MicroRNA induced cardiac reprogramming in vivo: evidence for mature cardiac myocytes and improved cardiac function

In situ reprogramming of cardiac fibroblasts into cardiomyocytes in mouse heart with chemicals

Cardiomyocytes are terminal differentiated cells and have limited ability to proliferate or regenerate. Condition like myocardial infarction causes massive death of cardiomyocytes and is the leading cause of death. Previous studies have demonstrated that cardiac fibroblasts can be induced to transdifferentiate into cardiomyocytes in vitro and in vivo by forced expression of cardiac transcription factors and microRNAs. Our previous study have demonstrated that full chemical cocktails could also induce fibroblast to cardiomyocyte transdifferentiation both in vitro and in vivo. With the development of tissue clearing techniques, it is possible to visualize the reprogramming at the whole-organ level. In this study, we investigated the effect of the chemical cocktail CRFVPTM in inducing in situ fibroblast to cardiomyocyte transdifferentiation with two strains of genetic tracing mice, and the reprogramming was observed at whole-heart level with CUBIC tissue clearing technique and 3D imaging. In addition, single-cell RNA sequencing (scRNA-seq) confirmed the generation of cardiomyocytes from cardiac fibroblasts which carries the tracing marker. Our study confirms the use of small molecule cocktails in inducing in situ fibroblast to cardiomyocyte reprogramming at the whole-heart level and proof-of-conceptly providing a new source of naturally incorporated cardiomyocytes to help heart regeneration.

Modifying miRs for effective reprogramming of fibroblasts to cardiomyocytes

Reprogramming scar fibroblasts into cardiomyocytes has been proposed to reverse the damage associated with myocardial infarction. However, the limited improvement in cardiac function calls for enhanced strategies. We reported enhanced efficacy of our miR reprogramming cocktail miR combo (miR-1, miR-133a, miR-208a, and miR-499) via RNA-sensing receptor stimulation. We hypothesized that we could combine RNA-sensing receptor activation with fibroblast reprogramming by chemically modifying miR combo. To test the hypothesis, miR combo was modified to enhance interaction with the RNA-sensing receptor Rig1 via the addition of a 5'-triphosphate (5'ppp) group. Importantly, when compared with unmodified miR combo, 5'ppp-modified miR combo markedly improved reprogramming efficacy in vitro. Enhanced reprogramming efficacy correlated with a type-I interferon immune response with strong and selective secretion of interferon β (IFNβ). Antibody blocking studies and media replacement experiments indicated that 5'ppp-miR combo utilized IFNβ to enhance fibroblast reprogramming efficacy. In conclusion, miRs can acquire powerful additional roles through chemical modification that potentially increases their clinical applications.

C166 EVs potentiate miR cardiac reprogramming via miR-148a-3p

We have demonstrated that directly reprogramming cardiac fibroblasts into new cardiomyocytes via miR combo improves cardiac function in the infarcted heart. However, major challenges exist with delivery and efficacy. During a screening based approach to improve delivery, we discovered that C166-derived EVs were effective delivery agents for miR combo both in vitro and in vivo. In the latter, EV mediated delivery of miR combo induced significant conversion of cardiac fibroblasts into cardiomyocytes (∼20%), reduced fibrosis and improved cardiac function in a myocardial infarction injury model. When compared to lipid-based transfection, C166 EV mediated delivery of miR combo enhanced reprogramming efficacy. Improved reprogramming efficacy was found to result from a miRNA within the exosome: miR-148a-3p. The target of miR-148a-3p was identified as Mdfic. Over-expression and targeted knockdown studies demonstrated that Mdfic was a repressor of cardiomyocyte specific gene expression. In conclusion, we have demonstrated that C166-derived EVs are an effective method for delivering reprogramming factors to cardiac fibroblasts and we have identified a novel miRNA contained within C166-derived EVs which enhances reprogramming efficacy.

RNA Therapeutics for the Cardiovascular System

RNA therapeutics hold significant promise in the treatment of cardiovascular diseases. RNAs are biologically diverse and functionally specific and can be used for gain- or loss-of-function purposes. The effectiveness of mRNA-based vaccines in the recent COVID-19 pandemic has undoubtedly proven the benefits of an RNA-based approach. RNA-based therapies are becoming more common as a treatment modality for cardiovascular disease. This is most evident in hypertension where several small interfering RNA-based drugs have proven to be effective in managing high blood pressure in several clinical trials. As befits a rapidly burgeoning field, there is significant interest in other classes of RNA. Revascularization of the infarcted heart through an mRNA drug is under clinical investigation. mRNA technology may provide the platform for the expression of paracrine factors for myocardial protection and regeneration. Emergent technologies on the basis of microRNAs and gene editing are tackling complex diseases in a novel fashion. RNA-based gene editing offers hope of permanent cures for monogenic cardiovascular diseases, and long-term control of complex diseases such as essential hypertension, as well. Likewise, microRNAs are proving effective in regenerating cardiac muscle. The aim of this review is to provide an overview of the current landscape of RNA-based therapies for the treatment of cardiovascular disease. The review describes the large number of RNA molecules that exist with a discussion of the clinical development of each RNA type. In addition, the review also presents a number of avenues for future development.

Partial Reprogramming as a Method for Regenerating Neural Tissues in Aged Organisms

Aging causes numerous age-related diseases, leading the human species to death. Nevertheless, rejuvenating strategies based on cell epigenetic modifications are a possible approach to counteract disease progression while getting old. Cell reprogramming of adult somatic cells toward pluripotency ought to be a promising tool for age-related diseases. However, researchers do not have control over this process as cells lose their fate, and cause potential cancerous cells or unexpected cell phenotypes. Direct and partial reprogramming were introduced in recent years with distinctive applications. Although direct reprogramming makes cells lose their identity, it has various applications in regeneration medicine. Temporary and regulated in vivo overexpression of Yamanaka factors has been shown in several experimental contexts to be achievable and is used to rejuvenate mice models. This regeneration can be accomplished by altering the epigenetic adult cell signature to the signature of a younger cell. The greatest advantage of partial reprogramming is that this method does not allow cells to lose their identity when they are resetting their epigenetic clock. It is a regimen of short-term Oct3/4, Sox2, Klf4, and c-Myc expression in vivo that prevents full reprogramming to the pluripotent state and avoids both tumorigenesis and the presence of unwanted undifferentiated cells. We know that many neurological age-related diseases, such as Alzheimer's disease, stroke, dementia, and Parkinson's disease, are the main cause of death in the last decades of life. Therefore, scientists have a special tendency regarding neuroregeneration methods to increase human life expectancy.

Direct cellular reprogramming techniques for cardiovascular regenerative therapeutics

Cardiovascular diseases remain a leading cause of hospitalization affecting approximately 38 million people worldwide. While pharmacological and revascularization techniques can improve the patient's survival and quality of life, they cannot help reversing myocardial infarction injury and heart failure. Direct reprogramming of somatic cells to cardiomyocyte and cardiac progenitor cells offers a new approach to cellular reprogramming and paves the way for translational regenerative medicine. Direct reprogramming can bypass the pluripotent stage with the potential advantage of non-immunogenic cell products, reduced carcinogenic risk, and no requirement for embryonic tissue. The process of directly reprogramming cardiac cells was first achieved through the overexpression of transcription factors such as GATA4, MEF2C, and TBX5. However, over the past decade, significant work has been focused on enhancing direct reprogramming using a mixture of transcription factors, microRNAs, and small molecules to achieve cardiac cell fate. This review discusses the evolution of direct reprogramming, recent progress in achieving efficient cardiac cell fate conversion, and describes the reprogramming mechanisms at a molecular level. We also explore various viral and non-viral delivery methods currently being used to aid in the delivery of reprogramming factors to improve efficiency. However, further studies will be needed to overcome molecular and epigenetic barriers to successfully achieve translational cardiac regenerative therapeutics.

Fibroblast Reprogramming in Cardiac Repair

Cardiovascular disease is one of the major causes of death worldwide. Limited proliferative capacity of adult mammalian cardiomyocytes has prompted researchers to exploit regenerative therapy after myocardial injury, such as myocardial infarction, to attenuate heart dysfunction caused by such injury. Direct cardiac reprogramming is a recently emerged promising approach to repair damaged myocardium by directly converting resident cardiac fibroblasts into cardiomyocyte-like cells. The achievement of in vivo direct reprogramming of fibroblasts has been shown, by multiple laboratories independently, to improve cardiac function and mitigate fibrosis post-myocardial infarction, which holds great potential for clinical application. There have been numerous pieces of valuable work in both basic and translational research to enhance our understanding and continued refinement of direct cardiac reprogramming in recent years. However, there remain many challenges to overcome before we can truly take advantage of this technique to treat patients with ischemic cardiac diseases. Here, we review recent progress of fibroblast reprogramming in cardiac repair, including the optimization of several reprogramming strategies, mechanistic exploration, and translational efforts, and we make recommendations for future research to further understand and translate direct cardiac reprogramming from bench to bedside. Challenges relating to these efforts will also be discussed.

Improved Cardiac Function in Postischemic Rats Using an Optimized Cardiac Reprogramming Cocktail Delivered in a Single Novel Adeno-Associated Virus

BACKGROUND

Cardiac reprogramming is a technique to directly convert nonmyocytes into myocardial cells using genes or small molecules. This intervention provides functional benefit to the rodent heart when delivered at the time of myocardial infarction or activated transgenically up to 4 weeks after myocardial infarction. Yet, several hurdles have prevented the advancement of cardiac reprogramming for clinical use.

METHODS

Through a combination of screening and rational design, we identified a cardiac reprogramming cocktail that can be encoded in a single adeno-associated virus. We also created a novel adeno-associated virus capsid that can transduce cardiac fibroblasts more efficiently than available parental serotypes by mutating posttranslationally modified capsid residues. Because a constitutive promoter was needed to drive high expression of these cell fate-altering reprogramming factors, we included binding sites to a cardiomyocyte-restricted microRNA within the 3' untranslated region of the expression cassette that limits expression to nonmyocytes. After optimizing this expression cassette to reprogram human cardiac fibroblasts into induced cardiomyocyte-like cells in vitro, we also tested the ability of this capsid/cassette combination to confer functional benefit in acute mouse myocardial infarction and chronic rat myocardial infarction models.

RESULTS

We demonstrated sustained, dose-dependent improvement in cardiac function when treating a rat model 2 weeks after myocardial infarction, showing that cardiac reprogramming, when delivered in a single, clinically relevant adeno-associated virus vector, can support functional improvement in the postremodeled heart. This benefit was not observed with GFP (green fluorescent protein) or a hepatocyte reprogramming cocktail and was achieved even in the presence of immunosuppression, supporting myocyte formation as the underlying mechanism.

CONCLUSIONS

Collectively, these results advance the application of cardiac reprogramming gene therapy as a viable therapeutic approach to treat chronic heart failure resulting from ischemic injury.

Amending the injured heart by in vivo reprogramming

Ischemic heart injury causes death of cardiomyocyte (CM), formation of a fibrotic scar, and often adverse cardiac remodeling, resulting in chronic heart failure. Therapeutic interventions have lowered myocardial damage and improved heart function, but pharmacological treatment of heart failure has only shown limited progress in recent years. Over the past two decades, different approaches have been pursued to regenerate the heart, by transplantation of newly generated CMs derived from pluripotent stem cells, generation of new CMs by reprogramming of cardiac fibroblasts, or by activating proliferation of preexisting CMs. Here, we summarize recent progress in the development of strategies for in situ generation of new CMs, review recent advances in understanding the underlying mechanisms, and discuss the challenges and future directions of the field.

Cellular reprogramming of fibroblasts in heart regeneration

Myocardial infarction causes the loss of cardiomyocytes and the formation of cardiac fibrosis due to the activation of cardiac fibroblasts, leading to cardiac dysfunction and heart failure. Unfortunately, current therapeutic interventions can only slow the disease progression. Furthermore, they cannot fully restore cardiac function, likely because the adult human heart lacks sufficient capacity to regenerate cardiomyocytes. Therefore, intensive efforts have focused on developing therapeutics to regenerate the damaged heart. Several strategies have been intensively investigated, including stimulation of cardiomyocyte proliferation, transplantation of stem cell-derived cardiomyocytes, and conversion of fibroblasts into cardiac cells. Resident cardiac fibroblasts are critical in the maintenance of the structure and contractility of the heart. Fibroblast plasticity makes this type of cells be reprogrammed into many cell types, including but not limited to induced pluripotent stem cells, induced cardiac progenitor cells, and induced cardiomyocytes. Fibroblasts have become a therapeutic target due to their critical roles in cardiac pathogenesis. This review summarizes the reprogramming of fibroblasts into induced pluripotent stem cell-derived cardiomyocytes, induced cardiac progenitor cells, and induced cardiomyocytes to repair a damaged heart, outlines recent findings in utilizing fibroblast-derived cells for heart regeneration, and discusses the limitations and challenges.

Direct cardiac reprogramming: A new technology for cardiac repair

Cardiovascular disease is one of the leading causes of morbidity and mortality worldwide, with myocardial infarctions being amongst the deadliest manifestations. Reduced blood flow to the heart can result in the death of cardiac tissue, leaving affected patients susceptible to further complications and recurrent disease. Further, contemporary management typically involves a pharmacopeia to manage the metabolic conditions contributing to atherosclerotic and hypertensive heart disease, rather than regeneration of the damaged myocardium. With modern healthcare extending lifespan, a larger demographic will be at risk for heart disease, driving the need for novel therapeutics that surpass those currently available in efficacy. Transdifferentiation and cellular reprogramming have been looked to as potential methods for the treatment of diseases throughout the body. Specifically targeting the fibrotic cells in cardiac scar tissue as a source to be reprogrammed into induced cardiomyocytes remains an appealing option. This review aims to highlight the history of and advances in cardiac reprogramming and describe its translational potential as a treatment for cardiovascular disease.

Regeneration of the heart: from molecular mechanisms to clinical therapeutics

Heart injury such as myocardial infarction leads to cardiomyocyte loss, fibrotic tissue deposition, and scar formation. These changes reduce cardiac contractility, resulting in heart failure, which causes a huge public health burden. Military personnel, compared with civilians, is exposed to more stress, a risk factor for heart diseases, making cardiovascular health management and treatment innovation an important topic for military medicine. So far, medical intervention can slow down cardiovascular disease progression, but not yet induce heart regeneration. In the past decades, studies have focused on mechanisms underlying the regenerative capability of the heart and applicable approaches to reverse heart injury. Insights have emerged from studies in animal models and early clinical trials. Clinical interventions show the potential to reduce scar formation and enhance cardiomyocyte proliferation that counteracts the pathogenesis of heart disease. In this review, we discuss the signaling events controlling the regeneration of heart tissue and summarize current therapeutic approaches to promote heart regeneration after injury.

Direct Reprogramming of Resident Non-Myocyte Cells and Its Potential for In Vivo Cardiac Regeneration

Cardiac diseases are the foremost cause of morbidity and mortality worldwide. The heart has limited regenerative potential; therefore, lost cardiac tissue cannot be replenished after cardiac injury. Conventional therapies are unable to restore functional cardiac tissue. In recent decades, much attention has been paid to regenerative medicine to overcome this issue. Direct reprogramming is a promising therapeutic approach in regenerative cardiac medicine that has the potential to provide in situ cardiac regeneration. It consists of direct cell fate conversion of one cell type into another, avoiding transition through an intermediary pluripotent state. In injured cardiac tissue, this strategy directs transdifferentiation of resident non-myocyte cells (NMCs) into mature functional cardiac cells that help to restore the native tissue. Over the years, developments in reprogramming methods have suggested that regulation of several intrinsic factors in NMCs can help to achieve in situ direct cardiac reprogramming. Among NMCs, endogenous cardiac fibroblasts have been studied for their potential to be directly reprogrammed into both induced cardiomyocytes and induced cardiac progenitor cells, while pericytes can transdifferentiate towards endothelial cells and smooth muscle cells. This strategy has been indicated to improve heart function and reduce fibrosis after cardiac injury in preclinical models. This review summarizes the recent updates and progress in direct cardiac reprogramming of resident NMCs for in situ cardiac regeneration.

Neonatal and adult cardiac fibroblasts exhibit inherent differences in cardiac regenerative capacity

Directly reprogramming fibroblasts into cardiomyocytes improves cardiac function in the infarcted heart. However, the low efficacy of this approach hinders clinical applications. Unlike the adult mammalian heart, the neonatal heart has an intrinsic regenerative capacity. Consequently, we hypothesized that birth imposes fundamental changes on cardiac fibroblasts which limit their regenerative capabilities. In support, we found that reprogramming efficacy in vitro was markedly lower with fibroblasts derived from adult mice versus those derived from neonatal mice. Notably, fibroblasts derived from adult mice expressed significantly higher levels of pro-angiogenic genes. Moreover, under conditions which promote angiogenesis, only fibroblasts derived from adult mice differentiated into tube-like structures. Targeted knockdown screening studies suggested a possible role for the transcription factor Epas1. Epas1 expression was higher in fibroblasts derived from adult mice and Epas1 knockdown improved reprogramming efficacy in cultured adult cardiac fibroblasts. Promoter activity assays indicated that Epas1 functions as both a transcription repressor and activator, inhibiting cardiomyocyte genes while activating angiogenic genes. Finally, the addition of an Epas1 targeting siRNA to the reprogramming cocktail markedly improved reprogramming efficacy in vivo with both the number of reprogramming events as well as cardiac function being markedly improved. Collectively, our results highlight differences between neonatal and adult cardiac fibroblasts and the dual transcriptional activities of Epas1 related to reprogramming efficacy.

Rig1 receptor plays a critical role in cardiac reprogramming via YY1 signaling

We discovered that innate immunity plays an important role in the reprogramming of fibroblasts into cardiomyocytes. In this report, we define the role of a novel retinoic acid-inducible gene 1 Yin Yang 1 (Rig1:YY1) pathway. We found that fibroblast to cardiomyocyte reprogramming efficacy was enhanced by specific Rig1 activators. To understand the mechanism of action, we performed various transcriptomic, nucleosome occupancy, and epigenomic approaches. Analysis of the datasets indicated that Rig1 agonists had no effect on reprogramming-induced changes in nucleosome occupancy or loss of inhibitory epigenetic motifs. Instead, Rig1 agonists were found to modulate cardiac reprogramming by promoting the binding of YY1 specifically to cardiac genes. To conclude, these results show that the Rig1:YY1 pathway plays a critical role in fibroblast to cardiomyocyte reprogramming.

Cardiac Regenerative Therapy Using Human Pluripotent Stem Cells for Heart Failure: A State-of-the-Art Review

Heart transplantation (HT) is the only definitive treatment available for patients with end-stage heart failure who are refractory to medical and device therapies. However, HT as a therapeutic option, is limited by a significant shortage of donors. To overcome this shortage, regenerative medicine using human pluripotent stem cells (hPSCs), such as human embryonic stem cells and human-induced pluripotent stem cells (hiPSCs), has been considered an alternative to HT. Several issues, including the methods of large-scale culture and production of hPSCs and cardiomyocytes, the prevention of tumorigenesis secondary to contamination of undifferentiated stem cells and non-cardiomyocytes, and the establishment of an effective transplantation strategy in large-animal models, need to be addressed to fulfill this unmet need. Although post-transplantation arrhythmia and immune rejection remain problems, the ongoing rapid technological advances in hPSC research have been directed toward the clinical application of this technology. Cell therapy using hPSC-derived cardiomyocytes is expected to serve as an integral component of realistic medicine in the near future and is being potentially viewed as a treatment that would revolutionize the management of patients with severe heart failure.

Development of direct cardiac reprogramming for clinical applications

The incidence of cardiovascular diseases is increasing worldwide, and cardiac regenerative therapy has great potential as a new treatment strategy, especially for ischemic heart disease. Direct cardiac reprogramming is a promising new cardiac regenerative therapy that uses defined factors to induce transdifferentiation of endogenous cardiac fibroblasts (CFs) into induced cardiomyocyte-like cells (iCMs). In vivo reprogramming is expected to restore lost cardiac function without necessitating cardiac transplantation by converting endogenous CFs that exist abundantly in cardiac tissues directly into iCMs. Indeed, we and other groups have demonstrated that in vivo cardiac reprogramming improves cardiac contractile function and reduces scar area after acute myocardial infarction (MI). Recently, we demonstrated that in vivo cardiac reprogramming is an innovative cardiac regenerative therapy that not only regenerates the myocardium, but also reverses fibrosis by inducing the quiescence of pro-fibrotic fibroblasts, thereby improving heart failure in chronic MI. In this review, we summarize the recent progresses in in vivo cardiac reprogramming, and discuss its prospects for future clinical applications and the challenges of direct human reprogramming, which has been a longstanding issue.

Direct cardiac reprogramming: basics and future challenges

BACKGROUND

Heart failure is the leading cause of morbidity and mortality worldwide and is characterized by reduced cardiac function. Currently, cardiac transplantation therapy is applied for end-stage heart failure, but it is limited by the number of available donors.

METHODS AND RESULTS

Following an assessment of available literature, a narrative review was conducted to summarizes the current status and challenges of cardiac reprogramming for clinical application. Scientists have developed different regenerative treatment strategies for curing heart failure, including progenitor cell delivery and pluripotent cell delivery. Recently, a novel strategy has emerged that directly reprograms cardiac fibroblast into a functional cardiomyocyte. In this treatment, transcription factors are first identified to reprogram fibroblast into a cardiomyocyte. After that, microRNA and small molecules show great potential to optimize the reprogramming process. Some challenges regarding cell reprogramming in humans are conversion efficiency, virus utilization, immature and heterogenous induced cardiomyocytes, technical reproducibility issues, and physiological effects of depleted fibroblasts on myocardial tissue.

CONCLUSION

Several strategies have shown positive results in direct cardiac reprogramming. However, direct cardiac reprogramming still needs improvement if it is used as a mainstay therapy in humans, and challenges need to be overcome before cardiac reprogramming can be considered a viable therapeutic strategy. Further advances in cardiac reprogramming studies are needed in cardiac regenerative therapy.

Three-dimensional heart extracellular matrix enhances chemically induced direct cardiac reprogramming

Direct cardiac reprogramming has emerged as a promising therapeutic approach for cardiac regeneration. Full chemical reprogramming with small molecules to generate cardiomyocytes may be more amenable than genetic reprogramming for clinical applications as it avoids safety concerns associated with genetic manipulations. However, challenges remain regarding low conversion efficiency and incomplete cardiomyocyte maturation. Furthermore, the therapeutic potential of chemically induced cardiomyocytes (CiCMs) has not been investigated. Here, we report that a three-dimensional microenvironment reconstituted with decellularized heart extracellular matrix can enhance chemical reprogramming and cardiac maturation of fibroblasts to cardiomyocytes. The resultant CiCMs exhibit elevated cardiac marker expression, sarcomeric organization, and improved electrophysiological features and drug responses. We investigated the therapeutic potential of CiCMs reprogrammed in three-dimensional heart extracellular matrix in a rat model of myocardial infarction. Our platform can facilitate the use of CiCMs for regenerative medicine, disease modeling, and drug screening.

Advances in Cellular Reprogramming-Based Approaches for Heart Regenerative Repair

Continuous loss of cardiomyocytes (CMs) is one of the fundamental characteristics of many heart diseases, which eventually can lead to heart failure. Due to the limited proliferation ability of human adult CMs, treatment efficacy has been limited in terms of fully repairing damaged hearts. It has been shown that cell lineage conversion can be achieved by using cell reprogramming approaches, including human induced pluripotent stem cells (hiPSCs), providing a promising therapeutic for regenerative heart medicine. Recent studies using advanced cellular reprogramming-based techniques have also contributed some new strategies for regenerative heart repair. In this review, hiPSC-derived cell therapeutic methods are introduced, and the clinical setting challenges (maturation, engraftment, immune response, scalability, and tumorigenicity), with potential solutions, are discussed. Inspired by the iPSC reprogramming, the approaches of direct cell lineage conversion are merging, such as induced cardiomyocyte-like cells (iCMs) and induced cardiac progenitor cells (iCPCs) derived from fibroblasts, without induction of pluripotency. The studies of cellular and molecular pathways also reveal that epigenetic resetting is the essential mechanism of reprogramming and lineage conversion. Therefore, CRISPR techniques that can be repurposed for genomic or epigenetic editing become attractive approaches for cellular reprogramming. In addition, viral and non-viral delivery strategies that are utilized to achieve CM reprogramming will be introduced, and the therapeutic effects of iCMs or iCPCs on myocardial infarction will be compared. After the improvement of reprogramming efficiency by developing new techniques, reprogrammed iCPCs or iCMs will provide an alternative to hiPSC-based approaches for regenerative heart therapies, heart disease modeling, and new drug screening.

Direct Cardiac Epigenetic Reprogramming through Codelivery of 5'Azacytidine and miR-133a Nanoformulation

Direct reprogramming of cardiac fibroblasts to induced cardiomyocytes (iCMs) is a promising approach to cardiac regeneration. However, the low yield of reprogrammed cells and the underlying epigenetic barriers limit its potential. Epigenetic control of gene regulation is a primary factor in maintaining cellular identities. For instance, DNA methylation controls cell differentiation in adults, establishing that epigenetic factors are crucial for sustaining altered gene expression patterns with subsequent rounds of cell division. This study attempts to demonstrate that 5'AZA and miR-133a encapsulated in PLGA-PEI nanocarriers induce direct epigenetic reprogramming of cardiac fibroblasts to cardiomyocyte-like cells. The results present a cardiomyocyte-like phenotype following seven days of the co-delivery of 5'AZA and miR-133a nanoformulation into human cardiac fibroblasts. Further evaluation of the global DNA methylation showed a decreased global 5-methylcytosine (5-medCyd) levels in the 5'AZA and 5'AZA/miR-133a treatment group compared to the untreated group and cells with void nanocarriers. These results suggest that the co-delivery of 5'AZA and miR-133a nanoformulation can induce the direct reprogramming of cardiac fibroblasts to cardiomyocyte-like cells in-vitro, in addition to demonstrating the influence of miR-133a and 5'AZA as epigenetic regulators in dictating cell fate.

The heart of cardiac reprogramming: The cardiac fibroblasts

Cardiovascular disease is the leading cause of death worldwide, outpacing pulmonary disease, infectious disease, and all forms of cancer. Myocardial infarction (MI) dominates cardiovascular disease, contributing to four out of five cardiovascular related deaths. Following MI, patients suffer adverse and irreversible myocardial remodeling associated with cardiomyocyte loss and infiltration of fibrotic scar tissue. Current therapies following MI only mitigate the cardiac physiological decline rather than restore damaged myocardium function. Direct cardiac reprogramming is one strategy that has promise in repairing injured cardiac tissue by generating new, functional cardiomyocytes from cardiac fibroblasts (CFs). With the ectopic expression of transcription factors, microRNAs, and small molecules, CFs can be reprogrammed into cardiomyocyte-like cells (iCMs) that display molecular signatures, structures, and contraction abilities similar to endogenous cardiomyocytes. The in vivo induction of iCMs following MI leads to significant reduction in fibrotic cardiac remodeling and improved heart function, indicating reprogramming is a viable option for repairing damaged heart tissue. Recent work has illustrated different methods to understand the mechanisms driving reprogramming, in an effort to improve the efficiency of iCM generation and create an approach translational into clinic. This review will provide an overview of CFs and describe different in vivo reprogramming methods.

Myocardial infarction from a tissue engineering and regenerative medicine point of view: A comprehensive review on models and treatments

In the modern world, myocardial infarction is one of the most common cardiovascular diseases, which are responsible for around 18 million deaths every year or almost 32% of all deaths. Due to the detrimental effects of COVID-19 on the cardiovascular system, this rate is expected to increase in the coming years. Although there has been some progress in myocardial infarction treatment, translating pre-clinical findings to the clinic remains a major challenge. One reason for this is the lack of reliable and human representative healthy and fibrotic cardiac tissue models that can be used to understand the fundamentals of ischemic/reperfusion injury caused by myocardial infarction and to test new drugs and therapeutic strategies. In this review, we first present an overview of the anatomy of the heart and the pathophysiology of myocardial infarction, and then discuss the recent developments on pre-clinical infarct models, focusing mainly on the engineered three-dimensional cardiac ischemic/reperfusion injury and fibrosis models developed using different engineering methods such as organoids, microfluidic devices, and bioprinted constructs. We also present the benefits and limitations of emerging and promising regenerative therapy treatments for myocardial infarction such as cell therapies, extracellular vesicles, and cardiac patches. This review aims to overview recent advances in three-dimensional engineered infarct models and current regenerative therapeutic options, which can be used as a guide for developing new models and treatment strategies.

Lipoplexes for effective in vitro delivery of microRNAs to adult human cardiac fibroblasts for perspective direct cardiac cell reprogramming

Design of nanocarriers for efficient miRNA delivery can significantly improve miRNA-based therapies. Lipoplexes based on helper lipid, dioleoyl phosphatidylethanolamine (DOPE) and cationic lipid [2-(2,3-didodecyloxypropyl)-hydroxyethyl] ammonium bromide (DE) were formulated to efficiently deliver miR-1 or a combination of four microRNAs (miRcombo) to adult human cardiac fibroblasts (AHCFs). Lipoplexes with amino-to-phosphate groups ratio of 3 (N/P 3) showed nanometric hydrodynamic size (372 nm), positive Z-potential (40 mV) and high stability under storage conditions. Compared to commercial DharmaFECT1 (DF), DE-DOPE/miRNA lipoplexes showed superior miRNA loading efficiency (99 % vs. 64 %), and faster miRNA release (99 % vs. 82 % at 48 h). DE-DOPE/miR-1 lipoplexes showed superior viability (80-100 % vs. 50 %) in AHCFs, a 2-fold higher miR-1 expression and Twinfilin-1 (TWF-1) mRNA downregulation. DE-DOPE/miRcombo lipoplexes significantly enhanced AHCFs reprogramming into induced cardiomyocytes (iCMs), as shown by increased expression of CM markers compared to DF/miRcombo.

Hybrid extracellular vesicles-liposome incorporated advanced bioink to deliver microRNA

In additive manufacturing, bioink formulations govern strategies to engineer 3D living tissues that mimic the complex architectures and functions of native tissues for successful tissue regeneration. Conventional 3D-printed tissues are limited in their ability to alter the fate of laden cells. Specifically, the efficient delivery of gene expression regulators (i.e. microRNAs (miRNAs)) to cells in bioprinted tissues has remained largely elusive. In this study, we explored the inclusion of extracellular vesicles (EVs), naturally occurring nanovesicles (NVs), into bioinks to resolve this challenge. EVs show excellent biocompatibility, rapid endocytosis, and low immunogenicity, which lead to the efficient delivery of miRNAs without measurable cytotoxicity. EVs were fused with liposomes to prolong and control their release by altering their physical interaction with the bioink. Hybrid EVs-liposome (hEL) NVs were embedded in gelatin-based hydrogels to create bioinks that could efficiently encapsulate and deliver miRNAs at the target site in a controlled and sustained manner. The regulation of cells' gene expression in a 3D bioprinted matrix was achieved using the hELs-laden bioink as a precursor for excellent shape fidelity and high cell viability constructs. Novel regulatory factors-loaded bioinks will expedite the translation of new bioprinting applications in the tissue engineering field.

Manipulating Cardiomyocyte Plasticity for Heart Regeneration

Pathological heart injuries such as myocardial infarction induce adverse ventricular remodeling and progression to heart failure owing to widespread cardiomyocyte death. The adult mammalian heart is terminally differentiated unlike those of lower vertebrates. Therefore, the proliferative capacity of adult cardiomyocytes is limited and insufficient to restore an injured heart. Although current therapeutic approaches can delay progressive remodeling and heart failure, difficulties with the direct replenishment of lost cardiomyocytes results in a poor long-term prognosis for patients with heart failure. However, it has been revealed that cardiac function can be improved by regulating the cell cycle or changing the cell state of cardiomyocytes by delivering specific genes or small molecules. Therefore, manipulation of cardiomyocyte plasticity can be an effective treatment for heart disease. This review summarizes the recent studies that control heart regeneration by manipulating cardiomyocyte plasticity with various approaches including differentiating pluripotent stem cells into cardiomyocytes, reprogramming cardiac fibroblasts into cardiomyocytes, and reactivating the proliferation of cardiomyocytes.

A novel Cbx1, PurB, and Sp3 complex mediates long-term silencing of tissue- and lineage-specific genes

miRNA-based cellular fate reprogramming offers an opportunity to investigate the mechanisms of long-term gene silencing. To further understand how genes are silenced in a tissue-specific manner, we leveraged our miRNA-based method of reprogramming fibroblasts into cardiomyocytes. Through screening approaches, we identified three proteins that were downregulated during reprogramming of fibroblasts into cardiomyocytes: heterochromatin protein Cbx1, transcriptional activator protein PurB, and transcription factor Sp3. We show that knockdown of Cbx1, PurB, and Sp3 was sufficient to induce cardiomyocyte gene expression in fibroblasts. Similarly, gene editing to ablate Cbx1, PurB, and Sp3 expression induced fibroblasts to convert into cardiomyocytes in vivo. Furthermore, high-throughput DNA sequencing and coimmunoprecipitation experiments indicated that Cbx1, PurB, and Sp3 also bound together as a complex and were necessary to localize nucleosomes to cardiomyocyte genes on the chromosome. Finally, we found that the expression of these genes led to nucleosome modification via H3K27me3 (trimethylated histone-H3 lysine-27) deposition through an interaction with the polycomb repressive PRC2 complex. In summary, we conclude that Cbx1, PurB, and Sp3 control cell fate by actively repressing lineage-specific genes.

Signaling cascades in the failing heart and emerging therapeutic strategies

Chronic heart failure is the end stage of cardiac diseases. With a high prevalence and a high mortality rate worldwide, chronic heart failure is one of the heaviest health-related burdens. In addition to the standard neurohormonal blockade therapy, several medications have been developed for chronic heart failure treatment, but the population-wide improvement in chronic heart failure prognosis over time has been modest, and novel therapies are still needed. Mechanistic discovery and technical innovation are powerful driving forces for therapeutic development. On the one hand, the past decades have witnessed great progress in understanding the mechanism of chronic heart failure. It is now known that chronic heart failure is not only a matter involving cardiomyocytes. Instead, chronic heart failure involves numerous signaling pathways in noncardiomyocytes, including fibroblasts, immune cells, vascular cells, and lymphatic endothelial cells, and crosstalk among these cells. The complex regulatory network includes protein-protein, protein-RNA, and RNA-RNA interactions. These achievements in mechanistic studies provide novel insights for future therapeutic targets. On the other hand, with the development of modern biological techniques, targeting a protein pharmacologically is no longer the sole option for treating chronic heart failure. Gene therapy can directly manipulate the expression level of genes; gene editing techniques provide hope for curing hereditary cardiomyopathy; cell therapy aims to replace dysfunctional cardiomyocytes; and xenotransplantation may solve the problem of donor heart shortages. In this paper, we reviewed these two aspects in the field of failing heart signaling cascades and emerging therapeutic strategies based on modern biological techniques.

MicroRNA Roles in Cell Reprogramming Mechanisms

Cell reprogramming is a groundbreaking technology that, in few decades, generated a new paradigm in biomedical science. To date we can use cell reprogramming to potentially generate every cell type by converting somatic cells and suitably modulating the expression of key transcription factors. This approach can be used to convert skin fibroblasts into pluripotent stem cells as well as into a variety of differentiated and medically relevant cell types, including cardiomyocytes and neural cells. The molecular mechanisms underlying such striking cell phenotypes are still largely unknown, but in the last decade it has been proven that cell reprogramming approaches are significantly influenced by non-coding RNAs. Specifically, this review will focus on the role of microRNAs in the reprogramming processes that lead to the generation of pluripotent stem cells, neurons, and cardiomyocytes. As highlighted here, non-coding RNA-forced expression can be sufficient to support some cell reprogramming processes, and, therefore, we will also discuss how these molecular determinants could be used in the future for biomedical purposes.

Cardiac Tissue-like 3D Microenvironment Enhances Route towards Human Fibroblast Direct Reprogramming into Induced Cardiomyocytes by microRNAs

The restoration of cardiac functionality after myocardial infarction represents a major clinical challenge. Recently, we found that transient transfection with microRNA combination (miRcombo: miR-1, miR-133, miR-208 and 499) is able to trigger direct reprogramming of adult human cardiac fibroblasts (AHCFs) into induced cardiomyocytes (iCMs) in vitro. However, achieving efficient direct reprogramming still remains a challenge. The aim of this study was to investigate the influence of cardiac tissue-like biochemical and biophysical stimuli on direct reprogramming efficiency. Biomatrix (BM), a cardiac-like extracellular matrix (ECM), was produced by in vitro culture of AHCFs for 21 days, followed by decellularization. In a set of experiments, AHCFs were transfected with miRcombo and then cultured for 2 weeks on the surface of uncoated and BM-coated polystyrene (PS) dishes and fibrin hydrogels (2D hydrogel) or embedded into 3D fibrin hydrogels (3D hydrogel). Cell culturing on BM-coated PS dishes and in 3D hydrogels significantly improved direct reprogramming outcomes. Biochemical and biophysical cues were then combined in 3D fibrin hydrogels containing BM (3D BM hydrogel), resulting in a synergistic effect, triggering increased CM gene and cardiac troponin T expression in miRcombo-transfected AHCFs. Hence, biomimetic 3D culture environments may improve direct reprogramming of miRcombo-transfected AHCFs into iCMs, deserving further study.

Reprogramming cellular identity in vivo

Cellular identity is established through complex layers of genetic regulation, forged over a developmental lifetime. An expanding molecular toolbox is allowing us to manipulate these gene regulatory networks in specific cell types in vivo. In principle, if we found the right molecular tricks, we could rewrite cell identity and harness the rich repertoire of possible cellular functions and attributes. Recent work suggests that this rewriting of cell identity is not only possible, but that newly induced cells can mitigate disease phenotypes in animal models of major human diseases. So, is the sky the limit, or do we need to keep our feet on the ground? This Spotlight synthesises key concepts emerging from recent efforts to reprogramme cellular identity in vivo. We provide our perspectives on recent controversies in the field of glia-to-neuron reprogramming and identify important gaps in our understanding that present barriers to progress.

Translational perspectives on cardiac reprogramming

Loss of cardiac muscle after cardiac injury is replaced by cardiac fibrosis, due to very limited regenerative capacity of the heart. Although initially beneficial, persistent cardiac fibrosis leads to pump failure and conduction abnormalities, common modes of death following cardiac injury. Thus, directly reprogramming cardiac fibroblasts into induced cardiomyocyte-like cells (iCMs) by forced expression of cardiogenic factors (referred to as cardiac reprogramming) is particularly attractive in that it targets cardiac fibroblasts, a major source of cardiac fibrosis, to induce new cardiac muscle. Over the last decade, remarkable progresses have been made on cardiac reprogramming, particularly focusing on how to enhance conversion of fibroblasts to iCMs in vitro. However, it still remains elusive whether this new regenerative approach can be translated into clinical practice. This review discusses progresses and challenges of cardiac reprogramming in the translational context.

Direct reprogramming as a route to cardiac repair

Ischemic heart disease is the leading cause of morbidity, mortality, and healthcare expenditure worldwide due to an inability of the heart to regenerate following injury. Thus, novel heart failure therapies aimed at promoting cardiomyocyte regeneration are desperately needed. In recent years, direct reprogramming of resident cardiac fibroblasts to induced cardiac-like myocytes (iCMs) has emerged as a promising therapeutic strategy to repurpose the fibrotic response of the injured heart toward a functional myocardium. Direct cardiac reprogramming was initially achieved through the overexpression of the transcription factors (TFs) Gata4, Mef2c, and Tbx5 (GMT). However, this combination of TFs and other subsequent cocktails demonstrated limited success in reprogramming adult human and mouse fibroblasts, constraining the clinical translation of this therapy. Over the past decade, significant effort has been dedicated to optimizing reprogramming cocktails comprised of cardiac TFs, epigenetic factors, microRNAs, or small molecules to yield efficient cardiac cell fate conversion. Yet, efficient reprogramming of adult human fibroblasts remains a significant challenge. Underlying mechanisms identified to accelerate this process have been centered on epigenetic remodeling at cardiac gene regulatory regions. Further studies to achieve a refined understanding and directed means of overcoming epigenetic barriers are merited to more rapidly translate these promising therapies to the clinic.

Resuscitating the Field of Cardiac Regeneration: Seeking Answers from Basic Biology

Heart failure (HF) is one of the leading causes for hospital admissions worldwide. HF patients are classified based on the chronic changes in left ventricular ejection fraction (LVEF) as preserved (LVEF ≥ 50%), reduced (LVEF ≤ 40%), or mid-ranged (40% < LVEF < 50%) HFs. Treatments nowadays can prevent HFrEF progress, whereas only a few of the treatments have been proven to be effective in improving the survival of HFpEF. In this review, numerous mediators involved in the pathogenesis of HF are summarized. The regional upstream signaling and their diagnostic and therapeutic potential are also discussed. Additionally, the recent challenges and development in cardiac regenerative therapy that hold opportunities for future research and clinical translation are discussed.

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.

Basic and Translational Research in Cardiac Repair and Regeneration: JACC State-of-the-Art Review

This paper aims to provide an important update on the recent preclinical and clinical trials using cell therapy strategies and engineered heart tissues for the treatment of postinfarction left ventricular remodeling and heart failure. In addition to the authors’ own works and opinions on the roadblocks of the field, they discuss novel approaches for cardiac remuscularization via the activation of proliferative mechanisms in resident cardiomyocytes or direct reprogramming of somatic cells into cardiomyocytes. This paper’s main mindset is to present current and future strategies in light of their implications for the design of future patient trials with the ultimate objective of facilitating the translation of discoveries in regenerative myocardial therapies to the clinic.

Bioengineering Methods in MicroRNA-Mediated Direct Reprogramming of Fibroblasts Into Cardiomyocytes

Ischemic heart disease is the major cause of mortality worldwide. Despite the most recent pharmacological progresses, cardiac regeneration is yet not possible, and heart transplantation is the only therapeutic option for end-stage heart failure. Traditional cardiac regenerative medicine approaches, such as cell therapies and tissue engineering, have failed in the obtainment of human functional cardiac tissue, mainly due to unavailability of high quantities of autologous functional cardiomyocytes (CMs), low grafting efficiency, and/or arrhythmic events. Direct reprogramming (DR) of fibroblasts into induced CMs (iCMs) has emerged as a new promising approach for myocardial regeneration by in situ transdifferentiation or providing additional CM source for cell therapy. Among available DR methods, non-viral transfection with microRNAs (miRcombo: miR-1, miR-133, miR-208, and miR-499) appears promising for future clinical translation. MiRcombo transfection of fibroblasts could be significantly improved by the development of safe nanocarriers, efficiently delivering their cargo to target cells at the required stoichiometric ratio and overall dose in due times. Newly designed in vitro 3D culture microenvironments, providing biomimetic biophysical and biochemical stimuli to miRcombo-transfected cells, significantly increase the yield of fibroblast transdifferentiation into iCMs, enhancing CM gene expression. Epigenetic regulation of gene expression programs, critical to cell lineage commitment, can also be promoted by the administration of specific anti-inflammatory and anti-fibrotic soluble factors, helping in suppressing fibroblast signature. The aim of this mini-review is to introduce the readers to a relatively unknown field of cardiac research integrating bioengineering tools as relevant for the progress of miRNA-mediated cardiac DR.

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.

Detection of Novel Potential Regulators of Stem Cell Differentiation and Cardiogenesis through Combined Genome-Wide Profiling of Protein-Coding Transcripts and microRNAs

In vitro differentiation of embryonic stem cells (ESCs) provides a convenient basis for the study of microRNA-based gene regulation that is relevant for early cardiogenic processes. However, to which degree insights gained from in vitro differentiation models can be readily transferred to the in vivo system remains unclear. In this study, we profiled simultaneous genome-wide measurements of mRNAs and microRNAs (miRNAs) of differentiating murine ESCs (mESCs) and integrated putative miRNA-gene interactions to assess miRNA-driven gene regulation. To identify interactions conserved between in vivo and in vitro, we combined our analysis with a recent transcriptomic study of early murine heart development in vivo. We detected over 200 putative miRNA-mRNA interactions with conserved expression patterns that were indicative of gene regulation across the in vitro and in vivo studies. A substantial proportion of candidate interactions have been already linked to cardiogenesis, supporting the validity of our approach. Notably, we also detected miRNAs with expression patterns that closely resembled those of key developmental transcription factors. The approach taken in this study enabled the identification of miRNA interactions in in vitro models with potential relevance for early cardiogenic development. Such comparative approaches will be important for the faithful application of stem cells in cardiovascular research.

miR-4732-3p in Extracellular Vesicles From Mesenchymal Stromal Cells Is Cardioprotective During Myocardial Ischemia

Extracellular vesicles (EVs) derived from mesenchymal stromal cells (MSCs) are an emerging alternative to cell-based therapies to treat many diseases. However, the complexity of producing homogeneous populations of EVs in sufficient amount hampers their clinical use. To address these limitations, we immortalized dental pulp-derived MSC using a human telomerase lentiviral vector and investigated the cardioprotective potential of a hypoxia-regulated EV-derived cargo microRNA, miR-4732-3p. We tested the compared the capacity of a synthetic miR-4732-3p mimic with EVs to confer protection to cardiomyocytes, fibroblasts and endothelial cells against oxygen-glucose deprivation (OGD). Results showed that OGD-induced cardiomyocytes treated with either EVs or miR-4732-3p showed prolonged spontaneous beating, lowered ROS levels, and less apoptosis. Transfection of the miR-4732-3p mimic was more effective than EVs in stimulating angiogenesis in vitro and in vivo and in reducing fibroblast differentiation upon transforming growth factor beta treatment. Finally, the miR-4732-3p mimic reduced scar tissue and preserved cardiac function when transplanted intramyocardially in infarcted nude rats. Overall, these results indicate that miR-4732-3p is regulated by hypoxia and exerts cardioprotective actions against ischemic insult, with potential application in cell-free-based therapeutic strategies.

Advanced Technologies to Target Cardiac Cell Fate Plasticity for Heart Regeneration

The adult human heart can only adapt to heart diseases by starting a myocardial remodeling process to compensate for the loss of functional cardiomyocytes, which ultimately develop into heart failure. In recent decades, the evolution of new strategies to regenerate the injured myocardium based on cellular reprogramming represents a revolutionary new paradigm for cardiac repair by targeting some key signaling molecules governing cardiac cell fate plasticity. While the indirect reprogramming routes require an in vitro engineered 3D tissue to be transplanted in vivo, the direct cardiac reprogramming would allow the administration of reprogramming factors directly in situ, thus holding great potential as in vivo treatment for clinical applications. In this framework, cellular reprogramming in partnership with nanotechnologies and bioengineering will offer new perspectives in the field of cardiovascular research for disease modeling, drug screening, and tissue engineering applications. In this review, we will summarize the recent progress in developing innovative therapeutic strategies based on manipulating cardiac cell fate plasticity in combination with bioengineering and nanotechnology-based approaches for targeting the failing heart.

A role for Sfrp2 in cardiomyogenesis in vivo

Cardiomyogenesis, the process by which the body generates cardiomyocytes, is poorly understood. We have recently shown that Sfrp2 promotes cardiomyogenesis in vitro. The objective of this study was to determine if Sfrp2 would similarly promote cardiomyogenesis in vivo. To test this hypothesis, we tracked multipotent cKit(+) cells in response to Sfrp2 treatment. In control adult mice, multipotent cKit(+) cells typically differentiated into endothelial cells but not cardiomyocytes. In contrast, Sfrp2 switched the fate of these cells. Following Sfrp2 injection, multipotent cKit(+) cells differentiated solely into cardiomyocytes. Sfrp2-derived cardiomyocytes integrated into the myocardium and exhibited identical physiological properties to preexisting native cardiomyocytes. The ability of Sfrp2 to promote cardiomyogenesis was further supported by tracking EdU-labeled cells. In addition, Sfrp2 did not promote the formation of new cardiomyocytes when the cKit(+) cell population was selectively ablated in vivo using a diphtheria toxin receptor-diphtheria toxin model. Notably, Sfrp2-induced cardiomyogenesis was associated with significant functional improvements in a cardiac injury model. In summary, our study further demonstrates the importance of Sfrp2 in cardiomyogenesis.

Regeneration of infarcted mouse hearts by cardiovascular tissue formed via the direct reprogramming of mouse fibroblasts

Fibroblasts can be directly reprogrammed into cardiomyocytes, endothelial cells or smooth muscle cells. Here we report the reprogramming of mouse tail-tip fibroblasts simultaneously into cells resembling these three cell types using the microRNA mimic miR-208b-3p, ascorbic acid and bone morphogenetic protein 4, as well as the formation of tissue-like structures formed by the directly reprogrammed cells. Implantation of the formed cardiovascular tissue into the infarcted hearts of mice led to the migration of reprogrammed cells to the injured tissue, reducing regional cardiac strain and improving cardiac function. The migrated endothelial cells and smooth muscle cells contributed to vessel formation, and the migrated cardiomyocytes, which initially displayed immature characteristics, became mature over time and formed gap junctions with host cardiomyocytes. Direct reprogramming of somatic cells to make cardiac tissue may aid the development of applications in cell therapy, disease modelling and drug discovery for cardiovascular diseases.

Direct Reprogramming Induces Vascular Regeneration Post Muscle Ischemic Injury

Reprogramming non-cardiomyocytes (non-CMs) into cardiomyocyte (CM)-like cells is a promising strategy for cardiac regeneration in conditions such as ischemic heart disease. Here, we used a modified mRNA (modRNA) gene delivery platform to deliver a cocktail, termed 7G-modRNA, of four cardiac-reprogramming genes—Gata4 (G), Mef2c (M), Tbx5 (T), and Hand2 (H)—together with three reprogramming-helper genes—dominant-negative (DN)-TGFβ, DN-Wnt8a, and acid ceramidase (AC)—to induce CM-like cells. We showed that 7G-modRNA reprogrammed 57% of CM-like cells in vitro. Through a lineage-tracing model, we determined that delivering the 7G-modRNA cocktail at the time of myocardial infarction reprogrammed ∼25% of CM-like cells in the scar area and significantly improved cardiac function, scar size, long-term survival, and capillary density. Mechanistically, we determined that while 7G-modRNA cannot create de novo beating CMs in vitro or in vivo, it can significantly upregulate pro-angiogenic mesenchymal stromal cells markers and transcription factors. We also demonstrated that our 7G-modRNA cocktail leads to neovascularization in ischemic-limb injury, indicating CM-like cells importance in other organs besides the heart. modRNA is currently being used around the globe for vaccination against COVID-19, and this study proves this is a safe, highly efficient gene delivery approach with therapeutic potential to treat ischemic diseases.

Direct in vivo reprogramming with non-viral sequential targeting nanoparticles promotes cardiac regeneration

microRNA-mediated direct cardiac reprogramming, directly converts fibroblasts into induced cardiomyocyte-like cells (iCMs), which holds great promise in cardiac regeneration therapy. However, effective approaches to deliver therapeutic microRNA into cardiac fibroblasts (CFs) to induce in vivo cardiac reprogramming remain to be explored. Herein, a non-viral biomimetic system to directly reprogram CFs for cardiac regeneration after myocardial injury was developed by coating FH peptide-modified neutrophil-mimicking membranes on mesoporous silicon nanoparticles (MSNs) loaded with microRNA1, 133, 208, and 499 (miR Combo). Through utilizing the natural inflammation-homing ability of neutrophil membrane protein and FH peptide's high affinity to tenascin-C (TN-C) produced by CFs, this nanoparticle could realize sequential targeting to CFs in the injured heart and precise intracellular delivery of miRCombo, which induced reprogramming resident CFs into iCMs. In a mouse model of myocardial ischemia/reperfusion injury, intravenous injection of the nanoparticles successfully delivered miRCombo into fibroblasts and led to efficient reprogramming, resulting in improved cardiac function and attenuated fibrosis. This delivery system is minimally invasive and bio-safe, providing a proof-of-concept for biomimetic and sequential targeting nanomedicine delivery system for microRNA-mediated reprogramming therapy in multiple diseases.

Non-viral approaches for somatic cell reprogramming into cardiomyocytes

Heart disease is the leading cause of human deaths worldwide. Due to lacking cardiomyocytes with replicative capacity and cardiac progenitor cells with differentiation potential in adult hearts, massive loss of cardiomyocytes after ischemic events produces permanent damage, ultimately leading to heart failure. Cellular reprogramming is a promising strategy to regenerate heart by induction of cardiomyocytes from other cell types, such as cardiac fibroblasts. In contrast to conventional virus-based cardiac reprogramming, non-viral approaches greatly reduce the potential risk that includes disruption of genome integrity by integration of foreign DNAs, expression of exogenous genes with oncogenic potential, and appearance of partially reprogrammed cells harmful for the physiological functions of tissues/organs, which impedes their in-vivo applications. Here, we review the recent progress in development of non-viral approaches to directly reprogram somatic cells towards cardiomyocytes and their therapeutic application for heart regeneration.

Cardiac regeneration by direct reprogramming in this decade and beyond

Japan faces an increasing incidence of heart disease, owing to a shift towards a westernized lifestyle and an aging demographic. In cases where conventional interventions are not appropriate, regenerative medicine offers a promising therapeutic option. However, the use of stem cells has limitations, and therefore, “direct cardiac reprogramming” is emerging as an alternative treatment. Myocardial regeneration transdifferentiates cardiac fibroblasts into cardiomyocytes in situ.

Three cardiogenic transcription factors: Gata4, Mef2c, and Tbx5 (GMT) can induce direct reprogramming of fibroblasts into induced cardiomyocytes (iCMs), in mice. However, in humans, additional factors, such as Mesp1 and Myocd, are required. Inflammation and immune responses hinder the reprogramming process in mice, and epigenetic modifiers such as TET1 are involved in direct cardiac reprogramming in humans. The three main approaches to improving reprogramming efficiency are (1) improving direct cardiac reprogramming factors, (2) improving cell culture conditions, and (3) regulating epigenetic factors. miR-133 is a potential candidate for the first approach. For the second approach, inhibitors of TGF-β and Wnt signals, Akt1 overexpression, Notch signaling pathway inhibitors, such as DAPT ((S)-tert-butyl 2-((S)-2-(2-(3,5-difluorophenyl) acetamido) propanamido)-2-phenylacetate), fibroblast growth factor (FGF)-2, FGF-10, and vascular endothelial growth factor (VEGF: FFV) can influence reprogramming. Reducing the expression of Bmi1, which regulates the mono-ubiquitination of histone H2A, alters histone modification, and subsequently the reprogramming efficiency, in the third approach. In addition, diclofenac, a non-steroidal anti-inflammatory drug, and high level of Mef2c overexpression could improve direct cardiac reprogramming.

Direct cardiac reprogramming needs improvement if it is to be used in humans, and the molecular mechanisms involved remain largely elusive. Further advances in cardiac reprogramming research are needed to bring us closer to cardiac regenerative therapy.

Direct reprogramming with Sendai virus vectors repaired infarct hearts at the chronic stage

Adult hearts have limited regenerative capacity. Hence, after acute myocardial infarction (MI), dead myocardial tissues are digested by immune cells and replaced by fibrosis, leading to ventricular remodeling and heart failure at the chronic stage. Direct reprogramming of the cardiac fibroblasts (CFs) into induced cardiomyocytes (iCMs) with cardiac transcription factors, including Gata4, Mef2c, and Tbx5 (GMT), may have significant potential for cardiac repair. Sendai virus (SeV) vectors expressing GMT have been reported to reprogram the mouse cardiac fibroblasts into iCMs without any risk of insertional mutagenesis. In vivo reprogramming improved the cardiac function after acute MI in immunodeficient mice. However, it is unknown whether the newly generated iCMs could exist in infarct hearts for a prolonged period and SeV-GMT can improve cardiac function after MI at the chronic stage in immunocompetent mice. Here, we show that SeV vectors efficiently infect CFs in vivo and reprogram them into iCMs, which existed for at least four weeks after MI, in fibroblast-linage tracing mice. Moreover, SeV-GMT improved cardiac function and reduced fibrosis and collagen I expression at 12 weeks after MI in immunocompetent mice. Thus, direct cardiac reprogramming with SeV vectors could be a promising therapy for MI.

In vivo reprogramming as a new approach to cardiac regenerative therapy

Cardiovascular diseases are a common cause of death worldwide. Adult cardiomyocytes have limited regenerative capacity after injury, and there is growing interest in cardiac regeneration as a new therapeutic strategy. There are several limitations of induced pluripotent stem cell-based transplantation therapy with respect to efficiency and risks of tumorigenesis. Direct reprogramming enables the conversion of terminally differentiated cells into target cell types using defined factors. In most cardiac diseases, activated fibroblasts proliferate in the damaged heart and contribute to the progression of heart failure. In vivo cardiac reprogramming, in which resident cardiac fibroblasts are converted into cardiomyocytes in situ, is expected to become a new cardiac regenerative therapy. Indeed, we and other groups have demonstrated that in vivo reprogramming improves cardiac function and reduces fibrosis after myocardial infarction. In this review, we summarize recent discoveries and developments related to in vivo reprogramming. In addition, issues that need to be resolved for clinical application are described.