Role of RNA degradation in germline stem cell to oocyte transition

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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, called maternal mRNAs, that help launch the next generation; any mistakes in this process could be detrimental to the offspring. However, it is not known if specific mRNAs are selected for deposition, or what, if any, guardians ensure that the right mRNAs are deposited. We have identified Drosophila twister (tst), the homologue of human SKI2VL, as one of the guardians of maternal inheritance. tst encodes an ATP-dependent RNA helicase that is part of the conserved Super Killer (Ski) complex, which funnels mRNA targets from mRNA surveillance pathways to the degradation machinery. We find that tst mutants are viable but females are sterile. Our data suggests that Tst promotes the degradation of mRNAs that are expressed and required in the undifferentiated cells such as germline stem cells (GSCs). We find that this Tst-mediated degradation happens during differentiation before oocytes are specified. Failure to degrade the GSC mRNAs in the differentiated stages results in loss of accumulation maternal mRNAs in the oocyte resulting in oocyte death. We find that Tst-regulated mRNAs contain polypyrimidine tracts (PTs) in the coding sequence (CDS) that can be bound by protein a PT binding protein, Half pint (Hfp), which we find also coregulates a subset of Tst-regulated mRNAs. We hypothesized that Hfp binding in the CDS can block translation by setting up barrier to ribosome movement on the coding sequence (CDS). In support of this hypothesis, we find that a key protein that monitors ribosome stalling on the CDS and can activate the No-Go decay (NGD) surveillance pathway, Pelota, also co-regulates both tst and hfp targets. Our central hypothesis is that the Ski complex detects a subset mRNAs are expressed and function in the GSCs and excludes these mRNAs from becoming part of the maternal inheritance by monitoring for stalled ribosomes. The objective of our proposal is to uncover how Tst and the NGD pathway regulates maternal mRNA inheritance via ribosome stalling. We will address this hypothesis with two specific aims: 1) Define the role of Twister, a component of RNA degradation promoting Super Killer complex, in regulating oogenesis. 2) Ascertain the role of No-Go decay pathway in controlling the selective elimination of mRNAs during oogenesis. The outcomes of our proposed studies are expected to improve scientific knowledge by revealing the extent, mechanism, and functional consequences of Tst and NGD mediated surveillance of the oocyte transcriptome. As loss of the tst human homolog, SKIV2L, results in trichohepatoenteric syndrome (THES) that has no known treatment. These findings will have a positive impact by providing insight into the etiology of diseases such as THES. Further, these studies may reveal potential therapeutic targets for many diseases arising from dysregulated RNA catabolism, including some spinal motor neuropathies.