Access Type

Open Access Embargo

Date of Award

January 2021

Degree Type

Thesis

Degree Name

M.S.

Department

Molecular Biology and Genetics

First Advisor

Shengyi Sun

Abstract

Ceruloplasmin (Cp) is a secreted ferroxidase produced by the hepatocytes that assists in the transport of iron throughout the human body. In human aceruloplasminemia patients, due to Cp deficiency excess intracellular iron buildup leads to ailments like liver cirrhosis, neurodegeneration, and blindness. We recently found that the biogenesis of Cp in the endoplasmic reticulum (ER) is regulated by a principal ER quality-control process, ER-associated degradation (ERAD). ERAD clears misfolded ER proteins for cytosolic proteasomal degradation, with the Sel-1 suppressor of lin-12-like (Sel1L)-HMG-CoA reductase degradation 1 (Hrd1) protein complex representing the most conserved branch in mammals. Interestingly, we found that Sel1L-Hrd1 ERAD in hepatocytes mediates the degradation of Cp, thereby regulating systemic iron homeostasis. However, the molecular mechanism underlying ERAD-mediated Cp degradation remains unknown. Osteosarcoma amplified 9 (OS9) and XTP3-transactivated gene B (XTP3B) are lectins in the ER that can recognize misfolded glycoproteins and transport them to Sel1L-Hrd1 ERAD for degradation. Here, we hypothesized that those two ER lectins, OS9 and XTP3B, mediate the ERAD of Cp. Using molecular and genetic approaches, we investigated whether and how OS9 and XTP3B mediate the degradation of a Cp mutant G176R found in human aceruloplasminemia patients. Specifically, we generated OS9- and/or XTP3B-deficient HEK293T cells using CRISPR/Cas9 technology. We found that the loss of both OS9 and XTP3B, but not either individually, leads to reduced ubiquitination and a stabilization of Cp, suggesting that the two lectins play a redundant and essential role in the efficient ERAD of Cp. Together, by investigating the molecular mechanism underlying the biogenesis of Cp, we provided novel insights into the regulation of iron metabolism under pathological conditions.

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