Embryonic stem cells and transgenic mice ubiquitously expressing a tau-tagged green fluorescent protein

SIP1 mediates cell-fate decisions between neuroectoderm and mesendoderm in human pluripotent stem cells

Human embryonic stem cells (hESCs) rely on fibroblast growth factor and Activin-Nodal signaling to maintain their pluripotency. However, Activin-Nodal signaling is also known to induce mesendoderm differentiation. The mechanisms by which Activin-Nodal signaling can achieve these contradictory functions remain unknown. Here, we demonstrate that Smad-interacting protein 1 (SIP1) limits the mesendoderm-inducing effects of Activin-Nodal signaling without inhibiting the pluripotency-maintaining effects exerted by SMAD2/3. In turn, Activin-Nodal signaling cooperates with NANOG, OCT4, and SOX2 to control the expression of SIP1 in hESCs, thereby limiting the neuroectoderm-promoting effects of SIP1. Similar results were obtained with mouse epiblast stem cells, implying that these mechanisms are evolutionarily conserved and may operate in vivo during mammalian development. Overall, our results reveal the mechanisms by which Activin-Nodal signaling acts through SIP1 to regulate the cell-fate decision between neuroectoderm and mesendoderm in the progression from pluripotency to primary germ layer differentiation.

Directed fiber outgrowth from transplanted embryonic cortex-derived neurospheres in the adult mouse brain

Neural transplantation has emerged as an attractive strategy for the replacement of neurons that have been lost in the central nervous system. Multipotent neural progenitor cells are potentially useful as donor cells to repopulate the degenerated regions. One important aspect of a transplantation strategy is whether transplanted cells are capable of fiber outgrowth with the aim of rebuilding axonal connections within the host brain. To address this issue, we expanded neuronal progenitor from the cortex of embryonic day 15 ubiquitously green fluorescent protein-expressing transgenic mice as neurospheres in vitro and grafted them into the entorhinal cortex of 8-week-old mice immediately after a perforant pathway lesion. After transplantation into a host brain with a lesion of the entorhino-hippocampal projection, the neurosphere-derived cells extended long fiber projections directed towards the dentate gyrus. Our results indicate that transplantation of neurosphere-derived cells might be a promising strategy to replace lost or damaged axonal projections.

Regenerating cortical connections in a dish: the entorhino-hippocampal organotypic slice co-culture as tool for pharmacological screening of molecules promoting axon regeneration

We present a method for using long-term organotypic slice co-cultures of the entorhino-hippocampal formation to analyze the axon-regenerative properties of a determined compound. The culture method is based on the membrane interphase method, which is easy to perform and is generally reproducible. The degree of axonal regeneration after treatment in lesioned cultures can be seen directly using green fluorescent protein (GFP) transgenic mice or by axon tracing and histological methods. Possible changes in cell morphology after pharmacological treatment can be determined easily by focal in vitro electroporation. The well-preserved cytoarchitectonics in the co-culture facilitate the analysis of identified cells or regenerating axons. The protocol takes up to a month.

Differentiation of neurons from neural precursors generated in floating spheres from embryonic stem cells

Background

Neural differentiation of embryonic stem (ES) cells is usually achieved by induction of ectoderm in embryoid bodies followed by the enrichment of neuronal progenitors using a variety of factors. Obtaining reproducible percentages of neural cells is difficult and the methods are time consuming.

Results

Neural progenitors were produced from murine ES cells by a combination of nonadherent conditions and serum starvation. Conversion to neural progenitors was accompanied by downregulation of Oct4 and NANOG and increased expression of nestin. ES cells containing a GFP gene under the control of the Sox1 regulatory regions became fluorescent upon differentiation to neural progenitors, and ES cells with a tau-GFP fusion protein became fluorescent upon further differentiation to neurons. Neurons produced from these cells upregulated mature neuronal markers, or differentiated to glial and oligodendrocyte fates. The neurons gave rise to action potentials that could be recorded after application of fixed currents.

Conclusion

Neural progenitors were produced from murine ES cells by a novel method that induced neuroectoderm cells by a combination of nonadherent conditions and serum starvation, in contrast to the embryoid body method in which neuroectoderm cells must be selected after formation of all three germ layers.

A role for NANOG in G1 to S transition in human embryonic stem cells through direct binding of CDK6 and CDC25A

In this study, we show that NANOG, a master transcription factor, regulates S-phase entry in human embryonic stem cells (hESCs) via transcriptional regulation of cell cycle regulatory components. Chromatin immunoprecipitation combined with reporter-based transfection assays show that the C-terminal region of NANOG binds to the regulatory regions of CDK6 and CDC25A genes under normal physiological conditions. Decreased CDK6 and CDC25A expression in hESCs suggest that both CDK6 and CDC25A are involved in S-phase regulation. The effects of NANOG overexpression on S-phase regulation are mitigated by the down-regulation of CDK6 or CDC25A alone. Overexpression of CDK6 or CDC25A alone can rescue the impact of NANOG down-regulation on S-phase entry, suggesting that CDK6 and CDC25A are downstream cell cycle effectors of NANOG during the G1 to S transition.

Glutamatergic neuronal differentiation of mouse embryonic stem cells after transient expression of neurogenin 1 and treatment with BDNF and GDNF: in vitro and in vivo studies

Differentiation of the pluripotent neuroepithelium into neurons and glia is accomplished by the interaction of growth factors and cell-type restricted transcription factors. One approach to obtaining a particular neuronal phenotype is by recapitulating the expression of these factors in embryonic stem (ES) cells. Toward the eventual goal of auditory nerve replacement, the aim of the current investigation was to generate auditory nerve-like glutamatergic neurons from ES cells. Transient expression of Neurog1 promoted widespread neuronal differentiation in vitro; when supplemented with brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF), 75% of ES cell-derived neurons attained a glutamatergic phenotype after 5 d in vitro. Mouse ES cells were also placed into deafened guinea pig cochleae and Neurog1 expression was induced for 48 h followed by 26 d of BDNF/GDNF infusion. In vivo differentiation resulted in 50-75% of ES cells bearing markers of early neurons, and a majority of these cells had a glutamatergic phenotype. This is the first study to report a high percentage of ES cell differentiation into a glutamatergic phenotype and sets the stage for cell replacement of auditory nerve.

Embryonic stem cell/fibroblast hybrid cells with near-tetraploid karyotype provide high yield of chimeras

Ten primary clones of hybrid cells were produced by the fusion of diploid embryonic stem (ES) cells, viz., line E14Tg2aSc4TP6.3 marked by green fluorescent protein (GFP), with diploid embryonic or adult fibroblasts derived from DD/c mice. All the hybrid clones had many characteristics similar to those of ES cells and were positive for GFP. Five hybrid clones having ploidy close to tetraploidy (over 80% of cells had 76-80 chromosomes) were chosen for the generation of chimeras via injection into C57BL blastocysts. These hybrid clones also contained microsatellites marking all ES cell and fibroblast chromosomes judging from microsatellite analysis. Twenty chimeric embryos at 11-13 days post-conception were obtained after injection of hybrid cells derived from two of three clones. Many embryos showed a high content of GFP-positive descendents of the tested hybrid cells. Twenty one adult chimeras were generated by the injection of hybrid cells derived from three clones. The contribution of GFP-labeled hybrid cells was significant and comparable with that of diploid E14Tg2aSc4TP6.3 cells. Cytogenetic and microsatellite analyses of cell cultures derived from chimeric embryos or adults indicated that the initial karyotype of the tested hybrid cells remained stable during the development of the chimeras, i.e., the hybrid cells were mainly responsible for the generation of the chimeras. Thus, ES cell/fibroblast hybrid cells with near-tetraploid karyotype are able to generate chimeras at a high rate, and many adult chimeras contain a high percentage of descendants of the hybrid 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.

Inhibition of Activin/Nodal signaling promotes specification of human embryonic stem cells into neuroectoderm

Nodal, a member of the TGF-beta family of signaling molecules, has been implicated in pluripotency in human embryonic stem cells (hESCs) [Vallier, L., Reynolds, D., Pedersen, R.A., 2004a. Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway. Dev. Biol. 275, 403-421], a finding that seems paradoxical given Nodal's central role in mesoderm/endoderm specification during gastrulation. In this study, we sought to clarify the role of Nodal signaling during hESC differentiation by constitutive overexpression of the endogenous Nodal inhibitors Lefty2 (Lefty) and truncated Cerberus (Cerb-S) and by pharmacological interference using the Nodal receptor antagonist SB431542. Compared to wildtype (WT) controls, embryoid bodies (EBs) derived from either Lefty or Cerb-S overexpressing hESCs showed increased expression of neuroectoderm markers Sox1, Sox3, and Nestin. Conversely, they were negative for a definitive endoderm marker (Sox17) and did not generate beating cardiomyocyte structures in conditions that allowed mesendoderm differentiation from WT hESCs. EBs derived from either Lefty or Cerb-S expressing hESCs also contained a greater abundance of neural rosette structures as compared to controls. Differentiating EBs derived from Lefty expressing hESCs generated a dense network of beta-tubulin III positive neurites, and when Lefty expressing hESCs were grown as a monolayer and allowed to differentiate, they generated significantly higher numbers of beta-tubulin positive neurons as compared to wildtype hESCs. SB431542 treatments reproduced the neuralising effects of Lefty overexpression in hESCs. These results show that inhibition of Nodal signaling promotes neuronal specification, indicating a role for this pathway in controlling early neural development of pluripotent cells.

Mbd3, a component of the NuRD co-repressor complex, is required for development of pluripotent cells

Mbd3 is a core component of the NuRD (Nucleosome Remodeling and Histone Deacetylation) co-repressor complex, and NuRD-mediated silencing has been implicated in cell fate decisions in a number of contexts. Mbd3-deficient embryonic stem (ES) cells made by gene targeting are viable but fail to form a stable NuRD complex, are severely compromised in the ability to differentiate, and show LIF-independent self-renewal. Mbd3 is known to be essential for postimplantation embryogenesis in mice, but the function of Mbd3 in vivo has not previously been addressed. Here we show that the inner cell mass (ICM) of Mbd3-deficient blastocysts fails to develop into mature epiblast after implantation. Unlike Mbd3-null ES cells, Mbd3-deficient ICMs grown ex vivo fail to expand their Oct4-positive, pluripotent cell population despite producing robust endoderm outgrowths. Additionally, we identify a set of genes showing stage-specific expression in ICM cells during preimplantation development, and show that Mbd3 is required for proper gene expression patterns in pre- and peri-implantation embryos and in ES cells. These results demonstrate the importance of Mbd3/NuRD for the development of pluripotent cells in vivo and for their ex vivo progression into embryonic stem cells, and highlight the differences between ES cells and the ICM cells from which they are derived.

Unipolar brush cells of the cerebellum are produced in the rhombic lip and migrate through developing white matter

Unipolar brush cells (UBCs) are glutamatergic interneurons in the cerebellar cortex and dorsal cochlear nucleus. We studied the development of UBCs, using transcription factor Tbr2/Eomes as a marker for UBCs and their progenitors in embryonic and postnatal mouse cerebellum. Tbr2+ UBCs appeared to migrate out of the upper rhombic lip via two cellular streams: a dorsal pathway into developing cerebellar white matter, where the migrating cells dispersed widely before entering the internal granular layer, and a rostral pathway along the cerebellar ventricular zone toward the brainstem. Ablation of the rhombic lip in organotypic slice cultures substantially reduced the production of Tbr2+ UBCs. In coculture experiments, Tbr2+ UBCs migrated from rhombic lip explants directly into the developing white matter of adjacent cerebellar slices. The origin of Tbr2+ UBCs was confirmed by colocalization with beta-galactosidase expressed from the Math1 locus, a molecular marker of rhombic lip lineages. Moreover, the production of Tbr2+ UBCs was Math1 dependent, as Tbr2+ UBCs were severely reduced in Math1-null cerebellum. In reeler mutant mice, Tbr2+ UBCs accumulated near the rhombic lip, consistent with impaired migration through developing white matter. Our results suggest that UBCs arise from the rhombic lip and migrate via novel pathways to their final destinations in the cerebellum and dorsal cochlear nucleus. Our findings support a model of cerebellar neurogenesis, in which glutamatergic and GABAergic neurons are produced from separate progenitor pools located mainly in the rhombic lip and the cerebellar ventricular zone, respectively.

Nanog promotes transfer of pluripotency after cell fusion

Through cell fusion, embryonic stem (ES) cells can erase the developmental programming of differentiated cell nuclei and impose pluripotency. Molecules that mediate this conversion should be identifiable in ES cells. One candidate is the variant homeodomain protein Nanog, which has the capacity to entrain undifferentiated ES cell propagation. Here we report that in fusions between ES cells and neural stem (NS) cells, increased levels of Nanog stimulate pluripotent gene activation from the somatic cell genome and enable an up to 200-fold increase in the recovery of hybrid colonies, all of which show ES cell characteristics. Nanog also improves hybrid yield when thymocytes or fibroblasts are fused to ES cells; however, fewer colonies are obtained than from ES x NS cell fusions, consistent with a hierarchical susceptibility to reprogramming among somatic cell types. Notably, for NS x ES cell fusions elevated Nanog enables primary hybrids to develop into ES cell colonies with identical frequency to homotypic ES x ES fusion products. This means that in hybrids, increased Nanog is sufficient for the NS cell epigenome to be reset completely to a state of pluripotency. We conclude that Nanog can orchestrate ES cell machinery to instate pluripotency with an efficiency of up to 100% depending on the differentiation status of the somatic cell.

Sensory neurite outgrowth on white matter astrocytes is influenced by intracellular and extracellular S100A4 protein

The central nervous system (CNS) is considered a nonpermissive environment for axonal regeneration because of the presence of myelin and associated repulsive molecules. However, neural cells transplanted to the CNS preferably migrate and extend their fibers in white matter areas. We previously showed that white matter astrocytes in vivo express the calcium-binding protein S100A4, which is strongly up-regulated in areas of white matter degeneration. To investigate the role of white matter astrocytes and their specific protein S100A4 in axonal regeneration, we developed white matter astrocyte cultures with strong S100A4 expression and grew dissociated adult dorsal root ganglion (DRG) cells on top of astrocytes for 24 hr. By using small interfering S100A4 RNA, we were able to eliminate S100A4 expression and compare growth of DRG cell neurites on S100A4-silenced and S100A4-expressing astrocytes. In addition, we studied whether extracellular S100A4 has an effect on neurite growth from adult DRG cells cultured on S100A4-expressing white matter astrocytes. Our data show that white matter astrocytes are permissive for neurite growth, although high levels of S100A4 in white matter astrocytes have a negative effect on this growth. Extracellular application of S100A4 induced extensive growth of DRG cell neurites on white matter astrocytes. These findings suggest that white matter astrocytes are able to support axonal regeneration and, furthermore, that administration of extracellular S100A4 provides strong additional support for axonal regeneration.

HOXB4 overexpression promotes hematopoietic development by human embryonic stem cells

Human embryonic stem cells (hESCs) are a potential source of hematopoietic cells for therapeutic transplantation and can provide a model for human hematopoiesis. Culture of hESCs on murine stromal layers or in stromal-free conditions as embryoid bodies results in low levels of hematopoietic cells. Here we demonstrate that overexpression of the transcription factor HOXB4 considerably augments hematopoietic development of hESCs. Stable HOXB4-expressing hESC clones were generated by lipofection and could be maintained in the undifferentiated state for prolonged passages. Moreover, differentiation of hESCs as embryoid bodies in serum-containing medium without the use of additional cytokines led to sequential expansion of first erythroid and then myeloid and monocytic progenitors from day 10 of culture. These cells retained the capacity to develop into formed blood elements during in vitro culture. Consistent with the development of committed hematopoietic cells, we observed the expression of transcription factors known to be critical for hematopoietic development. We thus demonstrate successful use of enforced gene expression to promote the differentiation of hESCs into a terminally differentiated tissue, thereby revealing an important role for HOXB4 in supporting their in vitro development along the hematopoietic pathway.

Development of the deep cerebellar nuclei: transcription factors and cell migration from the rhombic lip

The deep cerebellar nuclei (DCN) are the main output centers of the cerebellum, but little is known about their development. Using transcription factors as cell type-specific markers, we found that DCN neurons in mice are produced in the rhombic lip and migrate rostrally in a subpial stream to the nuclear transitory zone (NTZ). The rhombic lip-derived cells express transcription factors Pax6, Tbr2, and Tbr1 sequentially as they enter the NTZ. A subset of rhombic lip-derived cells also express reelin, a key regulator of Purkinje cell migrations. In organotypic slice cultures, the rhombic lip was necessary and sufficient to produce cells that migrate in the subpial stream, enter the NTZ, and express Pax6, Tbr2, Tbr1, and reelin. In later stages of development, the subpial stream is replaced by the external granular layer, and the NTZ organizes into distinct DCN nuclei. Tbr1 expression persists to adulthood in a subset of medial DCN projection neurons. In reeler mutant mice, which have a severe cerebellar malformation, rhombic lip-derived cells migrated to the NTZ, despite reelin deficiency. Studies in Tbr1 mutant mice suggested that Tbr1 plays a role in DCN morphogenesis but is not required for reelin expression, glutamatergic differentiation, or the initial formation of efferent axon pathways. Our findings reveal underlying similarities in the transcriptional programs for glutamatergic neuron production in the DCN and the cerebral cortex, and they support a model of cerebellar neurogenesis in which glutamatergic and GABAergic neurons are produced from separate progenitor compartments.

Fate of tetraploid cells in 4n<-->2n chimeric mouse blastocysts

Previous studies have shown that tetraploid (4n) cells rarely contribute to the derivatives of the epiblast lineage of mid-gestation 4n<-->2n mouse chimeras. The aim of the present study was to determine when and how 4n cells were excluded from the epiblast lineage of such chimeras. The contributions of GFP-positive cells to different tissues of 4n<-->2n chimeric blastocysts labelled with tauGFP were analysed at E3.5 and E4.5 using confocal microscopy. More advanced E5.5 and E7.5 chimeric blastocysts were analysed after a period of diapause to allow further growth without implantation. Tetraploid cells were not initially excluded from the epiblast in 4n<-->2n chimeric blastocysts and they contributed to all four blastocyst tissues at all of the blastocyst stages examined. Four steps affected the allocation and fate of 4n cells in chimeras, resulting in their exclusion from the epiblast lineage by mid-gestation. (1) Fewer 4n cells were allocated to the inner cell mass than trophectoderm. (2) The blastocyst cavity tended to form among the 4n cells, causing more 4n cells to be allocated to the hypoblast and mural trophectoderm than the epiblast and polar trophectoderm, respectively. (3) 4n cells were depleted from the hypoblast and mural trophectoderm, where initially they were relatively enriched. (4) After implantation 4n cells must be lost preferentially from the epiblast lineage. Relevance of these results to the aetiology of human confined placental mosaicism and possible implications for the interpretation of mouse tetraploid complementation studies of the site of gene action are discussed.

Activin/Nodal and FGF pathways cooperate to maintain pluripotency of human embryonic stem cells

Maintenance of pluripotency is crucial to the mammalian embryo's ability to generate the extra-embryonic and embryonic tissues that are needed for intrauterine survival and foetal development. The recent establishment of embryonic stem cells from human blastocysts (hESCs) provides an opportunity to identify the factors supporting pluripotency at early stages of human development. Using this in vitro model, we have recently shown that Nodal can block neuronal differentiation, suggesting that TGFbeta family members are involved in cell fate decisions of hESCs, including preservation of their pluripotency. Here, we report that Activin/Nodal signalling through Smad2/3 activation is necessary to maintain the pluripotent status of hESCs. Inhibition of Activin/Nodal signalling by follistatin and by overexpression of Lefty or Cerberus-Short, or by the Activin receptor inhibitor SB431542, precipitates hESC differentiation. Nevertheless, neither Nodal nor Activin is sufficient to sustain long-term hESC growth in a chemically defined medium without serum. Recent studies have shown that FGF2 can also maintain long-term expression of pluripotency markers, and we find that inhibition of the FGF signalling pathway by the tyrosine kinase inhibitor SU5402 causes hESC differentiation. However, this effect of FGF on hESC pluripotency depends on Activin/Nodal signalling, because it is blocked by SB431542. Finally, long-term maintenance of in-vitro pluripotency can be achieved with a combination of Activin or Nodal plus FGF2 in the absence of feeder-cell layers, conditioned medium or Serum Replacer. These findings suggest that the Activin/Nodal pathway maintains pluripotency through mechanism(s) in which FGF acts as a competence factor and therefore provide further evidence of distinct mechanisms for preservation of pluripotency in mouse and human ESCs.

Neural differentiation of mouse embryonic stem cells in chemically defined medium

Directed differentiation of embryonic stem (ES) cells has enormous potential to derive a wide variety of defined cell populations of therapeutic value. To achieve this, it is necessary to use protocols that promote cell differentiation under defined culture conditions. Furthermore, understanding the mechanisms of cell differentiation in vitro will allow the development of rationale approaches to systematically manipulate cell fates. Here we have analysed the differentiation of mouse ES cells to the neural lineage under serum and feeder cell-free conditions, using a previously described chemically defined medium (CDM). In CDM, ES cell differentiation is highly neurogenic. Cell differentiation was monitored by analysis of a gene expression array (Clontech-Atlas) and by semi-quantitative RT-PCR for a panel of genes involved in cell lineage specification and patterning of the epiblast. In addition to expression of neural markers, data identified a transient expression of several genes associated with the organising activities of the embryonic node and visceral endoderm, including regulators of WNT, BMP, Hedgehog and FGF signaling pathways. Neural differentiation in CDM does not occur by a simple default mechanism, but was dependent on endogenous FGF signaling, and could be blocked by adding BMP4, and LiCl to simulate WNT activation. Neural differentiation was also inhibited by antagonising endogenous hedgehog activity. Taken together the profile of gene expression changes seen in CDM cultures recapitulates those seen in the early embryo, and is suggestive of common developmental mechanisms.

Cell Proliferation in the Adult Hippocampal Formation of Rodents and its Modulation by Entorhinal and Fimbria–Fornix Afferents

New granule neurons are produced in the dentate gyrus (DG) of rodents throughout adult life. Recent studies have also reported adult neurogenesis in the cerebral cortex in normal animals or after brain injury. However, few of these studies focused on the hippocampal formation (HF), a cortical area involved in learning and memory in which extensive cell death occurs in neurodegenerative diseases. Thus, we studied cell proliferation in the HF of rodents and the intrinsic putative neurogenic potential of entorhinal cortex (EC) progenitors. We show that only the DG generates new neurons in the HF. In addition, neurospheres from the EC differentiate into neurons and glia in vitro and after transplantation in the adult DG. We also analyzed whether the absence of the synaptic input from the main hippocampal afferents induces neuronal generation in the HF outside the DG and/or regulates the proliferation of DG neuroprogenitors. We show that the denervation of the hippocampus does not induce neurogenesis in HF regions other than the DG. However, neuroprogenitor proliferation in the DG is reduced after fimbria-fornix lesions but not after entorhinal deafferentation, which supports the view that neuroprogenitor proliferation and/or differentiation in the DG are controlled from basal forebrain/septal neurons.

Unequal segregation of parental chromosomes in embryonic stem cell hybrids

Chromosome segregation was studied in 14 intra- and 20 inter-specific hybrid clones generated by fusion of Mus musculus embryonic stem (ES) cells with fibroblasts or splenocytes of DD/c mice or Mus caroli. As a control for in vitro evolution of tetraploid karyotype we used a set of hybrid clones obtained by fusion of ES cells (D3) with ES cells (TgTP6.3). Identification of the parental chromosomes in the clones was performed by microsatellite analysis and in situ hybridization with labeled species-specific probes. Both analyses have revealed three types of clones: (i) stable tetraploid, observed only for ES x ES cell hybrids; (ii) bilateral loss of chromosomes of both ES and somatic partners; (iii) unilateral segregation of chromosomes of the somatic partner. Observed unilateral segregation was extensive in ES-splenocyte cell hybrids, but lower in ES-fibroblast hybrid clones. Developmental state of the somatic partner is presumably responsible for directional chromosome loss. Nonrandom segregation implies that initial differences in the parental homologous chromosomes were not immediately equalized implying at least transient persistence of the differentiated epigenotype.

Developmental activation of the Rb-E2F pathway and establishment of cell cycle-regulated cyclin-dependent kinase activity during embryonic stem cell differentiation

To understand cell cycle control mechanisms in early development and how they change during differentiation, we used embryonic stem cells to model embryonic events. Our results demonstrate that as pluripotent cells differentiate, the length of G(1) phase increases substantially. At the molecular level, this is associated with a significant change in the size of active cyclin-dependent kinase (Cdk) complexes, the establishment of cell cycle-regulated Cdk2 activity and the activation of a functional Rb-E2F pathway. The switch from constitutive to cell cycle-dependent Cdk2 activity coincides with temporal changes in cyclin A2 and E1 protein levels during the cell cycle. Transcriptional mechanisms underpin the down-regulation of cyclin levels and the establishment of their periodicity during differentiation. As pluripotent cells differentiate and pRb/p107 kinase activities become cell cycle dependent, the E2F-pRb pathway is activated and imposes cell cycle-regulated transcriptional control on E2F target genes, such as cyclin E1. These results suggest the existence of a feedback loop where Cdk2 controls its own activity through regulation of cyclin E1 transcription. Changes in rates of cell division, cell cycle structure and the establishment of cell cycle-regulated Cdk2 activity can therefore be explained by activation of the E2F-pRb pathway.

LIF/STAT3 controls ES cell self-renewal and pluripotency by a Myc-dependent mechanism

Murine ES cells can be maintained as a pluripotent, self-renewing population by LIF/STAT3-dependent signaling. The downstream effectors of this pathway have not been previously defined. In this report, we identify a key target of the LIF self-renewal pathway by showing that STAT3 directly regulates the expression of the Myc transcription factor. Murine ES cells express elevated levels of Myc and following LIF withdrawal, Myc mRNA levels collapse and Myc protein becomes phosphorylated on threonine 58 (T58), triggering its GSK3beta dependent degradation. Maintained expression of stable Myc (T58A) renders self-renewal and maintenance of pluripotency independent of LIF. By contrast, expression of a dominant negative form of Myc antagonizes self-renewal and promotes differentiation. Transcriptional control by STAT3 and suppression of T58 phosphorylation are crucial for regulation of Myc activity in ES cells and therefore in promoting self-renewal. Together, our results establish a mechanism for how LIF and STAT3 regulate ES cell self-renewal and pluripotency.

Nodal inhibits differentiation of human embryonic stem cells along the neuroectodermal default pathway

Genetic studies in fish, amphibia, and mice have shown that deficiency of Nodal signaling blocks differentiation into mesoderm and endoderm. Thus, Nodal is considered as a major inducer of mesendoderm during gastrulation. On this basis, Nodal is a candidate for controlling differentiation of pluripotent human embryonic stem cells (hESCs) into tissue lineages with potential clinical value. We have investigated the effect of Nodal, both as a recombinant protein and as a constitutively expressed transgene, on differentiation of hESCs. When control hESCs were grown in chemically defined medium, their expression of markers of pluripotency progressively decreased, while expression of neuroectoderm markers was strongly upregulated, thus revealing a neuroectodermal default mechanism for differentiation in this system. hESCs cultured in recombinant Nodal, by contrast, showed prolonged expression of pluripotency marker genes and reduced induction of neuroectoderm markers. These Nodal effects were accentuated in hESCs expressing a Nodal transgene, with striking morphogenetic consequences. Nodal-expressing hESCs developing as embryoid bodies contained an outer layer of visceral endoderm-like cells surrounding an inner layer of epiblast-like cells, each layer having distinct gene expression patterns. Markers of neuroectoderm were not upregulated during development of Nodal-expressing embryoid bodies, nor was there induction of markers for definitive mesoderm or endoderm differentiation. Moreover, the inner layer expressed markers of pluripotency, characteristic of undifferentiated hESCs and of epiblast in mouse embryos. These results could be accounted for by an inhibitory effect of Nodal-induced visceral endoderm on pluripotent cell differentiation into mesoderm and endoderm, with a concomitant inhibition of neuroectoderm differentiation by Nodal itself. There could also be a direct effect of Nodal in the maintenance of pluripotency. In summary, analysis of the Nodal-expressing phenotype suggests a function for the transforming growth factor-beta (TGF-beta) growth factor superfamily in pluripotency and in early cell fate decisions leading to primary tissue layers during in vitro development of pluripotent human stem cells. The effects of Nodal on early differentiation illustrate how hESCs can augment mouse embryos as a model for analyzing mechanisms of early mammalian development.

Central migration of neuronal tissue and embryonic stem cells following transplantation along the adult auditory nerve

The regeneration of the auditory nerve remains a challenge in restoring hearing. An interesting approach would be to use a cell replacement therapy with the potential to establish connections from the inner ear to the central auditory system. This hypothesis was tested by xenografted (mouse to rat) implantation of embryonic dorsal root ganglion (DRG) neurons and embryonic stem (ES) cells along the auditory nerve in the adult host. DRG neurons were obtained at embryonic day 13-14 in transgenic animals expressing enhanced green fluorescence protein (EGFP). For embryonic stem cells, a tau-GFP ES cell line was used as a donor. The fibers of the auditory nerve in the adult rat were transected through the modiolus at the first cochlear turn, and the biological implants were transplanted into the transection. The transplanted DRG neurons and ES cells survived for a postoperative survival time ranging from 3 to 9 weeks, verified by EGFP/GFP fluorescence, and neurofilament or TUJ1 immunostaining. At 9 weeks following implantation, the implanted DRG neurons were found to have migrated along the auditory nerve in the internal meatus. At the same postoperative time, the ES cells had migrated into the brain stem close to the ventral cochlear nucleus. The results demonstrate not only the survival and migration of xenografted DRG neurons and stem cells along the adult auditory nerve but also the feasibility of a cell replacement therapy in the degenerated auditory system.

Neural development by transplanted human embryonal carcinoma stem cells expressing green fluorescent protein

For many years, researchers have investigated the fate and potential of neuroectodermal cells during the development of the central nervous system. Although several key factors that regulate neural differentiation have been identified, much remains unknown about the molecular mechanisms that control the fate and specification of neural subtypes, especially in humans. Human embryonal carcinoma (EC) stem cells are valuable research tools for the study of neural development; however, existing in vitro experiments are limited to inducing the differentiation of EC cells into only a handful of cell types. In this study, we developed and characterized a novel EC cell line (termed TERA2.cl.SP12-GFP) that carries the reporter molecule, green fluorescent protein (GFP). We demonstrate that TERA2.cl.SP12-GFP stem cells and their differentiated neural derivatives constitutively express GFP in cells grown both in vitro and in vivo. Cellular differentiation does not appear to be affected by insertion of the transgene. We propose that TERA2.cl.SP12-GFP cells provide a valuable research tool to track the fate of cells subsequent to transplantation into alternative environments and that this approach may be particularly useful to investigate the differentiation of human neural tissues in response to local environmental signals.

Evaluation of the mouse TgTP6.3 tauGFP transgene as a lineage marker in chimeras

The mouse TgTP6.3 transgene, encoding a tauGFP fusion protein, is becoming widely used but has yet to be fully characterized and evaluated as suitable lineage marker. The aim of the present study was to investigate the phenotype of TgTP6.3(+/+) homozygotes and TgTP6.3(+/-) hemizygotes, characterize the expression of the TgTP6.3 transgene in different tissues and critically evaluate its use as a lineage marker. TgTP6.3(+/+) homozygotes died between embryonic day 14.5 and weaning, whereas TgTP6.3(+/-) hemizygotes were mostly viable and fertile but smaller than non-transgenic siblings. TgTP6.3 expression began in the late two-cell stage, persisted in most fetal and adult tissues and was uniformly expressed in many (but not all) tissues. TgTP6.3(+/-) cells were readily identified in many chimeric tissues and their contribution appeared to be quantitatively and spatially normal. Overall, tauGFP expression in hemizygous TgTP6.3(+/-) cells fulfils the main criteria of a good lineage marker for many tissues. It provides a useful lineage marker, which should be particularly suitable for axons, blood vessels and pre-implantation embryos.

Human Embryonal Carcinoma Stem Cells Expressing Green Fluorescent Protein Form Functioning Neurons In Vitro: A Research Tool for Co-culture Studies

Neural differentiation is controlled by complex molecular mechanisms that determine cell fate and diversity within the nervous system. Interactions between developing tissues play an important role in regulating this process. In vitro co-culture experiments offer a method to study cell differentiation and function under controlled conditions, with the additional benefit of investigating how interactions between populations of cells influence cell growth and behavior. However, it can often be difficult to distinguish between populations of co-cultured cells. Here we report the development of a human embryonal carcinoma (EC) stem cell line (named TERA2.cl.SP12-GFP) that expresses the genetic marker, green fluorescent protein (GFP). Here, we demonstrate that TERA2.cl.SP12-GFP stem cells stably express GFP and that this remains detectable during retinoic acid-induced differentiation. Regulated expression of neural markers during cell development correlated with the formation of morphologically identifiable neurons. Populations of post-mitotic GFP-positive neurons were readily purified and electrophysiological characterization confirmed that such neurons were functionally active. Thus, cultured TERA2.cl.SP12-GFP cells can be readily distinguished from alternative cell types in vitro and provide an amenable system for live cell imaging to study the development and function of human neurons in isolation, and in co-culture with other tissue types.

Restricted growth of Schwann cells lacking Cajal bands slows conduction in myelinated nerves

Nerve impulses are propagated at nodes of Ranvier in the myelinated nerves of vertebrates. Internodal distances have been proposed to affect the velocity of nerve impulse conduction; however, direct evidence is lacking, and the cellular mechanisms that might regulate the length of the myelinated segments are unknown. Ramón y Cajal described longitudinal and transverse bands of cytoplasm or trabeculae in internodal Schwann cells and suggested that they had a nutritive function. Here we show that internodal growth in wild-type nerves is precisely matched to nerve extension, but disruption of the cytoplasmic bands in Periaxin-null mice impairs Schwann cell elongation during nerve growth. By contrast, myelination proceeds normally. The capacity of wild-type and mutant Schwann cells to elongate is cell-autonomous, indicating that passive stretching can account for the lengthening of the internode during limb growth. As predicted on theoretical grounds, decreased internodal distances strikingly decrease conduction velocities and so affect motor function. We propose that microtubule-based transport in the longitudinal bands of Cajal permits internodal Schwann cells to lengthen in response to axonal growth, thus ensuring rapid nerve impulse transmission.

Tracing axons of peripheral nerves in rats: a potential technique to study the equine recurrent laryngeal nerve

To study the fascicular anatomy of peripheral nerves, three different groups of retrograde axonal tracers were evaluated: fluorophores, horseradish peroxidase conjugated to subunit B of cholera toxin (CT-HRP), and adeno-associated virus (AAV). The hindlimb nerves in rats served as a model to identify the most efficient tracer in regard to labeling axons within peripheral nerves. The rat's tibial and common peroneal nerves were injected with the different tracers and the sciatic nerve was subsequently examined for evidence of labeled axons. The CT-HRP clearly provided the best results in this rat model. Subsequently, CT-HRP was injected into the recurrent laryngeal nerve (RLN) of two horses in order to identify the location and distribution pattern of the RLN axons within the course of the cervical vagus nerve trunk. No labeling could be observed in either of the two horses.

Regulation of Nogo and Nogo receptor during the development of the entorhino-hippocampal pathway and after adult hippocampal lesions

Axonal regeneration in the adult CNS is limited by the presence of several inhibitory proteins associated with myelin. Nogo-A, a myelin-associated inhibitor, is responsible for axonal outgrowth inhibition in vivo and in vitro. Here we study the onset and maturation of Nogo-A and Nogo receptor in the entorhino-hippocampal formation of developing and adult mice. We also provide evidence that Nogo-A does not inhibit embryonic hippocampal neurons, in contrast to other cell types such as cerebellar granule cells. Our results also show that Nogo and Nogo receptor mRNA are expressed in the adult by both principal and local-circuit hippocampal neurons, and that after lesion, Nogo-A is also transiently expressed by a subset of reactive astrocytes. Furthermore, we analyzed their regulation after kainic acid (KA) treatment and in response to the transection of the entorhino-hippocampal connection. We found that Nogo-A and Nogo receptor are differentially regulated after kainic acid or perforant pathway lesions. Lastly, we show that the regenerative potential of lesioned entorhino-hippocampal organotypic slice co-cultures is increased after blockage of Nogo-A with two IN-1 blocking antibodies. In conclusion, our results show that Nogo and its receptor might play key roles during development of hippocampal connections and that they are implicated in neuronal plasticity in the adult.

Repair of the entorhino-hippocampal projection in vitro

The repair of axonal projections and the reconstruction of neuronal circuits after CNS lesions or during neurodegenerative disease are major challenges in restorative neuroscience. We have explored the potential of transplanted immature neurons to repair a specific axonal projection in an entorhino-hippocampal slice culture model system. When slices of immature entorhinal cortex (EC) from tau-GFP transgenic mice were cultured next to slices from postnatal hippocampus, an axonal projection from the E18 embryonic entorhinal cortex to the dentate gyrus of the postnatal hippocampus developed, which was similar to that observed in control cultures. Even more immature neuronal precursors in slices from E15 developing cerebral cortex differentiated and established an axonal projection to the hippocampal slice. This projection terminated specifically in the outer molecular layer of the dentate gyrus, the normal target area of the entorhino-hippocampal projection. When embryonic tissue from the presumptive brainstem area was used, there was still a subpopulation of fibers with a specific termination in the outer molecular layer, but few specific fibers were found in cocultures with embryonic midbrain. Our results show that very immature cortical neurons are potentially able to form an entorhino-hippocampal projection that terminates in a correct lamina-specific fashion in the dentate gyrus. These findings support the idea that immature neuronal precursor cells could be used for the reconstruction of specific neuronal circuits.

A simple model for the study of effects of the extracellular matrix on the cell morphology in vitro

In the present work, a simple technique is proposed to study the effects of native extracellular matrix (ECM) of one cell type on the properties of other cell types. It is based on a procedure in which, after cells of one type are removed from the substrate, cells of another type are seeded on the same substrate. To obtain preparations of native ECM, cells were removed from the substrate by 0.02% EDTA only, without any proteolytic enzymes. Cells were placed on coverslips in standard Petri dishes and incubated in a culture medium for a time sufficient for adhesion and spreading, but not long enough to undergo mitosis. Up to four coverslips per Petri dish can be incubated, and various combinations of ECM and cell types can be used in one dish. It is important, therefore, that the different "ECM-cell" combinations are present in the same culture medium. For evaluation of ECM effects, the area occupied by the cell on a substrate and the perimeter of the cell were measured, and frequencies of cell distribution were calculated according to these parameters.

Green fluorescent protein gene-transfected peafowl somatic cells participate in the development of chicken embryos

This study was performed to investigate whether the embryonic somatic cells are capable of reconstituting and participating in the embryonic development of chickens to produce chimeras. In order to track the migration behavior of the donor cells, a cell line, originally isolated from an Indian peafowl embryo, was fluorescent-labeled by transfection of the cells with enhanced Green Fluorescent Protein (GFP) and Neomycin resistant (Neo) genes prior to injection into the stage X blastoderm of White Leghorn chickens. The injection was performed with a medium in the presence of 1-5% polyethylene glycol. The development of putative chimeric embryos between the stages three and 24 was examined for GFP expression under fluorescent light. To trace the peafowl cells in the developing chicken embryos, both a species-specific genetic marker originating from the mitochondrial DNA cytochrome b (cyt b) gene and a DNA fragment of GFP gene were used. Of the 185 fertile eggs manipulated, 173 developed into embryos. Fifty-five of them showed positive GFP patches in extra-embryonic tissues, and 15 expressed GFP in intra-embryonic tissues such as those of the head, heart, and gonad. PCR analysis revealed that PCR fragments for the peafowl mitochondrial DNA cyt b and GFP genes were detected in the samples of the GFP positive extra- and intra-embryonic tissues of the chimeras. The present results provide evidence that fluorescent-labeled peafowl embryonic cells carrying GFP and Neo genes are able to participate in the development of chicken embryos to generate chimeras.

Cdk6-cyclin D3 activity in murine ES cells is resistant to inhibition by p16(INK4a)

Through a screen aimed at identifying genes that are specifically upregulated in embryomic stem (ES) cells but not primitive ectoderm, we identified cyclin D3. This was surprising since cyclin D activity is generally believed to be inactive in ES cells even though retinoblastoma tumor suppressor protein (pRb) accumulates in a predominantly hyperphosphorylated state. Cdk6 is the major catalytic partner for cyclin D3 in ES cells and exhibits robust pRb kinase activity that is downregulated during the early stages of ES embryoid body differentiation. To investigate the basis underlying the insensitivity of ES cells to ectopic p16 expression, we show that Cdk6-cyclin D3 complexes are not subject to inhibition by p16, similar to Cdk-viral cyclin complexes. These observations show that specificity exists between Cdk4/6-cyclin D complexes and their ability to be targeted by p16. Our data suggest that Cdk6-cyclin D3 activity in other cell types, including tumors, may also be refractory to p16-mediated growth inhibition and raises the possibility of additional specificity within the INK4 family.

Enhancing and diminishing gene function in human embryonic stem cells

It is widely recognized that gain- and loss-of-function approaches are essential for understanding the functions of specific genes, and such approaches would be particularly valuable in studies involving human embryonic stem (hES) cells. We describe a simple and efficient approach using lipofection to transfect hES cells, which enabled us to generate hES cell lines expressing naturally fluorescent green or red proteins without affecting cell pluripotency. We used these cell lines to establish a means of diminishing gene function using small interfering (si)RNAs, which were effective at knocking down gene expression in hES cells. We then demonstrated that stable expression of siRNA could knock down the expression of endogenous genes. Application of these gain- and loss-of-function approaches should have widespread use, not only in revealing the developmental roles of specific human genes, but also for their utility in modulating differentiation.

Neuronal differentiation following transplantation of expanded mouse neurosphere cultures derived from different embryonic forebrain regions

In vitro, expanded neurospheres exhibit multipotent properties and can differentiate into neurons, astrocytes and oligodendrocytes. In vivo, cells from neurospheres derived from mouse fetal forebrain have previously been reported to predominantly differentiate into glial cells, and not into neurons. Here we isolated stem/progenitor cells from E13.5 lateral ganglionic eminence (LGE), medial ganglionic eminence (MGE) and cortical primordium, of a green fluorescent protein (GFP)-actin transgenic mouse. Free-floating neurospheres were expanded in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) and implanted after five to six passages into the striatum, hippocampus and cortex of neonatal rats. Cell suspensions of primary LGE tissue were prepared and grafted in parallel. Grafted cells derived from the primary tissue displayed widespread incorporation into all regions, as visualized with the mouse-specific antibody M2, or mouse satellite DNA in situ hybridization, and differentiated into both neurons, astrocytes and oligodendrocytes. Grafts of neurosphere cells derived from the LGE, MGE and cortical primordium differentiated primarily into astrocytes, but contained low but significant numbers of GFP-immunoreactive neurons. Neurons derived from LGE neurospheres were of three types: cells with the morphology of medium-sized densely spiny projection neurons in the striatum; cells with interneuron-like morphologies in striatum, cortex and hippocampus; and cells integrating into SVZ and migrating along the RMS to the olfactory bulb. MGE- or cortical primordium-derived neurospheres differentiated into interneuron-like cells in both striatum and hippocampus. The results demonstrate the ability of in vitro expanded neural stem/progenitor cells to generate both neurons and glia after transplantation into neonatal recipients, and differentiate in a region-specific manner into mature neurons with morphological features characteristic for each target site.

Technicolour transgenics: imaging tools for functional genomics in the mouse

Over the past decade, a battery of powerful tools that encompass forward and reverse genetic approaches have been developed to dissect the molecular and cellular processes that regulate development and disease. The advent of genetically-encoded fluorescent proteins that are expressed in wild type and mutant mice, together with advances in imaging technology, make it possible to study these biological processes in many dimensions. Importantly, these technologies allow direct visual access to complex events as they happen in their native environment, which provides greater insights into mammalian biology than ever before.

Visualization of molecular and cellular events with green fluorescent proteins in developing embryos: a review

During the past 5 years, green fluorescent protein (GFP) has become one of the most widely used in vivo protein markers for studying a number of different molecular processes during development, such as promoter activation, gene expression, protein trafficking and cell lineage determination. GFP fluorescence allows observation of dynamic developmental processes in real time, in both transiently and stably transformed cells, as well as in live embryos. In this review, we include the most up-to-date use of GFP during embryonic development and point out the unique contribution of GFP visualization, which resulted in novel discoveries.

Defined conditions for neural commitment and differentiation

The efficiency of monolayer differentiation establishes that commitment of ES cells to a neural fate needs neither multicellular aggregation nor extrinsic inducers. The entire process by which pluripotent ES cells acquire neural specification can be visualized and recorded at the level of individual colonies. Furthermore this simple culture system is amenable to cellular and molecular dissection, promising to yield new insights into the mechanism underlying neural determination in mammals and perhaps to deliver the goal of "directed" homogeneous differentiation of ES cells.

Normal timing of oligodendrocyte development from genetically engineered, lineage-selectable mouse ES cells

Oligodendrocytes are post-mitotic cells that myelinate axons in the vertebrate central nervous system (CNS). They develop from proliferating oligodendrocyte precursor cells (OPCs), which arise in germinal zones, migrate throughout the developing white matter and divide a limited number of times before they terminally differentiate. Thus far, it has been possible to purify OPCs only from the rat optic nerve, but the purified cells cannot be obtained in large enough numbers for conventional biochemical analyses. Moreover, the CNS stem cells that give rise to OPCs have not been purified, limiting one's ability to study the earliest stages of commitment to the oligodendrocyte lineage. Pluripotent, mouse embryonic stem (ES) cells can be propagated indefinitely in culture and induced to differentiate into various cell types. We have genetically engineered ES cells both to positively select neuroepithelial stem cells and to eliminate undifferentiated ES cells. We have then used combinations of known signal molecules to promote the development of OPCs from selected, ES-cell-derived, neuroepithelial cells. We show that the earliest stages of oligodendrocyte development follow an ordered sequence that is remarkably similar to that observed in vivo, suggesting that the ES-cell-derived neuroepithelial cells follow a normal developmental pathway to produce oligodendrocytes. These engineered ES cells thus provide a powerful system to study both the mechanisms that direct CNS stem cells down the oligodendrocyte pathway and those that influence subsequent oligodendrocyte differentiation. This strategy may also be useful for producing human cells for therapy and drug screening.

Efficient recombination in diverse tissues by a tamoxifen-inducible form of Cre: a tool for temporally regulated gene activation/inactivation in the mouse

In recent years, the Cre integrase from bacteriophage P1 has become an essential tool for conditional gene activation and/or inactivation in mouse. In an earlier report, we described a fusion protein between Cre and a mutated form of the ligand binding domain of the estrogen receptor (Cre-ER) that renders Cre activity tamoxifen (TM) inducible, allowing for conditional modification of gene activity in the mammalian neural tube in utero. In the current work, we have generated a transgenic mouse line in which Cre-ER is ubiquitously expressed to permit temporally regulated Cre-mediated recombination in diverse tissues of the mouse at embryonic and adult stages. We demonstrate that a single, intraperitoneal injection of TM into a pregnant mouse at 8.5 days postcoitum leads to detectable recombination in the developing embryo within 6 h of injection and efficient recombination of a reporter gene in derivatives of all three germ layers within 24 h of injection. In addition, by varying the dose of TM injected, the percentage of cells undergoing a recombination event in the embryo can be controlled. Dose-dependent excision induced by TM was also possible in diverse tissues in the adult mouse, including the central nervous system, and in cultured cells derived from the transgenic mouse line. This inducible Cre system will be a broadly useful tool to modulate gene activity in mouse embryos, adults, and culture systems where temporal control is an important consideration.

Changing potency by spontaneous fusion

Recent reports have suggested that mammalian stem cells residing in one tissue may have the capacity to produce differentiated cell types for other tissues and organs 1-9. Here we define a mechanism by which progenitor cells of the central nervous system can give rise to non-neural derivatives. Cells taken from mouse brain were co-cultured with pluripotent embryonic stem cells. Following selection for a transgenic marker carried only by the brain cells, undifferentiated stem cells are recovered in which the brain cell genome has undergone epigenetic reprogramming. However, these cells also carry a transgenic marker and chromosomes derived from the embryonic stem cells. Therefore the altered phenotype does not arise by direct conversion of brain to embryonic stem cell but rather through spontaneous generation of hybrid cells. The tetraploid hybrids exhibit full pluripotent character, including multilineage contribution to chimaeras. We propose that transdetermination consequent to cell fusion 10 could underlie many observations otherwise attributed to an intrinsic plasticity of tissue stem cells 9.

Biotinylated dextran amine as a marker for fetal hypothalamic homografts and their efferents

We have explored the use of biotinylated dextran amine (BDA) as a marker for labeling fetal brain grafts and their connections with the host. As a model system we used transplantation of the hamster suprachiasmatic nucleus, the site of an endogenous biological clock governing circadian rhythms. Similar transplants into arrhythmic hosts have been shown to restore behavioral function with a period specific to the donor. For locomotor rhythms, efferent connections are not necessary. For other responses, including endocrine rhythms, efferent connections may be necessary. In order to visualize homografts and their efferents, injections of BDA, an anterograde tracer, were made into the anterior hypothalamic (AH) region containing the SCN or into the dorsal cortex (CTX) of fetal hamster brains. The fetal AH or CTX was microdissected out and stereotaxically implanted into the third ventricle of intact, adult hamsters. After 2, 4, 8, or 12 weeks, hosts were sacrificed and their brains were processed for detection of BDA by either histochemistry or immunofluorescence. BDA intensely labeled graft neurons, their dendrites, and axons with minimal or no spread to the adjacent host brain. Labeled graft axons could be followed for long distances (>1 mm) into the host brain and graft-derived varicosities formed close contacts with host neurons. BDA-labeled graft neurons, located at the perimeter of the graft, also extended dendrite-like processes into the host parenchyma. We conclude that BDA is a useful marker for fetal homografts and their efferents for survival times of less than 2 months.

Radial migration in the cerebral cortex is enhanced by signals from thalamus

The six layered cerebral cortex derives from cells that divide in the ventricular zone and migrate to their final destination in the cortical plate (future cortex). In the mouse, cortical layer III and IV neurons undergo their final mitotic division at around E16, at which time thalamic axons are beginning to enter the cortex. We used bromodeoxyuridine-birth dating of cells in cortical slice cultures to show that the thalamus enhances the migration out of the ventricular zone of future layer III/IV cells. When cortical slices were cultured alone, less than 35% of cells born in vitro on E16 were present in the pial half of the slice after 48 h in culture. In contrast, when cortical slices were cocultured with thalamus, 69% of these cells were found in the pial half of the slice. Explants of other developing tissues did not mimic the effect of the thalamus. The thalamus had no obvious effect on cortical radial glial cells, cortical cell viability or maintenance of cortical slice structure. We found that most precursors born at a similar age but in vivo, shortly before cortical slices were isolated, migrated to the pial half of the cultured slices in the absence of a cocultured thalamic explant. Thus, E16 cortical slices cultured without thalamus permit migration of cells born in vivo and therefore already exposed to the thalamus. Our results indicate that the thalamus provides factors to E16-born cortical precursors that enhance their directed migration out of the ventricular zone to the cortical plate.

Directed differentiation of embryonic stem cells: genetic and epigenetic methods

Embryonic stem cells are derived from the inner cell mass of the pre-implantation blastocyst, and can both self-renew and differentiate into all the cells and tissues of the body. The embryonic stem cell is an unsurpassed starting material to begin to understand a critical, largely inaccessible, period of development, as well as an important source of cells for transplantation and gene therapy. Despite their potential, attempts to obtain specific cell types from embryonic stem cells have been only partially successful because many of the growth factor combinations and developmental control genes involved in cell type restricted differentiation are unknown. This article summarizes some of the recent advances in promoting lineage restricted differentiation of embryonic stem cells, focusing on growth factor manipulation, or genetically altering embryonic stem cells to produce a desired phenotype. The two approaches epitomize current scientific concerns regarding the therapeutic use of these cells; genetic alterations will produce more pure cells with the risk of increasing the likelihood of malignant transformation; epigenetic methods for the manipulation of stem cell phenotype are often incomplete and remaining pluripotent cells are likely to form teratomas. As more is known about lineage specification during development, it will be possible to more precisely control cell type specification.