Transcriptional regulation of Nfix by NFIB drives astrocytic maturation within the developing spinal cord

During mouse spinal cord development, ventricular zone progenitor cells transition from producing neurons to producing glia at approximately embryonic day 11.5, a process known as the gliogenic switch. The transcription factors Nuclear Factor I (NFI) A and B initiate this developmental transition, but the contribution of a third NFI member, NFIX, remains unknown. Here, we reveal that ventricular zone progenitor cells within the spinal cord express NFIX after the onset of NFIA and NFIB expression, and after the gliogenic switch has occurred. Mice lacking NFIX exhibit normal neurogenesis within the spinal cord, and, while early astrocytic differentiation proceeds normally, aspects of terminal astrocytic differentiation are impaired. Finally, we report that, in the absence of Nfia or Nfib, there is a marked reduction in the spinal cord expression of NFIX, and that NFIB can transcriptionally activate Nfix expression in vitro. These data demonstrate that NFIX is part of the downstream transcriptional program through which NFIA and NFIB coordinate gliogenesis within the spinal cord. This hierarchical organisation of NFI protein expression and function during spinal cord gliogenesis reveals a previously unrecognised auto-regulatory mechanism within this gene family.