and matured earlier than younger amputees, even though they had to regenerate more segments and recuperate the more costly germ cells which were already starting to undergo gametogenesis. To analyze germ cell regeneration across stages, we used Hybridization Chain Reaction for the germline marker vasa. We found that regenerated worms start repopulating new segments with germ cell clusters as early as 14 days post amputation. In addition, vasa expression is observed in a wide region of newly-regenerated segments, which appears different from expression patterns during normal growth or regeneration in worms before gonial cluster expansion. Future studies will focus on determining the exact sources of gonial clusters in regeneration.

Cannabidiol represses miR-143 to promote cardiomyocyte proliferation and heart regeneration after myocardial infarction

Mammalian heart is capable to regenerate almost completely early after birth through endogenous cardiomyocyte proliferation. However, this regenerative capacity diminishes gradually with growth and is nearly lost in adulthood. Cannabidiol (CBD) is a major component of cannabis and has various biological activities to regulate oxidative stress, fibrosis, inflammation, and cell death. The present study was conducted to investigate the pharmacological effects of CBD on heart regeneration in post-MI mice. MI models in adult mice were constructed via coronary artery ligation, which were administrated with or without CBD. Our results demonstrate that systemic administration (10 mg/kg) of CBD markedly increased cardiac regenerative ability, reduced infarct size, and restored cardiac function in MI mice. Consistently, in vitro study also showed that CBD was able to promote the proliferation of neonatal cardiomyocytes. Mechanistically, the expression of miR-143-3p related to cardiomyocyte proliferation was significantly down-regulated in CBD-treated cardiomyocytes, while the overexpression of miR-143-3p inhibited cardiomyocyte mitosis and eliminated CBD-induced cardiomyocyte proliferation. Moreover, CBD enhanced the expression of Yap and Ctnnd1, which were demonstrated as the target genes of miR-143-3p. Silencing of Yap and Ctnnd1 hindered the proliferative effects of CBD. We further revealed that inhibition of the cannabinoid receptor 2 impeded the regulatory effect of CBD on miR-143-3p and its downstream target Yap/Ctnnd1, which ultimately eliminated the pro-proliferative effect of CBD on neonatal and adult cardiomyocytes. Taken together, CBD promotes cardiomyocyte proliferation and heart regeneration after MI via miR-143-3p/Yap/Ctnnd1 signaling pathway, which provides a new strategy for cardiac repair in adult myocardium.

Two decades of heart regeneration research: Cardiomyocyte proliferation and beyond

Interest in vertebrate cardiac regeneration has exploded over the past two decades since the discovery that adult zebrafish are capable of complete heart regeneration, contrasting the limited regenerative potential typically observed in adult mammalian hearts. Undercovering the mechanisms that both support and limit cardiac regeneration across the animal kingdom may provide unique insights in how we may unlock this capacity in adult humans. In this review, we discuss key discoveries in the heart regeneration field over the last 20 years. Initially, seminal findings revealed that pre-existing cardiomyocytes are the major source of regenerated cardiac muscle, drawing interest into the intrinsic mechanisms regulating cardiomyocyte proliferation. Moreover, recent studies have identified the importance of intercellular interactions and physiological adaptations, which highlight the vast complexity of the cardiac regenerative process. Finally, we compare strategies that have been tested to increase the regenerative capacity of the adult mammalian heart. This article is categorized under: Cardiovascular Diseases > Stem Cells and Development.

Mechanisms regulating vascular and lymphatic regeneration in the heart after myocardial infarction

Myocardial infarction, caused by a thrombus or coronary vascular occlusion, leads to irreversible ischaemic injury. Advances in early reperfusion strategies have significantly reduced short-term mortality after myocardial infarction. However, survivors have an increased risk of developing heart failure, which confers a high risk of death at 1 year. The capacity of the injured neonatal mammalian heart to regenerate has stimulated extensive research into whether recapitulation of developmental regeneration programmes may be beneficial in adult cardiovascular disease. Restoration of functional blood and lymphatic vascular networks in the infarct and border regions via neovascularisation and lymphangiogenesis, respectively, is a key requirement to facilitate myocardial regeneration. An improved understanding of the endogenous mechanisms regulating coronary vascular and lymphatic expansion and function in development and in adult patients after myocardial infarction may inform future therapeutic strategies and improve translation from pre-clinical studies. In this review, we explore the underpinning research and key findings in the field of cardiovascular regeneration, with a focus on neovascularisation and lymphangiogenesis, and discuss the outcomes of therapeutic strategies employed to date. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Integrated metabolic and epigenetic mechanisms in cardiomyocyte proliferation

Heart disease continues to be the leading cause of mortality worldwide, primarily attributed to the restricted regenerative potential of the adult human heart following injury. In contrast to their adult counterparts, many neonatal mammals can spontaneously regenerate their myocardium in the first few days of life via extensive proliferation of the pre-existing cardiomyocytes. Reasons for the decline in regenerative capacity during postnatal development, and how to control it, remain largely unexplored. Accumulated evidence suggests that the preservation of regenerative potential depends on a conducive metabolic state in the embryonic and neonatal heart. Along with the postnatal increase in oxygenation and workload, the mammalian heart undergoes a metabolic transition, shifting its primary metabolic substrate from glucose to fatty acids shortly after birth for energy advantage. This metabolic switch causes cardiomyocyte cell-cycle arrest, which is widely regarded as a key mechanism for the loss of regenerative capacity. Beyond energy provision, emerging studies have suggested a link between this intracellular metabolism dynamics and postnatal epigenetic remodeling of the mammalian heart that reshapes the expression of many genes important for cardiomyocyte proliferation and cardiac regeneration, since many epigenetic enzymes utilize kinds of metabolites as obligate cofactors or substrates. This review summarizes the current state of knowledge of metabolism and metabolite-mediated epigenetic modifications in cardiomyocyte proliferation, with a particular focus on highlighting the potential therapeutic targets that hold promise to treat human heart failure via metabolic and epigenetic regulations.

Comparative Analysis of Heart Regeneration: Searching for the Key to Heal the Heart-Part I: Experimental Injury Models to Study Cardiac Regeneration

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

Regeneration of the heart: from molecular mechanisms to clinical therapeutics

Heart injury such as myocardial infarction leads to cardiomyocyte loss, fibrotic tissue deposition, and scar formation. These changes reduce cardiac contractility, resulting in heart failure, which causes a huge public health burden. Military personnel, compared with civilians, is exposed to more stress, a risk factor for heart diseases, making cardiovascular health management and treatment innovation an important topic for military medicine. So far, medical intervention can slow down cardiovascular disease progression, but not yet induce heart regeneration. In the past decades, studies have focused on mechanisms underlying the regenerative capability of the heart and applicable approaches to reverse heart injury. Insights have emerged from studies in animal models and early clinical trials. Clinical interventions show the potential to reduce scar formation and enhance cardiomyocyte proliferation that counteracts the pathogenesis of heart disease. In this review, we discuss the signaling events controlling the regeneration of heart tissue and summarize current therapeutic approaches to promote heart regeneration after injury.

Cardiomyocyte-fibroblast crosstalk in the postnatal heart

During the postnatal period in mammals, the heart undergoes significant remodeling in response to increased circulatory demands. In the days after birth, cardiac cells, including cardiomyocytes and fibroblasts, progressively lose embryonic characteristics concomitant with the loss of the heart’s ability to regenerate. Moreover, postnatal cardiomyocytes undergo binucleation and cell cycle arrest with induction of hypertrophic growth, while cardiac fibroblasts proliferate and produce extracellular matrix (ECM) that transitions from components that support cellular maturation to production of the mature fibrous skeleton of the heart. Recent studies have implicated interactions of cardiac fibroblasts and cardiomyocytes within the maturing ECM environment to promote heart maturation in the postnatal period. Here, we review the relationships of different cardiac cell types and the ECM as the heart undergoes both structural and functional changes during development. Recent advances in the field, particularly in several recently published transcriptomic datasets, have highlighted specific signaling mechanisms that underlie cellular maturation and demonstrated the biomechanical interdependence of cardiac fibroblast and cardiomyocyte maturation. There is increasing evidence that postnatal heart development in mammals is dependent on particular ECM components and that resulting changes in biomechanics influence cell maturation. These advances, in definition of cardiac fibroblast heterogeneity and function in relation to cardiomyocyte maturation and the extracellular environment provide, support for complex cell crosstalk in the postnatal heart with implications for heart regeneration and disease mechanisms.

A change of heart: understanding the mechanisms regulating cardiac proliferation and metabolism before and after birth

Mammalian cardiomyocytes undergo major maturational changes in preparation for birth and postnatal life. Immature cardiomyocytes contribute to cardiac growth via proliferation and thus the heart has the capacity to regenerate. To prepare for postnatal life, structural and metabolic changes associated with increased cardiac output and function must occur. This includes exit from the cell cycle, hypertrophic growth, mitochondrial maturation and sarcomeric protein isoform switching. However, these changes come at a price: the loss of cardiac regenerative capacity such that damage to the heart in postnatal life is permanent. This is a significant barrier to the development of new treatments for cardiac repair and contributes to heart failure. The transitional period of cardiomyocyte growth is a complex and multifaceted event. In this review, we focus on studies that have investigated this critical transition period as well as novel factors that may regulate and drive this process. We also discuss the potential use of new biomarkers for the detection of myocardial infarction and, in the broader sense, cardiovascular disease.

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.

Cardiac regeneration: Options for repairing the injured heart

Cardiac regeneration is one of the grand challenges in repairing injured human hearts. Numerous studies of signaling pathways and metabolism on cardiac development and disease pave the way for endogenous cardiomyocyte regeneration. New drug delivery approaches, high-throughput screening, as well as novel therapeutic compounds combined with gene editing will facilitate the development of potential cell-free therapeutics. In parallel, progress has been made in the field of cell-based therapies. Transplantation of human pluripotent stem cell (hPSC)-derived cardiomyocytes (hPSC-CMs) can partially rescue the myocardial defects caused by cardiomyocyte loss in large animals. In this review, we summarize current cell-based and cell-free regenerative therapies, discuss the importance of cardiomyocyte maturation in cardiac regenerative medicine, and envision new ways of regeneration for the injured heart.

How can the adult zebrafish and neonatal mice teach us about stimulating cardiac regeneration in the human heart?

The proliferative capacity of mammalian cardiomyocytes diminishes shortly after birth. In contrast, adult zebrafish and neonatal mice can regenerate cardiac tissues, highlighting new potential therapeutic avenues. Different factors have been found to promote cardiomyocyte proliferation in zebrafish and neonatal mice; these include maintenance of mononuclear and diploid cardiomyocytes and upregulation of the proto-oncogene c-Myc. The growth factor NRG-1 controls cell proliferation and interacts with the Hippo-Yap pathway to modulate regeneration. Key components of the extracellular matrix such as Agrin are also crucial for cardiac regeneration. Novel therapies explored in this review, include intramyocardial injection of Agrin or zebrafish-ECM and NRG-1 administration. These therapies may induce regeneration in patients and should be further explored.

Metabolic reprogramming and membrane glycan remodeling as potential drivers of zebrafish heart regeneration

The ability of the zebrafish heart to regenerate following injury makes it a valuable model to deduce why this capability in mammals is limited to early neonatal stages. Although metabolic reprogramming and glycosylation remodeling have emerged as key aspects in many biological processes, how they may trigger a cardiac regenerative response in zebrafish is still a crucial question. Here, by using an up-to-date panel of transcriptomic, proteomic and glycomic approaches, we identify a metabolic switch from mitochondrial oxidative phosphorylation to glycolysis associated with membrane glycosylation remodeling during heart regeneration. Importantly, we establish the N- and O-linked glycan structural repertoire of the regenerating zebrafish heart, and link alterations in both sialylation and high mannose structures across the phases of regeneration. Our results show that metabolic reprogramming and glycan structural remodeling are potential drivers of tissue regeneration after cardiac injury, providing the biological rationale to develop novel therapeutics to elicit heart regeneration in mammals.

Modified mRNA Therapeutics for Heart Diseases

Cardiovascular diseases (CVD) remain a substantial global health problem and the leading cause of death worldwide. Although many conventional small-molecule treatments are available to support the cardiac function of the patient with CVD, they are not effective as a cure. Among potential targets for gene therapy are severe cardiac and peripheral ischemia, heart failure, vein graft failure, and some forms of dyslipidemias. In the last three decades, multiple gene therapy tools have been used for heart diseases caused by proteins, plasmids, adenovirus, and adeno-associated viruses (AAV), but these remain as unmet clinical needs. These gene therapy methods are ineffective due to poor and uncontrolled gene expression, low stability, immunogenicity, and transfection efficiency. The synthetic modified mRNA (modRNA) presents a novel gene therapy approach which provides a transient, stable, safe, non-immunogenic, controlled mRNA delivery to the heart tissue without any risk of genomic integration, and achieves a therapeutic effect in different organs, including the heart. The mRNA translation starts in minutes, and remains stable for 8-10 days (pulse-like kinetics). The pulse-like expression of modRNA in the heart induces cardiac repair, cardiomyocyte proliferation and survival, and inhibits cardiomyocyte apoptosis post-myocardial infarction (MI). Cell-specific (cardiomyocyte) modRNA translation developments established cell-specific modRNA therapeutics for heart diseases. With these laudable characteristics, combined with its expression kinetics in the heart, modRNA has become an attractive therapeutic for the treatment of CVD. This review discusses new developments in modRNA therapy for heart diseases.

Construction of a Band-Aid Like Cardiac Patch for Myocardial Infarction with Controllable H2 S Release

Excessive or persistent inflammation incites cardiomyocytes necrosis by generating reactive oxygen species in myocardial infarction (MI). Hydrogen sulfide (H₂ S), a gaseous signal molecule, can quickly permeate cells and tissues, growing concerned for its cardioprotective effects. However, short resident time and strong side effects greatly restrict its application. Herein, a complex scaffold (AAB) is first developed to slowly release H₂ S for myocardial protection by integrating alginate modified with 2-aminopyridine-5-thiocarboxamide (H₂ S donor) into albumin electrospun fibers. Next, a band-aid like patch is constructed based on AAB (center) and nanocomposite scaffold which comprises albumin scaffold and black phosphorus nanosheets (BPNSs). With near-infrared laser (808 nm), thermal energy generated by BPNSs can locally change the molecular structure of fibrous scaffold, thereby attaching patch to the myocardium. In this study, it is also demonstrated that AAB can enhance regenerative M2 macrophage and attenuate inflammatory polarization of macrophages via reduction in intracellular ROS. Eventually, this engineered cardiac patch can relieve inflammation and promote angiogenesis after MI, and thereby recover heart function, providing a promising therapeutic strategy for MI treatment.

Kynurenine promotes neonatal heart regeneration by stimulating cardiomyocyte proliferation and cardiac angiogenesis

Indoleamine 2,3 dioxygenase-1 (IDO1) catalyzes tryptophan-kynurenine metabolism in many inflammatory and cancer diseases. Of note, acute inflammation that occurs immediately after heart injury is essential for neonatal cardiomyocyte proliferation and heart regeneration. However, the IDO1-catalyzed tryptophan metabolism during heart regeneration is largely unexplored. Here, we find that apical neonatal mouse heart resection surgery led to rapid and consistent increases in cardiac IDO1 expression and kynurenine accumulation. Cardiac deletion of Ido1 gene or chemical inhibition of IDO1 impairs heart regeneration. Mechanistically, elevated kynurenine triggers cardiomyocyte proliferation by activating the cytoplasmic aryl hydrocarbon receptor-SRC-YAP/ERK pathway. In addition, cardiomyocyte-derived kynurenine transports to endothelial cells and stimulates cardiac angiogenesis by promoting aryl hydrocarbon receptor nuclear translocation and enhancing vascular endothelial growth factor A expression. Notably, Ahr deletion prevents indoleamine 2,3 dioxygenase -kynurenine-associated heart regeneration. In summary, increasing indoleamine 2,3 dioxygenase-derived kynurenine level promotes cardiac regeneration by functioning as an endogenous regulator of cardiomyocyte proliferation and cardiac angiogenesis.

α-Gal Nanoparticles Mediated Homing of Endogenous Stem Cells for Repair and Regeneration of External and Internal Injuries by Localized Complement Activation and Macrophage Recruitment

This review discusses a novel experimental approach for the regeneration of original tissue structure by recruitment of endogenous stem-cells to injured sites following administration of α-gal nanoparticles, which harness the natural anti-Gal antibody. Anti-Gal is produced in large amounts in all humans, and it binds the multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) presented on α-gal nanoparticles. In situ binding of anti-Gal to α-gal nanoparticles activates the complement system and generates complement cleavage chemotactic-peptides that rapidly recruit macrophages. Macrophages reaching anti-Gal coated α-gal nanoparticles bind them via Fc/Fc receptor interaction and polarize into M2 pro-reparative macrophages. These macrophages secrete various cytokines that orchestrate regeneration of the injured tissue, including VEGF inducing neo-vascularization and cytokines directing homing of stem-cells to injury sites. Homing of stem-cells is also directed by interaction of complement cleavage peptides with their corresponding receptors on the stem-cells. Application of α-gal nanoparticles to skin wounds of anti-Gal producing mice results in decrease in healing time by half. Furthermore, α-gal nanoparticles treated wounds restore the normal structure of the injured skin without fibrosis or scar formation. Similarly, in a mouse model of occlusion/reperfusion myocardial-infarction, near complete regeneration after intramyocardial injection of α-gal nanoparticles was demonstrated, whereas hearts injected with saline display ~20% fibrosis and scar formation of the left ventricular wall. It is suggested that recruitment of stem-cells following anti-Gal/α-gal nanoparticles interaction in injured tissues may result in induction of localized regeneration facilitated by conducive microenvironments generated by pro-reparative macrophage secretions and “cues” provided by the extracellular matrix in the injury site.

Adult T-cells impair neonatal cardiac regeneration

AIMS

Newborn mice and humans display transient cardiac regenerative potential that rapidly declines postnatally. Patients who survive a myocardial infarction (MI) often develop chronic heart failure due to the heart's poor regeneration capacity. We hypothesized that the cardiac 'regenerative-to-scarring' transition might be driven by the perinatal shifts observed in the circulating T-cell compartment.

METHODS AND RESULTS

Post-MI immune responses were characterized in 1- (P1) vs. 7-day-old (P7) mice subjected to left anterior descending artery ligation. Myocardial infarction induced robust early inflammatory responses (36 h post-MI) in both age groups, but neonatal hearts exhibited rapid resolution of inflammation and full functional recovery. The perinatal loss of myocardial regenerative capacity was paralleled by a baseline increase in αβ-T cell (CD4+ and CD8+) numbers. Strikingly, P1-infarcted mice reconstituted with adult T-cells shifted to an adult-like healing phenotype, marked by irreversible cardiac functional impairment and increased fibrosis. Infarcted neonatal mice harbouring adult T-cells also had more monocyte-derived macrophage recruitment, as typically seen in adults. At the transcriptome level, infarcted P1 hearts that received isolated adult T-cells showed enriched gene sets linked to fibrosis, inflammation, and interferon-gamma (IFN-γ) signalling. In contrast, newborn mice that received isolated Ifng-/- adult T-cells prior to MI displayed a regenerative phenotype that resembled that of its age-matched untreated controls.

CONCLUSION

Physiological T-cell development or adoptive transfer of adult IFN-γ-producing T-cells into neonates contributed to impaired cardiac regeneration and promoted irreversible structural and functional cardiac damage. These findings reveal a trade-off between myocardial regenerative potential and the development of T-cell competence.

Myocardial-Treg Crosstalk: How to Tame a Wolf

The immune system plays a vital role in maintaining tissue integrity and organismal homeostasis. The sudden stress caused by myocardial infarction (MI) poses a significant challenge for the immune system: it must quickly substitute dead myocardial with fibrotic tissue while controlling overt inflammatory responses. In this review, we will discuss the central role of myocardial regulatory T-cells (Tregs) in orchestrating tissue repair processes and controlling local inflammation in the context of MI. We herein compile recent advances enabled by the use of transgenic mouse models with defined cardiac antigen specificity, explore whole-heart imaging techniques, outline clinical studies and summarize deep-phenotyping conducted by independent labs using single-cell transcriptomics and T-cell repertoire analysis. Furthermore, we point to multiple mechanisms and cell types targeted by Tregs in the infarcted heart, ranging from pro-fibrotic responses in mesenchymal cells to local immune modulation in myeloid and lymphoid lineages. We also discuss how both cardiac-specific and polyclonal Tregs participate in MI repair. In addition, we consider intriguing novel evidence on how the myocardial milieu takes control of potentially auto-aggressive local immune reactions by shaping myosin-specific T-cell development towards a regulatory phenotype. Finally, we examine the potential use of Treg manipulating drugs in the clinic after MI.

The regenerative capacity of neonatal tissues

It is well established that humans and other mammals are minimally regenerative compared with organisms such as zebrafish, salamander or amphibians. In recent years, however, the identification of regenerative potential in neonatal mouse tissues that normally heal poorly in adults has transformed our understanding of regenerative capacity in mammals. In this Review, we survey the mammalian tissues for which regenerative or improved neonatal healing has been established, including the heart, cochlear hair cells, the brain and spinal cord, and dense connective tissues. We also highlight common and/or tissue-specific mechanisms of neonatal regeneration, which involve cells, signaling pathways, extracellular matrix, immune cells and other factors. The identification of such common features across neonatal tissues may direct therapeutic strategies that will be broadly applicable to multiple adult tissues.

CTRP12 alleviates cardiomyocyte ischemia‑reperfusion injury via regulation of KLF15

Myocardial ischemia-reperfusion (I/R) serves a crucial role in myocardial infarction. C1q/TNF-related protein 12 (CTRP12) is a secretory protein involved in metabolism. It has been reported that CTRP12 participates in the regulation of numerous cardiovascular diseases. However, its role in myocardial I/R injury remains unclear. In the present study, the left anterior descending coronary artery in mice was ligated to establish a mouse I/R model. A myocardial hypoxia-reoxygenation (H/R) cell model was also established. Cardiomyocyte injury was evaluated using hematoxylin and eosin staining, Cell Counting Kit-8 and a lactate dehydrogenase (LDH) kit. The expression levels of CTRP12 and Krueppel-like factor 15 (KLF15) in murine myocardial tissues and H9c2 cells were determined using reverse transcription-quantitative PCR and western blotting, as KLF15 was previously reported to protect against I/R-induced cardiomyocyte damage. Furthermore, inflammatory factors TNF-α, IL-1β and IL-6 were analyzed using ELISA while apoptosis was assessed using TUNEL assays and western blotting. Moreover, the activity of the CTRP12 promoter was determined using a dual-luciferase reporter assay. The results demonstrated that I/R surgery markedly exacerbated myocardial tissue damage, whereas H/R treatment significantly reduced cell viability and significantly increased LDH activity as well as the release of inflammatory factors and apoptosis. I/R and H/R induction significantly reduced the expression levels of CTRP12 and KLF15. CTRP12 overexpression significantly alleviated H/R-induced cell injury and significantly inhibited inflammation and apoptosis. Further analysis demonstrated that KLF15 could significantly promote the activity of the CTRP12 promoter. However, following CTRP12 knockdown, KLF15 overexpression exacerbated cell injury, inflammation and apoptosis. In conclusion, the present study demonstrated that CTRP12 may mitigate inflammation and apoptosis in H/R-induced cardiomyocytes, possibly via the regulation of KLF15, which provided a theoretical basis for the potential treatment of I/R-induced myocardial infarction.

Hypoxia promotes a perinatal-like progenitor state in the adult murine epicardium

The epicardium is a reservoir of progenitors that give rise to coronary vasculature and stroma during development and mediates cardiac vascular repair. However, its role as a source of progenitors in the adult mammalian heart remains unclear due to lack of clear lineage markers and single-cell culture systems to elucidate epicardial progeny cell fate. We found that in vivo exposure of mice to physiological hypoxia induced adult epicardial cells to re-enter the cell cycle and to express a subset of developmental genes. Multiplex single cell transcriptional profiling revealed a lineage relationship between epicardial cells and smooth muscle, stromal cells, as well as cells with an endothelial-like fate. We found that physiological hypoxia promoted a perinatal-like progenitor state in the adult murine epicardium. In vitro clonal analyses of purified epicardial cells showed that cell growth and subsequent differentiation is dependent upon hypoxia, and that resident epicardial cells retain progenitor identity in the adult mammalian heart with self-renewal and multilineage differentiation potential. These results point to a source of progenitor cells in the adult heart that can be stimulated in vivo and provide an in vitro model for further studies.

Reconstruction of regulatory network predicts transcription factors driving the dynamics of zebrafish heart regeneration

The limited regenerative capacity in mammals has serious implications for cardiac tissue damage. Meanwhile, zebrafish has a high regenerative capacity, but the regulation of the heart healing process has yet to be elucidated. The dynamic nature of cardiac regeneration requires consideration of the inherent temporal dimension of this process. Here, we conducted a systematic review to find genes that define the regenerative cell state of the zebrafish heart. We then performed an in silico temporal gene regulatory network analysis using transcriptomic data from the zebrafish heart regenerative process obtained from databases. In this analysis, the genes found in the systematic review were used to represent the final cell state of the transition process from a non-regenerative cell state to a regenerative state. We found 135 transcription factors driving the cellular state transition process during zebrafish cardiac regeneration, including Hand2, Nkx2.5, Tbx20, Fosl1, Fosb, Junb, Vdr, Wt1, and Tcf21 previously reported for playing a key role in tissue regeneration. Furthermore, we demonstrate that most regulators are activated in the first days post-injury, indicating that the transition from a non-regenerative to a regenerative state occurs promptly.

Regulation of Epicardial Cell Fate during Cardiac Development and Disease: An Overview

The epicardium is the outermost cell layer in the vertebrate heart that originates during development from mesothelial precursors located in the proepicardium and septum transversum. The epicardial layer plays a key role during cardiogenesis since a subset of epicardial-derived cells (EPDCs) undergo an epithelial–mesenchymal transition (EMT); migrate into the myocardium; and differentiate into distinct cell types, such as coronary vascular smooth muscle cells, cardiac fibroblasts, endothelial cells, and presumably a subpopulation of cardiomyocytes, thus contributing to complete heart formation. Furthermore, the epicardium is a source of paracrine factors that support cardiac growth at the last stages of cardiogenesis. Although several lineage trace studies have provided some evidence about epicardial cell fate determination, the molecular mechanisms underlying epicardial cell heterogeneity remain not fully understood. Interestingly, seminal works during the last decade have pointed out that the adult epicardium is reactivated after heart damage, re-expressing some embryonic genes and contributing to cardiac remodeling. Therefore, the epicardium has been proposed as a potential target in the treatment of cardiovascular disease. In this review, we summarize the previous knowledge regarding the regulation of epicardial cell contribution during development and the control of epicardial reactivation in cardiac repair after damage.

Electrophysiologic Conservation of Epicardial Conduction Dynamics After Myocardial Infarction and Natural Heart Regeneration in Newborn Piglets

Newborn mammals, including piglets, exhibit natural heart regeneration after myocardial infarction (MI) on postnatal day 1 (P1), but this ability is lost by postnatal day 7 (P7). The electrophysiologic properties of this naturally regenerated myocardium have not been examined. We hypothesized that epicardial conduction is preserved after P1 MI in piglets. Yorkshire-Landrace piglets underwent left anterior descending coronary artery ligation at age P1 (n = 6) or P7 (n = 7), After 7 weeks, cardiac magnetic resonance imaging was performed with late gadolinium enhancement for analysis of fibrosis. Epicardial conduction mapping was performed using custom 3D-printed high-resolution mapping arrays. Age- and weight-matched healthy pigs served as controls (n = 6). At the study endpoint, left ventricular (LV) ejection fraction was similar for controls and P1 pigs (46.4 ± 3.0% vs. 40.3 ± 4.9%, p = 0.132), but significantly depressed for P7 pigs (30.2 ± 6.6%, p < 0.001 vs. control). The percentage of LV myocardial volume consisting of fibrotic scar was 1.0 ± 0.4% in controls, 9.9 ± 4.4% in P1 pigs (p = 0.002 vs. control), and 17.3 ± 4.6% in P7 pigs (p < 0.001 vs. control, p = 0.007 vs. P1). Isochrone activation maps and apex activation time were similar between controls and P1 pigs (9.4 ± 1.6 vs. 7.8 ± 0.9 ms, p = 0.649), but significantly prolonged in P7 pigs (21.3 ± 5.1 ms, p < 0.001 vs. control, p < 0.001 vs. P1). Conduction velocity was similar between controls and P1 pigs (1.0 ± 0.2 vs. 1.1 ± 0.4 mm/ms, p = 0.852), but slower in P7 pigs (0.7 ± 0.2 mm/ms, p = 0.129 vs. control, p = 0.052 vs. P1). Overall, our data suggest that epicardial conduction dynamics are conserved in the setting of natural heart regeneration in piglets after P1 MI.

Leukocyte-Mediated Cardiac Repair after Myocardial Infarction in Non-Regenerative vs. Regenerative Systems

Innate and adaptive leukocytes rapidly mobilize to ischemic tissues after myocardial infarction in response to damage signals released from necrotic cells. Leukocytes play important roles in cardiac repair and regeneration such as inflammation initiation and resolution; the removal of dead cells and debris; the deposition of the extracellular matrix and granulation tissue; supporting angiogenesis and cardiomyocyte proliferation; and fibrotic scar generation and resolution. By organizing and comparing the present knowledge of leukocyte recruitment and function after cardiac injury in non-regenerative to regenerative systems, we propose that the leukocyte response to cardiac injury differs in non-regenerative adult mammals such as humans and mice in comparison to cardiac regenerative models such as neonatal mice and adult zebrafish. Specifically, extensive neutrophil, macrophage, and T-cell persistence contributes to a lengthy inflammatory period in non-regenerative systems for adverse cardiac remodeling and heart failure development, whereas their quick removal supports inflammation resolution in regenerative systems for new contractile tissue formation and coronary revascularization. Surprisingly, other leukocytes have not been examined in regenerative model systems. With this review, we aim to encourage the development of improved immune cell markers and tools in cardiac regenerative models for the identification of new immune targets in non-regenerative systems to develop new therapies.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Natural cardiac regeneration conserves native biaxial left ventricular biomechanics after myocardial infarction in neonatal rats

After myocardial infarction (MI), adult mammals exhibit scar formation, adverse left ventricular (LV) remodeling, LV stiffening, and impaired contractility, ultimately resulting in heart failure. Neonatal mammals, however, are capable of natural heart regeneration after MI. We hypothesized that neonatal cardiac regeneration conserves native biaxial LV mechanics after MI. Wistar rat neonates (1 day old, n = 46) and adults (8-10 weeks old, n = 20) underwent sham surgery or permanent left anterior descending coronary artery ligation. At 6 weeks after neonatal MI, Masson's trichrome staining revealed negligible fibrosis. Echocardiography for the neonatal MI (n = 15) and sham rats (n = 14) revealed no differences in LV wall thickness or chamber diameter, and both groups had normal ejection fraction (72.7% vs 77.5%, respectively, p = 0.1946). Biaxial tensile testing revealed similar stress-strain curves along both the circumferential and longitudinal axes across a full range of physiologic stresses and strains. The circumferential modulus (267.9 kPa vs 274.2 kPa, p = 0.7847), longitudinal modulus (269.3 kPa vs 277.1 kPa, p = 0.7435), and maximum shear stress (3.30 kPa vs 3.95 kPa, p = 0.5418) did not differ significantly between the neonatal MI and sham groups, respectively. In contrast, transmural scars were observed at 4 weeks after adult MI. Adult MI hearts (n = 7) exhibited profound LV wall thinning (p < 0.0001), chamber dilation (p = 0.0246), and LV dysfunction (ejection fraction 45.4% vs 79.7%, p < 0.0001) compared to adult sham hearts (n = 7). Adult MI hearts were significantly stiffer than adult sham hearts in both the circumferential (321.5 kPa vs 180.0 kPa, p = 0.0111) and longitudinal axes (315.4 kPa vs 172.3 kPa, p = 0.0173), and also exhibited greater maximum shear stress (14.87 kPa vs 3.23 kPa, p = 0.0162). Our study is the first to show that native biaxial LV mechanics are conserved after neonatal heart regeneration following MI, thus adding biomechanical support for the therapeutic potential of cardiac regeneration in the treatment of ischemic heart disease.

Accelerated Growth, Differentiation, and Ploidy with Reduced Proliferation of Right Ventricular Cardiomyocytes in Children with Congenital Heart Defect Tetralogy of Fallot

The myocardium of children with tetralogy of Fallot (TF) undergoes hemodynamic overload and hypoxemia immediately after birth. Comparative analysis of changes in the ploidy and morphology of the right ventricular cardiomyocytes in children with TF in the first years of life demonstrated their significant increase compared with the control group. In children with TF, there was a predominantly diffuse distribution of Connexin43-containing gap junctions over the cardiomyocytes sarcolemma, which redistributed into the intercalated discs as cardiomyocytes differentiation increased. The number of Ki67-positive cardiomyocytes varied greatly and amounted to 7.0–1025.5/10⁶ cardiomyocytes and also were decreased with increased myocytes differentiation. Ultrastructural signs of immaturity and proliferative activity of cardiomyocytes in children with TF were demonstrated. The proportion of interstitial tissue did not differ significantly from the control group. The myocardium of children with TF under six months of age was most sensitive to hypoxemia, it was manifested by a delay in the intercalated discs and myofibril assembly and the appearance of ultrastructural signs of dystrophic changes in the cardiomyocytes. Thus, the acceleration of ontogenetic growth and differentiation of the cardiomyocytes, but not the reactivation of their proliferation, was an adaptation of the immature myocardium of children with TF to hemodynamic overload and hypoxemia.

Neonatal injury models: integral tools to decipher the molecular basis of cardiac regeneration

Myocardial injury often leads to heart failure due to the loss and insufficient regeneration of resident cardiomyocytes. The low regenerative potential of the mammalian heart is one of the main drivers of heart failure progression, especially after myocardial infarction accompanied by large contractile muscle loss. Preclinical therapies for cardiac regeneration are promising, but clinically still missing. Mammalian models represent an excellent translational in vivo platform to test drugs and treatments for the promotion of cardiac regeneration. Particularly, short-lived mice offer the possibility to monitor the outcome of such treatments throughout the life span. Importantly, there is a short period of time in newborn mice in which the heart retains full regenerative capacity after cardiac injury, which potentially also holds true for the neonatal human heart. Thus, in vivo neonatal mouse models of cardiac injury are crucial to gain insights into the molecular mechanisms underlying the cardiac regenerative processes and to devise novel therapeutic strategies for the treatment of diseased adult hearts. Here, we provide an overview of the established injury models to study cardiac regeneration. We summarize pioneering studies that demonstrate the potential of using neonatal cardiac injury models to identify factors that may stimulate heart regeneration by inducing endogenous cardiomyocyte proliferation in the adult heart. To conclude, we briefly summarize studies in large animal models and the insights gained in humans, which may pave the way toward the development of novel approaches in regenerative medicine.

Formyl peptide receptor 2 and heart disease

Formyl peptide receptor type 2 (FPR2) regulates the initiation and resolution phases of the inflammatory response. In the setting of heart injury and disease, dysregulated inflammation can potentiate maladaptive healing and pathological remodeling of the heart leading to cardiac dysfunction and failure. The potential to regulate and resolve adverse inflammation is postulated to improve outcome in the setting of heart disease. This review covers emerging concepts on the role of FPR2 in heart disease and strategies to activate pro-resolution processes to limit disease progression. We summarize key preclinical studies that support use of FPR2 agonists in heart disease. Finally, we briefly discuss the status of FPR2 agonists under evaluation in the clinic.

Biosynthesis of α-Gal Epitopes (Galα1-3Galβ1-4GlcNAc-R) and Their Unique Potential in Future α-Gal Therapies

The α-gal epitope is a carbohydrate antigen which appeared early in mammalian evolution and is synthesized in large amounts by the glycosylation enzyme α1,3galactosyltransferase (α1,3GT) in non-primate mammals, lemurs, and New-World monkeys. Ancestral Old-World monkeys and apes synthesizing α-gal epitopes underwent complete extinction 20–30 million years ago, and their mutated progeny lacking α-gal epitopes survived. Humans, apes, and Old-World monkeys which evolved from the surviving progeny lack α-gal epitopes and produce the natural anti-Gal antibody which binds specifically to α-gal epitopes. Because of this reciprocal distribution of the α-gal epitope and anti-Gal in mammals, transplantation of organs from non-primate mammals (e.g., pig xenografts) into Old-World monkeys or humans results in hyperacute rejection following anti-Gal binding to α-gal epitopes on xenograft cells. The in vivo immunocomplexing between anti-Gal and α-gal epitopes on molecules, pathogens, cells, or nanoparticles may be harnessed for development of novel immunotherapies (referred to as “α-gal therapies”) in various clinical settings because such immune complexes induce several beneficial immune processes. These immune processes include localized activation of the complement system which can destroy pathogens and generate chemotactic peptides that recruit antigen-presenting cells (APCs) such as macrophages and dendritic cells, targeting of antigens presenting α-gal epitopes for extensive uptake by APCs, and activation of recruited macrophages into pro-reparative macrophages. Some of the suggested α-gal therapies associated with these immune processes are as follows: 1. Increasing efficacy of enveloped-virus vaccines by synthesizing α-gal epitopes on vaccinating inactivated viruses, thereby targeting them for extensive uptake by APCs. 2. Conversion of autologous tumors into antitumor vaccines by expression of α-gal epitopes on tumor cell membranes. 3. Accelerating healing of external and internal injuries by α-gal nanoparticles which decrease the healing time and diminish scar formation. 4. Increasing anti-Gal–mediated protection against zoonotic viruses presenting α-gal epitopes and against protozoa, such as Trypanosoma, Leishmania, and Plasmodium, by vaccination for elevating production of the anti-Gal antibody. The efficacy and safety of these therapies were demonstrated in transgenic mice and pigs lacking α-gal epitopes and producing anti-Gal, raising the possibility that these α-gal therapies may be considered for further evaluation in clinical trials.

Microenvironment Stiffness Amplifies Post-ischemia Heart Regeneration in Response to Exogenous Extracellular Matrix Proteins in Neonatal Mice

The cardiogenesis of the fetal heart is absent in juveniles and adults. Cross-transplantation of decellularized extracellular matrix (dECM) can stimulate regeneration in myocardial infarct (MI) models. We have previously shown that dECM and tissue stiffness have cooperative regulation of heart regeneration in transiently regenerative day 1 neonatal mice. To investigate underlying mechanisms of mechano-signaling and dECM, we pharmacologically altered heart stiffness and administered dECM hydrogels in non-regenerative mice after MI. The dECM combined with softening exhibits preserved cardiac function, LV geometry, increased cardiomyocyte mitosis and lowered fibrosis while stiffening further aggravated ischemic damage. Transcriptome analysis identified a protein in cardiomyocytes, CLCA2, confirmed to be upregulated after MI and downregulated by dECM in a mechanosensitive manner. Synthetic knock-down of CLCA2 expression induced mitosis in primary rat cardiomyocytes in the dish. Together, our results indicate that therapeutic efficacy of extracellular molecules for heart regeneration can be modulated by heart microenvironment stiffness in vivo.

Towards precision medicine in heart failure

The number of therapies for heart failure (HF) with reduced ejection fraction has nearly doubled in the past decade. In addition, new therapies for HF caused by hypertrophic and infiltrative disease are emerging rapidly. Indeed, we are on the verge of a new era in HF in which insights into the biology of myocardial disease can be matched to an understanding of the genetic predisposition in an individual patient to inform precision approaches to therapy. In this Review, we summarize the biology of HF, emphasizing the causal relationships between genetic contributors and traditional structure-based remodelling outcomes, and highlight the mechanisms of action of traditional and novel therapeutics. We discuss the latest advances in our understanding of both the Mendelian genetics of cardiomyopathy and the complex genetics of the clinical syndrome presenting as HF. In the phenotypic domain, we discuss applications of machine learning for the subcategorization of HF in ways that might inform rational prescribing of medications. We aim to bridge the gap between the biology of the failing heart, its diverse clinical presentations and the range of medications that we can now use to treat it. We present a roadmap for the future of precision medicine in HF.

Reawakening the Intrinsic Cardiac Regenerative Potential: Molecular Strategies to Boost Dedifferentiation and Proliferation of Endogenous Cardiomyocytes

Despite considerable efforts carried out to develop stem/progenitor cell-based technologies aiming at replacing and restoring the cardiac tissue following severe damages, thus far no strategies based on adult stem cell transplantation have been demonstrated to efficiently generate new cardiac muscle cells. Intriguingly, dedifferentiation, and proliferation of pre-existing cardiomyocytes and not stem cell differentiation represent the preponderant cellular mechanism by which lower vertebrates spontaneously regenerate the injured heart. Mammals can also regenerate their heart up to the early neonatal period, even in this case by activating the proliferation of endogenous cardiomyocytes. However, the mammalian cardiac regenerative potential is dramatically reduced soon after birth, when most cardiomyocytes exit from the cell cycle, undergo further maturation, and continue to grow in size. Although a slow rate of cardiomyocyte turnover has also been documented in adult mammals, both in mice and humans, this is not enough to sustain a robust regenerative process. Nevertheless, these remarkable findings opened the door to a branch of novel regenerative approaches aiming at reactivating the endogenous cardiac regenerative potential by triggering a partial dedifferentiation process and cell cycle re-entry in endogenous cardiomyocytes. Several adaptations from intrauterine to extrauterine life starting at birth and continuing in the immediate neonatal period concur to the loss of the mammalian cardiac regenerative ability. A wide range of systemic and microenvironmental factors or cell-intrinsic molecular players proved to regulate cardiomyocyte proliferation and their manipulation has been explored as a therapeutic strategy to boost cardiac function after injuries. We here review the scientific knowledge gained thus far in this novel and flourishing field of research, elucidating the key biological and molecular mechanisms whose modulation may represent a viable approach for regenerating the human damaged myocardium.

RNA sequencing indicates age-dependent shifts in the cardiac fibroblast transcriptome between fetal, neonatal, and adult developmental ages

Cardiac fibroblasts are responsible for extracellular matrix turnover and repair in the cardiac environment and serve to help facilitate immune responses. However, it is well established that they have a significant phenotypic heterogeneity with respect to location, physiological conditions, and developmental age. The goal of this study was to provide an in-depth transcriptomic profile of cardiac fibroblasts derived from rat hearts at fetal, neonatal, and adult developmental ages to ascertain variations in gene expression that may drive functional differences in these cells at these specific stages of development. We performed RNA sequencing (RNA-seq) of cardiac fibroblasts isolated from fetal, neonatal, and adult rats and compared with the rat genome. Principal component analysis of RNA-seq data suggested that data variance was predominantly due to developmental age. Differential expression and gene set enrichment analysis against Gene Ontology and Kyoto Encyclopedia of Genes and Genomes datasets indicated an array of differences across developmental ages, including significant decreases in cardiac development and cardiac function-associated genes with age and a significant increase in immune- and inflammatory-associated functions, particularly immune cell signaling and cytokine and chemokine production, with respect to increasing developmental age. These results reinforce established evidence of diverse phenotypic heterogeneity of fibroblasts with respect to developmental age. Furthermore, based on our analysis of gene expression, age-specific alterations in cardiac fibroblasts may play a crucial role in observed differences in cardiac inflammation and immune response observed across developmental ages.