ORIGINAL ARTICLE Transforming growth factor a promotes sequential conversion of mature astrocytes into neural progenitors and stem cells A Sharif 1,2 , P Legendre 3 , V Pre´ vot 4 , C Allet 4 , L Romao 1,2,5 , J-M Studler 1,2 , H Chneiweiss 1,2 and M-P Junier 1,2 1 Inserm U114 and U752, Paris, France; 2 University of Paris 5, Paris, France; 3 UMR CNRS 7102, University Pierre et Marie Curie, Paris, France; 4 Inserm U816, Lille, France and 5 Departamento de Anatomia, Instituto de Ciencias Biomedicas, Universidade Federal do Rio de Janeiro, Centro de Ciencias da Saude, Rio de Janeiro, RJ, Brazil An instability of the mature cell phenotype is thought to participate to the formation of gliomas, primary brain tumors deriving from astrocytes and/or neural stem cells. Transforming growth factor a (TGFa) is an erbB1 ligand overexpressed in the earliest stages of gliomas, and exerts trophic effects on gliomal cells and astrocytes. Here, we questioned whether prolonged TGFa exposure affects the stability of the normal mature astrocyte phenotype. We first developed astrocyte cultures devoid of residual neural stem cells or progenitors. We demonstrate that days of TGFa treatment result in the functional conversion of a population of mature astrocytes into radial glial cells, a population of neural progenitors. TGFa-generated radial glial cells support embryonic neurons migration, and give birth to cells of the neuronal lineage, expressing neuronal markers and the electrophysiological properties of neuro- blasts. Lengthening TGFa treatment to months results in the delayed appearance of cells with neural stem cells properties: they form floating cellular spheres that are self-renewing, can be clonally derived from a single cell and differentiated into cells of the neuronal lineage. This study uncovers a novel population of mature astrocytes capable, in response to a single epigenetic factor, to regress progressively into a neural stem-like cell stage via an intermediate progenitor stage. Oncogene (2007) 26, 2695–2706. doi:10.1038/sj.onc.1210071; published online 23 October 2006 Keywords: erbB1; differentiation; radial glia; gliogenesis; oncogenesis; EGFR Introduction Gliomas are the most common primitive central nervous system (CNS) tumors thought to derive from macroglial cells (astrocytes and oligodendrocytes) and/or neural progenitors. Mechanisms underlying brain tumors formation and progression are largely unknown. Aside the rapid progression of glioblastomas with a poor prognosis, low-grade gliomas slowly progress over years before they change into more aggressive stages (Wechsler-Reya and Scott, 2001). Transforming growth factor a (TGFa) and its receptor erbB1 (or epidermal growth factor receptor) constitute one of the signalling modules most frequently deregulated in gliomas. TGFa overexpression is found in about 80% of them, and is observed from the initial steps of their develop- ment (Junier, 2000), whereas the overexpression of erbB1 appears in 20–40% of them in later phases (Wechsler-Reya and Scott, 2001). We previously showed that TGFa acts as a gliatrophin on mouse and human cortical astrocytes, promoting their growth as well as their survival (Sharif et al., 2006b), in agreement with the recent report of increased astrocyte apoptosis in erbB1 knockout mice (Wagner et al., 2006). These results suggest that TGFa might participate in the early stages of tumoral transformation through the deregu- lation of astrocyte proliferation and apoptosis. How- ever, the recent discoveries of cancerous neural stem-like cells in human gliomas, and the demonstration that neural progenitors are more sensitive than dif- ferentiated astrocytes to the transforming effects of various oncogenic manipulations, have raised the possibility that a trouble in cell differentiation might also participate to the formation of these tumors (Dai and Holland, 2003; Harris, 2004; Singh et al., 2004). Such a possibility is supported by the consequences of the introduction of a mutated, constitutively active form of erbB1 in cultured astrocytes (Bachoo et al., 2002). The activation of erbB1, insufficient alone to ensure wild-type astrocyte transformation, results in the dedifferentiation of cultured mutant astrocytes lacking the INK4a/ARF tumor suppressor gene locus, and their transformation into tumoral cells capable to form high-grade gliomas when transplanted in the brain (Bachoo et al., 2002). Interestingly, we and others noted that several days of TGFa exposure leads in vitro to morphological altera- tions of wild-type astrocytes, reminiscent of the bipolar shape of a population of neural progenitors, the radial glial cells (Miller et al., 1996; Zhou et al., 2001; Figiel etal., 2003; Liu and Neufeld, 2004; Sharif etal., 2006b). Received 24 May 2006; revised 20 July 2006; accepted 12 September 2006; published online 23 October 2006 Correspondence: Dr M-P Junier, College de France, INSERM U752, 11 Place Marcelin Berthelot, Paris 75005, France. E-mail: marie-pierre.junier@college-de-france.fr Oncogene (2007) 26, 2695–2706 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc