Disulfide cross-linked protein represents a significant fraction of ALS-associated Cu, Zn-superoxide dismutase aggregates in spinal cords of model mice

Discovery of Novel Inhibitors against ALS-Related SOD1(A4V) Aggregation through the Screening of a Chemical Library Using Differential Scanning Fluorimetry (DSF)

BACKGROUND

Cu/Zn Superoxide Dismutase 1 (SOD1) is a 32 kDa cytosolic dimeric metalloenzyme that neutralizes superoxide anions into oxygen and hydrogen peroxide. Mutations in SOD1 are associated with ALS, a disease causing motor neuron atrophy and subsequent mortality. These mutations exert their harmful effects through a gain of function mechanism, rather than a loss of function. Despite extensive research, the mechanism causing selective motor neuron death still remains unclear. A defining feature of ALS pathogenesis is protein misfolding and aggregation, evidenced by ubiquitinated protein inclusions containing SOD1 in affected motor neurons. This work aims to identify compounds countering SOD1(A4V) misfolding and aggregation, which could potentially aid in ALS treatment.

METHODS

The approach employed was in vitro screening of a library comprising 1280 pharmacologically active compounds (LOPAC®) in the context of drug repurposing. Using differential scanning fluorimetry (DSF), these compounds were tested for their impact on SOD1(A4V) thermal stability.

RESULTS AND CONCLUSIONS

Dimer stability was the parameter chosen as the criterion for screening, since the dissociation of the native SOD1 dimer is the step prior to its in vitro aggregation. The screening revealed one compound raising protein-ligand Tm by 6 °C, eleven inducing a higher second Tm, suggesting a stabilization effect, and fourteen reducing Tm from 10 up to 26 °C, suggesting possible interactions or non-specific binding.

Seeding activity of human superoxide dismutase 1 aggregates in familial and sporadic amyotrophic lateral sclerosis postmortem neural tissues by real-time quaking-induced conversion

Amyotrophic lateral sclerosis (ALS) is a rapidly progressive neurodegenerative disease with average lifespan of 2-5 years after diagnosis. The identification of novel prognostic and pharmacodynamic biomarkers are needed to facilitate therapeutic development. Metalloprotein human superoxide dismutase 1 (SOD1) is known to accumulate and form aggregates in patient neural tissue with familial ALS linked to mutations in their SOD1 gene. Aggregates of SOD1 have also been detected in other forms of ALS, including the sporadic form and the most common familial form linked to abnormal hexanucleotide repeat expansions in the Chromosome 9 open reading frame 72 (C9ORF72) gene. Here, we report the development of a real-time quaking-induced conversion (RT-QuIC) seed amplification assay using a recombinant human SOD1 substrate to measure SOD1 seeding activity in postmortem spinal cord and motor cortex tissue from persons with different ALS etiologies. Our SOD1 RT-QuIC assay detected SOD1 seeds in motor cortex and spinal cord dilutions down to 10-5. Importantly, we detected SOD1 seeding activity in specimens from both sporadic and familial ALS cases, with the latter having mutations in either their SOD1 or C9ORF72 genes. Analyses of RT-QuIC parameters indicated similar lag phases in spinal cords of sporadic and familial ALS patients, but higher ThT fluorescence maxima by SOD1 familial ALS specimens and sporadic ALS thoracic cord specimens. For a subset of sporadic ALS patients, motor cortex and spinal cords were examined, with seeding activity in both anatomical regions. Our results suggest SOD1 seeds are in ALS patient neural tissues not linked to SOD1 mutation, suggesting that SOD1 seeding activity may be a promising biomarker, particularly in sporadic ALS cases for whom genetic testing is uninformative.

Pan-neuronal expression of human mutant SOD1 in Drosophila impairs survival and motor performance, induces early neuroinflammation and chromosome aberrations

Several mutations in the SOD1 gene encoding for the antioxidant enzyme Superoxide Dismutase 1, are associated with amyotrophic lateral sclerosis, a rare and devastating disease characterized by motor neuron degeneration and patients' death within 2-5 years from diagnosis. Motor neuron loss and related symptomatology manifest mostly in adult life and, to date, there is still a gap of knowledge on the precise cellular and molecular events preceding neurodegeneration. To deepen our awareness of the early phases of the disease, we leveraged two Drosophila melanogaster models pan-neuronally expressing either the mutation A4V or G85R of the human gene SOD1 (hSOD1A4V or hSOD1G85R). We demonstrate that pan-neuronal expression of the hSOD1A4V or hSOD1G85R pathogenic construct impairs survival and motor performance in transgenic flies. Moreover, protein and transcript analysis on fly heads indicates that mutant hSOD1 induction stimulates the glial marker Repo, up-regulates the IMD/Toll immune pathways through antimicrobial peptides and interferes with oxidative metabolism. Finally, cytological analysis of larval brains demonstrates hSOD1-induced chromosome aberrations. Of note, these parameters are found modulated in a timeframe when neurodegeneration is not detected. The novelty of our work is twofold: we have expressed for the first time hSOD1 mutations in all neurons of Drosophila and confirmed some ALS-related pathological phenotypes in these flies, confirming the power of SOD1 mutations in generating ALS-like phenotypes. Moreover, we have related SOD1 pathogenesis to chromosome aberrations and antimicrobial peptides up-regulation. These findings were unexplored in the SOD1-ALS field.

Ebselen analogues delay disease onset and its course in fALS by on-target SOD-1 engagement

Amyotrophic lateral sclerosis (ALS) selectively affects motor neurons. SOD1 is the first causative gene to be identified for ALS and accounts for at least 20% of the familial (fALS) and up to 4% of sporadic (sALS) cases globally with some geographical variability. The destabilisation of the SOD1 dimer is a key driving force in fALS and sALS. Protein aggregation resulting from the destabilised SOD1 is arrested by the clinical drug ebselen and its analogues (MR6-8-2 and MR6-26-2) by redeeming the stability of the SOD1 dimer. The in vitro target engagement of these compounds is demonstrated using the bimolecular fluorescence complementation assay with protein-ligand binding directly visualised by co-crystallography in G93A SOD1. MR6-26-2 offers neuroprotection slowing disease onset of SOD1G93A mice by approximately 15 days. It also protected neuromuscular junction from muscle denervation in SOD1G93A mice clearly indicating functional improvement.

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.

Stress Granule Dysfunction via Chromophore-Associated Light Inactivation

Stress granules (SGs) are cytoplasmic condensates composed of various proteins and RNAs that protect translation-associated machinery from harmful conditions during stress. However, the method of spatiotemporal inactivation of condensates such as SGs in live cells to study cellular phenotypes is still in the process of being demonstrated. Here, we show that the inactivation of SGs by chromophore-associated light inactivation (CALI) using a genetically encoded red fluorescence protein (SuperNova-Red) as a photosensitizer leads to differences in cell viability during recovery from hyperosmotic stress. CALI delayed the disassembly kinetics of SGs during recovery from hyperosmotic stress. Consequently, CALI could inactivate the SGs, and the cellular fate due to SGs could be analyzed. Furthermore, CALI is an effective spatiotemporal knockdown method for intracellular condensates/aggregates and would contribute to the elucidation of importance of such condensates/aggregates.

Unveiling local and global conformational changes and allosteric communications in SOD1 systems using molecular dynamics simulation and network analyses

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a serious neurodegenerative disorder affecting nerve cells in the brain and spinal cord that is caused by mutations in the superoxide dismutase 1 (SOD1) enzyme. ALS-related mutations cause misfolding, dimerisation instability, and increased formation of aggregates. The underlying allosteric mechanisms, however, remain obscure as far as details of their fundamental atomistic structure are concerned. Hence, this gap in knowledge limits the development of novel SOD1 inhibitors and the understanding of how disease-associated mutations in distal sites affect enzyme activity.

METHODS

We combined microsecond-scale based unbiased molecular dynamics (MD) simulation with network analysis to elucidate the local and global conformational changes and allosteric communications in SOD1 Apo (unmetallated form), Holo, Apo_CallA (mutant and unmetallated form), and Holo_CallA (mutant form) systems. To identify hotspot residues involved in SOD1 signalling and allosteric communications, we performed network centrality, community network, and path analyses.

RESULTS

Structural analyses showed that unmetallated SOD1 systems and cysteine mutations displayed large structural variations in the catalytic sites, affecting structural stability. Inter- and intra H-bond analyses identified several important residues crucial for maintaining interfacial stability, structural stability, and enzyme catalysis. Dynamic motion analysis demonstrated more balanced atomic displacement and highly correlated motions in the Holo system. The rationale for structural disparity observed in the disulfide bond formation and R143 configuration in Apo and Holo systems were elucidated using distance and dihedral probability distribution analyses.

CONCLUSION

Our study highlights the efficiency of combining extensive MD simulations with network analyses to unravel the features of protein allostery.

MARCH5 promotes STING pathway activation by suppressing polymer formation of oxidized STING

Stimulator of interferon genes (STING) is a core DNA sensing adaptor in innate immune signaling. STING activity is regulated by a variety of post-translational modifications (PTMs), including phosphorylation, ubiquitination, sumoylation, palmitoylation, and oxidation, as well as the balance between active and inactive polymer formation. It remains unclear, though, how different PTMs and higher order structures cooperate to regulate STING activity. Here, we report that the mitochondrial ubiquitin ligase MARCH5 (Membrane Associated Ring-CH-type Finger 5, also known as MITOL) ubiquitinates STING and enhances its activation. A long-term MARCH5 deficiency, in contrast, leads to the production of reactive oxygen species, which then facilitate the formation of inactive STING polymers by oxidizing mouse STING cysteine 205. We show that MARCH5-mediated ubiquitination of STING prevents the oxidation-induced STING polymer formation. Our findings highlight that MARCH5 balances STING ubiquitination and polymer formation and its control of STING activation is contingent on oxidative conditions.

Extreme diversity of 12 cations in folding ALS-linked hSOD1 unveils novel hSOD1-dependent mechanisms for Fe2+/Cu2+-induced cytotoxicity

153-Residue copper-zinc superoxide dismutase 1 (hSOD1) is the first gene whose mutation was linked to FALS. To date, > 180 ALS-causing mutations have been identified within hSOD1, yet the underlying mechanism still remains mysterious. Mature hSOD1 is exceptionally stable constrained by a disulfide bridge to adopt a Greek-key β-barrel fold that accommodates copper/zinc cofactors. Conversely, nascent hSOD1 is unfolded and susceptible to aggregation and amyloid formation, requiring Zn2+ to initiate folding to a coexistence of folded and unfolded states. Recent studies demonstrate mutations that disrupt Zn2+-binding correlate with their ability to form toxic aggregates. Therefore, to decode the role of cations in hSOD1 folding provides not only mechanistic insights, but may bear therapeutic implications for hSOD1-linked ALS. Here by NMR, we visualized the effect of 12 cations: 8 essential for humans (Na+, K+, Ca2+, Zn2+, Mg2+, Mn2+, Cu2+, Fe2+), 3 mimicking zinc (Ni2+, Cd2+, Co2+), and environmentally abundant Al3+. Surprisingly, most cations, including Zn2+-mimics, showed negligible binding or induction for folding of nascent hSOD1. Cu2+ exhibited extensive binding to the unfolded state but led to severe aggregation. Unexpectedly, for the first time Fe2+ was deciphered to have Zn2+-like folding-inducing capacity. Zn2+ was unable to induce folding of H80S/D83S-hSOD1, while Fe2+ could. In contrast, Zn2+ could trigger folding of G93A-hSOD1, but Fe2+ failed. Notably, pre-existing Fe2+ disrupted the Zn2+-induced folding of G93A-hSOD1. Comparing with the ATP-induced folded state, our findings delineate that hSOD1 maturation requires: (1) intrinsic folding capacity encoded by the sequence; (2) specific Zn2+-coordination; (3) disulfide formation and Cu-load catalyzed by hCCS. This study unveils a previously-unknown interplay of cations in governing the initial folding of hSOD1, emphasizing the pivotal role of Zn2+ in hSOD1-related ALS and implying new hSOD1-dependent mechanisms for Cu2+/Fe2+-induced cytotoxicity, likely relevant to aging and other diseases.

Selective removal of misfolded SOD1 delays disease onset in a mouse model of amyotrophic lateral sclerosis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease. There is no cure currently. The discovery that mutations in the gene SOD1 are a cause of ALS marks a breakthrough in the search for effective treatments for ALS. SOD1 is an antioxidant that is highly expressed in motor neurons. Human SOD1 is prone to aberrant modifications. Familial ALS-linked SOD1 variants are particularly susceptible to aberrant modifications. Once modified, SOD1 undergoes conformational changes and becomes misfolded. This study aims to determine the effect of selective removal of misfolded SOD1 on the pathogenesis of ALS.

METHODS

Based on the chaperone-mediated protein degradation pathway, we designed a fusion peptide named CT4 and tested its efficiency in knocking down intracellularly misfolded SOD1 and its efficacy in modifying the pathogenesis of ALS.

RESULTS

Expression of the plasmid carrying the CT4 sequence in human HEK cells resulted in robust removal of misfolded SOD1 induced by serum deprivation. Co-transfection of the CT4 and the G93A-hSOD1 plasmids at various ratios demonstrated a dose-dependent knockdown efficiency on G93A-hSOD1, which could be further increased when misfolding of SOD1 was enhanced by serum deprivation. Application of the full-length CT4 peptide to primary cultures of neurons expressing the G93A variant of human SOD1 revealed a time course of the degradation of misfolded SOD1; misfolded SOD1 started to decrease by 2 h after the application of CT4 and disappeared by 7 h. Intravenous administration of the CT4 peptide at 10 mg/kg to the G93A-hSOD1 reduced human SOD1 in spinal cord tissue by 68% in 24 h and 54% in 48 h in presymptomatic ALS mice. Intraperitoneal administration of the CT4 peptide starting from 60 days of age significantly delayed the onset of ALS and prolonged the lifespan of the G93A-hSOD1 mice.

CONCLUSIONS

The CT4 peptide directs the degradation of misfolded SOD1 in high efficiency and specificity. Selective removal of misfolded SOD1 significantly delays the onset of ALS, demonstrating that misfolded SOD1 is the toxic form of SOD1 that causes motor neuron death. The study proves that selective removal of misfolded SOD1 is a promising treatment for ALS.

Molecular dynamics analysis of superoxide dismutase 1 mutations suggests decoupling between mechanisms underlying ALS onset and progression

Mutations in the superoxide dismutase 1 (SOD1) gene are the second most common known cause of ALS. SOD1 variants express high phenotypic variability and over 200 have been reported in people with ALS. It was previously proposed that variants can be broadly classified in two groups, 'wild-type like' (WTL) and 'metal binding region' (MBR) variants, based on their structural location and biophysical properties. MBR variants, but not WTL variants, were associated with a reduction of SOD1 enzymatic activity. In this study we used molecular dynamics and large clinical datasets to characterise the differences in the structural and dynamic behaviour of WTL and MBR variants with respect to the wild-type SOD1, and how such differences influence the ALS clinical phenotype. Our study identified marked structural differences, some of which are observed in both variant groups, while others are group specific. Moreover, collecting clinical data of approximately 500 SOD1 ALS patients carrying variants, we showed that the survival time of patients carrying an MBR variant is generally longer (∼6 years median difference, p < 0.001) with respect to patients with a WTL variant. In conclusion, our study highlighted key differences in the dynamic behaviour between WTL and MBR SOD1 variants, and between variants and wild-type SOD1 at an atomic and molecular level, that could be further investigated to explain the associated phenotypic variability. Our results support the hypothesis of a decoupling between mechanisms of onset and progression of SOD1 ALS, and an involvement of loss-of-function of SOD1 with the disease progression.

SOD1 gains pro-oxidant activity upon aberrant oligomerization: change in enzymatic activity by intramolecular disulfide bond cleavage

Copper-zinc superoxide dismutase (SOD1) has been proposed as one of the causative proteins of amyotrophic lateral sclerosis (ALS). The accumulation of non-native conformers, oligomers, and aggregates of SOD1 in motor neurons is considered responsible for this disease. However, it remains unclear which specific feature of these species induces the onset of ALS. In this study, we showed that disulfide-linked oligomers of denatured SOD1 exhibit pro-oxidant activity. Substituting all the cysteine residues in the free thiol state with serine resulted in the loss of both the propensity to oligomerize and the increase in pro-oxidant activity after denaturation. In contrast, these cysteine mutants oligomerized and acquired the pro-oxidant activity after denaturation in the presence of a reductant that cleaves the intramolecular disulfide bond. These results indicate that one of the toxicities of SOD1 oligomers is the pro-oxidant activity induced by scrambling of the disulfide bonds. Small oligomers such as dimers and trimers exhibit stronger pro-oxidant activity than large oligomers and aggregates, consistent with the trend of the cytotoxicity of oligomers and aggregates reported in previous studies. We propose that the cleavage of the intramolecular disulfide bond accompanied by the oligomerization reduces the substrate specificity of SOD1, leading to the non-native enzymatic activity.

Prionoids in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the third most frequent neurodegenerative disease after Alzheimer’s and Parkinson’s disease. ALS is characterized by the selective and progressive loss of motoneurons in the spinal cord, brainstem and cerebral cortex. Clinical manifestations typically occur in midlife and start with focal muscle weakness, followed by the rapid and progressive wasting of muscles and subsequent paralysis. As with other neurodegenerative diseases, the condition typically begins at an initial point and then spreads along neuroanatomical tracts. This feature of disease progression suggests the spreading of prion-like proteins called prionoids in the affected tissues, which is similar to the spread of prion observed in Creutzfeldt-Jakob disease. Intensive research over the last decade has proposed the ALS-causing gene products Cu/Zn superoxide dismutase 1, TAR DNA-binding protein of 43 kDa, and fused in sarcoma as very plausible prionoids contributing to the spread of the pathology. In this review, we will discuss the molecular and cellular mechanisms leading to the propagation of these prionoids in ALS.

A Single-Molecule Strategy to Capture Non-native Intramolecular and Intermolecular Protein Disulfide Bridges

Non-native disulfide bonds are dynamic covalent bridges that form post-translationally between two cysteines within the same protein (intramolecular) or with a neighboring protein (intermolecular), frequently due to changes in the cellular redox potential. The reversible formation of non-native disulfides is intimately linked to alterations in protein function; while they can provide a mechanism to protect against cysteine overoxidation, they are also involved in the early stages of protein multimerization, a hallmark of several protein aggregation diseases. Yet their identification using current protein chemistry technology remains challenging, mainly because of their fleeting reactivity. Here, we use single-molecule spectroscopy AFM and molecular dynamics simulations to capture both intra- and intermolecular disulfide bonds in γD-crystallin, a cysteine-rich, structural human lens protein involved in age-related eye cataracts. Our approach showcases the power of mechanical force as a conformational probe in dynamically evolving proteins and presents a platform to detect non-native disulfide bridges with single-molecule resolution.

SOD1 in ALS: Taking Stock in Pathogenic Mechanisms and the Role of Glial and Muscle Cells

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the loss of motor neurons in the brain and spinal cord. While the exact causes of ALS are still unclear, the discovery that familial cases of ALS are related to mutations in the Cu/Zn superoxide dismutase (SOD1), a key antioxidant enzyme protecting cells from the deleterious effects of superoxide radicals, suggested that alterations in SOD1 functionality and/or aberrant SOD1 aggregation strongly contribute to ALS pathogenesis. A new scenario was opened in which, thanks to the generation of SOD1 related models, different mechanisms crucial for ALS progression were identified. These include excitotoxicity, oxidative stress, mitochondrial dysfunctions, and non-cell autonomous toxicity, also implicating altered Ca2+ metabolism. While most of the literature considers motor neurons as primary target of SOD1-mediated effects, here we mainly discuss the effects of SOD1 mutations in non-neuronal cells, such as glial and skeletal muscle cells, in ALS. Attention is given to the altered redox balance and Ca2+ homeostasis, two processes that are strictly related with each other. We also provide original data obtained in primary myocytes derived from hSOD1(G93A) transgenic mice, showing perturbed expression of Ca2+ transporters that may be responsible for altered mitochondrial Ca2+ fluxes. ALS-related SOD1 mutants are also responsible for early alterations of fundamental biological processes in skeletal myocytes that may impinge on skeletal muscle functions and the cross-talk between muscle cells and motor neurons during disease progression.

Association of Oxidative Stress with Neurological Disorders

BACKGORUND

Oxidative stress is one of the main contributing factors involved in cerebral biochemical impairment. The higher susceptibility of the central nervous system to reactive oxygen species mediated damage could be attributed to several factors. For example, neurons use a greater quantity of oxygen, many parts of the brain have higher concentraton of iron, and neuronal mitochondria produce huge content of hydrogen peroxide. In addition, neuronal membranes have polyunsaturated fatty acids, which are predominantly vulnerable to oxidative stress (OS). OS is the imbalance between reactive oxygen species generation and cellular antioxidant potential. This may lead to various pathological conditions and diseases, especially neurodegenerative diseases such as, Parkinson's, Alzheimer's, and Huntington's diseases.

OBJECTIVES

In this study, we explored the involvement of OS in neurodegenerative diseases.

METHODS

We used different search terms like "oxidative stress and neurological disorders" "free radicals and neurodegenerative disorders" "oxidative stress, free radicals, and neurological disorders" and "association of oxidative stress with the name of disorders taken from the list of neurological disorders. We tried to summarize the source, biological effects, and physiologic functions of ROS.

RESULTS

Finally, it was noted that more than 190 neurological disorders are associated with oxidative stress.

CONCLUSION

More elaborated studies in the future will certainly help in understanding the exact mechanism involved in neurological diseases and provide insight into revelation of therapeutic targets.

Oxidative Modification of Proteins: From Damage to Catalysis, Signaling, and Beyond

Significance: The systematic investigation of oxidative modification of proteins by reactive oxygen species started in 1980. Later, it was shown that reactive nitrogen species could also modify proteins. Some protein oxidative modifications promote loss of protein function, cleavage or aggregation, and some result in proteo-toxicity and cellular homeostasis disruption. Recent Advances: Previously, protein oxidation was associated exclusively to damage. However, not all oxidative modifications are necessarily associated with damage, as with Met and Cys protein residue oxidation. In these cases, redox state changes can alter protein structure, catalytic function, and signaling processes in response to metabolic and/or environmental alterations. This review aims to integrate the present knowledge on redox modifications of proteins with their fate and role in redox signaling and human pathological conditions. Critical Issues: It is hypothesized that protein oxidation participates in the development and progression of many pathological conditions. However, no quantitative data have been correlated with specific oxidized proteins or the progression or severity of pathological conditions. Hence, the comprehension of the mechanisms underlying these modifications, their importance in human pathologies, and the fate of the modified proteins is of clinical relevance. Future Directions: We discuss new tools to cope with protein oxidation and suggest new approaches for integrating knowledge about protein oxidation and redox processes with human pathophysiological conditions. Antioxid. Redox Signal. 35, 1016-1080.

Impact of a frequent nearsplice SOD1 variant in amyotrophic lateral sclerosis: optimising SOD1 genetic screening for gene therapy opportunities

OBJECTIVE

Mutations in superoxide dismutase 1 gene (SOD1), encoding copper/zinc superoxide dismutase protein, are the second most frequent high penetrant genetic cause for amyotrophic lateral sclerosis (ALS) motor neuron disease in populations of European descent. More than 200 missense variants are reported along the SOD1 protein. To limit the production of these aberrant and deleterious SOD1 species, antisense oligonucleotide approaches have recently emerged and showed promising effects in clinical trials. To offer the possibility to any patient with SOD1-ALS to benefit of such a gene therapy, it is necessary to ascertain whether any variant of unknown significance (VUS), detected for example in SOD1 non-coding sequences, is pathogenic.

METHODS

We analysed SOD1 mutation distribution after SOD1 sequencing in a large cohort of 470 French familial ALS (fALS) index cases.

RESULTS

We identified a total of 27 SOD1 variants in 38 families including two SOD1 variants located in nearsplice or intronic regions of the gene. The pathogenicity of the c.358-10T>G nearsplice SOD1 variant was corroborated based on its high frequency (as the second most frequent SOD1 variant) in French fALS, the segregation analysis confirmed in eight affected members of a large pedigree, the typical SOD1-related phenotype observed (with lower limb onset and prominent lower motor neuron involvement), and findings on postmortem tissues showing SOD1 misaccumulation.

CONCLUSIONS

Our results highlighted nearsplice/intronic mutations in SOD1 are responsible for a significant portion of French fALS and suggested the systematic analysis of the SOD1 mRNA sequence could become the method of choice for SOD1 screening, not to miss these specific cases.

Liquid-liquid phase separation as a common organizing principle of intracellular space and biomembranes providing dynamic adaptive responses

This work is devoted to the phenomenon of liquid-liquid phase separation (LLPS), which has come to be recognized as fundamental organizing principle of living cells. We distinguish separation processes with different dimensions. Well-known 3D-condensation occurs in aqueous solution and leads to membraneless organelle (MLOs) formation. 2D-films may be formed near membrane surfaces and lateral phase separation (membrane rafts) occurs within the membranes themselves. LLPS may also occur on 1D structures like DNA and the cyto- and nucleoskeleton. Phase separation provides efficient transport and sorting of proteins and metabolites, accelerates the assembly of metabolic and signaling complexes, and mediates stress responses. In this work, we propose a model in which the processes of polymerization (1D structures), phase separation in membranes (2D structures), and LLPS in the volume (3D structures) influence each other. Disordered proteins and whole condensates may provide membrane raft separation or polymerization of specific proteins. On the other hand, 1D and 2D structures with special composition or embedded IDRs can nucleate condensates. We hypothesized that environmental change may trigger a LLPS which can propagate within the cell interior moving along the cytoskeleton or as an autowave. New phase propagation quickly and using a low amount of energy adjusts cell signaling and metabolic systems to new demands. Cumulatively, the interconnected phase separation phenomena in different dimensions represent a previously unexplored system of intracellular communication and regulation which cannot be ignored when considering both physiological and pathological cell processes.

The Role of Protein S-Nitrosylation in Protein Misfolding-Associated Diseases

Abnormal and excessive nitrosative stress contributes to neurodegenerative disease associated with the production of pathological levels of misfolded proteins. The accumulated findings strongly suggest that excessive NO production can induce and deepen these pathological processes, particularly by the S-nitrosylation of target proteins. Therefore, the relationship between S-nitrosylated proteins and the accumulation of misfolded proteins was reviewed. We particularly focused on the S-nitrosylation of E3-ubiquitin-protein ligase, parkin, and endoplasmic reticulum chaperone, PDI, which contribute to the accumulation of misfolded proteins. In addition to the target proteins being S-nitrosylated, NOS, which produces NO, and GSNOR, which inhibits S-nitrosylation, were also suggested as potential therapeutic targets for protein misfolding-associated diseases.

A Metal-Free, Disulfide Oxidized Form of Superoxide Dismutase 1 as a Primary Misfolded Species with Prion-Like Properties in the Extracellular Environments Surrounding Motor Neuron-Like Cells

Superoxide dismutase 1 (SOD1) is a metalloenzyme with high structural stability, but a lack of Cu and Zn ions decreases its stability and enhances the likelihood of misfolding, which is a pathological hallmark of amyotrophic lateral sclerosis (ALS). A growing body of evidence has demonstrated that misfolded SOD1 has prion-like properties such as transmissibility between cells and intracellular propagation of misfolding of natively folded SOD1. Recently, we found that SOD1 is misfolded in the cerebrospinal fluid of sporadic ALS patients, providing a route by which misfolded SOD1 spreads via the extracellular environment of the central nervous system. Unlike intracellular misfolded SOD1, it is unknown which extracellular misfolded species is most relevant to prion-like properties. Here, we determined a conformational feature of extracellular misfolded SOD1 that is linked to prion-like properties. Using culture media from motor neuron-like cells, NSC-34, extracellular misfolded wild-type, and four ALS-causing SOD1 mutants were characterized as a metal-free, disulfide oxidized form of SOD1 (apo-SOD1^S-S). Extracellular misfolded apo-SOD1^S-S exhibited cell-to-cell transmission from the culture medium to recipient cells as well as intracellular propagation of SOD1 misfolding in recipient cells. Furthermore, culture medium containing misfolded apo-SOD1^S-S exerted cytotoxicity to motor neuron-like cells, which was blocked by removal of misfolded apo-SOD1^S-S from the medium. We conclude that misfolded apo-SOD1^S-S is a primary extracellular species that is linked to prion-like properties.

Phase Separation and Neurodegenerative Diseases: A Disturbance in the Force

Protein aggregation is the main hallmark of neurodegenerative diseases. Many proteins found in pathological inclusions are known to undergo liquid-liquid phase separation, a reversible process of molecular self-assembly. Emerging evidence supports the hypothesis that aberrant phase separation behavior may serve as a trigger of protein aggregation in neurodegeneration, and efforts to understand and control the underlying mechanisms are underway. Here, we review similarities and differences among four main proteins, α-synuclein, FUS, tau, and TDP-43, which are found aggregated in different diseases and were independently shown to phase separate. We discuss future directions in the field that will help shed light on the molecular mechanisms of aggregation and neurodegeneration.

Emerging Evidence Highlighting the Importance of Redox Dysregulation in the Pathogenesis of Amyotrophic Lateral Sclerosis (ALS)

The cellular redox state, or balance between cellular oxidation and reduction reactions, serves as a vital antioxidant defence system that is linked to all important cellular activities. Redox regulation is therefore a fundamental cellular process for aerobic organisms. Whilst oxidative stress is well described in neurodegenerative disorders including amyotrophic lateral sclerosis (ALS), other aspects of redox dysfunction and their contributions to pathophysiology are only just emerging. ALS is a fatal neurodegenerative disease affecting motor neurons, with few useful treatments. Hence there is an urgent need to develop more effective therapeutics in the future. Here, we discuss the increasing evidence for redox dysregulation as an important and primary contributor to ALS pathogenesis, which is associated with multiple disease mechanisms. Understanding the connection between redox homeostasis, proteins that mediate redox regulation, and disease pathophysiology in ALS, may facilitate a better understanding of disease mechanisms, and lead to the design of better therapeutic strategies.

Protein disulfide isomerase ERp57 protects early muscle denervation in experimental ALS

Amyotrophic lateral sclerosis (ALS) is a progressive fatal neurodegenerative disease that affects motoneurons. Mutations in superoxide dismutase 1 (SOD1) have been described as a causative genetic factor for ALS. Mice overexpressing ALS-linked mutant SOD1 develop ALS symptoms accompanied by histopathological alterations and protein aggregation. The protein disulfide isomerase family member ERp57 is one of the main up-regulated proteins in tissue of ALS patients and mutant SOD1 mice, whereas point mutations in ERp57 were described as possible risk factors to develop the disease. ERp57 catalyzes disulfide bond formation and isomerization in the endoplasmic reticulum (ER), constituting a central component of protein quality control mechanisms. However, the actual contribution of ERp57 to ALS pathogenesis remained to be defined. Here, we studied the consequences of overexpressing ERp57 in experimental ALS using mutant SOD1 mice. Double transgenic SOD1G93A/ERp57WT animals presented delayed deterioration of electrophysiological activity and maintained muscle innervation compared to single transgenic SOD1G93A littermates at early-symptomatic stage, along with improved motor performance without affecting survival. The overexpression of ERp57 reduced mutant SOD1 aggregation, but only at disease end-stage, dissociating its role as an anti-aggregation factor from the protection of neuromuscular junctions. Instead, proteomic analysis revealed that the neuroprotective effects of ERp57 overexpression correlated with increased levels of synaptic and actin cytoskeleton proteins in the spinal cord. Taken together, our results suggest that ERp57 operates as a disease modifier at early stages by maintaining motoneuron connectivity.

Genetic screening of ANXA11 revealed novel mutations linked to amyotrophic lateral sclerosis

ANXA11 mutations have previously been discovered in amyotrophic lateral sclerosis (ALS) motor neuron disease. To confirm the contribution of ANXA11 mutations to ALS, a large exome data set obtained from 330 French patients, including 150 familial ALS index cases and 180 sporadic ALS cases, was analyzed, leading to the identification of 3 rare ANXA11 variants in 5 patients. The novel p.L254V variant was associated with early onset sporadic ALS. The novel p.D40Y mutation and the p.G38R variant concerned patients with predominant pyramidal tract involvement and cognitive decline. Neuropathologic findings in a p.G38R carrier associated the presence of ALS typical inclusions within the spinal cord, massive degeneration of the lateral tracts, and type A frontotemporal lobar degeneration. This mutant form of annexin A11 accumulated in various brain regions and in spinal cord motor neurons, although its stability was decreased in patients' lymphoblasts. Because most ANXA11 inclusions were not colocalized with transactive response DNA-binding protein 43 or p62 deposits, ANXA11 aggregation does not seem mandatory to trigger neurodegeneration with additional participants/partner proteins that could intervene.

Does wild-type Cu/Zn-superoxide dismutase have pathogenic roles in amyotrophic lateral sclerosis?

Amyotrophic lateral sclerosis (ALS) is characterized by adult-onset progressive degeneration of upper and lower motor neurons. Increasing numbers of genes are found to be associated with ALS; among those, the first identified gene, SOD1 coding a Cu/Zn-superoxide dismutase protein (SOD1), has been regarded as the gold standard in the research on a pathomechanism of ALS. Abnormal accumulation of misfolded SOD1 in affected spinal motor neurons has been established as a pathological hallmark of ALS caused by mutations in SOD1 (SOD1-ALS). Nonetheless, involvement of wild-type SOD1 remains quite controversial in the pathology of ALS with no SOD1 mutations (non-SOD1 ALS), which occupies more than 90% of total ALS cases. In vitro studies have revealed post-translationally controlled misfolding and aggregation of wild-type as well as of mutant SOD1 proteins; therefore, SOD1 proteins could be a therapeutic target not only in SOD1-ALS but also in more prevailing cases, non-SOD1 ALS. In order to search for evidence on misfolding and aggregation of wild-type SOD1 in vivo, we reviewed pathological studies using mouse models and patients and then summarized arguments for and against possible involvement of wild-type SOD1 in non-SOD1 ALS as well as in SOD1-ALS.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

The Cysteine (Cys) Residues Cys-6 and Cys-111 in Mutant Superoxide Dismutase 1 (SOD1) A4V Are Required for Induction of Endoplasmic Reticulum Stress in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the degeneration of motor neurons. Between 12 and 20% of inherited cases and approximately 1-2% of all cases are caused by mutations in the gene encoding dismutase 1 (SOD1). Mutant SOD1 A4V (alanine to valine) induces endoplasmic reticulum (ER) stress, which is increasingly implicated as a pathway to motor neuron degeneration and death in ALS. However, it remains unclear how ER stress is induced by mutant SOD1 A4V. Previous studies have established that it is induced early in pathophysiology and it precedes the formation of mutant SOD1 inclusions. SOD1 contains four cysteine residues, two of which form an intra-subunit disulphide bond involving Cys-57 and Cys-146. The remaining two cysteines, Cys-6 and Cys-111, remain unpaired and have been implicated in mutant SOD1 aggregation. In this study, we examined the relationship between the SOD1 A4V cysteine residues and aggregation, ER stress induction and toxicity. We report here that mutation of Cys-6 and Cys-111 in mutant SOD1 A4V, but not Cys-57 or Cys-146, ameliorates ER stress, inclusion formation and apoptosis in neuronal cell lines. These results imply that protein misfolding, induced by Cys-6 and Cys-111, is required for these pathological events in neuronal cells.

Oxidative misfolding of Cu/Zn-superoxide dismutase triggered by non-canonical intramolecular disulfide formation

Misfolded Cu/Zn-superoxide dismutase (SOD1) is a pathological species in a subset of amyotrophic lateral sclerosis (ALS). Oxidative stress is known to increase in affected spinal cords of ALS and is thus considered to cause damages on SOD1 leading to the misfolding and aggregation. Despite this, it still remains elusive what triggers misfolding of SOD1 under oxidizing environment. Here, we show that a thiol group of Cys111 in SOD1 is oxidized to a sulfenic acid with hydrogen peroxide and reveal that further dissociation of the bound metal ions from the oxidized SOD1 allows another free Cys residue (Cys6) to nucleophilically attack the sulfenylated Cys111. As a result, an intra-molecular disulfide bond forms between Cys6 and Cys111. Such an abnormal SOD1 with the non-canonical disulfide bond was conformationally extended with significant cytotoxicity as well as high propensity to aggregate. Taken together, we propose a new model of SOD1 misfolding under oxidizing environment, in which formation of the non-canonical intramolecular disulfide bond plays a pivotal role.

The biophysics of superoxide dismutase-1 and amyotrophic lateral sclerosis

Few proteins have come under such intense scrutiny as superoxide dismutase-1 (SOD1). For almost a century, scientists have dissected its form, function and then later its malfunction in the neurodegenerative disease amyotrophic lateral sclerosis (ALS). We now know SOD1 is a zinc and copper metalloenzyme that clears superoxide as part of our antioxidant defence and respiratory regulation systems. The possibility of reduced structural integrity was suggested by the first crystal structures of human SOD1 even before deleterious mutations in the sod1 gene were linked to the ALS. This concept evolved in the intervening years as an impressive array of biophysical studies examined the characteristics of mutant SOD1 in great detail. We now recognise how ALS-related mutations perturb the SOD1 maturation processes, reduce its ability to fold and reduce its thermal stability and half-life. Mutant SOD1 is therefore predisposed to monomerisation, non-canonical self-interactions, the formation of small misfolded oligomers and ultimately accumulation in the tell-tale insoluble inclusions found within the neurons of ALS patients. We have also seen that several post-translational modifications could push wild-type SOD1 down this toxic pathway. Recently we have come to view ALS as a prion-like disease where both the symptoms, and indeed SOD1 misfolding itself, are transmitted to neighbouring cells. This raises the possibility of intervention after the initial disease presentation. Several small-molecule and biologic-based strategies have been devised which directly target the SOD1 molecule to change the behaviour thought to be responsible for ALS. Here we provide a comprehensive review of the many biophysical advances that sculpted our view of SOD1 biology and the recent work that aims to apply this knowledge for therapeutic outcomes in ALS.

Effects of gem-dihydroperoxides against mutant copper‑zinc superoxide dismutase-mediated neurotoxicity

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by progressive muscle weakness, paralysis, and death. Although its neuropathology is well investigated, currently, effective treatments are unavailable. The mechanism of ALS involves the aggregation and accumulation of several mutant proteins, including mutant copper‑zinc superoxide dismutase (SOD1), TAR DNA binding protein 43 kDa (TDP-43) and fused in sarcoma (FUS) proteins. Previous reports have shown that excessive oxidative stress, associated with mitochondrial dysfunction and mutant protein accumulation, contributes to ALS pathology. The present study focuses on the promotion of SOD1 misfolding and aggregation by oxidative stress. Having recently synthesized novel organic gem-dihydroperoxides (DHPs) with high anti-oxidant activity, we now examined whether DHPs reduce the mutant SOD1-induced intracellular aggregates involved in oxidative stress. We found that, among DHPs, 12AC2O significantly inhibited mutant SOD1-induced cell death and reduced the intracellular mutant SOD1 aggregates. Moreover, immunofluorescence staining with redox-sensitive dyes showed that 12AC2O reduced the excessive level of intracellular mutant SOD1-induced reactive oxygen species (ROS). Additionally, ESR analysis showed that 12AC2O exerts a direct scavenging effect against the hydroxyl radical (OH) and the superoxide anion (O_2-). These results suggest that 12AC2O is a very useful agent in combination with other agents against ALS.

Endoplasmic reticulum stress leads to accumulation of wild-type SOD1 aggregates associated with sporadic amyotrophic lateral sclerosis

Abnormal modifications to mutant superoxide dismutase 1 (SOD1) are linked to familial amyotrophic lateral sclerosis (fALS). Misfolding of wild-type SOD1 (SOD1WT) is also observed in postmortem tissue of a subset of sporadic ALS (sALS) cases, but cellular and molecular mechanisms generating abnormal SOD1WT species are unknown. We analyzed aberrant human SOD1WT species over the lifetime of transgenic mice and found the accumulation of disulfide-cross-linked high-molecular-weight SOD1WT aggregates during aging. Subcellular fractionation of spinal cord tissue and protein overexpression in NSC-34 motoneuron-like cells revealed that endoplasmic reticulum (ER) localization favors oxidation and disulfide-dependent aggregation of SOD1WT We established a pharmacological paradigm of chronic ER stress in vivo, which recapitulated SOD1WTaggregation in young transgenic mice. These species were soluble in nondenaturing detergents and did not react with a SOD1 conformation-specific antibody. Interestingly, SOD1WT aggregation under ER stress correlated with astrocyte activation in the spinal cord of transgenic mice. Finally, the disulfide-cross-linked SOD1WT species were also found augmented in spinal cord tissue of sALS patients, correlating with the presence of ER stress markers. Overall, this study suggests that ER stress increases the susceptibility of SOD1WT to aggregate during aging, operating as a possible risk factor for developing ALS.

Targeting of Disordered Proteins by Small Molecules in Neurodegenerative Diseases

The formation of protein aggregates and inclusions in the brain and spinal cord is a common neuropathological feature of a number of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and many others. These are commonly referred as neurodegenerative proteinopathies or protein-misfolding diseases. The main characteristic of protein aggregates in these disorders is the fact that they are enriched in amyloid fibrils. Since protein aggregation is considered to play a central role for the onset of neurodegenerative proteinopathies, research is ongoing to develop strategies aimed at preventing or removing protein aggregation in the brain of affected patients. Numerous studies have shown that small molecule-based approaches may be potentially the most promising for halting protein aggregation in neurodegenerative diseases. Indeed, several of these compounds have been found to interact with intrinsically disordered proteins and promote their clearing in experimental models. This notwithstanding, at present small molecule inhibitors still awaits achievements for clinical translation. Hopefully, if we determine whether the formation of insoluble inclusions is effectively neurotoxic and find a valid biomarker to assess their protein aggregation-inhibitory activity in the human central nervous system, the use of small molecule inhibitors will be considered as a cure for neurodegenerative protein-misfolding diseases.

The cerebrospinal fluid values of advanced oxidation protein products and total thiol content in patients with amyotrophic lateral sclerosis

OBJECTIVES

Amyotrophic lateral sclerosis (ALS) is recognized as a progressive neurodegenerative disorder of unknown origin. Oxidative stress (OS) is considered as one of the most challenging hypothesis in the disease pathogenesis. The aim of this study was to contribute to the understanding of what extent there is involvement of OS in ALS.

PATIENTS AND METHODS

We assessed Advanced Oxidation Protein Products (AOPP) and total thiol (-SH) groups in cerebrospinal fluid (CSF) of 24 ALS patients (13 of them presented with spinal form while 11 patients had bulbar form) and 20 controls (CG).

RESULTS

The obtained AOPP levels in ALS patients were higher than those in CG (p <0.001), while -SH groups showed lower values compared to CG (p<0.001). The AOPP values were higher in ALS patients with bulbar compared with ALS patients with common spinal manifestation (p<0.001). There were no differences in -SH group's levels among these different clinical forms (p>0.05). The negative correlation between AOPP and the levels of -SH groups was confirmed (p <0.01). Significant mild correlations between tested parameters and functional rating scale as well as disease progression index were recorded for both of tested parameters in spinal form of ALS (p<0.01).

CONCLUSION

The data presented here clearly support the fact that OS is involved in patophysiology of ALS, where oxidation of -SH groups represents an important aspect of protein oxidation. The CSF AOPP level and -SH groups may serve as potential useful biomarker for functional disorder and progression of the disease in the spinal form of ALS.

Cu/Zn Superoxide Dismutase Forms Amyloid Fibrils under Near-Physiological Quiescent Conditions: The Roles of Disulfide Bonds and Effects of Denaturant

Cu/Zn superoxide dismutase (SOD1) forms intracellular aggregates that are pathological indicators of amyotrophic lateral sclerosis. A large body of research indicates that the entry point to aggregate formation is a monomeric, metal-ion free (apo), and disulfide-reduced species. Fibril formation by SOD1 in vitro has typically been reported only for harsh solvent conditions or mechanical agitation. Here we show that monomeric apo-SOD1 in the disulfide-reduced state forms fibrillar aggregates under near-physiological quiescent conditions. Monomeric apo-SOD1 with an intact intramolecular disulfide bond is highly resistant to aggregation under the same conditions. A cysteine-free variant of SOD1 exhibits fibrillization behavior and fibril morphology identical to those of disulfide-reduced SOD1, firmly establishing that intermolecular disulfide bonds or intramolecular disulfide shuffling are not required for aggregation and fibril formation. The decreased lag time for fibril formation resulting from reduction of the intramolecular disulfide bond thus primarily reflects the decreased stability of the folded state relative to partially unfolded states, rather than an active role of free sulfhydryl groups in mediating aggregation. Addition of urea to increase the amount of fully unfolded SOD1 increases the lag time for fibril formation, indicating that the population of this species does not dominate over other factors in determining the onset of aggregation. Our results contrast with previous results obtained for agitated samples, in which case amyloid formation was accelerated by denaturant. We reconcile these observations by suggesting that denaturants destabilize monomeric and aggregated species to different extents and thus affect nucleation and growth.

Onset of disorder and protein aggregation due to oxidation-induced intermolecular disulfide bonds: case study of RRM2 domain from TDP-43

We have investigated the behavior of second RNA-recognition motif (RRM2) of neuropathological protein TDP43 under the effect of oxidative stress as modeled in vitro. Toward this end we have used the specially adapted version of H/D exchange experiment, NMR relaxation and diffusion measurements, dynamic light scattering, controlled proteolysis, gel electrophoresis, site-directed mutagenesis and microsecond MD simulations. Under oxidizing conditions RRM2 forms disulfide-bonded dimers that experience unfolding and then assemble into aggregate particles (APs). These particles are strongly disordered, highly inhomogeneous and susceptible to proteolysis; some of them withstand the dithiothreitol treatment. They can recruit/release monomeric RRM2 through thiol-disulfide exchange reactions. By using a combination of dynamic light scattering and NMR diffusion data we were able to approximate the size distribution function for the APs. The key to the observed aggregation behavior is the diminished ability of disulfide-bonded RRM2 dimers to refold and their increased propensity to misfold, which makes them vulnerable to large thermal fluctuations. The emerging picture provides detailed insight on how oxidative stress can contribute to neurodegenerative disease, with unfolding, aggregation, and proteolytic cleavage as different facets of the process.

The oxidized thiol proteome in aging and cataractous mouse and human lens revealed by ICAT labeling

Age‐related cataractogenesis is associated with disulfide‐linked high molecular weight (HMW) crystallin aggregates. We recently found that the lens crystallin disulfidome was evolutionarily conserved in human and glutathione‐depleted mouse (LEGSKO) cataracts and that it could be mimicked by oxidation in vitro (Mol. Cell Proteomics, 14, 3211‐23 (2015)). To obtain a comprehensive blueprint of the oxidized key regulatory and cytoskeletal proteins underlying cataractogenesis, we have now used the same approach to determine, in the same specimens, all the disulfide‐forming noncrystallin proteins identified by ICAT proteomics. Seventy‐four, 50, and 54 disulfide‐forming proteins were identified in the human and mouse cataracts and the in vitro oxidation model, respectively, of which 17 were common to all three groups. Enzymes with oxidized cysteine at critical sites include GAPDH (hGAPDH, Cys247), glutathione synthase (hGSS, Cys294), aldehyde dehydrogenase (hALDH1A1, Cys126 and Cys186), sorbitol dehydrogenase (hSORD, Cys140, Cys165, and Cys179), and PARK7 (hPARK7, Cys46 and Cys53). Extensive oxidation was also present in lens‐specific intermediate filament proteins, such as BFSP1 and BFSP12 (hBFSP1 and hBFSP12, Cys167, Cys65, and Cys326), vimentin (mVim, Cys328), and cytokeratins, as well as microfilament and microtubule filament proteins, such as tubulin and actins. While the biological impact of these modifications for lens physiology remains to be determined, many of these oxidation sites have already been associated with either impaired metabolism or cytoskeletal architecture, strongly suggesting that they have a pathogenic role in cataractogenesis. By extrapolation, these findings may be of broader significance for age‐ and disease‐related dysfunctions associated with oxidant stress.

Production of Human Cu,Zn SOD with Higher Activity and Lower Toxicity in E. coli via Mutation of Free Cysteine Residues

Although, as an antioxidant enzyme, human Cu,Zn superoxide dismutase 1 (hSOD1) can mitigate damage to cell components caused by free radicals generated by aerobic metabolism, large-scale manufacturing and clinical use of hSOD1 are still limited by the challenge of rapid and inexpensive production of high-quality eukaryotic hSOD1 in recombinant forms. We have demonstrated previously that it is a promising strategy to increase the expression levels of soluble hSOD1 so as to increase hSOD1 yields in E. coli. In this study, a wild-type hSOD1 (wtSOD1) and three mutant SOD1s (mhSOD1s), in which free cysteines were substituted with serine, were constructed and their expression in soluble form was measured. Results show that the substitution of Cys111 (mhSOD1/C111S) increased the expression of soluble hSOD1 in E. coli whereas substitution of the internal Cys6 (mhSOD1/C6S) decreased it. Besides, raised levels of soluble expression led to an increase in hSOD1 yields. In addition, mhSOD1/C111S expressed at a higher soluble level showed lower toxicity and stronger whitening and antiradiation activities than those of wtSOD1. Taken together, our data demonstrate that C111S mutation in hSOD1 is an effective strategy to develop new SOD1-associated reagents and that mhSOD1/C111S is a satisfactory candidate for large-scale production.

A misfolded dimer of Cu/Zn-superoxide dismutase leading to pathological oligomerization in amyotrophic lateral sclerosis

Misfolding of mutant Cu/Zn-superoxide dismutase (SOD1) is a pathological hallmark in a familial form of amyotrophic lateral sclerosis. Pathogenic mutations have been proposed to monomerize SOD1 normally adopting a homodimeric configuration and then trigger abnormal oligomerization of SOD1 proteins. Despite this, a misfolded conformation of SOD1 leading to the oligomerization at physiological conditions still remains ambiguous. Here, we show that, around the body temperature (∼37°C), mutant SOD1 maintains a dimeric configuration but lacks most of its secondary structures. Also, such an abnormal SOD1 dimer with significant structural disorder was prone to irreversibly forming the oligomers crosslinked via disulfide bonds. The disulfide-crosslinked oligomers of SOD1 were detected in the spinal cords of the diseased mice expressing mutant SOD1. We hence propose an alternative pathway of mutant SOD1 misfolding that is responsible for oligomerization in the pathologies of the disease.

S-acylation of SOD1, CCS, and a stable SOD1-CCS heterodimer in human spinal cords from ALS and non-ALS subjects

Previously, we found that human Cu, Zn-superoxide dismutase (SOD1) is S-acylated (palmitoylated) in vitro and in amyotrophic lateral sclerosis (ALS) mouse models, and that S-acylation increased for ALS-causing SOD1 mutants relative to wild type. Here, we use the acyl resin-assisted capture (acyl-RAC) assay to demonstrate S-acylation of SOD1 in human post-mortem spinal cord homogenates from ALS and non-ALS subjects. Acyl-RAC further revealed that endogenous copper chaperone for SOD1 (CCS) is S-acylated in both human and mouse spinal cords, and in vitro in HEK293 cells. SOD1 and CCS formed a highly stable heterodimer in human spinal cord homogenates that was resistant to dissociation by boiling, denaturants, or reducing agents and was not observed in vitro unless both SOD1 and CCS were overexpressed. Cysteine mutations that attenuate SOD1 maturation prevented the SOD1-CCS heterodimer formation. The degree of S-acylation was highest for SOD1-CCS heterodimers, intermediate for CCS monomers, and lowest for SOD1 monomers. Given that S-acylation facilitates anchoring of soluble proteins to cell membranes, our findings suggest that S-acylation and membrane localization may play an important role in CCS-mediated SOD1 maturation. Furthermore, the highly stable S-acylated SOD1-CCS heterodimer may serve as a long-lived maturation intermediate in human spinal cord.

Immunochemical characterization on pathological oligomers of mutant Cu/Zn-superoxide dismutase in amyotrophic lateral sclerosis

Background

Dominant mutations in Cu/Zn-superoxide dismutase (SOD1) gene cause a familial form of amyotrophic lateral sclerosis (SOD1-ALS) with accumulation of misfolded SOD1 proteins as intracellular inclusions in spinal motor neurons. Oligomerization of SOD1 via abnormal disulfide crosslinks has been proposed as one of the misfolding pathways occurring in mutant SOD1; however, the pathological relevance of such oligomerization in the SOD1-ALS cases still remains obscure.

Methods

We prepared antibodies exclusively recognizing the SOD1 oligomers cross-linked via disulfide bonds in vitro. By using those antibodies, immunohistochemical examination and ELISA were mainly performed on the tissue samples of transgenic mice expressing mutant SOD1 proteins and also of human SOD1-ALS cases.

Results

We showed the recognition specificity of our antibodies exclusively toward the disulfide-crosslinked SOD1 oligomers by ELISA using various forms of purified SOD1 proteins in conformationally distinct states in vitro. Furthermore, the epitope of those antibodies was buried and inaccessible in the natively folded structure of SOD1. The antibodies were then found to specifically detect the pathological SOD1 species in the spinal motor neurons of the SOD1-ALS patients as well as the transgenic model mice.

Conclusions

Our findings here suggest that the SOD1 oligomerization through the disulfide-crosslinking associates with exposure of the SOD1 structural interior and is a pathological process occurring in the SOD1-ALS cases.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0145-9) contains supplementary material, which is available to authorized users.

Screening of Drugs Inhibiting In vitro Oligomerization of Cu/Zn-Superoxide Dismutase with a Mutation Causing Amyotrophic Lateral Sclerosis

Dominant mutations in Cu/Zn-superoxide dismutase (SOD1) gene have been shown to cause a familial form of amyotrophic lateral sclerosis (SOD1-ALS). A major pathological hallmark of this disease is abnormal accumulation of mutant SOD1 oligomers in the affected spinal motor neurons. While no effective therapeutics for SOD1-ALS is currently available, SOD1 oligomerization will be a good target for developing cures of this disease. Recently, we have reproduced the formation of SOD1 oligomers abnormally cross-linked via disulfide bonds in a test tube. Using our in vitro model of SOD1 oligomerization, therefore, we screened 640 FDA-approved drugs for inhibiting the oligomerization of SOD1 proteins, and three effective classes of chemical compounds were identified. Those hit compounds will provide valuable information on the chemical structures for developing a novel drug candidate suppressing the abnormal oligomerization of mutant SOD1 and possibly curing the disease.

Cellular Redox Systems Impact the Aggregation of Cu,Zn Superoxide Dismutase Linked to Familial Amyotrophic Lateral Sclerosis

Protein misfolding is implicated in neurodegenerative diseases such as ALS, where mutations of superoxide dismutase 1 (SOD1) account for about 20% of the inherited mutations. Human SOD1 (hSOD1) contains four cysteines, including Cys(57) and Cys(146), which have been linked to protein stability and folding via forming a disulfide bond, and Cys(6) and Cys(111) as free thiols. But the roles of the cellular oxidation-reduction (redox) environment in SOD1 folding and aggregation are not well understood. Here we explore the effects of cellular redox systems on the aggregation of hSOD1 proteins. We found that the known hSOD1 mutations G93A and A4V increased the capability of the thioredoxin and glutaredoxin systems to reduce hSOD1 compared with wild-type hSOD1. Treatment with inhibitors of these redox systems resulted in an increase of hSOD1 aggregates in the cytoplasm of cells transfected with mutants but not in cells transfected with wild-type hSOD1 or those containing a secondary C111G mutation. This aggregation may be coupled to changes in the redox state of the G93A and A4V mutants upon mild oxidative stress. These results strongly suggest that the thioredoxin and glutaredoxin systems are the key regulators for hSOD1 aggregation and may play critical roles in the pathogenesis of ALS.