The endo-lysosomal system is essential for maintaining cellular homeostasis by recycling organelles, proteins, and other intracellular or extracellular cargo. Moreover, lysosomes function as nutrient sensors, modulating cellular signaling pathways in response to metabolic status. To fulfill these roles, lysosomes rely on input from multiple trafficking routes: the biosynthetic pathway supplies them with membrane components and hydrolytic enzymes, while endocytosis and autophagy deliver cargo for degradation. The coordination and regulation of these pathways are vital for lysosomal integrity and cellular health.
In our recent work, we identified the HOPS tethering complex as a key regulator, or gatekeeper, of lysosomal trafficking. Loss of HOPS function disrupts fusion of endosomes and autophagosomes with lysosomes, depleting lysosomes from endocytic and autophagic substrates. Remarkably, HOPS also mediates biosynthetic trafficking of membrane components: the HOPS VPS41 subunit recruits vesicles carrying lysosomal membrane proteins, such as LAMPs, directly from the Trans-Golgi Network (TGN) to endo-lysosomes. A pathway we refer to as the "LAMP carrier pathway". In addition to fusion, HOPS depletion also impacts endosomal acidification and maturation, suggesting a broader role in endosomal biogenesis. We are currently exploring whether the LAMP carrier pathway is responsible for the delivery of newly synthesized V0-ATPase subunits to endosomes, which could explain the acidification and maturation defects observed in HOPS-depleted cells.
To study these processes, we employ correlative light and electron microscopy (CLEM). While fluorescence microscopy allows imaging of live-cells and a range of functional probes, electron microscopy (EM) reveals essential high-resolution information. CLEM merges these approaches, enabling us to visualize both the function and ultrastructure of lysosome biogenesis at the single-organelle level.