Reprogramming toward Heart Regeneration: Stem Cells and Beyond

Cardiac regeneration - Past advancements, current challenges, and future directions

Cardiovascular disease is the leading cause of mortality and morbidity worldwide. Despite improvements in the standard of care for patients with heart diseases, including innovation in pharmacotherapy and surgical interventions, none have yet been proven effective to prevent the progression to heart failure. Cardiac transplantation is the last resort for patients with severe heart failure, but donor shortages remain a roadblock. Cardiac regenerative strategies include cell-based therapeutics, gene therapy, direct reprogramming of non-cardiac cells, acellular biologics, and tissue engineering methods to restore damaged hearts. Significant advancements have been made over the past several decades within each of these fields. This review focuses on the advancements of: 1) cell-based cardiac regenerative therapies, 2) the use of noncoding RNA to induce endogenous cell proliferation, and 3) application of bioengineering methods to promote retention and integration of engrafted cells. Different cell sources have been investigated, including adult stem cells derived from bone marrow and adipose cells, cardiosphere-derived cells, skeletal myoblasts, and pluripotent stem cells. In addition to cell-based transplantation approaches, there have been accumulating interest over the past decade in inducing endogenous CM proliferation for heart regeneration, particularly with the use of noncoding RNAs such as miRNAs and lncRNAs. Bioengineering applications have focused on combining cell-transplantation approaches with fabrication of a porous, vascularized scaffold using biomaterials and advanced bio-fabrication techniques that may offer enhanced retention of transplanted cells, with the hope that these cells would better engraft with host tissue to improve cardiac function. This review summarizes the present status and future challenges of cardiac regenerative therapies.

Direct cardiac reprogramming: Toward the era of multi-omics analysis

Limited regenerative capacity of adult cardiomyocytes precludes heart repair and regeneration after cardiac injury. Direct cardiac reprograming that converts scar-forming cardiac fibroblasts (CFs) into functional induced-cardiomyocytes (iCMs) offers promising potential to restore heart structure and heart function. Significant advances have been achieved in iCM reprogramming using genetic and epigenetic regulators, small molecules, and delivery strategies. Recent researches on the heterogeneity and reprogramming trajectories elucidated novel mechanisms of iCM reprogramming at single cell level. Here, we review recent progress in iCM reprogramming with a focus on multi-omics (transcriptomic, epigenomic and proteomic) researches to investigate the cellular and molecular machinery governing cell fate conversion. We also highlight the future potential using multi-omics approaches to dissect iCMs conversion for clinal applications.

The generation of a lactate-rich environment stimulates cell cycle progression and modulates gene expression on neonatal and hiPSC-derived cardiomyocytes

In situ tissue engineering strategies are a promising approach to activate the endogenous regenerative potential of the cardiac tissue helping the heart to heal itself after an injury. However, the current use of complex reprogramming vectors for the activation of reparative pathways challenges the easy translation of these therapies into the clinic. Here, we evaluated the response of mouse neonatal and human induced pluripotent stem cell-derived cardiomyocytes to the presence of exogenous lactate, thus mimicking the metabolic environment of the fetal heart. An increase in cardiomyocyte cell cycle activity was observed in the presence of lactate, as determined through Ki67 and Aurora-B kinase. Gene expression and RNA-sequencing data revealed that cardiomyocytes incubated with lactate showed upregulation of BMP10, LIN28 or TCIM in tandem with downregulation of GRIK1 or DGKK among others. Lactate also demonstrated a capability to modulate the production of inflammatory cytokines on cardiac fibroblasts, reducing the production of Fas, Fraktalkine or IL-12p40, while stimulating IL-13 and SDF1a. In addition, the generation of a lactate-rich environment improved ex vivo neonatal heart culture, by affecting the contractile activity and sarcomeric structures and inhibiting epicardial cell spreading. Our results also suggested a common link between the effect of lactate and the activation of hypoxia signaling pathways. These findings support a novel use of lactate in cardiac tissue engineering, modulating the metabolic environment of the heart and thus paving the way to the development of lactate-releasing platforms for in situ cardiac regeneration.

scREMOTE: Using multimodal single cell data to predict regulatory gene relationships and to build a computational cell reprogramming model

Cell reprogramming offers a potential treatment to many diseases, by regenerating specialized somatic cells. Despite decades of research, discovering the transcription factors that promote cell reprogramming has largely been accomplished through trial and error, a time-consuming and costly method. A computational model for cell reprogramming, however, could guide the hypothesis formulation and experimental validation, to efficiently utilize time and resources. Current methods often cannot account for the heterogeneity observed in cell reprogramming, or they only make short-term predictions, without modelling the entire reprogramming process. Here, we present scREMOTE, a novel computational model for cell reprogramming that leverages single cell multiomics data, enabling a more holistic view of the regulatory mechanisms at cellular resolution. This is achieved by first identifying the regulatory potential of each transcription factor and gene to uncover regulatory relationships, then a regression model is built to estimate the effect of transcription factor perturbations. We show that scREMOTE successfully predicts the long-term effect of overexpressing two key transcription factors in hair follicle development by capturing higher-order gene regulations. Together, this demonstrates that integrating the multimodal processes governing gene regulation creates a more accurate model for cell reprogramming with significant potential to accelerate research in regenerative medicine.

Restoring Ravaged Heart: Molecular Mechanisms and Clinical Application of miRNA in Heart Regeneration

Human heart development is a complex and tightly regulated process, conserving proliferation, and multipotency of embryonic cardiovascular progenitors. At terminal stage, progenitor cell type gets suppressed for terminal differentiation and maturation. In the human heart, most cardiomyocytes are terminally differentiated and so have limited proliferation capacity. MicroRNAs (miRNAs) are non-coding single-stranded RNA that regulate gene expression and mRNA silencing at the post-transcriptional level. These miRNAs play a crucial role in numerous biological events, including cardiac development, and cardiomyocyte proliferation. Several cardiac cells specific miRNAs have been discovered. Inhibition or overexpression of these miRNAs could induce cardiac regeneration, cardiac stem cell proliferation and cardiomyocyte proliferation. Clinical application of miRNAs extends to heart failure, wherein the cell cycle arrest of terminally differentiated cardiac cells inhibits the heart regeneration. The regenerative capacity of the myocardium can be enhanced by cardiomyocyte specific miRNAs controlling the cell cycle. In this review, we focus on cardiac-specific miRNAs involved in cardiac regeneration and cardiomyocyte proliferation, and their potential as a new clinical therapy for heart regeneration.

Strategies and Challenges to Improve Cellular Programming-Based Approaches for Heart Regeneration Therapy

Limited adult cardiac cell proliferation after cardiovascular disease, such as heart failure, hampers regeneration, resulting in a major loss of cardiomyocytes (CMs) at the site of injury. Recent studies in cellular reprogramming approaches have provided the opportunity to improve upon previous techniques used to regenerate damaged heart. Using these approaches, new CMs can be regenerated from differentiation of iPSCs (similar to embryonic stem cells), the direct reprogramming of fibroblasts [induced cardiomyocytes (iCMs)], or induced cardiac progenitors. Although these CMs have been shown to functionally repair infarcted heart, advancements in technology are still in the early stages of development in research laboratories. In this review, reprogramming-based approaches for generating CMs are briefly introduced and reviewed, and the challenges (including low efficiency, functional maturity, and safety issues) that hinder further translation of these approaches into a clinical setting are discussed. The creative and combined optimal methods to address these challenges are also summarized, with optimism that further investigation into tissue engineering, cardiac development signaling, and epigenetic mechanisms will help to establish methods that improve cell-reprogramming approaches for heart regeneration.

Bioactive Lipid Signaling in Cardiovascular Disease, Development, and Regeneration

Cardiovascular disease (CVD) remains a leading cause of death globally. Understanding and characterizing the biochemical context of the cardiovascular system in health and disease is a necessary preliminary step for developing novel therapeutic strategies aimed at restoring cardiovascular function. Bioactive lipids are a class of dietary-dependent, chemically heterogeneous lipids with potent biological signaling functions. They have been intensively studied for their roles in immunity, inflammation, and reproduction, among others. Recent advances in liquid chromatography-mass spectrometry techniques have revealed a staggering number of novel bioactive lipids, most of them unknown or very poorly characterized in a biological context. Some of these new bioactive lipids play important roles in cardiovascular biology, including development, inflammation, regeneration, stem cell differentiation, and regulation of cell proliferation. Identifying the lipid signaling pathways underlying these effects and uncovering their novel biological functions could pave the way for new therapeutic strategies aimed at CVD and cardiovascular regeneration.

Fluorescent conjugated polymer nanovector for in vivo tracking and regulating the fate of stem cells for restoring infarcted myocardium

Stem cell therapy holds great promise for cardiac regeneration. However, the lack of ability to control stem cell fate after in vivo transplantation greatly restricts its therapeutic outcomes. MicroRNA delivery has emerged as a powerful tool to control stem cell fate for enhanced cardiac regeneration. However, the clinical translation of therapy based on gene-transfected stem cells remains challenging, due to the unknown in vivo behaviors of stem cells. Here, we developed a nano-platform (i.e., PFBT@miR-1-Tat NPs) that can achieve triggered release of microRNA-1 to promote cardiac differentiation of mesenchymal stem cells (MSCs), and long-term tracking of transplanted MSCs through bright and ultra-stable fluorescence of conjugated polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (PFBT). We found that PFBT@miR-1-Tat NP-treated MSCs significantly restored the infarcted myocardium by promoting stem cell cardiac differentiation and integration with the in situ cardiac tissues. Meanwhile, MSCs without gene delivery improved the infarcted heart functions mainly through a paracrine effect and blood vessel formation. The developed conjugated polymer nanovector should be a powerful tool for manipulating as well as revealing the fate of therapeutic cells in vivo, which is critical for optimizing the therapeutic route of gene and cell combined therapy and therefore for accelerating clinical translation. STATEMENT OF SIGNIFICANCE: The lack of controllability in stem cell fate and the unclear in vivo cellular behaviors restrict the therapeutic outcomes of stem cell therapy. Herein, we engineered fluorescent conjugated polymer nanoparticles as gene delivery nanovectors with controlled release and high intracellular delivery capability to harness the fate of mesenchymal stem cells (MSCs) in vivo, meanwhile to reveal the cellular mechanism of gene-treated stem cell therapy. As compared with only MSC treatment that improves infarcted myocardium functions through paracrine effect, treatment with conjugated polymer nanovector-treated MSCs significantly restored infarcted myocardium through enhancing MSC cardiac differentiation and integration with the in-situ cardiac tissues. These findings demonstrate that the conjugated polymer nanovector would be a powerful tool in optimizing gene and cell combined therapy.

Prospective Advances in Non-coding RNAs Investigation

Non-coding RNAs (ncRNAs) play significant roles in numerous physiological cellular processes and molecular alterations during pathological conditions including heart diseases, cancer, immunological disorders and neurological diseases. This chapter is focusing on the basis of ncRNA relation with their functions and prospective advances in non-coding RNAs particularly miRNAs investigation in the cardiovascular disease management.The field of ncRNAs therapeutics is a very fascinating and challenging too. Scientists have opportunity to develop more advanced therapeutics as well as diagnostic approaches for cardiovascular conditions. Advanced studies are critically needed to deepen the understanding of the molecular biology, mechanism and modulation of ncRNAs and chemical formulations for managing CVDs.

Analysis of Congenital Heart Defects in Mouse Embryos Using Qualitative and Quantitative Histological Methods

Congenital heart defects (CHD) are the most common type of birth defect in humans, affecting up to 1% of all live births. However, the underlying causes for CHD are still poorly understood. The developing mouse constitutes a valuable model for the study of CHD, because cardiac developmental programs between mice and humans are highly conserved. The protocol describes in detail how to produce mouse embryos of the desired gestational stage, methods to isolate and preserve the heart for downstream processing, quantitative methods to identify common types of CHD by histology (e.g., ventricular septal defects, atrial septal defects, patent ductus arteriosus), and quantitative histomorphometry methods to measure common muscular compaction phenotypes. These methods articulate all the steps involved in sample preparation, collection, and analysis, allowing scientists to correctly and reproducibly measure CHD.

Potential application of liposomal nanodevices for non-cancer diseases: an update on design, characterization and biopharmaceutical evaluation

Liposomes, lipid-based vesicular systems, have attracted major interest as a means to improve drug delivery to various organs and tissues in the human body. Recent literature highlights the benefits of liposomes for use as drug delivery systems, including encapsulating of both hydrophobic and hydrophilic cargos, passive and active targeting, enhanced drug bioavailability and therapeutic effects, reduced systemic side effects, improved cargo penetration into the target tissue and triggered contents release. Pioneering work of liposomes researchers led to introduction of long-circulating, ligand-targeted and triggered release liposomes, as well as, liposomes containing nucleic acids and vesicles containing combination of cargos. Altogether, these findings have led to widespread application of liposomes in a plethora of areas from cancer to conditions such as cardiovascular, neurologic, respiratory, skin, autoimmune and eye disorders. There are numerous review articles on the application of liposomes in treatment of cancer, which seems the primary focus, whereas other diseases also benefit from liposome-mediated treatments. Therefore, this article provides an illustrated detailed overview of liposomal formulations, in vitro characterization and their applications in different disorders other than cancer. Challenges and future directions, which must be considered to obtain the most benefit from applications of liposomes in these disorders, are discussed.

Loss of genomic integrity induced by lysosphingolipid imbalance drives ageing in the heart

Cardiac dysfunctions dramatically increase with age. Revealing a currently unknown contributor to cardiac ageing, we report the age-dependent, cardiac-specific accumulation of the lysosphingolipid sphinganine (dihydrosphingosine, DHS) as an evolutionarily conserved hallmark of the aged vertebrate heart. Mechanistically, the DHS-derivative sphinganine-1-phosphate (DHS1P) directly inhibits HDAC1, causing an aberrant elevation in histone acetylation and transcription levels, leading to DNA damage. Accordingly, the pharmacological interventions, preventing (i) the accumulation of DHS1P using SPHK2 inhibitors, (ii) the aberrant increase in histone acetylation using histone acetyltransferase (HAT) inhibitors, (iii) the DHS1P-dependent increase in transcription using an RNA polymerase II inhibitor, block DHS-induced DNA damage in human cardiomyocytes. Importantly, an increase in DHS levels in the hearts of healthy young adult mice leads to an impairment in cardiac functionality indicated by a significant reduction in left ventricular fractional shortening and ejection fraction, mimicking the functional deterioration of aged hearts. These molecular and functional defects can be partially prevented in vivo using HAT inhibitors. Together, we report an evolutionarily conserved mechanism by which increased DHS levels drive the decline in cardiac health.

Regulation of Cell Cycle to Stimulate Adult Cardiomyocyte Proliferation and Cardiac Regeneration

Human diseases are often caused by loss of somatic cells that are incapable of re-entering the cell cycle for regenerative repair. Here, we report a combination of cell-cycle regulators that induce stable cytokinesis in adult post-mitotic cells. We screened cell-cycle regulators expressed in proliferating fetal cardiomyocytes and found that overexpression of cyclin-dependent kinase 1 (CDK1), CDK4, cyclin B1, and cyclin D1 efficiently induced cell division in post-mitotic mouse, rat, and human cardiomyocytes. Overexpression of the cell-cycle regulators was self-limiting through proteasome-mediated degradation of the protein products. In vivo lineage tracing revealed that 15%-20% of adult cardiomyocytes expressing the four factors underwent stable cell division, with significant improvement in cardiac function after acute or subacute myocardial infarction. Chemical inhibition of Tgf-β and Wee1 made CDK1 and cyclin B dispensable. These findings reveal a discrete combination of genes that can efficiently unlock the proliferative potential in cells that have terminally exited the cell cycle.

Dual Roles of Graphene Oxide To Attenuate Inflammation and Elicit Timely Polarization of Macrophage Phenotypes for Cardiac Repair

Development of localized inflammatory environments by M1 macrophages in the cardiac infarction region exacerbates heart failure after myocardial infarction (MI). Therefore, the regulation of inflammation by M1 macrophages and their timely polarization toward regenerative M2 macrophages suggest an immunotherapy. Particularly, controlling cellular generation of reactive oxygen species (ROS), which cause M1 differentiation, and developing M2 macrophage phenotypes in macrophages propose a therapeutic approach. Previously, stem or dendritic cells were used in MI for their anti-inflammatory and cardioprotective potentials and showed inflammation modulation and M2 macrophage progression for cardiac repair. However, cell-based therapeutics are limited due to invasive cell isolation, time-consuming cell expansion, labor-intensive and costly ex vivo cell manipulation, and low grafting efficiency. Here, we report that graphene oxide (GO) can serve as an antioxidant and attenuate inflammation and inflammatory polarization of macrophages via reduction in intracellular ROS. In addition, GO functions as a carrier for interleukin-4 plasmid DNA (IL-4 pDNA) that propagates M2 macrophages. We synthesized a macrophage-targeting/polarizing GO complex (MGC) and demonstrated that MGC decreased ROS in immune-stimulated macrophages. Furthermore, DNA-functionalized MGC (MGC/IL-4 pDNA) polarized M1 to M2 macrophages and enhanced the secretion of cardiac repair-favorable cytokines. Accordingly, injection of MGC/IL-4 pDNA into mouse MI models attenuated inflammation, elicited early polarization toward M2 macrophages, mitigated fibrosis, and improved heart function. Taken together, the present study highlights a biological application of GO in timely modulation of the immune environment in MI for cardiac repair. Current therapy using off-the-shelf material GO may overcome the shortcomings of cell therapies for MI.

Therapeutic Potential of Adipose Stem Cells

Adipose stem cells (ASCs) have gained attention in the fields of stem cells regenerative medicine due to their multifaceted therapeutic capabilities. Promising preclinical evidence of ASCs has supported the substantial interest in the use of these cells as therapy for human disease. ASCs are an adult stem cell resident in adipose tissue with the potential to differentiation along mesenchymal lineages. They also are known to be recruited to sites of inflammation where they exhibit strong immunomodulatory capabilities to promote wound healing and regeneration. ASCs can be isolated from adipose tissue at a relatively high yield compared to their mesenchymal cell counterparts: bone marrow-derived mesenchymal stem cells (BM-MSCs). Like BM-MSCs, ASCs are easily culture expanded and have a reduced immunogenicity or are perhaps immune privileged, making them attractive options for cellular therapy. Additionally, the heterogeneous cellular product obtained after digestion of adipose tissue, called the stromal vascular fraction (SVF), contains ASCs and several populations of stromal and immune cells. Both the SVF and culture expanded ASCs have the potential to be therapeutic in various diseases. This review will focus on the preclinical and clinical evidence of SVF and ASCs, which make them potential candidates for therapy in regenerative medicine and inflammatory disease processes.

Single cardiomyocyte nuclear transcriptomes reveal a lincRNA-regulated de-differentiation and cell cycle stress-response in vivo

Cardiac regeneration may revolutionize treatment for heart failure but endogenous progenitor-derived cardiomyocytes in the adult mammalian heart are few and pre-existing adult cardiomyocytes divide only at very low rates. Although candidate genes that control cardiomyocyte cell cycle re-entry have been implicated, expression heterogeneity in the cardiomyocyte stress-response has never been explored. Here, we show by single nuclear RNA-sequencing of cardiomyocytes from both mouse and human failing, and non-failing adult hearts that sub-populations of cardiomyocytes upregulate cell cycle activators and inhibitors consequent to the stress-response in vivo. We characterize these subgroups by weighted gene co-expression network analysis and discover long intergenic non-coding RNAs (lincRNA) as key nodal regulators. KD of nodal lincRNAs affects expression levels of genes related to dedifferentiation and cell cycle, within the same gene regulatory network. Our study reveals that sub-populations of adult cardiomyocytes may have a unique endogenous potential for cardiac regeneration in vivo.

Adult mammalian cardiomyocytes are predominantly binucleated and unable to divide. Using single nuclear RNA-sequencing of cardiomyocytes from mouse and human failing and non-failing adult hearts, See et al. show that some cardiomyocytes respond to stress by dedifferentiation and cell cycle re-entry regulated by lncRNAs.

Dedifferentiation, Proliferation, and Redifferentiation of Adult Mammalian Cardiomyocytes After Ischemic Injury

BACKGROUND

Adult mammalian hearts have a limited ability to generate new cardiomyocytes. Proliferation of existing adult cardiomyocytes (ACMs) is a potential source of new cardiomyocytes. Understanding the fundamental biology of ACM proliferation could be of great clinical significance for treating myocardial infarction (MI). We aim to understand the process and regulation of ACM proliferation and its role in new cardiomyocyte formation of post-MI mouse hearts.

METHODS

β-Actin-green fluorescent protein transgenic mice and fate-mapping Myh6-MerCreMer-tdTomato/lacZ mice were used to trace the fate of ACMs. In a coculture system with neonatal rat ventricular myocytes, ACM proliferation was documented with clear evidence of cytokinesis observed with time-lapse imaging. Cardiomyocyte proliferation in the adult mouse post-MI heart was detected by cell cycle markers and 5-ethynyl-2-deoxyuridine incorporation analysis. Echocardiography was used to measure cardiac function, and histology was performed to determine infarction size.

RESULTS

In vitro, mononucleated and bi/multinucleated ACMs were able to proliferate at a similar rate (7.0%) in the coculture. Dedifferentiation proceeded ACM proliferation, which was followed by redifferentiation. Redifferentiation was essential to endow the daughter cells with cardiomyocyte contractile function. Intercellular propagation of Ca²⁺ from contracting neonatal rat ventricular myocytes into ACM daughter cells was required to activate the Ca²⁺-dependent calcineurin-nuclear factor of activated T-cell signaling pathway to induce ACM redifferentiation. The properties of neonatal rat ventricular myocyte Ca²⁺ transients influenced the rate of ACM redifferentiation. Hypoxia impaired the function of gap junctions by dephosphorylating its component protein connexin 43, the major mediator of intercellular Ca²⁺ propagation between cardiomyocytes, thereby impairing ACM redifferentiation. In vivo, ACM proliferation was found primarily in the MI border zone. An ischemia-resistant connexin 43 mutant enhanced the redifferentiation of ACM-derived new cardiomyocytes after MI and improved cardiac function.

CONCLUSIONS

Mature ACMs can reenter the cell cycle and form new cardiomyocytes through a 3-step process: dedifferentiation, proliferation, and redifferentiation. Intercellular Ca²⁺ signal from neighboring functioning cardiomyocytes through gap junctions induces the redifferentiation process. This novel mechanism contributes to new cardiomyocyte formation in post-MI hearts in mammals.

Non-coding microRNAs for cardiac regeneration: Exploring novel alternatives to induce heart healing

In recent years, different studies have revealed that adult mammalian cardiomyocytes have the capacity to self-renew under homeostatic conditions and after myocardial injury. Interestingly, data from animal models capable of regeneration, such as the adult zebrafish and neonatal mice, have identified different non-coding RNAs (ncRNAs) as functional RNA molecules driving cardiac regeneration and repair. In this review, we summarize the current knowledge of the roles that a specific subset of ncRNAs, namely microRNAs (miRNA), plays in these animal models. We also emphasize the importance of characterizing and manipulating miRNAs as a novel approach to awaken the dormant regenerative potential of the adult mammalian heart by the administration of miRNA mimics or inhibitors. Overall, the use of these strategies alone or in combination with current cardiac therapies may represent new avenues to pursue for cardiac regeneration.

Prospects of siRNA applications in regenerative medicine

Small interfering RNA (siRNA) has established its reputation in the field of tissue engineering owing to its ability to silence the proteins that inhibit tissue regeneration. siRNA is capable of regulating cellular behavior during tissue regeneration processes. The concept of using siRNA technology in regenerative medicine derived from its ability to inhibit the expression of target genes involved in defective tissues and the possibility to induce the expression of tissue-inductive factors that improve the tissue regeneration process. To date, siRNA has been used as a suppressive biomolecule in different tissues, such as nervous tissue, bone, cartilage, heart, kidney, and liver. Moreover, various delivery systems have been applied in order to deliver siRNA to the target tissues. This review will provide an in-depth discussion on the development of siRNA and their delivery systems and mechanisms of action in different tissues.

Meta-Analysis of Transcriptome Regulation During Induction to Cardiac Myocyte Fate From Mouse and Human Fibroblasts

Ectopic expression of a defined set of transcription factors (TFs) can directly convert fibroblasts into a cardiac myocyte cell fate. Beside inefficiency in generating induced cardiomyocytes (iCMs), the molecular mechanisms that regulate this process remained to be well defined. The main purpose of this study was to provide better insight on the transcriptome regulation and to introduce a new strategy for candidating TFs for the transdifferentiation process. Eight mouse and three human high quality microarray data sets were analyzed to find differentially expressed genes (DEGs), which we integrated with TF-binding sites and protein-protein interactions to construct gene regulatory and protein-protein interaction networks. Topological and biological analyses of constructed gene networks revealed the main regulators and most affected biological processes. The DEGs could be categorized into two distinct groups, first, up-regulated genes that are mainly involved in cardiac-specific processes and second, down-regulated genes that are mainly involved in fibroblast-specific functions. Gata4, Mef2a, Tbx5, Tead4 TFs were identified as main regulators of cardiac-specific gene expression program; and Trp53, E2f1, Myc, Sfpi1, Lmo2, and Meis1 were identified as TFs which mainly regulate the expression of fibroblast-specific genes. Furthermore, we compared gene expression profiles and identified TFs between mouse and human to find the similarities and differences. In summary, our strategy of meta-analyzing the data of high-throughput techniques by computational approaches, besides revealing the mechanisms involved in the regulation of the gene expression program, also suggests a new approach for increasing the efficiency of the direct reprogramming of fibroblasts into iCMs. J. Cell. Physiol. 232: 2053-2062, 2017. © 2016 Wiley Periodicals, Inc.

Targeting chronic cardiac remodeling with cardiac progenitor cells in a murine model of ischemia/reperfusion injury

BACKGROUND

Translational failure for cardiovascular disease is a substantial problem involving both high research costs and an ongoing lack of novel treatment modalities. Despite the progress already made, cell therapy for chronic heart failure in the clinical setting is still hampered by poor translation. We used a murine model of chronic ischemia/reperfusion injury to examine the effect of minimally invasive application of cardiac progenitor cells (CPC) in cardiac remodeling and to improve clinical translation.

METHODS

28 days after the induction of I/R injury, mice were randomized to receive either CPC (0.5 million) or vehicle by echo-guided intra-myocardial injection. To determine retention, CPC were localized in vivo by bioluminescence imaging (BLI) two days after injection. Cardiac function was assessed by 3D echocardiography and speckle tracking analysis to quantify left ventricular geometry and regional myocardial deformation.

RESULTS

BLI demonstrated successful injection of CPC (18/23), which were mainly located along the needle track in the anterior/septal wall. Although CPC treatment did not result in overall restoration of cardiac function, a relative preservation of the left ventricular end-diastolic volume was observed at 4 weeks follow-up compared to vehicle control (+5.3 ± 2.1 μl vs. +10.8 ± 1.5 μl). This difference was reflected in an increased strain rate (+16%) in CPC treated mice.

CONCLUSIONS

CPC transplantation can be adequately studied in chronic cardiac remodeling using this study set-up and by that provide a translatable murine model facilitating advances in research for new therapeutic approaches to ultimately improve therapy for chronic heart failure.

Neonatal epicardial-derived progenitors aquire myogenic traits in skeletal muscle, but not cardiac muscle

BACKGROUND/OBJECTIVES

Epicardium-derived progenitor cells (EPDCs) differentiate into all heart cell types in the embryonic heart, yet their differentiation into cardiomyocytes in the adult heart is limited and poorly described. This may be due to EPDCs lacking myogenic potential or the inert adult heart missing regenerative signals essential for directed differentiation of EPDCs. Herein, we aimed to evaluate the myogenic potential of neonatal EPDCs in adult and neonatal mouse myocardium, as well as in skeletal muscle. The two latter tissues have an intrinsic capability to develop and regenerate, in contrast to the adult heart.

METHODS

Highly purified mouse EPDCs were transplanted into damaged neonatal and adult myocardium as well as regenerating skeletal muscle. Co-cultures with skeletal myoblasts were used to distinguish fusion independent myogenic conversion.

RESULTS

No donor EPDC-derived cardiomyocytes were observed in hearts. In contrast, a remarkable contribution of EPDCs to skeletal muscle myofiber formation was evident in vivo. Furthermore, co-cultures of EPDCs with myoblasts showed that EPDCs became part of multinucleated fibers and appeared to acquire myogenic traits independent of a fusion event. Fluorescence activated cell sorting of EPDCs co-cultured with and without myoblasts and subsequent qRT-PCR of 64 transcripts established that the myogenic phenotype conversion was accomplished through induction of a transcriptional myogenic program.

CONCLUSION

These results suggest that EPDCs may be more myogenic than previously anticipated. But, the heart may lack factors for induction of myogenesis of EPDCs, a scenario that should be taken into consideration when aiming for repair of damaged myocardium by stem cell transplantation.

Zrf1 controls mesoderm lineage genes and cardiomyocyte differentiation

In the present study we addressed the function of the transcriptional activator Zrf1 in the generation of the 3 germ layers during in vitro development. Currently, Zrf1 is rather regarded as a factor that drives the expression of neuronal genes. Here, we have employed mouse embryonic stem cells and P19 cells to understand the role of Zrf1 in the generation of mesoderm-derived tissues like adipocytes, cartilage and heart. Our data shows that Zrf1 is essential for the transcriptional activation of genes that give rise to mesoderm and in particular heart development. In both, the mESC and P19 systems, we provide evidence that Zrf1 contributes to the generation of functional cardiomyocytes. We further demonstrate that Zrf1 binds to the transcription start sites (TSSs) of heart tissue-specific genes from the first and second heart field where it drives their temporal expression during differentiation. Taken together, we have identified Zrf1 as a novel regulator of the mesodermal lineage that might facilitate spatiotemporal expression of genes.

CIBZ Regulates Mesodermal and Cardiac Differentiation of by Suppressing T and Mesp1 Expression in Mouse Embryonic Stem Cells

The molecular mechanisms underlying mesodermal and cardiac specification from embryonic stem cells (ESCs) are not fully understood. Here, we showed that the BTB domain-containing zinc finger protein CIBZ is expressed in mouse ESCs but is dramatically downregulated during ESC differentiation. CIBZ deletion in ESCs induced specification toward mesoderm phenotypes and their differentiation into cardiomyocytes, whereas overexpression of CIBZ delayed these processes. During ESC differentiation, CIBZ loss-and-gain-of-function data indicate that CIBZ negatively regulates the expressions of Brachyury (T) and Mesp1, the key transcriptional factors responsible for the specification of mammalian mesoderm and cardiac progenitors, respectively. Chromatin immunoprecipitation assays showed that CIBZ binds to T and Mesp1 promoters in undifferentiated ESCs, and luciferase assays indicate that CIBZ suppresses T and Mesp1 promoters. These findings demonstrate that CIBZ is a novel regulator of mesodermal and cardiac differentiation of ESCs, and suggest that CIBZ-mediated cardiac differentiation depends on the regulation of these two genes.

Ciona as a Simple Chordate Model for Heart Development and Regeneration

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

Matrix metalloproteinase-2 in oncostatin M-induced sarcomere degeneration in cardiomyocytes

Cardiomyocyte dedifferentiation may be an important source of proliferating cardiomyocytes facilitating cardiac repair. Cardiomyocyte dedifferentiation and proliferation induced by oncostatin-M (OSM) is characterized by sarcomere degeneration. However, the mechanism underlying sarcomere degeneration remains unclear. We hypothesized that this process may involve matrix metalloproteinase-2 (MMP-2), a key protease localized at the sarcomere in cardiomyocytes. We tested the hypothesis that MMP-2 is involved in the sarcomere degeneration that characterizes cardiomyocyte dedifferentiation. Confocal immunofluorescence and biochemical methods were used to explore the role of MMP-2 in OSM-induced dedifferentiation of neonatal rat ventricular myocytes (NRVM). OSM caused a concentration- and time-dependent loss of sarcomeric α-actinin and troponin-I in NRVM. Upon OSM-treatment, the mature sarcomere transformed to a phenotype resembling a less-developed sarcomere, i.e., loss of sarcomeric proteins and Z-disk transformed into disconnected Z bodies, characteristic of immature myofibrils. OSM dose dependently increased MMP-2 activity. Both the pan-MMP inhibitor GM6001 and the selective MMP-2 inhibitor ARP 100 prevented sarcomere degeneration induced by OSM treatment. OSM also induced NRVM cell cycling and increased methyl-thiazolyl-tetrazolium (MTT) staining, preventable by MMP inhibition. These results suggest that MMP-2 mediates sarcomere degeneration in OSM-induced cardiomyocyte dedifferentiation and thus potentially contributes to cardiomyocyte regeneration.

PIWI-interacting RNA (piRNA) signatures in human cardiac progenitor cells

Cardiac progenitors, such as cardiospheres and cardiosphere-derived cells, represent an attractive cell source for cardiac regeneration. The PIWI-interacting RNAs, piRNAs, are an intriguing class of small non-coding RNAs, implicated in the regulation of epigenetic state, maintenance of genomic integrity and stem cell functions. Although non-coding RNAs are an exploiting field in cardiovascular research, the piRNA signatures of cardiac progenitors has not been evaluated yet.We profiled, through microarrays, 15,311 piRNAs expressed in cardiospheres, cardiosphere-derived cells and cardiac fibroblasts. Results showed a set of differentially expressed piRNAs (fold change ≥2, p<0.01): 641 piRNAs were upregulated and 1,301 downregulated in the cardiospheres compared to cardiosphere-derived cells, while 255 and 708 piRNAs resulted up- and down-regulated, respectively, if compared to cardiac fibroblasts. We also identified 181 piRNAs that are overexpressed and 129 are downregulated in cardiosphere-derived cells respect to cardiac fibroblasts.Bioinformatics analysis showed that the deregulated piRNAs were mainly distributed on few chromosomes, suggesting that piRNAs are organized in discrete genomic clusters.Furthermore, the bioinformatics search showed that the most upregulated piRNAs target transposons, especially belonged to LINE-1 class, as validated by qRT-PCR. This reduction is also associated to an activation of AKT signaling, which is beneficial for cardiac regeneration.The present study is the first to show a highly consistent piRNA expression pattern for human cardiac progenitors, likely responsible of their different regenerative power. Moreover, this piRNome analysis may provide new methods for characterize cardiac progenitors and may shed new light on the understanding the complex molecular mechanisms of cardiac regeneration.

De novo neurogenesis by targeted expression of atoh7 to Müller glia cells

Regenerative responses in the vertebrate CNS depend on quiescent radial glia stem cells, which re-enter the cell cycle and eventually differentiate into neurons. The entry into the cell cycle and the differentiation into neurons are events of opposite nature, and therefore efforts to force quiescent radial glia into neurons require different factors. Here, we use fish to show that a single neurogenic factor, Atoh7, directs retinal radial glia (Müller glia, MG) into proliferation. The resulting neurogenic clusters differentiate in vivo into various retinal neurons. We use signaling reporters to demonstrate that the Atoh7-induced regeneration-like response of MG cells is mimicked by Notch, resembling the behavior of early progenitors during retinogenesis. Activation of Notch signaling in MG cells is sufficient to trigger proliferation and differentiation. Our results uncover a new role for Atoh7 as a universal neurogenic factor, and illustrate how signaling modules are re-employed in diverse contexts to trigger different biological responses.

Highlighted article: Induced activation of atoh7 in Müller glia cells in vivo is sufficient to drive cell cycle re-entry and proliferation, followed by the formation of neurogenic clusters and de novo neurogenesis.

Combination of miRNA499 and miRNA133 exerts a synergic effect on cardiac differentiation

Several studies have demonstrated that miRNA are involved in cardiac development, stem cell maintenance, and differentiation. In particular, it has been shown that miRNA133, miRNA1, and miRNA499 are involved in progenitor cell differentiation into cardiomyocytes. However, it is unknown whether different miRNA may act synergistically to improve cardiac differentiation. We used mouse P19 cells as a cardiogenic differentiation model. miRNA499, miRNA1, or miRNA133 were transiently over-expressed in P19 cells individually or in different combinations. The over-expression of miRNA499 alone increased the number of beating cells and the association of miRNA499 with miRNA133 exerted a synergistic effect, further increasing the number of beating cells. Real-time polymerase chain reaction showed that the combination of miRNA499 + 133 enhanced the expression of cardiac genes compared with controls. Western blot and immunocytochemistry for connexin43 and cardiac troponin T confirmed these findings. Importantly, caffeine responsiveness, a clear functional parameter of cardiac differentiation, was increased by miRNA499 in association with miRNA133 and was directly correlated with the activation of the cardiac troponin I isoform promoter. Cyclic contractions were reversibly abolished by extracellular calcium depletion, nifedipine, ryanodine, and IP3R blockade. Finally, we demonstrated that the use of miRNA499 + 133 induced cardiac differentiation even in the absence of dimethyl sulfoxide. Our results show that the areas spontaneously contracting possess electrophysiological and pharmacological characteristics compatible with true cardiac excitation-contraction coupling. The translational relevance of our findings was reinforced by the demonstration that the over-expression of miRNA499 and miRNA133 was also able to induce the differentiation of human mesenchymal stromal cells toward the cardiac lineage. Stem Cells2015;33:1187–1199

Integration of mesenchymal stem cells with nanobiomaterials for the repair of myocardial infarction

The integration of nanobiomaterials with stem cells represents a promising strategy for the treatment of myocardial infarction. While stem cells and nanobiomaterials each demonstrated partial success in cardiac repair individually, the therapeutic efficacy of the clinical settings for each of these has been low. Hence, a combination of nanobiomaterials with stem cells is vigorously studied to create synergistic effects for treating myocardial infarction. To date, various types of nanomaterials have been incorporated with stem cells to control cell fate, modulate the therapeutic behavior of stem cells, and make them more suitable for cardiac repair. Here, we review the current stem cell therapies for cardiac repair and describe the combinatorial approaches of using nanobiomaterials and stem cells to improve therapeutic efficacy for the treatment of myocardial infarction.

The Isl1/Ldb1 Complex Orchestrates Genome-wide Chromatin Organization to Instruct Differentiation of Multipotent Cardiac Progenitors

Cardiac stem/progenitor cells hold great potential for regenerative therapies; however, the mechanisms regulating their expansion and differentiation remain insufficiently defined. Here we show that Ldb1 is a central regulator of genome organization in cardiac progenitor cells, which is crucial for cardiac lineage differentiation and heart development. We demonstrate that Ldb1 binds to the key regulator of cardiac progenitors, Isl1, and protects it from degradation. Furthermore, the Isl1/Ldb1 complex promotes long-range enhancer-promoter interactions at the loci of the core cardiac transcription factors Mef2c and Hand2. Chromosome conformation capture followed by sequencing identified specific Ldb1-mediated interactions of the Isl1/Ldb1 responsive Mef2c anterior heart field enhancer with genes that play key roles in cardiac progenitor cell function and cardiovascular development. Importantly, the expression of these genes was downregulated upon Ldb1 depletion and Isl1/Ldb1 haplodeficiency. In conclusion, the Isl1/Ldb1 complex orchestrates a network for heart-specific transcriptional regulation and coordination in three-dimensional space during cardiogenesis.

Glucagon is essential for alpha cell transdifferentiation and beta cell neogenesis

The interconversion of cell lineages via transdifferentiation is an adaptive mode of tissue regeneration and an appealing therapeutic target. However, its clinical exploitation is contingent upon the discovery of contextual regulators of cell fate acquisition and maintenance. In murine models of diabetes, glucagon-secreting alpha cells transdifferentiate into insulin-secreting beta cells following targeted beta cell depletion, regenerating the form and function of the pancreatic islet. However, the molecular triggers of this mode of regeneration are unknown. Here, using lineage-tracing assays in a transgenic zebrafish model of beta cell ablation, we demonstrate conserved plasticity of alpha cells during islet regeneration. In addition, we show that glucagon expression is upregulated after injury. Through gene knockdown and rescue approaches, we also find that peptides derived from the glucagon gene are necessary for alpha-to-beta cell fate switching. Importantly, whereas beta cell neogenesis was stimulated by glucose, alpha-to-beta cell conversion was not, suggesting that transdifferentiation is not mediated by glucagon/GLP-1 control of hepatic glucose production. Overall, this study supports the hypothesis that alpha cells are an endogenous reservoir of potential new beta cells. It further reveals that glucagon plays an important role in maintaining endocrine cell homeostasis through feedback mechanisms that govern cell fate stability.

Stemistry: the control of stem cells in situ using chemistry

A new paradigm for drug research has emerged, namely the deliberate search for molecules able to selectively affect the proliferation, differentiation, and migration of adult stem cells within the tissues in which they exist. Recently, there has been significant interest in medicinal chemistry toward the discovery and design of low molecular weight molecules that affect stem cells and thus have novel therapeutic activity. We believe that a successful agent from such a discover program would have profound effects on the treatment of many long-term degenerative disorders. Among these conditions are examples such as cardiovascular decay, neurological disorders including Alzheimer's disease, and macular degeneration, all of which have significant unmet medical needs. This perspective will review evidence from the literature that indicates that discovery of such agents is achievable and represents a worthwhile pursuit for the skills of the medicinal chemist.

In vivo activation of a conserved microRNA program induces mammalian heart regeneration

Heart failure is a leading cause of mortality and morbidity in the developed world, partly because mammals lack the ability to regenerate heart tissue. Whether this is due to evolutionary loss of regenerative mechanisms present in other organisms or to an inability to activate such mechanisms is currently unclear. Here we decipher mechanisms underlying heart regeneration in adult zebrafish and show that the molecular regulators of this response are conserved in mammals. We identified miR-99/100 and Let-7a/c and their protein targets smarca5 and fntb as critical regulators of cardiomyocyte dedifferentiation and heart regeneration in zebrafish. Although human and murine adult cardiomyocytes fail to elicit an endogenous regenerative response after myocardial infarction, we show that in vivo manipulation of this molecular machinery in mice results in cardiomyocyte dedifferentiation and improved heart functionality after injury. These data provide a proof of concept for identifying and activating conserved molecular programs to regenerate the damaged heart.

Recent development of temperature-responsive surfaces and their application for cell sheet engineering

Cell sheet engineering, which fabricates sheet-like tissues without biodegradable scaffolds, has been proposed as a novel approach for tissue engineering. Cells have been cultured and proliferate to confluence on a temperature-responsive cell culture surface at 37°C. By decreasing temperature to 20°C, an intact cell sheet can be harvested from the culture surface without enzymatic treatment. This new approach enables cells to keep their cell–cell junction, cell surface proteins and extracellular matrix. Therefore, recovered cell sheet can be easily not only transplanted to host tissue, but also constructed a three-dimensional (3D) tissue by layering cell sheets. Moreover, cell sheet manipulation technology and bioreactor have been combined with the cell sheet technology to fabricate a complex and functional 3D tissue in vitro. So far, cell sheet technology has been applied in regenerative medicine for several tissues, and a number of clinical studies have been performed. In this review, recent advances in the preparation of temperature-responsive cell culture surface, the fabrication of organ-like tissue and the clinical application of cell sheet engineering are summarized and discussed.

Adult stem cells and cardiac regeneration

There is worldwide demand for therapies to promote the robust repair and regeneration with maximum regain of function of particular tissues and organs damaged by disease or injury. The potential role of adult stem cells has been highlighted by an increasing number of in vitro and in vivo studies. Nowhere is this more evident than in adult stem cell-based therapies being explored to promote cardiac regeneration. In spite of encouraging advances, significant challenges remain.

Recent advances in heart regeneration

Although cardiac stem cells (CSCs) and tissue engineering are very promising for cardiac regenerative medicine, studies with model organisms for heart regeneration will provide alternative therapeutic targets and opportunities. Here, we present a review on heart regeneration, with a particular focus on the most recent work in mouse and zebrafish. We attempt to summarize the recent progresses and bottlenecks of CSCs and tissue engineering for heart regeneration; and emphasize what we have learned from mouse and zebrafish regenerative models on discovering crucial genetic and epigenetic factors for stimulating heart regeneration; and speculate the potential application of these regenerative factors for heart failure. A brief perspective highlights several important and promising research directions in this exciting field.

Do neonatal mouse hearts regenerate following heart apex resection?

The mammalian heart has generally been considered nonregenerative, but recent progress suggests that neonatal mouse hearts have a genuine capacity to regenerate following apex resection (AR). However, in this study, we performed AR or sham surgery on 400 neonatal mice from inbred and outbred strains and found no evidence of complete regeneration. Ideally, new functional cardiomyocytes, endothelial cells, and vascular smooth muscle cells should be formed in the necrotic area of the damaged heart. Here, damaged hearts were 9.8% shorter and weighed 14% less than sham controls. In addition, the resection border contained a massive fibrotic scar mainly composed of nonmyocytes and collagen disposition. Furthermore, there was a substantial reduction in the number of proliferating cardiomyocytes in AR hearts. Our results thus question the usefulness of the AR model for identifying molecular mechanisms underlying regeneration of the adult heart after damage.

Microvesicles as mediators of tissue regeneration

The use of stem cells in the treatment of various diseases and injuries has received increasing interest during the past decade. Injected stem cells, such as mesenchymal stem cells, stimulate tissue repair largely through the secretion of soluble factors that regulate various processes of tissue regeneration, including inflammatory responses, apoptosis, host cell proliferation, and angiogenesis. Recently, it has become apparent that stem cells also use membranous small vesicles, collectively called microvesicles, to repair damaged tissues. Microvesicles are released by many types of cells and exist in almost all types of body fluids. They serve as a vehicle to transfer protein, messenger RNA, and micro RNA to distant cells, altering the gene expression, proliferation, and differentiation of the recipient cells. Although animal models and in vitro studies have suggested promising applications for microvesicles-based regeneration therapy, its effectiveness and feasibility in clinical medicine remain to be established. Further studies of the basic mechanisms responsible for microvesicle-mediated tissue regeneration could lead to novel approaches in regenerative medicine.

Inhibition of TGFβ signaling increases direct conversion of fibroblasts to induced cardiomyocytes

Recent studies have been successful at utilizing ectopic expression of transcription factors to generate induced cardiomyocytes (iCMs) from fibroblasts, albeit at a low frequency in vitro. This work investigates the influence of small molecules that have been previously reported to improve differentiation to cardiomyocytes as well as reprogramming to iPSCs in conjunction with ectopic expression of the transcription factors Hand2, Nkx2.5, Gata4, Mef2C, and Tbx5 on the conversion to functional iCMs. We utilized a reporter system in which the calcium indicator GCaMP is driven by the cardiac Troponin T promoter to quantify iCM yield. The TGFβ inhibitor, SB431542 (SB), was identified as a small molecule capable of increasing the conversion of both mouse embryonic fibroblasts and adult cardiac fibroblasts to iCMs up to ∼5 fold. Further characterization revealed that inhibition of TGFβ by SB early in the reprogramming process led to the greatest increase in conversion of fibroblasts to iCMs in a dose-responsive manner. Global transcriptional analysis at Day 3 post-induction of the transcription factors revealed an increased expression of genes associated with the development of cardiac muscle in the presence of SB compared to the vehicle control. Incorporation of SB in the reprogramming process increases the efficiency of iCM generation, one of the major goals necessary to enable the use of iCMs for discovery-based applications and for the clinic.

Heart regeneration: opportunities and challenges for drug discovery with novel chemical and therapeutic methods or agents

Following a heart attack, more than a billion cardiac muscle cells (cardiomyocytes) can be killed, leading to heart failure and sudden death. Much research in this area is now focused on the regeneration of heart tissue through differentiation of stem cells, proliferation of existing cardiomyocytes and cardiac progenitor cells, and reprogramming of fibroblasts into cardiomyocytes. Different chemical modalities (i.e. methods or agents), ranging from small molecules and RNA approaches (including both microRNA and anti-microRNA) to modified peptides and proteins, are showing potential to meet this medical need. In this Review, we outline the recent advances in these areas and describe both the modality and progress, including novel screening strategies to identify hits, and the upcoming challenges and opportunities to develop these hits into pharmaceuticals, at which chemistry plays a key role.