Transcriptional regulation of the germline piRNA pathway in the Drosophila ovary

The gene-regulatory mechanisms controlling the expression of the germline PIWI-interacting RNA (piRNA) pathway components within the gonads of metazoan species remain largely unexplored. In contrast to the male germline piRNA pathway, which in mice is known to be activated by the testis-specific transcription factor A-MYB, the nature of the ovary-specific gene-regulatory network driving the female germline piRNA pathway remains a mystery. Here, using Drosophila as a model, we combined multiple genomics approaches to reveal the transcription factor Ovo as regulator of the germline piRNA pathway in ovarian germ cells. Ectopic expression of Ovo in ovarian somatic cells activates germline piRNA pathway components, including the ping-pong factors Aubergine, Argonaute-3, and Vasa, leading to assembly of perinuclear cellular structures resembling nuage bodies of germ cells. We found that in ovarian somatic cells, transcription of ovo is repressed by l(3)mbt, thus preventing expression of germline piRNA pathway genes in the soma. Cross-species ChIP-seq and motif analyses demonstrate that Ovo is binding to genomic CCGTTA motifs within the promoters of germline piRNA pathway genes, suggesting a regulation by Ovo in ovaries analogous to that of A-MYB in testes. Our results also show consistent engagement of the Ovo transcription factor family at ovarian piRNA clusters across metazoan species, reflecting a deep evolutionary conservation of this regulatory paradigm from insects to humans. Refer to individual Series

控制后生动物性腺内生殖系PIWI互作RNA（piRNA）通路组分表达的基因调控机制，目前仍未得到充分解析。与小鼠中由睾丸特异性转录因子A-MYB激活的雄性生殖系piRNA通路不同，驱动雌性生殖系piRNA通路的卵巢特异性基因调控网络的本质仍是未解之谜。本研究以果蝇（Drosophila）为模型，结合多种基因组学方法，揭示了转录因子Ovo可调控卵巢生殖细胞中的生殖系piRNA通路。在卵巢体细胞中异位表达Ovo，能够激活生殖系piRNA通路组分，包括乒乓循环因子Aubergine、Argonaute-3与Vasa，并促使形成类似生殖细胞nuage体的核周细胞结构。研究发现，在卵巢体细胞中，l(3)mbt可抑制ovo的转录，从而阻止体细胞中生殖系piRNA通路基因的表达。跨物种染色质免疫共沉淀测序（ChIP-seq）与基序分析结果表明，Ovo可结合生殖系piRNA通路基因启动子区域内的基因组CCGTTA基序，提示卵巢中Ovo的调控模式与睾丸中A-MYB的调控模式类似。本研究结果还显示，转录因子Ovo家族在不同后生动物的卵巢piRNA簇中均存在稳定结合，反映了从昆虫到人类的该调控范式具有高度的进化保守性。详见各独立系列。