Impact of ER stress and the unfolded protein response on Fabry disease

Abstract

Fabry disease (FD) is a lysosomal storage disorder caused by pathogenic missense and nonsense variants in the α-galactosidase A (GLA) gene, leading to absent or reduced enzyme activity. The resulting lysosomal accumulation of the substrate globotriaosylceramide leads to progressive renal failure, cardiomyopathy with (malignant) cardiac arrhythmias and progressive heart failure as well as recurrent strokes, which significantly limits the life expectancy of patients affected with FD. There is increasing evidence that pathogenic GLA missense variants as well as formally benign GLA variants can cause retention in the endoplasmic reticulum (ER), resulting in ER stress, which in turn triggers an unfolded protein response (UPR) leading to cellular dysregulation including inflammation, irreversible cell damage, and apoptosis. This review aims to provide an update on the pathogenetic significance of ER stress and UPR in FD, current treatment options, including pharmaceutical and chemical chaperones, and an outlook on current research and future treatment options in FD.

Keywords: Chaperones; Lysosomal storage disorder; Missense variants.

Fig. 1

The adaptive unfolded protein response (UPR) pathway. Following endoplasmic reticulum (ER) stress, three major UPR pathways can be activated. a) After activation, ATF6 passes from the ER into the Golgi apparatus, where it is cleaved by proteases. This produces an active cytosolic ATF6 fragment (ATF6α). This fragment migrates into the nucleus, binds to ATF6-responsive elements and activates the transcription of UPR target genes that are involved in the homoeostasis of ER protein folding, cell physiology, and protein secretion. In addition, unfolded or misfolded proteins that accumulate in the ER lumen can be degraded by the proteasome via proteasome-based ER-associated protein degradation (ERAD), which is regulated by the ATF6-mediated and/or IRE1α-X-box-binding protein 1 (XBP1)-mediated UPR branches. b) After dimerisation and phosphorylation, phosphorylated IRE1α splices cytosolic XBP1 mRNA, which encodes a potent transcription factor that activates the expression of UPR target genes involved in ER proteostasis and cell pathophysiology. IRE1α-RNase can also cleave ER-associated mRNAs or non-coding functional RNAs, leading to their degradation by regulated IRE1-dependent decay (RIDD), which modulates protein folding, cell metabolism, inflammation, and inflammasome signalling pathways. The cytosolic domain of IRE1α may also serve as a scaffold for the recruitment of adaptor proteins such as members of the tumour necrosis factor receptor-associated factor (TRAF) family, thus activating inflammatory responses under non-canonical ER stress conditions. c) After dimerisation and phosphorylation, PERK phosphorylates eukaryotic translation initiation factor 2 subunit-α (eIF2α), thereby reducing the overall frequency of mRNA translation initiation. However, selective mRNAs, such as ATF4 mRNA, are preferentially translated in the presence of phosphorylated eIF2α. Upon translocation to the nucleus, ATF4 also activates transcription of UPR target genes encoding factors involved in amino acid biosynthesis, antioxidant response, autophagy, and apoptosis.

Fig. 2

Schematic organisation and overview of endoplasmic reticulum-associated protein degradation (ERAD). An E3 ubiquitin-ligase complex in the ER membrane coordinates the different ERAD steps beginning with the recognition of a misfolded protein (red). Proteins will be recognised on either side (ER lumen or cytosolic) or within the ER membrane. After recognition, misfolded proteins in the lumen and membrane are retrotranslocated to the cytosolic side of the ER membrane, facilitated by the membrane domains of ubiquitin-ligase complex components. After reaching the cytosolic side of the ER membrane, substrates are ubiquitinated by ubiquitin ligase E2. The cytosolic Cdc48/p97 complex binds to ubiquitinated proteins, facilitating the last steps of retrotranslocation by extracting the protein out of the ER membrane. Finally, cytoplasmic proteins direct the newly polyubiquitinated protein to proteasomes for degradation.

Fig. 3

Overview of the intracellular α-galactosidase A trafficking and the potential impact of missense variants leading to reduced lysosomal enzymatic activities, enzyme retention within the ER or Golgi apparatus. AGAL: α-galactosidase A, CGN: cis-Golgi network, ER: endoplasmic reticulum, GA: Golgi apparatus, Gb₃: globotriaosylceramide, TGN: trans-Golgi network.

Fig. 4

Overview of the different chaperone classes. a) Molecular chaperones facilitate the correct folding of unfolded proteins so that they can leave the endoplasmic reticulum (ER) by vesicular transport via COPII vesicles. Pharmacological chaperones bind specific proteins and facilitate their correct folding, e.g., by binding to the active centre of an enzyme like migalastat. b) Chemical chaperones can be divided into osmolytic and hydrophobic components. Osmolytes act directly on unfolded proteins to stabilise their conformation, preventing protein-protein interactions and leading to a more stable state due to their hydration effect. Hydrophobic components can reduce the aggregation of unfolded proteins, increase the expression of molecular chaperones and promote the secretion of unfolded proteins via COPII vesicles. c) 4-Phenylbutyric acid (4-PBA) can act as a histone deacetylase (HDAC) inhibitor, modulating gene expression to promote molecular chaperone synthesis. Additionally, 4-PBA can reduce the accuracy of quality control mechanisms regulating the translocation of proteins from the ER by interacting with the COPII coat components. By inhibiting an element of the ER quality control machinery, 4-PBA enables the retained misfolded proteins to escape the ER, lowering ER stress by relieving the burden of accumulated proteins.