Elucidating the non-apoptotic role of the caspases in the Drosophila intestinal system

Abstract

The deregulation of caspase activity and stem cell properties are often correlated with the appearance and propagation of cancer, including colorectal cancer. However, the caspase-dependent regulation of the intestinal system remains poorly understood. Taking advantage of the powerful genetic tractability of Drosophila melanogaster, I aimed to decipher the functional role of the caspases within the intestinal system, in addition to their role in tumour formation. In this thesis, I demonstrated that using an apical-caspase reporter, I was able to observe widespread and stereotyped patterns of caspase activation within the intestinal system. Furthermore, this activation was non-apoptotic and occurred in non-regenerative conditions. Upon conditional knockout of the mammalian caspase-9/2 orthologue, Dronc, in intestinal progenitors, I demonstrated that Dronc was required to restrain proliferation and prevent the premature differentiation of Enteroblasts into Enteroctytes. Additionally, this activity of Dronc relied on its catalytic activity but not the canonical apoptotic pathway. Using a Dronc-protein reporter I was able to correlate these functions with its specific accumulation within the Enteroblasts. Molecularly, the non-apoptotic activity of Dronc in this context relies on the activity of the Notch and Insulin-TOR pathways. Using the Drop-seq single-cell sequencing technique, I observed the misexpression of several regional and cell-specific genes following loss of Dronc. Furthermore, these Dronc-specific effects may result in the deregulation of the intestinal microbiota. Collectively, this data provides novels insights into the caspase-dependent but non-apoptotic regulation of the intestinal system in non-regenerative conditions. Furthermore, these findings could explain the correlation between caspase-9 misexpression and the bad prognosis observed in patients suffering from colon cancer. </p