Stoichiometric optimization of Gata4, Hand2, Mef2c, and Tbx5 expression for contractile cardiomyocyte reprogramming

Induction of Senescence by Loss of Gata4 in Cardiac Fibroblasts

Cardiac fibroblasts are a major source of cardiac fibrosis during heart repair processes in various heart diseases. Although it has been shown that cardiac fibroblasts become senescent in response to heart injury, it is unknown how the senescence of cardiac fibroblasts is regulated in vivo. Gata4, a cardiogenic transcription factor essential for heart development, is also expressed in cardiac fibroblasts. However, it remains elusive about the role of Gata4 in cardiac fibroblasts. To define the role of Gata4 in cardiac fibroblasts, we generated cardiac fibroblast-specific Gata4 knockout mice by cross-breeding Tcf21-MerCreMer mice with Gata4fl/fl mice. Using this mouse model, we could genetically ablate Gata4 in Tcf21 positive cardiac fibroblasts in an inducible manner upon tamoxifen administration. We found that cardiac fibroblast-specific deletion of Gata4 spontaneously induces senescence in cardiac fibroblasts in vivo and in vitro. We also found that Gata4 expression in both cardiomyocytes and non-myocytes significantly decreases in the aged heart. Interestingly, when αMHC-MerCreMer mice were bred with Gata4fl/fl mice to generate cardiomyocyte-specific Gata4 knockout mice, no senescent cells were detected in the hearts. Taken together, our results demonstrate that Gata4 deficiency in cardiac fibroblasts activates a program of cellular senescence, suggesting a novel molecular mechanism of cardiac fibroblast senescence.

Fibroblast fate determination during cardiac reprogramming by remodeling of actin filaments

Fibroblasts can be reprogrammed into induced cardiomyocyte-like cells (iCMs) by forced expression of cardiogenic transcription factors. However, it remains unknown how fibroblasts adopt a cardiomyocyte (CM) fate during their spontaneous ongoing transdifferentiation toward myofibroblasts (MFs). By tracing fibroblast lineages following cardiac reprogramming in vitro, we found that most mature iCMs are derived directly from fibroblasts without transition through the MF state. This direct conversion is attributable to mutually exclusive induction of cardiac sarcomeres and MF cytoskeletal structures in the cytoplasm of fibroblasts during reprogramming. For direct fate switch from fibroblasts to iCMs, significant remodeling of actin isoforms occurs in fibroblasts, including induction of α-cardiac actin and decrease of the actin isoforms predominant in MFs. Accordingly, genetic or pharmacological ablation of MF-enriched actin isoforms significantly enhances cardiac reprogramming. Our results demonstrate that remodeling of actin isoforms is required for fibroblast to CM fate conversion by cardiac reprogramming.

Hippo Pathway Effector Tead1 Induces Cardiac Fibroblast to Cardiomyocyte Reprogramming

Background

The conversion of fibroblasts into induced cardiomyocytes may regenerate myocardial tissue from cardiac scar through in situ cell transdifferentiation. The efficiency transdifferentiation is low, especially for human cells. We explored the leveraging of Hippo pathway intermediates to enhance induced cardiomyocyte generation.

Methods and Results

We screened Hippo effectors Yap (yes‐associated protein), Taz (transcriptional activator binding domain), and Tead1 (TEA domain transcription factor 1; Td) for their reprogramming efficacy with cardio‐differentiating factors Gata4, Mef2C, and Tbx5 (GMT). Td induced nearly 3‐fold increased expression of cardiomyocyte marker cTnT (cardiac troponin T) by mouse embryonic and adult rat fibroblasts versus GMT administration alone (P<0.0001), while Yap and Taz failed to enhance cTnT expression. Serial substitution demonstrated that Td replacement of TBX5 induced the greatest cTnT expression enhancement and sarcomere organization in rat fibroblasts treated with all GMT substitutions (GMTd versus GMT: 17±1.2% versus 5.4±0.3%, P<0.0001). Cell contractility (beating) was seen in 6% of GMTd‐treated cells by 4 weeks after treatment, whereas no beating GMT‐treated cells were observed. Human cardiac fibroblasts likewise demonstrated increased cTnT expression with GMTd versus GMT treatment (7.5±0.3% versus 3.0±0.3%, P<0.01). Mechanistically, GMTd administration increased expression of the trimethylated lysine 4 of histone 3 (H3K4me3) mark at the promoter regions of cardio‐differentiation genes and mitochondrial biogenesis regulator genes in rat and human fibroblast, compared with GMT.

Conclusions

These data suggest that the Hippo pathway intermediate Tead1 is an important regulator of cardiac reprogramming that increases the efficiency of maturate induced cardiomyocytes generation and may be a vital component of human cardiodifferentiation strategies.

Improving Cardiac Reprogramming for Heart Regeneration in Translational Medicine

Direct reprogramming of fibroblasts into CM-like cells has emerged as an attractive strategy to generate induced CMs (iCMs) in heart regeneration. However, low conversion rate, poor purity, and the lack of precise conversion of iCMs are still present as significant challenges. In this review, we summarize the recent development in understanding the molecular mechanisms of cardiac reprogramming with various strategies to achieve more efficient iCMs. reprogramming. Specifically, we focus on the identified critical roles of transcriptional regulation, epigenetic modification, signaling pathways from the cellular microenvironment, and cell cycling regulation in cardiac reprogramming. We also discuss the progress in delivery system optimization and cardiac reprogramming in human cells related to preclinical applications. We anticipate that this will translate cardiac reprogramming-based heart therapy into clinical applications. In addition to optimizing the cardiogenesis related transcriptional regulation and signaling pathways, an important strategy is to modulate the pathological microenvironment associated with heart injury, including inflammation, pro-fibrotic signaling pathways, and the mechanical properties of the damaged myocardium. We are optimistic that cardiac reprogramming will provide a powerful therapy in heart regenerative medicine.

The Function of SUMOylation and Its Critical Roles in Cardiovascular Diseases and Potential Clinical Implications

Cardiovascular disease (CVD) is a common disease caused by many factors, including atherosclerosis, congenital heart disease, heart failure, and ischemic cardiomyopathy. CVD has been regarded as one of the most common diseases and has a severe impact on the life quality of patients. The main features of CVD include high morbidity and mortality, which seriously threaten human health. SUMO proteins covalently conjugate lysine residues with a large number of substrate proteins, and SUMOylation regulates the function of target proteins and participates in cellular activities. Under certain pathological conditions, SUMOylation of proteins related to cardiovascular development and function are greatly changed. Numerous studies have suggested that SUMOylation of substrates plays critical roles in normal cardiovascular development and function. We reviewed the research progress of SUMOylation in cardiovascular development and function, and the regulation of protein SUMOylation may be applied as a potential therapeutic strategy for CVD treatment.

Cell Transdifferentiation and Reprogramming in Disease Modeling: Insights into the Neuronal and Cardiac Disease Models and Current Translational Strategies

Cell transdifferentiation and reprogramming approaches in recent times have enabled the manipulation of cell fate by enrolling exogenous/artificial controls. The chemical/small molecule and regulatory components of transcription machinery serve as potential tools to execute cell transdifferentiation and have thereby uncovered new avenues for disease modeling and drug discovery. At the advanced stage, one can believe these methods can pave the way to develop efficient and sensitive gene therapy and regenerative medicine approaches. As we are beginning to learn about the utility of cell transdifferentiation and reprogramming, speculations about its applications in translational therapeutics are being largely anticipated. Although clinicians and researchers are endeavoring to scale these processes, we lack a comprehensive understanding of their mechanism(s), and the promises these offer for targeted and personalized therapeutics are scarce. In the present report, we endeavored to provide a detailed review of the original concept, methods and modalities enrolled in the field of cellular transdifferentiation and reprogramming. A special focus is given to the neuronal and cardiac systems/diseases towards scaling their utility in disease modeling and drug discovery.

Chamber-Specific Protein Expression during Direct Cardiac Reprogramming

Forced expression of core cardiogenic transcription factors can directly reprogram fibroblasts to induced cardiomyocyte-like cells (iCMs) in vitro and in vivo. This cardiac reprogramming approach provides a proof of concept for induced heart regeneration by converting a fibroblast fate to a cardiomyocyte fate. However, it remains elusive whether chamber-specific cardiomyocytes can be generated by cardiac reprogramming. Therefore, we assessed the ability of the cardiac reprogramming approach for chamber specification in vitro and in vivo. We found that in vivo cardiac reprogramming post-myocardial infarction exclusively induces a ventricular-like phenotype, while a major fraction of iCMs generated in vitro failed to determine their chamber identities. Our results suggest that in vivo cardiac reprogramming may have an inherent advantage of generating chamber-matched new cardiomyocytes as a potential heart regenerative approach.

The histone reader PHF7 cooperates with the SWI/SNF complex at cardiac super enhancers to promote direct reprogramming

Direct cardiac reprogramming of fibroblasts to cardiomyocytes presents an attractive therapeutic strategy to restore cardiac function following injury. Cardiac reprogramming was initially achieved through overexpression of the transcription factors Gata4, Mef2c and Tbx5; later, Hand2 and Akt1 were found to further enhance this process^1-5. Yet, staunch epigenetic barriers severely limit the ability of these cocktails to reprogramme adult fibroblasts^6,7. We undertook a screen of mammalian gene regulatory factors to discover novel regulators of cardiac reprogramming in adult fibroblasts and identified the histone reader PHF7 as the most potent activating factor⁸. Mechanistically, PHF7 localizes to cardiac super enhancers in fibroblasts, and through cooperation with the SWI/SNF complex, it increases chromatin accessibility and transcription factor binding at these sites. Furthermore, PHF7 recruits cardiac transcription factors to activate a positive transcriptional autoregulatory circuit in reprogramming. Importantly, PHF7 achieves efficient reprogramming in the absence of Gata4. Here, we highlight the underexplored necessity of cardiac epigenetic readers, such as PHF7, in harnessing chromatin remodelling and transcriptional complexes to overcome critical barriers to direct cardiac reprogramming.

Cardiac Regeneration: New Insights Into the Frontier of Ischemic Heart Failure Therapy

Ischemic heart disease is the leading cause of morbidity and mortality in the world. While pharmacological and surgical interventions developed in the late twentieth century drastically improved patient outcomes, mortality rates over the last two decades have begun to plateau. Following ischemic injury, pathological remodeling leads to cardiomyocyte loss and fibrosis leading to impaired heart function. Cardiomyocyte turnover rate in the adult heart is limited, and no clinical therapies currently exist to regenerate cardiomyocytes lost following ischemic injury. In this review, we summarize the progress of therapeutic strategies including revascularization and cell-based interventions to regenerate the heart: transiently inducing cardiomyocyte proliferation and direct reprogramming of fibroblasts into cardiomyocytes. Moreover, we highlight recent mechanistic insights governing these strategies to promote heart regeneration and identify current challenges in translating these approaches to human patients.

Age-Related Pathways in Cardiac Regeneration: A Role for lncRNAs?

Aging imposes a barrier for tissue regeneration. In the heart, aging leads to a severe rearrangement of the cardiac structure and function and to a subsequent increased risk of heart failure. An intricate network of distinct pathways contributes to age-related alterations during healthy heart aging and account for a higher susceptibility of heart disease. Our understanding of the systemic aging process has already led to the design of anti-aging strategies or to the adoption of protective interventions. Nevertheless, our understanding of the molecular determinants operating during cardiac aging or repair remains limited. Here, we will summarize the molecular and physiological alterations that occur during aging of the heart, highlighting the potential role for long non-coding RNAs (lncRNAs) as novel and valuable targets in cardiac regeneration/repair.

Assessing Cardiac Reprogramming using High Content Imaging Analysis

The goal of this protocol is to describe a method for quantifying induced cardiomyocyte-like cells (iCMs), which are directly reprogrammed in vitro by a reprogramming technique. Cardiac reprogramming provides a strategy to generate new cardiomyocytes. By introducing core cardiogenic transcription factors into fibroblasts; fibroblasts can be converted to iCMs without transition through the pluripotent stem cell state. However, the conversion rate of fibroblasts to iCMs still remains low. Accordingly, there have been numerous additional approaches to enhance cardiac reprogramming efficiency. Most of these studies assessed cardiac reprogramming efficiency using flow cytometry, while at the same time performed immunocytochemistry to visualize iCMs. Thus, at least two separate sets of reprogramming experiments are required to demonstrate the success of iCM reprogramming. In contrast, automated high content imaging analysis will provide both quantification and qualification of iCM reprogramming with a relatively small number of cells. With this method, it is possible to directly assess the quantity and quality of iCMs with a single reprogramming experiment. This approach will be able to facilitate future cardiac reprogramming studies that require large-scale reprogramming experiments such as screening genetic or pharmacological factors for enhancing reprogramming efficiency. In addition, the application of high content imaging analysis protocol is not limited to cardiac reprogramming. It can be applied to reprogramming of other cell lineages as well as any immunostaining experiments which need both quantification and visualization of immunostained cells.

The Future of Direct Cardiac Reprogramming: Any GMT Cocktail Variety?

Direct cardiac reprogramming has emerged as a novel therapeutic approach to treat and regenerate injured hearts through the direct conversion of fibroblasts into cardiac cells. Most studies have focused on the reprogramming of fibroblasts into induced cardiomyocytes (iCMs). The first study in which this technology was described, showed that at least a combination of three transcription factors, GATA4, MEF2C and TBX5 (GMT cocktail), was required for the reprogramming into iCMs in vitro using mouse cells. However, this was later demonstrated to be insufficient for the reprogramming of human cells and additional factors were required. Thereafter, most studies have focused on implementing reprogramming efficiency and obtaining fully reprogrammed and functional iCMs, by the incorporation of other transcription factors, microRNAs or small molecules to the original GMT cocktail. In this respect, great advances have been made in recent years. However, there is still no consensus on which of these GMT-based varieties is best, and robust and highly reproducible protocols are still urgently required, especially in the case of human cells. On the other hand, apart from CMs, other cells such as endothelial and smooth muscle cells to form new blood vessels will be fundamental for the correct reconstruction of damaged cardiac tissue. With this aim, several studies have centered on the direct reprogramming of fibroblasts into induced cardiac progenitor cells (iCPCs) able to give rise to all myocardial cell lineages. Especially interesting are reports in which multipotent and highly expandable mouse iCPCs have been obtained, suggesting that clinically relevant amounts of these cells could be created. However, as of yet, this has not been achieved with human iCPCs, and exactly what stage of maturity is appropriate for a cell therapy product remains an open question. Nonetheless, the major concern in regenerative medicine is the poor retention, survival, and engraftment of transplanted cells in the cardiac tissue. To circumvent this issue, several cell pre-conditioning approaches are currently being explored. As an alternative to cell injection, in vivo reprogramming may face fewer barriers for its translation to the clinic. This approach has achieved better results in terms of efficiency and iCMs maturity in mouse models, indicating that the heart environment can favor this process. In this context, in recent years some studies have focused on the development of safer delivery systems such as Sendai virus, Adenovirus, chemical cocktails or nanoparticles. This article provides an in-depth review of the in vitro and in vivo cardiac reprograming technology used in mouse and human cells to obtain iCMs and iCPCs, and discusses what challenges still lie ahead and what hurdles are to be overcome before results from this field can be transferred to the clinical settings.

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.

Preconditioned and Genetically Modified Stem Cells for Myocardial Infarction Treatment

Ischemic heart disease and myocardial infarction remain leading causes of mortality worldwide. Existing myocardial infarction treatments are incapable of fully repairing and regenerating the infarcted myocardium. Stem cell transplantation therapy has demonstrated promising results in improving heart function following myocardial infarction. However, poor cell survival and low engraftment at the harsh and hostile environment at the site of infarction limit the regeneration potential of stem cells. Preconditioning with various physical and chemical factors, as well as genetic modification and cellular reprogramming, are strategies that could potentially optimize stem cell transplantation therapy for clinical application. In this review, we discuss the most up-to-date findings related to utilizing preconditioned stem cells for myocardial infarction treatment, focusing mainly on preconditioning with hypoxia, growth factors, drugs, and biological agents. Furthermore, genetic manipulations on stem cells, such as the overexpression of specific proteins, regulation of microRNAs, and cellular reprogramming to improve their efficiency in myocardial infarction treatment, are discussed as well.

Enhanced Generation of Induced Cardiomyocytes Using a Small-Molecule Cocktail to Overcome Barriers to Cardiac Cellular Reprogramming

Background

Given known inefficiencies in reprogramming of fibroblasts into mature induced cardiomyocytes (iCMs), we sought to identify small molecules that would overcome these barriers to cardiac cell transdifferentiation.

Methods and Results

We screened alternative combinations of compounds known to impact cell reprogramming using morphologic and functional cell differentiation assays in vitro. After screening 6 putative reprogramming factors, we found that a combination of the histone deacetylase inhibitor sodium butyrate, the WNT inhibitor ICG‐001, and the cardiac growth regulator retinoic acid (RA) maximally enhanced iCM generation from primary rat cardiac fibroblasts when combined with administration of the cardiodifferentiating transcription factors Gata4, Mef2C, and Tbx5 (GMT) compared with GMT administration alone (23±1.5% versus 3.3±0.2%; P<0.0001). Expression of the cardiac markers cardiac troponin T, Myh6, and Nkx2.5 was upregulated as early as 10 days after GMT–sodium butyrate, ICG‐001, and RA treatment. Human iCM generation was likewise enhanced when administration of the human cardiac reprogramming factors GMT, Hand2, and Myocardin plus miR‐590 was combined with sodium butyrate, ICG‐001, and RA compared with GMT, Hand2, and Myocardin plus miR‐590 treatment alone (25±1.3% versus 5.7±0.4%; P<0.0001). Rat and human iCMs also more frequently demonstrated spontaneous beating in coculture with neonatal cardiomyocytes with the addition of sodium butyrate, ICG‐001, and RA to transcription factor cocktails compared with transcription factor treatment alone.

Conclusions

The combined administration of histone deacetylase and WNT inhibitors with RA enhances rat and human iCM generation induced by transcription factor administration alone. These findings suggest opportunities for improved translational approaches for cardiac regeneration.