Summary Sox 2 deficiency causes neurodegeneration and impaired neurogenesis in the adult mouse brain

Sox (Sry-related HMG box) (Gubbay et al., 1990) genes encode transcription factors regulating crucial developmental decisions in different systems, such as early gonad, crystallin, chondrocytes and glia (Kamachi et al., 2000). These genes are also involved in hereditary diseases in man (Kamachi et al., 2000). Sox2is expressed in, and is essential for, totipotent inner cell mass stem cells and other early multipotent cell lineages, and its ablation causes early embryonic lethality (Avilion et al., 2003). In many different species, Sox2 is a marker of the nervous system from the beginning of its development (Uwanogho et al., 1995; Wood and Episkopou, 1999; Kishi et al., 2000; Kamachi et al., 2000; Avilion et al., 2003); an important role in early neural development is further suggested by experiments in chick andXenopus(Kamachi et al., 2000; Kishi et al., 2000). We previously showed that mouse Sox2 is expressed in embryonic neural stem cells (Zappone et al., 2000). This observation, together with the crucial role of Sox2 in early embryonic stem cells (Avilion et al., 2003), suggests that Sox2 function may be related to important aspects of multipotent cell biology. To investigate the role of Sox2in the nervous system, we generated different mouse mutant alleles: a null allele (Sox2 βgeo‘knock-in’) (Zappone et al., 2000; Avilion et al., 2003), and a regulatory mutant allele (Sox2 ∆ENH), in which a neural cellspecific enhancer (Zappone et al., 2000) is deleted. Compound Sox2β-geo/∆ENH heterozygotes are born with important cerebral malformations and neural cell pathology. In addition to proliferative defects of adult stem/progenitor cells, our results unexpectedly reveal a crucial role for Sox2 in neuronal maintenance in selected brain areas. In many species, the Sox2transcription factor is a marker of the nervous system from the beginning of its development, and we have previously shown that Sox2 is expressed in embryonic neural stem cells. It is also expressed in, and is essential for, totipotent inner cell mass stem cells and other multipotent cell lineages, and its ablation causes early embryonic lethality. To investigate the role of Sox2in the nervous system, we generated different mouse mutant alleles: a null allele (Sox2 β-geo ‘knock-in’), and a regulatory mutant allele (Sox2∆ENH), in which a neural cell-specific enhancer is deleted. Sox2 is expressed in embryonic early neural precursors of the ventricular zone and, in the adult, in ependyma (a descendant of the ventricular zone). It is also expressed in the vast majority of dividing precursors in the neurogenic regions, and in a small proportion of differentiated neurones, particularly in the thalamus, striatum and septum. Compound Sox2β-geo/∆ENH heterozygotes show important cerebral malformations, with parenchymal loss and ventricle enlargement, and L-dopa-rescuable circling behaviour and epilepsy. We observed striking abnormalities in neurones; degeneration and cytoplasmic protein aggregates, a feature common to diverse human neurodegenerative diseases, are observed in thalamus, striatum and septum. Furthermore, ependymal cells show ciliary loss and pathological lipid inclusions. Finally, precursor cell proliferation and the generation of new neurones in adult neurogenic regions are greatly decreased, and GFAP/nestin-positive hippocampal cells, which include the earliest neurogenic precursors, are strikingly diminished. These findings highlight a crucial and unexpected role for Sox2in the maintenance of neurones in selected brain areas, and suggest a contribution of neural cell proliferative defects to the pathological phenotype.