Toti-/pluripotential stem cells and epigenetic modifications

Determinants of Chromatin Organization in Aging and Cancer-Emerging Opportunities for Epigenetic Therapies and AI Technology

This review article critically examines the pivotal role of chromatin organization in gene regulation, cellular differentiation, disease progression and aging. It explores the dynamic between the euchromatin and heterochromatin, coded by a complex array of histone modifications that orchestrate essential cellular processes. We discuss the pathological impacts of chromatin state misregulation, particularly in cancer and accelerated aging conditions such as progeroid syndromes, and highlight the innovative role of epigenetic therapies and artificial intelligence (AI) in comprehending and harnessing the histone code toward personalized medicine. In the context of aging, this review explores the use of AI and advanced machine learning (ML) algorithms to parse vast biological datasets, leading to the development of predictive models for epigenetic modifications and providing a framework for understanding complex regulatory mechanisms, such as those governing cell identity genes. It supports innovative platforms like CEFCIG for high-accuracy predictions and tools like GridGO for tailored ChIP-Seq analysis, which are vital for deciphering the epigenetic landscape. The review also casts a vision on the prospects of AI and ML in oncology, particularly in the personalization of cancer therapy, including early diagnostics and treatment optimization for diseases like head and neck and colorectal cancers by harnessing computational methods, AI advancements and integrated clinical data for a transformative impact on healthcare outcomes.

HERV-H RNA is abundant in human embryonic stem cells and a precise marker for pluripotency

Background

Certain post-translational modifications to histones, including H3K4me3, as well as binding sites for the transcription factor STAT1, predict the site of integration of exogenous gamma-retroviruses with great accuracy and cell-type specificity. Statistical methods that were used to identify chromatin features that predict exogenous gamma-retrovirus integration site selection were exploited here to determine whether cell type-specific chromatin markers are enriched in the vicinity of endogenous retroviruses (ERVs).

Results

Among retro-elements in the human genome, the gamma-retrovirus HERV-H was highly associated with H3K4me3, though this association was only observed in embryonic stem (ES) cells (p < 10^-300) and, to a lesser extent, in induced pluripotent stem (iPS) cells. No significant association was observed in nearly 40 differentiated cell types, nor was any association observed with other retro-elements. Similar strong association was observed between HERV-H and the binding sites within ES cells for the pluripotency transcription factors NANOG, OCT4, and SOX2. NANOG binding sites were located within the HERV-H 5^′LTR itself. OCT4 and SOX2 binding sites were within 1 kB and 2 kB of the 5^′LTR, respectively. In keeping with these observations, HERV-H RNA constituted 2% of all poly A RNA in ES cells. As ES cells progressed down a differentiation pathway, the levels of HERV-H RNA decreased progressively. RNA-Seq datasets showed HERV-H transcripts to be over 5 kB in length and to have the structure 5^′LTR-gag-pro-3^′LTR, with no evidence of splicing and no intact open reading frames.

Conclusion

The developmental regulation of HERV-H expression, the association of HERV-H with binding sites for pluripotency transcription factors, and the extremely high levels of HERV-H RNA in human ES cells suggest that HERV-H contributes to pluripotency in human cells. Proximity of HERV-H to binding sites for pluripotency transcription factors within ES cells might be due to retention of the same chromatin features that determined the site of integration of the ancestral, exogenous, gamma-retrovirus that gave rise to HERV-H in the distant past. Retention of these markers, or, alternatively, recruitment of them to the site of the established provirus, may have acted post-integration to fix the provirus within the germ-line of the host species. Either way, HERV-H RNA provides a specific marker for pluripotency in human cells.

Transdifferentiation: a cell and molecular reprogramming process

Evidence has emerged recently indicating that differentiation is not entirely a one-way process, and that it is possible to convert one cell type to another, both in vitro and in vivo. This phenomenon is called transdifferentiation, and is generally defined as the stable switch of one cell type to another. Transdifferentiation plays critical roles during development and in regeneration pathways in nature. Although this phenomenon occurs rarely in nature, recent studies have been focused on transdifferentiation and the reprogramming ability of cells to produce specific cells with new phenotypes for use in cell therapy and regenerative medicine. Thus, understanding the principles and the mechanism of this process is important for producing desired cell types. Here some well-documented examples of transdifferentiation, and their significance in development and regeneration are reviewed. In addition, transdifferentiation pathways are considered and their potential molecular mechanisms, especially the role of master switch genes, are considered. Finally, the significance of transdifferentiation in regenerative medicine is discussed.

Technological progress in generation of induced pluripotent stem cells for clinical applications

Reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) is achieved by viral-mediated transduction of defined transcription factors. Generation of iPSCs is of great medical interest as they have the potential to be a source of patient-specific cells. For the eventual goal of clinical application, it is necessary to overcome the limitations of low reprogramming efficiency and chromosomal abnormalities due to viral DNA integration. In this paper, we summarize the current state of reprogramming technology for generation of iPSCs and also discuss potential approaches to the development of safe iPSCs for personalized cell-based replacement therapy.

De novo myocardial regeneration: advances and pitfalls

The capability of adult tissue-derived stem cells for cardiogenesis has been extensively studied in experimental animals and clinical studies for treatment of postischemic cardiomyopathy. The less-than-anticipated improvement in the heart function in most clinical studies with skeletal myoblasts and bone marrow cells has warranted a search for alternative sources of stem cells. Despite their multilineage differentiation potential, ethical issues, teratogenicity, and tissue rejection are main obstacles in developing clinically feasible methods for embryonic stem cell transplantation into patients. A decade-long research on embryonic stem cells has paved the way for discovery of alternative approaches for generating pluripotent stem cells. Genetic manipulation of somatic cells for pluripotency genes reprograms the cells to pluripotent status. Efforts are currently focused to make reprogramming protocols safer for clinical applications of the reprogrammed cells. We summarize the advancements and complicating features of stem cell therapy and discuss the decade-and-a-half-long efforts made by stem cell researchers for moving the field from bench to the bedside as an adjunct therapy or as an alternative to the contemporary therapeutic modalities for routine clinical application. The review also provides a special focus on the advancements made in the field of somatic cell reprogramming.

Cell-type-specific level of DNA nucleotide excision repair in primary human mammary and ovarian epithelial cell cultures

DNA repair, a fundamental function of cellular metabolism, has long been presumed to be constitutive and equivalent in all cells. However, we have previously shown that normal levels of nucleotide excision repair (NER) can vary by 20-fold in a tissue-specific pattern. We have now successfully established primary cultures of normal ovarian tissue from seven women by using a novel culture system originally developed for breast epithelial cells. Epithelial cells in these cultures aggregated to form three-dimensional structures called "attached ovarian epispheres". The availability of these actively proliferating cell cultures allowed us to measure NER functionally and quantitatively by the unscheduled DNA synthesis (UDS) assay, a clinical test used to diagnose constitutive deficiencies in NER capacity. We determined that ovarian epithelial cells manifested an intermediate level of NER capacity in humans, viz., only 25% of that of foreskin fibroblasts, but still 2.5-fold higher than that of peripheral blood lymphocytes. This level of DNA repair capacity was indistinguishable from that of normal breast epithelial cells, suggesting that it might be characteristic of the epithelial cell type. Similar levels of NER activity were observed in cultures established from a disease-free known carrier of a BRCA1 truncation mutation, consistent with previous normal results shown in breast epithelium and blood lymphocytes. These results establish that at least three "normal" levels of such DNA repair occur in human tissues, and that NER capacity is epigenetically regulated during cell differentiation and development.

Cell fusion-mediated nuclear reprogramming of somatic cells

Pluripotential embryonic stem cells (ESC) possess a unique property of being able to carry out nuclear reprogramming of somatic nuclei, as shown after cell fusion. The nuclear reprogramming activity has been applied for producing pluripotential stem cells from personal somatic cells through several new technologies, including cytoplasmic cell fusion and ES cell factor introduction. Targeted elimination of ESC-derived chromosome(s) following cell fusion-mediated reprogramming of somatic chromosomes is one of the new technologies for producing personalized stem cells. A universal chromosome elimination cassette (CEC) has been developed that confers drug resistance and GFP (green fluorescent protein) fluorescence, flanked by oppositely orientated loxP sites, to induce sister chromatid recombination and targeted chromosome loss. GFP-positive ESC generated with a CEC-integrated chromosome were hybridized with adult thymocytes and then exposed to Cre recombinase. This led to loss of GFP expression and elimination of the CEC-tagged chromosome. Targeted elimination of a pair of ESC-derived chromosome 6s, which are key chromosomes for maintaining pluripotency, demonstrated that the reprogrammed somatic factors are sufficient for the continued pluripotentiality of hybrid cells. Targeted chromosome elimination technology therefore offers a means for developing major histocompatibility complex-personalized or completely personalized pluripotential stem cell populations for use in a range of therapeutic applications.

Epigenetic reprogramming of nonreplicating somatic cells for long-term proliferation by temporary cell-cell contact

Embryonic stem (ES) cells are potential sources of tissue regeneration; however, transplanted ES cells produce tumors in the host tissues. In addition, transplantation between genetically unrelated individuals often results in graft rejection. Although the development of patient specific stem cell lines by somatic cell nuclear transfer (SCNT) represents a means of overcoming the problem of rejection, its human application has ethical dilemmas. Adult stem (AS) cells can also differentiate into specialized cells and may provide an alternative source of cells for human applications. In common with other somatic cells, AS cells have limited capacity for proliferation and cannot be produced in large quantities without genetic manipulation. We demonstrate here that nonreplicating mammalian cells can be reprogrammed for long-term proliferation by temporary cell-cell contact through coculture of AS cells with the GM05267-derived F7 mouse cell line. Subsequent elimination of F7 cells from the co-culture allows proliferation of previously nonreplicating cells, colonies of which can be isolated to produce cell lines. We also demonstrate that the epigenetically reprogrammed AS cells, without the physical transfer of either nuclear or cytoplasmic material from other cells, are capable of long-term proliferation and able to relay signals to other nonreplicating cells to reinitiate proliferation with no addition of recombinant factors. The reported cell amplification procedure is methodologically simple and can be easily reproduced. This procedure allows the production of an unlimited number of cells from a limited number of AS cells, making them an ideal source of cells for applications involving autologous cell transplantation.

Nuclear reprogramming of somatic nucleus hybridized with embryonic stem cells by electrofusion

Cell fusion is a powerful tool for understanding the molecular mechanisms of epigenetic reprogramming. In hybrid cells of somatic cells and pluripotential stem cells, including embryonic stem (ES) and embryonic germ cells, somatic nuclei acquire pluripotential competence. ES and embryonic germ cells retain intrinsic trans activity to induce epigenetic reprogramming. For generating hybrid cells, we have used the technique of electrofusion. Electrofusion is a highly effective, reproducible, and biomedically safe in vitro system. For successful cell fusion, two sequential steps of electric pulse stimulation are required for the alignment (pearl chain formation) of two different types of cells between electrodes in response to alternating current stimulation and for the fusion of cytoplasmic membranes by direct current stimulation. Optimal conditions for electrofusion with a pulse generator are introduced for ES and somatic cell fusion. Topics in the field of stem cell research include the successful production of cloned animals via the epigenetic reprogramming of somatic cells and contribution of spontaneous cell fusion to generating intrinsic plasticity of tissue stem cells. Cell fusion technology may make important contributions to the fields of epigenetic reprogramming and regenerative medicine.

Functional characterization of cell hybrids generated by induced fusion of primary porcine mesenchymal stem cells with an immortal murine cell line

Bone marrow mesenchymal stem cells (MSC) integrate into various organs and contribute to the regeneration of diverse tissues. However, the mechanistic basis of the plasticity of MSC is not fully understood. The change of cell fate has been suggested to occur through cell fusion. We have generated hybrid cell lines by polyethylene-glycol-mediated cell fusion of primary porcine MSC with the immortal murine fibroblast cell line F7, a derivative of the GM05267 cell line. The hybrid cell lines display fibroblastic morphology and proliferate like immortal cells. They contain tetraploid to hexaploid porcine chromosomes accompanied by hypo-diploid murine chromosomes. Interestingly, many hybrid cell lines also express high levels of tissue-nonspecific alkaline phosphatase, which is considered to be a marker of undifferentiated embryonic stem cells. All tested hybrid cell lines retain osteogenic differentiation, a few of them also retain adipogenic potential, but none retain chondrogenic differentiation. Conditioned media from hybrid cells enhance the proliferation of both early-passage and late-passage porcine MSC, indicating that the hybrid cells secrete diffusible growth stimulatory factors. Murine F7 cells thus have the unique property of generating immortal cell hybrids containing unusually high numbers of chromosomes derived from normal cells. These hybrid cells can be employed in various studies to improve our understanding of regenerative biology. This is the first report, to our knowledge, describing the generation of experimentally induced cell hybrids by using normal primary MSC.

Toti-/Pluripotential Stem Cells and Epigenetic Modifications

Nuclear reprogramming induces global changes of epigenetic profile and confers pluripotency on specialized somatic nuclei. Embryonic stem (ES) cells retain nuclear reprogramming activity as shown by cell fusion with adult somatic cells. The reprogrammed somatic nuclei resemble ES cell nuclei in pluripotential competence. Changes of histone tail modifications in somatic cell-derived genome by cell fusion demonstrate that the molecular process of nuclear reprogramming is separable at least in two steps: erasure of somatic epigenetic modification (genome-wide reprogramming) and establishment of pluripotential epigenetic modification (gene-specific reprogramming). In the latter step, the newly identified transcriptional factor Nanog functions in maintaining pluripotency in cooperation with other key gene Oct4. Somatic-derived Nanog is reactivated in the reprogrammed nuclei in hybrid cells and also in cloned blastocysts. It is unclear which key molecules are responsible for the nuclear reprogramming. It is, however, evident that adult somatic cell nuclei are capable of being reprogrammed in vitro by cell fusion with ES cells. A technological innovation for eliminating ES-derived chromosomes from the hybrid nuclei could make the production of personalized pluripotential stem cells without the need for therapeutic cloning possible.

Mechanisms of self-renewal in human embryonic stem cells

Embryonic stem cells (ESCs) are the pluripotent cell population derived from the inner cell mass of pre-implantation embryos and are characterised by prolonged self-renewal and the potential to differentiate into cells representing all three germ layers both in vitro and in vivo. Preservation of the undifferentiated status of the ESC population requires the maintenance of self-renewal whilst inhibiting differentiation and regulating senescence and apoptosis. In this review, we discuss the intrinsic and extrinsic factors associated with self-renewal process, together with possible signalling pathway interactions and mechanisms of regulation.

Identification of candidate regulators of embryonic stem cell differentiation by comparative phosphoprotein affinity profiling

Embryonic stem cells are a unique cell population capable both of self-renewal and of differentiation into all tissues in the adult organism. Despite the central importance of these cells, little information is available regarding the intracellular signaling pathways that govern self-renewal or early steps in the differentiation program. Embryonic stem cell growth and differentiation correlates with kinase activities, but with the exception of the JAK/STAT3 pathway, the relevant substrates are unknown. To identify candidate phosphoproteins with potential relevance to embryonic stem cell differentiation, a systems biology approach was used. Proteins were purified using phosphoprotein affinity columns, then separated by two-dimensional gel electrophoresis, and detected by silver stain before being identified by tandem mass spectrometry. By comparing preparations from undifferentiated and differentiating mouse embryonic stem cells, a set of proteins was identified that exhibited altered post-translational modifications that correlated with differentiation state. Evidence for altered post-translational modification included altered gel mobility, altered recovery after affinity purification, and direct mass spectra evidence. Affymetrix microarray analysis indicated that gene expression levels of these same proteins had minimal variability over the same differentiation period. Bioinformatic annotations indicated that this set of proteins is enriched with chromatin remodeling, catabolic, and chaperone functions. This set of candidate phosphoprotein regulators of stem cell differentiation includes products of genes previously noted to be enriched in embryonic stem cells at the mRNA expression level as well as proteins not associated previously with stem cell differentiation status.

Octamer and Sox elements are required for transcriptional cis regulation of Nanog gene expression

The pluripotential cell-specific gene Nanog encodes a homeodomain-bearing transcription factor required for maintaining the undifferentiated state of stem cells. However, the molecular mechanisms that regulate Nanog gene expression are largely unknown. To address this important issue, we used luciferase assays to monitor the relative activities of deletion fragments from the 5'-flanking region of the gene. An adjacent pair of highly conserved Octamer- and Sox-binding sites was found to be essential for activating pluripotential state-specific gene expression. Furthermore, the 5'-end fragment encompassing the Octamer/Sox element was sufficient for inducing the proper expression of a green fluorescent protein reporter gene even in human embryonic stem (ES) cells. The potential of OCT4 and SOX2 to bind to this element was verified by electrophoretic mobility shift assays with extracts from F9 embryonal carcinoma cells and embryonic germ cells derived from embryonic day 12.5 embryos. However, in ES cell extracts, a complex of OCT4 with an undefined factor preferentially bound to the Octamer/Sox element. Thus, Nanog transcription may be regulated through an interaction between Oct4 and Sox2 or a novel pluripotential cell-specific Sox element-binding factor which is prominent in ES cells.

Pluripotential competence of cells associated with Nanog activity

Nanog is a novel pluripotential cell-specific gene that plays a crucial role in maintaining the undifferentiated state of early postimplantation embryos and embryonic stem (ES) cells. We have explored the expression pattern and function of Nanog and a Nanog-homologue, Nanog-ps1.Nanog-ps1 was mapped on Chromosome 7 and shown to be a pseudogene. Immunocytochemical analysis in vivo showed that the NANOG protein was absent in unfertilized oocytes, and was detected in cells of morula-stage embryos, the inner cell mass of blastocysts and the epiblast of E6.5 and E7.5 embryos, but not in primordial germ cells of early postimplantation embryos. In monkey and human ES cells, NANOG expression was restricted to undifferentiated cells. Furthermore, reactivation of the somatic cell-derived Nanog was tightly linked with nuclear reprogramming induced by cell hybridization with ES cells and by nuclear transplantation into enucleated oocytes. Notably, mouse Nanog (+/-) ES cells, which produced approximately half the amount of NANOG produced by wild-type ES cells, readily differentiated to multi-lineage cells in culture medium including LIF. The labile undifferentiated state was fully rescued by constitutive expression of exogenous Nanog. Thus, the activity of Nanog is tightly correlated with an undifferentiated state of cells even in nuclear reprogrammed somatic cells. Nanog may function as a key regulator for sustaining pluripotency in a dose-dependent manner.

Identification of potential nuclear reprogramming and differentiation factors by a novel selection method for cloning chromatin-binding proteins

Nuclear reprogramming is critical for animal cloning and stem cell creation through nuclear transfer, which requires extensive remodeling of chromosomal architecture involving dramatic changes in chromatin-binding proteins. To understand the mechanism of nuclear reprogramming, it is critical to identify chromatin-binding factors specify the reprogramming process. In this report, we have developed a high-throughput selection method, based on T7 phage display and chromatin immunoprecipitation, to isolate chromatin-binding factors expressed in mouse embryonic stem cells using primary mouse embryonic fibroblast chromatin. Seven chromatin-binding proteins have been isolated by this method. We have also isolated several chromatin-binding proteins involved in hepatocyte differentiation. Our method provides a powerful tool to rapidly and selectively identify chromatin-binding proteins. The method can be used to study epigenetic modification of chromatin during nuclear reprogramming, cell differentiation, and transdifferentiation.

Subzonal Older Adult Fibroblast Insertion in Both In Vivo–Fertilized and Nuclear Transfer Rabbit Zygotes and Embryos: Effects on Further In Vitro Embryo Development

In the present work, we evaluated the effect on further in vitro embryo development of inserting rabbit adult fibroblasts into in vivo-fertilized rabbit embryos. To this end, we inserted either 4 or 15-20 rabbit adult fibroblasts in two different early embryo stages of development, 1-cell stage and 4-8-cell stage embryos. We observed that fibroblast insertion not only did not negatively affect further embryo development, but also may have exerted a positive effect on development on it. Therefore, in forthcoming works were where we intend to study a possible cell helper role on early embryo development. The early embryo microenvironment may reprogram somatic cell gene expression of fibroblasts inserted within the embryo, making them suitable as nuclear donors.

Genetic and Cellular Therapies for Cerebral Infarction

Neurosurgeons, working as surgical scientists, can have a prominent role in developing and implementing genetic and cellular therapies for cerebral ischemia. The rapid emergence of both genetic and cellular therapies for neural regeneration warrants a careful analysis before implementation of human studies to understand the pitfalls and promises of this strategy. In this article, we review the topic of genetic and cellular therapy for stroke to provide a foundation for practicing neurosurgeons and clinical scientists who may become involved in this type of work. In Part 1, we review preclinical approaches with gene transfer, such as 1) improved energy delivery, 2) reduction of intracellular calcium availability, 3) abrogation of effects of reactive oxygen species, 4) reduction of proinflammatory cytokine signaling, 5) inhibition of apoptosis mediators, and 6) restorative gene therapy, that are paving the way to develop new strategies to treat cerebral infarction. In Part 2, we discuss the results of studies that address the possibility of using cellular therapies for stroke in animal models and in human trials by reviewing 1) the basics of stem cell biology, 2) exogenous and 3) and endogenous cell sources for therapy, and 4) clinical considerations in cell therapy applications. These emerging technologies based on the advancements made in recent years in the fields of genetics, therapeutic cloning, neuroscience, stem cell biology, and gene therapy provide significant potential for new therapies for stroke.

Stem cell therapy for neurodegenerative diseases: the issue of transdifferentiation

In the past few years research on stem cells has exploded as a tool to develop potential therapies to treat incurable neurodegenerative diseases. Stem cell transplantation has been effective in several animal models, but the underlying restorative mechanisms are still unknown. Several events such as cell fusion, neurotrophic factor release, endogenous stem cell proliferation, and transdifferentiation (adult cell acquisition of new unexpected identities) may explain therapeutic success, in addition to replacement of lost cells. This issue needs to be clarified further to maximize the potential for effective therapies. Preliminary stem transplantation trials have already been performed for some neurodegenerative diseases. There is no effective pharmacological treatment for amyotrophic lateral sclerosis, but recent preliminary data both in experimental and clinical settings have targeted it as an ideal candidate disease for the development of stem cell therapy in humans. This review summarizes recent advances gained in stem cell research applied to neurodegenerative diseases with a special emphasis to the criticisms put forward.

Nuclear reprogramming and stem cell creation

The transplantation of a somatic cell nucleus to an enucleated egg results in a major reprogramming of gene expression and switch in cell fate. We review the efficiency of nuclear reprogramming by nuclear transfer. The serial transplantation of nuclei from defective first-transfer embryos and the grafting of cells from such embryos to normal host embryos greatly increases the proportion of nuclei that can be seen to have been reprogrammed. We discuss possible reasons for the early failure of most nuclear transfers from differentiated cells and describe the potential value of growing oocytes, rather than unfertilized eggs, as a source of nuclear reprogramming molecules and for the eventual identification of these molecules. Nuclear transfer provides a possible route for the creation of stem cells from adult somatic cells.

Pluripotency of reprogrammed somatic genomes in embryonic stem hybrid cells

Somatic nuclei can be epigenetically reprogrammed by factors present in undifferentiated embryonic stem (ES) cells. The acquisition of pluripotency by somatic genomes could render such cells a viable source of personalized cell type(s) for therapeutic application, avoiding the need for controversial therapeutic cloning. To investigate this possibility, we first determined the origin of transcripts in teratomas generated from mouse (ES x somatic cell) hybrid clones. Transcription of markers from the somatic genome demonstrated efficient in vivo differentiation down independent lineages. The induction of dopaminergic neurons by coculture with stromal PA6 feeder cells also demonstrated efficient capacity to differentiate in vitro. Hybrid clone-derived neurons expressed appropriate markers, and transcription of Pitx3 from the somatic genome was confirmed. When transplanted into mouse brains, the dopaminergic neurons were successfully integrated and expressed tyrosine hydroxylase. Thus, it should be possible to produce personalized ES-like cells with the reprogrammed somatic genomes.

Nuclei of adult mammalian somatic cells are directly reprogrammed to oct-4 stem cell gene expression by amphibian oocytes

Nuclear reprogramming by the transplantation of somatic cell nuclei to eggs (in second meiotic metaphase) is always followed by a phase of chromosome replication and cell division before new gene expression is seen. To help understand the mechanism of nuclear reprogramming, we have asked whether the nuclei of normal, nontransformed, nondividing, and terminally differentiated mammalian cells can be directly reprogrammed, without DNA replication, by Xenopus oocytes. We find that nuclei of adult mouse thymocytes and of adult human blood lymphocytes, injected into Xenopus oocytes, are induced to extinguish a differentiation marker and to strongly express oct-4, the most diagnostic mammalian stem cell/pluripotency marker. In the course of 2 days at 18 degrees C, the mammalian oct-4 transcripts are spliced to mature mRNA. We conclude that normal mammalian nuclei can be directly reprogrammed by the nucleus (germinal vesicle) of amphibian oocytes to express oct-4 at a rate comparable to that of oct-4 in mouse ES cells. To our knowledge, this is the first demonstration of a stem cell marker being induced in a differentiated adult human cell nucleus. This is an early step toward the long-term aim of developing a procedure for reprogramming readily accessible human adult cells for cell replacement therapy.

The chiaroscuro stem cell: a unified stem cell theory

Hematopoiesis has been considered hierarchical in nature, but recent data suggest that the system is not hierarchical and is, in fact, quite functionally plastic. Existing data indicate that engraftment and progenitor phenotypes vary inversely with cell cycle transit and that gene expression also varies widely. These observations suggest that there is no progenitor/stem cell hierarchy, but rather a reversible continuum. This may, in turn, be dependent on shifting chromatin and gene expression with cell cycle transit. If the phenotype of these primitive marrow cells changes from engraftable stem cell to progenitor and back to engraftable stem cell with cycle transit, then this suggests that the identity of the engraftable stem cell may be partially masked in nonsynchronized marrow cell populations. A general model indicates a marrow cell that can continually change its surface receptor expression and thus responds to external stimuli differently at different points in the cell cycle.