Department of Molecular Genetics of the Cell, Institute of Molecular Genetics, Russian Academy of Science, Moscow 123182, Russia.
Proc Natl Acad Sci U S A. 2011 Nov 15;108(46):18760-5. doi: 10.1073/pnas.1106676108. Epub 2011 Nov 7.
Piwi-interacting RNAs (piRNAs) and Piwi proteins have the evolutionarily conserved function of silencing of repetitive genetic elements in germ lines. The founder of the Piwi subfamily, Drosophila nuclear Piwi protein, was also shown to be required for the maintenance of germ-line stem cells (GSCs). Hence, null mutant piwi females exhibit two types of abnormalities, overexpression of transposons and severely underdeveloped ovaries. It remained unknown whether the failure of GSC maintenance is related to transposon derepression or if GSC self-renewal and piRNA silencing are two distinct functions of the Piwi protein. We have revealed a mutation, piwi(Nt), removing the nuclear localization signal of the Piwi protein. piwi(Nt) females retain the ability of GSC self-renewal and a near-normal number of egg chambers in the ovarioles but display a drastic transposable element derepression and nuclear accumulation of their transcripts in the germ line. piwi(Nt) mutants are sterile most likely because of the disturbance of piRNA-mediated transposon silencing. Analysis of chromatin modifications in the piwi(Nt) ovaries indicated that Piwi causes chromatin silencing only of certain types of transposons, whereas others are repressed in the nuclei without their chromatin modification. Thus, Piwi nuclear localization that is required for its silencing function is not essential for the maintenance of GSCs. We suggest that the Piwi function in GSC self-renewal is independent of transposon repression and is normally realized in the cytoplasm of GSC niche cells.
Piwi 相互作用 RNA（piRNAs）和 Piwi 蛋白具有在生殖系中沉默重复遗传元件的进化保守功能。Piwi 亚家族的创始人，果蝇核 Piwi 蛋白，也被证明是维持生殖干细胞（GSCs）所必需的。因此，piwi 基因缺失突变体的雌性表现出两种异常，转座子的过度表达和卵巢严重发育不全。尚不清楚 GSC 维持的失败是否与转座子去抑制有关，或者 GSC 自我更新和 piRNA 沉默是否是 Piwi 蛋白的两个不同功能。我们已经揭示了一种突变，piwi（Nt），去除了 Piwi 蛋白的核定位信号。piwi（Nt）的雌性保留了 GSC 自我更新的能力和卵原细胞中接近正常数量的卵室，但表现出转座元件的强烈去抑制和其转录物在生殖系中的核积累。piwi（Nt）突变体不育很可能是因为 piRNA 介导的转座子沉默受到干扰。piwi（Nt）卵巢中的染色质修饰分析表明，Piwi 仅引起某些类型转座子的染色质沉默，而其他转座子在没有染色质修饰的情况下在核内被抑制。因此，Piwi 的核定位对于其沉默功能不是必需的，但其对于维持 GSCs 是必需的。我们认为，Piwi 在 GSC 自我更新中的功能独立于转座子的抑制，并且通常在 GSC 巢细胞的细胞质中实现。
