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Background: Bone marrow stromal cells and radial glia are two stem cell types with neural phenotypic plasticity. Bone marrow mesenchymal stem cells can differentiate into osteocytes, chondrocytes and adipocytes, but can also differentiate into non-mesenchymal cell, i.e. neural cells in appropriate in vivo and in vitro experimental conditions. Likewise, radial glial cells are the progenitors of many neurons in the developing cortex, but can also generate astrocytes. Both cell types express nestin, an intermediate filament protein which is the hallmark of neural precursors.

Hesx1 that are involved in specifying the anterior neurectoderm are unknown.We have previously shown that mouse embryonic stem cells differentiated as aggregates in HepG2 cell conditioned medium, synchronously and homogeneously differentiate to a midbrainlike population of neurectoderm cells. Analysis of ES cellderived neurectoderm suggest that it has not been exposed to patterning signals, such as the ventralising signal Sonic Hedgehog, making it a powerful tool for identifying and characterising molecules involved in neural tube patterning. We have exploited these characteristics to understand the signalling molecules that emanate from the AVE and anteriorise the neurectoderm. Hesx1 was over-expressed in HepG2 cells (HepG2:Hesx1) and the conditioned medium tested for the ability to anteriorise ES cell-derived neurectoderm. After 2 days, in presence of HepG2:Hesx1 CM anterior markers were up-regulated in the neurectoderm, suggesting that the CM contained signals that anteriorise neurectoderm.Microarray analysis was performed on HepG2 and HepG2:Hesx1 cells to identify differentially expressed genes. Of the four secreted molecules identified one growth factor was chosen for further characterisation. Addition of recombinant growth factor to ES cell-derived neurectoderm resulted in up-regulation of anterior markders, indicating that this growth factor is involved in anteriorising neural progenitors. In addition, expression analysis showed this growth factor is present in the AVE of 6.5 and 7.5 dpc mouse embryos suggesting a mechanism by which Hesx1 expression in the AVE regulates the positional specification of the anterior neurectoderm and forebrain. During central nervous system development, the proliferation and the differentiation of neural stem cells (NSC) are regulated by several intrinsic and extrinsic factors. Among these, neuregulins have been already characterized as stimulators of proliferation but are also implicated in the differentiation process, especially regarding the oligodendroglial pathway. Neuregulin-1 NRG1 gene is transcribed and translated into several isoforms, which are either secreted or membrane-anchored. In vitro, neural stem cells express mainly the CRD-NRG isoform, a membraneanchored type III form following the classical nomenclature. This isoform exhibits a cystein-rich-domain, which is responsible for a second transmembrane domain. Transmembrane neuregulins can undergo a proteolytic cleavage to release a signalling domain at the cell surface. We previously showed that cultivated NSC express CRD-NRG which is important for proliferation of NSC and their oligodendroglial fate choice (Calaora et al., 2001). We hypothesize that NSC proliferation and survival is linked to the extracellular domain containing the EGFlike motif (CRD-ECD) interacting with erbB receptors and that oligodendroglial differentiation is related to a retrograde intracellular domain (CRD-ICD) signalling. We thus looked first at the distribution of NRG isoforms in mouse NSC. Using specific antibodies, we observed that ...

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