De-differentiation of mouse interfollicular keratinocytes by the embryonic transcription factor Oct-4

Oct4 is a gatekeeper of epithelial identity by regulating cytoskeletal organization in skin keratinocytes

Oct4 is a pioneer transcription factor regulating pluripotency. However, it is not well known whether Oct4 has an impact on epidermal cells. We generated OCT4 knockout clonal cell lines using immortalized human skin keratinocytes to identify a functional role for the protein. Here, we report that Oct4-deficient cells transitioned into a mesenchymal-like phenotype with enlarged size and shape, exhibited accelerated migratory behavior, decreased adhesion, and appeared arrested at the G2/M cell cycle checkpoint. Oct4 absence had a profound impact on cortical actin organization, with loss of microfilaments from the cell membrane, increased puncta deposition in the cytoplasm, and stress fiber formation. E-cadherin, β-catenin, and ZO1 were almost absent from cell-cell contacts, while fibronectin deposition was markedly increased in the extracellular matrix (ECM). Mapping of the transcriptional and chromatin profiles of Oct4-deficient cells revealed that Oct4 controls the levels of cytoskeletal, ECM, and differentiation-related genes, whereas epithelial identity is preserved through transcriptional and non-transcriptional mechanisms.

Somatic Cell Reprogramming for Nervous System Diseases: Techniques, Mechanisms, Potential Applications, and Challenges

Nervous system diseases present significant challenges to the neuroscience community due to ethical and practical constraints that limit access to appropriate research materials. Somatic cell reprogramming has been proposed as a novel way to obtain neurons. Various emerging techniques have been used to reprogram mature and differentiated cells into neurons. This review provides an overview of somatic cell reprogramming for neurological research and therapy, focusing on neural reprogramming and generating different neural cell types. We examine the mechanisms involved in reprogramming and the challenges that arise. We herein summarize cell reprogramming strategies to generate neurons, including transcription factors, small molecules, and microRNAs, with a focus on different types of cells.. While reprogramming somatic cells into neurons holds the potential for understanding neurological diseases and developing therapeutic applications, its limitations and risks must be carefully considered. Here, we highlight the potential benefits of somatic cell reprogramming for neurological disease research and therapy. This review contributes to the field by providing a comprehensive overview of the various techniques used to generate neurons by cellular reprogramming and discussing their potential applications.

Probing cell identity hierarchies by fate titration and collision during direct reprogramming

Despite the therapeutic promise of direct reprogramming, basic principles concerning fate erasure and the mechanisms to resolve cell identity conflicts remain unclear. To tackle these fundamental questions, we established a single-cell protocol for the simultaneous analysis of multiple cell fate conversion events based on combinatorial and traceable reprogramming factor expression: Collide-seq. Collide-seq revealed the lack of a common mechanism through which fibroblast-specific gene expression loss is initiated. Moreover, we found that the transcriptome of converting cells abruptly changes when a critical level of each reprogramming factor is attained, with higher or lower levels not contributing to major changes. By simultaneously inducing multiple competing reprogramming factors, we also found a deterministic system, in which titration of fates against each other yields dominant or colliding fates. By investigating one collision in detail, we show that reprogramming factors can disturb cell identity programs independent of their ability to bind their target genes. Taken together, Collide-seq has shed light on several fundamental principles of fate conversion that may aid in improving current reprogramming paradigms.

The Contributions of Cancer-Testis and Developmental Genes to the Pathogenesis of Keratinocyte Carcinomas

Simple Summary

In addition to mutations, ectopically-expressed genes are emerging as important contributors to cancer development. Efforts to characterize the expression patterns in cancers of gamete-restricted cancer-testis antigens and developmentally-restricted genes are underway, revealing these genes to be putative biomarkers and therapeutic targets for various malignancies. Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC) are two highly-prevalent non-melanoma skin cancers that result in considerable burden on patients and our health system. To optimize disease prognostication and treatment, it is necessary to further classify the molecular complexity of these malignancies. This review describes the expression patterns and functions of cancer-testis antigens and developmentally-restricted genes in BCC and cSCC tumors. A large number of cancer-testis antigens and developmental genes exhibit substantial expression levels in BCC and cSCC. These genes have been shown to contribute to several aspects of cancer biology, including tumorigenesis, differentiation, invasion and responses to anti-cancer therapy.

Abstract

Keratinocyte carcinomas are among the most prevalent malignancies worldwide. Basal cell carcinoma (BCC) and cutaneous squamous cell carcinoma (cSCC) are the two cancers recognized as keratinocyte carcinomas. The standard of care for treating these cancers includes surgery and ablative therapies. However, in recent years, targeted therapies (e.g., cetuximab for cSCC and vismodegib/sonidegib for BCC) have been used to treat advanced disease as well as immunotherapy (e.g., cemiplimab). These treatments are expensive and have significant toxicities with objective response rates approaching ~50–65%. Hence, there is a need to dissect the molecular pathogenesis of these cancers to identify novel biomarkers and therapeutic targets to improve disease management. Several cancer-testis antigens (CTA) and developmental genes (including embryonic stem cell factors and fetal genes) are ectopically expressed in BCC and cSCC. When ectopically expressed in malignant tissues, functions of these genes may be recaptured to promote tumorigenesis. CTAs and developmental genes are emerging as important players in the pathogenesis of BCC and cSCC, positioning themselves as attractive candidate biomarkers and therapeutic targets requiring rigorous testing. Herein, we review the current research and offer perspectives on the contributions of CTAs and developmental genes to the pathogenesis of keratinocyte carcinomas.

MicroRNA-mediated regulation of BM-MSCs differentiation into sweat gland-like cells: targeting NF-κB

Sweat gland regeneration is important for patients with an extensive deep burn injury. In previous study, we reported that bone marrow-mesenchymal stem cells (BM-MSCs) could differentiate into sweat gland-like cells (SGLCs), but the underlying molecular mechanism remains unclear. Recently, microRNAs (miRNAs or miRs) are reported to manipulate many biological processes. However, whether the process of MSCs differentiation into sweat gland cells (SGCs) is regulated by miRNAs has not been reported. In this study, BM-MSCs were induced into SGLCs by co-culturing with SGCs. Differential expressions of miRNAs between BM-MSC and SGLCs were determined through miRNAs microarray and 68 miRNAs were found significantly changed in miRNA profile including hsa-miR-138-5p. Bioinformatics analysis showed that hsa-miR-138-5p targeted a group of nuclear factor-κB (NF-κB) related genes which play an important role in skin appendage development. As expected, hsa-miR-138-5p inhibitor transfected into BM-MSCs partly mimicked the effects of co-culture and increased the number of SGLCs by increasing the expression of NF-κB related genes. These results suggest that hsa-miR-138-5p and NF-κB are involved in the regulation of BM-MSCs differentiation into SGLCs. This study may also offer a new approach to yield SGCs for burn patients.

Lung cancer stem cells and low-intensity laser irradiation: a potential future therapy?

Lung cancer is notably a significant threat when considering worldwide cancer-related deaths. Despite significant advances in treatment modalities, death rates as a result of cancer relapse remain high. Relapse can occur as a result of metastasis. Cancer stem cells (CSCs) have been implicated as an important contributory factor in the development of metastasis. CSCs have the same characteristics as normal stem cells; that is, they can proliferate indefinitely and are capable of both self-renewal and differentiating into specialized cells. The molecular and cellular characteristics of stem cells and CSCs are coded for by cell-specific genes, which can be analyzed by using molecular assays setting the standard to work from. Low-intensity laser irradiation (LILI) has been applied in the treatment of numerous diseases and pathological conditions. LILI has been shown to stimulate proliferation of cells, capillary growth, and cellular metabolism as observed by adenosine triphosphate activation. It has been shown, by using different dosing levels of LILI, to either stimulate or inhibit cellular functions. One treatment strategy used on cancer cells is photodynamic therapy (PDT), in which cancer cells are treated with a photosensitizer (PS) in combination with laser irradiation. PSs are non-toxic by themselves but, with light activation, cause reactive oxygen species generation, which causes cancer cell death. Cell-specific PSs are being developed for future cancer treatment. In this review, we look at the potential effects of LILI and PDT on lung CSCs.

A promising approach to iPSC-based cell therapy for diabetic wound treatment: direct lineage reprogramming

Successful reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) has ushered in a new era of regenerative medicine. Several studies on iPSCs have corroborated their immense promise and potential for use in cell therapy and disease modeling. However, several shortcomings need to be overcome before they can be used in clinical therapy. Investigation of iPSC fate and physiology in vivo and ultimately, the feasibility of their application in cell transplantation therapy, requires more in-depth studies in living subjects. One recently established alternative approach to reprogramming involves the direct conversion of a terminally differentiated somatic cell of one type into another, without dedifferentiating into a pluripotent state. This direct lineage reprogramming strategy is significantly faster, has the potential to generate an enriched population of a specific subtype of cells, and hence, has wide implications in regenerative cell therapy. Here, we review recent advances in iPSC technology and summarize the research on the generation of patient-specific induced cell types using direct lineage conversion. Specifically, we focus on the scope of application of this approach in autologous cell replacement therapy for diabetic wound treatment.

Human skin keratinocytes can be reprogrammed to express neuronal genes and proteins after a single treatment with decitabine

Patient-specific cell replacement therapy is fast becoming the future of medicine, requiring safe, effective methods for reprogramming a patient's own cells. Previously, we showed that a single transient transfection with a plasmid encoding Oct4 was sufficient to reprogram human skin keratinocytes (HSKs), and that this transfection resulted in a decrease in global DNA methylation. In more recent work we showed that decreasing global DNA methylation using the U.S. Food and Drug Administration-approved cancer treatment drug decitabine was sufficient to induce expression of endogenous Oct4. Here we report that a single treatment with decitabine, followed by 5 days in a defined neuronal transformation medium, then 7 days in a neuronal maintenance medium is sufficient to convert HSKs into cells that change their morphology substantially, gain expression of neuronal markers, and lose expression of keratinocyte markers. Within 1 week of treatment the cells express mRNA for β3-tubulin and doublecortin, and at the end of 2 weeks express mRNA for NeuN, FOXP2, and NCAM1. Additionally, at the end of this protocol, neurofilament-1, nestin, synapsin, FOXP2, and GluR1 proteins are detectable by immunostaining. Thus, we demonstrate a simple method that begins the process for producing cells for cell replacement therapies without using exogenously introduced DNA.

Biochemistry of epidermal stem cells

BACKGROUND

The epidermis is an important protective barrier that is essential for maintenance of life. Maintaining this barrier requires continuous cell proliferation and differentiation. Moreover, these processes must be balanced to produce a normal epidermis. The stem cells of the epidermis reside in specific locations in the basal epidermis, hair follicle and sebaceous glands and these cells are responsible for replenishment of this tissue.

SCOPE OF REVIEW

A great deal of effort has gone into identifying protein epitopes that mark stem cells, in identifying stem cell niche locations, and in understanding how stem cell populations are related. We discuss these studies as they apply to understanding normal epidermal homeostasis and skin cancer.

MAJOR CONCLUSIONS

An assortment of stem cell markers have been identified that permit assignment of stem cells to specific regions of the epidermis, and progress has been made in understanding the role of these cells in normal epidermal homeostasis and in conditions of tissue stress. A key finding is the multiple stem cell populations exist in epidermis that give rise to different structures, and that multiple stem cell types may contribute to repair in damaged epidermis.

GENERAL SIGNIFICANCE

Understanding epidermal stem cell biology is likely to lead to important therapies for treating skin diseases and cancer, and will also contribute to our understanding of stem cells in other systems. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

WITHDRAWN: The mesenchymal stem cell potential of human dental pulp derived cells transfected with embryonic transcription factor Oct-4

This article has been withdrawn at the request of the author(s) and/or editor. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at http://www.elsevier.com/locate/withdrawalpolicy.

Ovarian stem cells and aging

OBJECTIVE

To review successes to date in the field of ovarian stem cell research and discuss the evidence supporting their potential to rejuvenate the follicular pool during adult life; to present factors that may contribute to their competence; and to address the question of why menopause is an inevitable outcome of advanced age if ovarian stem cells exist.

METHOD

In a review of the literature, relevant articles were identified through a PubMed literature search from inception to July 2010.

RESULTS

The current concept that mammalian ovaries possess a static ovarian reserve is at odds with the experimental results discussed in this review. Ovarian stem cells are likely to be the source of germline stem cells during fetal and adult life, due to their potential to differentiate into competent oocytes given a suitable environment.

CONCLUSIONS

Stem cells in different compartments share properties such as pluripotency, self-renewal, and diminished regenerative potential in old age. Our model of ovarian stem cell aging suggests that menopause is driven by an age-related decline in ovarian stem cell function rather than depletion of a non-renewable follicular reserve. Understanding how ovarian stem cells interact with their surrounding environment moves us a step closer to controlling the female biological clock when it might be clinically desirable.

Dedifferentiation of human primary thyrocytes into multilineage progenitor cells without gene introduction

While identification and isolation of adult stem cells have potentially important implications, recent reports regarding dedifferentiation/reprogramming from differentiated cells have provided another clue to gain insight into source of tissue stem/progenitor cells. In this study, we developed a novel culture system to obtain dedifferentiated progenitor cells from normal human thyroid tissues. After enzymatic digestion, primary thyrocytes, expressing thyroglobulin, vimentin and cytokeratin-18, were cultured in a serum-free medium called SAGM. Although the vast majority of cells died, a small proportion (∼0.5%) survived and proliferated. During initial cell expansion, thyroglobulin/cytokeratin-18 expression was gradually declined in the proliferating cells. Moreover, sorted cells expressing thyroid peroxidase gave rise to proliferating clones in SAGM. These data suggest that those cells are derived from thyroid follicular cells or at least thyroid-committed cells. The SAGM-grown cells did not express any thyroid-specific genes. However, after four-week incubation with FBS and TSH, cytokeratin-18, thyroglobulin, TSH receptor, PAX8 and TTF1 expressions re-emerged. Moreover, surprisingly, the cells were capable of differentiating into neuronal or adipogenic lineage depending on differentiating conditions. In summary, we have developed a novel system to generate multilineage progenitor cells from normal human thyroid tissues. This seems to be achieved by dedifferentiation of thyroid follicular cells. The presently described culture system may be useful for regenerative medicine, but the primary importance will be as a tool to elucidate the mechanisms of thyroid diseases.

Embryonic stem cell factors undifferentiated transcription factor-1 (UFT-1) and reduced expression protein-1 (REX-1) are widely expressed in human skin and may be involved in cutaneous differentiation but not in stem cell fate determination

Undifferentiated transcription factor-1 (UTF-1) and reduced expression protein-1 (REX-1) are used as markers for the undifferentiated state of pluripotent stem cells. Because no highly specific cytochemical marker for epidermal stem cells has yet been identified, we investigated the expression pattern of these markers in human epidermis and skin tumours by immunohistochemistry and in keratinocyte cell cultures. Both presumed stem cell markers were widely expressed in the epidermis and skin appendages. Distinct expression was found in the matrix cells of the hair shaft. Differentiation of human primary keratinocytes (KC) in vitro strongly downregulated UTF-1 and REX-1 expression. In addition, REX-1 was upregulated in squamous cell carcinomas, indicating a possible role of this transcription factor in malignant tumour formation. Our data point to a role for these proteins not only in maintaining KC stem cell populations, but also in proliferation and differentiation of matrix cells of the shaft and also suprabasal KC.

The expression of stem cell-related indicators as a prognostic factor in human lung adenocarcinoma

INTRODUCTION

The purpose of the present study was to detect the presence of BASC-like stem cell-related indicators, such as clara cell secretory protein (CCSP), Octamer-4 (OCT4) and Bmi-1, and evaluate their implications in the prognosis of patients with lung adenocarcinoma.

METHODS

Specimens of 134 cases of lung adenocarcinoma were collected after radical surgery from January 1999 to June 2004.

RESULTS

One hundred and twenty-six cases showed cells that were positive for CCSP, 99 cases positive for OCT4, 91 cases simultaneous expression of CCSP and OCT4 and 74 cases positive for Bmi-1. Bmi-1 was significantly higher in patients at stage III compared to patients at stages I and II. The pattern of survival curves showed that Bmi-1 was a significant prognostic factor of poor overall survival in lung adenocarcinoma patients (P = 0.0000), and the patients with OCT4(+) expression showed a greater increase in mortality than OCT4(-) patients (P = 0.0103). The results of univariate and multivariate Cox analysis revealed that the pathological stages of tumor node metastases (P = 0.037), OCT4 (P = 0.046) and Bmi-1 expression (P = 0.001) were independent prognostic factors.

CONCLUSIONS

OCT4 and Bmi-1 may be good biomarkers to predict the prognosis of patients with completely resected lung adenocarcinoma.

Advances in reprogramming somatic cells to induced pluripotent stem cells

Traditionally, nuclear reprogramming of cells has been performed by transferring somatic cell nuclei into oocytes, by combining somatic and pluripotent cells together through cell fusion and through genetic integration of factors through somatic cell chromatin. All of these techniques changes gene expression which further leads to a change in cell fate. Here we discuss recent advances in generating induced pluripotent stem cells, different reprogramming methods and clinical applications of iPS cells. Viral vectors have been used to transfer transcription factors (Oct4, Sox2, c-myc, Klf4, and nanog) to induce reprogramming of mouse fibroblasts, neural stem cells, neural progenitor cells, keratinocytes, B lymphocytes and meningeal membrane cells towards pluripotency. Human fibroblasts, neural cells, blood and keratinocytes have also been reprogrammed towards pluripotency. In this review we have discussed the use of viral vectors for reprogramming both animal and human stem cells. Currently, many studies are also involved in finding alternatives to using viral vectors carrying transcription factors for reprogramming cells. These include using plasmid transfection, piggyback transposon system and piggyback transposon system combined with a non viral vector system. Applications of these techniques have been discussed in detail including its advantages and disadvantages. Finally, current clinical applications of induced pluripotent stem cells and its limitations have also been reviewed. Thus, this review is a summary of current research advances in reprogramming cells into induced pluripotent stem cells.

Transient expression of OCT4 is sufficient to allow human keratinocytes to change their differentiation pathway

In this study, we describe a simple system in which human keratinocytes can be redirected to an alternative differentiation pathway. We transiently transfected freshly isolated human skin keratinocytes with the single transcription factor OCT4. Within 2 days these cells displayed expression of endogenous embryonic genes and showed reduced genomic methylation. More importantly, these cells could be specifically converted into neuronal and contractile mesenchymal cell types. Redirected differentiation was confirmed by expression of neuronal and mesenchymal cell mRNA and protein, and through a functional assay in which the newly differentiated mesenchymal cells contracted collagen gels as efficiently as authentic myofibroblasts. Thus, to generate patient-specific cells for therapeutic purposes, it may not be necessary to completely reprogram somatic cells into induced pluripotent stem cells before altering their differentiation and grafting them into new tissues.

Master stem cell transcription factors and signaling regulation

The present article reviews master stem cell transcription factors, their expression regulation network, and related signaling pathways with the aim of understanding the molecular mechanisms of pluripotent cell fate decisions. Oct4, Sox2, and Nanog are master transcription factors for maintenance of the undifferentiated state and self-renewal of embryonic stem cells (ESCs). In the mouse, they form a regulatory circuitry with coregulators, such as beta-catenin, Stat3, Myc, Klfs, Sall4, and Esrrb to control the expression of pluripotency-related genes including themselves. The threshold expression of Oct4, Sox2, and Nanog for sustaining ESC properties depends on the synergistic effects among Stat3, beta-catenin, and Smad signaling pathway under the specific conditions of the ESC cytoplasmic microenvironment. Some of the salient differences in human ESC signaling pathways affecting their fate commitment are highlighted.

Protein chip analysis of pluripotency-associated proteins in NIH3T3 fibroblast

Specific transcription factors regulate the totipotent and pluripotent capability of embryonic stem cells. Amongst these regulatory transcription factors in embryonic stem cells, Oct4 and Nanog are master factors that also have unique characteristic ability of cell-specific pluripotency and self-renewal. The expression of Nanog in fibroblasts confirms increased cell proliferation and transformation of foci-forming phenotype indicative of its oncogenic potential. The expression of Oct4, interestingly, leads to transformation of non-tumorgenic mouse into tumorigenic mouse. Our current investigation ascertains that the resultant increase in DNA synthesis and cell proliferation is the consequence of transforming the phenotype into foci formation. We used a manually curetted ProteoChip to carry out the signaling protein microarray analysis, which revealed up-regulated expression of various proteins including FAK1, MEK1 and Raf1. Some of the proteins explain the mechanism by which Oct4 and Nanog transform the phenotype. In NIH3T3 cells expressed with mouse Oct4 (mOct4), mouse Nanog (mNanog) separately as well as together, the specific knockdown of mFAK1 inhibited morphological transformation of the cells, and their invasion activity. The mFAK1 overexpression leads to morphological transformation as shown with mOct4 and mNanog. Additionally, we showed that the ERK1/2 pathway is involved in the up-regulation of c-myc and cyclin D1 expression mediated by mFAK1. Our results signify that the combinatorial signaling protein-array using biomolecular approach may possibly provide us with a new tool to understand cellular homeostasis.

Rat alveolar type I cells proliferate, express OCT-4, and exhibit phenotypic plasticity in vitro

Alveolar type I (TI) cells are large, squamous cells that cover 95-99% of the internal surface area of the lung. Although TI cells are believed to be terminally differentiated, incapable of either proliferation or phenotypic plasticity, TI cells in vitro both proliferate and express phenotypic markers of other differentiated cell types. Rat TI cells isolated in purities of >99% proliferate in culture, with a sixfold increase in cell number before the cells reach confluence; >50% of the cultured TI cells are Ki67+. At cell densities of 1-2 cells/well, approximately 50% of the cells had the capacity to form colonies. Under the same conditions, type II cells do not proliferate. Cultured TI cells express RTI40 and aquaporin 5, phenotypic markers of the TI cell phenotype. By immunofluorescence, Western blotting, and Q-PCR, TI cells express OCT-4A (POU5F1), a transcription factor associated with maintenance of the pluripotent state in stem cells. Based on the expression patterns of various marker proteins, TI cells are distinct from either of two recently described putative pulmonary multipotent cell populations, the bronchoalveolar stem cell or the OCT-4+ stem/progenitor cell. Although TI cells in adult rat lung tissue do not express either surfactant protein C (SP-C) or CC10, respective markers of the TII and Clara cell phenotypes, in culture TI cells can be induced to express both SP-C and CC10. Together, the findings that TI cells proliferate and exhibit phenotypic plasticity in vitro raise the possibility that TI cells may have similar properties in vivo.

Human immature dental pulp stem cells' contribution to developing mouse embryos: production of human/mouse preterm chimaeras

OBJECTIVES

In this study, we aimed at determining whether human immature dental pulp stem cells (hIDPSC) would be able to contribute to different cell types in mouse blastocysts without damaging them. Also, we analysed whether these blastocysts would progress further into embryogenesis when implanted to the uterus of foster mice, and develop human/mouse chimaera with retention of hIDPSC derivates and their differentiation.

MATERIALS AND METHODS

hIDPSC and mouse blastocysts were used in this study. Fluorescence staining of hIDPSC and injection into mouse blastocysts, was performed. Histology, immunohistochemistry, fluorescence in situ hybridization and confocal microscopy were carried out.

RESULTS AND CONCLUSION

hIDPSC showed biological compatibility with the mouse host environment and could survive, proliferate and contribute to the inner cell mass as well as to the trophoblast cell layer after introduction into early mouse embryos (n = 28), which achieved the hatching stage following 24 and 48 h in culture. When transferred to foster mice (n = 5), these blastocysts with hIDPSC (n = 57) yielded embryos (n = 3) and foetuses (n = 6); demonstrating presence of human cells in various organs, such as brain, liver, intestine and hearts, of the human/mouse chimaeras. We verified whether hIDPSC would also be able to differentiate into specific cell types in the mouse environment. Contribution of hIDPSC in at least two types of tissues (muscles and epithelial), was confirmed. We showed that hIDPSC survived, proliferated and differentiated in mouse developing blastocysts and were capable of producing human/mouse chimaeras.

Stem cells and nuclear reprogramming

Derivation of human embryonic stem (ES) cells from preimplantation embryos ten years ago raised great hopes that they may be an excellent source of cells for cell replacement therapy. However, serious ethical concerns and the risk of immune rejection of allotransplanted cells have hindered the translation of ES cell-based therapies into the clinic. In an attempt to circumvent these barriers, a number of methods have been developed for converting adult somatic cells into a pluripotent state from which ethically acceptable patient-specific mature cells of interest could be derived. These efforts, backed by advances in elucidating the molecular basis of pluripotency, have culminated in successful reprogramming of fibroblasts into ES cell-like cells, termed induced pluripotent stem (iPS) cells, by ectopic expression of only a handful of "stemness" factors. iPS cells possess morphological, molecular and developmental features of conventional blastocyst-derived ES cells and have the potential to serve as a source of therapeutic cells for customized tissue repair, gene therapy, drug discovery, toxicological testing and for studying the molecular basis of human disease. The goal of this review is to provide the current state-of-the-art in this very exciting and dynamic field and to discuss barriers that remain to be removed before the therapeutic potential of iPS cells can be fully realized.

Epidermal stem cells in skin homeostasis and cutaneous carcinomas

Skin and squamous epithelia regulate water and heat homeostasis and constitute our first barrier of protection against pathogens. Cells from the outermost layer of the skin, the cornified envelope (stratum corneum), are constantly being shed, imposing a constant demand for replenishment to maintain homeostasis. Hair follicles and sebaceous glands provide protective hair growth and skin sebum, and continuously undergo cycles of growth and regression. The outstanding ability of the epidermis, hair follicles and sebaceous glands to self-renew relies on a population of adult stem cells that are maintained throughout our life span. In this review we will provide an overview of our current knowledge about epidermal stem cells, and some of the molecular mechanisms that identify them and dictate their behaviour. We will also summarise our view on the possible link between adult epidermal stem cells and cancer stem cells within skin and squamous neoplasias. The potential of epidermal stem cells in regenerative medicine and for designing targeted antitumoral therapies will be discussed.

Embryonic stem cell biology

ES cell research represents an exploding field of exploration. Initially predicted to provide rapid cures for numerous human diseases, the clinical usefulness of ES cell-derived cells remains untested in humans. However, ES cells have rapidly expanded our knowledge of human development and the molecular details of differentiation. Our ability to generate relatively pure populations of specifically differentiated cells for transplantation has markedly improved. It is hoped that soon researchers will overcome the biologic impediments to successful treatment of human disease with ES cell-derived cells.

Cells isolated from the epidermis by Hoechst dye exclusion, small size, and negative selection for hematopoietic markers can generate B lymphocyte precursors

Transdifferentiation has become a common claim for somatic stem cells, yet how such cells can be directed toward a specified cell lineage has not been well investigated. We previously demonstrated that when isolated epidermal stem cells were placed into an embryonic environment, their potential was extended beyond the keratinocyte lineage. Here, we present evidence that cells isolated using a modification of our published method for epidermal stem cells can be specifically directed to differentiate into B lymphocyte precursors. We found that these isolated cells co-cultured with S17 bone marrow stromal cells in cytokine-supplemented medium changed their cell surface marker profile and gene expression pattern to one characteristic of B lymphocyte precursors. Such cells also underwent variable, diversity, joining rearrangement at the immunoglobulin heavy-chain locus, a permanent genetic change unique to lymphocytes. This feature is limited to the cells isolated using the modified epidermal stem cell method, as cells isolated using the modified transit amplifying cell method could not be re-directed or reprogrammed. Such results demonstrate that cells from the epidermis can be directed to cross lineage boundaries to become mesodermally derived lymphocytes.

Skin keratinocytes pre-treated with embryonic stem cell-conditioned medium or BMP4 can be directed to an alternative cell lineage

OBJECTIVES

In this study, we have investigated whether secreted factors from embryonic stem cells (ESCs) could reprogramme keratinocytes and increase their potential to be directed into alternative cell lineages.

MATERIALS AND METHODS

Contact and non-contact co-cultures of skin keratinocytes and murine ESCs were used initially to confirm any reprogramming ability of ESC-conditioned medium (CM). Immunofluoresence was used to assess nuclear expression of octamer-4 (Oct-4), as well as to confirm neuronal protein expression in neuroectodermally directed keratinocytes. Transcript expression changes were evaluated using semiquantitative reverse transcription-polymerase chain reaction. Western blotting, accompanied by densitometry analysis, was used to evaluate protein expression following morphology changes.

RESULTS

We found that keratinocytes treated with ESC-CM changed their morphology and were stimulated to express the pluripotency regulator, Oct-4, and its target transcripts, Sox-2, Nanog, Utf1 and Rex-1. We demonstrate that at least one of the reprogramming factors is bone morphogenetic factor-4 (BMP4). Pre-treated keratinocytes could be specifically directed to differentiate into cells of the neuronal lineage. The majority of responsive keratinocytes were the epidermal stem cell population, with a small percentage of transit-amplifying cells also being affected.

CONCLUSIONS

Our results suggest that ESC-CM contains a number of factors, including BMP4, which are capable of reprogramming mouse skin keratinocytes to make them more developmentally potent, as evidenced by their ability to be re-differentiated into cells of the neuronal lineage. Our findings also imply a continuum of differentiation within the basal keratinocyte population. An increase in developmental potential combined with directed differentiation could increase the therapeutic relevancy of somatic cells.

Epidermal stem cell fate: what can we learn from embryonic stem cells?

Because of its constant renewal and high propensity for repair, the epidermis is, together with the gut and the hematopoietic system, a tissue of choice to explore stem cell biology. Previous research over many years has revealed the complexity of the epidermis: the heterogeneity of the stem cell compartment, with its rare, slowly cycling, multipotent, hair-follicle, "bulge" stem cells and the more restricted interfollicular, follicle-matrix, and sebaceous-gland stem cells, which in turn generate the large pool of transit-amplifying progeny. Stem cell activity has been used for some considerable time to repair skin injuries, but ex-vivo keratinocyte amplification has its limitations, and grafted skin homeostasis is not totally satisfactory. Human embryonic stem cells raise the hope that the understanding of the developmental steps leading to the generation of epidermal stem cells and the characterization of the key signaling pathways involved in skin morphogenesis (such as p63) will be translated into therapeutic benefit. Our recent results suggest the feasibility not only of identifying but also of amplifying human ES cells, early ectodermal progenitors with an intact multipotent potential that might improve the quality and functionality of grafts, provided that preclinical in vivo studies confirm our expectations from in vitro analysis.

Oct-4: The Almighty POUripotent Regulator?

Oct-4 plays an essential role as a central regulator of the undifferentiated state. Grinnell et al. demonstrate for the first time that Oct-4 by itself has the ability to reprogram committed somatic cells, inducing their dedifferentiation by reverting them to a more developmentally potent state. This study provides evidence that Oct-4 might be the master regulator of the pluripotent state in mammalian cells.