Retention of Mutant Low Density Lipoprotein Receptor in Endoplasmic Reticulum (ER) Leads to ER Stress

Stine Sørensen,Trine Ranheim,Kari Solberg Bakken,Trond P Leren,Mari Ann Kulseth

doi:10.1074/jbc.m507071200

Stine Sørensen, Trine Ranheim + Show 3 more

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https://doi.org/10.1074/jbc.m507071200

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Abstract

Familial hypercholesterolemia is an autosomal dominant disease caused by mutations in the gene encoding the low density lipoprotein receptor (LDLR). More than 50% of these mutations lead to receptor proteins that are completely or partly retained in the endoplasmic reticulum (ER). The mechanisms involved in the intracellular processing and retention of mutant LDLR are poorly understood. In the present study we show that the G544V mutant LDLR associates with the chaperones Grp78, Grp94, ERp72, and calnexin in the ER of transfected Chinese hamster ovary cells. Retention of the mutant LDLR was shown to cause ER stress and activation of the unfolded protein response. We observed a marked increase in the activity of two ER stress sensors, IRE1 and PERK. These results show that retention of mutant LDLR in ER induces cellular responses, which might be important for the clinical outcome of familial hypercholesterolemia.

Highlights

The low density lipoprotein receptor (LDLR) is a cell surface glycoprotein composed of several structural domains that mediates the specific binding and uptake of apoBand apoE-containing lipoproteins by receptor-mediated endocytosis [1]
We have shown that a fusion protein of LDLR and enhanced yellow-green fluorescence protein (EYFP) can be used in functional studies of a disease-causing mutation found in the LDLR gene
The G544V mutation has previously been described as a class 2a mutation [20, 21]. In accordance with these previous observations, we showed that the G544V mutant LDLR is expressed in the precursor form co-localized with an endoplasmic reticulum (ER) marker

Summary

Introduction

The LDLR is a cell surface glycoprotein composed of several structural domains that mediates the specific binding and uptake of apoBand apoE-containing lipoproteins by receptor-mediated endocytosis [1]. Different mutations in the LDLR gene have different effects on the receptor protein function. Properly folded proteins must be guided from the ER to their final destination within the cell, and on the other hand, misfolded proteins that may be toxic to the cell must be disposed of without compromising normal cell function This quality control system is based on common structural and biophysical features that distinguish native from nonnative protein conformations. It is possible that differences in the quality control systems within the ER, as well as differences in the ERAD [19], could explain some of the phenotypic variation To test this hypothesis there is a need to understand how class 2 mutants interact with the ER quality control components

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