The basic helix-loop-helix transcription factor Nato3 controls neurogenic activity in mesencephalic floor plate cells

Abstract

SUMMARYFloor plate (FP) cells, the ventral midline cells of the developing neural tube, have long been thought to be non-neurogenic organizer cells that control neuronal patterning and axonal guidance. Recent studies have revealed that mesencephalic FP (mesFP) cells have neurogenic activity and generate dopaminergic neurons. However, the mechanisms underlying the control of neurogenic potential in FP cells are not yet fully understood. Here we identified the bHLH factor Nato3 as an FP-specific transcription factor. In Nato3-null mutant mice, FP cells in the spinal cord were correctly specified, but could not properly mature. By contrast, in the developing mesencephalon, loss of Nato3 did not affect FP differentiation, but led to loss of neurogenic activity in the medial subpopulation of mesFP cells by suppressing proneural gene expression and inducing cell cycle arrest. As a consequence, the number of midbrain dopaminergic neurons generated was decreased in mutants. We also found that Hes1, which is known to be required for non-dividing organizer cell development in the neural tube, was aberrantly upregulated in the mesFP cells of Nato3 mutants. Consistently, forced expression of Nato3 repressed Hes1 expression and consequently induced premature neurogenesis. Finally, we showed that forced expression of Hes1 in mesFP cells induced cell cycle arrest and downregulation of proneural factors. Taken together, these results suggest that Nato3 confers neurogenic potential on mesFP cells by suppressing classical non-neurogenic FP cell differentiation, at least in part, through repressing Hes1.

Nato3 plays an integral role in dorsoventral patterning of the spinal cord by segregating floor plate/p3 fates via Nkx2.2 suppression and Foxa2 maintenance

“…S1). The knockout mice were viable, fertile and appeared normal, in agreement with published results (Ono et al, 2010).…”

“…1G,H). These observations from chick are in agreement with findings from mouse, in which Nato3 expression was shown to be upregulated in parallel with the downregulation of Nkx2.2 in the FP (Ono et al, 2010;Ribes et al, 2010). Taken together, in both avian and rodent models, the dynamic expression of Nato3 is temporally correlated with the establishment of the definitive FP identity (Foxa2 + /Nkx2.2 -) and the spatial separation of FP and p3 marker-expressing cells (Fig.…”

“…In both chick (this study) and mouse (Ono et al, 2010) embryos, Foxa2 expression precedes that of Nato3 (Fig. 8, t1-3), thus ruling out the possibility that Nato3 is involved in the induction of FP cells.…”

“…In the developing mesencephalon, loss of Nato3 does not affect FP differentiation, but suppresses proneural gene expression and induces cell cycle arrest. As a consequence, a loss of neurogenic activity in the mesencephalic FP causes a decrease in the number of midbrain dopaminergic neurons generated in Nato3 mutants (Ono et al, 2010). By contrast, FP cells Nato3 plays an integral role in dorsoventral patterning of the spinal cord by segregating floor plate/p3 fates via Nkx2.2 suppression and Foxa2 maintenance in the spinal cord are correctly specified and display only a temporary downregulation of medial FP-specific genes.…”

“…By contrast, FP cells Nato3 plays an integral role in dorsoventral patterning of the spinal cord by segregating floor plate/p3 fates via Nkx2.2 suppression and Foxa2 maintenance in the spinal cord are correctly specified and display only a temporary downregulation of medial FP-specific genes. Thus, the function of Nato3 in the spinal FP is unclear (Ono et al, 2010).…”