Instituto de Medicina Molecular, Faculdade de Medicina de Lisboa, Lisboa, Portugal.
PLoS One. 2010 Dec 2;5(12):e15459. doi: 10.1371/journal.pone.0015459.
Notch signalling plays a central role in the mechanisms regulating neuronal differentiation in the vertebrate nervous system. The transcriptional repressors encoded by Hes genes are the main effectors of this pathway, acting in neural progenitors during the lateral inhibition process to repress proneural genes and inhibit differentiation. However, Hes6 genes seem to behave differently: they are expressed in differentiating neurons and facilitate the activity of proneural genes in promoting neurogenesis. Still, the molecular mechanisms underlying this unique function of Hes6 genes are not yet understood.
METHODOLOGY/PRINCIPAL FINDINGS: Here, we identify two subgroups of Hes6 genes that seem conserved in most vertebrate species and characterize a novel Hes6 gene in chicken: cHes6-1. The embryonic expression pattern of cHes6-1 suggests roles for this gene in the formation of the pancreas, nervous system and in the generation of body asymmetry. We show that cHes6-1 is negatively regulated by Notch signalling in the developing embryonic spinal cord and in pancreatic progenitors, but requires Notch for the observed asymmetric expression at the lateral mesoderm. Functional studies by ectopic expression in the chick embryonic neural tube revealed that cHES6-1 up-regulates the expression of cDelta1 and cHes5 genes, in contrast with overexpression of cHES6-2, which represses the same genes. We show that this activity of cHES6-2 is dependent on its capacity to bind DNA and repress transcription, while cHES6-1 seems to function by sequestering other HES proteins and inhibit their activity as transcriptional repressors.
CONCLUSIONS/SIGNIFICANCE: Our results indicate that the two chick HES6 proteins act at different phases of neuronal differentiation, contributing to the progression of neurogenesis by different mechanisms: while cHES6-2 represses the transcription of Hes genes, cHES6-1 acts later, sequestering HES proteins. Together, the two cHES6 proteins progressively shut down the Notch-mediated progenitor program and ensure that neuronal differentiation can proceed.
Notch 信号通路在脊椎动物神经系统中调控神经元分化的机制中起着核心作用。Hes 基因编码的转录抑制因子是该通路的主要效应物，在神经前体细胞的侧向抑制过程中发挥作用，抑制 proneural 基因并抑制分化。然而，Hes6 基因的行为似乎有所不同：它们在分化神经元中表达，并促进 proneural 基因的活性，从而促进神经发生。尽管如此，Hes6 基因这种独特功能的分子机制尚不清楚。
方法/主要发现：在这里，我们鉴定了两个似乎在大多数脊椎动物物种中保守的 Hes6 基因亚组，并在鸡中表征了一个新的 Hes6 基因：cHes6-1。cHes6-1 的胚胎表达模式表明该基因在胰腺、神经系统的形成以及体轴不对称的产生中发挥作用。我们表明，cHes6-1 在发育中的胚胎脊髓和胰腺祖细胞中受到 Notch 信号的负调控，但 Notch 对于 lateral mesoderm 中观察到的不对称表达是必需的。通过在鸡胚神经管中的异位表达进行的功能研究表明，cHES6-1 上调 cDelta1 和 cHes5 基因的表达，与 cHES6-2 的过表达相反，后者抑制相同的基因。我们表明，cHES6-2 的这种活性依赖于其结合 DNA 和抑制转录的能力，而 cHES6-1 似乎通过隔离其他 HES 蛋白并抑制其作为转录抑制因子的活性来发挥作用。
结论/意义：我们的结果表明，两种鸡 HES6 蛋白在神经元分化的不同阶段发挥作用，通过不同的机制促进神经发生的进展：cHES6-2 抑制 Hes 基因的转录，而 cHES6-1 则在后期发挥作用，隔离 HES 蛋白。两种 cHES6 蛋白共同逐渐关闭 Notch 介导的祖细胞程序，并确保神经元分化能够进行。
