SOX3 overexpression experiments in chicken spinal cord at E4. 48 h post‐electroporation, transfected side showed decreased expression of FGFR2, NFIA, and Fgfr3. Scale bar: 50μm.
24 h after miss‐expression of a dominant negative version of SOXB1 (dnSOXB1), premature expression of FGFR2, NFIA, and Fgfr3 was found in the transfected side. Scale bar: 40 μm.
24 h after SOX9 overexpression in chicken spinal cord (at E4), increased expression of FGFR2, NFIA, and Fgfr3 was shown compared to the un‐transfected control side. Scale bar: 40 μm.
Sox3 and Sox9 co‐electroporation in chicken spinal cord at E4 could rescue the effect of SOX9 overexpression, and FGFR2, NFIA, and Fgfr3 levels were comparable to the un‐transfected control side. Embryos were analyzed 24 h post‐electroporation. Scale bar: 40 μm.
Proposed model of sequentially acting SOX proteins in neuronal and glial lineage specification. Silent neuronal, astrocyte, and oligodendrocyte genes are all prebound by SOX3 in NPCs. In GPCs, transactivating astrocytic enhancers (CRM A), enriched for NFI motifs, become additionally targeted by SOX9, whereas non‐activating astrocytic enhancers (CRM B) or CRMs around oligodendrocyte genes become prebound by SOX9 only. While SOX3 represses the activation of prebound genes, neuronal and glial gene expression become activated when SOX3 is downregulated and a cellular context of activating transcription factors has developed, which according to the model can consist of SOX11 in neurons, SOX10 in oligodendrocytes, and NFIA and SOX9 in astrocytes. It should be noted that we do not provide any evidence that SOX3/SOX9 or NFIA/SOX9 physically interact or co‐bind DNA.
