Piwi-interacting RNAs (piRNAs) are essential for silencing of transposable elements in the germline, but their biogenesis is poorly understood. Here we demonstrate that MOV10L1, a germ cell-specific putative RNA helicase, is associated with Piwi proteins. Genetic disruption of the MOV10L1 RNA helicase domain in mice renders both MILI and MIWI2 devoid of piRNAs. Absence of a functional piRNA pathway in Mov10l1 mutant testes causes loss of DNA methylation and subsequent derepression of retrotransposons in germ cells. The Mov10l1 mutant males are sterile owing to complete meiotic arrest. This mouse mutant expresses Piwi proteins but lacks piRNAs, suggesting that MOV10L1 is required for piRNA biogenesis and/or loading to Piwi proteins.T he Piwi clade of Argonaute proteins associates with a class of 26-31-nt germline-specific small RNAs called "piRNAs". Together they participate in suppression of transposable elements in all animals studied (1-4). In mice, the Piwi clade contains three members: Miwi2, Mili, and Miwi. These three Piwi members exhibit distinct developmental expression patterns. Miwi2 is expressed in perinatal male germ cells (5), whereas Mili is more broadly expressed from embryonic germ cells to postnatal round spermatids (6). Miwi expression begins in pachytene spermatocytes and persists in haploid round spermatids (7). The overlapping temporal expression of Mili with Miwi and Miwi2 points to the pivotal role of MILI in the piRNA pathway, as further supported by the fact that MILI is associated with developmental stage-dependent pools of piRNAs: prenatal, prepachytene, and pachytene piRNAs (5,8,9).The mechanisms of piRNA biogenesis are largely unclear (1-4). One feature of piRNAs in all species is their highly clustered genomic origins. Several of these clusters produce piRNAs only from one strand. This leads to a hypothesized primary processing pathway whereby an unknown nuclease cleaves off mature piRNAs from a long single-stranded precursor transcript. On the other hand, some piRNAs in prenatal and prepachytene pools display signatures indicative of a proposed RNA-mediated amplification loop that uses primary piRNAs to generate secondary piRNAs from precursor transcripts (ping-pong mechanism) (10, 11). Apart from the Piwi proteins themselves, factors directly impacting piRNA production are unknown.We previously identified Mov10l1 as a gene specifically expressed in mouse germ cells, which encodes a putative RNA helicase of unknown function (12). Whereas the N-terminal half of MOV10L1 is not homologous to any other mouse proteins, its C-terminal RNA helicase domain exhibits low homology (45% amino acid identity) with MOV10. MOV10, the vertebrate homolog of Drosophila Armi, is ubiquitously expressed. In mammalian cells, MOV10 is associated with Argonaute proteins in the RNA-induced silencing complex (RISC) and is functionally required for RNA interference (13, 14). Here we demonstrate that MOV10L1 is an essential factor in the piRNA pathway.
Results
MOV10L1 Is Associated with Piwi Proteins.To identi...