Department of Molecular and Cellular Biology, University of California, Davis, California 95616, USA.
Department of Cell Biology, Yale Stem Cell Center, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
RNA. 2020 May;26(5):550-563. doi: 10.1261/rna.072835.119. Epub 2020 Feb 19.
Transposable elements (TEs) can damage genomes, thus organisms use a variety of mechanisms to repress TE expression. The PIWI-piRNA pathway is a small RNA pathway that represses TE expression in the germline of animals. Here we explore the function of the pathway in the somatic stem cells of , a long-lived freshwater cnidarian. have three stem cell populations, all of which express PIWI proteins; endodermal and ectodermal epithelial stem cells (ESCs) are somatic, whereas the interstitial stem cells have germline competence. To study somatic function of the pathway, we isolated piRNAs from that lack the interstitial lineage and found that these somatic piRNAs map predominantly to TE transcripts and display the conserved sequence signatures typical of germline piRNAs. Three lines of evidence suggest that the PIWI-piRNA pathway represses TEs in ESCs. First, epithelial knockdown of the gene resulted in up-regulation of TE expression. Second, degradome sequencing revealed evidence of PIWI-mediated cleavage of TE RNAs in epithelial cells using the ping-pong mechanism. Finally, we demonstrated a direct association between Hywi protein and TE transcripts in epithelial cells using RNA immunoprecipitation. Altogether, our data reveal that the PIWI-piRNA pathway represses TE expression in the somatic cell lineages of , which we propose contributes to the extreme longevity of the organism. Furthermore, our results, in combination with others, suggest that somatic TE repression is an ancestral function of the PIWI-piRNA pathway.
转座元件 (TEs) 可能会破坏基因组，因此生物会利用多种机制来抑制 TE 的表达。PIWI-piRNA 通路是一种小 RNA 通路，可抑制动物生殖细胞中 TE 的表达。在这里，我们探索了该通路在长寿淡水腔肠动物 的体干细胞中的功能。 具有三个干细胞群体，所有这些群体都表达 PIWI 蛋白；内胚层和外胚层上皮干细胞 (ESCs) 是体细胞，而间质干细胞具有生殖细胞能力。为了研究该通路的体干细胞功能，我们从缺乏间质谱系的 中分离出 piRNAs，并发现这些体细胞 piRNAs 主要映射到 TE 转录本，并显示出与生殖细胞 piRNAs 典型的保守序列特征。有三条证据表明 PIWI-piRNA 通路在 ESCs 中抑制 TEs。首先，上皮细胞中 的基因 敲低导致 TE 表达上调。其次，降解组测序显示，上皮细胞中存在 PIWI 介导的 TE RNA 切割的证据，使用乒乓机制。最后，我们使用 RNA 免疫沉淀证明了 Hywi 蛋白与上皮细胞中 TE 转录本的直接关联。总的来说，我们的数据表明 PIWI-piRNA 通路抑制了 的体细胞谱系中的 TE 表达，我们认为这有助于该生物体的极端长寿。此外，我们的结果与其他结果结合表明，体细胞 TE 抑制是 PIWI-piRNA 通路的一个古老功能。
