Misfolded SOD1 and ALS: zeroing in on mitochondria

CRISPR/Cas9 system and its applications in nervous system diseases

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system is an acquired immune system of many bacteria and archaea, comprising CRISPR loci, Cas genes, and its associated proteins. This system can recognize exogenous DNA and utilize the Cas9 protein's nuclease activity to break DNA double-strand and to achieve base insertion or deletion by subsequent DNA repair. In recent years, multiple laboratory and clinical studies have revealed the therapeutic role of the CRISPR/Cas9 system in neurological diseases. This article reviews the CRISPR/Cas9-mediated gene editing technology and its potential for clinical application against neurological diseases.

Methodological advances and strategies for high resolution structure determination of cellular protein aggregates

Aggregation of proteins is at the nexus of molecular processes crucial to aging, disease, and employing proteins for biotechnology and medical applications. There has been much recent progress in determining the structural features of protein aggregates that form in cells; yet, owing to prevalent heterogeneity in aggregation, many aspects remain obscure and often experimentally intractable to define. Here, we review recent results of structural studies for cell-derived aggregates of normally globular proteins, with a focus on high-resolution methods for their analysis and prediction. Complementary results obtained by solid-state NMR spectroscopy, FTIR spectroscopy and microspectroscopy, cryo-EM, and amide hydrogen/deuterium exchange measured by NMR and mass spectrometry, applied to bacterial inclusion bodies and disease inclusions, are uncovering novel information on in-cell aggregation patterns as well as great diversity in the structural features of useful and aberrant protein aggregates. Using these advances as a guide, this review aims to advise the reader on which combination of approaches may be the most appropriate to apply to their unique system.

Ceramide and Sphingosine-1-Phosphate in Neurodegenerative Disorders and Their Potential Involvement in Therapy

Neurodegenerative disorders (ND) are progressive diseases of the nervous system, often without resolutive therapy. They are characterized by a progressive impairment and loss of specific brain regions and neuronal populations. Cellular and animal model studies have identified several molecular mechanisms that play an important role in the pathogenesis of ND. Among them are alterations of lipids, in particular sphingolipids, that play a crucial role in neurodegeneration. Overall, during ND, ceramide-dependent pro-apoptotic signalling is promoted, whereas levels of the neuroprotective spingosine-1-phosphate are reduced. Moreover, ND are characterized by alterations of the metabolism of complex sphingolipids. The finding that altered sphingolipid metabolism has a role in ND suggests that its modulation might provide a useful strategy to identify targets for possible therapies. In this review, based on the current literature, we will discuss how bioactive sphingolipids (spingosine-1-phosphate and ceramide) are involved in some ND (Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis) and their possible involvement in therapies.

Gene Therapy in Amyotrophic Lateral Sclerosis

Since the discovery of Cu/Zn superoxide dismutase (SOD1) gene mutation, in 1993, as the first genetic abnormality in amyotrophic lateral sclerosis (ALS), over 50 genes have been identified as either cause or modifier in ALS and ALS/frontotemporal dementia (FTD) spectrum disease. Mutations in C9orf72, SOD1, TAR DNA binding protein 43 (TARDBP), and fused in sarcoma (FUS) genes are the four most common ones. During the last three decades, tremendous effort has been made worldwide to reveal biological pathways underlying the pathogenesis of these gene mutations in ALS/FTD. Accordingly, targeting etiologic genes (i.e., gene therapies) to suppress their toxic effects have been investigated widely. It includes four major strategies: (i) removal or inhibition of abnormal transcribed RNA using microRNA or antisense oligonucleotides (ASOs), (ii) degradation of abnormal mRNA using RNA interference (RNAi), (iii) decrease or inhibition of mutant proteins (e.g., using antibodies against misfolded proteins), and (iv) DNA genome editing with methods such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (CRISPR/Cas). The promising results of these studies have led to the application of some of these strategies into ALS clinical trials, especially for C9orf72 and SOD1. In this paper, we will overview advances in gene therapy in ALS/FTD, focusing on C9orf72, SOD1, TARDBP, and FUS genes.

Misfolding-Associated Exposure of Natively Buried Residues in Mutant SOD1 Facilitates Binding to TRAF6

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease primarily impacting motor neurons. Mutations in superoxide dismutase 1 (SOD1) are the second most common cause of familial ALS. Several of these mutations lead to misfolding or toxic gain of function in the SOD1 protein. Recently, we reported that misfolded SOD1 interacts with TNF receptor-associated factor 6 (TRAF6) in the SOD1^G93A rat model of ALS. Further, we showed in cultured cells that several mutant SOD1 proteins, but not wildtype SOD1 protein, interact with TRAF6 via the MATH domain. Here, we sought to uncover the structural details of this interaction through molecular dynamics (MD) simulations of a dimeric model system, coarse grained using the AWSEM force field. We used direct MD simulations to identify buried residues, and predict binding poses by clustering frames from the trajectories. Metadynamics simulations were also used to deduce preferred binding regions on the protein surfaces from the potential of the mean force in orientation space. Well-folded SOD1 was found to bind TRAF6 via co-option of its native homodimer interface. However, if loops IV and VII of SOD1 were disordered, as typically occurs in the absence of stabilizing Zn²⁺ ion binding, these disordered loops now participated in novel interactions with TRAF6. On TRAF6, multiple interaction hot-spots were distributed around the equatorial region of the MATH domain beta barrel. Expression of TRAF6 variants with mutations in this region in cultured cells demonstrated that TRAF6^T475 facilitates interaction with different SOD1 mutants. These findings contribute to our understanding of the disease mechanism and uncover potential targets for the development of therapeutics.

Accumulation of misfolded SOD1 outlines distinct patterns of motor neuron pathology and death during disease progression in a SOD1G93A mouse model of amyotrophic lateral sclerosis

Early misfolded superoxide dismutase 1 (mfSOD1) accumulation, motor neuron (MN) degeneration, and microgliosis are hallmark pathological features in SOD1^G93A amyotrophic lateral sclerosis (ALS) mice. Because of the different vulnerabilities of distinct MN subtypes, degenerating and surviving MNs coexist in different proportions during disease progression. By examining the expression of misfolded conformers of SOD1 using specific antibodies, we defined distinct MN phenotypes that were evaluated during disease progression and the local neuroinflammatory reaction. The most severe phenotype corresponded to somata of fast‐twitch subtype MNs, which exhibited highly positive mfSOD1 immunostaining and an extreme degree of vacuolar degeneration. Vacuoles, which are of mitochondrial origin, contain mfSOD1 in conjunction with nonmitochondrial proteins, such as chromogranin, CD81, and flotillin. The fusion of ER‐derived vesicles enriched in mfSOD1 with outer mitochondrial membranes is thought to be the primary mechanism for vacuole formation. In addition, the ulterior coalescence of enlarged mitochondria may lead to the formation of giant vacuoles. Vacuolar degeneration is a transient degenerative process occurring early during the presymptomatic stages of the disease in ALS mice. Some vacuolated MNs are also positive for pMLKL, the effector protein of necroptosis. This indicates a newly described mechanism in which extracellular vesicles derived from damaged MNs, via cellular secretion or necroptotic disruption, may be the triggers for initiating neuroinflammation, glial‐mediated neurotoxicity, and disease spreading. Furthermore, as MN degeneration in mutant SOD1 mice is noncell autonomous, the effects of experimentally increasing or decreasing the microglial response on the expression of MN phenotypes were also evaluated, demonstrating bidirectional cross talk signaling between the degree of expression of mfSOD1 and local neuroinflammation. More detailed knowledge regarding these processes occurring long before the end stages of the disease is necessary to identify novel molecular targets for future preclinical testing.

Neurodegenerative Disorders: Spotlight on Sphingolipids

Neurodegenerative diseases are incurable diseases of the nervous system that lead to a progressive loss of brain areas and neuronal subtypes, which is associated with an increase in symptoms that can be linked to the affected brain areas. The key findings that appear in many neurodegenerative diseases are deposits of proteins and the damage of mitochondria, which mainly affect energy production and mitophagy. Several causative gene mutations have been identified in various neurodegenerative diseases; however, a large proportion are considered sporadic. In the last decade, studies linking lipids, and in particular sphingolipids, to neurodegenerative diseases have shown the importance of these sphingolipids in the underlying pathogenesis. Sphingolipids are bioactive lipids consisting of a sphingoid base linked to a fatty acid and a hydrophilic head group. They are involved in various cellular processes, such as cell growth, apoptosis, and autophagy, and are an essential component of the brain. In this review, we will cover key findings that demonstrate the relevance of sphingolipids in neurodegenerative diseases and will focus on neurodegeneration with brain iron accumulation and Parkinson’s disease.

Progressive Mitochondrial SOD1G93A Accumulation Causes Severe Structural, Metabolic and Functional Aberrations through OPA1 Down-Regulation in a Mouse Model of Amyotrophic Lateral Sclerosis

In recent years, the “non-autonomous motor neuron death” hypothesis has become more consolidated behind amyotrophic lateral sclerosis (ALS). It postulates that cells other than motor neurons participate in the pathology. In fact, the involvement of the autonomic nervous system is fundamental since patients die of sudden death when they become unable to compensate for cardiorespiratory arrest. Mitochondria are thought to play a fundamental role in the physiopathology of ALS, as they are compromised in multiple ALS models in different cell types, and it also occurs in other neurodegenerative diseases. Our study aimed to uncover mitochondrial alterations in the sympathoadrenal system of a mouse model of ALS, from a structural, bioenergetic and functional perspective during disease instauration. We studied the adrenal chromaffin cell from mutant SOD1^G93A mouse at pre-symptomatic and symptomatic stages. The mitochondrial accumulation of the mutated SOD1^G93A protein and the down-regulation of optic atrophy protein-1 (OPA1) provoke mitochondrial ultrastructure alterations prior to the onset of clinical symptoms. These changes affect mitochondrial fusion dynamics, triggering mitochondrial maturation impairment and cristae swelling, with increased size of cristae junctions. The functional consequences are a loss of mitochondrial membrane potential and changes in the bioenergetics profile, with reduced maximal respiration and spare respiratory capacity of mitochondria, as well as enhanced production of reactive oxygen species. This study identifies mitochondrial dynamics regulator OPA1 as an interesting therapeutic target in ALS. Additionally, our findings in the adrenal medulla gland from presymptomatic stages highlight the relevance of sympathetic impairment in this disease. Specifically, we show new SOD1^G93A toxicity pathways affecting cellular energy metabolism in non-motor neurons, which offer a possible link between cell specific metabolic phenotype and the progression of ALS.

Improving mitochondria and ER stability helps eliminate upper motor neuron degeneration that occurs due to mSOD1 toxicity and TDP-43 pathology

BACKGROUND

Upper motor neurons (UMNs) are a key component of motor neuron circuitry. Their degeneration is a hallmark for diseases, such as hereditary spastic paraplegia (HSP), primary lateral sclerosis (PLS), and amyotrophic lateral sclerosis (ALS). Currently there are no preclinical assays investigating cellular responses of UMNs to compound treatment, even for diseases of the UMNs. The basis of UMN vulnerability is not fully understood, and no compound has yet been identified to improve the health of diseased UMNs: two major roadblocks for building effective treatment strategies.

METHODS

Novel UMN reporter models, in which UMNs that are diseased because of misfolded superoxide dismutase protein (mSOD1) toxicity and TDP-43 pathology are labeled with eGFP expression, allow direct assessment of UMN response to compound treatment. Electron microscopy reveals very precise aspects of endoplasmic reticulum (ER) and mitochondrial damage. Administration of NU-9, a compound initially identified based on its ability to reduce mSOD1 toxicity, has profound impact on improving the health and stability of UMNs, as identified by detailed cellular and ultrastructural analyses.

RESULTS

Problems with mitochondria and ER are conserved in diseased UMNs among different species. NU-9 has drug-like pharmacokinetic properties. It lacks toxicity and crosses the blood brain barrier. NU-9 improves the structural integrity of mitochondria and ER, reduces levels of mSOD1, stabilizes degenerating UMN apical dendrites, improves motor behavior measured by the hanging wire test, and eliminates ongoing degeneration of UMNs that become diseased both because of mSOD1 toxicity and TDP-43 pathology, two distinct and important overarching causes of motor neuron degeneration.

CONCLUSIONS

Mechanism-focused and cell-based drug discovery approaches not only addressed key cellular defects responsible for UMN loss, but also identified NU-9, the first compound to improve the health of diseased UMNs, neurons that degenerate in ALS, HSP, PLS, and ALS/FTLD patients.

Amyotrophic Lateral Sclerosis Genes in Drosophila melanogaster

Amyotrophic lateral sclerosis (ALS) is a devastating adult-onset neurodegenerative disease characterized by the progressive degeneration of upper and lower motoneurons. Most ALS cases are sporadic but approximately 10% of ALS cases are due to inherited mutations in identified genes. ALS-causing mutations were identified in over 30 genes with superoxide dismutase-1 (SOD1), chromosome 9 open reading frame 72 (C9orf72), fused in sarcoma (FUS), and TAR DNA-binding protein (TARDBP, encoding TDP-43) being the most frequent. In the last few decades, Drosophila melanogaster emerged as a versatile model for studying neurodegenerative diseases, including ALS. In this review, we describe the different Drosophila ALS models that have been successfully used to decipher the cellular and molecular pathways associated with SOD1, C9orf72, FUS, and TDP-43. The study of the known fruit fly orthologs of these ALS-related genes yielded significant insights into cellular mechanisms and physiological functions. Moreover, genetic screening in tissue-specific gain-of-function mutants that mimic ALS-associated phenotypes identified disease-modifying genes. Here, we propose a comprehensive review on the Drosophila research focused on four ALS-linked genes that has revealed novel pathogenic mechanisms and identified potential therapeutic targets for future therapy.

The Timing and Extent of Motor Neuron Vulnerability in ALS Correlates with Accumulation of Misfolded SOD1 Protein in the Cortex and in the Spinal Cord

Understanding the cellular and molecular basis of selective vulnerability has been challenging, especially for motor neuron diseases. Developing drugs that improve the health of neurons that display selective vulnerability relies on in vivo cell-based models and quantitative readout measures that translate to patient outcome. We initially developed and characterized UCHL1-eGFP mice, in which motor neurons are labeled with eGFP that is stable and long-lasting. By crossing UCHL1-eGFP to amyotrophic lateral sclerosis (ALS) disease models, we generated ALS mouse models with fluorescently labeled motor neurons. Their examination over time began to reveal the cellular basis of selective vulnerability even within the related motor neuron pools. Accumulation of misfolded SOD1 protein both in the corticospinal and spinal motor neurons over time correlated with the timing and extent of degeneration. This further proved simultaneous degeneration of both upper and lower motor neurons, and the requirement to consider both upper and lower motor neuron populations in drug discovery efforts. Demonstration of the direct correlation between misfolded SOD1 accumulation and motor neuron degeneration in both cortex and spinal cord is important for building cell-based assays in vivo. Our report sets the stage for shifting focus from mice to diseased neurons for drug discovery efforts, especially for motor neuron diseases.

TNF receptor-associated factor 6 interacts with ALS-linked misfolded superoxide dismutase 1 and promotes aggregation

Amyotrophic lateral sclerosis (ALS) is a fatal disease, characterized by the selective loss of motor neurons leading to paralysis. Mutations in the gene encoding superoxide dismutase 1 (SOD1) are the second most common cause of familial ALS, and considerable evidence suggests that these mutations result in an increase in toxicity due to protein misfolding. We previously demonstrated in the SOD1^G93A rat model that misfolded SOD1 exists as distinct conformers and forms deposits on mitochondrial subpopulations. Here, using SOD1^G93A rats and conformation-restricted antibodies specific for misfolded SOD1 (B8H10 and AMF7-63), we identified the interactomes of the mitochondrial pools of misfolded SOD1. This strategy identified binding proteins that uniquely interacted with either AMF7-63 or B8H10-reactive SOD1 conformers as well as a high proportion of interactors common to both conformers. Of this latter set, we identified the E3 ubiquitin ligase TNF receptor-associated factor 6 (TRAF6) as a SOD1 interactor, and we determined that exposure of the SOD1 functional loops facilitates this interaction. Of note, this conformational change was not universally fulfilled by all SOD1 variants and differentiated TRAF6 interacting from TRAF6 noninteracting SOD1 variants. Functionally, TRAF6 stimulated polyubiquitination and aggregation of the interacting SOD1 variants. TRAF6 E3 ubiquitin ligase activity was required for the former but was dispensable for the latter, indicating that TRAF6-mediated polyubiquitination and aggregation of the SOD1 variants are independent events. We propose that the interaction between misfolded SOD1 and TRAF6 may be relevant to the etiology of ALS.

In vitro evidence consistent with an interaction between wild-type and mutant SOD1 protein associated with canine degenerative myelopathy

Canine degenerative myelopathy (DM) is a progressive neurological disorder that may be considered to be a large animal model for specific forms of the fatal human disease, familial amyotrophic lateral sclerosis (fALS). DM is associated with a c118G>A mutation of the superoxide dismutase 1 (Sod1) gene, and a significant proportion of cases are inherited in an autosomal recessive manner in contrast to the largely, but not exclusively, dominant mode of inheritance in fALS. The consensus view is that these Sod1/SOD1 mutations result in a toxic gain of function but the mechanisms remain unclear. Here we used an in vitro neuroblastoma cell line transfection system to monitor wild-type and mutant forms of SOD1 fusion proteins containing either a Cherry or an enhanced green fluorescent protein (EGFP) tag. These fusion proteins retained SOD1 enzymatic activity on a native gel assay system. We demonstrate that SOD1 aggregate density is significantly higher in DM transfectants compared to wild-type. In addition, we show by co-immunoprecipitation and confocal microscopy, evidence for a potential interaction between wild-type and mutant forms of SOD1 in co-transfected cells. While in vitro studies have shown SOD1 heterodimer formation in fALS models, this is the first report for DM SOD1. Therefore, despite for the majority of cases there is a difference in the mode of inheritance between fALS and DM, a similar interaction between wild-type and mutant SOD1 forms can occur. Clarifying the role of SOD1 in DM may also be of benefit to understanding the role of SOD1 in fALS.

The Protective Effects of Levetiracetam on a Human iPSCs-Derived Spinal Muscular Atrophy Model

Spinal muscular atrophy (SMA) is an inherited disease characterized by progressive motor neuron death and subsequent muscle weakness and is caused by deletion or mutation of survival motor neuron (SMN) 1 gene. Protecting spinal motor neuron is an effective clinical strategy for SMA. The purpose of this study was to investigate the potential effect of an anti-epileptic drug levetiracetam on SMA. In the present study, we used differentiated spinal motor neurons (MNs) from SMA patient-derived induced pluripotent stem cells (SMA-iPSCs) to investigate the effect of levetiracetam. Levetiracetam promoted neurite elongation in SMA-iPSCs-MNs. TUNEL-positive spinal motor neurons were significantly reduced by levetiracetam in SMA-iPSCs-MNs. In addition, the expression level of cleaved-caspase 3 was decreased by levetiracetam in SMA-iPSCs-MNs. Furthermore, levetiracetam improved impaired mitochondrial function in SMA-iPSCs-MNs. On the other hand, levetiracetam did not affect the expression level of SMN protein in SMA-iPSCs-MNs. These findings indicate that levetiracetam has a neuroprotective effect for SMA.

Misfolded SOD1 Accumulation and Mitochondrial Association Contribute to the Selective Vulnerability of Motor Neurons in Familial ALS: Correlation to Human Disease

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder, with a 10% genetic linkage, of which 20% of these cases may be attributed to mutations in superoxide dismutase (SOD1). Specific mutations in SOD1 have been associated with disease duration, which can be highly variable ranging from a life expectancy of 3 to beyond 10 years. SOD1 neurotoxicity has been attributed to aberrant accumulation of misfolded SOD1, which in its soluble form binds to intracellular organelles disrupting their function or forms insoluble toxic aggregates. To understand whether these biophysical properties of the mutant protein may influence disease onset and duration, we generated 19 point mutations in the SOD1 gene, based on available clinical data of disease onset and progression from patients. By overexpressing these mutants in motor-neuron-like NSC-34 cells, we demonstrate a variability in misfolding capacity between the different mutants with a correlation between the degree of protein misfolding and mutation severity. We also show a clear variation of the different SOD1 mutants to associate with mitochondrial-enriched fractions with a correlation between mutation severity and this association. In summary, these findings reveal a correlation between the accumulation of misfolded SOD1 species and their mitochondrial association with disease duration but not with disease onset, and they have implications for the potential therapeutic role of suppressing the accumulation of misfolded SOD1.

Misfolded SOD1 is not a primary component of sporadic ALS

A common feature of inherited and sporadic ALS is accumulation of abnormal proteinaceous inclusions in motor neurons and glia. SOD1 is the major protein component accumulating in patients with SOD1 mutations, as well as in mutant SOD1 mouse models. ALS-linked mutations of SOD1 have been shown to increase its propensity to misfold and/or aggregate. Antibodies specific for monomeric or misfolded SOD1 have detected misfolded SOD1 accumulating predominantly in spinal cord motor neurons of ALS patients with SOD1 mutations. We now use seven different conformationally sensitive antibodies to misfolded human SOD1 (including novel high affinity antibodies currently in pre-clinical development) coupled with immunohistochemistry, immunofluorescence and immunoprecipitation to test for the presence of misfolded SOD1 in high quality human autopsy samples. Whereas misfolded SOD1 is readily detectable in samples from patients with SOD1 mutations, it is below detection limits for all of our measures in spinal cord and cortex tissues from patients with sporadic or non-SOD1 inherited ALS. The absence of evidence for accumulated misfolded SOD1 supports a conclusion that SOD1 misfolding is not a primary component of sporadic ALS.

Neurodegeneration and NLRP3 inflammasome expression in the anterior thalamus of SOD1(G93A) ALS mice

Nowadays, amyotrophic lateral sclerosis (ALS) is considered as a multisystem disorder, characterized by a primary degeneration of motor neurons as well as neuropathological changes in non-motor regions. Neurodegeneration in subcortical areas, such as the thalamus, are believed to contribute to cognitive and behavioral abnormalities in ALS patients. In the present study, we investigated neurodegenerative changes including neuronal loss and glia pathology in the anterodorsal thalamic nucleus (AD) of SOD1(G93A) mice, a widely used animal model for ALS. We detected massive dendrite swelling and neuronal loss in SOD1(G93A) animals, which was accompanied by a mild gliosis. Furthermore, misfolded SOD1 protein and autophagy markers were accumulating in the AD. Since innate immunity and activation inflammasomes seem to play a crucial role in ALS, we examined protein expression of Nod-like receptor protein 3 (NLRP3), apoptosis-associated speck-like protein containing a caspase-1 recruitment domain (ASC) and the cytokine interleukin 1 beta (IL1β) in AD glial cells and neurons. NLRP3 and ASC were significantly up-regulated in the AD of SOD1(G93A) mice. Finally, co-localization studies revealed expression of NLRP3, ASC and IL1β in neurons. Our study yielded two main findings: (i) neurodegenerative changes already occur at an early symptomatic stage in the AD and (ii) increased inflammasome expression may contribute to neuronal cell death. In conclusion, neurodegeneration in the anterior thalamus may critically account for cognitive changes in ALS pathology.

ALS: Recent Developments from Genetics Studies

Amyotrophic lateral sclerosis (ALS) is a fatal disorder that is characterized by a progressive degeneration of the upper and lower motor neurons. Most cases appear to be sporadic, but 5-10 % of cases have a family history of the disease. High-throughput DNA sequencing and related genomic capture tools are methodological advances which have rapidly contributed to an acceleration in the discovery of genetic risk factors for both familial and sporadic ALS. It is interesting to note that as the number of ALS genes grows, many of the proteins they encode are in shared intracellular processes. This review will summarize some of the recent advances and gene discovery made in ALS.

New perspectives on amyotrophic lateral sclerosis: the role of glial cells at the neuromuscular junction

Amyotrophic lateral sclerosis (ALS) is a disease leading to the death of motor neurons (MNs). It is also recognized as a non-cell autonomous disease where glial cells in the CNS are involved in its pathogenesis and progression. However, although denervation of neuromuscular junctions (NMJs) represents an early and major event in ALS, the importance of glial cells at this synapse receives little attention. An interesting possibility is that altered relationships between glial cells and MNs in the spinal cord in ALS may also take place at the NMJ. Perisynaptic Schwann cells (PSCs), which are glial cells at the NMJ, show great morphological and functional adaptability to ensure NMJ stability, maintenance and repair. More specifically, PSCs change their properties according to the state of innervation. Hence, abnormal changes or lack of changes can have detrimental effects on NMJs in ALS. This review will provide an overview of known and hypothesized interactions between MN nerve terminals and PSCs at NMJs during development, aging and ALS-induced denervation. These neuron-PSC interactions may be crucial to the understanding of how degenerative changes begin and progress at NMJs in ALS, and represent a novel therapeutic target.

Drosophila expressing human SOD1 successfully recapitulates mitochondrial phenotypic features of familial amyotrophic lateral sclerosis

Mitochondrial pathology is a seminal pathogenic hallmark of familial amyotrophic lateral sclerosis (FALS) which is extensively manifested by human patients and mutant SOD1(G93A) mammalian models. Rodents expressing human FALS-associated mutations successfully mimic several human disease features; although they are not as amenable to genetic and therapeutic compound screenings as non-mammalian models. In this study, we report a newly generated and characterized Drosophila model that expresses human SOD1(G93A) in muscle fibers. Presence of SOD1(G93A) in thoracic muscles causes mitochondrial pathology and impairs normal motor behavior in these flies. Use of this new FALS-24B-SOD1(G93A) fly model holds promise for better understanding of the mitochondrial affectation process in FALS and for the discovery of novel therapeutic compounds able to reverse mitochondrial dysfunction in this fatal disease.

ALS-linked misfolded SOD1 species have divergent impacts on mitochondria

Approximately 20 % of familial Amyotrophic Lateral Sclerosis (ALS) is caused by mutations in superoxide dismutase (SOD1), which leads to misfolding of the SOD1 protein, resulting in a toxic gain of function. Several conformation-restricted antibodies have been generated that specifically recognize misfolded SOD1 protein, and have been used as therapeutics in pre-clinical models. Misfolded SOD1 selectively associates with spinal cord mitochondria in SOD1 rodent models. Using the SOD1(G93A) rat model, we find that SOD1 conformational specific antibodies AMF7-63 and DSE2-3H1 labeled a fibrillar network concentrated in the anterior horn; while A5C3, B8H10, C4F6 and D3H5 labeled motor neurons as well as puncta in the neuropil. There is a time-dependent accumulation of misfolded SOD1 at the surface of spinal cord mitochondria with AMF7-63-labeled mitochondria having increased volume in contrast to a mitochondrial subset labeled with B8H10. In spinal cord homogenates and isolated mitochondria, AMF7-63, DSE2-3H1 and B8H10 detect misfolded SOD1 aggregates. SOD1 that lacks its metal cofactors has an increased affinity for naïve mitochondria and misfolded SOD1 antibodies B8H10 and DSE2-3H1 readily detect demetalated mutant and wild-type SOD1. Together, these data suggest that multiple non-native species of misfolded SOD1 may exist, some of which are associated with mitochondrial damage. Conformational antibodies are invaluable tools to identify and characterize the variation in misfolded SOD1 species with regards to biochemical characteristics and toxicity. This information is highly relevant to the further development of these reagents as therapeutics.

Adverse effects of a SOD1-peptide immunotherapy on SOD1 G93A mouse slow model of amyotrophic lateral sclerosis

Previous reports from our lab had shown that some anti-purinergic receptor P2X4 antibodies cross-reacted with misfolded forms of mutant Cu/Zn superoxide dismutase 1 (SOD1), linked to amyotrophic lateral sclerosis (ALS). Cross-reactivity could be caused by the abnormal exposure of an epitope located in the inner hydrophobic region of SOD1 that shared structural homology with the P2X4-immunizing peptide. We had previously raised antibodies against human SOD1 epitope mimicked by the P2X4 immunizing peptide. One of these antibodies, called AJ10, was able to recognize mutant/misfolded forms of ALS-linked mutant SOD1. Here, we used the AJ10 antigen as a vaccine to target neurotoxic species of mutant SOD1 in a slow mouse model of ALS. However, the obtained results showed no improvement in life span, disease onset or weight loss in treated animals; we observed an increased microglial neuroinflammatory response and high amounts of misfolded SOD1 accumulated within spinal cord neurons after AJ10 immunization. An increase of immunoglobulin G deposits was also found due to the treatment. Finally, a significantly worse clinical evolution was displayed by an impairment on motor function as a consequence of AJ10 peptide immunization.

The blood-epididymis barrier and inflammation

The blood-epididymis barrier (BEB) is a critical structure for male fertility. It enables the development of a specific luminal environment that allows spermatozoa to acquire both the ability to swim and fertilize an ovum. The presence of tight junctions and specific cellular transporters can regulate the composition of the epididymal lumen to favor proper sperm maturation. The BEB is also at the interface between the immune system and sperm. Not only does the BEB protect maturing spermatozoa from the immune system, it is also influenced by cytokines released during inflammation, which can result in the loss of barrier function. Such a loss is associated with an immune response, decreased sperm functions, and appears to be a contributing factor to post-testicular male infertility. Alterations in the BEB may be responsible for the formation of inflammatory conditions such as sperm granulomas. The present review summarizes current knowledge on the morphological, physiological and pathological components associated with the BEB, the role of immune function on the regulation of the BEB, and how disturbance of these factors can result in inflammatory lesions of the epididymis.

Many roads lead to Rome? Multiple modes of Cu,Zn superoxide dismutase destabilization, misfolding and aggregation in amyotrophic lateral sclerosis

ALS (amyotrophic lateral sclerosis) is a fatal neurodegenerative syndrome characterized by progressive paralysis and motor neuron death. Although the pathological mechanisms that cause ALS remain unclear, accumulating evidence supports that ALS is a protein misfolding disorder. Mutations in Cu,Zn-SOD1 (copper/zinc superoxide dismutase 1) are a common cause of familial ALS. They have complex effects on different forms of SOD1, but generally destabilize the protein and enhance various modes of misfolding and aggregation. In addition, there is some evidence that destabilized covalently modified wild-type SOD1 may be involved in disease. Among the multitude of misfolded/aggregated species observed for SOD1, multiple species may impair various cellular components at different disease stages. Newly developed antibodies that recognize different structural features of SOD1 represent a powerful tool for further unravelling the roles of different SOD1 structures in disease. Evidence for similar cellular targets of misfolded/aggregated proteins, loss of cellular proteostasis and cell–cell transmission of aggregates point to common pathological mechanisms between ALS and other misfolding diseases, such as Alzheimer's, Parkinson's and prion diseases, as well as serpinopathies. The recent progress in understanding the molecular basis for these devastating diseases provides numerous avenues for developing urgently needed therapeutics.

Altered thiol chemistry in human amyotrophic lateral sclerosis-linked mutants of superoxide dismutase 1

Neurodegenerative diseases share a common characteristic, the presence of intracellular or extracellular deposits of protein aggregates in nervous tissues. Amyotrophic Lateral Sclerosis (ALS) is a severe and fatal neurodegenerative disorder, which affects preferentially motoneurons. Changes in the redox state of superoxide dismutase 1 (SOD1) are associated with the onset and development of familial forms of ALS. In human SOD1 (hSOD1), a conserved disulfide bond and two free cysteine residues can engage in anomalous thiol/disulfide exchange resulting in non-native disulfides, a hallmark of ALS that is related to protein misfolding and aggregation. Because of the many competing reaction pathways, traditional bulk techniques fall short at quantifying individual thiol/disulfide exchange reactions. Here, we adapt recently developed single-bond chemistry techniques to study individual disulfide isomerization reactions in hSOD1. Mechanical unfolding of hSOD1 leads to the formation of a polypeptide loop held by the disulfide. This loop behaves as a molecular jump rope that brings reactive Cys-111 close to the disulfide. Using force-clamp spectroscopy, we monitor nucleophilic attack of Cys-111 at either sulfur of the disulfide and determine the selectivity of the reaction. Disease-causing mutations G93A and A4V show greatly altered reactivity patterns, which may contribute to the progression of familial ALS.

Deciphering genetic interactions between ALS genes using C. elegans

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder causing selective death of motor neurons in which it is speculated that 10% of cases have a familial history. In the past 20 years, many genes causative for ALS have been discovered, but the link between them and their roles in neurodegeneration remain unknown. The identification of genes associated with both ALS and frontotemporal dementia (FTD), along with the observation of patients affected by both diseases, have suggested that they are part of the same neurodegenerative spectrum. Investigating possible genetic interactions among ALS/FTD genes could help understand the role of these genes in neurodegeneration. To pursue this goal, our group has developed several ALS models to study potential genetic interactions. More recently, we characterized the deletion mutant alfa-1, the ortholog of C9ORF72, to evaluate the potential genetic interactions between C9ORF72/alfa-1 and other ALS genes. Here, we discuss the genetic interactions identified in our models and how some of these proteins may also be linked to other neurodegenerative disorders.

Worming forward: amyotrophic lateral sclerosis toxicity mechanisms and genetic interactions in Caenorhabditis elegans

Neurodegenerative diseases share pathogenic mechanisms at the cellular level including protein misfolding, excitotoxicity and altered RNA homeostasis among others. Recent advances have shown that the genetic causes underlying these pathologies overlap, hinting at the existence of a genetic network for neurodegeneration. This is perhaps best illustrated by the recent discoveries of causative mutations for amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD). Once thought to be distinct entities, it is now recognized that these diseases exist along a genetic spectrum. With this wealth of discoveries comes the need to develop new genetic models of ALS and FTD to investigate not only pathogenic mechanisms linked to causative mutations, but to uncover potential genetic interactions that may point to new therapeutic targets. Given the conservation of many disease genes across evolution, Caenorhabditis elegans is an ideal system to investigate genetic interactions amongst these genes. Here we review the use of C. elegans to model ALS and investigate a putative genetic network for ALS/FTD that may extend to other neurological disorders.

Immunodetection of disease-associated conformers of mutant cu/zn superoxide dismutase 1 selectively expressed in degenerating neurons in amyotrophic lateral sclerosis

We previously showed that some antipurinergic receptor P2X4 antibodies cross react with misfolded forms of amyotrophic lateral sclerosis (ALS)-linked mutant Cu/Zn superoxide dismutase (SOD1). Cross reactivity might be caused by abnormal exposure of an epitope in the inner hydrophobic region of SOD1 that shares structural homology with the P2X4-immunizing peptide. Here, we raised antibodies against the human SOD1 epitope mimicked by the P2X4 immunizing peptide. One of these antibodies, AJ10, is a recognized mutant/misfolded form of ALS-linked mutant SOD1. This was demonstrated in the hybrid motoneuron cell line NSC34 expressing enhanced green fluorescent protein-tagged G943A or A4V mutant SOD1. We also found AJ10 immunoreactivity to be selectively associated with degenerating neurons but not with glial cells in mice overexpressing either SOD1 or SOD1 mutants. Neurons with strongly positive AJ10 immunostaining were often associated with activated microglia displaying neuronophagic activity. AJ10-immunopositive SOD1 aggregates were also found in spinal cord tissue from a patient with a SOD1-linked familial ALS. AJ10-immunoreactive mutant SOD1 conformers were localized in large intracellular protein aggregates with a filamentous amyloid-like organization by ultrastructural immunolabeling and were also detected in neuronal organelles. These data are consistent with the ability of the AJ10 antibody to recognize misfolded conformations of SOD1 shared by different ALS-linked SOD1 mutations but not with the native protein. The neuronal mutant SOD1 conformers detected with AJ10 may promote neuroinflammation and may define a new epitope in SOD1 for ALS research.

Rodent models of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterised by the degeneration of upper and lower motor neurons. Recent advances in our understanding of some of the genetic causes of ALS, such as mutations in SOD1, TARDBP, FUS and VCP have led to the generation of rodent models of the disease, as a strategy to help our understanding of the pathophysiology of ALS and to assist in the development of therapeutic strategies. This review provides detailed descriptions of TDP-43, FUS and VCP models of ALS, and summarises potential therapeutics which have been recently trialled in rodent models of the disease. This article is part of a Special Issue entitled: Animal Models of Disease.

Enhancing mitochondrial calcium buffering capacity reduces aggregation of misfolded SOD1 and motor neuron cell death without extending survival in mouse models of inherited amyotrophic lateral sclerosis

Mitochondria have been proposed as targets for toxicity in amyotrophic lateral sclerosis (ALS), a progressive, fatal adult-onset neurodegenerative disorder characterized by the selective loss of motor neurons. A decrease in the capacity of spinal cord mitochondria to buffer calcium (Ca(2+)) has been observed in mice expressing ALS-linked mutants of SOD1 that develop motor neuron disease with many of the key pathological hallmarks seen in ALS patients. In mice expressing three different ALS-causing SOD1 mutants, we now test the contribution of the loss of mitochondrial Ca(2+)-buffering capacity to disease mechanism(s) by eliminating ubiquitous expression of cyclophilin D, a critical regulator of Ca(2+)-mediated opening of the mitochondrial permeability transition pore that determines mitochondrial Ca(2+) content. A chronic increase in mitochondrial buffering of Ca(2+) in the absence of cyclophilin D was maintained throughout disease course and was associated with improved mitochondrial ATP synthesis, reduced mitochondrial swelling, and retention of normal morphology. This was accompanied by an attenuation of glial activation, reduction in levels of misfolded SOD1 aggregates in the spinal cord, and a significant suppression of motor neuron death throughout disease. Despite this, muscle denervation, motor axon degeneration, and disease progression and survival were unaffected, thereby eliminating mutant SOD1-mediated loss of mitochondrial Ca(2+) buffering capacity, altered mitochondrial morphology, motor neuron death, and misfolded SOD1 aggregates, as primary contributors to disease mechanism for fatal paralysis in these models of familial ALS.

Redox regulation in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that results from the death of upper and lower motor neurons. Due to a lack of effective treatment, it is imperative to understand the underlying mechanisms and processes involved in disease progression. Regulations in cellular reduction/oxidation (redox) processes are being increasingly implicated in disease. Here we discuss the possible involvement of redox dysregulation in the pathophysiology of ALS, either as a cause of cellular abnormalities or a consequence. We focus on its possible role in oxidative stress, protein misfolding, glutamate excitotoxicity, lipid peroxidation and cholesterol esterification, mitochondrial dysfunction, impaired axonal transport and neurofilament aggregation, autophagic stress, and endoplasmic reticulum (ER) stress. We also speculate that an ER chaperone protein disulphide isomerase (PDI) could play a key role in this dysregulation. PDI is essential for normal protein folding by oxidation and reduction of disulphide bonds, and hence any disruption to this process may have consequences for motor neurons. Addressing the mechanism underlying redox regulation and dysregulation may therefore help to unravel the molecular mechanism involved in ALS.