Ciliary neurotrophic factor maintains the pluripotentiality of embryonic stem cells

Hair Follicle Dermal Cells Support Expansion of Murine and Human Embryonic and Induced Pluripotent Stem Cells and Promote Haematopoiesis in Mouse Cultures

In the hair follicle, the dermal papilla (DP) and dermal sheath (DS) support and maintain proliferation and differentiation of the epithelial stem cells that produce the hair fibre. In view of their regulatory properties, in this study, we investigated the interaction between hair follicle dermal cells (DP and DS) and embryonic stem cells (ESCs); induced pluripotent stem cells (iPSCs); and haematopoietic stem cells. We found that coculture of follicular dermal cells with ESCs or iPSCs supported their prolonged maintenance in an apparently undifferentiated state as established by differentiation assays, immunocytochemistry, and RT-PCR for markers of undifferentiated ESCs. We further showed that cytokines that are involved in ESC support are also expressed by cultured follicle dermal cells, providing a possible explanation for maintenance of ES cell stemness in cocultures. The same cytokines were expressed within follicles in situ in a pattern more consistent with a role in follicle growth activities than stem cell maintenance. Finally, we show that cultured mouse follicle dermal cells provide good stromal support for haematopoiesis in an established coculture model. Human follicular dermal cells represent an accessible and readily propagated source of feeder cells for pluripotent and haematopoietic cells and have potential for use in clinical applications.

The many faces of Pluripotency: in vitro adaptations of a continuum of in vivo states

Pluripotency defines the propensity of a cell to differentiate into, and generate, all somatic, as well as germ cells. The epiblast of the early mammalian embryo is the founder population of all germ layer derivatives and thus represents the bona fide in vivo pluripotent cell population. The so-called pluripotent state spans several days of development and is lost during gastrulation as epiblast cells make fate decisions towards a mesoderm, endoderm or ectoderm identity. It is now widely recognized that the features of the pluripotent population evolve as development proceeds from the pre- to post-implantation period, marked by distinct transcriptional and epigenetic signatures. During this period of time epiblast cells mature through a continuum of pluripotent states with unique properties. Aspects of this pluripotent continuum can be captured in vitro in the form of stable pluripotent stem cell types. In this review we discuss the continuum of pluripotency existing within the mammalian embryo, using the mouse as a model, and the cognate stem cell types that can be derived and propagated in vitro. Furthermore, we speculate on embryonic stage-specific characteristics that could be utilized to identify novel, developmentally relevant, pluripotent states.

Lentivirus-mediated Persephin over-expression in Parkinson's disease rats

Persephin, together with glial cell line-derived neurotrophic factor and neurturin, has a neurotrophic effect and promotes the survival of motor neurons cultured in vitro. In this study, dopaminergic neurons in the substantia nigra of rats were transfected with the Persephin gene. One week later 6-hydroxydopamine was injected into the anterior medial bundle to establish a Parkinson's disease model in the rats. Results found that the number of dopaminergic neurons in the substantia nigra increased, tyrosine hydroxylase expression was upregulated and concentrations of dopamine and its metabolites in corpus striatum were increased after pretreatment with Persephin gene. In addition, the rotating effect of the induced Parkinson's disease rats was much less in the group pretreated with the Persephin gene. Persephin has a neuroprotective effect on the 6-hydroxydopamine-induced Parkinson's disease through protecting dopaminergic neurons.

LIF signal in mouse embryonic stem cells

Since the establishment of mouse embryonic stem cells (mESCs) in the 1980s, a number of important notions on the self-renewal of pluripotent stem cells in vitro have been found. In serum containing conventional culture, an exogenous cytokine, leukemia inhibitory factor (LIF), is absolutely essential for the maintenance of pluripotency. In contrast, in serum-free culture with simultaneous inhibition of Map-kinase and Gsk3 (so called 2i-culture), LIF is no longer required. However, recent findings also suggest that LIF may have a role not covered by the 2i for the maintenance of naïve pluripotency. These suggest that LIF functions for the maintenance of naïve pluripotency in a context dependent manner. We summarize how LIF-signal pathway is converged to maintain the naïve state of pluripotency.

LIF supports primitive endoderm expansion during pre-implantation development

Embryonic stem cells (ESCs) are pluripotent cell lines that can be maintained indefinitely in an early developmental state. ESC culture conditions almost all require the cytokine LIF to maintain self-renewal. As ESCs are not homogeneous, but contain multiple populations reminiscent of the blastocyst, identifying the target cells of LIF is necessary to understand the propagation of pluripotency. We recently found that LIF acts under self-renewing conditions to stimulate the fraction of ESCs that express extraembryonic markers, but has little impact on pluripotent gene expression. Here we report that LIF has two distinct roles. It blocks early epiblast differentiation and supports the expansion of primitive endoderm (PrE) primed ESCs and PrE in vivo. We find that activation of JAK/STAT signalling downstream of LIF occurs initially throughout the pre-implantation embryo, but later marks the PrE. Moreover, the addition of LIF to cultured embryos increases the GATA6+ PrE population while inhibition of JAK/STAT reduces both NANOG+ epiblast (Epi) and GATA6+ PrE. The reduction of the NANOG+ Epi may be explained by its precocious differentiation to later Epi derivatives, while the increase in PrE is mediated both by an increase in proliferation and inhibition of PrE apoptosis that is normally triggered in embryos with an excess of GATA6+ cells. Thus, it appears that the relative size of the PrE is determined by the number of LIF-producing cells in the embryo. This suggests a mechanism by which the embryo adjusts the relative ratio of the primary lineages in response to experimental manipulation.

Ubiquitin-mediated regulation of JAK-STAT signaling in embryonic stem cells

LIF activates several intracellular signaling pathways including JAK-STAT, PI3K/AKT and MAPK pathways. LIF is an important cytokine for maintenance of pluripotency and self-renewal of mouse ES cells. The JAK-STAT signal plays a key role in maintenance of the pluripotency of ESCs. Recent evidence shows that several post-translational modifications regulate activation or inhibition of intracellular signal transductions. The JAK-STAT signal is also modulated by several modifications including phosphorylation, acetylation and ubiquitination. In this review, we discuss regulation of the LIF-mediated-JAK-STAT signaling pathway that contributes to self-renewal of pluripotent ESCs.

Matrix remodeling maintains embryonic stem cell self-renewal by activating Stat3

While a variety of natural and synthetic matrices have been used to influence embryonic stem cell (ESC) self-renewal or differentiation, and ESCs also deposit a rich matrix of their own, the mechanisms behind how extracellular matrix affects cell fate are largely unexplored. The ESC matrix is continuously remodeled by matrix metalloproteinases (MMPs), a process that we find is enhanced by the presence of mouse embryonic fibroblast feeders in a paracrine manner. Matrix remodeling by MMPs aids in the self-renewal of ESCs, as inhibition of MMPs inhibits the ability of ESCs to self-renew. We also find that addition of the interstitial collagenase MMP1 is sufficient to maintain long-term leukemia inhibitory factor (LIF)-independent mouse ESC (mESC) self-renewal in a dose-dependent manner. This remarkable ability is due to the presence of endogenously produced self-renewal-inducing signals, including the LIF-family ligand ciliary neurotrophic factor, that are normally trapped within the ECM and become exposed upon MMP-induced matrix remodeling to signal through JAK and Stat3. These results uncover a new role for feeder cells in maintaining self-renewal and show that mESCs normally produce sufficient levels of autocrine-acting pro-self-renewal ligands.

The States of Pluripotency: Pluripotent Lineage Development in the Embryo and in the Dish

The pluripotent cell lineage of the embryo comprises a series of temporally and functionally distinct intermediary cell states, the epiblast precursor cell of the newly formed blastocyst, the epiblast population of the inner cell mass, and the early and late epiblast of the postimplantation embryo, referred to here as early and late primitive ectoderm. Pluripotent cell populations representative of the embryonic populations can be formed in culture. Although multiple pluripotent cell states are now recognised, little is known about the signals and pathways that progress cells from the epiblast precursor cell to the late primitive ectoderm in the embryo or in culture. The characterisation of cell states is most advanced in mouse where conditions for culturing distinct pluripotent cell states are well established and embryonic material is accessible. This review will focus on the pluripotent cell states present during embryonic development in the mouse and what is known of the mechanisms that regulate the progression of the lineage from the epiblast precursor cell and the ground state of pluripotency to the late primitive ectoderm present immediately prior to cell differentiation.

The contribution of leukemia inhibitory factor (LIF) for embryo implantation differs among strains of mice

Despite of the claim that maternal leukemia inhibitory factor (LIF) - a member of interleukin 6 (IL6) family of cytokines - plays indispensable roles for murine embryo implantation, these roles remain undefined in humans because the potency of LIF on implantation appears to vary among individuals. Here, we showed that the contribution of LIF for murine implantation was dependent on the strains of mice (ICR, C57BL/6J (B6), ddY, BALB/c, DBA/2Cr and MF1 strains). Inhibition of LIF during the implantation period caused severe disruption of embryo implantation in B6 and MF1 strains. Implantation was partly disrupted in other strains, but some embryos were implanted successfully. We speculated that other IL6 family members compensate for LIF actions on implantation in ICR, ddY, BALB/c, and DBA/2Cr strains. Indeed, the expression level of Ctf1 was upregulated by blockage of LIF function. CT-1 (encoded by Ctf1) treatment induced successful implantation without LIF in delayed implantation mice (ICR and B6) via phosphorylation of the signal transducer and activator of transcription 3 (STAT3) in the uterine luminal epithelium. Simultaneous inhibition of LIF and CT-1 did not block implantation completely in ICR mice, indicating that embryo implantation in this strain was robustly protected by LIF, CT-1 and other potential STAT3 activators. The present study might provide an explanation for the individual variation in the potency of LIF for embryo implantation in humans.

Genetic and epigenetic instability of stem cells

Recently, research on stem cells has been receiving an increasing amount of attention, both for its advantages and disadvantages. Genetic and epigenetic instabilities among stem cells have been a recurring obstacle to progress in regenerative medicine using stem cells. Various reports have stated that these instabilities can transform stem cells when transferred in vivo and thus have the potential to develop tumors. Previous research has shown that various extrinsic and intrinsic factors can contribute to the stability of stem cells. The extrinsic factors include growth supplements, growth factors, oxygen tension, passage technique, and cryopreservation. Controlling these factors based on previous reports may assist researchers in developing strategies for the production and clinical application of "safe" stem cells. On the other hand, the intrinsic factors can be unpredictable and uncontrollable; therefore, to ensure the successful use of stem cells in regenerative medicine, it is imperative to develop and implement appropriate strategies and technique for culturing stem cells and to confirm the genetic and epigenetic safety of these stem cells before employing them in clinical trials.

Regulation of embryonic stem cell self-renewal and pluripotency by leukaemia inhibitory factor

LIF (leukaemia inhibitory factor) is a key cytokine for maintaining self-renewal and pluripotency of mESCs (mouse embryonic stem cells). Upon binding to the LIF receptor, LIF activates three major intracellular signalling pathways: the JAK (Janus kinase)/STAT3 (signal transducer and activator of transcription 3), PI3K (phosphoinositide 3-kinase)/AKT and SHP2 [SH2 (Src homology 2) domain-containing tyrosine phosphatase 2]/MAPK (mitogen-activated protein kinase) pathways. These pathways converge to orchestrate the gene expression pattern specific to mESCs. Among the many signalling events downstream of the LIF receptor, activation and DNA binding of the transcription factor STAT3 plays a central role in transducing LIF's functions. The fundamental role of LIF for pluripotency was highlighted further by the discovery that LIF accelerates the conversion of epiblast-derived stem cells into a more fully pluripotent state. In the present review, we provide an overview of the three major LIF signalling pathways, the molecules that interact with STAT3 and the current interpretations of the roles of LIF in pluripotency.

Embryo implantation is blocked by intraperitoneal injection with anti-LIF antibody in mice

Leukemia inhibitory factor (LIF) is essential for embryo implantation in mice and plays an important role in other mammals including humans. Intraperitoneal (i.p.) injections with anti-LIF antibody (7.5 µg/g body weight, 3 times) between D3 (D1 = day of vaginal plug detection) and D4 effectively blocked embryo implantation; complete inhibition was achieved in C57BL/6J mice, and implantation was dramatically reduced in ICR mice (reduced to 27%). Normal rabbit IgG used as the control did not disturb embryo implantation. Anti-LIF antibody was localized not only in the stroma, but also in the luminal epithelium and the glandular lumen after i.p. injections. Growth-arrested blastocysts were recovered from the uterus without any implantation sites in both strains. Blastocysts made contact with the LE on the antimesometrial side; however, uterine stromal cells did not undergo secondary decidual reaction, and the uterine lumen was open, even at D7. Several regions of decidualization in ICR mice treated with anti-LIF antibody were smaller than those of the control, and development of blastocysts was delayed. The expression of LIF-regulated genes, such as immune-responsive gene-1 and insulin-like growth factor binding protein-3, was significantly decreased in C57BL/6J mice treated with anti-LIF antibody compared with the control, but not in ICR mice. The present study demonstrated that simple ip injections of an antibody are sufficient to block one of the important factors involved in embryo implantation in mice, and this method should also be easily applicable to the investigation of other factors involved in implantation.

Dynamic suspension culture for scalable expansion of undifferentiated human pluripotent stem cells

Human pluripotent (embryonic or induced) stem cells (hPSCs) have many potential applications, not only for research purposes but also for clinical and industrial uses. While culturing these cells as undifferentiated lines, an adherent cell culture based on supportive layers or matrices is most often used. However, the use of hPSCs for industrial or clinical applications requires a scalable, reproducible and controlled process. Here we present a suspension culture system for undifferentiated hPSCs, based on a serum-free medium supplemented with interleukins and basic fibroblast growth factor, suitable for the mass production of these cells. The described system supports a suspension culture of hPSC lines, in both static and dynamic cultures. Results showed that hPSCs cultured with the described dynamic method maintained all hPSC features after 20 passages, including stable karyotype and pluripotency, and increased in cell numbers by 25-fold in 10 d. Thus, the described suspension method is suitable for large-scale culture of undifferentiated hPSCs.

Effect of long-term culture of mouse embryonic stem cells under low oxygen concentration as well as on glycosaminoglycan hyaluronan on cell proliferation and differentiation

OBJECTIVES

Maintaining undifferentiated stem cells in defined conditions is of critical importance to improve their in vitro culture. We have evaluated the effects of culturing mouse stem (mES) cells under physiological oxygen concentration as well as by replacing fibroblast feeder layer (mEF) with gelatin or glycosaminoglycan hyaluronan (HA), on cell proliferation and differentiation.

MATERIALS AND METHODS

After 3 days culture or after long-term cell culture under different conditions, levels of apoptotic cell death were determined by cell cycle and TUNEL (TdT-mediated dUTP nick end labelling) assays and levels of cell proliferation by CFSE (5-(and-6)-carboxyfluorescein diacetate succinimidyl ester) labelling. We assessed spontaneous differentiation into cardiomyocytes and mRNA expression of pluripotency and differentiation biomarkers.

RESULTS

After 3 days culture under hypoxic conditions, levels of proliferation and apoptosis of mES cells were higher, in correlation with increase in intracellular reactive oxygen species. However, when cells were continuously grown for 1 month under those conditions, the level of apoptosis was, in all cases, under 4%. Hypoxia reduced spontaneous differentiation of mES into cardiomyocytes. Long-term culture on HA was more effective in maintaining the pluripotent state of the mES cells when compared to that on gelatin. Level of terminal differentiation was highest on mEF, intermediate on HA and lowest on gelatin.

CONCLUSIONS

Our data suggest that hypoxia is not necessary for maintaining pluripotency of mES cells and appeared to be detrimental during ES differentiation. Moreover, HA may offer a valuable alternative for long-term culture of mES cells in vitro.

Suspension culture of undifferentiated human embryonic and induced pluripotent stem cells

Alongside their contribution to research, human embryonic stem cells (hESC) may also prove valuable for cell-based therapies. Traditionally, these cells have been grown in adhesion culture either with or without feeder cells, allowing for their continuous growth as undifferentiated cells. However, to be applicable in therapy and industry they must be produced in a scalable and controlled process. Here we present for the first time a suspension culture system for undifferentiated hESC and induced pluripotent stem cells (iPSC), based on medium supplemented with the IL6RIL6 chimera (interleukin-6 receptor fused to interleukin-6), and basic fibroblast growth factor. Four hESC lines cultured in this system maintained all ESC features after 20 passages, including stable karyotype and pluripotency. Similar results were obtained when hESC were replaced with iPSC from two different cell lines. We demonstrate that the IL6RIL6 chimera supports the self-renewal and expansion of undifferentiated hESC and iPSC in suspension, and thus present another efficient system for large-scale propagation of undifferentiated pluripotent cells for clinical and translational applications.

Development of a pluripotent ES-like cell line from Asian sea bass (Lates calcarifer)--an oviparous stem cell line mimicking viviparous ES cells

We report a pluripotent embryonic stem cell-like cell line designated as SBES from blastula stage embryos of Asian sea bass (Lates calcarifer), which is an economically important cultivable and edible marine fish species in India. The SBES cells were cultured at 28 degrees C in Leibovitz L-15 medium supplemented with 20% fetal bovine serum without a feeder layer. The ES-like cells were round or polygonal and grew exponentially in culture. The SBES cells exhibited an intense alkaline phosphatase activity and expression of transcription factor Oct 4. The undifferentiated state of these cells was maintained at low seeding densities and the cells formed embryoid bodies when seeded in bacteriological plates. On treatment with all-trans retinoic acid, these cells differentiated into neuron-like cells, muscle cells, and beating cardiomyocytes, indicating their pluripotency. This embryonic ES-like cell line derived from an oviparous fish blastula conserved several peculiar features of viviparous mammalian embryonic stem cell lines. The present study highlights the importance and potential of piscine ES-like cell line for stem cell research without evoking ethical issues and invasive interventions sparing mammalian embryos.

Analysis of leukemia inhibitory factor and leukemia inhibitory factor receptor in embryonic and adult zebrafish (Danio rerio)

Leukemia inhibitory factor (LIF) is a member of the IL-6 cytokine family that functions in the survival, repair and formation of neurons as well as in the maintenance of neural and embryonic stem cells. The functions of LIF have been well documented in mammals, however until recently, the presence of IL-6 family cytokines in ectothermic vertebrates has only been speculated. We report on the identification of lif and lifr transcripts in the zebrafish and document the expression of these molecules in the developing embryos and tissues of adult zebrafish. We also examined the phylogenetic relationship between these molecules and other IL-6 cytokine family members known in mammals. In adult zebrafish, lif is expressed in the kidney and brain while lifr is expressed in the kidney, gill, brain, spleen and liver. During zebrafish embryogenesis, lif and lifr are both expressed as early as 12 hours postfertilization (hpf). In developing zebrafish, lif is expressed in the otic vesicle, retina and cranial sensory ganglia, and lifr is strongly expressed in the notochord, forebrain, otic vesicle, cranial ganglia and the retina. Morpholino knockdown of Lif and Lifr in developing embryos suggests that Lifr, but not Lif is required for proper neural development. lifr morpholino-injected embryos exhibit defects in the trigeminal, facial and vagal branchiomotor neurons, and improper axonal development as measured by acetylated tubulin staining. These embryos also display severe hydrocephaly by 48 hpf. This suggests that Lifrs are involved in proper neural development in zebrafish. This is the first evidence of the expression and role of an LIFR-like molecule in developing fish.

Concise Review: No Breakthroughs for Human Mesenchymal and Embryonic Stem Cell Culture: Conditioned Medium, Feeder Layer, or Feeder-Free; Medium with Fetal Calf Serum, Human Serum, or Enriched Plasma; Serum-Free, Serum Replacement Nonconditioned Medium, or Ad Hoc Formula? All That Glitters Is Not Gold!

The choice of an optimal strategy of stem cell culture is at the moment an impossible task, and the elaboration of a culture medium adapted to the production of embryonic and adult mesenchymal stem cells for the clinical application of cell therapy remains a crucial matter. To make an informed choice, it is crucial to not underestimate the theoretical health risk of using xenogenic compounds, to limit the immunological reactions once stem cells are transplanted, to not overestimate the controversial results obtained with human serum, plasma, and blood derivatives, as well as to carefully examine the pros and cons of serum-free and ad hoc formulation strategies; besides that, to also maintain multipotentiality, self-renewal, and transplantability. The extent to which we are able to achieve effective cell therapies will depend on assimilating a rapidly developing base of scientific knowledge with the practical considerations of design, delivery, and host response. Although clinical studies have already started, many questions remain unsolved, and concomitantly even more evidence on suitable and safe off-the-shelf products (mainly xeno-free) for embryonic and mesenchymal stem cells is cropping up, even though there should be no rush to enter the clinical stage while the underlying basic research is still not so solid; this solely will lead to high-quality translational research, without making blunders stemming from the assumption that all that glitters is not gold. Disclosure of potential conflicts of interest is found at the end of this article.

Retinoic acid and nerve growth factor induce differential regulation of nicotinic acetylcholine receptor subunit expression in SN56 cells

Retinoic acid (RA) and nerve growth factor (NGF) have multiple functions in the regulation of neuronal development. In the present study, we characterized the expression of different nicotinic acetylcholine receptor (nAChR) subtypes in the cholinergic SN56 cell line and investigated the roles of RA and NGF in the expression of choline acetyltransferase (ChAT) and different nAChR subtypes. The nAChR agonist [(3)H]epibatidine was bound to two sites, with apparent affinities of 13 and 380 pM. RT-PCR analysis revealed expression of alpha3, alpha4, alpha5, alpha7, beta2, and beta4 nAChR subunits. RA treatment induced morphological changes, and the mRNA level of ChAT was maximally elevated after 4 days of exposure. The density of [(3)H]epibatidine binding sites and the mRNA and protein level of the alpha3 and beta2 nAChR subunits were also increased by RA-induced differentiation. RA down-regulated the mRNA and protein level of the alpha4 nAChR subunit, whereas no significant change was observed in the mRNA and protein level of the alpha7 nAChR subunit. NGF treatment increased the mRNA and protein level of the alpha3 and beta2 nAChR subunits. No morphological effects of NGF were observed, and the mRNA level of ChAT and mRNA and protein level of the alpha4 and alpha7 nAChR subunits were not significantly altered. Validation was performed with real-time RT-PCR. The present results show that RA and NGF have different effects on the expression of ChAT and the morphology and the expression pattern of different nAChR subunits in cholinergic SN56 cells.

Cellular composition of long-term human spinal cord- and forebrain-derived neurosphere cultures

In vitro expanded neural precursor cells (NPCs) may provide a stable source for cell therapy. In search of the optimal cell source for spinal cord repair, we investigated influences of gestational age, regional heterogeneity, and long-term in vitro propagation. The cellular content of neurosphere cultures prior to and after in vitro differentiation was studied by immunocytochemistry and flow cytometry. Human forebrain and spinal cord NPCs deriving from first-trimester tissue were cultured as neurospheres in the presence of epidermal growth factor, basic fibroblast growth factor, and ciliary neurotrophic factor. Proteins characteristic for embryonic stem cells, i.e., Tra-1-60, Tra-1-81, and SSEA-4, were present in approximately 0.5% of the cells in donor tissues and neurospheres. The proportions of nestin- and proliferating cell nuclear antigen-immunoreactive (IR) cells were also maintained, whereas the CD133-IR population increased in vitro. Glial fibrillary acidic protein-IR cells increased in number, and in contrast the fraction of beta-tubulin III-IR cells decreased, at and beyond passage 5 in spinal cord but not forebrain cultures. However, dissociated and in vitro-differentiated forebrain- and spinal cord-derived neurospheres generated similar proportions of neurons, astrocytes, and oligodendrocytes. Gestational age of the donor tissue, which ranged from 4.5 to 12 weeks for forebrain and from 4.5 to 9.5 weeks for spinal cord, did not affect the proportion of cells with different phenotypes in culture. Thus, cellular composition of human neurosphere cultures differs as a result of long-term in vitro propagation and regional heterogeneity of source tissue, despite expansion under equal culture conditions. This could in turn imply that human spinal cord and forebrain NPCs present different repair potentials in in vivo settings.

Spatial organization of embryonic stem cell responsiveness to autocrine gp130 ligands reveals an autoregulatory stem cell niche

Highly ordered aggregates of cells, or niches, regulate stem cell fate. Specific tissue location need not be an obligatory requirement for a stem cell niche, particularly during embryogenesis, where cells exist in a dynamic environment. We investigated autoregulatory fixed-location-independent processes controlling cell fate by analyzing the spatial organization of embryonic stem cells (ESCs) using quantitative single-cell immunocytochemistry and a computational approach involving Delaunay triangulation. ESC colonies demonstrated radial organization of phosphorylated signal transducer and activator of transcription 3, Nanog, and Oct4 (among others) in the presence and absence of exogenous leukemia inhibitory factor (LIF). Endogenous self-renewal signaling resulted from autocrine non-LIF gp130 ligands, which buffered cells against differentiation upon exogenous LIF deprivation. Together with a radial organization of differential responsiveness to gp130 ligands within colonies, autocrine signaling produced a radial organization of self-renewal, generating a fixed-location-independent autoregulatory niche. These findings reveal fundamental properties of niches and elucidate mechanisms colonies of cells use to transition between fates during morphogenesis.

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.

Basic Fibroblast Growth Factor Supports Undifferentiated Human Embryonic Stem Cell Growth Without Conditioned Medium

Previous studies have shown that prolonged propagation of undifferentiated human embryonic stem cells (hESCs) requires conditioned medium from mouse embryonic feeders (MEF-CM) as well as matrix components. Because hESCs express growth factor receptors, including those for basic fibroblast growth factor (bFGF), stem cell factor (SCF), and fetal liver tyrosine kinase-3 ligand (Flt3L), we evaluated these and other growth factors for their ability to maintain undifferentiated hESCs in the absence of conditioned medium. We found cultures maintained in bFGF alone or in combination with other factors showed characteristics similar to MEF-CM control cultures, including morphology, surface marker and transcription factor expression, telomerase activity, differentiation, and karyotypic stability. In contrast, cells in media containing Flt-3L, thrombopoietin, and SCF, individually or in combination, showed almost complete differentiation after 6 weeks in culture. These data demonstrate that hESCs can be maintained in nonconditioned medium using growth factors.

Avian pluripotent stem cells

Pluripotent embryonic stem cells are undifferentiated cells capable of proliferation and self-renewal and have the capacity to differentiate into all somatic cell types and the germ line. They provide an in vitro model of early embryonic differentiation and are a useful means for targeted manipulation of the genome. Pluripotent stem cells in the chick have been derived from stage X blastoderms and 5.5 day gonadal primordial germ cells (PGCs). Blastoderm-derived embryonic stem cells (ESCs) have the capacity for in vitro differentiation into embryoid bodies and derivatives of the three primary germ layers. When grafted onto the chorioallantoic membrane, the ESCs formed a variety of differentiated cell types and attempted to organize into complex structures. In addition, when injected into the unincubated stage X blastoderm, the ESCs can be found in numerous somatic tissues and the germ line. The potential give rise to somatic and germ line chimeras is highly dependent upon the culture conditions and decreases with passage. Likewise, PGC-derived embryonic germ cells (EGCs) can give rise to simple embryoid bodies and can undergo some differentiation in vitro. Interestingly, chicken EG cells contribute to somatic lineages when injected into the stage X blastoderm, but only germ line chimeras have resulted from EGCs injected into the vasculature of the stage 16 embryo. To date, no lines of transgenic chickens have been generated using ESCs or EGCs. Nevertheless, progress towards the culture of avian pluripotent stem cells has been significant. In the future, the answers to fundamental questions regarding segregation of the avian germ line and the molecular basis of pluripotency should foster the full use of avian pluripotent stem cells.

Differences between human and mouse embryonic stem cells

We compared gene expression profiles of mouse and human ES cells by immunocytochemistry, RT-PCR, and membrane-based focused cDNA array analysis. Several markers that in concert could distinguish undifferentiated ES cells from their differentiated progeny were identified. These included known markers such as SSEA antigens, OCT3/4, SOX-2, REX-1 and TERT, as well as additional markers such as UTF-1, TRF1, TRF2, connexin43, and connexin45, FGFR-4, ABCG-2, and Glut-1. A set of negative markers that confirm the absence of differentiation was also developed. These include genes characteristic of trophoectoderm, markers of germ layers, and of more specialized progenitor cells. While the expression of many of the markers was similar in mouse and human cells, significant differences were found in the expression of vimentin, beta-III tubulin, alpha-fetoprotein, eomesodermin, HEB, ARNT, and FoxD3 as well as in the expression of the LIF receptor complex LIFR/IL6ST (gp130). Profound differences in cell cycle regulation, control of apoptosis, and cytokine expression were uncovered using focused microarrays. The profile of gene expression observed in H1 cells was similar to that of two other human ES cell lines tested (line I-6 and clonal line-H9.2) and to feeder-free subclones of H1, H7, and H9, indicating that the observed differences between human and mouse ES cells were species-specific rather than arising from differences in culture conditions.

Enhancement of Oligodendrocyte Differentiation from Murine Embryonic Stem Cells by an Activator of gp130 Signaling

Embryonic stem (ES) cells derived from the inner cell mass of blastocyst-stage embryos are a potential large scale source of oligodendrocytes and of their progenitors for transplantation into the central nervous system for the repair of demyelinating lesions. We found previously that interleukin-6 (IL-6) fused to its soluble receptor (IL-6R), a potent activator of the gp130 receptor, induces myelin gene expression in Schwann cells of embryonic dorsal root ganglia. Like leukemia inhibitory factor, IL-6R/IL-6 inhibits the differentiation of murine ES cells into embryoid bodies. In the present study, we show that this recombinant cytokine may be efficiently used to stimulate the differentiation of oligodendrocytes if added to ES cell-derived neural precursors. IL-6R/IL-6 leads to an increase in early chondroitin sulfate proteoglycan positive and late O4 positive progenitors and to a stimulation of maturation into O1 and myelin basic protein expressing oligodendrocytes. Expression of the genes for transcription factor genes Olig-1 and Sox10, which appear early in the oligodendrocyte lineage, was stimulated by IL-6R/IL-6 addition. We conclude that this cytokine can significantly enhance the derivation of oligodendrocytes from ES cells.

Identification of a biological activity that supports maintenance and proliferation of pluripotent cells from the primitive ectoderm of the mouse

Pluripotent cell development in the mammalian embryo results in the sequential formation of several developmentally distinct populations, inner cell mass, primitive ectoderm, and the primordial germ lineage. Factors within medium conditioned by HepG2 cells (MEDII) have been implicated in the formation and maintenance of primitive ectoderm from inner cell mass cells both in vitro and in vivo. Here we demonstrate that MEDII, but not LIF, is able to support the maintenance and proliferation in culture of pluripotent cells derived from primitive ectoderm formed in vitro or during embryonic development. This distinguishes primitive ectoderm and inner cell mass (ICM) on the basis of cytokine responsiveness and validates the biological activity proposed for factors within MEDII in primitive ectoderm establishment and maintenance. Further, it potentially provides an alternative technology for the isolation of pluripotent cells from the mammalian embryo.

Gene expression profiling of ciliary neurotrophic factor-overexpressing rat striatal progenitor cells (ST14A) indicates improved stress response during the early stage of differentiation

Neuronal progenitor cells delivering neurotrophic factors are a promising therapeutic tool for treatment of neurodegenerative diseases. Although several promising results have come from studies in different animal models, detailed knowledge of the action of neurotrophic factors in the CNS is still lacking. A clonally derived, immortalized rat striatal cell line (ST14A) expressing ciliary neurotrophic factor (CNTF) offers a stable and controlled background with which to analyze CNTF actions on the transcriptional level in CNS progenitor cells. To identify early transcriptional changes induced by CNTF expression, we transfected the CNTF gene into ST14A cells, which differentiate at the nonpermissive temperature of 39 degrees C via suppression of the immortalizing SV40 large T antigen. This shows a CNTF-dependent hypoxic/ischemic stress response during the earliest stage of differentiation, with expression of specific transcripts and evidence of translational repression leading to decreased protein synthesis in the transfected cells. This process is mediated by the Ras/MAP kinase pathway and is accompanied by impaired proliferation and metabolism as well as signs of neuronal differentiation. The stress-like response in the early stage of differentiation improves the ability of the transfected cells to respond to and cope with a stressful environment in vivo. The present data indicate higher viability, longer life, and greater differentiation capacity of CNTF-ST14A cells if they are used for transplantation. We conclude that the stress-like response during the early stage of differentiation improves the ability of the CNTF-ST14A cells to respond and adapt to a stressful environment, which renders them useful candidate cells for in vivo trials of treatment for neurodegenerative diseases in animal models, e.g., of Huntington's disease.

Early intracellular events induced by in vivo leptin treatment in mouse skeletal muscle

Experimental evidence suggests that leptin may exert direct effects on peripheral tissues. In this study we investigated some transductional molecules in skeletal muscle, after intraperitoneal leptin injection in wild-type and ob/ob mice. By immunoprecipitation and immunoblotting with anti-phosphotyrosine antibodies, we observed a modified pattern of phosphotyrosine proteins. We then identified an increase in JAK2, IRS1 and IRS2 tyrosine-phosphorylation and in their association with p85, a subunit of PI3K. The increase in PI3K activity in immunoprecipitated p85 did not reach statistical significance, however, both Akt and GSK3 resulted significantly hyper-phosphorylated. Bad, an Akt substrate involved in cell survival, appeared modified in its phosphorylation. ERK1, ERK2 and p38 MAP kinase phosphorylation significantly increased, even if the latter only in wild-type animals. Finally, by EMSA experiments, we documented that leptin increased the DNA binding capacity of Stat3 homodimers and AP-1. Thus, leptin appears to activate, within minutes, some insulin signalling molecules. Stat3 and AP-1 activation by gene expression remodelling could subsequently trigger more leptin-specific effects. Further, leptin might play a still underestimated role in cell survival.

Neural stem cells in aging and disease

Aging in the central nervous system is associated with progressive loss of function which is exacerbated by neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. The two primary cell replacement strategies involve transplantation of exogenous tissue, and activation of proliferation of endogenous cells. Transplanted tissue is used to either directly replace lost tissue, or to implant genetically engineered cells that secrete factors which promote survival and/or proliferation. However, successful application of any cell replacement therapy requires knowledge of the complex relationships between neural stem cells and the more restricted neural and glial progenitor cells. This review focuses on recent advances in the field of stem cell biology of the central nervous system, with an emphasis on cellular and molecular approaches to replacing cells lost in neurodegenerative disorders.

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.

Pluripotent stem cells--model of embryonic development, tool for gene targeting, and basis of cell therapy

Embryonic stem (ES) cells are pluripotent cell lines with the capacity of self-renewal and a broad differentiation plasticity. They are derived from pre-implantation embryos and can be propagated as a homogeneous, uncommitted cell population for an almost unlimited period of time without losing their pluripotency and their stable karyotype. Murine ES cells are able to reintegrate fully into embryogenesis when returned into an early embryo, even after extensive genetic manipulation. In the resulting chimeric offspring produced by blastocyst injection or morula aggregation, ES cell descendants are represented among all cell types, including functional gametes. Therefore, mouse ES cells represent an important tool for genetic engineering, in particular via homologous recombination, to introduce gene knock-outs and other precise genomic modifications into the mouse germ line. Because of these properties ES cell technology is of high interest for other model organisms and for livestock species like cattle and pigs. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have yet been established for vertebrate species other than the mouse (Evans and Kaufman, 1981; Martin, 1981) and chicken (Pain et al., 1996). The in vitro differentiation capacity of ES cells provides unique opportunities for experimental analysis of gene regulation and function during cell commitment and differentiation in early embryogenesis. Recently, pluripotent stem cells were established from human embryos (Thomson et al., 1998) and early fetuses (Shamblott et al., 1998), opening new scenarios both for research in human developmental biology and for medical applications, i.e. cell replacement strategies. At about the same time, research activities focused on characteristics and differentiation potential of somatic stem cells, unravelling an unexpected plasticity of these cell types. Somatic stem cells are found in differentiated tissues and can renew themselves in addition to generating the specialized cell types of the tissue from which they originate. Additional to discoveries of somatic stem cells in tissues that were previously not thought to contain these kinds of cells, they also appear to be capable of developing into cell types of other tissues, but have a reduced differentiation potential as compared to embryo-derived stem cells. Therefore, somatic stem cells are referred to as multipotent rather than pluripotent. This review summarizes characteristics of pluripotent stem cells in the mouse and in selected livestock species, explains their use for genetic engineering and basic research on embryonic development, and evaluates their potential for cell therapy as compared to somatic stem cells.

Embryonic stem cells provide a powerful and versatile model system

Embryonic stem (ES) cells are pluripotent stem cells that differentiate both in vitro and in vivo into cell types derived from each of the three embryonic germ layers. ES cells and their close relatives, embryonal carcinoma (EC) cells and embryonic germ (EG) cells, have been used extensively as model systems for studying early mammalian development. This work has led to important insights into the mechanisms that control embryogenesis at the molecular and cellular levels. This chapter focuses on the use of ES cells as an in vitro model system for studying cellular differentiation and reviews several areas where important progress has been made. Impressive progress has been made in the isolation and characterization of ES cells from many species, including humans. Significant progress has also been made in the development of culture conditions that help direct the differentiation of ES cells to specific cell types that form during myogenesis, angiogenesis, hematopoiesis, neurogenesis, and cardiogenesis. The ability to inactivate virtually any gene in ES cells by gene targeting has vastly improved our understanding of the roles played by specific genes at the cellular and organismic levels. Moreover, ES cells and EC cells have been used widely to investigate how specific genes are turned on and turned off in the course of differentiation. In this connection, DNA array technology has been used to identify genes regulated when ES cells differentiate. The final section of this chapter discusses how work with ES cells is shaping our understanding of stem cells, mammalian development, and cell replacement therapy.

Mouse ES cells: experimental exploitation of pluripotent differentiation potential

Pluripotent ES cells can be used to generate a wide variety of cell populations in vitro in a manner resembling embryonic development. Recent advances in controlling ES cell differentiation, combined with the power of genetic and biochemical manipulation, are providing insights into cell biology and the determination of cell fate.

The ciliary neurotrophic factor and its receptor, CNTFR alpha

Ciliary neurotrophic factor (CNTF) is expressed in glial cells within the central and peripheral nervous systems. CNTF stimulates gene expression, cell survival or differentiation in a variety of neuronal cell types such as sensory, sympathetic, ciliary and motor neurons. In addition, effects of CNTF on oligodendrocytes as well as denervated and intact skeletal muscle have been documented. CNTF itself lacks a classical signal peptide sequence of a secreted protein, but is thought to convey its cytoprotective effects after release from adult glial cells by some mechanism induced by injury. Interestingly, mice that are homozygous for an inactivated CNTF gene develop normally and initially thrive. Only later in adulthood do they exhibit a mild loss of motor neurons with resulting muscle weakness, leading to the suggestion that CNTF is not essential for neural development, but instead acts in response to injury or other stresses. The CNTF receptor complex is most closely related to, and shares subunits with the receptor complexes for interleukin-6 and leukemia inhibitory factor. The specificity conferring alpha subunit of the CNTF complex (CNTFR alpha), is extremely well conserved across species, and has a distribution localized predominantly to the nervous system and skeletal muscle. CNTFR alpha lacks a conventional transmembrane domain and is thought to be anchored to the cell membrane by a glycosyl-phosphatidylinositol linkage. Mice lacking CNTFR alpha die perinatally, perhaps indicating the existence of a second developmentally important CNTF-like ligand. Signal transduction by CNTF requires that it bind first to CNTFR alpha, permitting the recruitment of gp130 and LIFR beta, forming a tripartite receptor complex. CNTF-induced heterodimerization of the beta receptor subunits leads to tyrosine phosphorylation (through constitutively associated JAKs), and the activated receptor provides docking sites for SH2-containing signaling molecules, such as STAT proteins. Activated STATs dimerize and translocate to the nucleus to bind specific DNA sequences, resulting in enhanced transcription of responsive genes. The neuroprotective effects of CNTF have been demonstrated in a number of in vitro cell models as well as in vivo in mutant mouse strains which exhibit motor neuron degeneration. Intracerebral administration of CNTF and CNTF analogs has also been shown to protect striatal output neurons in rodent and primate models of Huntington's disease. Treatment of humans and animals with CNTF is also known to induce weight loss characterized by a preferential loss of body fat. When administered systemically, CNTF activates downstream signaling molecules such as STAT-3 in areas of the hypothalamus which regulate food intake. In addition to its neuronal actions, CNTF and analogs have been shown to act on non-neuronal cells such as glia, hepatocytes, skeletal muscle, embryonic stem cells and bone marrow stromal cells.

Establishment of SSEA-1- and Oct-4-expressing rat embryonic stem-like cell lines and effects of cytokines of the IL-6 family on clonal growth

Here, we demonstrate long-term cultivation of alkaline phosphatase-positive rat embryonic stem-like (RES) cell lines. RES cells were characterized by their typical growth in highly compacted cell clusters, which were found to be sensitive against enzymatic dissociation. RES cells expressed stage-specific embryonic antigen-1 (SSEA-1) and transcription factor Oct-4, but Oct-4 mRNA was detected at lower levels compared to mouse ES cells. Once established to tissue culture, RES cells were able to grow in the absence of feeder cells under clonal conditions. Cytokines of the interleukin-6 family known to maintain the undifferentiated state of mouse ES cells were comparatively analyzed for their capacity to maintain the undifferentiated growth of two cell lines, RES-1 and RES-15, in a clonal assay. Rat ciliary neurotrophic factor (rCNTF), human oncostatin M (hOSM), and interleukin-6 and soluble interleukin-6 receptor (IL-6/sIL-6R) were found to support clonal growth of RES cells, but the cytokines did not reach the efficiency of the colony forming ability of leukemia inhibitory factor (LIF). When RES-1 and RES-15 cells were cultivated without feeder cells, SSEA-1 expression was maintained after clonal growth in the presence of LIF and LIF + rCNTF, respectively. Oct-4 mRNA was significantly detected in RES-15 cells when cultivated in the absence of feeder cells in media substituted by LIF and/or IL-6/sIL-6R, as well as without cytokines. In summary, rat embryonic stem-like cell lines could be established from rat blastocysts and were able to proliferate as undifferentiated alkaline phosphatase-positive cells. Embryonal stem cell properties, such as SSEA-1 and Oct-4 expression, were maintained by members of the IL-6 family of cytokines, but most significantly by LIF.

Functional interaction of STAT3 transcription factor with the cell cycle inhibitor p21WAF1/CIP1/SDI1

Signal transducers and activators of transcription (STAT) factors are cytoplasmic proteins that induce gene activation in response to cytokine receptor stimulation. Following tyrosine phosphorylation, STAT proteins dimerize, translocate into the nucleus, and activate specific target genes. Activation is transient, and down-regulation of STAT signaling occurs within a few hours. In the present study, we show that the cyclin-dependent kinase inhibitor p21(WAF1/CIP1/SDI1) inhibits STAT3 transcriptional activation. Following leukemia inhibitory factor stimulation, p21(WAF1/CIP1/SDI1) was found to associate with STAT3 proteins in coimmunoprecipitation and pull down assays. In vivo, overexpression of p21(WAF1/CIP1/SDI1) reduced transcriptional activation by STAT3 proteins but did not modify DNA binding activity. Interestingly, pull down experiments showed that p21(WAF1/CIP1/SDI1) could interact with the CREB-binding coactivator protein, and inhibition of STAT3 activity by p21(WAF1/CIP1/SDI1) did not occur when CREB-binding protein was overexpressed. These results suggest a model by which p21(WAF1/CIP1/SDI1) functions as an inhibitor of STAT3 signaling and highlight a new activity for this cyclin-dependent kinase inhibitor.

The ciliary neurotrophic factor and its receptor, CNTFR alpha

Ciliary neurotrophic factor (CNTF) is expressed in glial cells within the central and peripheral nervous systems. CNTF stimulates gene expression, cell survival or differentiation in a variety of neuronal cell types such as sensory, sympathetic, ciliary and motor neurons. In addition, effects of CNTF on oligodendrocytes as well as denervated and intact skeletal muscle have been documented. CNTF itself lacks a classical signal peptide sequence of a secreted protein, but is thought to convey its cytoprotective effects after release from adult glial cells by some mechanism induced by injury. Interestingly, mice that are homozygous for an inactivated CNTF gene develop normally and initially thrive. Only later in adulthood do they exhibit a mild loss of motor neurons with resulting muscle weakness, leading to the suggestion that CNTF is not essential for neural development, but instead acts in response to injury or other stresses. The CNTF receptor complex is most closely related to, and shares subunits with the receptor complexes for interleukin-6 and leukemia inhibitory factor. The specificity conferring alpha subunit of the CNTF complex (CNTFR alpha), is extremely well conserved across species, and has a distribution localized predominantly to the nervous system and skeletal muscle. CNTFR alpha lacks a conventional transmembrane domain and is thought to be anchored to the cell membrane by a glycosyl-phosphatidylinositol linkage. Mice lacking CNTFR alpha die perinatally, perhaps indicating the existence of a second developmentally important CNTF-like ligand. Signal transduction by CNTF requires that it bind first to CNTFR alpha, permitting the recruitment of gp130 and LIFR beta, forming a tripartite receptor complex. CNTF-induced heterodimerization of the beta receptor subunits leads to tyrosine phosphorylation (through constitutively associated JAKs), and the activated receptor provides docking sites for SH2-containing signaling molecules, such as STAT proteins. Activated STATs dimerize and translocate to the nucleus to bind specific DNA sequences, resulting in enhanced transcription of responsive genes. The neuroprotective effects of CNTF have been demonstrated in a number of in vitro cell models as well as in vivo in mutant mouse strains which exhibit motor neuron degeneration. Intracerebral administration of CNTF and CNTF analogs has also been shown to protect striatal output neurons in rodent and primate models of Huntington's disease. Treatment of humans and animals with CNTF is also known to induce weight loss characterized by a preferential loss of body fat. When administered systemically, CNTF activates downstream signaling molecules such as STAT-3 in areas of the hypothalamus which regulate food intake. In addition to its neuronal actions, CNTF and analogs have been shown to act on non-neuronal cells such as glia, hepatocytes, skeletal muscle, embryonic stem cells and bone marrow stromal cells.

Human embryonic stem cell and embryonic germ cell lines

Undifferentiated human embryonic stem (ES) cells and embryonic germ (EG) cells can be cultured indefinitely and yet maintain the potential to form many or all of the differentiated cells in the body. Human ES and EG cells provide an exciting new model for understanding the differentiation and function of human tissue, offer new strategies for drug discovery and testing, and promise new therapies based on the transplantation of ES and EG cell-derived tissues.

Cytokine signal transduction in P19 embryonal carcinoma cells: regulation of Stat3-mediated transactivation occurs independently of p21ras-Erk signaling

Ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) are members of a subfamily of related cytokines that share gp130 as common signal-transducing receptor component. CNTF has recently been demonstrated to induce increased survival and neuronal differentiation of P19 embryonal carcinoma (EC) cells; however, the molecular mechanisms underlying these effects are still elusive. Here we report that CNTF and LIF, but not interleukin-6, activated signal transducers and activators of transcription (STAT)-reporter constructs in P19 EC cells. Supershift analysis revealed that the STAT-element binding complex contained the transcription factor Stat3. Binding of Stat3 was inhibited by protein tyrosine kinase inhibitors, but not by the broad serine/threonine protein kinase inhibitor, H7. However, H7 inhibited CNTF-induced Stat3 transactivation. Using a dominant-negative p21ras construct and a specific inhibitor of mitogen-activated protein kinase kinase (MEK; PD098059) we demonstrated that CNTF-induced Stat3 transactivation was independent of the p21ras-mitogen-activated protein kinase (MAPK) pathway, while CNTF-induced MAPK activation was p21ras- and MEK-dependent. Taken together, our results demonstrate the activation of the p21ras-MAPK and STAT signal transduction pathways in response to CNTF and LIF in P19 EC cells and reveal that there is no modulating crosstalk between these pathways. Furthermore, our data suggest that CNTF- and LIF-induced Stat3 activation in P19 EC cells involves an H7-sensitive p21ras/MAPK- and Ca(2+)-independent kinase.

In vitro expansion of a multipotent population of human neural progenitor cells

The isolation and expansion of human neural progenitor cells have important potential clinical applications, because these cells may be used as graft material in cell therapies to regenerate tissue and/or function in patients with central nervous system (CNS) disorders. This paper describes a continuously dividing multipotent population of progenitor cells in the human embryonic forebrain that can be propagated in vitro. These cells can be maintained and expanded using a serum-free defined medium containing basic fibroblast growth factor (bFGF), leukemia inhibitory factor (LIF), and epidermal growth factor (EGF). Using these three factors, the cell cultures expand and remain multipotent for at least 1 year in vitro. This period of expansion results in a 10(7)-fold increase of this heterogeneous population of cells. Upon differentiation, they form neurons, astrocytes, and oligodendrocytes, the three main phenotypes in the CNS. Moreover, GABA-immunoreactive and tyrosine hydroxylase-immunoreactive neurons can be identified. These results demonstrate the feasibility of long-term in vitro expansion of human neural progenitor cells. The advantages of such a population of neural precursors for allogeneic transplantation include the ability to provide an expandable, well-characterized, defined cell source which can form specific neuronal or glial subtypes.

Formation of a primitive ectoderm like cell population, EPL cells, from ES cells in response to biologically derived factors

The primitive ectoderm of the mouse embryo arises from the inner cell mass between 4.75 and 5.25 days post coitum, around the time of implantation. Positioned at a pivotal time in development, just prior to formation of the three germ layers of the embryo proper, the primitive ectoderm responds directly to the signals generated during gastrulation. We have identified a conditioned medium, MEDII, which caused the homogeneous conversion of ES cells to a morphologically distinct cell population, termed early primitive ectoderm-like (EPL) cells. EPL cells expressed the pluripotent cell markers Oct4, SSEA1 and alkaline phosphatase. However, the formation of EPL cells was accompanied by alterations in Fgf5, Gbx2 and Rex1 expression, a loss in chimaera forming ability, changes in factor responsiveness and modified differentiation capabilities, all consistent with the identification of EPL cells as equivalent to the primitive ectoderm population of the 5.5 to 6.0 days post coitum embryo. EPL cell formation could be reversed in the presence of LIF and withdrawal of MEDII, which suggested that EPL cell formation was not a terminal differentiation event but reflected the ability of pluripotent cells to adopt distinct cell states in response to specific factors. Partial purification of MEDII revealed the presence of two separable biological activities, both of which were required for the induction and maintenance of EPL cells. We show here the first demonstration of uniform differentiation of ES cells in response to biological factors. The formation of primitive ectoderm, both in vivo and in vitro, appears to be an obligatory step in the differentiation of the inner cell mass or ES cells into cell lineages of the embryonic germ layers. EPL cells potentially represent a model for the development of lineage specific differentiation protocols and analysis of gastrulation at a molecular level. An understanding of the active components of MEDII may provide a route for the identification of factors which induce primitive ectoderm formation in vivo.

Paracrine induction of stem cell renewal by LIF-deficient cells: a new ES cell regulatory pathway

The propagation of pluripotential mouse embryonic stem (ES) cells is sustained by leukemia inhibitory factor (LIF) or related cytokines that act through a common receptor complex comprising the LIF receptor subunit (LIF-R) and the signal transducer gp130. However, the findings that embryos lacking LIF-R or gp130 can develop beyond gastrulation argue for the existence of an alternative pathway(s) governing the maintenance of pluripotency in vivo. In order to define those factors that contribute to self-renewal in ES cell cultures, we have generated ES cells in which both copies of the lif gene are deleted. These cells showed a significantly reduced capacity for regeneration of stem cell colonies when induced to differentiate, confirming that LIF is the major endogenous regulatory cytokine in ES cell cultures. However, self-renewal was not abolished and undifferentiated ES cell colonies were still obtained in the complete absence of LIF. A differentiated, LIF-deficient, parietal endoderm-like cell line was derived and shown to support ES cell propagation via production of a soluble, macromolecular, trypsin-sensitive activity. This activity, which we name ES cell renewal factor (ESRF), is distinct from members of the IL-6/LIF family because (i) it is effective on ES cells lacking LIF-R; (ii) it is not blocked by anti-gp130 neutralizing antibodies; and (iii) it acts without activation of STAT3. ES cells propagated clonally using ESRF alone can contribute fully to chimaeras and engender germline transmission. These findings establish that ES cell pluripotency can be sustained via a LIF-R/gp130-independent, STAT-3 independent, signaling pathway. Operation of this pathway in vivo could play an important role in the regulation of pluripotency in the epiblast and account for the viability of lifr -/- and gp130 -/- embryos.

Self-renewal of pluripotent embryonic stem cells is mediated via activation of STAT3

The propagation of embryonic stem (ES) cells in an undifferentiated pluripotent state is dependent on leukemia inhibitory factor (LIF) or related cytokines. These factors act through receptor complexes containing the signal transducer gp130. The downstream mechanisms that lead to ES cell self-renewal have not been delineated, however. In this study, chimeric receptors were introduced into ES cells. Biochemical and functional studies of transfected cells demonstrated a requirement for engagement and activation of the latent trancription factor STAT3. Detailed mutational analyses unexpectedly revealed that the four STAT3 docking sites in gp130 are not functionally equivalent. The role of STAT3 was then investigated using the dominant interfering mutant, STAT3F. ES cells that expressed this molecule constitutively could not be isolated. An episomal supertransfection strategy was therefore used to enable the consequences of STAT3F expression to be examined. In addition, an inducible STAT3F transgene was generated. In both cases, expression of STAT3F in ES cells growing in the presence of LIF specifically abrogated self-renewal and promoted differentiation. These complementary approaches establish that STAT3 plays a central role in the maintenance of the pluripotential stem cell phenotype. This contrasts with the involvement of STAT3 in the induction of differentiation in somatic cell types. Cell type-specific interpretation of STAT3 activation thus appears to be pivotal to the diverse developmental effects of the LIF family of cytokines. Identification of STAT3 as a key transcriptional determinant of ES cell self-renewal represents a first step in the molecular characterization of pluripotency.

The neurotrophins and neuropoietic cytokines: two families of growth factors acting on neural and hematopoietic cells

Recent progress has revealed similarities between the receptors and signaling systems used by neurotrophic factors as compared to other growth factors and cytokines. The neurotrophins use a family of receptor tyrosine kinases known as the Trk receptors, whereas ciliary neurotrophic factor (CNTF) uses a "cytokine receptor" system that shares receptor components with a number of distantly related cytokines. We have used a human embryonal carcinoma cell line and human leukemia cell lines to examine the actions of the neurotrophins and CNTF on cellular differentiation. Our findings demonstrate that specific combinations of neurotrophic factors are required to influence the neuronal progenitor cells to become postmitotic mature CNS neurons. Such synergistic interactions may play an important role in modulating the differentiation of a wide assortment of neuronal precursors in the developing nervous system. Furthermore, our studies with leukemia cells suggest that neurotrophic factors may play a similar role in hematopoietic differentiation and that these factors may have therapeutic application in leukemia differentiation.

Transcription factor Stat5 is an early marker of differentiation of murine embryonic stem cells

Embryonic stem (ES) cells are pluripotent descendants of the inner cell mass of blastocysts capable of differentiating into progenitor cells of most if not all tissues. The pluripotency of ES cells is maintained by leukemia inhibitory factor (LIF), a member of the family of interleukin-6-type cytokines. These cytokines activate Janus tyrosine kinases and signal transducer and activator of transcription factors (Stat) via the signalling receptor component gp130. Pluripotent ES1 cells proliferating in the presence of LIF were known from previous studies to contain Stat3 and Stat1 capable of transcriptional activation. Here we report that the level of tyrosine-phosphorylated Stat3 decreases rapidly during differentiation induced by treatment of ES1 cells either with retinoic acid (RA) or by withdrawal of LIF. In line with this finding, the DNA-binding activity of Stat3 decreased during differentiation. In contrast, Stat5 was absent from pluripotent proliferating ES cells, but appeared early after induction of differentiation. The positive correlation between induction of differentiation and expression of Stat5 mRNA was confirmed for three independent ES cell lines. Stat5 transcripts were detectable in ES1 cells as early as 12 h after treatment with RA and 36 h after withdrawal of LIF. Stat5 protein was detectable 2 days after the onset of differentiation. These results establish Stat5 as a novel marker of very early stages of differentiation of ES cells.

Primate embryonic stem cells

Primate embryonic stem (ES) cells are derived from preimplantation embryos, have a normal karyotype, and are capable of indefinite, undifferentiated proliferation. Even after culture for more than a year, primate ES cells maintain the potential to differentiate to trophoblast and derivatives of embryonic endoderm, mesoderm, and ectoderm. In this review, we compare the characteristics of ES cell lines from two primate species, the rhesus monkey (Macaca mulatta) and the common marmoset (Callithrix jacchus), with the characteristics of mouse ES cells and human embryonal carcinoma cells. We also discuss the implications of using primate ES cells to understand early human development and discuss the practical and ethical implications for the understanding and treatment of human disease.