Drosophila non-gonadal PIWI protein Aubergine regulates regenerative stem cell proliferation and tumourigenesis in the adult intestine [ncRNA-seq]
Somatic stem cells are executors of physiological and pathological proliferation of adult self-renewing tissues, such as the intestine. The actions of intestinal stem cells (ISCs) rely on the integration of cell intrinsic and niche-derived signals, which are necessary to achieve a balanced response to the multiple stimuli that constantly challenge tissue homeostasis and organismal health. Disruption of such balance is causative of age-associated tissue dysfunction and hyperproliferative conditions, including inflammation and cancer.  The highly conserved PIWI-interacting RNAs (piRNAs) biosynthesis pathway, also known as the PIWI pathway, has been classically studied in the Drosophila germline for its role in the repression of transposable elements (TEs) and the regulation of germline stem cell homeostasis. Recent reports have emerged on a role of Piwi, the founding member of the pathway, in the maintenance of ISC homeostasis in the adult Drosophila midgut. The implications of these findings regarding a general role of the PIWI pathway and piRNAs in the intestine remain to be addressed. Here, we characterise a cell autonomous role of the PIWI family protein Aubergine (Aub) in ISCs. We show that, while dispensable for homeostatic self-renewal of the midgut, inducible Aub is essential to regulate ISC proliferation following acute damage of the intestinal epithelium and in oncogenic settings. Our work suggests that the role of Aub in ISCs is independent of its piRNAs regulatory function. Instead, Aub drives ISC proliferation through the induction of regenerative stem cell factors Myc and Sox21a, and through the initiation of protein translation by the eukaryotic initiation factor 3 complex (eIF3). In summary, our results discover a role of Aub in damage induced proliferation of the adult Drosophila intestine involving the regulation of major regenerative signalling pathways and the protein translation machinery. Furthermore, we present genetically distinct hyperplastic settings fostering a role of Aub as an intestinal tumour promoter or suppressor. Overall design: The study aims at understanding the role of the PIWI family protein Aubergine in the proliferation of intestinal stem cells (ISCs) upon damage in Drosophila melanogaster. We generated oxidised small RNA sequencing libraries from freshly dissected midguts, sorted ISCs/Enteroblasts (marked by escargot-Gal4) and ovaries. We also generated RNA sequencing libraries from poly-A plus RNA to determine the abundance of transposon mRNAs expressed in the ISCs/Enteroblasts.