Reduced secreted clusterin as a mechanism for Alzheimer-associated CLU mutations

TL;DR

This study shows that certain CLU gene mutations linked to Alzheimer's disease reduce the secretion of the CLU protein, potentially contributing to disease progression.

Contribution

The paper identifies specific CLU mutations that impair protein secretion and provides functional evidence for their role in Alzheimer's disease.

Findings

Three CLU mutations (p.I303NfsX13, p.R338W, p.I360N) disrupt protein localization and reduce secretion.

Mutated CLU accumulates in the endoplasmic reticulum and is poorly transported to the Golgi.

Reduced CLU secretion is proposed as a mechanism for these mutations in Alzheimer's disease.

Abstract

The clusterin (CLU) gene has been identified as an important risk locus for Alzheimer’s disease (AD). Although the actual risk–increasing polymorphisms at this locus remain to be identified, we previously observed an increased frequency of rare non-synonymous mutations and small insertion-deletions of CLU in AD patients, which specifically clustered in the β-chain domain of CLU. Nonetheless the pathogenic nature of these variants remained unclear.

Here we report a novel non-synonymous CLU mutation (p.I360N) in a Belgian Alzheimer patient and have explored the pathogenic nature of this and 10 additional CLU mutations on protein localization and secretion in vitro using immunocytochemistry, immunodetection and ELISAs.

Three patient-specific CLU mutations in the β-chain (p.I303NfsX13, p.R338W and p.I360N) caused an alteration of the subcellular CLU localization and diminished CLU transport through the secretory pathway, indicative of possible degradation mechanisms. For these mutations, significantly reduced CLU intensity was observed in the Golgi while almost all CLU protein was exclusively present in the endoplasmic reticulum. This was further confirmed by diminished CLU secretion in HEK293T and HEK293 FLp-In cell lines.

Our data lend further support to the contribution of rare coding CLU mutations in the pathogenesis of neurodegenerative diseases. Functional analyses suggest reduced secretion of the CLU protein as the mode of action for three of the examined CLU mutations. One of those is a frameshift mutation leading to a loss of secreted protein, and the other two mutations are amino acid substitutions in the disulfide bridge region, possibly interfering with heterodimerization of the α- and β-chain of CLU.

The online version of this article (doi:10.1186/s13024-015-0024-9) contains supplementary material, which is available to authorized users.

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