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Review
. 2014 Jan 16;505(7483):353-359.
doi: 10.1038/nature12987.

VSports - PIWI proteins and PIWI-interacting RNAs in the soma

Robert J Ross #  1 , Molly M Weiner #  1 , Haifan Lin  1
Affiliations
Review

PIWI proteins and PIWI-interacting RNAs in the soma

Robert J Ross (V体育平台登录) et al. Nature. .

Abstract

The discovery of millions of PIWI-interacting RNAs revealed a fascinating and unanticipated dimension of biology. The PIWI-piRNA pathway has been commonly perceived as germline-specific, even though the somatic function of PIWI proteins was documented when they were first discovered. Recent studies have begun to re-explore this pathway in somatic cells in diverse organisms, particularly lower eukaryotes. These studies have illustrated the multifaceted somatic functions of the pathway not only in transposon silencing but also in genome rearrangement and epigenetic programming, with biological roles in stem-cell function, whole-body regeneration, memory and possibly cancer VSports手机版. .

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Figures

Figure 1
Figure 1. piRNA biogenesis in the Drosophila ovarian soma
piRNAs are generated from specific genomic loci known as piRNA clusters, which include flamenco, the 5′ UTRs of mRNAs and traffic jam in the soma. The long single-stranded piRNA precursor (red) is then exported from the nucleus. In the cytoplasm, the precursors are processed into mature piRNAs. The precursors are cleaved by an unknown endonuclease to generate their 5′ end, and transported into the perinuclear Yb body for further processing. The precursors are loaded onto Piwi in a process that is dependent on Yb, Armitage (Armi) and Vreteno. Overlapping proteins indicate protein–protein interaction. The co-chaperone Shutdown plays an uncharacterized, but crucial, part in piRNA loading. The putative endonuclease Zucchini (Zuc) is required for piRNA maturation and for nuclear localization of Piwi. Subsequently, piRNAs are trimmed to the appropriate length by an unidentified exonuclease and 2′-O-methylated at the piRNA 3′ end, rendering them more stable. The Piwi–piRNA complex is then transported into the nucleus, where it modulates chromatin state.
Figure 2
Figure 2. Piwi–piRNA mediated epigenetic regulation
Simplified illustrations of the currently proposed models of Piwi-mediated transcriptional gene silencing. a, In heterochromatin, Piwi may be guided to its target sequences by the complementarity of its bound piRNA (red) to genomic DNA. On binding, Piwi recruits the epigenetic modifier HP1a that then recruits the major Drosophila histone methyltransferase Su(var)3-9, which then deposits a methyl group on the unmethylated histone 3 lysine 9 (H3K9). Through an unknown mechanism, a critical mass of the H3K9 repressive chromatin marks inhibits Pol II transcription, effectively silencing the Piwi–piRNA target. b, In euchromatin, Piwi targets nascent transcripts (green) by piRNA sequence complementarity. Subsequent to this, there are multiple working models. Piwi may directly recruit HP1a, which in turn recruits a histone methyltransferase (Su(var)3-9 is shown) that then deposits methyl groups on the unmethylated H3K9. Alternatively, Piwi may directly recruit the histone methyltransferase, and subsequently HP1a may then bind the methylated H3K9. Regardless of the mechanism, the net effect is that these repressive marks, in concert with Maelstrom (not shown), inhibit RNA Pol II transcription.
Figure 3
Figure 3. Somatic genome rearrangement in ciliates
Unicellular ciliates posses two nuclei: the germline micronucleus and the somatic macronucleus. After mating, the developing macronucleus is extensively edited to remove germline-specific sequences. The new mature somatic nucleus contains only genes needed for somatic (vegetative) growth. The rest of the germline-specific genome (red boxes) undergoes DNA elimination, an extraordinary method to purge the somatic genome of repetitive sequences and transposons. This process, called somatic genome rearrangement, differs between Tetrahymena and Oxytricha. a, Tetrahymena piRNAs (red lines) are generated by the germ line, and target germline-specific sequences of the developing somatic macronucleus for elimination. b, Oxytricha piRNAs (blue, orange and purple lines) are generated by the parent somatic macronucleus, and direct the retention of somatic genes in the mature somatic macronucleus (blue, orange and purple boxes).
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References

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