Profiling of Mili and Miwi2 associated piRNAs in WT, MiliDAH, Miwi2DAH and Miwi2KO from E16.5 gonadocytes.. Profiling of Mili and Miwi2 associated piRNAs in WT, MiliDAH, Miwi2DAH and Miwi2KO from E16.5 gonadocytes.

Piwi proteins and piRNAs have conserved functions in transposon silencing. The murine Piwi proteins Mili and Miwi2 direct epigenetic LINE1 (L1) and intracisternal A particle (IAP) transposon silencing during genome reprogramming in the embryonic male germline. Piwi proteins are proposed to be piRNA-guided endonucleases that initiate secondary piRNA biogenesis . However the actual contribution of their endonuclease activities to piRNA biogenesis and transposon silencing remain unknown. To investigate the role of Piwi-catalyzed endonucleolytic activity, we engineered point mutations in the mouse that substitute the second D to an A in the catalytic triad (DDH) of Mili and Miwi2, generating the MiliDAH and Miwi2DAH alleles, respectively. Analysis of Mili-bound piRNAs from homozygous MiliDAH fetal gonadocytes revealed the failure of transposon piRNA amplification resulting in the stark reduction of piRNA bound within Miwi2 ribonuclear particles (RNPs). We find that Mili-mediated piRNA amplification is selectively required for L1 but not IAP silencing. The defective piRNA pathway in MiliDAH mice results in spermatogenic failure and sterility. Surprisingly, homozygous Miwi2DAH mice are fertile, transposon silencing is established normally and no defects in secondary piRNA biogenesis are observed. In addition, the hallmarks of piRNA amplification are observed in Miwi2-deficient gonadocytes. We conclude that cycles of intra-Mili secondary piRNA biogenesis fuel piRNA amplification that is selectively required for L1 silencing.