Isolation and characterization of a leukemia inhibitory factor-independent embryonic stem cell line

Mechanisms of self-renewal in human embryonic stem cells

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

Cytokine signalling in embryonic stem cells

Cytokines play a central role in maintaining self-renewal in mouse embryonic stem (ES) cells through a member of the interleukin-6 type cytokine family termed leukemia inhibitory factor (LIF). LIF activates the JAK-STAT3 pathway through the class I cytokine receptor gp130, which forms a trimeric complex with LIF and the class I cytokine receptor LIF receptor beta. STAT3 has been shown to play a crucial role in self-renewal in mouse ES cells probably by induction of c-myc expression. Thus, ablation of STAT3 activation leads to differentiation. However, important connections between STAT3 and other signalling pathways have been documented. In addition, gp130 activation leads to both PI3K and Src activation. The canonical Wnt pathway is sufficient to maintain self-renewal of both human ES cells and mouse ES cells. It seems quite possible that the main pathway maintaining self-renewal in ES cells is the Wnt pathway, while the LIF-JAK-STAT3 pathway is present in mouse cells as an adaptation for sustaining self-renewal during embryonic diapause, a condition of delayed implantation in mammals. In keeping with this scenario, the Wnt pathway has been shown to elevate the level of c-myc. Thus, the two pathways seem to converge on c-myc as a common target to promote self-renewal. Whereas LIF does not seem to stimulate self-renewal in human embryonic stem cells it cannot be excluded that other cytokines are involved. The pleiotropic actions of the increasing number of cytokines and receptors signalling via JAKs, STATs and SOCS exhibit considerable redundancy, compensation and plasticity in stem cells in accordance with the view that stem cells are governed by quantitative variations in strength and duration of signalling events known from other cell types rather than qualitatively different stem cell-specific factors.

Conserved and divergent paths that regulate self-renewal in mouse and human embryonic stem cells

The past few years have seen remarkable progress in our understanding of embryonic stem cell (ES cell) biology. The necessity of examining human ES cells in culture, coupled with the wealth of genomic data and the multiplicity of cell lines available, has enabled researchers to identify critical conserved pathways regulating self-renewal and identify markers that tightly correlate with the ES cell state. Comparison across species has suggested additional pathways likely to be important in long-term self-renewal of ES cells including heterochronic genes, microRNAs, genes involved in telomeric regulation, and polycomb repressors. In this review, we have discussed information on molecules known to be important in ES cell self-renewal or blastocyst development and highlighted known differences between mouse and human ES cells. We suggest that several additional pathways required for self-renewal remain to be discovered and these likely include genes involved in antisense regulation, microRNAs, as well as additional global repressive pathways and novel genes. We suggest that cross species comparisons using large-scale genomic analysis tools are likely to reveal conserved and divergent paths required for ES cell self-renewal and will allow us to derive ES lines from species and strains where this has been difficult.

Nestin-positive mesenchymal stem cells favour the astroglial lineage in neural progenitors and stem cells by releasing active BMP4

BACKGROUND

Spontaneous repair is limited after CNS injury or degeneration because neurogenesis and axonal regrowth rarely occur in the adult brain. As a result, cell transplantation has raised much interest as potential treatment for patients with CNS lesions. Several types of cells have been considered as candidates for such cell transplantation and replacement therapies. Foetal brain tissue has already been shown to have significant effects in patients with Parkinson's disease. Clinical use of the foetal brain tissue is, however, limited by ethical and technical problems as it requires high numbers of grafted foetal cells and immunosuppression. Alternatively, several reports suggested that mesenchymal stem cells, isolated from adult bone marrow, are multipotent cells and could be used in autograft approach for replacement therapies.

RESULTS

In this study, we addressed the question of the possible influence of mesenchymal stem cells on neural stem cell fate. We have previously reported that adult rat mesenchymal stem cells are able to express nestin in defined culture conditions (in the absence of serum and after 25 cell population doublings) and we report here that nestin-positive (but not nestin-negative) mesenchymal stem cells are able to favour the astroglial lineage in neural progenitors and stem cells cultivated from embryonic striatum. The increase of the number of GFAP-positive cells is associated with a significant decrease of the number of Tuj1- and O4-positive cells. Using quantitative RT-PCR, we demonstrate that mesenchymal stem cells express LIF, CNTF, BMP2 and BMP4 mRNAs, four cytokines known to play a role in astroglial fate decision. In this model, BMP4 is responsible for the astroglial stimulation and oligodendroglial inhibition, as 1) this cytokine is present in a biologically-active form only in nestin-positive mesenchymal stem cells conditioned medium and 2) anti-BMP4 antibodies inhibit the nestin-positive mesenchymal stem cells conditioned medium inducing effect on astrogliogenesis.

CONCLUSIONS

When thinking carefully about mesenchymal stem cells as candidates for cellular therapy in neurological diseases, their effects on resident neural cell fate have to be considered.

Ligand/receptor signaling threshold (LIST) model accounts for gp130-mediated embryonic stem cell self-renewal responses to LIF and HIL-6

We previously demonstrated that embryonic stem (ES) cell self-renewal required sustained signaling by leukemia inhibitory factor (LIF) in a concentration-dependent manner, allowing us to hypothesize that thresholds in ligand-receptor signaling modulate stem cell differentiation control. To test this hypothesis, we have experimentally and computationally compared the abilities of two gp130-signaling cytokines (LIF and Hyper-interleukin-6 [HIL-6]) to sustain ES cell self-renewal. Quantitative measurements of ES cell phenotypic markers (stage-specific embryonic antigen-1 and E-cadherin), functional assays (alkaline phosphatase activity and embryoid body formation efficiency), and transcription factor (Oct-4) expression over a range of LIF and HIL-6 concentrations demonstrated a superior ability of LIF to maintain ES cell pluripotentiality at higher concentrations (> or =500 pM). Additionally, we observed distinct qualitative differences in the ES cell self-renewal dose response profiles between the two cytokines. A computational model permitted calculation of the number of signaling complexes as a function of receptor expression, ligand concentration, and ligand/receptor-binding properties, generating predictions for the degree of self-renewal as a function of cytokine concentration by comparison of these calculated complex numbers to experimentally determined threshold cytokine concentrations. Model predictions, consistent with experimental data, indicated that differences in the potencies of these two cytokines were based primarily on differences in receptor-binding stoichiometries and properties. These results support a ligand/receptor signaling threshold model of ES cell fate modulation through appropriate types and levels of cytokine stimulation. Insights from these results may be more generally applicable to tissue-specific stem cells and could aid in the development of stem cell-based technologies.

Forced expression of the homeobox-containing gene Pem blocks differentiation of embryonic stem cells

Similarities in the differentiation of mouse embryos and ES cell embryoid bodies suggest that aspects of early mammalian embryogenesis can be studied in ES cell embryoid bodies. In an effort to understand the regulation of cellular differentiation during early mouse embryogenesis, we altered the expression of the Pem homeobox-containing gene in ES cells. Pem is normally expressed in the preimplantation embryo and expressed in a lineage-restricted fashion following implantation, suggesting a role for Pem in regulating cellular differentiation in the early embryo. Here, we show that the forced expression of Pem from the mouse Pgk-1 promoter in ES cells blocks the in vitro and in vivo differentiation of the cells. In particular, embryoid bodies produced from these Pgk-Pem ES cells do not differentiate into primitive endoderm or embryonic ectoderm, which are prominent features of early embryoid bodies from normal ES cells. This Pgk-Pem phenotype is also different from the null phenotype, as embryoid bodies derived from ES cells in which endogenous Pem gene expression has been blocked show a pattern of differentiation similar to that of normal ES cells. When the Pgk-Pem ES cells were introduced into subcutaneous sites of nude mice, only undifferentiated EC-like cells were found in the teratomas derived from the injected cells. The Pem-dependent block of ES cell differentiation appears to be cell autonomous; Pgk-Pem ES cells did not differentiate when mixed with normal, differentiating ES cells. A block to ES cell differentiation, resulting from the forced expression of Pem, can also be produced by the forced expression of the nonhomeodomain region of Pem. These studies are consistent with a role for Pem in regulating the transition between undifferentiated and differentiated cells of the early mouse embryo.