Global Proteotoxicity Caused by Human β2 Microglobulin Variants Impairs the Unfolded Protein Response in C. elegans.

Sarah C Good,Sheena E Radford,Katherine M Dewison,Patricija Van Oosten-Hawle

doi:10.3390/ijms221910752

Abstract

Aggregation of β2 microglobulin (β2m) into amyloid fibrils is associated with systemic amyloidosis, caused by the deposition of amyloid fibrils containing the wild-type protein and its truncated variant, ΔN6 β2m, in haemo-dialysed patients. A second form of familial systemic amyloidosis caused by the β2m variant, D76N, results in amyloid deposits in the viscera, without renal dysfunction. Although the folding and misfolding mechanisms of β2 microglobulin have been widely studied in vitro and in vivo, we lack a comparable understanding of the molecular mechanisms underlying toxicity in a cellular and organismal environment. Here, we established transgenic C. elegans lines expressing wild-type (WT) human β2m, or the two highly amyloidogenic naturally occurring variants, D76N β2m and ΔN6 β2m, in the C. elegans bodywall muscle. Nematodes expressing the D76N β2m and ΔN6 β2m variants exhibit increased age-dependent and cell nonautonomous proteotoxicity associated with reduced motility, delayed development and shortened lifespan. Both β2m variants cause widespread endogenous protein aggregation contributing to the increased toxicity in aged animals. We show that expression of β2m reduces the capacity of C. elegans to cope with heat and endoplasmic reticulum (ER) stress, correlating with a deficiency to upregulate BiP/hsp-4 transcripts in response to ER stress in young adult animals. Interestingly, protein secretion in all β2m variants is reduced, despite the presence of the natural signal sequence, suggesting a possible link between organismal β2m toxicity and a disrupted ER secretory metabolism.

Highlights

Amyloid diseases such as Alzheimer’s Disease, Parkinson’s Disease and DialysisRelated Amyloidosis are characterised by the self-assembly of proteins into insoluble amyloid fibrils containing a cross-β fold that leads to age-dependent cellular toxicity [1,2,3,4].β2 Microglobulin (β2 m), is a 99 amino acid protein with a β-sandwich immunoglobulin fold in its native state [5,6], that comprises the non-covalently bound light chain of the major histocompatibility complex class I (MHC I) [7]
∆N6 β2 m and D76N β2 m in the C. elegans bodywall muscle under control of the myo-3p
The results demonstrate that the expression of the amyloidogenic β2 m variants D76N β2 m and ∆N6 β2 m cause widespread aggregation of endogenous C. elegans proteins in an age-dependent manner

Summary

Introduction

Amyloid diseases such as Alzheimer’s Disease, Parkinson’s Disease and DialysisRelated Amyloidosis are characterised by the self-assembly of proteins into insoluble amyloid fibrils containing a cross-β fold that leads to age-dependent cellular toxicity [1,2,3,4].β2 Microglobulin (β2 m), is a 99 amino acid protein with a β-sandwich immunoglobulin fold in its native state [5,6], that comprises the non-covalently bound light chain of the major histocompatibility complex class I (MHC I) [7]. Amyloid diseases such as Alzheimer’s Disease, Parkinson’s Disease and DialysisRelated Amyloidosis are characterised by the self-assembly of proteins into insoluble amyloid fibrils containing a cross-β fold that leads to age-dependent cellular toxicity [1,2,3,4]. WT β2 m self-associates, forming amyloid fibrils that deposit in osteoarticular tissues, causing a pathology known as dialysis-related amyloidosis (DRA) [8]. Amyloid deposits of β2 m are mainly composed of WT β2 m (~70%), and contain truncation products formed by proteolysis, of which 30% represent a six amino acid. In contrast with WT β2 m, the ∆N6 β2 m variant is highly amyloidogenic, and can readily aggregate into amyloid fibrils in vitro at the physiological pH of 6–7, in the absence of additives or fibril seeds [10,11,12]. Point mutations in β2 m can increase its aggregation propensity and cause disease, such as the Asp76Asn (D76N)

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Results

Conclusion