Deciphering endosomal sorting mechanisms in epithelial cells
Cell Biology
Final Report Abstract
Fas2 and E-cadherin are two transmembrane proteins that connect epithelial cells. These cellcell contacts must allow a high degree of plasticity because new cells have to be be integrated into the epithelium and the epithelium has to change its shape during development. Plasticity is achieved by constantly endocytosing and recycling the adhesion proteins. The regulation of this process is complex, as adhesion proteins are also constantly degraded and new proteins are incorporated into the cycle. Endosomes are membrane systems within cells, in which endocytosed transmembrane proteins are sorted either for recycling or for degradation. Proper sorting into endosomes ensures homeostasis of epithelial tissues and disruptions are the basis of many diseases, such as cancer and fibrosis. We use the follicular epithelium of the Drosophila ovary to better understand endosomal sorting. In this project, we describe three new aspects of endosomal sorting. First, we identify a new mechanism in which the small GTPase RabX1 supplies the part of endosomes, where transmembrane proteins are degraded with enzymes that accomplish degradation. We show that RabX1 induces the formation of membrane tubules that briefly connect endosomes to lysosomes, the storage sites of hydrolytic enzymes. We also show how this mechanism is upregulated during epithelial morphogenesis. A second aspect of the project contributes to a better understanding of the role of Rab7 in recycling. We show that Rab7 has different functions in the recycling of Fas2 and E-cadherin. On the one hand, Rab7 is required to recruit the retromer complex to Fas2, which facilitates Fas2 recycling to the lateral membrane. On the other hand, in the case of E-cadherin, Rab7 recruits SNX16, which then transports E- cadherin to a different area of the endosome, which facilitates secretion to the zonula adherens. In the third aspect, we shed light on the question to what extent endosomes are also involved in the sorting of newly synthesised proteins. For E-cadherin, we had already shown that both newly formed and endocytosed protein is sorted in endosomes for transport to the zonula adherens. For Fas2, we have now applied a new method, RUSH, to follow the path of Fas2 after translation. This analysis suggests that, unlike E-cadherin, newly synthesised Fas2 is not sorted in endosomes for its secretion to the lateral membrane. Thus, endosomes do not appear to be a general sorting station for secreted proteins.
Publications
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RabX1 Organizes a Late Endosomal Compartment that Forms Tubular Connections to Lysosomes Consistent with a “Kiss and Run” Mechanism. Current Biology, 30(7), 1177-1188.e5.
Laiouar, Sara; Berns, Nicola; Brech, Andreas & Riechmann, Veit
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Myosin V facilitates polarised E‐cadherin secretion. Traffic, 23(7), 374-390.
Tanasic, Dajana; Berns, Nicola & Riechmann, Veit
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Acute manipulation and real-time visualization of membrane trafficking and exocytosis in Drosophila. Developmental Cell, 58(8), 709-723.e7.
Glashauser, Jade; Camelo, Carolina; Hollmann, Manuel; Backer, Wilko; Jacobs, Thea; Sanchez, Jone Isasti; Schleutker, Raphael; Förster, Dominique; Berns, Nicola; Riechmann, Veit & Luschnig, Stefan
