Ishizu, Hirotsugu



School of Medicine, Department of Molecular Biology (Shinanomachi)


Assistant Professor/Senior Assistant Professor


Papers 【 Display / hide

  • Requirements for multivalent Yb body assembly in transposon silencing in Drosophila

    Hirakata S., Ishizu H., Fujita A., Tomoe Y., Siomi M.

    EMBO Reports (EMBO Reports)  20 ( 7 )  2019.07

    ISSN  1469221X

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    © 2019 The Authors Female sterile (1) Yb (Yb) is a primary component of Yb bodies, perinuclear foci considered to be the site of PIWI-interacting RNA (piRNA) biogenesis in Drosophila ovarian somatic cells (OSCs). Yb consists of three domains: Helicase C-terminal (Hel-C), RNA helicase, and extended Tudor (eTud) domains. We previously showed that the RNA helicase domain is necessary for Yb–RNA interaction, Yb body formation, and piRNA biogenesis. Here, we investigate the functions of Hel-C and eTud and reveal that Hel-C is dedicated to Yb–Yb homotypic interaction, while eTud is necessary for Yb–RNA association, as is the RNA helicase domain. All of these domains are indispensable for Yb body formation and transposon-repressing piRNA production. Strikingly, however, genic piRNAs unrelated to transposon silencing are produced in OSCs where Yb bodies are disassembled. We also reveal that Yb bodies are liquid-like multivalent condensates whose assembly depends on Yb–Yb homotypic interaction and Yb binding particularly with flamenco RNA transcripts, the source of transposon-repressing piRNAs. New insights into Yb body assembly and biological relevance of Yb bodies in transposon silencing have emerged.

  • Distinct and Collaborative Functions of Yb and Armitage in Transposon-Targeting piRNA Biogenesis

    Ishizu H., Kinoshita T., Hirakata S., Komatsuzaki C., Siomi M.

    Cell Reports (Cell Reports)  27 ( 6 ) 1822 - 1835.e8 2019.05

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    © 2019 The Authors PIWI-interacting RNAs (piRNAs) repress transposons to maintain germline genome integrity. Previous studies showed that artificial tethering of Armitage (Armi) to reporter RNAs induced piRNA biogenesis. However, the lack of female sterile (1) Yb (Yb) in Drosophila ovarian somatic cells (OSCs) impaired the production of transposon-targeting piRNAs, even in the presence of Armi. Here, we show that the specific interaction of Armi with RNA transcripts of the flamenco piRNA cluster, the primary source of transposon-targeting piRNAs in OSCs, is strictly regulated by Yb. The lack of Yb allowed Armi to bind RNAs promiscuously, leading to the production of piRNAs unrelated to transposon silencing. The ATP hydrolysis-defective mutants of Armi failed to unwind RNAs and were retained on them, abolishing piRNA production. These findings shed light on distinct and collaborative requirements of Yb and Armi in transposon-targeting piRNA biogenesis. We also provide evidence supporting the direct involvement of Armi but not Yb in Zucchini-dependent piRNA phasing. Ishizu et al. show that the selective binding of Armi to piRNA precursors is controlled by Yb and the ATP hydrolysis activity of Armi in Drosophila ovarian somatic cells. The data presented here also suggest that Armi is directly involved in Zuc-dependent phased piRNAs biogenesis.

  • The nucleolar transcriptome regulates Piwi shuttling between the nucleolus and the nucleoplasm

    Mikhaleva E., Leinsoo T., Ishizu H., Gvozdev V., Klenov M.

    Chromosome Research (Chromosome Research)  27 ( 1-2 ) 141 - 152 2019.03

    ISSN  09673849

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    © 2018, Springer Nature B.V. The nucleolus contains a lot of proteins unrelated to ribosome biogenesis. Some of these proteins shuttle between the nucleolus and the nucleoplasm regulating the cell cycle and stress response. The piRNA binding protein Piwi is involved in silencing of transposable elements (TEs) in the Drosophila gonads. Here we used cultured ovarian somatic cells (OSC) to characterize Piwi as a visitor to the nucleolus. Dynamic Piwi localization was shown to vary from its uniform distribution between the nucleoplasm and the nucleolus to pronounced nucleolar immobilization. We were intrigued by this localization behavior and revealed that nascent nucleolar transcripts recruit Piwi for nucleolar retention. Piwi eviction from the nucleolus was observed upon RNase treatment and after RNA polymerase (Pol) I inhibition, but not after Pol II inactivation. On the contrary, heat shock caused drastic Piwi redistribution from the nucleoplasm to the nucleolus, which occurred only in the presence of Pol I-mediated transcription. These results allow us to hypothesize that specific stress-induced transcripts made by Pol I promote the nucleolar sequestration of proteins in Drosophila, similar to previous observations in mammalian cells. We also found that in OSC, Piwi partially restricts expression of the rDNA copies containing R1 and R2 retrotransposon insertions especially upon heat shock-induced activation of these copies. Therefore, we suggest that Piwi intranuclear shuttling may have a functional role in ensuring a balance between silencing of rDNA-specific TEs under stress and the canonical Piwi function in non-nucleolar TE repression.

  • Nuclear RNA export factor variant initiates piRNA-guided co-transcriptional silencing

    Murano K., Iwasaki Y., Ishizu H., Mashiko A., Shibuya A., Kondo S., Adachi S., Suzuki S., Saito K., Natsume T., Siomi M., Siomi H.

    EMBO Journal (EMBO Journal)   2019

    ISSN  02614189

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    © 2019 The Authors The PIWI-interacting RNA (piRNA) pathway preserves genomic integrity by repressing transposable elements (TEs) in animal germ cells. Among PIWI-clade proteins in Drosophila, Piwi transcriptionally silences its targets through interactions with cofactors, including Panoramix (Panx) and forms heterochromatin characterized by H3K9me3 and H1. Here, we identified Nxf2, a nuclear RNA export factor (NXF) variant, as a protein that forms complexes with Piwi, Panx, and p15. Panx–Nxf2–P15 complex formation is necessary in the silencing by stabilizing protein levels of Nxf2 and Panx. Notably, ectopic targeting of Nxf2 initiates co-transcriptional repression of the target reporter in a manner independent of H3K9me3 marks or H1. However, continuous silencing requires HP1a and H1. In addition, Nxf2 directly interacts with target TE transcripts in a Piwi-dependent manner. These findings suggest a model in which the Panx–Nxf2–P15 complex enforces the association of Piwi with target transcripts to trigger co-transcriptional repression, prior to heterochromatin formation in the nuclear piRNA pathway. Our results provide an unexpected connection between an NXF variant and small RNA-mediated co-transcriptional silencing.

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  • piRNA生合成経路の解析


    MEXT,JSPS, Grant-in-Aid for Scientific Research, 石津 大嗣, Grant-in-Aid for Scientific Research (B), Principal Investigator


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