Impaired extracellular secretion of mutant superoxide dismutase 1 associates with neurotoxicity in familial amyotrophic lateral sclerosis

Amyloidogenic regions in beta-strands II and III modulate the aggregation and toxicity of SOD1 in living cells

Mutations in the protein superoxide dismutase-1 (SOD1) promote its misfolding and aggregation, ultimately causing familial forms of the debilitating neurodegenerative disease amyotrophic lateral sclerosis (ALS). Currently, over 220 (mostly missense) ALS-causing mutations in the SOD1 protein have been identified, indicating that common structural features are responsible for aggregation and toxicity. Using in silico tools, we predicted amyloidogenic regions in the ALS-associated SOD1-G85R mutant, finding seven regions throughout the structure. Introduction of proline residues into β-strands II (I18P) or III (I35P) reduced the aggregation propensity and toxicity of SOD1-G85R in cells, significantly more so than proline mutations in other amyloidogenic regions. The I18P and I35P mutations also reduced the capability of SOD1-G85R to template onto previously formed non-proline mutant SOD1 aggregates as measured by fluorescence recovery after photobleaching. Finally, we found that, while the I18P and I35P mutants are less structurally stable than SOD1-G85R, the proline mutants are less aggregation-prone during proteasome inhibition, and less toxic to cells overall. Our research highlights the importance of a previously underappreciated SOD1 amyloidogenic region in β-strand II (15QGIINF20) to the aggregation and toxicity of SOD1 in ALS mutants, and suggests that β-strands II and III may be good targets for the development of SOD1-associated ALS therapies.

Human CD4+CD25+ T cells expressing a chimeric antigen receptor against aberrant superoxide dismutase 1 trigger antigen-specific immunomodulation

BACKGROUND AIMS

Amyotrophic lateral sclerosis (ALS) is a fatal disease associated with motor neuron degeneration, accumulation of aggregated misfolded proteins and neuroinflammation in motor regions of the central nervous system (CNS). Clinical trials using regulatory T cells (Tregs) are ongoing because of Tregs' immunomodulatory function, ability to traffic to the CNS, high numbers correlating with slower disease in ALS and disease-modifying activity in ALS mouse models. In the current study, a chimeric antigen receptor (CAR) was developed and characterized in human Tregs to enhance their immunomodulatory activity when in contact with an ALS protein aggregate.

METHODS

A CAR (DG05-28-3z) consisting of a human superoxide dismutase 1 (hSOD1)-binding single-chain variable fragment, CD28 hinge, transmembrane and co-stimulatory domain and CD3ζ signaling domain was created and expressed in human Tregs. Human Tregs were isolated by either magnetic enrichment for CD4+CD25hi cells (Enr-Tregs) or cell sorting for CD4+CD25hiCD127lo cells (FP-Tregs), transduced and expanded for 17 days.

RESULTS

The CAR bound preferentially to the ALS mutant G93A-hSOD1 protein relative to the wild-type hSOD1 protein. The CAR Tregs produced IL-10 when cultured with aggregated G93A-hSOD1 proteins or spinal cord explants from G93A-hSOD1 transgenic mice. Co-culturing DG05-28-3z CAR Tregs with human monocytes/macrophages inhibited production of tumor necrosis factor alpha and reactive oxygen species. Expanded FP-Tregs resulted in more robust Tregs compared with Enr-Tregs. FP-Tregs produced similar IL-10 and less interferon gamma, had lower Treg-specific demethylated region methylation and expressed higher FoxP3 and CD39.

CONCLUSIONS

Taken together, this study demonstrates that gene-modified Tregs can be developed to target an aggregated ALS-relevant protein to elicit CAR-mediated Treg effector functions and provides an approach for generating Treg therapies for ALS with the goal of enhanced disease site-specific immunomodulation.

Hepatoprotective Mechanisms Induced by Spinach Methanolic Extract in Rats with Hyperglycemia-An Immunohistochemical Analysis

Spinach methanolic extract (SME) has a hepatoprotective effect due to its polyphenolic antioxidants; however, its action in parenchymal (PQ) and non-parenchymal (nPQ) cells remains unknown. This study investigates the hepatoprotective effect of SME on streptozotocin-induced hyperglycemic rats (STZ), focusing on immunohistochemical analyses. Methods: The extract was prepared, and the total polyphenols and antioxidant activity were quantified. Adult male Wistar rats were divided into four groups (n = 8): normoglycemic rats (NG), STZ-induced hyperglycemic (STZ), STZ treated with 400 mg/kg SME (STZ-SME), and NG treated with SME (SME) for 12 weeks. Serum liver transaminases and lipid peroxidation levels in tissue were determined. The distribution pattern and relative levels of markers related to oxidative stress [reactive oxygen species (ROS), superoxide dismutase-1, catalase, and glutathione peroxidase-1], of cytoprotective molecules [nuclear NRF2 and heme oxygenase-1 (HO-1)], of inflammatory mediators [nuclear NF-κB, TNF-α], proliferation (PCNA), and of fibrogenesis markers [TGF-β, Smad2/3, MMP-9, and TIMP1] were evaluated. Results: SME had antioxidant capacity, and it lowered serum transaminase levels in STZ-SME compared to STZ. It reduced NOX4 staining, and lipid peroxidation levels were related to low formation of ROS. In STZ-SME, the immunostaining for antioxidant enzymes increased in nPQ cells compared to STZ. However, enzymes were also localized in extra and intracellular vesicles in STZ. Nuclear NRF2 staining and HO-1 expression in PQ and nPQ were higher in STZ-SME than in STZ. Inflammatory factors were decreased in STZ-SME and were related to the percentage decrease in NF-κB nuclear staining in nPQ cells. Similarly, TGF-β (in the sinusoids) and MMP-9 (in nPQ) were increased in the STZ-SME group compared to the other groups; however, staining for CTGF, TIMP1, and Smad2/3 was lower. Conclusions: SME treatment in hyperglycemic rats induced by STZ may have hepatoprotective properties due to its scavenger capacity and the regulation of differential expression of antioxidant enzymes between the PQ and nPQ cells, reducing inflammatory and fibrogenic biomarkers in liver tissue.

Insights on the Multifaceted Roles of Wild-Type and Mutated Superoxide Dismutase 1 in Amyotrophic Lateral Sclerosis Pathogenesis

Amyotrophic Lateral Sclerosis (ALS) is a progressive motor neurodegenerative disease. Cell damage in ALS is the result of many different, largely unknown, pathogenetic mechanisms. Astrocytes and microglial cells play a critical role also for their ability to enhance a deranged inflammatory response. Excitotoxicity, due to excessive glutamate levels and increased intracellular Ca2+ concentration, has also been proposed to play a key role in ALS pathogenesis/progression. Reactive Oxygen Species (ROS) behave as key second messengers for multiple receptor/ligand interactions. ROS-dependent regulatory networks are usually mediated by peroxides. Superoxide Dismutase 1 (SOD1) physiologically mediates intracellular peroxide generation. About 10% of ALS subjects show a familial disease associated with different gain-of-function SOD1 mutations. The occurrence of sporadic ALS, not clearly associated with SOD1 defects, has been also described. SOD1-dependent pathways have been involved in neuron functional network as well as in immune-response regulation. Both, neuron depolarization and antigen-dependent T-cell activation mediate SOD1 exocytosis, inducing increased interaction of the enzyme with a complex molecular network involved in the regulation of neuron functional activity and immune response. Here, alteration of SOD1-dependent pathways mediating increased intracellular Ca2+ levels, altered mitochondria functions and defective inflammatory process regulation have been proposed to be relevant for ALS pathogenesis/progression.

SRF deletion results in earlier disease onset in a mouse model of amyotrophic lateral sclerosis

Changes in neuronal activity modulate the vulnerability of motoneurons (MNs) in neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). So far, the molecular basis of neuronal activity's impact in ALS is poorly understood. Herein, we investigated the impact of deleting the neuronal activity-stimulated transcription factor (TF) serum response factor (SRF) in MNs of SOD1G93A mice. SRF was present in vulnerable MMP9+ MNs. Ablation of SRF in MNs induced an earlier disease onset starting around 7-8 weeks after birth, as revealed by enhanced weight loss and decreased motor ability. This earlier disease onset in SRF-depleted MNs was accompanied by a mild elevation of neuroinflammation and neuromuscular synapse degeneration, whereas overall MN numbers and mortality were unaffected. In SRF-deficient mice, MNs showed impaired induction of autophagy-encoding genes, suggesting a potentially new SRF function in transcriptional regulation of autophagy. Complementary, constitutively active SRF-VP16 enhanced autophagy-encoding gene transcription and autophagy progression in cells. Furthermore, SRF-VP16 decreased ALS-associated aggregate induction. Chemogenetic modulation of neuronal activity uncovered SRF as having important TF-mediating activity-dependent effects, which might be beneficial to reduce ALS disease burden. Thus, our data identify SRF as a gene regulator connecting neuronal activity with the cellular autophagy program initiated in degenerating MNs.

Human SOD1 is secreted via a conventional secretion pathway in Saccharomyces cerevisiae

Soluble proteins sorted through the secretory pathway contain an N-terminal signal peptide that induces their translocation into the endoplasmic reticulum (ER) from the cytosol. However, a few proteins that lack a signal peptide are still translocated into the ER, such as SOD1. SOD1 is a causative gene of amyotrophic lateral sclerosis (ALS). A relationship has been suggested between the secretion of SOD1 and the pathogenesis of ALS; however, the transport mechanism of SOD1 remains unclear. We herein report that SOD1 was translocated into the ER lumen through the translocon Sec61 and was then secreted extracellularly. The present results indicate the potential of suppressing the secretion of SOD1 as a therapeutic target for ALS.

Extracellular Vesicles in Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis is a progressive neurodegenerative disease and is the most common adult motor neuron disease. The disease pathogenesis is complex with the perturbation of multiple pathways proposed, including mitochondrial dysfunction, RNA processing, glutamate excitotoxicity, endoplasmic reticulum stress, protein homeostasis and endosomal transport/extracellular vesicle (EV) secretion. EVs are nanoscopic membrane-bound particles that are released from cells, involved in the intercellular communication of proteins, lipids and genetic material, and there is increasing evidence of their role in ALS. After discussing the biogenesis of EVs, we review their roles in the propagation of pathological proteins in ALS, such as TDP-43, SOD1 and FUS, and their contribution to disease pathology. We also discuss the ALS related genes which are involved in EV formation and vesicular trafficking, before considering the EV protein and RNA dysregulation found in ALS and how these have been investigated as potential biomarkers. Finally, we highlight the potential use of EVs as therapeutic agents in ALS, in particular EVs derived from mesenchymal stem cells and EVs as drug delivery vectors for potential treatment strategies.

P2X7 receptor activation mediates superoxide dismutase 1 (SOD1) release from murine NSC-34 motor neurons

Mutant superoxide dismutase 1 (SOD1) can be constitutively released from motor neurons and transmitted to naïve motor neurons to promote the progression of amyotrophic lateral sclerosis (ALS). However, the biological impacts of this process and the precise mechanisms of SOD1 release remain to be fully resolved. Using biochemical and fluorescent techniques, this study aimed to determine if P2X7 receptor activation could induce mutant SOD1 release from motor neurons and whether this released SOD1 could be transmitted to motor neurons or microglia to mediate effects associated with neurodegeneration in ALS. Aggregated SOD1^G93A, released from murine NSC-34 motor neurons transiently transfected with SOD1^G93A, could be transmitted to naïve NSC-34 cells and murine EOC13 microglia to induce endoplasmic reticulum (ER) stress and tumour necrosis factor-alpha (TNFα) release, respectively. Immunoblotting revealed NSC-34 cells expressed P2X7. Extracellular ATP induced cation dye uptake into these cells, which was blocked by the P2X7 antagonist AZ10606120, demonstrating these cells express functional P2X7. Moreover, ATP induced the rapid release of aggregated SOD1^G93A from NSC-34 cells transiently transfected with SOD1^G93A, a process blocked by AZ10606120 and revealing a role for P2X7 in this process. ATP-induced SOD1^G93A release coincided with membrane blebbing. Finally, aggregated SOD1^G93A released via P2X7 activation could also be transmitted to NSC-34 and EOC13 cells to induce ER stress and TNFα release, respectively. Collectively, these results identify a novel role for P2X7 in the prion-like propagation of SOD1 in ALS and provide a possible explanation for the therapeutic benefits of P2X7 antagonism previously observed in ALS SOD1^G93A mice.

Assessment of protein inclusions in cultured cells using automated image analysis

Proteinaceous inclusions are associated with neurodegenerative diseases and cell models are often used to determine genetic and chemical modifiers of their formation. This protocol involves the usage of automated microscopy and machine learning-based image analysis to accurately quantify the levels of protein inclusion formation in cultured cells from fluorescence microscopy images. This protocol is highly scalable and can be applied to a few images or large datasets. For complete details on the use and execution of this protocol, please refer to McAlary et al. (2022).

Sequence grammar underlying the unfolding and phase separation of globular proteins

Aberrant phase separation of globular proteins is associated with many diseases. Here, we use a model protein system to understand how the unfolded states of globular proteins drive phase separation and the formation of unfolded protein deposits (UPODs). We find that for UPODs to form, the concentrations of unfolded molecules must be above a threshold value. Additionally, unfolded molecules must possess appropriate sequence grammars to drive phase separation. While UPODs recruit molecular chaperones, their compositional profiles are also influenced by synergistic physicochemical interactions governed by the sequence grammars of unfolded proteins and cellular proteins. Overall, the driving forces for phase separation and the compositional profiles of UPODs are governed by the sequence grammars of unfolded proteins. Our studies highlight the need for uncovering the sequence grammars of unfolded proteins that drive UPOD formation and cause gain-of-function interactions whereby proteins are aberrantly recruited into UPODs.

Cells Overexpressing ALS-Associated SOD1 Variants Are Differentially Susceptible to CuATSM-Associated Toxicity

CuATSM has repeatedly demonstrated to be therapeutically effective in SOD1 mouse models of amyotrophic lateral sclerosis (ALS), leading to current clinical trials. CuATSM acts to stabilize ALS-associated mutant SOD1 protein by supplying copper. However, in vitro work has demonstrated that CuATSM is only therapeutic for wild-type-like SOD1 mutants, not metal-binding-region mutants, suggesting that CuATSM may have genotype-specific effects. Furthermore, relatively high doses of CuATSM have been shown to produce adverse events in humans and mice. Here, we investigated the genotype-specific therapeutic window of CuATSM. NSC-34 cells transiently expressing copper-binding or non-binding mutations of SOD1 were treated with a broad range of CuATSM concentrations and examined for survival via time-lapse microscopy. Determination of the no-observed-adverse-effect level and the LC₅₀ suggest that CuATSM-associated toxicity is dependent on the amount of copper-depleted SOD1 available as well as the mutant's ability to bind copper. Our results suggest that the particular variant of SOD1 mutant is crucial in not only determining the level of efficacy achieved but also potential adverse events.

A copper chaperone-mimetic polytherapy for SOD1-associated amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which motor neurons progressively and rapidly degenerate, eventually leading to death. The first protein found to contain ALS-associated mutations was copper/zinc superoxide dismutase 1 (SOD1), which is conformationally stable when it contains its metal ligands and has formed its native intramolecular disulfide. Mutations in SOD1 reduce protein folding stability via disruption of metal binding and/or disulfide formation, resulting in misfolding, aggregation, and ultimately cellular toxicity. A great deal of effort has focused on preventing the misfolding and aggregation of SOD1 as a potential therapy for ALS; however, the results have been mixed. Here, we utilize a small-molecule polytherapy of diacetylbis(N(4)-methylthiosemicarbazonato)copper(II) (CuATSM) and ebselen to mimic the metal delivery and disulfide bond promoting activity of the cellular chaperone of SOD1, the “copper chaperone for SOD1.” Using microscopy with automated image analysis, we find that polytherapy using CuATSM and ebselen is highly effective and acts in synergy to reduce inclusion formation in a cell model of SOD1 aggregation for multiple ALS-associated mutants. Polytherapy reduces mutant SOD1-associated cell death, as measured by live-cell microscopy. Measuring dismutase activity via zymography and immunoblotting for disulfide formation showed that polytherapy promoted more effective maturation of transfected SOD1 variants beyond either compound alone. Our data suggest that a polytherapy of CuATSM and ebselen may merit more study as an effective method of treating SOD1-associated ALS.

Na+/Ca2+ exchanger isoform 1 takes part to the Ca2+-related prosurvival pathway of SOD1 in primary motor neurons exposed to beta-methylamino-L-alanine

Background

The cycad neurotoxin beta-methylamino-l-alanine (L-BMAA), one of the environmental trigger factor for amyotrophic lateral sclerosis/Parkinson-dementia complex (ALS/PDC), may cause neurodegeneration by disrupting organellar Ca²⁺ homeostasis. Through the activation of Akt/ERK1/2 pathway, the Cu,Zn-superoxide dismutase (SOD1) and its non-metallated form, ApoSOD1, prevent endoplasmic reticulum (ER) stress-induced cell death in motor neurons exposed to L-BMAA. This occurs through the rapid increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i) in part flowing from the extracellular compartment and in part released from ER. However, the molecular components of this mechanism remain uncharacterized.

Methods

By an integrated approach consisting on the use of siRNA strategy, Western blotting, confocal double- labeling immunofluorescence, patch-clamp electrophysiology, and Fura 2-/SBFI-single-cell imaging, we explored in rat motor neuron-enriched cultures the involvement of the plasma membrane proteins Na⁺/Ca²⁺ exchanger (NCX) and purinergic P₂X₇ receptor as well as that of the intracellular cADP-ribose (cADPR) pathway, in the neuroprotective mechanism of SOD1.

Results

We showed that SOD1-induced [Ca²⁺]_i rise was prevented neither by A430879, a P₂X₇ receptor specific antagonist or 8-bromo-cADPR, a cell permeant antagonist of cADP-ribose, but only by the pan inhibitor of NCX, CB-DMB. The same occurred for the ApoSOD1. Confocal double labeling immunofluorescence showed a huge expression of plasmalemmal NCX1 and intracellular NCX3 isoforms. Furthermore, we identified NCX1 reverse mode as the main mechanism responsible for the neuroprotective ER Ca²⁺ refilling elicited by SOD1 and ApoSOD1 through which they promoted translocation of active Akt in the nuclei of a subset of primary motor neurons. Finally, the activation of NCX1 by the specific agonist CN-PYB2 protected motor neurons from L-BMAA-induced cell death, mimicking the effect of SOD1.

Conclusion

Collectively, our data indicate that SOD1 and ApoSOD1 exert their neuroprotective effect by modulating ER Ca²⁺ content through the activation of NCX1 reverse mode and Akt nuclear translocation in a subset of primary motor neurons.

The Transcriptional Complex Sp1/KMT2A by Up-Regulating Restrictive Element 1 Silencing Transcription Factor Accelerates Methylmercury-Induced Cell Death in Motor Neuron-Like NSC34 Cells Overexpressing SOD1-G93A

Methylmercury (MeHg) exposure has been related to amyotrophic lateral sclerosis (ALS) pathogenesis and molecular mechanisms of its neurotoxicity has been associated to an overexpression of the Restrictive Element 1 Silencing Transcription factor (REST). Herein, we evaluated the possibility that MeHg could accelerate neuronal death of the motor neuron-like NSC34 cells transiently overexpressing the human Cu²⁺/Zn²⁺superoxide dismutase 1 (SOD1) gene mutated at glycine 93 (SOD1-G93A). Indeed, SOD1-G93A cells exposed to 100 nM MeHg for 24 h showed a reduction in cell viability, as compared to cells transfected with empty vector or with unmutated SOD1 construct. Interestingly, cell survival reduction in SOD1-G93A cells was associated with an increase of REST mRNA and protein levels. Furthermore, MeHg increased the expression of the transcriptional factor Sp1 and promoted its binding to REST gene promoter sequence. Notably, Sp1 knockdown reverted MeHg-induced REST increase. Co-immunoprecipitation experiments demonstrated that Sp1 physically interacted with the epigenetic writer Lysine-Methyltransferase-2A (KMT2A). Moreover, knocking-down of KMT2A reduced MeHg-induced REST mRNA and protein increase in SOD1-G93A cells. Finally, we found that MeHg-induced REST up-regulation triggered necropoptotic cell death, monitored by RIPK1 increased protein expression. Interestingly, REST knockdown or treatment with the necroptosis inhibitor Necrostatin-1 (Nec) decelerated MeH-induced cell death in SOD1-G93A cells. Collectively, this study demonstrated that MeHg hastens necroptotic cell death in SOD1-G93A cells via Sp1/KMT2A complex, that by epigenetic mechanisms increases REST gene expression.

Mutant Cu/Zn Superoxide Dismutase (A4V) Turnover Is Altered in Cells Containing Inclusions

SOD1 mutations account for ∼20% of familial amyotrophic lateral sclerosis (ALS) cases in which the hallmark pathological feature is insoluble SOD1 aggregates within motor neurons. Here, we investigated the degradation and synthesis of mutant SOD1 to determine whether the aggregation of mutant SOD1^A4V affects these processes. We confirm that, in general, the degradation of mutant SOD1^A4V occurs at a significantly faster rate than wild-type SOD1. We also report that the turnover and synthesis of mutant SOD1^A4V is impaired in the presence of insoluble SOD1^A4V aggregates. However, the timing of aggregation of SOD1^A4V did not coincide with UPS dysfunction. Together, these results reveal the impact of SOD1 aggregation on protein degradation pathways, highlighting the importance of the UPS in preventing neurodegenerative disorders such as ALS.

Proteostatic imbalance and protein spreading in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder whose exact causative mechanisms are still under intense investigation. Several lines of evidence suggest that the anatomical and temporal propagation of pathological protein species along the neural axis could be among the main driving mechanisms for the fast and irreversible progression of ALS pathology. Many ALS-associated proteins form intracellular aggregates as a result of their intrinsic prion-like properties and/or following impairment of the protein quality control systems. During the disease course, these mutated proteins and aberrant peptides are released in the extracellular milieu as soluble or aggregated forms through a variety of mechanisms. Internalization by recipient cells may seed further aggregation and amplify existing proteostatic imbalances, thus triggering a vicious cycle that propagates pathology in vulnerable cells, such as motor neurons and other susceptible neuronal subtypes. Here, we provide an in-depth review of ALS pathology with a particular focus on the disease mechanisms of seeding and transmission of the most common ALS-associated proteins, including SOD1, FUS, TDP-43, and C9orf72-linked dipeptide repeats. For each of these proteins, we report historical, biochemical, and pathological evidence of their behaviors in ALS. We further discuss the possibility to harness pathological proteins as biomarkers and reflect on the implications of these findings for future research.

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.

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.

Amyotrophic Lateral Sclerosis: Proteins, Proteostasis, Prions, and Promises

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of the motor neurons that innervate muscle, resulting in gradual paralysis and culminating in the inability to breathe or swallow. This neuronal degeneration occurs in a spatiotemporal manner from a point of onset in the central nervous system (CNS), suggesting that there is a molecule that spreads from cell-to-cell. There is strong evidence that the onset and progression of ALS pathology is a consequence of protein misfolding and aggregation. In line with this, a hallmark pathology of ALS is protein deposition and inclusion formation within motor neurons and surrounding glia of the proteins TAR DNA-binding protein 43, superoxide dismutase-1, or fused in sarcoma. Collectively, the observed protein aggregation, in conjunction with the spatiotemporal spread of symptoms, strongly suggests a prion-like propagation of protein aggregation occurs in ALS. In this review, we discuss the role of protein aggregation in ALS concerning protein homeostasis (proteostasis) mechanisms and prion-like propagation. Furthermore, we examine the experimental models used to investigate these processes, including in vitro assays, cultured cells, invertebrate models, and murine models. Finally, we evaluate the therapeutics that may best prevent the onset or spread of pathology in ALS and discuss what lies on the horizon for treating this currently incurable disease.

Insulin-like growth factor 2 (IGF2) protects against Huntington's disease through the extracellular disposal of protein aggregates

Impaired neuronal proteostasis is a salient feature of many neurodegenerative diseases, highlighting alterations in the function of the endoplasmic reticulum (ER). We previously reported that targeting the transcription factor XBP1, a key mediator of the ER stress response, delays disease progression and reduces protein aggregation in various models of neurodegeneration. To identify disease modifier genes that may explain the neuroprotective effects of XBP1 deficiency, we performed gene expression profiling of brain cortex and striatum of these animals and uncovered insulin-like growth factor 2 (Igf2) as the major upregulated gene. Here, we studied the impact of IGF2 signaling on protein aggregation in models of Huntington's disease (HD) as proof of concept. Cell culture studies revealed that IGF2 treatment decreases the load of intracellular aggregates of mutant huntingtin and a polyglutamine peptide. These results were validated using induced pluripotent stem cells (iPSC)-derived medium spiny neurons from HD patients and spinocerebellar ataxia cases. The reduction in the levels of mutant huntingtin was associated with a decrease in the half-life of the intracellular protein. The decrease in the levels of abnormal protein aggregation triggered by IGF2 was independent of the activity of autophagy and the proteasome pathways, the two main routes for mutant huntingtin clearance. Conversely, IGF2 signaling enhanced the secretion of soluble mutant huntingtin species through exosomes and microvesicles involving changes in actin dynamics. Administration of IGF2 into the brain of HD mice using gene therapy led to a significant decrease in the levels of mutant huntingtin in three different animal models. Moreover, analysis of human postmortem brain tissue and blood samples from HD patients showed a reduction in IGF2 level. This study identifies IGF2 as a relevant factor deregulated in HD, operating as a disease modifier that buffers the accumulation of abnormal protein species.

Transport Alteration of 4-Phenyl Butyric Acid Mediated by a Sodium- and Proton-Coupled Monocarboxylic Acid Transporter System in ALS Model Cell Lines (NSC-34) Under Inflammatory States

4-Phenyl butyric acid (PBA) has histone deacetylase inhibitory and neuroprotective effects. We aimed to examine the transport alteration activity of PBA in control (WT) and disease (MT) model cell lines of an amyotrophic lateral sclerosis (ALS) model. The transport characteristics of PBA were examined uptake rates and mRNA expression levels in NSC-34 cell lines. PBA uptake was pH, sodium, and concentration dependent. The K_m and V_max values for PBA uptake in the MT were more than two-fold higher than those in the WT. The presence of monocarboxylic acids (MA) and inhibitors of MA transporter (MCT) inhibited the uptake of PBA. PBA showed competitive inhibition in the presence of MAs in both cell lines. SiRNA transfection studies showed that PBA can be transported to NSC-34 cell lines through sodium-coupled MCT1. TNF-α and H₂O₂ increased, but LPS and glutamate reduced the uptake rate after the pretreatment of the MT cell lines. SMCT1 mRNA expression levels, in the presence of oxidative stress inducing agents, showed consistent results with the uptake results. These results demonstrate that PBA can be transported to the ALS model NSC-34 cell lines by sodium- and proton-coupled MCTs, and MA plays a vital role in the prevention of neurodegenerative diseases.

Metabolism Regulation and Redox State: Insight into the Role of Superoxide Dismutase 1

Energy metabolism and redox state are strictly linked; energy metabolism is a source of reactive oxygen species (ROS) that, in turn, regulate the flux of metabolic pathways. Moreover, to assure redox homeostasis, metabolic pathways and antioxidant systems are often coordinately regulated. Several findings show that superoxide dismutase 1 (SOD1) enzyme has effects that go beyond its superoxide dismutase activity and that its functions are not limited to the intracellular compartment. Indeed, SOD1 is secreted through unconventional secretory pathways, carries out paracrine functions and circulates in the blood bound to lipoproteins. Striking experimental evidence links SOD1 to the redox regulation of metabolism. Important clues are provided by the systemic effects on energy metabolism observed in mutant SOD1-mediated amyotrophic lateral sclerosis (ALS). The purpose of this review is to analyze in detail the involvement of SOD1 in redox regulation of metabolism, nutrient sensing, cholesterol metabolism and regulation of mitochondrial respiration. The scientific literature on the relationship between ALS, mutated SOD1 and metabolism will also be explored, in order to highlight the metabolic functions of SOD1 whose biological role still presents numerous unexplored aspects that deserve further investigation.

The prion-like nature of amyotrophic lateral sclerosis

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

Neurodegenerative disease-associated protein aggregates are poor inducers of the heat shock response in neuronal cells

Protein aggregates that result in inclusion formation are a pathological hallmark common to many neurodegenerative diseases, including amyotrophic lateral sclerosis, Parkinson's disease and Huntington's disease. Under conditions of cellular stress, activation of the heat shock response (HSR) results in an increase in the levels of molecular chaperones and is a first line of cellular defence against inclusion formation. It remains to be established whether neurodegenerative disease-associated proteins and inclusions are themselves capable of inducing an HSR in neuronal cells. To address this, we generated a neuroblastoma cell line that expresses a fluorescent reporter protein under conditions of heat shock transcription factor 1 (HSF1)-mediated HSR induction. We show that the HSR is not induced by exogenous treatment with aggregated forms of recombinant α-synuclein or the G93A mutant of superoxide dismutase-1 (SOD1^G93A) nor intracellular expression of SOD1^G93A or a pathogenic form of polyglutamine-expanded huntingtin (Htt^72Q). These results suggest that pathogenic proteins evade detection or impair induction of the HSR in neuronal cells. A failure of protein aggregation to induce an HSR might contribute to the development of inclusion pathology in neurodegenerative diseases.This article has an associated First Person interview with the first author of the paper.

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.

Cu/Zn-superoxide dismutase and wild-type like fALS SOD1 mutants produce cytotoxic quantities of H2O2 via cysteine-dependent redox short-circuit

The Cu/Zn−superoxide dismutase (SOD1) is a ubiquitous enzyme that catalyzes the dismutation of superoxide radicals to oxygen and hydrogen peroxide. In addition to this principal reaction, the enzyme is known to catalyze, with various efficiencies, several redox side-reactions using alternative substrates, including biological thiols, all involving the catalytic copper in the enzyme’s active-site, which is relatively surface exposed. The accessibility and reactivity of the catalytic copper is known to increase upon SOD1 misfolding, structural alterations caused by a mutation or environmental stresses. These competing side-reactions can lead to the formation of particularly toxic ROS, which have been proposed to contribute to oxidative damage in amyotrophic lateral sclerosis (ALS), a neurodegenerative disease that affects motor neurons. Here, we demonstrated that metal-saturated SOD1^WT (holo-SOD1^WT) and a familial ALS (fALS) catalytically active SOD1 mutant, SOD1^G93A, are capable, under defined metabolic circumstances, to generate cytotoxic quantities of H₂O₂ through cysteine (CSH)/glutathione (GSH) redox short-circuit. Such activity may drain GSH stores, therefore discharging cellular antioxidant potential. By analyzing the distribution of thiol compounds throughout the CNS, the location of potential hot-spots of ROS production can be deduced. These hot-spots may constitute the origin of oxidative damage to neurons in ALS.

Network approach identifies Pacer as an autophagy protein involved in ALS pathogenesis

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a multifactorial fatal motoneuron disease without a cure. Ten percent of ALS cases can be pointed to a clear genetic cause, while the remaining 90% is classified as sporadic. Our study was aimed to uncover new connections within the ALS network through a bioinformatic approach, by which we identified C13orf18, recently named Pacer, as a new component of the autophagic machinery and potentially involved in ALS pathogenesis.

METHODS

Initially, we identified Pacer using a network-based bioinformatic analysis. Expression of Pacer was then investigated in vivo using spinal cord tissue from two ALS mouse models (SOD1^G93A and TDP43^A315T) and sporadic ALS patients. Mechanistic studies were performed in cell culture using the mouse motoneuron cell line NSC34. Loss of function of Pacer was achieved by knockdown using short-hairpin constructs. The effect of Pacer repression was investigated in the context of autophagy, SOD1 aggregation, and neuronal death.

RESULTS

Using an unbiased network-based approach, we integrated all available ALS data to identify new functional interactions involved in ALS pathogenesis. We found that Pacer associates to an ALS-specific subnetwork composed of components of the autophagy pathway, one of the main cellular processes affected in the disease. Interestingly, we found that Pacer levels are significantly reduced in spinal cord tissue from sporadic ALS patients and in tissues from two ALS mouse models. In vitro, Pacer deficiency lead to impaired autophagy and accumulation of ALS-associated protein aggregates, which correlated with the induction of cell death.

CONCLUSIONS

This study, therefore, identifies Pacer as a new regulator of proteostasis associated with ALS pathology.

CuATSM Protects Against the In Vitro Cytotoxicity of Wild-Type-Like Copper-Zinc Superoxide Dismutase Mutants but not Mutants That Disrupt Metal Binding

Mutations in the SOD1 gene are associated with some forms of familial amyotrophic lateral sclerosis (fALS). There are more than 150 different mutations in the SOD1 gene that have various effects on the copper-zinc superoxide dismutase (SOD1) enzyme structure, including the loss of metal binding and a decrease in dimer affinity. The copper-based therapeutic CuATSM has been proven to be effective at rescuing neuronal cells from SOD1 mutant toxicity and has also increased the life expectancy of mice expressing the human transgenes SOD1^G93A and SOD1^G37R. Furthermore, CuATSM is currently the subject of a phase I/II clinical trial in Australia as a treatment for ALS. To determine if CuATSM protects against a broad variety of SOD1 mutations, we used a well-established cell culture model of SOD1-fALS. NSC-34 cells expressing SOD1-EGFP constructs were treated with CuATSM and examined by time-lapse microscopy. Our results show a concentration-dependent protection of cells expressing mutant SOD1^A4V over the experimental time period. We tested the efficacy of CuATSM on 10 SOD1-fALS mutants and found that while protection was observed in cells expressing pathogenic wild-type-like mutants, cells expressing a truncation mutant or metal binding region mutants were not. We also show that CuATSM rescue is associated with an increase in human SOD1 activity and a decrease in the level of SOD1 aggregation in vitro. In conclusion, CuATSM has shown to be a promising therapeutic for SOD1-associated ALS; however, our in vitro results suggest that the protection afforded varies depending on the SOD1 variant, including negligible protection to mutants with deficient copper binding.

Experimental Mutations in Superoxide Dismutase 1 Provide Insight into Potential Mechanisms Involved in Aberrant Aggregation in Familial Amyotrophic Lateral Sclerosis

Mutations in more than 80 different positions in superoxide dismutase 1 (SOD1) have been associated with amyotrophic lateral sclerosis (fALS). There is substantial evidence that a common consequence of these mutations is to induce the protein to misfold and aggregate. How these mutations perturb native structure to heighten the propensity to misfold and aggregate is unclear. In the present study, we have mutagenized Glu residues at positions 40 and 133 that are involved in stabilizing the β-barrel structure of the native protein and a critical Zn binding domain, respectively, to examine how specific mutations may cause SOD1 misfolding and aggregation. Mutations associated with ALS as well as experimental mutations were introduced into these positions. We used an assay in which mutant SOD1 was fused to yellow fluorescent protein (SOD1:YFP) to visualize the formation of cytosolic inclusions by mutant SOD1. We then used existing structural data on SOD1, to predict how different mutations might alter local 3D conformation. Our findings reveal an association between mutant SOD1 aggregation and amino acid substitutions that are predicted to introduce steric strain, sometimes subtly, in the 3D conformation of the peptide backbone.

Loss of charge mutations in solvent exposed Lys residues of superoxide dismutase 1 do not induce inclusion formation in cultured cell models

Mutations in superoxide dismutase 1 (SOD1) associated with familial amyotrophic lateral sclerosis (fALS) induce the protein to misfold and aggregate. Missense mutations at more than 80 different amino acid positions have been associated with disease. How these mutations heighten the propensity of SOD1 to misfold and aggregate is unclear. With so many mutations, it is possible that more than one mechanism of aggregation may be involved. Of many possible mechanisms to explain heightened aggregation, one that has been suggested is that mutations that eliminate charged amino acids could diminish repulsive forces that would inhibit aberrant protein:protein interactions. Mutations at twenty-one charged residues in SOD1 have been associated with fALS, but of the 11 Lys residues in the protein, only 1 has been identified as mutated in ALS patients. Here, we examined whether loss of positively charged surface Lys residues in SOD1 would induce misfolding and formation of intracellular inclusions. We mutated four different Lys residues (K30, K36, K75, K91) in SOD1 that are not particularly well conserved, and expressed these variants as fusion proteins with yellow fluorescent protein (YFP) to assess inclusion formation. We also assessed whether these mutations induced binding to a conformation-restricted SOD1 antibody, designated C4F6, which recognizes non-natively folded protein. Although we observed some mutations to cause enhanced C4F6 binding, we did not observe that mutations that reduce charge at these positions caused the protein to form intracellular inclusions. Our findings may have implications for the low frequency of mutations at Lys residues SOD1 in ALS patients.

Tryptophan 32-mediated SOD1 aggregation is attenuated by pyrimidine-like compounds in living cells

Over 160 mutations in superoxide dismutase 1 (SOD1) are associated with familial amyotrophic lateral sclerosis (fALS), where the main pathological feature is deposition of SOD1 into proteinaceous cytoplasmic inclusions. We previously showed that the tryptophan residue at position 32 (W32) mediates the prion-like propagation of SOD1 misfolding in cells, and that a W32S substitution blocks this phenomenon. Here, we used in vitro protein assays to demonstrate that a W32S substitution in SOD1-fALS mutants significantly diminishes their propensity to aggregate whilst paradoxically decreasing protein stability. We also show SOD1-W32S to be resistant to seeded aggregation, despite its high abundance of unfolded protein. A cell-based aggregation assay demonstrates that W32S substitution significantly mitigates inclusion formation. Furthermore, this assay reveals that W32 in SOD1 is necessary for the formation of a competent seed for aggregation under these experimental conditions. Following the observed importance of W32 for aggregation, we established that treatment of living cells with the W32-interacting 5-Fluorouridine (5-FUrd), and its FDA approved analogue 5-Fluorouracil (5-FU), substantially attenuate inclusion formation similarly to W32S substitution. Altogether, we highlight W32 as a significant contributor to SOD1 aggregation, and propose that 5-FUrd and 5-FU present promising lead drug candidates for the treatment of SOD1-associated 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 (SOD1^WT) is also observed in postmortem tissue of a subset of sporadic ALS (sALS) cases, but cellular and molecular mechanisms generating abnormal SOD1^WT species are unknown. We analyzed aberrant human SOD1^WT species over the lifetime of transgenic mice and found the accumulation of disulfide-cross-linked high-molecular-weight SOD1^WT 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 SOD1^WT We established a pharmacological paradigm of chronic ER stress in vivo, which recapitulated SOD1^WTaggregation in young transgenic mice. These species were soluble in nondenaturing detergents and did not react with a SOD1 conformation-specific antibody. Interestingly, SOD1^WT aggregation under ER stress correlated with astrocyte activation in the spinal cord of transgenic mice. Finally, the disulfide-cross-linked SOD1^WT 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 SOD1^WT to aggregate during aging, operating as a possible risk factor for developing ALS.

Unconventional Secretion and Intercellular Transfer of Mutant Huntingtin

The mechanism of intercellular transmission of pathological agents in neurodegenerative diseases has received much recent attention. Huntington’s disease (HD) is caused by a monogenic mutation in the gene encoding Huntingtin (HTT). Mutant HTT (mHTT) harbors a CAG repeat extension which encodes an abnormally long polyglutamine (polyQ) repeat at HTT’s N-terminus. Neuronal pathology in HD is largely due to the toxic gain-of-function by mHTT and its proteolytic products, which forms both nuclear and cytoplasmic aggregates that perturb nuclear gene transcription, RNA splicing and transport as well cellular membrane dynamics. The neuropathological effects of mHTT have been conventionally thought to be cell-autonomous in nature. Recent findings have, however, indicated that mHTT could be secreted by neurons, or transmitted from one neuronal cell to another via different modes of unconventional secretion, as well as via tunneling nanotubes (TNTs). These modes of transmission allow the intercellular spread of mHTT and its aggregates, thus plausibly promoting neuropathology within proximal neuronal populations and between neurons that are connected within neural circuits. Here, the various possible modes for mHTT’s neuronal cell exit and intercellular transmission are discussed.

SOD1A4V aggregation alters ubiquitin homeostasis in a cell model of ALS

A hallmark of amyotrophic lateral sclerosis (ALS) pathology is the accumulation of ubiquitylated protein inclusions within motor neurons. Recent studies suggest the sequestration of ubiquitin (Ub) into inclusions reduces the availability of free Ub, which is essential for cellular function and survival. However, the dynamics of the Ub landscape in ALS have not yet been described. Here, we show that Ub homeostasis is altered in a cell model of ALS induced by expressing mutant SOD1 (SOD1^A4V). By monitoring the distribution of Ub in cells expressing SOD1^A4V, we show that Ub is present at the earliest stages of SOD1^A4V aggregation, and that cells containing SOD1^A4V aggregates have greater ubiquitin-proteasome system (UPS) dysfunction. Furthermore, SOD1^A4V aggregation is associated with the redistribution of Ub and depletion of the free Ub pool. Ubiquitomics analysis indicates that expression of SOD1^A4V is associated with a shift of Ub to a pool of supersaturated proteins, including those associated with oxidative phosphorylation and metabolism, corresponding with altered mitochondrial morphology and function. Taken together, these results suggest that misfolded SOD1 contributes to UPS dysfunction and that Ub homeostasis is an important target for monitoring pathological changes in ALS.This article has an associated First Person interview with the first author of the paper.

Effect of Mutated Cu, Zn Superoxide Dismutase (SOD1G93A) on Modulation of Transductional Pathway Mediated by M1 Muscarinic Receptor in SK-N-BE and NSC-34 Cells

The constitutive secretion of antioxidant Cu-Zn Superoxide dismutase (SOD1) has been widely demonstrated in many cellular lines. In addition, we showed that as well as the basal SOD1 secretion, this enzyme is also exported through depolarization of excitable cells by high extracellular K concentration. Recent data showed that SOD1 was able to activate muscarinic M1 receptor producing the activation, via phospholipase C, of ERK1-2 and AKT pathways. It is also known that about 20% of familial amyotrophic lateral sclerosis (fALS) is due to mutations in the gene coding for SOD1. The aim of the present research is to evaluate whether, analogously to wild type SOD1 (SOD1^wt), the mutated form of SOD1^G93A is able to activate ERK1-2 and AKT through muscarinic M1 receptor in SK-N-BE as well as in motoneuron like NSC-34. Our results demonstrated that in NSC-34 and SK-N-BE cells mutated SOD1^G93A carried out a more evident activation of ERK1-2 and AKT and a stronger increase of intracellular calcium levels compared to SOD1^WT; we also demonstrated that these effects are mediated by the M1 receptor as shown using pirenzepine, a specific M1 inhibitor and the calcium chelator BAPTA. Of note, M1 receptor pathway activation by SOD1G93A, but not by SOD1WT, is associated with both an increase of reactive oxygen species and a cytotoxic effect.

Prion-like mechanisms in amyotrophic lateral sclerosis

The prion hypothesis - a protein conformation capable of replicating without a nucleic acid genome - was heretical at the time of its discovery. However, the characteristics of the disease-misfolded prion protein and its ability to transmit disease, replicate, and spread are now widely accepted throughout the scientific community. In fact, in the last decade a wealth of evidence has emerged supporting similar properties observed for many of the misfolded proteins implicated in other neurodegenerative diseases, such as Alzheimer disease, Parkinson disease, tauopathies, and as described in this chapter, amyotrophic lateral sclerosis (ALS). Multiple studies have now demonstrated the ability for superoxide dismutase-1, 43-kDa transactive response (TAR) DNA-binding protein, fused-in sarcoma, and most recently, C9orf72-encoded polypeptides to display properties similar to those of prions. The majority of these are cell-free and in vitro assays, while superoxide dismutase-1 remains the only ALS-linked protein to demonstrate several prion-like properties in vivo. In this chapter, we provide an introduction to ALS and review the recent literature linking several proteins implicated in the familial forms of the disease to properties of the prion protein.

Eukaryotic copper-only superoxide dismutases (SODs): A new class of SOD enzymes and SOD-like protein domains

The copper-containing superoxide dismutases (SODs) represent a large family of enzymes that participate in the metabolism of reactive oxygen species by disproportionating superoxide anion radical to oxygen and hydrogen peroxide. Catalysis is driven by the redox-active copper ion, and in most cases, SODs also harbor a zinc at the active site that enhances copper catalysis and stabilizes the protein. Such bimetallic Cu,Zn-SODs are widespread, from the periplasm of bacteria to virtually every organelle in the human cell. However, a new class of copper-containing SODs has recently emerged that function without zinc. These copper-only enzymes serve as extracellular SODs in specific bacteria (i.e. Mycobacteria), throughout the fungal kingdom, and in the fungus-like oomycetes. The eukaryotic copper-only SODs are particularly unique in that they lack an electrostatic loop for substrate guidance and have an unusual open-access copper site, yet they can still react with superoxide at rates limited only by diffusion. Copper-only SOD sequences similar to those seen in fungi and oomycetes are also found in the animal kingdom, but rather than single-domain enzymes, they appear as tandem repeats in large polypeptides we refer to as CSRPs (copper-only SOD-repeat proteins). Here, we compare and contrast the Cu,Zn versus copper-only SODs and discuss the evolution of copper-only SOD protein domains in animals and fungi.

Rilmenidine promotes MTOR-independent autophagy in the mutant SOD1 mouse model of amyotrophic lateral sclerosis without slowing disease progression

Macroautophagy/autophagy is the main intracellular catabolic pathway in neurons that eliminates misfolded proteins, aggregates and damaged organelles associated with ageing and neurodegeneration. Autophagy is regulated by both MTOR-dependent and -independent pathways. There is increasing evidence that autophagy is compromised in neurodegenerative disorders, which may contribute to cytoplasmic sequestration of aggregation-prone and toxic proteins in neurons. Genetic or pharmacological modulation of autophagy to promote clearance of misfolded proteins may be a promising therapeutic avenue for these disorders. Here, we demonstrate robust autophagy induction in motor neuronal cells expressing SOD1 or TARDBP/TDP-43 mutants linked to amyotrophic lateral sclerosis (ALS). Treatment of these cells with rilmenidine, an anti-hypertensive agent and imidazoline-1 receptor agonist that induces autophagy, promoted autophagic clearance of mutant SOD1 and efficient mitophagy. Rilmenidine administration to mutant SOD1G93A mice upregulated autophagy and mitophagy in spinal cord, leading to reduced soluble mutant SOD1 levels. Importantly, rilmenidine increased autophagosome abundance in motor neurons of SOD1G93A mice, suggesting a direct action on target cells. Despite robust induction of autophagy in vivo, rilmenidine worsened motor neuron degeneration and symptom progression in SOD1G93A mice. These effects were associated with increased accumulation and aggregation of insoluble and misfolded SOD1 species outside the autophagy pathway, and severe mitochondrial depletion in motor neurons of rilmenidine-treated mice. These findings suggest that rilmenidine treatment may drive disease progression and neurodegeneration in this mouse model due to excessive mitophagy, implying that alternative strategies to beneficially stimulate autophagy are warranted in ALS.

Extracellular Vesicles in Brain Tumors and Neurodegenerative Diseases

Extracellular vesicles (EVs) can be classified into apoptotic bodies, microvesicles (MVs), and exosomes, based on their origin or size. Exosomes are the smallest and best characterized vesicles which derived from the endosomal system. These vesicles are released from many different cell types including neuronal cells and their functions in the nervous system are investigated. They have been proposed as novel means for intercellular communication, which takes part not only to the normal neuronal physiology but also to the transmission of pathogenic proteins. Indeed, exosomes are fundamental to assemble and transport proteins during development, but they can also transfer neurotoxic misfolded proteins in pathogenesis. The present review will focus on their roles in neurological diseases, specifically brain tumors, such as glioblastoma (GBM), neuroblastoma (NB), medulloblastoma (MB), and metastatic brain tumors and chronic neurodegenerative diseases, such as Alzheimer, Parkinson, multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), Huntington, and Prion diseseases highlighting their involvement in spreading neurotoxicity, in therapeutics, and in pathogenesis.

A diacidic motif determines unconventional secretion of wild-type and ALS-linked mutant SOD1

Starvation-induced unconventional secretion of Acb1 requires ESCRT-I, -II, and -III and Grh1. Cruz-Garcia et al. report that SOD1 and its mutant form linked to amyotrophic lateral sclerosis are also secreted upon nutrient starvation in a Grh1- and ESCRT-I–, -II–, and -III–dependent process. The authors identify a conserved diacidic motif in Acb1 and SOD1 that is necessary for their export in yeast and human cells.

The nutrient starvation-specific unconventional secretion of Acb1 in Saccharomyces cerevisiae requires ESCRT-I, -II, and -III and Grh1. In this study, we report that another signal sequence lacking cytoplasmic protein, superoxide dismutase 1 (SOD1), and its mutant form linked to amyotrophic lateral sclerosis (ALS), is also secreted by yeast upon nutrient starvation in a Grh1- and ESCRT-I–, -II–, and -III–dependent process. Our analyses reveal that a conserved diacidic motif (Asp-Glu) in these proteins is necessary for their export. Importantly, secretion of wild-type human SOD1 and the ALS-linked mutant in human cells also require the diacidic residues. Altogether, these findings reveal information encoded within the cytoplasmic proteins required for their unconventional secretion and provide a means to unravel the significance of the cytoplasmic versus the secreted form of mutant SOD1 in the pathology of ALS. We also propose how cells, based on a signal-induced change in cytoplasmic physiology, select a small pool of a subset of cytoplasmic proteins for unconventional secretion.

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.

The Endoplasmic Reticulum Unfolded Protein Response in Neurodegenerative Disorders and Its Potential Therapeutic Significance

In eukaryotic cells, the endoplasmic reticulum (ER) is the cell compartment involved in secretory protein translocation and quality control of secretory protein folding. Different conditions can alter ER function, resulting in the accumulation of unfolded or misfolded proteins within the ER lumen. Such a condition, known as ER stress, elicits an integrated adaptive response known as the unfolded protein response (UPR) that aims to restore proteostasis within the secretory pathway. Conversely, in prolonged cell stress or insufficient adaptive response, UPR signaling causes cell death. ER dysfunctions are involved and contribute to neuronal degeneration in several human diseases, including Alzheimer, Parkinson and Huntington disease and amyotrophic lateral sclerosis. The correlations between ER stress and its signal transduction pathway known as the UPR with neuropathological changes are well established. In addition, much evidence suggests that genetic or pharmacological modulation of UPR could represent an effective strategy for minimizing the progressive neuronal loss in neurodegenerative diseases. Here, we review recent results describing the main cellular mechanisms linking ER stress and UPR to neurodegeneration. Furthermore, we provide an up-to-date panoramic view of the currently pursued strategies for ameliorating the toxic effects of protein unfolding in disease by targeting the ER UPR pathway.

Protein Quality Control and the Amyotrophic Lateral Sclerosis/Frontotemporal Dementia Continuum

Protein homeostasis, or proteostasis, has an important regulatory role in cellular function. Protein quality control mechanisms, including protein folding and protein degradation processes, have a crucial function in post-mitotic neurons. Cellular protein quality control relies on multiple strategies, including molecular chaperones, autophagy, the ubiquitin proteasome system, endoplasmic reticulum (ER)-associated degradation (ERAD) and the formation of stress granules (SGs), to regulate proteostasis. Neurodegenerative diseases are characterized by the presence of misfolded protein aggregates, implying that protein quality control mechanisms are dysfunctional in these conditions. Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are neurodegenerative diseases that are now recognized to overlap clinically and pathologically, forming a continuous disease spectrum. In this review article, we detail the evidence for dysregulation of protein quality control mechanisms across the whole ALS-FTD continuum, by discussing the major proteins implicated in ALS and/or FTD. We also discuss possible ways in which protein quality mechanisms could be targeted therapeutically in these disorders and highlight promising protein quality control-based therapeutics for clinical trials.

ApoSOD1 lacking dismutase activity neuroprotects motor neurons exposed to beta-methylamino-L-alanine through the Ca2+/Akt/ERK1/2 prosurvival pathway

Amyotrophic lateral sclerosis (ALS) is a severe human adult-onset neurodegenerative disease affecting lower and upper motor neurons. In >20% of cases, the familial form of ALS is caused by mutations in the gene encoding Cu,Zn-superoxide dismutase (SOD1). Interestingly, administration of wild-type SOD1 to SOD1^G93A transgenic rats ameliorates motor symptoms through an unknown mechanism. Here we investigated whether the neuroprotective effects of SOD1 are due to the Ca²⁺-dependent activation of such prosurvival signaling pathway and not to its catalytic activity. To this aim, we also examined the mechanism of neuroprotective action of ApoSOD1, the metal-depleted state of SOD1 that lacks dismutase activity, in differentiated motor neuron-like NSC-34 cells and in primary motor neurons exposed to the cycad neurotoxin beta-methylamino-L-alanine (L-BMAA). Preincubation of ApoSOD1 and SOD1, but not of human recombinant SOD1^G93A, prevented cell death in motor neurons exposed to L-BMAA. Moreover, ApoSOD1 elicited ERK1/2 and Akt phosphorylation in motor neurons through an early increase of intracellular Ca²⁺ concentration ([Ca²⁺]_i). Accordingly, inhibition of ERK1/2 by siMEK1 and PD98059 counteracted ApoSOD1- and SOD1-induced neuroprotection. Similarly, transfection of the dominant-negative form of Akt in NSC-34 motor neurons and treatment with the selective PI3K inhibitor LY294002 prevented ApoSOD1- and SOD1-mediated neuroprotective effects in L-BMAA-treated motor neurons. Furthermore, ApoSOD1 and SOD1 prevented the expression of the two markers of L-BMAA-induced ER stress GRP78 and caspase-12. Collectively, our data indicate that ApoSOD1, which is devoid of any catalytic dismutase activity, exerts a neuroprotective effect through an early activation of Ca²⁺/Akt/ERK1/2 pro-survival pathway that, in turn, prevents ER stress in a neurotoxic model of ALS.

The Cu, Zn Superoxide Dismutase: Not Only a Dismutase Enzyme

The Cu,Zn superoxide dismutase (SOD1) is an ubiquitary cytosolic dimeric carbohydrate free molecule, belonging to a family of isoenzymes involved in the scavenger of superoxide anions. This effect certainly represents the main and well known function ascribed to this enzyme. Here we highlight new aspects of SOD1 physiology that point out some inedited effects of this enzyme in addition to the canonic role of oxygen radical enzymatic dismutation. In the last two decades our research group produced many data obtained in in vitro studies performed in many cellular lines, mainly neuroblastoma SK-N-BE cells, indicating that this enzyme is secreted either constitutively or after depolarization induced by high extracellular K⁺ concentration. In addition, we gave many experimental evidences showing that SOD1 is able to stimulate, through muscarinic M1 receptor, pathways involving ERK1/2, and AKT activation. These effects are accompanied with an intracellular calcium increase. In the last part of this review we describe researches that link deficient extracellular secretion of mutant SOD1^G93A to its intracellular accumulation and toxicity in NSC-34 cells. Alternatively, SOD1^G93A toxicity has been attributed to a decrease of K_m for H₂O₂ with consequent OH radical formation. Interestingly, this last inedited effect of SOD1^G93A could represent a gain of function that could be involved in the pathogenesis of familial Amyotrophic Lateral Sclerosis (fALS).

Susceptibility of Mutant SOD1 to Form a Destabilized Monomer Predicts Cellular Aggregation and Toxicity but Not In vitro Aggregation Propensity

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the rapid and progressive degeneration of upper and lower motor neurons in the spinal cord, brain stem and motor cortex. The first gene linked to ALS was the gene encoding the free radical scavenging enzyme superoxide dismutase-1 (SOD1) that currently has over 180, mostly missense, ALS-associated mutations identified. SOD1-associated fALS patients show remarkably broad mean survival times (<1 year to ~17 years death post-diagnosis) that are mutation dependent. A hallmark of SOD1-associated ALS is the deposition of SOD1 into large insoluble aggregates in motor neurons. This is thought to be a consequence of mutation induced structural destabilization and/or oxidative damage leading to the misfolding and aggregation of SOD1 into a neurotoxic species. Here we aim to understand the relationship between SOD1 variant toxicity, structural stability, and aggregation propensity using a combination of cell culture and purified protein assays. Cell based assays indicated that aggregation of SOD1 variants correlate closely to cellular toxicity. However, the relationship between cellular toxicity and disease severity was less clear. We next utilized mass spectrometry to interrogate the structural consequences of metal loss and disulfide reduction on fALS-associated SOD1 variant structure. All variants showed evidence of unfolded, intermediate, and compact conformations, with SOD1^G37R, SOD1^G93A and SOD1^V148G having the greatest abundance of intermediate and unfolded SOD1. SOD1^G37R was an informative outlier as it had a high propensity to unfold and form oligomeric aggregates, but it did not aggregate to the same extent as SOD1^G93A and SOD1^V148G in in vitro aggregation assays. Furthermore, seeding the aggregation of DTT/EDTA-treated SOD1^G37R with preformed SOD1^G93A fibrils elicited minimal aggregation response, suggesting that the arginine substitution at position-37 blocks the templating of SOD1 onto preformed fibrils. We propose that this difference may be explained by multiple strains of SOD1 aggregate and this may also help explain the slow disease progression observed in patients with SOD1^G37R.

Hexokinase I N-terminal based peptide prevents the VDAC1-SOD1 G93A interaction and re-establishes ALS cell viability

Superoxide Dismutase 1 mutants associate with 20–25% of familial Amyotrophic Lateral Sclerosis (ALS) cases, producing toxic aggregates on mitochondria, notably in spinal cord. The Voltage Dependent Anion Channel isoform 1 (VDAC1) in the outer mitochondrial membrane is a docking site for SOD1 G93A mutant in ALS mice and the physiological receptor of Hexokinase I (HK1), which is poorly expressed in mouse spinal cord. Our results demonstrate that HK1 competes with SOD1 G93A for binding VDAC1, suggesting that in ALS spinal cord the available HK1-binding sites could be used by SOD1 mutants for docking mitochondria, producing thus organelle dysfunction. We tested this model by studying the action of a HK1-N-terminal based peptide (NHK1). This NHK1 peptide specifically interacts with VDAC1, inhibits the SOD1 G93A binding to mitochondria and restores the viability of ALS model NSC34 cells. Altogether, our results suggest that NHK1 peptide could be developed as a therapeutic tool in ALS, predicting an effective role also in other proteinopathies.