The endo-lysosomal system and its organization by tethers: Advances in microscopy connect structure and function

This dissertation investigates the organization of the endo-lysosomal system, with a particular focus on organelle fusion and the role of tethers in this process. Endosomes and lysosomes are essential cellular organelles involved in sorting and degrading molecules. Central to this research is the fusion of endosomes with lysosomes, a process in which the HOPS tethering complex plays a crucial role.
The first half of the dissertation focuses on the development of new microscopy techniques, including improvements in immunogold labeling and an optimized protocol for correlative light and electron microscopy (CLEM). These methods make it possible to visualize important proteins such as LC3, Rab5, Rab7, EEA1, and APPL1 in their cellular context. Among other findings, these techniques reveal that the tether EEA1 is also present on late endosomes — an unexpected discovery that challenges conventional definitions of endosomal stages.
The second half focuses on the function of the HOPS complex. Loss of HOPS subunits disrupts the fusion between endosomes and lysosomes and leads to poorly organized endosomes with mixed protein compositions. HOPS is also shown to be important for the transition from early to late endosomes. In cancer cells, loss of the HOPS subunit Vps39 reduces their invasive capacity, while loss of the subunit Vps18 is compensated through alternative pathways — a striking and unexpected difference between the two HOPS subunits.
Finally, the dissertation demonstrates that autophagosomes likely always fuse first with endosomes (forming amphisomes) before merging with lysosomes — a step dependent on HOPS. These insights contribute to a revision of the classical model of organelle fusion and open new directions for cell biological research.