Identification and characterisation of cell intrinsic mechanisms regulating intestinal stem cell behaviour during acute damage

The maintenance and regeneration of tissues and organs is essential to ensure the health of an organism and its survival. These processes are characteristics of self-renewing tissues and are typically undertaken by dedicated stem cells. Therefore, understanding the signals within these stem cells that drive their proliferation in various contexts is crucial for understanding stem cell-dependent health and disease associated processes, such as cancer. We use the adult Drosophila midgut as a model to study stem cell biology and the processes by which stem cells maintain the tissues they reside in, regenerate tissues in response to damage, and how these processes can lead to malignancy following deregulation or co-option of normal developmental programmes. The PIWI pathway is composed of a family of proteins discovered and extensively characterised in the Drosophila germline, which are known for binding to retrotransposons and suppressing their insertion into DNA during meiosis, thereby maintaining genomic integrity. This process has been shown to be essential for maintaining germline stem cell populations. Biological functions of PIWI proteins outside the germline are largely unknown. However, reports in various species, including humans, point to a potential functional role of this family of proteins in somatic tissues. PIWI proteins are required for planarian wound healing and are highly expressed in many human cancers including colorectal cancer. We utilised the adult Drosophila midgut to study the role of the Drosophila PIWI family members, Piwi, Aubergine (Aub) and Ago3, as potential drivers of proliferation in somatic stem-cells. We find that piwi, aub and ago3 are expressed in the midgut and, using whole tissue mutants and cell specific gene manipulations, we have determined an exclusive role for Aub in the intestinal stem and progenitor cells (ISC/EBs). Our results indicate that Aub is induced upon stress in the midgut and knockdown of aub within intestinal stem cells (ISCs) and progenitor enteroblast (EB) cells results in abolishment of the proliferative response of ISCs during damage and ageing. Reciprocally, overexpression of aub within ISC/EBs is sufficient to induce tissue hyper-proliferation and accumulation of stem cells. Small RNA sequencing has shown that Aub is not having an impact on transposable elements (TEs), suggesting a potential non-canonical role for Aub in the regenerative response of the intestine following damage. Consistently, we find that knockdown of aub from ISC/EBs abolishes the upregulation of Wingless (Wg), a critical stem cell niche component required for the intestinal regeneration. We have also looked in to the role of the PIWI homologues in mice and found that the PIWIL1 protein, a homologue of Aub and Piwi, appears to be upregulated in colorectal cancer. Elucidation of this novel somatic role of Aub in the Drosophila midgut may have broader implications into the role of mammalian PIWI proteins in regeneration, ageing and hyperplastic conditions, such as cancer, affecting somatic self-renewing tissues