Cardiac muscle plasticity in adult and embryo by heart-derived progenitor cells

Stem cell antigen-1+cell-derived fibroblasts are crucial for cardiac fibrosis during heart failure

AIMS

Mesenchymal stem cells (MSCs) present in the heart cannot differentiate into cardiomyocytes, but may play a role in pathological conditions. Therefore, the aim of this study was to scrutinise the role and mechanism of MSC differentiation in vivo during heart failure.

METHODS AND RESULTS

We performed single-cell RNA sequencing of total non-cardiomyocytes from murine and adult human hearts. By analysing the transcriptomes of single cells, we illustrated the dynamics of the cell landscape during the progression of heart hypertrophy, including those of stem cell antigen-1 (Sca1)+ stem/progenitor cells and fibroblasts. By combining genetic lineage tracing and bone marrow transplantation models, we demonstrated that non-bone marrow-derived Sca1+ cells give rise to fibroblasts. Interestingly, partial depletion of Sca1+ cells alleviated the severity of myocardial fibrosis and led to a significant improvement in cardiac function in Sca1-CreERT2;Rosa26-eGFP-DTA mice. Similar non-cardiomyocyte cell composition and heterogeneity were observed in human patients with heart failure. Mechanistically, our study revealed that Sca1+ cells can transform into fibroblasts and affect the severity of fibrosis through the Wnt4-Pdgfra pathway.

CONCLUSIONS

Our study describes the cellular landscape of hypertrophic hearts and reveals that fibroblasts derived from Sca1+ cells with a non-bone marrow source largely account for cardiac fibrosis. These findings provide novel insights into the pathogenesis of cardiac fibrosis and have potential therapeutic implications for heart failure. Non-bone marrow-derived Sca1+ cells differentiate into fibroblasts involved in cardiac fibrosis via Wnt4-PDGFRα pathway.

Cardiac Cell Therapy for Heart Repair: Should the Cells Be Left Out?

Cardiovascular disease (CVD) is still the leading cause of death worldwide. Coronary artery occlusion, or myocardial infarction (MI) causes massive loss of cardiomyocytes. The ischemia area is eventually replaced by a fibrotic scar. From the mechanical dysfunctions of the scar in electronic transduction, contraction and compliance, pathological cardiac dilation and heart failure develops. Once end-stage heart failure occurs, the only option is to perform heart transplantation. The sequential changes are termed cardiac remodeling, and are due to the lack of endogenous regenerative actions in the adult human heart. Regenerative medicine and biomedical engineering strategies have been pursued to repair the damaged heart and to restore normal cardiac function. Such strategies include both cellular and acellular products, in combination with biomaterials. In addition, substantial progress has been made to elucidate the molecular and cellular mechanisms underlying heart repair and regeneration. In this review, we summarize and discuss current therapeutic approaches for cardiac repair and provide a perspective on novel strategies that holding potential opportunities for future research and clinical translation.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Reprogramming of Keratinocytes as Donor or Target Cells Holds Great Promise for Cell Therapy and Regenerative Medicine

One of the most crucial branches of regenerative medicine is cell therapy, in which cellular material is injected into the patient to initiate the regenerative process. Cells obtained by reprogramming of the patient's own cells offer ethical and clinical advantages could provide a new source of material for therapeutic applications. Studies to date have shown that only a subset of differentiated cell types can be reprogrammed. Among these, keratinocytes, which are the most abundant proliferating cell type in the epidermis, have gained increasing attention as both donor and target cells for reprogramming and have become a new focus of regenerative medicine. As target cells for the treatment of skin defects, keratinocytes can be differentiated or reprogrammed from embryonic stem cells, induced pluripotent stem cells, fibroblasts, adipose tissue stem cells, and mesenchymal cells. As donor cells, keratinocytes can be reprogrammed or direct reprogrammed into a number of cell types, including induced pluripotent stem cells, neural cells, and Schwann cells. In this review, we discuss recent advances in keratinocyte reprogramming, focusing on the induction methods, potential molecular mechanisms, conversion efficiency, and safety for clinical applications. Graphical Abstract KCs as target cells can be reprogrammed or differentiated from fibroblasts, iPSCs, ATSCs, and mesenchymal cells. And as donor cells, KCs can be reprogrammed or directly reprogrammded into iPSCs, neural cells, Schwann cells, and epidermal stem cells.

Cardiac progenitors and paracrine mediators in cardiogenesis and heart regeneration

The mammalian hearts have the least regenerative capabilities among tissues and organs. As such, heart regeneration has been and continues to be the ultimate goal in the treatment against acquired and congenital heart diseases. Uncovering such a long-awaited therapy is still extremely challenging in the current settings. On the other hand, this desperate need for effective heart regeneration has developed various forms of modern biotechnologies in recent years. These involve the transplantation of pluripotent stem cell-derived cardiac progenitors or cardiomyocytes generated in vitro and novel biochemical molecules along with tissue engineering platforms. Such newly generated technologies and approaches have been shown to effectively proliferate cardiomyocytes and promote heart repair in the diseased settings, albeit mainly preclinically. These novel tools and medicines give somehow credence to breaking down the barriers associated with re-building heart muscle. However, in order to maximize efficacy and achieve better clinical outcomes through these cell-based and/or cell-free therapies, it is crucial to understand more deeply the developmental cellular hierarchies/paths and molecular mechanisms in normal or pathological cardiogenesis. Indeed, the morphogenetic process of mammalian cardiac development is highly complex and spatiotemporally regulated by various types of cardiac progenitors and their paracrine mediators. Here we discuss the most recent knowledge and findings in cardiac progenitor cell biology and the major cardiogenic paracrine mediators in the settings of cardiogenesis, congenital heart disease, and heart regeneration.

Genetic Lineage Tracing of Sca-1+ Cells Reveals Endothelial but Not Myogenic Contribution to the Murine Heart

BACKGROUND

The adult mammalian heart displays a cardiomyocyte turnover rate of ≈1%/y throughout postnatal life and after injuries such as myocardial infarction (MI), but the question of which cell types drive this low level of new cardiomyocyte formation remains contentious. Cardiac-resident stem cells marked by stem cell antigen-1 (Sca-1, gene name Ly6a) have been proposed as an important source of cardiomyocyte renewal. However, the in vivo contribution of endogenous Sca-1⁺ cells to the heart at baseline or after MI has not been investigated.

METHODS

Here we generated Ly6a gene-targeted mice containing either a constitutive or an inducible Cre recombinase to perform genetic lineage tracing of Sca-1⁺ cells in vivo.

RESULTS

We observed that the contribution of endogenous Sca-1⁺ cells to the cardiomyocyte population in the heart was <0.005% throughout all of cardiac development, with aging, or after MI. In contrast, Sca-1⁺ cells abundantly contributed to the cardiac vasculature in mice during physiological growth and in the post-MI heart during cardiac remodeling. Specifically, Sca-1 lineage-traced endothelial cells expanded postnatally in the mouse heart after birth and into adulthood. Moreover, pulse labeling of Sca-1⁺ cells with an inducible Ly6a-MerCreMer allele also revealed a preferential expansion of Sca-1 lineage-traced endothelial cells after MI injury in the mouse.

CONCLUSIONS

Cardiac-resident Sca-1⁺ cells are not significant contributors to cardiomyocyte renewal in vivo. However, cardiac Sca-1⁺ cells represent a subset of vascular endothelial cells that expand postnatally with enhanced responsiveness to pathological stress in vivo.

Regenerating the field of cardiovascular cell therapy

The retraction of >30 falsified studies by Anversa et al. has had a disheartening impact on the cardiac cell therapeutics field. The premise of heart muscle regeneration by the transdifferentiation of bone marrow cells or putative adult resident cardiac progenitors has been largely disproven. Over the past 18 years, a generation of physicians and scientists has lost years chasing these studies, and patients have been placed at risk with little scientific grounding. Funding agencies invested hundreds of millions of dollars in irreproducible work, and both academic institutions and the scientific community ignored troubling signals over a decade of questionable work. Our collective retrospective analysis identifies preventable problems at the level of the editorial and peer-review process, funding agencies and academic institutions. This Perspective provides a chronology of the forces that led to this scientific debacle, integrating direct knowledge of the process. We suggest a science-driven path forward that includes multiple novel approaches to the problem of heart muscle regeneration.

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.

Dnmt3a-mediated inhibition of Wnt in cardiac progenitor cells improves differentiation and remote remodeling after infarction

Adult cardiac progenitor cells (CPCs) display a low capacity to differentiate into cardiomyocytes in injured hearts, strongly limiting the regenerative capacity of the mammalian myocardium. To identify new mechanisms regulating CPC differentiation, we used primary and clonally expanded Sca-1+ CPCs from murine adult hearts in homotypic culture or coculture with cardiomyocytes. Expression kinetics analysis during homotypic culture differentiation showed downregulation of Wnt target genes concomitant with increased expression of the Wnt antagonist, Wnt inhibitory factor 1 (Wif1), which is necessary to stimulate CPC differentiation. We show that the expression of the Wif1 gene is repressed by DNA methylation and regulated by the de novo DNA methyltransferase Dnmt3a. In addition, miR-29a is upregulated early during CPC differentiation and downregulates Dnmt3a expression, thereby decreasing Wif1 gene methylation and increasing the efficiency of differentiation of Sca-1+ CPCs in vitro. Extending these findings in vivo, transient silencing of Dnmt3a in CPCs subsequently injected in the border zone of infarcted mouse hearts improved CPC differentiation in situ and remote cardiac remodeling. In conclusion, miR-29a and Dnmt3a epigenetically regulate CPC differentiation through Wnt inhibition. Remote effects on cardiac remodeling support paracrine signaling beyond the local injection site, with potential therapeutic interest for cardiac repair.

Approaches to augment vascularisation and regeneration of the adult heart via the reactivated epicardium

Survival rates following myocardial infarction have increased in recent years but current treatments for post-infarction recovery are inadequate and cannot induce regeneration of damaged hearts. Regenerative medicine could provide disease-reversing treatments by harnessing modern concepts in cell and developmental biology. A recently-established paradigm in regenerative medicine is that regeneration of a tissue can be achieved by reactivation of the coordinated developmental processes that originally formed the tissue. In the heart, the epicardium has emerged as an important regulator of cardiac development and reactivation of epicardial developmental processes may provide a means to enable cardiac regeneration. Indeed, in adult mouse hearts, treatment with thymosin β4 and other drug-like molecules reactivates the epicardium and improves outcomes after myocardial infarction by inducing regenerative paracrine signalling, neovascularisation and de novo cardiomyocyte production. However, there are considerable limitations to current methods of epicardial reactivation that prevent direct translation into clinical practice. Here, we describe the rationale for targeting the epicardium and the successes and limitations of this approach. We consider how several recent advances in epicardial biology could be used to overcome these limitations. These advances include insight into epicardial signalling and heterogeneity, epicardial modulation of inflammation and epicardial remodelling of extracellular matrix.

PROTECTIVE EFFECT OF Ailanthus excelsa ROXB IN MYOCARDIAL INFARCTION POST MESENCHYMAL STEM CELL TRANSPLANTATION: STUDY IN CHRONIC ISCHEMIC RAT MODEL

Background:

Thia study evaluates the effects of Ailanthus excelsa Roxb methanolic extract (AER-ME) in rats induced with Myocardial Infarction (MI) followed by transplantation of MSCs.

Material and Methods:

Rats were induced with MI by ligation technique of left coronary artery. The sham-operated the control and AER-ME treated group of rats received transplantation of PKH-26 and marked MSCs followed by normal saline and AER-ME treatment (200mg/kg/day of AER-ME extract) respectively for 30 days. Parameters such as cardiac function, inflammation, oxidative stress, apoptosis and differentiation of MSCs (angiogenesis) were evaluated. Histological studies of infracted myocardium reveled anti-inflammatory activity of AER-ME treatment.

Result and Discussion:

Oxidative stress parameters revealed decrease in levels of malondialdehyde (MDA) and increase in superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSHpx) activity significantly indicating antioxidant activity of the extract. There was a reduction in cell death rate of treated rats due to the decrease in apoptotic index with prolongation of MI when compared to both control and sham-operated groups. The expression of Fas protein was parallel to apoptotic index. The vascular density increased significantly in extract treated group. The treatment showed improved cardiac activity with decreased left ventricular end diastolic (LVEDP) and arterial pressure while the left ventricular end systolic pressure (LVEP) and dp/dtmax increased significantly when compared to both control and sham-operated groups respectively showing the protective effect of the extract as necessitated by the transplantation of MSCs. The study marked the protective outcomes of AER-ME treatment for MSCs in microenvironment of infracted myocardium by improving their viability and increasing differentiation into cardiomyocytes.

Paracrine nitric oxide induces expression of cardiac sarcomeric proteins in adult progenitor cells through soluble guanylyl cyclase/cyclic-guanosine monophosphate and Wnt/β-catenin inhibition

AIM

Cardiac progenitor cells (CPC) from adult hearts can differentiate to several cell types composing the myocardium but the underlying molecular pathways are poorly characterized. We examined the role of paracrine nitric oxide (NO) in the specification of CPC to the cardiac lineage, particularly through its inhibition of the canonical Wnt/β-catenin pathway, a critical step preceding cardiac differentiation.

METHODS AND RESULTS

Sca1 + CPC from adult mouse hearts were isolated by magnetic-activated cell sorting and clonally expanded. Pharmacologic NO donors increased their expression of cardiac myocyte-specific sarcomeric proteins in a concentration and time-dependent manner. The optimal time window for NO efficacy coincided with up-regulation of CPC expression of Gucy1a3 (coding the alpha1 subunit of guanylyl cyclase). The effect of paracrine NO was reproduced in vitro upon co-culture of CPC with cardiac myocytes expressing a transgenic NOS3 (endothelial nitric oxide synthase) and in vivo upon injection of CPC in infarcted hearts from cardiac-specific NOS3 transgenic mice. In mono- and co-cultures, this effect was abrogated upon inhibition of soluble guanylyl cyclase or nitric oxide synthase, and was lost in CPC genetically deficient in Gucy1a3. Mechanistically, NO inhibits the constitutive activity of the canonical Wnt/β-catenin in CPC and in cell reporter assays in a guanylyl cyclase-dependent fashion. This was paralleled with decreased expression of β-catenin and down-regulation of Wnt target genes in CPC and abrogated in CPC with a stabilized, non-inhibitable β-catenin.

CONCLUSIONS

Exogenous or paracrine sources of NO promote the specification towards the myocyte lineage and expression of cardiac sarcomeric proteins of adult CPC. This is contingent upon the expression and activity of the alpha1 subunit of guanylyl cyclase in CPC that is necessary for NO-mediated inhibition of the canonical Wnt/β-catenin pathway.

Differentiation of Enhanced Green Fluorescent Protein-Labeled Mouse Amniotic Fluid-Derived Stem Cells into Cardiomyocyte-Like Beating Cells

BACKGROUND

Amniotic fluid-derived stem cells (AFSCs) possess optimal differentiation potential and are a promising resource for cell therapy and tissue engineering. Mouse is a good model to be studied for pre-clinical research.

METHODS

In this study, we successfully established enhanced green fluorescent protein mouse-derived amniotic fluid stem cells (EGFP-mAFSCs) and investigated whether EGFP-mAFSCs possess the ability to differentiate into cardiomyocytes by in vitro culture. We evaluated stem-cell differentiation using immunofluorescence.

RESULTS

This study showed that EGFP-mAFSCs can give rise to spontaneously beating cardiomyocyte-like cells expressing the specific markers c-kit, myosin heavy chain, and cardiac troponin I.

CONCLUSIONS

We demonstrated that mAFSCs have the in vitro propensity to acquire a cardiomyogenic phenotype and to a certain extent cardiomyocytes; however the process efficiency which gives rise to cardiomyocyte-like cells remains quite low (2 out of 10 were found).

KEY WORDS

Amniotic fluid; Cardiomyocytes; In vitro differentiation; Stem cells.

Position Paper of the European Society of Cardiology Working Group Cellular Biology of the Heart: cell-based therapies for myocardial repair and regeneration in ischemic heart disease and heart failure

Despite improvements in modern cardiovascular therapy, the morbidity and mortality of ischaemic heart disease (IHD) and heart failure (HF) remain significant in Europe and worldwide. Patients with IHD may benefit from therapies that would accelerate natural processes of postnatal collateral vessel formation and/or muscle regeneration. Here, we discuss the use of cells in the context of heart repair, and the most relevant results and current limitations from clinical trials using cell-based therapies to treat IHD and HF. We identify and discuss promising potential new therapeutic strategies that include ex vivo cell-mediated gene therapy, the use of biomaterials and cell-free therapies aimed at increasing the success rates of therapy for IHD and HF. The overall aim of this Position Paper of the ESC Working Group Cellular Biology of the Heart is to provide recommendations on how to improve the therapeutic application of cell-based therapies for cardiac regeneration and repair.

Identification of cardiac stem cells within mature cardiac myocytes

Cardiac stem cells are described in a number of mammalian species including humans. Cardiac stem cell clusters consisting of both lineage-negative and partially committed cells are generally identified between contracting cardiac myocytes. In the present study, c-kit(+), Sca(+), and Isl1(+) stem cells were revealed to be located inside the sarcoplasm of cardiac myocytes in myocardial cell cultures derived from newborn, 20-, and 40-day-old rats. Intracellularly localized cardiac stem cells had a coating or capsule with a few pores that opened into the host cell sarcoplasm. The similar structures were also identified in the suspension of freshly isolated myocardial cells (ex vivo) of 20- and 40-day-old rats. The results from this study provide direct evidence for the replicative division of encapsulated stem cells, followed by their partial cardiomyogenic differentiation. The latter is substantiated by the release of multiple transient amplifying cells following the capsule rupture. In conclusion, functional cardiac stem cells can reside not only exterior to but also within cardiomyocytes.

Endogenous resident c-Kit cardiac stem cells increase in mice with an exercise-induced, physiologically hypertrophied heart

Physical activity evokes well-known adaptations in the cardiovascular system. Although exercise training induces cardiac remodeling, whether multipotent stem cells play a functional role in the hypertrophic process remains unknown. To evaluate this possibility, C57BL/6 mice were subjected to swimming training aimed at achieving cardiac hypertrophy, which was morphologically and electrocardiographically characterized. Subsequently, c-Kit(+)Lin(-) and Sca-1(+)Lin(-) cardiac stem cells (CSCs) were quantified using flow cytometry while cardiac muscle-derived stromal cells (CMSCs, also known as cardiac-derived mesenchymal stem cells) were assessed using in vitro colony-forming unit fibroblast assay (CFU-F). Only the number of c-Kit(+)Lin(-) cells increased in the hypertrophied heart. To investigate a possible extracardiac origin of these cells, a parabiotic eGFP transgenic/wild-type mouse model was used. The parabiotic pairs were subjected to swimming, and the wild-type heart in particular was tested for eGFP(+) stem cells. The results revealed a negligible number of extracardiac stem cells in the heart, allowing us to infer a cardiac origin for the increased amount of detected c-Kit(+) cells. In conclusion, the number of resident Sca-1(+)Lin(-) cells and CMSCs was not changed, whereas the number of c-Kit(+)Lin(-) cells was increased during physiological cardiac hypertrophy. These c-Kit(+)Lin(-) CSCs may contribute to the physiological cardiac remodeling that result from exercise training.

Stem cells in the management of heart failure: what have we learned from clinical trials?

Research shows that various types of stem cells (SCs) have the ability to rebuild damaged heart tissue. The TIME and Late TIME human trials shed light on the optimum timing of SC therapy administration after myocardial damage. The FOCUS study failed to show a substantial positive effect of bone marrow-derived mononuclear cells in patients suffering from ischemic heart failure; however, some completed human trials do show promise, with improvement in cardiac function. Recent clinical trials have identified a subset of marrow cells that was able to stimulate endogenous adult cardiac SCs where cardiac SCs administration showed promise in the SCIPIO trial. This review addresses some of the lessons learned from clinical trials with SC therapy in ischemic heart failure.

Polymorphisms of the drug transporters ABCB1, ABCG2, ABCC2 and ABCC3 and their impact on drug bioavailability and clinical relevance

INTRODUCTION

Human ATP-binding cassette (ABC) transporters act as translocators of numerous substrates across extracellular and intracellular membranes, thereby contributing to bioavailability and consequently therapy response. Genetic polymorphisms are considered as critical determinants of expression level or activity and subsequently response to selected drugs.

AREAS COVERED

Here the influence of polymorphisms of the prominent ABC transporters P-glycoprotein (MDR1, ABCB1), breast cancer resistance protein (BCRP, ABCG2) and the multidrug resistance-associated protein (MRP) 2 (ABCC2) as well as MRP3 (ABCC3) on the pharmacokinetic of drugs and associated consequences on therapy response and clinical outcome is discussed.

EXPERT OPINION

ABC transporter genetic variants were assumed to affect interindividual differences in pharmacokinetics and subsequently clinical response. However, decades of medical research have not yielded in distinct and unconfined reproducible outcomes. Despite some unique results, the majority were inconsistent and dependent on the analyzed cohort or study design. Therefore, variability of bioavailability and drug response may be attributed only by a small amount to polymorphisms in transporter genes, whereas transcriptional regulation or post-transcriptional modification seems to be more critical. In our opinion, currently identified genetic variants of ABC efflux transporters can give some hints on the role of transporters at interfaces but are less suitable as biomarkers to predict therapeutic outcome.

Sca-1+ cardiac progenitor cells and heart-making: a critical synopsis

The identification, in the adult, of cardiomyocyte turnover events and of cardiac progenitor cells (CPCs) has revolutionized the field of cardiovascular medicine. However, the low rate of CPCs differentiation events reported both in vitro and in vivo, even after injury, raised concerns on the biological significance of these subsets. In this Comprehensive Review, we discuss the current understanding of cardiac Lin(-)Sca-1(+) cells in light of what is also known for cellular compartments with similar phenotypes in other organs. The Lin(-)Sca-1(+) heart subset is heterogeneous and displays a mesenchymal profile, characterized by a limited ability to generate cardiomyocytes in vitro and in vivo, even after injury. There is no evidence for Sca-1 expression in embryonic cardiovascular progenitors. In other organs, Sca-1 expression is mainly observed on mesoderm-derived cells, although it is not restricted to stem/progenitor cell populations. It is urgent to determine, at a single cell level, to which extent cardiac Lin(-)Sca-1(+) cells overlap with the fibroblast compartment.

THY-1 receptor expression differentiates cardiosphere-derived cells with divergent cardiogenic differentiation potential

Despite over a decade of intense research, the identity and differentiation potential of human adult cardiac progenitor cells (aCPC) remains controversial. Cardiospheres have been proposed as a means to expand aCPCs in vitro, but the identity of the progenitor cell within these 3D structures is unknown. We show that clones derived from cardiospheres could be subdivided based on expression of thymocyte differentiation antigen 1 (THY-1/CD90) into two distinct populations that exhibit divergent cardiac differentiation potential. One population, which is CD90(+), expressed markers consistent with a mesenchymal/myofibroblast cell. The second clone type was CD90(-) and could form mature, functional myocytes with sarcomeres albeit at a very low rate. These two populations of cardiogenic clones displayed distinct cell surface markers and unique transcriptomes. Our study suggests that a rare aCPC exists in cardiospheres along with a mesenchymal/myofibroblast cell, which demonstrates incomplete cardiac myocyte differentiation.

Fate choice of post-natal mesoderm progenitors: skeletal versus cardiac muscle plasticity

Regenerative medicine for skeletal and cardiac muscles still constitutes a fascinating and ambitious frontier. In this perspective, understanding the possibilities of intrinsic cell plasticity, present in post-natal muscles, is vital to define and improve novel therapeutic strategies for acute and chronic diseases. In addition, many somatic stem cells are now crossing the boundaries of basic/translational research to enter the first clinical trials. However, it is still an open question whether a lineage switch between skeletal and cardiac adult myogenesis is possible. Therefore, this review focuses on resident somatic stem cells of post-natal skeletal and cardiac muscles and their plastic potential toward the two lineages. Furthermore, examples of myogenic lineage switch in adult stem cells are also reported and discussed.

Cardiac-derived stem cell-based therapy for heart failure: progress and clinical applications

Stem cell-based therapy is emerging as a promising strategy to treat end-stage heart failure, a leading cause of morbidity and mortality. Stem cells can be isolated from a variety of sources and exhibit unique characteristics that impact their potential therapeutic utility. The adult heart contains small populations of committed, multipotent cardiac stem cells (CSC), which are adapted to the cardiac microenvironment and participate in postnatal physiological and pathological cardiac renewal or repair. These cells can be isolated, expanded in culture, and administered therapeutically to improve cardiac function in the setting of heart failure. CSC can be differentiated into three distinct cardiovascular lineages and exhibit enhanced paracrine factor production and engraftment as compared with other types of mesenchymal stem cells, which in turn may translate into improved therapeutic efficacy. The cell surface marker expression and phenotype of these CSC, however, depends on the method of isolation, selection and propagation, which likely explains the variable experimental results obtained to date. Moreover, invasive procedures are required to obtain CSC from humans. Early trials using autologous CSC in patients with ischemic cardiomyopathy have demonstrated feasibility and safety, along with variable degrees of therapeutic efficacy in terms of enhancing myocardial viability and cardiac function. Further studies are needed to optimize methods of CSC isolation, manipulation and delivery. If fully realized, the potential of CSC therapy could fundamentally change the approach to the treatment of end-stage heart failure.

Dissecting the molecular relationship among various cardiogenic progenitor cells

RATIONALE

Multiple progenitors derived from the heart and bone marrow (BM) have been used for cardiac repair. Despite this, not much is known about the molecular identity and relationship among these progenitors. To develop a robust stem cell therapy for the heart, it is critical to understand the molecular identity of the multiple cardiogenic progenitor cells.

OBJECTIVE

This study is the first report of high-throughput transcriptional profiling of cardiogenic progenitor cells carried out on an identical platform.

METHOD AND RESULTS

Microarray-based transcriptional profiling was carried out for 3 cardiac (ckit(+), Sca1(+), and side population) and 2 BM (ckit(+) and mesenchymal stem cell) progenitors, obtained from age- and sex-matched wild-type C57BL/6 mice. Analysis indicated that cardiac-derived ckit(+) population was very distinct from Sca1(+) and side population cells in the downregulation of genes encoding for cell-cell and cell-matrix adhesion proteins, and in the upregulation of developmental genes. Significant enrichment of transcripts involved in DNA replication and repair was observed in BM-derived progenitors. The BM ckit(+) cells seemed to have the least correlation with the other progenitors, with enrichment of immature neutrophil-specific molecules.

CONCLUSIONS

Our study indicates that cardiac ckit(+) cells represent the most primitive population in the rodent heart. Primitive cells of cardiac versus BM origin differ significantly with respect to stemness and cardiac lineage-specific genes, and molecules involved in DNA replication and repair. The detailed molecular profile of progenitors reported here will serve as a useful reference to determine the molecular identity of progenitors used in future preclinical and clinical studies.

Migration of resident cardiac stem cells in myocardial infarction

Ischemic heart disease is a major cause of morbidity and mortality worldwide. Stem cell-based therapy, which aims to restore cardiac structure and function by regeneration of functional myocardium, has recently been proposed as a novel alternative treatment modality. Resident cardiac stem cells (CSCs) in adult hearts are a key cell type under investigation. CSCs have been shown to be able to repair damaged myocardium and improve myocardial function in both human and animal studies. This approach relies not only on the proliferation of the CSCs, but also upon their migration to the site of injury within the heart. Here, we briefly review reported CSC populations and discuss signaling factors and pathways required for the migration of CSCs.

Cardiac side population cells: moving toward the center stage in cardiac regeneration

Over the past decade, extensive work in animal models and humans has identified the presence of adult cardiac progenitor cells, capable of cardiomyogenic differentiation and likely contributors to cardiomyocyte turnover during normal development and disease. Among cardiac progenitor cells, there is a distinct subpopulation, termed "side population" (SP) progenitor cells, identified by their unique ability to efflux DNA binding dyes through an ATP-binding cassette transporter. This review highlights the literature on the isolation, characterization, and functional relevance of cardiac SP cells. We review the initial discovery of cardiac SP cells in adult myocardium as well as their capacity for functional cardiomyogenic differentiation and role in cardiac regeneration after myocardial injury. Finally, we discuss recent advances in understanding the molecular regulators of cardiac SP cell proliferation and differentiation, as well as likely future areas of investigation required to realize the goal of effective cardiac regeneration.

Stem cell therapy for cardiac disease

Congenital heart disease occurs in 1% of liveborn infants, making it the most common birth defect worldwide. Many of these children develop heart failure. In addition, both genetic and acquired forms of dilated cardiomyopathy are a significant source of heart failure in the pediatric population. Heart failure occurs when the myocardium is unable to meet the body's metabolic demands. Unlike some organs, the heart has limited, if any, capacity for repair after injury. Heart transplantation remains the ultimate approach to treating heart failure, but this is costly and excludes patients who are poor candidates for transplantation given their comorbidities, or for whom a donor organ is unavailable. Stem cell therapy represents the first realistic strategy for reversing the effects of what has until now been considered terminal heart damage. We will discuss potential sources of cardiac-specific stem cells, including mesenchymal, resident cardiac, embryonic, and induced pluripotent stem cells. We will consider efforts to enhance cardiac stem cell engraftment and survival in damaged myocardium, the incorporation of cardiac stem cells into tissue patches, and techniques for creating bioartificial myocardial tissue as well as whole organs. Finally, we will review progress being made in assessing functional improvement in animals and humans after cellular transplant.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

Fetal cells traffic to injured maternal myocardium and undergo cardiac differentiation

RATIONALE

Fetal cells enter the maternal circulation during pregnancy and may persist in maternal tissue for decades as microchimeras.

OBJECTIVE

Based on clinical observations of peripartum cardiomyopathy patients and the high rate of recovery they experience from heart failure, our objective was to determine whether fetal cells can migrate to the maternal heart and differentiate to cardiac cells.

METHODS AND RESULTS

We report that fetal cells selectively home to injured maternal hearts and undergo differentiation into diverse cardiac lineages. Using enhanced green fluorescent protein (eGFP)-tagged fetuses, we demonstrate engraftment of multipotent fetal cells in injury zones of maternal hearts. In vivo, eGFP+ fetal cells form endothelial cells, smooth muscle cells, and cardiomyocytes. In vitro, fetal cells isolated from maternal hearts recapitulate these differentiation pathways, additionally forming vascular tubes and beating cardiomyocytes in a fusion-independent manner; ≈40% of fetal cells in the maternal heart express Caudal-related homeobox2 (Cdx2), previously associated with trophoblast stem cells, thought to solely form placenta.

CONCLUSIONS

Fetal maternal stem cell transfer appears to be a critical mechanism in the maternal response to cardiac injury. Furthermore, we have identified Cdx2 cells as a novel cell type for potential use in cardiovascular regenerative therapy.

In vitro and in vivo cardiomyogenic differentiation of amniotic fluid stem cells

Cell therapy has developed as a complementary treatment for myocardial regeneration. While both autologous and allogeneic uses have been advocated, the ideal candidate has not been identified yet. Amniotic fluid-derived stem (AFS) cells are potentially a promising resource for cell therapy and tissue engineering of myocardial injuries. However, no information is available regarding their use in an allogeneic context. c-kit-sorted, GFP-positive rat AFS (GFP-rAFS) cells and neonatal rat cardiomyocytes (rCMs) were characterized by cytocentrifugation and flow cytometry for the expression of mesenchymal, embryonic and cell lineage-specific antigens. The activation of the myocardial gene program in GFP-rAFS cells was induced by co-culture with rCMs. The stem cell differentiation was evaluated using immunofluorescence, RT-PCR and single cell electrophysiology. The in vivo potential of Endorem-labeled GFP-rAFS cells for myocardial repair was studied by transplantation in the heart of animals with ischemia/reperfusion injury (I/R), monitored by magnetic resonance imaging (MRI). Three weeks after injection a small number of GFP-rAFS cells acquired an endothelial or smooth muscle phenotype and to a lesser extent CMs. Despite the low GFP-rAFS cells count in the heart, there was still an improvement of ejection fraction as measured by MRI. rAFS cells have the in vitro propensity to acquire a cardiomyogenic phenotype and to preserve cardiac function, even if their potential may be limited by poor survival in an allogeneic setting.

miR669a and miR669q prevent skeletal muscle differentiation in postnatal cardiac progenitors

Postnatal heart stem and progenitor cells are a potential therapeutic tool for cardiomyopathies, but little is known about the mechanisms that control cardiac differentiation. Recent work has highlighted an important role for microribonucleic acids (miRNAs) as regulators of cardiac and skeletal myogenesis. In this paper, we isolated cardiac progenitors from neonatal β-sarcoglycan (Sgcb)-null mouse hearts affected by dilated cardiomyopathy. Unexpectedly, Sgcb-null cardiac progenitors spontaneously differentiated into skeletal muscle fibers both in vitro and when transplanted into regenerating muscles or infarcted hearts. Differentiation potential correlated with the absence of expression of a novel miRNA, miR669q, and with down-regulation of miR669a. Other miRNAs are known to promote myogenesis, but only miR669a and miR669q act upstream of myogenic regulatory factors to prevent myogenesis by directly targeting the MyoD 3' untranslated region. This finding reveals an added level of complexity in the mechanism of the fate choice of mesoderm progenitors and suggests that using endogenous cardiac stem cells therapeutically will require specially tailored procedures for certain genetic diseases.

Myocyte proliferation in the developing heart

Regulation of organ growth is critical during embryogenesis. At the cellular level, mechanisms controlling the size of individual embryonic organs include cell proliferation, differentiation, migration, and attrition through cell death. All these mechanisms play a role in cardiac morphogenesis, but experimental studies have shown that the major determinant of cardiac size during prenatal development is myocyte proliferation. As this proliferative capacity becomes severely restricted after birth, the number of cell divisions that occur during embryogenesis limits the growth potential of the postnatal heart. We summarize here current knowledge concerning regional control of myocyte proliferation as related to cardiac morphogenesis and dysmorphogenesis. There are significant spatial and temporal differences in rates of cell division, peaking during the preseptation period and then gradually decreasing toward birth. Analysis of regional rates of proliferation helps to explain the mechanics of ventricular septation, chamber morphogenesis, and the development of the cardiac conduction system. Proliferation rates are influenced by hemodynamic loading, and transduced by autocrine and paracrine signaling by means of growth factors. Understanding the biological response of the developing heart to such factors and physical forces will further our progress in engineering artificial myocardial tissues for heart repair and designing optimal treatment strategies for congenital heart disease.

Developmental and regenerative biology of multipotent cardiovascular progenitor cells

Our limited ability to improve the survival of patients with heart failure is attributable, in part, to the inability of the mammalian heart to meaningfully regenerate itself. The recent identification of distinct families of multipotent cardiovascular progenitor cells from endogenous, as well as exogenous, sources, such as embryonic and induced pluripotent stem cells, has raised much hope that therapeutic manipulation of these cells may lead to regression of many forms of cardiovascular disease. Although the exact source and cell type remains to be clarified, our greater understanding of the scientific underpinning behind developmental cardiovascular progenitor cell biology has helped to clarify the origin and properties of diverse cells with putative cardiogenic potential. In this review, we highlight recent advances in the understanding of cardiovascular progenitor cell biology from embryogenesis to adulthood and their implications for therapeutic cardiac regeneration. We believe that a detailed understanding of cardiogenesis will inform future applications of cardiovascular progenitor cells in heart failure therapy and regenerative medicine.

In vitro and in vivo evidence for the importance of breast cancer resistance protein transporters (BCRP/MXR/ABCP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) is a member of the G-subfamiliy of the ATP-binding cassette (ABC)-transporter superfamily. This half-transporter is assumed to function as an important mechanism limiting cellular accumulation of various compounds. In context of its tissue distribution with localization in the sinusoidal membrane of hepatocytes, and in the apical membrane of enterocytes ABCG2 is assumed to function as an important mechanism facilitating hepatobiliary excretion and limiting oral bioavailability, respectively. Indeed functional assessment performing mouse studies with genetic deletion or chemical inhibition of the transporter, or performing pharmacogenetic studies in humans support this assumption. Furthermore the efflux function of ABCG2 has been linked to sanctuary blood tissue barriers as described for placenta and the central nervous system. However, in lactating mammary glands ABCG2 increases the transfer of substrates into milk thereby increasing the exposure to potential noxes of a breastfed newborn. With regard to its broad substrate spectrum including various anticancer drugs and environmental carcinogens the function of ABCG2 has been associated with multidrug resistance and tumor development/progression. In terms of cancer biology current research is focusing on the expression and function of ABCG2 in immature stem cells. Recent findings support the notion that the physiological function of ABCG2 is involved in the elimination of uric acid resulting in higher risk for developing gout in male patients harboring genetic variants. Taken together ABCG2 is implicated in various pathophysiological and pharmacological processes.

Embryonic stem cell-derived cardiomyocytes harbor a subpopulation of niche-forming Sca-1+ progenitor cells

The adult mammalian heart is known to contain a population of cardiac progenitor cells. It has not been unambiguously determined, however, whether these cells form as part of the developmental program of the heart or migrate there by way of the circulatory system. This study was done in order to determine the origin of this population of cells. A population of cardiomyocytes was established from mouse embryonic stem (ES) cells using a genetic selection technique. In order to determine whether cardiac progenitor cells exist within this ES cell-derived cardiomyocyte population, the cells were analyzed by fluorescence activated cell sorting (FACS) using an antibody directed against stem cell antigen-1 (Sca-1). We observed that approximately 4% of the cardiomyocyte population was composed of Sca-1(+) cells. When the Sca-1(+) cells were isolated by magnetic cell sorting and differentiated as cellular aggregates, contractions were observed in 100% of the aggregates. Gene expression studies using quantitative RT-PCR showed that these cells expressed terminally differentiated cardiac-specific genes. When three-dimensional cellular aggregates were formed from ES cell-derived cardiomyocytes co-cultured with adult HL-1 cardiomyocytes, the Sca-1(+) cells were found to "sort out" and form niches within the cell aggregates. Our data demonstrate that cardiac progenitor cells in the adult heart originate as part of the developmental program of the heart and that Sca-1(+) progenitor cells can provide an important in vitro model system to study the formation of cellular niches in the heart.

Strategies for ensuring that regenerative cardiomyocytes function properly and in cooperation with the host myocardium

In developed countries, in which people have nutrient-rich diets, convenient environments, and access to numerous medications, the disease paradigm has changed. Nowadays, heart failure is one of the major causes of death. In spite of this, the therapeutic efficacies of medications are generally unsatisfactory. Although whole heart transplantation is ideal for younger patients with heart failure, many patients are deemed to be unsuitable for this type of surgery due to complications and/or age. The need for therapeutic alternatives to heart transplantation is great. Regenerative therapy is a strong option. For this purpose, several cell sources have been investigated, including intrinsic adult stem or progenitor cells and extrinsic pluripotent stem cells. Most intrinsic stem cells seem to contribute to a regenerative environment via paracrine factors and/or angiogenesis, whereas extrinsic pluripotent stem cells are unlimited sources of cardiomyocytes. In this review, we summarize the various strategies for using regenerative cardiomyocytes including our recent progressions: non-genetic approaches for the purification of cardiomyocytes and efficient transplantation. We expect that use of intrinsic and extrinsic stem cells in combination will enhance therapeutic effectiveness.

Strategies for ensuring that regenerative cardiomyocytes function properly and in cooperation with the host myocardium

In developed countries, in which people have nutrient-rich diets, convenient environments, and access to numerous medications, the disease paradigm has changed. Nowadays, heart failure is one of the major causes of death. In spite of this, the therapeutic efficacies of medications are generally unsatisfactory. Although whole heart transplantation is ideal for younger patients with heart failure, many patients are deemed to be unsuitable for this type of surgery due to complications and/or age. The need for therapeutic alternatives to heart transplantation is great. Regenerative therapy is a strong option. For this purpose, several cell sources have been investigated, including intrinsic adult stem or progenitor cells and extrinsic pluripotent stem cells. Most intrinsic stem cells seem to contribute to a regenerative environment via paracrine factors and/or angiogenesis, whereas extrinsic pluripotent stem cells are unlimited sources of cardiomyocytes. In this review, we summarize the various strategies for using regenerative cardiomyocytes including our recent progressions: non-genetic approaches for the purification of cardiomyocytes and efficient transplantation. We expect that use of intrinsic and extrinsic stem cells in combination will enhance therapeutic effectiveness.

Local mesenchymal stem/progenitor cells are a preferential target for initiation of adult soft tissue sarcomas associated with p53 and Rb deficiency

The cell of origin and pathogenesis of the majority of adult soft tissue sarcomas (STS) remains poorly understood. Because mutations in both the P53 and RB tumor suppressor genes are frequent in STS in humans, we inactivated these genes by Cre-loxP-mediated recombination in mice with floxed p53 and Rb. Ninety-three percent of mice developed spindle cell/pleomorphic sarcomas after a single subcutaneous injection of adenovirus carrying Cre-recombinase. Similar to human STS, these sarcomas overexpress Cxcr4, which contributes to their invasive properties. Using irradiation chimeras generated by transplanting bone marrow cells from mice carrying either the Rosa26StoploxPLacZ or the Z/EG reporter, as well as the floxed p53 and Rb genes, into irradiated p53loxP/loxPRbloxP/loxP mice, it was determined that sarcomas do not originate from bone marrow-derived cells, such as macrophages, but arise from the local resident cells. At the same time, dermal mesenchymal stem cells isolated by strict plastic adherence and low levels of Sca-1 expression (Sca-1low, CD31negCD45neg) have shown enhanced potential for malignant transformation according to soft agar, invasion, and tumorigenicity assays, after the conditional inactivation of both p53 and Rb. Sarcomas formed after transplantation of these cells have features typical for undifferentiated high-grade pleomorphic sarcomas. Taken together, our studies indicate that local Sca-1low dermal mesenchymal stem/progenitor cells are preferential targets for malignant transformation associated with deficiencies in both p53 and Rb.

Dedifferentiation and proliferation of mammalian cardiomyocytes

Background

It has long been thought that mammalian cardiomyocytes are terminally-differentiated and unable to proliferate. However, myocytes in more primitive animals such as zebrafish are able to dedifferentiate and proliferate to regenerate amputated cardiac muscle.

Methodology/Principal Findings

Here we test the hypothesis that mature mammalian cardiomyocytes retain substantial cellular plasticity, including the ability to dedifferentiate, proliferate, and acquire progenitor cell phenotypes. Two complementary methods were used: 1) cardiomyocyte purification from rat hearts, and 2) genetic fate mapping in cardiac explants from bi-transgenic mice. Cardiomyocytes isolated from rodent hearts were purified by multiple centrifugation and Percoll gradient separation steps, and the purity verified by immunostaining and RT-PCR. Within days in culture, purified cardiomyocytes lost their characteristic electrophysiological properties and striations, flattened and began to divide, as confirmed by proliferation markers and BrdU incorporation. Many dedifferentiated cardiomyocytes went on to express the stem cell antigen c-kit, and the early cardiac transcription factors GATA4 and Nkx2.5. Underlying these changes, inhibitory cell cycle molecules were suppressed in myocyte-derived cells (MDCs), while microRNAs known to orchestrate proliferation and pluripotency increased dramatically. Some, but not all, MDCs self-organized into spheres and re-differentiated into myocytes and endothelial cells in vitro. Cell fate tracking of cardiomyocytes from 4-OH-Tamoxifen-treated double-transgenic MerCreMer/ZEG mouse hearts revealed that green fluorescent protein (GFP) continues to be expressed in dedifferentiated cardiomyocytes, two-thirds of which were also c-kit⁺.

Conclusions/Significance

Contradicting the prevailing view that they are terminally-differentiated, postnatal mammalian cardiomyocytes are instead capable of substantial plasticity. Dedifferentiation of myocytes facilitates proliferation and confers a degree of stemness, including the expression of c-kit and the capacity for multipotency.

MicroRNA-155 prevents necrotic cell death in human cardiomyocyte progenitor cells via targeting RIP1

To improve regeneration of the injured myocardium, cardiomyocyte progenitor cells (CMPCs) have been put forward as a potential cell source for transplantation therapy. Although cell transplantation therapy displayed promising results, many issues need to be addressed before fully appreciating their impact. One of the hurdles is poor graft-cell survival upon injection, thereby limiting potential beneficial effects. Here, we attempt to improve CMPCs survival by increasing microRNA-155 (miR-155) levels, potentially to improve engraftment upon transplantation. Using quantitative PCR, we observed a 4-fold increase of miR-155 when CMPCs were exposed to hydrogen-peroxide stimulation. Flow cytometric analysis of cell viability, apoptosis and necrosis showed that necrosis is the main cause of cell death. Overexpressing miR-155 in CMPCs revealed that miR-155 attenuated necrotic cell death by 40 ± 2.3%via targeting receptor interacting protein 1 (RIP1). In addition, inhibiting RIP1, either by pre-incubating the cells with a RIP1 specific inhibitor, Necrostatin-1 or siRNA mediated knockdown, reduced necrosis by 38 ± 2.5% and 33 ± 1.9%, respectively. Interestingly, analysing gene expression using a PCR-array showed that increased miR-155 levels did not change cell survival and apoptotic related gene expression. By targeting RIP1, miR-155 repressed necrotic cell death of CMPCs, independent of activation of Akt pro-survival pathway. MiR-155 provides the opportunity to block necrosis, a conventionally thought non-regulated process, and might be a potential novel approach to improve cell engraftment for cell therapy.

Endogenous retinoic acid regulates cardiac progenitor differentiation

Retinoic acid (RA) has several established functions during cardiac development, including actions in the fetal epicardium required for myocardial growth. An open question is if retinoid effects are limited to growth factor stimulation pathway(s) or if additional actions on uncommitted progenitor/stem populations might drive cardiac differentiation. Here we report the dual effects of RA deficiency on cardiac growth factor signaling and progenitor/stem biology using the mouse retinaldehyde dehydrogenase 2 (Raldh2) knockout model. Although early heart defects in Raldh2(-/-) embryos result from second-heart-field abnormalities, it is unclear whether this role is transient or whether RA has sustained effects on cardiac progenitors. To address this, we used transient maternal RA supplementation to overcome early Raldh2(-/-) lethality. By embryonic day 11.5-14.5, Raldh2(-/-) hearts exhibited reduced venticular compact layer outgrowth and altered coronary vessel development. Although reductions in Fgf2 and target pERK levels occurred, no alterations in Wnt/beta-catenin expression were observed. Cell proliferation is increased in compact zone myocardium, whereas cardiomyocyte differentiation is reduced, alterations that suggest progenitor defects. We report that the fetal heart contains a reservoir of stem/progenitor cells, which can be isolated by their ability to efflux a fluorescent dye and that retinoid signaling acts on this fetal cardiac side population (SP). Raldh2(-/-) hearts display increased SP cell numbers, with selective increases in expression of cardiac progenitor cell markers and reduced differentiation marker levels. Hence, although lack of RA signaling increases cardiac SP numbers, simultaneous reductions in Fgf signaling reduce cardiomyocyte differentiation, possibly accounting for long-term defects in myocardial growth.

Predifferentiated Adult Stem Cells and Matrices for Cardiac Cell Therapy

Stem cell therapy is a major field of research worldwide, with increasing clinical application, especially in cardiovascular pathology. However, the best stem cell source and type with optimal safety for functional engraftment remains unclear. An intermediate cardiac precommitted phenotype expressing some of the key proteins of a mature cardiomyocyte would permit better integration into the cardiac environment. The predifferentiated cells would receive signals from the environment, thus achieving gradual and complete differentiation. In cell transplantation, survival and engraftment within the environment of the ischemic myocardium represents a challenge for all types of cells, regardless of their state of differentiation. An alternative strategy is to embed cells in a 3-dimensional structure replicating the extracellular matrix, which is crucial for full tissue restoration and prevention of ventricular remodeling. The clinical translation of cell therapy requires avoidance of potentially harmful drugs and cytokines, and rapid off-the-shelf availability of cells. The combination of predifferentiated cells with a functionalized scaffold, locally releasing molecules tailored to promote in-situ completion of differentiation and improve homing, survival, and function, could be an exciting approach that might circumvent the potential undesired effects of growth factor administration and improve tissue restoration.

Cardiomyocyte cell cycle control and growth estimation in vivo--an analysis based on cardiomyocyte nuclei

AIMS

Adult mammalian cardiomyocytes are traditionally viewed as being permanently withdrawn from the cell cycle. Whereas some groups have reported none, others have reported extensive mitosis in adult myocardium under steady-state conditions. Recently, a highly specific assay of 14C dating in humans has suggested a continuous generation of cardiomyocytes in the adult, albeit at a very low rate. Mice represent the most commonly used animal model for these studies, but their short lifespan makes them unsuitable for 14C studies. Herein, we investigate the cellular growth pattern for murine cardiomyocyte growth under steady-state conditions, addressed with new analytical and technical strategies, and we furthermore relate this to gene expression patterns.

METHODS AND RESULTS

The observed levels of DNA synthesis in early life were associated with cardiomyocyte proliferation. Mitosis was prolonged into early life, longer than the most conservative previous estimates. DNA synthesis in neonatal life was attributable to bi-nucleation, therefore suggesting that cardiomyocytes withdraw from the cell cycle shortly after birth. No cell cycle activity was observed in adult cardiomyocytes and significant polyploidy was observed in cardiomyocyte nuclei.

CONCLUSION

Gene analyses identified 32 genes whose expression was predicted to be particular to day 3-4 neonatal myocytes, compared with embryonic or adult cells. These cell cycle-associated genes are crucial to the understanding of the mechanisms of bi-nucleation and physiological cellular growth in the neonatal period.

Cellular cardiomyoplasty: what have we learned?

Restoring blood flow, improving perfusion, reducing clinical symptoms, and augmenting ventricular function are the goals after acute myocardial infarction. Other than cardiac transplantation, no standard clinical procedure is available to restore damaged myocardium. Since we first reported cellular cardiomyoplasty in 1989, successful outcomes have been confirmed by experimental and clinical studies, but definitive long-term efficacy requires large-scale placebo-controlled double-blind randomized trials. On meta-analysis, stem cell-treated groups had significantly improved left ventricular ejection fraction, reduced infarct scar size, and decreased left ventricular end-systolic volume. Fewer myocardial infarctions, deaths, readmissions for heart failure, and repeat revascularizations were additional benefits. Encouraging clinical findings have been reported using satellite or bone marrow stem cells, but understanding of the benefit mechanisms demands additional studies. Adult mammalian ventricular myocardium lacks adequate regeneration capability, and cellular cardiomyoplasty offers a new way to overcome this; the poor retention and engraftment rate and high apoptotic rate of the implanted stem cells limit outcomes. The ideal type and number of cells, optimal timing of cell therapy, and ideal cell delivery method depend on determining the beneficial mechanisms. Cellular cardiomyoplasty has progressed rapidly in the last decade. A critical review may help us to better plan the future direction.