发育编程涉及信号水平和动力学到转录输出的准确转换。Notch途径中的转录中继依赖于含有共激活因子Mastermind(Mam)的核复合物。通过实时跟踪这些复合体,我们发现它们促进NotchON核中动态转录中心的形成,该中心集中了包括MediatorCDK模块在内的关键因素。集线器的组成是不稳定的,并且在Notch退出后仍然存在,从而赋予了能够快速重整的记忆。令人惊讶的是,只有三分之一的NotchON集线器发展到具有新生转录的状态,这与聚合酶II和核心介体募集相关。该概率增加了第二信号。靶基因转录是概率性的发现具有深远的意义,因为它暗示Notch途径输出的随机差异可能出现在受体激活的下游。
为了正确地产生未来的组织,胚胎中的细胞必须接收并响应正确的信号,在正确的时间,以正确的方式。这涉及基因被快速打开,细胞通常确保一系列分子参与者在细胞核的“转录中心”物理上聚集在一起,细胞核是容纳遗传信息的隔室。这些中心被认为促进了微环境,促进了将激活所需基因并将其复制到信使RNA分子中的机器的组装。由此产生的“mRNA”充当生产相应蛋白质的模板,允许细胞充分响应信号。例如,一种叫做Notch的分子在细胞表面的激活引发了一系列事件,导致重要的发育基因在几分钟内被转录。这个过程涉及一组专门的蛋白质,被称为Notch核复合物,在细胞核中快速聚集并与转录机制相互作用。他们如何在正确的基因位置有效地做到这一点,然而,仍然知之甚少。特别是,尚不清楚Notch核复合物是否参与转录中心的形成,以及这些如何影响mRNA的产生以及细胞“记住”暴露于Notch活性的方式。为了调查这些问题,DeHaro-Arbona等人。基因工程果蝇,使它们的Notch核复合物和Notch靶基因都携带可见的标签,可以在活细胞中实时跟踪。苍蝇组织的显微镜成像显示,由于它们的特点,Notch复合物与转录机制聚集在一起,并在其靶基因附近形成转录中心。所有暴露于Notch的细胞都显示出这些集线器,但是只有三分之一产生了与Notch靶基因相关的mRNAs;添加第二信号(昆虫激素)显着增加了比例。这说明了“机会”和合作如何影响生物体对Notch信号的反应方式。最后,实验表明,集线器在消除Notch信号后至少持续了一天。当再次呈现Notch活性时,这种“分子记忆”导致细胞反应更快。DeHaro-Arbona的工作揭示了单个细胞如何响应Notch信号,以及影响其靶基因激活的因素。当试图更好地了解这种途径所涉及的疾病时,这种知识可能会被证明是有用的,比如癌症。
Developmental programming involves the accurate conversion of signalling levels and dynamics to transcriptional outputs. The transcriptional relay in the Notch pathway relies on nuclear complexes containing the co-activator Mastermind (Mam). By tracking these complexes in real time, we reveal that they promote the formation of a dynamic transcription hub in Notch ON nuclei which concentrates key factors including the Mediator CDK module. The composition of the hub is labile and persists after Notch withdrawal conferring a memory that enables rapid reformation. Surprisingly, only a third of Notch ON hubs progress to a state with nascent transcription, which correlates with polymerase II and core Mediator recruitment. This probability is increased by a second signal. The discovery that target-gene transcription is probabilistic has far-reaching implications because it implies that stochastic differences in Notch pathway output can arise downstream of receptor activation.
To correctly give rise to future tissues, cells in an embryo must receive and respond to the right signals, at the right time, in the right way. This involves genes being switched on quickly, with cells often ensuring that a range of molecular actors physically come together at ‘transcription hubs’ in the nucleus – the compartment that houses genetic information. These hubs are thought to foster a microenvironment that facilitates the assembly of the machinery that will activate and copy the required genes into messenger RNA molecules. The resulting ‘mRNAs’ act as templates for producing the corresponding proteins, allowing cells to adequately respond to signals. For example, the activation at the cell surface of a molecule called Notch triggers a series of events that lead to important developmental genes being transcribed within minutes. This process involves a dedicated group of proteins, known as Notch nuclear complexes, quickly getting together in the nucleus and interacting with the transcriptional machinery. How they do this efficiently at the right gene locations is, however, still poorly understood. In particular, it remained unclear whether Notch nuclear complexes participate in the formation of transcription hubs, as well as how these influence mRNA production and the way cells ‘remember’ having been exposed to Notch activity. To investigate these questions, DeHaro-Arbona et al. genetically engineered fruit flies so that their Notch nuclear complexes and Notch target genes both carried visible tags that could be tracked in living cells in real time. Microscopy imaging of fly tissues revealed that, due to their characteristics, Notch complexes clustered with the transcription machinery and formed transcription hubs near their target genes. All cells exposed to Notch exhibited these hubs, but only a third produced the mRNAs associated with Notch target genes; adding a second signal (an insect hormone) significantly increased the proportion. This illustrates how ‘chance’ and collaboration influence the way the organism responds to Notch signalling. Finally, the experiments revealed that the hubs persisted for at least a day after removing the Notch signal. This ‘molecular memory’ led to cells responding faster when presented with Notch activity again. The work by DeHaro-Arbona sheds light on how individual cells respond to Notch signalling, and the factors that influence the activation of its target genes. This knowledge may prove useful when trying to better understand diseases in which this pathway is implicated, such as cancer.