Reply to Shoji and Katsuma

Effects of transgene insertion loci and copy number on Dnmt3L gene silencing through antisense transgene-derived PIWI-interacting RNAs

PIWI-interacting RNAs (piRNAs), which are germ cell-specific small RNAs, are essential for spermatogenesis. In fetal mouse germ cells, piRNAs are synthesized from sense and antisense RNAs of transposable element sequences for retrotransposon silencing. In a previous study, we reported that transgenic mice expressing antisense-Dnmt3L under the control of the Miwi2 promoter (Tg-Miwi2P-asDnmt3L) exhibited piRNA-mediated DNMT3L down-regulation. In this study, two transgene integration loci (B3 and E1) were identified on chromosome 18 of the Tg-Miwi2P-asDnmt3L mice; these loci were weak piRNA clusters. Crossbreeding was performed to obtain mice with the transgene cassette inserted into a single locus. DNMT3L was silenced and spermatogenesis was severely impaired in mice with the transgene cassette inserted at the B3 locus (Tg-B mice). In contrast, spermatogenesis in mice bearing the transgene at the E1 locus (Tg-E mice) was normal. The number of piRNAs for Dnmt3L in Tg-B mice was eightfold higher than that in Tg-E mice. Therefore, both gene silencing and impaired spermatogenesis depended on the transgene copy number rather than on the insertion loci. Additionally, the endogenous Dnmt3L promoter was not methylated in Tg mice, suggesting that Dnmt3L silencing was caused by post-transcriptional gene silencing. Based on these data, we discuss a piRNA-dependent gene silencing mechanism against novel gene insertions.

Artificial "ping-pong" cascade of PIWI-interacting RNA in silkworm cells

PIWI-interacting RNAs (piRNAs) play essential roles in the defense system against selfish elements in animal germline cells by cooperating with PIWI proteins. A subset of piRNAs is predicted to be generated via the "ping-pong" cascade, which is mainly controlled by two different PIWI proteins. Here we established a cell-based artificial piRNA production system using a silkworm ovarian cultured cell line that is believed to possess a complete piRNA pathway. In addition, we took advantage of a unique silkworm sex-determining one-to-one ping-pong piRNA pair, which enabled us to precisely monitor the behavior of individual artificial piRNAs. With this novel strategy, we successfully generated artificial piRNAs against endogenous protein-coding genes via the expected back-and-forth traveling mechanism. Furthermore, we detected "primary" piRNAs from the upstream region of the artificial "ping-pong" site in the endogenous gene. This artificial piRNA production system experimentally confirms the existence of the "ping-pong" cascade of piRNAs. Also, this system will enable us to identify the factors involved in both, or each, of the "ping" and "pong" cascades and the sequence features that are required for efficient piRNA production.