Investigation of Sip1 gene interactions in the development of the mammalian telencephalon

Abstract

The transcriptional repressor Sip1, or Smad- interacting protein 1, has been shown to play a key role in early neurodevelopment. Mowat- Wilson Syndrome in humans is associated with mutations in Sip1. In the developing mouse cerebral cortex, it is expressed mostly in the differentiating field, and weakly in the proliferative zones. We found that stage and tissue specific ablation of Sip1 leads to several defects in the development of the neocortex. Deletion of Sip1 in cortical progenitors as well as exclusively in postmitotic cortical plate neurons, led to a reduction in the size of deep layer neuronal populations and precocious generation of cortical layers 2-5. Further studies showed that while the onset of neurogenesis is unaffected, this process ends prematurely in conditional Sip1 mutants. We also observed an increase in astrocyte production, and traced its origin to increased and ectopic proliferation of astrocytic progenitors at E17.5, and premature specification of glial precursors at E16.5. Furthermore, elevated proliferation in the cingulate cortex continued until early postnatal stages in the mutant. Our data suggest a possible non- cell autonomous effect of Sip1 on the specification of deep layer versus upper layer neuronal fate, and neuronal versus astrocytic fate of progenitors in the germinal zone, at early and late stages of development, respectively. Comparison of gene expression profile between wildtype and Sip1 conditional knockouts enabled us to identify the growth factors, Neurotrophin-3 (NT3) and Fgf9 as putative downstream targets of Sip1 in the cortex. During early corticogenesis, ablation of Sip1 induces interplay of NT3 and other as yet unconfirmed factor(s) leading to a premature switch in the fate of cortical neuronal precursors. Interestingly, treatment of cortical slices with Fgf9 in vitro led to enhanced production of Olig2 expressing glial precursors at the germinal zone, an effect that recapitulates the phenotyp! e of a Sip1- deficient neocortex. Our data indicate that lack of Sip1 in the cortical plate triggers an Fgf9- mediated signaling cascade back to the germinal zone, possibly via interaction with Fgfr2/3 on radial glial processes, to influence the onset of astrocytic specification. Altogether, our work provides the first evidence of neuron to progenitor feedback signaling in the neocortex, to regulate the fate of uncommitted precursors and ensure production of appropriate numbers of different neurons and glia. Moreover, we have established Sip1 as a key mediator of this mechanism by its negative regulation of expression of certain signaling molecules.