Endoplasmic Reticulum-associated Degradation of Niemann-Pick C1

Abstract

Background: Mutant Niemann-Pick C1 (NPC1) is degraded by the proteasome. Results: NPC1 is one of the clients of HSP/CHIP (heat shock protein/carboxyl terminus of Hsp70-interacting protein) complexes and accepts ubiquitin at multiple lysine residues. Conclusion: HSPs are critical in the quality control of NPC1. Significance: Unraveling the mechanisms of NPC1 degradation is crucial for development of a chaperone therapy.

Lysosome and endoplasmic reticulum quality control pathways in Niemann-Pick type C disease

“…Possibly due to its large size and complicated structure, it is estimated that slightly more than half of the synthesized wild type (WT) NPC1 protein misfolds and is degraded (Gelsthorpe et al, 2008; Nakasone et al, 2014). This is consistent with the notion that 30–75% of all proteins entering the ER fail to achieve a native or near-native state (Ellgaard and Helenius, 2003).…”

“…One pool, with a shorter, 9 hour half-life, may represent misfolded forms which are rapidly degraded, while the long-lived fraction with a half-life of 42 hours may represent functional NPC1 in late endosomes/lysosomes. Other efforts to quantify NPC1 half-life in mouse or human cells have similarly detected the short-lived protein pool, but have had more difficultly detecting long-lived proteins (Nakasone et al, 2014) perhaps due to technical differences between approaches. Interestingly, cholesterol depletion causes rapid ubiquitination and degradation of NPC1 (Nakasone et al, 2014; Ohsaki et al, 2006).…”

“…Other efforts to quantify NPC1 half-life in mouse or human cells have similarly detected the short-lived protein pool, but have had more difficultly detecting long-lived proteins (Nakasone et al, 2014) perhaps due to technical differences between approaches. Interestingly, cholesterol depletion causes rapid ubiquitination and degradation of NPC1 (Nakasone et al, 2014; Ohsaki et al, 2006). This observation suggests that binding of ligand to the newly synthesized protein in the ER may favor its folding and trafficking.…”

“…Endogenous or over-expressed human or mouse NPC1-I1061T have half-lives measured between 4–6.5 hours (Gelsthorpe et al, 2008; Nakasone et al, 2014; Praggastis et al, 2015). These data are consistent with immunofluorescence analyses and endoglycosidase H sensitivity studies which revealed that NPC1-I1061T fails to traffic through the Golgi (Gelsthorpe et al, 2008; Praggastis et al, 2015), indicating that misfolded forms in the endoplasmic reticulum (ER) are targeted for degradation.…”

“…However, it is possible that a small fraction of the remaining I1061T protein traffics to the lysosome and is functional. While this notion is suggested by the observation that Npc1-I1061T knock-in mice have a milder disease phenotype than null mice (Praggastis et al, 2015), NPC1-I1061T protein levels do not increase when human fibroblasts are treated with the lysosomal inhibitors chloroquine, NH4Cl or leupeptin (Gelsthorpe et al, 2008; Nakasone et al, 2014). A possible explanation of this discrepancy is that these small molecules are only weakly effective at altering lysosomal pH to inhibit protein degradation (Yoshimori et al, 1991).…”