Rapid clearance of PcG histone modifications from Hox genes anticipates motor neuron differentiation: ChIP-chip

[original Title] Rapid and synchronous clearance of PcG histone modifications from Hox genes anticipates motor neuron differentiation. Hox genes are expressed in patterns that are spatially and temporally collinear with their chromosomal organization. This feature is an evolutionarily conserved hallmark of embryonic development, and in vertebrates it is critical, among others, for the specification of motor neuron subtypes and the wiring of sensory-motor circuits. We show here that the differentiation of motor neurons from stem cells is accompanied by a synchronous, domain-wide clearance of “repressive” Polycomb (PcG)-dependent histone methylation from Hox gene chromatin domains. These findings argue against the idea, advanced recently, that the collinear dynamics of Hox gene expression invariably reflects the progressive clearance of repressive chromatin modifications. The rapid establishment of stable chromatin domains in response to a transient patterning signal likely serves as a molecular correlate of enduring rostro-caudal neural identity, which underlies the specification of postmitotic motor neuron subtype diversity and neuronal circuit assembly. The differentiation of ventral motor neurons is induced by treating embryonic stem cell cultures with retinoic acid and hedgehog agonist. Here, ChIP-chip using a custom Agilent array is used to profile the occupancy of H3K27me3, H3K4me3, and H3K79me2 at various defined stages during the differentiation process.

【原文标题】 从同源异型基因（Hox genes）同步快速清除Polycomb组蛋白（Polycomb group, PcG）依赖的组蛋白修饰，预示运动神经元分化
同源异型基因（Hox genes）的表达模式与其染色体排布呈现空间与时间上的共线性特征。这一特征是胚胎发育过程中进化保守的标志性现象，在脊椎动物中，其对运动神经元亚型的特化以及感觉-运动环路的构建至关重要。
本研究证实，干细胞向运动神经元分化的过程中，同源异型基因染色质结构域会同步发生全结构域范围的“抑制性”Polycomb组蛋白（PcG）依赖的组蛋白甲基化清除。
本研究结果驳斥了近期提出的一种观点：即同源异型基因表达的共线性动态变化始终对应抑制性染色质修饰的逐步清除。
响应瞬时模式化信号的稳定染色质结构域的快速建立，可能是持久的前后轴神经身份特征的分子关联物，而这一特征是有丝分裂后运动神经元亚型多样性特化以及神经元环路组装的基础。
通过在胚胎干细胞培养体系中添加维甲酸（retinoic acid）与刺猬信号通路激动剂（hedgehog agonist），可诱导腹侧运动神经元的分化。
本研究使用定制安捷伦（Agilent）芯片开展染色质免疫沉淀芯片（Chromatin Immunoprecipitation coupled with microarray, ChIP-chip）实验，以检测分化过程中各明确阶段下H3K27me3、H3K4me3与H3K79me2的结合占据情况。