Direct In Vivo Reprogramming with Sendai Virus Vectors Improves Cardiac Function after Myocardial Infarction

Tbx6 induces cardiomyocyte proliferation in postnatal and adult mouse hearts

Cardiovascular disease is a leading cause of death worldwide. Mammalian cardiomyocytes (CMs) proliferate during embryonic development, whereas they largely lose their regenerative capacity after birth. Defined factors expressed in cardiac progenitors or embryonic CMs may activate the cell cycle and induce CM proliferation in postnatal and adult hearts. Here, we report that the overexpression of Tbx6, enriched in the cardiac mesoderm (progenitor cells), induces CM proliferation in postnatal and adult mouse hearts. By screening 24 factors enriched in cardiac progenitors or embryonic CMs, we found that only Tbx6 could induce CM proliferation in primary cultured postnatal rat CMs. Intriguingly, it did not induce the proliferation of cardiac fibroblasts. We next generated a recombinant adeno-associated virus serotype 9 vector encoding Tbx6 (AAV9-Tbx6) for transduction into mouse CMs in vivo. The subcutaneous injection of AAV9-Tbx6 into neonatal mice induced CM proliferation in postnatal and adult mouse hearts. Mechanistically, Tbx6 overexpression upregulated multiple cell cycle activators including Aurkb, Mki67, Ccna1, and Ccnb2 and suppressed the tumor suppressor Rb1. Thus, Tbx6 promotes CM proliferation in postnatal and adult mouse hearts by modifying the expression of cell cycle regulators.

Role of cyclooxygenase-2-mediated prostaglandin E2-prostaglandin E receptor 4 signaling in cardiac reprogramming

Direct cardiac reprogramming from fibroblasts can be a promising approach for disease modeling, drug screening, and cardiac regeneration in pediatric and adult patients. However, postnatal and adult fibroblasts are less efficient for reprogramming compared with embryonic fibroblasts, and barriers to cardiac reprogramming associated with aging remain undetermined. In this study, we screened 8400 chemical compounds and found that diclofenac sodium (diclofenac), a non-steroidal anti-inflammatory drug, greatly enhanced cardiac reprogramming in combination with Gata4, Mef2c, and Tbx5 (GMT) or GMT plus Hand2. Intriguingly, diclofenac promoted cardiac reprogramming in mouse postnatal and adult tail-tip fibroblasts (TTFs), but not in mouse embryonic fibroblasts (MEFs). Mechanistically, diclofenac enhanced cardiac reprogramming by inhibiting cyclooxygenase-2, prostaglandin E2/prostaglandin E receptor 4, cyclic AMP/protein kinase A, and interleukin 1β signaling and by silencing inflammatory and fibroblast programs, which were activated in postnatal and adult TTFs. Thus, anti-inflammation represents a new target for cardiac reprogramming associated with aging.

Fibroblasts can be directly reprogrammed to cardiomyocytes, but reprogramming is less efficient for adult compared to embryonic fibroblasts. Here, the authors find that inhibition of inflammation and Cox-2-prostaglandin-cAMP-IL-1β signaling enhances reprogramming efficiency of adult, but not embryonic fibroblasts.

Sendai virus-mediated transduction of mammalian spermatogonial stem cells†

Spermatogonial stem cells (SSCs) provide the foundation of spermatogenesis. However, because of their small number and slow self-renewal, transfection of SSCs has met with limited success. Although several viral vectors can infect SSCs, genome integration and an inability to maintain long-term gene expression have hampered studies on SSCs. Here we report successful SSC infection by Sendai virus (SV), an RNA virus in the Paramyxoviridae. The SV efficiently transduced germline stem (GS) cells, cultured spermatogonia with enriched SSC activity, and maintained gene expression for at least 5 months. It also infected freshly isolated SSCs from adult testes. The transfected GS cells reinitiated spermatogenesis following spermatogonial transplantation into seminiferous tubules of infertile mice, suggesting that SV transfection does not interfere with spermatogenesis progression. On the other hand, microinjection of SV into the seminiferous tubules of immature mice transduced SSCs and Sertoli cells, but did not transduce Leydig or peritubular cells by interstitial virus injection. SV-infected hamster GS cells, and freshly isolated rabbit or monkey SSC-like cells were identified following xenogeneic spermatogonial transplantation, suggesting that SV transduces SSCs from several mammalian species. Thus, SV is a useful vector that can transduce both SSCs and Sertoli cells and overcome problems associated with other viral vectors.

Vascular Regeneration With New Sources of Endothelial Cells

Two new sources of ECs were generated from human induced pluripotent stem cells (hiPSCs), and by direct reprogramming of somatic cells without undergoing the stages of stem or progenitor cell. These two types of ECs will advance our understanding of EC biology and can become a novel therapeutic option for treating ischemic cardiovascular diseases.

Direct cellular reprogramming and inner ear regeneration

INTRODUCTION

Sound is integral to communication and connects us to the world through speech and music. Cochlear hair cells are essential for converting sounds into neural impulses. However, these cells are highly susceptible to damage from an array of factors, resulting in degeneration and ultimately irreversible hearing loss in humans. Since the discovery of hair cell regeneration in birds, there have been tremendous efforts to identify therapies that could promote hair cell regeneration in mammals.

AREAS COVERED

Here, we will review recent studies describing spontaneous hair cell regeneration and direct cellular reprograming as well as other factors that mediate mammalian hair cell regeneration.

EXPERT OPINION

Numerous combinatorial approaches have successfully reprogrammed non-sensory supporting cells to form hair cells, albeit with limited efficacy and maturation. Studies on epigenetic regulation and transcriptional network of hair cell progenitors may accelerate discovery of more promising reprogramming regimens.

Efficient in vivo direct conversion of fibroblasts into cardiomyocytes using a nanoparticle-based gene carrier

The reprogramming of induced cardiomyocytes (iCMs) has shown potential in regenerative medicine. However, in vivo reprogramming of iCMs is significantly inefficient, and novel gene delivery systems are required to more efficiently and safely induce in vivo reprogramming of iCMs for therapeutic applications in heart injury. In this study, we show that cationic gold nanoparticles (AuNPs) loaded with Gata4, Mef2c, and Tbx5 function as nanocarriers for cardiac reprogramming. The AuNP/GMT/PEI nanocomplexes show high reprogramming efficiency in human and mouse somatic cells with low cytotoxicity and direct conversion into iCMs without integrating factors into the genome. Importantly, AuNP/GMT/PEI nanocomplexes led to efficient in vivo conversion into cardiomyocytes after myocardial infarction (MI), resulting in the effective recovery of cardiac function and scar area. Taken together, these results show that the AuNP/GMT/PEI nanocarrier can be used to develop effective therapeutics for heart regeneration in cardiac disease patients.

Progress and Challenge of Cardiac Regeneration to Treat Heart Failure

Cardiac muscle has limited proliferative capacity, and regenerative therapies are highly in demand as a new treatment strategy. Pharmacological and non-pharmacological therapies have been developed, but these medical therapies have limited effects to cure patients with severe heart failure. Moreover, heart transplantation is limited due to the low number of donor organs. Thus, heart regeneration holds great potential to offer innovative therapy to treat heart failure patients. Currently, there are several strategies for heart regeneration. Transplantation of somatic stem cells was safe and modestly improved cardiac function after myocardial infarction mainly through paracrine mechanisms. Alternatively, new cardiomyocytes could be generated from induced pluripotent stem cells (iPSCs) to transplant into injured hearts. However, several issues remain to be resolved prior to using iPSC-derived cardiomyocytes, such as a potential risk of tumorigenesis and poor survival of transplanted cells in the injured heart. More recently, direct cardiac reprogramming has emerged as a novel technology to regenerate damaged myocardium by directly converting endogenous cardiac fibroblasts into induced cardiomyocyte-like cells to restore cardiac function. Following our first report of cardiac reprogramming, an improvement in cardiac reprogramming efficiency, in vivo direct cardiac reprogramming, and cardiac reprogramming in human cells were reported by many investigators. While these previous studies have advanced regenerative research, many challenges remain. Here, we review the current status of cardiac regenerative technology, a great hope to treat cardiovascular diseases.

Therapeutic approaches for cardiac regeneration and repair

Ischaemic heart disease is a leading cause of death worldwide. Injury to the heart is followed by loss of the damaged cardiomyocytes, which are replaced with fibrotic scar tissue. Depletion of cardiomyocytes results in decreased cardiac contraction, which leads to pathological cardiac dilatation, additional cardiomyocyte loss, and mechanical dysfunction, culminating in heart failure. This sequential reaction is defined as cardiac remodelling. Many therapies have focused on preventing the progressive process of cardiac remodelling to heart failure. However, after patients have developed end-stage heart failure, intervention is limited to heart transplantation. One of the main reasons for the dramatic injurious effect of cardiomyocyte loss is that the adult human heart has minimal regenerative capacity. In the past 2 decades, several strategies to repair the injured heart and improve heart function have been pursued, including cellular and noncellular therapies. In this Review, we discuss current therapeutic approaches for cardiac repair and regeneration, describing outcomes, limitations, and future prospects of preclinical and clinical trials of heart regeneration. Substantial progress has been made towards understanding the cellular and molecular mechanisms regulating heart regeneration, offering the potential to control cardiac remodelling and redirect the adult heart to a regenerative state.

Direct Cardiac Reprogramming: A Novel Approach for Heart Regeneration

Cardiac diseases are among the most common causes of death globally. Cardiac muscle has limited proliferative capacity, and regenerative therapies are highly in demand as a new treatment strategy. Although pluripotent reprogramming has been developed, it has obstacles, such as a potential risk of tumor formation, poor survival of the transplanted cells, and high cost. We previously reported that fibroblasts can be directly reprogrammed to cardiomyocytes by overexpressing a combination of three cardiac-specific transcription factors (Gata4, Mef2c, Tbx5 (together, GMT)). We and other groups have promoted cardiac reprogramming by the addition of certain miRNAs, cytokines, and epigenetic factors, and unraveled new molecular mechanisms of cardiac reprogramming. More recently, we discovered that Sendai virus (SeV) vector expressing GMT could efficiently and rapidly reprogram fibroblasts into integration-free cardiomyocytes in vitro via robust transgene expression. Gene delivery of SeV-GMT also improves cardiac function and reduces fibrosis after myocardial infarction in mice. Through direct cardiac reprogramming, new cardiomyocytes can be generated and scar tissue reduced to restore cardiac function, and, thus, direct cardiac reprogramming may serve as a powerful strategy for cardiac regeneration. Here, we provide an overview of the previous reports and current challenges in this field.

Dedifferentiation, transdifferentiation and cell fusion: in vivo reprogramming strategies for regenerative medicine

Regenerative capacities vary enormously across the animal kingdom. In contrast to most cold-blooded vertebrates, mammals, including humans, have very limited regenerative capacity when it comes to repairing damaged or degenerating tissues. Here, we review the main mechanisms of tissue regeneration, underlying the importance of cell dedifferentiation and reprogramming. We discuss the significance of cell fate and identity changes in the context of regenerative medicine, with a particular focus on strategies aiming at the promotion of the body's self-repairing mechanisms. We also introduce some of the most recent advances that have resulted in complete reprogramming of cell identity in vivo. Lastly, we discuss the main challenges that need to be addressed in the near future to develop in vivo reprogramming approaches with therapeutic potential.

In vivo somatic cell reprogramming for tissue regeneration: the emerging role of the local microenvironment

The past few years have witnessed an exponential increase of interest in the reprogramming process. This has been motivated by the enthusiasm of unravelling key aspects not only of cell identity and dedifferentiation, but also of the endogenous regenerative capacities of mammalian organs. Here, we present the most recent advances in the field of reprogramming, stressing how they are re-defining the rules of cell fate and plasticity in vivo. Specifically, we focus on the emerging role of the tissue microenvironment, with particular emphasis on tissue damage, inflammation and senescence that can facilitate in vivo reprogramming and regeneration through cell-extrinsic mechanisms.

New Myocyte Formation in the Adult Heart: Endogenous Sources and Therapeutic Implications

Death of adult cardiac myocytes and supportive tissues resulting from cardiovascular diseases such as myocardial infarction is the proximal driver of pathological ventricular remodeling that often culminates in heart failure. Unfortunately, no currently available therapeutic barring heart transplantation can directly replenish myocytes lost from the injured heart. For decades, the field has struggled to define the intrinsic capacity and cellular sources for endogenous myocyte turnover in pursuing more innovative therapeutic strategies aimed at regenerating the injured heart. Although controversy persists to this day as to the best therapeutic regenerative strategy to use, a growing consensus has been reached that the very limited capacity for new myocyte formation in the adult mammalian heart is because of proliferation of existing cardiac myocytes but not because of the activity of an endogenous progenitor cell source of some sort. Hence, future therapeutic approaches should take into consideration the fundamental biology of myocyte renewal in designing strategies to potentially replenish these cells in the injured heart.

Direct Reprograming to Regenerate Myocardium and Repair Its Pacemaker and Conduction System

The regenerative medicine field has been revolutionized by the direct conversion of one cell type to another by ectopic expression of lineage-specific transcription factors. The direct reprogramming of fibroblasts to induced cardiac myocytes (iCMs) by core cardiac transcription factors (Gata4, Mef2c, Tbx5) both in vitro and in vivo has paved the way in cardiac regeneration and repair. Several independent research groups have successfully reported the direct reprogramming of fibroblasts in injured myocardium to cardiac myocytes employing a variety of approaches that rely on transcription factors, small molecules, and micro RNAs (miRNAs). Recently, this technology has been considered for local repair of the pacemaker and the cardiac conduction system. To address this, we will first discuss the direct reprograming advancements in the setting of working myocardium regeneration, and then elaborate on how this technology can be applied to repair the cardiac pacemaker and the conduction system.

Harnessing cell pluripotency for cardiovascular regenerative medicine

Human pluripotent stem cells (hPSCs), in particular embryonic stem cells and induced pluripotent stem cells, have received enormous attention in cardiovascular regenerative medicine owing to their ability to expand and differentiate into functional cardiomyocytes and other cardiovascular cell types. Despite the potential applications of hPSCs for tissue regeneration in patients suffering from cardiovascular disease, whether hPSC-based therapies can be safe and efficacious remains inconclusive, with strong evidence from clinical trials lacking. Critical factors limiting therapeutic efficacy are the degree of maturity and purity of the hPSC-derived differentiated progeny, and the tumorigenic risk associated with residual undifferentiated cells. In this Review, we discuss recent advances in cardiac-cell differentiation from hPSCs and in the direct reprogramming of non-myocyte cells for cardiovascular regenerative applications. We also discuss approaches for the delivery of cells to diseased tissue, and how such advances are contributing to progress in cardiac tissue engineering for tackling heart disease.

Small molecule-induced cellular fate reprogramming: promising road leading to Rome

Cellular fate reprogramming holds great promise to generate functional cell types for replenishing new cells and restoring functional loss. Inspired by transcription factor-induced reprogramming, the field of cellular reprogramming has greatly advanced and developed into divergent streams of reprogramming approaches. Remarkably, increasing studies have shown the power and advantages of small molecule-based approaches for cellular fate reprogramming, which could overcome the limitations of conventional transgenic-based reprogramming. In this concise review, we discuss these findings and highlight the future potentiality with particular focus on this new trend of chemical reprogramming.

S-phase Synchronization Facilitates the Early Progression of Induced-Cardiomyocyte Reprogramming through Enhanced Cell-Cycle Exit

Direct reprogramming of fibroblasts into induced cardiomyocytes (iCMs) holds a great promise for regenerative medicine and has been studied in several major directions. However, cell-cycle regulation, a fundamental biological process, has not been investigated during iCM-reprogramming. Here, our time-lapse imaging on iCMs, reprogrammed by Gata4, Mef2c, and Tbx5 (GMT) monocistronic retroviruses, revealed that iCM-reprogramming was majorly initiated at late-G1- or S-phase and nearly half of GMT-reprogrammed iCMs divided soon after reprogramming. iCMs exited cell cycle along the process of reprogramming with decreased percentage of 5-ethynyl-20-deoxyuridine (EdU)⁺/α-myosin heavy chain (αMHC)-GFP⁺ cells. S-phase synchronization post-GMT-infection could enhance cell-cycle exit of reprogrammed iCMs and yield more GFP^high iCMs, which achieved an advanced reprogramming with more expression of cardiac genes than GFP^low cells. However, S-phase synchronization did not enhance the reprogramming with a polycistronic-viral vector, in which cell-cycle exit had been accelerated. In conclusion, post-infection synchronization of S-phase facilitated the early progression of GMT-reprogramming through a mechanism of enhanced cell-cycle exit.