The endonuclease activity of Mili fuels piRNA amplification that silences LINE1 elements.
Piwi proteins and Piwi-interacting RNAs (piRNAs) have conserved functions in transposon silencing. The murine Piwi proteins Mili and Miwi2 (also called Piwil2 and Piwil4, respectively) direct epigenetic LINE1 and intracisternal A particle transposon silencing during genome reprogramming in the embryonic male germ line. 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-catalysed endonucleolytic activity, we engineered point mutations in mice that substitute the second aspartic acid to an alanine in the DDH catalytic triad of Mili and Miwi2, generating the Mili(DAH) and Miwi2(DAH) alleles, respectively. Analysis of Mili-bound piRNAs from homozygous Mili(DAH) fetal gonadocytes revealed a failure of transposon piRNA amplification, resulting in the marked reduction of piRNA bound within Miwi2 ribonuclear particles. We find that Mili-mediated piRNA amplification is selectively required for LINE1, but not intracisternal A particle, silencing. The defective piRNA pathway in Mili(DAH) mice results in spermatogenic failure and sterility. Surprisingly, homozygous Miwi2(DAH) 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 absolutely required for LINE1 silencing.
European Molecular Biology Laboratory, Mouse Biology Unit, Via Ramarini 32, Monterotondo Scalo 00015, Italy.
Serena
S
De Fazio
Nenad
N
Bartonicek
Monica
M
Di Giacomo
Cei
C
Abreu-Goodger
Aditya
A
Sankar
Charlotta
C
Funaya
Claude
C
Antony
Pedro N
PN
Moreira
Anton J
AJ
Enright
Dónal
D
O'Carroll
D000483:Alleles; D000818:Animals; D060565:Argonaute Proteins / Q000235:genetics / Q000378:metabolism*; D004251:DNA Transposable Elements / Q000235:genetics; D020868:Gene Silencing; D020084:Long Interspersed Nucleotide Elements / Q000235:genetics*; D008297:Male; D051379:Mice; D034741:RNA, Small Interfering / Q000096:biosynthesis* / Q000235:genetics* / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D013091:Spermatogenesis / Q000235:genetics
D060565:Argonaute Proteins; D004251:DNA Transposable Elements; C546575:PIWIL4 protein, mouse; C448521:Piwil2 protein, mouse; D034741:RNA, Small Interfering; D016601:RNA-Binding Proteins