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Genes Dev. 2019 Sep 19. doi: 10.1101/gad.322776.118. [Epub ahead of print]

Planarians recruit piRNAs for mRNA turnover in adult stem cells.

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Gene regulation by Non-coding RNA, Elite Network of Bavaria and University of Bayreuth, 95447 Bayreuth, Germany.
Stowers Institute for Medical Research, Kansas City, Missouri 64110, USA.
Howard Hughes Medical Institute, Stowers Institute for Medical Research, Kansas City, Missouri 64110, USA.


PIWI proteins utilize small RNAs called piRNAs to silence transposable elements, thereby protecting germline integrity. In planarian flatworms, PIWI proteins are essential for regeneration, which requires adult stem cells termed neoblasts. Here, we characterize planarian piRNAs and examine the roles of PIWI proteins in neoblast biology. We find that the planarian PIWI proteins SMEDWI-2 and SMEDWI-3 cooperate to degrade active transposons via the ping-pong cycle. Unexpectedly, we discover that SMEDWI-3 plays an additional role in planarian mRNA surveillance. While SMEDWI-3 degrades numerous neoblast mRNAs in a homotypic ping-pong cycle, it is also guided to another subset of neoblast mRNAs by antisense piRNAs and binds these without degrading them. Mechanistically, the distinct activities of SMEDWI-3 are primarily dictated by the degree of complementarity between target mRNAs and antisense piRNAs. Thus, PIWI proteins enable planarians to repurpose piRNAs for potentially critical roles in neoblast mRNA turnover.


PIWI proteins; RNA-binding proteins; S. mediterranea; mRNA surveillance; mRNA turnover; piRNAs; planarians; posttranscriptional gene silencing; regeneration; stem cells


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