Copper-independent lysosomal localisation of the Wilson disease protein ATP7B

In hepatocytes, the Wilson disease protein ATP7B resides on the trans-Golgi network (TGN) and traffics to peripheral lysosomes to export excess intracellular copper through lysosomal exocytosis. We found that in basal copper or even upon copper chelation, a significant amount of ATP7B persists in the endolysosomal compartment of hepatocytes but not in non-hepatic cells. These ATP7B-harbouring lysosomes lie in close proximity of ~10 nm to the TGN. ATP7B constitutively distributes itself between the sub-domain of the TGN with a lower pH and the TGN-proximal lysosomal compartments. The presence of ATP7B on TGN-lysosome colocalising sites upon Golgi disruption suggested a possible exchange of ATP7B directly between the TGN and its proximal lysosomes. Manipulating lysosomal positioning significantly alters the localisation of ATP7B in the cell. Contrary to previous understanding, we found that upon copper chelation in a copper-replete hepatocyte, ATP7B is not retrieved back to TGN from peripheral lysosomes; rather, ATP7B recycles to these TGN-proximal lysosomes to initiate the next cycle of copper transport. We report a hitherto unknown copper-independent lysosomal localisation of ATP7B and the importance of TGN-proximal lysosomes but not TGN as the terminal acceptor organelle of ATP7B in its retrograde pathway.     

Mechanistic insights in recov-ER-phagy: micro-ER-phagy to recover from stress

ABSTRACT

Physiological and pathological stresses may cause swelling of the endoplasmic reticulum (ER), a biosynthetic organelle in eukaryotic cells. Upon conclusion of the stress, ER size and content return to physiological levels. The translocon component SEC62 decorates the portions of excess ER that must be cleared from cells. Our recent paper highlights the role of endosomal sorting complex required for transport (ESCRT)-III-driven micro-ER-phagy in ER remodeling during cell recovery from ER stress.     

ER‐to‐lysosome‐associated degradation of proteasome‐resistant ATZ polymers occurs via receptor‐mediated vesicular transport

Maintenance of cellular proteostasis relies on efficient clearance of defective gene products. For misfolded secretory proteins, this involves dislocation from the endoplasmic reticulum (ER) into the cytosol followed by proteasomal degradation. However, polypeptide aggregation prevents cytosolic dislocation and instead activates ill‐defined lysosomal catabolic pathways. Here, we describe an ER‐to‐lysosome‐associated degradation pathway (ERLAD) for proteasome‐resistant polymers of alpha1‐antitrypsin Z (ATZ). ERLAD involves the ER‐chaperone calnexin (CNX) and the engagement of the LC3 lipidation machinery by the ER‐resident ER‐phagy receptor FAM134B, echoing the initiation of starvation‐induced, receptor‐mediated ER‐phagy. However, in striking contrast to ER‐phagy, ATZ polymer delivery from the ER lumen to LAMP1/RAB7‐positive endolysosomes for clearance does not require ER capture within autophagosomes. Rather, it relies on vesicular transport where single‐membrane, ER‐derived, ATZ‐containing vesicles release their luminal content within endolysosomes upon membrane:membrane fusion events mediated by the ER‐resident SNARE STX17 and the endolysosomal SNARE VAMP8. These results may help explain the lack of benefits of pharmacologic macroautophagy enhancement that has been reported for some luminal aggregopathies.