Institute of Molecular Biology & Biotechnology, Foundation for Research & Technology Hellas, 70013, Heraklion, Crete, Greece.
Department of Biology, University of Crete, 70013, Heraklion, Crete, Greece.
BMC Biol. 2022 May 13;20(1):107. doi: 10.1186/s12915-022-01300-8.
In both Drosophila and mammals, the achaete-scute (ASC/ASCL) proneural bHLH transcription factors are expressed in the developing central and peripheral nervous systems, where they function during specification and maintenance of the neural stem cells in opposition to Notch signaling. In addition to their role in nervous system development, ASC transcription factors are oncogenic and exhibit chromatin reprogramming activity; however, the impact of ASC on chromatin dynamics during neural stem cell generation remains elusive. Here, we investigate the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology.
We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and less accessible, Zelda-independent regions. Both classes of cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in a variety of biological processes necessary for neuroblast function/homeostasis. We identified an ASC-Notch regulated TF network that includes likely prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, unable to divide, and lacking expression of many proneural targets. When mutant neuroblasts eventually start proliferating, they produce compromised progeny. Reporter lines driven by proneural-bound enhancers display ASC dependency, suggesting that the partial neuroblast identity seen in the absence of ASC genes is likely driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild-type embryos, the induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to consolidate the chromatin dynamics that promote neural specification.
We demonstrate that ASC proneural transcription factors are indispensable for the timely initiation of the neural stem cell program at the chromatin level by regulating a large number of enhancers in the vicinity of neural genes. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.
在果蝇和哺乳动物中，achaete-scute（ASC/ASCL）原神经细胞 bHLH 转录因子在中枢和周围神经系统中表达，在那里它们在神经干细胞的规范和维持中发挥作用，与 Notch 信号相反。除了在神经系统发育中的作用外，ASC 转录因子还具有致癌性和染色质重编程活性；然而，ASC 在神经干细胞生成过程中对染色质动力学的影响仍然难以捉摸。在这里，我们使用整合遗传学和基因组学方法研究了伴随神经承诺的染色质变化。
我们发现，ASC 因子同样强烈地结合到两种不同类别的顺式调控元件上：Zelda 在母源到合子过渡期间更早重塑的开放区域和较少可及的、Zelda 独立的区域。这两类顺式元件在神经规范过程中表现出增强的染色质可及性，并与涉及各种生物学过程的基因的转录调控相关，这些过程对于神经母细胞的功能/动态平衡是必要的。我们确定了一个 ASC-Notch 调节的 TF 网络，其中包括神经母细胞功能的潜在主要调节因子。使用一组 ASC 靶基因，我们报告说 ASC 缺失的神经母细胞的规范指定存在缺陷，最初停滞不前，无法分裂，并且缺乏许多原神经靶基因的表达。当突变的神经母细胞最终开始增殖时，它们产生的后代受损。由原神经结合增强子驱动的报告基因显示出对 ASC 的依赖性，这表明在缺乏 ASC 基因的情况下观察到的部分神经母细胞身份可能是由其他原神经独立的顺式元件驱动的。通过外胚层诱导单个 ASC 基因可以纠正神经母细胞的损伤和 ASC 突变体的后期分化缺陷，但不能纠正 ASC 下游的 TF 网络的单个成员。然而，在野生型胚胎中，该网络的单个成员的诱导会引起中枢神经系统过度增生，这表明它们与 ASC 的激活功能协同作用，巩固了促进神经规范的染色质动力学。
我们证明，ASC 原神经转录因子通过调节神经基因附近的大量增强子，在染色质水平上对神经干细胞程序的及时启动是必不可少的。这种早期的染色质重塑对于神经母细胞的动态平衡以及未来后代的保真度都是至关重要的。
