MIWI2 and MILI have differential effects on piRNA biogenesis and DNA methylation
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ABSTRACT: In developing male germ cells, prospermatogonia, two Piwi proteins, MILI and MIWI2, use piRNA guides to repress transposable element (TE) expression and ensure genome stability and proper gametogenesis. In addition to their roles in post-transcriptional TE repression, both proteins are required for DNA methylation of TE sequences. Here we analyzed the effect of Miwi2 deficiency on piRNA biogenesis and transposon repression. Miwi2-deficiency had only a minor impact on piRNA biogenesis; however, the piRNA profile of Miwi2-knockout mice indicated overexpression of several LINE1 TE families that led to activation of the ping-pong piRNA cycle. Furthermore, we found that MILI and MIWI2 have distinct functions in TE repression in the nucleus. MILI is responsible for DNA methylation of a larger subset of TE families than MIWI2 suggesting that the proteins have independent roles in establishing DNA methylation patterns.
Project description:In developing male germ cells, prospermatogonia, two Piwi proteins, MILI and MIWI2, use piRNA guides to repress transposable element (TE) expression and ensure genome stability and proper gametogenesis. In addition to their roles in post-transcriptional TE repression, both proteins are required for DNA methylation of TE sequences. Here we analyzed the effect of Miwi2 deficiency on piRNA biogenesis and transposon repression. Miwi2-deficiency had only a minor impact on piRNA biogenesis; however, the piRNA profile of Miwi2-knockout mice indicated overexpression of several LINE1 TE families that led to activation of the ping-pong piRNA cycle. Furthermore, we found that MILI and MIWI2 have distinct functions in TE repression in the nucleus. MILI is responsible for DNA methylation of a larger subset of TE families than MIWI2 suggesting that the proteins have independent roles in establishing DNA methylation patterns. Small RNA profiles (19-30 nt range) of embryonic (E16.5) and post-natal (P10) testis of Miwi2 mutant mice and matched heterozygote controls. mRNA profiles of embryonic testis (E16.5) of heterozygote control mice and of postnatal testis (P10) of Miwi2 and Mili mutants and heterozygote controls. CpG methylation BS-seq profile of postnatal (P10) spermatocytes of Miwi2 mutant mice and matched heterozygote controls.
Project description:Piwi proteins and piRNAs have conserved functions in transposonM- silencing. The murine Piwi proteins Mili and Miwi2 direct epigeneticM- LINE1 (L1) and intracisternal A particle (IAP) transposon silencingM- during genome reprogramming in the embryonic male germline. PiwiM- proteins are proposed to be piRNA-guided endonucleases that initiateM- secondary piRNA biogenesis . However the actual contribution of theirM- endonuclease activities to piRNA biogenesis and transposon silencingM- remain unknown. To investigate the role of Piwi-catalyzedM- endonucleolytic activity, we engineered point mutations in the mouseM- that substitute the second D to an A in the catalytic triad (DDH) ofM- Mili and Miwi2, generating the MiliDAH and Miwi2DAH alleles,M- M- respectively. Analysis of Mili-bound piRNAs from homozygous MiliDAHM- fetal gonadocytes revealed the failure of transposon piRNA amplification resulting in the stark reduction of piRNA bound withinM- Miwi2 ribonuclear particles (RNPs). We find that Mili-mediated piRNA amplification is selectively required for L1 but not IAP silencing.M- The defective piRNA pathway in MiliDAH mice results in spermatogenic failure and sterility. Surprisingly, homozygous Miwi2DAH mice areM- fertile, transposon silencing is established normally and no defectsM- in secondary piRNA biogenesis are observed. In addition, the hallmarks of piRNA amplification are observed in Miwi2-deficient gonadocytes. WeM- conclude that cycles of intra-Mili secondary piRNA biogenesis fuelM- piRNA amplification that is selectively required for L1 silencing.M-
Project description:PIWI proteins and their associated small RNAs called PIWI-interacting RNAs (piRNAs) restrict transposon activity in animal gonads to ensure fertility. Distinct biogenesis pathways load piRNAs into the PIWI proteins MILI and MIWI2 in the mouse male embryonic germline. While most of MILI piRNAs derive via a slicer-independent pathway, a MILI slicer endonuclease-initiated pathway loads nuclear MIWI2 with a series of phased piRNAs. Tudor domain-containing 12 (TDRD12) and its interaction partner Exonuclease domain-containing 1 (EXD1) are required for loading MIWI2, but only Tdrd12 is essential for fertility, leaving us with no explanation for the physiological role of Exd1. Using an artificial piRNA precursor, we demonstrate that MILI-triggered piRNA biogenesis is greatly reduced in the Exd1 mutant. The situation deteriorates in the sensitized Exd1 mutant (Exd1-/-; Tdrd12+/-), where diminished MIWI2 piRNA levels de-repress LINE1 retrotransposons, causing infertility. Thus, EXD1 enhances slicing-triggered MIWI2 piRNA biogenesis via a functional interaction with TDRD12.
Project description:The PIWI protein MIWI2 and its associated PIWI-interacting RNAs (piRNAs) instruct DNA methylation of young active transposable elements (TEs) in the male germline. Here we show that MIWI2 associates with TEX15 in foetal gonocytes. TEX15 is predominantly a nuclear protein that is not required for piRNA biogenesis but is essential for piRNA-directed TE de novo methylation and silencing. In summary, TEX15 is an essential executor of mammalian piRNA-directed DNA methylation.
Project description:In mice, the PIWI-piRNA pathway is essential to re-establish transposon silencing during male germline reprogramming. The cytoplasmic PIWI protein MILI mediates piRNA-guided transposon RNA cleavage as well as piRNA amplification. MIWI2-bound piRNAs and its nuclear localization are proposed to be dependent upon MILI function. Here, we demonstrate the existence of a piRNA biogenesis pathway that in the absence of MILI that sustains partial MIWI2 function and reprogramming activity.
Project description:The PIWI protein MIWI2 and its associated PIWI-interacting RNAs (piRNAs) instruct DNA methylation of young active transposable elements (TEs) in the male germline. Here we show that MIWI2 associates with TEX15 in foetal gonocytes. TEX15 is predominantly a nuclear protein that is not required for piRNA biogenesis but is essential for piRNA-directed TE de novo methylation and silencing. In summary, TEX15 is an essential executor of mammalian piRNA-directed DNA methylation.
Project description:In mammals, the acquisition of the germline from the soma provides the germline with an essential challenge, the necessity to erase and reset genomic methylation. In the male germline RNA-directed DNA methylation silences young active transposable elements (TEs). The PIWI protein MIWI2 (PIWIL4) and its associated PIWI-interacting RNAs (piRNAs) are proposed to tether MIWI2 to nascent TE transcripts and instruct DNA methylation. The mechanism by which MIWI2 directs de novo TE methylation is poorly understood but central to the immortality of the germline. Here, we define the interactome of MIWI2 in foetal gonocytes that are undergoing de novo genome methylation and identify a novel MIWI2-associated factor, SPOCD1, that is essential for young TE methylation and silencing. The loss of Spocd1 in mice results in male specific infertility and does not impact on piRNA biogenesis nor localization of MIWI2 to the nucleus. SPOCD1 is a nuclear protein and its expression is restricted to the period of de novo genome methylation. We found SPOCD1 co-purified in vivo with DNMT3L and DNMT3A, components of the de novo methylation machinery as well as constituents of the NURD and BAF chromatin remodelling complexes. We propose a model whereby tethering of MIWI2 to a nascent TE transcript recruits repressive chromatin remodelling activities and the de novo methylation apparatus through its association with SPOCD1. In summary, we have identified a novel and essential executor of mammalian piRNA-directed DNA methylation.
Project description:The PIWI protein MIWI2 and its associated PIWI-interacting RNAs (piRNAs) instruct DNA methylation of young active transposable elements (TEs) in the male germline. Here we show that MIWI2 associates with TEX15 in foetal gonocytes. TEX15 is predominantly a nuclear protein that is not required for piRNA biogenesis but is essential for piRNA-directed TE de novo methylation and silencing. In summary, TEX15 is an essential executor of mammalian piRNA-directed DNA methylation.
Project description:In mammals, the acquisition of the germline from the soma provides the germline with an essential challenge, the necessity to erase and reset genomic methylation. De novo genome methylation re-encodes the epigenome including transposable element (TE) silencing. In the male germline RNA-directed DNA methylation silences young active TEs. The PIWI protein MIWI2 (PIWIL4) and its associated PIWI-interacting RNA (piRNAs) act to tether MIWI2 to nascent TE transcripts and instruct DNA methylation of the active TE. The mechanism by which MIWI2 directs de novo TE methylation is poorly understood but central to the immortality of the germline. Here, we define the interactome of MIWI2 in fetal gonocytes that are undergoing de novo genome methylation and identify a novel MIWI2-associated factor SPOCD1 that is essential for TE silencing. The loss of Spocd1 in mice phenocopies that of Miwi2-deficient mice and does not impact on piRNA biogenesis nor localization of MIWI2 to the nucleus. SPOCD1 is a nuclear protein and its expression is restricted to the period of de novo genome methylation. We found SPOCD1 co-purified in vivo with constituents of several repressive chromatin remodelling complexes (NURD and BAF) as well as DNMT3L and DNMT3A, components of the de novo methylation machinery. We propose a model whereby tethering of MIWI2 to a nascent TE transcript recruits repressive chromatin remodelling activities and the de novo methylation apparatus through its association with SPOCD1. In summary, we have identified a novel and essential executor of mammalian piRNA-directed DNA methylation.