Virus-free induction of pluripotency and subsequent excision of reprogramming factors

A small-molecule inhibitor of tgf-Beta signaling replaces sox2 in reprogramming by inducing nanog

The combined activity of three transcription factors can reprogram adult cells into induced pluripotent stem cells (iPSCs). However, the transgenic methods used for delivering reprogramming factors have raised concerns regarding the future utility of the resulting stem cells. These uncertainties could be overcome if each transgenic factor were replaced with a small molecule that either directly activated its expression from the somatic genome or in some way compensated for its activity. To this end, we have used high-content chemical screening to identify small molecules that can replace Sox2 in reprogramming. We show that one of these molecules functions in reprogramming by inhibiting Tgf-beta signaling in a stable and trapped intermediate cell type that forms during the process. We find that this inhibition promotes the completion of reprogramming through induction of the transcription factor Nanog.

Stem cell-derived hepatocytes and their use in toxicology

Better prediction of safety risk and understanding of mechanism of action of drug candidates remain a major challenge in order to prevent late stage attrition. Continuous efforts are made to improve and develop new models, especially in some areas such as hepatotoxicity. Besides primary hepatocytes and transformed liver cell lines, stem cells either isolated from embryos or adult tissues or obtained by reprogramming somatic cells are emerging as a new potential source of unlimited numbers of hepatocytes. Presently, only hepatocyte-like cells expressing low levels of liver-specific markers, especially drug metabolizing and detoxifying enzymes, are usually obtained, making them still unsuitable as metabolically competent cells for toxicity studies. The only exceptions are some hepatoma cell lines, particularly the HepaRG cell line that can differentiate from a bipotent progenitor stage to attain the functional capacity of normal adult hepatocytes in primary culture without losing the indefinite growth property of transformed cells. Since the research field on stem cells is growing fast marked advances might be expected in the next future.

iPS cells produce viable mice through tetraploid complementation

Since the initial description of induced pluripotent stem (iPS) cells created by forced expression of four transcription factors in mouse fibroblasts, the technique has been used to generate embryonic stem (ES)-cell-like pluripotent cells from a variety of cell types in other species, including primates and rat. It has become a popular means to reprogram somatic genomes into an embryonic-like pluripotent state, and a preferred alternative to somatic-cell nuclear transfer and somatic-cell fusion with ES cells. However, iPS cell reprogramming remains slow and inefficient. Notably, no live animals have been produced by the most stringent tetraploid complementation assay, indicative of a failure to create fully pluripotent cells. Here we report the generation of several iPS cell lines that are capable of generating viable, fertile live-born progeny by tetraploid complementation. These iPS cells maintain a pluripotent potential that is very close to ES cells generated from in vivo or nuclear transfer embryos. We demonstrate the practicality of using iPS cells as useful tools for the characterization of cellular reprogramming and developmental potency, and confirm that iPS cells can attain true pluripotency that is similar to that of ES cells.

Neuronal cell replacement in Parkinson's disease

Transplantation of foetal dopamine neurons into the striatum of Parkinson's disease patients can provide restoration of the dopamine system and alleviate motor deficits. However, cellular replacement is associated with several problems. As with pharmacological treatments, cell therapy can lead to disabling abnormal involuntary movements (dyskinesias). The exclusion of serotonin and GABA neurons, and enrichment of substantia nigra (A9) dopamine neurons, may circumvent this problem. Furthermore, although grafted foetal dopamine neurons can survive in Parkinson's patients for more than a decade, the occurrence of Lewy bodies within such transplanted cells and reduced dopamine transporter and tyrosine hydroxylase expression levels indicate that grafted cells are associated with pathology. It will be important to understand if such abnormalities are host- or graft induced and to develop methods to ensure survival of functional dopamine neurons. Careful preparation of cellular suspensions to minimize graft-induced inflammatory responses might influence the longevity of transplanted cells. Finally, a number of practical and ethical issues are associated with the use of foetal tissue sources. Thus, future cell therapy is aiming towards the use of embryonic stem cell or induced pluripotent stem cell derived dopamine neurons.

Enhanced generation of induced pluripotent stem cells from a subpopulation of human fibroblasts

Background

The derivation of induced pluripotent stem cells (iPSCs) provides new possibilities for basic research and novel cell-based therapies. Limitations, however, include our current lack of understanding regarding the underlying mechanisms and the inefficiency of reprogramming.

Methodology/Principal Findings

Here, we report identification and isolation of a subpopulation of human dermal fibroblasts that express the pluripotency marker stage specific embryonic antigen 3 (SSEA3). Fibroblasts that expressed SSEA3 demonstrated an enhanced iPSC generation efficiency, while no iPSC derivation was obtained from the fibroblasts that did not express SSEA3. Transcriptional analysis revealed NANOG expression was significantly increased in the SSEA3 expressing fibroblasts, suggesting a possible mechanistic explanation for the differential reprogramming.

Conclusions/Significance

To our knowledge, this study is the first to identify a pluripotency marker in a heterogeneous population of human dermal fibroblasts, to isolate a subpopulation of cells that have a significantly increased propensity to reprogram to pluripotency and to identify a possible mechanism to explain this differential reprogramming. This discovery provides a method to significantly increase the efficiency of reprogramming, enhancing the feasibility of the potential applications based on this technology, and a tool for basic research studies to understand the underlying reprogramming mechanisms.

Transcriptional signature and memory retention of human-induced pluripotent stem cells

Genetic reprogramming of somatic cells to a pluripotent state (induced pluripotent stem cells or iPSCs) by over-expression of specific genes has been accomplished using mouse and human cells. However, it is still unclear how similar human iPSCs are to human Embryonic Stem Cells (hESCs). Here, we describe the transcriptional profile of human iPSCs generated without viral vectors or genomic insertions, revealing that these cells are in general similar to hESCs but with significant differences. For the generation of human iPSCs without viral vectors or genomic insertions, pluripotent factors Oct4 and Nanog were cloned in episomal vectors and transfected into human fetal neural progenitor cells. The transient expression of these two factors, or from Oct4 alone, resulted in efficient generation of human iPSCs. The reprogramming strategy described here revealed a potential transcriptional signature for human iPSCs yet retaining the gene expression of donor cells in human reprogrammed cells free of viral and transgene interference. Moreover, the episomal reprogramming strategy represents a safe way to generate human iPSCs for clinical purposes and basic research.

Cardiomyocytes from human pluripotent stem cells in regenerative medicine and drug discovery

Stem cells derived from pre-implantation human embryos or from somatic cells by reprogramming are pluripotent and self-renew indefinitely in culture. Pluripotent stem cells are unique in being able to differentiate to any cell type of the human body. Differentiation towards the cardiac lineage has attracted significant attention, initially with a strong focus on regenerative medicine. Although an important research area, the heart has proven challenging to repair by cardiomyocyte replacement. However, the ability to reprogramme adult cells to pluripotent stem cells and genetically manipulate stem cells presented opportunities to develop models of human disease. The availability of human cardiomyocytes from stem cell sources is expected to accelerate the discovery of cardiac drugs and safety pharmacology by offering more clinically relevant human culture models than presently available. Here we review the state-of-the-art using stem cell-derived human cardiomyocytes in drug discovery, drug safety pharmacology, and regenerative medicine.

Feeder-free derivation of induced pluripotent stem cells from adult human adipose stem cells

Ectopic expression of transcription factors can reprogram somatic cells to a pluripotent state. However, most of the studies used skin fibroblasts as the starting population for reprogramming, which usually take weeks for expansion from a single biopsy. We show here that induced pluripotent stem (iPS) cells can be generated from adult human adipose stem cells (hASCs) freshly isolated from patients. Furthermore, iPS cells can be readily derived from adult hASCs in a feeder-free condition, thereby eliminating potential variability caused by using feeder cells. hASCs can be safely and readily isolated from adult humans in large quantities without extended time for expansion, are easy to maintain in culture, and therefore represent an ideal autologous source of cells for generating individual-specific iPS cells.

Efficient generation of hepatocyte-like cells from human induced pluripotent stem cells

Human induced pluripotent stem (iPS) cells are similar to embryonic stem (ES) cells, and can proliferate intensively and differentiate into a variety of cell types. However, the hepatic differentiation of human iPS cells has not yet been reported. In this report, human iPS cells were induced to differentiate into hepatic cells by a stepwise protocol. The expression of liver cell markers and liver-related functions of the human iPS cell-derived cells were monitored and compared with that of differentiated human ES cells and primary human hepatocytes. Approximately 60% of the differentiated human iPS cells at day 7 expressed hepatic markers alpha fetoprotein and Alb. The differentiated cells at day 21 exhibited liver cell functions including albumin Asecretion, glycogen synthesis, urea production and inducible cytochrome P450 activity. The expression of hepatic markers and liver-related functions of the iPS cell-derived hepatic cells were comparable to that of the human ES cell-derived hepatic cells. These results show that human iPS cells, which are similar to human ES cells, can be efficiently induced to differentiate into hepatocyte-like cells.

Can cellular models revolutionize drug discovery in Parkinson's disease?

The study of mechanisms that underlie Parkinson's disease (PD), as well as translational drug development, has been hindered by the lack of appropriate models. Both cell culture systems and animal models have limitations, and to date none faithfully recapitulate all of the clinical and pathological phenotypes of the disease. In this review we examine the various cell culture model systems of PD, with a focus on different stem cell models that can be used for investigating disease mechanisms as well as drug discovery for PD. We conclude with a discussion of recent discoveries in the field of stem cell biology that have led to the ability to reprogram somatic cells to a pluripotent state via the use of a combination of genetic factors; these reprogrammed cells are termed "induced pluripotent stem cells" (iPSCs). This groundbreaking technique allows for the derivation of patient-specific cell lines from individuals with sporadic forms of PD and also those with known disease-causing mutations. Such cell lines have the potential to serve as a human cellular model of neurodegeneration and PD when differentiated into dopaminergic neurons. The hope is that these iPSC-derived dopaminergic neurons can be used to replicate the key molecular aspects of neural degeneration associated with PD. If so, this approach could lead to transformative new tools for the study of disease mechanisms. In addition, such cell lines can be potentially used for high-throughput drug screening. While not the focus of this review, ultimately it is envisioned that techniques for reprogramming of somatic cells may be optimized to a point sufficient to provide potential new avenues for stem cell-based restorative therapies.

Reclaiming a natural beauty: whole-organ engineering with natural extracellular materials

The ability to engineer whole organs as replacements for allografts and xenografts is an ongoing pursuit in regenerative medicine. While challenges remain, including systemic tissue integration with angiogenesis, lymphatiogenesis and neurogenesis, ongoing efforts are working to develop novel technologies to produce implantable engineered scaffolds and potentially engineered whole organs. Natural extracellular matrix materials, commonly utilized in vitro, are now being used as effective, natural, acellular allografts, and are being integrated into nanoscale scaffolds and matrices with programmable responsiveness. Based on the significant use of natural scaffolds for tissue regeneration and bioengineering strategies, this review focuses on recent and ongoing efforts to engineer whole organs, such as the tooth, featuring natural extracellular matrix molecules.

Induced pluripotent stem cells in regenerative medicine: an argument for continued research on human embryonic stem cells

Human embryonic stem cells (ESCs) can be induced to differentiate into a wide range of tissues that soon could be used for therapeutic applications in regenerative medicine. Despite their developmental potential, sources used to generate human ESC lines raise serious ethical concerns, which recently prompted efforts to reprogram somatic cells back to a pluripotent state. These efforts resulted in the generation of induced pluripotent stem (iPS) cells that are functionally similar to ESCs. However, the genetic manipulations required to generate iPS cells may complicate their growth and developmental characteristics, which poses serious problems in predicting how they will behave when used for tissue-regenerative purposes. In this article we summarize the recently developed methodologies used to generate iPS cells, including those that minimize their genetic manipulation, and discuss several important complicating features of iPS cells that may compromise their future use for therapies in regenerative medicine.

Regulation of stem cell pluripotency and differentiation involves a mutual regulatory circuit of the NANOG, OCT4, and SOX2 pluripotency transcription factors with polycomb repressive complexes and stem cell microRNAs

Coordinated transcription factor networks have emerged as the master regulatory mechanisms of stem cell pluripotency and differentiation. Many stem cell-specific transcription factors, including the pluripotency transcription factors, OCT4, NANOG, and SOX2 function in combinatorial complexes to regulate the expression of loci, which are involved in embryonic stem (ES) cell pluripotency and cellular differentiation. This review will address how these pathways form a reciprocal regulatory circuit whereby the equilibrium between stem cell self-renewal, proliferation, and differentiation is in perpetual balance. We will discuss how distinct epigenetic repressive pathways involving polycomb complexes, DNA methylation, and microRNAs cooperate to reduce transcriptional noise and to prevent stochastic and aberrant induction of differentiation. We will provide a brief overview of how these networks cooperate to modulate differentiation along hematopoietic and neuronal lineages. Finally, we will describe how aberrant functioning of components of the stem cell regulatory network may contribute to malignant transformation of adult stem cells and the establishment of a "cancer stem cell" phenotype and thereby underlie multiple types of human malignancies.

Senescence impairs successful reprogramming to pluripotent stem cells

Somatic cells can be reprogrammed into induced pluripotent stem (iPS) cells by overexpressing combinations of factors such as Oct4, Sox2, Klf4, and c-Myc. Reprogramming is slow and stochastic, suggesting the existence of barriers limiting its efficiency. Here we identify senescence as one such barrier. Expression of the four reprogramming factors triggers senescence by up-regulating p53, p16(INK4a), and p21(CIP1). Induction of DNA damage response and chromatin remodeling of the INK4a/ARF locus are two of the mechanisms behind senescence induction. Crucially, ablation of different senescence effectors improves the efficiency of reprogramming, suggesting novel strategies for maximizing the generation of iPS cells.

Stable gene transfer and expression in cord blood-derived CD34+ hematopoietic stem and progenitor cells by a hyperactive Sleeping Beauty transposon system

Here we report stable gene transfer in cord blood-derived CD34(+) hematopoietic stem cells using a hyperactive nonviral Sleeping Beauty (SB) transposase (SB100X). In colony-forming assays, SB100X mediated the highest efficiency (24%) of stable Discosoma sp red fluorescent protein (DsRed) reporter gene transfer in committed hematopoietic progenitors compared with both the early-generation hyperactive SB11 transposase and the piggyBac transposon system (1.23% and 3.8%, respectively). In vitro differentiation assays further demonstrated that SB100X-transfected CD34(+) cells can develop into DsRed(+) CD4(+)CD8(+) T (3.17%-21.84%; median, 7.97%), CD19(+) B (3.83%-18.66%; median, 7.84%), CD56(+)CD3(-) NK (3.53%-79.98%; median, 7.88%), and CD33(+) myeloid (7.59%-15.63%; median, 9.48%) cells. SB100X-transfected CD34(+) cells achieved approximately 46% engraftment in NOD-scid IL2gammac(null) (NOG) mice. Twelve weeks after transplantation, 0.57% to 28.96% (median, 2.79%) and 0.49% to 34.50% (median, 5.59%) of total human CD45(+) cells in the bone marrow and spleen expressed DsRed, including CD19(+) B, CD14(+) monocytoid, and CD33(+) myeloid cell lineages. Integration site analysis revealed SB transposon sequences in the human chromosomes of in vitro differentiated T, B, NK, and myeloid cells, as well as in human CD45(+) cells isolated from bone marrow and spleen of transplanted NOG mice. Our results support the continuing development of SB-based gene transfer into human hematopoietic stem cells as a modality for gene therapy.

Pluripotency can be rapidly and efficiently induced in human amniotic fluid-derived cells

Direct reprogramming of human somatic cells into pluripotency has broad implications in generating patient-specific induced pluripotent stem (iPS) cells for disease modeling and cellular replacement therapies. However, the low efficiency and safety issues associated with generation of human iPS cells have limited their usage in clinical settings. Cell types can significantly influence reprogramming efficiency and kinetics. To date, human iPS cells have been obtained only from a few cell types. Here, we report for the first time rapid and efficient generation of iPS cells from human amniotic fluid-derived cells (hAFDCs) via ectopic expression of four human factors: OCT4/SOX2/KLF4/C-MYC. Significantly, typical single iPS cell colonies can be picked up 6 days after viral infection with high efficiency. Eight iPS cell lines have been derived. They can be continuously propagated in vitro and express pluripotency markers such as AKP, OCT4, SOX2, SSEA4, TRA-1-60 and TRA-1-81, maintaining the normal karyotype. Transgenes are completely inactivated and the endogenous OCT4 promoter is adequately demethylated in the established iPS cell lines. Moreover, various cells and tissues from all three germ layers are found in embryoid bodies and teratomas, respectively. In addition, microarray analysis demonstrates a high correlation coefficient between hAFDC-iPS cells and human embryonic stem cells, but a low correlation coefficient between hAFDCs and hAFDC-iPS cells. Taken together, these data identify an ideal human somatic cell resource for rapid and efficient generation of iPS cells, allowing us to establish human iPS cells using more advanced approaches and possibly to establish disease- or patient-specific iPS cells.

Deciphering the stem cell machinery as a basis for understanding the molecular mechanism underlying reprogramming

Stem cells provide fascinating prospects for biomedical applications by combining the ability to renew themselves and to differentiate into specialized cell types. Since the first isolation of embryonic stem (ES) cells about 30 years ago, there has been a series of groundbreaking discoveries that have the potential to revolutionize modern life science. For a long time, embryos or germ cell-derived cells were thought to be the only source of pluripotency--a dogma that has been challenged during the last decade. Several findings revealed that cell differentiation from (stem) cells to mature cells is not in fact an irreversible process. The molecular mechanism underlying cellular reprogramming is poorly understood thus far. Identifying how pluripotency maintenance takes place in ES cells can help us to understand how pluripotency induction is regulated. Here, we review recent advances in the field of stem cell regulation focusing on key transcription factors and their functional interplay with non-coding RNAs.

Toward clinical therapies using hematopoietic cells derived from human pluripotent stem cells

Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) provide remarkable cellular platforms to better understand human hematopoiesis and to develop clinically applicable hematopoietic cell-based therapies. Over the past decade, hESCs have been used to characterize molecular and cellular mechanisms underpinning the differentiation of hematopoietic progenitors and mature, functional hematopoietic cells. These advances are now poised to lead to clinical translation of hESC- and iPSC-derived hematopoietic cells for novel therapies in the next few years. On the basis of areas of recent success, initial clinical use of hematopoietic cells derived from human pluripotent stem cells will probably be in the areas of transfusion therapies (erythrocytes and platelets) and immune therapies (natural killer cells). In contrast, efficient development and isolation of hematopoietic stem cells capable of long-term, multilineage engraftment still remains a significant challenge. Technical, safety, and regulatory concerns related to clinical applications of human PSCs must be appropriately addressed. However, proper consideration of these issues should facilitate and not inhibit clinical translation of new therapies. This review outlines the current status of hematopoietic cell development and what obstacles must be surmounted to bring hematopoietic cell therapies from human PSCs from "bench to bedside."

Current progress and prospects of induced pluripotent stem cells

Induced pluripotent stem (iPS) cells are derived from somatic cells by ectopic expression of few transcription factors. Like embryonic stem (ES) cells, iPS cells are able to self-renew indefinitely and to differentiate into all types of cells in the body. iPS cells hold great promise for regenerative medicine, because iPS cells circumvent not only immunological rejection but also ethical issues. Since the first report on the derivation of iPS cells in 2006, many laboratories all over the world started research on iPS cells and have made significant progress. This paper reviews recent progress in iPS cell research, including the methods to generate iPS cells, the molecular mechanism of reprogramming in the formation of iPS cells, and the potential applications of iPS cells in cell replacement therapy. Current problems that need to be addressed and the prospects for iPS research are also discussed.

Salvianolic acid B-vitamin C synergy in cardiac differentiation from embryonic stem cells

Inefficient cardiomyocyte differentiation limits the therapeutic use of embryonic stem (ES) cell-derived cardiomyocytes. While large collections of proprietary chemicals had been screened to improve ES cell differentiation into cardiomyocytes, the natural product library remained unexplored. Using a mouse ES cell line transfected with a cardiomyocyte-specific alpha-myosin heavy chain promoter-driven enhanced green fluorescent protein (EGFP) reporter, we screened 24 natural products with known cardioprotective actions. Salvianolic acid B (saB), while produced minimal effect on its own, concentration-dependently synergized with vitamin C in inducing cardiomyocyte differentiation, as demonstrated by an increase in EGFP(+) cells, beating area in embryoid bodies, and expression of cardiomyocyte maturity markers. This synergy is specific to cardiomyocyte differentiation, and is involved with collagen synthesis. The present study demonstrates the saB-vitamin C synergy in inducing ES cell differentiation into matured and functional cardiomyocytes, and this may lead to a practicable cocktail approach to generate ES cell-derived cardiomyocytes for cardiac stem cell therapy.

Molecules that promote or enhance reprogramming of somatic cells to induced pluripotent stem cells

Reprogramming of somatic cells to induced pluripotent stem cells (iPSCs) can be achieved by viral-mediated transduction of defined transcription factors. Moving toward the eventual goal of clinical application, it is necessary to overcome limitations such as low reprogramming efficiency and genomic alterations due to viral integration. Here, we review recent progress made in the usage of genetic factors, chemical inhibitors, and signaling molecules that can either replace core reprogramming factors or enhance reprogramming efficiency. Current iPSC studies will provide a paradigm for the combinatorial use of genetic factors and chemicals for the broader applications to alter cellular states of potency.

Induced pluripotent stem cells and embryonic stem cells are distinguished by gene expression signatures

Induced pluripotent stem cells (iPSCs) outwardly appear to be indistinguishable from embryonic stem cells (ESCs). A study of gene expression profiles of mouse and human ESCs and iPSCs suggests that, while iPSCs are quite similar to their embryonic counterparts, a recurrent gene expression signature appears in iPSCs regardless of their origin or the method by which they were generated. Upon extended culture, hiPSCs adopt a gene expression profile more similar to hESCs; however, they still retain a gene expression signature unique from hESCs that extends to miRNA expression. Genome-wide data suggested that the iPSC signature gene expression differences are due to differential promoter binding by the reprogramming factors. High-resolution array profiling demonstrated that there is no common specific subkaryotypic alteration that is required for reprogramming and that reprogramming does not lead to genomic instability. Together, these data suggest that iPSCs should be considered a unique subtype of pluripotent cell.

The molecular mechanism of induced pluripotency: a two-stage switch

Pluripotent stem cells are basic cells with an indefinite self-renewal capacity and the potential to generate all the cell types of the three germinal layers. So far, the major source for pluripotent stem cells is the inner cell mass of the blastocysts: embryonic stem (ES) cells. Potential clinical application of ES cells is faced with many practical and ethical concerns. So, a major breakthrough was achieved in 2006, when it was shown that pluripotent stem cells could be obtained by transducing mouse embryonic and adult fibroblasts with a limited set of defined transcription factors. These reprogrammed cells, named induced pluripotent stem (iPS) cells, resembled ES cells in many of their characteristics. Since this initial study, iPS cell research has taken an incredible flight, and to date iPS cells have been generated from cells from several species using different sets of reprogramming factors. Given the potential to generate patient-specific cell populations without the need for human embryonic cells, iPS cell technology has been received with great excitement by research and medical communities. However, many questions regarding the actual molecular process of induced reprogramming remain unanswered and need to be addressed before iPS cells can go to the clinic. In this review, we start by summarizing recent advances in iPS cell research and inventory the hurdles that still need to be taken before safe clinical application. Our major aim, however, is to review the available data on the molecular processes underlying pluripotency reprogramming and present a two-stage switch model.

Stoichiometric and temporal requirements of Oct4, Sox2, Klf4, and c-Myc expression for efficient human iPSC induction and differentiation

Human-induced pluripotent stem cells (hiPSCs) are generated from somatic cells by ectopic expression of the 4 reprogramming factors (RFs) Oct-4, Sox2, Klf4, and c-Myc. To better define the stoichiometric requirements and dynamic expression patterns required for successful hiPSC induction, we generated 4 bicistronic lentiviral vectors encoding the 4 RFs co-expressed with discernable fluorescent proteins. Using this system, we define the optimal stoichiometry of RF expression to be highly sensitive to Oct4 dosage, and we demonstrate the impact that variations in the relative ratios of RF expression exert on the efficiency of hiPSC induction. Monitoring of expression of each individual RF in single cells during the course of reprogramming revealed that vector silencing follows acquisition of pluripotent cell markers. Pronounced lentiviral vector silencing was a characteristic of successfully reprogrammed hiPSC clones, but lack of complete silencing did not hinder hiPSC induction, maintenance, or directed differentiation. The vector system described here presents a powerful tool for mechanistic studies of reprogramming and the optimization of hiPSC generation.

Pluripotency rush! Molecular cues for pluripotency, genetic reprogramming of adult stem cells, and widely multipotent adult cells

In the last few years, pluripotent stem cells have been the objective of intense investigation efforts. These cells are of paramount therapeutic interest, since they could be utilized: as in vitro models of disease, for pharmaceutical screening purposes, and for the regeneration of damaged organs. Over the years, pluripotent cells have been cultured from teratomas, the inner cell mass, and primordial germ cells. Accumulating informations have partially decrypted the molecular machinery responsible for the maintenance of a very primitive state, permitting the reprogramming of differentiated cells. Although the debate is still open, an extreme excitement is arising from two strictly related possibilities: pluripotent cells could be obtained from adult tissues with minimal manipulations or very rare pluripotent cells could be identified in adult tissues. This intriguing option will trigger new researches aimed both at identifying the possible biological role of pluripotent adult stem cells and at exploiting their potential clinical use. The present review article will summarize current knowledge of the molecular cues for pluripotency but also discusses whether pluripotent stem cells could be obtained from adult tissues.

Induced pluripotent stem cells and the stability of the differentiated state

For much of the last century, the differentiated state that characterizes the many cell types of an adult organism was thought to be stable and abrogated only in rare instances by transdifferentiation, metaplasia or cancer. This stability was thought to reside in the autoregulatory molecular circuitry that exists between the cytoplasm and the nucleus, a status quo that could be disrupted during somatic cell nuclear transfer, to reprogramme cells to a pluripotent state. Pioneering work in the 1980s showed that transdifferentiation of cell lineages could be induced by the addition of transcription factors. However, these conversions were usually confined to cell types from the same germ layer, and proof of conversion was difficult to obtain. This deficiency has now been overturned by demonstrations that exogenously added transcription factors can convert differentiated cell types into embryonic-like induced pluripotent stem cells. Here, we highlight the recent progress, and the implications of this work for our understanding of the relationship between the pluripotent and more differentiated cell states.

Complexity in biomaterials for tissue engineering

The molecular and physical information coded within the extracellular milieu is informing the development of a new generation of biomaterials for tissue engineering. Several powerful extracellular influences have already found their way into cell-instructive scaffolds, while others remain largely unexplored. Yet for commercial success tissue engineering products must be not only efficacious but also cost-effective, introducing a potential dichotomy between the need for sophistication and ease of production. This is spurring interest in recreating extracellular influences in simplified forms, from the reduction of biopolymers into short functional domains, to the use of basic chemistries to manipulate cell fate. In the future these exciting developments are likely to help reconcile the clinical and commercial pressures on tissue engineering.

From genes to function: the next challenge to understanding multiple sclerosis

Susceptibility to multiple sclerosis is jointly determined by genetic and environmental factors, and progress has been made in defining some of these genetic associations, as well as their possible interactions with the environment. However, definitive proof for the involvement of specific genetic determinants in the disease will only come from studies that examine their functional roles in disease pathogenesis. New and combined approaches are needed to analyse the complexity of gene regulation and the functional contribution of each genetic determinant to disease susceptibility or pathophysiology. These studies should proceed in parallel with the use of genetically defined human populations to explore how both genetic and environmental factors affect the function of the pathways in individuals with and without disease, and how these determine the inherited risk of multiple sclerosis.

Induced pluripotent stem cells offer new approach to therapy in thalassemia and sickle cell anemia and option in prenatal diagnosis in genetic diseases

The innovation of reprogramming somatic cells to induced pluripotent stem cells provides a possible new approach to treat beta-thalassemia and other genetic diseases such as sickle cell anemia. Induced pluripotent stem (iPS) cells can be made from these patients' somatic cells and the mutation in the beta-globin gene corrected by gene targeting, and the cells differentiated into hematopoietic cells to be returned to the patient. In this study, we reprogrammed the skin fibroblasts of a patient with homozygous beta(0) thalassemia into iPS cells, and showed that the iPS cells could be differentiated into hematopoietic cells that synthesized hemoglobin. Prenatal diagnosis and selective abortion have been effective in decreasing the number of beta-thalassemia births in some countries that have instituted carrier screening and genetic counseling. To make use of the cells from the amniotic fluid or chorionic villus sampling that are used for prenatal diagnosis, we also showed that these cells could be reprogrammed into iPS cells. This raises the possibility of providing a new option following prenatal diagnosis of a fetus affected by a severe illness. Currently, the parents would choose either to terminate the pregnancy or continue it and take care of the sick child after birth. The cells for prenatal diagnosis can be converted into iPS cells for treatment in the perinatal periods. Early treatment has the advantage of requiring much fewer cells than adult treatment, and can also prevent organ damage in those diseases in which damage can begin in utero or at an early age.

Emerging potential of transposons for gene therapy and generation of induced pluripotent stem cells

Effective gene therapy requires robust delivery of the desired genes into the relevant target cells, long-term gene expression, and minimal risks of secondary effects. The development of efficient and safe nonviral vectors would greatly facilitate clinical gene therapy studies. However, nonviral gene transfer approaches typically result in only limited stable gene transfer efficiencies in most primary cells. The use of nonviral gene delivery approaches in conjunction with the latest generation transposon technology based on Sleeping Beauty (SB) or piggyBac transposons may potentially overcome some of these limitations. In particular, a large-scale genetic screen in mammalian cells yielded a novel hyperactive SB transposase, resulting in robust and stable gene marking in vivo after hematopoietic reconstitution with CD34(+) hematopoietic stem/progenitor cells in mouse models. Moreover, the first-in-man clinical trial has recently been approved to use redirected T cells engineered with SB for gene therapy of B-cell lymphoma. Finally, induced pluripotent stem cells could be generated after genetic reprogramming with piggyBac transposons encoding reprogramming factors. These recent developments underscore the emerging potential of transposons in gene therapy applications and induced pluripotent stem generation for regenerative medicine.

Reprogramming of murine fibroblasts to induced pluripotent stem cells with chemical complementation of Klf4

Ectopic expression of defined transcription factors can reprogram somatic cells to induced pluripotent stem (iPS) cells, but the utility of iPS cells is hampered by the use of viral delivery systems. Small molecules offer an alternative to replace virally transduced transcription factors with chemical signaling cues responsible for reprogramming. In this report we describe a small-molecule screening platform applied to identify compounds that functionally replace the reprogramming factor Klf4. A series of small-molecule scaffolds were identified that activate Nanog expression in mouse fibroblasts transduced with a subset of reprogramming factors lacking Klf4. Application of one such molecule, kenpaullone, in lieu of Klf4 gave rise to iPS cells that are indistinguishable from murine embryonic stem cells. This experimental platform can be used to screen large chemical libraries in search of novel compounds to replace the reprogramming factors that induce pluripotency. Ultimately, such compounds may provide mechanistic insight into the reprogramming process.

Human induced pluripotent stem cells free of vector and transgene sequences

Reprogramming differentiated human cells to induced pluripotent stem (iPS) cells has applications in basic biology, drug development, and transplantation. Human iPS cell derivation previously required vectors that integrate into the genome, which can create mutations and limit the utility of the cells in both research and clinical applications. We describe the derivation of human iPS cells with the use of nonintegrating episomal vectors. After removal of the episome, iPS cells completely free of vector and transgene sequences are derived that are similar to human embryonic stem (ES) cells in proliferative and developmental potential. These results demonstrate that reprogramming human somatic cells does not require genomic integration or the continued presence of exogenous reprogramming factors and removes one obstacle to the clinical application of human iPS cells.

Induced pluripotent stem cells free of exogenous reprogramming factors

The development of novel approaches for reprogramming mouse and human somatic cells has enabled the generation of induced pluripotent stem cells that are free of exogenous genes.

Towards personalized cell-replacement therapies for brain repair

The inability of the CNS to efficiently repair damage caused by trauma and neurodegenerative or demyelinating diseases has underlined the necessity for developing novel therapeutic strategies. Cell transplantation to replace lost neurons and the grafting of myelinating cells to repair demyelinating lesions are promising approaches for treating CNS injuries and demyelination. In this review, we will address the prospects of using stem cells or myelinating glial cells of the PNS, as well as olfactory ensheathing cells, in cell-replacement therapies. The recent generation of induced pluripotent stem cells from adult somatic cells by introduction of three or four genes controlling ‘stemness’ and their subsequent differentiation to desired phenotypes, constitutes a significant advancement towards personalized cell-replacement therapies.

Generation of transgene-free induced pluripotent mouse stem cells by the piggyBac transposon

Induced pluripotent stem cells (iPSCs) have been generated from somatic cells by transgenic expression of Oct4 (Pou5f1), Sox2, Klf4 and Myc. A major difficulty in the application of this technology for regenerative medicine, however, is the delivery of reprogramming factors. Whereas retroviral transduction increases the risk of tumorigenicity, transient expression methods have considerably lower reprogramming efficiencies. Here we describe an efficient piggyBac transposon-based approach to generate integration-free iPSCs. Transposons carrying 2A peptide-linked reprogramming factors induced reprogramming of mouse embryonic fibroblasts with equivalent efficiencies to retroviral transduction. We removed transposons from these primary iPSCs by re-expressing transposase. Transgene-free iPSCs could be identified by negative selection. piggyBac excised without a footprint, leaving the iPSC genome without any genetic alteration. iPSCs fulfilled all criteria of pluripotency, such as pluripotency gene expression, teratoma formation and contribution to chimeras. piggyBac transposon-based reprogramming may be used to generate therapeutically applicable iPSCs.

Stem cells to pancreatic beta-cells: new sources for diabetes cell therapy

The number of patients worldwide suffering from the chronic disease diabetes mellitus is growing at an alarming rate. Insulin-secreting beta-cells in the islet of Langerhans are damaged to different extents in diabetic patients, either through an autoimmune reaction present in type 1 diabetic patients or through inherent changes within beta-cells that affect their function in patients suffering from type 2 diabetes. Cell replacement strategies via islet transplantation offer potential therapeutic options for diabetic patients. However, the discrepancy between the limited number of donor islets and the high number of patients who could benefit from such a treatment reflects the dire need for renewable sources of high-quality beta-cells. Human embryonic stem cells (hESCs) are capable of self-renewal and can differentiate into components of all three germ layers, including all pancreatic lineages. The ability to differentiate hESCs into beta-cells highlights a promising strategy to meet the shortage of beta-cells. Here, we review the different approaches that have been used to direct differentiation of hESCs into pancreatic and beta-cells. We will focus on recent progress in the understanding of signaling pathways and transcription factors during embryonic pancreas development and how this knowledge has helped to improve the methodology for high-efficiency beta-cell differentiation in vitro.

Ethical issues in stem cell research

Stem cell research offers great promise for understanding basic mechanisms of human development and differentiation, as well as the hope for new treatments for diseases such as diabetes, spinal cord injury, Parkinson's disease, and myocardial infarction. However, human stem cell (hSC) research also raises sharp ethical and political controversies. The derivation of pluripotent stem cell lines from oocytes and embryos is fraught with disputes about the onset of human personhood. The reprogramming of somatic cells to produce induced pluripotent stem cells avoids the ethical problems specific to embryonic stem cell research. In any hSC research, however, difficult dilemmas arise regarding sensitive downstream research, consent to donate materials for hSC research, early clinical trials of hSC therapies, and oversight of hSC research. These ethical and policy issues need to be discussed along with scientific challenges to ensure that stem cell research is carried out in an ethically appropriate manner. This article provides a critical analysis of these issues and how they are addressed in current policies.