Dicer is required for the maintenance of notch signaling and gliogenic competence during mouse retinal development.

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 (: 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 intra-cellular 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，它们调节信使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进行了条件性基因敲除（CKO），并表明miRNAs在视网膜祖细胞从早期到晚期分化能力状态的转变中起着至关重要的作用（《神经科学杂志》30:4048 - 4061）。值得注意的是，Dicer CKO祖细胞未能表达Ascl1，并在其正常分化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，我们假设在Dicer CKO视网膜中Notch信号被下调。我们在此表明，在Dicer CKO视网膜中Notch信号严重降低，但视网膜祖细胞仍然保留低水平的Notch信号。通过组成性表达Notch细胞内结构域（NICD）增加Dicer CKO祖细胞中的Notch信号，我们发现Notch信号的转基因拯救对视网膜祖细胞的分化能力或神经节细胞的过度产生几乎没有影响，这表明Notch信号的缺失不是这些表型的主要决定因素。然而，转基因NICD表达恢复了水平细胞，这表明在这种细胞类型的发育过程中miRNAs和Notch信号之间存在相互作用。此外，虽然在对照视网膜中NICD过表达导致强烈的神经胶质诱导，但NICD过表达不足以驱动Dicer缺失的视网膜祖细胞向神经胶质命运分化。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。