Embryonic stem cell—derived astrocytes expressing drug-inducible transgenes: differentiation and transplantion into the mouse brain

OBJECT

Embryonic stem cell (ESC)-derived astrocytes have many theoretical and practical advantages as vectors for delivery of gene therapy to the central nervous system (CNS). The aim of this study was to generate highly pure populations of ESC-derived astrocytes expressing drug-inducible transgenes, while minimizing contamination by undifferentiated ESCs

METHODS

Embryonic stem cells carrying a doxycycline-inducible green fluorescent protein (GFP) transgene were induced to differentiate into astrocytes by using feeder cell-free conditions that are completely defined. More than 95% of these cells expressed the astrocyte markers glial fibrillary acidic protein and GLT-1 glutamate transporter, and the morphological characteristics of these cells were typical of astrocytes. The expression of additional astrocyte markers was detected using reverse transcription-polymerase chain reaction. Undifferentiated ESCs comprised fewer than 0.1% of the cells after 10 days in this culture. Positive and negative selection techniques based on fluorescence-activated cell sorting were successfully used to decrease further the numbers of undifferentiated ESCs. Fully differentiated astrocytes expressed a GFP transgene under the tight control of a doxycycline-responsive promoter, and maintained their astrocytic phenotype 24 hours after transplantation into the mouse brain.

CONCLUSIONS

This study shows that transgenic ESCs can be induced to differentiate into highly pure populations of astrocytes. The astrocytes continue to express the transgene under the tight control of a drug-inducible promoter and are suitable for transplantation into the mouse brain. The number of potentially hazardous ESCs can be minimized using cell-sorting techniques. This strategy may be used to generate cellular vectors for delivering gene therapy to the CNS.

Embryonic stem cell differentiation: emergence of a new era in biology and medicine

The discovery of mouse embryonic stem (ES) cells >20 years ago represented a major advance in biology and experimental medicine, as it enabled the routine manipulation of the mouse genome. Along with the capacity to induce genetic modifications, ES cells provided the basis for establishing an in vitro model of early mammalian development and represented a putative new source of differentiated cell types for cell replacement therapy. While ES cells have been used extensively for creating mouse mutants for more than a decade, their application as a model for developmental biology has been limited and their use in cell replacement therapy remains a goal for many in the field. Recent advances in our understanding of ES cell differentiation, detailed in this review, have provided new insights essential for establishing ES cell-based developmental models and for the generation of clinically relevant populations for cell therapy.

Committing embryonic stem cells to early endocrine pancreas in vitro

A panel of genetic markers was used to assess the in vitro commitment of murine embryonic stem (ES) cells toward the endoderm-derived pancreas and to distinguish insulin-expressing cells of this lineage from other lineages such as neuron, liver, and yolk sac. There are two nonallelic insulin genes in mice. Neuronal cells express only insulin II, whereas the pancreas expresses both insulin I and II. Yolk sac and fetal liver express predominately insulin II, small amounts of insulin I, and no glucagon. We found that ES-derived embryoid bodies cultured in the presence of stage-specific concentrations of monothio-glycerol and 15% fetal calf serum, followed by serum-free conditions, give rise to a population that expresses insulin I, insulin II, pdx-1 (a pancreas marker), and Sox17 (an endoderm marker). Immunohistochemical staining shows intracellular insulin particles, and its de novo production was confirmed by staining for C-peptide. Most, but not all, of the insulin+ or C-peptide+ cells coexpress glucagon, demonstrating a differentiation pathway to pancreas rather than yolk sac or fetal liver. Addition of beta-cell specification and differentiation factors activin beta B, nicotinamide, and exendin-4 to later-stage culture increased insulin-positive cells to 2.73% of the total population, compared with the control culture, which gave rise to less than 1% insulin-staining cells. These findings suggest that stepwise culture manipulations can direct ES cells to become early endocrine pancreas.

Tracking mesoderm formation and specification to the hemangioblast in vitro

During development of the mouse embryo, blood cells are generated from mesodermal precursors at specific times and locations. Using various in vivo and in vitro systems, we are now starting to understand the cascade of molecular events leading to the commitment of mesoderm and the formation of the first blood precursors, the hemangioblast. The in vitro differentiation of embryonic stem (ES) cells has proved to be an invaluable model to study lineage commitment, because one can access and easily manipulate large quantities of early progenitor cells. To help us track mesodermal subpopulations and study their specification toward blood lineages, we have engineered an ES cell line that expresses the green fluorescence protein exclusively in the mesoderm germ layer, under the control of the Brachyury regulatory sequences.

The homeobox gene HEX regulates proliferation and differentiation of hemangioblasts and endothelial cells during ES cell differentiation

In this report we have investigated the role of the homeobox gene Hex in the development and differentiation of the blast colony-forming cell (BL-CFC), a progenitor with hemangioblast characteristics generated in embryonic stem (ES) cell-derived embryoid bodies (EBs). Molecular analysis showed that Hex is expressed in mesoderm, in populations that contain BL-CFCs, and in blast cell colonies, the progeny of the BL-CFCs. Hex(-/-) EBs displayed a defect in macrophage development but generated higher numbers of BL-CFCs than did wild-type EBs. In addition to differences in these progenitor populations, we also found that endothelial cells from the Hex(-/-) EBs showed enhanced proliferative potential compared with those from wild-type EBs. Forced expression of Hex at the onset of ES cell differentiation resulted in reduced EB cellularity, fetal liver kinase-1 (Flk-1) expression, and BL-CFC development. Taken together, these findings demonstrate that Hex functions at multiple stages of development within the differentiating EBs and uncover a novel role for this transcription factor as a negative regulator of the hemangioblast and the endothelial lineage.

SCL/Tal-1 is essential for hematopoietic commitment of the hemangioblast but not for its development

In this report, we have defined the stage at which Scl functions in the establishment of the hematopoietic system and provide evidence that its primary role is in the generation of the hematopoietic lineages from a progenitor called the blast colony-forming cell (BL-CFC), a cell considered to be the in vitro equivalent of the hemangioblast. Using an embryonic stem (ES) cell line in which lacZ cDNA has been targeted to the Scl locus, we show that most of the BL-CFCs are detected in the SCL/lacZ- population, indicating that this progenitor does not express Scl. In the blast colony assay, Scl-/- cells initiate colony growth but are unable to generate endothelial and hematopoietic progeny and thus form colonies consisting of vascular smooth muscle cells only. The capacity to give rise to blast colonies can be rescued by retroviral transduction of a wild-type Scl gene into Scl-/- FLK-1+ cells, suggesting that the BL-CFC is generated in this population. Finally, we show that Scl-/- endothelial cells display a growth deficiency in monolayer cultures that can be partially overcome by maintaining this population as 3-dimensional aggregates indicating that specific cellular interactions are required for maintenance of the Scl-/- endothelial lineage in vitro.

Haemangioblast commitment is initiated in the primitive streak of the mouse embryo

Haematopoietic and vascular cells are thought to arise from a common progenitor called the haemangioblast. Support for this concept has been provided by embryonic stem (ES) cell differentiation studies that identified the blast colony-forming cell (BL-CFC), a progenitor with both haematopoietic and vascular potential. Using conditions that support the growth of BL-CFCs, we identify comparable progenitors that can form blast cell colonies (displaying haematopoietic and vascular potential) in gastrulating mouse embryos. Cell mixing and limiting dilution analyses provide evidence that these colonies are clonal, indicating that they develop from a progenitor with haemangioblast potential. Embryo-derived haemangioblasts are first detected at the mid-streak stage of gastrulation and peak in number during the neural plate stage. Analysis of embryos carrying complementary DNA of the green fluorescent protein targeted to the brachyury locus demonstrates that the haemangioblast is a subpopulation of mesoderm that co-expresses brachyury (also known as T) and Flk-1 (also known as Kdr). Detailed mapping studies reveal that haemangioblasts are found at highest frequency in the posterior region of the primitive streak, indicating that initial stages of haematopoietic and vascular commitment occur before blood island development in the yolk sac.

Hypoxia affects mesoderm and enhances hemangioblast specification during early development

Hypoxia Inducible Factor (HIF), consisting of HIF1alpha and ARNT (HIF1beta) subunits, activates multiple genes in response to oxygen (O(2)) deprivation. Arnt(-/-) mice exhibit substantial defects in blood cell and vessel development. We demonstrate that hypoxia accelerates the expression of Brachyury (a mesoderm-specific transcription factor), BMP4 (a mesoderm-promoting growth factor) and FLK1 (a marker of hemangioblasts, the bipotential progenitor of endothelial and hematopoietic cells) in differentiating ES cell cultures. Significantly, proliferation of embryonic hemangioblasts (BL-CFCs) is regulated by hypoxia, as Arnt(+/+) ES cells generate increased numbers of FLK1(+) cells, and BL-CFCs with accelerated kinetics in response to low O(2). This response is HIF-dependent as Arnt(-/-) ES cells produce fewer FLK1(+) cells and BL-CFCs, under both normoxic and hypoxic conditions. Interestingly, this defect is rescued when Arnt(-/-) ES cells are co-cultured with Arnt(+/+) ES cells. Vegf(+/-)or Vegf(-/-) ES cells generate proper numbers of FLK1(+) cells but fewer BL-CFCs, suggesting that additional factors regulated by HIF (other than VEGF) are involved in these early events. Thus, hypoxic responses are important for the establishment of various progenitor cells, including early mesoderm and its differentiation into hemangioblasts. Together these data suggest that ineffective responses to hypoxia in Arnt(-/-) embryos abrogate proper cardiovascular development during early embryogenesis, including the pathways controlling hemangioblast differentiation.

Mutual experimental and theoretical validation of bulk photoemission spectra of Sr1-xCaxVO3

We report high-resolution high-energy photoemission spectra together with parameter-free LDA + DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d(1) system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk sensitivity of the high-energy photoemission spectra.

[Molecular pathology in hereditary colorectal cancer. Recommendations of the Collaborative German Study Group on hereditary colorectal cancer funded by the German Cancer Aid (Deutsche Krebshilfe)]

Although twin studies indicate that inherited genetic factors contribute to about 35% of colorectal cancers (CRC), the exact genetic background has currently been elucidated in only 5-10% of cases. These comprise several hereditary cancer predisposition syndromes that present with a high number of syn- or metachronous neoplasms within an affected person and/or family. Many of these tumors exhibit typical histopathological changes. In general, one should discriminate between cancer syndromes associated with adenomatous and non-adenomatous (i.e., hamartomatous) polyps, the latter being quite rare. The patient's age often serves as a substantial hint to hereditary cancer. The next step of diagnostic work-up includes analysis of microsatellite instability (MSI) together with immunohistochemical detection of a loss of expression in one of the most frequently affected mismatch repair genes (MSH2, MSH6; MLH1, PMS2). Finally, the molecular demonstration of a gene mutation in the blood or germline is the most expensive and tedious procedure. This requires a signed informed consent from the patient after appropriate genetic counseling.

Self-association process of a peptide in solution: from beta-sheet filaments to large embedded nanotubes

Lanreotide is a synthetic octapeptide used in the therapy against acromegaly. When mixed with pure water at 10% (w/w), Lanreotide (acetate salt) forms liquid crystalline and monodisperse nanotubes with a radius of 120 A. The molecular and supramolecular organization of these structures has been determined in a previous work as relying on the lateral association of 26 beta-sheet filaments made of peptide noncovalent dimers, the basic building blocks. The work presented here has been devoted to the corresponding self-association mechanisms, through the characterization of the Lanreotide structures formed in water, as a function of peptide (acetate salt) concentration (from 2% to 70% (w/w)) and temperature (from 15 degrees C to 70 degrees C). The corresponding states of water were also identified and quantified from the thermal behavior of water in the Lanreotide mixtures. At room temperature and below 3% (w/w) Lanreotide acetate in water, soluble aggregates were detected. From 3% to 20% (w/w) long individual and monodisperse nanotubes crystallized in a hexagonal lattice were evidenced. Their molecular and supramolecular organizations are identical to the ones characterized for the 10% (w/w) sample. Heating induces the dissolution of the nanotubes into soluble aggregates of the same structural characteristics as the room temperature ones. The solubilization temperature increases from 20 degrees C to 70 degrees C with the peptide concentration and reaches a plateau between 15% and 25% (w/w) in peptide. These aggregates are proposed to be the beta-sheet filaments that self-associate to build the walls of the nanotubes. Above 20% (w/w) of Lanreotide acetate in water, polydisperse embedded nanotubes are formed and the hexagonal lattice is lost. These embedded nanotubes exhibit the same molecular and supramolecular organizations as the individual monodisperse nanotubes formed at lower peptide concentration. The embedded nanotubes do not melt in the range of temperature studied indicating a higher thermodynamic stability than individual nanotubes. In parallel, the thermal behaviors of water in mixtures containing 2-80% (w/w) in peptide have been studied by differential scanning calorimetry, and three different types of water were characterized: 1), bulk water melting at 0 degrees C, 2), nonfreezing water, and 3), interfacial water melting below 0 degrees C. The domains of existence and coexistence of these different water states are related to the different Lanreotide supramolecular structures. All these results were compiled into a binary Lanreotide-water phase diagram and allowed to propose a self-association mechanism of Lanreotide filaments into monodisperse individual nanotubes and embedded nanotubes.

Development of definitive endoderm from embryonic stem cells in culture

The cellular and molecular events regulating the induction and tissue-specific differentiation of endoderm are central to our understanding of the development and function of many organ systems. To define and characterize key components in this process, we have investigated the potential of embryonic stem (ES) cells to generate endoderm following their differentiation to embryoid bodies (EBs) in culture. We found that endoderm can be induced in EBs, either by limited exposure to serum or by culturing in the presence of activin A (activin) under serum-free conditions. By using an ES cell line with the green fluorescent protein (GFP) cDNA targeted to the brachyury locus, we demonstrate that endoderm develops from a brachyury(+) population that also displays mesoderm potential. Transplantation of cells generated from activin-induced brachyury(+) cells to the kidney capsule of recipient mice resulted in the development of endoderm-derived structures. These findings demonstrate that ES cells can generate endoderm in culture and, as such, establish this differentiation system as a unique murine model for studying the development and specification of this germ layer.

SCL interacts with VEGF to suppress apoptosis at the onset of hematopoiesis

During development, hematopoiesis initiates in the yolk sac through a process that depends on VEGF/Flk1 signaling and on the function of the SCL/Tal1 transcription factor. Here we show that VEGF modifies the developmental potential of primitive erythroid progenitors and prolongs their life span. Furthermore, the survival of yolk sac erythrocytes in vivo depends on the dose of VEGF. Interestingly, in Vegflo/lo embryos carrying a hypomorph allele, Flk1-positive cells reach the yolk sac at E8.5,but are severely compromised in their ability to generate primitive erythroid precursors. These observations indicate that during embryonic development,different thresholds of VEGF are required for the migration and clonal expansion of hematopoietic precursors. The near absence of primitive erythroid precursors in Vegflo/lo embryos correlates with low levels of Scl in the yolk sac. Strikingly, gain-of-function of SCL partially complements the hematopoietic defect caused by the hypomorph Vegflo allele, and re-establishes the survival of erythroid cells and the expression of erythroid genes (Gata1 andβH1). This indicates that SCL functions downstream of VEGF to ensure an expansion of the hematopoietic compartment.

Haploinsufficiency of Runx1 results in the acceleration of mesodermal development and hemangioblast specification upon in vitro differentiation of ES cells

The AML1 gene (recently renamed Runx1), which encodes the DNA-binding subunit of a transcription factor of the core binding factor (CBF) family, is required for the establishment of definitive hematopoiesis. We have previously demonstrated that Runx1 is expressed in yolk sac mesodermal cells prior to the establishment of the blood islands and in the embryoid body (EB)–derived blast-colony–forming cells (BL-CFCs), the in vitro equivalent of the hemangioblast. Analysis of Runx1-deficient embryonic stem (ES) cells demonstrated that this gene is essential for the generation of normal numbers of blast colonies, the progeny of the BL-CFCs. In the present study, we analyzed the potential of Runx1+/– ES cells to determine if heterozygosity at the Runx1 locus impacts early developmental events leading to the commitment of the BL-CFCs. Our results indicate that Runx1 heterozygosity leads to an acceleration of mesodermal commitment and specification to the BL-CFCs and to the hematopoietic lineages in EBs.

Unsuspected role of the brain morphogenetic gene Otx1 in hematopoiesis

Otx1 belongs to the paired class of homeobox genes and plays a pivotal role in brain development. Here, we show that Otx1 is expressed in hematopoietic pluripotent and erythroid progenitor cells. Moreover, bone marrow cells from mice lacking Otx1 exhibit a cell-autonomous impairment of the erythroid compartment. In agreement with these results, molecular analysis revealed decreased levels of erythroid genes that include the SCL and GATA-1 transcription factors. Accordingly, a gain of function of SCL rescues the erythroid deficiency in Otx1-/- mice. Taken together, our findings indicate a function for Otx1 in the regulation of blood cell production.

Tracking mesoderm induction and its specification to the hemangioblast during embryonic stem cell differentiation

The hematopoietic and endothelial lineages derive from mesoderm and are thought to develop through the maturation of a common progenitor, the hemangioblast. To investigate the developmental processes that regulate mesoderm induction and specification to the hemangioblast, we generated an embryonic stem cell line with the green fluorescent protein (GFP) targeted to the mesodermal gene, brachyury. After the in vitro differentiation of these embryonic stem cells to embryoid bodies, developing mesodermal progenitors could be separated from those with neuroectoderm potential based on GFP expression. Co-expression of GFP with the receptor tyrosine kinase Flk1 revealed the emergence of three distinct cell populations, GFP(-)Flk1(-), GFP(+)Flk1(-) and GFP(+)Flk1(+) cells, which represent a developmental progression ranging from pre-mesoderm to prehemangioblast mesoderm to the hemangioblast.

Stochastic resonance in pattern recognition by a holographic neuron model

The recognition rate of holographic neural synapses, performing a pattern recognition task, is significantly higher when applied to natural, rather than artificial, images. This shortcoming of artificial images can be largely compensated for, if noise is added to the input pattern. The effect is the result of a trade-off between optimal representation of the stimulus (for which noise is favorable) and keeping as much as possible of the stimulus-specific information (for which noise is detrimental). The observed mechanism may play a prominent role for simple biological sensors.