Protein Homeostasis in Amyotrophic Lateral Sclerosis: Therapeutic Opportunities?

Abstract

Protein homeostasis (proteostasis), the correct balance between production and degradation of proteins, is essential for the health and survival of cells. Proteostasis requires an intricate network of protein quality control pathways (the proteostasis network) that work to prevent protein aggregation and maintain proteome health throughout the lifespan of the cell. Collapse of proteostasis has been implicated in the etiology of a number of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), the most common adult onset motor neuron disorder. Here, we review the evidence linking dysfunctional proteostasis to the etiology of ALS and discuss how ALS-associated insults affect the proteostasis network. Finally, we discuss the potential therapeutic benefit of proteostasis network modulation in ALS.

Keywords: amyotrophic lateral sclerosis (ALS); autophagy; chaperonins; motor neuron disease; protein aggregation; protein homeostasis; proteostasis; unfolded protein response (UPR).

FIGURE 1

The proteostasis network and ALS. Protein folding occurs co-translationally at the ribosome with the aid of molecular chaperones, including Hsp70. Correct folding is essential for protein function. Protein folding and refolding continues in the cytosol and the endoplasmic reticulum (ER) lumen. Chronic misfolding in the cytosol leads to targeting of misfolded substrates to the ubiquitin proteasome system (UPS). Poly-ubiquitin chains target substrates for degradation by the proteasome. Overwhelming of the UPS can lead to poly-ubiquitinated aggregate formation, which are cleared by the autophagosome–lysosome pathway. Chronic misfolding in the ER leads to the induction of ER stress and activation of the unfolded protein response (UPR). The UPR leads to altered gene transcription, upregulating ER associated degradation (ERAD) and autophagy. The proteostasis network seeks to restore protein homeostasis, but failure of the pathway leads to the aggregation of potentially toxic species. Disruption of the proteostasis network is prevalent in the pathogenesis of ALS. A large number of ALS-associated genes (indicated in red) directly or indirectly regulate the proteostasis network. In addition, some ALS-associated proteins such as FUS, TDP-43, and SOD1 are also substrates of these pathways. For further details please refer to the main text.

FIGURE 2

Chaperone dysfunction in ALS. Protein folding occurs co-translationally at the ribosome. Correct folding and re-folding continues in the cytoplasm or ER with the help of molecular chaperones and leads to correctly folded, fully functional proteins. Chaperone dysfunction has been implicated in ALS. Aggregating ALS mutant SOD1 and mislocalised TDP-43 interact with chaperones of the heat shock protein family, namely Hsp70 and Hsp90. While the recruitment of chaperones to the aggregates is likely a protective mechanism, their sequestration potentially depletes the levels of available chaperones, decreasing chaperone folding activity, therefore leading to toxicity. The ALS-associated protein aggregates and other ALS-associated defects to the chaperone system are indicated in red. For further details please refer to the main text.

FIGURE 3

Unfolded protein response in the ER leads to ER stress. The accumulation of protein aggregates is sensed by three ER-stress transducers: IRE1α, PERK, and ATF6. ER stress causes IRE1α dimerisation, which activates its intrinsic RNAse activity and leads to alternative splicing of XBP1 mRNA. Spliced XBP1 forms a functional transcription factor. XBP1 increases expression of chaperone related genes and those involved in ERAD. PERK also dimerises due to ER stress. PERK dimerization leads to phosphorylation of the eukaryotic initiation factor eIF2α, thus inhibiting general protein synthesis. Inhibition of protein synthesis allows the translation of stress response transcription factor, ATF4. ATF4 increases expression of genes related to autophagy and apoptosis. Via the action of coat protein complex II (COPII), ATF6 translocates from the ER membrane to the Golgi during ER stress where it is processed by the Site 1 (Sp1) and Site 2 (Sp2) proteases. Cleavage produces a functional cytosolic fragment of ATF6. The ATF6 transcription factor induces expression of genes related to ERAD, but also XBP1, thereby promoting UPR. Chronic ER stress and UPR activation indicates the cell has failed to respond to ER stress. Under such conditions all three ER stress transducers lead to the increased expression of CHOP, which promotes apoptosis. For further details please refer to the main text.

FIGURE 4

Endoplasmic reticulum stress the UPR and ALS. ALS-associated genes have been implicated in ER stress and the UPR. The ALS-associated genes and their positions within the UPR pathway are indicated in red, as are other ALS-associated defects to the UPR. Briefly, ALS-associated protein aggregates, including C9orf72-related DPR proteins, TDP-43 and SOD1, are sensed by the ER-stress transducers leading to chronic activation of the UPR, caspase-12 cleavage and apoptosis, while disruption of ER/mitochondria contact sites leads to dysfunctional calcium homeostasis and, in turn, elevated ER stress. Finally, mutant SOD1 aggregates interact with Derlin-1, a member of the ERAD pathway, and disrupts the proteasome-dependent degradation of misfolded ER proteins, thus promoting further ER stress. For further details please refer to the main text.

FIGURE 5

Proteasome dysfunction in ALS. The ubiquitin proteasome system is responsible for the degradation of poly-ubiquitinated protein substrates. Misfolded proteins are poly-ubiquitinated by the action of the E1, E2, and E3 ubiquitin ligases. Proteasome dysfunction has been implicated in ALS. Altered substrate delivery to the proteasome, mutant protein interaction with the proteasome, as in the case of mutant SOD1, and reduced proteasome function have all been implicated in ALS pathogenesis, ultimately leading to poly-ubiquitinated protein aggregate formation. The ALS-associated genes and their positions in the UPS are indicated in red, as are other ALS-associated defects. Interestingly, not only can mutant SOD1 interact with the 19S subunit of the proteasome, but mutant SOD1 is also a substrate for proteasome clearance. For further details please refer to the main text.

FIGURE 6

The steps of autophagy. The four stages of autophagy are indicated. (1) Translocation of the ULK1 initiation complex to the phagophore is the first step in autophagy initiation. Inhibition of mTOR, releases the ULK1 complex allowing activation and translocation of the complex. (2) Elongation of the phagophore membrane is mediated by the Class III PI3 kinase complex. Cargo recruitment to the growing phagophore is mediated by the autophagy receptors, p62/sequestosome-1 and optineurin. Autophagy receptors bind both poly-ubiquitin chains on autophagy substrates via the ubiquitin-like (Ubl) domains and LC3-II on the growing phagophore via LC3-interacting regions (LIRs). (3) After substrate recruitment and closure, completed autophagosomes are transported to allow fusion with the lysosome. (4) Autophagosome–lysosome fusion allows the degradation of the autophagic substrates by the action of acid hydrolases, present within the lysosome. Degradation allows the recycling of nutrients back to the cytosol. For further details please refer to the main text. Figure adapted from Webster et al. (2016b) under the terms of the Creative Commons Attribution License (CC BY).

FIGURE 7

Autophagy dysfunction and ALS. Many ALS-associated genes, indicated in red, are implicated in the autophagy pathway. The location of gene names indicates the likely part of the pathway affected in ALS. Protein aggregates are a common feature of ALS pathology. The autophagy cargoes detailed here include a number of ALS-associated proteins, namely SOD1 and TDP-43, both of which are considered autophagy substrates. Mutant forms of these proteins, alterations to the pathway as a whole, or aberrant production of autophagy substrates, potentially in the case of C9orf72-related DPR proteins, may disrupt efficient substrate clearance, overwhelming the autophagy pathway and further promoting autophagy dysfunction. Importantly, TDP-43 is also important for autophagy gene transcription, thus participating as both a substrate and a regulator of the autophagy. For further details please refer to the main text. Figure adapted from Webster et al. (2016b) under the terms of the Creative Commons Attribution License (CC BY).