@misc{oai:ir.soken.ac.jp:00001444,
 author = {渡部, 聡朗 and ワタナベ, トシアキ and WATANABE, Toshiaki},
 month = {2016-02-17, 2016-02-17},
 note = {Small RNAs ranging in size between 20 and 35 nucleotides (nt) are found in many organisms including yeasts, plants, and animals. Small RNAs are associated with Argonaute proteins, which are effectors of silencing, and involved in regulation of gene expression through translational repression, mRNA degradation, and chromatin modification in a sequence dependent manner. In mammals, three classes of small RNAs have been found; microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi interacting RNAs (piRNAs). miRNAs are ubiquitously expressed in many tissues. In contrast, piRNAs and siRNAs are expressed mainly in germline cells. 
　　　　In the first chapter, I describe the results obtained from the analysis of mouse fetal piRNAs. piRNAs are 24-30 nt small RNAs that associate with Piwi proteins, which are a subfamily of Argonaute proteins. piRNAs and Piwi proteins are mainly expressed in the germline and implicated in germline development and silencing of retrotransposons. A recent study in mouse showed that DNA methylation of the 5’-untranslated region of LINE-1 is decreased in Piwi knockout (KO) male germ cells, raising the possibility that DNA methylation is mediated by piRNAs. Although piRNAs have been identified in neonatal and adult testes, they remained uncharacterized in fetal testes, where de novo methylation takes place. To examine the link between piRNAs and DNA methylation, I have analyzed piRNAs in fetal testes. 
　　　　I cloned more than 100,000 small RNAs from fetal testes. A distinct set of piRNAs were expressed in fetal testes and most of them were derived from retrotransposons. L1Md and IAP1 retrotransposons, of which DNA methylation levels are reduced in Piwi KO, were the major classes in fetal piRNAs. This suggests that piRNAs define genomic retrotransposon sequences to be subject to DNA methylation. To further investigate the link between methylation and piRNAs, I generated and analyzed Zucchini KO mice. The Zucchini gene has been thought to be involved in the piRNA pathway in Drosophila. In Zucchini mutant mice, the expression level of piRNAs and methylation levels of L1Md and IAP1 were decreased in fetal testes. Furthermore, decreased methylation was also observed at a specific region of the Rasgrf1 locus. In this region, piRNAs were specifically and densely mapped, and RNAs transcribed from the Rasgrf1 locus were targeted by these piRNAs. The results of my study are in concordance with the piRNA-mediated DNA methylation in fetal mouse testes and thus help to understand the mechanism of piRNA-mediated DNA methylation. 
　　　　In the second chapter, I describe the results obtained from the analysis of mouse oocyte endogenous siRNAs. siRNAs are generated from long double-stranded RNAs (dsRNAs) by a ribonuclease called Dicer and are mainly involved in defense against molecular parasites including viruses, transposons, and transgenes through RNA interference (RNAi) in plants and worms. RNA dependent RNA polymerase (RdRP) is involved in the generation of precursor dsRNAs from single stranded RNAs. Gene regulation by endogenous siRNAs has been observed only in organisms possessing RdRP. Despite no report of RdRP activity in mammalian cells, endogenous siRNA molecules were previously observed in mouse fully grown oocytes. However, only a small number of endogenous siRNAs have been identified and their biogenesis and function largely remained unclear. In order to obtain a comprehensive picture of endogenous siRNAs, I have analyzed small RNAs expressed in mouse growing oocytes through deep sequencing. 
　　　　I identified a large number of both ~25-27 nt piRNAs and ~21 nt siRNAs corresponding to mRNAs or retrotransposons in growing oocytes. piRNAs in oocytes play a role in the regulation of retrotransposons. siRNAs were exclusively mapped to retrotransposons or other genomic regions that produce transcripts capable of forming dsRNA structures. Inverted repeat structures, bidirectional transcription and antisense transcripts from various loci are sources of the dsRNAs. Some precursor transcripts of siRNAs were derived from expressed pseudogenes, suggesting that one role of pseudogenes is to adjust the level of the founding source mRNA through RNAi. Loss of Dicer or Ago2 resulted in decreased levels of siRNAs and increased levels of retrotransposons and protein-coding transcripts complementary to the siRNAs. Thus, the RNAi pathway regulates both protein-coding transcripts and retrotransposons in mouse oocytes. 
　　　　The results obtained here establish the existence of endogenous siRNAs in mammals, and provide the information on small RNAs expressed in mouse germline cells and new insights into the pathway and function of small RNAs., 総研大甲第1251号},
 title = {Roles and biogenesis pathways of endogenous siRNAs and piRNAs in mouse germline cells},
 year = {}
}