Adhesion is prerequisite, but alone insufficient, to elicit stem cell pluripotency

The road to generating transplantable organs: from blastocyst complementation to interspecies chimeras

Growing human organs in animals sounds like something from the realm of science fiction, but it may one day become a reality through a technique known as interspecies blastocyst complementation. This technique, which was originally developed to study gene function in development, involves injecting donor pluripotent stem cells into an organogenesis-disabled host embryo, allowing the donor cells to compensate for missing organs or tissues. Although interspecies blastocyst complementation has been achieved between closely related species, such as mice and rats, the situation becomes much more difficult for species that are far apart on the evolutionary tree. This is presumably because of layers of xenogeneic barriers that are a result of divergent evolution. In this Review, we discuss the current status of blastocyst complementation approaches and, in light of recent progress, elaborate on the keys to success for interspecies blastocyst complementation and organ generation.

Building a central nervous system: The neural stem cell lineage revealed

Neural stem cells (NSCs) are a multipotent, self-renewing source of undifferentiated cells in the periventricular region of the mammalian central nervous system (CNS). Since their original discovery 25 years ago, much has been learned about their development, persistence, localization, properties and potential. Herein we discuss the current state of knowledge pertaining to neural stem cells with a focus on the lineage relationship between two NSC populations along the neuraxis and their regionally distinct niches in the CNS.

Surfaceome profiling reveals regulators of neural stem cell function

The composition of cell-surface proteins changes during lineage specification, altering cellular responses to their milieu. The changes that characterize maturation of early neural stem cells (NSCs) remain poorly understood. Here we use mass spectrometry-based cell surface capture technology to profile the cell surface of early NSCs and demonstrate functional requirements for several enriched molecules. Primitive NSCs arise from embryonic stem cells upon removal of Transforming growth factor-β signaling, while definitive NSCs arise from primitive NSCs upon Lif removal and FGF addition. In vivo aggregation assays revealed that N-cadherin upregulation is sufficient for the initial exclusion of definitive NSCs from pluripotent ectoderm, while c-kit signaling limits progeny of primitive NSCs. Furthermore, we implicate EphA4 in primitive NSC survival signaling and Erbb2 as being required for NSC proliferation. This work elucidates several key mediators of NSC function whose relevance is confirmed on forebrain-derived populations and identifies a host of other candidates that may regulate NSCs.

Primitive neural stem cells in the adult mammalian brain give rise to GFAP-expressing neural stem cells

Adult forebrain definitive neural stem cells (NSCs) comprise a subpopulation of GFAP-expressing subependymal cells that arise from embryonic fibroblast growth factor (FGF)-dependent NSCs that are first isolated from the developing brain at E8.5. Embryonic FGF-dependent NSCs are derived from leukemia inhibitory factor (LIF)-responsive, Oct4-expressing primitive NSCs (pNSCs) that are first isolated at E5.5. We report the presence of a rare population of pNCSs in the periventricular region of the adult forebrain. Adult-derived pNSCs (AdpNSCs) are GFAP⁻, LIF-responsive stem cells that display pNSC properties, including Oct4 expression and the ability to integrate into the inner cell mass of blastocysts. AdpNSCs generate self-renewing, multipotent colonies that give rise to definitive GFAP⁺ NSCs in vitro and repopulate the subependyma after the ablation of GFAP⁺ NSCs in vivo. These data support the hypothesis that a rare population of pNSCs is present in the adult brain and is upstream of the GFAP⁺ NSCs.

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Rare, multipotent, self-renewing, Oct4⁺ AdpNSCs in the adult brain

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AdpNSCs lie upstream of definitive, GFAP-expressing adult NSCs

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AdpNSCs repopulate the SE after ablation of GFAP-expressing NSCs

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The AdpNSC pool is activated and expands after injury or LIF infusion in vivo

The neural stem cell lineage reveals novel relationships among spermatogonial germ stem cells and other pluripotent stem cells

The embryonic stem cell (ESC) derived from the inner cell mass is viewed as the core pluripotent cell (PC) type from which all other cell types emanate. This familiar perspective derives from an embryological time line in which PCs are ordered according to their time of appearance. However, this schema does not take into account their potential for interconversion, thereby excluding this critical quality of PCs. The persistence of bona fide pluripotent adult stem cells has garnered increasing attention in recent years. Adult pluripotent spermatogonial germ stem cells (aSGSCs) arise from primordial germ cells (pGCs) that emerge from the epiblast during gastrulation. Adult definitive neural stem cells (dNSCs) arise clonally from pluripotent embryonic primitive neural stem cells (pNSCs), which can also be derived clonally from ESCs. To test for stem cell-type convertibility, we employed differentiation in the clonal lineage from ESCs to pNSCs to dNSCs, and revealed the relationships and lineage positioning among various PC populations, including spermatogonial germ cells (aSGSCs), epiblast-derived stem cells (Epi-SCs) and the bFGF, Activin, and BIO-derived stem cell (FAB-SC). Adult, murine aSGSCs assumed a 'pseudo-ESC' state in vitro, and then differentiated into dNSCs, but not pNSCs. Similarly, Epi-SCs and FAB-SCs only gave rise to dNSCs and not to pNSCs. The results of these experiments suggest a new pluripotency lineage model describing the relationship(s) among PCs that better reflects the transitions between these cell types in vitro.

Bone morphogenic protein signalling suppresses differentiation of pluripotent cells by maintaining expression of E-Cadherin

Bone morphogenic protein (BMP) signalling contributes towards maintenance of pluripotency and favours mesodermal over neural fates upon differentiation, but the mechanisms by which BMP controls differentiation are not well understood. We report that BMP regulates differentiation by blocking downregulation of Cdh1, an event that accompanies the earliest stages of neural and mesodermal differentiation. We find that loss of Cdh1 is a limiting requirement for differentiation of pluripotent cells, and that experimental suppression of Cdh1 activity rescues the BMP-imposed block to differentiation. We further show that BMP acts prior to and independently of Cdh1 to prime pluripotent cells for mesoderm differentiation, thus helping to reinforce the block to neural differentiation. We conclude that differentiation depends not only on exposure to appropriate extrinsic cues but also on morphogenetic events that control receptivity to those differentiation cues, and we explain how a key pluripotency signal, BMP, feeds into this control mechanism.

DOI:http://dx.doi.org/10.7554/eLife.01197.001

The human body is made up of about 200 different types of cell, all of which are descended from a single fertilised egg. As an embryo develops, its cells divide and specialise into distinct lineages. Cells in each lineage go on to form a restricted number of cell types that are required to make a specific tissue. As such, during early development, cells switch from being ‘pluripotent’, with the potential to become the many different cell types, to committing to one particular cell lineage.

Controlling this process involves a huge number of signalling proteins and pathways. One such protein is bone morphogenetic protein, or BMP for short, which has a number of different roles in embryo development: for example, it stops pluripotent cells turning into nerve tissue, and it also encourages embryonic stem cells to contribute to the ‘mesoderm’ of the early embryo (which goes on to form the muscles, connective tissues and some blood cells). How these two actions are linked, and whether they depend on similar signalling pathways, was unknown.

BMP is also known to trigger the production of proteins known as ‘Id factors’—which stands for ‘inhibitor of differentiation’. Now, Malaguti et al. have investigated the roles of BMP and Id factors in controlling mouse embryo development and found, somewhat surprisingly, that these proteins needed help from a third protein to stop pluripotent cells turning into nerve tissue. This third protein, which is called E-Cadherin, normally helps cells to adhere to other cells. Malaguti et al. showed that losing this protein encourages cells to become either nerve or mesoderm tissues, and that a drop in E-Cadherin levels must occur before nerve tissue can form.

Malaguti et al. also showed that encouraging cells to become part of the mesoderm requires BMP to activate another pathway, which does not require E-Cadherin. The two effects of BMP can be uncoupled by adjusting the levels of this protein. At low concentrations, BMP can keep cells pluripotent, but it cannot encourage cells to commit to a mesoderm fate. At higher doses, however, BMP ‘primes’ cells to respond to the signals that trigger their development into mesoderm tissue.

The findings of Malaguti et al. suggest that manipulating both E-Cadherin and BMP signalling could improve our ability to generate useful cell types, such as neurons, from stem cells grown in laboratory cultures.

DOI:http://dx.doi.org/10.7554/eLife.01197.002

TWIST1 promotes invasion through mesenchymal change in human glioblastoma

Background

Tumor cell invasion into adjacent normal brain is a mesenchymal feature of GBM and a major factor contributing to their dismal outcomes. Therefore, better understandings of mechanisms that promote mesenchymal change in GBM are of great clinical importance to address invasion. We previously showed that the bHLH transcription factor TWIST1 which orchestrates carcinoma metastasis through an epithelial mesenchymal transition (EMT) is upregulated in GBM and promotes invasion of the SF767 GBM cell line in vitro.

Results

To further define TWIST1 functions in GBM we tested the impact of TWIST1 over-expression on invasion in vivo and its impact on gene expression. We found that TWIST1 significantly increased SNB19 and T98G cell line invasion in orthotopic xenotransplants and increased expression of genes in functional categories associated with adhesion, extracellular matrix proteins, cell motility and locomotion, cell migration and actin cytoskeleton organization. Consistent with this TWIST1 reduced cell aggregation, promoted actin cytoskeletal re-organization and enhanced migration and adhesion to fibronectin substrates. Individual genes upregulated by TWIST1 known to promote EMT and/or GBM invasion included SNAI2, MMP2, HGF, FAP and FN1. Distinct from carcinoma EMT, TWIST1 did not generate an E- to N-cadherin "switch" in GBM cell lines. The clinical relevance of putative TWIST target genes SNAI2 and fibroblast activation protein alpha (FAP) identified in vitro was confirmed by their highly correlated expression with TWIST1 in 39 human tumors. The potential therapeutic importance of inhibiting TWIST1 was also shown through a decrease in cell invasion in vitro and growth of GBM stem cells.

Conclusions

Together these studies demonstrated that TWIST1 enhances GBM invasion in concert with mesenchymal change not involving the canonical cadherin switch of carcinoma EMT. Given the recent recognition that mesenchymal change in GBMs is associated with increased malignancy, these findings support the potential therapeutic importance of strategies to subvert TWIST1-mediated mesenchymal change.

Developmental potential of rat extraembryonic stem cells

We have previously found that certain stem cells that are derived from rat blastocysts and named extraembryonic endoderm precursor (XEN-P) cells show a unique molecular signature sharing some of the characteristics of embryonic stem cells (ES), trophoblast stem cells (TS), and extraembryonic endoderm stem cells (XEN). These XEN-P cells are positive for AP, SSEA1, Oct4, and Rex1 markers similar to ES cells and also express signature markers of TS-eomesodermin (Eomes) and XEN-Gata6. Here we show that these cells integrate into the visceral and parietal extraembryonic endoderm lineages as well as into the inner cell mass (ICM), the primitive endoderm, and the polar and mural trophectoderm (TE) of cultured embryos. In addition, we find that the XEN-P cells colonize yolk sac and contribute to trophoblast lineages of postimplantation embryos following transfer to surrogate mothers. We also find that the XEN-P cell culture propagates by shedding cell clusters into the media in addition to typical expansion of colonies. Interestingly, the cell cultures exist as mixed populations of two interconvertible phenotypes of flat and round cells with preferential expression of stem cell markers Oct4 and SSEA1 in round cells. We believe these cells represent a metastable stage during ICM cellular segregation. These results are important for developing hypotheses of cell fate plasticity in the ICM and provide a model for the study of development and differentiation along the extraembryonic lineages.

Developmental cues and persistent neurogenic potential within an in vitro neural niche

Background

Neurogenesis, the production of neural cell-types from neural stem cells (NSCs), occurs during development as well as within select regions of the adult brain. NSCs in the adult subependymal zone (SEZ) exist in a well-categorized niche microenvironment established by surrounding cells and their molecular products. The components of this niche maintain the NSCs and their definitive properties, including the ability to self-renew and multipotency (neuronal and glial differentiation).

Results

We describe a model in vitro NSC niche, derived from embryonic stem cells, that produces many of the cells and products of the developing subventricular zone (SVZ) and adult SEZ NSC niche. We demonstrate a possible role for apoptosis and for components of the extracellular matrix in the maintenance of the NSC population within our niche cultures. We characterize expression of genes relevant to NSC self-renewal and the process of neurogenesis and compare these findings to gene expression produced by an established neural-induction protocol employing retinoic acid.

Conclusions

The in vitro NSC niche shows an identity that is distinct from the neurally induced embryonic cells that were used to derive it. Molecular and cellular components found in our in vitro NSC niche include NSCs, neural progeny, and ECM components and their receptors. Establishment of the in vitro NSC niche occurs in conjunction with apoptosis. Applications of this culture system range from studies of signaling events fundamental to niche formation and maintenance as well as development of unique NSC transplant platforms to treat disease or injury.

Suppression of Oct4 by germ cell nuclear factor restricts pluripotency and promotes neural stem cell development in the early neural lineage

The earliest murine neural stem cells are leukemia inhibitory factor (LIF)-dependent, primitive neural stem cells, which can be isolated from embryonic stem cells or early embryos. These primitive neural stem cells have the ability to differentiate to non-neural tissues and transition into FGF2-dependent, definitive neural stem cells between embryonic day 7.5 and 8.5 in vivo, accompanied by a decrease in non-neural competency. We found that Oct4 is expressed in LIF-dependent primitive neural stem cells and suppressed in FGF-dependent definitive neural stem cells. In mice lacking germ cell nuclear factor (GCNF), a transcriptional repressor of Oct4, generation of definitive neural stem cells was dramatically suppressed, accompanied by a sustained expression of Oct4 in the early neuroectoderm. Knockdown of Oct4 in GCNF(-/-) neural stem cells rescued the GCNF(-/-) phenotype. Overexpression of Oct4 blocked the differentiation of primitive to definitive neural stem cells, but did not induce the dedifferentiation of definitive to primitive neural stem cells. These results suggested that primitive neural stem cells develop into definitive neural stem cells by means of GCNF induced suppression of Oct4. The Oct4 promoter was methylated during the development from primitive neural stem cell to definitive neural stem cell, while these neural stem cells lose their pluripotency through a GCNF dependent mechanism. Thus, the suppression of Oct4 by GCNF is important for the transition from primitive to definitive neural stem cells and restriction of the non-neural competency in the early neural stem cell lineage.