Multiple epigenetic mechanisms and the piRNA pathway enforce LINE1 silencing during adult spermatogenesis. Multiple epigenetic mechanisms and the piRNA pathway enforce LINE1 silencing during adult spermatogenesis

The Piwi protein Mili and the Piwi-interacting RNA (piRNA) pathway are required to establish L1 DNA methylation during epigenetic reprogramming that accompanies male germ cell development. Although Mili is expressed throughout most of adult spermatogenesis, its function therein remains unknown. We find that Mili displays a hitherto unappreciated dynamic expression profile during spermatogenesis, expressed in the mitotic spermatogonia but absent at the early meiotic stages only to reappear in late zygotene spermatocytes. Here we show using conditional mutagenesis that Mili's endonuclease activity post-transcriptionally cleaves L1 transcripts commencing upon its reappearance in late zygotene spermatocytes even in the presence of normal L1 DNA methylation