Neurobiology and Behavior Program, Department of Biological Structure, School of Medicine, University of Washington, Seattle, USA.
Dev Neurobiol. 2011 Dec;71(12):1153-69. doi: 10.1002/dneu.20899.
MicroRNAs (miRNAs) are 19-25 nucleotide RNAs that regulate messenger RNA translation and stability. Recently, we performed a conditional knockout (CKO) of the miRNA-processing enzyme Dicer during mouse retinal development and showed an essential role for miRNAs in the transition of retinal progenitors from an early to a late competence state (Georgi and Reh [2010]: J Neurosci 30:4048-4061). Notably, Dicer CKO progenitors failed to express Ascl1 and generated ganglion cells beyond their normal competence window. Because Ascl1 regulates multiple Notch signaling components, we hypothesized that Notch signaling is downregulated in Dicer CKO retinas. We show here that Notch signaling is severely reduced in Dicer CKO retinas, but that retinal progenitors still retain a low level of Notch signaling. By increasing Notch signaling in Dicer CKO progenitors through constitutive expression of the Notch intracellular domain (NICD), we show that transgenic rescue of Notch signaling has little effect on the competence of retinal progenitors or the enhanced generation of ganglion cells, suggesting that loss of Notch signaling is not a major determinant of these phenotypes. Nevertheless, transgenic NICD expression restored horizontal cells, suggesting an interaction between miRNAs and Notch signaling in the development of this cell type. Furthermore, while NICD overexpression leads to robust glial induction in control retinas, NICD overexpression was insufficient to drive Dicer-null retinal progenitors to a glial fate. Surprisingly, the presence of transgenic NICD expression did not prevent the differentiation of some types of retinal neurons, suggesting that Notch inactivation is not an absolute requirement for the initial stages of neuronal differentiation.
微小 RNA（miRNAs）是 19-25 个核苷酸的 RNA，可调节信使 RNA 的翻译和稳定性。最近，我们在小鼠视网膜发育过程中对 miRNA 加工酶 Dicer 进行了条件性敲除（CKO），并表明 miRNAs 在视网膜祖细胞从早期向晚期能力状态的转变中起着重要作用（Georgi 和 Reh [2010]：J Neurosci 30:4048-4061）。值得注意的是，Dicer CKO 祖细胞未能表达 Ascl1，并在其正常能力窗口之外产生神经节细胞。由于 Ascl1 调节多个 Notch 信号成分，我们假设 Notch 信号在 Dicer CKO 视网膜中下调。我们在这里表明，Notch 信号在 Dicer CKO 视网膜中严重降低，但视网膜祖细胞仍保留低水平的 Notch 信号。通过在 Dicer CKO 祖细胞中通过组成型表达 Notch 细胞内结构域（NICD）来增加 Notch 信号，我们表明，Notch 信号的转基因挽救对视网膜祖细胞的能力或增强的神经节细胞的产生几乎没有影响，表明 Notch 信号的缺失不是这些表型的主要决定因素。尽管如此，转基因 NICD 表达恢复了水平细胞，表明 miRNAs 和 Notch 信号在该细胞类型的发育中相互作用。此外，虽然 NICD 过表达在对照视网膜中导致强烈的胶质诱导，但 NICD 过表达不足以驱使 Dicer 缺失的视网膜祖细胞向胶质命运分化。令人惊讶的是，转基因 NICD 表达的存在并没有阻止某些类型的视网膜神经元的分化，这表明 Notch 失活不是神经元分化初始阶段的绝对要求。
