A novel membrane remodeling pathway maintains membrane homeostasis during giant vesicle secretion

Dynamic membrane trafficking comprises the essence of cellular homeostasis in every living eukaryotic cell. However, membrane dynamics become especially challenging in secretory tissues, where the continuous fusion of vesicles with the cell surface takes place. This is most dramatically demonstrated in exocrine tissues that secrete viscous cargoes from giant vesicles, ranging up to 8 microns in diameter. Secretion of such giant vesicles adds large amounts of membrane to the apical surface of cells. Yet, it remains unclear how homeostasis of the cell surface in terms of size, shape, and composition is maintained under these extreme circumstances. To address this question, we used the Drosophila larval salivary gland, which secrets a viscous mucin-like protein called ‘glue’ via giant vesicles. We observed that after fusion the vesicular membrane does not appear to simply collapse into the apical surface as previously thought. Using a combination of correlative fluorescence and 3D-electron microscopy (CLEM) approaches, we determined the precise ultrastructure of the vesicular membrane at different phases of secretion. We observed that the vesicle membrane becomes increasingly crumpled as secretion progresses. We quantified the surface area of the crumpled vesicles and found that the compacted membrane accounts for most of the original vesicular membrane before secretion. Furthermore, we found that the sequestered membrane recruits the clathrin-mediated endocytosis machinery, which recovers the membrane over a prolonged period of time post secretion. Our results indicate that membrane homeostasis is maintained by a novel mechanism mediating crumpling and sequestration of the vesicular membrane, essential for exocrine tissue physiology.