Functional Testing of Primitive Oocyte-like Cells Developed in Ovarian Surface Epithelium Cell Culture from Small VSEL-like Stem Cells: Can They Be Fertilized One Day?

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

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

Autologous Human Mesenchymal Stem Cell-Based Therapy in Infertility: New Strategies and Future Perspectives

Infertility could be associated with a few factors including problems with physical and mental health, hormonal imbalances, lifestyles, and genetic factors. Given that there is a concern about the rise of infertility globally, increased focus has been given to its treatment for the last several decades. Traditional assisted reproductive technology (ART) has been the prime option for many years in solving various cases of infertility; however, it contains significant risks and does not solve the fundamental problem of infertility such as genetic disorders. Attention toward the utilization of MSCs has been widely regarded as a promising option in the development of stem-cell-based infertility treatments. This narrative review briefly presents the challenges in the current ART treatment of infertility and the various potential applications of autologous MSCs in the treatment of these reproductive diseases.

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

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

Inhibition of EED activity enhances cell survival of female germline stem cell and improves the oocytes production during oogenesis in vitro

Ovarian organoids, based on female germline stem cells (FGSCs), are nowadays widely applied for reproductive medicine screening and exploring the potential mechanisms during mammalian oogenesis. However, there are still key issues that urgently need to be resolved in ovarian organoid technology, one of which is to establish a culture system that effectively expands FGSCs in vitro, as well as maintaining the unipotentcy of FGSCs to differentiate into oocytes. Here, FGSCs were EED226 treated and processed for examination of proliferation and differentiation in vitro. According to the results, EED226 specifically increased FGSC survival by decreasing the enrichment of H3K27me3 on Oct4 promoter and exon, as well as enhancing OCT4 expression and inhibiting P53 and P63 expression. Notably, we also found that FGSCs with EED226 treatment differentiated into more oocytes during oogenesis in vitro, and the resultant oocytes maintained a low level of P63 versus control at early stage development. These results demonstrated that inhibition of EED activity appeared to promote the survival of FGSCs and markedly inhibited their apoptosis during in vitro differentiation. As a result of our study, we propose an effective culture strategy to culture FGSCs and obtain oocytes in vitro, which provides a new vision for oogenesis in vitro.

Proteomic Analysis of Murine Bone Marrow Very Small Embryonic-like Stem Cells at Steady-State Conditions and after In Vivo Stimulation by Nicotinamide and Follicle-Stimulating Factor Reflects their Germ-Lineage Origin and Multi Germ Layer Differentiation Potential

Very small embryonic-like stem cells (VSELs) are a dormant population of development early stem cells deposited in adult tissues that as demonstrated contribute to tissue/organ repair and regeneration. We postulated developmental relationship of these cells to migrating primordial germ cells (PGCs) and explained the quiescent state of these cells by the erasure of differently methylated regions (DMRs) at some of the paternally imprinted genes involved in embryogenesis. Recently, we reported that VSELs began to proliferate and expand in vivo in murine bone marrow (BM) after exposure to nicotinamide (NAM) and selected pituitary and gonadal sex hormones. In the current report, we performed proteomic analysis of VSELs purified from murine bone marrow (BM) after repeated injections of NAM + Follicle-Stimulating Hormone (FSH) that in our previous studies turned out to be an effective combination to expand these cells. By employing the Gene Ontology (GO) resources, we have performed a combination of standard GO annotations (GO-CAM) to produce a network between BM steady-state conditions VSELs (SSC-VSELS) and FSH + NAM expanded VSELs (FSH + NAM VSELs). We have identified several GO biological processes regulating development, organogenesis, gene expression, signal transduction, Wnt signaling, insulin signaling, cytoskeleton organization, cell adhesion, inhibiting apoptosis, responses to extra- and intracellular stimuli, protein transport and stabilization, protein phosphorylation and ubiquitination, DNA repair, immune response, and regulation of circadian rhythm. We report that VSELs express a unique panel of proteins that only partially overlapped with the proteome of BM - derived hematopoietic stem cells (HSCs) and hematopoietic mononuclear cells (MNCs) and respond to FSH + NAM stimulation by expressing proteins involved in the development of all three germ layers. Thus, our current data supports further germ-lineage origin and multi germ layer differentiation potential of these cells.

Artificial Oocyte: Development and Potential Application

Millions of people around the world suffer from infertility, with the number of infertile couples and individuals increasing every year. Assisted reproductive technologies (ART) have been widely developed in recent years; however, some patients are unable to benefit from these technologies due to their lack of functional germ cells. Therefore, the development of alternative methods seems necessary. One of these methods is to create artificial oocytes. Oocytes can be generated in vitro from the ovary, fetal gonad, germline stem cells (GSCs), ovarian stem cells, or pluripotent stem cells (PSCs). This approach has raised new hopes in both basic research and medical applications. In this article, we looked at the principle of oocyte development, the landmark studies that enhanced our understanding of the cellular and molecular mechanisms that govern oogenesis in vivo, as well as the mechanisms underlying in vitro generation of functional oocytes from different sources of mouse and human stem cells. In addition, we introduced next-generation ART using somatic cells with artificial oocytes. Finally, we provided an overview of the reproductive application of in vitro oogenesis and its use in human fertility.

Overview of follicle stimulating hormone and its receptors in reproduction and in stem cells and cancer stem cells

Follicle stimulating hormone (FSH) and its receptor (FSHR) have been reported to be responsible for several physiological functions and cancers. The responsiveness of stem cells and cancer stem cells towards the FSH-FSHR system make the function of FSH and its receptors more interesting in the context of cancer biology. This review is comprised of comprehensive information on FSH-FSHR signaling in normal physiology, gonadal stem cells, cancer cells, and potential options of utilizing FSH-FSHR system as an anti-cancer therapeutic target.

Stem Cells in Adult Mice Ovaries Form Germ Cell Nests, Undergo Meiosis, Neo-oogenesis and Follicle Assembly on Regular Basis During Estrus Cycle

Very small embryonic-like (VSELs) and ovarian (OSCs) stem cells are located in adult mammalian ovary surface epithelium (OSE). OSCs can expand long-term and differentiate into oocyte-like structures in vitro and have resulted in birth of fertile pups. Lineage tracing studies have provided evidence to suggest OSCs differentiation into oocytes in vivo. But how these stem cells function under normal physiological conditions has not yet been well worked out. Besides studying STRA-8 and SCP-3 expression in enzymatically isolated OSE cells smears, mice were injected BrdU to track mitosis, meiosis and follicle assembly. H&E stained OSE cells during late diestrus and proestrus showed VSELs undergoing asymmetrical cell divisions to give rise to slightly bigger OSCs which in turn underwent symmetrical cell divisions followed by clonal expansion (rapid expansion with incomplete cytokinesis) during early estrus to form germ cell nests (GCN). OCT-4, SSEA-1, MVH and DAZL positive cells in GCN expressed Erα, Erβ and FSHR, were interconnected by ring canals (TEX-14), showed mitochondrial aggregation (Cytochrome C) and Balbiani Body (TRAL). Apoptosis in 'nurse' cells was marked by PARP and putative oocytes were clearly visualized. BrdU was detected in cells undergoing mitosis/meiosis and also in an oocyte of secondary follicle. FACS sorted, green fluorescent protein (GFP) positive VSELs upon transplantation resulted in GFP positive GCN suggesting crucial role for VSELs in adult ovaries. Results suggest that various events described during oogenesis and follicle assembly in fetal ovaries are recapitulated on regular basis in adult ovary and result in the formation of follicles.

SARS-CoV-2 Entry Receptor ACE2 Is Expressed on Very Small CD45- Precursors of Hematopoietic and Endothelial Cells and in Response to Virus Spike Protein Activates the Nlrp3 Inflammasome

Angiotensin-converting enzyme 2 (ACE2) plays an important role as a member of the renin–angiotensin–aldosterone system (RAAS) in regulating the conversion of angiotensin II (Ang II) into angiotensin (1–7) (Ang [1–7]). But at the same time, while expressed on the surface of human cells, ACE2 is the entry receptor for SARS-CoV-2. Expression of this receptor has been described in several types of cells, including hematopoietic stem cells (HSCs) and endothelial progenitor cells (EPCs), which raises a concern that the virus may infect and damage the stem cell compartment. We demonstrate for the first time that ACE2 and the entry-facilitating transmembrane protease TMPRSS2 are expressed on very small CD133⁺CD34⁺Lin⁻CD45⁻ cells in human umbilical cord blood (UCB), which can be specified into functional HSCs and EPCs. The existence of these cells known as very small embryonic-like stem cells (VSELs) has been confirmed by several laboratories, and some of them may correspond to putative postnatal hemangioblasts. Moreover, we demonstrate for the first time that, in human VSELs and HSCs, the interaction of the ACE2 receptor with the SARS-CoV-2 spike protein activates the Nlrp3 inflammasome, which if hyperactivated may lead to cell death by pyroptosis. Based on this finding, there is a possibility that human VSELs residing in adult tissues could be damaged by SARS-CoV-2, with remote effects on tissue/organ regeneration. We also report that ACE2 is expressed on the surface of murine bone marrow-derived VSELs and HSCs, although it is known that murine cells are not infected by SARS-CoV-2. Finally, human and murine VSELs express several RAAS genes, which sheds new light on the role of these genes in the specification of early-development stem cells.

Graphical Abstract

Advances in Female Germ Cell Induction from Pluripotent Stem Cells

Germ cells are capable of maintaining species continuity through passing genetic and epigenetic information across generations. Female germ cells mainly develop during the embryonic stage and pass through subsequent developmental stages including primordial germ cells, oogonia, and oocyte. However, due to the limitation of using early human embryos as in vivo research model, in vitro research models are needed to reveal the early developmental process and related mechanisms of female germ cells. After birth, the number of follicles gradually decreases with age. Various conditions which damage ovarian functions would cause premature ovarian failure. Alternative treatments to solve these problems need to be investigated. Germ cell differentiation from pluripotent stem cells in vitro can simulate early embryonic development of female germ cells and clarify unresolved issues during the development process. In addition, pluripotent stem cells could potentially provide promising applications for female fertility preservation after proper in vitro differentiation. Mouse female germ cells have been successfully reconstructed in vitro and delivered to live offspring. However, the derivation of functional human female germ cells has not been fully achieved due to technical limitations and ethical issues. To provide an updated and comprehensive information, this review centers on the major studies on the differentiation of mouse and human female germ cells from pluripotent stem cells and provides references to further studies of developmental mechanisms and potential therapeutic applications of female germ cells.

Testicular Stem Cell Dysfunction Due to Environmental Insults Could Be Responsible for Deteriorating Reproductive Health of Men

Reproductive health of men has declined over time including reduced semen quality specifically sperm count, increased incidence of infertility, and testicular cancers. Our recent findings suggest that these disease states possibly arise as a result of disruption of testicular stem cells biology by perinatal insults including exposure to endocrine disrupting chemicals. Testicular stem cells include relatively quiescent, very small embryonic-like stem cells (VSELs), and actively dividing spermatogonial stem cells (SSCs). Both VSELs and SSCs express estrogen receptors and are directly vulnerable to endocrine disruption. Exposing mice pups to estradiol (20 μg/pup/day on days 5-7) or diethylstilbestrol (2 μg/pup/day on days 1-5) affected spermatogenesis during adult life with reduced numbers of tubules in stage VIII, tetraploid cells and sperm. These mice were infertile and majority of diethylstilbestrol treated mice revealed testicular cancer-like changes. An increase in VSEL numbers, observed by both flow cytometry and qRT-PCR, was associated with marked reduction of c-KIT positive spermatogonial cells. VSELs undergo epigenetic changes due to endocrine disruption that results in blocked differentiation (impaired spermatogenesis) leading to reduced sperm count and infertility, and their excessive self-renewal initiates cancer-like changes in adult life. Thus, testicular dysgenesis syndrome (TDS) has a stem cell rather than a genetic basis.

Bone morphogenetic protein 15 induces differentiation of mesenchymal stem cells derived from human follicular fluid to oocyte-like cell

Follicular fluid (FF) is essential for developing ovarian follicles. Besides the oocytes, FF has abundant undifferentiated somatic cells containing stem cell properties, which are discarded in daily medical procedures. Earlier studies have shown that FF cells could differentiate into primordial germ cells via forming embryoid bodies, which produced oocyte-like cells (OLC). This study aimed at isolating mesenchymal stem cells (MSC) from FF and evaluating the impacts of bone morphogenetic protein 15 (BMP15) on the differentiation of these cells into OLCs. Human FF-derived cells were collected from 78 women in the assisted fertilization program and cultured in human recombinant BMP15 medium for 21 days. Real-time polymerase chain reaction and immunocytochemistry staining characterized MSCs and OLCs. MSCs expressed germline stem cell (GSC) markers, such as OCT4 and Nanog. In the control group, after 15 days, OLCs were formed and expressed zona pellucida markers (ZP2 and ZP3), and reached 20-30 µm in diameter. Ten days after induction with BMP15, round cells developed, and the size of OLCs reached 115 µm. A decrease ranged from 0.04 to 4.5 in the expression of pluripotency and oocyte-specific markers observed in the cells cultured in a BMP15-supplemented medium. FF-derived MSCs have an innate potency to differentiate into OLCs, and BMP15 is effective in promoting the differentiation of these cells, which may give an in vitro model to examine germ cell development.

In Vitro Generation of Oocyte Like Cells and Their In Vivo Efficacy: How Far We have been Succeeded

In the last few decades, stem cell therapy has grown as a boon for many pathological complications including female reproductive disorders. In this review, a brief description of available strategies that are related to stem cell-based in vitro oocyte-like cell (OLC) development are given. We have tried to cover all the aspects and latest updates of the in vitro OLC developmental methodologies, marker profiling, available disease models, and in vivo efficacies, with a special focus on mesenchymal stem cells (MSCs), induced pluripotent stem cells (iPSCs), and embryonic stem cells (ESCs) usage. The differentiation abilities of both the ovarian and non-ovarian stem cell sources under various induction conditions have shown different effects on morphological alterations, proliferation- and size-associated developments, hormonal secretions under gonadotropic stimulations, and their neo-oogenesis or folliculogenesis abilities after in vivo transplantations. The attainment of characters like oocyte-like morphology, size expansion, and meiosis initiation have been found to be major obstacles during in vitro oogenesis. A number of reports have either lacked in vivo studies or have shown their functional incapability to produce viable and healthy offspring. Though researchers have gained many valuable insights regarding in vitro gametogenesis, still there are many things to do to make stem cell-derived OLCs fully functional.

Hematopoietic Stem and Progenitor Cells (HSPCs)

Hematopoietic stem/progenitor cells (HSPCs) isolated from bone marrow have been successfully employed for 50 years in hematological transplantations. Currently, these cells are more frequently isolated from mobilized peripheral blood or umbilical cord blood. In this chapter, we overview several topics related to these cells including their phenotype, methods for isolation, and in vitro and in vivo assays to evaluate their proliferative potential. The successful clinical application of HSPCs is widely understood to have helped establish the rationale for the development of stem cell therapies and regenerative medicine.

Role of Adult Tissue-Derived Pluripotent Stem Cells in Bone Regeneration

Background

Bone marrow-derived mononuclear cells (BM-MNC) consist of a heterogeneous mix of mesenchymal stem cells (MSC), hematopoietic progenitor cells (HPC), endothelial progenitor cells (EPC), monocytes, lymphocytes and pluripotent stem cells. Whereas the importance of MSC and EPC has been well documented in bone healing and regeneration studies, the role of pluripotent stem cells is still poorly understood. In the present study we evaluated if and how Very Small Embryonic Like cells (VSEL), isolated from rat BM-MNC, contribute to bone healing.

Methods

Large bone defects were made in the femurs of 38 Sprague Dawley female rats and treated with β-TCP scaffold granules seeded with male VSEL; BM-MNC, VSEL-depleted BM-MNC or scaffold alone, and bone healing was evaluated at 8 weeks post-surgery.

Results

Bone healing was significantly increased in defects treated with VSEL and BM-MNC, compared to defects treated with VSEL-depleted BM-MNC. Donor cells were detected in new bone tissue, in all the defects treated with cells, and in fibrous tissue only in defects treated with VSEL-depleted BM-MNC. The number of CD68+ cells was the highest in the VSEL-depleted group, whereas the number of TRAP positive cells was the lowest in this group.

Conclusions

Based on the results, we can conclude that VSEL play a role in BM-MNC induced bone formation. In our rat femur defect model, in defects treated with VSEL-depleted BM-MNC, osteoclastogenesis and bone formation were decreased, and foreign body reaction was increased.

Similar Population of CD133+ and DDX4+ VSEL-Like Stem Cells Sorted from Human Embryonic Stem Cell, Ovarian, and Ovarian Cancer Ascites Cell Cultures: The Real Embryonic Stem Cells?

A population of small stem cells with diameters of up to 5 μm resembling very small embryonic-like stem cells (VSELs) were sorted from human embryonic stem cell (hESC) cultures using magnetic-activated cell sorting (MACS) based on the expression of a stem-cell-related marker prominin-1 (CD133). These VSEL-like stem cells had nuclei that almost filled the whole cell volume and expressed stem-cell-related markers (CD133, SSEA-4) and markers of germinal lineage (DDX4/VASA, PRDM14). They were comparable to similar populations of small stem cells sorted from cell cultures of normal ovaries and were the predominant cells in ascites of recurrent ovarian cancer. The sorted populations of CD133+ VSEL-like stem cells were quiescent in vitro, except for ascites, and were highly activated after exposure to valproic acid and follicle-stimulating hormone (FSH), indicating a new tool to study these cells in vitro. These VSEL-like stem cells spontaneously formed clusters resembling tumour-like structures or grew into larger, oocyte-like cells and were differentiated in vitro into adipogenic, osteogenic and neural lineages after sorting. We propose the population of VSEL-like stem cells from hESC cultures as potential original embryonic stem cells, which are present in the human embryo, persist in adult human ovaries from the embryonic period of life and are involved in cancer manifestation.

Heterogeneity of Stem Cells in the Ovary

Every organ in the body is thought to harbor two populations of stem cells, including the quiescent and the actively dividing, that leads to heterogeneity among them. It is generally believed that the ovary harbors a fixed number of follicles at birth that differentiate during fetal development from the primordial germ cells. The numbers of follicles decrease by age, leading to menopause. However, in 2004, it was suggested that ovary may harbor stem cells that are possibly involved in the formation of new follicles throughout reproductive life. Research over little more than a decade shows that ovarian stem cells include a quiescent population of very small embryonic-like stem cells (VSELs) and slightly bigger, actively dividing ovarian stem cells (OSCs). This heterogeneity among ovarian stem cells is similar to the presence of VSELs along with spermatogonial stem cells (SSCs) in the testis or hematopoietic stem cells (HSCs) in the hematopoietic system. VSELs express embryonic markers, including nuclear OCT-4, and are lodged in the ovary surface epithelium (OSE). Ovarian VSELs undergo asymmetric cell division to self-renew and give rise to OSCs that in turn undergo symmetric cell divisions and clonal expansion (germ cell nest) followed by meiosis to form an oocyte that gets assembled as a primordial follicle. Both VSELs and OSCs also express receptors for follicle-stimulating hormone (FSHR) and are directly activated by FSH to undergo neo-oogenesis and primordial follicle assembly. Whether stimulation of ovaries by FSH in Infertility Clinics activates the stem cells leading to the formation of multiple follicles needs further investigation. Epithelial cells lining the surface of ovary provide a niche to the stem cells under normal circumstances and undergo epithelial-mesenchymal transition (EMT) to form granulosa cells for primordial follicle assembly. Compromised function of the epithelial cells with age possibly leads to inability of stem cells to form follicles, leading to menopause. More than 90% of ovarian cancers arise in the OSE, possibly due to excessive self-renewal of VSELs. Altered biology of the OSE cells results in the formation of myofibroblasts by EMT and may provide a cancerous niche that supports excessive expansion of the stem cells lodged in the OSE, leading to ovarian cancer. Ovarian cancer cells express markers like OCT-4 and FSHR, which are also expressed by the VSELs lodged in the OSE, whereas the epithelial cells are distinctly negative for the same. Lot more research is required in the field to gain further understanding of ovarian stem cell biology.