Generation of germline-competent induced pluripotent stem cells

MLL translocations, histone modifications and leukaemia stem-cell development

Translocations that involve the mixed lineage leukaemia (MLL) gene identify a unique group of acute leukaemias, and often predict a poor prognosis. The MLL gene encodes a DNA-binding protein that methylates histone H3 lysine 4 (H3K4), and positively regulates gene expression including multiple Hox genes. Leukaemogenic MLL translocations encode MLL fusion proteins that have lost H3K4 methyltransferase activity. A key feature of MLL fusion proteins is their ability to efficiently transform haematopoietic cells into leukaemia stem cells. The link between a chromatin modulator and leukaemia stem cells provides support for epigenetic landscapes as an important part of leukaemia and normal stem-cell development.

Insertional mutagenesis and clonal dominance: biological and statistical considerations

Improvements of (retroviral) gene transfer vectors, stem cell isolation and culture techniques as well as transduction protocols eventually resulted not only in the successful genetic modification of cells capable of reconstituting the haematopoietic system in various animal models, but also human beings. This was a conditio sine qua non for the successful application of gene therapy for inherited diseases as meanwhile achieved for severe combined immune deficiencies (SCID-X1, ADA-SCID) and chronic granulomatous disease (CGD). Unexpectedly, in long-term animal experiments as well as in the follow up of patients from the CGD trial, haematopoietic clones bearing insertions in certain gene loci became dominant, which was most apparent in the myeloid blood compartment. Accumulating data strongly suggest that this clonal dominance was due to some growth and/or survival advantage conferred by gene-activating or -suppressing effects of the integrated retroviral vector (insertional mutagenesis). Importantly, such induced clonal dominance seems not to lead to malignant transformation of affected cell clones inadvertently. The latter finding has become the basis for the concept of 'induced haematopoietic stem cells', a potentially powerful tool to investigate genes involved in the regulation of mechanisms underlying competitive advantages of stem cells, but also in the multi-step nature of malignant transformation. Here we discuss promises and open issues of this concept as well as the important question of common insertion sites statistics and its pitfalls.

The context of embryonic development and its ethical relevance

Research on human stem cells and embryos creates ethical issues. Here I discuss ten frequently used arguments against research and point out their weaknesses. These arguments include the possessed potentiality of the embryo per se and, in contrast to other cell systems, the "slippery slope" argument, the right of disposal of parents, totipotency versus pluripotency, the burden of proof for research, natural versus artificial, and three arguments based on the precaution principle (the open biological questions, uncertainty regarding clinically applicable therapies, and the problem solving rule). I finally suggest a different answer to the ethical questions concerning research on human embryos and embryonic stem cells, which takes into consideration their biological context.

Concise Review: Roles of Polycomb Group Proteins in Development and Disease: A Stem Cell Perspective

The acquisition and maintenance of cell fate are essential for metazoan growth and development. A strict coordination between genetic and epigenetic programs regulates cell fate determination and maintenance. Polycomb group (PcG) genes are identified as essential in these epigenetic developmental processes. These genes encode components of multimeric transcriptional repressor complexes that are crucial in maintaining cell fate. PcG proteins have also been shown to play a central role in stem cell maintenance and lineage specification. PcG proteins, together with a battery of components including sequence-specific DNA binding/accessory factors, chromatin remodeling factors, signaling pathway intermediates, noncoding small RNAs, and RNA interference machinery, generally define a dynamic cellular identity through tight regulation of specific gene expression patterns. Epigenetic modification of chromatin structure that results in expression silencing of specific genes is now emerging as an important molecular mechanism in this process. In embryonic stem (ES) cells and adult stem cells, such specific genes represent those associated with differentiation and development, and silencing of these genes in a PcG protein-dependent manner confers stemness. ES cells also contain novel chromatin motifs enriched in epigenetic modifications associated with both activation and repression of genes, suggesting that certain genes are poised for activation or repression. Interestingly, these chromatin domains are highly coincident with the promoters of developmental regulators, which are also found to be occupied by PcG proteins. The epigenetic integrity is compromised, however, by mutations or other alterations that affect the function of PcG proteins in stem cells leading to aberrant cell proliferation and tissue transformation, a hallmark of cancer. Disclosure of potential conflicts of interest is found at the end of this article.

Conversion of mature B cells into T cells by dedifferentiation to uncommitted progenitors

Lineage commitment and differentiation to a mature cell type are considered to be unidirectional and irreversible processes under physiological conditions. The commitment of haematopoietic progenitors to the B-cell lineage and their development to mature B lymphocytes critically depend on the transcription factor encoded by the paired box gene 5 (Pax5). Here we show that conditional Pax5 deletion in mice allowed mature B cells from peripheral lymphoid organs to dedifferentiate in vivo back to early uncommitted progenitors in the bone marrow, which rescued T lymphopoiesis in the thymus of T-cell-deficient mice. These B-cell-derived T lymphocytes carried not only immunoglobulin heavy- and light-chain gene rearrangements but also participated as functional T cells in immune reactions. Mice lacking Pax5 in mature B cells also developed aggressive lymphomas, which were identified by their gene expression profile as progenitor cell tumours. Hence, the complete loss of Pax5 in late B cells could initiate lymphoma development and uncovered an extraordinary plasticity of mature peripheral B cells despite their advanced differentiation stage.

[Adult stem cells: their scientific interest and therapeutic future]

Fascinating and provocative findings have shaken the stem cell field during these past years, which may be exploited in the future in cell replacement therapies. Continuous renewal of blood, skin, and gut cells, has long be attributed to stem cells, but it was more unexpected to identify cells that fulfil the requirements for stem-progenitor cells in many tissues with a slow turnover such as heart, kidney, muscle and brain. However, despite their lack of risk and immunological barrier, adult stem cells are yet of poor therapeutic value in many diseases, because they are available in scarce number, are poorly amplified, and loose potential with ageing, among many obstacles. Thus, the identification in adult, and more recently fetal tissues, of cells with a high proliferative capacity and multi-lineage differentiation potential has been wellcome, although their existence is still a matter of controversy. An alternative would be to activate stem cells in situ, by acting on components of the niche as recently exemplified in the hematopoetic system. Finally, as fiction meets reality, it may become possible to reprogram human adult cells in pluripotent ES cells-like, as recently demonstrated in mice.

Direct reprogramming of genetically unmodified fibroblasts into pluripotent stem cells

In vitro reprogramming of somatic cells into a pluripotent embryonic stem cell-like state has been achieved through retroviral transduction of murine fibroblasts with Oct4, Sox2, c-myc and Klf4. In these experiments, the rare 'induced pluripotent stem' (iPS) cells were isolated by stringent selection for activation of a neomycin-resistance gene inserted into the endogenous Oct4 (also known as Pou5f1) or Nanog loci. Direct isolation of pluripotent cells from cultured somatic cells is of potential therapeutic interest, but translation to human systems would be hindered by the requirement for transgenic donors in the present iPS isolation protocol. Here we demonstrate that reprogrammed pluripotent cells can be isolated from genetically unmodified somatic donor cells solely based upon morphological criteria.

Pedigreed primate embryonic stem cells express homogeneous familial gene profiles

Human embryonic stem cells (hESCs) hold great biomedical promise, but experiments comparing them produce heterogeneous results, raising concerns regarding their reliability and utility, although these variations may result from their disparate and anonymous origins. To determine whether primate ESCs have intrinsic biological limitations compared with mouse ESCs, we examined expression profiles and pluripotency of newly established nonhuman primate ESC (nhpESCs). Ten pedigreed nhpESC lines, seven full siblings (fraternal quadruplets and fraternal triplets), and nine half siblings were derived from 41 rhesus embryos; derivation success correlated with embryo quality. Each line has been growing continuously for approximately 1 year with stable diploid karyotype (except for one stable trisomy) and expresses in vitro pluripotency markers, and eight have already formed teratomas. Unlike the heterogeneous gene expression profiles found among hESCs, these nhpESCs display remarkably homogeneous profiles (>97%), with full-sibling lines nearly identical (>98.2%). Female nhpESCs express genes distinct from their brother lines; these sensitive analyses are enabled because of the very low background differences. Experimental comparisons among these primate ESCs may prove more reliable than currently available hESCs, since they are akin to inbred mouse strains in which genetic variables are also nearly eliminated. Finally, contrasting the biological similarities among these lines with the heterogeneous hESCs might suggest that additional, more uniform hESC lines are justified. Taken together, pedigreed primate ESCs display homogeneous and reliable expression profiles. These similarities to mouse ESCs suggest that heterogeneities found among hESCs likely result from their disparate origins rather than intrinsic biological limitations with primate embryonic stem cells.

Reprogramming somatic cells to their embryonic state

Embryonic stem (ES) cells have the capacity to form every type of cell in our adult bodies due to their pluripotency. The prospective use of ES cells in regenerative therapies for human diseases such as Parkinson's disease and diabetes has raised the interest in identifying the mechanisms that allow these cells to maintain pluripotent fate and differentiate along many lineages. However, ethical questions regarding the use of human eggs andor embryos for medical research have limited the ability of scientists to develop therapies with human ES cells. Three recent papers in Nature and Cell Stem Cell have revealed novel methods of reprogramming somatic cells into cells with the same pluripotent potential as ES cells via the expression of only four transcription factors. These scientific advances illuminate the mechanisms that drive pluripotent fate in embryonic cells. In addition, by giving scientists a model to study ES-like cells that are not derived from embryos, these newly identified models have the potential to progress therapies for regenerative medicine.

Towards the generation of patient-specific pluripotent stem cells for combined gene and cell therapy of hematologic disorders

Hematopoietic stem cell transplantation (HSCT) has proven successful for the treatment of a host of genetic and malignant diseases of the blood, but immune barriers to allogeneic tissue transplantation have hindered wider application. Likewise, gene therapy now appears effective in the treatment of various forms of immune deficiency, and yet insertional mutagenesis from viral gene transfer has raised safety concerns. One strategy for addressing the limitations of both gene therapy and allogeneic transplantation entails the creation of pluripotent stem cells from a patient's own somatic cells, thereby enabling precise in situ gene repair via homologous recombination in cultured cells, followed by autologous tissue transplantation. In murine model systems, the methods of somatic cell nuclear transfer, parthenogenesis, and direct somatic cell reprogramming with defined genetic factors have been used to generate pluripotent stem cells, and initial efforts at therapeutic gene repair and tissue transplantation suggest that the technology is feasible. Generating patient-specific autologous pluripotent stem cells provides an opportunity to combine gene therapy with autologous cell therapy to treat a host of human conditions. However, a number of technical hurdles must be overcome before therapies based on pluripotent human stem cells will appear in the clinic.

Stem cell plasticity: Learning from hepatogenic differentiation strategies

Many studies on stem cell plasticity are challenging the concept that stem cells contain an intrinsically predefined, unidirectional differentiation program. This means that the developmental fate of a stem cell is dependent on the general potential of the cell (pre-determined stem cell fate) as well as on microenvironmental cues, such as stimuli from growth factors (stem cell niche). Here, we reviewed reports that examined the hepatocyte differentiation ability of stem cells from two different sources: embryonic stem cells and adult stem cells. All of those stem cells revealed the ability to give rise to hepatocyte-like cells using different induction strategies. However, it is still not clear which of those stem cells would be the best source for hepatocyte replacement or which would be the best protocol. We herein present the current knowledge regarding available protocols and factors used in order to obtain functional hepatocytes from stem cells.

A protein interaction network for pluripotency of embryonic stem cells

Embryonic stem (ES) cells are pluripotent and of therapeutic potential in regenerative medicine. Understanding pluripotency at the molecular level should illuminate fundamental properties of stem cells and the process of cellular reprogramming. Through cell fusion the embryonic cell phenotype can be imposed on somatic cells, a process promoted by the homeodomain protein Nanog, which is central to the maintenance of ES cell pluripotency. Nanog is thought to function in concert with other factors such as Oct4 (ref. 8) and Sox2 (ref. 9) to establish ES cell identity. Here we explore the protein network in which Nanog operates in mouse ES cells. Using affinity purification of Nanog under native conditions followed by mass spectrometry, we have identified physically associated proteins. In an iterative fashion we also identified partners of several Nanog-associated proteins (including Oct4), validated the functional relevance of selected newly identified components and constructed a protein interaction network. The network is highly enriched for nuclear factors that are individually critical for maintenance of the ES cell state and co-regulated on differentiation. The network is linked to multiple co-repressor pathways and is composed of numerous proteins whose encoding genes are putative direct transcriptional targets of its members. This tight protein network seems to function as a cellular module dedicated to pluripotency.

The fatty acid constitution and ordering state of membranes in dominant temperature-sensitive lethal mutation and wild-type Drosophila melanogaster larvae

The ordering state and changes in fatty acid composition of microsomal (MS) and mitochondrial (MC) membranes of two dominant temperature-sensitive (DTS) lethal mutations and the wild-type Oregon-R strain larvae of Drosophila melanogaster have been studied at 18 and 29 degrees C and after temperature-shift experiments. The membranes of wild-type larvae have a stable ordering state, with "S" values between 0.6 (18 degrees C) and 0.5 (29 degrees C) in both membranes which remained unchanged in shift experiments, although the ratios of saturated/unsaturated fatty acids were changed as expected. The strongly DTS mutation 1(2) 10DTS forms very rigid membranes at the restrictive temperature (29 degrees C) which cannot be normalized after shift down, while shift up or development at the permissive temperature results in normal ordering state. This mutant is less able to adjust MS and MC fatty acid composition in response to the growth temperature than the wild type. The less temperature-sensitive 1(2)2DTS allele occupies an intermediate state between Oregon-R and 1(2)10DTS in both respects. We assume and the genetical data suggest that the DTS mutant gene product is in competition with the wild-type product, resulting in a membrane structure which is not able to accommodate to the restrictive temperature.

Participation of neural crest-derived cells in the genesis of the skull in birds

The differentiation of cephalic neural crest cells into skeletal tissue in birds has been observed using the quail-chick nuclear marking system, which is based on specific differences in the distribution of the nuclear DNA. Chimaeras were formed by replacing a fragment of cephalic neural primordium of a 2- to 12-somite chicken embryo by the corresponding fragment isolated from an equivalent quail embryo. The participation of the graft-derived cells in the formation of the skull of these embryos was studied on histological sections after Feulgen and Rossenbeck staining. Cells from the prosencephalic neural crest migrate into the frontal nasal process and mix with the mesencephalic neural crest cells in the lateral nasal processes, around the optic cupule and beneath the diencephalon. In addition, the mesencephalic neural crest cells form the bulk of the mesenchyme of the maxillary processes and mandibular arch, whereas the rhombencephalic neural crest cells become located in the branchial arches. The origin of cartilages of the chondrocranium and bones of the neurocranium and viscerocranium has been shown in the chimaeric embryos: the basal plate cartilages, occipital bones, sphenoid bones and the cranial vault are mainly of mesodermal origin. However some parts have a dual origin: rhombo-mesencephalic neural crest cells are found in the otic capsule, and the frontal bone, the rostrum of parasphenoid and the orbital cartilages contain diverse amounts of prosencephalo-mesencephalic neural crest cells. The squamosals and the columella auris are formed from mesectodermic cells as are the nasal skeleton, the palatines and the maxillar bones. The mesectodermal origin of mandibular and hyoid bones and cartilages was already known. From these results it appears that the cephalic neural crest is particularly important in the formation of the facial part of the skull, while the vault and dorsal part are mesodermal and cartilages and bones found in the intermediary region are of mixed origin. The presence of mixed structures implies that the mesoderm and the mesectoderm are equally competent towards the specific inducers of these bones and cartilages. This correlates with the equivalence in differentiation capacities already shown for cephalic mesodermal and mesectodermal mesenchymes.