UBC Theses and Dissertations
Regulation of receptor signaling and membrane trafficking by beta1,6-branched n-glycans and caveolin-1/cholesterol membrane domain organization Lajoie, Patrick
Modification by glycosylation gives proteins a range of diverse functions reflecting their structural variability. N-glycans regulate many biological outcomes in mammalian cells under both normal and pathological conditions. They play a major role in various pathologies such as cancer and lysosomal storage diseases. Interplay between N-glycans and other regulators, such as membrane lipid domains, in the control of signaling pathways remains poorly understood. My thesis therefore focuses on how N-glycans and membrane lipid domains oppose and/or work together at different cellular levels to regulate various processes such as receptor signaling and diffusion, endocytosis and lysosomal organelle biogenesis. Mgat5 encodes for ß1,6-N-acetylglucosaminyltransferase V that produces N-glycans, the preferred ligand for galectins. In tumor cells, galectins bind glycosylated receptors at the cell surface forming a lattice, that restricts receptor endocytosis and enhances its residency at the plasma membrane. In the first part of my thesis, I report that Galectin/receptor crosslinking opposes receptor sequestration by oligomerized caveolin-1 (Cav1) domains overriding its negative regulation of epidermal growth factor receptor (EGFR) signaling, cell surface diffusion and tumor growth. These results identify Cav1 as a conditional tumor suppressor. I also demonstrate that Cav1 is a negative regulator of lipid raft-mediated endocytosis. Cav1 indirectly regulates the internalization of cholera toxin b subunit to the Golgi apparatus independently of caveolae formation. That identifies a new role for caveolin-1 outside caveolae in the regulation of raft-dependent endocytosis Finally, Mgat5 overexpression in pneumocytes is associated with the expression of a lysosomal organelle, the multilamellar body (MLB), via autophagy. MLB expression is also a characteristic of various lysosomal storage diseases. I demonstrate that cholesterol accumulation can override the need for Mgat5 overexpression in MLB formation indicating that they may form via multiple mechanisms. However, I also demonstrate that a contribution of the autophagic pathway is a common determinant of biogenesis of MLB of various lipid compositions. In conclusion, Mgat5-dependent protein glycosylation and Cav1/raft domains therefore both function as regulators of plasma membrane interactions, endocytosis and lysosomal organelle biogenesis. Understanding of this interplay is crucial for the understanding of the mechanisms involve in various pathologies such as cancer and lysosomal storage diseases.
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