How time is measured by neural stem cells during temporal neurogenesis has remained unresolved. By combining experiments and computational modeling, we define a Shh/Gli-driven three-node timer underlying the sequential generation of motor neurons (MNs) and serotonergic neurons in the brainstem. The timer is founded on temporal decline of Gli-activator and Gli-repressor activities established through down-regulation of Gli transcription. The circuitry conforms an incoherent feed-forward loop, whereby Gli proteins not only promote expression of Phox2b and thereby MN-fate but also account for a delayed activation of a self-promoting transforming growth factor–β (Tgfβ) node triggering a fate switch by repressing Phox2b. Hysteresis and spatial averaging by diffusion of Tgfβ counteract noise and increase temporal accuracy at the population level, providing a functional rationale for the intrinsically programmed activation of extrinsic switch signals in temporal patterning. Our study defines how time is reliably encoded during the sequential specification of neurons.

在颞神经发生过程中，神经干细胞如何测量时间仍未得到解决。通过结合实验和计算建模，我们定义了一个 Shh/Gli 驱动的三节点计时器，该计时器是脑干中运动神经元 (MN) 和血清素能神经元的顺序生成的基础。计时器建立在通过下调 Gli 转录建立的 Gli 激活剂和 Gli 阻遏物活性的时间下降之上。该电路符合一个不连贯的前馈回路，由此 Gli 蛋白不仅促进了 Phox2b 的表达，从而促进了 MN-fate，而且还解释了自我促进的转化生长因子-β (Tgfβ) 节点的延迟激活，从而触发了命运转换。抑制 Phox2b。通过 Tgfβ 扩散的滞后和空间平均可以抵消噪声并提高总体水平的时间准确性，为时间模式中外部开关信号的内在程序激活提供了功能原理。我们的研究定义了在神经元的顺序规范期间如何可靠地编码时间。