Harnessing the Induction of Cardiomyocyte Proliferation for Cardiac Regenerative Medicine

Cell Reprogramming, Transdifferentiation, and Dedifferentiation Approaches for Heart Repair

Cardiovascular disease (CVD) remains the leading cause of death globally, with myocardial infarction (MI) being a major contributor. The current therapeutic approaches are limited in effectively regenerating damaged cardiac tissue. Up-to-date strategies for heart regeneration/reconstitution aim at cardiac remodeling through repairing the damaged tissue with an external cell source or by stimulating the existing cells to proliferate and repopulate the compromised area. Cell reprogramming is addressed to this challenge as a promising solution, converting fibroblasts and other cell types into functional cardiomyocytes, either by reverting cells to a pluripotent state or by directly switching cell lineage. Several strategies such as gene editing and the application of miRNA and small molecules have been explored for their potential to enhance cardiac regeneration. Those strategies take advantage of cell plasticity by introducing reprogramming factors that regress cell maturity in vitro, allowing for their later differentiation and thus endorsing cell transplantation, or promote in situ cell proliferation, leveraged by scaffolds embedded with pro-regenerative factors promoting efficient heart restoration. Despite notable advancements, important challenges persist, including low reprogramming efficiency, cell maturation limitations, and safety concerns in clinical applications. Nonetheless, integrating these innovative approaches offers a promising alternative for restoring cardiac function and reducing the dependency on full heart transplants.

Heart rhythm in vitro: measuring stem cell-derived pacemaker cells on microelectrode arrays

Background

Cardiac arrhythmias have markedly increased in recent decades, highlighting the urgent need for appropriate test systems to evaluate the efficacy and safety of new pharmaceuticals and the potential side effects of established drugs.

Methods

The Microelectrode Array (MEA) system may be a suitable option, as it provides both real-time and non-invasive monitoring of cellular networks of spontaneously active cells. However, there is currently no commercially available cell source to apply this technology in the context of the cardiac conduction system (CCS). In response to this problem, our group has previously developed a protocol for the generation of pure functional cardiac pacemaker cells from mouse embryonic stem cells (ESCs). In addition, we compared the hanging drop method, which was previously utilized, with spherical plate-derived embryoid bodies (EBs) and the pacemaker cells that are differentiated from these.

Results

We described the application of these pacemaker cells on the MEA platform, which required a number of crucial optimization steps in terms of coating, dissociation, and cell density. As a result, we were able to generate a monolayer of pure pacemaker cells on an MEA surface that is viable and electromechanically active for weeks. Furthermore, we introduced spherical plates as a convenient and scalable method to be applied for the production of induced sinoatrial bodies.

Conclusion

We provide a tool to transfer modeling and analysis of cardiac rhythm diseases to the cell culture dish. Our system allows answering CCS-related queries within a cellular network, both under baseline conditions and post-drug exposure in a reliable and affordable manner. Ultimately, our approach may provide valuable guidance not only for cardiac pacemaker cells but also for the generation of an MEA test platform using other sensitive non-proliferating cell types.

Characterization of iCell cardiomyocytes using single-cell RNA-sequencing methods

INTRODUCTION

Human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes are being evaluated for their use in pharmacological and toxicological testing, particularly for electrophysiological side effects. However, little is known about the composition of the commercially available iCell cardiomyocyte (Fuijifilm Cellular Dynamics) cultures and the transcriptomic phenotype of individual cells.

METHODS

We characterized iCell cardiomyocytes (assumed to be a mixture of nodal-, atrial-, and ventricular-like cardiomyocytes together with potential residual non-myocytes) using bulk RNA-sequencing, followed by investigation of cellular heterogeneity using two different single-cell RNA-sequencing platforms.

RESULTS

Bulk RNA-sequencing identified key cardiac markers (TNNT2, MYL7) as well as fibroblast associated genes (P4HB, VIM), and cardiac ion channels in the iCell cardiomyocyte culture. High-resolution single cell RNA-sequencing demonstrated that both, cardiac and fibroblast-related genes were co-expressed throughout the cell population. This approach resolved two cell clusters within iCell cardiomyocytes. Interestingly, these clusters could not be associated with known cardiac subtypes. However, transcripts of ion channels potentially useful as functional markers for cardiac subtypes were below the detection limits of the single-cell approaches used. Instead, one cluster (10.8% of the cells) is defined by co-expression of cardiac and cell cycle-related genes (e.g. TOP2A). Incorporation of bromodeoxyuridine further confirmed the capability of iCell cardiomyocytes to enter cell cycle.

DISCUSSION

The co-expression of cardiac related genes with cell cycle or fibroblast related genes may be interpreted either as aberrant or as an immature feature. However, this excludes the presence of a non-cardiomyocyte sub-population and indicates that some cardiomyocytes themselves enter cell cycle.

Novel insights into cardiac regeneration based on differential fetal and adult ovine heart transcriptomic analysis

The adult mammalian cardiomyocyte has a very limited capacity to reenter the cell cycle and advance into mitosis. Therefore, diseases characterized by lost contractile tissue usually evolve into myocardial remodeling and heart failure. Analyzing the cardiac transcriptome at different developmental stages in a large mammal closer to the human than laboratory rodents may serve to disclose positive and negative cardiomyocyte cell cycle regulators potentially targetable to induce cardiac regeneration in the clinical setting. Thus we aimed at characterizing the transcriptomic profiles of the early fetal, late fetal, and adult sheep heart by employing RNA-seq technique and bioinformatic analysis to detect protein-encoding genes that in some of the stages were turned off, turned on, or differentially expressed. Genes earlier proposed as positive cell cycle regulators such as cyclin A, cdk2, meis2, meis3, and PCNA showed higher expression in fetal hearts and lower in AH, as expected. In contrast, genes previously proposed as cell cycle inhibitors, such as meis1, p16, and sav1, tended to be higher in fetal than in adult hearts, suggesting that these genes are involved in cell processes other than cell cycle regulation. Additionally, we described Gene Ontology (GO) enrichment of different sets of genes. GO analysis revealed that differentially expressed gene sets were mainly associated with metabolic and cellular processes. The cell cycle-related genes fam64a, cdc20, and cdk1, and the metabolism-related genes pitx and adipoq showed strong differential expression between fetal and adult hearts, thus being potent candidates to be targeted in human cardiac regeneration strategies.NEW & NOTEWORTHY We characterized the transcriptomic profiles of the fetal and adult sheep hearts employing RNAseq technique and bioinformatic analyses to provide sets of transcripts whose variation in expression level may link them to a specific role in cell cycle regulation. It is important to remark that this study was performed in a large mammal closer to humans than laboratory rodents. In consequence, the results can be used for further translational studies in cardiac regeneration.

Cardiomyocytes from CCND2-overexpressing human induced-pluripotent stem cells repopulate the myocardial scar in mice: A 6-month study

BACKGROUND

Cardiomyocytes that have been differentiated from CCND2-overexpressing human induced-pluripotent stem cells (hiPSC-CCND2OE CMs) can proliferate when transplanted into mouse hearts after myocardial infarction (MI). However, it is unknown whether remuscularization can replace the thin LV scar and if the large muscle graft can electrophysiologically synchronize to the recipient myocardium. Our objectives are to evaluate the structural and functional potential of hiPSC-CCND2OE CMs in replacing the LV thin scar.

METHODS

NOD/SCID mice were treated with hiPSC-CCND2OE CMs (i.e., the CCND2OE group), hiPSC-CCND2WT CMs (the CCND2WT group), or an equal volume of PBS immediately after experimentally-induced myocardial infarction. The treatments were administered to one site in the infarcted zone (IZ), two sites in the border zone (BZ), and a fourth group of animals underwent Sham surgery.

RESULTS

Six months later, engrafted cells occupied >50% of the scarred region in CCND2OE animals, and exceeded the number of engrafted cells in CCND2WT animals by ~8-fold. Engrafted cells were also more common in the IZ than in the BZ for both cell-treatment groups. Measurements of cardiac function, infarct size, wall thickness, and cardiomyocyte hypertrophy were significantly improved in CCND2OE animals compared to animals from the CCND2WT or PBS-treatment groups. Measurements in the CCND2WT and PBS groups were similar, and markers for cell cycle activation and proliferation were significantly higher in hiPSC-CCND2OE CMs than in hiPSC-CCND2WT CMs. Optical mapping of action potential propagation indicated that the engrafted hiPSC-CCND2OE CMs were electrically coupled to each other and to the cells of the native myocardium. No evidence of tumor formation was observed in any animals.

CONCLUSIONS

Six months after the transplantation, CCND2-overexpressing hiPSC-CMs proliferated and replaced >50% of the myocardial scar tissue. The large graft hiPSC-CCND2OE CMs also electrically integrated with the host myocardium, which was accompanied by a significant improvement in LV function.

Pluripotent Stem Cell-Derived Cardiomyocyte Transplantation for Heart Disease Treatment

PURPOSE OF REVIEW

Cardiovascular disease is the leading cause of mortality worldwide. Pluripotent stem cell-derived cardiomyocytes (PSC-CMs) have great potential to treat heart disease, owing to their capacity of engraftment and remuscularization in the host heart after transplantation. In the current review, we provide an overview of PSC-CMs for clinical transplantation.

RECENT FINDINGS

Studies have shown that PSC-CMs can survive, engraft, and form gap junctions after transplantation, with functional benefit. Engrafted PSC-CMs matured gradually in host hearts. Only in a large animal model, transient ventricular arrhythmias were detected, mainly because of the ectopic pacing from the grafted PSC-CMs. Although intense immunosuppression is unavoidable in xenotransplantation, immunosuppression remains necessary for MHC-matched allogenic non-human primate PSC-CMs transplantation. This review offers insights on how PSC-CMs contribute to functional benefit after transplantation to injured non-human primate hearts. We believe that PSC-CM transplantation represents a potentially novel treatment for ischemic heart diseases, provided that several technological and biological limitations can be overcome.

High-content phenotypic assay for proliferation of human iPSC-derived cardiomyocytes identifies L-type calcium channels as targets

Over 5 million people in the United States suffer from heart failure, due to the limited ability to regenerate functional cardiac tissue. One potential therapeutic strategy is to enhance proliferation of resident cardiomyocytes. However, phenotypic screening for therapeutic agents is challenged by the limited ability of conventional markers to discriminate between cardiomyocyte proliferation and endoreplication (e.g. polyploidy and multinucleation). Here, we developed a novel assay that combines automated live-cell microscopy and image processing algorithms to discriminate between proliferation and endoreplication by quantifying changes in the number of nuclei, changes in the number of cells, binucleation, and nuclear DNA content. We applied this assay to further prioritize hits from a primary screen for DNA synthesis, identifying 30 compounds that enhance proliferation of human induced pluripotent stem cell-derived cardiomyocytes. Among the most active compounds from the phenotypic screen are clinically approved L-type calcium channel blockers from multiple chemical classes whose activities were confirmed across different sources of human induced pluripotent stem cell-derived cardiomyocytes. Identification of compounds that stimulate human cardiomyocyte proliferation may provide new therapeutic strategies for heart failure.

Targeting the Cardiomyocyte Cell Cycle for Heart Regeneration

Adult mammalian cardiomyocytes (CMs) exhibit limited proliferative capacity, as cell cycle activity leads to an increase in DNA content, but mitosis and cytokinesis are infrequent. This makes the heart highly inefficient in replacing with neoformed cardiomyocytes lost contractile cells as occurs in diseases such as myocardial infarction and dilated cardiomyopathy. Regenerative therapies based on the implant of stem cells of diverse origin do not warrant engraftment and electromechanical connection of the new cells with the resident ones, a fundamental condition to restore the physiology of the cardiac syncytium. Consequently, there is a growing interest in identifying factors playing relevant roles in the regulation of the CM cell cycle to be targeted in order to induce the resident cardiomyocytes to divide into daughter cells and thus achieve myocardial regeneration with preservation of physiologic syncytial performance.

Stage-specific Effects of Bioactive Lipids on Human iPSC Cardiac Differentiation and Cardiomyocyte Proliferation

Bioactive lipids such as sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA) regulate diverse processes including cell proliferation, differentiation, and migration. However, their roles in cardiac differentiation and cardiomyocyte proliferation have not been explored. Using a 96-well differentiation platform for generating human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) we found that S1P and LPA can independently enhance cardiomyocyte generation when administered at an early stage of differentiation. We showed that the combined S1P and LPA treatment of undifferentiated hiPSCs resulted in increased nuclear accumulation of β-catenin, the canonical Wnt signaling pathway mediator, and synergized with CHIR99021, a glycogen synthase kinase 3 beta inhibitor, to enhance mesodermal induction and subsequent cardiac differentiation. At later stages of cardiac differentiation, the addition of S1P and LPA resulted in cell cycle initiation in hiPSC-CMs, an effect mediated through increased ERK signaling. Although the addition of S1P and LPA alone was insufficient to induce cell division, it was able to enhance β-catenin-mediated hiPSC-CM proliferation. In summary, we demonstrated a developmental stage-specific effect of bioactive lipids to enhance hiPSC-CM differentiation and proliferation via modulating the effect of canonical Wnt/β-catenin and ERK signaling. These findings may improve hiPSC-CM generation for cardiac disease modeling, precision medicine, and regenerative therapies.

Nanotechnology-Based Cardiac Targeting and Direct Cardiac Reprogramming: The Betrothed

Cardiovascular diseases represent the first cause of morbidity in Western countries, and chronic heart failure features a significant health care burden in developed countries. Efforts in the attempt of finding new possible strategies for the treatment of CHF yielded several approaches based on the use of stem cells. The discovery of direct cardiac reprogramming has unveiled a new approach to heart regeneration, allowing, at least in principle, the conversion of one differentiated cell type into another without proceeding through a pluripotent intermediate. First developed for cancer treatment, nanotechnology-based approaches have opened new perspectives in many fields of medical research, including cardiovascular research. Nanotechnology could allow the delivery of molecules with specific biological activity at a sustained and controlled rate in heart tissue, in a cell-specific manner. Potentially, all the mediators and structural molecules involved in the fibrotic process could be selectively targeted by nanocarriers, but to date, only few experiences have been made in cardiac research. This review highlights the most prominent concepts that characterize both the field of cardiac reprogramming and a nanomedicine-based approach to cardiovascular diseases, hypothesizing a possible synergy between these two very promising fields of research in the treatment of heart failure.

Biomarkers of Human Pluripotent Stem Cell-Derived Cardiac Lineages

Human pluripotent stem cells (hPSCs) offer a practical source for the de novo generation of cardiac tissues and a unique opportunity to investigate cardiovascular lineage commitment. Numerous strategies have focused on the in vitro production of cardiomyocytes, smooth muscle, and endothelium from hPSCs. However, these differentiation protocols often yield undesired cell types. Thus, establishing a set of stage-specific markers for pure cardiac subpopulations will assist in defining the hierarchy of cardiac differentiation, aid in the development of cellular therapy, and facilitate drug screening and disease modeling. The recent characterization of many such markers is enabling the isolation of major cardiac lineages and subpopulations from differentiating hPSCs. We provide here a comprehensive review detailing the suite of biomarkers used to differentiate cardiac lineages from mixed hPSC-derived populations.

Direct Cardiac Cellular Reprogramming for Cardiac Regeneration

OPINION STATEMENT

Direct cardiac cellular reprogramming of endogenous cardiac fibroblasts directly into induced cardiomyocytes is a highly feasible, promising therapeutic option for patients with advanced heart failure. The most successful cardiac reprogramming strategy will likely be a multimodal approach involving an optimal combination of cardio-differentiating factors, suppression of fibroblast gene expression, and induction of angiogenic factors.

Induction of Human iPSC-Derived Cardiomyocyte Proliferation Revealed by Combinatorial Screening in High Density Microbioreactor Arrays

Inducing cardiomyocyte proliferation in post-mitotic adult heart tissue is attracting significant attention as a therapeutic strategy to regenerate the heart after injury. Model animal screens have identified several candidate signalling pathways, however, it remains unclear as to what extent these pathways can be exploited, either individually or in combination, in the human system. The advent of human cardiac cells from directed differentiation of human pluripotent stem cells (hPSCs) now provides the ability to interrogate human cardiac biology in vitro, but it remains difficult with existing culture formats to simply and rapidly elucidate signalling pathway penetrance and interplay. To facilitate high-throughput combinatorial screening of candidate biologicals or factors driving relevant molecular pathways, we developed a high-density microbioreactor array (HDMA) – a microfluidic cell culture array containing 8100 culture chambers. We used HDMAs to combinatorially screen Wnt, Hedgehog, IGF and FGF pathway agonists. The Wnt activator CHIR99021 was identified as the most potent molecular inducer of human cardiomyocyte proliferation, inducing cell cycle activity marked by Ki67, and an increase in cardiomyocyte numbers compared to controls. The combination of human cardiomyocytes with the HDMA provides a versatile and rapid tool for stratifying combinations of factors for heart regeneration.