Cardiomyocyte proliferation contributes to heart growth in young humans

The Cardiac Sarcomere and Cell Cycle

PURPOSE OF REVIEW

The lack of adult human cardiomyocyte proliferative capacity impairs cardiac regeneration such as after myocardial injury. The sarcomere, a specialized actin cytoskeletal structure that is essential for twitch contraction in cardiomyocytes, has been considered a critical factor limiting adult human cardiomyocyte proliferation through incompletely understood mechanisms.

RECENT FINDINGS

This review summarizes known and emerging regulatory mechanisms connecting the human cardiomyocyte sarcomere to cell cycle regulation including structural and signaling mechanisms. Cardiac regeneration could be augmented through targeting the inhibitory effects of the sarcomere on cardiomyocyte proliferation.

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

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

Human Engineered Heart Tissue Models for Disease Modeling and Drug Discovery

The emergence of human induced pluripotent stem cells (hiPSCs) and efficient differentiation of hiPSC-derived cardiomyocytes (hiPSC-CMs) induced from diseased donors have the potential to recapitulate the molecular and functional features of the human heart. Although the immaturity of hiPSC-CMs, including the structure, gene expression, conduct, ion channel density, and Ca²⁺ kinetics, is a major challenge, various attempts to promote maturation have been effective. Three-dimensional cardiac models using hiPSC-CMs have achieved these functional and morphological maturations, and disease models using patient-specific hiPSC-CMs have furthered our understanding of the underlying mechanisms and effective therapies for diseases. Aside from the mechanisms of diseases and drug responses, hiPSC-CMs also have the potential to evaluate the safety and efficacy of drugs in a human context before a candidate drug enters the market and many phases of clinical trials. In fact, novel drug testing paradigms have suggested that these cells can be used to better predict the proarrhythmic risk of candidate drugs. In this review, we overview the current strategies of human engineered heart tissue models with a focus on major cardiac diseases and discuss perspectives and future directions for the real application of hiPSC-CMs and human engineered heart tissue for disease modeling, drug development, clinical trials, and cardiotoxicity tests.

Nanoparticles Targeting the Molecular Pathways of Heart Remodeling and Regeneration

Cardiovascular diseases are the main cause of death worldwide, a trend that will continue to grow over the next decade. The heart consists of a complex cellular network based mainly on cardiomyocytes, but also on endothelial cells, smooth muscle cells, fibroblasts, and pericytes, which closely communicate through paracrine factors and direct contact. These interactions serve as valuable targets in understanding the phenomenon of heart remodeling and regeneration. The advances in nanomedicine in the controlled delivery of active pharmacological agents are remarkable and may provide substantial contribution to the treatment of heart diseases. This review aims to summarize the main mechanisms involved in cardiac remodeling and regeneration and how they have been applied in nanomedicine.

Polyploidy as a Fundamental Phenomenon in Evolution, Development, Adaptation and Diseases

DNA replication during cell proliferation is 'vertical' copying, which reproduces an initial amount of genetic information. Polyploidy, which results from whole-genome duplication, is a fundamental complement to vertical copying. Both organismal and cell polyploidy can emerge via premature cell cycle exit or via cell-cell fusion, the latter giving rise to polyploid hybrid organisms and epigenetic hybrids of somatic cells. Polyploidy-related increase in biological plasticity, adaptation, and stress resistance manifests in evolution, development, regeneration, aging, oncogenesis, and cardiovascular diseases. Despite the prevalence in nature and importance for medicine, agri- and aquaculture, biological processes and epigenetic mechanisms underlying these fundamental features largely remain unknown. The evolutionarily conserved features of polyploidy include activation of transcription, response to stress, DNA damage and hypoxia, and induction of programs of morphogenesis, unicellularity, and longevity, suggesting that these common features confer adaptive plasticity, viability, and stress resistance to polyploid cells and organisms. By increasing cell viability, polyploidization can provide survival under stressful conditions where diploid cells cannot survive. However, in somatic cells it occurs at the expense of specific function, thus promoting developmental programming of adult cardiovascular diseases and increasing the risk of cancer. Notably, genes arising via evolutionary polyploidization are heavily involved in cancer and other diseases. Ploidy-related changes of gene expression presumably originate from chromatin modifications and the derepression of bivalent genes. The provided evidence elucidates the role of polyploidy in evolution, development, aging, and carcinogenesis, and may contribute to the development of new strategies for promoting regeneration and preventing cardiovascular diseases and cancer.

Porcine Models of Heart Regeneration

Swine are popular large mammals for cardiac preclinical testing due to their similarities with humans in terms of organ size and physiology. Recent studies indicate an early neonatal regenerative capacity for swine hearts similar to small mammal laboratory models such as rodents, inspiring exciting possibilities for studying cardiac regeneration with the goal of improved clinical translation to humans. However, while swine hearts are anatomically similar to humans, fundamental differences exist in growth mechanisms, nucleation, and the maturation of pig cardiomyocytes, which could present difficulties for the translation of preclinical findings in swine to human therapeutics. In this review, we discuss the maturational dynamics of pig cardiomyocytes and their capacity for proliferative cardiac regeneration during early neonatal development to provide a perspective on swine as a preclinical model for developing cardiac gene- and cell-based regenerative therapeutics.

Heart regeneration: 20 years of progress and renewed optimism

Cardiovascular disease is a leading cause of death worldwide, and thus there remains great interest in regenerative approaches to treat heart failure. In the past 20 years, the field of heart regeneration has entered a renaissance period with remarkable progress in the understanding of endogenous heart regeneration, stem cell differentiation for exogenous cell therapy, and cell-delivery methods. In this review, we highlight how this new understanding can lead to viable strategies for human therapy. For the near term, drugs, electrical and mechanical devices, and heart transplantation will remain mainstays of cardiac therapies, but eventually regenerative therapies based on fundamental regenerative biology may offer more permanent solutions for patients with heart failure.

Advances in Manufacturing Cardiomyocytes from Human Pluripotent Stem Cells

The emergence of human pluripotent stem cell (hPSC) technology over the past two decades has provided a source of normal and diseased human cells for a wide variety of in vitro and in vivo applications. Notably, hPSC-derived cardiomyocytes (hPSC-CMs) are widely used to model human heart development and disease and are in clinical trials for treating heart disease. The success of hPSC-CMs in these applications requires robust, scalable approaches to manufacture large numbers of safe and potent cells. Although significant advances have been made over the past decade in improving the purity and yield of hPSC-CMs and scaling the differentiation process from 2D to 3D, efforts to induce maturation phenotypes during manufacturing have been slow. Process monitoring and closed-loop manufacturing strategies are just being developed. We discuss recent advances in hPSC-CM manufacturing, including differentiation process development and scaling and downstream processes as well as separation and stabilization. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Cardiac regeneration following myocardial infarction: the need for regeneration and a review of cardiac stromal cell populations used for transplantation

Myocardial infarction is a leading cause of death globally due to the inability of the adult human heart to regenerate after injury. Cell therapy using cardiac-derived progenitor populations emerged about two decades ago with the aim of replacing cells lost after ischaemic injury. Despite early promise from rodent studies, administration of these populations has not translated to the clinic. We will discuss the need for cardiac regeneration and review the debate surrounding how cardiac progenitor populations exert a therapeutic effect following transplantation into the heart, including their ability to form de novo cardiomyocytes and the release of paracrine factors. We will also discuss limitations hindering the cell therapy field, which include the challenges of performing cell-based clinical trials and the low retention of administered cells, and how future research may overcome them.

Targeting ACSL1 promotes cardiomyocyte proliferation and cardiac regeneration

BACKGROUND

Neonatal hearts have considerable regenerative potential within 7 days post birth (P7), but the rate of regeneration is extremely low after P7. Interestingly, lipid metabolism increases dramatically after P7. The similarities in these age profiles suggests a possible link between cardiac regeneration and lipid metabolism. Acyl CoA synthase long chain family member 1 (ACSL1) is the key enzyme that regulates lipid metabolism. The aim of this study was to identify the role of ACSL1 in the regeneration of cardiomyocytes.

METHODS AND RESULTS

The uptake of fatty acids in hearts increased after P7; however, myocardial regeneration was decreased. We profiled an RNA-sequence array of hearts from mice of different ages, including E10.5 (embryonic stage)-, 3-, 7-, 21-, 30-, and 60-day-old mice, and found that the expression of ACSL1 was significantly increased after P7. By establishing ACSL1 knockdown mice with adeno-associated virus (AAV9). Then, we verified that knockdown of ACSL1 enhanced the capacity for myocardial regeneration both in mice and in primary cardiomyocytes. Indeed, ACSL1 knockdown in primary cardiomyocytes promoted the cell cycle progression from G0 to G2 phase by regulating specific factors, which may correlate with the activation of AKT by ACSL1 and withdrawal of FOXO1 from the nucleus. In vivo, knockdown of ACSL1 effectively restored cardiac function and myocardial regeneration in adult mice with myocardial infarction (MI).

CONCLUSIONS

ACSL1 possibly induces the loss of the myocardial regenerative potential beginning at P7 in mice, and inhibition of ACSL1 effectively promoted myocardial repair after MI in mice.

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.

Molecular Mechanisms of Cardiac Injury Associated With Myocardial SARS-CoV-2 Infection

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread around the world. The development of cardiac injury is a common condition in patients with COVID-19, but the pathogenesis remains unclear. The RNA-Seq dataset (GSE150392) comparing expression profiling of mock human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and SARS-CoV-2-infected hiPSC-CMs was obtained from Gene Expression Omnibus (GEO). We identified 1,554 differentially expressed genes (DEGs) based on GSE150392. Gene set enrichment analysis (GSEA), Gene ontology (GO) analysis, and Kyoto encyclopedia of genes and genomes (KEGG) pathway analysis showed that immune-inflammatory responses were activated by SARS-CoV-2, while muscle contraction, cellular respiration, and cell cycle of hiPSC-CMs were inhibited. A total of 15 hub genes were identified according to protein-protein interaction (PPI), among which 11 upregulated genes were mainly involved in cytokine activation related to the excessive inflammatory response. Moreover, we identified potential drugs based on these hub genes. In conclusion, SARS-CoV-2 infection of cardiomyocytes caused a strong defensive response, leading to excessive immune inflammation, cell hypoxia, functional contractility reduction, and apoptosis, ultimately resulting in myocardial injury.

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.

Modulation of VEGFA Signaling During Heart Regeneration in Zebrafish

Over the last decades, myocardial infarction and heart failure have accounted every year for millions of deaths worldwide. After a coronary occlusion, the lack of blood supply to downstream muscle leads to cell death and scarring. To date, several pro-angiogenic factors have been tested to stimulate reperfusion of the affected myocardium, VEGFA being one of the most extensively studied. Given the unsuccessful outcomes of clinical trials, understanding how cardiac revascularization takes place in models with endogenous regenerative capacity holds the key to devising more efficient therapies. Here, we summarize the main findings on VEGFA's role during cardiac repair and regeneration, with a particular focus on zebrafish as a regenerative model. Moreover, we provide a comprehensive overview of available tools to modulate Vegfa expression and action in zebrafish regeneration studies. Understanding the role of Vegfa during zebrafish heart regeneration may help devise efficient therapies and circumvent current limitations in using VEGFA for therapeutic angiogenesis approaches.

Can Pediatric Heart Failure Therapy Be Improved? Yes It Can, But…

Given the heterogenous etiology of pediatric heart failure (pHF), evidence-based studies improving pHF are unlikely. A paradigm shift towards updated medicine-based evidence is therefore necessary. In view of the life expectancy of children, cardiac regeneration strategies are required. Therefore, age- and disease-related differences in myocardial (receptor) physiology require individualized precision medicine. First-line diuretic therapy, adopted from the treatment of adults with HF with no chance for recovery, should be questioned in the treatment of pHF with potential for recovery. Inadequate use of diuretics is a common reason for additional stimulation of the neurohumoral axis. Consecutive intravascular volume depletion led to an inadequate treatment with β-blocker and renin-angiotensin-aldosterone antagonists. Given the age-related catecholamine-driven cardiovascular (patho-) physiology, highly selective β1-blockers (bisoprolol) protect against β1-(noradrenaline)-related myocytic apoptosis and necrosis, but allow β2-receptor-mediated myocardial regeneration. Based on its high safety-efficacy profile with rarely seen adverse effects but easily monitorable efficacy by the surrogate of heart rate (reduction), bisoprolol is our first-line drug in infancy. Reduced heart rate economizes the heart and full body oxygen consumption and extends the diastolic filling and coronary perfusion time. Based on our many years of institutional experience, physicians should be encouraged to use β1-selected blockers in infants with dilated cardiomyopathy and hypoplastic left heart syndrome after stage-1 procedure, but also to treat ventricular septal defects with a significant left-to-right shunt. In summary, individualized pHF therapy is the prerequisite for a causal treatment to improve HF symptoms, but above all for the most functional regeneration possible.

Human Induced Pluripotent Stem Cell as a Disease Modeling and Drug Development Platform-A Cardiac Perspective

A comprehensive understanding of the pathophysiology and cellular responses to drugs in human heart disease is limited by species differences between humans and experimental animals. In addition, isolation of human cardiomyocytes (CMs) is complicated because cells obtained by biopsy do not proliferate to provide sufficient numbers of cells for preclinical studies in vitro. Interestingly, the discovery of human-induced pluripotent stem cell (hiPSC) has opened up the possibility of generating and studying heart disease in a culture dish. The combination of reprogramming and genome editing technologies to generate a broad spectrum of human heart diseases in vitro offers a great opportunity to elucidate gene function and mechanisms. However, to exploit the potential applications of hiPSC-derived-CMs for drug testing and studying adult-onset cardiac disease, a full functional characterization of maturation and metabolic traits is required. In this review, we focus on methods to reprogram somatic cells into hiPSC and the solutions for overcome immaturity of the hiPSC-derived-CMs to mimic the structure and physiological properties of the adult human CMs to accurately model disease and test drug safety. Finally, we discuss how to improve the culture, differentiation, and purification of CMs to obtain sufficient numbers of desired types of hiPSC-derived-CMs for disease modeling and drug development platform.

Identification and characterization of distinct cell cycle stages in cardiomyocytes using the FUCCI transgenic system

Understanding the regulatory mechanism by which cardiomyocyte proliferation transitions to endoreplication and cell cycle arrest during the neonatal period is crucial for identifying proproliferative factors and developing regenerative therapies. We used a transgenic mouse model based on the fluorescent ubiquitination-based cell cycle indicator (FUCCI) system to isolate and characterize cycling cardiomyocytes at different cell cycle stages at a single-cell resolution. Single-cell transcriptome analysis of cycling and noncycling cardiomyocytes was performed at postnatal days 0 (P0) and 7 (P7). The FUCCI system proved to be efficient for the identification of cycling cardiomyocytes with the highest mitotic activity at birth, followed by a gradual decline in the number of cycling and mitotic cardiomyocytes during the neonatal period. Cardiomyocytes showed premature cell cycle exit at G1/S shortly after birth and delayed G1/S progression during endoreplication at P7. Single-cell RNA-seq confirmed previously described signaling pathways involved in cardiomyocyte proliferation (Erbb2 and Hippo/YAP), and maturation-related transcriptional changes during postnatal development, including the metabolic switch from glycolysis to fatty acid oxidation in cardiomyocytes. Importantly, we generated transcriptional profiles specific to cell division and endoreplication in cardiomyocytes at different developmental stages that may facilitate the identification of genes important for adult cardiomyocyte proliferation and heart regeneration. In conclusion, the FUCCI mouse provides a valuable system to study cardiomyocyte cell cycle activity at single cell resolution that can help to decipher the switch from cardiomyocyte proliferation to endoreplication, and to revert this process to facilitate endogenous repair.

Neuregulin-1 enhances cell-cycle activity, delays cardiac fibrosis, and improves cardiac performance in rat pups with right ventricular pressure load

OBJECTIVES

Right ventricular (RV) failure is a leading cause of death in patients with congenital heart disease. RV failure is kept at bay during childhood. Limited proliferation of cardiomyocytes is present in the postnatal heart. We propose that cardiomyocyte proliferation improves RV adaptation to pressure load (PL). We studied adaptation in response to increased RV PL and the role of increased cardiomyocyte cell cycle activity (CCA) in rat pups growing into adulthood.

METHODS

We induced RV PL at day of weaning in rats (3 weeks; 30-40 g) by pulmonary artery banding and followed rats into adulthood (300 g). We performed histological analyses and RNA sequencing analysis. To study the effects of increased cardiomyocyte cell cycle activity, we administered neuregulin-1 (NRG1), a growth factor involved in cardiac development.

RESULTS

PL induced an increase in CCA, with subsequent decline of CCA (sham/PL at 4 weeks: 0.14%/0.83%; P = .04 and 8 weeks: 0.00%/0.00%; P = .484) and cardiac function (cardiac index: control/PL 4 weeks: 4.41/3.29; P = .468 and 8 weeks: 3.57/1.44; P = .024). RNA sequencing analysis revealed delayed maturation and increased CCA pathways. NRG1 stimulated CCA (PL vehicle/NRG1 at 2 weeks: 0.62%/2.28%; P = .003), improved cardiac function (cardiac index control vs vehicle/NRG1 at 2 weeks: 4.21 vs 3.07/4.17; P = .009/.705) and postponed fibrosis (control vs vehicle/NRG1 at 4 weeks: 1.66 vs 4.82%/2.97%; P = .009/.078) in RV PL rats during childhood.

CONCLUSIONS

RV PL during growth induces a transient CCA increase. Further CCA stimulation improves cardiac function and delays fibrosis. This proof-of-concept study shows that stimulation of CCA can improve RV adaptation to PL in the postnatal developing heart and might provide a new approach to preserve RV function in patients with congenital heart disease.

Can polarization of macrophage metabolism enhance cardiac regeneration?

While largely appreciated for their antimicrobial and repair functions, macrophages have emerged as indispensable for the development, homeostasis, and regeneration of tissue, including regeneration of the neonatal heart. Upon activation, mammalian neonatal macrophages express and secrete factors that coordinate angiogenesis, resolution of inflammation, and ultimately cardiomyocyte proliferation. This is contrary to adult macrophages in the adult heart, which are incapable of inducing significant levels of cardiac regeneration. The underlying mechanisms by which pro-regenerative macrophages are activated and regulated remain vague. A timely hypothesis is that macrophage metabolism contributes to this proliferative and regenerative potential. This is because we now appreciate the significant contributions of metabolites to immune cell programming and function, beyond solely bioenergetics. After birth, the metabolic milieu of the neonate is subject to significant alterations in oxygenation and nutrient supply, which will affect how metabolic substrates are catabolized. In this context, we discuss potential roles for select macrophage metabolic pathways during cardiac regeneration.

Label-free imaging of age-related cardiac structural changes in non-human primates using multiphoton nonlinear microscopy

Heart failure is one of the most common causes of morbidity and mortality. Both maturational abnormalities and age-associated cardiac pathologies contribute to heart failure. Imaging-based assessment to discern detailed cardiac structure at various maturational stages is imperative for understanding mechanisms behind cardiac growth and aging. Using multiphoton nonlinear optical microscopy (NLOM) based label-free imaging, we investigated cardiac structural composition in a human-relevant aging model, the common marmoset monkey (Callithrix jacchus). Animals were divided into three different age groups including neonatal, young adult and old. By devising a unique strategy for segregating collagen and myosin emitted second harmonic generation (SHG) signals, we performed a volumetric assessment of collagen and total scattering tissue (collagen + myosin). Aged marmoset hearts exhibited an increase in collagen and total scattering tissue volume at the sites of severe tissue remodelling indicating age-related cardiac fibrosis. Significantly low scattering tissue volume in neonatal marmoset hearts was attributed to a lack of binding between the myofibrils in maturing cardiac tissue. Comprehensive quantitative assessment of structural composition during maturation and aging of marmoset hearts revealed significant differences in myofibril length, alignment, curvature and angular distribution. In conclusion, label-free high-resolution NLOM facilitates visualization and quantification of subcellular structural features for understanding vital age-related morphological alterations in the marmoset heart.

Innate Mechanisms of Heart Regeneration

Heart regeneration is a remarkable process whereby regrowth of damaged cardiac tissue rehabilitates organ anatomy and function. Unfortunately, the human heart is highly resistant to regeneration, which creates a shortage of cardiomyocytes in the wake of ischemic injury, and explains, in part, why coronary artery disease remains a leading cause of death worldwide. Luckily, a detailed blueprint for achieving therapeutic heart regeneration already exists in nature because several lower vertebrate species successfully regenerate amputated or damaged heart muscle through robust cardiomyocyte proliferation. A growing number of species are being interrogated for cardiac regenerative potential, and several commonalities have emerged between those animals showing high or low innate capabilities. In this review, we provide a historical perspective on the field, discuss how regenerative potential is influenced by cardiomyocyte properties, mitogenic signals, and chromatin accessibility, and highlight unanswered questions under active investigation. Ultimately, delineating why heart regeneration occurs preferentially in some organisms, but not in others, will uncover novel therapeutic inroads for achieving cardiac restoration in humans.

Mechanism-Based Cardiac Regeneration Strategies in Mammals

Heart failure in adults is a leading cause of morbidity and mortality worldwide. It can arise from a variety of diseases, with most resulting in a loss of cardiomyocytes that cannot be replaced due to their inability to replicate, as well as to a lack of resident cardiomyocyte progenitor cells in the adult heart. Identifying and exploiting mechanisms underlying loss of developmental cardiomyocyte replicative capacity has proved to be useful in developing therapeutics to effect adult cardiac regeneration. Of course, effective regeneration of myocardium after injury requires not just expansion of cardiomyocytes, but also neovascularization to allow appropriate perfusion and resolution of injury-induced inflammation and interstitial fibrosis, but also reversal of adverse left ventricular remodeling. In addition to overcoming these challenges, a regenerative therapy needs to be safe and easily translatable. Failure to address these critical issues will delay the translation of regenerative approaches. This review critically analyzes current regenerative approaches while also providing a framework for future experimental studies aimed at enhancing success in regenerating the injured heart.

The molecular mosaic of regulated cell death in the cardiovascular system

Cell death is now understood to be a highly regulated process that contributes to normal development and tissue homeostasis, alongside its role in the etiology of various pathological conditions. Through detailed molecular analysis, we have come to know that all cells do not always die in the same way, and that there are at least 7 processes involved, including: apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, and autophagy-mediated cell death. These processes act as pieces in the mosaic of cardiomyocyte cell death, which come together depending on context and stimulus. This review details each individual process, as well as highlights how they come together to produce various cardiac pathologies. By knowing how the pieces go together we can aim towards the development of efficacious therapeutics, which will enable us to prevent cardiomyocyte loss in the face of stress, both reducing mortality and improving quality of life.

Human Erbb2-induced Erk activity robustly stimulates cycling and functional remodeling of rat and human cardiomyocytes

Multiple mitogenic pathways capable of promoting mammalian cardiomyocyte (CM) proliferation have been identified as potential candidates for functional heart repair following myocardial infarction. However, it is unclear whether the effects of these mitogens are species-specific and how they directly compare in the same cardiac setting. Here, we examined how CM-specific lentiviral expression of various candidate mitogens affects human induced pluripotent stem cell-derived CMs (hiPSC-CMs) and neonatal rat ventricular myocytes (NRVMs) in vitro. In 2D-cultured CMs from both species, and in highly mature 3D-engineered cardiac tissues generated from NRVMs, a constitutively active mutant form of the human gene Erbb2 (cahErbb2) was the most potent tested mitogen. Persistent expression of cahErbb2 induced CM proliferation, sarcomere loss, and remodeling of tissue structure and function, which were attenuated by small molecule inhibitors of Erk signaling. These results suggest transient activation of Erbb2/Erk axis in CMs as a potential strategy for regenerative heart repair.

Intracoronary Delivery of Porcine Cardiac Progenitor Cells Overexpressing IGF-1 and HGF in a Pig Model of Sub-Acute Myocardial Infarction

Human cardiac progenitor cells (hCPC) are considered a good candidate in cell therapy for ischemic heart disease, demonstrating capacity to improve functional recovery after myocardial infarction (MI), both in small and large preclinical animal models. However, improvements are required in terms of cell engraftment and efficacy. Based on previously published reports, insulin-growth factor 1 (IGF-1) and hepatocyte growth factor (HGF) have demonstrated substantial cardioprotective, repair and regeneration activities, so they are good candidates to be evaluated in large animal model of MI. We have validated porcine cardiac progenitor cells (pCPC) and lentiviral vectors to overexpress IGF-1 (co-expressing eGFP) and HGF (co-expressing mCherry). pCPC were transduced and IGF1-eGFP^pos and HGF-mCherry^pos populations were purified by cell sorting and further expanded. Overexpression of IGF-1 has a limited impact on pCPC expression profile, whereas results indicated that pCPC-HGF-mCherry cultures could be counter selecting high expresser cells. In addition, pCPC-IGF1-eGFP showed a higher cardiogenic response, evaluated in co-cultures with decellularized extracellular matrix, compared with native pCPC or pCPC-HGF-mCherry. In vivo intracoronary co-administration of pCPC-IGF1-eGFP and pCPC-HFG-mCherry (1:1; 40 × 10⁶/animal), one week after the induction of an MI model in swine, revealed no significant improvement in cardiac function.

Heart failure therapy based on interventricular mechanics and cardio-vascular communications

The heart should not be divided in right and left, whether in health nor in disease. However, the morphological and functional differences between the right and left ventricle should be known and the impact of the ventricle's position considered. Further, the parameters beyond heart rate, contractility, pre- and afterload guaranteeing a sufficient systemic cardiac output have to be integrated in therapeutic measures; preferentially the influence of interventricular mechanics. Despite of recent developments of specific drug therapies, heart failure is associated with a high rate of morbidity and mortality in children. During the progression of heart failure, pulmonary vascular disease is the consequence or the reason for further failing. Clinical symptoms are associated with congestion and low cardiac output at rest or exercise. Improved understanding of the pathophysiological mechanisms particularly of ventricular failure has resulted in the development of innovative therapies that target atrial/ventricular/arterial interactions. Recent advances in interventional and surgical approaches provide promising new strategies to deal with right and left ventricular deterioration. These techniques may delay listing for heart and (heart-) lung transplantation or even make redundant in individual cases. The beneficial effects of these ventricular interaction strategies are mainly based on the mechanics of the interventricular septum and improvement of systolic and diastolic ventricular performance.

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.

Endocrine Influence on Cardiac Metabolism in Development and Regeneration

Mammalian cardiomyocytes mostly utilize oxidation of fatty acids to generate ATP. The fetal heart, in stark contrast, mostly uses anaerobic glycolysis. During perinatal development, thyroid hormone drives extensive metabolic remodeling in the heart for adaptation to extrauterine life. These changes coincide with critical functional maturation and exit of the cell cycle, making the heart a post-mitotic organ. Here, we review the current understanding on the perinatal shift in metabolism, hormonal status, and proliferative potential in cardiomyocytes. Thyroid hormone and glucocorticoids have roles in adult cardiac metabolism, and both pathways have been implicated as regulators of myocardial regeneration. We discuss the evidence that suggests these processes could be interrelated and how this can help explain variation in cardiac regeneration across ontogeny and phylogeny, and we note what breakthroughs are still to be made.

Surgical strategies for the management of end-stage heart failure in infants and children: A 15-year experience with a patient-tailored approach

End‐stage heart failure (ESHF) in pediatric age is an ongoing challenge. Heart transplantation is the final option, but its long‐term outcomes are still suboptimal in children. An alternative patient‐tailored surgical protocol to manage ESHF in children is described. Retrospective, single‐center analysis of pediatric patients admitted to our institution between April 2004 and February 2021 for ESHF. Our current protocol is as follows: (a) Patients <1 year with isolated left ventricular dysfunction due to dilated cardiomyopathy underwent pulmonary artery banding (PAB). (b) Patients <10 years and <20 kg, who did not meet previous criteria were managed with Berlin Heart EXCOR. (c) Patients >10 years or >20 kg, underwent placement of intracorporeal Heartware. Primary outcomes were survival, transplant incidence, and postoperative adverse events. A total of 24 patients (mean age 5.3 ± 5.9 years) underwent 26 procedures: PAB in 6 patients, Berlin Heart in 11, and Heartware in 7. Two patients shifted from PAB to Berlin Heart. Overall survival at 1‐year follow‐up and 5‐year follow‐up was 78.7% (95%CI = 62%‐95.4%) and 74.1% (95%CI = 56.1%‐92.1%), respectively. Berlin Heart was adopted in higher‐risk settings showing inferior outcomes, whereas a PAB enabled 67% of patients to avoid transplantation, with no mortality. An integrated, patient‐tailored surgical strategy, comprehensive of PAB and different types of ventricular assist devices, can provide satisfactory medium‐term results for bridging to transplant or recovery. The early postoperative period is critical and requires strict clinical vigilance. Selected infants can benefit from PAB that has demonstrated to be a safe bridge to recovery.

Effect of Preterm Birth on Cardiac and Cardiomyocyte Growth and the Consequences of Antenatal and Postnatal Glucocorticoid Treatment

Preterm birth coincides with a key developmental window of cardiac growth and maturation, and thus has the potential to influence long-term cardiac function. Individuals born preterm have structural cardiac remodelling and altered cardiac growth and function by early adulthood. The evidence linking preterm birth and cardiovascular disease in later life is mounting. Advances in the perinatal care of preterm infants, such as glucocorticoid therapy, have improved survival rates, but at what cost? This review highlights the short-term and long-term impact of preterm birth on the structure and function of the heart and focuses on the impact of antenatal and postnatal glucocorticoid treatment on the immature preterm heart.

Epigenetic Regulation of Cardiomyocyte Differentiation from Embryonic and Induced Pluripotent Stem Cells

With the intent to achieve the best modalities for myocardial cell therapy, different cell types are being evaluated as potent sources for differentiation into cardiomyocytes. Embryonic stem cells and induced pluripotent stem cells have great potential for future progress in the treatment of myocardial diseases. We reviewed aspects of epigenetic mechanisms that play a role in the differentiation of these cells into cardiomyocytes. Cardiomyocytes proliferate during fetal life, and after birth, they undergo permanent terminal differentiation. Upregulation of cardiac-specific genes in adults induces hypertrophy due to terminal differentiation. The repression or expression of these genes is controlled by chromatin structural and epigenetic changes. However, few studies have reviewed and analyzed the epigenetic aspects of the differentiation of embryonic stem cells and induced pluripotent stem cells into cardiac lineage cells. In this review, we focus on the current knowledge of epigenetic regulation of cardiomyocyte proliferation and differentiation from embryonic and induced pluripotent stem cells through histone modification and microRNAs, the maintenance of pluripotency, and its alteration during cardiac lineage differentiation.

Downregulated developmental processes in the postnatal right ventricle under the influence of a volume overload

The molecular atlas of postnatal mouse ventricular development has been made available and cardiac regeneration is documented to be a downregulated process. The right ventricle (RV) differs from the left ventricle. How volume overload (VO), a common pathologic state in children with congenital heart disease, affects the downregulated processes of the RV is currently unclear. We created a fistula between the abdominal aorta and inferior vena cava on postnatal day 7 (P7) using a mouse model to induce a prepubertal RV VO. RNAseq analysis of RV (from postnatal day 14 to 21) demonstrated that angiogenesis was the most enriched gene ontology (GO) term in both the sham and VO groups. Regulation of the mitotic cell cycle was the second-most enriched GO term in the VO group but it was not in the list of enriched GO terms in the sham group. In addition, the number of Ki67-positive cardiomyocytes increased approximately 20-fold in the VO group compared to the sham group. The intensity of the vascular endothelial cells also changed dramatically over time in both groups. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of the downregulated transcriptome revealed that the peroxisome proliferators-activated receptor (PPAR) signaling pathway was replaced by the cell cycle in the top-20 enriched KEGG terms because of the VO. Angiogenesis was one of the primary downregulated processes in postnatal RV development, and the cell cycle was reactivated under the influence of VO. The mechanism underlying the effects we observed may be associated with the replacement of the PPAR-signaling pathway with the cell-cycle pathway.

A word on netting of angiotensin-converting enzyme inhibitor therapy in hypoplastic left heart syndrome following stage-I

NPC-QIC Registry data showed that angiotensin-converting enzyme inhibitors were described in almost 38% for patients with single ventricle physiology after stage-I Norwood palliation. However, mortality and ventricular dysfunction or atrioventricular valve insufficiency seems to be not improved by oral application of angiotensin-converting enzyme inhibitors. The final conclusion was that despite limited evidence of benefit for patients with hypoplastic left heart syndrome, prescription of angiotensin-converting enzyme inhibitors during interstage is still common. Taking into account of the predominant cardiovascular regulation in newborns and young infants by circulating catecholamines, no real improvement is to be expected from angiotensin-converting enzyme inhibitor monotherapy. The goals of drug therapy after stage-I Norwood palliation in hypoplastic left heart syndrome are prevention of systemic right ventricle failure, balancing pulmonary and systemic blood flow, and reduction of oxygen consumption with regard to limitations of oxygen supply by the single ventricle, furthermore, avoiding harmful effects of endogenous catecholamine production in the long term on somatic and cognitive development. In this light of knowledge, we want to recommend the use of a long-acting and highly specific ß1-adrenoreceptor blocker for almost all patients after stage-I Norwood palliation and a combination with angiotensin-converting enzyme inhibitors only by indication after exclusion of potential side effects.

Role of cardiac magnetic resonance strain analysis in patients with hypoplastic left heart syndrome in evaluating right ventricular (dys)function: a systematic review

OBJECTIVES

Right ventricular dysfunction predicts death in patients with hypoplastic left heart syndrome (HLHS), but differences in morphology and loading conditions make calculation of the ejection fraction (EF), a challenging measure of its function. Our goal was to evaluate how strain measurements with cardiac magnetic resonance feature tracking could be used to evaluate right ventricular function in patients with HLHS.

METHODS

A systematic search of the literature was performed by 2 independent researchers using the terms 'population', 'intervention', 'comparison', 'outcome' and 'time criteria'. PubMed and the Ovid database were searched according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.

RESULTS

Our review included 8 studies with 608 participants with ventricular strain values obtained using cardiac magnetic resonance feature tracking. After stage I palliation, global strain was reduced in patients after a hybrid procedure and a right ventricle-to-pulmonary artery conduit compared with a modified Blalock-Taussig shunt despite similar EFs. Global longitudinal strain did not differ between stage II and stage III (Fontan) palliation. Fontan patients had significantly impaired global longitudinal and circumferential strain compared to the left ventricular strain of the controls. Studies of Fontan patients that included patients with HLHS who were part of a cohort with a single right ventricle showed impaired global circumferential strain compared with the cohort with a single left ventricle, with controls, and over time. In this group, impaired global circumferential strain was associated with major adverse cardiac events.

CONCLUSIONS

Cardiac magnetic resonance feature tracking can be used in patients with HLHS to evaluate RV strain and demonstrate differences between surgical strategies, over time and compared with controls. It could be used alongside clinical symptoms and EF values to detect ventricular dysfunction.

Polyploid cardiomyocytes: implications for heart regeneration

Terminally differentiated cells are generally thought to have arrived at their final form and function. Many terminally differentiated cell types are polyploid, i.e. they have multiple copies of the normally diploid genome. Mammalian heart muscle cells, termed cardiomyocytes, are one such example of polyploid cells. Terminally differentiated cardiomyocytes are bi- or multi-nucleated, or have polyploid nuclei. Recent mechanistic studies of polyploid cardiomyocytes indicate that they can limit cellular proliferation and, hence, heart regeneration. In this short Spotlight, we present the mechanisms generating bi- and multi-nucleated cardiomyocytes, and the mechanisms generating polyploid nuclei. Our aim is to develop hypotheses about how these mechanisms might relate to cardiomyocyte proliferation and cardiac regeneration. We also discuss how these new findings could be applied to advance cardiac regeneration research, and how they relate to studies of other polyploid cells, such as cancer cells.

Design and rationale of a clinical trial to increase cardiomyocyte division in infants with tetralogy of Fallot

BACKGROUND

Patients with Tetralogy of Fallot with pulmonary stenosis (ToF/PS), the most common form of cyanotic congenital heart disease (CHD), develop adverse right ventricular (RV) remodeling, leading to late heart failure and arrhythmia. We recently demonstrated that overactive β-adrenergic receptor signaling inhibits cardiomyocyte division in ToF/PS infants, providing a conceptual basis for the hypothesis that treatment with the β-adrenergic receptor blocker, propranolol, early in life would increase cardiomyocyte division. No data are available in ToF/PS infants on the efficacy of propranolol as a possible novel therapeutic option to increase cardiomyocyte division and potentially reduce adverse RV remodeling.

METHODS

Using a randomized, double-blind, placebo-controlled trial, we will evaluate the effect of propranolol administration on reactivating cardiomyocyte proliferation to prevent adverse RV remodeling in 40 infants with ToF/PS. Propranolol administration (1 mg/kg po QID) will begin at 1 month of age and last until surgical repair. The primary endpoint is cardiomyocyte division, quantified after ¹⁵N-thymidine administration with Multi-isotope Imaging Mass Spectrometry (MIMS) analysis of resected myocardial specimens. The secondary endpoints are changes in RV myocardial and cardiomyocyte hypertrophy.

CONCLUSION

This trial will be the first study in humans to assess whether cardiomyocyte proliferation can be pharmacologically increased. If successful, the results could introduce a paradigm shift in the management of patients with ToF/PS from a purely surgical approach, to synergistic medical and surgical management. It will provide the basis for future multi-center randomized controlled trials of propranolol administration in infants with ToF/PS and other types of CHD with RV hypertension.

CLINICAL TRIAL REGISTRATION

The trial protocol was registered at clinicaltrials.gov (NCT04713657).

Fosl1 is vital to heart regeneration upon apex resection in adult Xenopus tropicalis

Cardiovascular disease is the leading cause of death in the world due to losing regenerative capacity in the adult heart. Frogs possess remarkable capacities to regenerate multiple organs, including spinal cord, tail, and limb, but the response to heart injury and the underlying molecular mechanism remains largely unclear. Here we demonstrated that cardiomyocyte proliferation greatly contributes to heart regeneration in adult X. tropicalis upon apex resection. Using RNA-seq and qPCR, we found that the expression of Fos-like antigen 1 (Fosl1) was dramatically upregulated in early stage of heart injury. To study Fosl1 function in heart regeneration, its expression was modulated in vitro and in vivo. Overexpression of X. tropicalis Fosl1 significantly promoted the proliferation of cardiomyocyte cell line H9c2. Consistently, endogenous Fosl1 knockdown suppressed the proliferation of H9c2 cells and primary cardiomyocytes isolated from neonatal mice. Taking use of a cardiomyocyte-specific dominant-negative approach, we show that blocking Fosl1 function leads to defects in cardiomyocyte proliferation during X. tropicalis heart regeneration. We further show that knockdown of Fosl1 can suppress the capacity of heart regeneration in neonatal mice, but overexpression of Fosl1 can improve the cardiac function in adult mouse upon myocardium infarction. Co-immunoprecipitation, luciferase reporter, and ChIP analysis reveal that Fosl1 interacts with JunB and promotes the expression of Cyclin-T1 (Ccnt1) during heart regeneration. In conclusion, we demonstrated that Fosl1 plays an essential role in cardiomyocyte proliferation and heart regeneration in vertebrates, at least in part, through interaction with JunB, thereby promoting expression of cell cycle regulators including Ccnt1.

Hypertrophic Cardiomyopathy in Infants from the Perspective of Cardiomyocyte Maturation

Hypertrophic cardiomyopathy (HCM) in infancy is rare and many fulminant cases are fatal. Infantile HCM shows a rapid progressive clinical course and different characteristics compared with late-onset HCM presenting during the prepubertal age. There are also spontaneously resolving phenotypes of HCM that are diagnosed in neonates being treated for bronchopulmonary dysplasia with corticosteroids or in those with other problems related to maternal endocrine diseases. The pathophysiology of infantile HCM has not been well described. Therefore, this review updates the pathophysiology of infantile HCM and includes molecular studies on maturation of cardiomyocytes from a clinician's point of view.

Hypertrophic cardiomyopathy (HCM) is characterized by ventricular wall hypertrophy with diastolic dysfunction. Pediatric HCM is distinguished from the adult in many aspects. Most children with HCM do not present clinically until the adolescent period, even when they are born with genetic mutations. Some infants with early-onset HCM present with massive progressive myocardial hypertrophy in the first few months of life, which is often fatal. The mortality of pediatric HCM peaks during the infantile and adolescent periods. These periods roughly correlate with children's growth spurt. Non-sarcomeric causes of HCM are more frequent in pediatric HCM, while sarcomeric causes are more common in adults. From the perspective of cardiac development, the fetal heart has immature cardiomyocytes, which are characterized by proliferation and exit their cell cycles with a decreased regenerative property after birth. In the perinatal period, there is a dynamic change in maturation of cardiomyocytes from immature to mature cells. Infants who are treated with steroids or born to mothers with diabetes or hyperthyroidism often show phenotypes of HCM, which gradually resolve. With remarkable advancement of molecular biology, understanding on maturation of cardiomyocytes has increased. Neonates undergo abrupt environmental changes during the transitional circulation, which is affected by oxygen, metabolic and hormonal fluctuations. Derangement in physiological transition to the normal postnatal environment may influence maturation of proliferative immature cardiomyocytes during early infancy. This article reviews updates of infantile HCM and recent molecular studies related to maturation of cardiomyocytes from the clinical point of view of identifying distinct characteristics of infantile HCM.

Next generation of heart regenerative therapies: progress and promise of cardiac tissue engineering

The adult heart is a vital and highly specialized organ of the human body, with limited capability of self-repair and regeneration in case of injury or disease. Engineering biomimetic cardiac tissue to regenerate the heart has been an ambition in the field of tissue engineering, tracing back to the 1990s. Increased understanding of human stem cell biology and advances in process engineering have provided an unlimited source of cells, particularly cardiomyocytes, for the development of functional cardiac muscle, even though pluripotent stem cell-derived cardiomyocytes poorly resemble those of the adult heart. This review outlines key biology-inspired strategies reported to improve cardiomyocyte maturation features and current biofabrication approaches developed to engineer clinically relevant cardiac tissues. It also highlights the potential use of this technology in drug discovery science and disease modeling as well as the current efforts to translate it into effective therapies that improve heart function and promote regeneration.

Utilizing Developmentally Essential Secreted Peptides Such as Thymosin Beta-4 to Remind the Adult Organs of Their Embryonic State-New Directions in Anti-Aging Regenerative Therapies

Our dream of defeating the processes of aging has occupied the curious and has challenged scientists globally for hundreds of years. The history is long, and sadly, the solution is still elusive. Our endeavors to reverse the magnitude of damaging cellular and molecular alterations resulted in only a few, yet significant advancements. Furthermore, as our lifespan increases, physicians are facing more mind-bending questions in their routine practice than ever before. Although the ultimate goal is to successfully treat the body as a whole, steps towards regenerating individual organs are even considered significant. As our initial approach to enhance the endogenous restorative capacity by delivering exogenous progenitor cells appears limited, we propose, utilizing small molecules critical during embryonic development may prove to be a powerful tool to increase regeneration and to reverse the processes associated with aging. In this review, we introduce Thymosin beta-4, a 43aa secreted peptide fulfilling our hopes and capable of numerous regenerative achievements via systemic administration in the heart. Observing the broad capacity of this small, secreted peptide, we believe it is not the only molecule which nature conceals to our benefit. Hence, the discovery and postnatal administration of developmentally relevant agents along with other approaches may result in reversing the aging process.

Preterm Birth With Neonatal Interventions Accelerates Collagen Deposition in the Left Ventricle of Lambs Without Affecting Cardiomyocyte Development

Background

Adults born preterm (< 37 weeks’ gestation) exhibit altered cardiac growth and are susceptible to cardiac dysfunction. Sheep studies have shown that moderate preterm birth results in maladaptive structural remodelling of the cardiac ventricles. The aim of this study was to examine ventricular structure in lambs born at a greater severity of preterm birth and ventilated postnatally.

Methods

Former-preterm lambs delivered at 128 days’ gestation, and mechanically ventilated for a week after birth, were compared with unventilated lambs born at term (150 days’ gestation), at 2 months (term: n = 10, former-preterm: n = 8), and 5 months (term: n = 9, former-preterm: n = 8) term-equivalent age. The right ventricle and left ventricle plus septum were analysed using immunohistochemistry, histology, and stereology.

Results

Cardiomyocyte number, cross-sectional area, proliferation, and apoptosis were not affected by preterm birth or age. Left ventricle plus septum interstitial collagen levels increased with age (P = 0.0015) and were exacerbated by preterm birth (P = 0.0006; 2 months term: 0.57% ± 0.07%, former-preterm: 1.44% ± 0.18%; 5 months term: 1.37% ± 0.25%, former-preterm: 2.15% ± 0.31%). Right ventricle interstitial collagen levels increased with age (P = 0.012) but were not affected by preterm birth.

Conclusion

This study is the first to explore the effect of preterm birth combined with modern neonatal interventions on the ventricular myocardium in lambs. There was no adverse impact on cardiomyocyte growth in early postnatal life. Of concern, however, there was increased collagen deposition in the preterm hearts, which has the potential to induce cardiac dysfunction, especially if it becomes exaggerated with ageing.

Maturation of human pluripotent stem cell derived cardiomyocytes in vitro and in vivo

Human pluripotent stem cell derived cardiomyocytes (hPSC-CMs) represent an inexhaustible cell source for in vitro disease modeling, drug discovery and toxicity screening, and potential therapeutic applications. However, currently available differentiation protocols yield populations of hPSC-CMs with an immature phenotype similar to cardiomyocytes in the early fetal heart. In this review, we consider the developmental processes and signaling cues involved in normal human cardiac maturation, as well as how these insights might be applied to the specific maturation of hPSC-CMs. We summarize the state-of-the-art and relative merits of reported hPSC-CM maturation strategies including prolonged duration in culture, metabolic manipulation, treatment with soluble or substrate-based cues, and tissue engineering approaches. Finally, we review the evidence that hPSC-CMs mature after implantation in injured hearts as such in vivo remodeling will likely affect the safety and efficacy of a potential hPSC-based cardiac therapy.

The Role of TGF-β Signaling in Cardiomyocyte Proliferation

PURPOSE OF REVIEW

The loss of contractile function after heart injury remains one of the major healthcare issues of our time. One strategy to deal with this problem would be to increase the number of cardiomyocytes to enhance cardiac function. In the last couple of years, reactivation of cardiomyocyte proliferation has repeatedly demonstrated to aid in functional recovery after cardiac injury.

RECENT FINDINGS

The Tgf-β superfamily plays key roles during development of the heart and populating the embryonic heart with cardiomyocytes. In this review, we discuss the role of Tgf-β signaling in regulating cardiomyocyte proliferation during development and in the setting of cardiac regeneration. Although various pathways to induce cardiomyocyte proliferation have been established, the extent to which cardiomyocyte proliferation requires or involves activation of the Tgf-β superfamily is not entirely clear. More research is needed to better understand cross-talk between pathways that regulate cardiomyocyte proliferation.

Sarcomere function activates a p53-dependent DNA damage response that promotes polyploidization and limits in vivo cell engraftment

Human cardiac regeneration is limited by low cardiomyocyte replicative rates and progressive polyploidization by unclear mechanisms. To study this process, we engineer a human cardiomyocyte model to track replication and polyploidization using fluorescently tagged cyclin B1 and cardiac troponin T. Using time-lapse imaging, in vitro cardiomyocyte replication patterns recapitulate the progressive mononuclear polyploidization and replicative arrest observed in vivo. Single-cell transcriptomics and chromatin state analyses reveal that polyploidization is preceded by sarcomere assembly, enhanced oxidative metabolism, a DNA damage response, and p53 activation. CRISPR knockout screening reveals p53 as a driver of cell-cycle arrest and polyploidization. Inhibiting sarcomere function, or scavenging ROS, inhibits cell-cycle arrest and polyploidization. Finally, we show that cardiomyocyte engraftment in infarcted rat hearts is enhanced 4-fold by the increased proliferation of troponin-knockout cardiomyocytes. Thus, the sarcomere inhibits cell division through a DNA damage response that can be targeted to improve cardiomyocyte replacement strategies.

The role of hormones and neurons in cardiomyocyte maturation

The heart undergoes profound morphological and functional changes as it continues to mature postnatally. However, this phase of cardiac development remains understudied. More recently, cardiac maturation research has attracted a lot of interest due to the need for more mature stem cell-derived cardiomyocytes for disease modeling, drug screening and heart regeneration. Additionally, neonatal heart injury models have been utilized to study heart regeneration, and factors regulating postnatal heart development have been associated with adult cardiac disease. Critical components of cardiac maturation are systemic and local biochemical cues. Specifically, cardiac innervation and the concentration of various metabolic hormones appear to increase perinatally and they have striking effects on cardiomyocytes. Here, we first report some of the key parameters of mature cardiomyocytes and then discuss the specific effects of neurons and hormonal cues on cardiomyocyte maturation. We focus primarily on the structural, electrophysiologic, metabolic, hypertrophic and hyperplastic effects of each factor. This review highlights the significance of underappreciated regulators of cardiac maturation and underscores the need for further research in this exciting field.

Biodiversity-based development and evolution: the emerging research systems in model and non-model organisms

Evolutionary developmental biology, or Evo-Devo for short, has become an established field that, broadly speaking, seeks to understand how changes in development drive major transitions and innovation in organismal evolution. It does so via integrating the principles and methods of many subdisciplines of biology. Although we have gained unprecedented knowledge from the studies on model organisms in the past decades, many fundamental and crucially essential processes remain a mystery. Considering the tremendous biodiversity of our planet, the current model organisms seem insufficient for us to understand the evolutionary and physiological processes of life and its adaptation to exterior environments. The currently increasing genomic data and the recently available gene-editing tools make it possible to extend our studies to non-model organisms. In this review, we review the recent work on the regulatory signaling of developmental and regeneration processes, environmental adaptation, and evolutionary mechanisms using both the existing model animals such as zebrafish and Drosophila, and the emerging nonstandard model organisms including amphioxus, ascidian, ciliates, single-celled phytoplankton, and marine nematode. In addition, the challenging questions and new directions in these systems are outlined as well.

GLUT1 overexpression enhances glucose metabolism and promotes neonatal heart regeneration

The mammalian heart switches its main metabolic substrate from glucose to fatty acids shortly after birth. This metabolic switch coincides with the loss of regenerative capacity in the heart. However, it is unknown whether glucose metabolism regulates heart regeneration. Here, we report that glucose metabolism is a determinant of regenerative capacity in the neonatal mammalian heart. Cardiac-specific overexpression of Glut1, the embryonic form of constitutively active glucose transporter, resulted in an increase in glucose uptake and concomitant accumulation of glycogen storage in postnatal heart. Upon cryoinjury, Glut1 transgenic hearts showed higher regenerative capacity with less fibrosis than non-transgenic control hearts. Interestingly, flow cytometry analysis revealed two distinct populations of ventricular cardiomyocytes: Tnnt2-high and Tnnt2-low cardiomyocytes, the latter of which showed significantly higher mitotic activity in response to high intracellular glucose in Glut1 transgenic hearts. Metabolic profiling shows that Glut1-transgenic hearts have a significant increase in the glucose metabolites including nucleotides upon injury. Inhibition of the nucleotide biosynthesis abrogated the regenerative advantage of high intra-cardiomyocyte glucose level, suggesting that the glucose enhances the cardiomyocyte regeneration through the supply of nucleotides. Our data suggest that the increase in glucose metabolism promotes cardiac regeneration in neonatal mouse heart.