Evolution of the c-kit-positive cell response to pathological challenge in the myocardium

Cardiac progenitor cell therapy: mechanisms of action

Heart failure (HF) is an end-stage of many cardiac diseases and one of the main causes of death worldwide. The current management of this disease remains suboptimal. The adult mammalian heart was considered a post-mitotic organ. However, several reports suggest that it may possess modest regenerative potential. Adult cardiac progenitor cells (CPCs), the main players in the cardiac regeneration, constitute, as it may seem, a heterogenous group of cells, which remain quiescent in physiological conditions and become activated after an injury, contributing to cardiomyocytes renewal. They can mediate their beneficial effects through direct differentiation into cardiac cells and activation of resident stem cells but majorly do so through paracrine release of factors. CPCs can secrete cytokines, chemokines, and growth factors as well as exosomes, rich in proteins, lipids and non-coding RNAs, such as miRNAs and YRNAs, which contribute to reparation of myocardium by promoting angiogenesis, cardioprotection, cardiomyogenesis, anti-fibrotic activity, and by immune modulation. Preclinical studies assessing cardiac progenitor cells and cardiac progenitor cells-derived exosomes on damaged myocardium show that administration of cardiac progenitor cells-derived exosomes can mimic effects of cell transplantation. Exosomes may become new promising therapeutic strategy for heart regeneration nevertheless there are still several limitations as to their use in the clinic. Key questions regarding their dosage, safety, specificity, pharmacokinetics, pharmacodynamics and route of administration remain outstanding. There are still gaps in the knowledge on basic biology of exosomes and filling them will bring as closer to translation into clinic.

Bone marrow-derived c-kit positive stem cell administration protects against diabetes-induced nephropathy in a rat model by reversing PI3K/AKT/GSK-3β pathway and inhibiting cell apoptosis

Stem cell-based therapy has been proposed as a novel therapeutic strategy for diabetic nephropathy. This study was designed to evaluate the effect of systemic administration of rat bone marrow-derived c-kit positive (c-kit+) cells on diabetic nephropathy in male rats, focusing on PI3K/AKT/GSK-3β pathway and apoptosis as a possible therapeutic mechanism. Twenty-eight animals were randomly classified into four groups: Control group (C), diabetic group (D), diabetic group, intravenously received 50 μl phosphate-buffered saline (PBS) containing 3 × 105 c-kit- cells (D + ckit-); and diabetic group, intravenously received 50 μl PBS containing 3 × 105 c-Kit positive cells (D + ckit+). Control and diabetic groups intravenously received 50 μl PBS. C-kit+ cell therapy could reduce renal fibrosis, which was associated with attenuation of inflammation as indicated by decreased TNF-α and IL-6 levels in the kidney tissue. In addition, c-kit+ cells restored the expression levels of PI3K, pAKT, and GSK-3β proteins. Furthermore, renal apoptosis was decreased following c-kit+ cell therapy, evidenced by the lower apoptotic index in parallel with the increased Bcl-2 and decreased Bax and Caspase-3 levels. Our results showed that in contrast to c-kit- cells, the administration of c-kit+ cells ameliorate diabetic nephropathy and suggested that c-kit+ cells could be an alternative cell source for attenuating diabetic nephropathy.

Transcriptomic Characterization of Genes Regulating the Stemness in Porcine Atrial Cardiomyocytes during Primary In Vitro Culture

Heart failure remains a major cause of death worldwide. There is a need to establish new management options as current treatment is frequently suboptimal. Clinical approaches based on autologous stem cell transplant is potentially a good alternative. The heart was long considered an organ unable to regenerate and renew. However, several reports imply that it may possess modest intrinsic regenerative potential. To allow for detailed characterization of cell cultures, whole transcriptome profiling was performed after 0, 7, 15, and 30 days of in vitro cell cultures (IVC) from the right atrial appendage and right atrial wall utilizing microarray technology. In total, 4239 differentially expressed genes (DEGs) with ratio > abs |2| and adjusted p-value ≤ 0.05 for the right atrial wall and 4662 DEGs for the right atrial appendage were identified. It was shown that a subset of DEGs, which have demonstrated some regulation of expression levels with the duration of the cell culture, were enriched in the following GO BP (Gene Ontology Biological Process) terms: "stem cell population maintenance" and "stem cell proliferation". The results were validated by RT-qPCR. The establishment and detailed characterization of in vitro culture of myocardial cells may be important for future applications of these cells in heart regeneration processes.

'Youthful' phenotype of c-Kit+ cardiac fibroblasts

Cardiac fibroblast (CF) population heterogeneity and plasticity present a challenge for categorization of biological and functional properties. Distinct molecular markers and associated signaling pathways provide valuable insight for CF biology and interventional strategies to influence injury response and aging-associated remodeling. Receptor tyrosine kinase c-Kit mediates cell survival, proliferation, migration, and is activated by pathological injury. However, the biological significance of c-Kit within CF population has not been addressed. An inducible reporter mouse detects c-Kit promoter activation with Enhanced Green Fluorescent Protein (EGFP) expression in cardiac cells. Coincidence of EGFP and c-Kit with the DDR2 fibroblast marker was confirmed using flow cytometry and immunohistochemistry. Subsequently, CFs expressing DDR2 with or without c-Kit was isolated and characterized. A subset of DDR2+ CFs also express c-Kit with coincidence in ~ 8% of total cardiac interstitial cells (CICs). Aging is associated with decreased number of c-Kit expressing DDR2+ CFs, whereas pathological injury induces c-Kit and DDR2 as well as the frequency of coincident expression in CICs. scRNA-Seq profiling reveals the transcriptome of c-Kit expressing CFs as cells with transitional phenotype. Cultured cardiac DDR2+ fibroblasts that are c-Kit+ exhibit morphological and functional characteristics consistent with youthful phenotypes compared to c-Kit- cells. Mechanistically, c-Kit expression correlates with signaling implicated in proliferation and cell migration, including phospho-ERK and pro-caspase 3. The phenotype of c-kit+ on DDR2+ CFs correlates with multiple characteristics of 'youthful' cells. To our knowledge, this represents the first evaluation of c-Kit biology within DDR2+ CF population and provides a fundamental basis for future studies to influence myocardial biology, response to pathological injury and physiological aging.

In vitro CSC-derived cardiomyocytes exhibit the typical microRNA-mRNA blueprint of endogenous cardiomyocytes

miRNAs modulate cardiomyocyte specification by targeting mRNAs of cell cycle regulators and acting in cardiac muscle lineage gene regulatory loops. It is unknown if or to-what-extent these miRNA/mRNA networks are operative during cardiomyocyte differentiation of adult cardiac stem/progenitor cells (CSCs). Clonally-derived mouse CSCs differentiated into contracting cardiomyocytes in vitro (iCMs). Comparison of "CSCs vs. iCMs" mRNome and microRNome showed a balanced up-regulation of CM-related mRNAs together with a down-regulation of cell cycle and DNA replication mRNAs. The down-regulation of cell cycle genes and the up-regulation of the mature myofilament genes in iCMs reached intermediate levels between those of fetal and neonatal cardiomyocytes. Cardiomyo-miRs were up-regulated in iCMs. The specific networks of miRNA/mRNAs operative in iCMs closely resembled those of adult CMs (aCMs). miR-1 and miR-499 enhanced myogenic commitment toward terminal differentiation of iCMs. In conclusions, CSC specification/differentiation into contracting iCMs follows known cardiomyo-MiR-dependent developmental cardiomyocyte differentiation trajectories and iCMs transcriptome/miRNome resembles that of CMs.

Hypoxia Inducible Factors as Central Players in the Pathogenesis and Pathophysiology of Cardiovascular Diseases

Cardiovascular (CV) diseases are the major cause of death in industrialized countries. The main function of the CV system is to deliver nutrients and oxygen to all tissues. During most CV pathologies, oxygen and nutrient delivery is decreased or completely halted. Several mechanisms, including increased oxygen transport and delivery, as well as increased blood flow are triggered to compensate for the hypoxic state. If the compensatory mechanisms fail to sufficiently correct the hypoxia, irreversible damage can occur. Thus, hypoxia plays a central role in the pathogenesis and pathophysiology of CV diseases. Hypoxia inducible factors (HIFs) orchestrate the gene transcription for hundreds of proteins involved in erythropoiesis, glucose transport, angiogenesis, glycolytic metabolism, reactive oxygen species (ROS) handling, cell proliferation and survival, among others. The overall regulation of the expression of HIF-dependent genes depends on the severity, duration, and location of hypoxia. In the present review, common CV diseases were selected to illustrate that HIFs, and proteins derived directly or indirectly from their stabilization and activation, are related to the development and perpetuation of hypoxia in these pathologies. We further classify CV diseases into acute and chronic hypoxic states to better understand the temporal relevance of HIFs in the pathogenesis, disease progression and clinical outcomes of these diseases. We conclude that HIFs and their derived factors are fundamental in the genesis and progression of CV diseases. Understanding these mechanisms will lead to more effective treatment strategies leading to reduced morbidity and mortality.

Atrial myxoma: the cardiac chameleon

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Identification and Comparison of Hyperglycemia-Induced Extracellular Vesicle Transcriptome in Different Mouse Stem Cells

Extracellular vesicles (EVs) derived from stem /progenitor cells harbor immense potential to promote cardiomyocyte survival and neovascularization, and to mitigate ischemic injury. However, EVs’ parental stem/progenitor cells showed modest benefits in clinical trials, suggesting autologous stem cell/EV quality might have been altered by stimuli associated with the co-morbidities such as hyperglycemia associated with diabetes. Hyperglycemia is a characteristic of diabetes and a major driving factor in cardiovascular disease. The functional role of stem/progenitor cell-derived EVs and the molecular signature of their secreted EV cargo under hyperglycemic conditions remain elusive. Therefore, we hypothesized that hyperglycemic stress causes transcriptome changes in stem/progenitor cell-derived EVs that may compromise their reparative function. In this study, we performed an unbiased analysis of EV transcriptome signatures from 3 different stem/progenitor cell types by RNA sequencing. The analysis revealed differential expression of a variety of RNA species in EVs. Specifically, we identified 241 common-dysregulated mRNAs, 21 ncRNAs, and 16 miRNAs in three stem cell-derived EVs. Gene Ontology revealed that potential function of common mRNAs mostly involved in metabolism and transcriptional regulation. This study provides potential candidates for preventing the adverse effects of hyperglycemia-induced stem/progenitor cell-derived EV dysfunction, and reference data for future biological studies and application of stem/progenitor cell-derived EVs.

C-Kit+ progenitors restore rat asthmatic lung function by modulation of T-bet and GATA-3 expression

NEW FINDINGS

What is the central question of this study? The aim of the experiment was to highlight the regenerative capacity of bone marrow Kit⁺ cells in the restoration of asthmatic pulmonary function in the rat model. What is the main finding and its importance? Data showed that these cells were recruited successfully to the asthmatic niche after intratracheal administration and accelerated the regeneration of asthmatic lungs by the modulation of inflammation via the control of Gata3 and Tbx21 expression, leading to decreased tracheal responsiveness to methacholine and reduction of pathological remodelling.

ABSTRACT

Allergic asthma is a T helper (Th) 2 immunological disorder with consequential uncontrolled inflammatory responses. There is an increasing demand to use new methods for the treatment of asthma based on modulation of the Th2-to-Th1 ratio in favour of the Th1 population. Accordingly, we decided to evaluate the effects of intratracheal administration of Kit⁺ bone marrow cells on tracheal responsiveness and the expression of Gata3 and Tbx21 genes. Forty male Wistar rats were allocated  randomly  into four experimental groups: healthy rats (control group), sensitized rats (OVA group), sensitized rats receiving Kit^- cells (OVA+Kit^- group) and sensitized rats receiving Kit⁺ cells (OVA+Kit⁺ group). Total and differential white blood cell counts, tracheal responsiveness to cumulative methacholine concentrations and histopathological analysis were evaluated. The results showed a statistically significant increase in total white blood cell, eosinophil and neutrophil counts, tracheal contractility, Gata3 expression and prototypical histopathology of asthma. Along with these conditions, we found that the number of lymphocytes was decreased and expression of Tbx21 diminished in sensitized rats compared with control animals. Monitoring of labelled tagged cells confirmed successful engraftment of transplanted cells in pulmonary tissue. Juxtaposition of Kit⁺ cells changed the blood leucogram closer to the control values. Kit⁺ cells increased the expression of Tbx21 and suppressed Gata3 (P < 0.05). In the OVA+Kit⁺ group, tracheal responsiveness was improved coincident with increased pulmonary regeneration. In conclusion, this study showed that intratracheal administration of bone marrow-derived Kit⁺ cells, but not Kit^- cells, could be effective in the alleviation of asthma, presumably by the modulation of Gata3 and Tbx21.

An overview of the myocardial regeneration potential of cardiac c-Kit+ progenitor cells via PI3K and MAPK signaling pathways

In recent years, several studies have investigated cell transplantation as an innovative strategy to restore cardiac function following heart failure. Previous studies have also shown cardiac progenitor cells as suitable candidates for cardiac cell therapy compared with other stem cells. Cellular kit (c-kit) plays an important role in the survival and migration of cardiac progenitor cells. Like other types of cells, in the heart, cellular responses to various stimuli are mediated via coordinated pathways. Activation of c-kit⁺ cells leads to subsequent activation of several downstream mediators such as PI3K and the MAPK pathways. This review aims to outline current research findings on the role of PI3K/AKT and the MAPK pathways in myocardial regeneration potential of c-kit⁺.

Cardiac regenerative therapy: Many paths to repair

Cardiovascular disease remains the primary cause of death in the United States and in most nations worldwide, despite ongoing intensive efforts to promote cardiac health and treat heart failure. Replacing damaged myocardium represents perhaps the most promising treatment strategy, but also the most challenging given that the adult mammalian heart is notoriously resistant to endogenous repair. Cardiac regeneration following pathologic challenge would require proliferation of surviving tissue, expansion and differentiation of resident progenitors, or transdifferentiation of exogenously applied progenitor cells into functioning myocardium. Adult cardiomyocyte proliferation has been the focus of investigation for decades, recently enjoying a renaissance of interest as a therapeutic strategy for reversing cardiomyocyte loss due in large part to ongoing controversies and frustrations with myocardial cell therapy outcomes. The promise of cardiac cell therapy originated with reports of resident adult cardiac stem cells that could be isolated, expanded and reintroduced into damaged myocardium, producing beneficial effects in preclinical animal models. Despite modest functional improvements, Phase I clinical trials using autologous cardiac derived cells have proven safe and effective, setting the stage for an ongoing multi-center Phase II trial combining autologous cardiac stem cell types to enhance beneficial effects. This overview will examine the history of these two approaches for promoting cardiac repair and attempt to provide context for current and future directions in cardiac regenerative research.

Cardiac interstitial tetraploid cells can escape replicative senescence in rodents but not large mammals

Cardiomyocyte ploidy has been described but remains obscure in cardiac interstitial cells. Ploidy of c-kit+ cardiac interstitial cells was assessed using confocal, karyotypic, and flow cytometric technique. Notable differences were found between rodent (rat, mouse) c-kit+ cardiac interstitial cells possessing mononuclear tetraploid (4n) content, compared to large mammals (human, swine) with mononuclear diploid (2n) content. In-situ analysis, confirmed with fresh isolates, revealed diploid content in human c-kit+ cardiac interstitial cells and a mixture of diploid and tetraploid content in mouse. Downregulation of the p53 signaling pathway provides evidence why rodent, but not human, c-kit+ cardiac interstitial cells escape replicative senescence. Single cell transcriptional profiling reveals distinctions between diploid versus tetraploid populations in mouse c-kit+ cardiac interstitial cells, alluding to functional divergences. Collectively, these data reveal notable species-specific biological differences in c-kit+ cardiac interstitial cells, which could account for challenges in extrapolation of myocardial from preclinical studies to clinical trials.

Single-cell imaging and transcriptomic analyses of endogenous cardiomyocyte dedifferentiation and cycling

While it is recognized that there are low levels of new cardiomyocyte (CM) formation throughout life, the source of these new CM generates much debate. One hypothesis is that these new CMs arise from the proliferation of existing CMs potentially after dedifferentiation although direct evidence for this is lacking. Here we explore the mechanisms responsible for CM renewal in vivo using multi-reporter transgenic mouse models featuring efficient adult CM (ACM) genetic cell fate mapping and real-time cardiomyocyte lineage and dedifferentiation reporting. Our results demonstrate that non-myocytes (e.g., cardiac progenitor cells) contribute negligibly to new ACM formation at baseline or after cardiac injury. In contrast, we found a significant increase in dedifferentiated, cycling CMs in post-infarct hearts. ACM cell cycling was enhanced within the dedifferentiated CM population. Single-nucleus transcriptomic analysis demonstrated that CMs identified with dedifferentiation reporters had significant down-regulation in gene networks for cardiac hypertrophy, contractile, and electrical function, with shifts in metabolic pathways, but up-regulation in signaling pathways and gene sets for active cell cycle, proliferation, and cell survival. The results demonstrate that dedifferentiation may be an important prerequisite for CM proliferation and explain the limited but measurable cardiac myogenesis seen after myocardial infarction (MI).

Prolonged elevated levels of c-kit+ progenitor cells after a myocardial infarction by beta 2 adrenergic receptor priming

Endogenous progenitor cells may participate in cardiac repair after a myocardial infarction (MI). The beta 2 adrenergic receptor (ß2-AR) pathway induces proliferation of c-kit+ cardiac progenitor cells (CPC) in vitro. We investigated if ß2-AR pharmacological stimulation could ameliorate endogenous CPC-mediated regeneration after a MI. C-kit+ CPC ß1-AR and ß2-AR expression was evaluated in vivo and in vitro. A significant increase in the percentage of CPCs expressing ß1-AR and ß2-AR was measured 7 days post-MI. Accordingly, 24 hrs of low serum and hypoxia in vitro significantly increased CPC ß2-AR expression. Cell viability and differentiation assays validated a functional role of CPC ß2-AR. The effect of pharmacological activation of ß2-AR was studied in C57 mice using fenoterol administered in the drinking water 1 week before MI or sham surgery or at the time of the surgery. MI induced a significant increase in the percentage of c-kit+ progenitor cells at 7 days, whereas pretreatment with fenoterol prolonged this response resulting in a significant elevated number of CPC up to 21 days post-MI. This increased number of CPC correlated with a decrease in infarct size. The immunofluorescence analysis of the heart tissue for proliferation, apoptosis, macrophage infiltration, cardiomyocytes surface area, and vessel density showed significant changes on the basis of surgery but no benefit due to fenoterol treatment. Cardiac function was not ameliorated by fenoterol administration when evaluated by echocardiography. Our results suggest that ß2-AR stimulation may improve the cardiac repair process by supporting an endogenous progenitor cell response but is not sufficient to improve the cardiac function.

Role of c-Kit in Myocardial Regeneration and Aging

c-Kit, a type III receptor tyrosine kinase (RTK), is involved in multiple intracellular signaling whereby it is mainly considered a stem cell factor receptor, which participates in vital functions of the mammalian body, including the human. Furthermore, c-kit is a necessary yet not sufficient marker to detect and isolate several types of tissue-specific adult stem cells. Accordingly, c-kit was initially used as a marker to identify and enrich for adult cardiac stem/progenitor cells (CSCs) that were proven to be clonogenic, self-renewing and multipotent, being able to differentiate into cardiomyocytes, endothelial cells and smooth muscle cells in vitro as well as in vivo after myocardial injury. Afterwards it was demonstrated that c-kit expression labels a heterogenous cardiac cell population, which is mainly composed by endothelial cells while only a very small fraction represents CSCs. Furthermore, c-kit as a signaling molecule is expressed at different levels in this heterogenous c-kit labeled cardiac cell pool, whereby c-kit low expressers are enriched for CSCs while c-kit high expressers are endothelial and mast cells. This heterogeneity in cell composition and expression levels has been neglected in recent genetic fate map studies focusing on c-kit, which have claimed that c-kit identifies cells with robust endothelial differentiation potential but with minimal if not negligible myogenic commitment potential. However, modification of c-kit gene for Cre Recombinase expression in these Cre/Lox genetic fate map mouse models produced a detrimental c-kit haploinsufficiency that prevents efficient labeling of true CSCs on one hand while affecting the regenerative potential of these cells on the other. Interestingly, c-kit haploinsufficiency in c-kit-deficient mice causes a worsening myocardial repair after injury and accelerates cardiac aging. Therefore, these studies have further demonstrated that adult c-kit-labeled CSCs are robustly myogenic and that the adult myocardium relies on c-kit expression to regenerate after injury and to counteract aging effects on cardiac structure and function.

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.

Angiotensin II receptor blocker reverses heart failure by attenuating local oxidative stress and preserving resident stem cells in rats with myocardial infarction

The present study aimed to test whether angiotensin receptor blockers (ARBs) are cardioprotective after myocardial infarction (MI) by preventing augmented local renin-angiotensin-system (RAS)-induced oxidative stress injury and senescence, preserving resident stem cells, and restoring the insulin-like growth factor (IGF-1)/IGF-1 receptor (IGF-R) pathway. Sprague-Dawley rats with ligated or unligated coronary arteries were treated with losartan (20 mg/kg/d) or vehicle for 3 or 9 weeks. Heart function and molecular and histological changes were assessed. It was found MI induced left ventricular dysfunction and remodeling, increased levels of the oxidative stress marker 8-hydroxy-2'-deoxyguanosine and cell senescence marker p16^ink4a, and downregulated the IGF-1/IGF-1R/Akt pathway after both 3 and 9 weeks post-MI. MI induced an increase in stem cells identified by immunostaining for c-kit and Wilms' tumor-1 predominantly after 3 weeks. Losartan significantly inhibited local cardiac RAS activation and improved left ventricular function and remodeling at both timepoints. Losartan also preserved c-kit- and Wilms' tumor-1-positive cells (particularly at 3 weeks), attenuated 8-hydroxy-2'-deoxyguanosine- and p16^ink4a-positive cardiomyocytes, and restored the IGF-1/IGF-1R/Akt pathway at both 3 and 9 weeks. In conclusion, ARBs aided cardiac repair post-MI through short-term preservation of stem cells and persistent anti-oxidative stress and anti-senescence effects, partially by attenuating activation of cardiac RAS and restoring the local IGF-1/IGF-1R/Akt pathway.

Cardiac c-Kit Biology Revealed by Inducible Transgenesis

RATIONALE

Biological significance of c-Kit as a cardiac stem cell marker and role(s) of c-Kit+ cells in myocardial development or response to pathological injury remain unresolved because of varied and discrepant findings. Alternative experimental models are required to contextualize and reconcile discordant published observations of cardiac c-Kit myocardial biology and provide meaningful insights regarding clinical relevance of c-Kit signaling for translational cell therapy.

OBJECTIVE

The main objectives of this study are as follows: demonstrating c-Kit myocardial biology through combined studies of both human and murine cardiac cells; advancing understanding of c-Kit myocardial biology through creation and characterization of a novel, inducible transgenic c-Kit reporter mouse model that overcomes limitations inherent to knock-in reporter models; and providing perspective to reconcile disparate viewpoints on c-Kit biology in the myocardium.

METHODS AND RESULTS

In vitro studies confirm a critical role for c-Kit signaling in both cardiomyocytes and cardiac stem cells. Activation of c-Kit receptor promotes cell survival and proliferation in stem cells and cardiomyocytes of either human or murine origin. For creation of the mouse model, the cloned mouse c-Kit promoter drives Histone2B-EGFP (enhanced green fluorescent protein; H2BEGFP) expression in a doxycycline-inducible transgenic reporter line. The combination of c-Kit transgenesis coupled to H2BEGFP readout provides sensitive, specific, inducible, and persistent tracking of c-Kit promoter activation. Tagging efficiency for EGFP+/c-Kit+ cells is similar between our transgenic versus a c-Kit knock-in mouse line, but frequency of c-Kit+ cells in cardiac tissue from the knock-in model is 55% lower than that from our transgenic line. The c-Kit transgenic reporter model reveals intimate association of c-Kit expression with adult myocardial biology. Both cardiac stem cells and a subpopulation of cardiomyocytes express c-Kit in uninjured adult heart, upregulating c-Kit expression in response to pathological stress.

CONCLUSIONS

c-Kit myocardial biology is more complex and varied than previously appreciated or documented, demonstrating validity in multiple points of coexisting yet heretofore seemingly irreconcilable published findings.

The effect of transient oxygenation on stem cell mobilization and ischemia/reperfusion heart injury

For general anesthesia, pre-oxygenation is routinely performed prior to intubation. It is well-known that ischemic/hypoxic preconditioning induces stem cell mobilization and protects against ischemia/reperfusion (I/R) injury. In this study, we investigated the effect of transient oxygenation on stem cell mobilization and I/R injury of the heart. Mice were exposed to 100% oxygen for 5 or 20 minutes. We evaluated the number of c-kit+ stem/progenitor cells and the levels of SDF-1α and VEGF in peripheral blood at 1, 3, 6, and 24 hours after oxygenation. We also induced I/R injury of the heart at 3 hours post-oxygenation for 5 minutes and then examined stem cell recruitment and fibrotic changes in the heart 3 or 14 days later. The number of c-kit+ cells in peripheral blood was significantly increased at 1 or 24 hours after oxygenation for either 5 or 20 minutes. Oxygenation for 5 or 20 minutes did not significantly change the SDF-1α level measured in plasma. However, the plasma VEGF level was decreased at 3 hours post-oxygenation for 20 minutes (p = 0.051). Oxygenation for 5 minutes did not significantly alter the fibrotic area or cell apoptosis. Although oxygenation for 5 minutes increased the number of c-kit+ cells in hearts damaged by I/R injury, this difference was not significant between groups due to large variation between individuals (p = 0.14). Although transient oxygenation induces stem cell mobilization, it does not appear to protect against I/R injury of the heart in mice.

β-arrestin2 Affects Cardiac Progenitor Cell Survival through Cell Mobility and Tube Formation in Severe Hypoxia

Background and Objectives

β-arrestin2 (β-arr2) basically regulates multiple signaling pathways in mammalian cells by desensitization and internalization of G-protein coupled receptors (GPCRs). We investigated impacts of β-arr2 on survival, mobility, and tube formation of cardiac progenitor cells (CPCs) obtained from wild-type (WT) mouse (CPC-WT), and β-arr2 knock-out (KO) mouse (CPC-KO) cultured in presence or absence of serum and oxygen as non-canonical roles in GPCR system.

Methods

CPCs were cultured in Dulbecco's Modified Eagle Medium/Nutrient Mixture F-12 -based media containing fetal bovine serum and growth factors. Survival of 2 types of CPCs in hypoxia and/or serum deprivation was measured by fluorescence-activated cell sorting. Wound healing ability, and tube formation ability on Matrigel of 2 kinds of CPCs were compared in normoxic and hypoxic cultures. Protein expression related to survival and mobility were measured with the Western blot for each culture conditions.

Results

CPC-KO showed significantly worse mobility in the wound healing assay and in tube formation on Matrigel especially in hypoxic culture than did the CPC-WT. Also, CPC-KO showed significantly higher apoptosis fraction in both normoxic and hypoxic cultures than did the CPC-WT. Expression of proteins associated with cell survival and mobility, e.g., protein kinase B (Akt), β-catenin, and glycogen synthase kinase-3β (GSK-3β) was significantly worse in CPC-KO.

Conclusions

The CPC-KO had significantly worse cell mobility, tube formation ability, and survival than the CPC-WT, especially in the hypoxic cultures. Apparently, β-arr2 is important on CPC survival by means of mobility and tube formation in myocardial ischemia.

c-kit Positive Cardiac Outgrowth Cells Demonstrate Better Ability for Cardiac Recovery Against Ischemic Myopathy

Objective

Resident cardiac stem cells are expected to be a therapeutic option for patients who suffer from severe heart failure. However, uncertainty remains over whether sorting cells for c-kit, a stem cell marker, improves therapeutic outcomes.

Materials and methods

Cardiac outgrowth cells cultured from explants of rat heart atrium were sorted according to their positivity (+) or negativity (−) for c-kit. These cells were exposed to hypoxia for 3 d, and subsequently harvested for mRNA expression measurement. The cell medium was also collected to assess cytokine secretion. To test for a functional benefit in animals, myocardial infarction (MI) was induced in rats, and c-kit+ or c-kit− cells were injected. The left ventricular ejection fraction (LVEF) was measured for up to 4 weeks, after which the heart was harvested for biological and histological analyses.

Results and conclusion

Expression of the angiogenesis-related genes, VEGF and ANGPTL2, was significantly higher in c-kit+ cells after 3 d of hypoxic culture, although we found no such difference prior to hypoxia. Secretion of VEGF and ANGPTL2 was greater in the c-kit+ group than in the c-kit− group, while hypoxia tended to increase cytokine expression in both groups. In addition, IGF-1 was significantly increased in the c-kit+ group, consistent with the relatively low expression of cleaved-caspase 3 revealed by western blot assay, and the relatively low count of apoptotic cells revealed by histochemical analysis. Administration of c-kit+cells into the MI heart improved the LVEF and increased neovascularization. These results indicate that c-kit+cells may be useful in cardiac stem cell therapy.

Chasing c-Kit through the heart: Taking a broader view

Stem cell mediated cardiac repair is an exciting and controversial area of cardiovascular research that holds the potential to produce novel, revolutionary therapies for the treatment of heart disease. Extensive investigation to define cell types contributing to cardiac formation, homeostasis and regeneration has produced several candidates, including adult cardiac c-Kit+ expressing stem and progenitor cells that have even been employed in a Phase I clinical trial demonstrating safety and feasibility of this therapeutic approach. However, the field of cardiac cell based therapy remains deeply divided due to strong disagreement among researchers and clinicians over which cell types, if any, are the best candidates for these applications. Research models that identify and define specific cardiac cells that effectively contribute to heart repair are urgently needed to resolve this debate. In this review, current c-Kit reporter models are discussed with respect to myocardial c-Kit cell biology and function, and future designs imagined to better represent endogenous myocardial c-Kit expression.

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.

Transcription factor-induced activation of cardiac gene expression in human c-kit+ cardiac progenitor cells

Although transplantation of c-kit+ cardiac progenitor cells (CPCs) significantly alleviates post-myocardial infarction left ventricular dysfunction, generation of cardiomyocytes by exogenous CPCs in the recipient heart has often been limited. Inducing robust differentiation would be necessary for improving the efficacy of the regenerative cardiac cell therapy. We assessed the hypothesis that differentiation of human c-kit+ CPCs can be enhanced by priming them with cardiac transcription factors (TFs). We introduced five different TFs (Gata4, MEF2C, NKX2.5, TBX5, and BAF60C) into CPCs, either alone or in combination, and then examined the expression of marker genes associated with the major cardiac cell types using quantitative RT-PCR. When introduced individually, Gata4 and TBX5 induced a subset of myocyte markers. Moreover, Gata4 alone significantly induced smooth muscle cell and fibroblast markers. Interestingly, these gene expression changes brought by Gata4 were also accompanied by morphological changes. In contrast, MEF2C and NKX2.5 were largely ineffective in initiating cardiac gene expression in CPCs. Surprisingly, introduction of multiple TFs in different combinations mostly failed to act synergistically. Likewise, addition of BAF60C to Gata4 and/or TBX5 did not further potentiate their effects on cardiac gene expression. Based on our results, it appears that GATA4 is able to potentiate gene expression programs associated with multiple cardiovascular lineages in CPCs, suggesting that GATA4 may be effective in priming CPCs for enhanced differentiation in the setting of stem cell therapy.

TNF receptor signaling inhibits cardiomyogenic differentiation of cardiac stem cells and promotes a neuroadrenergic-like fate

Despite expansion of resident cardiac stem cells (CSCs; c-kit⁺Lin^-) after myocardial infarction, endogenous repair processes are insufficient to prevent adverse cardiac remodeling and heart failure (HF). This suggests that the microenvironment in post-ischemic and failing hearts compromises CSC regenerative potential. Inflammatory cytokines, such as tumor necrosis factor-α (TNF), are increased after infarction and in HF; whether they modulate CSC function is unknown. As the effects of TNF are specific to its two receptors (TNFRs), we tested the hypothesis that TNF differentially modulates CSC function in a TNFR-specific manner. CSCs were isolated from wild-type (WT), TNFR1-/-, and TNFR2-/- adult mouse hearts, expanded and evaluated for cell competence and differentiation in vitro in the absence and presence of TNF. Our results indicate that TNF signaling in murine CSCs is constitutively related primarily to TNFR1, with TNFR2 inducible after stress. TNFR1 signaling modestly diminished CSC proliferation, but, along with TNFR2, augmented CSC resistance to oxidant stress. Deficiency of either TNFR1 or TNFR2 did not impact CSC telomerase activity. Importantly, TNF, primarily via TNFR1, inhibited cardiomyogenic commitment during CSC differentiation, and instead promoted smooth muscle and endothelial fates. Moreover, TNF, via both TNFR1 and TNFR2, channeled an alternate CSC neuroadrenergic-like fate (capable of catecholamine synthesis) during differentiation. Our results suggest that elevated TNF in the heart restrains cardiomyocyte differentiation of resident CSCs and may enhance adrenergic activation, both effects that would reduce the effectiveness of endogenous cardiac repair and the response to exogenous stem cell therapy, while promoting adverse cardiac remodeling.

Reduction of c-kit positive cardiac stem cells in patients with atrial fibrillation

BACKGROUND

We aimed to determine expression patterns of cardiac stem cells in the left atrium (LA) tissue from patients with atrial fibrillation.

METHODS

LA appendages were obtained during open-heart surgery and processed for explant cell culture and tissue analysis (n=319). The total number of grown cells and c-kit positive cells were analyzed by flow cytometry after 4 weeks of culture. The remaining tissue was used for Masson's trichrome staining to determine the area of the fibrosis.

RESULTS

The diameter of the LA, as measured by echocardiography, was significantly larger in the AF group than in the sinus rhythm group. Reverse transcription polymerase chain reaction analysis revealed higher expression of collagen in the AF group and an increase in the expression of basic fibrosis growth factor and transforming growth factor-2 and -3. Masson's trichrome staining showed progression of fibrosis in the AF tissue. In addition, the expression of apoptosis-related genes were significantly higher in AF group. There was no difference in the expression of connexin-40 between groups, while the expression of connexin-43 was decreased and that of connexin-45 was increased in the AF group. The total numbers of grown cells as well as c-kit positive cells after 4 weeks of cardiac tissue culture were significantly lower in the AF group.

CONCLUSION

Progression of remodeling in LA tissue was observed in AF patients. The number of c-kit positive cells cultured from LA appendages was reduced in AF patients, suggesting impairments in self-renewal.

Sphingosine 1-phosphate elicits RhoA-dependent proliferation and MRTF-A mediated gene induction in CPCs

Although c-kit(+) cardiac progenitor cells (CPCs) are currently used in clinical trials there remain considerable gaps in our understanding of the molecular mechanisms underlying their proliferation and differentiation. G-protein coupled receptors (GPCRs) play an important role in regulating these processes in mammalian cell types thus we assessed GPCR mRNA expression in c-kit(+) cells isolated from adult mouse hearts. Our data provide the first comprehensive overview of the distribution of this fundamental class of cardiac receptors in CPCs and reveal notable distinctions from that of adult cardiomyocytes. We focused on GPCRs that couple to RhoA activation in particular those for sphingosine-1-phosphate (S1P). The S1P2 and S1P3 receptors are the most abundant S1P receptor subtypes in mouse and human CPCs while cardiomyocytes express predominantly S1P1 receptors. Treatment of CPCs with S1P, as with thrombin and serum, increased proliferation through a pathway requiring RhoA signaling, as evidenced by significant attenuation when Rho was inhibited by treatment with C3 toxin. Further analysis demonstrated that both S1P- and serum-induced proliferation are regulated through the S1P2 and S1P3 receptor subtypes which couple to Gα12/13 to elicit RhoA activation. The transcriptional co-activator MRTF-A was activated by S1P as assessed by its nuclear accumulation and induction of a RhoA/MRTF-A luciferase reporter. In addition S1P treatment increased expression of cardiac lineage markers Mef2C and GATA4 and the smooth muscle marker GATA6 through activation of MRTF-A. In conclusion, we delineate an S1P-regulated signaling pathway in CPCs that introduces the possibility of targeting S1P2/3 receptors, Gα12/13 or RhoA to influence the proliferation and commitment of c-kit(+) CPCs and improve the response of the myocardium following injury.

Activated c-Kit receptor in the heart promotes cardiac repair and regeneration after injury

The role of endogenous c-Kit receptor activation on cardiac cell homeostasis and repair remains largely unexplored. Transgenic mice carrying an activating point mutation (TgD814Y) in the kinase domain of the c-Kit gene were generated. c-Kit^TgD814Y receptor was expressed in the heart during embryonic development and postnatal life, in a similar timing and expression pattern to that of the endogenous gene, but not in the hematopoietic compartment allowing the study of a cardiac-specific phenotype. c-Kit^TgD814Y mutation produced a constitutive active c-Kit receptor in cardiac tissue and cells from transgenic mice as demonstrated by the increased phosphorylation of ERK1/2 and AKT, which are the main downstream molecular effectors of c-Kit receptor signaling. In adult transgenic hearts, cardiac morphology, size and total c-Kit⁺ cardiac cell number was not different compared with wt mice. However, when c-Kit^TgD814Y mice were subjected to transmural necrotic heart damage by cryoinjury (CI), all transgenic survived, compared with half of wt mice. In the sub-acute phase after CI, transgenic and wt mice showed similar heart damage. However, 9 days after CI, transgenic mice exhibited an increased number of c-Kit⁺CD31⁺ endothelial progenitor cells surrounding the necrotic area. At later follow-up, a consistent reduction of fibrotic area, increased capillary density and increased cardiomyocyte replenishment rate (as established by BrdU incorporation) were observed in transgenic compared with wt mice. Consistently, CD45⁻c-Kit⁺ cardiac stem cells isolated from transgenic c-Kit^TgD814Y mice showed an enhanced endothelial and cardiomyocyte differentiation potential compared with cells isolated from the wt. Constitutive activation of c-Kit receptor in mice is associated with an increased cardiac myogenic and vasculogenic reparative potential after injury, with a significant improvement of survival.

Type 2 Diabetes Dysregulates Glucose Metabolism in Cardiac Progenitor Cells

Type 2 diabetes is associated with increased mortality and progression to heart failure. Recent studies suggest that diabetes also impairs reparative responses after cell therapy. In this study, we examined potential mechanisms by which diabetes affects cardiac progenitor cells (CPCs). CPCs isolated from the diabetic heart showed diminished proliferation, a propensity for cell death, and a pro-adipogenic phenotype. The diabetic CPCs were insulin-resistant, and they showed higher energetic reliance on glycolysis, which was associated with up-regulation of the pro-glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3). In WT CPCs, expression of a mutant form of PFKFB, which mimics PFKFB3 activity and increases glycolytic rate, was sufficient to phenocopy the mitochondrial and proliferative deficiencies found in diabetic cells. Consistent with activation of phosphofructokinase in diabetic cells, stable isotope carbon tracing in diabetic CPCs showed dysregulation of the pentose phosphate and glycero(phospho)lipid synthesis pathways. We describe diabetes-induced dysregulation of carbon partitioning using stable isotope metabolomics-based coupling quotients, which relate relative flux values between metabolic pathways. These findings suggest that diabetes causes an imbalance in glucose carbon allocation by uncoupling biosynthetic pathway activity, which could diminish the efficacy of CPCs for myocardial repair.

Hiding inside? Intracellular expression of non-glycosylated c-kit protein in cardiac progenitor cells

Cardiac progenitor cells including c-kit(+) cells and cardiosphere-derived cells (CDCs) play important roles in cardiac repair and regeneration. CDCs were reported to contain only small subpopulations of c-kit(+) cells and recent publications suggested that depletion of the c-kit(+) subpopulation of cells has no effect on regenerative properties of CDCs. However, our current study showed that the vast majority of CDCs from murine heart actually express c-kit, albeit, in an intracellular and non-glycosylated form. Immunostaining and flow cytometry showed that the fluorescent signal indicative of c-kit immunostaining significantly increased when cell membranes were permeabilized. Western blots further demonstrated that glycosylation of c-kit was increased during endothelial differentiation in a time dependent manner. Glycosylation inhibition by 1-deoxymannojirimycin hydrochloride (1-DMM) blocked c-kit glycosylation and reduced expression of endothelial cell markers such as Flk-1 and CD31 during differentiation. Pretreatment of these cells with a c-kit kinase inhibitor (imatinib mesylate) also attenuated Flk-1 and CD31 expression. These results suggest that c-kit glycosylation and its kinase activity are likely needed for these cells to differentiate into an endothelial lineage. In vivo, we found that intracellular c-kit expressing cells are located in the wall of cardiac blood vessels in mice subjected to myocardial infarction. In summary, our work demonstrated for the first time that c-kit is not only expressed in CDCs but may also directly participate in CDC differentiation into an endothelial lineage.

Electrospun nanofibrous sheets of collagen/elastin/polycaprolactone improve cardiac repair after myocardial infarction

Electrospun nanofibrous sheets get increasing attention in myocardial infarction (MI) treatment due to their good cytocompatibility to deliver transplanted stem cells to infarcted areas and due to mechanical characteristics to support damaged tissue. Cardiac extracellular matrix is essential for implanted cells since it provides the cardiac microenvironment. In this study, we hypothesized high concentrations of cardiac nature protein (NP), namely elastin and collagen, in hybrid polycaprolactone (PCL) electrospun nanofibrous sheets could be effective as cardiac-mimicking patch. Optimal ratio of elastin and collagen with PCL in electrospun sheets (80% NP/PCL) was selected based on cytocompatibility and mechanical characteristics. Bone-marrow (BM) c-kit(+) cells anchoring onto NP/PCL sheets exhibited increased proliferative capacity compared with those seeded on PCL in vitro. Moreover, we examined the improvement of cardiac function in MI mice by cell-seeded cardiac patch. Green Fluorescent Protein (GFP)-labeled BM c-kit(+) cells were loaded on 80% NP/PCL sheets which was transplanted into MI mice. Both 80% NP/PCL and c-kit(+)-seeded 80% NP/PCL effectively improved cardiac function after 4 weeks of transplantation, with reduced infarction area and restricted LV remodeling. C-kit(+)-seeded 80% NP/PCL was even superior to the 80% NP/PCL alone and both superior to PCL. GFP(+) cells were identified both in the sheets and local infarcted area where transplanted cells underwent cardiac differentiation after 4 weeks. To the best of our knowledge, this is the first report that sheets with high concentrations of nature proteins loaded with BM c-kit(+) cells might be a novel promising candidate for tissue-engineered cardiac patch to improve cardiac repair after MI.

O-GlcNAcylation Negatively Regulates Cardiomyogenic Fate in Adult Mouse Cardiac Mesenchymal Stromal Cells

In both preclinical and clinical studies, cell transplantation of several cell types is used to promote repair of damaged organs and tissues. Nevertheless, despite the widespread use of such strategies, there remains little understanding of how the efficacy of cell therapy is regulated. We showed previously that augmentation of a unique, metabolically derived stress signal (i.e., O-GlcNAc) improves survival of cardiac mesenchymal stromal cells; however, it is not known whether enhancing O-GlcNAcylation affects lineage commitment or other aspects of cell competency. In this study, we assessed the role of O-GlcNAc in differentiation of cardiac mesenchymal stromal cells. Exposure of these cells to routine differentiation protocols in culture increased markers of the cardiomyogenic lineage such as Nkx2.5 and connexin 40, and augmented the abundance of transcripts associated with endothelial and fibroblast cell fates. Differentiation significantly decreased the abundance of O-GlcNAcylated proteins. To determine if O-GlcNAc is involved in stromal cell differentiation, O-GlcNAcylation was increased pharmacologically during the differentiation protocol. Although elevated O-GlcNAc levels did not significantly affect fibroblast and endothelial marker expression, acquisition of cardiomyocyte markers was limited. In addition, increasing O-GlcNAcylation further elevated smooth muscle actin expression. In addition to lineage commitment, we also evaluated proliferation and migration, and found that increasing O-GlcNAcylation did not significantly affect either; however, we found that O-GlcNAc transferase--the protein responsible for adding O-GlcNAc to proteins--is at least partially required for maintaining cellular proliferative and migratory capacities. We conclude that O-GlcNAcylation contributes significantly to cardiac mesenchymal stromal cell lineage and function. O-GlcNAcylation and pathological conditions that may affect O-GlcNAc levels (such as diabetes) should be considered carefully in the context of cardiac cell therapy.

Stimulating endogenous cardiac repair

The healthy adult heart has a low turnover of cardiac myocytes. The renewal capacity, however, is augmented after cardiac injury. Participants in cardiac regeneration include cardiac myocytes themselves, cardiac progenitor cells, and peripheral stem cells, particularly from the bone marrow compartment. Cardiac progenitor cells and bone marrow stem cells are augmented after cardiac injury, migrate to the myocardium, and support regeneration. Depletion studies of these populations have demonstrated their necessary role in cardiac repair. However, the potential of these cells to completely regenerate the heart is limited. Efforts are now being focused on ways to augment these natural pathways to improve cardiac healing, primarily after ischemic injury but in other cardiac pathologies as well. Cell and gene therapy or pharmacological interventions are proposed mechanisms. Cell therapy has demonstrated modest results and has passed into clinical trials. However, the beneficial effects of cell therapy have primarily been their ability to produce paracrine effects on the cardiac tissue and recruit endogenous stem cell populations as opposed to direct cardiac regeneration. Gene therapy efforts have focused on prolonging or reactivating natural signaling pathways. Positive results have been demonstrated to activate the endogenous stem cell populations and are currently being tested in clinical trials. A potential new avenue may be to refine pharmacological treatments that are currently in place in the clinic. Evidence is mounting that drugs such as statins or beta blockers may alter endogenous stem cell activity. Understanding the effects of these drugs on stem cell repair while keeping in mind their primary function may strike a balance in myocardial healing. To maximize endogenous cardiac regeneration, a combination of these approaches could ameliorate the overall repair process to incorporate the participation of multiple cellular players.

Nuclear Calcium/Calmodulin-dependent Protein Kinase II Signaling Enhances Cardiac Progenitor Cell Survival and Cardiac Lineage Commitment

Ca(2+)/Calmodulin-dependent protein kinase II (CaMKII) signaling in the heart regulates cardiomyocyte contractility and growth in response to elevated intracellular Ca(2+). The δB isoform of CaMKII is the predominant nuclear splice variant in the adult heart and regulates cardiomyocyte hypertrophic gene expression by signaling to the histone deacetylase HDAC4. However, the role of CaMKIIδ in cardiac progenitor cells (CPCs) has not been previously explored. During post-natal growth endogenous CPCs display primarily cytosolic CaMKIIδ, which localizes to the nuclear compartment of CPCs after myocardial infarction injury. CPCs undergoing early differentiation in vitro increase levels of CaMKIIδB in the nuclear compartment where the kinase may contribute to the regulation of CPC commitment. CPCs modified with lentiviral-based constructs to overexpress CaMKIIδB (CPCeδB) have reduced proliferative rate compared with CPCs expressing eGFP alone (CPCe). Additionally, stable expression of CaMKIIδB promotes distinct morphological changes such as increased cell surface area and length of cells compared with CPCe. CPCeδB are resistant to oxidative stress induced by hydrogen peroxide (H2O2) relative to CPCe, whereas knockdown of CaMKIIδB resulted in an up-regulation of cell death and cellular senescence markers compared with scrambled treated controls. Dexamethasone (Dex) treatment increased mRNA and protein expression of cardiomyogenic markers cardiac troponin T and α-smooth muscle actin in CPCeδB compared with CPCe, suggesting increased differentiation. Therefore, CaMKIIδB may serve as a novel modulatory protein to enhance CPC survival and commitment into the cardiac and smooth muscle lineages.

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.

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.

Accumulation of Mitochondrial DNA Mutations Disrupts Cardiac Progenitor Cell Function and Reduces Survival

Transfer of cardiac progenitor cells (CPCs) improves cardiac function in heart failure patients. However, CPC function is reduced with age, limiting their regenerative potential. Aging is associated with numerous changes in cells including accumulation of mitochondrial DNA (mtDNA) mutations, but it is unknown how this impacts CPC function. Here, we demonstrate that acquisition of mtDNA mutations disrupts mitochondrial function, enhances mitophagy, and reduces the replicative and regenerative capacities of the CPCs. We show that activation of differentiation in CPCs is associated with expansion of the mitochondrial network and increased mitochondrial oxidative phosphorylation. Interestingly, mutant CPCs are deficient in mitochondrial respiration and rely on glycolysis for energy. In response to differentiation, these cells fail to activate mitochondrial respiration. This inability to meet the increased energy demand leads to activation of cell death. These findings demonstrate the consequences of accumulating mtDNA mutations and the importance of mtDNA integrity in CPC homeostasis and regenerative potential.

Contemporary perspective on endogenous myocardial regeneration

Considering the complex nature of the adult heart, it is no wonder that innate regenerative processes, while maintaining adequate cardiac function, fall short in myocardial jeopardy. In spite of these enchaining limitations, cardiac rejuvenation occurs as well as restricted regeneration. In this review, the background as well as potential mechanisms of endogenous myocardial regeneration are summarized. We present and analyze the available evidence in three subsequent steps. First, we examine the experimental research data that provide insights into the mechanisms and origins of the replicating cardiac myocytes, including cell populations referred to as cardiac progenitor cells (i.e., c-kit+ cells). Second, we describe the role of clinical settings such as acute or chronic myocardial ischemia, as initiators of pathways of endogenous myocardial regeneration. Third, the hitherto conducted clinical studies that examined different approaches of initiating endogenous myocardial regeneration in failing human hearts are analyzed. In conclusion, we present the evidence in support of the notion that regaining cardiac function beyond cellular replacement of dysfunctional myocardium via initiation of innate regenerative pathways could create a new perspective and a paradigm change in heart failure therapeutics. Reinitiating cardiac morphogenesis by reintroducing developmental pathways in the adult failing heart might provide a feasible way of tissue regeneration. Based on our hypothesis “embryonic recall”, we present first supporting evidence on regenerative impulses in the myocardium, as induced by developmental processes.

Glutamine Regulates Cardiac Progenitor Cell Metabolism and Proliferation

Autologous transplantation of cardiac progenitor cells (CPCs) alleviates myocardial dysfunction in the damaged heart; however, the mechanisms that contribute to their reparative qualities remain poorly understood. In this study, we examined CPC metabolism to elucidate the metabolic pathways that regulate their proliferative capacity. In complete growth medium, undifferentiated CPCs isolated from adult mouse heart proliferated rapidly (Td  = 13.8 hours). CPCs expressed the Glut1 transporter and their glycolytic rate was increased by high extracellular glucose (Glc) concentration, in the absence of insulin. Although high Glc concentrations did not stimulate proliferation, glutamine (Gln) increased CPC doubling time and promoted survival under conditions of oxidative stress. In comparison with Glc, pyruvate (Pyr) or BSA-palmitate, Gln, when provided as the sole metabolic substrate, increased ATP-linked and uncoupled respiration. Although fatty acids were not used as respiratory substrates when present as a sole carbon source, Gln-induced respiration was doubled in the presence of BSA-palmitate, suggesting that Gln stimulates fatty acid oxidation. Additionally, Gln promoted rapid phosphorylation of the mTORC1 substrate, p70S6k, as well as retinoblastoma protein, followed by induction of cyclin D1 and cdk4. Inhibition of either mTORC1 or glutaminolysis was sufficient to diminish CPC proliferation, and provision of cell permeable α-ketoglutarate in the absence of Gln increased both respiration and cell proliferation, indicating a key role of Gln anaplerosis in cell growth. These findings suggest that Gln, by enhancing mitochondrial function and stimulating mTORC1, increases CPC proliferation, and that interventions to increase Gln uptake or oxidation may improve CPC therapy.

c-kit(+) cells: the tell-tale heart of cardiac regeneration?

Cardiovascular disease is the leading cause of morbidity and mortality in the developed world. Although ongoing therapeutic strategies ameliorate symptoms and prolong life for patients with cardiovascular diseases, they do not solve the critical issue related to the loss of cardiac tissue. Accordingly, stem/progenitor cell therapy has emerged as a paramount approach for cardiac repair and regeneration. In this regard, c-kit(+) cells have animated much interest and controversy. These cells are self-renewing, clonogenic, and multipotent and display a noteworthy potential to differentiate into all cardiovascular lineages. However, their functional contribution to cardiomyocyte turnover is one of the centrally debated issues concerning their regenerative potential. Regardless, plentiful preclinical and clinical studies have been conducted which provide evidence for the capacity of c-kit(+) cells to improve cardiac function. The purpose of this review is to give a comprehensive, impartial, critical description and evaluation of the literature on c-kit(+) cells from bench to bedside in order to address their true potential, benefits and controversies.

Notch activation enhances lineage commitment and protective signaling in cardiac progenitor cells

Phase I clinical trials applying autologous progenitor cells to treat heart failure have yielded promising results; however, improvement in function is modest, indicating a need to enhance cardiac stem cell reparative capacity. Notch signaling plays a crucial role in cardiac development, guiding cell fate decisions that underlie myocyte and vessel differentiation. The Notch pathway is retained in the adult cardiac stem cell niche, where level and duration of Notch signal influence proliferation and differentiation of cardiac progenitors. In this study, Notch signaling promotes growth, survival and differentiation of cardiac progenitor cells into smooth muscle lineages in vitro. Cardiac progenitor cells expressing tamoxifen-regulated intracellular Notch1 (CPCeK) are significantly larger and proliferate more slowly than control cells, exhibit elevated mTORC1 and Akt signaling, and are resistant to oxidative stress. Vascular smooth muscle and cardiomyocyte markers increase in CPCeK and are augmented further upon ligand-mediated induction of Notch signal. Paracrine signals indicative of growth, survival and differentiation increase with Notch activity, while markers of senescence are decreased. Adoptive transfer of CPCeK into infarcted mouse myocardium enhances preservation of cardiac function and reduces infarct size relative to hearts receiving control cells. Greater capillary density and proportion of vascular smooth muscle tissue in CPCeK-treated hearts indicate improved vascularization. Finally, we report a previously undescribed signaling mechanism whereby Notch activation stimulates CPC growth, survival and differentiation via mTORC1 and paracrine factor expression. Taken together, these findings suggest that regulated Notch activation potentiates the reparative capacity of CPCs in the treatment of cardiac disease.

Functional Effect of Pim1 Depends upon Intracellular Localization in Human Cardiac Progenitor Cells

Human cardiac progenitor cells (hCPC) improve heart function after autologous transfer in heart failure patients. Regenerative potential of hCPCs is severely limited with age, requiring genetic modification to enhance therapeutic potential. A legacy of work from our laboratory with Pim1 kinase reveals effects on proliferation, survival, metabolism, and rejuvenation of hCPCs in vitro and in vivo. We demonstrate that subcellular targeting of Pim1 bolsters the distinct cardioprotective effects of this kinase in hCPCs to increase proliferation and survival, and antagonize cellular senescence. Adult hCPCs isolated from patients undergoing left ventricular assist device implantation were engineered to overexpress Pim1 throughout the cell (PimWT) or targeted to either mitochondrial (Mito-Pim1) or nuclear (Nuc-Pim1) compartments. Nuc-Pim1 enhances stem cell youthfulness associated with decreased senescence-associated β-galactosidase activity, preserved telomere length, reduced expression of p16 and p53, and up-regulation of nucleostemin relative to PimWT hCPCs. Alternately, Mito-Pim1 enhances survival by increasing expression of Bcl-2 and Bcl-XL and decreasing cell death after H2O2 treatment, thereby preserving mitochondrial integrity superior to PimWT. Mito-Pim1 increases the proliferation rate by up-regulation of cell cycle modulators Cyclin D, CDK4, and phospho-Rb. Optimal stem cell traits such as proliferation, survival, and increased youthful properties of aged hCPCs are enhanced after targeted Pim1 localization to mitochondrial or nuclear compartments. Targeted Pim1 overexpression in hCPCs allows for selection of the desired phenotypic properties to overcome patient variability and improve specific stem cell characteristics.

Origin of cardiomyocytes in the adult heart

This review article discusses the mechanisms of cardiomyogenesis in the adult heart. They include the re-entry of cardiomyocytes into the cell cycle; dedifferentiation of pre-existing cardiomyocytes, which assume an immature replicating cell phenotype; transdifferentiation of hematopoietic stem cells into cardiomyocytes; and cardiomyocytes derived from activation and lineage specification of resident cardiac stem cells. The recognition of the origin of cardiomyocytes is of critical importance for the development of strategies capable of enhancing the growth response of the myocardium; in fact, cell therapy for the decompensated heart has to be based on the acquisition of this fundamental biological knowledge.

Complement component 3 is necessary to preserve myocardium and myocardial function in chronic myocardial infarction

Activation of the complement cascade (CC) with myocardial infarction (MI) acutely initiates immune cell infiltration, membrane attack complex formation on injured myocytes, and exacerbates myocardial injury. Recent studies implicate the CC in mobilization of stem/progenitor cells and tissue regeneration. Its role in chronic MI is unknown. Here, we consider complement component C3, in the chronic response to MI. C3 knockout (KO) mice were studied after permanent coronary artery ligation. C3 deficiency exacerbated myocardial dysfunction 28 days after MI compared to WT with further impaired systolic function and LV dilation despite similar infarct size 24 hours post-MI. Morphometric analysis 28 days post-MI showed C3 KO mice had more scar tissue with less viable myocardium within the infarct zone which correlated with decreased c-kit(pos) cardiac stem/progenitor cells (CPSC), decreased proliferating Ki67(pos) CSPCs and decreased formation of new BrdU(pos) /α-sarcomeric actin(pos) myocytes, and increased apoptosis compared to WT. Decreased CSPCs and increased apoptosis were evident 7 days post-MI in C3 KO hearts. The inflammatory response with MI was attenuated in the C3 KO and was accompanied by attenuated hematopoietic, pluripotent, and cardiac stem/progenitor cell mobilization into the peripheral blood 72 hours post-MI. These results are the first to demonstrate that CC, through C3, contributes to myocardial preservation and regeneration in response to chronic MI. Responses in the C3 KO infer that C3 activation in response to MI expands the resident CSPC population, increases new myocyte formation, increases and preserves myocardium, inflammatory response, and bone marrow stem/progenitor cell mobilization to preserve myocardial function.

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.

Small molecules targeting in vivo tissue regeneration

The field of regenerative medicine has boomed in recent years thanks to milestone discoveries in stem cell biology and tissue engineering, which has been driving paradigm shifts in the pharmacotherapy of degenerative and ischemic diseases. Small molecule-mediated replenishment of lost and/or dysfunctional tissue in vivo, however, is still in its infancy due to a limited understanding of mechanisms that control such endogenous processes of tissue homeostasis or regeneration. Here, we discuss current progress using small molecules targeting in vivo aspects of regeneration, including adult stem cells, stem cell niches, and mechanisms of homing, mobilization, and engraftment as well as somatic cell proliferation. Many of these compounds derived from both knowledge-based design and screening campaigns, illustrating the feasibility of translating in vitro discovery to in vivo regeneration. These early examples of drug-mediated in vivo regeneration provide a glimpse of the future directions of in vivo regenerative medicine approaches.

CENP-A is essential for cardiac progenitor cell proliferation

Centromere protein A (CENP-A) is a homolog of histone H3 that epigenetically marks the heterochromatin of chromosomes. CENP-A is a critical component of the cell cycle machinery that is necessary for proper assembly of the mitotic spindle. However, the role of CENP-A in the heart and cardiac progenitor cells (CPCs) has not been previously studied. This study shows that CENP-A is expressed in CPCs and declines with age. Silencing CENP-A results in a decreased CPC growth rate, reduced cell number in phase G₂/M of the cell cycle, and increased senescence associated β-galactosidase activity. Lineage commitment is not affected by CENP-A silencing, suggesting that cell cycle arrest induced by loss of CENP-A is a consequence of senescence and not differentiation. CENP-A knockdown does not exacerbate cell death in undifferentiated CPCs, but increases apoptosis upon lineage commitment. Taken together, these results indicate that CPCs maintain relatively high levels of CENP-A early in life, which is necessary for sustaining proliferation, inhibiting senescence, and promoting survival following differentiation of CPCs.