Early endosomal Rab21 in enterocytes contributes to gut tissue maintenance
Authors: Sonya Nassari; Steve Jean
Affiliation: Université de Sherbrooke
Keywords: c. endocytosis; d. apoptosis-induced proliferation
Membrane trafficking describes the vesicular transport of proteins and macromolecules by cells. Trafficking defects impair signaling events and have dramatic consequences on cell homeostasis. Enterocytes (ECs), intestinal differentiated cells, are the main cell type of the gut epithelium. Given their absorptive functions, they display a high membrane trafficking flux. Notably, defects in endocytic pathways can affect ECs function and lead to intestinal bowel diseases. However, how does trafficking regulate intestinal tissue maintenance is poorly understood. Rab21 is a small GTPase involved in early endosomal trafficking. It was first identified in human intestinal cells and is expressed in ECs. Importantly, Rab21 levels are drastically decreased in ECs upon intestinal inflammation.
Using the Drosophila intestine as an in vivo model system, we investigated EC-specific functions for Rab21 in gut tissue maintenance. We monitored and assessed Rab21 loss of function in ECs of adult flies. Rab21 depleted guts showed severe intestinal morphology abnormalities. Furthermore, cellular equilibrium was affected, and a gain in mitotic cells, associated with increased cell death was observed. Interestingly, we found that an increased level of the IL-6 like cytokine, Upd3, in ECs was responsible for overproliferation, through activation of the JAK/STAT pathway mediated via yki activity and apoptosis.
To further characterize Rab21 functions, we performed a TMT-based quantitative proteomic analysis for Rab21 depleted guts and identified 101 proteins significantly modulated. We investigated the functional link between Rab21 and two overabundant proteins: ApoLpp, an apolipoprotein involved in diacylglycerol mobilization and Tret1-1, a trehalose transporter. We observed that Rab21 silencing in EC affects lipid content in the gut, as well as in the fat body, while circulating lipids did not appear affected, highlighting an unappreciated lipid-related function for Rab21. Furthermore, we demonstrated that loss of Rab21 in EC impaired trehalose levels in hemolymph, suggesting the involvement of Rab21 in the regulation of trehalose distribution, likely via specific trafficking of the trehalose transporter.
Altogether, our data indicate that EC Rab21 plays an important role in intestinal tissue maintenance and functions. Although Rab21 specific roles on lipids and trehalose regulation remain to be defined, we show that Rab21 contributes to ECs survival and their correct function.