Bubble guts–Controlling apical membrane morphology in the C. elegans intestine

Abstract

Establishment and maintenance of tissue morphogenesis are key processes in building a functional organism’s body. In this thesis, I have used the small roundworm Caenorhabditis elegans as a model to broaden our understanding of the maintenance of tube morphogenesis in vivo—with a special role for protein-protein interactions. Chapter 2 describes a timeline of older and recent advances in the identification of the C. elegans protein interactome. Chapter 3 introduces a novel protein-protein interaction assay to investigate whether pairs of proteins interact in vivo. C. elegans light-induced co-clustering (CeLINC) traps fluorescently-tagged bait proteins into artificial clusters to observe whether candidate interacting prey proteins co-cluster with the bait protein. Chapter 4 identifies the small coiled-coil protein BBLN-1 as a regulator of lumen morphology in the C. elegans intestine. Loss of BBLN-1 leads to the aggregation of the intermediate filament (IF) network into bundles, that subsequently causes cytoplasmic membrane invaginations. We additionally identify bublin (BBLN) as the mammalian ortholog of BBLN-1. Chapter 5 exploits CeLINC to confirm the protein-protein interaction between BBLN-1 and subunits of the Vacuolar-type H+ ATPase (V-ATPase) identified in chapter 4. We address their molecular link and show that loss of V-ATPase function can result in bubble-shaped membrane protrusions. Chapter 6 investigates how IFs and MAPK signaling shape the C. elegans intestinal lumen. The IF polypeptide IFB-2 proved to be the main effector of the MAPK kinase sma-5 phenotype, highlighting the tight linkage between the IF cytoskeleton and signaling. In chapter 7 we manipulate intestinal polarity protein localization using optogenetic techniques to establish ectopic lumens laterally, but we conclude that lateral localization of PAR-6 alone is insufficient to manipulate apical-basal polarity in the established C. elegans intestine. To conclude, the work described in this thesis has focused on the role of IFs in tube formation and maintenance, guided by the identification of the IF regulator BBLN-1. All in all, these findings reveal novel insights into the process of C. elegans intestinal tube morphogenesis and the maintenance of apical membrane morphology.