Patient profile modulates cardiac c-kit(+) progenitor cell availability and amplification potential

Stem cell therapy for heart failure in the clinics: new perspectives in the era of precision medicine and artificial intelligence

Stem/progenitor cells have been widely evaluated as a promising therapeutic option for heart failure (HF). Numerous clinical trials with stem/progenitor cell-based therapy (SCT) for HF have demonstrated encouraging results, but not without limitations or discrepancies. Recent technological advancements in multiomics, bioinformatics, precision medicine, artificial intelligence (AI), and machine learning (ML) provide new approaches and insights for stem cell research and therapeutic development. Integration of these new technologies into stem/progenitor cell therapy for HF may help address: 1) the technical challenges to obtain reliable and high-quality therapeutic precursor cells, 2) the discrepancies between preclinical and clinical studies, and 3) the personalized selection of optimal therapeutic cell types/populations for individual patients in the context of precision medicine. This review summarizes the current status of SCT for HF in clinics and provides new perspectives on the development of computation-aided SCT in the era of precision medicine and AI/ML.

The impact of autophagy modulation on phenotype and survival of cardiac stromal cells under metabolic stress

Cardiac stromal cells (CSCs) embrace multiple phenotypes and are a contributory factor in tissue homeostasis and repair. They can be exploited as therapeutic mediators against cardiac fibrosis and remodeling, but their survival and cardioprotective properties can be decreased by microenvironmental cues. We evaluated the impact of autophagy modulation by different pharmacological/genetic approaches on the viability and phenotype of murine CSCs, which had been subjected to nutrient deprivation or hyperglycemia, in order to mimic relevant stress conditions and risk factors of cardiovascular diseases. Our results show that autophagy is activated in CSCs by nutrient deprivation, and that autophagy induction by trehalose or autophagy-related protein 7 (ATG7)-overexpression can significantly preserve CSC viability. Furthermore, autophagy induction is associated with a higher proportion of primitive, non-activated stem cell antigen 1 (Sca1)-positive cells, and with a reduced fibrotic fraction (positive for the discoidin domain-containing receptor 2, DDR2) in the CSC pool after nutrient deprivation. Hyperglycemia, on the other hand, is associated with reduced autophagic flux in CSCs, and with a significant reduction in primitive Sca1+ cells. Autophagy induction by adenoviral-mediated ATG7-overexpression maintains a cardioprotective, anti-inflammatory and pro-angiogenic paracrine profile of CSCs exposed to hyperglycemia for 1 week. Finally, autophagy induction by ATG7-overexpression during hyperglycemia can significantly preserve cell viability in CSCs, which were subsequently exposed to nutrient deprivation, reducing hyperglycemia-induced impairment of cell resistance to stress. In conclusion, our results show that autophagy stimulation preserves CSC viability and function in response to metabolic stressors, suggesting that it may boost the beneficial functions of CSCs in cardiac repair mechanisms.

Stem cell therapy for heart failure: Medical breakthrough, or dead end?

Heart failure continues to be one of the leading causes of morbidity and mortality worldwide. Myocardial infarction is the primary causative agent of chronic heart failure resulting in cardiomyocyte necrosis and the subsequent formation of fibrotic scar tissue. Current pharmacological and non-pharmacological therapies focus on managing symptoms of heart failure yet remain unable to reverse the underlying pathology. Heart transplantation usually cannot be relied on, as there is a major discrepancy between the availability of donors and recipients. As a result, heart failure carries a poor prognosis and high mortality rate. As the heart lacks significant endogenous regeneration potential, novel therapeutic approaches have incorporated the use of stem cells as a vehicle to treat heart failure as they possess the ability to self-renew and differentiate into multiple cell lineages and tissues. This review will discuss past, present, and future clinical trials, factors that influence stem cell therapy outcomes as well as ethical and safety considerations. Preclinical and clinical studies have shown a wide spectrum of outcomes when applying stem cells to improve cardiac function. This may reflect the infancy of clinical trials and the limited knowledge on the optimal cell type, dosing, route of administration, patient parameters and other important variables that contribute to successful stem cell therapy. Nonetheless, the field of stem cell therapeutics continues to advance at an unprecedented pace. We remain cautiously optimistic that stem cells will play a role in heart failure management in years to come.

Cell therapy for the treatment of heart disease: Renovation work on the broken heart is still in progress

Cardiovascular disease (CVD) continues to be the number one killer in the aging population. Heart failure (HF) is also an important cause of morbidity and mortality in patients with congenital heart disease (CHD). Novel therapeutic approaches that could restore stable heart function are much needed in both paediatric and adult patients. Regenerative medicine holds promises to provide definitive solutions for correction of congenital and acquired cardiac defects. In this review article, we recap some important aspects of cardiovascular cell therapy. First, we report quantifiable data regarding the scientific advancements in the field and how this has been translated into tangible outcomes according clinical studies and related meta-analyses. We then comment on emerging trends and technologies, such as the use of second-generation cell products, including pericyte-like vascular progenitors, and reprogramming of cells by different approaches including modulation of oxidative stress. The more affordable and feasible strategy of repurposing clinically available drugs to awaken the intrinsic healing potential of the heart will be discussed in the light of current social, financial, and ethical context. Cell therapy remains a work in progress field. Uncertainty in the ability of the experts and policy makers to solve urgent medical problems is growing in a world that is significantly influenced by them. This is particularly true in the field of regenerative medicine, due to great public expectations, polarization of leadership and funding, and insufficient translational vision. Cardiovascular regenerative medicine should be contextualized in a holistic program with defined priorities to allow a complete realization. Reshaping the notion of medical expertise is fundamental to fill the current gap in translation.

Differences in Mitochondrial Membrane Potential Identify Distinct Populations of Human Cardiac Mesenchymal Progenitor Cells

Adult human cardiac mesenchymal progenitor cells (hCmPC) are multipotent resident populations involved in cardiac homeostasis and heart repair. Even if the mechanisms have not yet been fully elucidated, the stem cell differentiation is guided by the mitochondrial metabolism; however, mitochondrial approaches to identify hCmPC with enhanced stemness and/or differentiation capability for cellular therapy are not established. Here we demonstrated that hCmPCs sorted for low and high mitochondrial membrane potential (using a lipophilic cationic dye tetramethylrhodamine methyl ester, TMRM), presented differences in energy metabolism from preferential glycolysis to oxidative rates. TMRM-high cells are highly efficient in terms of oxygen consumption rate, basal and maximal respiration, and spare respiratory capacity compared to TMRM-low cells. TMRM-high cells showed characteristics of pre-committed cells and were associated with higher in vitro differentiation capacity through endothelial, cardiac-like, and, to a lesser extent, adipogenic and chondro/osteogenic cell lineage, when compared with TMRM-low cells. Conversely, TMRM-low showed higher self-renewal potential. To conclude, we identified two hCmPC populations with different metabolic profile, stemness maturity, and differentiation potential. Our findings suggest that metabolic sorting can isolate cells with higher regenerative capacity and/or long-term survival. This metabolism-based strategy to select cells may be broadly applicable to therapies.

Isolation, growth kinetics, and immunophenotypic characterization of adult human cardiac progenitor cells

The discovery of cardiac progenitor cells (CPCs) has raised expectations for the development of cell-based therapy of the heart. Although cell therapy is emerging as a novel treatment for heart failure, several issues still exist concerning an unambiguous definition of the phenotype of CPC types. There is a need to define and validate the methods for the generation of quality CPC populations used in cell therapy applications. Considering the critical roles of cardiac cell progenitors in cellular therapy, we speculate that long term culture might modulate the immunophenotypes of CPCs. Hence, a strategy to validate the isolation and cell culture expansion of cardiac cell populations was devised. Isolation of three subpopulations of human CPCs was done from a single tissue sample using explant, enzymatic isolation, and c-kit⁺ immunomagnetic sorting methods. The study assessed the effects of ex vivo expansion on proliferation, immunophenotypes, and differentiation of CPCs. Additionally, we report that an explant culture can take over 2 months to achieve similar cell yields, and cell sorting requires a much larger starting population to match this expansion time frame. In comparison, an enzymatic method is expected to yield equivalent quantities of CPCs in 2-3 weeks, notably at a significantly lower cost, which may intensify their use in therapeutic approaches. We determined that ex vivo expansion caused changes in cellular characteristics, and hence propose validated molecular signatures should be established to evaluate the impact of ex vivo expansion for a safe cell therapy product.

Preferential myofibroblast differentiation of cardiac mesenchymal progenitor cells in the presence of atrial fibrillation

Atrial fibrillation (AF) is characterized by electrical, contractile, and structural remodeling mediated by interstitial fibrosis. It has been shown that human cardiac mesenchymal progenitor cells (CMPCs) can be differentiated into endothelial, smooth muscle, and fibroblast cells. Here, we have investigated, for the first time, the contribution of CMPCs in the fibrotic process occurring in AF. As expected, right auricolae samples displayed significantly higher fibrosis in AF vs control (CTR) patients. In tissue samples of AF patients only, double staining for c-kit and the myofibroblast marker α-smooth muscle actin (α-SMA) was detected. The number of c-kit-positive CMPC was higher in atrial subepicardial regions of CTR than AF cells. AF-derived CMPC (AF-CMPC) and CTR-derived CMPC (Ctr-CMPC) were phenotypically similar, except for CD90 and c-kit, which were significantly more present in AF and CTR cells, respectively. Moreover, AF showed a lower rate of population doubling and fold enrichment vs Ctr-CMPC. When exogenously challenged with the profibrotic transforming growth factor-β1 (TGF-β1), AF-CMPC showed a significantly higher nuclear translocation of SMAD2 than Ctr-CMPC. In addition, TGF-β1 treatment induced the upregulation of COL1A1 and COL1A2 in AF-CMPC only. Further, both a marked production of soluble collagen and α-SMA upregulation have been observed in AF-CMPC only. Finally, electrophysiological studies showed that the inwardly rectifying potassium current (I_K1) was evenly present in AF- and Ctr-CMPC in basal conditions and similarly disappeared after TGF-β1 exposure. All together, these data suggest that AF steers the resident atrial CMPC compartment toward an electrically inert profibrotic phenotype.

Cell therapy for heart disease after 15 years: Unmet expectations

Over the past two decades cardiac cell therapy (CCT) has emerged as a promising new strategy to cure heart diseases at high unmet need. Thousands of patients have entered clinical trials for acute or chronic heart conditions testing different cell types, including autologous or allogeneic bone marrow (BM)-derived mononuclear or selected cells, BM- or adipose tissue-derived mesenchymal cells, or cardiac resident progenitors based on their potential ability to regenerate scarred or dysfunctional myocardium. Nowadays, the original enthusiasm surrounding the regenerative medicine field has been cushioned by a cumulative body of evidence indicating an inefficient or modest efficacy of CCT in improving cardiac function, along with the continued lack of indisputable proof for long-term prognostic benefit. In this review, we have firstly comprehensively outlined the positive and negative results of cell therapy studies in patients with acute myocardial infarction, refractory angina and chronic heart failure. Next, we have discussed cell therapy- and patient-related variables (e.g. cell intrinsic and extrinsic characteristics as well as criteria of patient selection and proposed methodologies) that might have dampened the efficacy of past cell therapy trials. Finally, we have addressed critical factors to be considered before embarking on further clinical trials.

Beta2-adrenergic signaling affects the phenotype of human cardiac progenitor cells through EMT modulation

Human cardiac progenitor cells (CPCs) offer great promises to cardiac cell therapy for heart failure. Many in vivo studies have shown their therapeutic benefits, paving the way for clinical translation. The 3D model of cardiospheres (CSs) represents a unique niche-like in vitro microenvironment, which includes CPCs and supporting cells. CSs have been shown to form through a process mediated by epithelial-to-mesenchymal transition (EMT). β2-Adrenergic signaling significantly affects stem/progenitor cells activation and mobilization in multiple tissues, and crosstalk between β2-adrenergic signaling and EMT processes has been reported. In the present study, we aimed at investigating the biological response of CSs to β2-adrenergic stimuli, focusing on EMT modulation in the 3D culture system of CSs. We treated human CSs and CS-derived cells (CDCs) with the β2-blocker butoxamine (BUT), using either untreated or β2 agonist (clenbuterol) treated CDCs as control. BUT-treated CS-forming cells displayed increased migration capacity and a significant increase in their CS-forming ability, consistently associated with increased expression of EMT-related genes, such as Snai1. Moreover, long-term BUT-treated CDCs contained a lower percentage of CD90+ cells, and this feature has been previously correlated with higher cardiogenic and therapeutic potential of the CDCs population. In addition, long-term BUT-treated CDCs had an increased ratio of collagen-III/collagen-I gene expression levels, and showed decreased release of inflammatory cytokines, overall supporting a less fibrosis-prone phenotype. In conclusion, β2 adrenergic receptor block positively affected the stemness vs commitment balance within CSs through the modulation of type1-EMT (so called "developmental"). These results further highlight type-1 EMT to be a key process affecting the features of resident cardiac progenitor cells, and mediating their response to the microenvironment.

Β-blockers treatment of cardiac surgery patients enhances isolation and improves phenotype of cardiosphere-derived cells

Β-blockers (BB) are a primary treatment for chronic heart disease (CHD), resulting in prognostic and symptomatic benefits. Cardiac cell therapy represents a promising regenerative treatment and, for autologous cell therapy, the patients clinical history may correlate with the biology of resident progenitors and the quality of the final cell product. This study aimed at uncovering correlations between clinical records of biopsy-donor CHD patients undergoing cardiac surgery and the corresponding yield and phenotype of cardiospheres (CSs) and CS-derived cells (CDCs), which are a clinically relevant population for cell therapy, containing progenitors. We describe a statistically significant association between BB therapy and improved CSs yield and CDCs phenotype. We show that BB-CDCs have a reduced fibrotic-like CD90 + subpopulation, with reduced expression of collagen-I and increased expression of cardiac genes, compared to CDCs from non-BB donors. Moreover BB-CDCs had a distinctive microRNA expression profile, consistent with reduced fibrotic features (miR-21, miR-29a/b/c downregulation), and enhanced regenerative potential (miR-1, miR-133, miR-101 upregulation) compared to non-BB. In vitro adrenergic pharmacological treatments confirmed cytoprotective and anti-fibrotic effects of β1-blocker on CDCs. This study shows anti-fibrotic and pro-commitment effects of BB treatment on endogenous cardiac reparative cells, and suggests adjuvant roles of β-blockers in cell therapy applications.

Cardiac mesenchymal stromal cells are a source of adipocytes in arrhythmogenic cardiomyopathy

AIM

Arrhythmogenic cardiomyopathy (ACM) is a genetic disorder mainly due to mutations in desmosomal genes, characterized by progressive fibro-adipose replacement of the myocardium, arrhythmias, and sudden death. It is still unclear which cell type is responsible for fibro-adipose substitution and which molecular mechanisms lead to this structural change. Cardiac mesenchymal stromal cells (C-MSC) are the most abundant cells in the heart, with propensity to differentiate into several cell types, including adipocytes, and their role in ACM is unknown. The aim of the present study was to investigate whether C-MSC contributed to excess adipocytes in patients with ACM.

METHODS AND RESULTS

We found that, in ACM patients' explanted heart sections, cells actively differentiating into adipocytes are of mesenchymal origin. Therefore, we isolated C-MSC from endomyocardial biopsies of ACM and from not affected by arrhythmogenic cardiomyopathy (NON-ACM) (control) patients. We found that both ACM and control C-MSC express desmosomal genes, with ACM C-MSC showing lower expression of plakophilin (PKP2) protein vs.

CONTROLS

Arrhythmogenic cardiomyopathy C-MSC cultured in adipogenic medium accumulated more lipid droplets than controls. Accordingly, the expression of adipogenic genes was higher in ACM vs. NON-ACM C-MSC, while expression of cell cycle and anti-adipogenic genes was lower. Both lipid accumulation and transcription reprogramming were dependent on PKP2 deficiency.

CONCLUSIONS

Cardiac mesenchymal stromal cells contribute to the adipogenic substitution observed in ACM patients' hearts. Moreover, C-MSC from ACM patients recapitulate the features of ACM adipogenesis, representing a novel, scalable, patient-specific in vitro tool for future mechanistic studies.

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.

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.

Interleukin-6 contributes to the paracrine effects of cardiospheres cultured from human, murine and rat hearts

Cardiosphere-derived cells (CDCs) were cultured from human, murine, and rat hearts. Diluted supernatant (conditioned-medium) of the cultures improved the contractile behavior of isolated rat cardiomyocytes (CMCs). This effect is mediated by the paracrine release of cytokines. The present study tested the hypothesis, that the cardiovascular state of the donor's heart influences this effect on CMCs and tries to identify the responsible factors. CDCs were cultured from human tissue samples of cardiac surgery and from murine and rat hearts. The supernatants of cultured CDCs from hypertensive humans and rats showed a higher improvement of the contractile behavior of CMCs compared to CDCs of normotensive origin. Subsequently, the cytokine profile of the supernatants was analyzed. Among the cytokines elevated in supernatants originating from hypertensive humans or rats was Interleukin-6. CDCs were also generated from Interleukin-6(-/-) -mice and their wildtype littermates. The supernatant of the cultured Interleukin-6(-/-) -CDCs had no effect on the contractile behavior, whereas the supernatant of the Interleukin-6(+/+) -CDCs showed a positive effect. To confirm the hypothesis that Interleukin-6 contributes to the paracrine effects, CMCs were incubated with Interleukin-6. It improved the contractile function in a concentration dependent way. Finally, the effect of the supernatant of cultured CDCs derived from a hypertensive human sample could be abolished by simultaneous incubation with a specific Interleukin-6 antibody. CDCs release cytokines that improve the contractile behavior of CMCs. This effect is more intense in CDCs from hypertensive donors. Interleukin-6 is involved in this phenomenon.

Expression of dual nucleotides/cysteinyl-leukotrienes receptor GPR17 in early trafficking of cardiac stromal cells after myocardial infarction

GPR17 is a G_i-coupled dual receptor activated by uracil-nucleotides and cysteinyl-leukotrienes. These mediators are massively released into hypoxic tissues. In the normal heart, GPR17 expression has been reported. By contrast, its role in myocardial ischaemia has not yet been assessed. In the present report, the expression of GPR17 was investigated in mice before and at early stages after myocardial infarction by using immunofluorescence, flow cytometry and RT-PCR. Before induction of ischaemia, results indicated the presence of the receptor in a population of stromal cells expressing the stem-cell antigen-1 (Sca-1). At early stages after ligation of the coronary artery, the receptor was expressed in Sca-1⁺ cells, and cells stained with Isolectin-B4 and anti-CD45 antibody. GPR17⁺ cells also expressed mesenchymal marker CD44. GPR17 function was investigated in vitro in a Sca-1⁺/CD31⁻ cell line derived from normal hearts. These experiments showed a migratory function of the receptor by treatment with UDP-glucose and leukotriene LTD4, two GPR17 pharmacological agonists. The GPR17 function was finally assessed in vivo by treating infarcted mice with Cangrelor, a pharmacological receptor antagonist, which, at least in part, inhibited early recruitment of GPR17⁺ and CD45⁺ cells. These findings suggest a regulation of heart-resident mesenchymal cells and blood-borne cellular species recruitment following myocardial infarction, orchestrated by GPR17.

Characteristic of c-Kit+ progenitor cells in explanted human hearts

According to literature data, self-renewing, multipotent, and clonogenic cardiac c-Kit⁺ progenitor cells occur within human myocardium. The aim of this study was to isolate and characterize c-Kit⁺ progenitor cells from explanted human hearts. Experimental material was obtained from 19 adult and 7 pediatric patients. Successful isolation and culture was achieved for 95 samples (84.1 %) derived from five different regions of the heart: right and left ventricles, atrium, intraventricular septum, and apex. The average percentage of c-Kit⁺ cells, as assessed by FACS, ranged between 0.7 and 0.9 %. In contrast to published data we do not observed statistically significant differences in the number of c-Kit⁺ cells between disease-specific groups, parts of the heart or sexes. Nevertheless, c-Kit⁺ cells were present in significant numbers (11–24 %) in samples derived from three explanted pediatric hearts. c-Kit⁺ cells were also positive for CD105 and a majority of them was positive for CD31 and CD34 (83.7 ± 8.6 and 75.7 ± 11.4 %, respectively). Immunohistochemical analysis of the heart tissue revealed that most cells possessing the c-Kit antigen were also positive for tryptase, a specific mast cell marker. However, flow cytometry analysis has shown cultured c-Kit⁺ cells to be negative for hematopoietic marker CD45 and mast cell marker CD33. Isolated c-Kit⁺ cells display mesenchymal stem cell features and are thought to differentiate into endothelial cells.

Epicardial adipose tissue and atrial fibrillation

Atrial fibrillation (AF) is the most frequent cardiac arrhythmia in clinical practice. AF is often associated with profound functional and structural alterations of the atrial myocardium that compose its substrate. Recently, a relationship between the thickness of epicardial adipose tissue (EAT) and the incidence and severity of AF has been reported. Adipose tissue is a biologically active organ regulating the metabolism of neighbouring organs. It is also a major source of cytokines. In the heart, EAT is contiguous with the myocardium without fascia boundaries resulting in paracrine effects through the release of adipokines. Indeed, Activin A, which is produced in abundance by EAT during heart failure or diabetes, shows a marked fibrotic effect on the atrial myocardium. The infiltration of adipocytes into the atrial myocardium could also disorganize the depolarization wave front favouring micro re-entry circuits and local conduction block. Finally, EAT contains progenitor cells in abundance and therefore could be a source of myofibroblasts producing extracellular matrix. The study on the role played by adipose tissue in the pathogenesis of AF is just starting and is highly likely to uncover new biomarkers and therapeutic targets for AF.

Endogenous cardiac stem cells for the treatment of heart failure

Stem cell-based therapies hold promise for regenerating the myocardium after injury. Recent data obtained from phase I clinical trials using endogenous cardiovascular progenitors isolated directly from the heart suggest that cell-based treatment for heart patients using stem cells that reside in the heart provides significant functional benefit and an improvement in patient outcome. Methods to achieve improved engraftment and regeneration may extend this therapeutic benefit. Endogenous cardiovascular progenitors have been tested extensively in small animals to identify cells that improve cardiac function after myocardial infarction. However, the relative lack of large animal models impedes translation into clinical practice. This review will exclusively focus on the latest research pertaining to humans and large animals, including both endogenous and induced sources of cardiovascular progenitors.

Functional multipotency of stem cells: what do we need from them in the heart?

After more than ten years of human research in the field of cardiac regenerative medicine, application of stem cells in different phases of ischemic heart disease has come to age. Randomized clinical trials have demonstrated that stem cell therapy can improve cardiac recovery after the acute phase of myocardial ischemia and in patients with chronic ischemic heart disease, and several efficacy phase III trials with clinical endpoints are on their way. Nevertheless, a complete knowledge on the mechanisms of action of stem cells still remains elusive. Of the three main mechanisms by which stem cells could exert their benefit, paracrine signaling from the administered cells and stimulation of endogenous repair are nowadays the most plausible ones. However, in this review we will define and discuss the concept of stem cell potency and differentiation, will examine the evidence available, and will depict future directions of research.