P41. Primary piRNA biogenesis occurs through Yb bodies in the Drosophila ovarian somatic cells

Abstract

PIWI proteins, a subset of Argonaute proteins that are the key factors in small RNA-mediated gene silencing, are expressed predominantly in the germline and are associated specifically with PIWI-interacting RNAs (piRNAs), small RNAs expressed predominantly in the germline. The PIWI-piRNA complex silences transposable elements and other genes that show complementarities to the piRNA sequences. Without functions of PIWI proteins and piRNAs, transposable elements become active, which has high potential to injure the genome. Thus, both PIWI proteins and piRNAs are essential for maintenance of genomic integrity and germline development. piRNA in Drosophila ovarian somatic cells (OSCs) are generated only through the primary processing pathway and are loaded onto Piwi, a member of PIWI proteins. However, factors required for the primary piRNA processing remain elusive. We focused on elucidating the mechanism of the primary processing pathway using the OSC line, which we have established from the female germline stem cell/ovarian somatic sheet (fGS/OSS) line, and found that primary piRNA biogenesis occur through Yb bodies, cytoplasmic structures that have been previously implicated in RNA processing. armitage (armi), a gene encoding an ATP-dependent RNA helicase, is necessary for primary piRNA biogenesis in OSCs. Armi localizes Piwi to Yb bodies by associating with Piwi and Yb, the main component of Yb bodies. piRNA intermediates are loaded onto the Armi-Piwi complex and processed into mature piRNAs. Loss of Yb caused depletion of Yb bodies and resulted in a reduction in overall piRNA levels. Yb bodies also impacted piRNA intermediates production, suggesting that they play important roles in primary piRNA biogenesis. zucchini (zuc), a gene encoding a putative cytoplasmic nuclease, is involved in processing piRNA intermediates into mature piRNAs. Without Armi, Yb or Zuc, Piwi is not loaded with piRNAs and does not localize to nucleus. A Piwi mutant that lacks the nuclear localization signal and therefore is not localized in the nucleus failed to silence transposons. These data suggest that Armi, Yb and Zuc are required for primary piRNA biogenesis and Piwi function. We further identified components of Yb bodies other than Armi and Yb and cloned piRNA intermediates. We will discuss in detail primary piRNA biogenesis based on our analyses.