Protocadherin 8 and transcription factor Dbx1 regulate the formation of developing cerebral cortex
Normal development of the mammalian central nervous system requires correct tissue patterning, production of the appropriate cell types and establishment of functional neuronal circuits. Transcription factors (TFs) play essential roles in these processes, regulating the expression of target genes responsible for neuronal subtypes specific features. Cell adhesion molecules are key components of neuronal identities that control cell sorting, migration, neurite outgrowth/guidance and synaptogenesis. So far, the link between cell adhesion and cell fate is not known TFs are known to control neuronal adhesion but not the opposite.   Here, using acute gain- and loss-of-function experiments by in utero electroporation in the developing mouse telencephalon we demonstrate that ectopic expression of Dbx1, a homeodomain TF acting as a cell fate determinant, leads to increased expression of Protocadherin 8 (Pcdh8) and cell aggregation, together with the induction of neuronal fate markers Nr4a2 (Nurr1)   and Pax6. These effects were are modulated depending on the region and timing of electroporation. Furthermore, we found that Pcdh8 expression is required for Dbx1-induced fate specification. Surprisingly, Pcdh8 overexpression also proved sufficient to induce Dbx1 expression as well as a complete reorganisation of the apico-basal polarity and dorso-ventral patterning. Finally, we present evidence that these effects are mediated through regulation of the expression of Notch ligands and promotion of cell cycle exit. Altogether, our work therefore points to cell adhesion molecules as unexpectedimportant, yet unexpected important, players in the regulation of cell identity and, in particular, Pcdh8 through its bidirectional action crossregulation  with the Dbx1 transcription factor. Overall design: Explants encompassing the septum were dissected from eight PGKCre ; Rosa26YFP E11.5 mouse embryos originating from two distinct litters and four Dbx1Cre ; Rosa26tdTomato E12.5 embryos from two litters. Cells were dissociated, pooled and a Single Cell 3' Kit v2 library was produced. The telencephalic vesicle was dissected from three wild-type E12 mouse embryos originating from two distinct litters. Cells were dissociated, pooled and a Single Cell 3' Kit v3.1 library was produced.