Sca-1 knockout impairs myocardial and cardiac progenitor cell function

Electroacupuncture improves cardiac function after myocardial infarction by regulating the mobilization and migration of endogenous stem cells

OBJECTIVE

The aim of this study was to explore the role and mechanisms of electroacupuncture (EA) in the regulation of chemokines in endogenous stem cell mobilization and myocardial regeneration after myocardial infarction (MI).

METHODS

An MI model was constructed in adult male Sprague-Dawley rats by ligating the left anterior descending coronary artery. After 4 weeks of treatment, echocardiography was used to detect changes in cardiac function, and Masson's trichrome staining was used to detect collagen deposition. In addition, immunofluorescence staining was applied to examine von Willebrand factor (vWF)-positive vessels, the expression of cardiac troponin T (cTnT) and proliferation marker Ki67, and the number of c-kit-positive, C-X-C chemokine receptor type 4 (CXCR4)-positive, and Sca-1-positive endogenous stem cells in the infarcted area. In addition, the expression of stromal cell-derived factor (SDF)-1 and stem cell factor (SCF) was detected.

RESULTS

EA increased the ejection fraction after MI, reduced collagen deposition and cellular apoptosis, and increased the number of blood vessels compared with an untreated model group. EA significantly promoted cellular proliferation, except for myocardial cells, and significantly increased the number of c-kit-, CXCR4- and Sca-1-positive stem cells. Moreover, the expression of SDF-1 and SCF in myocardial tissue in the EA group was significantly higher than that in the (untreated) MI group.

CONCLUSIONS

EA appears to promote angiogenesis and reduce collagen deposition, thus improving the cardiac function of rats with MI. The underlying mechanism of action may involve endogenous stem cell mobilization mediated by SDF-1/CXCR4 and SCF/c-kit.

Transplantation of Skeletal Muscle-Derived Sca-1+/PW1+/Pax7- Interstitial Cells (PICs) Improves Cardiac Function and Attenuates Remodeling in Mice Subjected to Myocardial Infarction

We have previously shown that skeletal muscle-derived Sca-1+/PW1+/Pax7- interstitial cells (PICs) are multi-potent and enhance endogenous repair and regeneration. Here, we investigated the regenerative potential of PICs following intramyocardial transplantation in mice subjected to an acute myocardial infarction (MI). MI was induced through the ligation of the left anterior descending coronary artery in 8-week old male C57BL/6 mice. 5 × 105 eGFP-labelled PICs (MI + PICs; n = 7) or PBS (MI-PBS; n = 7) were injected intramyocardially into the border zone. Sham mice (n = 8) were not subjected to MI, or the transplantation of PICs or PBS. BrdU was administered via osmotic mini-pump for 14 days. Echocardiography was performed prior to surgery (baseline), and 1-, 3- and 6-weeks post-MI and PICs transplantation. Mice were sacrificed at 6 weeks post-MI + PICs transplantation, and heart sections were analysed for fibrosis, hypertrophy, engraftment, proliferation, and differentiation of PICs. A significant (p < 0.05) improvement in ejection fraction (EF) and fractional shortening was observed in the MI-PICs group, compared to MI + PBS group at 6-weeks post MI + PICs transplantation. Infarct size/fibrosis of the left ventricle significantly (p < 0.05) decreased in the MI-PICs group (14.0 ± 2.5%), compared to the MI-PBS group (32.8 ± 2.2%). Cardiomyocyte hypertrophy in the border zone significantly (p < 0.05) decreased in the MI-PICs group compared to the MI-PBS group (330.0 ± 28.5 µM2 vs. 543.5 ± 26.6 µm2), as did cardiomyocyte apoptosis (0.6 ± 0.9% MI-PICs vs. 2.8 ± 0.8% MI-PBS). The number of BrdU+ cardiomyocytes was significantly (p < 0.05) increased in the infarct/border zone of the MI-PICs group (7.0 ± 3.3%), compared to the MI-PBS group (1.7 ± 0.5%). The proliferation index (total BrdU+ cells) was significantly increased in the MI-PICs group compared to the MI-PBS group (27.0 ± 3.4% vs. 7.6 ± 1.0%). PICs expressed and secreted pro-survival and reparative growth factors, supporting a paracrine effect of PICs during recovery/remodeling. Skeletal muscle-derived PICs show significant reparative potential, attenuating cardiac remodelling following transplantation into the infarcted myocardium. PICs can be easily sourced from skeletal muscle and therefore show promise as a potential cell candidate for supporting the reparative and regenerative effects of cell therapies.

Existence of multiple organ aging in animal model of emphysema induced by cigarette smoke extract

INTRODUCTION

It is commonly considered that COPD or at least emphysema represents accelerated lung aging induced in part by oxidative damage from cigarette smoke components. However, the issue if there are any aging signs in other organs in patients with COPD or emphysema remains unclear. The aim of this study is to explore whether there is multiple organ aging in the animal model of emphysema induced by cigarette smoke extract (CSE), and to ascertain the possible mechanisms, if any.

METHODS

The animal model of emphysema was induced by CSE. Histomorphological changes in lung, heart, liver, kidney and spleen tissues were measured after staining with hematoxylin and eosin (H&E). The concentrations of stem cell factor (SCF), CyclinD1 and superoxide dismutase (SOD) in serum were determined by ELISA kit. The expressions of p16 (INK4a), Sca-1, eNOS proteins and mRNA in lung, heart, liver, kidney and spleen tissues were detected by Western blotting and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), respectively. Decitabine (Dec) was applied to examine whether it could alter the changes caused by CSE.

RESULTS

The histomorphology of lung tissue was significantly changed, while other organs exhibited normal structure and histomorphology. The concentrations of SCF, CyclinD1 and SOD in serum were lower in the CSE group than in the control group. The expression levels of p16(INK4a) protein and mRNA in lung, heart, liver, kidney and spleen tissues were higher in the CSE group than in the control group, while the expression levels of Sca-1 and eNOS proteins and mRNA were lower in the CSE group than in the control group, in the tissues described above. Dec could partly alleviate the damages caused by CSE and the degree of alleviation resulted by Dec varied from organ to organ.

CONCLUSIONS

In addition to the aging of the lung tissue in the emphysema animal model induced by CSE, the tissues of the heart, liver, kidney and spleen were also in the progress of aging, but the sensibility and affinity of lung to CSE were higher than those of the other organs. Multiple organ aging may also exist in the animal model of emphysema induced by CSE. DEC can partly alleviate the multiple organ aging caused by CSE.

Mitochondrial protein hyperacetylation underpins heart failure with preserved ejection fraction in mice

Over 50% of patients with heart failure have preserved ejection fraction (HFpEF), rather than reduced ejection fraction (HFrEF). The prevalence of HFpEF continues to increase, while the pathogenic mechanisms underlying HFpEF remain largely elusive and evidence-based therapies are still lacking. This study was designed to investigate the metabolic signature of HFpEF and test the potential therapeutic intervention in a mouse model. By utilizing a "3-Hit" HFpEF mouse model, we observed a global protein hyperacetylation in the HFpEF hearts as compared to the pressure overload-induced HFrEF and adult/aged non-heart failure (NHF) hearts. Acetylome analysis identified that a large proportion of the hyperacetylated proteins (74%) specific to the HFpEF hearts are in mitochondria, and enriched in tricarboxylic acid (TCA) cycle, oxidative phosphorylation (OXPHOS), and fatty acid oxidation. Further study showed that the elevated protein acetylation in the HFpEF hearts was correlated with reduced NAD⁺/NADH ratio, impaired mitochondrial function, and depleted TCA cycle metabolites. Normalization of NAD⁺/NADH ratio by supplementation of nicotinamide riboside (NR) for 30 days downregulated the acetylation level, improved mitochondrial function and ameliorated HFpEF phenotypes. Therefore, our study identified a distinct protein acetylation pattern in the HFpEF hearts, and proposed NR as a promising agent in lowering acetylation and mitigating HFpEF phenotypes in mice.

Stem cell therapies in cardiac diseases: Current status and future possibilities

Cardiovascular diseases represent the world's leading cause of death. In this heterogeneous group of diseases, ischemic cardiomyopathies are the most devastating and prevalent, estimated to cause 17.9 million deaths per year. Despite all biomedical efforts, there are no effective treatments that can replace the myocytes lost during an ischemic event or progression of the disease to heart failure. In this context, cell therapy is an emerging therapeutic alternative to treat cardiovascular diseases by cell administration, aimed at cardiac regeneration and repair. In this review, we will cover more than 30 years of cell therapy in cardiology, presenting the main milestones and drawbacks in the field and signaling future challenges and perspectives. The outcomes of cardiac cell therapies are discussed in three distinct aspects: The search for remuscularization by replacement of lost cells by exogenous adult cells, the endogenous stem cell era, which pursued the isolation of a progenitor with the ability to induce heart repair, and the utilization of pluripotent stem cells as a rich and reliable source of cardiomyocytes. Acellular therapies using cell derivatives, such as microvesicles and exosomes, are presented as a promising cell-free therapeutic alternative.

Smooth muscle cells in atherosclerosis: clones but not carbon copies

Our knowledge of the contribution of vascular smooth muscle cells (SMCs) to atherosclerosis has greatly advanced in the previous decade with the development of techniques allowing for the unambiguous identification and phenotypic characterization of SMC populations within the diseased vascular wall. By performing fate mapping or single-cell transcriptomics studies, or a combination of both, the field has made key observations: SMCs populate atherosclerotic lesions by the selective expansion and investment of a limited number of medial SMCs, which undergo profound and diverse modifications of their original phenotype and function. Thus, if SMCs residing within atherosclerotic lesions and contributing to the disease are clones, they are not carbon copies and can play atheroprotective or atheropromoting roles, depending on the nature of their phenotypic transitions. Tremendous progress has been made in identifying the transcriptional mechanisms biasing SMC fate. In the present review, we have summarized the recent advances in characterizing SMC investment and phenotypic diversity and the molecular mechanisms controlling SMC fate in atherosclerotic lesions. We have also discussed some of the remaining questions associated with these breakthrough observations. These questions include the underlying mechanisms regulating the phenomenon of SMC oligoclonal expansion; whether single-cell transcriptomics is reliable and sufficient to ascertain SMC functions and contributions during atherosclerosis development and progression; and how SMC clonality and phenotypic plasticity affects translational research and the therapeutic approaches developed to prevent atherosclerosis complications. Finally, we have discussed the complementary approaches the field should lean toward by combining single-cell phenotypic categorization and functional studies to understand further the complex SMC behavior and contribution in atherosclerosis.

The role of stem cell antigen-1/Lymphocyte antigen 6A-2/6E-1 knock out in murine epidermis

Stem Cell Antigen-1 (SCA-1) is a central positive marker for isolating stem cells in several tissues in the mouse. However, for the epidermis, this appears to be the opposite since lack of SCA-1 has been shown to identify keratinocyte populations with progenitor characteristics. This study investigates the effect of SCA-1 knockout in murine keratinocytes. We compared Sca-1^EGFP/EGFP knockout and wildtype mice with respect to the three-dimensional morphology of the epidermis, performed functional assays, and generated gene expression profiles on FACS sorted cells. There were no morphological abnormalities on skin, fur, or hair follicles in transgenic knockout mice compared to wild type mice. SCA-1 knockout keratinocytes showed significantly reduced colony-forming efficiency, colony size and proliferation rate in vitro, however, SCA-1 knockout did not alter wound healing efficiency or keratinocyte proliferation rate in vivo. Moreover, gene expression profiling shows that the effect from knockout of SCA-1 in keratinocytes is dissimilar from what has been observed in other tissues. Additionally, tumor assay indicated that SCA-1 knockout decreases the number of formed papillomas. The results indicate a more complex role for SCA-1, which might differ between epidermal keratinocytes during homeostasis and activated conditions.

Cardioprotective effects of genetically engineered cardiac stem cells by spheroid formation on ischemic cardiomyocytes

BACKGROUND

Sca-1+ cardiac stem cells and their limited proliferative potential were major limiting factors for use in various studies.

METHODS

Therefore, the effects of sphere genetically engineered cardiac stem cells (S-GECS) inserted with telomerase reverse transcriptase (TERT) were investigated to examine cardiomyocyte survival under hypoxic conditions. GECS was obtained from hTERT-immortalized Sca-1+ cardiac stem cell (CSC) lines, and S-GECS were generated using poly-HEMA.

RESULTS

The optimal conditions for S-GECS was determined to be 1052 GECS cells/mm² and a 48 h culture period to produce spheroids. Compared to adherent-GECS (A-GECS) and S-GECS showed significantly higher mRNA expression of SDF-1α and CXCR4. S-GECS conditioned medium (CM) significantly reduced the proportion of early and late apoptotic cardiomyoblasts during CoCl₂-induced hypoxic injury; however, gene silencing via CXCR4 siRNA deteriorated the protective effects of S-GECS against hypoxic injury. As downstream pathways of SDF-1α/CXCR4, the Erk and Akt signaling pathways were stimulated in the presence of S-GECS CM. S-GECS transplantation into a rat acute myocardial infarction model improved cardiac function and reduced the fibrotic area. These cardioprotective effects were confirmed to be related with the SDF-1α/CXCR4 pathway.

CONCLUSIONS

Our findings suggest that paracrine factors secreted from transplanted cells may protect host cardiomyoblasts in the infarcted myocardium, contributing to beneficial left ventricle (LV) remodeling after acute myocardial infarction (AMI).

Stem cell antigen/Ly6a protects against bladder fibrosis in mice

We have defined a population of stem cell antigen (Sca)-1⁺/CD34⁺/lin^- mesenchymal stem cells in the mouse urinary bladder. These cells are reduced after partial bladder outlet obstruction (PO). To test the role of Sca-1 expressed by these cells, we analyzed bladders from Sca-1 knockout (KO) mice in both uninjured male mice and male mice subjected to PO. We found that loss of Sca-1 alone had little effect on bladder development or function but reduced the total number of mesenchymal stem cells by 30%. After PO, bladders from Sca-1-null KO male mice were larger, with more collagen and less muscle, than obstructed wild-type mice. Steady-state levels of caldesmon were significantly reduced and levels of fibroblast-specific protein 1 were significantly increased in Sca-1 KO mice compared with wild-type mice after PO. In investigating the effects of PO on cell proliferation, we found that loss of Sca-1 changed the timing of cell division in CD34⁺/lin^-, collagen-producing, and smooth muscle cells. PO in combination with loss of Sca-1 drastically reduced the ability of CD34⁺/lin^- cells to form colonies in vitro. Our findings therefore support the hypothesis that Sca-1 protects the bladder from fibrotic remodeling after obstruction, in part by influencing the proliferation of cells responding to the injury.

Heterogeneity of Adult Cardiac Stem Cells

Cardiac biology and heart regeneration have been intensively investigated and debated in the last 15 years. Nowadays, the well-established and old dogma that the adult heart lacks of any myocyte-regenerative capacity has been firmly overturned by the evidence of cardiomyocyte renewal throughout the mammalian life as part of normal organ cell homeostasis, which is increased in response to injury. Concurrently, reproducible evidences from independent laboratories have convincingly shown that the adult heart possesses a pool of multipotent cardiac stem/progenitor cells (CSCs or CPCs) capable of sustaining cardiomyocyte and vascular tissue refreshment after injury. CSC transplantation in animal models displays an effective regenerative potential and may be helpful to treat chronic heart failure (CHF), obviating at the poor/modest results using non-cardiac cells in clinical trials. Nevertheless, the degree/significance of cardiomyocyte turnover in the adult heart, which is insufficient to regenerate extensive damage from ischemic and non-ischemic origin, remains strongly disputed. Concurrently, different methodologies used to detect CSCs in situ have created the paradox of the adult heart harboring more than seven different cardiac progenitor populations. The latter was likely secondary to the intrinsic heterogeneity of any regenerative cell agent in an adult tissue but also to the confusion created by the heterogeneity of the cell population identified by a single cell marker used to detect the CSCs in situ. On the other hand, some recent studies using genetic fate mapping strategies claimed that CSCs are an irrelevant endogenous source of new cardiomyocytes in the adult. On the basis of these contradictory findings, here we critically reviewed the available data on adult CSC biology and their role in myocardial cell homeostasis and repair.

Changing Metabolism in Differentiating Cardiac Progenitor Cells-Can Stem Cells Become Metabolically Flexible Cardiomyocytes?

The heart is a metabolic omnivore and the adult heart selects the substrate best suited for each circumstance, with fatty acid oxidation preferred in order to fulfill the high energy demand of the contracting myocardium. The fetal heart exists in an hypoxic environment and obtains the bulk of its energy via glycolysis. After birth, the "fetal switch" to oxidative metabolism of glucose and fatty acids has been linked to the loss of the regenerative phenotype. Various stem cell types have been used in differentiation studies, but most are cultured in high glucose media. This does not change in the majority of cardiac differentiation protocols. Despite the fact that metabolic state affects marker expression and cellular function and activity, the substrate composition is currently being overlooked. In this review we discuss changes in cardiac metabolism during development, the various protocols used to differentiate progenitor cells to cardiomyocytes, what is known about stem cell metabolism and how consideration of metabolism can contribute toward maturation of stem cell-derived cardiomyocytes.

[Stem cells: searching predisposition to cardiac commitment by surface markers expression]

It is well-known that cardiovascular diseases are the leading cause of death worldwide, and represent an important economic burden to health systems. In an attempt to solve this problem, stem cell therapy has emerged as a therapeutic option. Within the last 20 years, a great variety of stem cells have been used in different myocardial infarction models. Up until now, the use of cardiac stem cells (CSCs) has seemed to be the best option, but the inaccessibility and scarcity of these cells make their use unreliable. Additionally, there is a high risk as they have to be obtained directly from the heart of the patient. Unlike CSCs, adult stem cells originating from bone marrow or adipose tissue, among others, appear to be an attractive option due to their easier accessibility and abundance, but particularly due to the probable existence of cardiac progenitors among their different sub-populations. In this review an analysis is made of the surface markers present in CSCs compared with other adult stem cells. This suggested the pre-existence of cells sharing specific surface markers with CSCs, a predictable immunophenotype present in some cells, although in low proportions, and with a potential of cardiac differentiation that could be similar to CSCs, thus increasing their therapeutic value. This study highlights new perspectives regarding MSCs that would enable some of these sub-populations to be differentiated at cardiac tissue level.

The current status and future of cardiac stem/progenitor cell therapy for congenital heart defects from diabetic pregnancy

Pregestational maternal diabetes induces congenital heart defects (CHDs). Cardiac dysfunction after palliative surgical procedures contributes to the high mortality of CHD patients. Autologous or allogeneic stem cell therapies are effective for improving cardiac function in animal models and clinical trials. c-kit+ cardiac progenitor cells (CPCs), the most recognized CPCs, have the following basic properties of stem cells: self-renewal, multicellular clone formation, and differentiation into multiple cardiac lineages. However, there is ongoing debate regarding whether c-kit+ CPCs can give rise to sufficient cardiomyocytes. A new hypothesis to address the beneficial effect of c-kit+ CPCs is that these cells stimulate endogenous cardiac cells through a paracrine function in producing a robust secretome and exosomes. The values of other cardiac CPCs, including Sca1+ CPCs and cardiosphere-derived cells, are beginning to be revealed. These cells may be better choices than c-kit+ CPCs for generating cardiomyocytes. Adult mesenchymal stem cells are considered immune-incompetent and effective for improving cardiac function. Autologous CPC therapy may be limited by the observation that maternal diabetes adversely affects the biological function of embryonic stem cells and CPCs. Future studies should focus on determining the mechanistic action of these cells, identifying new CPC markers, selecting highly effective CPCs, and engineering cell-free products.

Cardiac stem cells for myocardial regeneration: promising but not ready for prime time

Remarkable strides have been made in the treatment of ischemic heart disease in decades. As the initial loss of cardiomyocytes associated with myocardial infarction serves as an impetus for myocardial remodeling, the ability to replace these cells with healthy counterparts would represent an effective treatment for many forms of cardiovascular disease. The discovery of cardiac stem cells (that can differentiate into multiple lineages) highlighted the possibility for development of cell-based therapeutics to achieve this ultimate goal. Recent research features cardiac stem cell maintenance, proliferation, and differentiation, as well as direct reprogramming of various somatic cells into cardiomyocytes, all within the context of the holy grail of regeneration of the injured heart. Much work remains to be done, but the future looks bright!

Pericytes and cardiac stem cells: Common features and peculiarities

Clinical data and basic research indicate that the structural and functional alterations that characterize the evolution of cardiac disease towards heart failure may be, at least in part, reversed. This paradigm shift is due to the accumulation of evidence indicating that, in peculiar settings, cardiomyocytes may be replenished. Moving from the consideration that cardiomyocytes are rapidly withdrawn from the cell cycle after birth, independent laboratories have tested the hypothesis that cardiac resident stem/progenitor cells resided in mammalian hearts and were important for myocardial repair. After almost two decades of intensive investigation, several (but partially overlapping) cardiac resident stem/progenitor cell populations have been identified. These primitive cells are characterized by mesenchymal features, unique properties that distinguish them from mesodermal progenitors residing in other tissues, and heterogeneous embryological origins (that include the neural crest and the epicardium). A further layer of complexity is related to the nature, in vivo localization and properties of mesodermal progenitors residing in adult tissues. Intriguingly, these latter, whose possible perivascular pericyte/mural cell origin has been shown, have been identified in human hearts too. However, their exact anatomical localization, pathophysiological role, and their relationship with cardiac stem/progenitor cells are emerging only recently. Therefore, aim of this review is to discuss the different origin, the distinct nature, and the complementary effect of cardiac stem cells and pericytes supporting regenerative strategies based on the combined use of both myogenic and angiogenic factors.

Dual roles for bone marrow-derived Sca-1 cells in cardiac function

Recruitment of stem cells from the bone marrow (BM) is an important aspect of cardiac healing that becomes inefficient with age. We investigated the role of young stem cell antigen 1 (Sca-1)-positive BM cells on the aged heart by microarray analysis after BM reconstitution. Sca-1⁺ and Sca-1^- BM cells from young green fluorescent protein (GFP)-positive mice were used to reconstitute the BM of aged mice. Myocardial infarction (MI) was induced 3 mo later. GFP⁺ cells were more abundant in the BM, blood, and heart of Sca-1⁺ mice, which corresponded to preserved cardiac function after MI. At baseline, Sca-1⁺ BM reconstitution increased cardiac expression of serum response factor, vascular endothelial growth factor A, and myogenic genes, but reduced the expression of Il-1β. After MI, inflammation was identified as a key difference between Sca-1^- and Sca-1⁺ groups, as cytokine expression and cell surface markers associated with inflammatory cells were up-regulated with Sca-1⁺ reconstitution. Mac-3 and F4/80 staining showed that the postinfarction heart was composed of a mixture of GFP⁺ (donor) macrophages, GFP^- (host) macrophages, and GFP⁺ cells that did not contribute to the macrophage population. This study demonstrates that Sca-1⁺ BM cells regulate cardiac healing though an acute inflammatory response and also before injury by stimulating formation of a beneficial cardiac niche.-Tobin, S. W., Li, S.-H., Li, J., Wu, J., Yeganeh, A., Yu, P., Weisel, R. D., Li, R.-K. Dual roles for bone marrow-derived Sca-1 cells in cardiac function.

Cardiac regenerative medicine: At the crossroad of microRNA function and biotechnology

There is an urgent need to develop new therapeutic strategies to stimulate cardiac repair after damage, such as myocardial infarction. Already for more than a century scientist are intrigued by studying the regenerative capacity of the heart. While moving away from the old classification of the heart as a post-mitotic organ, and being inspired by the stem cell research in other scientific fields, mainly three different strategies arose in order to develop regenerative medicine, namely; the use of cardiac stem cells, reprogramming of fibroblasts into cardiomyocytes or direct stimulation of endogenous cardiomyocyte proliferation. MicroRNAs, known to play a role in orchestrating cell fate processes such as proliferation, differentiation and reprogramming, gained a lot of attention in this context the latest years. Indeed, several research groups have independently demonstrated that microRNA-based therapy shows promising results to induce heart tissue regeneration and improve cardiac pump function after myocardial injury. Nowadays, a whole new biotechnology field has been unveiled to investigate the possibilities for efficient, safe and specific delivery of microRNAs towards the heart.

Young Bone-Marrow Sca-1+ Stem Cells Rejuvenate the Aged Heart and Improve Function after Injury through PDGFRβ-Akt pathway

Bone marrow (BM) reconstitution with young BM cells in aged recipients restores the functionality of cardiac resident BM-derived progenitors. This study investigated the cell type primarily responsible for this effect. We reconstituted old mice with BM cells from young or old mice and found that the number of stem cell antigen 1 (Sca-1) cells homing to the heart was significantly greater in young than old chimeras. We then reconstituted old mice with young BM Sca-1⁺ or Sca-1⁻ cells. We found that Sca-1 cells repopulated the recipient BM and homed to the heart. The number of BM-derived cells in the aged myocardium co-expressing PDGFRβ was 3 times greater in Sca-1⁺ than Sca-1⁻ chimeric mice. Sca-1⁺ chimeras had more active cell proliferation in the infarcted heart and improved ventricular function after MI. The improved regeneration involved activation of the PDGFRβ/Akt/p27^Kip1 pathway. Sca-1⁺ stem cells rejuvenated cardiac tissue in aged mice. Restoration of the Sca-1⁺ subset of stem cells by BM reconstitution improved cardiac tissue regeneration after injury in aged mice.

Stem Cells in Regenerative Cardiology

The common prevalence of heart failure and limitations in its treatment are leading cause of attention and interest towards the induction of cardiac regeneration with novel approaches. Recent studies provide growing evidence regarding bona fide cardiac regeneration post genetic manipulations, administration of stimulatory factors and myocardial injuries in animal models and human studies. To this end, stem cells of different sources have been tested to treat heart failure for the development of cellular therapies. Endogenous and exogenous stem cells sources used in regenerative cardiology have provided a proof of concept and applicability of cellular therapies in myocardial improvement. Recent clinical studies, especially, based on the endogenous cardiac progenitor and stem cells highlighted the possibility to regenerate lost cardiomyocytes in the myocardium. This review discusses emerging concepts in cardiac stem cell therapy, their sources and route of administration, and plausibility of de novo cardiomyocyte formation.

Developmental origin and lineage plasticity of endogenous cardiac stem cells

Over the past two decades, several populations of cardiac stem cells have been described in the adult mammalian heart. For the most part, however, their lineage origins and in vivo functions remain largely unexplored. This Review summarizes what is known about different populations of embryonic and adult cardiac stem cells, including KIT(+), PDGFRα(+), ISL1(+)and SCA1(+)cells, side population cells, cardiospheres and epicardial cells. We discuss their developmental origins and defining characteristics, and consider their possible contribution to heart organogenesis and regeneration. We also summarize the origin and plasticity of cardiac fibroblasts and circulating endothelial progenitor cells, and consider what role these cells have in contributing to cardiac repair.

Stem Cell Therapy for the Heart: Blind Alley or Magic Bullet?

When stressed by ageing or disease, the adult human heart is unable to regenerate, leading to scarring and hypertrophy and eventually heart failure. As a result, stem cell therapy has been proposed as an ultimate therapeutic strategy, as stem cells could limit adverse remodelling and give rise to new cardiomyocytes and vasculature. Unfortunately, the results from clinical trials to date have been largely disappointing. In this review, we discuss the current status of the field and describe various limitations and how future work may attempt to resolve these to make way to successful clinical translation.

Bmi1 (+) cardiac progenitor cells contribute to myocardial repair following acute injury

Background

The inability of the adult mammalian heart to replace cells lost after severe cardiac injury compromises organ function. Although the heart is one of the least regenerative organs in the body, evidence accumulated in recent decades indicates a certain degree of renewal after injury. We have evaluated the role of cardiac Bmi1⁺ progenitor cells (Bmi1-CPC) following acute myocardial infarction (AMI).

Methods

Bmi1^Cre/+;Rosa26^YFP/+ (Bmi1-YFP) mice were used for lineage tracing strategy. After tamoxifen (TM) induction, yellow fluorescent protein (YFP) is expressed under the control of Rosa26 regulatory sequences in Bmi1⁺ cells. YFP⁺ cells were tracked following myocardial infarction. Additionally, whole transcriptome analysis of isolated YFP⁺ cells was performed in unchallenged hearts and after myocardial infarction.

Results

Deep-sequencing analysis of Bmi1-CPC from unchallenged hearts suggests that this population expresses high levels of pluripotency markers. Conversely, transcriptome evaluation of Bmi1-CPC following AMI shows a rich representation of genes related to cell proliferation, movement, and cell cycle. Lineage-tracing studies after cardiac infarction show that the progeny of Bmi1-expressing cells contribute to de novo cardiomyocytes (CM) (13.8 ± 5 % new YFP⁺ CM compared to 4.7 ± 0.9 % in age-paired non-infarcted hearts). However, apical resection of TM-induced day 1 Bmi1-YFP pups indicated a very minor contribution of Bmi1-derived cells to de novo CM.

Conclusions

Cardiac Bmi1 progenitor cells respond to cardiac injury, contributing to the generation of de novo CM in the adult mouse heart.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0355-7) contains supplementary material, which is available to authorized users.

Bone Marrow Is a Reservoir for Cardiac Resident Stem Cells

Resident cardiac stem cells (CSCs) represent a responsive stem cell reservoir within the adult myocardium and have a significant function in myocardial homeostasis and injury. However, the distribution, origin, homing and possible therapeutic benefits of CSCs are still under discussion. Here we investigated whether bone marrow (BM) stem cells could contribute to repopulating the pool of CSCs in heart. The engraftment of BM cells in heart was detected at a low level after BM transplantation (BMT) and ischemia/reperfusion (I/R) could increase BM cells engraftment but not significant. We clarified that more than 50% CSCs are derived from BM and confirmed that BM-derived CSCs have similar characteristics with the host CSCs. Furthermore, we transplanted BM-derived CSCs into heart ischemia models and presented evidence for the first time that BM-derived CSCs can differentiate into cardiomyocytes in vivo. In conclusions, BM stem cells could be a potential back-up source of CSCs for restoring heart function after injury or maintaining homeostasis of CSCs.

Effects of vitamin A deficiency in the postnatal mouse heart: role of hepatic retinoid stores

To determine whether hepatic depletion of vitamin A (VA) stores has an effect on the postnatal heart, studies were carried out with mice lacking liver retinyl ester stores fed either a VA-sufficient (LRVAS) or VA-deficient (LRVAD) diet (to deplete circulating retinol and extrahepatic stores of retinyl esters). There were no observable differences in the weights or gross morphology of hearts from LRVAS or LRVAD mice relative to sex-matched, age-matched, and genetically matched wild-type (WT) controls fed the VAS diet (WTVAS), but changes in the transcription of functionally relevant genes were consistent with a state of VAD in LRVAS and LRVAD ventricles. In silico analysis revealed that 58/67 differentially expressed transcripts identified in a microarray screen are products of genes that have DNA retinoic acid response elements. Flow cytometric analysis revealed a significant and cell-specific increase in the number of proliferating Sca-1 cardiac progenitor cells in LRVAS animals relative to WTVAS controls. Before myocardial infarction, LRVAS and WTVAS mice had similar cardiac systolic function and structure, as measured by echocardiography, but, unexpectedly, repeat echocardiography demonstrated that LRVAS mice had less adverse remodeling by 1 wk after myocardial infarction. Overall, the results demonstrate that the adult heart is responsive to retinoids, and, most notably, reducing hepatic VA stores (while maintaining circulating levels of VA) impacts ventricular gene expression profiles, progenitor cell numbers, and response to injury.

Cardiac Stem Cell Secretome Protects Cardiomyocytes from Hypoxic Injury Partly via Monocyte Chemotactic Protein-1-Dependent Mechanism

Cardiac stem cells (CSCs) were known to secrete diverse paracrine factors leading to functional improvement and beneficial left ventricular remodeling via activation of the endogenous pro-survival signaling pathway. However, little is known about the paracrine factors secreted by CSCs and their roles in cardiomyocyte survival during hypoxic condition mimicking the post-myocardial infarction environment. We established Sca-1+/CD31- human telomerase reverse transcriptase-immortalized CSCs (Sca-1+/CD31- CSCs(hTERT)), evaluated their stem cell properties, and paracrine potential in cardiomyocyte survival during hypoxia-induced injury. Sca-1+/CD31- CSCs(hTERT) sustained proliferation ability even after long-term culture exceeding 100 population doublings, and represented multi-differentiation potential into cardiomyogenic, endothelial, adipogenic, and osteogenic lineages. Dominant factors secreted from Sca-1+/CD31- CSCs(hTERT) were EGF, TGF-β1, IGF-1, IGF-2, MCP-1, HGF R, and IL-6. Among these, MCP-1 was the most predominant factor in Sca-1+/CD31- CSCs(hTERT) conditioned medium (CM). Sca-1+/CD31- CSCs(hTERT) CM increased survival and reduced apoptosis of HL-1 cardiomyocytes during hypoxic injury. MCP-1 silencing in Sca-1+/CD31- CSCs(hTERT) CM resulted in a significant reduction in cardiomyocyte apoptosis. We demonstrated that Sca-1+/CD31- CSCs(hTERT) exhibited long-term proliferation capacity and multi-differentiation potential. Sca-1+/CD31- CSCs(hTERT) CM protected cardiomyocytes from hypoxic injury partly via MCP-1-dependent mechanism. Thus, they are valuable sources for in vitro and in vivo studies in the cardiovascular field.

Organization, evolution and functions of the human and mouse Ly6/uPAR family genes

Members of the lymphocyte antigen-6 (Ly6)/urokinase-type plasminogen activator receptor (uPAR) superfamily of proteins are cysteine-rich proteins characterized by a distinct disulfide bridge pattern that creates the three-finger Ly6/uPAR (LU) domain. Although the Ly6/uPAR family proteins share a common structure, their expression patterns and functions vary. To date, 35 human and 61 mouse Ly6/uPAR family members have been identified. Based on their subcellular localization, these proteins are further classified as GPI-anchored on the cell membrane, or secreted. The genes encoding Ly6/uPAR family proteins are conserved across different species and are clustered in syntenic regions on human chromosomes 8, 19, 6 and 11, and mouse Chromosomes 15, 7, 17, and 9, respectively. Here, we review the human and mouse Ly6/uPAR family gene and protein structure and genomic organization, expression, functions, and evolution, and introduce new names for novel family members.

MiR218 Modulates Wnt Signaling in Mouse Cardiac Stem Cells by Promoting Proliferation and Inhibiting Differentiation through a Positive Feedback Loop

MiRNA expression was determined in both proliferating and differentiated cardiac stem cells (CSCs) through a comprehensive miRNA microarray analysis. We selected miR218 for functional follow-up studies to examine its significance in CSCs. First, we observed that the expression of miR218 was altered in CSCs during differentiation into cardiomyocytes, and transfection of an miR218 mimic or miR218 inhibitor affected the myocardial differentiation of CSCs. Furthermore, we observed that a negative regulator of Wnt signaling, sFRP2, was a direct target of miR218, and the protein levels of sFRP2 were increased in cells transfected with the synthetic miR218 inhibitor. In contrast, transfection with the miR218 mimic decreased the expression of sFRP2 and potentiated Wnt signaling. The subsequent down-regulation of sFRP2 by shRNA potentiated Wnt signaling, contributing to a gene expression program that is important for CSC proliferation and cardiac differentiation. Specifically, canonical Wnt signaling induced miR218 transcription. Thus, miR218 and Wnt signaling were coupled through a feed-forward positive feedback loop, forming a biological regulatory circuit. Together, these results provide the first evidence that miR218 plays an important role in CSC proliferation and differentiation through the canonical Wnt signaling pathway.

[Ageing: a matter of heart?]

Ageing is considered as a major risk factor for the development of chronic diseases. Among these, heart failure seems to be particularly important for both triggering and accelerating pathological ageing. In the present review, we give a general overview of the most relevant results concerning the mechanism of normal and premature senescence of cardiomyocytes and cardiac stromal cells. In particular, we will address the role of telomere dysfunction, DNA damage response, impairment of mitochondrial function, miRNAs and secretome of senescent cells in cardiac ageing and failure.

miR-300 mediates Bmi1 function and regulates differentiation in primitive cardiac progenitors

B lymphoma Mo-MLV insertion region 1 (Bmi1) is a polycomb-family transcriptional factor critical for self-renewal in many adult stem cells and human neoplasia. We sought to identify microRNAs regulated by Bmi1 that could play a role in multipotent cardiac progenitor cell (CPC) decisions. We found that miR-300, a poorly characterized microRNA mapping in the Dlk1-Dio3 microRNA cluster, was positively regulated by Bmi1 in CPCs. Forced expression of miR-300 in CPCs promoted an improved stemness signature with a significant increase in Oct4 levels, a reduction in senescence progression and an enhanced proliferative status via p19 activation and inhibition of p16 accumulation. Endothelial and cardiogenic differentiation were clearly compromised by sustained miR-300 expression. Additionally, RNA and protein analysis revealed a significant reduction in key cardiac transcription factors, including Nkx2.5 and Tbx5. Collectively, these results suggest that some functions attributed to Bmi1 are due to induction of miR-300, which decreases the cardiogenic differentiation potential of multipotent CPCs in vitro and promotes self-renewal.

Cardiac Bmi1(+) cells contribute to myocardial renewal in the murine adult heart

INTRODUCTION

The mammalian adult heart maintains a continuous, low cardiomyocyte turnover rate throughout life. Although many cardiac stem cell populations have been studied, the natural source for homeostatic repair has not yet been defined. The Polycomb protein BMI1 is the most representative marker of mouse adult stem cell systems. We have evaluated the relevance and role of cardiac Bmi1 (+) cells in cardiac physiological homeostasis.

METHODS

Bmi1 (CreER/+);Rosa26 (YFP/+) (Bmi1-YFP) mice were used for lineage tracing strategy. After tamoxifen (TM) induction, yellow fluorescent protein (YFP) is expressed under the control of Rosa26 regulatory sequences in Bmi1 (+) cells. These cells and their progeny were tracked by FACS, immunofluorescence and RT-qPCR techniques from 5 days to 1 year.

RESULTS

FACS analysis of non-cardiomyocyte compartment from TM-induced Bmi1-YFP mice showed a Bmi1 (+)-expressing cardiac progenitor cell (Bmi1-CPC: B-CPC) population, SCA-1 antigen-positive (95.9 ± 0.4 %) that expresses some stemness-associated genes. B-CPC were also able to differentiate in vitro to the three main cardiac lineages. Pulse-chase analysis showed that B-CPC remained quite stable for extended periods (up to 1 year), which suggests that this Bmi1 (+) population contains cardiac progenitors with substantial self-maintenance potential. Specific immunostaining of Bmi1-YFP hearts serial sections 5 days post-TM induction indicated broad distribution of B-CPC, which were detected in variably sized clusters, although no YFP(+) cardiomyocytes (CM) were detected at this time. Between 2 to 12 months after TM induction, YFP(+) CM were clearly identified (3 ± 0.6 % to 6.7 ± 1.3 %) by immunohistochemistry of serial sections and by flow cytometry of total freshly isolated CM. B-CPC also contributed to endothelial and smooth muscle (SM) lineages in vivo.

CONCLUSIONS

High Bmi1 expression identifies a non-cardiomyocyte resident cardiac population (B-CPC) that contributes to the main lineages of the heart in vitro and in vivo.

Cardioprotective Actions of TGFβRI Inhibition Through Stimulating Autocrine/Paracrine of Survivin and Inhibiting Wnt in Cardiac Progenitors

Heart failure due to myocardial infarction (MI) is a major cause of morbidity and mortality in the world. We found previously that A83-01, a TGFβRI inhibitor, could facilitate cardiac repair in post-MI mice and induce the expansion of a Nkx2.5 + cardiomyoblast population. This study aimed to investigate the key autocrine/paracrine factors regulated by A83-01 in the injured heart and the mechanism of cardioprotection by this molecule. Using a previously described transgenic Nkx2.5 enhancer-green fluorescent protein (GFP) reporter mice, we isolated cardiac progenitor cells (CPC) including Nkx2.5-GFP + (Nkx2.5+), sca1+, and Nkx2.5+/sca1 + cells. A83-01 was found to induce proliferation of these three subpopulations mainly through increasing Birc5 expression in the MEK/ERK-dependent pathway. Survivin, encoded by Birc5, could also directly proliferate Nkx2.5 + cells and enhance cultured cardiomyocytes viability. A83-01 could also reverse the downregulation of Birc5 in postinjured mice hearts (n = 6) to expand CPCs. Moreover, the increased Wnt3a in postinjured hearts could decrease CPCs, which could be reversed by A83-01 via inhibiting Fzd6 and Wnt1-induced signaling protein 1 expressions in CPCs. Next, we used inducible αMHC-cre/mTmG mice to label cardiomyocytes with GFP and nonmyocytes with RFP. We found A83-01 preserved more GFP + myocytes (68.6% ± 3.1% vs. 80.9% ± 3.0%; p < .05, n = 6) and fewer renewed RFP + myocytes (0.026% ± 0.005% vs. 0.062% ± 0.008%; p < .05, n = 6) in parallel with less cardiac fibrosis in isoprenaline-injected mice treated with A83-01. TGFβRI inhibition in an injured adult heart could both stimulate the autocrine/paracrine activity of survivin and inhibit Wnt in CPCs to mediate cardioprotection and improve cardiac function.

Making it stick: chasing the optimal stem cells for cardiac regeneration

Despite the increasing use of stem cells for regenerative-based cardiac therapy, the optimal stem cell population(s) remains in a cloud of uncertainty. In the past decade, the field has witnessed a surge of researchers discovering stem cell populations reported to directly and/or indirectly contribute to cardiac regeneration through processes of cardiomyogenic commitment and/or release of cardioprotective paracrine factors. This review centers upon defining basic biological characteristics of stem cells used for sustaining cardiac integrity during disease and maintenance of communication between the cardiac environment and stem cells. Given the limited successes achieved so far in regenerative therapy, the future requires development of unprecedented concepts involving combinatorial approaches to create and deliver the optimal stem cell(s) that will enhance myocardial healing.

Decitabine enhances stem cell antigen-1 expression in cigarette smoke extract-induced emphysema in animal model

Stem cell antigen-1 (Sca-1) is a mouse glycosyl phosphatidylinositol-anchored protein and a cell surface marker found on hematopoietic stem cells (HSCs). Despite decades of study, its biological functions remain little known. Sca-1 is a typical marker of bone marrow-derived HSCs, it is also expressed by a mixture of tissue-resident stem, progenitor cells in nonhematopoietic organs. Endothelial progenitor cell (EPC) is a subtype of HSC and contributes to endothelial repair by homing in on locations of injury. Abnormal genetic methylation has been detected in smoking-related diseases. The present study aimed to investigate the lung function and histomorphology, the expression of Sca-1 gene in lung tissues, and bone marrow-derived EPCs in cigarette smoke extract (CSE)-induced emphysema mice, and to further determine whether Decitabine (Dec), the most widely used inhibitor of DNA methylation, could protect against the damages caused by CSE. The results of the present study demonstrated that Dec could partly protect against CSE-induced emphysema in mice, enhance Sca-1 expression in lung tissue, and bone marrow-derived EPCs. The results suggested that the depletion of the progenitor cell pool and DNA methylation of Sca-1 gene may be involved in the progression of emphysema in mice.

Inhibition of Wnt6 by Sfrp2 regulates adult cardiac progenitor cell differentiation by differential modulation of Wnt pathways

Wnt signaling has recently emerged as an important regulator of cardiac progenitor cell proliferation and differentiation, but the exact mechanisms by which Wnt signaling modulates these effects are not known. Understanding these mechanisms is essential for advancing our knowledge of cardiac progenitor cell biology and applying this knowledge to enhance cardiac therapy. Here, we explored the effects of Sfrp2, a canonical Wnt inhibitor, in adult cardiac progenitor cell (CPC) differentiation and investigated the molecular mechanisms involved. Our data show that Sfrp2 treatment can promote differentiation of CPCs after ischemia-reperfusion injury. Treatment of CPCs with Sfrp2 inhibited CPC proliferation and primed them for cardiac differentiation. Sfrp2 binding to Wnt6 and inhibition of Wnt6 canonical pathway was essential for the inhibition of CPC proliferation. This inhibition of Wnt6 canonical signaling by Sfrp2 was important for activation of the non-canonical Wnt/Planar Cell Polarity (PCP) pathway through JNK, which in turn induced expression of cardiac transcription factors and CPC differentiation. Taken together, these results demonstrate a novel role of Sfrp2 and Wnt6 in regulating the dynamic process of CPC proliferation and differentiation, as well as providing new insights into the mechanisms of Wnt signaling in cardiac differentiation.

Negative Regulation of TGFβ Signaling by Stem Cell Antigen-1 Protects against Ischemic Acute Kidney Injury

Acute kidney injury, often caused by an ischemic insult, is associated with significant short-term morbidity and mortality, and increased risk of chronic kidney disease. The factors affecting the renal response to injury following ischemia and reperfusion remain to be clarified. We found that the Stem cell antigen-1 (Sca-1), commonly used as a stem cell marker, is heavily expressed in renal tubules of the adult mouse kidney. We evaluated its potential role in the kidney using Sca-1 knockout mice submitted to acute ischemia reperfusion injury (IRI), as well as cultured renal proximal tubular cells in which Sca-1 was stably silenced with shRNA. IRI induced more severe injury in Sca-1 null kidneys, as assessed by increased expression of Kim-1 and Ngal, rise in serum creatinine, abnormal pathology, and increased apoptosis of tubular epithelium, and persistent significant renal injury at day 7 post IRI, when recovery of renal function in control animals was nearly complete. Serum creatinine, Kim-1 and Ngal were slightly but significantly elevated even in uninjured Sca-1-/- kidneys. Sca-1 constitutively bound both TGFβ receptors I and II in cultured normal proximal tubular epithelial cells. Its genetic loss or silencing lead to constitutive TGFβ receptor—mediated activation of canonical Smad signaling even in the absence of ligand and to KIM-1 expression in the silenced cells. These studies demonstrate that by normally repressing TGFβ-mediated canonical Smad signaling, Sca-1 plays an important in renal epithelial cell homeostasis and in recovery of renal function following ischemic acute kidney injury.

Forward Programming of Cardiac Stem Cells by Homogeneous Transduction with MYOCD plus TBX5

Adult cardiac stem cells (CSCs) express many endogenous cardiogenic transcription factors including members of the Gata, Hand, Mef2, and T-box family. Unlike its DNA-binding targets, Myocardin (Myocd)-a co-activator not only for serum response factor, but also for Gata4 and Tbx5-is not expressed in CSCs. We hypothesised that its absence was a limiting factor for reprogramming. Here, we sought to investigate the susceptibility of adult mouse Sca1+ side population CSCs to reprogramming by supplementing the triad of GATA4, MEF2C, and TBX5 (GMT), and more specifically by testing the effect of the missing co-activator, Myocd. Exogenous factors were expressed via doxycycline-inducible lentiviral vectors in various combinations. High throughput quantitative RT-PCR was used to test expression of 29 cardiac lineage markers two weeks post-induction. GMT induced more than half the analysed cardiac transcripts. However, no protein was detected for the induced sarcomeric genes Actc1, Myh6, and Myl2. Adding MYOCD to GMT affected only slightly the breadth and level of gene induction, but, importantly, triggered expression of all three proteins examined (α-cardiac actin, atrial natriuretic peptide, sarcomeric myosin heavy chains). MYOCD + TBX was the most effective pairwise combination in this system. In clonal derivatives homogenously expressing MYOCD + TBX at high levels, 93% of cardiac transcripts were up-regulated and all five proteins tested were visualized.

IN SUMMARY

(1) GMT induced cardiac genes in CSCs, but not cardiac proteins under the conditions used. (2) Complementing GMT with MYOCD induced cardiac protein expression, indicating a more complete cardiac differentiation program. (3) Homogeneous transduction with MYOCD + TBX5 facilitated the identification of differentiating cells and the validation of this combinatorial reprogramming strategy. Together, these results highlight the pivotal importance of MYOCD in driving CSCs toward a cardiac muscle fate.

Cardiac stem cells: translation to human studies

The discovery of multiple classes of cardiac progenitor cells in the adult mammalian heart has generated hope for their use as a therapeutic in heart failure. However, successful results from animal models have not always yielded similar findings in human studies. Recent Phase I/II trials of c-Kit (SCIPIO) and cardiosphere-based (CADUCEUS) cardiac progenitor cells have demonstrated safety and some therapeutic efficacy. Gaps remain in our understanding of the origins, function and relationships between the different progenitor cell families, many of which are heterogeneous populations with overlapping definitions. Another challenge lies in the limitations of small animal models in replicating the human heart. Cryopreserved human cardiac tissue provides a readily available source of cardiac progenitor cells and may help address these questions. We review important findings and relative unknowns of the main classes of cardiac progenitor cells, highlighting differences between animal and human studies.

Cardiac stem cell niches

The critical role that stem cell niches have in cardiac homeostasis and myocardial repair following injury is the focus of this review. Cardiac niches represent specialized microdomains where the quiescent and activated state of resident stem cells is regulated. Alterations in niche function with aging and cardiac diseases result in abnormal sites of cardiomyogenesis and inadequate myocyte formation. The relevance of Notch1 signaling, gap-junction formation, HIF-1α and metabolic state in the regulation of stem cell growth and differentiation within the cardiac niches are discussed.

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.

Targeting pleiotropic signaling pathways to control adult cardiac stem cell fate and function

The identification of different pools of cardiac progenitor cells resident in the adult mammalian heart opened a new era in heart regeneration as a means to restore the loss of functional cardiac tissue and overcome the limited availability of donor organs. Indeed, resident stem cells are believed to participate to tissue homeostasis and renewal in healthy and damaged myocardium although their actual contribution to these processes remain unclear. The poor outcome in terms of cardiac regeneration following tissue damage point out at the need for a deeper understanding of the molecular mechanisms controlling CPC behavior and fate determination before new therapeutic strategies can be developed. The regulation of cardiac resident stem cell fate and function is likely to result from the interplay between pleiotropic signaling pathways as well as tissue- and cell-specific regulators. Such a modular interaction-which has already been described in the nucleus of a number of different cells where transcriptional complexes form to activate specific gene programs-would account for the unique responses of cardiac progenitors to general and tissue-specific stimuli. The study of the molecular determinants involved in cardiac stem/progenitor cell regulatory mechanisms may shed light on the processes of cardiac homeostasis in health and disease and thus provide clues on the actual feasibility of cardiac cell therapy through tissue-specific progenitors.

Re-activated adult epicardial progenitor cells are a heterogeneous population molecularly distinct from their embryonic counterparts

Cardiovascular disease remains the major cause of mortality, and cardiac cell therapy has recently emerged as a paradigm for heart repair. The epicardium is a layer of mesothelial cells covering the heart that during development contributes to different cardiovascular lineages, including cardiomyocytes, but which becomes quiescent after birth. We previously revealed that the peptide thymosin beta 4 (Tβ4) can reactivate adult epicardium-derived cells (EPDCs) after myocardial infarction (MI), to proliferate, and differentiate into cardiovascular derivatives. The aim of this study was to provide a lineage characterization of the adult EPDCs relative to the embryonic epicardial lineage and to determine prospective cell fate biases within the activated adult population during cardiovascular repair. Wt1(GFPCre/+) mice were primed with Tβ4 and MI induced by ligation of the left anterior descending coronary artery. Adult WT1(+) GFP(+) EPDCs were fluorescence-activated cell sorted (FACS) at 2, 4, and 7 days after MI. Embryonic WT1(+) GFP(+) EPDCs were isolated from embryonic hearts (E12.5) by FACS, and sorted cells were characterized by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-qPCR) and immunostaining. Adult WT1(+) GFP(+) EPDCs were highly heterogeneous, expressing cardiac progenitor and mesenchymal stem markers. Based on the expression of stem cell antigen-1 (Sca-1), CD44, and CD90, we identified different subpopulations of EPDCs of varying cardiovascular potential, according to marker gene profiles, with a molecular phenotype distinct from the source embryonic epicardial cells at E12.5. Thus, adult WT1(+) GFP(+) cells are a heterogeneous population that when activated can restore an embryonic gene programme, but do not revert entirely to adopt an embryonic phenotype. Potential biases in cardiovascular cell fate suggest that discrete subpopulations of EPDCs might be clinically relevant for regenerative therapy.

Stimulation of endogenous cardioblasts by exogenous cell therapy after myocardial infarction

Controversy surrounds the identity, origin, and physiologic role of endogenous cardiomyocyte progenitors in adult mammals. Using an inducible genetic labeling approach to identify small non-myocyte cells expressing cardiac markers, we find that activated endogenous cardioblasts are rarely evident in the normal adult mouse heart. However, myocardial infarction results in significant cardioblast activation at the site of injury. Genetically labeled isolated cardioblasts express cardiac transcription factors and sarcomeric proteins, exhibit spontaneous contractions, and form mature cardiomyocytes in vivo after injection into unlabeled recipient hearts. The activated cardioblasts do not arise from hematogenous seeding, cardiomyocyte dedifferentiation, or mere expansion of a preformed progenitor pool. Cell therapy with cardiosphere-derived cells amplifies innate cardioblast-mediated tissue regeneration, in part through the secretion of stromal cell-derived factor 1 by transplanted cells. Thus, stimulation of endogenous cardioblasts by exogenous cells mediates therapeutic regeneration of injured myocardium.

Cardiogenic genes expressed in cardiac fibroblasts contribute to heart development and repair

RATIONALE

Cardiac fibroblasts are critical to proper heart function through multiple interactions with the myocardial compartment, but appreciation of their contribution has suffered from incomplete characterization and lack of cell-specific markers.

OBJECTIVE

To generate an unbiased comparative gene expression profile of the cardiac fibroblast pool, identify and characterize the role of key genes in cardiac fibroblast function, and determine their contribution to myocardial development and regeneration.

METHODS AND RESULTS

High-throughput cell surface and intracellular profiling of cardiac and tail fibroblasts identified canonical mesenchymal stem cell and a surprising number of cardiogenic genes, some expressed at higher levels than in whole heart. While genetically marked fibroblasts contributed heterogeneously to interstitial but not cardiomyocyte compartments in infarcted hearts, fibroblast-restricted depletion of one highly expressed cardiogenic marker, T-box 20, caused marked myocardial dysmorphology and perturbations in scar formation on myocardial infarction.

CONCLUSIONS

The surprising transcriptional identity of cardiac fibroblasts, the adoption of cardiogenic gene programs, and direct contribution to cardiac development and repair provoke alternative interpretations for studies on more specialized cardiac progenitors, offering a novel perspective for reinterpreting cardiac regenerative therapies.

Resident cardiac stem cells and their role in stem cell therapies for myocardial repair

Despite advances in treatment, heart failure remains one of the top killers in Canada. This recognition motivated a new research focus to harness the fundamental repair properties of the human heart. Since then, cardiac stem cells (CSCs) have emerged as a promising cell candidate to regenerate damaged hearts. The rationale of this approach is simple with ex vivo amplification of CSCs from clinical-grade biopsies, followed by delivery to areas of injury, where they engraft and regenerate the heart. In this review we will summarize recent advances and discuss future developments in CSC-mediated cardiac repair to treat the growing number of Canadians living with and dying from heart failure.

Prostaglandin E₂ promotes post-infarction cardiomyocyte replenishment by endogenous stem cells

Although self-renewal ability of adult mammalian heart has been reported, few pharmacological treatments are known to promote cardiomyocyte regeneration after injury. In this study, we demonstrate that the critical period of stem/progenitor cell-mediated cardiomyocyte replenishment is initiated within 7 days and saturates on day 10 post-infarction. Moreover, blocking the inflammatory reaction with COX-2 inhibitors may also reduce the capability of endogenous stem/progenitor cells to repopulate lost cells. Injection of the COX-2 product PGE₂ enhances cardiomyocyte replenishment in young mice and recovers cell renewal through attenuating TGF-β1 signaling in aged mice. Further analyses suggest that cardiac stem cells are PGE₂-responsive and that PGE₂ may regulate stem cell activity directly through the EP2 receptor or indirectly by modulating its micro-environment in vivo. Our findings provide evidence that PGE₂ holds great potential for cardiac regeneration.