β-Methylamino-L-alanine substitution of serine in SOD1 suggests a direct role in ALS etiology

Protein aggregation and therapeutic strategies in SOD1- and TDP-43- linked ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease with severe socio-economic impact. A hallmark of ALS pathology is the presence of aberrant cytoplasmic inclusions composed of misfolded and aggregated proteins, including both wild-type and mutant forms. This review highlights the critical role of misfolded protein species in ALS pathogenesis, particularly focusing on Cu/Zn superoxide dismutase (SOD1) and TAR DNA-binding protein 43 (TDP-43), and emphasizes the urgent need for innovative therapeutic strategies targeting these misfolded proteins directly. Despite significant advancements in understanding ALS mechanisms, the disease remains incurable, with current treatments offering limited clinical benefits. Through a comprehensive analysis, the review focuses on the direct modulation of the misfolded proteins and presents recent discoveries in small molecules and peptides that inhibit SOD1 and TDP-43 aggregation, underscoring their potential as effective treatments to modify disease progression and improve clinical outcomes.

Chronic exposure to l-BMAA cyanotoxin induces cytoplasmic TDP-43 accumulation and glial activation, reproducing an amyotrophic lateral sclerosis-like phenotype in mice

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a progressive and often fatal neurodegenerative disease characterized by the loss of Motor Neurons (MNs) in spinal cord, motor cortex and brainstem. Despite significant efforts in the field, the exact pathogenetic mechanisms underlying both familial and sporadic forms of ALS have not been fully elucidated, and the therapeutic possibilities are still very limited. Here we investigate the molecular mechanisms of neurodegeneration induced by chronic exposure to the environmental cyanotoxin L-BMAA, which causes a form of ALS/Parkinson's disease (PD) in several populations consuming food and/or water containing high amounts of this compound.

METHODS

In this effort, mice were chronically exposed to L-BMAA and analyzed at different time points to evaluate cellular and molecular alterations and behavioral deficits, performing MTT assay, immunoblot, immunofluorescence and immunohistochemistry analysis, and behavioral tests.

RESULTS

We found that cyanotoxin L-BMAA determines apoptotic cell death and a marked astrogliosis in spinal cord and motor cortex, and induces neurotoxicity by favoring TDP-43 cytoplasmic accumulation.

CONCLUSIONS

Overall, our results characterize a new versatile neurotoxic animal model of ALS that may be useful for the identification of new druggable targets to develop innovative therapeutic strategies for this disease.

Global translational control by the transcriptional repressor TrcR in the filamentous cyanobacterium Anabaena sp. PCC 7120

Transcriptional and translational regulations are important mechanisms for cell adaptation to environmental conditions. In addition to house-keeping tRNAs, the genome of the filamentous cyanobacterium Anabaena sp. strain PCC 7120 (Anabaena) has a long tRNA operon (trn operon) consisting of 26 genes present on a megaplasmid. The trn operon is repressed under standard culture conditions, but is activated under translational stress in the presence of antibiotics targeting translation. Using the toxic amino acid analog β-N-methylamino-L-alanine (BMAA) as a tool, we isolated and characterized several BMAA-resistance mutants from Anabaena, and identified one gene of unknown function, all0854, named as trcR, encoding a transcription factor belonging to the ribbon-helix-helix (RHH) family. We provide evidence that TrcR represses the expression of the trn operon and is thus the missing link between the trn operon and translational stress response. TrcR represses the expression of several other genes involved in translational control, and is required for maintaining translational fidelity. TrcR, as well as its binding sites, are highly conserved in cyanobacteria, and its functions represent an important mechanism for the coupling of the transcriptional and translational regulations in cyanobacteria.

Cell cycle of microalga Isochrysis galbana arrested by neurotoxin β-N-methylamino-l-alanine and corresponding molecular mechanisms

The phycotoxin β-N-methylamino-l-alanine (BMAA) has attracted attention due to its risks to marine organisms and human health. In this study, approximately 85 % of synchronized cells of the marine microalga Isochrysis galbana were arrested at the cell cycle G1 phase by BMAA at 6.5 μM for a 24-h exposure. The concentration of chlorophyll a (Chl a) gradually decreased, while the maximum quantum yield of PSII (F_v/F_m), the maximum relative electron transport rate (rETR_max), light utilization efficiency (α) and half-saturated light irradiance (I_k) reduced early and recovered gradually in I. galbana exposed to BMAA in 96-h batch cultures. Transcriptional expression of I. galbana analyzed at 10, 12, and 16 h disclosed multiple mechanisms of BMAA to suppress the microalgal growth. Production of ammonia and glutamate was limited by the down-regulation of nitrate transporters, glutamate synthase, glutamine synthetase, cyanate hydrolase, and formamidase. Diverse extrinsic proteins related to PSII, PSI, cytochrome b6f complex, and ATPase were influenced by BMAA at transcriptional level. Suppression of the DNA replication and mismatch repair pathways increased the accumulation of misfolded proteins, which was reflected by the up-regulated expression of proteasome to accelerate proteolysis. This study improves our understanding of the chemical ecology impacts of BMAA in marine ecosystems.

A computational strategy for therapeutic development against superoxide dismutase (SOD1) amyloid formation: effect of polyphenols on the various events in the aggregation pathway

Pathology of superoxide dismutase 1 (SOD1) aggregation is linked to a neurodegenerative disease known as amyotrophic lateral sclerosis (ALS). Without suitable post-translational modifications (PTMs), the protein structure tends to become aggregation-prone. Understanding the role of PTMs and targeting the aggregation-prone SOD1 with small molecules can be used to design a strategy to inhibit its aggregation. Microsecond long molecular dynamics (MD) simulations followed by free energy surface (FES) analyses show that the loss of structure in the apo monomer happens locally and stepwise. Removing the disulfide bond from apoprotein leads to further instability in the zinc-binding loop, giving rise to non-native protein conformations. Further, it was found that these non-native conformations have a higher propensity to form a non-native dimer. We chose three structurally similar polyphenols based on their binding energies and investigated their impact on SOD1 aggregation kinetics. MD simulations of apo-SOD1^SH/corkscrew fibril-polyphenol complexes were also carried out. The effect of polyphenols was seen on fibril elongation as well. Based on the experiments and MD simulation results, it can be inferred that the choice of inhibitors is influenced not only by the binding energy but also by dimer interface stabilization, the proclivity to form non-native dimers, the propensity to break fibrils, and the propensity to decrease the rate of elongation. The polyphenols with 3' and 4' hydroxyl groups are better inhibitors of SOD1 aggregation.

Urbanization, air pollution, and water pollution: Identification of potential environmental risk factors associated with amyotrophic lateral sclerosis using systematic reviews

Introduction

Despite decades of research, causes of ALS remain unclear. To evaluate recent hypotheses of plausible environmental factors, the aim of this study was to synthesize and appraise literature on the potential associations between the surrounding environment, including urbanization, air pollution and water pollution, and ALS.

Methods

We conducted a series (n = 3) of systematic reviews in PubMed and Scopus to identify epidemiological studies assessing relationships between urbanization, air pollution and water pollution with the development of ALS.

Results

The combined search strategy led to the inclusion of 44 articles pertaining to at least one exposure of interest. Of the 25 included urbanization studies, four of nine studies on living in rural areas and three of seven studies on living in more highly urbanized/dense areas found positive associations to ALS. There were also three of five studies for exposure to electromagnetic fields and/or proximity to powerlines that found positive associations to ALS. Three case-control studies for each of diesel exhaust and nitrogen dioxide found positive associations with the development of ALS, with the latter showing a dose-response in one study. Three studies for each of high selenium content in drinking water and proximity to lakes prone to cyanobacterial blooms also found positive associations to ALS.

Conclusion

Whereas markers of air and water pollution appear as potential risk factors for ALS, results are mixed for the role of urbanization.

Synucleinopathy in Amyotrophic Lateral Sclerosis: A Potential Avenue for Antisense Therapeutics?

Amyotrophic lateral sclerosis (ALS) is the most common adult-onset motor neuron disease classified as both a neurodegenerative and neuromuscular disorder. With a complex aetiology and no current cure for ALS, broadening the understanding of disease pathology and therapeutic avenues is required to progress with patient care. Alpha-synuclein (αSyn) is a hallmark for disease in neurodegenerative disorders, such as Parkinson's disease, Lewy body dementia, and multiple system atrophy. A growing body of evidence now suggests that αSyn may also play a pathological role in ALS, with αSyn-positive Lewy bodies co-aggregating alongside known ALS pathogenic proteins, such as SOD1 and TDP-43. This review endeavours to capture the scope of literature regarding the aetiology and development of ALS and its commonalities with "synucleinopathy disorders". We will discuss the involvement of αSyn in ALS and motor neuron disease pathology, and the current theories and strategies for therapeutics in ALS treatment, as well as those targeting αSyn for synucleinopathies, with a core focus on small molecule RNA technologies.

Non-Proteinogenic Amino Acid β-N-Methylamino-L-Alanine (BMAA): Bioactivity and Ecological Significance

Research interest in a non-protein amino acid β-N-methylamino-L-alanine (BMAA) arose due to the discovery of a connection between exposure to BMAA and the occurrence of neurodegenerative diseases. Previous reviews on this topic either considered BMAA as a risk factor for neurodegenerative diseases or focused on the problems of detecting BMAA in various environmental samples. Our review is devoted to a wide range of fundamental biological problems related to BMAA, including the molecular mechanisms of biological activity of BMAA and the complex relationships between producers of BMAA and the environment in various natural ecosystems. At the beginning, we briefly recall the most important facts about the producers of BMAA (cyanobacteria, microalgae, and bacteria), the pathways of BMAA biosynthesis, and reliable methods of identification of BMAA. The main distinctive feature of our review is a detailed examination of the molecular mechanisms underlying the toxicity of BMAA to living cells. A brand new aspect, not previously discussed in any reviews, is the effect of BMAA on cyanobacterial cells. These recent studies, conducted using transcriptomics and proteomics, revealed potent regulatory effects of BMAA on the basic metabolism and cell development of these ancient photoautotrophic prokaryotes. Exogenous BMAA strongly influences cell differentiation and primary metabolic processes in cyanobacteria, such as nitrogen fixation, photosynthesis, carbon fixation, and various biosynthetic processes involving 2-oxoglutarate and glutamate. Cyanobacteria were found to be more sensitive to exogenous BMAA under nitrogen-limited growth conditions. We suggest a hypothesis that this toxic diaminoacid can be used by phytoplankton organisms as a possible allelopathic tool for controlling the population of cyanobacterial cells during a period of intense competition for nitrogen and other resources in various ecosystems.

Motor dysfunction in Drosophila melanogaster as a biomarker for developmental neurotoxicity

Adequate alternatives to conventional animal testing are needed to study developmental neurotoxicity (DNT). Here, we used kinematic analysis to assess DNT of known (toluene (TOL) and chlorpyrifos (CPS)) and putative (β-N-methylamino-L-alanine (BMAA)) neurotoxic compounds. Drosophila melanogaster was exposed to these compounds during development and evaluated for survival and adult kinematic parameters using the FlyWalker system, a kinematics evaluation method. At concentrations that do not induce general toxicity, the solvent DMSO had a significant effect on kinematic parameters. Moreover, while TOL did not significantly induce lethality or kinematic dysfunction, CPS not only induced developmental lethality but also significantly impaired coordination in comparison to DMSO. Interestingly, BMAA, which was not lethal during development, induced motor decay in young adult animals, phenotypically resembling aged flies, an effect later attenuated upon aging. Furthermore, BMAA induced abnormal development of leg motor neuron projections. Our results suggest that our kinematic approach can assess potential DNT of chemical compounds.

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Alternatives to mammalian testing are needed to detect developmental neurotoxicity

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The pesticide chlorpyrifos causes partial lethality and motor dysfunction

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Non-lethal levels of BMAA induce motor dysfunction in a dose-dependent manner

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Kinematic profiling of adult Drosophila can identify developmental neurotoxicity

Toxic SOD1 trimers are off-pathway in the formation of amyloid-like fibrils in ALS

Accumulation of insoluble amyloid fibrils is widely studied as a critical factor in the pathology of multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease. Misfolded Cu, Zn superoxide dismutase (SOD1) was the first protein linked to ALS, and non-native SOD1 trimeric oligomers were recently linked to cytotoxicity, while larger oligomers were protective to cells. The balance between trimers and larger aggregates in the process of SOD1 aggregation is, thus, a critical determinant of potential therapeutic approaches to treat ALS. Yet, it is unknown whether these trimeric oligomers are a necessary intermediate for larger aggregate formation or a distinct off-pathway species competing with fibril formation. Depending on the on- or off-pathway scenario of trimer formation, we expect drastically different therapeutic approaches. Here, we show that the toxic SOD1 trimer is an off-pathway intermediate competing with protective fibril formation. We design mutant SOD1 constructs that remain in a trimeric state (super stable trimers) and show that stabilizing the trimeric SOD1 prevents formation of fibrils in vitro and in a motor neuron like cell model (NSC-34). Using size exclusion chromatography, we track the aggregation kinetics of purified SOD1 and show direct competition of trimeric SOD1 with larger oligomer and fibril formation. Finally, we show the trimer is structurally independent of both larger soluble oligomers and insoluble fibrils using circular dichroism spectroscopy and limited proteolysis.

The pathogenesis of amyotrophic lateral sclerosis: Mitochondrial dysfunction, protein misfolding and epigenetics

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with multiple complex mechanisms involved. Among them, mitochondrial dysfunction plays an important role in ALS. Multiple studies have shown that mitochondria are closely associated with reactive oxygen species production and oxidative stress and exhibit different functional states in different genetic backgrounds. In this review we explored the roles of Ca²⁺, autophagy, mitochondrial quality control in the regulation of mitochondrial homeostasis and their relationship with ALS. In addition, we also summarized and analyzed the roles of protein misfolding and abnormal aggregation in the pathogenesis of ALS. Moreover, we also discussed how epigenetic mechanisms such as DNA methylation and protein post-translational modification affect initiation and progression of ALS. Nevertheless, existing events still cannot fully explain the pathogenesis of ALS at present, more studies are required to explore pathological mechanisms of ALS.

A tRNA t6A modification system contributes to the sensitivity towards the toxin β-N-methylamino-L-alanine (BMAA) in the cyanobacterium Anabaena sp. PCC 7120

Cyanobacteria are oxygen-evolving photosynthetic autotrophs essential for nutrient cycling in the environment. They possess multiple control mechanisms for their cellular activities in order to adapt to the environment. While protein translation is essential for cell survival and adaptation, the regulation and the flexibility of this process are poorly understood in cyanobacteria. β-N-methylamino-L-alanine (BMAA), an amino acid analog proposed as an environmental neurotoxin, is highly toxic to the filamentous diazotrophic cyanobacterium Anabaena PCC 7120. In this study, through genetic analysis of BMAA-resistant mutants, we demonstrate that the system responsible for modification of ANN-decoding tRNAs with N(6)-threonylcarbamoyl adenosine (t⁶A) is involved in BMAA sensitivity through the control of translation. Both BMAA and inactivation of the t⁶A biosynthesis pathway affect translational fidelity and ribosome assembly. However, the two factors display either additive effects on translational elongation, or attenuate each other over translational fidelity or the resistance/sensitivity to antibiotics that inhibit different steps of the translational process. BMAA has a broad effect on translation and transcription, and once BMAA enters the cells, the presence of the t⁶A biosynthesis pathway increases the sensitivity of the cells towards this toxin. BMAA-resistant mutants screening is an effective method for getting insight into the toxic mechanisms of BMAA. In addition, BMAA is a useful tool for probing translational flexibility of cyanobacteria, and the characterization of the corresponding resistant mutants should help us to reveal translational mechanism allowing cyanobacteria to adapt to changing environments.

Glioprotective Effects of Resveratrol Against BMAA-Induced Astroglial Dysfunctions

Astroglial cells play important roles in maintaining central nervous system (CNS) homeostasis. The neurotoxin β-N-methylamino-L-alanine (BMAA) has usually been associated with neurodegeneration due to its toxic effects on neurons. However, little is known about the effects of BMAA on astroglial cells. Resveratrol, a natural polyphenol, represents a potential protective strategy against brain injuries. In the present study, we sought to investigate BMAA-induced astroglial dysfunctions and the glioprotective roles of resveratrol. BMAA did not impair astroglial cellular viability, but increased glutamate uptake, glutamate metabolism into glutamine, and reactive oxygen species production, while decreased glutathione (GSH) and superoxide dismutase (SOD)-based antioxidant defenses and triggers an inflammatory response. In contrast, resveratrol was able to prevent most of these BMAA-induced functional changes in astroglial cells. Moreover, both BMAA and resveratrol modulated the gene expression of molecular pathways associated with glutamate metabolism, redox homeostasis, and inflammatory response, which characterize their roles on astroglial functions. In this regard, BMAA downregulated adenosine receptors, peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α), phosphoinositide-3-kinase (PI3K), and Akt, while resveratrol prevented these effects and upregulated nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1). Our study, for the first time, demonstrates that BMAA directly impacts key astroglial functions, contributing to elucidating the cellular and molecular mechanisms of this toxin in the CNS. In addition, we reinforce the glioprotective effects of resveratrol against BMAA-induced astroglial dysfunctions.

Amyloid Oligomers: A Joint Experimental/Computational Perspective on Alzheimer's Disease, Parkinson's Disease, Type II Diabetes, and Amyotrophic Lateral Sclerosis

Protein misfolding and aggregation is observed in many amyloidogenic diseases affecting either the central nervous system or a variety of peripheral tissues. Structural and dynamic characterization of all species along the pathways from monomers to fibrils is challenging by experimental and computational means because they involve intrinsically disordered proteins in most diseases. Yet understanding how amyloid species become toxic is the challenge in developing a treatment for these diseases. Here we review what computer, in vitro, in vivo, and pharmacological experiments tell us about the accumulation and deposition of the oligomers of the (Aβ, tau), α-synuclein, IAPP, and superoxide dismutase 1 proteins, which have been the mainstream concept underlying Alzheimer's disease (AD), Parkinson's disease (PD), type II diabetes (T2D), and amyotrophic lateral sclerosis (ALS) research, respectively, for many years.

Food web biomagnification of the neurotoxin β-N-methylamino-L-alanine in a diatom-dominated marine ecosystem in China

The neurotoxin β-N-methylamino-L-alanine (BMAA) reported in some cyanobacteria and eukaryote microalgae is a cause of concern due to its potential risk of human neurodegenerative diseases. Here, BMAA distribution in phytoplankton, zooplankton, and other marine organisms was investigated in Jiaozhou Bay, China, a diatom-dominated marine ecosystem, during four seasons in 2019. Results showed that BMAA was biomagnified in the food web from phytoplankton to higher trophic levels. Trophic magnification factors (TMFs) for zooplankton, bivalve mollusks, carnivorous crustaceans and carnivorous gastropod mollusks were ca. 4.58, 30.1, 42.5, and 74.4, respectively. Putative identification of β-amino-N-methylalanine (BAMA), an isomer of BMAA, was frequently detected in phytoplankton samples. A total of 56 diatom strains of the genera Pseudo-nitzschia, Thalassiosira, Chaetoceros, Planktoniella, and Minidiscus isolated from the Chinese coast were cultured in the laboratory, among which 21 strains contained BMAA mainly in precipitated bound form at toxin concentrations ranging from 0.11 to 3.95 µg/g dry weight. Only 2,4-diaminobutyric acid (DAB) but not BMAA or BAMA was detected in seven species of bacteria isolated from the gut of gastropod Neverita didyma, suggesting that this benthic vector of BMAA may have accumulated this compound via trophic transfer.

Is Exposure to BMAA a Risk Factor for Neurodegenerative Diseases? A Response to a Critical Review of the BMAA Hypothesis

In a literature survey, Chernoff et al. (2017) dismissed the hypothesis that chronic exposure to β-N-methylamino-L-alanine (BMAA) may be a risk factor for progressive neurodegenerative disease. They question the growing scientific literature that suggests the following: (1) BMAA exposure causes ALS/PDC among the indigenous Chamorro people of Guam; (2) Guamanian ALS/PDC shares clinical and neuropathological features with Alzheimer's disease, Parkinson's disease, and ALS; (3) one possible mechanism for protein misfolds is misincorporation of BMAA into proteins as a substitute for L-serine; and (4) chronic exposure to BMAA through diet or environmental exposures to cyanobacterial blooms can cause neurodegenerative disease. We here identify multiple errors in their critique including the following: (1) their review selectively cites the published literature; (2) the authors reported favorably on HILIC methods of BMAA detection while the literature shows significant matrix effects and peak coelution in HILIC that may prevent detection and quantification of BMAA in cyanobacteria; (3) the authors build alternative arguments to the BMAA hypothesis, rather than explain the published literature which, to date, has been unable to refute the BMAA hypothesis; and (4) the authors erroneously attribute methods to incorrect studies, indicative of a failure to carefully consider all relevant publications. The lack of attention to BMAA research begins with the review's title which incorrectly refers to BMAA as a "non-essential" amino acid. Research regarding chronic exposure to BMAA as a cause of human neurodegenerative diseases is emerging and requires additional resources, validation, and research. Here, we propose strategies for improvement in the execution and reporting of analytical methods and the need for additional and well-executed inter-lab comparisons for BMAA quantitation. We emphasize the need for optimization and validation of analytical methods to ensure that they are fit-for-purpose. Although there remain gaps in the literature, an increasingly large body of data from multiple independent labs using orthogonal methods provides increasing evidence that chronic exposure to BMAA may be a risk factor for neurological illness.

SOD1 oligomers in amyotrophic lateral sclerosis

Identifying nonnative, trimeric forms of SOD1 trimers as the toxic species, rather than large aggregates revolutionizes our understanding of ALS pathophysiology. Large protein aggregates, what was previously thought as the central cause of neurodegeneration, play protective role and are not responsible for neuronal death. SOD1 trimers are implicated at the molecular, cellular, and organismal level. Understanding the formation of the nonnative trimer and its role in the cell, leading to cell death, holds the key to developing a new standard of therapeutics for ALS and for other neurodegenerative diseases. This review highlights recent advances of knowledge for the role of SOD1 oligomers in ALS.

l-Serine Reduces Spinal Cord Pathology in a Vervet Model of Preclinical ALS/MND

The early neuropathological features of amyotrophic lateral sclerosis/motor neuron disease (ALS/MND) are protein aggregates in motor neurons and microglial activation. Similar pathology characterizes Guamanian ALS/Parkinsonism dementia complex, which may be triggered by the cyanotoxin β-N-methylamino-l-alanine (BMAA). We report here the occurrence of ALS/MND-type pathological changes in vervets (Chlorocebus sabaeus; n = 8) fed oral doses of a dry powder of BMAA HCl salt (210 mg/kg/day) for 140 days. Spinal cords and brains from toxin-exposed vervets were compared to controls fed rice flour (210 mg/kg/day) and to vervets coadministered equal amounts of BMAA and l-serine (210 mg/kg/day). Immunohistochemistry and quantitative image analysis were used to examine markers of ALS/MND and glial activation. UHPLC-MS/MS was used to confirm BMAA exposures in dosed vervets. Motor neuron degeneration was demonstrated in BMAA-dosed vervets by TDP-43⁺ proteinopathy in anterior horn cells, by reactive astrogliosis, by activated microglia, and by damage to myelinated axons in the lateral corticospinal tracts. Vervets dosed with BMAA + l-serine displayed reduced neuropathological changes. This study demonstrates that chronic dietary exposure to BMAA causes ALS/MND-type pathological changes in the vervet and coadministration of l-serine reduces the amount of reactive gliosis and the number of protein inclusions in motor neurons.

The prion-like nature of amyotrophic lateral sclerosis

The misfolding, aggregation, and deposition of specific proteins is the key hallmark of most progressive neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). ALS is characterized by the rapid and progressive degenerations of motor neurons in the spinal cord and motor cortex, resulting in paralysis of those who suffer from it. Pathologically, there are three major aggregating proteins associated with ALS, including TAR DNA-binding protein of 43kDa (TDP-43), superoxide dismutase-1 (SOD1), and fused in sarcoma (FUS). While there are ALS-associated mutations found in each of these proteins, the most prevalent aggregation pathology is that of wild-type TDP-43 (97% of cases), with the remaining split between mutant forms of SOD1 (~2%) and FUS (~1%). Considering the progressive nature of ALS and its association with the aggregation of specific proteins, a growing notion is that the spread of pathology and symptoms can be explained by a prion-like mechanism. Prion diseases are a group of highly infectious neurodegenerative disorders caused by the misfolding, aggregation, and spread of a transmissible conformer of prion protein (PrP). Pathogenic PrP is capable of converting healthy PrP into a toxic form through template-directed misfolding. Application of this finding to other neurodegenerative disorders, and in particular ALS, has revolutionized our understanding of cause and progression of these disorders. In this chapter, we first provide a background on ALS pathology and genetic origin. We then detail and discuss the evidence supporting a prion-like propagation of protein misfolding and aggregation in ALS with a particular focus on SOD1 and TDP-43 as these are the most well-established models in the field.

Proteome Homeostasis Dysfunction: A Unifying Principle in ALS Pathogenesis

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease but currently has no effective treatment. Growing evidence suggests that proteome homeostasis underlies ALS pathogenesis. Protein production, trafficking, and degradation all shape the proteome. We present a hypothesis that proposes all genetic lesions associated with ALS (including in mRNA-binding proteins) cause widespread imbalance to an already metastable proteome. The impact of such dysfunction is felt across the entire proteome and is not restricted to a small subset of proteins. Proteome imbalance may cause functional defects, such as excitability alterations, and eventually cell death. While this idea is a unifying principle for all of ALS, we propose that stratification will appear that might dictate the efficacy of therapeutics based on the proteostasis network.