Advances in Medical Sciences 59 (2014) 273–280

Contents lists available at ScienceDirect

Advances in Medical Sciences journal homepage: www.elsevier.com/locate/advms

Review Article

Very small embryonic-like stem cells as a novel developmental concept and the hierarchy of the stem cell compartment Mariusz Z. Ratajczak a,b,*, Krzysztof Marycz c,d, Agata Poniewierska-Baran a,b, Katarzyna Fiedorowicz b, Monika Zbucka-Kretowska e, Marcin Moniuszko f,g a

Stem Cell Institute at James Graham Brown Cancer Center, University of Louisville, Louisville, KY, USA Department of Physiology, Pomeranian Medical University, Szczecin, Poland University of Environmental and Life Sciences, Electron Microscopy Laboratory, Wroclaw, Poland d Wroclaw Research Centre EIT+, Wroclaw, Poland e Department of Reproduction and Gynecological Endocrinology, Medical University of Bialystok, Bialystok, Poland f Department of Regenerative Medicine and Immune Regulation, Medical University of Bialystok, Bialystok, Poland g Department of Allergology and Internal Medicine, Medical University of Bialystok, Bialystok, Poland b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 April 2014 Accepted 4 August 2014 Available online 14 August 2014

Our current understanding of stem cells suffers from a lack of precision, as the stem cell compartment is a broad continuum between early stages of development and adult postnatal tissues, and it is not fully understood how this transition occurs. The definition of stem cell pluripotency is adapted from embryology and excludes the possibility that some early-development stem cells with pluri- and/or multipotential differentiation potential may reside in postnatal tissues in a dormant state in which they are protected from uncontrolled proliferation and thus do not form teratomas or have the ability to complement blastocyst development. We will discuss the concept that a population of very small embryonic-like stem cells (VSELs) could be a link between early-development stages and adult stem cell compartments and reside in a quiescent state in adult tissues. The epigenetic mechanism identified that changes expression of certain genes involved in insulin/insulin-like growth factor signaling (IIS) in VSELs, on the one hand, keeps these cells quiescent in adult tissues and, on the other hand, provides a novel view of the stem cell compartment, IIS, tissue/organ rejuvenation, aging, and cancerogenesis. ß 2014 Medical University of Bialystok. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

Keywords: Pluripotent stem cells Multipotent stem cells Stem cell hierarchy Development

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definitions of stem cell pluripotency and multipotency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Developmental concept of PSCs and MultSCs in adult tissues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Stem cell plasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Stress-induced PSCs and MultiSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Developmental deposition of PSCs and MultiSCs in adult tissue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. Somatic imprinting as a mechanism that keeps early-development stem cells in adult tissues in a quiescent state . 2.3. Germline markers in early-development stem cells in adult tissues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Mobilization of VSELs into PB in response to stress and tissue/organ injury. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Potential approaches to regulating VSEL quiescence – implications for longevity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. VSELs and their potential involvement in cancerogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

. . . . . . . . . . . . . .

274 274 274 275 275 275 275 275 276 276 277 277 277 278

* Corresponding author at: Stem Cell Institute at James Graham Brown Cancer Center, University of Louisville, 500 South Floyd Street, Rm. 107, Louisville, KY 40202, USA. Tel.: +1 502 852 1788; fax: +1 502 852 3032. E-mail address: [email protected] (M.Z. Ratajczak). http://dx.doi.org/10.1016/j.advms.2014.08.001 1896-1126/ß 2014 Medical University of Bialystok. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

274

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280

1. Introduction

2. Review

Stem cell research and regenerative medicine, perhaps more than any other topics in current biology and medicine, generate controversy beyond the bounds of scientific inquiry. The causes of these controversies are patent disputes stemming from the financial interests of biotech companies, religious beliefs, and political issues. However, scientists should remain open to new ideas, and science should stay free of these collateral problems and dogmas that, in many cases, have slowed progress in uncovering scientific truth. In this review, we will discuss the accumulating evidence that the stem cell compartment in adult tissues is a continuum of embryonic development, and some early-development stem cells with multi-tissue differentiation potential may survive into adulthood [1–11]. Such cells have been described by many investigators and, depending on the methods for how they were isolated, assigned different names, for example, spore-like stem cells, multipotent adult stem cells (MASCs) [12], mesenchymal stem cells (MSCs) [13–15], multilineage-differentiating stressenduring (Muse) cells [16–18], multipotent adult progenitor cells (MAPCs) [19,20], unrestricted somatic stem cells (USSCs) [21], marrow-isolated adult multilineage-inducible (MIAMI) cells, multipotent progenitor cells (MPCs) [12,23] and, as described by us a decade ago, very small embryonic-like stem cells (VSELs) [24–27]. This has created a kind of nomenclatural chaos, and probably several of these stem cells described as separate entities are in fact overlapping populations of similar cells. Furthermore, we envision that VSELs are on the top of hierarchy of all these various overlapping populations of stem cells endowed with pluti/multipotent differentiation potential [1]. In this review, we will discuss the concept that adult tissues contain cells from the early-development stem cell compartment and that in adult tissues there is a developmental continuum including stem cells with characteristics of pluripotent stem cells (PSCs) or multipotent stem cells (MultiSCs). These cells coexist in a dormant state together with already differentiated tissue-committed stem cells (TCSCs). It is logical that such early-development PSCs and MultiSCs are protected from uncontrolled proliferation, because otherwise they would form teratomas. We have demonstrated that one of the mechanisms that keep most-primitive stem cells quiescent in adult tissues is based on epigenetic modification of somatically imprinted genes that govern pathways related to development and insulin/insulin-like growth factor signaling (IIS) [28–30]. This mechanism, known very well to regulate the quiescence of primordial germ cells (PGCs) [31–33], was also reported by us for the first time to operate in stem cells isolated from adult tissues, as seen in case of VSELs [34,35]. This original report has been supported by recent papers showing that early-development stem cells residing in adult tissues are also regulated by epigenetic modification of the imprinted genes involved in IIS [36]. Identification of early-development stem cells in adult tissues raises several questions, such as: (i) Are these cells functional and do they play a role in tissue/organ rejuvenation? (ii) Are they involved in regulating life span of the individual? (iii) Are they involved in regeneration of damaged tissues? (iv) If regulatory mechanisms fail, could these cells give rise to malignancies? We will try to address these questions in this review as well as to address whether these cells could be a potential target for manipulations such as pharmacological, dietary and physical exercise approaches to extending our quality of life and life span.

2.1. Definitions of stem cell pluripotency and multipotency A pluripotent stem cell (PSC) is a stem cell endowed with the ability to differentiate into cells from all three germ layers (meso-, ecto-, and endoderm) as well as into germline cells. Based on research with embryonic stem cells (ESCs), several in vitro and in vivo criteria to classify a given stem cell as pluripotent have been proposed. According to the proposed definition, PSCs display undifferentiated morphology, undifferentiated euchromatin, a high nuclear/cytoplasm ratio, PSC markers (e.g., Oct-4, Nanog, SSEA), and bivalent domains reviewed in our recent publications [37,38]. Moreover, female PSCs reactivate the X chromosome and, as mentioned above, all PSCs must be able to differentiate into cells from all three germ layers (meso-, ecto- and endoderm) as a sign of their multilineage differentiation potential. Finally, based on research with ESCs isolated from embryos, special emphasis has been put on in vivo criteria, such as the ability of these cells to complement blastocyst development and to grow teratomas after inoculation into immunodeficient mice. We would like to point out that these in vivo criteria proposed by embryologists do not take into consideration that at a certain point in development, PSCs may undergo epigenetic modifications and, despite the fact that they express several markers of PSCs, may be kept quiescent, with their pluripotency locked to reduce the risk of teratoma and tumor formation [39,40]. However, the most important question here is whether this quiescent state is fully reversible and whether such cells could regain pluripotency to comply with the definition proposed by embryologists. It is well known that even normal somatic cells can be made to revert to a state of pluripotency, as seen in the case of induced pluripotent stem cells (iPSCs), which are generated in vitro by appropriate genetic manipulations. It is obvious that the requirements for unlocking the mechanism that enables PSCs to reside in adult tissues in a dormant state should be much simpler to achieve than in the case of iPSCs and should not affect genomic stability, as seen in the case of iPSCs generated by transduction with retroviruses encoding a cocktail of genes. In support of this expectation, primordial germ cells (PGCs), which due to epigenetic changes based on erasure of regulatory regions in some parentally imprinted genes are locked in a dormant state, can revert to full pluripotency after coculture on murine fetal fibroblasts in the presence of recombinant kit ligand (KL), leukemia inhibitory factor (LIF), and basic fibroblast growth factor (bFGF-2) [41,42]. This interesting phenomenon will be discussed latter in this review. Furthermore, the definitions of in vitro and in vivo criteria of pluripotency are somewhat vague, as demonstrated in the case of epiblast-generated, embryo-derived pluripotent stem cell lines. In many cases, depending on the developmental stage of the embryo, such cells can give rise to all three germ layers in vitro but neither complete blastocyst development nor grow tertomas in experimental animals [31–34,37,43]. In contrast to PSCs, multipotent stem cells (MultSCs) are already more committed in development and are able to give rise not to three germ layers as for PSCs but to cells from two germ layers only. As already mentioned, evidence has accumulated that cells with broader pluripotent or multipotent differentiation potential can be isolated from adult tissues, and their potential origin will be discussed below. These cells are precursors of TCSCs and example of cells that are already committed to one developmental lineage are hematopoietic stem cells (HSCs), skeletal muscle satellite stem cells or epidermal stem cells [1].

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280

2.2. Developmental concept of PSCs and MultSCs in adult tissues As mentioned above, the presence of stem cells with a broader multi-germ layer differentiation potential has been reported by numerous investigators. The basic question is, however, what is the origin of these cells? Is it due to stem cell plasticity when adult stem cells change their fate epigenetics of these cells so that they become pluripotent, as seen in case of the recently described phenomenon of stimulus-triggered fate conversion of somatic cells into pluripotency-in STAP cells [44,45] or stress-induced Muse cells [17,18]? Finally, what we believe is most likely: are such cells deposited in adult tissues during development and are overlapping populations of early-development stem cells (e.g., spore-like stem cells, MASCs, MAPCs, USSCs, MIAMI cells, MPCs, or VSELs)? We will discuss all three possibilities. 2.2.1. Stem cell plasticity The concept of stem cell plasticity was proposed more than a decade ago, based on in vivo evidence showing that bone marrow (BM)-derived cells that are mostly hematopoietic stem cells (HSCs) may contribute to regeneration of damaged tissues, e.g., heart or brain [46–48]. According to this concept, TCSCs, such as HSCs obtained from bone marrow, would be able to dedifferentiate into stem cells typical of other organs, such as myocardium, the central nervous system, or liver [49–54]. Based on this concept, there were great expectations associated with the potential use of HSCs as a source of plastic stem cells. However, despite initially promising results, a direct role of these cells in the regeneration of injured organs by reversal of their phenotype has not been proven. Specifically, a series of studies on the use of phenotypically defined and purified subpopulations of HSCs have been disappointing, yielding negative results in models of regeneration of myocardium and brain [55,56]. Several alternative explanations have been proposed to explain these results. First, it is possible that some of the stem cell plasticity data can be explained by the phenomenon of cell fusion [57]. Specifically, transplanted HSCs might undergo fusion with the cells of the injured organ to form heterokaryons. Alternatively, the positive beneficial effects in regeneration of damaged organs by BM-derived HSCs might be explained by stem cell-derived paracrine effects [58,59]. Stem cells employed in therapy are a rich source of growth factors, cytokines, chemokines, and bioactive lipids, which may inhibit apoptosis and promote neovascularization in the damaged tissues [60–64]. The function and phenotype of cells in the damaged tissues may also be modified by the transfer of cell receptors, cytoplasmic proteins, and mRNAs from surrounding cells by microvesicles (MVs), which are spherical structures in which a part of the cell cytoplasm enriched for mRNA, miRNA, and functional proteins is encapsulated by cell membrane [59,65]. MVs released from the surface of cells employed to regenerate damaged organs may deliver these cargo molecules to damaged tissues. Evidence has accumulated that MV cargo has positive effects on cell survival and angiogenesis. Thus, paracrine effects associated with MVs most likely make the major contribution to the positive results reported in clinical trials employing adult stem cells. Finally, cells employed for therapy that are derived, for example, from hematopoietic tissues, may from the beginning contain heterogeneous populations of stem cells, including some rare PSCs or MultiSCs, that possess a broader differentiation potential. 2.2.2. Stress-induced PSCs and MultiSCs We also cannot exclude the possibility that some factors present in the environment of damaged organs induce epigenetic changes in genes that regulate the pluripotency of adult cells (involving changes in DNA methylation or acetylation of histones). This mechanism could be involved, for example, in generation of

275

the recently reported STAP cells as the result of stimulus-triggered fate conversion of somatic cells [44,45] or multilineage-differentiating stress-enduring cells (Muse cells) [16–18]. However, these interesting stem cell populations derived by such mechanisms have recently come into question, and further studies from independent laboratories are required to assess how efficient and reproducible are these mechanisms. 2.2.3. Developmental deposition of PSCs and MultiSCs in adult tissue The possibility that during embryonic development some PSCs and MultiSCs could be deposited in adult tissues as a potential reserve population of stem cells to back up TCSCs was proposed by us several years ago. This concept is based on two possible scenarios that occur during the specification of PSCs into TCSCs. The first scenario is based on the assumption that PSCs during their specification into more differentiated stem cells such as TCSCs disappear completely from the developing embryo and are not present in postnatal tissues. The second scenario is based on the concept that, after giving rise to TCSCs, some of these cells are retained in adult tissues as a backup population of quiescent cells activated when needed for tissue/organ regeneration [66]. This possibility, however, requires the involvement of epigenetic mechanisms that keep these cells quiescent (in a dormant state). Later in the review we will discuss this mechanism affecting expression of certain paternally imprinted genes, which is very similar to the mechanism regulating the quiescence of PGCs. As a consequence of this mechanism, we are proposing that there is a continuum in stem cell hierarchical development and that some dormant PSCs and MultiSCs persist from the early stages of development in adult tissues. In support of this hypothesis, we identified a population of cells in adult murine and human tissues that express several markers of pluripotency that we named very small embryonic-like stem cells (VSELs) [24–27,67]. Murine VSELs (i) are slightly smaller than erythrocytes, (ii) are purified as SSEA-1+/Oct-4+/Sca-1+/CXCR4+/ Lin /CD45 cells, (iii) express bivalent domains at developmentally important homeobox domain-containing genes, and (iv) have a high nuclear/cytoplasm ratio and primitive euchromatin. Murine BM-derived VSELs that do not exhibit hematopoietic activity immediately after isolation acquire hematopoietic potential, similar to stem cells from established embryonic stem cell (ESC) lines or inducible pluripotent stem cells (iPSCs), following coculture/activation over OP9-supported stromal cells [68]. As recently demonstrated by others, these cells are also high up in the mesenchymal lineage hierarchy [8] and give rise to endothelium [35], and lung epithelium [69,70]. Moreover, VSELs isolated from gonads give rise to the female and male gametes [5,71–73]. The corresponding populations of SSEA-4+/Oct-4+/CD133+/CD34+/ CXCR4+/Lin /CD45 cells were also identified by us in human umbilical cord blood (UCB). Human VSELs are also (i) slightly smaller than erythrocytes, (ii) have a high nuclear/cytoplasm ratio and euchromatin, and (iii) display the transcription factor signature of PSCs, including Oct-4 and Nanog [34]. In discussing the stem cell hierarchy in adult tissues (e.g., BM) we propose that VSELs are the most-primitive population of stem cells of all the early-development stem cells present in adult tissues described so far (e.g., MASCs, MAPCs, USSCs, MIAMI cells, or MPCs). This possibility, however, requires further study. 2.3. Somatic imprinting as a mechanism that keeps earlydevelopment stem cells in adult tissues in a quiescent state We have demonstrated that murine Oct-4+SSEA-1+Sca1 Lin CD45 VSELs are kept quiescent in BM in the G0 phase of the cell cycle by epigenetic modification of the somatic imprint in the differentially methylated regions (DMRs) of certain crucial +

276

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280

paternally imprinted genes (Igf2-H19, RasGRF1, IGF2R, and p57Kip2) that regulate proliferation of embryonic stem cells, initiation of embryogenesis, and insulin/insulin-like growth factor signaling (IIS) [28,34,74,75]. Specifically, we observed that murine BMsorted VSELs erase the paternally methylated imprints within the DMRs for Igf2-H19 and RasGrf1, while they hypermethylate the maternally methylated DMR for IGF2R. As a result of these changes, VSELs, like PGCs, are resistant to IIS signaling, and this epigenetic modification of imprinted loci (including Igf2-H19, RasGRF1, and IGFR) prevents VSELs from uncontrolled proliferation and teratoma formation. To explain this at the signaling level, the changes in expression of imprinted genes in murine VSELs lead to perturbation of IIS by downregulation of (i) insulin-like growth factor 2 (IGF2), which is an autocrine factor involved in proliferation of VSELs, and (ii) RasGRF1, which is a GTP-exchange factor (GEF) crucial for signaling from the activated insulin-like growth factor 1 receptor (IGF-IR) and the insulin receptor (InsR). In addition, since the insulin-like growth factor 2 receptor (IGF2R) serves as a decoy receptor that prevents IGF-2 from binding to IGF-IR, hyperemethylation of the DMRs on the maternal chromosome encoding IGF-2R, which leads to overexpression of this gene, has an additional negative affect on IIS in VSELs [28]. This epigenetic reprogramming of genomic imprinting negatively affects IIS signaling, maintaining the quiescent state of murine VSELs, protecting them from premature depletion in the tissues, and preventing their involvement in tumor formation. We also observed that, in addition to changes in expression of imprinted genes, VSELs express several miRNAs that attenuate IIS signaling in these cells (e.g., mir681, mir470, and mir669b) or upregulate the expression of p57KIP2 (e.g., mir25.1, mir19b, and mir92). Our recent data suggest that a very similar mechanism is also most likely responsible for the quiescent state of human VSELs in adult tissues. Thus, the epigenetic modification of certain imprinted genes explains why murine VSELs, despite expressing several markers of pluripotency (e.g., an open chromatin structure at the promoters for Oct-4 and Nanog), the presence of bivalent domains at developmentally important homeobox domain-containing genes, the reactivation of the X chromosome in female VSELs, and in vitro differentiation into cells from all three germ layers [7,67–70,76], do not fulfill all the criteria required by the definition of PSCs, specifically, that they neither complement blastocyst development after injection into the pre-implantation blastocyst nor grow teratomas in immunodeficient mice [34,37,77]. On the other hand, reversal of this imprinting mechanism will be crucial to employing VSELs as a population of PSCs in regenerative medicine. Currently, we are testing whether downregulation of the expression of H19 enhances VSEL expansion, as has recently been demonstrated for PSCs derived by parthenogenesis [35,78]. 2.4. Germline markers in early-development stem cells in adult tissues As mentioned above, the mechanism that keeps VSELs in a quiescent state in adult tissues is similar to the one that prevents PGCs from uncontrolled proliferation and teratoma formation. Therefore, we became interested in a potential link between VSELs and the germline [79,80]. Our careful gene expression studies on highly purified populations of murine VSELs has revealed that they express several epiblast and germline markers, and we hypothesize that VSELs originate from early epiblast-derived migrating primordial germ cells (PGCs), are deposited during development in adult tissues as a source of TCSCs, and play a role in organ rejuvenation [81]. Thus, according to our proposal, the germline is not only the origin of all adult stem cells, but the earlydevelopment primitive stem cells in adult tissues express several germline markers.

In support of this notion, molecular analysis of murine BMderived VSELs has revealed that these cells express several genes that are characteristic of epiblast SCs (Gbx2, Fgf5, and Nodal) and, more importantly, germline specification (Stella, Prdm14, Fragilis, Blimp1, Nanos3, and Dnd1) [79,82]. The expression of some of these crucial genes has been confirmed subsequently by demonstrating the presence of transcriptionally active promoters. Finally, in direct in vivo and in vitro experiments, we confirmed that the quiescent population of BM-residing VSELs, like HSCs, expands in response to stimulation by androgens (danazol) and pituitary gonadotropins (FSH, and LH). In further support of these results, we observed that 10-day administration of these sex hormones directly stimulated expansion of VSELs in BM, as measured by an increase in the total number of these cells (2–3) and enhanced BrdU incorporation (the percentage of proliferating BrdU+Sca-1+Lin CD45 VSELs increased from 2% to 15–35%) (manuscript in preparation). In fact, several reports have reported that early-development stem cells isolated from adult tissues express germline markers [79], and examples of such cells are listed in Table 1. Moreover, in a very elegant study, VSELs isolated from the female and male gonads have been demonstrated to be precursors of the gametes. 2.5. Mobilization of VSELs into PB in response to stress and tissue/organ injury It is well known that VSELs circulate at very low levels in PB under steady-state conditions and can be mobilized efficiently into peripheral blood in mice and adult patients injected with granulocyte-colony stimulating factor (G-CSF). This observation laid the foundation for the concept that G-CSF mobilization can be employed to harvest these cells from patients for therapeutic purposes in regenerative medicine. Several reports also provided evidence that VSELs are mobilized into PB during strenuous exercise, infections, and sepsis as well as in patients suffering from heart infarct, stroke, skin burns, active inflammatory bowel disease, and several types of cancer [83–91] More importantly, based on our preliminary data, we also envision that in several clinical situations the number of VSELs circulating in PB in patients could be of prognostic value. This possibility, however, requires further study and long-term clinical correlations.

Table 1 Selected reports from other groups on stem cells with germline potential in adult non-gonadal tissues. Cells as they were originally described in the literature

References

Stem cells with germline potential isolated from newborn mouse skin – Oct-4+ cells isolated by FACS from Oct-4 – GFP mice that are able to give rise in vitro and in vivo to early oocytes. Multipotent stem/stromal cells isolated from porcine skin – Oct-3/4+, Nanog+, Sox-2+ cells isolated from porcine skin and adipose tissue able to differentiate into oocyte-like cells. SSEA-1+ murine BM cells – isolated from murine BM by anti-SSEA-1 immunomagnetic beads. In the presence of BMP4 (bone morphogenetic protein 4), differentiate into Oct-4+Stella+Mvh+ gamete precursors. BM-derived germ cell candidates – Oct-4+Mvh+Dazl+Stella+ cells present in BM that may affect recurrence of oogenesis in mice sterilized by chemotherapy. BM-derived male germ cells – Oct-4+, Mvh+, Stella+ cells isolated as Stra8 – GFP cells from BM from Stra8 – GFP transgenic mice. These cells express several molecular markers of spermatogonial stem cells and spermatogonia. Chicken BM-derived precursors of male germ cells – GFP+ transgenic chicken Oct-4+SSEA-1/3/4+ BM cells that after injection into testes give rise to functional sperm.

[115]

[116]

[117]

[118,119]

[120]

[121]

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280

Mobilization of VSELs into PB has also been observed in several situations, listed in Table 2, that lead to an increase in the proliferation rate of VSELs in BM. These observations support the concept that VSELs are a mobile population of stem cells highly responsible for responding to stress situations and thus are most likely to be involved in repair mechanisms that require the contribution of stem cells. In further support of this concept, we observed that VSELs respond by chemotaxis to several factors that are released in damaged tissues, such as stromal derived factor-1 (SDF-1), hepatocyte growth factor (HGF), and leukemia inhibitory factor (LIF), as well as to phosphosphingolipids such as sphingosine-1-phosphate (S1P) and ceramic-1-phosphate (C1P) [62– 64,92–94]. 2.6. Potential approaches to regulating VSEL quiescence – implications for longevity We hypothesize that the number of VSELs in adult tissues correlates with tissue/organ rejuvenation potential. The fact that the mechanism that maintains the pool of these cells in adult tissues depends on their epigenetically regulated resistance to IIS may explain our recent data on the beneficial effect of physical exercise and calorie restriction on tissue-residing VSELs. Both of these situations lead to a decrease in IIS, which seems to have additional beneficial effects on these cells. We also observed that the number of VSELs correlates with longevity in experimental mice [29]. Specifically, our data on longliving murine strains (e.g., Laron dwarf or Ames dwarf mice) that have very low levels of circulating IGF-1 in peripheral blood have demonstrated that these animals have greater numbers of VSELs than age-matched normal wild type animals. By contrast, shortliving mice with overexpression of growth hormone (GH) and thus high levels of circulating IGF-1 have reduced numbers of VSELs in their tissues [29,30,75,95,96]. Based on these findings, we tested the effect of prolonged exercise and calorie restriction [97] of VSELs in adult mice and observed that both of these approaches have beneficial effects on the number of these cells compared with control animals that are fed ad libitum and do not exercise. These observations in experimental mice may have important implications for humans. However, further studies in humans are needed. From these studies there is also the important message for modern pharmacology that there is a need to develop novel drugs that will interfere at the VSEL level with unwanted IIS and thus decelerate the age-related decrease of these cells in adult tissues and organs. VSELs, which are a quiescent population of cells in BM, could also be activated and enter the cell cycle by the several approaches listed in Table 2. Beside the abovementioned strenuous exercise, several hormones, such as erythropoietin, pituitary gonadotropins (FSH and LH), prolactin, and androgens, as well as excessive bleeding, irradiation, and administration of 5-FU also induce proliferation of these cells, as demonstrated by the BrdU incorporation assay.

Table 2 Factors that induce proliferation of BM-residing VSELs. 1. 2. 3. 4. 5. 6. 7. 8.

Strenuous exercise Erythropoietin Pituitary gonadotropins (FSH and LH) Prolactin Androgens Bleeding Irradiation Administration of 5-FU

277

2.7. VSELs and their potential involvement in cancerogenesis The other side of the coin is that VSELs could be responsible for the development of malignancies and the progression of growing tumors. The concept that adult tissues contain developmentally primitive cells with embryonic features that can lead to tumors is, surprisingly, not so novel. During the 19th and early 20th centuries, it was proposed that cancer may develop in populations of cells that are left in a dormant state in developing organs during embryogenesis. This so-called ‘‘embryonic rest hypothesis of cancer origin’’ was proposed by Virchow [98], Durante [99], and Conheim [100]. According to these pathologists, adult tissues contain embryonic remnants that normally lie dormant but that can be ‘‘activated’’ to become cancerous. In agreement with these theories, Wright [101] proposed a germinal cell origin of nephroblastoma and Beard [102] postulated that tumors arise from displaced and activated trophoblasts or germ cells. Nevertheless, the putative cells responsible for these effects were at that time neither clearly identified nor purified from adult tissues. We propose that VSELs could be the missing link in this hypothesis by being the origin of these malignancies. There are several pieces of evidence supporting the embryonic rest hypothesis of cancer development and the potential involvement of VSELs. First, there is the existence of classical germline tumors seen in patients with seminomas, ovarian tumors, yolk sac tumors, mediastinal or brain germ cell tumors, teratomas, and teratocarcinomas [103,104]. Second, several types of cancer cells express cancer testis (C/T) antigens (40 identified), which are encoded by genes that are normally expressed only in the human germline but are also often expressed unexpectedly in various non-gonadal tumor types (e.g., gastric, lung, liver, and bladder carcinomas) [105,106]. Third, in several types of malignancies, embryonic markers, such as chorionic gonadotropin (hCG) and/or carcinoembryonic antigen (CEA), are detected in patient plasma [107]. Finally, it is known that several solid tumors (e.g., gastric, lung, bladder, and oral mucosa carcinomas as well as germinal tumors) express the embryonic/germline transcription factor Oct-4 [108,109]. VSELs, besides being a source of tumor-initiating cells in the most-primitive anaplastic types of solid tumors or sarcomas, may also play a role in tumor progression by providing endothelial and stromal precursors. The role of VSELs in stromalization of growing prostate cancers has recently been demonstrated by one of our groups [83]. However, more studies are needed to shed further light on the role of VSELs in cancerogenesis [81,110]. 3. Conclusions Despite significant progress, there are still many important problems with VSELs that must be solved. First, since most of the published data has been generated in murine BM-derived VSELs, we do not know whether phenotypically similar cells that reside in other murine organs are VSELs. It is very likely that, depending on tissue location, VSELs carry some level of epigenetic memory of the host tissue. Second, these small cells could be at different levels of tissue specification and development. Third, more studies are needed on human VSELs to see whether they have the same molecular signature as their murine counterparts. We should also compare VSELs side-by-side with other early-development stem cells isolated from adult tissues, such as spore-like stem cells, MASCs, MAPCs, USSCs, MIAMI cells, and MPCs [12–15,19–23]. Furthermore, it is still a problem to expand murine and human VSELs in vitro. We believe that epigenetic modification of some imprinted genes in these cells here plays a crucial role. Therefore, we need to explore the possibility that reversing these modifications of imprinted genes in VSELs could help to expand

278

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280

these cells. We expect that the coming years will bring answers to all of these questions. Finally, we ask the scientific community to follow our well-tested isolation protocols and directly contact our group if there are problems with gating and sorting of these very rare cells [25,27,36,111–114]. Doing so will avoid confusion in the field and situations in which cells are wrongly identified as VSELs but lack a true VSEL phenotype. Conflict of interests The University of Louisville is the owner of patents on VSELs and some areas of VSEL technology, which are licensed to Neostem Inc., New York. None of the authors have any stock in Neostem Inc., New York or any other biotechnological stem cell company. Financial disclosure Supported with EU structural funds and the Innovative Economy Operational Program POIG.01.01.02-00-109/09 grant. The research was partially supported by Wroclaw Research Centre EIT+ under the project ‘‘Biotechnologies and advanced medical technologies’’ – BioMed (POIG.01.01.02-02-003/08) financed from the European Regional Development Fund (Operational Programmed Innovative Economy, 1.1.2.). References [1] Ratajczak MZ, Zuba-Surma E, Kucia M, Poniewierska A, Suszynska M, Ratajczak J. Pluripotent and multipotent stem cells in adult tissues. Adv Med Sci 2012;57(1):1–17. [2] Ratajczak MZ, Machalinski B, Wojakowski W, Ratajczak J, Kucia M. A hypothesis for an embryonic origin of pluripotent Oct-4(+) stem cells in adult bone marrow and other tissues. Leukemia 2007;21(5):860–7. [3] Ratajczak MZ, Zuba-Surma EK, Shin DM, Ratajczak J, Kucia M. Very small embryonic-like (VSEL) stem cells in adult organs and their potential role in rejuvenation of tissues and longevity. Exp Gerontol 2008;43(11):1009–17. [4] McGuckin CP, Forraz N, Baradez MO, Navran S, Zhao J, Urban R, et al. Production of stem cells with embryonic characteristics from human umbilical cord blood. Cell Prolif 2005;38(4):245–55. [5] Parte S, Bhartiya D, Telang J, Daithankar V, Salvi V, Zaveri K, et al. Detection, characterization, and spontaneous differentiation in vitro of very small embryonic-like putative stem cells in adult mammalian ovary. Stem Cells Dev 2011;20(8):1451–64. [6] Jones RJ, Collector MI, Barber JP, Vala MS, Fackler MJ, May WS, et al. Characterization of mouse lymphohematopoietic stem cells lacking spleen colonyforming activity. Blood 1996;88(2):487–91. [7] Howell JC, Lee WH, Morrison P, Zhong J, Yoder MC, Srour EF. Pluripotent stem cells identified in multiple murine tissues. Ann N Y Acad Sci 2003;996:158– 73. [8] Havens AM, Shiozawa Y, Jung Y, Sun H, Wang J, McGee S, et al. Human very small embryonic-like cells generate skeletal structures, in vivo. Stem Cells Dev 2013;22(4):622–30. [9] Kassmer SH, Krause DS. Very small embryonic-like cells: biology and function of these potential endogenous pluripotent stem cells in adult tissues. Mol Reprod Dev 2013;80(8):677–90. [10] Kajstura J, Rota M, Hall SR, Hosoda T, D’Amario D, Sanada F, et al. Evidence for human lung stem cells. N Engl J Med 2011;364(19):1795–806. [11] Vacanti MP, Roy A, Cortiella J, Bonassar L, Vacanti CA. Identification and initial characterization of spore-like cells in adult mammals. J Cell Biochem 2001;80(3):455–60. [12] Beltrami AP, Cesselli D, Bergamin N, Marcon P, Rigo S, Puppato E, et al. Multipotent cells can be generated in vitro from several adult human organs (heart, liver, and bone marrow). Blood 2007;110(9):3438–46. [13] Peister A, Mellad JA, Larson BL, Hall BM, Gibson LF, Prockop DJ. Adult stem cells from bone marrow (MSCs) isolated from different strains of inbred mice vary in surface epitopes, rates of proliferation, and differentiation potential. Blood 2004;103(5):1662–8. [14] Prockop DJ. Marrow stromal cells as stem cells for nonhematopoietic tissues. Science 1997;276(5309):71–4. [15] Dominici M, Le Blanc K, Mueller I, Slaper-Cortenbach I, Marini F, Krause D, et al. Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy 2006;8(4):315–7. [16] Wakao S, Kitada M, Kuroda Y, Shigemoto T, Matsuse D, Akashi H, et al. Multilineage-differentiating stress-enduring (Muse) cells are a primary source of induced pluripotent stem cells in human fibroblasts. Proc Natl Acad Sci U S A 2011;108(24):9875–80.

[17] Wakao S, Akashi H, Kushida Y, Dezawa M. Muse cells, newly found nontumorigenic pluripotent stem cells, reside in human mesenchymal tissues. Pathol Int 2014;64(1):1–9. [18] Kuroda Y, Wakao S, Kitada M, Murakami T, Nojima M, Dezawa M. Isolation, culture and evaluation of multilineage-differentiating stress-enduring (Muse) cells. Nat Protoc 2013;8(7):1391–415. [19] Jiang Y, Jahagirdar BN, Reinhardt RL, Schwartz RE, Keene CD, Ortiz-Gonzalez XR, et al. Pluripotency of mesenchymal stem cells derived from adult marrow. Nature 2002;418(6893):41–9. [20] Jiang Y, Vaessen B, Lenvik T, Blackstad M, Reyes M, Verfaillie CM. Multipotent progenitor cells can be isolated from postnatal murine bone marrow, muscle, and brain. Exp Hematol 2002;30(8):896–904. [21] Kogler G, Sensken S, Airey JA, Trapp T, Muschen M, Feldhahn N, et al. A new human somatic stem cell from placental cord blood with intrinsic pluripotent differentiation potential. J Exp Med 2004;200(2):123–35. [22] D’Ippolito G, Diabira S, Howard GA, Menei P, Roos BA, Schiller PC. Marrowisolated adult multilineage inducible (MIAMI) cells, a unique population of postnatal young and old human cells with extensive expansion and differentiation potential. J Cell Sci 2004;117(Pt 14):2971–81. [23] Cesselli D, Beltrami AP, Rigo S, Bergamin N, D’Aurizio F, Verardo R, et al. Multipotent progenitor cells are present in human peripheral blood. Circ Res 2009;104(10):1225–34. [24] Kucia M, Wu W, Ratajczak MZ. Bone marrow-derived very small embryoniclike stem cells: their developmental origin and biological significance. Dev Dyn 2007;236(12):3309–20. [25] Kucia M, Reca R, Campbell FR, Zuba-Surma E, Majka M, Ratajczak J, et al. A population of very small embryonic-like (VSEL) CXCR4(+)SSEA-1(+)Oct-4+ stem cells identified in adult bone marrow. Leukemia 2006;20(5):857–69. [26] Kucia M, Wysoczynski M, Ratajczak J, Ratajczak MZ. Identification of very small embryonic like (VSEL) stem cells in bone marrow. Cell Tissue Res 2008;331(1):125–34. [27] Kucia M, Halasa M, Wysoczynski M, Baskiewicz-Masiuk M, Moldenhawer S, Zuba-Surma E, et al. Morphological and molecular characterization of novel population of CXCR4+ SSEA-4+ Oct-4+ very small embryonic-like cells purified from human cord blood: preliminary report. Leukemia 2007;21(2): 297–303. [28] Kucia M, Masternak M, Liu R, Shin DM, Ratajczak J, Mierzejewska K, et al. The negative effect of prolonged somatotrophic/insulin signaling on an adult bone marrow-residing population of pluripotent very small embryonic-like stem cells (VSELs). Age 2013;35(2):315–30. [29] Ratajczak J, Shin DM, Wan W, Liu R, Masternak MM, Piotrowska K, et al. Higher number of stem cells in the bone marrow of circulating low Igf-1 level Laron dwarf mice-novel view on Igf-1, stem cells and aging. Leukemia 2011;25(4):729–33. [30] Piotrowska K, Borkowska SJ, Wiszniewska B, Laszczynska M, SluczanowskaGlabowska S, Havens AM, et al. The effect of low and high plasma levels of insulin-like growth factor-1 (IGF-1) on the morphology of major organs: studies of Laron dwarf and bovine growth hormone transgenic (bGHTg) mice. Histol Histopathol 2013;28(10):1325–36. [31] Reik W, Walter J. Genomic imprinting: parental influence on the genome. Nat Rev Genet 2001;2(1):21–32. [32] Hayashi K, Surani MA. Resetting the epigenome beyond pluripotency in the germline. Cell Stem Cell 2009;4(6):493–8. [33] Wylie C. Germ cells. Cell 1999;96(2):165–74. [34] Shin DM, Zuba-Surma EK, Wu W, Ratajczak J, Wysoczynski M, Ratajczak MZ, et al. Novel epigenetic mechanisms that control pluripotency and quiescence of adult bone marrow-derived Oct4(+) very small embryonic-like stem cells. Leukemia 2009;23(11):2042–51. [35] Vassena R, Montserrat N, Carrasco Canal B, Aran B, de Onate L, Veiga A, et al. Accumulation of instability in serial differentiation and reprogramming of parthenogenetic human cells. Hum Mol Genet 2012;21(15):3366–73. [36] Suszynska M, Zuba-Surma EK, Maj M, Mierzejewska K, Ratajczak J, Kucia M, et al. The proper criteria for identification and sorting of very small embryonic-like stem cells, and some nomenclature issues. Stem Cells Dev 2014;23(7):702–13. [37] Ratajczak MZ, Shin DM, Liu R, Mierzejewska K, Ratajczak J, Kucia M, et al. 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. Aging 2012;4(4):235–46. [38] Ratajczak MZ, Liu R, Ratajczak J, Kucia M, Shin DM. The role of pluripotent embryonic-like stem cells residing in adult tissues in regeneration and longevity. Differentiation 2011;81(3):153–61. [39] Muller FJ, Goldmann J, Loser P, Loring JF. A call to standardize teratoma assays used to define human pluripotent cell lines. Cell Stem Cell 2010;6(5):412–4. [40] Smith KP, Luong MX, Stein GS. Pluripotency: toward a gold standard for human ES and iPS cells. J Cell Physiol 2009;220(1):21–9. [41] Donovan PJ. Growth factor regulation of mouse primordial germ cell development. Curr Top Dev Biol 1994;29:189–225. [42] Resnick JL, Ortiz M, Keller JR, Donovan PJ. Role of fibroblast growth factors and their receptors in mouse primordial germ cell growth. Biol Reprod 1998;59(5):1224–9. [43] Hayashi K, de Sousa Lopes SM, Surani MA. Germ cell specification in mice. Science 2007;316(5823):394–6. [44] Obokata H, Wakayama T, Sasai Y, Kojima K, Vacanti MP, Niwa H, et al. Stimulus-triggered fate conversion of somatic cells into pluripotency. Nature 2014;505(7485):641–7.

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280 [45] Williams JS, Xiao Y, Brownell I. Low pH reprograms somatic murine cells into pluripotent stem cells: a novel technique with therapeutic implications. Cancer Biol Ther 2014;15(6). [46] Mezey E, Chandross KJ, Harta G, Maki RA, McKercher SR. Turning blood into brain: cells bearing neuronal antigens generated in vivo from bone marrow. Science 2000;290(5497):1779–82. [47] Hess DC, Abe T, Hill WD, Studdard AM, Carothers J, Masuya M, et al. Hematopoietic origin of microglial and perivascular cells in brain. Exp Neurol 2004;186(2):134–44. [48] Jin K, Galvan V. Endogenous neural stem cells in the adult brain. J Neuroimmune Pharmacol 2007;2(3):236–42. [49] Orlic D, Kajstura J, Chimenti S, Jakoniuk I, Anderson SM, Li B, et al. Bone marrow cells regenerate infarcted myocardium. Nature 2001;410(6829): 701–705. [50] Lagasse E, Connors H, Al-Dhalimy M, Reitsma M, Dohse M, Osborne L, et al. Purified hematopoietic stem cells can differentiate into hepatocytes in vivo. Nat Med 2000;6(11):1229–34. [51] Di Campli C, Piscaglia AC, Pierelli L, Rutella S, Bonanno G, Alison MR, et al. A human umbilical cord stem cell rescue therapy in a murine model of toxic liver injury. Dig Liver Dis 2004;36(9):603–13. [52] Taupin P. Neural progenitor and stem cells in the adult central nervous system. Ann Acad Med Singap 2006;35(11):814–20. [53] Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells. Hepatology 2004;39(6):1477–87. [54] Strick-Marchand H, Morosan S, Charneau P, Kremsdorf D, Weiss MC. Bipotential mouse embryonic liver stem cell lines contribute to liver regeneration and differentiate as bile ducts and hepatocytes. Proc Natl Acad Sci U S A 2004;101(22):8360–5. [55] Castro RF, Jackson KA, Goodell MA, Robertson CS, Liu H, Shine HD. Failure of bone marrow cells to transdifferentiate into neural cells in vivo. Science 2002;297(5585):1299. [56] Murry CE, Soonpaa MH, Reinecke H, Nakajima H, Nakajima HO, Rubart M, et al. Haematopoietic stem cells do not transdifferentiate into cardiac myocytes in myocardial infarcts. Nature 2004;428(6983):664–8. [57] Terada N, Hamazaki T, Oka M, Hoki M, Mastalerz DM, Nakano Y, et al. Bone marrow cells adopt the phenotype of other cells by spontaneous cell fusion. Nature 2002;416(6880):542–5. [58] Rafii S, Lyden D. Therapeutic stem and progenitor cell transplantation for organ vascularization and regeneration. Nat Med 2003;9(6):702–12. [59] Ratajczak MZ, Kucia M, Jadczyk T, Greco NJ, Wojakowski W, Tendera M, et al. Pivotal role of paracrine effects in stem cell therapies in regenerative medicine: can we translate stem cell-secreted paracrine factors and microvesicles into better therapeutic strategies? Leukemia 2012;26(6):1166–73. [60] Reca R, Cramer D, Yan J, Laughlin MJ, Janowska-Wieczorek A, Ratajczak J, et al. A novel role of complement in mobilization: immunodeficient mice are poor granulocyte-colony stimulating factor mobilizers because they lack complement-activating immunoglobulins. Stem Cells 2007;25(12):3093– 100. [61] Wysoczynski M, Kucia M, Ratajczak J, Ratajczak MZ. Cleavage fragments of the third complement component (C3) enhance stromal derived factor-1 (SDF-1)-mediated platelet production during reactive postbleeding thrombocytosis. Leukemia 2007;21(5):973–82. [62] Takahashi T, Kalka C, Masuda H, Chen D, Silver M, Kearney M, et al. Ischemiaand cytokine-induced mobilization of bone marrow-derived endothelial progenitor cells for neovascularization. Nat Med 1999;5(4):434–8. [63] Togel F, Isaac J, Hu Z, Weiss K, Westenfelder C. Renal SDF-1 signals mobilization and homing of CXCR4-positive cells to the kidney after ischemic injury. Kidney Int 2005;67(5):1772–84. [64] Wojakowski W, Tendera M, Michalowska A, Majka M, Kucia M, Maslankiewicz K, et al. Mobilization of CD34/CXCR4+, CD34/CD117+, c-met+ stem cells, and mononuclear cells expressing early cardiac, muscle, and endothelial markers into peripheral blood in patients with acute myocardial infarction. Circulation 2004;110(20):3213–20. [65] Camussi G, Deregibus MC, Tetta C. Paracrine/endocrine mechanism of stem cells on kidney repair: role of microvesicle-mediated transfer of genetic information. Curr Opin Nephrol Hypertens 2010;19(1):7–12. [66] Ratajczak MZ, Kucia M, Reca R, Majka M, Janowska-Wieczorek A, Ratajczak J. Stem cell plasticity revisited: CXCR4-positive cells expressing mRNA for early muscle, liver and neural cells ‘hide out’ in the bone marrow. Leukemia 2004;18(1):29–40. [67] Zuba-Surma EK, Kucia M, Wu W, Klich I, Lillard Jr JW, Ratajczak J, et al. Very small embryonic-like stem cells are present in adult murine organs: ImageStream-based morphological analysis and distribution studies. Cytometry A 2008;73A(12):1116–27. [68] Ratajczak J, Wysoczynski M, Zuba-Surma E, Wan W, Kucia M, Yoder MC, et al. Adult murine bone marrow-derived very small embryonic-like stem cells differentiate into the hematopoietic lineage after coculture over OP9 stromal cells. Exp Hematol 2011;39(2):225–37. [69] Kassmer SH, Jin H, Zhang PX, Bruscia EM, Heydari K, Lee JH, et al. Very small embryonic-like stem cells from the murine bone marrow differentiate into epithelial cells of the lung. Stem Cells 2013;31(12):2759–66. [70] Kassmer SH, Bruscia EM, Zhang PX, Krause DS. Nonhematopoietic cells are the primary source of bone marrow-derived lung epithelial cells. Stem Cells 2012;30(3):491–9. [71] Virant-Klun I, Zech N, Rozman P, Vogler A, Cvjeticanin B, Klemenc P, et al. Putative stem cells with an embryonic character isolated from the ovarian

[72]

[73]

[74] [75]

[76]

[77]

[78]

[79]

[80]

[81]

[82]

[83] [84]

[85]

[86]

[87]

[88]

[89]

[90]

[91]

[92]

[93]

[94]

[95]

279

surface epithelium of women with no naturally present follicles and oocytes. Differentiation 2008;76(8):843–56. Bhartiya D, Kasiviswananthan S, Shaikh A. Cellular origin of testis-derived pluripotent stem cells: a case for very small embryonic-like stem cells. Stem Cells Dev 2012;21(5):670–4. Anand S, Bhartiya D, Sriraman K, Patel H, Manjramkar D, Bakshi G, et al. Quiescent very small embryonic-like stem cells resist oncotherapy and can restore spermatogenesis in germ cell depleted mammalian testis. Stem Cells Dev 2013. http://dx.doi.org/10.1089/scd.2013.0059. Ratajczak MZ, Kucia M, Liu R, Shin DM, Bryndza E, Masternak MM, et al. RasGrf1: genomic imprinting, VSELs, and aging. Aging 2011;3(7):692–7. Ratajczak MZ, Shin DM, Ratajczak J, Kucia M, Bartke A. A novel insight into aging: are there pluripotent very small embryonic-like stem cells (VSELs) in adult tissues overtime depleted in an Igf-1-dependent manner? Aging 2010;2(11):875–83. Taichman RS, Wang Z, Shiozawa Y, Jung Y, Song J, Balduino A, et al. Prospective identification and skeletal localization of cells capable of multilineage differentiation in vivo. Stem Cells Dev 2010;19(10):1557–70. Yamazaki Y, Mann MR, Lee SS, Marh J, McCarrey JR, Yanagimachi R, et al. Reprogramming of primordial germ cells begins before migration into the genital ridge, making these cells inadequate donors for reproductive cloning. Proc Natl Acad Sci U S A 2003;100(21):12207–12. Didie M, Christalla P, Rubart M, Muppala V, Doker S, Unsold B, et al. Parthenogenetic stem cells for tissue-engineered heart repair. J Clin Invest 2013;123(3):1285–98. Shin DM, Liu R, Klich I, Wu W, Ratajczak J, Kucia M, et al. Molecular signature of adult bone marrow-purified very small embryonic-like stem cells supports their developmental epiblast/germ line origin. Leukemia 2010;24(8):1450–61. Suszynska M, Poniewierska-Baran A, Gunjal P, Ratajczak J, Marycz K, Kakar SS, et al. Expression of the erythropoietin receptor by germline-derived cells – further support for a potential developmental link between the germline and hematopoiesis. J Ovarian Res 2014;7:66. Ratajczak MZ, Shin DM, Kucia M. Very small embryonic/epiblast-like stem cells: a missing link to support the germ line hypothesis of cancer development? Am J Pathol 2009;174(6):1985–92. Shin DM, Liu R, Klich I, Ratajczak J, Kucia M, Ratajczak MZ. Molecular characterization of isolated from murine adult tissues very small embryonic/epiblast like stem cells (VSELs). Mol Cells 2010;29(6):533–8. Bhartiya D, Unni S, Parte S, Anand S. Very small embryonic-like stem cells: implications in reproductive biology. Biomed Res Int 2013;2013:682326. Ratajczak MZ, Liu R, Marlicz W, Blogowski W, Starzynska T, Wojakowski W, et al. Identification of very small embryonic/epiblast-like stem cells (VSELs) circulating in peripheral blood during organ/tissue injuries. Methods Cell Biol 2011;103:31–54. Wojakowski W, Tendera M, Kucia M, Zuba-Surma E, Paczkowska E, Ciosek J, et al. Mobilization of bone marrow-derived Oct-4+ SSEA-4+ very small embryonic-like stem cells in patients with acute myocardial infarction. J Am Coll Cardiol 2009;53(1):1–9. Paczkowska E, Kucia M, Koziarska D, Halasa M, Safranow K, Masiuk M, et al. Clinical evidence that very small embryonic-like stem cells are mobilized into peripheral blood in patients after stroke. Stroke 2009;40(4):1237–44. Marlicz W, Zuba-Surma E, Kucia M, Blogowski W, Starzynska T, Ratajczak MZ. Various types of stem cells, including a population of very small embryoniclike stem cells, are mobilized into peripheral blood in patients with Crohn’s disease. Inflamm Bowel Dis 2012;18(9):1711–22. Drukala J, Paczkowska E, Kucia M, Mlynska E, Krajewski A, Machalinski B, et al. Stem cells, including a population of very small embryonic-like stem cells, are mobilized into peripheral blood in patients after skin burn injury. Stem Cell Rev 2012;8(1):184–94. Zuba-Surma EK, Guo Y, Taher H, Sanganalmath SK, Hunt G, Vincent RJ, et al. Transplantation of expanded bone marrow-derived very small embryoniclike stem cells (VSEL-SCs) improves left ventricular function and remodelling after myocardial infarction. J Cell Mol Med 2011;15(6):1319–28. Kucia M, Dawn B, Hunt G, Guo Y, Wysoczynski M, Majka M, et al. Cells expressing early cardiac markers reside in the bone marrow and are mobilized into the peripheral blood after myocardial infarction. Circ Res 2004;95(12):1191–9. Grymula K, Tarnowski M, Piotrowska K, Suszynska M, Mierzejewska K, Borkowska S, et al. Evidence that the population of quiescent bone marrow-residing very small embryonic/epiblast-like stem cells (VSELs) expands in response to neurotoxic treatment. J Cell Mol Med 2014. http://dx.doi.org/ 10.1111/jcmm.12315. Karapetyan AV, Klyachkin YM, Selim S, Sunkara M, Ziada KM, Cohen DA, et al. Bioactive lipids and cationic antimicrobial peptides as new potential regulators for trafficking of bone marrow-derived stem cells in patients with acute myocardial infarction. Stem Cells Dev 2013;22(11):1645–56. Kucharska-Mazur J, Tarnowski M, Dolegowska B, Budkowska M, Pedziwiatr D, Jablonski M, et al. Novel evidence for enhanced stem cell trafficking in antipsychotic-naive subjects during their first psychotic episode. J Psychiatr Res 2014;49:18–24. Starzynska T, Dabkowski K, Blogowski W, Zuba-Surma E, Budkowska M, Salata D, et al. An intensified systemic trafficking of bone marrow-derived stem/progenitor cells in patients with pancreatic cancer. J Cell Mol Med 2013;17(6):792–9. Sluczanowska-Glabowska S, Laszczynska M, Piotrowska K, Glabowski W, Kopchick JJ, Bartke A, et al. Morphology of ovaries in laron dwarf mice, with

280

[96]

[97]

[98] [99] [100] [101] [102] [103] [104]

[105]

[106] [107]

[108]

M.Z. Ratajczak et al. / Advances in Medical Sciences 59 (2014) 273–280 low circulating plasma levels of insulin-like growth factor-1 (IGF-1), and in bovine GH-transgenic mice, with high circulating plasma levels of IGF-1. J Ovarian Res 2012;5:18. Sluczanowska-Glabowska S, Laszczynska M, Piotrowska K, Glabowski W, Rumianowski B, Masternak M, et al. The effect of calorie restriction on the presence of apoptotic ovarian cells in normal wild type mice and lowplasma-IGF-1 Laron dwarf mice. J Ovarian Res 2013;6(1):67. Grymula K, Piotrowska K, Sluczanowska-Glabowska S, Mierzejewska K, Tarnowski M, Tkacz M, et al. Positive effects of prolonged caloric restriction on the population of very small embryonic-like stem cells – hematopoietic and ovarian implications. J Ovarian Res 2014;7:68. Virchow R. Editorial Archive fuer pathologische. Anat Physiol Med 1855;8: 23–54. Durante F. Nesso fisio-pathologico tra la struttura dei nei materni e la genesi di alcuni tumori maligni. Arch Memor Osser Chir Pract 1874;11:217–26. Conheim J. Congenitales, quergestreiftes Muskelsarkon der Nireren. Virchows Arch 1875;65:64. Wright JH. Neurocytoma or nauroblastoma, a kind of tumor not generally recognized. J Exp Med 1910;12:556–61. Beard J. The enzyme treatment of cancer. London: Chatto and Windus; 1911. Macchiarini P, Ostertag H. Uncommon primary mediastinal tumours. Lancet Oncol 2004;5(2):107–18. Andrews PW, Matin MM, Bahrami AR, Damjanov I, Gokhale P, Draper JS. Embryonic stem (ES) cells and embryonal carcinoma (EC) cells: opposite sides of the same coin. Biochem Soc Trans 2005;33(Pt 6):1526–30. Sigalotti L, Covre A, Zabierowski S, Himes B, Colizzi F, Natali PG, et al. Cancer testis antigens in human melanoma stem cells: expression, distribution, and methylation status. J Cell Physiol 2008;215(2):287–91. Simpson AJ, Caballero OL, Jungbluth A, Chen YT, Old LJ. Cancer/testis antigens, gametogenesis and cancer. Nat Rev Cancer 2005;5(8):615–25. Hotakainen K, Ljungberg B, Haglund C, Nordling S, Paju A, Stenman UH. Expression of the free beta-subunit of human chorionic gonadotropin in renal cell carcinoma: prognostic study on tissue and serum. Int J Cancer 2003;104(5):631–5. Ratajczak MZ, Shin DM, Liu R, Marlicz W, Tarnowski M, Ratajczak J, et al. Epiblast/germ line hypothesis of cancer development revisited: lesson from the presence of Oct-4+ cells in adult tissues. Stem Cell Rev 2010; 6(2):307–16.

[109] Cheng L. Establishing a germ cell origin for metastatic tumors using OCT4 immunohistochemistry. Cancer 2004;101(9):2006–10. [110] Lee AS, Tang C, Rao MS, Weissman IL, Wu JC. Tumorigenicity as a clinical hurdle for pluripotent stem cell therapies. Nat Med 2013;19(8):998–1004. [111] Ratajczak MZ, Zuba-Surma E, Wojakowski W, Suszynska M, Mierzejewska K, Liu R, et al. 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. Leukemia 2014;28(3):473–84. [112] Zuba-Surma EK, Ratajczak MZ. Overview of very small embryonic-like stem cells (VSELs) and methodology of their identification and isolation by flow cytometric methods. Curr Protoc Cytom 2010. Chapter 9: Unit 9.29. [113] Halasa M, Baskiewicz-Masiuk M, Dabkowska E, Machalinski B. An efficient two-step method to purify very small embryonic-like (VSEL) stem cells from umbilical cord blood (UCB). Folia Histochem Cytobiol 2008;46(2):239–43. [114] Ratajczak MZ, Kucia M, Ratajczak J, Zuba-Surma EK. A multi-instrumental approach to identify and purify very small embryonic like stem cells (VSELs) from adult tissues. Micron 2009;40(3):386–93. [115] Dyce PW, Liu J, Tayade C, Kidder GM, Betts DH, Li J. In vitro and in vivo germ line potential of stem cells derived from newborn mouse skin. PLoS ONE 2011;6(5):e20339. [116] Song SH, Kumar BM, Kang EJ, Lee YM, Kim TH, Ock SA, et al. Characterization of porcine multipotent stem/stromal cells derived from skin, adipose, and ovarian tissues and their differentiation in vitro into putative oocyte-like cells. Stem Cells Dev 2011;20(8):1359–70. [117] Shirazi R, Zarnani AH, Soleimani M, Abdolvahabi MA, Nayernia K, Ragerdi Kashani I. BMP4 can generate primordial germ cells from bone-marrowderived pluripotent stem cells. Cell Biol Int 2012;36(12):1185–93. [118] Johnson J, Bagley J, Skaznik-Wikiel M, Lee HJ, Adams GB, Niikura Y, et al. Oocyte generation in adult mammalian ovaries by putative germ cells in bone marrow and peripheral blood. Cell 2005;122(2):303–15. [119] Selesniemi K, Lee HJ, Niikura T, Tilly JL. Young adult donor bone marrow infusions into female mice postpone age-related reproductive failure and improve offspring survival. Aging 2009;1(1):49–57. [120] Nayernia K, Lee JH, Drusenheimer N, Nolte J, Wulf G, Dressel R, et al. Derivation of male germ cells from bone marrow stem cells. Lab Invest 2006;86(7):654–63. [121] Heo YT, Lee SH, Yang JH, Kim T, Lee HT. Bone marrow cell-mediated production of transgenic chickens. Lab Invest 2011;91(8):1229–40.