An expedition in the jungle of pluripotent stem cells of non-human primates

For nearly three decades, more than 80 embryonic stem cell lines and more than 100 induced pluripotent stem cell lines have been derived from New World monkeys, Old World monkeys, and great apes. In this comprehensive review, we examine these cell lines originating from marmoset, cynomolgus macaque, rhesus macaque, pig-tailed macaque, Japanese macaque, African green monkey, baboon, chimpanzee, bonobo, gorilla, and orangutan. We outline the methodologies implemented for their establishment, the culture protocols for their long-term maintenance, and their basic molecular characterization. Further, we spotlight any cell lines that express fluorescent reporters. Additionally, we compare these cell lines with human pluripotent stem cell lines, and we discuss cell lines reprogrammed into a pluripotent naive state, detailing the processes used to attain this. Last, we present the findings from the application of these cell lines in two emerging fields: intra- and interspecies embryonic chimeras and blastoids.

Major transcriptomic, epigenetic and metabolic changes underlie the pluripotency continuum in rabbit preimplantation embryos

Despite the growing interest in the rabbit model for developmental and stem cell biology, the characterization of embryos at the molecular level is still poorly documented. We conducted a transcriptome analysis of rabbit preimplantation embryos from E2.7 (morula stage) to E6.6 (early primitive streak stage) using bulk and single-cell RNA-sequencing. In parallel, we studied oxidative phosphorylation and glycolysis, and analysed active and repressive epigenetic modifications during blastocyst formation and expansion. We generated a transcriptomic, epigenetic and metabolic map of the pluripotency continuum in rabbit preimplantation embryos, and identified novel markers of naive pluripotency that might be instrumental for deriving naive pluripotent stem cell lines. Although the rabbit is evolutionarily closer to mice than to primates, we found that the transcriptome of rabbit epiblast cells shares common features with those of humans and non-human primates.

Induced Cognitive Impairments Reversed by Grafts of Neural Precursors: A Longitudinal Study in a Macaque Model of Parkinson's Disease

Parkinson's disease (PD) evolves over an extended and variable period in humans; years prior to the onset of classical motor symptoms, sleep and biological rhythm disorders develop, significantly impacting the quality-of-life of patients. Circadian-rhythm disorders are accompanied by mild cognitive deficits that progressively worsen with disease progression and can constitute a severe burden for patients at later stages. The gold-standard 6-methyl-1-methyl-4-phenyl-1,2,3,6-tetrahydropyridin (MPTP) macaque model of PD recapitulates the progression of motor and nonmotor symptoms over contracted periods of time. Here, this multidisciplinary/multiparametric study follows, in five animals, the steady progression of motor and nonmotor symptoms and describes their reversal following grafts of neural precursors in diverse functional domains of the basal ganglia. Results show unprecedented recovery from cognitive symptoms in addition to a strong clinical motor recuperation. Both motor and cognitive recovery and partial circadian rhythm recovery correlate with the degree of graft integration, and in a subset of animals, with in vivo levels of striatal dopaminergic innervation and function. The present study provides empirical evidence that integration of neural precursors following transplantation efficiently restores function at multiple levels in parkinsonian nonhuman primates and, given interindividuality of disease progression and recovery, underlines the importance of longitudinal multidisciplinary assessments in view of clinical translation.

[Interspecies systemic chimeras]

Inter-species chimeras are both fantastic and monstrous creatures from Greek or Egyptian mythology, and a long-established research tool. Recent advances in the field of pluripotent stem cells have made it possible to extend the repertoire of inter-species chimeras to "systemic" chimeras, in which the mixing of cells from both species involves all organs including the germline. These chimeric embryos and fetuses open up new research avenues and potential medical applications. We will review the latest advances in the field. We will discuss the concepts of developmental complementation and developmental equivalence. We will discuss the methodological hurdles to be unlocked, as well as the biological and ethical limits of these new technologies.

[Chimeric embryos and pseudo-embryos: An alternative to human embryos for research]

The study of human development is essential to further our knowledge and to improve our therapeutic strategies in the fields of reproductive and regenerative medicine. Given the limited access to supernumerary embryos and the prohibition on creating new ones for research, two alternative strategies can be proposed to study human embryonic development. The first is to create pseudo-embryos or blastoids. The second is to create human/animal chimeric embryos by injecting pluripotent stem cells, ES or iPS, into animal embryos. We explain herein the importance of these new experimental paradigms for studying human development and their complementarity.

Apoptosis, G1 Phase Stall, and Premature Differentiation Account for Low Chimeric Competence of Human and Rhesus Monkey Naive Pluripotent Stem Cells

After reprogramming to naive pluripotency, human pluripotent stem cells (PSCs) still exhibit very low ability to make interspecies chimeras. Whether this is because they are inherently devoid of the attributes of chimeric competency or because naive PSCs cannot colonize embryos from distant species remains to be elucidated. Here, we have used different types of mouse, human, and rhesus monkey naive PSCs and analyzed their ability to colonize rabbit and cynomolgus monkey embryos. Mouse embryonic stem cells (ESCs) remained mitotically active and efficiently colonized host embryos. In contrast, primate naive PSCs colonized host embryos with much lower efficiency. Unlike mouse ESCs, they slowed DNA replication after dissociation and, after injection into host embryos, they stalled in the G1 phase and differentiated prematurely, regardless of host species. We conclude that human and non-human primate naive PSCs do not efficiently make chimeras because they are inherently unfit to remain mitotically active during colonization.

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Mouse ESCs are highly effective in colonizing rabbit and non-human primate embryos

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Rhesus monkey and human naive PSCs ineffectively colonize rabbit and monkey embryos

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Most rhesus/human naive PSCs differentiate prematurely upon injection into embryos

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Rhesus monkey PSCs stall in the G1 phase after transfer into rabbit embryos

Regulation of Cyclin E by transcription factors of the naïve pluripotency network in mouse embryonic stem cells

Continuous, non-cell cycle-dependent expression of cyclin E is a characteristic feature of mouse embryonic stem cells (mESCs). We studied the 5′ regulatory region of Cyclin E, also known as Ccne1, and identified binding sites for transcription factors of the naïve pluripotency network, including Esrrb, Klf4, and Tfcp2l1 within 1 kilobase upstream of the transcription start site. Luciferase assay and chromatin immunoprecipitation-quantitative polymerase chain reaction (ChiP–qPCR) study highlighted one binding site for Esrrb that is essential to transcriptional activity of the promoter region, and three binding sites for Klf4 and Tfcp2l1. Knockdown of Esrrb, Klf4, and Tfcp2l1 reduced Cyclin E expression whereas overexpression of Esrrb and Klf4 increased it, indicating a strong correlation between the expression level of these factors and that of cyclin E. We observed that cyclin E overexpression delays differentiation induced by Esrrb depletion, suggesting that cyclin E is an important target of Esrrb for differentiation blockade. We observed that mESCs express a low level of miR-15a and that transfection of a miR-15a mimic decreases Cyclin E mRNA level. These results lead to the conclusion that the high expression level of Cyclin E in mESCs can be attributed to transcriptional activation by Esrrb as well as to the absence of its negative regulator, miR-15a.

[Fifty shades of pluripotency]

Since the derivation of the first pluripotent embryonic stem cell lines in mice in the early 1980s, a plethora of lines has been obtained from various mammalian species including rodents, lagomorphs and primates. These lines are distinguished by their molecular and functional characteristics and correspond to the different pluripotency states observed in the developing embryo between the "blastocyst" and "gastrula" stages. These cell lines are positioned along a gradient, or continuum of pluripotency, the ends of which are epitomized by the naïve and primed states, respectively. Conventional human pluripotent stem cells self-renew in the primed state of pluripotency (ie, at the bottom of the gradient), a position that is undoubtedly the cause of their natural instability. Recent studies aim to generate naive human pluripotent stem cells (at the top of the gradient). The importance of this research in the perspective of medical applications will be discussed.

Reprogramming of rabbit induced pluripotent stem cells toward epiblast and chimeric competency using Krüppel-like factors

Rabbit induced pluripotent stem cells (rbiPSCs) possess the characteristic features of primed pluripotency as defined in rodents and primates. In the present study, we reprogrammed rbiPSCs using human Krüppel-like factors (KLFs) 2 and 4 and cultured them in a medium supplemented with fetal calf serum and leukemia inhibitory factor. These cells (designated rbEKA) were propagated by enzymatic dissociation for at least 30 passages, during which they maintained a normal karyotype. This new culturing protocol resulted in transcriptional and epigenetic reconfiguration, as substantiated by the expression of transcription factors and the presence of histone modifications associated with naïve pluripotency. Furthermore, microarray analysis of rbiPSCs, rbEKA cells, rabbit ICM cells, and rabbit epiblast showed that the global gene expression profile of the reprogrammed rbiPSCs was more similar to that of rabbit ICM and epiblast cells. Injection of rbEKA cells into 8-cell stage rabbit embryos resulted in extensive colonization of ICM in 9% early-blastocysts (E3.5), epiblast in 10% mid-blastocysts (E4.5), and embryonic disk in 1.4% pre-gastrulae (E6). Thus, these results indicate that KLF2 and KLF4 triggered the conversion of rbiPSCs into epiblast-like, embryo colonization-competent PSCs. Our results highlight some of the requirements to achieve bona fide chimeric competency.

A Panel of Embryonic Stem Cell Lines Reveals the Variety and Dynamic of Pluripotent States in Rabbits

Conventional rabbit embryonic stem cell (ESC) lines are derived from the inner cell mass (ICM) of pre-implantation embryos using methods and culture conditions that are established for primate ESCs. In this study, we explored the capacity of the rabbit ICM to give rise to ESC lines using conditions similar to those utilized to generate naive ESCs in mice. On single-cell dissociation and culture in fibroblast growth factor 2 (FGF2)-free, serum-supplemented medium, rabbit ICMs gave rise to ESC lines lacking the DNA-damage checkpoint in the G1 phase like mouse ESCs, and with a pluripotency gene expression profile closer to the rabbit ICM/epiblast profiles. These cell lines can be converted to FGF2-dependent ESCs after culture in conventional conditions. They can also colonize the rabbit pre-implantation embryo. These results indicate that rabbit epiblast cells can be coaxed toward different types of pluripotent stem cells and reveal the dynamics of pluripotent states in rabbit ESCs.

Generation of Embryonic Stem Cells in Rabbits

Here we have described a procedure to generate embryonic stem cell (ESC) lines from rabbit preimplantation blastocysts. We have also provided detailed procedures to characterize the resulting ESC lines, such as the analysis of pluripotency marker expression by reverse transcription quantitative polymerase chain reaction, immunolabeling, and fluorescence-associated cell sorting; evaluation of pluripotency by teratoma production; and assessment of genetic stability by karyotyping.

Generation of Induced Pluripotent Stem Cells in Rabbits

We describe a procedure for generating induced pluripotent stem cell lines in rabbits, using retroviral vectors expressing Oct4, Sox2, Klf4, and c-Myc of human origin to reprogram rabbit fibroblasts prepared from an ear skin biopsy. We also provide detailed procedures for characterizing the resulting iPSC lines, including the analysis of pluripotency marker expression by RT-qPCR, immunolabeling, and fluorescent-associated cell sorting, the evaluation of pluripotency by teratoma production and genetic stability by karyotyping.

Klf4 and Klf5 differentially inhibit mesoderm and endoderm differentiation in embryonic stem cells

Krüppel-like factors (Klf) 4 and 5 are two closely related members of the Klf family, known to play key roles in cell cycle regulation, somatic cell reprogramming and pluripotency. Here we focus on the functional divergence between Klf4 and Klf5 in the inhibition of mouse embryonic stem (ES) cell differentiation. Using microarrays and chromatin immunoprecipitation coupled to ultra-high-throughput DNA sequencing, we show that Klf4 negatively regulates the expression of endodermal markers in the undifferentiated ES cells, including transcription factors involved in the commitment of pluripotent stem cells to endoderm differentiation. Knockdown of Klf4 enhances differentiation towards visceral and definitive endoderm. In contrast, Klf5 negatively regulates the expression of mesodermal markers, some of which control commitment to the mesoderm lineage, and knockdown of Klf5 specifically enhances differentiation towards mesoderm. We conclude that Klf4 and Klf5 differentially inhibit mesoderm and endoderm differentiation in murine ES cells.

Induced pluripotent stem cells derived from rabbits exhibit some characteristics of naïve pluripotency

Not much is known about the molecular and functional features of pluripotent stem cells (PSCs) in rabbits. To address this, we derived and characterized 2 types of rabbit PSCs from the same breed of New Zealand White rabbits: 4 lines of embryonic stem cells (rbESCs), and 3 lines of induced PSCs (rbiPSCs) that were obtained by reprogramming adult skin fibroblasts. All cell lines required fibroblast growth factor 2 for their growth and proliferation. All rbESC lines showed molecular and functional properties typically associated with primed pluripotency. The cell cycle of rbESCs had a prolonged G1 phase and a DNA damage checkpoint before entry into the S phase, which are the 2 features typically associated with the somatic cell cycle. In contrast, the rbiPSC lines exhibited some characteristics of naïve pluripotency, including resistance to single-cell dissociation by trypsin, robust activity of the distal enhancer of the mouse Oct4 gene, and expression of naïve pluripotency-specific genes, as defined in rodents. According to gene expression profiles, rbiPSCs were closer to the rabbit inner cell mass (ICM) than rbESCs. Furthermore, rbiPSCs were capable of colonizing the ICM after aggregation with morulas. Therefore, we propose that rbiPSCs self-renew in an intermediate state between naïve and primed pluripotency, which represents a key step toward the generation of bona fide naïve PSC lines in rabbits.

Induced pluripotent stem cells derived from rabbits exhibit some characteristics of naïve pluripotency

Not much is known about the molecular and functional features of pluripotent stem cells (PSCs) in rabbits. To address this, we derived and characterized 2 types of rabbit PSCs from the same breed of New Zealand White rabbits: 4 lines of embryonic stem cells (rbESCs), and 3 lines of induced PSCs (rbiPSCs) that were obtained by reprogramming adult skin fibroblasts. All cell lines required fibroblast growth factor 2 for their growth and proliferation. All rbESC lines showed molecular and functional properties typically associated with primed pluripotency. The cell cycle of rbESCs had a prolonged G1 phase and a DNA damage checkpoint before entry into the S phase, which are the 2 features typically associated with the somatic cell cycle. In contrast, the rbiPSC lines exhibited some characteristics of naïve pluripotency, including resistance to single-cell dissociation by trypsin, robust activity of the distal enhancer of the mouse Oct4 gene, and expression of naïve pluripotency-specific genes, as defined in rodents. According to gene expression profiles, rbiPSCs were closer to the rabbit inner cell mass (ICM) than rbESCs. Furthermore, rbiPSCs were capable of colonizing the ICM after aggregation with morulas. Therefore, we propose that rbiPSCs self-renew in an intermediate state between naïve and primed pluripotency, which represents a key step toward the generation of bona fide naïve PSC lines in rabbits.

Directed pancreatic acinar differentiation of mouse embryonic stem cells via embryonic signalling molecules and exocrine transcription factors

Pluripotent embryonic stem cells (ESC) are a promising cellular system for generating an unlimited source of tissue for the treatment of chronic diseases and valuable in vitro differentiation models for drug testing. Our aim was to direct differentiation of mouse ESC into pancreatic acinar cells, which play key roles in pancreatitis and pancreatic cancer. To that end, ESC were first differentiated as embryoid bodies and sequentially incubated with activin A, inhibitors of Sonic hedgehog (Shh) and bone morphogenetic protein (BMP) pathways, fibroblast growth factors (FGF) and retinoic acid (RA) in order to achieve a stepwise increase in the expression of mRNA transcripts encoding for endodermal and pancreatic progenitor markers. Subsequent plating in Matrigel® and concomitant modulation of FGF, glucocorticoid, and folllistatin signalling pathways involved in exocrine differentiation resulted in a significant increase of mRNAs encoding secretory enzymes and in the number of cells co-expressing their protein products. Also, pancreatic endocrine marker expression was down-regulated and accompanied by a significant reduction in the number of hormone-expressing cells with a limited presence of hepatic marker expressing-cells. These findings suggest a selective activation of the acinar differentiation program. The newly differentiated cells were able to release α-amylase and this feature was greatly improved by lentiviral-mediated expression of Rbpjl and Ptf1a, two transcription factors involved in the maximal production of digestive enzymes. This study provides a novel method to produce functional pancreatic exocrine cells from ESC.

A short G1 phase is an intrinsic determinant of naïve embryonic stem cell pluripotency

A short G1 phase is a characteristic feature of mouse embryonic stem cells (ESCs). To determine if there is a causal relationship between G1 phase restriction and pluripotency, we made use of the Fluorescence Ubiquitination Cell Cycle Indicator (FUCCI) reporter system to FACS-sort ESCs in the different cell cycle phases. Hence, the G1 phase cells appeared to be more susceptible to differentiation, particularly when ESCs self-renewed in the naïve state of pluripotency. Transitions from ground to naïve, then from naïve to primed states of pluripotency were associated with increased durations of the G1 phase, and cyclin E-mediated alteration of the G1/S transition altered the balance between self-renewal and differentiation. LIF withdrawal resulted in a lengthening of the G1 phase in naïve ESCs, which occurred prior to the appearance of early lineage-specific markers, and could be reversed upon LIF supplementation. We concluded that the short G1 phase observed in murine ESCs was a determinant of naïve pluripotency and was partially under the control of LIF signaling.

GENERATION OF RABBIT PLURIPOTENT STEM CELL LINES

Pluripotent stem cells have the capacity to divide indefinitely and to differentiate to all the somatic tissues. They can be genetically manipulated in vitro by knocking in and out genes, therefore they serve as an excellent tool for gene-function studies and for the generation of models for human diseases. Since 1981, when the first mouse embryonic stem cell (ESC) line was generated, several attempts have been made to generate pluripotent stem cells from other species as it would help us to understand the differences and similarities of signaling pathways involved in pluripotency and differentiation, and would reveal whether the fundamental mechanism controlling self-renewal of pluripotent cells is conserved among different species. This review gives an overlook of embryonic and induced pluripotent stem cell (iPSCs) research in the rabbit which is one of the most relevant non-rodent species for animal models. To date, several lines of putative ESCs and iPSCs have been described in the rabbit. All expressed stem cell-associated markers and exhibited longevity and pluripotency in vitro, but none have been proven to exhibit full pluripotency in vivo. Moreover, similarly to several domestic species, markers used to characterize the putative ESCs are not fully adequate because studies in domestic species have revealed that they are not specific to the pluripotent inner cell mass. Future validation of rabbit pluripotent stem cells would benefit greatly from a reliable panel of molecular markers specific to pluripotent cells of the developing rabbit embryo. The status of isolation and characterization of the putative pluripotency genes in rabbit will be discussed. Using rabbit specific pluripotency genes we might be able to reprogram somatic cells and generate induced pluripotent stem cells more efficiently thus overcome some of the challenges towards harnessing the potential of this technology. This study was financed by EU FP7 (PartnErS, PIAP-GA-2008-218205; InduHeart, PEOPLE-IRG-2008-234390; InduVir, PEOPLE-IRG-2009-245808; RabPstem, PERG07-GA-2010-268422; PluriSys, HEALTH-2007-B-223485; AniStem, PIAP-GA-2011-286264), NKTH-OTKA-EU-7KP HUMAN-MB08-C-80-205; Plurabbit, OMFB-00130-00131/2010 ANR-NKTH/09-GENM-010-01.

Forced expression of LIM homeodomain transcription factor 1b enhances differentiation of mouse embryonic stem cells into serotonergic neurons

The LIM homeodomain transcription factor 1b (Lmx1b) is a key factor in the specification of the serotonergic neurotransmitter phenotype. Here, we explored the capacity of Lmx1b to direct differentiation of mouse embryonic stem (mES) cells into serotonergic neurons. mES cells stably expressing human Lmx1b were generated by lentiviral vector infection. Clones expressing Lmx1b at a low level showed increased neurogenesis and elevated production of neurons expressing serotonin, serotonin transporter, tryptophan hydroxylase 2, and transcription factor Pet1, the landmarks of serotonergic differentiation. To explore the role of Lmx1b in the specification of the serotonin neurotransmission phenotype further, a conditional system making use of a floxed inducible vector targeted into the ROSA26 locus and a hormone-dependent Cre recombinase was engineered. This novel strategy was tested with the reporter gene encoding human placental alkaline phosphatase, and demonstrated its capacity to drive transgene expression in nestin(+) neural progenitors (NPs) and in Tuj1(+) neurons. When it was applied to inducible expression of human Lmx1b, it resulted in elevated expression of serotonergic markers. Treatment of neural precursors with the floor plate signal Sonic hedgehog further enhanced differentiation of Lmx1b-overexpressing NPs into neurons expressing 5-HT, serotonin transporter, tryptophan hydroxylase 2, and Pet1, when compared with Lmx1b-nonexpressing progenitors. Together, our results demonstrate the capacity of Lmx1b to specify a serotonin neurotransmitter phenotype when overexpressed in mES cell-derived NPs.

Krüppel-like transcription factors and control of pluripotency

Recent papers have demonstrated a role for Krüppel-like transcription factors 2, 4 and 5 in the control of mouse embryonic stem cell pluripotency. However, it is not clear whether each factor has a unique role or whether they are functionally redundant. A paper by Parisi and colleagues in BMC Biology now sheds light on the mechanism by which Klf5 regulates pluripotency.

See research article http://www.biomedcentral.com/1741-7007/8/128

Novel STAT3 target genes exert distinct roles in the inhibition of mesoderm and endoderm differentiation in cooperation with Nanog

Leukemia inhibitory factor (LIF) activates the transcription factor signal transducer and activator of transcription 3 (STAT3), which results in the maintenance of mouse embryonic stem cells in the pluripotent state by inhibiting both mesodermal and endodermal differentiation. How the LIF/STAT3 pathway inhibits commitment to both mesoderm and endoderm lineages is presently unknown. Using a hormone-dependent STAT3 and with microarray analysis, we identified 58 targets of STAT3 including 20 unknown genes. Functional analysis showed that 22 among the 23 STAT3 target genes analyzed contribute to the maintenance of the undifferentiated state, as evidenced by an increase in the frequency of differentiated colonies in a self-renewal assay and a concomitant elevation of early differentiation markers upon knockdown. Fourteen of them, including Dact1, Klf4, Klf5, Rgs16, Smad7, Ccrn4l, Cnnm1, Ocln, Ier3, Pim1, Cyr61, and Sgk, were also regulated by Nanog. Analysis of lineage-specific markers showed that the STAT3 target genes fell into three distinct categories, depending on their capacity to inhibit either mesoderm or endoderm differentiation or both. The identification of genes that harness self-renewal and are downstream targets of both STAT3 and Nanog shed light on the mechanisms underlying functional redundancy between STAT3 and Nanog in mouse embryonic stem cells.

The FunGenES database: a genomics resource for mouse embryonic stem cell differentiation

Embryonic stem (ES) cells have high self-renewal capacity and the potential to differentiate into a large variety of cell types. To investigate gene networks operating in pluripotent ES cells and their derivatives, the “Functional Genomics in Embryonic Stem Cells” consortium (FunGenES) has analyzed the transcriptome of mouse ES cells in eleven diverse settings representing sixty-seven experimental conditions. To better illustrate gene expression profiles in mouse ES cells, we have organized the results in an interactive database with a number of features and tools. Specifically, we have generated clusters of transcripts that behave the same way under the entire spectrum of the sixty-seven experimental conditions; we have assembled genes in groups according to their time of expression during successive days of ES cell differentiation; we have included expression profiles of specific gene classes such as transcription regulatory factors and Expressed Sequence Tags; transcripts have been arranged in “Expression Waves” and juxtaposed to genes with opposite or complementary expression patterns; we have designed search engines to display the expression profile of any transcript during ES cell differentiation; gene expression data have been organized in animated graphs of KEGG signaling and metabolic pathways; and finally, we have incorporated advanced functional annotations for individual genes or gene clusters of interest and links to microarray and genomic resources. The FunGenES database provides a comprehensive resource for studies into the biology of ES cells.

Viral-mediated coexpression of Pdx1 and p48 regulates exocrine pancreatic differentiation in mouse ES cells

Embryonic stem cells (ES) can spontaneously activate a pancreatic differentiation program in vitro, although with low efficiency. The aim was to improve such process by using viral mediated gene transduction. In this study, we have examined the suitability of using viral vectors to express key transcriptional factors involved in pancreatic development. ES cell lines that constitutively express Pdx1, a homeodomain protein involved in both exocrine and endocrine pancreatic development and differentiation, were established using a lentiviral vector. These cells were additionally infected with an adenovirus expressing p48, a bHLH factor that is also crucial for pancreatic development and acinar differentiation. Quantitative RT-PCR analysis demonstrated an increase in the expression of exocrine genes, including those coding for both digestive enzymes and transcription factors. Immunocytochemical staining also revealed an increase in the number of amylase-expressing cell clusters. However, other important genes involved in acinar cell maturation (i.e., Mist1) were not modulated under these conditions, suggesting that the cells display features of immature exocrine cells or because of an uncoupled gene expression of the exocrine differentiation program. Importantly, this effect was selective for the acinar lineage as the expression of a large set of endocrine markers remained unchanged. Therefore, combined expression of key genes involved in pancreatic development may be a promising approach to generate mature pancreatic exocrine cells.

Derivation and cloning of a novel rhesus embryonic stem cell line stably expressing tau-green fluorescent protein

Embryonic stem cells (ESC) have the ability of indefinite self-renewal and multilineage differentiation, and they carry great potential in cell-based therapies. The rhesus macaque is the most relevant preclinical model for assessing the benefit, safety, and efficacy of ESC-based transplantations in the treatment of neurodegenerative diseases. In the case of neural cell grafting, tracing both the neurons and their axonal projections in vivo is essential for studying the integration of the grafted cells in the host brain. Tau-Green fluorescent protein (tau-GFP) is a powerful viable lineage tracer, allowing visualization of cell bodies, dendrites, and axons in exquisite detail. Here, we report the first rhesus monkey ESC line that ubiquitously and stably expresses tau-GFP. First, we derived a new line of rhesus monkey ESC (LYON-ES1) that show marker expression and cell cycle characteristics typical of primate ESCs. LYON-ES1 cells are pluripotent, giving rise to derivatives of the three germ layers in vitro and in vivo through teratoma formation. They retain all their undifferentiated characteristics and a normal karyotype after prolonged culture. Using lentiviral infection, we then generated a monkey ESC line stably expressing tau-GFP that retains all the characteristics of the parental wild-type line and is clonogenic. We show that neural precursors derived from the tau-GFP ESC line are multipotent and that their fate can be precisely mapped in vivo after grafting in the adult rat brain. Disclosure of potential conflicts of interest is found at the end of this article.

Self-renewal of murine embryonic stem cells is supported by the serine/threonine kinases Pim-1 and Pim-3

pim-1 and pim-3 encode serine/threonine kinases involved in the regulation of cell proliferation and apoptosis in response to cytokine stimulation. We analyzed the regulation of pim-1 and pim-3 by the leukemia inhibitory factor (LIF)/gp130/signal transducer and activator of transcription-3 (STAT3) pathway and the role of Pim-1 and Pim-3 kinases in mouse embryonic stem (ES) cell self-renewal. Making use of ES cells expressing a granulocyte colony-stimulating factor:gp130 chimeric receptor and a hormone-dependent signal transducer and activator of transcription-3 estrogen receptor (STAT3-ER(T2)), we showed that expression of pim-1 and pim-3 was upregulated by LIF/gp130-dependent signaling and the STAT3 transcription factor. ES cells overexpressing pim-1 and pim-3 had a greater capacity to self-renew and displayed a greater resistance to LIF starvation based on a clonal assay. In contrast, knockdown of pim-1 and pim-3 increased the rate of spontaneous differentiation in a self-renewal assay. Knockdown of pim-1 and pim-3 was also detrimental to the growth of undifferentiated ES cell colonies and increased the rate of apoptosis. These findings provide a novel role of Pim-1 and Pim-3 kinases in the control of self-renewal of ES cells. Disclosure of potential conflicts of interest is found at the end of this article.

Regulation of Nanog expression by phosphoinositide 3-kinase-dependent signaling in murine embryonic stem cells

Embryonic stem (ES) cell pluripotency is regulated by a combination of extrinsic and intrinsic factors. Previously we have demonstrated that phosphoinositide 3-kinase (PI3K)-dependent signaling is required for efficient self-renewal of murine ES cells. In the study presented here, we have investigated the downstream molecular mechanisms that contribute to the ability of PI3Ks to regulate pluripotency. We show that inhibition of PI3K activity with either pharmacological or genetic tools results in decreased expression of RNA for the homeodomain transcription factor Nanog and decreased Nanog protein levels. Inhibition of glycogen synthase kinase 3 (GSK-3) activity by PI3Ks plays a key role in regulation of Nanog expression, because blockade of GSK-3 activity effectively reversed the effects of PI3K inhibition on Nanog RNA, and protein expression and self-renewal under these circumstances were restored. Furthermore, GSK-3 mutants mimicked the effects of PI3K or GSK-3 inhibition on Nanog expression. Importantly, expression of an inducible form of Nanog prevented the loss of self-renewal observed upon inhibition of PI3Ks, supporting a functional relationship between PI3Ks and Nanog expression. In addition, expression of a number of putative Nanog target genes was sensitive to PI3K inhibition. Thus, the new evidence provided in this study shows that PI3K-dependent regulation of ES cell self-renewal is mediated, at least in part, by the ability of PI3K signaling to maintain Nanog expression. Regulation of GSK-3 activity by PI3Ks appears to play a key role in this process.

Human bone marrow mesenchymal stem cells can express insulin and key transcription factors of the endocrine pancreas developmental pathway upon genetic and/or microenvironmental manipulation in vitro

Multipotential stem cells can be selected from the bone marrow by plastic adhesion, expanded, and cultured. They are able to differentiate not only into multiple cell types, including cartilage, bone, adipose and fibrous tissues, and myelosupportive stroma, but also into mesodermal (endothelium), neuroectodermal, or endodermal (hepatocytes) lineages. Our goal was to characterize the multipotential capacities of human mesenchymal stem cells (hMSCs) and to evaluate their ability to differentiate into insulin-secreting cells in vitro. hMSCs were obtained from healthy donors, selected by plastic adhesion, and phenotyped by fluorescence-activated cell sorter and reverse transcription-polymerase chain reaction analysis before and after infection with adenoviruses coding for mouse IPF1, HLXB9, and FOXA2 transcription factors involved early in the endocrine developmental pathway. We found that native hMSCs have a pluripotent phenotype (OCT4 expression and high telomere length) and constitutively express NKX6-1 at a low level but lack all other transcription factors implicated in beta-cell differentiation. In all hMSCs, we detected mRNA of cytokeratin 18 and 19, epithelial markers present in pancreatic ductal cells, whereas proconvertase 1/3 mRNA expression was detected only in some hMSCs. Ectopic expression of IPF1, HLXB9, and FOXA2 with or without islet coculture or islet-conditioned medium results in insulin gene expression. In conclusion, our results demonstrated that in vitro human bone marrow stem cells are able to differentiate into insulin-expressing cells by a mechanism involving several transcription factors of the beta-cell developmental pathway when cultured in an appropriate microenvironment.

Cell cycle features of primate embryonic stem cells

Using flow cytometry measurements combined with quantitative analysis of cell cycle kinetics, we show that rhesus monkey embryonic stem cells (ESCs) are characterized by an extremely rapid transit through the G1 phase, which accounts for 15% of the total cell cycle duration. Monkey ESCs exhibit a non-phasic expression of cyclin E, which is detected during all phases of the cell cycle, and do not growth-arrest in G1 after gamma-irradiation, reflecting the absence of a G1 checkpoint. Serum deprivation or pharmacological inhibition of mitogen-activated protein kinase kinase (MEK) did not result in any alteration in the cell cycle distribution, indicating that ESC growth does not rely on mitogenic signals transduced by the Ras/Raf/MEK pathway. Taken together, these data indicate that rhesus monkey ESCs, like their murine counterparts, exhibit unusual cell cycle features in which cell cycle control mechanisms operating during the G1 phase are reduced or absent.

G1 phase regulation, area-specific cell cycle control, and cytoarchitectonics in the primate cortex

We have investigated the cell cycle-related mechanisms that lead to the emergence of primate areas 17 and 18. These areas are characterized by striking differences in cytoarchitectonics and neuron number. We show in vivo that (1) area 17 precursors of supragranular neurons exhibit a shorter cell cycle duration, a reduced G1 phase, and a higher rate of cell cycle reentry than area 18 precursors; (2) area 17 and area 18 precursors show contrasting and specific levels of expression of cyclin E (high in area 17, low in area 18) and p27Kip1 (low in area 17, high in area 18); (3) ex vivo up- and downmodulation of cyclin E and p27Kip1 show that both regulators influence cell cycle kinetics by modifying rates of cell cycle progression and cell cycle reentry; (4) modeling the areal differences in cell cycle parameters suggests that they contribute to areal differences in numbers of precursors and neuron production.

Transforming growth factor (TGF)beta, fibroblast growth factor (FGF) and retinoid signalling pathways promote pancreatic exocrine gene expression in mouse embryonic stem cells

Extracellular signalling cues play a major role in the activation of differentiation programmes. Mouse embryonic stem (ES) cells are pluripotent and can differentiate into a wide variety of specialized cells. Recently, protocols designed to induce endocrine pancreatic differentiation in vitro have been designed but little information is currently available concerning the potential of ES cells to differentiate into acinar pancreatic cells. By using conditioned media of cultured foetal pancreatic rudiments, we demonstrate that ES cells can respond in vitro to signalling pathways involved in exocrine development and differentiation. In particular, modulation of the hedgehog, transforming growth factor beta, retinoid, and fibroblast growth factor pathways in ES cell-derived embryoid bodies (EB) resulted in increased levels of transcripts encoding pancreatic transcription factors and cytodifferentiation markers, as demonstrated by RT-PCR. In EB undergoing spontaneous differentiation, expression of the majority of the acinar genes (i.e. amylase, carboxypeptidase A and elastase) was induced after the expression of endocrine genes, as occurs in vivo during development. These data indicate that ES cells can undergo exocrine pancreatic differentiation with a kinetic pattern of expression reminiscent of pancreas development in vivo and that ES cells can be coaxed to express an acinar phenotype by activation of signalling pathways known to play a role in pancreatic development and differentiation.

[Embryonic stem cells and cell replacement therapies in the nervous system]

Embryonic stem (ES) cells are pluripotential cells derived from the pre-implantation embryo. They can proliferate indefinitely in vitro while retaining pluripotency. ES cells can also be made to differentiate into a large variety of cell types in vitro. This has paved the way to research aimed at using ES-derived cells for cell replacement therapies. Hence, mouse ES cells can efficiently differentiate into neural precursors which can further generate functional neurons, astrocytes, and oligodendrocytes. Methods have also been developed to coax mouse ES-derived neural stem cells to differentiate into either dopaminergic neurons or motoneurons. Mouse ES-derived neural stem cells, or their fully differentiated progeny, have been shown to survive, integrate, and to some extent, function following transplantation within appropriate rodent host tissue. Research on human ES cells is still in its infancy. Considerable work has to be done: (1) to master growth and genetic manipulation of human ES cells; (2) to master their differentiation into specific cell types; and (3) to demonstrate that they can provide long term therapeutical benefits upon grafting into damaged tissues in humans. From the ethical point of view, the establishment of appropriate primate model will be an obligatory prerequisite to clinical trials based on ES cells derivatives grafting.

[Embryonal stem cells do not undergo cell cycle arrest upon exposure to damaging factors]

As shown recently (Malashicheva et al., 2000), embryonic teratocarcinoma F9 mouse cells do not stop on the G1/S border after the treatment with agents causing G1 arrest in normal fibroblast cells. Since after a prolonged cultivation in vitro F9 cells could lose some properties characteristic of the stem cells, we studied here the capability of mouse ES cells to undergo cell cycle blocks following gamma-irradiation, adriamycin and PALA treatment as well as upon cultivation in the presence of nocodazol, an inhibitor of spindle assembly. The results obtained show that ES cells, similarly as their tumorigenic derivative F9 cells, do not demonstrate any delay on the G1/S boundary of the cell cycle. Moreover, nocodazol treatment for 48 h leads to accumulation of polyploid cells. Immunoblot experiments reveal a low level of p21/Waf1 expression both in F9 and in ES cells. Interestingly, the content of p21/Waf1 has been found to increase after cell treatment with proteasome inhibitor lactacystin, implying that p21/Waf1 level is regulated by proteasomal degradation. Thus, the p21/Waf1--dependent mechanisms of cell cycle control (checkpoint control) do not function properly in embryonic stem cells.

Signalling, cell cycle and pluripotency in embryonic stem cells

Pluripotent mouse embryonic stem (ES) cells can be expanded in large numbers in vitro owing to a process of symmetrical self-renewal. Self-renewal entails proliferation with a concomitant suppression of differentiation. Here we describe how the cytokine leukaemia inhibitory factor (LIF) sustains self-renewal through activation of the transcription factor STAT3, and how two other signals - extracellular-signal-related kinase (ERK) and phosphatidylinositol-3-OH kinase (PI3K) - can influence differentiation and propagation, respectively. We relate these observations to the unusual cell-cycle properties of ES cells and speculate on the role of the cell cycle in maintaining pluripotency.

[Cell cycle control and self-renewal of embryonic stem cells]

On one hand, self-renewal of mouse embryonic stem (ES) cells rely exclusively upon the LIFR beta/gp130-signaling pathway and the subsequent activation of the STAT3 transcription factor. On the other hand, the much-studied cellular machinery, that is organized to collect extracellular signals, transduce them via tyrosine kinase receptors and the SOS-RAS-RAF-MEK-MAPK pathway, ultimately leading to regulation of D-type cyclin expression, while regulation of the retinoblastoma (RB) protein phosphorylation is likely not to be operative in ES cells. We hypothetize that ES cells are blinkered by the lack of RB-dependent control of the G1/S transition, and that commitment into differentiation triggers the birth of a regulatable G1 phase. We discuss how the LIFR beta/gp130-signaling pathway and the ES cell-cycle machinery may functionally interact to promote self-renewal.

Differential contributions of ERK and PI3-kinase to the regulation of cyclin D1 expression and to the control of the G1/S transition in mouse embryonic stem cells

Mouse embryonic stem (ES) cells are known to express D-type cyclins at very low levels and these levels increase dramatically during in vitro and in vivo differentiation. Here, we investigate some of the signalling pathways regulating expression of cyclin D1 and progression to S phase, the Ras/Extracellular signal-regulated protein kinase (ERK) pathway and the phosphatidylinositol 3-kinase (PI3-kinase) pathway. We demonstrate that ERK phosphorylation is fully dispensable for the regulation of cyclin D1 level and for the progression from G1 to S phase in ES cells. By contrast, PI3-kinase activity is required for both. Differentiation induced by retinoic acid results in the gain of ERK-dependent control of cyclin D1 expression and of S phase progression. Differentiation is also paralleled by an increase in PI3-kinase activity. This leads (a) to an increase in the p70 S6 kinase-dependent regulation of the steady-state level of cyclin D1, and (b) to a concomitant decrease in the GSK3beta-dependent rate of cyclin D1 degradation. Altogether, these multiple pathways account for the dramatic increase in the level of cyclin D1 protein which parallels ES cell differentiation. Our studies suggest that PI3-kinase is an important regulator of the ES cell cycle and that its activity is not regulated by mitogen stimulation.

An efficient system for conditional gene expression in embryonic stem cells and in their in vitro and in vivo differentiated derivatives

We have developed a universally applicable system for conditional gene expression in embryonic stem (ES) cells that relies on tamoxifen-dependent Cre recombinase-loxP site-mediated recombination and bicistronic gene-trap expression vectors that allow transgene expression from endogenous cellular promoters. Two vectors were introduced into the genome of recipient ES cells, successively: (i) a bicistronic gene-trap vector encoding the beta-galactosidase/neo(R) fusion protein and the Cre-ER(T2) (Cre recombinase fused to a mutated ligand-binding domain of the human estrogen receptor) and (ii) a bicistronic gene-trap vector encoding the hygro(R) protein and the human alkaline phosphatase (hAP), the expression of which is prevented by tandemly repeated stop-of-transcription sequences flanked by loxP sites. In selected clones, hAP expression was shown to be regulated accurately by 4'hydroxy-tamoxifen. Strict hormone-dependent expression of hAP was achieved (i) in vitro in undifferentiated ES cells and embryoid bodies, (ii) in vivo in virtually all the tissues of the 10-day-old chimeric fetus (after injection of 4'hydroxy-tamoxifen to foster mothers), and (iii) ex vivo in primary embryonic fibroblasts isolated from chimeric fetuses. Therefore, this approach can be applied to drive conditional expression of virtually any transgene in a large variety of cell types, both in vitro and in vivo.

Cell-cycle kinetics of neocortical precursors are influenced by embryonic thalamic axons

Thalamic afferents are known to exert a control over the differentiation of cortical areas at late stages of development. Here, we show that thalamic afferents also influence early stages of corticogenesis at the level of the ventricular zone. Using an in vitro approach, we show that embryonic day 14 mouse thalamic axons release a diffusable factor that promotes the proliferation of cortical precursors over a restricted developmental window. The thalamic mitogenic effect on cortical precursors (1) shortens the total cell-cycle duration via a reduction of the G(1) phase; (2) facilitates the G(1)/S transition leading to an increase in proliferative divisions; (3) is significantly reduced by antibodies directed against bFGF; and (4) influences the proliferation of both glial and neuronal precursors and does not preclude the action of signals that induce differentiation in these two lineages. We have related these in vitro findings to the in vivo condition: the organotypic culture of cortical explants in which anatomical thalamocortical innervation is preserved shows significantly increased proliferation rates compared with cortical explants devoid of subcortical afferents. These results are in line with a number of studies at subcortical levels showing the control of neurogenesis via afferent fibers in both vertebrates and invertebrates. Specifically, they indicate the mechanisms whereby embryonic thalamic afferents contribute to the known early regionalization of the ventricular zone, which plays a major role in the specification of neocortical areas.

Mouse embryonic stem cells with aberrant transforming growth factor beta signalling exhibit impaired differentiation in vitro and in vivo

Embryonic stem (ES) cells are resistant to transforming growth factor beta (TGF beta). We have shown previously that they lack type-II binding receptors (T beta RII) and in this respect resemble the inner cell mass and ectoderm cells of mouse embryos 4.5-7.5 days post coitum (dpc); they do however express type-I (alk-5) signalling receptors. Here we show that in contrast to several tumour cell lines, stable transfection of wtT beta RII is not sufficient for ES cells to become biologically sensitive to TGF beta. We analysed the expression of several down-stream molecules known to be involved in TGF beta signalling (Smads) and TGF beta-mediated cell cycle regulation (cyclins D) during the differentiation of control and wtT beta RII-expressing ES cells and showed that upregulation of these molecules correlated with (i) an increase in plasminogen activator inhibitor-1 (PAI-1) synthesis and (ii) growth inhibition, following addition of TGF beta 1. These TGF beta responses were reduced in an ES cell line expressing a dominant negative (truncated) T beta RII (delta T beta RII). The differentiation pattern of control and wtT beta RII-expressing ES cells was indistinguishable in monolayer culture and as embryoid bodies, but in delta T beta RII ES cells, the capacity to form mesodermal derivatives in monolayer cultures in response to the addition of retinoic acid (RA) and removal of leukemia inhibitory factor (LIF) was lost, and only endoderm-like cells formed. The T beta RII and delta T beta RII ES cells were, however, both distinguishable from control ES cells when allowed to differentiate in chimaeric embryos following aggregation with morula-stage hosts. Conceptuses containing mutant cells, recovered from pseudopregnant females at the equivalent of 9.5 dpc, exhibited highly defective yolk sac development; most strikingly, no blood vessels were present and in addition the yolk sacs with derivatives of ES cells containing wtT beta RII were blistered and lacked haematopoietic cells. The implications for understanding TGF beta signalling in early mouse development are discussed.

G1-phase regulators, cyclin D1, cyclin D2, and cyclin D3: up-regulation at gastrulation and dynamic expression during neurulation

Gastrulation in rodents is associated with an increase in the rate of growth and with the start of differentiation within the embryo proper. In an effort to understand the role played by the cell cycle control in these processes, expression of cyclin D1, D2, and D3--three major positive regulators of the G1/S transition--has been investigated by in situ hybrization and RT-PCR. Cyclin D1 and D2 transcripts are first detected in the epiblast at gastrulation, when a proliferative burst occurs, and subsequently in its differentiated derivatives within the embryo proper, indicating that activation of their expression takes place prior to the differentiation of epiblast progenitors. In contrast, cyclin D3 transcript is undetectable in the epiblast itself and its expression is activated exclusively in extraembryonic tissues of both epiblast and trophoblast origin. During neurulation, expression of each cyclin D RNA is dynamically regulated along the anterior-posterior axis. In the hindbrain, cyclin D1 and D2 show distinct segment-specific restricted expression and this pattern is conserved between mouse and chick. These results strongly suggest that D-type cyclins act as developmental regulators.

Identification of transcripts initiated from an internal promoter in the c-erbA alpha locus that encode inhibitors of retinoic acid receptor-alpha and triiodothyronine receptor activities

The thyroid hormone receptor-coding locus, c-erbA alpha, generates several mRNAs originating from a single primary transcript that undergoes alternative splicing. We have identified for the first time two new transcripts, called TRdelta alpha1 and TRdelta alpha2 [mRNA for isoform alpha1 and alpha2 of the T3 receptor (TR), respectively], whose transcription is initiated from an internal promoter located within intron 7 of the c-erbA alpha gene. These two new transcripts exhibit tissue-specific patterns of expression in the mouse. These two patterns are in sharp contrast with the expression patterns of the full-length transcripts generated from the c-erbA alpha locus. TR alpha1 and TRdelta alpha2 mRNAs encode N-terminally truncated isoforms of T3R alpha1 and T3R alpha2, respectively. The protein product of TRdelta alpha1 antagonizes the transcriptional activation elicited by T3 and retinoic acid. This protein inhibits the ligand-induced activating functions of T3R alpha1 and 9-cis-retinoic acid receptor-alpha but does not affect the retinoic acid-dependent activating function of retinoic acid receptor-alpha. We predict that these truncated proteins may work as down-regulators of transcriptional activity of nuclear hormone receptors in vivo.

Identification of BTG2, an antiproliferative p53-dependent component of the DNA damage cellular response pathway

Cell cycle regulation is critical for maintenance of genome integrity. A prominent factor that guarantees genomic stability of cells is p53 (ref. 1). The P53 gene encodes a transcription factor that has a role as a tumour suppressor. Identification of p53-target genes should provide greater insight into the molecular mechanisms that mediate the tumour suppressor activities of p53. The rodent Pc3/Tis21 gene was initially described as an immediate early gene induced by tumour promoters and growth factors in PC12 and Swiss 3T3 cells. It is expressed in a variety of cell and tissue types and encodes a remarkably labile protein. Pc3/Tis21 has a strong sequence similarity to the human antiproliferative BTG1 gene cloned from a chromosomal translocation of a B-cell chronic lymphocytic leukaemia. This similarity led us to speculate that BTG1 and the putative human homologue of Pc3/Tis21 (named BTG2) were members of a new family of genes involved in growth control and/or differentiation. This hypothesis was recently strengthened by the identification of a new antiproliferative protein, named TOB, which shares sequence similarity with BTG1 and PC3/TIS21 (ref. 7). Here, we cloned and localized the human BTG2 gene. We show that BTG2 expression is induced through a p53-dependent mechanism and that BTG2 function may be relevant to cell cycle control and cellular response to DNA damage.

Effect of nerve growth factor on the expression of cell cycle regulatory proteins in PC12 cells: dissection of the neurotrophic response from the anti-mitogenic response

PC12 cells treated with nerve growth factor (NGF) undergo a G1 block and differentiate. Expression of selected cell cycle regulatory proteins was studied under culture conditions which permit observation of a differentiation response independently from a mitogenic or anti-mitogenic response. The expression of all cell cycle regulatory proteins studied is modulated by NGF addition to exponentially-growing cultures in the presence of serum. While levels of most of these proteins decrease, accumulation of cyclin D1 and the cyclin-dependent kinase inhibitor p21 Cip1/WAF1 is observed. Cyclin D1 associated kinase activity is inhibited, correlating with an increase in p21 protein. PC12 cells, synchronized by serum starvation, undergo morphological and functional differentiation in the presence of NGF. Neither cyclin D1 nor p21 are present in such cultures, nor is their expression upregulated by NGF, indicating that they are not required for this process. Removal of serum from differentiated PC12 cells results in loss of these proteins, but has no effect on differentiation or the nonproliferative state in presence of NGF. Together, the results indicate that cyclin D1 and p21 are not necessary for differentiation per se, nor are they required for maintenance of the differentiated state in the absence of serum.

Withdrawal of differentiation inhibitory activity/leukemia inhibitory factor up-regulates D-type cyclins and cyclin-dependent kinase inhibitors in mouse embryonic stem cells

The expression of E and D-type cyclins, Cyclin-Dependent Kinase (CDK) 2 and 4, as well as CDK inhibitors p21Cip1 and p27Kip1 were examined during in vitro differentiation of mouse embryonic stem (ES) cells. ES cells cultured in presence of Differentiation Inhibitory Activity/Leukemia Inhibitory Factor (DIA/LIF) express very low levels of cyclin E/CDK2 complexes, p21Cip1 and p27Kip1 CDK inhibitors, while cyclin D/CDK4-associated kinase activity is undetectable. Withdrawal of DIA/LIF, which induces differentiation, results in the progressive up-regulation of all. Up-regulation of D cyclins occurs through an increase in the steady-state levels of mRNA, concomitantly with the activation of Brachyury and Goosecoid, two early markers of mesoderm differentiation. Similarly, cells from the epiblast of the early postimplantation mouse embryo do not express any cyclin D/CDK4 complexes. These are progressively upregulated at gastrulation and early organogenesis. DIA/LIF-stimulated ES cells are not growth-arrested by overexpression of p16Ink4a, a specific inhibitor of CDK4 and CDK6. We propose that the G1/S transition may be regulated by a minimal mechanism in mouse embryonic stem cells. Induction of differentiation triggers the establishment of a more sophisticated mechanism involving both cyclin D/CDK4- and CDK inhibitor-associated control of G1-phase progression.

In vitro differentiation of embryonic stem cells into glial cells and functional neurons

Mouse embryonic stem cells were induced to differentiate in culture with retinoic acid. Putative precursors of neurons and glial cells (nestin-positive cells) were clearly identified as early as three days after the onset of differentiation. At day 6, neuron-like cells could be clearly identified, either as isolated cells or as cellular networks. Some of these cells were positive for astrocyte- or oligodendrocyte-specific antigens (GFAP or O4 antigens, respectively). Other cells were positive for neuron-specific antigens (cytoskeleton proteins MAP2, MAP5 and NF200, as well as synaptophysin). Some neuronal-like cells were also positive for acetylcholinesterase activity or glutamic acid decarboxylase expression, indicating that ES cells could differentiate into GABAergic and possibly cholinergic neurons. Electrophysiological analyses performed in voltage clamp conditions showed that cell membranes contained voltage-dependent channels. Overshooting action potentials could be triggered by current injection. Taken together, these data provide evidence that embryonic stem cells can differentiate first into neuron-glia progenitors, and later into glial cells and functional neurons, in vitro. This technique provides an unique system to study early steps of neuronal differentiation in vitro.