Building and re-building the heart by cardiomyocyte proliferation

Loss of microRNA-128 promotes cardiomyocyte proliferation and heart regeneration

The goal of replenishing the cardiomyocyte (CM) population using regenerative therapies following myocardial infarction (MI) is hampered by the limited regeneration capacity of adult CMs, partially due to their withdrawal from the cell cycle. Here, we show that microRNA-128 (miR-128) is upregulated in CMs during the postnatal switch from proliferation to terminal differentiation. In neonatal mice, cardiac-specific overexpression of miR-128 impairs CM proliferation and cardiac function, while miR-128 deletion extends proliferation of postnatal CMs by enhancing expression of the chromatin modifier SUZ12, which suppresses p27 (cyclin-dependent kinase inhibitor) expression and activates the positive cell cycle regulators Cyclin E and CDK2. Furthermore, deletion of miR-128 promotes cell cycle re-entry of adult CMs, thereby reducing the levels of fibrosis, and attenuating cardiac dysfunction in response to MI. These results suggest that miR-128 serves as a critical regulator of endogenous CM proliferation, and might be a novel therapeutic target for heart repair.

Telomeres and telomerase in heart regeneration

Although recent advances have overturned the old view of the human heart as an inert postmitotic organ, it is clear that the adult heart´s capacity to regenerate after an ischemic episode is very limited. Unlike humans, zebrafish and other lower vertebrates vigorously regenerate damaged myocardium after cardiac injury. Understanding how the zebrafish is able to conserve life-long cardiac regeneration capacity while mammals lose it soon after birth is crucial for the development of new treatments for myocardial infarction. Mammals and lower vertebrates differ markedly in their rates of cardiomyocyte proliferation and levels of telomerase activity. Here, we review recent discoveries identifying lack of telomerase activity and concomitant telomere dysfunction as natural barriers to cardiomyocyte proliferation and cardiac regeneration.

Complement Receptor C5aR1 Plays an Evolutionarily Conserved Role in Successful Cardiac Regeneration

BACKGROUND

Defining conserved molecular pathways in animal models of successful cardiac regeneration could yield insight into why adult mammals have inadequate cardiac regeneration after injury. Insight into the transcriptomic landscape of early cardiac regeneration from model organisms will shed light on evolutionarily conserved pathways in successful cardiac regeneration.

METHODS

Here we describe a cross-species transcriptomic screen in 3 model organisms for cardiac regeneration: axolotl, neonatal mice, and zebrafish. Apical resection to remove ≈10% to 20% of ventricular mass was carried out in these model organisms. RNA-sequencing analysis was performed on the hearts harvested at 3 time points: 12, 24, and 48 hours after resection. Sham surgery was used as internal control.

RESULTS

Genes associated with inflammatory processes were found to be upregulated in a conserved manner. Complement receptors (activated by complement components, part of the innate immune system) were found to be highly upregulated in all 3 species. This approach revealed induction of gene expression for complement 5a receptor 1 in the regenerating hearts of zebrafish, axolotls, and mice. Inhibition of complement 5a receptor 1 significantly attenuated the cardiomyocyte proliferative response to heart injury in all 3 species. Furthermore, after left ventricular apical resection, the cardiomyocyte proliferative response was diminished in mice with genetic deletion of complement 5a receptor 1.

CONCLUSIONS

These data reveal that activation of complement 5a receptor 1 mediates an evolutionarily conserved response that promotes cardiomyocyte proliferation after cardiac injury and identify complement pathway activation as a common pathway of successful heart regeneration.

Fetal Origins of Hypertension

Hypertension is a common noncommunicable disease. According to the World Health Organization, 1.13 billion people were suffering from hypertension in the year 2015. High blood pressure, hypertension, has a multifactorial etiology. Arterial atherosclerotic changes, systolic or diastolic dysfunction of the heart, and other noncardiac factors are involved. Epidemiological evidence has revealed that perinatal growth disturbance elevates the prevalence of hypertension. However, the specific effects of developmental disturbances on the pathological process of hypertension are poorly understood. Recently, it has become apparent that the perinatal period plays many essential roles in cardiovascular development. In this chapter, we focus on the perinatal development of the cardiovascular system, especially in murine models. Individual organs, blood, blood vessels, and the heart show unique growth characteristics during this period. We also introduce evidence from related clinical studies regarding the developmental origins of hypertension. Finally, evidence from several animal models is presented to reveal the effects of developmental disturbance or stress on arterial pathology. Improving our understanding of both developmental events and the results of clinical studies will give fresh insight into the fetal origins of hypertension.

Neonatal Rat Cardiomyocytes Isolation, Culture, and Determination of MicroRNAs' Effects in Proliferation

Cardiomyocytes loss is a major contributor for many cardiovascular diseases including heart failure and myocardial infarction. Although extremely limited, adult cardiomyocytes are able to proliferate. Understanding the molecular mechanisms controlling cardiomyocytes proliferation is extremely important for enhancing cardiomyocyte proliferation to promote cardiac regeneration and repair. MicroRNAs (miRNAs, miRs) are powerful controllers of many essential biological processes including cell proliferation. Here, we described in detail a protocol for isolation and culture of neonatal rat cardiomyocytes and the determination of miRNAs' effects in proliferation based on two well-established methods including EdU and Ki67 immunofluorescent stainings.

Drug Discovery in Fish, Flies, and Worms

Nonmammalian model organisms such as the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the zebrafish Danio rerio provide numerous experimental advantages for drug discovery including genetic and molecular tractability, amenability to high-throughput screening methods and reduced experimental costs and increased experimental throughput compared to traditional mammalian models. An interdisciplinary approach that strategically combines the study of nonmammalian and mammalian animal models with diverse experimental tools has and will continue to provide deep molecular and genetic understanding of human disease and will significantly enhance the discovery and application of new therapies to treat those diseases. This review will provide an overview of C. elegans, Drosophila, and zebrafish biology and husbandry and will discuss how these models are being used for phenotype-based drug screening and for identification of drug targets and mechanisms of action. The review will also describe how these and other nonmammalian model organisms are uniquely suited for the discovery of drug-based regenerative medicine therapies.

DanHong injection targets endothelin receptor type B and angiotensin II receptor type 1 in protection against cardiac hypertrophy

Cardiac hypertrophy (CH) is an independent risk factor for cardiovascular diseases (CVDs). Mitigating or preventing CH is the most effective strategy for the treatment of CVDs. DanHong injection (DH) is a Chinese herbal medicine preparation (CHMP) widely used in clinical treatment of several CVDs in China. However, the direct targets and cellular mechanisms for these protective effects remain unclear. This study was designed to illustrate the direct targets of DH in protecting against CH and investigate CH molecular pathogenesis. A hypertrophic cell model was induced by endothelin-1 (ET-1) on human induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs). Real time cellular analysis (RTCA) cardio system and high content analysis (HCA) were used to detect the changes in contractile function, morphology and protein level of hypertrophic hiPS-CMs. Agonist and antagonist assay on receptors were performed using calcium mobilization high-throughput screening (HTS). DH significantly attenuated CH by modulating myocardial contractility, suppressing cell area enlargement and down-regulating ET-1-induced brain natriuretic peptide (BNP), actinin alpha 2 (ACTN2) and cardiac muscle troponin T (TNNT2) protein expression (P < 0.05). Endothelin receptor type B (ETBR) and angiotensin II receptor type 1 (AT1R) were DH direct targets, with IC50 value of 25.67 μL/mL and 1.10 μL/mL, respectively. Proteomics analysis showed that proteins involved in cell cycle inhibition, RNA processing, mitochondrial translation and cytoskeleton are significant regulated by DH treatment. These data revealed that ETBR and AT1R are DH direct targets on protecting against CH, providing a strategy to explore direct targets of CHMPs.

Multicellular Transcriptional Analysis of Mammalian Heart Regeneration

BACKGROUND

The inability of the adult mammalian heart to regenerate following injury represents a major barrier in cardiovascular medicine. In contrast, the neonatal mammalian heart retains a transient capacity for regeneration, which is lost shortly after birth. Defining the molecular mechanisms that govern regenerative capacity in the neonatal period remains a central goal in cardiac biology. Here, we assemble a transcriptomic framework of multiple cardiac cell populations during postnatal development and following injury, which enables comparative analyses of the regenerative (neonatal) versus nonregenerative (adult) state for the first time.

METHODS

Cardiomyocytes, fibroblasts, leukocytes, and endothelial cells from infarcted and noninfarcted neonatal (P1) and adult (P56) mouse hearts were isolated by enzymatic dissociation and fluorescence-activated cell sorting at day 3 following surgery. RNA sequencing was performed on these cell populations to generate the transcriptome of the major cardiac cell populations during cardiac development, repair, and regeneration. To complement our transcriptomic data, we also surveyed the epigenetic landscape of cardiomyocytes during postnatal maturation by performing deep sequencing of accessible chromatin regions by using the Assay for Transposase-Accessible Chromatin from purified mouse cardiomyocyte nuclei (P1, P14, and P56).

RESULTS

Profiling of cardiomyocyte and nonmyocyte transcriptional programs uncovered several injury-responsive genes across regenerative and nonregenerative time points. However, the majority of transcriptional changes in all cardiac cell types resulted from developmental maturation from neonatal stages to adulthood rather than activation of a distinct regeneration-specific gene program. Furthermore, adult leukocytes and fibroblasts were characterized by the expression of a proliferative gene expression network following infarction, which mirrored the neonatal state. In contrast, cardiomyocytes failed to reactivate the neonatal proliferative network following infarction, which was associated with loss of chromatin accessibility around cell cycle genes during postnatal maturation.

CONCLUSIONS

This work provides a comprehensive framework and transcriptional resource of multiple cardiac cell populations during cardiac development, repair, and regeneration. Our findings define a regulatory program underpinning the neonatal regenerative state and identify alterations in the chromatin landscape that could limit reinduction of the regenerative program in adult cardiomyocytes.

Multimodal Regulation of Cardiac Myocyte Proliferation

Efficient cardiac regeneration is closely associated with the ability of cardiac myocytes to proliferate. Fetal or neonatal mouse hearts containing proliferating cardiac myocytes regenerate even extensive injuries, whereas adult hearts containing mostly post-mitotic cardiac myocytes have lost this ability. The same correlation is seen in some homoiotherm species such as teleost fish and urodelian amphibians leading to the hypothesis that cardiac myocyte proliferation is a major driver of heart regeneration. Although cardiomyocyte proliferation might not be the only prerequisite to restore full organ function after cardiac damage, induction of cardiac myocyte proliferation is an attractive therapeutic option to cure the injured heart and prevent heart failure. To (re)initiate cardiac myocyte proliferation in adult mammalian hearts, a thorough understanding of the molecular circuitry governing cardiac myocyte cell cycle regulation is required. Here, we review the current knowledge in the field focusing on the withdrawal of cardiac myocytes from the cell cycle during the transition from neonatal to adult stages.

Prenatal Mechanistic Target of Rapamycin Complex 1 (m TORC1) Inhibition by Rapamycin Treatment of Pregnant Mice Causes Intrauterine Growth Restriction and Alters Postnatal Cardiac Growth, Morphology, and Function

Background

Fetal growth impacts cardiovascular health throughout postnatal life in humans. Various animal models of intrauterine growth restriction exhibit reduced heart size at birth, which negatively influences cardiac function in adulthood. The mechanistic target of rapamycin complex 1 (mTORC1) integrates nutrient and growth factor availability with cell growth, thereby regulating organ size. This study aimed at elucidating a possible involvement of mTORC1 in intrauterine growth restriction and prenatal heart growth.

Methods and Results

We inhibited mTORC1 in fetal mice by rapamycin treatment of pregnant dams in late gestation. Prenatal rapamycin treatment reduces mTORC1 activity in various organs at birth, which is fully restored by postnatal day 3. Rapamycin‐treated neonates exhibit a 16% reduction in body weight compared with vehicle‐treated controls. Heart weight decreases by 35%, resulting in a significantly reduced heart weight/body weight ratio, smaller left ventricular dimensions, and reduced cardiac output in rapamycin‐ versus vehicle‐treated mice at birth. Although proliferation rates in neonatal rapamycin‐treated hearts are unaffected, cardiomyocyte size is reduced, and apoptosis increased compared with vehicle‐treated neonates. Rapamycin‐treated mice exhibit postnatal catch‐up growth, but body weight and left ventricular mass remain reduced in adulthood. Prenatal mTORC1 inhibition causes a reduction in cardiomyocyte number in adult hearts compared with controls, which is partially compensated for by an increased cardiomyocyte volume, resulting in normal cardiac function without maladaptive left ventricular remodeling.

Conclusions

Prenatal rapamycin treatment of pregnant dams represents a new mouse model of intrauterine growth restriction and identifies an important role of mTORC1 in perinatal cardiac growth.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

Programming cells for cardiac repair

Because the heart is a poorly regenerative organ, there has been considerable interest in developing novel cell-based approaches to restore lost contractile function after myocardial infarction (MI). While a wide variety of candidate cell types have been tested in animal MI models, the vast majority of clinical trials have used adult stem cell types, usually derived from bone marrow. These studies have generally yielded disappointing results, an outcome that may reflect in part the limited cardiogenic potential of the adult stem cell sources employed. Post-MI heart failure is ultimately a disease of cardiomyocyte deficiency, so better outcomes may be possible with more cardiogenic approaches that may 'remuscularize' the infarct scar with new, electrically-integrated myocardium. In this review, we summarize work in the field to 'program' exogenous or endogenous cells into such a cardiogenic state, as well as efforts to test their capacity to mediate true heart regeneration.

Cardiac regeneration strategies: Staying young at heart

The human heart is continually operating as a muscular pump, contracting, on average, 80 times per minute to propel 8000 liters of blood through body tissues each day. Whereas damaged skeletal muscle has a profound capacity to regenerate, heart muscle, at least in mammals, has poor regenerative potential. This deficiency is attributable to the lack of resident cardiac stem cells, combined with roadblocks that limit adult cardiomyocytes from entering the cell cycle and completing division. Insights for regeneration have recently emerged from studies of animals with an elevated innate capacity for regeneration, the innovation of stem cell and reprogramming technologies, and a clearer understanding of the cardiomyocyte genetic program and key extrinsic signals. Methods to augment heart regeneration now have potential to counteract the high morbidity and mortality of cardiovascular disease.

The Elusive Progenitor Cell in Cardiac Regeneration: Slip Slidin' Away

The adult human heart is unable to regenerate after various forms of injury, suggesting that this organ lacks a biologically meaningful endogenous stem cell pool. However, injecting the infarcted area of the adult mammalian heart with exogenously prepared progenitor cells of various types has been reported to create new myocardium by the direct conversion of these progenitor cells into cardiomyocytes. These reports remain controversial because follow-up studies from independent laboratories failed to observe such an effect. Also, the exact nature of various putative myocyte-producing progenitor cells remains elusive and undefined across laboratories. By comparison, the field has gradually worked toward a consensus viewpoint that proposes that the adult mammalian myocardium can undergo a low level of new cardiomyocyte renewal of ≈1% per year, which is primarily because of proliferation of existing cardiomyocytes but not from the differentiation of putative progenitor cells. This review will weigh the emerging evidence, suggesting that the adult mammalian heart lacks a definable myocyte-generating progenitor cell of biological significance.

Antagonistic regulation of cell-cycle and differentiation gene programs in neonatal cardiomyocytes by homologous MEF2 transcription factors

Cardiomyocytes acquire their primary specialized function (contraction) before exiting the cell cycle. In this regard, proliferation and differentiation must be precisely coordinated for proper cardiac morphogenesis. Here, we have investigated the complex transcriptional mechanisms employed by cardiomyocytes to coordinate antagonistic cell-cycle and differentiation gene programs through the molecular dissection of the core cardiac transcription factor, MEF2. Knockdown of individual MEF2 proteins, MEF2A, -C, and -D, in primary neonatal cardiomyocytes resulted in radically distinct and opposite effects on cellular homeostasis and gene regulation. MEF2A and MEF2D were absolutely required for cardiomyocyte survival, whereas MEF2C, despite its major role in cardiac morphogenesis and direct reprogramming, was dispensable for this process. Inhibition of MEF2A or -D also resulted in the activation of cell-cycle genes and down-regulation of markers of terminal differentiation. In striking contrast, the regulation of cell-cycle and differentiation gene programs by MEF2C was antagonistic to that of MEF2A and -D. Computational analysis of regulatory regions from MEF2 isoform-dependent gene sets identified the Notch and Hedgehog signaling pathways as key determinants in coordinating MEF2 isoform-specific control of antagonistic gene programs. These results reveal that mammalian MEF2 family members have distinct transcriptional functions in cardiomyocytes and suggest that these differences are critical for proper development and maturation of the heart. Analysis of MEF2 isoform-specific function in neonatal cardiomyocytes has yielded insight into an unexpected transcriptional regulatory mechanism by which these specialized cells utilize homologous members of a core cardiac transcription factor to coordinate cell-cycle and differentiation gene programs.

The careg element reveals a common regulation of regeneration in the zebrafish myocardium and fin

The existence of common mechanisms regulating organ regeneration is an intriguing concept. Here we report on a regulatory element that is transiently activated during heart and fin regeneration in zebrafish. This element contains a ctgfa upstream sequence, called careg, which is induced by TGFβ/Activin-β signalling in the peri-injury zone of the myocardium and the fin mesenchyme. In addition, this reporter demarcates a primordial cardiac layer and intraray osteoblasts. Using genetic fate mapping, we show the regenerative competence of careg-expressing cells. The analysis of the heart reveals that the primordial cardiac layer is incompletely restored after cryoinjury, whereas trabecular and cortical cardiomyocytes contribute to myocardial regrowth. In regenerating fins, the activated mesenchyme of the stump gives rise to the blastema. Our findings provide evidence of a common regenerative programme in cardiomyocytes and mesenchyme that opens the possibility to further explore conserved mechanisms of the cellular plasticity in diverse vertebrate organs.

Myocardial infarction in a neonate. Lessons for neonatal and internal medicine

Due to the lack of evidence-based guidelines, management strategies for neonatal MI should be individualized and administered largely at the discretion of responsible treating teams. Supportive care with a focus on preserving adequate circulation and antithrombotic therapy with a view to restoring vascular patency are the mainstays of treatment. Thrombolytic therapy of neonatal MI includes a chance to completely restore myocardial function. Understanding the resilience of the neonatal heart and mechanism of cardiac cell repair in neonates may spark novel treatment strategies for severe MI in the large number of affected individuals in an aging population.

The protein tyrosine phosphatase 1B inhibitor MSI-1436 stimulates regeneration of heart and multiple other tissues

Regenerative medicine holds substantial promise for repairing or replacing tissues and organs damaged by disease, injury, and degeneration. Much of the field has focused on development of cell-based therapeutics, gene-based therapeutics, and tissue engineering-based therapeutics. In contrast, development of small molecule regenerative medicine therapies is an emerging area. Using the adult zebrafish as a novel screening platform, we identified MSI-1436 as a first-in-class regenerative medicine drug candidate. MSI-1436 is a naturally occurring aminosterol that inhibits protein tyrosine phosphatase 1B. Treatment of adult zebrafish by intraperitoneal injection of MSI-1436 increased the rate of regeneration of the amputated caudal fin, which is comprised of bone, connective, skin, vascular and nervous tissues and also increased the rate of adult zebrafish heart regeneration. Intraperitoneal administration of MSI-1436 to adult mice for 4 weeks after induction of myocardial infarction increased survival, improved heart function, reduced infarct size, reduced ventricular wall thinning and increased cardiomyocyte proliferation. Satellite cell activation in injured mouse skeletal muscle was stimulated by MSI-1436. MSI-1436 was well tolerated by patients in Phase 1 and 1b obesity and type 2 diabetes clinical trials. Doses effective at stimulating regeneration are 5–50-times lower than the maximum well tolerated human dose. The demonstrated safety and well established pharmacological properties of MSI-1436 underscore the potential of this molecule as a novel treatment for heart attack and multiple other degenerative diseases.

A naturally occurring small molecule shows promise as a drug for tissue and organ repair and regeneration. Viravuth Yin of the Kathryn W. Davis Center for Regenerative Biology and Medicine with colleagues in the US found that treating zebrafish with an intraperitoneal injection of MSI-1436, which inhibits the enzyme ‘protein tyrosine phosphatase 1B’, increased the rate of regeneration of an amputated caudal fin and of partially removed heart muscle without apparent tissue malformation. Intraperitoneal injection of MSI-1436 in adult mice also reduced the size of an induced heart infarction, improved survivability, triggered new heart muscle formation and stimulated regeneration after skeletal muscle injury. Effective doses for tissue regeneration in both animals were much lower than the maximum tolerated doses found for humans in clinical trials for potential treatment of obesity and diabetes.

Blockade to pathological remodeling of infarcted heart tissue using a porcupine antagonist

The secreted Wnt signaling molecules are essential to the coordination of cell-fate decision making in multicellular organisms. In adult animals, the secreted Wnt proteins are critical for tissue regeneration and frequently contribute to cancer. Small molecules that disable the Wnt acyltransferase Porcupine (Porcn) are candidate anticancer agents in clinical testing. Here we have systematically assessed the effects of the Porcn inhibitor (WNT-974) on the regeneration of several tissue types to identify potentially unwanted chemical effects that could limit the therapeutic utility of such agents. An unanticipated observation from these studies is proregenerative responses in heart muscle induced by systemic chemical suppression of Wnt signaling. Using in vitro cultures of several cell types found in the heart, we delineate the Wnt signaling apparatus supporting an antiregenerative transcriptional program that includes a subunit of the nonfibrillar collagen VI. Similar to observations seen in animals exposed to WNT-974, deletion of the collagen VI subunit, COL6A1, has been shown to decrease aberrant remodeling and fibrosis in infarcted heart tissue. We demonstrate that WNT-974 can improve the recovery of heart function after left anterior descending coronary artery ligation by mitigating adverse remodeling of infarcted tissue. Injured heart tissue exposed to WNT-974 exhibits decreased scarring and reduced Col6 production. Our findings support the development of Porcn inhibitors as antifibrotic agents that could be exploited to promote heart repair following injury.

Zebrafish as a Vertebrate Model System to Evaluate Effects of Environmental Toxicants on Cardiac Development and Function

Environmental pollution is a serious problem of the modern world that possesses a major threat to public health. Exposure to environmental pollutants during embryonic development is particularly risky. Although many pollutants have been verified as potential toxicants, there are new chemicals in the environment that need assessment. Heart development is an extremely sensitive process, which can be affected by environmentally toxic molecule exposure during embryonic development. Congenital heart defects are the most common life-threatening global health problems, and the etiology is mostly unknown. The zebrafish has emerged as an invaluable model to examine substance toxicity on vertebrate development, particularly on cardiac development. The zebrafish offers numerous advantages for toxicology research not found in other model systems. Many laboratories have used the zebrafish to study the effects of widespread chemicals in the environment on heart development, including pesticides, nanoparticles, and various organic pollutants. Here, we review the uses of the zebrafish in examining effects of exposure to external molecules during embryonic development in causing cardiac defects, including chemicals ubiquitous in the environment and illicit drugs. Known or potential mechanisms of toxicity and how zebrafish research can be used to provide mechanistic understanding of cardiac defects are discussed.

Epithelial self-healing is recapitulated by a 3D biomimetic E-cadherin junction

Epithelial monolayers undergo self-healing when wounded. During healing, cells collectively migrate into the wound site, and the converging tissue fronts collide and form a stable interface. To heal, migrating tissues must form cell-cell adhesions and reorganize from the front-rear polarity characteristic of cell migration to the apical-basal polarity of an epithelium. However, identifying the "stop signal" that induces colliding tissues to cease migrating and heal remains an open question. Epithelial cells form integrin-based adhesions to the basal extracellular matrix (ECM) and E-cadherin-mediated cell-cell adhesions on the orthogonal, lateral surfaces between cells. Current biological tools have been unable to probe this multicellular 3D interface to determine the stop signal. We addressed this problem by developing a unique biointerface that mimicked the 3D organization of epithelial cell adhesions. This "minimal tissue mimic" (MTM) comprised a basal ECM substrate and a vertical surface coated with purified extracellular domain of E-cadherin, and was designed for collision with the healing edge of an epithelial monolayer. Three-dimensional imaging showed that adhesions formed between cells, and the E-cadherin-coated MTM resembled the morphology and dynamics of native epithelial cell-cell junctions and induced the same polarity transition that occurs during epithelial self-healing. These results indicate that E-cadherin presented in the proper 3D context constitutes a minimum essential stop signal to induce self-healing. That the Ecad:Fc MTM stably integrated into an epithelial tissue and reduced migration at the interface suggests that this biointerface is a complimentary approach to existing tissue-material interfaces.

Tbx20 Is an Essential Regulator of Embryonic Heart Growth in Zebrafish

The molecular mechanisms that regulate cardiomyocyte proliferation during embryonic heart growth are not completely deciphered yet. In a forward genetic N-ethyl-N-nitrosourea (ENU) mutagenesis screen, we identified the recessive embryonic-lethal zebrafish mutant line weiches herz (whz). Homozygous mutant whz embryos display impaired heart growth due to diminished embryonic cardiomyocyte proliferation resulting in cardiac hypoplasia and weak cardiac contraction. By positional cloning, we found in whz mutant zebrafish a missense mutation within the T-box 20 (Tbx20) transcription factor gene leading to destabilization of Tbx20 protein. Morpholino-mediated knock-down of Tbx20 in wild-type zebrafish embryos phenocopies whz, indicating that the whz phenotype is due to loss of Tbx20 function, thereby leading to significantly reduced cardiomyocyte numbers by impaired proliferation of heart muscle cells. Ectopic overexpression of wild-type Tbx20 in whz mutant embryos restored cardiomyocyte proliferation and heart growth. Interestingly, ectopic overexpression of Tbx20 in wild-type zebrafish embryos resulted, similar to the situation in the embryonic mouse heart, in significantly reduced proliferation rates of ventricular cardiomyocytes, suggesting that Tbx20 activity needs to be tightly fine-tuned to guarantee regular cardiomyocyte proliferation and embryonic heart growth in vivo.

5'-Hydroxymethylcytosine Precedes Loss of CpG Methylation in Enhancers and Genes Undergoing Activation in Cardiomyocyte Maturation

Background

Cardiomyocytes undergo major changes in DNA methylation during maturation and transition to a non-proliferative state after birth. 5’-hydroxylation of methylated cytosines (5hmC) is not only involved in DNA loss of CpG methylation but is also thought to be an epigenetic mark with unique distribution and functions. Here, we sought to get insight into the dynamics of 5’-hydroxymethylcytosine in newborn and adult cardiomyocytes.

Methods

Cardiomyocyte nuclei from newborn and adult C57BL/6 mice were purified by flow cytometric sorting. 5hmC-containing DNA was captured by selective chemical labeling, followed by deep sequencing. Sequencing reads of library replicates were mapped independently (n = 3 for newborn, n = 2 for adult mice) and merged for further analysis steps. 5hmC coverage was normalized to read length and the total number of mapped reads (RPKM). MethylC-Seq, ChIP-Seq and RNA-Seq data sets of newborn and adult cardiomyocytes served to elucidate specific features of 5hmC at gene bodies and around low methylated regions (LMRs) representing regulatory genomic regions with enhancer function.

Results

163,544 and 315,220 5hmC peaks were identified in P1 and adult cardiomyocytes, respectively. Of these peaks, 66,641 were common between P1 and adult cardiomyocytes with more than 50% reciprocal overlap. P1 and adult 5hmC peaks were overrepresented in genic features such as exons, introns, 3’- and 5’-untranslated regions (UTRs), promotors and transcription end sites (TES). During cardiomyocyte maturation, 5hmC was found to be enriched at sites of subsequent DNA loss of CpG methylation such as gene bodies of upregulated genes (i.e. Atp2a2, Tnni3, Mb, Pdk4). Additionally, centers of postnatally established enhancers were premarked by 5hmC before DNA loss of CpG methylation.

Conclusions

Simultaneous analysis of 5hmC-Seq, MethylC-Seq, RNA-Seq and ChIP-Seq data at two defined time points of cardiomyocyte maturation demonstrates that 5hmC is positively associated with gene expression and decorates sites of subsequent DNA loss of CpG methylation.

Melatonin promotes cardiomyogenesis of embryonic stem cells via inhibition of HIF-1α stabilization

Melatonin, a molecule involved in the regulation of circadian rhythms, has protective effects against myocardial injuries. However, its capability to regulate the maturation of cardiac progenitor cells is unclear. Recently, several studies have shown that melatonin inhibits the stabilization of hypoxia-inducible factors (HIFs), important signaling molecules with cardioprotective effects. In this study, by employing differentiating mouse embryonic stem cells, we report that melatonin significantly upregulated the expression of cardiac cell-specific markers (myosin heavy chains six and seven) as well as the percentage of myosin heavy chain-positive cells. Importantly, melatonin decreased HIF-1α stabilization and transcriptional activity and, in contrast, induced HIF-2α stabilization. Interestingly, the deletion of HIF-1α completely inhibited the pro-cardiomyogenic effect of melatonin as well as the melatonin-mediated HIF-2α stabilization. Moreover, melatonin increased Sirt-1 levels in a HIF-1α-dependent manner. Taken together, we provide new evidence of a time-specific inhibition of HIF-1α stabilization as an essential feature of melatonin-induced cardiomyogenesis and unexpected different roles of HIF-1α stabilization during various stages of cardiac development. These results uncover new mechanisms underlying the maturation of cardiac progenitor cells and can help in the development of novel strategies for using melatonin in cardiac regeneration therapy.

Decellularized zebrafish cardiac extracellular matrix induces mammalian heart regeneration

Heart attack is a global health problem that leads to significant morbidity, mortality, and health care burden. Adult human hearts have very limited regenerative capability after injury. However, evolutionarily primitive species generally have higher regenerative capacity than mammals. The extracellular matrix (ECM) may contribute to this difference. Mammalian cardiac ECM may not be optimally inductive for cardiac regeneration because of the fibrotic, instead of regenerative, responses in injured adult mammalian hearts. Given the high regenerative capacity of adult zebrafish hearts, we hypothesize that decellularized zebrafish cardiac ECM (zECM) made from normal or healing hearts can induce mammalian heart regeneration. Using zebrafish and mice as representative species of lower vertebrates and mammals, we show that a single administration of zECM, particularly the healing variety, enables cardiac functional recovery and regeneration of adult mouse heart tissues after acute myocardial infarction. zECM-treated groups exhibit proliferation of the remaining cardiomyocytes and multiple cardiac precursor cell populations and reactivation of ErbB2 expression in cardiomyocytes. Furthermore, zECM exhibits pro-proliferative and chemotactic effects on human cardiac precursor cell populations in vitro. These contribute to the structural preservation and correlate with significantly higher cardiac contractile function, notably less left ventricular dilatation, and substantially more elastic myocardium in zECM-treated hearts than control animals treated with saline or decellularized adult mouse cardiac ECM. Inhibition of ErbB2 activity abrogates beneficial effects of zECM administration, indicating the possible involvement of ErbB2 signaling in zECM-mediated regeneration. This study departs from conventional focuses on mammalian ECM and introduces a new approach for cardiac tissue regeneration.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Probing early heart development to instruct stem cell differentiation strategies

Scientists have studied organs and their development for centuries and, along that path, described models and mechanisms explaining the developmental principles of organogenesis. In particular, with respect to the heart, new fundamental discoveries are reported continuously that keep changing the way we think about early cardiac development. These discoveries are driven by the need to answer long-standing questions regarding the origin of the earliest cells specified to the cardiac lineage, the differentiation potential of distinct cardiac progenitor cells, and, very importantly, the molecular mechanisms underlying these specification events. As evidenced by numerous examples, the wealth of developmental knowledge collected over the years has had an invaluable impact on establishing efficient strategies to generate cardiovascular cell types ex vivo, from either pluripotent stem cells or via direct reprogramming approaches. The ability to generate functional cardiovascular cells in an efficient and reliable manner will contribute to therapeutic strategies aimed at alleviating the increasing burden of cardiovascular disease and morbidity. Here we will discuss the recent discoveries in the field of cardiac progenitor biology and their translation to the pluripotent stem cell model to illustrate how developmental concepts have instructed regenerative model systems in the past and promise to do so in the future. Developmental Dynamics 245:1130-1144, 2016. © 2016 Wiley Periodicals, Inc.

Innate heart regeneration: endogenous cellular sources and exogenous therapeutic amplification

INTRODUCTION

The -once viewed as heretical- concept of the adult mammalian heart as a dynamic organ capable of endogenous regeneration has recently gained traction. However, estimated rates of myocyte turnover vary wildly and the underlying mechanisms of cardiac plasticity remain controversial. It is still unclear whether the adult mammalian heart gives birth to new myocytes through proliferation of resident myocytes, through cardiomyogenic differentiation of endogenous progenitors or through both mechanisms.

AREAS COVERED

In this review, the authors discuss the cellular origins of postnatal mammalian cardiomyogenesis and touch upon therapeutic strategies that could potentially amplify innate cardiac regeneration.

EXPERT OPINION

The adult mammalian heart harbors a limited but detectable capacity for spontaneous endogenous regeneration. During normal aging, proliferation of pre-existing cardiomyocytes is the dominant mechanism for generation of new cardiomyocytes. Following myocardial injury, myocyte proliferation increases modestly, but differentiation of endogenous progenitor cells appears to also contribute to cardiomyogenesis (although agreement on the latter point is not universal). Since cardiomyocyte deficiency underlies almost all types of heart disease, development of therapeutic strategies that amplify endogenous regeneration to a clinically-meaningful degree is of utmost importance.

Ciona as a Simple Chordate Model for Heart Development and Regeneration

Cardiac cell specification and the genetic determinants that govern this process are highly conserved among Chordates. Recent studies have established the importance of evolutionarily-conserved mechanisms in the study of congenital heart defects and disease, as well as cardiac regeneration. As a basal Chordate, the Ciona model system presents a simple scaffold that recapitulates the basic blueprint of cardiac development in Chordates. Here we will focus on the development and cellular structure of the heart of the ascidian Ciona as compared to other Chordates, principally vertebrates. Comparison of the Ciona model system to heart development in other Chordates presents great potential for dissecting the genetic mechanisms that underlie congenital heart defects and disease at the cellular level and might provide additional insight into potential pathways for therapeutic cardiac regeneration.

Myocardial plasticity: cardiac development, regeneration and disease

The adult mammalian heart is unable to recover from myocardial cell loss due to cardiac ischemia and infarction because terminally differentiated cardiomyocytes proliferate at a low rate. However, cardiomyocytes in other vertebrate animal models such as zebrafish, axolotls, newts and mammalian mouse neonates are capable of de-differentiating in order to promote cardiomyocyte proliferation and subsequent cardiac regeneration after injury. Although de-differentiation may occur in adult mammalian cardiomyocytes, it is typically associated with diseased hearts and pathologic remodeling rather than repair and regeneration. Here, we review recent studies of cardiac development, regeneration and disease that highlight how changes in myocardial identity (plasticity) is regulated and impacts adaptive and maladaptive cardiac responses.

Cell migration during heart regeneration in zebrafish

Zebrafish possess the remarkable ability to regenerate injured hearts as adults, which contrasts the very limited ability in mammals. Although very limited, mammalian hearts do in fact have measurable levels of cardiomyocyte regeneration. Therefore, elucidating mechanisms of zebrafish heart regeneration would provide information of naturally occurring regeneration to potentially apply to mammalian studies, in addition to addressing this biologically interesting phenomenon in itself. Studies over the past 13 years have identified processes and mechanisms of heart regeneration in zebrafish. After heart injury, pre-existing cardiomyocytes dedifferentiate, enter the cell cycle, and repair the injured myocardium. This process requires interaction with epicardial cells, endocardial cells, and vascular endothelial cells. Epicardial cells envelope the heart, while endocardial cells make up the inner lining of the heart. They provide paracrine signals to cardiomyocytes to regenerate the injured myocardium, which is vascularized during heart regeneration. In addition, accumulating results suggest that local migration of these major cardiac cell types have roles in heart regeneration. In this review, we summarize the characteristics of various heart injury methods used in the research community and regeneration of the major cardiac cell types. Then, we discuss local migration of these cardiac cell types and immune cells during heart regeneration. Developmental Dynamics 245:774-787, 2016. © 2016 Wiley Periodicals, Inc.

Zebrafish fin and heart: what's special about regeneration?

Many organs regenerate well in adult zebrafish, but most research has been directed toward fin and heart regeneration. Cells have been found to remain generally lineage-restricted during regeneration, and proliferative regenerative progenitors can be formed by dedifferentiation from differentiated cells. Recent studies begin to shed light on the molecular underpinnings of differences between development and regeneration. Retinoic acid, BMP and NF-κB signaling are emerging as regulators of cellular dedifferentiation. Reactive oxygen species promote regeneration, and the dynamics of ROS signaling might help explain differences between wound healing and regeneration. Finally, the heart has been added to those organs that require a nerve supply to regenerate, and a trade-off between regeneration and tumor suppression has been proposed to help explain why mammals regenerate poorly.

Heartbreak hotel: a convergence in cardiac regeneration

In February 2016, The Company of Biologists hosted an intimate gathering of leading international researchers at the forefront of experimental cardiovascular regeneration, with its emphasis on ‘Transdifferentiation and Tissue Plasticity in Cardiovascular Rejuvenation’. As I review here, participants at the workshop revealed how understanding cardiac growth and lineage decisions at their most fundamental level has transformed the strategies in hand that presently energize the prospects for human heart repair.