Developmental changes in contractility and sarcomeric proteins from the early embryonic to the adult stage in the mouse heart

Myoglobin modulates the Hippo pathway to promote cardiomyocyte differentiation

The endogenous mechanisms that propagate cardiomyocyte differentiation and prevent de-differentiation remain unclear. While the expression of the heme protein myoglobin increases by over 50% during cardiomyocyte differentiation, a role for myoglobin in regulating cardiomyocyte differentiation has not been tested. Here, we show that deletion of myoglobin in cardiomyocyte models decreases the gene expression of differentiation markers and stimulates cellular proliferation, consistent with cardiomyocyte de-differentiation. Mechanistically, the heme prosthetic group of myoglobin catalyzes the oxidation of the Hippo pathway kinase LATS1, resulting in phosphorylation and inactivation of yes-associated protein (YAP). In vivo, myoglobin-deficient zebrafish hearts show YAP dephosphorylation and accelerated cardiac regeneration after apical injury. Similarly, myoglobin knockdown in neonatal murine hearts shows increased YAP dephosphorylation and cardiomyocyte cycling. These data demonstrate a novel role for myoglobin as an endogenous driver of cardiomyocyte differentiation and highlight myoglobin as a potential target to enhance cardiac development and improve cardiac repair and regeneration.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Switching of hypertrophic signalling towards enhanced cardiomyocyte identity and maturity by a GATA4-targeted compound

BACKGROUND

The prevalence of heart failure is constantly increasing, and the prognosis of patients remains poor. New treatment strategies to preserve cardiac function and limit cardiac hypertrophy are therefore urgently needed. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are increasingly used as an experimental platform for cardiac in vitro studies. However, in contrast to adult cardiomyocytes, hiPSC-CMs display immature morphology, contractility, gene expression and metabolism and hence express a naive phenotype that resembles more of a foetal cardiomyocyte.

METHODS

A library of 14 novel compounds was synthesized in-house and screened for GATA4-NKX2-5 reporter activity and cellular toxicity. The most potent compound, 3i-1262, along with previously reported GATA4-acting compounds, were selected to investigate their effects on hypertrophy induced by endothelin-1 or mechanical stretch. Morphological changes and protein expression were characterized using immunofluorescence staining and high-content analysis. Changes in gene expression were studied using qPCR and RNA sequencing.

RESULTS

The prototype compound 3i-1262 inhibited GATA4-NKX2-5 synergy in a luciferase reporter assay. Additionally, the isoxazole compound 3i-1262 inhibited the hypertrophy biomarker B-type natriuretic peptide (BNP) by reducing BNP promoter activity and proBNP expression in neonatal rat ventricular myocytes and hiPSC-CMs, respectively. Treatment with 3i-1262 increased metabolic activity and cardiac troponin T expression in hiPSC-CMs without affecting GATA4 protein levels. RNA sequencing analysis revealed that 3i-1262 induces gene expression related to metabolic activity and cell cycle exit, indicating a change in the identity and maturity status of hiPSC-CMs. The biological processes that were enriched in upregulated genes in response to 3i-1262 were downregulated in response to mechanical stretch, and conversely, the downregulated processes in response to 3i-1262 were upregulated in response to mechanical stretch.

CONCLUSIONS

There is currently a lack of systematic understanding of the molecular modulation and control of hiPSC-CM maturation. In this study, we demonstrated that the GATA4-interfering compound 3i-1262 reorganizes the cardiac transcription factor network and converts hypertrophic signalling towards enhanced cardiomyocyte identity and maturity. This conceptually unique approach provides a novel structural scaffold for further development as a modality to promote cardiomyocyte specification and maturity.

Cardiomyocyte maturation and its reversal during cardiac regeneration

Cardiovascular disease is a leading cause of death worldwide. Due to the limited proliferative and regenerative capacity of adult cardiomyocytes, the lost myocardium is not replenished efficiently and is replaced by a fibrotic scar, which eventually leads to heart failure. Current therapies to cure or delay the progression of heart failure are limited; hence, there is a pressing need for regenerative approaches to support the failing heart. Cardiomyocytes undergo a series of transcriptional, structural, and metabolic changes after birth (collectively termed maturation), which is critical for their contractile function but limits the regenerative capacity of the heart. In regenerative organisms, cardiomyocytes revert from their terminally differentiated state into a less mature state (ie, dedifferentiation) to allow for proliferation and regeneration to occur. Importantly, stimulating adult cardiomyocyte dedifferentiation has been shown to promote morphological and functional improvement after myocardial infarction, further highlighting the importance of cardiomyocyte dedifferentiation in heart regeneration. Here, we review several hallmarks of cardiomyocyte maturation, and summarize how their reversal facilitates cardiomyocyte proliferation and heart regeneration. A detailed understanding of how cardiomyocyte dedifferentiation is regulated will provide insights into therapeutic options to promote cardiomyocyte de-maturation and proliferation, and ultimately heart regeneration in mammals.

Integrated Proteomics Identifies Troponin I Isoform Switch as a Regulator of a Sarcomere-Metabolism Axis During Cardiac Regeneration

Adult mammalian cardiomyocytes have limited proliferative potential, and after myocardial infarction (MI), injured cardiac tissue is replaced with fibrotic scar rather than with functioning myocardium. In contrast, the neonatal mouse heart possesses a regenerative capacity governed by cardiomyocyte proliferation; however, a metabolic switch from glycolysis to fatty acid oxidation during postnatal development results in loss of this regenerative capacity. Interestingly, a sarcomere isoform switch also takes place during postnatal development where slow skeletal troponin I (ssTnI) is replaced with cardiac troponin I (cTnI). In this study, we first employ integrated quantitative bottom-up and top-down proteomics to comprehensively define the proteomic and sarcomeric landscape during postnatal heart maturation. Utilizing a cardiomyocyte-specific ssTnI transgenic mouse model, we found that ssTnI overexpression increased cardiomyocyte proliferation and the cardiac regenerative capacity of the postnatal heart following MI compared to control mice by histological analysis. Our global proteomic analysis of ssTnI transgenic mice following MI reveals that ssTnI overexpression induces a significant shift in the cardiac proteomic landscape. This shift is characterized by an upregulation of key proteins involved in glycolytic metabolism. Collectively, our data suggest that the postnatal TnI isoform switch may play a role in the metabolic shift from glycolysis to fatty acid oxidation during postnatal maturation. This underscores the significance of a sarcomere-metabolism axis during cardiomyocyte proliferation and heart regeneration.

Cardiomyocyte-fibroblast crosstalk in the postnatal heart

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

ER stress induces upregulation of transcription factor Tbx20 and downstream Bmp2 signaling to promote cardiomyocyte survival

In the mammalian heart, fetal cardiomyocytes proliferate prior to birth; however, they exit the cell cycle shortly after birth. Recent studies show that adult cardiomyocytes re-enters the cell cycle postinjury to promote cardiac regeneration. The endoplasmic reticulum (ER) orchestrates the production and assembly of different types of proteins, and a disruption in this machinery leads to the generation of ER stress, which activates the unfolded protein response. There is a very fine balance between ER stress-mediated protective and proapoptotic responses. T-box transcription factor 20 (Tbx20) promotes embryonic and adult cardiomyocyte proliferation postinjury to restore cardiac homeostasis. However, the function and regulatory interactions of Tbx20 in ER stress-induced cardiomyopathy have not yet been reported. We show here that ER stress upregulates Tbx20, which activates downstream bone morphogenetic protein 2 (Bmp2)-pSmad1/5/8 signaling to induce cardiomyocyte proliferation and limit apoptosis. However, augmenting ER stress reverses this protective response. We also show that increased expression of tbx20 during ER stress is mediated by the activating transcription factor 6 arm of the unfolded protein response. Cardiomyocyte-specific loss of Tbx20 results in decreased cardiomyocyte proliferation and increased apoptosis. Administration of recombinant Bmp2 protein during ER stress upregulates Tbx20 leading to augmented proliferation, indicating a feed-forward loop mechanism. In in vivo ER stress, as well as in diabetic cardiomyopathy, the activity of Tbx20 is increased with concomitant increased cardiomyocyte proliferation and decreased apoptosis. These data support a critical role of Tbx20-Bmp2 signaling in promoting cardiomyocyte survival during ER stress-induced cardiomyopathies.

Gene-Edited Human-Induced Pluripotent Stem Cell Lines to Elucidate DAND5 Function throughout Cardiac Differentiation

(1) Background: The contribution of gene-specific variants for congenital heart disease, one of the most common congenital disabilities, is still far from our complete understanding. Here, we applied a disease model using human-induced pluripotent stem cells (hiPSCs) to evaluate the function of DAND5 on human cardiomyocyte (CM) differentiation and proliferation. (2) Methods: Taking advantage of our DAND5 patient-derived iPSC line, we used CRISPR-Cas9 gene-editing to generate a set of isogenic hiPSCs (DAND5-corrected and DAND5 full-mutant). The hiPSCs were differentiated into CMs, and RT-qPCR and immunofluorescence profiled the expression of cardiac markers. Cardiomyocyte proliferation was analysed by flow cytometry. Furthermore, we used a multi-electrode array (MEA) to study the functional electrophysiology of DAND5 hiPSC-CMs. (3) Results: The results indicated that hiPSC-CM proliferation is affected by DAND5 levels. Cardiomyocytes derived from a DAND5 full-mutant hiPSC line are more proliferative when compared with gene-corrected hiPSC-CMs. Moreover, parallel cardiac differentiations showed a differential cardiac gene expression profile, with upregulated cardiac progenitor markers in DAND5-KO hiPSC-CMs. Microelectrode array (MEA) measurements demonstrated that DAND5-KO hiPSC-CMs showed prolonged field potential duration and increased spontaneous beating rates. In addition, conduction velocity is reduced in the monolayers of hiPSC-CMs with full-mutant genotype. (4) Conclusions: The absence of DAND5 sustains the proliferation of hiPSC-CMs, which alters their electrophysiological maturation properties. These results using DAND5 hiPSC-CMs consolidate the findings of the in vitro and in vivo mouse models, now in a translational perspective. Altogether, the data will help elucidate the molecular mechanism underlying this human heart disease and potentiates new therapies for treating adult CHD.

Extracellular matrix and cyclic stretch alter fetal cardiomyocyte proliferation and maturation in a rodent model of heart hypoplasia

Introduction

Birth defects, particularly those that affect development of the heart, are a leading cause of morbidity and mortality in infants and young children. Babies born with heart hypoplasia (heart hypoplasia) disorders often have a poor prognosis. It remains unclear whether cardiomyocytes from hypoplastic hearts retain the potential to recover growth, although this knowledge would be beneficial for developing therapies for heart hypoplasia disorders. The objective of this study was to determine the proliferation and maturation potential of cardiomyocytes from hypoplastic hearts and whether these behaviors are influenced by biochemical signaling from the extracellular matrix (ECM) and cyclic mechanical stretch.

Method

Congenital diaphragmatic hernia (CDH)-associated heart hypoplasia was induced in rat fetuses by maternal exposure to nitrofen. Hearts were isolated from embryonic day 21 nitrofen-treated fetuses positive for CDH (CDH+) and from fetuses without nitrofen administration during gestation.

Results and discussion

CDH+ hearts were smaller and had decreased myocardial proliferation, along with evidence of decreased maturity compared to healthy hearts. In culture, CDH+ cardiomyocytes remained immature and demonstrated increased proliferative capacity compared to their healthy counterparts. Culture on ECM derived from CDH+ hearts led to a significant reduction in proliferation for both CDH+ and healthy cardiomyocytes. Healthy cardiomyocytes were dosed with exogenous nitrofen to examine whether nitrofen may have an aberrant effect on the proliferative ability of cardiomyocyte, yet no significant change in proliferation was observed. When subjected to stretch, CDH+ cardiomyocytes underwent lengthening of sarcomeres while healthy cardiomyocyte sarcomeres were unaffected. Taken together, our results suggest that alterations to environmental cues such as ECM and stretch may be important factors in the pathological progression of heart hypoplasia.

Three-dimensional remodelling of the cellular energy distribution system during postnatal heart development

The heart meets the high energy demands of constant muscle contraction and calcium cycling primarily through the conversion of fatty acids into adenosine triphosphate (ATP) by a large volume of mitochondria. As such, the spatial relationships among lipid droplets (LDs), mitochondria, the sarcotubular system and the contractile apparatus are critical to the efficient distribution of energy within the cardiomyocyte. However, the connectivity among components of the cardiac cellular energy distribution system during postnatal development remains unclear. Here, we use volume electron microscopy to demonstrate that the sarcomere branches uniting the myofibrillar network occur more than twice as frequently during early postnatal development as in mature cardiomyocytes. Moreover, we show that the mitochondrial networks arranged in parallel to the contractile apparatus are composed of larger, more compact mitochondria with greater connectivity to adjacent mitochondria in mature as compared with early postnatal cardiomyocytes. Finally, we find that connectivity among mitochondria, LDs and the sarcotubular network is greater in developing than in mature muscles. These data suggest that physical connectivity among cellular structures may facilitate the communication needed to coordinate developmental processes within the cardiac muscle cell. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Sarcomere maturation: function acquisition, molecular mechanism, and interplay with other organelles

During postnatal cardiac development, cardiomyocytes mature and turn into adult ones. Hence, all cellular properties, including morphology, structure, physiology and metabolism, are changed. One of the most important aspects is the contractile apparatus, of which the minimum unit is known as a sarcomere. Sarcomere maturation is evident by enhanced sarcomere alignment, ultrastructural organization and myofibrillar isoform switching. Any maturation process failure may result in cardiomyopathy. Sarcomere function is intricately related to other organelles, and the growing evidence suggests reciprocal regulation of sarcomere and mitochondria on their maturation. Herein, we summarize the molecular mechanism that regulates sarcomere maturation and the interplay between sarcomere and other organelles in cardiomyocyte maturation. This article is part of the theme issue 'The cardiomyocyte: new revelations on the interplay between architecture and function in growth, health, and disease'.

Fine-tuned KDM1A alternative splicing regulates human cardiomyogenesis through an enzymatic-independent mechanism

The histone demethylase KDM1A is a multi-faceted regulator of vital developmental processes, including mesodermal and cardiac tube formation during gastrulation. However, it is unknown whether the fine-tuning of KDM1A splicing isoforms, already shown to regulate neuronal maturation, is crucial for the specification and maintenance of cell identity during cardiogenesis. Here, we discovered a temporal modulation of ubKDM1A and KDM1A+2a during human and mice fetal cardiac development and evaluated their impact on the regulation of cardiac differentiation. We revealed a severely impaired cardiac differentiation in KDM1A^−/− hESCs that can be rescued by re-expressing ubKDM1A or catalytically impaired ubKDM1A-K661A, but not by KDM1A+2a or KDM1A+2a-K661A. Conversely, KDM1A+2a^−/− hESCs give rise to functional cardiac cells, displaying increased beating amplitude and frequency and enhanced expression of critical cardiogenic markers. Our findings prove the existence of a divergent scaffolding role of KDM1A splice variants, independent of their enzymatic activity, during hESC differentiation into cardiac cells.

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ubKDM1A and KDM1A+2a isoforms are fine-tuned during fetal cardiac development

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Depletion of KDM1A isoforms impairs hESC differentiation into cardiac cells

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KDM1A+2a ablation enhances the expression of key cardiac markers

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KDM1A isoforms exhibit enzymatic-independent divergent roles during cardiogenesis

Cell maturation: Hallmarks, triggers, and manipulation

How cells become specialized, or "mature," is important for cell and developmental biology. While maturity is usually deemed a terminal fate, it may be more helpful to consider maturation not as a switch but as a dynamic continuum of adaptive phenotypic states set by genetic and environment programing. The hallmarks of maturity comprise changes in anatomy (form, gene circuitry, and interconnectivity) and physiology (function, rhythms, and proliferation) that confer adaptive behavior. We discuss efforts to harness their chemical (nutrients, oxygen, and growth factors) and physical (mechanical, spatial, and electrical) triggers in vitro and in vivo and how maturation strategies may support disease research and regenerative medicine.

Inhibition of β1-AR/Gαs signaling promotes cardiomyocyte proliferation in juvenile mice through activation of RhoA-YAP axis

The regeneration potential of the mammalian heart is incredibly limited, as cardiomyocyte proliferation ceases shortly after birth. β-adrenergic receptor (β-AR) blockade has been shown to improve heart functions in response to injury; however, the underlying mechanisms remain poorly understood. Here, we inhibited β-AR signaling in the heart using metoprolol, a cardio-selective β blocker for β1-adrenergic receptor (β1-AR) to examine its role in heart maturation and regeneration in postnatal mice. We found that metoprolol enhanced cardiomyocyte proliferation and promoted cardiac regeneration post myocardial infarction, resulting in reduced scar formation and improved cardiac function. Moreover, the increased cardiomyocyte proliferation was also induced by the genetic deletion of Gnas, the gene encoding G protein alpha subunit (Gαs), a downstream effector of β-AR. Genome wide transcriptome analysis revealed that the Hippo-effector YAP, which is associated with immature cardiomyocyte proliferation, was upregulated in the cardiomyocytes of β-blocker treated and Gnas cKO hearts. Moreover, the increased YAP activity is modulated by RhoA signaling. Our pharmacological and genetic studies reveal that β1-AR-Gαs-YAP signaling axis is involved in regulating postnatal cardiomyocyte proliferation. These results suggest that inhibiting β-AR-Gαs signaling promotes the regenerative capacity and extends the cardiac regenerative window in juvenile mice by activating YAP-mediated transcriptional programs.

Recent Advances in Maturation of Pluripotent Stem Cell-Derived Cardiomyocytes Promoted by Mechanical Stretch

Stem cells have significant potential use in tissue regeneration, especially for treating cardiac diseases because of their multi-directional differentiation capability. By mimicking the in vivo physiological environment of native cardiomyocytes during their development and maturation, researchers have been able to induce pluripotent stem cell-derived cardiomyocytes (PSC-CMs) at high purity. However, the phenotype of these PSC-CMs is immature compared with that of adult cardiomyocytes. Various strategies have been explored to improve the maturity of PSC-CMs, such as long-term culturing, mechanical stimuli, chemical stimuli, and combinations of these strategies. Among these strategies, mechanical stretch as a key mechanical stimulus plays an important role in PSC-CM maturation. In this review, the optimal parameters of mechanical stretch, the effects of mechanical stretch on maturation of PSC-CMs, underlying molecular mechanisms as well as existing problems are discussed. Mechanical stretch is a powerful approach to promote the maturation of SC-CMs in terms of morphology, structure, and functionality. Nonetheless, further research efforts are needed to reach a satisfactory standard for clinical applications of PSC-CMs in treating cardiac diseases.

Knockdown of DNA methyltransferase 1 reduces DNA methylation and alters expression patterns of cardiac genes in embryonic cardiomyocytes

We previously found that DNA methyltransferase 3a (DNMT3a) plays an important role in regulating embryonic cardiomyocyte gene expression, morphology, and function. In this study, we investigated the role of the most abundant DNMT in mammalian cells, DNMT1, in these processes. It is known that DNMT1 is essential for embryonic development, during which it is involved in regulating cardiomyocyte DNA methylation and gene expression. We used siRNA to knock down DNMT1 expression in primary cultures of mouse embryonic cardiomyocytes. Immunofluorescence staining and multielectrode array were, respectively, utilized to evaluate cardiomyocyte growth and electrophysiology. RNA sequencing (RNA‐Seq) and multiplex bisulfite sequencing were, respectively, performed to examine gene expression and promoter methylation. At 72 h post‐transfection, reduction of DNMT1 expression decreased the number and increased the size of embryonic cardiomyocytes. Beat frequency and the amplitude of field action potentials were decreased by DNMT1 siRNA. RNA‐Seq analysis identified 801 up‐regulated genes and 494 down‐regulated genes in the DNMT1 knockdown cells when compared to controls. Pathway analysis of the differentially expressed genes revealed pathways that were associated with cell death and survival, cell morphology, cardiac function, and cardiac disease. Alternative splicing analysis identified 929 differentially expressed exons, including 583 up‐regulated exons and 308 down‐regulated exons. Moreover, decreased methylation levels were found in the promoters of cardiac genes Myh6, Myh7, Myh7b, Tnnc1, Tnni3, Tnnt2, Nppa, Nppb, mef2c, mef2d, Camta2, Cdkn1A, and Cdkn1C. Of these 13 genes, 6 (Myh6, Tnnc1, Tnni3, Tnnt2, Nppa, Nppb) and 1 (Cdkn1C) had increased or decreased gene expression, respectively. Altogether, these data show that DNMT1 is important in embryonic cardiomyocytes by regulating DNA methylation, gene expression, gene splicing, and cell function.

Developmental and lifelong dioxin exposure induces measurable changes in cardiac structure and function in adulthood

Congenital heart disease (CHD) is the most common congenital abnormality. A precise etiology for CHD remains elusive, but likely results from interactions between genetic and environmental factors during development, when the heart adapts to physiological and pathophysiological conditions. Further, it has become clearer that early exposure to toxins that do not result in overt CHD may be associated with adverse cardiac outcomes that are not manifested until later life. Previously, interference with endogenous developmental functions of the aryl hydrocarbon receptor (AHR), either by gene ablation or by in utero exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent AHR ligand, was shown to cause structural, molecular and functional cardiac abnormalities and altered heart physiology in mouse embryos. Here, we show that continuous exposure to TCDD from fertilization throughout adulthood caused male mice to underperform at exercise tolerance tests compared to their control and female counterparts, confirming previous observations of a sexually dimorphic phenotype. Renin-angiotensin stimulation by angiotensin II (Ang II) caused measurable increases in blood pressure and left ventricle mass, along with decreased end diastolic volume and preserved ejection fraction. Interestingly, TCDD exposure caused measurable reductions in the myocardial hypertrophic effects of Ang II, suggesting that endogenous AHR signaling present in adulthood may play a role in the pathogenesis of hypertrophy. Overall, the findings reported in this pilot study highlight the complex systems underlying TCDD exposure in the development of cardiac dysfunction in later life.

Therapeutic efficacy of large aligned cardiac tissue derived from induced pluripotent stem cell in a porcine ischemic cardiomyopathy model

BACKGROUND

Although induced pluripotent stem (iPS) cell-derived cardiac constructs may have a potential in cardiomyogenesis of a distressed myocardium, obtaining polarity in cardiac constructs, such as via myocyte alignment, may be crucial to achieve a maximum contractile force for better clinical outcomes. We herein hypothesized that transplantation of an aligned cardiac tissue derived from iPS cells has therapeutic effects in a porcine ischemic cardiomyopathy model as a preclinical trial.

METHODS

Aligned cardiac tissues were developed by culturing high-purity iPS cell-derived cardiomyocytes in xeno-free conditions and transplanting them into infarct porcine hearts (iPS-CM group, n = 7; control, n = 6). Three months after treatment, therapeutic efficacy was evaluated functionally and histologically.

RESULTS

In vitro assessment revealed that the aligned cardiac tissue containing high purity cardiomyocytes contracted homogeneously and had excellent mechanical properties. In the in vivo study, the left ventricular ejection fraction of the iPS-CM group was significantly greater than that of the control group, 3 months after transplantation (37.8% ± 2.3% vs 28.3% ± 2.5%, p < 0.05). Pathologically, attenuated interstitial fibrosis, attenuation of hypertrophied cardiomyocytes, and an increased capillary density were also prominent in the iPS-CM group. A limited amount of engraftment of the transplanted tissue maintaining tissue alignment was observed at 2 weeks after transplantation.

CONCLUSIONS

The creation of large-scale functional aligned cardiac tissue was feasible, and the transplantation of the aligned tissue improved cardiac function with angiogenesis and antifibrotic effects in a porcine cardiomyopathy model.

LncRNAs in Cardiomyocyte Maturation: New Window for Cardiac Regenerative Medicine

Cardiomyocyte (CM) maturation, which is characterized by structural, functional, and metabolic specializations, is the last phase of CM development that prepares the cells for efficient and forceful contraction throughout life. Over the past decades, CM maturation has gained increased attention due to the fact that pluripotent stem cell-derived CMs are structurally, transcriptionally, and functionally immature and embryonic-like, which causes a defect in cell replacement therapy. The current challenge is to discover and understand the molecular mechanisms, which control the CM maturation process. Currently, emerging shreds of evidence emphasize the role of long noncoding RNAs (lncRNAs) in regulating different aspects of CM maturation, including myofibril maturation, electrophysiology, and Ca²⁺ handling maturation, metabolic maturation and proliferation to hypertrophy transition. Here, we describe the structural and functional characteristics of mature CMs. Furthermore, this review highlights the lncRNAs as crucial regulators of different aspects in CM maturation, which have the potential to be used for mature CM production. With the current advances in oligonucleotide delivery; lncRNAs may serve as putative therapeutic targets to produce highly mature CMs for research and regenerative medicine.

LINC complex regulation of genome organization and function

The regulation of genomic function is in part mediated through the physical organization and architecture of the nucleus. Disruption to nuclear organization and architecture is increasingly being recognized by its contribution to many diseases. The LINC complexes - protein structures traversing the nuclear envelope, that physically connect the nuclear interior, and hence the genome, to cytoplasmic cytoskeletal networks are an important component in the physical organization of the genome and its function. This connection, potentially allows for the constant detection of environmental mechanical stimuli, resulting in altered regulation of nuclear architecture and genome function, either directly or via the process of mechanotransduction. Here, we review the influences LINC complexes exert on genome functions and their impact on cellular/organismal health.

Modulating hESC-derived cardiomyocyte and endothelial cell function with triple-helical peptides for heart tissue engineering

In this study, we investigated the role of cardiomyocyte (CM) and endothelial cell (EC) specific interactions with collagen in the assembly of an operational myocardium in vitro. Engineered cardiac patches represent valuable tools for myocardial repair following infarction and are generally constituted of a suitable biomaterial populated by CMs and supportive cell types. Among those, ECs are required for tissue vascularization and positively modulate CM function. To direct the function of human embryonic stem cell (hESC)-derived CM and EC seeded on biomaterials, we replicated cell-collagen interactions, which regulate cellular behaviour in the native myocardium, using triple-helical peptides (THPs) that are ligands for collagen-binding proteins. THPs enhanced proliferation and activity of CMs and ECs separately and in co-culture, drove CM maturation and enabled coordinated cellular contraction on collagen films. These results highlight the importance of collagen interactions on cellular response and establish THP-functionalized biomaterials as novel tools to produce engineered cardiac tissues.

Dioxin Disrupts Dynamic DNA Methylation Patterns in Genes That Govern Cardiomyocyte Maturation

Congenital heart disease (CHD), the leading birth defect worldwide, has a largely unknown etiology, likely to result from complex interactions between genetic and environmental factors during heart development, at a time when the heart adapts to diverse physiological and pathophysiological conditions. Crucial among these is the regulation of cardiomyocyte development and postnatal maturation, governed by dynamic changes in DNA methylation. Previous work from our laboratory has shown that exposure to the environmental toxicant tetrachlorodibenzo-p-dioxin (TCDD) disrupts several molecular networks responsible for heart development and function. To test the hypothesis that the disruption caused by TCDD in the heart results from changes in DNA methylation and gene expression patterns of cardiomyocytes, we established a stable mouse embryonic stem cell line expressing a puromycin resistance selectable marker under control of the cardiomyocyte-specific Nkx2-5 promoter. Differentiation of these cells in the presence of puromycin induces the expression of a large suite of cardiomyocyte-specific markers. To assess the consequences of TCDD treatment on gene expression and DNA methylation in these cardiomyocytes, we subjected them to transcriptome and methylome analyses in the presence of TCDD. Unlike control cardiomyocytes maintained in vehicle, the TCDD-treated cardiomyocytes showed extensive gene expression changes, with a significant correlation between differential RNA expression and DNA methylation in 111 genes, many of which are key elements of pathways that regulate cardiovascular development and function. Our findings provide an important clue toward the elucidation of the complex interactions between genetic and epigenetic mechanisms after developmental TCDD exposure that may contribute to CHD.

Design of synthetic microenvironments to promote the maturation of human pluripotent stem cell derived cardiomyocytes

Cardiomyocyte like cells derived from human pluripotent stem cells (hPSC-CMs) have a good application perspective in many fields such as disease modeling, drug screening and clinical treatment. However, these are severely hampered by the fact that hPSC-CMs are immature compared to adult human cardiomyocytes. Therefore, many approaches such as genetic manipulation, biochemical factors supplement, mechanical stress, electrical stimulation and three-dimensional culture have been developed to promote the maturation of hPSC-CMs. Recently, establishing in vitro synthetic artificial microenvironments based on the in vivo development program of cardiomyocytes has achieved much attention due to their inherent properties such as stiffness, plasticity, nanotopography and chemical functionality. In this review, the achievements and deficiency of reported synthetic microenvironments that mainly discussed comprehensive biological, chemical, and physical factors, as well as three-dimensional culture were mainly discussed, which have significance to improve the microenvironment design and accelerate the maturation of hPSC-CMs.

Electrospun Scaffolds and Induced Pluripotent Stem Cell-Derived Cardiomyocytes for Cardiac Tissue Engineering Applications

Tissue engineering (TE) combines cells, scaffolds, and growth factors to assemble functional tissues for repair or replacement of tissues and organs. Cardiac TE is focused on developing cardiac cells, tissues, and structures-most notably the heart. This review presents the requirements, challenges, and research surrounding electrospun scaffolds and induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMs) towards applications to TE hearts. Electrospinning is an attractive fabrication method for cardiac TE scaffolds because it produces fibers that demonstrate the optimal potential for mimicking the complex structure of the cardiac extracellular matrix (ECM). iPSCs theoretically offer the capacity to generate limitless numbers of CMs for use in TE hearts, however these iPSC-CMs are electrophysiologically, morphologically, mechanically, and metabolically immature compared to adult CMs. This presents a functional limitation to their use in cardiac TE, and research aiming to address this limitation is presented in this review.

The long and winding road of cardiomyocyte maturation

Knowledge about the molecular mechanisms regulating cardiomyocyte (CM) proliferation and differentiation has increased exponentially in recent years. Such insights together with the availability of more efficient protocols for generation of CMs from induced pluripotent stem cells (iPSCs) have raised expectations for new therapeutic strategies to treat congenital and non-congenital heart diseases. However, the poor regenerative potential of the postnatal heart and the incomplete maturation of iPSC-derived CMs represent important bottlenecks for such therapies in future years. CMs undergo dramatic changes at the doorstep between prenatal and postnatal life, including terminal cell cycle withdrawal, change in metabolism, and further specialization of the cellular machinery required for high-performance contraction. Here, we review recent insights into pre- and early postnatal developmental processes that regulate CM maturation, laying specific focus on genetic and metabolic pathways that control transition of CMs from the embryonic and perinatal to the fully mature adult CM state. We recapitulate the intrinsic features of CM maturation and highlight the importance of external factors, such as energy substrate availability and endocrine regulation in shaping postnatal CM development. We also address recent approaches to enhance maturation of iPSC-derived CMs in vitro, and summarize new discoveries that might provide useful tools for translational research on repair of the injured human heart.

Mending a broken heart: current strategies and limitations of cell-based therapy

The versatility of pluripotent stem cells, attributable to their unlimited self-renewal capacity and plasticity, has sparked a considerable interest for potential application in regenerative medicine. Over the past decade, the concept of replenishing the lost cardiomyocytes, the crux of the matter in ischemic heart disease, with pluripotent stem cell-derived cardiomyocytes (PSC-CM) has been validated with promising pre-clinical results. Nevertheless, clinical translation was hemmed in by limitations such as immature cardiac properties, long-term engraftment, graft-associated arrhythmias, immunogenicity, and risk of tumorigenicity. The continuous progress of stem cell-based cardiac therapy, incorporated with tissue engineering strategies and delivery of cardio-protective exosomes, provides an optimistic outlook on the development of curative treatment for heart failure. This review provides an overview and current status of stem cell-based therapy for heart regeneration, with particular focus on the use of PSC-CM. In addition, we also highlight the associated challenges in clinical application and discuss the potential strategies in developing successful cardiac-regenerative therapy.

Four Dimensions of the Cardiac Myocyte Epigenome: from Fetal to Adult Heart

PURPOSE OF REVIEW

Development, physiological growth and the response of the heart to injury are accompanied by changes of the transcriptome and epigenome of cardiac myocytes. Recently, cell sorting and next generation sequencing techniques have been applied to determine cardiac myocyte-specific transcriptional and epigenetic mechanisms. This review provides a comprehensive overview of studies analysing the transcriptome and epigenome of cardiac myocytes in mouse and human hearts during development, physiological growth and disease.

RECENT FINDINGS

Adult cardiac myocytes express > 12,600 genes, and their expression levels correlate positively with active histone marks and inversely with gene body DNA methylation. DNA methylation accompanied the perinatal switch in sarcomere or metabolic isoform gene expression in cardiac myocytes, but remained rather stable in heart disease. DNA methylation and histone marks identified > 100,000 cis-regulatory regions in the cardiac myocyte epigenome with a dynamic spectrum of transcription factor binding sites. The ETS-related transcription factor ETV1 was identified as an atrial-specific element involved in the pathogenesis of atrial fibrillation. Thus, dynamic development of the atrial vs. ventricular cardiac myocyte epigenome provides a basis to identify location and time-dependent mechanisms of epigenetic control to shape pathological gene expression during heart disease. Identifying the four dimensions of the cardiac myocyte epigenome, atrial vs. ventricular location, time during development and growth, and disease-specific signals, may ultimately lead to new treatment strategies for heart disease.

Infantile restrictive cardiomyopathy: cTnI-R170G/W impair the interplay of sarcomeric proteins and the integrity of thin filaments

TNNI3 encoding cTnI, the inhibitory subunit of the troponin complex, is the main target for mutations leading to restrictive cardiomyopathy (RCM). Here we investigate two cTnI-R170G/W amino acid replacements, identified in infantile RCM patients, which are located in the regulatory C-terminus of cTnI. The C-terminus is thought to modulate the function of the inhibitory region of cTnI. Both cTnI-R170G/W strongly enhanced the Ca²⁺-sensitivity of skinned fibres, as is typical for RCM-mutations. Both mutants strongly enhanced the affinity of troponin (cTn) to tropomyosin compared to wildtype cTn, whereas binding to actin was either strengthened (R170G) or weakened (R170W). Furthermore, the stability of reconstituted thin filaments was reduced as revealed by electron microscopy. Filaments containing R170G/W appeared wavy and showed breaks. Decoration of filaments with myosin subfragment S1 was normal in the presence of R170W, but was irregular with R170G. Surprisingly, both mutants did not affect the Ca²⁺-dependent activation of reconstituted cardiac thin filaments. In the presence of the N-terminal fragment of cardiac myosin binding protein C (cMyBPC-C0C2) cooperativity of thin filament activation was increased only when the filaments contained wildtype cTn. No effect was observed in the presence of cTn containing R170G/W. cMyBPC-C0C2 significantly reduced binding of wildtype troponin to actin/tropomyosin, but not of both mutant cTn. Moreover, we found a direct troponin/cMyBPC-C0C2 interaction using microscale thermophoresis and identified cTnI and cTnT, but not cTnC as binding partners for cMyBPC-C0C2. Only cTn containing cTnI-R170G showed a reduced affinity towards cMyBPC-C0C2. Our results suggest that the RCM cTnI variants R170G/W impair the communication between thin and thick filament proteins and destabilize thin filaments.

Cardiomyocyte maturation: advances in knowledge and implications for regenerative medicine

Our knowledge of pluripotent stem cell (PSC) biology has advanced to the point where we now can generate most cells of the human body in the laboratory. PSC-derived cardiomyocytes can be generated routinely with high yield and purity for disease research and drug development, and these cells are now gradually entering the clinical research phase for the testing of heart regeneration therapies. However, a major hurdle for their applications is the immature state of these cardiomyocytes. In this Review, we describe the structural and functional properties of cardiomyocytes and present the current approaches to mature PSC-derived cardiomyocytes. To date, the greatest success in maturation of PSC-derived cardiomyocytes has been with transplantation into the heart in animal models and the engineering of 3D heart tissues with electromechanical conditioning. In conventional 2D cell culture, biophysical stimuli such as mechanical loading, electrical stimulation and nanotopology cues all induce substantial maturation, particularly of the contractile cytoskeleton. Metabolism has emerged as a potent means to control maturation with unexpected effects on electrical and mechanical function. Different interventions induce distinct facets of maturation, suggesting that activating multiple signalling networks might lead to increased maturation. Despite considerable progress, we are still far from being able to generate PSC-derived cardiomyocytes with adult-like phenotypes in vitro. Future progress will come from identifying the developmental drivers of maturation and leveraging them to create more mature cardiomyocytes for research and regenerative medicine.

Glucocorticoid Maturation of Fetal Cardiovascular Function

The last decade has seen rapid advances in the understanding of the central role of glucocorticoids in preparing the fetus for life after birth. However, relative to other organ systems, maturation by glucocorticoids of the fetal cardiovascular system has been ignored. Here, we review the effects of glucocorticoids on fetal basal cardiovascular function and on the fetal cardiovascular defense responses to acute stress. This is important because glucocorticoid-driven maturational changes in fetal cardiovascular function under basal and stressful conditions are central to the successful transition from intra- to extrauterine life. The cost-benefit balance for the cardiovascular health of the preterm baby of antenatal glucocorticoid therapy administered to pregnant women threatened with preterm birth is also discussed.

Enhanced structural maturation of human induced pluripotent stem cell-derived cardiomyocytes under a controlled microenvironment in a microfluidic system

The lack of a fully developed human cardiac model in vitro hampers the progress of many biomedical research fields including pharmacology, developmental biology, and disease modeling. Currently, available methods may only differentiate human induced pluripotent stem cells (iPSCs) into immature cardiomyocytes. To achieve cardiomyocyte maturation, appropriate modulation of cellular microenvironment is needed. This study aims to optimize a microfluidic system that enhances maturation of human iPSC-derived cardiomyocytes (iPSC-CMs) through cyclic pulsatile hemodynamic forces. Human iPSC-CMs cultured in the microfluidic system show increased alignment and contractility and appear more rod-like shaped with increased cell size and increased sarcomere length when compared to static cultures. Increased complexity and density of the mitochondrial network in iPSC-CMs cultured in the microfluidic system are in line with expression of mitochondrial marker genes MT-CO1 and OPA1. Moreover, the optimized microfluidic system is capable of stably maintaining controlled oxygen levels and inducing hypoxia, revealed by increased expression of HIF1α and EGLN2 as well as changes in contraction parameters in iPSC-CMs. In summary, this microfluidic system boosts the structural maturation of iPSC-CM culture and could serve as an advanced in vitro cardiac model for biomedical research in the future. STATEMENT OF SIGNIFICANCE: The availability of in vitro human cardiomyocytes generated from induced pluripotent stem cells (iPSCs) opens the possibility to develop human in vitro heart models for disease modeling and drug testing. However, iPSC-derived cardiomyocytes remain structurally and functionally immature, which hinders their application. In this manuscript, we present an optimized and complete microfluidic system that enhances maturation of iPSC-derived cardiomyocytes through physiological cyclic pulsatile hemodynamic forces. Furthermore, we improved our microfluidic system by using a closed microfluidic recirculation and oxygen exchangers to achieve and maintain low oxygen in the culture chambers, which is suitable for mimicking the hypoxic condition and studying the pathophysiological mechanisms of human diseases in vitro. In the future, a variety of technologies including 3D tissue engineering could be integrated into our system, which may greatly extend the use of iPSC-derived cardiac models in drug development and disease modeling.

The role of CXC receptors signaling in early stages of mouse embryonic stem cell differentiation

Interplay between CXCR7 and other CXC receptors, namely CXCR4 or CXCR3, binding such ligands as SDF-1 or ITAC, was shown to regulate multiple cellular processes. The developmental role of signaling pathways mediated by these receptors was proven by the phenotypes of mice lacking either functional CXCR4, or CXCR7, or SDF-1, showing that formation of certain lineages relies on these factors. In this study, using in vitro differentiating mouse embryonic stem cells that lacked the function of CXCR7, we asked the question about the role of CXCR mediated signaling during early steps of differentiation. Our analysis showed that interaction of SDF-1 or ITAC with CXC receptors is necessary for the regulation of crucial developmental regulators expression and that CXCR7 is involved in the control of ESC pluripotency and differentiation into mesodermal lineages.

Enhancement of human iPSC-derived cardiomyocyte maturation by chemical conditioning in a 3D environment

Recent advances in the understanding and use of pluripotent stem cells have produced major changes in approaches to the diagnosis and treatment of human disease. An obstacle to the use of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for regenerative medicine, disease modeling and drug discovery is their immature state relative to adult myocardium. We show the effects of a combination of biochemical factors, thyroid hormone, dexamethasone, and insulin-like growth factor-1 (TDI) on the maturation of hiPSC-CMs in 3D cardiac microtissues (CMTs) that recapitulate aspects of the native myocardium. Based on a comparison of the gene expression profiles and the structural, ultrastructural, and electrophysiological properties of hiPSC-CMs in monolayers and CMTs, and measurements of the mechanical and pharmacological properties of CMTs, we find that TDI treatment in a 3D tissue context yields a higher fidelity adult cardiac phenotype, including sarcoplasmic reticulum function and contractile properties consistent with promotion of the maturation of hiPSC derived cardiomyocytes.

Learn from Your Elders: Developmental Biology Lessons to Guide Maturation of Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cells (hPSCs) offer a multifaceted platform to study cardiac developmental biology, understand disease mechanisms, and develop novel therapies. Remarkable progress over the last two decades has led to methods to obtain highly pure hPSC-derived cardiomyocytes (hPSC-CMs) with reasonable ease and scalability. Nevertheless, a major bottleneck for the translational application of hPSC-CMs is their immature phenotype, resembling that of early fetal cardiomyocytes. Overall, bona fide maturation of hPSC-CMs represents one of the most significant goals facing the field today. Developmental biology studies have been pivotal in understanding the mechanisms to differentiate hPSC-CMs. Similarly, evaluation of developmental cues such as electrical and mechanical activities or neurohormonal and metabolic stimulations revealed the importance of these pathways in cardiomyocyte physiological maturation. Those signals cooperate and dictate the size and the performance of the developing heart. Likewise, this orchestra of stimuli is important in promoting hPSC-CM maturation, as demonstrated by current in vitro maturation approaches. Different shades of adult-like phenotype are achieved by prolonging the time in culture, electromechanical stimulation, patterned substrates, microRNA manipulation, neurohormonal or metabolic stimulation, and generation of human-engineered heart tissue (hEHT). However, mirroring this extremely dynamic environment is challenging, and reproducibility and scalability of these approaches represent the major obstacles for an efficient production of mature hPSC-CMs. For this reason, understanding the pattern behind the mechanisms elicited during the late gestational and early postnatal stages not only will provide new insights into postnatal development but also potentially offer new scalable and efficient approaches to mature hPSC-CMs.

An Unbiased Proteomics Method to Assess the Maturation of Human Pluripotent Stem Cell-Derived Cardiomyocytes

RATIONALE

Human pluripotent stem cell (hPSC)-derived cardiomyocytes exhibit the properties of fetal cardiomyocytes, which limits their applications. Various methods have been used to promote maturation of hPSC-cardiomyocytes; however, there is a lack of an unbiased and comprehensive method for accurate assessment of the maturity of hPSC-cardiomyocytes.

OBJECTIVE

We aim to develop an unbiased proteomics strategy integrating high-throughput top-down targeted proteomics and bottom-up global proteomics for the accurate and comprehensive assessment of hPSC-cardiomyocyte maturation.

METHODS AND RESULTS

Utilizing hPSC-cardiomyocytes from early- and late-stage 2-dimensional monolayer culture and 3-dimensional engineered cardiac tissue, we demonstrated the high reproducibility and reliability of a top-down proteomics method, which enabled simultaneous quantification of contractile protein isoform expression and associated post-translational modifications. This method allowed for the detection of known maturation-associated contractile protein alterations and, for the first time, identified contractile protein post-translational modifications as promising new markers of hPSC-cardiomyocytes maturation. Most notably, decreased phosphorylation of α-tropomyosin was found to be associated with hPSC-cardiomyocyte maturation. By employing a bottom-up global proteomics strategy, we identified candidate maturation-associated markers important for sarcomere organization, cardiac excitability, and Ca²⁺ homeostasis. In particular, upregulation of myomesin 1 and transmembrane 65 was associated with hPSC-cardiomyocyte maturation and validated in cardiac development, making these promising markers for assessing maturity of hPSC-cardiomyocytes. We have further validated α-actinin isoforms, phospholamban, dystrophin, αB-crystallin, and calsequestrin 2 as novel maturation-associated markers, in the developing mouse cardiac ventricles.

CONCLUSIONS

We established an unbiased proteomics method that can provide accurate and specific assessment of the maturity of hPSC-cardiomyocytes and identified new markers of maturation. Furthermore, this integrated proteomics strategy laid a strong foundation for uncovering the molecular pathways involved in cardiac development and disease using hPSC-cardiomyocytes.

Dynamics of Non-Canonical Amino Acid-Labeled Intra- and Extracellular Proteins in the Developing Mouse

Introduction

Mapping protein synthesis and turnover during development will provide insight into functional tissue assembly; however, quantitative in vivo characterization has been hindered by a lack of tools. To address this gap, we previously demonstrated murine embryos can be labeled with the non-canonical amino acid azidohomoalanine (Aha), which enables the enrichment and identification of newly synthesized proteins. Using this technique, we now show how protein turnover varies as a function of both time and cellular compartment during murine development.

Methods

Pregnant C57BL/6 mice were injected with Aha or PBS (control) at different embryonic time points. Aha-labeled proteins from homogenized E12.5 and E15.5 embryos were conjugated with diazo biotin-alkyne, bound to NeutrAvidin beads, selectively released, then processed for either SDS-PAGE or LC-MS/MS. For turnover studies, embryos were harvested 0-48 h after Aha injection at E12.5, separated into different cellular fractions based on solubility, and analyzed via western blotting.

Results

We developed an enhanced method for isolating Aha-labeled proteins from embryos that minimizes background signal from unlabeled proteins and avidin contamination. Approximately 50% of all identified proteins were found only in Aha samples. Comparing proteins present in both Aha and PBS samples, 90% were > 2-fold enriched in Aha-treated embryos. Furthermore, this method could resolve differences in the Aha-labeled proteome between developmental time points. Newly synthesized Aha-labeled proteins were observed by 3 h and peak labeling was around 6 h. Notably, extracellular matrix and cytoskeletal turnover appeared lower than the cytosolic fraction.

Conclusions

The methods developed in this work enable the identification and quantification of protein synthesis and turnover in different tissue fractions during development. This will provide insight into functional tissue assembly and ultimately inform the design of regenerative therapies that seek to promote growth and repair.

Conserved functions of RNA-binding proteins in muscle

Animals require different types of muscle for survival, for example for circulation, motility, reproduction and digestion. Much emphasis in the muscle field has been placed on understanding how transcriptional regulation generates diverse types of muscle during development. Recent work indicates that alternative splicing and RNA regulation are as critical to muscle development, and altered function of RNA-binding proteins causes muscle disease. Although hundreds of genes predicted to bind RNA are expressed in muscles, many fewer have been functionally characterized. We present a cross-species view summarizing what is known about RNA-binding protein function in muscle, from worms and flies to zebrafish, mice and humans. In particular, we focus on alternative splicing regulated by the CELF, MBNL and RBFOX families of proteins. We discuss the systemic nature of diseases associated with loss of RNA-binding proteins in muscle, focusing on mis-regulation of CELF and MBNL in myotonic dystrophy. These examples illustrate the conservation of RNA-binding protein function and the marked utility of genetic model systems in understanding mechanisms of RNA regulation.

Regulation of cardiomyocyte maturation during critical perinatal window

A primary limitation in the use of pluripotent stem cell-derived cardiomyocytes (PSC-CMs) for both patient health and scientific investigation is the failure of these cells to achieve full functional maturity. In vivo, cardiomyocytes undergo numerous adaptive structural, functional and metabolic changes during maturation. By contrast, PSC-CMs fail to fully undergo these developmental processes, instead remaining arrested at an embryonic stage of maturation. There is thus a significant need to understand the biological processes underlying proper CM maturation in vivo. Here, we discuss what is known regarding the initiation and coordination of CM maturation. We postulate that there is a critical perinatal window, ranging from embryonic day 18.5 to postnatal day 14 in mice, in which the maturation process is exquisitely sensitive to perturbation. While the initiation mechanisms of this process are unknown, it is increasingly clear that maturation proceeds through interconnected regulatory circuits that feed into one another to coordinate concomitant structural, functional and metabolic CM maturation. We highlight PGC1α, SRF and the MEF2 family as transcription factors that may potentially mediate this cross-talk. We lastly discuss several emerging technologies that will facilitate future studies into the mechanisms of CM maturation. Further study will not only produce a better understanding of its key processes, but provide practical insights into developing a robust strategy to produce mature PSC-CMs.

Bioengineered Cardiac Tissue Based on Human Stem Cells for Clinical Application

Engineered cardiac tissue might enable novel therapeutic strategies for the human heart in a number of acquired and congenital diseases. With recent advances in stem cell technologies, namely the availability of pluripotent stem cells, the generation of potentially autologous tissue grafts has become a realistic option. Nevertheless, a number of limitations still have to be addressed before clinical application of engineered cardiac tissue based on human stem cells can be realized. We summarize current progress and pending challenges regarding the optimal cell source, cardiomyogenic lineage specification, purification, safety of genetic cell engineering, and genomic stability. Cardiac cells should be combined with clinical grade scaffold materials for generation of functional myocardial tissue in vitro. Scale-up to clinically relevant dimensions is mandatory, and tissue vascularization is most probably required both for preclinical in vivo testing in suitable large animal models and for clinical application. Graphical Abstract.

Exploring the Crosstalk Between LMNA and Splicing Machinery Gene Mutations in Dilated Cardiomyopathy

Mutations in the LMNA gene, which encodes for the nuclear lamina proteins lamins A and C, are responsible for a diverse group of diseases known as laminopathies. One type of laminopathy is Dilated Cardiomyopathy (DCM), a heart muscle disease characterized by dilation of the left ventricle and impaired systolic function, often leading to heart failure and sudden cardiac death. LMNA is the second most commonly mutated gene in DCM. In addition to LMNA, mutations in more than 60 genes have been associated with DCM. The DCM-associated genes encode a variety of proteins including transcription factors, cytoskeletal, Ca2+-regulating, ion-channel, desmosomal, sarcomeric, and nuclear-membrane proteins. Another important category among DCM-causing genes emerged upon the identification of DCM-causing mutations in RNA binding motif protein 20 (RBM20), an alternative splicing factor that is chiefly expressed in the heart. In addition to RBM20, several essential splicing factors were validated, by employing mouse knock out models, to be embryonically lethal due to aberrant cardiogenesis. Furthermore, heart-specific deletion of some of these splicing factors was found to result in aberrant splicing of their targets and DCM development. In addition to splicing alterations, advances in next generation sequencing highlighted the association between splice-site mutations in several genes and DCM. This review summarizes LMNA mutations and splicing alterations in DCM and discusses how the interaction between LMNA and splicing regulators could possibly explain DCM disease mechanisms.

Investigation of human iPSC-derived cardiac myocyte functional maturation by single cell traction force microscopy

Recent advances have made it possible to readily derive cardiac myocytes from human induced pluripotent stem cells (hiPSC-CMs). HiPSC-CMs represent a valuable new experimental model for studying human cardiac muscle physiology and disease. Many laboratories have devoted substantial effort to examining the functional properties of isolated hiPSC-CMs, but to date, force production has not been adequately characterized. Here, we utilized traction force microscopy (TFM) with micro-patterning cell printing to investigate the maximum force production of isolated single hiPSC-CMs under varied culture and assay conditions. We examined the role of length of differentiation in culture and the effects of varied extracellular calcium concentration in the culture media on the maturation of hiPSC-CMs. Results show that hiPSC-CMs developing in culture for two weeks produced significantly less force than cells cultured from one to three months, with hiPSC-CMs cultured for three months resembling the cell morphology and function of neonatal rat ventricular myocytes in terms of size, dimensions, and force production. Furthermore, hiPSC-CMs cultured long term in conditions of physiologic calcium concentrations were larger and produced more force than hiPSC-CMs cultured in standard media with sub-physiological calcium. We also examined relationships between cell morphology, substrate stiffness and force production. Results showed a significant relationship between cell area and force. Implementing directed modifications of substrate stiffness, by varying stiffness from embryonic-like to adult myocardium-like, hiPSC-CMs produced maximal forces on substrates with a lower modulus and significantly less force when assayed on increasingly stiff adult myocardium-like substrates. Calculated strain energy measurements paralleled these findings. Collectively, these findings further establish single cell TFM as a valuable approach to illuminate the quantitative physiological maturation of force in hiPSC-CMs.

Differences in Contractile Function of Myofibrils within Human Embryonic Stem Cell-Derived Cardiomyocytes vs. Adult Ventricular Myofibrils Are Related to Distinct Sarcomeric Protein Isoforms

Characterizing the contractile function of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is key for advancing their utility for cellular disease models, promoting cell based heart repair, or developing novel pharmacological interventions targeting cardiac diseases. The aim of the present study was to understand whether steady-state and kinetic force parameters of β-myosin heavy chain (βMyHC) isoform-expressing myofibrils within human embryonic stem cell-derived cardiomyocytes (hESC-CMs) differentiated in vitro resemble those of human ventricular myofibrils (hvMFs) isolated from adult donor hearts. Contractile parameters were determined using the same micromechanical method and experimental conditions for both types of myofibrils. We identified isoforms and phosphorylation of main sarcomeric proteins involved in the modulation of force generation of both, chemically demembranated hESC-CMs (d-hESC-CMs) and hvMFs. Our results indicate that at saturating Ca²⁺ concentration, both human-derived contractile systems developed forces with similar rate constants (0.66 and 0.68 s⁻¹), reaching maximum isometric force that was significantly smaller for d-hESC-CMs (42 kPa) than for hvMFs (94 kPa). At submaximal Ca²⁺-activation, where intact cardiomyocytes normally operate, contractile parameters of d-hESC-CMs and hvMFs exhibited differences. Ca²⁺ sensitivity of force was higher for d-hESC-CMs (pCa₅₀ = 6.04) than for hvMFs (pCa₅₀ = 5.80). At half-maximum activation, the rate constant for force redevelopment was significantly faster for d-hESC-CMs (0.51 s⁻¹) than for hvMFs (0.28 s⁻¹). During myofibril relaxation, kinetics of the slow force decay phase were significantly faster for d-hESC-CMs (0.26 s⁻¹) than for hvMFs (0.21 s⁻¹), while kinetics of the fast force decay were similar and ~20x faster. Protein analysis revealed that hESC-CMs had essentially no cardiac troponin-I, and partially non-ventricular isoforms of some other sarcomeric proteins, explaining the functional discrepancies. The sarcomeric protein isoform pattern of hESC-CMs had features of human cardiomyocytes at an early developmental stage. The study indicates that morphological and ultrastructural maturation of βMyHC isoform-expressing hESC-CMs is not necessarily accompanied by ventricular-like expression of all sarcomeric proteins. Our data suggest that hPSC-CMs could provide useful tools for investigating inherited cardiac diseases affecting contractile function during early developmental stages.

HIF-1, Metabolism, and Diabetes in the Embryonic and Adult Heart

The heart is able to metabolize any substrate, depending on its availability, to satisfy its energy requirements. Under normal physiological conditions, about 95% of ATP is produced by oxidative phosphorylation and the rest by glycolysis. Cardiac metabolism undergoes reprograming in response to a variety of physiological and pathophysiological conditions. Hypoxia-inducible factor 1 (HIF-1) mediates the metabolic adaptation to hypoxia and ischemia, including the transition from oxidative to glycolytic metabolism. During embryonic development, HIF-1 protects the embryo from intrauterine hypoxia, its deletion as well as its forced expression are embryonically lethal. A decrease in HIF-1 activity is crucial during perinatal remodeling when the heart switches from anaerobic to aerobic metabolism. In the adult heart, HIF-1 protects against hypoxia, although its deletion in cardiomyocytes affects heart function even under normoxic conditions. Diabetes impairs HIF-1 activation and thus, compromises HIF-1 mediated responses under oxygen-limited conditions. Compromised HIF-1 signaling may contribute to the teratogenicity of maternal diabetes and diabetic cardiomyopathy in adults. In this review, we discuss the function of HIF-1 in the heart throughout development into adulthood, as well as the deregulation of HIF-1 signaling in diabetes and its effects on the embryonic and adult heart.

Engineered Microenvironments for Maturation of Stem Cell Derived Cardiac Myocytes

Through the use of stem cell-derived cardiac myocytes, tissue-engineered human myocardial constructs are poised for modeling normal and diseased physiology of the heart, as well as discovery of novel drugs and therapeutic targets in a human relevant manner. This review highlights the recent bioengineering efforts to recapitulate microenvironmental cues to further the maturation state of newly differentiated cardiac myocytes. These techniques include long-term culture, co-culture, exposure to mechanical stimuli, 3D culture, cell-matrix interactions, and electrical stimulation. Each of these methods has produced various degrees of maturation; however, a standardized measure for cardiomyocyte maturation is not yet widely accepted by the scientific community.

HIC2 regulates isoform switching during maturation of the cardiovascular system

Physiological changes during embryonic development are associated with changes in the isoform expression of both myocyte sarcomeric proteins and of erythrocyte haemoglobins. Cell type-specific isoform expression of these genes also occurs. Although these changes appear to be coordinated, it is unclear how changes in these disparate cell types may be linked. The transcription factor Hic2 is required for normal cardiac development and the mutant is embryonic lethal. Hic2 embryos exhibit precocious expression of the definitive-lineage haemoglobin Hbb-bt in circulating primitive erythrocytes and of foetal isoforms of cardiomyocyte genes (creatine kinase, Ckm, and eukaryotic elongation factor Eef1a2) as well as ectopic cardiac expression of fast-twitch skeletal muscle troponin isoforms. We propose that HIC2 regulates a switching event within both the contractile machinery of cardiomyocytes and the oxygen carrying systems during the developmental period where demands on cardiac loading change rapidly.

Titin isoforms are increasingly protected against oxidative modifications in developing rat cardiomyocytes

During the perinatal adaptation process N2BA titin isoforms are switched for N2B titin isoforms leading to an increase in cardiomyocyte passive tension (F_passive). Here we attempted to reveal how titin isoform composition and oxidative insults (i.e. sulfhydryl (SH)-group oxidation or carbonylation) influence F_passive of left ventricular (LV) cardiomyocytes during rat heart development. Moreover, we also examined a hypothetical protective role for titin associated small heat shock proteins (sHSPs), Hsp27 and αB-crystallin in the above processes. Single, permeabilized LV cardiomyocytes of the rat (at various ages following birth) were exposed either to 2,2'-dithiodipyridine (DTDP) to provoke SH-oxidation or Fenton reaction reagents (iron(II), hydrogen peroxide (H₂O₂), ascorbic acid) to induce protein carbonylation of cardiomyocytes in vitro. Thereafter, cardiomyocyte force measurements for F_passive determinations and Western immunoblot assays were carried out for the semiquantitative determination of oxidized SH-groups or carbonyl-groups of titin isoforms and to monitor sHSPs' expressions. DTDP or Fenton reagents increased F_passive in 0- and 7-day-old rats to relatively higher extents than in 21-day-old and adult animals. The degrees of SH-group oxidation or carbonylation declined with cardiomyocyte age to similar extents for both titin isoforms. Moreover, the above characteristics were mirrored by increasing levels of HSP27 and αB-crystallin expressions during cardiomyocyte development. Our data implicate a gradual build-up of a protective mechanism against titin oxidation through the upregulation of HSP27 and αB-crystallin expressions during postnatal cardiomyocyte development.

A cardiac patch from aligned microvessel and cardiomyocyte patches

Cardiac tissue engineering aims to produce replacement tissue patches in the lab to replace or treat infarcted myocardium. However, current patches lack preformed microvascularization and are therefore limited in thickness and force production. In this study, we sought to assess whether a bilayer patch composed of a layer made from human induced pluripotent stem cell-derived cardiomyocytes and a microvessel layer composed of self-assembled human blood outgrowth endothelial cells and pericytes was capable of engrafting on the epicardial surface of a nude rat infarct model and becoming perfused by the host 4 weeks after acute implantation. The bilayer configuration was found to increase the twitch force production, improve human induced pluripotent stem cell-derived cardiomyocyte survival and maturation, and increase patent microvessel lumens compared with time-matched single layer controls after 2 weeks of in vitro culture. Upon implantation, the patch microvessels sprouted into the cardiomyocyte layer of the patch and inosculated with the host vasculature as evidenced by species-specific perfusion labels and erythrocyte staining. Our results demonstrate that the added microvessel layer of a bilayer patch substantially improves in vitro functionality and that the bilayer patch is capable of engraftment with rapid microvessel inosculation on injured myocardium. The bilayer format will allow for scaling up in size through the addition of layers to obtain thicker tissues generating greater force in the future.

Does anthracycline-based chemotherapy in pregnant women with cancer offer safe cardiac and neurodevelopmental outcomes for the developing fetus?

BACKGROUND

Cancer treatment during pregnancy is a growing problem especially now that women delay childbearing. Systemic treatment of these malignancies during pregnancy centers mainly on the anticancer drugs anthracyclines, widely used in treating hematological and breast cancer during pregnancy and sometimes associated with early and late toxicity for the fetus. Owing to concern about their cardiac and neurodevelopmental toxicity more information is needed on which anthracycline to prefer and whether they can safely guarantee a cardiotoxicity-free outcome in the fetus.

DISCUSSION

The major research findings underline anthracycline-induced dose-dependent effects, including cardiotoxicity, many avoidable. Partly because the placenta acts mainly as a barrier, research findings indicate low transplacental anthracycline transfer. Anthracycline-induced teratogenicity depends closely on when patients receive chemotherapy. Anthracycline cardiac toxicity may depend on the association with drugs that inhibit or induce placental P-glycoprotein (P-gp). P-gp-induced drug interactions may alter placental P-gp barrier function and subsequently change fetal exposure. Though many anthracyclines have acceptable safety profiles clinical studies suggest giving idarubicin with special caution. Patients and doctors who care for pregnant women should whenever possible avoid prematurity and hence reduce prematurity-induced medical complications at birth and in the long-term. Information is lacking on long-term anthracycline-induced effects.

CONCLUSION

Pregnant women receiving anthracycline-based chemotherapy should undergo regular, state-of-the-art diagnostic imaging to detect fetal drug-induced cardiac damage early, and allow alternative therapeutic options. Recognizing drug-induced interactions and understanding the most vulnerable fetuses will help in choosing tailored therapy. Future research on placental transport, blood-brain barrier drug passage and pharmacokinetics will improve the way we manage these difficult-to-treat patients and their fetuses.

Naturally Engineered Maturation of Cardiomyocytes

Ischemic heart disease remains one of the most prominent causes of mortalities worldwide with heart transplantation being the gold-standard treatment option. However, due to the major limitations associated with heart transplants, such as an inadequate supply and heart rejection, there remains a significant clinical need for a viable cardiac regenerative therapy to restore native myocardial function. Over the course of the previous several decades, researchers have made prominent advances in the field of cardiac regeneration with the creation of in vitro human pluripotent stem cell-derived cardiomyocyte tissue engineered constructs. However, these engineered constructs exhibit a functionally immature, disorganized, fetal-like phenotype that is not equivalent physiologically to native adult cardiac tissue. Due to this major limitation, many recent studies have investigated approaches to improve pluripotent stem cell-derived cardiomyocyte maturation to close this large functionality gap between engineered and native cardiac tissue. This review integrates the natural developmental mechanisms of cardiomyocyte structural and functional maturation. The variety of ways researchers have attempted to improve cardiomyocyte maturation in vitro by mimicking natural development, known as natural engineering, is readily discussed. The main focus of this review involves the synergistic role of electrical and mechanical stimulation, extracellular matrix interactions, and non-cardiomyocyte interactions in facilitating cardiomyocyte maturation. Overall, even with these current natural engineering approaches, pluripotent stem cell-derived cardiomyocytes within three-dimensional engineered heart tissue still remain mostly within the early to late fetal stages of cardiomyocyte maturity. Therefore, although the end goal is to achieve adult phenotypic maturity, more emphasis must be placed on elucidating how the in vivo fetal microenvironment drives cardiomyocyte maturation. This information can then be utilized to develop natural engineering approaches that can emulate this fetal microenvironment and thus make prominent progress in pluripotent stem cell-derived maturity toward a more clinically relevant model for cardiac regeneration.