Regulated RNA degradation is required for Drosophila oogenesis

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Abstract The germ line generates gametes that link generations by passing genetic information from parent to offspring. During oogenesis, the egg receives critical mRNAs from the mother that help launch the next generation; any mistakes in this process could be detrimental to the offspring. However, it is not known if the maternally provided mRNAs are surveilled for quality or how aberrantly expressed mRNAs impact oogenesis. Twister (Tst), an RNA helicase conserved from yeast to humans, promotes decay of defective mRNAs. Loss of the tst human homolog, SKIV2L, results in trichohepatoenteric (THE) syndrome, which has no known treatment. Tst is part of the Ski complex that unwinds RNA duplexes, which are then subject to degradation by the exosome complex. mRNA targets of quality control mechanisms such as nonsense-mediated decay (NMD), no-go decay (NGD) and nonstop decay (NSD) are substrates for the Ski complex and the exosome. We find that the tst homozygous flies are viable but female sterile, exhibiting an oogenesis defect. We find tst is required in the germ line but not the soma of the gonad for proper oogenesis. Using RNA-Seq and bioinformatic analyses, we found that in tst mutants most of the transcriptome is not affected, but a distinct subset is upregulated and only a few transcripts are down regulated. This is consistent with Tst playing a central role in RNA degradation. The transcripts upregulated in tst mutants are poorly expressed in wild type conditions and these mRNAs are typically shorter in their 5’UTR, ORF and 3’UTR regions. Surprisingly, we find that the Tst targets do not have features of the canonical mRNA decay pathways such as NMD, NGD and NSD. We hypothesize that Tst plays a central role in surveilling the mRNAs supplied to the developing oocyte and facilitates the turnover of developmentally expressed mRNAs through their 3’UTRs. The main goals of this research are to: 1) determine how mRNA degradation mediated by Tst promotes proper oogenesis, 2) identify mRNAs targets of Tst, and 3) elucidate the mechanism by which Tst is recruited to these mRNA targets to initiate their degradation. These studies aim to establish the contributions of Tst to egg development and female fertility through turnover of developmentally regulated mRNAs. If Tst mediates turnover of developmentally regulated transcripts, this can provide insight to diseases such as THE. Our work in a genetically tractable organism will allow us to elucidate a target to design treatments against to alleviate symptoms resulting from insufficient mRNA metabolism and turnover. !