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

Yuichi Ono,Minoru Kumai,Tomoya Nakatani,Yasuko Minaki

doi:10.1242/dev.042572

Abstract

Floor 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.

Highlights

Floor plate (FP) cells are morphologically specialized cell populations that develop at the ventral midline of the neural tube (Placzek and Briscoe, 2005; Strahle et al, 2004)
Nato3 is selectively expressed in FP cells To identify genes that regulate FP and mesencephalic dopaminergic (mesDA) development we searched for genes selectively expressed in FP cells by comparing gene expression levels in the ventral midline region and in the basal plate region of the developing mesencephalon by subtractive PCR (Ono et al, 2007)
The percentages of mesDA progenitors expressing Ngn2 and Mash1 in the Nato3 mutants were reduced from 39.8% ± 1.5% and 33.9% ± 3.2% to 25.4% ± 2.4% and 25.8% ± 3.4%, respectively (Fig. 4E). These results suggest that Nato3 is required for efficient neurogenesis in mesencephalic FP (mesFP) cells, and that reduced neurogenic activity is a cause of the mesDA neuron reduction in Nato3 mutant mice

Summary

Introduction

Floor plate (FP) cells are morphologically specialized cell populations that develop at the ventral midline of the neural tube (Placzek and Briscoe, 2005; Strahle et al, 2004). FP cells organize ventral cell fate patterning and the projection of commissural axons by secreting diffusible factors such as Shh and netrin 1 and contacting axons via cell adhesion molecules. FP cells have long been thought to be non-proliferative cells that never give rise to neurons by themselves (Jessell, 2000; Placzek and Briscoe, 2005). Consistent with this non-neurogenic property of FP cells, persistent expression of Hes, which suppresses proneural gene expression, is required for the establishment of FP cell fate (Baek et al, 2006). Recent cell-sorting and lineage-tracing studies revealed that FP cells in the developing mesencephalon have neurogenic potential and generate

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