Regeneration of ischemic cardiac muscle and vascular endothelium by adult stem cells

Stem cell therapy: challenges ahead

Stem cells have generated great interest for their potential therapeutic use because of their capacity to self-renew indefinitely and to generate all cell lineages (pluripotency). Many diseases such as neurodegenerative disorders or diabetes are caused by loss of functionality or deficiency of a particular cell type. Stem cells differentiated into a specific cell type such as pancreatic β-cells or neurons, for example, thus hold great promise for regenerative medicine. However, many challenges have to be overcome before stem cell therapy can become a viable clinical approach.

Circulating cells contribute to cardiomyocyte regeneration after injury

RATIONALE

The contribution of bone marrow-borne hematopoietic cells to the ischemic myocardium has been documented. However, a pivotal study reported no evidence of myocardial regeneration from hematopoietic-derived cells. The study did not take into account the possible effect of early injury-induced signaling as the test mice were parabiotically paired to partners immediately after surgery-induced myocardial injury when cross-circulation has not yet developed.

OBJECTIVE

To re-evaluate the role of circulating cells in the injured myocardium.

METHODS AND RESULTS

By combining pulse-chase labeling and parabiosis model, we show that circulating cells derived from the parabiont expressed cardiac-specific markers in the injured myocardium. Genetic fate mapping also revealed that circulating hematopoietic cells acquired cardiac cell fate by means of cell fusion and transdifferentiation.

CONCLUSIONS

These results suggest that circulating cells participate in cardiomyocyte regeneration in a mouse model of parabiosis when the circulatory system is fully developed before surgery-induced heart injury.

An emerging consensus on cardiac regeneration

Cardiac regeneration is a rapidly evolving and controversial field of research. The identification some 12 years ago of progenitor cells that reside within the heart spurred enthusiasm for cell-based regenerative therapies. However, recent evidence has called into question both the presence of a biologically important stem cell population in the heart and the ability of exogenously derived cells to promote regeneration through direct formation of new cardiomyocytes. Here, we discuss recent developments that suggest an emerging consensus on the ability of different cell types to regenerate the adult mammalian heart.

Unique expression pattern and functional role of periostin in human limbal stem cells

Periostin is a non-structural matricellular protein. Little is known about periostin in human limbal stem cells (LSCs). This study was to explore the unique expression pattern and functional role of periostin in maintaining the properties of human LSCs. Fresh donor corneal tissues were used to make cryosections for evaluation of periostin expression on ex vivo tissues. Primary human limbal epithelial cells (HLECs) were generated from limbal explant culture. In vitro culture models for proliferation and epithelial regeneration were performed to explore functional role of periostin in LSCs. The mRNA expression was determined by reverse transcription and quantitative real-time PCR (RT-qPCR), and the protein production and localization were detected by immunofluorescent staining and Western blot analysis. Periostin protein was found to be exclusively immunolocalized in the basal layer of human limbal epithelium. Periostin localization was well matched with nuclear factor p63, but not with corneal epithelial differentiation marker Keratin 3. Periostin transcripts was also highly expressed in limbal than corneal epithelium. In primary HLECs, periostin expression at mRNA and protein levels was significantly higher in 50% and 70% confluent cultures at exponential growth stage than in 100% confluent cultures at slow growth or quiescent condition. This expression pattern was similar to other stem/progenitor cell markers (p63, integrin β1 and TCF4). Periostin expression at transcripts, protein and immunoreactivity levels increased significantly during epithelial regeneration in wound healing process, especially in 16-24 hours at wound edge, which was accompanied by similar upregulation and activation of p63, integrin β1 and TCF4. Our findings demonstrated that periostin is exclusively produced by limbal basal epithelium and co-localized with p63, where limbal stem cells reside. Periostin promotes HLEC proliferation and regeneration with accompanied activation of stem/progenitor cell markers p63, integrin β1 and TCF4, suggesting its novel role in maintaining the phenotype and functional properties of LSC.

The Role of MicroRNAs in Cardiac Stem Cells

Stem cells are considered as the next generation drug treatment in patients with cardiovascular disease who are resistant to conventional treatment. Among several stem cells used in the clinical setting, cardiac stem cells (CSCs) which reside in the myocardium and epicardium of the heart have been shown to be an effective option for the source of stem cells. In normal circumstances, CSCs primarily function as a cell store to replace the physiologically depleted cardiovascular cells, while under the diseased condition they have been shown to experimentally regenerate the diseased myocardium. In spite of their major functional role, molecular mechanisms regulating the CSCs proliferation and differentiation are still unknown. MicroRNAs (miRs) are small, noncoding RNA molecules that regulate gene expression at the posttranscriptional level. Recent studies have demonstrated the important role of miRs in regulating stem cell proliferation and differentiation, as well as other physiological and pathological processes related to stem cell function. This review summarises the current understanding of the role of miRs in CSCs. A deeper understanding of the mechanisms by which miRs regulate CSCs may lead to advances in the mode of stem cell therapies for the treatment of cardiovascular diseases.

Strategies for cardiac regeneration and repair

Heart failure is a growing epidemic caused by cardiomyocyte depletion. Current therapies prolong survival by protecting remaining cardiomyocytes but are unable to overcome the fundamental problem of regenerating lost cardiomyocytes. Several strategies for promoting heart regeneration have emerged from decades of intensive study. Although some of these strategies remain confined to basic research, others are beginning to be tested in humans. We review strategies for cardiac regeneration and summarize progress of related clinical trials.

High Mobility Group Box 1 Promotes Angiogenesis from Bone Marrow-derived Endothelial Progenitor Cells after Myocardial Infarction

AIMS

High mobility group box 1 (HMGB1) is a DNA-binding protein secreted into the extracellular space from necrotic cells that acts as a cytokine. We examined the role of HMGB1 in angiogenesis from bone marrow-derived cells in the heart using transgenic mice exhibiting the cardiac-specific overexpression of HMGB1 (HMGB1-TG).

METHODS

HMGB1-TG mice and wild-type littermate (WT) mice were lethally irradiated and injected with bone marrow cells from green fluorescent protein mice through the tail vein. After bone marrow transplantation, the left anterior descending artery was ligated to induce myocardial infarction (MI).

RESULTS

Flow cytometry revealed that the levels of circulating endothelial progenitor cells (EPCs) mobilized from the bone marrow increased after MI in the HMGB-TG mice versus the WT mice. In addition, the size of MI was smaller in the HMGB1-TG mice than in the WT mice, and immunofluorescence staining demonstrated that the number of engrafted vascular endothelial cells derived from bone marrow in the border zones of the MI areas was increased in the HMGB1-TG mice compared to that observed in the WT mice. Moreover, the levels of cardiac vascular endothelial growth factor after MI were higher in the HMGB1-TG mice than in the WT mice.

CONCLUSIONS

The present study demonstrated that HMGB1 promotes angiogenesis and reduces the MI size by enhancing the mobilization and differentiation of bone marrow cells to EPCs as well as their migration to the border zones of the MI areas and engraftment as vascular endothelial cells in new capillaries or arterioles in the infarcted heart.

The potential role of genetically-modified pig mesenchymal stromal cells in xenotransplantation

Mesenchymal stromal cells (MSCs) are known to have regenerative, anti-inflammatory, and immunodulatory effects. There are extensive indications that pig MSCs function satisfactorily across species barriers. Pig MSCs might have considerable therapeutic potential, particularly in xenotransplantation, where they have several potential advantages. (i) pMSCs can be obtained from the specific organ- or cell-source donor pig or from an identical (cloned) pig. (ii) They are easy to obtain in large numbers, negating the need for prolonged ex vivo expansion. (iii) They can be obtained from genetically-engineered pigs, and the genetic modification can be related to the therapeutic goal of the MSCs. We have reviewed our own studies on MSCs from genetically-engineered pigs, and summarize them here. We have successfully harvested and cultured MSCs from wild-type and genetically-engineered pig bone marrow and adipose tissue. We have identified several pig (p)MSC surface markers (positive for CD29, CD44, CD73, CD105, CD166, and negative for CD31, CD45), have demonstrated their proliferation and differentiation (into adipocytes, osteoblasts, and chondroblasts), and evaluated their antigenicity and immune suppressive effects on human peripheral blood mononuclear cells and CD4(+)T cells. They have identical or very similar characteristics to MSCs from other mammals. Genetically-modified pMSCs are significantly less immunogenic than wild-type pMSCs, and downregulate the human T cell response to pig antigens as efficiently as do human MSCs. We hypothesized that pMSCs can immunomodulate human T cells through induction of apoptosis or anergy, or cause T cell phenotype switching with induction of regulatory T cells, but we could find no evidence for these mechanisms. However, pMSCs upregulated the expression of CD69 on human CD4(+) and CD8(+) T cells, the relevance of which is currently under investigation. We conclude that MSCs from genetically-engineered pigs should continue to be investigated for their immunomodulatory (and regenerative and anti-inflammatory) effects in pig-to-nonhuman primate organ and cell transplantation models.

Mesenchymal stem cell insights: prospects in cardiovascular therapy

Ischemic heart damage usually triggers cardiomyopathological remodeling and fibrosis, thus promoting the development of heart functional failure. Mesenchymal stem cells (MSCs) are a heterogeneous group of cells in culture, with multipotent and hypoimmunogenic characters to aid tissue repair and avoid immune responses, respectively. Numerous experimental findings have proven the feasibility, safety, and efficiency of MSC therapy for cardiac regeneration. Despite that the exact mechanism remains unclear, the therapeutic ability of MSCs to treat ischemia heart diseases has been tested in phase I/II clinical trials. Based on encouraging preliminary findings, MSCs might become a potentially efficacious tool in the therapeutic options available to treat ischemic and nonischemic cardiovascular disorders. The molecular mechanism behind the efficacy of MSCs on promoting engraftment and accelerating the speed of heart functional recovery is still waiting for clarification. It is hypothesized that cardiomyocyte regeneration, paracrine mechanisms for cardiac repair, optimization of the niche for cell survival, and cardiac remodeling by inflammatory control are involved in the interaction between MSCs and the damaged myocardial environment. This review focuses on recent experimental and clinical findings related to cellular cardiomyoplasticity. We focus on MSCs, highlighting their roles in cardiac tissue repair, transdifferentiation, the MSC niche in myocardial tissues, discuss their therapeutic efficacy that has been tested for cardiac therapy, and the current bottleneck of MSC-based cardiac therapies.

Selective Migration of Subpopulations of Bone Marrow Cells along an SDF-1α and ATP Gradient

Both stem cell chemokine stromal cell-derived factor-1α (SDF-1α) and extracellular nucleotides such as adenosine triphosphate (ATP) are increased in ischemic myocardium. Since ATP has been reported to influence cell migration, we analysed the migratory response of bone marrow cells towards a combination of SDF-1 and ATP. Total nucleated cells (BM-TNCs) were isolated from bone marrow of cardiac surgery patients. Migration assays were performed in vitro. Subsequently, migrated cells were subjected to multicolor flow cytometric analysis of CD133, CD34, CD117, CD184, CD309, and CD14 expression. BM-TNCs migrated significantly towards a combination of SDF-1 and ATP. The proportions of CD34+ cells as well as subpopulations coexpressing multiple stem cell markers were selectively enhanced after migration towards SDF-1 or SDF-1 + ATP. After spontaneous migration, significantly fewer stem cells and CD184+ cells were detected. Direct incubation with SDF-1 led to a reduction of CD184+ but not stem cell marker-positive cells, while incubation with ATP significantly increased CD14+ percentage. In summary, we found that while a combination of SDF-1 and ATP elicited strong migration of BM-TNCs in vitro, only SDF-1 was responsible for selective attraction of hematopoietic stem cells. Meanwhile, spontaneous migration of stem cells was lower compared to BM-TNCs or monocytes.

Donor mesenchymal stromal cells (MSCs) undergo variable cardiac reprogramming in vivo and predominantly co-express cardiac and stromal determinants after experimental acute myocardial infarction

We previously showed the emergence of predominantly non-fused murine cells co-expressing cardiac and stromal determinants in co-cultures of murine mesenchymal stromal cells (MSCs) and rat embryonic cardiomyocytes. To determine whether a similar phenotype is detectable in vivo in ischemic myocardium, we infused green fluorescence protein (GFP)-marked MSCs intravenously into wild-type mice in an acute myocardial infarction (AMI) model generated by ischemia/reperfusion (I/R) or fixed coronary artery ligation. We found that infused GFP+ cells were confined strictly to ischemic areas and represented approximately 10% of total cellularity. We showed that over 60% of the cells co-expressed collagen type IV and troponin T or myosin heavy chain, characteristic of MSCs and cardiomyocytes, respectively, and were CD45(-). Nonetheless, up to 25% of the GFP+ donor cells expressed one of two cardiomyocyte markers, either myosin heavy chain or troponin T, in the absence of MSC determinants. We also observed a marked reduction in OCT4 expression in MSCs pre-infusion compared with those lodged in the myocardium, suggesting reduced stem cell properties. Despite the low frequency of lodged donor MSCs, left-ventricular end-diastolic pressure was significantly better in experimental versus saline animals for both AMI (12.10 ± 1.81 vs. 20.50 ± 1.53 mmHg, p < 0.001) and I/R models (8.75 ± 2.95 vs. 17.53 ± 3.85 mmHg, p = 0.004) when measured 21 days after MSC infusion and is consistent with a paracrine effect. Our data indicate that donor MSCs undergo variable degrees of cardiomyocyte reprogramming with the majority co-expressing cardiomyocyte and stromal markers. Further studies are needed to elucidate the factors mediating the extent of cardiomyocyte reprogramming and importance of the cellular changes on tissue repair.

Biology and flow cytometry of proangiogenic hematopoietic progenitors cells

During development, hematopoiesis and neovascularization are closely linked to each other via a common bipotent stem cell called the hemangioblast that gives rise to both hematopoietic cells and endothelial cells. In postnatal life, this functional connection between the vasculature and hematopoiesis is maintained by a subset of hematopoietic progenitor cells endowed with the capacity to differentiate into potent proangiogenic cells. These proangiogenic hematopoietic progenitors comprise a specific subset of bone marrow (BM)-derived cells that homes to sites of neovascularization and possess potent paracrine angiogenic activity. There is emerging evidence that this subpopulation of hematopoietic progenitors plays a critical role in vascular health and disease. Their angiogenic activity is distinct from putative "endothelial progenitor cells" that become structural cells of the endothelium by differentiation into endothelial cells. Proangiogenic hematopoietic progenitor cell research requires multidisciplinary expertise in flow cytometry, hematology, and vascular biology. This review provides a comprehensive overview of proangiogenic hematopoietic progenitor cell biology and flow cytometric methods to detect these cells in the peripheral blood circulation and BM.

Targeted delivery of genes to endothelial cells and cell- and gene-based therapy in pulmonary vascular diseases

Pulmonary arterial hypertension (PAH) is a devastating disease that, despite significant advances in medical therapies over the last several decades, continues to have an extremely poor prognosis. Gene therapy is a method to deliver therapeutic genes to replace defective or mutant genes or supplement existing cellular processes to modify disease. Over the last few decades, several viral and nonviral methods of gene therapy have been developed for preclinical PAH studies with varying degrees of efficacy. However, these gene delivery methods face challenges of immunogenicity, low transduction rates, and nonspecific targeting which have limited their translation to clinical studies. More recently, the emergence of regenerative approaches using stem and progenitor cells such as endothelial progenitor cells (EPCs) and mesenchymal stem cells (MSCs) have offered a new approach to gene therapy. Cell-based gene therapy is an approach that augments the therapeutic potential of EPCs and MSCs and may deliver on the promise of reversal of established PAH. These new regenerative approaches have shown tremendous potential in preclinical studies; however, large, rigorously designed clinical studies will be necessary to evaluate clinical efficacy and safety.

Stem cell therapy and breast cancer treatment: review of stem cell research and potential therapeutic impact against cardiotoxicities due to breast cancer treatment

A new problem has emerged with the ever-increasing number of breast cancer survivors. While early screening and advances in treatment have allowed these patients to overcome their cancer, these treatments often have adverse cardiovascular side effects that can produce abnormal cardiovascular function. Chemotherapeutic and radiation therapy have both been linked to cardiotoxicity; these therapeutics can cause a loss of cardiac muscle and deterioration of vascular structure that can eventually lead to heart failure (HF). This cardiomyocyte toxicity can leave the breast cancer survivor with a probable diagnosis of dilated or restrictive cardiomyopathy (DCM or RCM). While current HF standard of care can alleviate symptoms, other than heart transplantation, there is no therapy that replaces cardiac myocytes that are killed during cancer therapies. There is a need to develop novel therapeutics that can either prevent or reverse the cardiac injury caused by cancer therapeutics. These new therapeutics should promote the regeneration of lost or deteriorating myocardium. Over the last several decades, the therapeutic potential of cell-based therapy has been investigated for HF patients. In this review, we discuss the progress of pre-clinical and clinical stem cell research for the diseased heart and discuss the possibility of utilizing these novel therapies to combat cardiotoxicity observed in breast cancer survivors.

Endothelial progenitor cells--potential new avenues to improve neoangiogenesis and reendothelialization

The term endothelial progenitor cell (EPC) was established more than 10 years ago and is used to refer to a group of circulating cells that display endothelial lineage qualities and are able to home to areas of ischemia or vascular injury and to facilitate the repair of damaged blood vessels or develop new vessels as needed. This chapter reviews the current lineage relationships among all the cells called EPC and will clear the terminology used in EPC research. Furthermore, an overview of the clinical and in vitro research, as well as cytokine and drug interactions and potential EPC applications, is given.

Growth factor- and cytokine-stimulated endothelial progenitor cells in post-ischemic cerebral neovascularization

Endothelial progenitor cells are resident in the bone marrow blood sinusoids and circulate in the peripheral circulation. They mobilize from the bone marrow after vascular injury and home to the site of injury where they differentiate into endothelial cells. Activation and mobilization of endothelial progenitor cells from the bone marrow is induced via the production and release of endothelial progenitor cell-activating factors and includes specific growth factors and cytokines in response to peripheral tissue hypoxia such as after acute ischemic stroke or trauma. Endothelial progenitor cells migrate and home to specific sites following ischemic stroke via growth factor/cytokine gradients. Some growth factors are less stable under acidic conditions of tissue ischemia, and synthetic analogues that are stable at low pH may provide a more effective therapeutic approach for inducing endothelial progenitor cell mobilization and promoting cerebral neovascularization following ischemic stroke.

Regenerative principles enrich cardiac rehabilitation practice

Cardiovascular morbidity imposes a high degree of disability and mortality, with limited therapeutic options available in end-stage disease. Integral to standard of care, cardiac rehabilitation aims on improving quality-of-life and prolonging survival. The recent advent of regenerative technologies paves the way for a transformative era in rehabilitation medicine whereby, beyond controlling risk factors and disease progression, the prospect of curative solutions is increasingly tangible. To date, the spectrum of clinical experience in cardiac regenerative medicine relies on stem cell-based therapies delivered to the diseased myocardium either acutely/subacutely, after a coronary event, or in the setting of chronic heart failure. Application of autologous/allogeneic stem cell platforms has established safety and feasibility, with encouraging signals of efficacy. Newer protocols aim to purify cell populations in an attempt to eliminate nonregenerative and enrich for regenerative cell types before use. Most advanced technologies have been developed to isolate resident cell populations directly from the heart or, alternatively, condition cells from noncardiac sources to attain a disease-targeted lineage-specified phenotype for optimized outcome. Because a multiplicity of cell-based technologies has undergone phase I/II evaluation, pivotal trials are currently underway in larger patient populations. Translation of regenerative principles into clinical practice will increasingly involve rehabilitation providers across the continuum of patient care. Regenerative rehabilitation is thus an emerging multidisciplinary field, full of opportunities and ready to be explored.

Chemical genetics and its potential in cardiac stem cell therapy

Over the last decade or so, intensive research in cardiac stem cell biology has led to significant discoveries towards a potential therapy for cardiovascular disease; the main cause of morbidity and mortality in humans. The major goal within the field of cardiovascular regenerative medicine is to replace lost or damaged cardiac muscle and coronaries following ischaemic disease. At present, de novo cardiomyocytes can be generated either in vitro, for cell transplantation or disease modelling using directed differentiation of embryonic stem cells or induced pluripotent stem cells, or in vivo via direct reprogramming of resident adult cardiac fibroblast or ectopic stimulation of resident cardiac stem or progenitor cells. A major bottleneck with all of these approaches is the low efficiency of cardiomyocyte differentiation alongside their relative functional immaturity. Chemical genetics, and the application of phenotypic screening with small molecule libraries, represent a means to enhance understanding of the molecular pathways controlling cardiovascular cell differentiation and, moreover, offer the potential for discovery of new drugs to invoke heart repair and regeneration. Here, we review the potential of chemical genetics in cardiac stem cell therapy, highlighting not only the major contributions to the field so far, but also the future challenges.

Stem cell therapy for cardiovascular disease: the demise of alchemy and rise of pharmacology

Regenerative medicine holds great promise as a way of addressing the limitations of current treatments of ischaemic disease. In preclinical models, transplantation of different types of stem cells or progenitor cells results in improved recovery from ischaemia. Furthermore, experimental studies indicate that cell therapy influences a spectrum of processes, including neovascularization and cardiomyogenesis as well as inflammation, apoptosis and interstitial fibrosis. Thus, distinct strategies might be required for specific regenerative needs. Nonetheless, clinical studies have so far investigated a relatively small number of options, focusing mainly on the use of bone marrow-derived cells. Rapid clinical translation resulted in a number of small clinical trials that do not have sufficient power to address the therapeutic potential of the new approach. Moreover, full exploitation has been hindered so far by the absence of a solid theoretical framework and inadequate development plans. This article reviews the current knowledge on cell therapy and proposes a model theory for interpretation of experimental and clinical outcomes from a pharmacological perspective. Eventually, with an increased association between cell therapy and traditional pharmacotherapy, we will soon need to adopt a unified theory for understanding how the two practices additively interact for a patient's benefit.

Revisiting cardiovascular regeneration with bone marrow-derived angiogenic and vasculogenic cells

Cell-based therapy has emerged as a promising therapy for cardiovascular disease. Particularly, bone marrow (BM)-derived cells have been most extensively investigated and have shown encouraging results in preclinical studies. Clinical trials, however, have demonstrated split results in post-myocardial infarction cardiac repair. Mechanistically, transdifferentiation of BM-derived cells into cardiovascular tissue demonstrated by earlier studies is now known to play a minor role in functional recovery, and humoral and paracrine effects turned out to be main mechanisms responsible for tissue regeneration and functional recovery. With this advancement in the mechanistic insight of BM-derived cells, new efforts have been made to identify cell population, which can be readily isolated and obtained in sufficient quantity without mobilization and have higher therapeutic potential. Recently, haematopoietic CD31(+) cells, which are more prevalent in bone marrow and peripheral blood, have been revealed to have angiogenic and vasculogenic activities and strong potential for therapeutic neovascularization in ischaemic tissues. This article will cover the recent advances in BM-derived cell-based therapy and implication of CD31(+) cells.

The Role of Stem Cells in Wound Angiogenesis

Significance: Revascularization plays a critical role in wound healing and is regulated by a complex milieu of growth factors and cytokines. Deficiencies in revascularization contribute to the development of chronic nonhealing wounds. Recent Advances: Stem-cell-based therapy provides a novel strategy to enhance angiogenesis and improve wound healing. With bioethical concerns associated with embryonic stem cells, focus has shifted to different populations of vascular precursors, isolated from adult somatic tissue. Three main populations have been identified: endothelial progenitor cells, mesenchymal stem cells, and induced-pluripotent stem cells. These populations demonstrate great promise to positively influence neovascularization and wound repair. Critical Issues: Further studies to more definitively define each population are necessary to efficiently translate stem-cell-based therapeutic angiogenesis to the bedside. Better understanding of the physiologic pathways of how stem cells contribute to angiogenesis in normal tissue repair will help identify targets for successful therapeutic angiogenesis. Future Directions: Active studies in both animal models and clinical trials are being conducted to develop effective delivery routes, including dosing, route, and timing. Stem-cell-based therapy holds significant potential as a strategy for therapeutic angiogenesis in the care of patients with chronic nonhealing wounds.

Metastatic cancer stem cells: from the concept to therapeutics

Metastatic cancer stem cells (MCSCs) refer to a subpopulation of cancer cells with both stem cell properties and invasion capabilities that contribute to cancer metastasis. MCSCs have capability of self-renewal, potentials of multiple differentiation and development and/or reconstruction of cancer tissues. As compared with stationary cancer stem cells, MCSCs are capable of invasion to normal tissues such as vasculatures, resistance to chemo- and/or radio-therapies, escape from immune surveillance, survival in circulation and formation of metastasis. MCSCs are derived from invasive cancer stem cells (iCSCs) due to the plasticity of cancer stem cells, which is one of the characteristics of cancer cell heterogeneity. Both stages of iCSCs and MSCSs are the potential therapeutic targets for cancer metastasis in the future strategies of personalized cancer therapy.

Impact of prior chronic statin therapy and high-intensity statin therapy at discharge on circulating endothelial progenitor cell levels in patients with acute myocardial infarction: a prospective observational study

BACKGROUND

Endothelial progenitor stem cells (EPCs) are mobilized to the peripheral circulation in response to myocardial ischemia, playing a crucial role in vascular repair. Statins have been shown to stimulate EPCs. However, neither the impact of previous statin therapy on EPC response of acute myocardial infarction (AMI) patients nor the effect of post-AMI high-intensity statin therapy on the evolution of circulating EPC levels has yet been addressed. Therefore, we aimed to compare circulating EPC levels between patients receiving long-term statin therapy before the AMI and statin-naive patients and to assess the impact of high-intensity statin therapy at discharge on the evolution of circulating EPCs post-AMI.

METHODS

This is a prospective observational study of 100 AMI patients. Circulating EPCs (CD45dimCD34 + KDR + cells) and their subpopulation coexpressing the homing marker CXCR4 were quantified by the high-performance flow cytometer FACSCanto II in whole blood, in two different moments: within the first 24 h of admission and 3 months post-AMI. Patients were followed up clinically for 2 years.

RESULTS

Patients previously treated with statins had significantly higher levels of EPCs coexpressing CXCR4 (1.9 ± 1.4 vs. 1.3 ± 1.0 cells/1,000,000 events, p = 0.031) than statin-naive patients. In addition, the subanalysis of diabetics (N = 38) also revealed that patients previously on statins had significantly greater numbers of both CD45dimCD34 + KDR + CXCR4+ cells (p = 0.024) and CD45dimCD34 + KDR + CD133+ cells (p = 0.022) than statin-naive patients. Regarding the evolution of EPC levels after the AMI, patients not on a high-intensity statin therapy at discharge had a significant reduction of CD45dimCD34 + KDR + and CD45dimCD34 + KDR + CXCR4+ cells from baseline to 3 months follow-up (p = 0.031 and p = 0.005, respectively). However, patients discharged on a high-intensity statin therapy maintained circulating levels of all EPC populations, presenting at 3 months of follow-up significantly higher EPC levels than patients not on an intensive statin therapy. Moreover, the high-intensity statin treatment group had significantly better clinical outcomes during the 2-year follow-up period than patients not discharged on a high-intensity statin therapy.

CONCLUSION

Chronic statin therapy prior to an AMI strongly enhances the response of EPCs to myocardial ischemia, even in diabetic patients. Furthermore, high-intensity statin therapy after an AMI prevents the expected decrease of circulating EPC levels during follow-up. These results reinforce the importance of an early and intensive statin therapy in AMI patients.

A neonatal blueprint for cardiac regeneration

Adult mammals undergo minimal regeneration following cardiac injury, which severely compromises cardiac function and contributes to the ongoing burden of heart failure. In contrast, the mammalian heart retains a transient capacity for cardiac regeneration during fetal and early neonatal life. Recent studies have established the importance of several evolutionarily conserved mechanisms for heart regeneration in lower vertebrates and neonatal mammals including induction of cardiomyocyte proliferation, epicardial cell activation, angiogenesis, extracellular matrix deposition and immune cell infiltration. In this review, we provide an up-to-date account of the molecular and cellular basis for cardiac regeneration in lower vertebrates and neonatal mammals. The historical context for these recent findings and their ramifications for the future development of cardiac regenerative therapies are also discussed.

The potential of stem cells in the treatment of cardiovascular diseases

Although the adult mammalian heart was once believed to be a post-mitotic organ without any capacity for regeneration, recent findings have challenged this dogma. A modified view assigns to the mammalian heart a measurable capacity for regeneration throughout life. The ultimate goals of the cardiac regeneration field have been pursued by multiple strategies, including understanding the developmental biology of cardiomyocytes and cardiac stem and progenitor cells, applying chemical genetics, and engineering biomaterials and delivery methods that facilitate cell transplantation. Successful stimulation of endogenous regenerative capacity in injured adult mammalian hearts can benefit from studies of natural cardiac regeneration.

Specific protein markers for stem cell cross-talk with neighboring cells in the environment

A stem cell interacts with the neighboring cells in its environment. To maintain a living organism's metabolism, either cell-cell or cell-environment interactions may be significant. Usually, these cells communicate with each other through biological signaling by interactive behaviors of primary proteins or complementary chemicals. The signaling intermediates offer the stem cell's functionality on its metabolism. With the rapid advent of omics technologies, various specific markers by which stem cells cooperate with their surroundings have been discovered and established. In this article, we review several stem cell markers used to communicate with either cancer or immune cells in the human body.

JAK-STAT signaling in cardiomyogenesis of cardiac stem cells

Recently various kinds of cardiac stem/progenitor cells have been identified and suggested to be involved in cardiac repair and regeneration in injured myocardium. In this review, we focus on the roles of JAK-STAT signaling in cardiac stem/progenitor cells in cardiomyogenesis. JAK-STAT signaling plays important roles in the differentiation of stem cells into cardiac lineage cells. The activation of JAK-STAT signal elicits the mobilization of mesenchymal stem cells as well, contributing to the maintenance of cardiac function. Thus we propose that JAK-STAT could be a target signaling pathway in cardiac regenerative therapy.

Clinical application of stem cells for therapeutic angiogenesis in patients with peripheral arterial disease

Peripheral arterial disease (PAD) may ultimately cause to the loss of the affected limb due to gangrene or infection. Some patients with PAD may have severe coexisting diseases and diffuse involvement of their distal arteries, and so they are poor candidates for revascularization procedures. Angiogenesis has recently been suggested to be a new emerging treatment strategy for patients with PAD. Angiogenesis is defined as the sprouting of new capillaries from pre-existing vascular structures; this process plays a major role in the development of collateral vessels in an ischemic limb. Yet, the exact mechanism of angiogenesis is currently poorly understood. It has been established that angiogenesis is initiated by hypoxia and it requires various pro-angiogenic factors such as vascular endothelial growth factor. Therapeutic angiogenesis is aimed at enhancing natural angiogenesis by the administration of the cells or genes that can trigger angiogenesis and this can lead to pain relief and wound healing by the development of collateral vessels. Most of the recent clinical trials have reported that stem cell therapy for promoting angiogenesis in patients with PAD improves the ischemic symptoms and enhances wound healing. However, there are several limitations to approve a standard treatment for PAD such as small sample size in several prevous studies, their diverse inclusion criteria and the lack of standard assessment methods for the safety and outcome. Therefore, multicenter, large-scale and randomized controlled studies are needed to prove the safety and efficacy of the clinically applying stem cells for therapeutic angiogenesis in patients with PAD.

Effect of exogenous Oct4 overexpression on cardiomyocyte differentiation of human amniotic mesenchymal cells

Regenerative therapy is a new strategy for the end-stage heart failure; however, the ideal cell source has not yet been established for this therapy. We expected that the amnion might be an ideal cell source for cardiac regenerative therapy and that the differentiation potency of the human amnion mesenchymal cells (hAMCs) could be improved by overexpression of Oct4, a key factor that maintains the undifferentiated state. A plasmid vector was made by insertion of the Oct4 open reading frame (ORF) under control of a cytomegalovirus (CMV) promoter (pCMV-hOct4) and transfected into hAMCs by electroporation. The optimum induction time was investigated by comparing the quantity of stem cell-specific mRNAs, cardiac-specific mRNAs, and cardiac-specific proteins with time. hAMCs already expressed cardiac-specific proteins such as Nkx2.5 and Connexin43. After pCMV-hOct4 transfection, endogenous Oct4 mRNA and other stem cell markers showed a transient increase. With 5-azacytidine treatment, quantities of the cardiac-specific mRNAs, such as GATA4 and myosin light-chain-2v (Mlc-2v), were increased significantly. After Oct4 overexpression, the highest expression of cardiac-specific mRNAs and stem cell makers was seen at almost the same time. Furthermore, more mature myocardial contraction proteins were observed when hAMCs were induced at specific optimal times after gene transfection. In conclusion, hAMCs were activated to an undifferentiated state by overexpression of Oct4, and their cardiac differentiation potency was improved. Thus, the single-time transfection of the Oct4 expression vector may be a useful strategy for effective cell therapy. The use of cryopreserved hAMCs in cell therapy still requires more investigation.

Impact of short-term liquid storage on human CD133(+) stem cells

Stem cell transplantation is a viable strategy for regenerative medicine. However, it is inevitable to have cells undergo storage for several hours or days due to processing and transportation. Therefore, it is crucial to have rigidly controlled conditions ensuring the therapeutic benefit of isolated stem cells. In the present study, we investigated the impact of short-term storage on human CD133(+) cells. CD133(+) cells were isolated from human bone marrow and kept at standardized nonfreezing storage conditions for up to 72 h. Cell viability (apoptosis/necrosis) and expression of CD133 and CXCR4 were analyzed by flow cytometry. Metabolic activity was determined using an MTT assay; colony-forming ability, as well as endothelial-like differentiation, was further evaluated. A qRT-PCR array was employed to investigate the expression of stemness genes. CD133 and CXCR4 expressions were preserved at all time points. After 30 h, cell number and metabolic activity decreased, although no significant changes were detected in cell viability and proliferation as well as endothelial-like differentiation. Cell viability and proliferation decreased significantly only after 72 h of storage. Our results indicate that storage of isolated human CD133(+) bone marrow stem cells in liquid allows for high viability and functionality. However, storage time should be limited in order to avoid cell loss.

Clinical applications of mesenchymal stem cells in chronic diseases

Extraordinary progress in understanding several key features of stem cells has been made in the last ten years, including definition of the niche, and identification of signals regulating mobilization and homing as well as partial understanding of the mechanisms controlling self-renewal, commitment, and differentiation. This progress produced invaluable tools for the development of rational cell therapy protocols that have yielded positive results in preclinical models of genetic and acquired diseases and, in several cases, have entered clinical experimentation with positive outcome. Adult mesenchymal stem cells (MSCs) are nonhematopoietic cells with multilineage potential to differentiate into various tissues of mesodermal origin. They can be isolated from bone marrow and other tissues and have the capacity to extensively proliferate in vitro. Moreover, MSCs have also been shown to produce anti-inflammatory molecules which can modulate humoral and cellular immune responses. Considering their regenerative potential and immunoregulatory effect, MSC therapy is a promising tool in the treatment of degenerative, inflammatory, and autoimmune diseases. It is obvious that much work remains to be done to increase our knowledge of the mechanisms regulating development, homeostasis, and tissue repair and thus to provide new tools to implement the efficacy of cell therapy trials.

Transdifferentiation between bone and fat on bone metabolism

The transdifferentiation of bone and fat is a new insight in studying the increasingly bone marrow fat in the process of osteoporosis of elderly or menopause crowd which is increasing in prevalence. The loss of bone mass in osteoporosis is multifactorially determined and includes genetic, hormonal and environmental determinants. Although it has long been considered whether the transdifferentiation process does exist in vivo and whether it could be find in the same individual, interaction between skeleton and adipose tissue has been proved pre-clinically and clinically by increasing evidence. Here we focus on the current understanding of the transdifferentiation between bone and fat, the molecular interactions and future clinical implications of recent studies linking the transdifferentiation to bone metabolism diseases. Furthermore, a set of recommendations of bone and fat transdifferentiation on bone metabolism are also presented to facilitate evaluation of this magic process.

Sciatic nerve regeneration induced by transplantation of in vitro bone marrow stromal cells into an inside-out artery graft in rat

Traumatic injury to peripheral nerves results in considerable motor and sensory disability. Several research groups have tried to improve the regeneration of traumatized nerves by invention of favorable microsurgery. Effect of undifferentiated bone marrow stromal cells (BMSCs) combined with artery graft on peripheral nerve regeneration was studied using a rat sciatic nerve regeneration model. A 10-mm sciatic nerve defect was bridged using an artery graft (IOAG) filled with undifferentiated BMSCs (2 × 10(7) cells/mL). In control group, the graft was filled with phosphated buffer saline alone. The regenerated fibers were studied 4, 8 and 12 weeks after surgery. Assessment of nerve regeneration was based on behavioral, functional (Walking Track Analysis), electrophysiological, histomorphometric and immuohistochemical (Schwann cell detection by S-100 expression) criteria. The behavioral, functional and electrophysiological studies confirmed significant recovery of regenerated axons in IOAG/BMSC group (P < 0.05). Quantitative morphometric analyses of regenerated fibers showed the number and diameter of myelinated fibers in IOAG/BMSC group were significantly higher than in the control group (P < 0.05). This demonstrates the potential of using undifferentiated BMSCs combined with artery graft in peripheral nerve regeneration without limitations of donor-site morbidity associated with isolation of Schwann cells. It is also cost saving due to reduction in interval from tissue collection until cell injection, simplicity of laboratory procedures compared to differentiated BMSCs and may have clinical implications for the surgical management of patients after facial nerve transection.

Regenerative therapy for cardiovascular disease

Recent insights into myocardial biology uncovered a hereto unknown regenerative capacity of the adult heart. The discovery of dividing cardiomyocytes and the identification and characterization of cardiac stem and progenitor cells with myogenic and angiogenic potential have generated new hopes that cardiac regeneration and repair might become a therapeutic option. During the past decade, multiple candidate cells have been proposed for cardiac regeneration, and their mechanisms of action in the myocardium have been explored. Initial clinical trials have focused on the use of bone marrow-derived cells to promote myocardial regeneration in ischemic heart disease and have yielded very mixed results, with no clear signs of clinically meaningful functional improvement. Although the efficiency of bona fide cardiomyocyte generation is generally low, stem cells delivered into the myocardium act mainly via paracrine mechanisms. More recent studies taking advantage of cardiac committed cells (eg, resident cardiac progenitor cells or primed cardiogenic mesenchymal stem cells) showed promising results in first clinical pilot trials. Also, transplantation of cardiomyogenic cells generated by induced pluripotent stem cells and genetic reprogramming of dividing nonmyocytes into cardiomyocytes may constitute attractive new regenerative approaches in cardiovascular medicine in the future. We discuss advantages and limitations of specific cell types proposed for cell-based therapy in cardiology and give an overview of the first clinical trials using this novel therapeutic approach in patients with cardiovascular disease.

The effect of bone marrow mononuclear stem cell therapy on left ventricular function and myocardial perfusion

BACKGROUND

Bone marrow stem cell (BMC) transfer is an emerging therapy with potential to salvage cardiomyocytes during acute myocardial infarction and promote regeneration and endogenous repair of damaged myocardium in patients with left ventricular (LV) dysfunction. We performed a meta-analysis to examine the association between administration of BMC and LV functional recovery as assessed by imaging.

METHODS AND RESULTS

Our meta-analysis included data from 32 trials comprising information on 1,300 patients in the treatment arm and 1,006 patients in the control arm. Overall, BMC therapy was associated with a significant increase in LV ejection fraction by 4.6% ± 0.7% (P < .001) (control-adjusted increase of 2.8% ± 0.9%, P = .001), and a significant decrease in perfusion defect size by 9.5% ± 1.4% (P < .001) (control-adjusted decrease of 3.8% ± 1.2%, P = .002). The effect of BMC therapy was similar whether the cells were administered via intra-coronary or intra-myocardial routes and was not influenced by baseline ejection fraction or perfusion defect size.

CONCLUSIONS

BMC transfer appears to have a positive impact on LV recovery in patients with acute coronary syndrome and those with stable coronary disease with or without heart failure. Most studies were small and a minority used a core laboratory for image analysis.

Plasminogen regulates cardiac repair after myocardial infarction through its noncanonical function in stem cell homing to the infarcted heart

OBJECTIVES

The purpose of this study was to investigate the role of plasminogen (Plg) in stem cell-mediated cardiac repair and regeneration after myocardial infarction (MI).

BACKGROUND

An MI induces irreversible tissue damage, eventually leading to heart failure. Bone marrow (BM)-derived stem cells promote tissue repair and regeneration after MI. Thrombolytic treatment with Plg activators significantly improves the clinical outcome in MI by restoring cardiac perfusion. However, the role of Plg in stem cell-mediated cardiac repair remains unclear.

METHODS

An MI was induced in Plg-deficient (Plg(-/-)) and wild-type (Plg(+/+)) mice by ligation of the left anterior descending coronary artery. Stem cells were visualized by in vivo tracking of green fluorescent protein (GFP)-expressing BM cells after BM transplantation. Cardiac function, stem cell homing, and signaling pathways downstream of Plg were examined.

RESULTS

Granulocyte colony-stimulating factor, a stem cell mobilizer, significantly promoted BM-derived stem cell (GFP(+)c-kit(+) cell) recruitment into the infarcted heart and stem cell-mediated cardiac repair in Plg(+/+) mice. However, Plg deficiency markedly inhibited stem cell homing and cardiac repair, suggesting that Plg is critical for stem cell-mediated cardiac repair. Moreover, Plg regulated C-X-C chemokine receptor type 4 (CXCR4) expression in stem cells in vivo and in vitro through matrix metalloproteinase-9. Lentiviral reconstitution of CXCR4 expression in BM cells successfully rescued stem cell homing to the infarcted heart in Plg-deficient mice, indicating that CXCR4 has a critical role in Plg-mediated stem cell homing after MI.

CONCLUSIONS

These findings have identified a novel role for Plg in stem cell-mediated cardiac repair after MI. Thus, targeting Plg may offer a new therapeutic strategy for stem cell-mediated cardiac repair after MI.

Polycaprolactone-thiophene-conjugated carbon nanotube meshes as scaffolds for cardiac progenitor cells

The myocardium is unable to regenerate itself after infarct, resulting in scarring and thinning of the heart wall. Our objective was to develop a patch to buttress and bypass the scarred area, while allowing regeneration by incorporated cardiac stem/progenitor cells (CPCs). Polycaprolactone (PCL) was fabricated as both sheets by solvent casting, and fibrous meshes by electrospinning, as potential patches, to determine the role of topology in proliferation and phenotypic changes to the CPCs. Thiophene-conjugated carbon nanotubes (T-CNTs) were incorporated to enhance the mechanical strength. We showed that freshly isolated CPCs from murine hearts neither attached nor spread on the PCL sheets, both with and without T-CNT. As electrospun meshes, however, both PCL and PCL/T-CNT supported CPC adhesion, proliferation, and differentiation. The incorporation of T-CNT into PCL resulted in a significant increase in mechanical strength but no morphological changes to the meshes. In turn, proliferation, but not differentiation, of CPCs into cardiomyocytes was enhanced in T-CNT containing meshes. We have shown that changing the topology of PCL, a known hydrophobic material, dramatically altered its properties, in this case, allowing CPCs to survive and differentiate. With further development, PCL/T-CNT meshes or similar patches may become a viable strategy to aid restoration of the postmyocardial infarction myocardium.

Cell therapy for cardiac repair--lessons from clinical trials

The global impetus to identify curative therapies has been fuelled by the unmet needs of patients in the context of a growing heart failure pandemic. To date, regeneration trials in patients with cardiovascular disease have used stem-cell-based therapy in the period immediately after myocardial injury, in an attempt to halt progression towards ischaemic cardiomyopathy, or in the setting of congestive heart failure, to target the disease process and prevent organ decompensation. Worldwide, several thousand patients have now been treated using autologous cell-based therapy; the safety and feasibility of this approach has been established, pitfalls have been identified, and optimization procedures envisioned. Furthermore, the initiation of phase III trials to further validate the therapeutic value of cell-based regenerative medicine and address the barriers to successful clinical implementation has led to resurgence in the enthusiasm for such treatments among patients and health-care providers. In particular, poor definition of cell types used, diversity in cell-handling procedures, and functional variability intrinsic to autologously-derived cells have been identified as the main factors limiting adoption of cell-based therapies. In this Review, we summarize the experience obtained from trials of 'first-generation' cell-based therapy, and emphasize the advances in the purification and lineage specification of stem cells that have enabled the development of 'next-generation' stem-cell-based therapies targeting cardiovascular disease.

Very small embryonic-like stem cells (VSELs) represent a real challenge in stem cell biology: recent pros and cons in the midst of a lively debate

The concept that adult tissue, including bone marrow (BM), contains early-development cells with broader differentiation potential has again been recently challenged. In response, we would like to review the accumulated evidence from several independent laboratories that adult tissues, including BM, harbor a population of very rare stem cells that may cross germ layers in their differentiation potential. Thus, the BM stem cell compartment hierarchy needs to be revisited. These dormant, early-development cells that our group described as very small embryonic-like stem cells (VSELs) most likely overlap with similar populations of stem cells that have been identified in adult tissues by other investigators as the result of various experimental strategies and have been given various names. As reported, murine VSELs have some pluripotent stem cell characteristics. Moreover, they display several epiblast/germline markers that suggest their embryonic origin and developmental deposition in adult BM. Moreover, at the molecular level, changes in expression of parentally imprinted genes (for example, Igf2-H19) and resistance to insulin/insulin-like growth factor signaling (IIS) regulates their quiescent state in adult tissues. In several emergency situations related to organ damage, VSELs can be activated and mobilized into peripheral blood, and in appropriate animal models they contribute to tissue organ/regeneration. Interestingly, their number correlates with lifespan in mice, and they may also be involved in some malignancies. VSELs have been successfully isolated in several laboratories; however, some investigators experience problems with their isolation.

Soluble factors secreted by differentiating embryonic stem cells stimulate exogenous cell proliferation and migration

INTRODUCTION

Stem cells are being investigated as catalysts of tissue regeneration to either directly replace or promote cellularity lost as a result of traumatic injury or degenerative disease. In many reports, despite low numbers of stably integrated cells, the transient presence of cells delivered or recruited to sites of tissue remodeling globally benefits functional recovery. Such findings have motivated the need to determine how paracrine factors secreted from transplanted cells may be capable of positively impacting endogenous repair processes and somatic cell responses.

METHODS

Embryonic stem cells were differentiated as embryoid bodies (EBs) in vitro and media conditioned by EBs were collected at different intervals of time. Gene and protein expression analysis of several different growth factors secreted by EBs were examined by polymerase chain reaction and enzyme-linked immunosorbent assay analysis, respectively, as a function of time. The proliferation and migration of fibroblasts and endothelial cells treated with EB conditioned media was examined compared with unconditioned and growth media controls.

RESULTS

The expression of several growth factors, including bone morphogenic protein-4, insulin-like growth factors and vascular endothelial growth factor-A, increased during the course of embryonic stem cell (ESC) differentiation as EBs. Conditioned media collected from EBs at different stages of differentiation stimulated proliferation and migration of both fibroblasts and endothelial cells, based on 5-bromo-2'-deoxyuridine incorporation and transwell assays, respectively.

CONCLUSIONS

Overall, these results demonstrate that differentiating ESCs express increasing amounts of various growth factors over time that altogether are capable of stimulating mitogenic and motogenic activity of exogenous cell populations.

Reoxygenation-derived toxic reactive oxygen/nitrogen species modulate the contribution of bone marrow progenitor cells to remodeling after myocardial infarction

Background

The core region of a myocardial infarction is notoriously unsupportive of cardiomyocyte survival. However, there has been less investigation of the potentially beneficial spontaneous recruitment of endogenous bone marrow progenitor cells (BMPCs) within infarcted areas. In the current study we examined the role of tissue oxygenation and derived toxic species in the control of BMPC engraftment during postinfarction heart remodeling.

Methods and Results

For assessment of cellular origin, local oxygenation, redox status, and fate of cells in the infarcted region, myocardial infarction in mice with or without LacZ⁺ bone marrow transplantation was induced by coronary ligation. Sham‐operated mice served as controls. After 1 week, LacZ⁺ BMPC‐derived cells were found inhomogeneously distributed into the infarct zone, with a lower density at its core. Electron paramagnetic resonance (EPR) oximetry showed that pO₂ in the infarct recovered starting on day 2 post–myocardial infarction, concomitant with wall thinning and erythrocytes percolating through muscle microruptures. Paralleling this reoxygenation, increased generation of reactive oxygen/nitrogen species was detected at the infarct core. This process delineated a zone of diminished BMPC engraftment, and at 1 week infiltrating cells displayed immunoreactive 3‐nitrotyrosine and apoptosis. In vivo treatment with a superoxide dismutase mimetic significantly reduced reactive oxygen species formation and amplified BMPC accumulation. This treatment also salvaged wall thickness by 43% and left ventricular ejection fraction by 27%, with significantly increased animal survival.

Conclusions

BMPC engraftment in the infarct inversely mirrored the distribution of reactive oxygen/nitrogen species. Antioxidant treatment resulted in increased numbers of engrafted BMPCs, provided functional protection to the heart, and decreased the incidence of myocardial rupture and death.

Hemogenic endothelial cell specification requires c-Kit, Notch signaling, and p27-mediated cell-cycle control

Delineating the mechanism or mechanisms that regulate the specification of hemogenic endothelial cells from primordial endothelium is critical for optimizing their derivation from human stem cells for clinical therapies. We previously determined that retinoic acid (RA) is required for hemogenic specification, as well as cell-cycle control, of endothelium during embryogenesis. Herein, we define the molecular signals downstream of RA that regulate hemogenic endothelial cell development and demonstrate that cell-cycle control is required for this process. We found that re-expression of c-Kit in RA-deficient (Raldh2(-/-)) primordial endothelium induced Notch signaling and p27 expression, which restored cell-cycle control and rescued hemogenic endothelial cell specification and function. Re-expression of p27 in RA-deficient and Notch-inactivated primordial endothelial cells was sufficient to correct their defects in cell-cycle regulation and hemogenic endothelial cell development. Thus, RA regulation of hemogenic endothelial cell specification requires c-Kit, notch signaling, and p27-mediated cell-cycle control.