Restraining Wnt activation and intestinal tumorigenesis by a Rab35 dependent GTPase relay

Abstract

ABSTRACTMaintenance of homeostatic processes ensure curtailment of intestinal tumorigenesis. Inactivating mutations to Adenomatous Polyposis Coli (Apc) result in aberrantly activated Wnt signalling and initiates colorectal cancer (CRC) in approx. 80% of cases, yet our understanding of the subcellular mechanisms that modulate dysregulated pathway activity is limited. Here, using a conditionalin vivogenetic screen, we identify Rab35 GTPase as a novel tumour suppressor that modulates regional Wnt activity after loss ofApcin progenitor cells. Single cell analysis revealed that progenitor cells respond toApcdepletion by increasing the expression of a GTPase activating protein, which we namedblackbelt, and triggering Rab35 disassociation from the plasma membrane. Mechanistically, we demonstrate that Rab35 controls the localisation and activation of the Rho GTPase, Cdc42, which functions as a relay to regulate JNK signalling. This in turn tunes the Wnt pathway upstream of β-catenin to direct proliferation and differentiation of progenitor cells. Importantly, we show that maintaining active JNK signalling is important for the propagation ofApcmutant mouse colon organoids. Our findings highlight a novel GTPase cascade that sustains aberrant Wnt activity in specific segments of the intestine and provides impetus to therapeutically exploit this pathway to target CRC.