Elucidating the ER-to-Lysosome-Associated Degradation Pathway of a Toxic Renal Proteinopathy

The majority of misfolded proteins synthesized in the ER are cleared through an ER-associated degradation, or ERAD, pathway. However, certain proteins are “ERAD-resistant” and must be degraded by an alternative mechanism. ER-to-lysosome-associated degradation, or ERLAD, is a newly characterized pathway employed by some ERAD-resistant proteins. Frameshift Mucin-1 (MUC1-fs) is an ERAD-resistant, misfolded protein which causes Mucin-1 Kidney Disease (MKD), a genetic disease that has no cure and ultimately results in renal failure. Using patient-derived renal cell lines, we have discovered that MUC1-fs employs an ERLAD pathway, and that MUC1-fs is shuttled to the lysosome independently of autophagosome machinery. Interestingly, MUC1-fs undergoes a strong and specific interaction with calnexin en route to the lysosome, indicating that a previously reported, calnexin-dependent ERLAD pathway may be involved. While studying which factors enhance or prevent degradation of MUC1-fs, we determined that the cargo receptor TMED9 impedes degradation of MUC1-fs, entrapping the misfolded protein and promoting its retention in TMED9-enriched vesicles between the cis-Golgi and the ER. We also identified two pharmacological agents which relieve TMED9 entrapment and enhance MUC1-fs degradation through ERLAD. This study sheds light on the molecular mechanism associated with ERLAD, provides novel tools for the investigation of this pathway, and furthers our understanding of the biology behind an understudied toxic proteinopathy. Importantly, our work offers new potential therapeutic avenues that may have beneficial effects in other toxic proteinopathies with similar etiologies.