Human CD34+ very small embryonic-like stem cells can give rise to endothelial colony-forming cells with a multistep differentiation strategy using UM171 and nicotinamide acid

Murine and Human-Purified very Small Embryonic-like Stem Cells (VSELs) Express Purinergic Receptors and Migrate to Extracellular ATP Gradient

Purinergic signaling is an ancient primordial signaling system regulating tissue development and specification of various types of stem cells. Thus, functional purinergic receptors are present in several types of cells in the body, including multiple populations of stem cells. However, one stem cell type that has not been evaluated for expression of purinergic receptors is very small embryonic stem cells (VSELs) isolated from postnatal tissues. Herein, we report that human umbilical cord blood (UCB) and murine bone marrow (BM) purified VSELs express mRNA for P1 and P2 purinergic receptors and CD39 and CD73 ectonucleotidases converting extracellular ATP (eATP) into its signaling metabolite extracellular adenosine (eAdo), that antagonizes eATP effects. More importantly, we demonstrate that human and murine VSELs respond by chemotaxis to eATP, and eAdo inhibits this migration. These responses to eATP are mediated by activation of Nlrp3 inflammasome, and exposure of VSELs to its specific inhibitor MCC950 abolished the chemotactic response to ATP. We conclude that purinergic signaling plays an essential, underappreciated role in the biology of these cells and their potential role in response to tissue/organ injuries.

Thrombin receptor PAR1 silencing in endothelial colony-forming cells modifies stemness and vasculogenic properties

BACKGROUND

The involvement of thrombin receptor PAR1 in blood vessel development has been largely demonstrated in knockout mice; however, its implication in adult mouse angiogenesis seems very moderate.

OBJECTIVES

We aimed to explore the potential relationships between PAR1, stemness, and angiogenic properties of human endothelial colony-forming cells (ECFCs).

METHODS AND RESULTS

PAR1 activation on ECFCs using the selective PAR1-activating peptide induced a significant decrease in CD133 expression (RTQ-PCR analysis). In line, silencing of PAR1 gene expression with siRNA increased CD133 mRNA as well as intracellular CD133 protein expression. To confirm the link between CD133 and PAR1, we explored the association between PAR1 and CD133 levels in fast and slow fibroblasts prone to reprogramming. An imbalance between PAR1 and CD133 levels was evidenced, with a decreased expression of PAR1 in fast reprogramming fibroblasts expressing a high CD133 level. Regarding in vitro ECFC angiogenic properties, PAR1 silencing with specific siRNA induced cell proliferation evidenced by the overexpression of Ki67. However, it did not impact migration properties nor ECFC adhesion on smooth muscle cells or human arterial endothelial cells. In a mouse model of hind-limb ischemia, PAR1 silencing in ECFCs significantly increased postischemic revascularization compared to siCtrl-ECFCs along with a significant increase in cutaneous blood flows (P < .0001), microvessel density (P = .02), myofiber regeneration (P < .0001), and human endothelial cell incorporation in muscle (P < .0001).

CONCLUSION

In conclusion, our work describes for the first time a link between PAR1, stemness, and vasculogenesis in human ECFCs.

Development of a potency assay for CD34+ cell-based therapy

We have previously shown that intracardiac delivery of autologous CD34+ cells after acute myocardial infarction (AMI) is safe and leads to long term improvement. We are now conducting a multicenter, randomized, controlled Phase I/IIb study in post-AMI to investigate the safety and efficacy of intramyocardial injection of expanded autologous CD34+ cells (ProtheraCytes) (NCT02669810). Here, we conducted a series of in vitro studies characterizing the growth factor secretion, exosome secretion, gene expression, cell surface markers, differentiation potential, and angiogenic potential of ProtheraCytes clinical batches to develop a potency assay. We show that ProtheraCytes secrete vascular endothelial growth factor (VEGF) and its concentration is significantly correlated with the number of CD34+ cells obtained after expansion. ProtheraCytes also secrete exosomes containing proangiogenic miRNAs (126, 130a, 378, 26a), antiapoptotic miRNAs (21 and 146a), antifibrotic miRNAs (133a, 24, 29b, 132), and miRNAs promoting myocardial regeneration (199a and 590). We also show that ProtheraCytes have in vitro angiogenic activity, express surface markers of endothelial progenitor cells, and can differentiate in vitro into endothelial cells. After the in vitro characterization of multiple ProtheraCytes clinical batches, we established that measuring the concentration of VEGF provided the most practical, reliable, and consistent potency assay.

Nestin is a New Partner in Endothelial Colony Forming Cell Angiogenic Potential

Nestin, an intermediate filament protein expressed by progenitor cells, is associated with tissue regeneration. Although nestin expression has been reported in poorly differentiated and newly formed blood vessels, its role in endothelial cells remains unclear. In this study, we investigated the involvement of nestin in the angiogenic properties of endothelial colony-forming cells (ECFCs) derived from human umbilical cord blood. Our results demonstrate that ECFCs express high levels of nestin, and that its inhibition by small interfering RNAs decreased ECFC proliferation, migration in response to SDF-1 and VEGF-A, tubulogenesis, and adhesion on collagen. These effects are associated with modulation of focal adhesion kinase phosphorylation. Furthermore, nestin silencing resulted in reduced revascularization in a mouse hindlimb ischemia model. In conclusion, these findings provide evidence that nestin more than being a structural protein, is an active player in ECFC angiogenic properties.

Results of an international survey about methods used to isolate human endothelial colony-forming cells: guidance from the SSC on Vascular Biology of the ISTH

BACKGROUND

Assessment of endothelial colony-forming cell (ECFC) number and vasculogenic properties is crucial for exploring vascular diseases and regeneration strategies. A previous survey of the Scientific and Standardization Committee on Vascular Biology of the International Society on Thrombosis and Haemostasis clarified key methodological points but highlighted a lack of standardization associated with ECFC culture.

OBJECTIVES

The aim of this study was to provide expert consensus guidance on ECFC isolation and culture.

METHODS

We surveyed 21 experts from 10 different countries using a questionnaire proposed during the 2019 International Society on Thrombosis and Haemostasis Congress in Melbourne (Australia) to attain a consensus on ECFC isolation and culture.

RESULTS

We report here the consolidated results of the questionnaire. There was agreement on several general statements, mainly the technical aspects of ECFC isolation and cell culture. In contrast, on the points concerning the definition of a colony of ECFCs, the quantification of ECFCs, and the estimation of their age (in days or number of passages), the expert opinions were widely dispersed.

CONCLUSION

Our survey clearly indicates an unmet need for rigorous standardization, multicenter comparison of results, and validation of ECFC isolation and culture procedures for clinical laboratory practice and robustness of results. To this end, we propose a standardized protocol for the isolation and expansion of ECFCs from umbilical cord and adult peripheral blood.

Molecular analysis and comparison of CD34+ and CD133+ very small embryonic-like stem cells purified from umbilical cord blood

Very small embryonic like stem cells (VSELs) are a dormant population of stem cells that, as proposed, are deposited during embryogenesis in various tissues, including bone marrow (BM). These cells are released under steady state conditions from their tissue locations and circulate at a low level in peripheral blood (PB). Their number increases in response to stressors as well as tissue/organ damage. This increase is evident during neonatal delivery, as delivery stress prompts enrichment of umbilical cord blood (UCB) with VSELs. These cells could be purified from BM, PB, and UCB by multiparameter sorting as a population of very small CXCR4+ Lin- CD45- cells that express the CD34 or CD133 antigen. In this report, we evaluated a number of CD34+ Lin- CD45- and CD133+ Lin- CD45- UCB-derived VSELs. We also performed initial molecular characterization of both cell populations for expression of selected pluripotency markers and compared these cells at the proteomic level. We noticed that CD133+ Lin- CD45- population is more rare and express, at a higher level, mRNA for pluripotency markers Oct-4 and Nanog as well as the stromal-derived factor-1 (SDF-1) CXCR4 receptor that regulates trafficking of these cells, however both cells population did not significantly differ in the expression of proteins assigned to main biological processes.

Very small embryonic-like stem cells have the potential to win the three-front war on tissue damage, cancer, and aging

The concept of dedifferentiation and reprogramming of mature somatic cells holds much promise for the three-front "war" against tissue damage, cancer, and aging. It was hoped that reprogramming human somatic cells into the induced pluripotent state, along with the use of embryonic stem cells, would transform regenerative medicine. However, despite global efforts, clinical applications remain a distant dream, due to associated factors such as genomic instability, tumorigenicity, immunogenicity, and heterogeneity. Meanwhile, the expression of embryonic (pluripotent) markers in multiple cancers has baffled the scientific community, and it has been suggested that somatic cells dedifferentiate and "reprogram" into the pluripotent state in vivo to initiate cancer. It has also been suggested that aging can be reversed by partial reprogramming in vivo. However, better methods are needed; using vectors or Yamanaka factors in vivo, for example, is dangerous, and many potential anti-aging therapies carry the same risks as those using induced pluripotent cells, as described above. The present perspective examines the potential of endogenous, pluripotent very small embryonic-like stem cells (VSELs). These cells are naturally present in multiple tissues; they routinely replace diseased tissue and ensure regeneration to maintain life-long homeostasis, and they have the ability to differentiate into adult counterparts. Recent evidence suggests that cancers initiate due to the selective expansion of epigenetically altered VSELs and their blocked differentiation. Furthermore, VSEL numbers have been directly linked to lifespan in studies of long- and short-lived transgenic mice, and VSEL dysfunction has been found in the ovaries of aged mice. To conclude, a greater interest in VSELs, with their potential to address all three fronts of this war, could be the "light at the end of the tunnel."

CD34 positive cells as endothelial progenitor cells in biology and medicine

CD34 is a cell surface antigen expressed in numerous stem/progenitor cells including hematopoietic stem cells (HSCs) and endothelial progenitor cells (EPCs), which are known to be rich sources of EPCs. Therefore, regenerative therapy using CD34⁺ cells has attracted interest for application in patients with various vascular, ischemic, and inflammatory diseases. CD34⁺ cells have recently been reported to improve therapeutic angiogenesis in a variety of diseases. Mechanistically, CD34⁺ cells are involved in both direct incorporation into the expanding vasculature and paracrine activity through angiogenesis, anti-inflammatory, immunomodulatory, and anti-apoptosis/fibrosis roles, which support the developing microvasculature. Preclinical, pilot, and clinical trials have well documented a track record of safety, practicality, and validity of CD34⁺ cell therapy in various diseases. However, the clinical application of CD34⁺ cell therapy has triggered scientific debates and controversies in last decade. This review covers all preexisting scientific literature and prepares an overview of the comprehensive biology of CD34⁺ cells as well as the preclinical/clinical details of CD34⁺ cell therapy for regenerative medicine.

Industrialized GMP Production of CD34+ Cells (ProtheraCytes®) at Clinical Scale for Treatment of Ischemic Cardiac Diseases Is Feasible and Safe

Regenerative medicine now needs to pass a crucial turning point, from academic research to the market. Several sources/types of cells have been experimented with, more or less successfully. CD34⁺ cells have demonstrated multipotent or even pluripotent capacities, making them good candidates for regenerative medicine, particularly for treating heart diseases. Strongly encouraged by the results we achieved in a pilot study using CD34⁺ stem cells in patients with poor-prognosis acute myocardial infarcts (AMIs), we soon began the development of an industrialized platform making use of a closed automated device (StemXpand^®) and a disposable kit (StemPack^®) for the large-scale expansion of CD34⁺ cells with reproducible good manufacturing practice (GMP). This scalable platform can produce expanded CD34⁺ cells (ProtheraCytes^®) of sufficient quality that, interestingly, express early markers of the cardiac and endothelial pathways and early cardiac-mesoderm markers. They also contain CD34⁺ pluripotent cells characterized as very small embryonic-like stem cells (VSELs), capable of differentiating under appropriate stimuli into different tissue lineages, including endothelial and cardiomyocytic ones.