Dynamic extrinsic pacing of the HOX clock in human axial progenitors control motor neuron subtype specification. Dynamic extrinsic pacing of the HOX clock in human axial progenitors control motor neuron subtype specification

Rostro-caudal patterning of vertebrates depends on the temporally progressive activation of HOX genes within axial stem cells that fuel axial embryo elongation. Whether HOX genes sequential activation, the “HOX clock”, is paced by intrinsic chromatin-based timing mechanisms or by temporal changes in extrinsic cues remains unclear. Here, we studied HOX clock pacing in human pluripotent stem cells differentiating into spinal cord motor neuron subtypes which are progenies of axial progenitors. We show that the progressive activation of caudal HOX genes is controlled by a dynamic increase in FGF signaling. Blocking FGF pathway stalled induction of HOX genes, while precocious increase in FGF alone, or with GDF11 ligand, accelerated the HOX clock. Cells differentiated under accelerated HOX induction generated appropriate posterior motor neuron subtypes found along the human embryonic spinal cord. The HOX clock is thus dynamically paced by exposure parameters to secreted cues. Its manipulation by extrinsic factors alleviates temporal requirements to provide unprecedented synchronized access to human cells of multiple, defined, rostro-caudal identities for basic and translational applications. Overall design: human ES cells SA001

脊椎动物的颅尾轴模式建成依赖于轴干细胞内同源框基因（HOX genes）的时序性逐步激活，该过程驱动胚胎轴的延伸。目前学界尚未明确，同源框基因的循序激活——即“HOX时钟”——究竟是由基于染色质的内在计时机制调控，还是由外在信号的时序变化所驱动。本研究针对人类多能干细胞（human pluripotent stem cells）分化为源自轴祖细胞的脊髓运动神经元亚型这一模型，探究了HOX时钟的调控机制。研究发现，尾侧同源框基因的逐步激活受成纤维细胞生长因子（FGF）信号的动态增强所调控。阻断FGF信号通路会阻滞同源框基因的诱导激活，而仅提前上调FGF信号，或联合生长分化因子11（GDF11）配体，均可加速HOX时钟的运转。在HOX诱导加速条件下分化得到的细胞，可生成沿人类胚胎脊髓分布的对应后侧运动神经元亚型。由此可见，HOX时钟的运转速率动态地由细胞接触分泌型信号分子的参数所决定。通过外在因素操控HOX时钟，可突破固定的时间限制，从而为基础研究与转化应用提供前所未有的同步获取途径，以获得多种具有明确颅尾轴身份的人类细胞。实验整体设计：人类胚胎干细胞SA001