Human superoxide dismutase 1 (hSOD1) maturation through interaction with human copper chaperone for SOD1 (hCCS)

The physiological and pathophysiological roles of copper in the nervous system

Copper is a critical trace element in biological systems due the vast number of essential enzymes that require the metal as a cofactor, including cytochrome c oxidase, superoxide dismutase and dopamine-β-hydroxylase. Due its key role in oxidative metabolism, antioxidant defence and neurotransmitter synthesis, copper is particularly important for neuronal development and proper neuronal function. Moreover, increasing evidence suggests that copper also serves important functions in synaptic and network activity, the regulation of circadian rhythms, and arousal. However, it is important to note that because of copper's ability to redox cycle and generate reactive species, cellular levels of the metal must be tightly regulated to meet cellular needs while avoiding copper-induced oxidative stress. Therefore, it is essential that the intricate system of copper transporters, exporters, copper chaperones and copper trafficking proteins function properly and in coordinate fashion. Indeed, disorders of copper metabolism such as Menkes disease and Wilson disease, as well as diseases linked to dysfunction of copper-requiring enzymes, such as SOD1-linked amyotrophic lateral sclerosis, demonstrate the dramatic neurological consequences of altered copper homeostasis. In this review, we explore the physiological importance of copper in the nervous system as well as pathologies related to improper copper handling.

Current therapy in amyotrophic lateral sclerosis (ALS): A review on past and future therapeutic strategies

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that affects the first and second motoneurons (MNs), associated with muscle weakness, paralysis and finally death. The exact etiology of the disease still remains unclear. Currently, efforts to develop novel ALS treatments which target specific pathomechanisms are being studied. The mechanisms of ALS pathogenesis involve multiple factors, such as protein aggregation, glutamate excitotoxicity, oxidative stress, mitochondrial dysfunction, apoptosis, inflammation etc. Unfortunately, to date, there are only two FDA-approved drugs for ALS, riluzole and edavarone, without curative treatment for ALS. Herein, we give an overview of the many pathways and review the recent discovery and preclinical characterization of neuroprotective compounds. Meanwhile, drug combination and other therapeutic approaches are also reviewed. In the last part, we analyze the reasons of clinical failure and propose perspective on the treatment of ALS in the future.

Copper homeostasis in chronic kidney disease and its crosstalk with ferroptosis

Chronic kidney disease (CKD) has become a global public health problem with high morbidity and mortality. Renal fibrosis can lead to end-stage renal disease (ESRD). However, there is still no effective treatment to prevent or delay the progression of CKD into ESRD. Therefore, exploring the pathogenesis of CKD is essential for preventing and treating CKD. There are a variety of trace elements in the human body that interact with each other within a complex regulatory network. Iron and copper are both vital trace elements in the body. They are critical for maintaining bodily functions, and the dysregulation of their metabolism can cause many diseases, including kidney disease. Ferroptosis is a new form of cell death characterized by iron accumulation and lipid peroxidation. Studies have shown that ferroptosis is closely related to kidney disease. However, the role of abnormal copper metabolism in kidney disease and its relationship with ferroptosis remains unclear. Here, our current knowledge regarding copper metabolism, its regulatory mechanism, and the role of abnormal copper metabolism in kidney diseases is summarized. In addition, we discuss the relationship between abnormal copper metabolism and ferroptosis to explore the possible pathogenesis and provide a potential therapeutic target for CKD.

Structural insights into the modulation Of SOD1 aggregation By a fungal metabolite Phialomustin-B: Therapeutic potential in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal human motor neuron disease leading to muscle atrophy and paralysis. Mutations in superoxide dismutase 1 (SOD1) are associated with familial ALS (fALS). The SOD1 mutants in ALS have a toxic-gain of function by destabilizing the functional SOD1 homodimer, consequently inducing fibril-like aggregation with a cytotoxic non-native trimer intermediate. Therefore, reducing SOD1 oligomerization via chemical modulators is an optimal therapy in ALS. Here, we report the discovery of Phialomustin-B, an unsaturated secondary metabolite from the endophytic fungus Phialophora mustea, as a modulator of SOD1 aggregation. The crystal structure of the SOD1-Phialomustin complex refined to 1.90 Å resolution demonstrated for the first time that the ligand binds to the dimer interface and the lateral region near the electrostatic loop. The aggregation analyses of SOD1WT and the disease mutant SOD1A4V revealed that Phialomustin-B reduces cytotoxic trimerization. We propose that Phialomustin-B is a potent lead molecule with therapeutic potential in fALS.

Can Some Anticancer Drugs Be Repurposed to Treat Amyotrophic Lateral Sclerosis? A Brief Narrative Review

Amyotrophic lateral sclerosis (ALS) is a rare progressive motor neuron disease that, due to its high complexity, still lacks effective treatments. Development of a new drug is a highly costly and time-consuming process, and the repositioning of approved drugs can represent an efficient strategy to provide therapeutic opportunities. This is particularly true for rare diseases, which are characterised by small patient populations and therefore attract little commercial interest. Based on the overlap between the biological background of cancer and neurodegeneration, the repurposing of antineoplastic drugs for ALS has been suggested. The objective of this narrative review was to summarise the current experimental evidence on the use of approved anticancer drugs in ALS. Specifically, anticancer drugs belonging to different classes were found to act on mechanisms involved in the ALS pathogenesis, and some of them proved to exert beneficial effects in ALS models. However, additional studies are necessary to confirm the real therapeutic potential of anticancer drugs for repositioning in ALS treatment.

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

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

Advanced Techniques for Detecting Protein Misfolding and Aggregation in Cellular Environments

Protein misfolding and aggregation, a key contributor to the progression of numerous neurodegenerative diseases, results in functional deficiencies and the creation of harmful intermediates. Detailed visualization of this misfolding process is of paramount importance for improving our understanding of disease mechanisms and for the development of potential therapeutic strategies. While in vitro studies using purified proteins have been instrumental in delivering significant insights into protein misfolding, the behavior of these proteins in the complex milieu of living cells often diverges significantly from such simplified environments. Biomedical imaging performed in cell provides cellular-level information with high physiological and pathological relevance, often surpassing the depth of information attainable through in vitro methods. This review highlights a variety of methodologies used to scrutinize protein misfolding within biological systems. This includes optical-based methods, strategies leaning on mass spectrometry, in-cell nuclear magnetic resonance, and cryo-electron microscopy. Recent advancements in these techniques have notably deepened our understanding of protein misfolding processes and the features of the resulting misfolded species within living cells. The progression in these fields promises to catalyze further breakthroughs in our comprehension of neurodegenerative disease mechanisms and potential therapeutic interventions.

ATP induces folding of ALS-causing C71G-hPFN1 and nascent hSOD1

ALS-causing C71G-hPFN1 coexists in both folded and unfolded states, while nascent hSOD1 is unfolded. So far, the mechanisms underlying their ALS-triggering potential remain enigmatic. Here we show by NMR that ATP completely converts C71G-hPFN1 into the folded state at a 1:2 ratio, while inducing nascent hSOD1 into two co-existing states at a 1:8 ratio. Surprisingly, the inducing capacity of ATP comes from its triphosphate, but free triphosphate triggers aggregation. The inducing capacity ranks as: ATP = ATPP = PPP > ADP = AMP-PNP = AMP-PCP = PP, while AMP, adenosine, P, and NaCl show no conversion. Mechanistically, ATP and triphosphate appear to enhance the intrinsic folding capacity encoded in the sequences, as unveiled by comparing conformations and dynamics of ATP- and Zn2+-induced hSOD1 folded states. Our study provides a mechanism for the finding that some single-cell organisms employ polyphosphates as primordial chaperones, and sheds light on the enigma of age-related onset of familial ALS and risk increase of neurodegenerative diseases.

A double point mutation of SOD1 targeting net charge promotes aggregation under destabilizing conditions: Correlation of charge distribution and ALS-provoking mutation

A progressive neurodegenerative illness such as amyotrophic lateral sclerosis (ALS) is characterized by the misfolding and aggregation of human CuZn superoxide dismutase (hSOD1) into amyloid aggregates. Thus, designing strategies for the choice of WT-SOD1 and double mutant (G12D/G138E) with an increased net negative charge can be a good idea to elucidate the pathological mechanism of SOD1 in ALS under some destabilizing conditions. Consequently, we show evidence that protein charge, together with other destabilizing conditions, plays an important role in ALS disease. To achieve this purpose, we use methods, such as spectroscopy and transmission electron microscopy (TEM) to monitor the formation of amyloid aggregation. The specific activity of WT-SOD1 was approximately 1.72 times higher than that of the double mutant. Under amyloidogenic circumstances, structural properties such as local, secondary, oligomeric, and fibrillar structures were explored. The double mutant's far-UV CD spectra displayed a broad minimum peak in the region 213 to 218 nm, suggesting the production of β-rich amyloid fibrils. FTIR spectra of the double mutant samples at different incubation times showed a low-frequency peak around 1630-1640 cm^-1, attributed to a parallel β-sheet. Moreover, CR-binding assay and TEM analysis revealed and confirmed that mutation with an increased repulsive charge promotes the formation of fibrous aggregates. Consequently, ALS mutations with a higher repulsive charge are the apparent exceptions that validate the rule. This findings revealed that the double mutant increases protein aggregation through a novel mechanism, likely involving destabilization of structure and a change in the net negative charge.

Effect of Mineral Element Imbalance on Neutrophil Respiratory Burst Function and Inflammatory and Antioxidant Responses in Sheep

This study established a model of mineral element homeostatic imbalance and examined the respiratory burst function of peripheral blood neutrophils and inflammatory and antioxidant indicators before and after the imbalance in sheep. The results showed that after an EDTA injection, the number of activated neutrophils in the peripheral blood was significantly higher than that in the control group (p < 0.01). In addition, the serum IL-6 level was significantly increased (p < 0.05) and matrix metalloproteinase 7 (MMP7) was inhibited (p < 0.05), but returned to a normal level one week after the injection. Tissue inhibitor of metalloproteinase 1 (TIMP1) levels were consistently higher after the injection and significantly higher than in the control group (p < 0.05). CuZn-SOD, TNOS activity, serum creatinine and urea nitrogen levels were significantly higher than before the injection (p < 0.05). Combining the results of previous studies, the EDTA injection altered the metabolism and transcription of peripheral blood neutrophils. These changes enhance the respiratory burst function of neutrophils and alter the status of inflammatory and antioxidant indicators such as IL-6 and CuZn-SOD.

Metal trafficking in the cell: Combining atomic resolution with cellular dimension

Metals are widely present in biological systems as simple ions or complex cofactors, and are involved in a variety of processes essential for life. Their transport inside cells and insertion into the binding sites of the proteins that need metals to function occur through complex and selective pathways involving dedicated multiprotein machineries specifically and transiently interacting with each other, often sharing the coordination of metal ions and/or cofactors. The understanding of these machineries requires integrated approaches, ranging from bioinformatics to experimental investigations, possibly in the cellular context. In this review, we report two case studies where the use of integrated in vitro and in cellulo approaches is necessary to clarify at atomic resolution essential aspects of metal trafficking in cells.

Superoxide dismutase (SOD) family in durum wheat: promising candidates for improving crop resilience

The SOD family has been extensively analyzed at genome wide level in several crops. However, little is known about this family in durum wheat. In this study, a total of 14 TdSOD genes were identified in whole durum wheat genome including 8 TdCu-ZnSODs, 2 TdMnSODs, and 4 TdFeSODs. In silico analysis evinced that TdSOD family members displayed a closer evolutionary relationship, similar gene structure and protein features with their homologs from other plant species. Furthermore, the analysis of their promoter regions revealed the presence of a great number of cis-regulatory elements related to plant development, abiotic and biotic stresses, phytohormones, and several potential binding sites for transcription factors. Interestingly, 3D structure analysis revealed that TdCu-ZnSOD2A-2 and TdCu-ZnSOD2B-2, belonging to the Cu-Zn group, were modeled as copper chaperone for SOD like their homologs from rice and Arabidopsis. The expression profile of eight TdSOD candidate genes was investigated under salt, drought, cold, and ABA treatments. Notably, TdCu-ZnSOD2A-1, TdFeSOD4A-1, and TdFeSOD7A-1 were significantly up-regulated under all stress treatments. On the other hand, TdCu-ZnSOD7B and TdMnSOD2B were strongly expressed in roots and leaves under cold stress and TdCu-ZnSOD2B-2 was particularly up-regulated in leaves under ABA treatment. Ultimately, these findings provide valuable information for the identification of attractive candidate genes to improve wheat resilience.

Zinc-mediated interaction of copper chaperones through their heavy-metal associated domains

BACKGROUND

A copper chaperone CCS is a multi-domain protein that supplies a copper ion to Cu/Zn-superoxide dismutase (SOD1). Among the domains of CCS, the N-terminal domain (CCS^dI) belongs to a heavy metal-associated (HMA) domain, in which a Cys-x-x-Cys (CxxC) motif binds a heavy metal ion. It has hence been expected that the HMA domain in CCS has a role in the metal trafficking; however, the CxxC motif in the domain is dispensable for supplying a copper ion to SOD1, leaving an open question on roles of CCS^dI in CCS.

METHODS

To evaluate protein-protein interactions of CCS through CCS^dI, yeast two-hybrid assay, a pull-down assay using recombinant proteins, and the analysis with fluorescence resonance energy transfer were performed.

RESULTS

We found that CCS specifically interacted with another copper chaperone HAH1, a HMA domain protein, through CCS^dI. The interaction between CCS^dI and HAH1 was not involved in the copper supply from CCS to SOD1 but was mediated by a zinc ion ligated with Cys residues of the CxxC motifs in CCS^dI and HAH1.

CONCLUSION

While physiological significance of the interaction between copper chaperones awaits further investigation, we propose that CCS^dI would have a role in the metal-mediated interaction with other proteins including heterologous copper chaperones.

Structural analysis of the overoxidized Cu/Zn-superoxide dismutase in ROS-induced ALS filament formation

Eukaryotic Cu, Zn-superoxide dismutase (SOD1) is primarily responsible for cytotoxic filament formation in amyotrophic lateral sclerosis (ALS) neurons. Two cysteine residues in SOD1 form an intramolecular disulfide bond. This study aims to explore the molecular mechanism of SOD1 filament formation by cysteine overoxidation in sporadic ALS (sALS). In this study, we determined the crystal structure of the double mutant (C57D/C146D) SOD1 that mimics the overoxidation of the disulfide-forming cysteine residues. The structure revealed the open and relaxed conformation of loop IV containing the mutated Asp57. The double mutant SOD1 produced more contagious filaments than wild-type protein, promoting filament formation of the wild-type SOD1 proteins. Importantly, we further found that HOCl treatment to the wild-type SOD1 proteins facilitated their filament formation. We propose a feasible mechanism for SOD1 filament formation in ALS from the wild-type SOD1, suggesting that overoxidized SOD1 is a triggering factor of sALS. Our findings extend our understanding of other neurodegenerative disorders associated with ROS stresses at the molecular level.

Characterization of the structure of an overoxidation-mimicking double mutant of superoxide dismutase SOD1 shows the production of more cytotoxic filaments seen in amyotrophic lateral sclerosis (ALS) neurons.

Copper delivery to an endospore coat protein of Bacillus subtilis

A family of cytosolic copper (Cu) storage proteins (the Csps) bind large quantities of Cu(I) via their Cys-lined four-helix bundles, and the majority are cytosolic (Csp3s). The presence of Csp3s in many bacteria appears inconsistent with the current dogma that bacteria, unlike eukaryotes, have evolved not to maintain intracellular pools of Cu due to its potential toxicity. Sporulation in Bacillus subtilis has been used to investigate if a Csp3 binds Cu(I) in the cytosol for a target enzyme. The activity of the Cu-requiring endospore multi-Cu oxidase BsCotA (a laccase) increases under Cu-replete conditions in wild type B. subtilis. In the strain lacking BsCsp3 lower BsCotA activity is observed and is unaffected by Cu levels. BsCsp3 loaded with Cu(I) readily activates apo-BsCotA in vitro. Experiments with a high affinity Cu(I) chelator demonstrate that Cu(I) transfer from Cu(I)-BsCsp3 must occur via an associative mechanism. BsCsp3 and BsCotA are both upregulated during late sporulation. We hypothesise that BsCsp3 acquires cuprous ions in the cytosol of B. subtilis for BsCotA.

Radio Signals from Live Cells: The Coming of Age of In-Cell Solution NMR

A detailed knowledge of the complex processes that make cells and organisms alive is fundamental in order to understand diseases and to develop novel drugs and therapeutic treatments. To this aim, biological macromolecules should ideally be characterized at atomic resolution directly within the cellular environment. Among the existing structural techniques, solution NMR stands out as the only one able to investigate at high resolution the structure and dynamic behavior of macromolecules directly in living cells. With the advent of more sensitive NMR hardware and new biotechnological tools, modern in-cell NMR approaches have been established since the early 2000s. At the coming of age of in-cell NMR, we provide a detailed overview of its developments and applications in the 20 years that followed its inception. We review the existing approaches for cell sample preparation and isotopic labeling, the application of in-cell NMR to important biological questions, and the development of NMR bioreactor devices, which greatly increase the lifetime of the cells allowing real-time monitoring of intracellular metabolites and proteins. Finally, we share our thoughts on the future perspectives of the in-cell NMR methodology.

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.

Dynamic and cell-specific transport networks for intracellular copper ions

Copper (Cu) homeostasis is essential for the development and function of many organisms. In humans, Cu misbalance causes serious pathologies and has been observed in a growing number of diseases. This Review focuses on mammalian Cu(I) transporters and highlights recent studies on regulation of intracellular Cu fluxes. Cu is used by essential metabolic enzymes for their activity. These enzymes are located in various intracellular compartments and outside cells. When cells differentiate, or their metabolic state is otherwise altered, the need for Cu in different cell compartments change, and Cu has to be redistributed to accommodate these changes. The Cu transporters SLC31A1 (CTR1), SLC31A2 (CTR2), ATP7A and ATP7B regulate Cu content in cellular compartments and maintain Cu homeostasis. Increasing numbers of regulatory proteins have been shown to contribute to multifaceted regulation of these Cu transporters. It is becoming abundantly clear that the Cu transport networks are dynamic and cell specific. The comparison of the Cu transport machinery in the liver and intestine illustrates the distinct composition and dissimilar regulatory response of their Cu transporters to changing Cu levels.

Phenotypic diversity in ALS and the role of poly-conformational protein misfolding

In many types of familial amyotrophic lateral sclerosis (fALS), mutations cause proteins to gain toxic properties that mediate neurodegenerative processes. It is becoming increasingly clear that the proteins involved in ALS, and those responsible for a host of other neurodegenerative diseases, share many characteristics with a growing number of prion diseases. ALS is a heterogenous disease in which the majority of cases are sporadic in their etiology. Studies investigating the inherited forms of the disease are now beginning to provide evidence that some of this heterogeneity may be due to the existence of distinct conformations that ALS-linked proteins can adopt to produce the equivalent of prion strains. In this review, we discuss the in vitro and in vivo evidence that has been generated to better understand the characteristics of these proteins and how their tertiary structure may impact the disease phenotype.

Molecular and pharmacological chaperones for SOD1

The efficacy of superoxide dismutase-1 (SOD1) folding impacts neuronal loss in motor system neurodegenerative diseases. Mutations can prevent SOD1 post-translational processing leading to misfolding and cytoplasmic aggregation in familial amyotrophic lateral sclerosis (ALS). Evidence of immature, wild-type SOD1 misfolding has also been observed in sporadic ALS, non-SOD1 familial ALS and Parkinson's disease. The copper chaperone for SOD1 (hCCS) is a dedicated and specific chaperone that assists SOD1 folding and maturation to produce the active enzyme. Misfolded or misfolding prone SOD1 also interacts with heat shock proteins and macrophage migration inhibitory factor to aid folding, refolding or degradation. Recognition of specific SOD1 structures by the molecular chaperone network and timely dissociation of SOD1-chaperone complexes are, therefore, important steps in SOD1 processing. Harnessing these interactions for therapeutic benefit is actively pursued as is the modulation of SOD1 behaviour with pharmacological and peptide chaperones. This review highlights the structural and mechanistic aspects of a selection of SOD1-chaperone interactions together with their impact on disease models.

Bacterial evolutionary precursors of eukaryotic copper-zinc superoxide dismutases

Internalization of a bacteria by an archaeal cell expedited eukaryotic evolution. An important feature of the species that diversified into the great variety of eukaryotic life visible today was the ability to combat oxidative stress with a copper–zinc superoxide dismutase (CuZnSOD) enzyme activated by a specific, high-affinity copper chaperone. Adoption of a single protein interface that facilitates homodimerization and heterodimerization was essential; however, its evolution has been difficult to rationalize given the structural differences between bacterial and eukaryotic enzymes. In contrast, no consistent strategy for the maturation of periplasmic bacterial CuZnSODs has emerged. Here, 34 CuZnSODs are described that closely resemble the eukaryotic form but originate predominantly from aquatic bacteria. Crystal structures of a Bacteroidetes bacterium CuZnSOD portray both prokaryotic and eukaryotic characteristics and propose a mechanism for self-catalyzed disulfide maturation. Unification of a bacterial but eukaryotic-like CuZnSOD along with a ferredoxin-fold MXCXXC copper-binding domain within a single polypeptide created the advanced copper delivery system for CuZnSODs exemplified by the human copper chaperone for superoxide dismutase-1. The development of this system facilitated evolution of large and compartmentalized cells following endosymbiotic eukaryogenesis.

Superoxide Dismutase 1 in Health and Disease: How a Frontline Antioxidant Becomes Neurotoxic

Cu/Zn superoxide dismutase (SOD1) is a frontline antioxidant enzyme catalysing superoxide breakdown and is important for most forms of eukaryotic life. The evolution of aerobic respiration by mitochondria increased cellular production of superoxide, resulting in an increased reliance upon SOD1. Consistent with the importance of SOD1 for cellular health, many human diseases of the central nervous system involve perturbations in SOD1 biology. But far from providing a simple demonstration of how disease arises from SOD1 loss-of-function, attempts to elucidate pathways by which atypical SOD1 biology leads to neurodegeneration have revealed unexpectedly complex molecular characteristics delineating healthy, functional SOD1 protein from that which likely contributes to central nervous system disease. This review summarises current understanding of SOD1 biology from SOD1 genetics through to protein function and stability.

Superoxide dismutase 1 (SOD1) and cadmium: A three models approach to the comprehension of its neurotoxic effects

Cadmium (Cd) is a widespread toxic environmental contaminant, released by anthropogenic activities. It interferes with essential metal ions homeostasis and affects protein structures and functions by substituting zinc, copper and iron. In this study, the effect of cadmium on SOD1, a CuZn metalloenzyme catalyzing superoxide conversion into hydrogen peroxide, has been investigated in three different biological models. We first evaluated the effects of cadmium combined with copper and/or zinc on the recombinant GST-SOD1, expressed in E. coli BL21. The enzyme activity and expression were investigated in the presence of fixed copper and/or zinc doses with different cadmium concentrations, in the cellular medium. Cadmium caused a dose-dependent reduction in SOD1 activity, while the expression remains constant. Similar results were obtained in the cellular model represented by the human SH-SY5Y neuronal cell line. After cadmium treatment for 24 and 48 h, SOD1 enzymatic activity decreased in a dose- and time-dependent way, while the protein expression remained constant. Finally, a 16 h cadmium treatment caused a 25 % reduction of CuZn-SOD activity without affecting the protein expression in the Caenorhabditis elegans model. Taken together our results show an inhibitory effect of cadmium on SOD1 enzymatic activity, without affecting the protein expression, in all the biological models used, suggesting that cadmium can displace zinc from the enzyme catalytic site.

Role of Oxidative Stress in the Pathogenesis of Amyotrophic Lateral Sclerosis: Antioxidant Metalloenzymes and Therapeutic Strategies

Amyotrophic lateral sclerosis (ALS) affects motor neurons in the cerebral cortex, brainstem and spinal cord and leads to death due to respiratory failure within three to five years. Although the clinical symptoms of this disease were first described in 1869 and it is the most common motor neuron disease and the most common neurodegenerative disease in middle-aged individuals, the exact etiopathogenesis of ALS remains unclear and it remains incurable. However, free oxygen radicals (i.e., molecules containing one or more free electrons) are known to contribute to the pathogenesis of this disease as they very readily bind intracellular structures, leading to functional impairment. Antioxidant enzymes, which are often metalloenzymes, inactivate free oxygen radicals by converting them into a less harmful substance. One of the most important antioxidant enzymes is Cu²⁺Zn²⁺ superoxide dismutase (SOD1), which is mutated in 20% of cases of the familial form of ALS (fALS) and up to 7% of sporadic ALS (sALS) cases. In addition, the proper functioning of catalase and glutathione peroxidase (GPx) is essential for antioxidant protection. In this review article, we focus on the mechanisms through which these enzymes are involved in the antioxidant response to oxidative stress and thus the pathogenesis of ALS and their potential as therapeutic targets.

Sclerotinia sclerotiorum utilizes host-derived copper for ROS detoxification and infection

Necrotrophic plant pathogen induces host reactive oxygen species (ROS) production, which leads to necrosis in the host, allowing the pathogen to absorb nutrients from the dead tissues. Sclerotinia sclerotiorum is a typical necrotrophic pathogen that causes Sclerotinia stem rot in more than 400 species, resulting in serious economic losses. Here, we found that three S. sclerotiorum genes involved in copper ion import/transport, SsCTR1, SsCCS and SsATX1, were significantly up-regulated during infection of Brassica oleracea. Function analysis revealed that these genes involved in fungal ROS detoxification and virulence. On the host side, four genes putatively involved in copper ion homeostasis, BolCCS, BolCCH, BolMT2A and BolDRT112, were significantly down-regulated in susceptible B. oleracea, but stably expressed in resistant B. oleracea during infection. Their homologs were found to promote resistance to S. sclerotiorum and increase antioxidant activity in Arabidopsis thaliana. Furthermore, copper concentration analysis indicated that copper flow from healthy area into the necrotic area during infection. A model was proposed that S. sclerotiorum utilizes host copper to detoxify ROS in its cells, whereas the resistant hosts may restrict the supply of essential copper nutrients to S. sclerotiorum by maintaining copper ion homeostasis during infection.

Molecular machinery providing copper bioavailability for spermatozoa along the epididymial tubule in mouse

Progressive functional maturation of spermatozoa is completed during the transit of these cells through the epididymis, a tubule structure connecting a testicle to a vas deferens. Epididymal epithelial cells by means of their secretory and absorptive functions determine a highly specialized luminal microenvironment containing multiple organic and inorganic components. The latter include copper ions, which due to their redox properties are indispensable for critical homeostatic processes occurring in spermatozoa floating in different part of epididymis but can be potentially toxic. Main purpose of our study was to determine epididymal region-dependent expression and localization of copper transporters ensuring a tight control of copper concentration in epididymal fluid. We also aimed at identifying proteins responsible for copper uptake by spermatozoa and verifying whether this process is coordinated with copper supply to superoxide dismutase 1 (SOD1), a copper-dependent antioxidant enzyme. Our study identifies two ATPases-ATP7A, ATP7B and Slc31a1, major copper importers/exporters depending on their differential expression on epididymal polarized epithelial cells of the caput, corpus, and cauda. Next, ceruloplasmin seems to be a chief protein transporting copper in the epididymal fluid and providing this biometal to spermatozoa. The entry of copper to germ cells is mediated by Slc31a1 and is correlated with both expressions of copper chaperone for superoxide dismutase (CCS), copper chaperone directly providing copper ions to SOD1 and with the expression and activity of the latter. Our results outline a network of cooperating copper binding proteins expressed in epididymal epithelium and in spermatozoa that orchestrate bioavailability of this microelement for gametes and protect them against copper toxicity.

Real-Time Quantitative In-Cell NMR: Ligand Binding and Protein Oxidation Monitored in Human Cells Using Multivariate Curve Resolution

In-cell NMR can investigate protein conformational changes at atomic resolution, such as those changes induced by drug binding or chemical modifications, directly in living human cells, and therefore has great potential in the context of drug development as it can provide an early assessment of drug potency. NMR bioreactors can greatly improve the cell sample stability over time and, more importantly, allow for recording in-cell NMR data in real time to monitor the evolution of intracellular processes, thus providing unique insights into the kinetics of drug-target interactions. However, current implementations are limited by low cell viability at >24 h times, the reduced sensitivity compared to “static” experiments and the lack of protocols for automated and quantitative analysis of large amounts of data. Here, we report an improved bioreactor design which maintains human cells alive and metabolically active for up to 72 h, and a semiautomated workflow for quantitative analysis of real-time in-cell NMR data relying on Multivariate Curve Resolution. We apply this setup to monitor protein–ligand interactions and protein oxidation in real time. High-quality concentration profiles can be obtained from noisy 1D and 2D NMR data with high temporal resolution, allowing further analysis by fitting with kinetic models. This unique approach can therefore be applied to investigate complex kinetic behaviors of macromolecules in a cellular setting, and could be extended in principle to any real-time NMR application in live cells.

Copper Sources for Sod1 Activation

Copper ions (i.e., copper) are a critical part of several cellular processes, but tight regulation of copper levels and trafficking are required to keep the cell protected from this highly reactive transition metal. Cu, Zn superoxide dismutase (Sod1) protects the cell from the accumulation of radical oxygen species by way of the redox cycling activity of copper in its catalytic center. Multiple posttranslational modification events, including copper incorporation, are reliant on the copper chaperone for Sod1 (Ccs). The high-affinity copper uptake protein (Ctr1) is the main entry point of copper into eukaryotic cells and can directly supply copper to Ccs along with other known intracellular chaperones and trafficking molecules. This review explores the routes of copper delivery that are utilized to activate Sod1 and the usefulness and necessity of each.

Exploring the Extended Biological Functions of the Human Copper Chaperone of Superoxide Dismutase 1

The human copper chaperone of SOD1 (designated as CCS) was discovered more than two decades ago. It is an important copper binding protein and a homolog of Saccharomyces cerevisiae LYS7. To date, no studies have systematically or specifically elaborated on the functional development of CCS. This review summarizes the essential information about CCS, such as its localization, 3D structure, and copper binding ability. An emphasis is placed on its interacting protein partners and its biological functions in vivo and in vitro. Three-dimensional structural analysis revealed that CCS is composed of three domains. Its primary molecular function is the delivery of copper to SOD1 and activation of SOD1. It has also been reported to bind to XIAP, Mia40, and X11α, and other proteins. Through these protein partners, CCS is implicated in several vital biological processes in vivo, such as copper homeostasis, apoptosis, angiogenesis and oxidative stress. This review is anticipated to assist scientists in systematically understanding the latest research developments of CCS for facilitating the development of new therapeutics targeting CCS in the future.

Silence superoxide dismutase 1 (SOD1): a promising therapeutic target for amyotrophic lateral sclerosis (ALS)

Introduction: Amyotrophic lateral sclerosis (ALS) is a progressive and incurable neurodegenerative disorder that targets upper and lower motor neurons and leads to fatal muscle paralysis. Mutations in the superoxide dismutase 1 (SOD1) gene are responsible for 15% of familial ALS cases, but several studies have indicated that SOD1 dysfunction may also play a pathogenic role in sporadic ALS. SOD1 induces numerous toxic effects through the pathological misfolding and aggregation of mutant SOD1 species, hence a reduction of the levels of toxic variants appears to be a promising therapeutic strategy for SOD1-related ALS. Several methods are used to modulate gene expression in vivo; these include RNA interference, antisense oligonucleotides (ASOs) and CRISPR/Cas9 technology.Areas covered: This paper examines the current approaches for gene silencing and the progress made in silencing SOD1 in vivo. It progresses to shed light on the key results and pitfalls of these studies and highlights the future challenges and new perspectives for this exciting research field.Expert opinion: Gene silencing strategies targeting SOD1 may represent effective approaches for familial and sporadic ALS-related neurodegeneration; however, the risk of off-target effects must be minimized, and effective and minimally invasive delivery strategies should be fine-tuned.

Mutations in Superoxide Dismutase 1 (Sod1) Linked to Familial Amyotrophic Lateral Sclerosis Can Disrupt High-Affinity Zinc-Binding Promoted by the Copper Chaperone for Sod1 (Ccs)

Zinc (II) ions (hereafter simplified as zinc) are important for the structural and functional activity of many proteins. For Cu, Zn superoxide dismutase (Sod1), zinc stabilizes the native structure of each Sod1 monomer, promotes homo-dimerization and plays an important role in activity by "softening" the active site so that copper cycling between Cu(I) and Cu(II) can rapidly occur. Previously, we have reported that binding of Sod1 by its copper chaperone (Ccs) stabilizes a conformation of Sod1 that promotes site-specific high-affinity zinc binding. While there are a multitude of Sod1 mutations linked to the familial form of amyotrophic lateral sclerosis (fALS), characterizations by multiple research groups have been unable to realize strong commonalities among mutants. Here, we examine a set of fALS-linked Sod1 mutations that have been well-characterized and are known to possess variation in their biophysical characteristics. The zinc affinities of these mutants are evaluated here for the first time and then compared with the previously established value for wild-type Sod1 zinc affinity. Ccs does not have the same ability to promote zinc binding to these mutants as it does for the wild-type version of Sod1. Our data provides a deeper look into how (non)productive Sod1 maturation by Ccs may link a diverse set of fALS-Sod1 mutations.

Nucleation and kinetics of SOD1 aggregation in human cells for ALS1

Aberrant structural formations of Cu/Zn superoxide dismutase enzyme (SOD1) are the probable mechanism by which circumscribed mutations in the SOD1 gene cause familial amyotrophic lateral sclerosis (ALS1). SOD1 forms aberrant structures which can proceed by nucleation to insoluble aggregates. Here, the SOD1 aggregation reaction was investigated predominantly by time-course studies on ALS1 variants G85R, G37R, D101G, and D101N in human embryonic kidney cells (HEK293FT), with analysis by detergent ultracentrifugation extractions and high-resolution PAGE methodologies. Nucleation was found to be pseudo-zeroth order and dependent on time and concentration at constant 37.0 °C and pH 7.4. The predominant subsets of the total SOD1 expression set which comprised the nucleation phase were both soluble and insoluble inactive monomers, trimers, and hexamers with reduced intra-disulfide bonds. Superoxide exposure via paraquat initiated the formation of SOD1 trimers in untransfected SH-SY5Y cells and increased the aggregation propensity of G85R in HEK293FT. These data show the kinetic formation of aberrant SOD1 subsets implicated in ALS1 and indicate that superoxide substrate may initiate its radical polymerization. In an instance of the utility of methodological reductionism in molecular theory: though many ALS1 variants retain their global enzymatic activity, the SOD1 subsets most implicated in causing ALS1 do not retain their specific activity.

Methylglyoxal interaction with superoxide dismutase 1

Methylglyoxal (MG) is a highly reactive aldehyde spontaneously formed in human cells mainly as a by-product of glycolysis. Such endogenous metabolite reacts with proteins, nucleotides and lipids forming advanced glycation end-products (AGEs). MG binds to arginine, lysine and cysteine residues of proteins causing the formation of stable adducts that can interfere with protein function. Among the proteins affected by glycation, MG has been found to react with superoxide dismutase 1 (SOD1), a fundamental anti-oxidant enzyme that is abundantly expressed in neurons. Considering the high neuronal susceptibility to MG-induced oxidative stress, we sought to investigate by mass spectrometry and NMR spectroscopy which are the structural modifications induced on SOD1 by the reaction with MG. We show that MG reacts preferentially with the disulfide-reduced, demetallated form of SOD1, gradually causing its unfolding, and to a lesser extent, with the intermediate state of maturation – the reduced, zinc-bound homodimer – causing its gradual monomerization. These results suggest that MG could impair the correct maturation of SOD1 in vivo, thus both increasing cellular oxidative stress and promoting the cytotoxic misfolding and aggregation process of SOD1.

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MG forms stable adducts with the immature forms of SOD1.

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MG causes the unfolding of the apo-SOD1^SH form.

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MG causes the monomerization of the E,Zn-SOD1^SH form.

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In both forms, arginine 143 is more prone to interact with MG.

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The structural modifications caused by MG impair the correct maturation of SOD1.

Copper-only superoxide dismutase enzymes and iron starvation stress in Candida fungal pathogens

Copper (Cu)-only superoxide dismutases (SOD) represent a newly characterized class of extracellular SODs important for virulence of several fungal pathogens. Previous studies of the Cu-only enzyme SOD5 from the opportunistic fungal pathogen Candida albicans have revealed that the active-site structure and Cu binding of SOD5 strongly deviate from those of Cu/Zn-SODs in its animal hosts, making Cu-only SODs a possible target for future antifungal drug design. C. albicans also expresses a Cu-only SOD4 that is highly similar in sequence to SOD5, but is poorly characterized. Here, we compared the biochemical, biophysical, and cell biological properties of C. albicans SOD4 and SOD5. Analyzing the recombinant proteins, we found that, similar to SOD5, Cu-only SOD4 can react with superoxide at rates approaching diffusion limits. Both SODs were monomeric and they exhibited similar binding affinities for their Cu cofactor. In C. albicans cultures, SOD4 and SOD5 were predominantly cell wall proteins. Despite these similarities, the SOD4 and SOD5 genes strongly differed in transcriptional regulation. SOD5 was predominantly induced during hyphal morphogenesis, together with a fungal burst in reactive oxygen species. Conversely, SOD4 expression was specifically up-regulated by iron (Fe) starvation and controlled by the Fe-responsive transcription factor SEF1. Interestingly, Candida tropicalis and the emerging fungal pathogen Candida auris contain a single SOD5-like SOD rather than a pair, and in both fungi, this SOD was induced by Fe starvation. This unexpected link between Fe homeostasis and extracellular Cu-SODs may help many fungi adapt to Fe-limited conditions of their hosts.

Evolutionary Analyses of Sequence and Structure Space Unravel the Structural Facets of SOD1

Superoxide dismutase (SOD) is the primary enzyme of the cellular antioxidant defense cascade. Misfolding, concomitant oligomerization, and higher order aggregation of human cytosolic SOD are linked to amyotrophic lateral sclerosis (ALS). Although, with two metal ion cofactors SOD1 is extremely robust, the de-metallated apo form is intrinsically disordered. Since the rise of oxygen-based metabolism and antioxidant defense systems are evolutionary coupled, SOD is an interesting protein with a deep evolutionary history. We deployed statistical analysis of sequence space to decode evolutionarily co-varying residues in this protein. These were validated by applying graph theoretical modelling to understand the impact of the presence of metal ion co-factors in dictating the disordered (apo) to hidden disordered (wild-type SOD1) transition. Contact maps were generated for different variants, and the selected significant residues were mapped on separate structure networks. Sequence space analysis coupled with structure networks helped us to map the evolutionarily coupled co-varying patches in the SOD1 and its metal-depleted variants. In addition, using structure network analysis, the residues with a major impact on the internal dynamics of the protein structure were investigated. Our results reveal that the bulk of these evolutionarily co-varying residues are localized in the loop regions and positioned differentially depending upon the metal residence and concomitant steric restrictions of the loops.

The biophysics of superoxide dismutase-1 and amyotrophic lateral sclerosis

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

Prion-Like Propagation of Protein Misfolding and Aggregation in Amyotrophic Lateral Sclerosis

The discovery that prion protein can misfold into a pathological conformation that encodes structural information capable of both propagation and inducing severe neuropathology has revolutionized our understanding of neurodegenerative disease. Many neurodegenerative diseases with a protein misfolding component are now classified as “prion-like” owing to the propagation of both symptoms and protein aggregation pathology in affected individuals. The neuromuscular disorder amyotrophic lateral sclerosis (ALS) is characterized by protein inclusions formed by either TAR DNA-binding protein of 43 kDa (TDP-43), Cu/Zn superoxide dismutase (SOD1), or fused in sarcoma (FUS), in both upper and lower motor neurons. Evidence from in vitro, cell culture, and in vivo studies has provided strong evidence to support the involvement of a prion-like mechanism in ALS. In this article, we review the evidence suggesting that prion-like propagation of protein aggregation is a primary pathomechanism in ALS, focusing on the key proteins and genes involved in disease (TDP-43, SOD1, FUS, and C9orf72). In each case, we discuss the evidence ranging from biophysical studies to in vivo examinations of prion-like spreading. We suggest that the idiopathic nature of ALS may stem from its prion-like nature and that elucidation of the specific propagating protein assemblies is paramount to developing effective therapies.

Targeting of copper-trafficking chaperones causes gene-specific systemic pathology in Drosophila melanogaster: prospective expansion of mutational landscapes that regulate tumor resistance to cisplatin

Copper, a transition metal, is an essential component for normal growth and development. It acts as a critical co-factor of many enzymes that play key roles in diverse cellular processes. The present study attempts to investigate the regulatory functions decisively controlling copper trafficking during development and aging of the Drosophila model system. Hence, through engagement of the GAL4/UAS genetic platform and RNAi technology, we herein examined the in vivo significance of Atox1 and CCS genes, products of which pivotally govern cellular copper trafficking in fly tissue pathophysiology. Specifically, we analyzed the systemic effects of their targeted downregulation on the eye, wing, neuronal cell populations and whole-body tissues of the fly. Our results reveal that, in contrast to the eye, suppression of their expression in the wing leads to a notable increase in the percentage of malformed organs observed. Furthermore, we show that Atox1 or CCS gene silencing in either neuronal or whole-body tissues can critically affect the viability and climbing capacity of transgenic flies, while their double-genetic targeting suggests a rather synergistic mode of action of the cognate protein products. Interestingly, pharmacological intervention with the anti-cancer drug cisplatin indicates the major contribution of CCS copper chaperone to cisplatin's cellular trafficking, and presumably to tumor resistance often acquired during chemotherapy. Altogether, it seems that Atox1 and CCS proteins serve as tissue/organ-specific principal regulators of physiological Drosophila development and aging, while their tissue-dependent downregulation can provide important insights for Atox1 and CCS potential exploitation as predictive gene biomarkers of cancer-cell chemotherapy responses.

Summary: We demonstrate the essential roles of Atox1 and CCS copper-trafficking chaperones in Drosophila development and aging. We also provide insights for their therapeutic exploitation as cisplatin regulators during cancer chemotherapy.

Network mapping of the conformational heterogeneity of SOD1 by deploying statistical cluster analysis of FTIR spectra

A crucial contribution to the heterogeneity of the conformational landscape of a protein comes from the way an intermediate relates to another intermediate state in its journey from the unfolded to folded or misfolded form. Unfortunately, it is extremely hard to decode this relatedness in a quantifiable manner. Here, we developed an application of statistical cluster analyses to explore the conformational heterogeneity of a metalloenzyme, human cytosolic copper-zinc superoxide dismutase (SOD1), using the inputs from infrared spectroscopy. This study provides a quantifiable picture of how conformational information at one particular site (for example, the copper-binding pocket) is related to the information at the second site (for example, the zinc-binding pocket), and how this relatedness is transferred to the global conformational information of the protein. The distance outputs were used to quantitatively generate a network capturing the folding sub-stages of SOD1.

Crosstalk Between Oxidative Stress and Mitochondrial Damage: Focus on Amyotrophic Lateral Sclerosis

Proteins oxidation by reactive species is implicated in the aetiology or progression of a panoply of disorders and diseases such as neurodegenerative disorders. It is becoming increasingly evident that redox imbalance in the brain mediates neurodegeneration. Free radicals, as reactive species of oxygen (ROS) but also reactive nitrogen species (RNS) and reactive sulfur species (RSS), are generated in vivo from several sources. Within the cell the mitochondria represent the main source of ROS and mitochondrial dysfunction is both the major contributor to oxidative stress (OS) as well its major consequence.To date there are no doubts that a condition of OS added to other factors as mitochondrial damage in mtDNA or mitochondrial respiratory chain, may contribute to trigger or amplify mechanisms leading to neurodegenerative disorders.In this chapter, we aim at illustrate the molecular interplay occurring between mitochondria and OS focusing on Amyotrophic Lateral Sclerosis, describing a phenotypic reprogramming mechanism of mitochondria in complex neurological disorder.

Folding of copper proteins: role of the metal?

Copper is a redox-active transition metal ion required for the function of many essential human proteins. For biosynthesis of proteins coordinating copper, the metal may bind before, during or after folding of the polypeptide. If the metal binds to unfolded or partially folded structures of the protein, such coordination may modulate the folding reaction. The molecular understanding of how copper is incorporated into proteins requires descriptions of chemical, thermodynamic, kinetic and structural parameters involved in the formation of protein-metal complexes. Because free copper ions are toxic, living systems have elaborate copper-transport systems that include particular proteins that facilitate efficient and specific delivery of copper ions to target proteins. Therefore, these pathways become an integral part of copper protein folding in vivo. This review summarizes biophysical-molecular in vitro work assessing the role of copper in folding and stability of copper-binding proteins as well as protein-protein copper exchange reactions between human copper transport proteins. We also describe some recent findings about the participation of copper ions and copper proteins in protein misfolding and aggregation reactions in vitro.

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.

Copper-zinc superoxide dismutase (Sod1) activation terminates interaction between its copper chaperone (Ccs) and the cytosolic metal-binding domain of the copper importer Ctr1

Copper-zinc superoxide dismutase (Sod1) is a critical antioxidant enzyme that rids the cell of reactive oxygen through the redox cycling of a catalytic copper ion provided by its copper chaperone (Ccs). Ccs must first acquire this copper ion, directly or indirectly, from the influx copper transporter, Ctr1. The three proteins of this transport pathway ensure careful trafficking of copper ions from cell entry to target delivery, but the intricacies remain undefined. Biochemical examination of each step in the pathway determined that the activation of the target (Sod1) regulates the Ccs·Ctr1 interaction. Ccs stably interacts with the cytosolic C-terminal tail of Ctr1 (Ctr1c) in a copper-dependent manner. This interaction becomes tripartite upon the addition of an engineered immature form of Sod1 creating a stable Cu(I)-Ctr1c·Ccs·Sod1 heterotrimer in solution. This heterotrimer can also be made by the addition of a preformed Sod1·Ccs heterodimer to Cu(I)-Ctr1c, suggestive of multiple routes to the same destination. Only complete Sod1 activation (i.e. active site copper delivery and intra-subunit disulfide bond formation) breaks the Sod1·Ccs·Ctr1c complex. The results provide a new and extended view of the Sod1 activation pathway(s) originating at cellular copper import.

What Are We Missing by Not Measuring the Net Charge of Proteins?

The net electrostatic charge (Z) of a folded protein in solution represents a bird's eye view of its surface potentials-including contributions from tightly bound metal, solvent, buffer, and cosolvent ions-and remains one of its most enigmatic properties. Few tools are available to the average biochemist to rapidly and accurately measure Z at pH≠pI. Tools that have been developed more recently seem to go unnoticed. Most scientists are content with this void and estimate the net charge of a protein from its amino acid sequence, using textbook values of pK_a . Thus, Z remains unmeasured for nearly all folded proteins at pH≠pI. When marveling at all that has been learned from accurately measuring the other fundamental property of a protein-its mass-one wonders: what are we missing by not measuring the net charge of folded, solvated proteins? A few big questions immediately emerge in bioinorganic chemistry. When a single electron is transferred to a metalloprotein, does the net charge of the protein change by approximately one elementary unit of charge or does charge regulation dominate, that is, do the pK_a values of most ionizable residues (or just a few residues) adjust in response to (or in concert with) electron transfer? Would the free energy of charge regulation (ΔΔG_z ) account for most of the outer sphere reorganization energy associated with electron transfer? Or would ΔΔG_z contribute more to the redox potential? And what about metal binding itself? When an apo-metalloprotein, bearing minimal net negative charge (e.g., Z=-2.0) binds one or more metal cations, is the net charge abolished or inverted to positive? Or do metalloproteins regulate net charge when coordinating metal ions? The author's group has recently dusted off a relatively obscure tool-the "protein charge ladder"-and used it to begin to answer these basic questions.

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.

Molecular recognition and maturation of SOD1 by its evolutionarily destabilised cognate chaperone hCCS

Superoxide dismutase-1 (SOD1) maturation comprises a string of posttranslational modifications which transform the nascent peptide into a stable and active enzyme. The successive folding, metal ion binding, and disulphide acquisition steps in this pathway can be catalysed through a direct interaction with the copper chaperone for SOD1 (CCS). This process confers enzymatic activity and reduces access to noncanonical, aggregation-prone states. Here, we present the functional mechanisms of human copper chaperone for SOD1 (hCCS)-catalysed SOD1 activation based on crystal structures of reaction precursors, intermediates, and products. Molecular recognition of immature SOD1 by hCCS is driven by several interface interactions, which provide an extended surface upon which SOD1 folds. Induced-fit complexation is reliant on the structural plasticity of the immature SOD1 disulphide sub-loop, a characteristic which contributes to misfolding and aggregation in neurodegenerative disease. Complexation specifically stabilises the SOD1 disulphide sub-loop, priming it and the active site for copper transfer, while delaying disulphide formation and complex dissociation. Critically, a single destabilising amino acid substitution within the hCCS interface reduces hCCS homodimer affinity, creating a pool of hCCS available to interact with immature SOD1. hCCS substrate specificity, segregation between solvent and biological membranes, and interaction transience are direct results of this substitution. In this way, hCCS-catalysed SOD1 maturation is finessed to minimise copper wastage and reduce production of potentially toxic SOD1 species.

Cadmium effects on superoxide dismutase 1 in human cells revealed by NMR

Cadmium is a toxic pollutant that in recent decades has become more widespread in the environment due to anthropogenic activities, significantly increasing the risk of exposure. Concurrently, a continually growing body of research has begun to enumerate the harmful effects that this heavy metal has on human health. Consequently, additional research is required to better understand the mechanism and effects of cadmium at the molecular level. The main mechanism of cadmium toxicity is based on the indirect induction of severe oxidative stress, through several processes that unbalance the anti-oxidant cellular defence system, including the displacement of metals such as zinc from its native binding sites. Such mechanism was thought to alter the in vivo enzymatic activity of SOD1, one of the main antioxidant proteins of many tissues, including the central nervous system. SOD1 misfolding and aggregation is correlated with cytotoxicity in neurodegenerative diseases such as amyotrophic lateral sclerosis. We assessed the effect of cadmium on SOD1 folding and maturation pathway directly in human cells through in-cell NMR. Cadmium does not directly bind intracellular SOD1, instead causes the formation of its intramolecular disulfide bond in the zinc-bound form. Metallothionein overexpression is strongly induced by cadmium, reaching NMR-detectable levels. The intracellular availability of zinc modulates both SOD1 oxidation and metallothionein overexpression, strengthening the notion that zinc-loaded metallothioneins help maintaining the redox balance under cadmium-induced acute stress.

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Cadmium does not bind to superoxide dismutase 1 (SOD1) in human cells.

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In defect of zinc, cadmium causes the premature oxidation of SOD1.

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Cadmium induces the overexpression of metallothioneins to levels detectable by NMR.

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Zinc modulates metallothionein expression and attenuates SOD1 oxidation.

The yeast copper chaperone for copper-zinc superoxide dismutase (CCS1) is a multifunctional chaperone promoting all levels of SOD1 maturation

Copper (Cu) is essential for the survival of aerobic organisms through its interaction with molecular oxygen (O₂). However, Cu's chemical properties also make it toxic, requiring specific cellular mechanisms for Cu uptake and handling, mediated by Cu chaperones. CCS1, the budding yeast (S. cerevisiae) Cu chaperone for Cu-zinc (Zn) superoxide dismutase (SOD1) activates by directly promoting both Cu delivery and disulfide formation in SOD1. The complete mechanistic details of this transaction along with recently proposed molecular chaperone-like functions for CCS1 remain undefined. Here, we present combined structural, spectroscopic, kinetic, and thermodynamic data that suggest a multifunctional chaperoning role(s) for CCS1 during SOD1 activation. We observed that CCS1 preferentially binds a completely immature form of SOD1 and that the SOD1·CCS1 interaction promotes high-affinity Zn(II) binding in SOD1. Conserved aromatic residues within the CCS1 C-terminal domain are integral in these processes. Previously, we have shown that CCS1 delivers Cu(I) to an entry site at the SOD1·CCS1 interface upon binding. We show here that Cu(I) is transferred from CCS1 to the entry site and then to the SOD1 active site by a thermodynamically driven affinity gradient. We also noted that efficient transfer from the entry site to the active site is entirely dependent upon the oxidation of the conserved intrasubunit disulfide bond in SOD1. Our results herein provide a solid foundation for proposing a complete molecular mechanism for CCS1 activity and reclassification as a first-of-its-kind "dual chaperone."