Fetal liver very small embryonic/epiblast like stem cells follow developmental migratory pathway of hematopoietic stem cells

Isolation of a novel embryonic stem cell cord blood-derived population with in vitro hematopoietic capacity in the presence of Wharton's jelly-derived mesenchymal stromal cells

BACKGROUND

Recent studies highlight the existence of a population of cord blood (CB)-derived stem cells that bare embryonic features (very small embryonic-like stem cells [VSELs]) as the most primitive CB-stem cell population. In the present study, we present for the first time a novel and high purity isolation method of VSELs with in vitro hematopoietic capacity in the presence of Wharton's jelly-derived mesenchymal stromal cells (WJ-MSCs).

METHODS

The experimental procedure includes isolation upon gradually increased centrifugation spins and chemotaxis to Stromal cell-derived factor 1a (SDF-1a). Τhis cell population is characterized with flow cytometry, alkaline phosphatase (ALP) staining and qRT-PCR. The functional role of the isolated VSELs is assayed following co-culture with WJ-MSCs or bone marrow-derived mesenchymal stromal cells (BM-MSCs), whereas the stimulation of the quiescent VSEL population is verified via cell cycle analysis. The in vitro hematopoietic capacity is evaluated in methylcellulose cultures and also through induction of erythroid differentiation.

RESULTS

The final isolated subpopulation is characterized as a small-sized CD45/Lineage-/CXCR4+/CD133+/SSEA-4+cell population, positive in ALP staining and overexpressing the Oct3/4, Nanog and Sox-2 transcription factors. Upon the co-culture with MSCs, a stimulation of the quiescent VSEL population is observed. An impressive increase in the co-expression of the CD34+/CD45+ markers is observed following the co-culture with the WJ-MSCs, which is confirmed by the intense clonogenic ability suggesting in vitro differentiation toward all of the hematopoietic cell lineages and successful differentiation toward erythrocytes.

DISCUSSION

Conclusively, we propose a novel, rapid and rather simplified isolation method of CB-VSELs, capable of in vitro hematopoiesis.

Endogenous, very small embryonic-like stem cells: critical review, therapeutic potential and a look ahead

BACKGROUND

Both pluripotent very small embryonic-like stem cells (VSELs) and induced pluripotent stem (iPS) cells were reported in 2006. In 2012, a Nobel Prize was awarded for iPS technology whereas even today the very existence of VSELs is not well accepted. The underlying reason is that VSELs exist in low numbers, remain dormant under homeostatic conditions, are very small in size and do not pellet down at 250-280g. The VSELs maintain life-long tissue homeostasis, serve as a backup pool for adult stem cells and are mobilized under stress conditions. An imbalance in VSELs function (uncontrolled proliferation) may result in cancer.

SEARCH METHODS

The electronic database 'Medline/Pubmed' was systematically searched with the subject heading term 'very small embryonic-like stem cells'.

OBJECTIVE AND RATIONALE

The most primitive stem cells that undergo asymmetric cell divisions to self-renew and give rise to progenitors still remain elusive in the hematopoietic system and testes, while the presence of stem cells in ovary is still being debated. We propose to review the available literature on VSELs, the methods of their isolation and characterization, their ontogeny, how they compare with embryonic stem (ES) cells, primordial germ cells (PGCs) and iPS cells, and their role in maintaining tissue homeostasis. The review includes a look ahead on how VSELs will result in paradigm shifts in basic reproductive biology.

OUTCOMES

Adult tissue-specific stem cells including hematopoietic, spermatogonial, ovarian and mesenchymal stem cells have good proliferation potential and are indeed committed progenitors (with cytoplasmic OCT-4), which arise by asymmetric cell divisions of pluripotent VSELs (with nuclear OCT-4). VSELs are the most primitive stem cells and postulated to be an overlapping population with the PGCs. Rather than migrating only to the gonads, PGCs migrate and survive in various adult body organs throughout life as VSELs. VSELs express both pluripotent and PGC-specific markers and are epigenetically and developmentally more mature compared with ES cells obtained from the inner cell mass of a blastocyst-stage embryo. As a result, VSELs readily differentiate into three embryonic germ layers and spontaneously give rise to both sperm and oocytes in vitro. Like PGCs, VSELs do not divide readily in culture, nor produce teratoma or integrate in the developing embryo. But this property of being relatively quiescent allows endogenous VSELs to survive various kinds of toxic insults. VSELs that survive oncotherapy can be targeted to induce endogenous regeneration of non-functional gonads. Transplanting healthy niche (mesenchymal) cells have resulted in improved gonadal function and live births.

WIDER IMPLICATIONS

Being quiescent, VSELs possibly do not accumulate genomic (nuclear or mitochondrial) mutations and thus may be ideal endogenous, pluripotent stem cell candidates for regenerative and reproductive medicine. The presence of VSELs in adult gonads and the fact that they survive oncotherapy may obviate the need to bank gonadal tissue for fertility preservation prior to oncotherapy. VSELs and their ability to undergo spermatogenesis/neo-oogenesis in the presence of a healthy niche will help identify newer strategies toward fertility restoration in cancer survivors, delaying menopause and also enabling aged mothers to have better quality eggs.

Identification of New Rat Bone Marrow-Derived Population of Very Small Stem Cell with Oct-4A and Nanog Expression by Flow Cytometric Platforms

Very small embryonic-like stem cells (VSELs) represent a unique rare population of adult stem cells (SCs) sharing several structural, genetic, biochemical, and functional properties with embryonic SCs and have been identified in several adult murine and human tissues. However, rat bone marrow- (BM-) derived SCs closely resembling murine or human VSELs have not been described. Thus, we employed multi-instrumental flow cytometric approach including classical and imaging cytometry and we established that newly identified population of nonhematopoietic cells expressing CD106 (VCAM-I) antigen contains SCs with very small size, expressing markers of pluripotency (Oct-4A and Nanog) on both mRNA and protein levels that indicate VSEL population. Based on our experience in both murine and human VSEL isolation procedures by fluorescence-activated cell sorting (FACS), we also optimized sorting protocol for separation of CD45⁻/Lin⁻/CD106⁺ rat BM-derived VSELs from wild type and eGFP-expressing rats, which are often used as donor animals for cell transplantations in regenerative studies in vivo. Thus, this is a first study identifying multiantigenic phenotype and providing sorting protocols for isolation VSELs from rat BM tissue for further examining of their functional properties in vitro as well as regenerative capacity in distinct in vivo rat models of tissue injury.

Human haematopoietic stem/progenitor cells express several functional sex hormone receptors

Evidence has accumulated that murine haematopoietic stem/progenitor cells (HSPCs) share several markers with the germline, a connection supported by recent reports that pituitary and gonadal sex hormones (SexHs) regulate development of murine HSPCs. It has also been reported that human HSPCs, like their murine counterparts, respond to certain SexHs (e.g. androgens). However, to better address the effects of SexHs, particularly pituitary SexHs, on human haematopoiesis, we tested for expression of receptors for pituitary SexHs, including follicle‐stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL), as well as the receptors for gonadal SexHs, including progesterone, oestrogens, and androgen, on HSPCs purified from human umbilical cord blood (UCB) and peripheral blood (PB). We then tested the functionality of these receptors in ex vivo signal transduction studies and in vitro clonogenic assays. In parallel, we tested the effect of SexHs on human mesenchymal stromal cells (MSCs). Finally, based on our observation that at least some of the UCB‐derived, CD45⁻ very small embryonic‐like stem cells (VSELs) become specified into CD45⁺HSPCs, we also evaluated the expression of pituitary and gonadal SexH receptors on these cells. We report for the first time that human HSPCs and VSELs, like their murine counterparts, express pituitary and gonadal SexH receptors at the mRNA and protein levels. Most importantly, SexH if added to suboptimal doses of haematopoietic cytokines and growth factors enhance clonogenic growth of human HSPCs as well as directly stimulate proliferation of MSCs.

The molecular nature of very small embryonic-like stem cells in adult tissues

Pluripotent stem cells (PSCs) have been considered as the most important cells in regenerative medicine as they are able to differentiate into all types of cells in the human body. PSCs have been established from several sources of embryo tissue or by reprogramming of terminally differentiated adult tissue by transduction of so-called Yamanaka factors (Oct4, Sox2, Klf4, and cMyc). Interestingly, accumulating evidence has demonstrated the residence of PSCs in adult tissue and with the ability to differentiate into multiple types of tissue-committed stem cells (TCSCs). We also recently demonstrated that a population of pluripotent Oct4⁺ SSEA-1⁺Sca-1⁺Lin⁻CD45⁻ very small embryonic-like stem cells (VSELs) resides in the adult murine bone marrow (BM) and in other murine tissue. These very small (∼3–6 μm) cells express pluripotent markers such as Oct4, Nanog, and SSEA-1. VSELs could be specified into several tissue-residing TCSCs in response to tissue/organ injury, and thus suggesting that these cells have a physiological role in the rejuvenation of a pool of TCSCs under steady-state conditions. In this review article, we discuss the molecular nature of the rare population of VSELs which have a crucial role in regulating the pluripotency, proliferation, differentiation, and aging of these cells.

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.

Do pluripotent stem cells exist in adult mice as very small embryonic stem cells?

Very small embryonic-like stem cells (VSELs) isolated from bone marrow (BM) have been reported to be pluripotent. Given their nonembryonic source, they could replace blastocyst-derived embryonic stem cells in research and medicine. However, their multiple-germ-layer potential has been incompletely studied. Here, we show that we cannot find VSELs in mouse BM with any of the reported stem cell potentials, specifically for hematopoiesis. We found that: (1) most events within the "VSEL" flow-cytometry gate had little DNA and the cells corresponding to these events (2) could not form spheres, (3) did not express Oct4, and (4) could not differentiate into blood cells. These results provide a failure to confirm the existence of pluripotent VSELs.

Murine bone marrow Lin⁻Sca⁻1⁺CD45⁻ very small embryonic-like (VSEL) cells are heterogeneous population lacking Oct-4A expression

Murine very small embryonic-like (VSEL) cells, defined by the Lin(-)Sca-1(+)CD45(-) phenotype and small size, were described as pluripotent cells and proposed to be the most primitive hematopoietic precursors in adult bone marrow. Although their isolation and potential application rely entirely on flow cytometry, the immunophenotype of VSELs has not been extensively characterized. Our aim was to analyze the possible heterogeneity of Lin(-)Sca(+)CD45(-) population and investigate the extent to which VSELs characteristics may overlap with that of hematopoietic stem cells (HSCs) or endothelial progenitor cells (EPCs). The study evidenced that murine Lin(-)Sca-1(+)CD45(-) population was heterogeneous in terms of c-Kit and KDR expression. Accordingly, the c-Kit(+)KDR(-), c-Kit(-)KDR(+), and c-Kit(-)KDR(-) subpopulations could be distinguished, while c-Kit(+)KDR(+) events were very rare. The c-Kit(+)KDR(-) subset contained almost solely small cells, meeting the size criterion of VSELs, in contrast to relatively bigger c-Kit(-)KDR(+) cells. The c-Kit(-)KDR(-)FSC(low) subset was highly enriched in Annexin V-positive, apoptotic cells, hence omitted from further analysis. Importantly, using qRT-PCR, we evidenced lack of Oct-4A and Oct-4B mRNA expression either in whole adult murine bone marrow or in the sorted of Lin(-)Sca-1(+)CD45(-)FSC(low) population, even by single-cell qRT-PCR. We also found that the Lin(-)Sca-1(+)CD45(-)c-Kit(+) subset did not exhibit hematopoietic potential in a single cell-derived colony in vitro assay, although it comprised the Sca-1(+)c-Kit(+)Lin(-) (SKL) CD34(-)CD45(-)CD105(+) cells, expressing particular HSC markers. Co-culture of Lin(-)Sca-1(+)CD45(-)FSC(low) with OP9 cells did not induce hematopoietic potential. Further investigation revealed that SKL CD45(-)CD105(+) subset consisted of early apoptotic cells with fragmented chromatin, and could be contaminated with nuclei expelled from erythroblasts. Concluding, murine bone marrow Lin(-)Sca-1(+)CD45(-)FSC(low) cells are heterogeneous population, which do not express the pluripotency marker Oct-4A. Despite expression of some hematopoietic markers by a Lin(-)Sca-1(+)CD45(-)c-Kit(+)KDR(-) subset of VSELs, they do not display hematopoietic potential in a clonogenic assay and are enriched in early apoptotic cells.

Very small embryonic/epiblast-like stem cells (VSELs) and their potential role in aging and organ rejuvenation--an update and comparison to other primitive small stem cells isolated from adult tissues

Very small embryonic-like stem cells (VSELs) are a population of developmentally early stem cells residing in adult tissues. These rare cells, which are slightly smaller than red blood cells, i) become mobilized during stress situations into peripheral blood, ii) are enriched in the Sca1⁺Lin⁻CD45⁻ cell fraction in mice and the CD133⁺ Lin⁻CD45⁻ cell fraction in humans, iii) express markers of pluripotent stem cells (e.g., Oct4, Nanog, and SSEA), and iv) display a distinct morphology characterized by a high nuclear/cytoplasmic ratio and undifferentiated chromatin. Recent evidence indicates that murine VSELs are kept quiescent in adult tissues and protected from teratoma formation by epigenetic modification of imprinted genes that regulate insulin/insulin like growth factor signaling (IIS). The successful reversal of these epigenetic changes in VSELs that render them quiescent will be crucial for efficient expansion of these cells. The most recent data in vivo from our and other laboratories demonstrated that both murine and human VSELs exhibit some characteristics of long-term repopulating hematopoietic stem cells (LT-HSCs), are at the top of the hierarchy in the mesenchymal lineage, and may differentiate into organ-specific cells (e.g., cardiomyocytes). Moreover, as recently demonstrated the number of these cells positively correlates in several murine models with longevity. Finally, while murine BM-derived VSELs have been extensively characterized more work is needed to better characterize these small cells at the molecular level in humans.

Imaging flow cytometry: coping with heterogeneity in biological systems

Imaging flow cytometry (IFC) platforms combine features of flow cytometry and fluorescent microscopy with advances in data-processing algorithms. IFC allows multiparametric fluorescent and morphological analysis of thousands of cellular events and has the unique capability of identifying collected events by their real images. IFC allows the analysis of heterogeneous cell populations, where one of the cellular components has low expression (<0.03%) and can be described by Poisson distribution. With the help of IFC, one can address a critical question of statistical analysis of subcellular distribution of proteins in a cell. Here the authors review advantages of IFC in comparison with more traditional technologies, such as Western blotting and flow cytometry (FC), as well as new high-throughput fluorescent microscopy (HTFM), and discuss further developments of this novel analytical technique.

Global gene expression analysis of very small embryonic-like stem cells reveals that the Ezh2-dependent bivalent domain mechanism contributes to their pluripotent state

Recently, we identified a population of Oct4(+)Sca-1(+)Lin(-)CD45(-) very small embryonic-like stem cells (VSELs) in murine and human adult tissues. VSELs can differentiate in vitro into cells from all 3 germ layers and in vivo tissue-committed stem cells. Open chromatin structure of core pluripotency transcription factors (TFs) supports the pluripotent state of VSELs. However, it has been difficult to determine how primitive VSELs maintain pluripotency, owing to their limited number in adult tissues. Here, we demonstrate by genome-wide gene-expression analysis with a small number of highly purified murine bone marrow-derived VSELs that Oct4(+) VSELs (i) express a similar, yet nonidentical, transcriptome as embryonic stem cells (ESCs), (ii) highly express cell cycle checkpoint genes, (iii) express at a low level genes involved in protein turnover and mitogenic pathways, and (iv) highly express enhancer of zeste drosophila homolog 2 (Ezh2), a polycomb group protein. Furthermore, as a result of high expression of Ezh2, VSELs, like ESCs, exhibit bivalently modified nucleosomes (trimethylated H3K27 and H3K4) at promoters of important homeodomain-containing developmental TFs, thus preventing premature activation of the lineage-committing factors. Notably, spontaneous or RNA interference-enforced downregulation of Ezh2 during VSEL differentiation removes the bivalent domain (BD) structure, which leads to de-repression of several BD-regulated genes. Therefore, we suggest that Oct4(+) VSELs, like other pluripotent stem cells, maintain their pluripotent state through an Ezh2-dependent BD-mediated epigenetic mechanism. Furthermore, our global survey of VSEL gene expression signature would not only advance our understanding of biological process for their pluripotency, differentiation, and quiescence but should also help to develop better protocols for ex vivo expansion of VSELs.

The great migration of bone marrow-derived stem cells toward the ischemic brain: therapeutic implications for stroke and other neurological disorders

Accumulating laboratory studies have implicated the mobilization of bone marrow (BM)-derived stem cells in brain plasticity and stroke therapy. This mobilization of bone cells to the brain is an essential concept in regenerative medicine. Over the past ten years, mounting data have shown the ability of bone marrow-derived stem cells to mobilize from BM to the peripheral blood (PB) and eventually enter the injured brain. This homing action is exemplified in BM stem cell mobilization following ischemic brain injury. Various BM-derived cells, such as hematopoietic stem cells (HSCs), mesenchymal stem cells (MSCs), endothelial progenitor cells (EPCs) and very small embryonic-like cells (VSELs) have been demonstrated to exert therapeutic benefits in stroke. Here, we discuss the current status of these BM-derived stem cells in stroke therapy, with emphasis on possible cellular and molecular mechanisms of action that mediate the cells' beneficial effects in the ischemic brain. When possible, we also discuss the relevance of this therapeutic regimen in other central nervous system (CNS) disorders.

Very small embryonic-like stem cells: biology and therapeutic potential for heart repair

Very small embryonic-like stem cells (VSELs) represent a population of extremely small nonhematopoietic pluripotent cells that are negative for lineage markers and express Sca-1 in mice and CD133 in humans. Their embryonic-like characteristics include the expression of markers of pluripotency; the ability to give rise to cellular derivatives of all three germ-layers; and the ability to form embryoid-like bodies. Indeed, quiescent VSELs may represent the remnants of epiblast-derived cells in adult organs. After tissue injury, including acute myocardial infarction (MI), bone marrow-derived VSELs are mobilized into the peripheral blood and home to the damaged organ. Given the ability of VSELs to differentiate into cardiomyocytes and endothelial cells, and their ability to secrete various cardioprotective growth factors/cytokines, VSELs may serve as an ideal cellular source for cardiac repair. Consistently, transplantation of VSELs after an acute MI improves left ventricular (LV) structure and function, and these benefits remain stable during long-term follow-up. Although the mechanisms remain under investigation, effects of secreted factors, regeneration of cellular constituents, and stimulation of endogenous stem/progenitors may play combinatorial roles. The purpose of this review is to summarize the current evidence regarding the biologic features of VSELs, and to discuss their potential as cellular substrates for therapeutic cardiac repair.

The role of pluripotent embryonic-like stem cells residing in adult tissues in regeneration and longevity

From the point of view of regenerative potential, the most important cells are pluripotent stem cells (PSCs). Such cells must fulfill certain in vitro as well as in vivo criteria that have been established by work with PSCs isolated from embryos, which are known as embryonic stem cells (ESCs). According to these criteria, pluripotent stem cells should: (i) give rise to cells from all three germ layers, (ii) complete blastocyst development, and (iii) form teratomas after inoculation into experimental animals. Unfortunately, in contrast to immortalized embryonic ESC lines or induced PSCs (iPSCs), these last two criteria have thus far not been obtained in a reproducible manner for any potential PSC candidates isolated from adult tissues. There are two possible explanations for this failure. The first is that PSCs isolated from adult tissues are not fully pluripotent; the second is that there are some physiological mechanisms involved in keeping these cells quiescent in adult tissues that preclude their "unleashed proliferation", thereby avoiding the risk of teratoma formation. In this review we present an evidence that adult tissues contain remnants from development; a population of PSCs that is deposited in various organs as a backup for primitive stem cells, plays a role in rejuvenation of the pool of more differentiated tissue-committed stem cells (TCSCs), and is involved in organ regeneration. These cells share several markers with epiblast/germ line cells and have been named very small embryonic-like stem cells (VSELs). We suggest that, on one hand, VSELs maintain mammalian life span but, on the other hand, they may give rise to several malignancies if they mutate. We provide an evidence that the quiescent state of these cells in adult tissues, which prevents teratoma formation, is the result of epigenetic changes in some of the imprinted genes.

Identification and isolation from either adult human bone marrow or G-CSF-mobilized peripheral blood of CD34(+)/CD133(+)/CXCR4(+)/ Lin(-)CD45(-) cells, featuring morphological, molecular, and phenotypic characteristics of very small embryonic-like (VSEL) stem cells

OBJECTIVE

Recently, we demonstrated that normal human bone marrow (hBM)-derived CD34(+) cells, released into the peripheral blood after granulocyte colony-stimulating factor mobilization, contain cell subpopulations committed along endothelial and cardiac differentiation pathways. These subpopulations could play a key role in the regeneration of post-ischemic myocardial lesion after their direct intracardiac delivery. We hypothesized that these relevant cells might be issued from very small embryonic-like stem cells deposited in the BM during ontogenesis and reside lifelong in the adult BM, and that they could be mobilized into peripheral blood by granulocyte colony-stimulating factor.

MATERIALS AND METHODS

Samples of normal hBM and leukapheresis products harvested from cancer patients after granulocyte colony-stimulating factor mobilization were analyzed and sorted by multiparameter flow cytometry strategy. Immunofluorescence and reverse transcription quantitative polymerase chain reaction assays were performed to analyze the expression of typical pluripotent stem cells markers.

RESULTS

A population of CD34(+)/CD133(+)/CXCR4(+)/Lin(-) CD45(-) immature cells was first isolated from the hBM or from leukapheresis products. Among this population, very small (2-5 μm) cells expressing Oct-4, Nanog, and stage-specific embryonic antigen-4 at protein and messenger RNA levels were identified.

CONCLUSIONS

Our study supports the hypothesis that very small embryonic-like stem cells constitute a "mobile" pool of primitive/pluripotent stem cells that could be released from the BM into the peripheral blood under the influence of various physiological or pathological stimuli. In order to fully support that hBM- and leukapheresis product-derived very small embryonic-like stem cells are actually pluripotent, we are currently testing their ability to differentiate in vitro into cells from all three germ layers.

Analytical capabilities of the ImageStream cytometer

Imaging cytometry has recently become an important achievement in development of flow cytometric technologies. The ImageStream cytometer combines the vast features of classical flow cytometry including an impartial analysis of great number of cells in short period of time which results in strong statistical data output, with essential features of fluorescence microscopy such us collecting of real multiparameter images of analyzed objects. In this chapter, we would like to introduce an overview of imaging cytometry platform and emphasize the potential advantages of using this system for several experimental purposes. Moreover, both well established as well as potential applications of imaging cytometry will be described. Eventually, we would like to illustrate the unique use of ImageStream cytometer for identification and characterization of subpopulations of stem/ progenitor cells present in different biological specimens.

Circulating very small embryonic-like stem cells in cardiovascular disease

Very small embryonic-like cells (VSELs) are a population of stem cells residing in the bone marrow (BM) and several organs, which undergo mobilization into peripheral blood (PB) following acute myocardial infarction and stroke. These cells express markers of pluripotent stem cells (PSCs), such as Oct-4, Nanog, and SSEA-1, as well as early cardiac, endothelial, and neural tissue developmental markers. VSELs can be effectively isolated from the BM, umbilical cord blood, and PB. Peripheral blood and BM-derived VSELs can be expanded in co-culture with C2C12 myoblast feeder layer and undergo differentiation into cells from all three germ layers, including cardiomyocytes and vascular endothelial cells. Isolation of VSLEs using fluorescence-activated cell sorting multiparameter live cell sorting system is dependent on gating strategy based on their small size and expression of PSC and absence of hematopoietic lineage markers. VSELs express early cardiac and endothelial lineages markers (GATA-4, Nkx2.5/Csx, VE-cadherin, and von Willebrand factor), SDF-1 chemokine receptor CXCR4, and undergo rapid mobilization in acute MI and ischemic stroke. Experiments in mice showed differentiation of BM-derived VSELs into cardiac myocytes and effectiveness of expanded and pre-differentiated VSLEs in improvement of left ventricular ejection fraction after myocardial infarction.

Very small embryonic-like stem cells in cardiovascular repair

Adult bone marrow (BM) harbors several small populations of cells which may contribute to cardiac and endothelial repair, such as endothelial progenitor cells (EPCs), mesenchymal stromal cells (MSCs) and very small embryonic-like cells (VSELs) expressing several markers of pluripotent stem cells (PSCs), such as Oct-4, Nanog and SSEA-1. Such cells were identified in mice bone marrow, peripheral blood and solid organs as well as in umbilical cord blood (UCB) and peripheral blood (PB) in humans. The adult BM-derived VSELs may undergo differentiation into cells derived for all three germ layers, including cardiomyocytes and vascular endothelial cells. VSELs can be isolated using a multiparameter live cell sorting technique with special gating strategy based on their small size, expression of stem cell markers (Sca-1 in mice, CXCR4 and CD133 in humans) and absence of hematopoietic lineage markers (CD45(-) Lin(-)). Experiments in murine models of myocardial infarction (MI) demonstrated population of VSELs expressed also early markers of cardiac and endothelial lineages (GATA-4, Nkx2.5/Csx, VE-cadherin, von Willebrand factor) which migrated to stromal-derived factor-1 (SDF-1) and other chemoattractant gradient and underwent rapid mobilization into peripheral blood in experimental MI mice models. Recently, we demonstrated the mobilization of VSELs expressing PSC, early cardiac and endothelial markers in patients with acute MI. In addition to BM, VSELs were also identified in several murine solid organs including the heart and brain, as well as in umbilical cord blood and peripheral blood in adult humans. We hypothesized that VSELs are quiescent progeny of epiblast-derived PSCs that are deposited during organogenesis in developing organs. In experimental MI intramyocardial injection of VSELs was more efficient than that of HSCs at improving left ventricular ejection fraction and attenuation of myocardial hypertrophy. VSELs can be useful in translational studies of cardiovascular repair.

Adult murine bone marrow-derived very small embryonic-like stem cells differentiate into the hematopoietic lineage after coculture over OP9 stromal cells

OBJECTIVE

We recently identified a population of small Sca-1(+)/Lin(-)/CD45(-) cells in adult murine bone marrow that express several epiblast/germ line and pluripotent stem cell markers (e.g., Oct-4 and SSEA-4) that we named "very small embryonic-like stem cells" (VSELs). In this report, we test the hypothesis that VSELs can differentiate along the hemato/lymphopoietic lineage.

MATERIALS AND METHODS

Purified from bone marrow, VSELs were primed/cocultured over OP9 stroma cell line and subsequently tested in vitro and in vivo assays for their hematopoietic potential. In parallel, cells derived from VSELs were evaluated for expression of hematopoietic genes and surface markers.

RESULTS

Although we observed that freshly isolated VSELs do not exhibit in vitro and in vivo hematopoietic potential, they may, after coculture over OP9 stromal cells, differentiate along the hematopoietic lineage in a similar way as embryonic stem cells or inducible pluripotent stem cells. "OP9-primed," VSEL-derived cells acquired expression of several hemato/lymphopoiesis-specific genes and markers, gave rise to hematopoietic colonies in vitro, and protected lethally irradiated mice in both primary and secondary transplant models on transplantation. We also observed that, compared to hematopoietic stem/progenitor cells, VSELs are highly resistant to total body irradiation.

CONCLUSIONS

Based on these observations, we postulate that VSELs are the most primitive murine bone marrow-residing population of stem cells that have the potential to become specified into the hematopoietic lineage and may share some of the characteristics of long-term repopulating HSCs.

Molecular characterization of isolated from murine adult tissues very small embryonic/epiblast like stem cells (VSELs)

Pluripotent very small embryonic/epiblast derived stem cells (VSELs) as we hypothesize are deposited at begin of gastrulation in developing tissues and play an important role as backup population of pluripotent stem cells (PSCs) for tissue committed stem cells (TCSCs). We envision that during steady state conditions these cells may be involved in tissue rejuvenation and in processes of regeneration/repair after organ injuries. Molecular analysis of adult bone marrow (BM)-derived purified VSELs revealed that they i) express pluripotent stem cells markers e.g., Oct4, Nanog, Klf-4, SSEA-1 ii) share several markers characteristic for epiblast as well as migratory primordial germ cells (PGCs), and iii) possess a unique pattern of genomic imprinting (e.g., erasure of differently methylated regions at Igf2-H19 and Rasgrf1 loci and hypermethylation at KCNQ1 and Igf2R loci). This supports that VSELs are related to epiblast-derived migrating PGC-like cells and, despite their pluripotent stem cell character, changes in the epigenetic signature of imprinted genes keep these cells quiescent in adult tissues and prevent them from teratoma formation. In contrast epigenetic changes/mutations that lead to activation of imprinted genes could potentially lead to tumor formation by these cells. Mounting evidence accumulates that perturbation of expression of imprinted genes is a common phenomenon observed in developing tumors.

Molecular signature of adult bone marrow-purified very small embryonic-like stem cells supports their developmental epiblast/germ line origin

We postulated that Oct4(+)SSEA-1(+)Sca-1(+)Lin(-)CD45(-) very small embryonic-like stem cells (VSELs) isolated from adult bone marrow (BM) could be a reserve population for tissue-committed stem cells. The aim of this study was to elucidate the developmental origin of these cells. We report that during embryogenesis, VSELs are enriched in embryonic day (E)12.5 murine fetal livers (FLs) and subsequently follow the developmental route of hematopoietic stem cells (H)SCs to colonize BM. Molecular analysis of purified VSELs revealed that both FL-derived VSELs and their adult BM-derived counterparts express: (i) several epiblast/primordial germ cell (PGC) markers; (ii) migrating PGC-like epigenetic reprogramming profiles of Oct4, Nanog and Stella loci; as well as (iii) a unique pattern of genomic imprinting. Thus, these data suggest that VSELs may originate from epiblast/migrating PGC-like cells and, in spite of the expression of pluripotent stem cell markers, changes in the epigenetic signature of imprinted genes keep these cells quiescent in adult tissues and prevent them from teratoma formation.