Ebselen as template for stabilization of A4V mutant dimer for motor neuron disease therapy

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

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

Evaluating protein cross-linking as a therapeutic strategy to stabilize SOD1 variants in a mouse model of familial ALS

Mutations in the gene encoding Cu-Zn superoxide dismutase 1 (SOD1) cause a subset of familial amyotrophic lateral sclerosis (fALS) cases. A shared effect of these mutations is that SOD1, which is normally a stable dimer, dissociates into toxic monomers that seed toxic aggregates. Considerable research effort has been devoted to developing compounds that stabilize the dimer of fALS SOD1 variants, but unfortunately, this has not yet resulted in a treatment. We hypothesized that cyclic thiosulfinate cross-linkers, which selectively target a rare, 2 cysteine-containing motif, can stabilize fALS-causing SOD1 variants in vivo. We created a library of chemically diverse cyclic thiosulfinates and determined structure-cross-linking-activity relationships. A pre-lead compound, "S-XL6," was selected based upon its cross-linking rate and drug-like properties. Co-crystallographic structure clearly establishes the binding of S-XL6 at Cys 111 bridging the monomers and stabilizing the SOD1 dimer. Biophysical studies reveal that the degree of stabilization afforded by S-XL6 (up to 24°C) is unprecedented for fALS, and to our knowledge, for any protein target of any kinetic stabilizer. Gene silencing and protein degrading therapeutic approaches require careful dose titration to balance the benefit of diminished fALS SOD1 expression with the toxic loss-of-enzymatic function. We show that S-XL6 does not share this liability because it rescues the activity of fALS SOD1 variants. No pharmacological agent has been proven to bind to SOD1 in vivo. Here, using a fALS mouse model, we demonstrate oral bioavailability; rapid engagement of SOD1G93A by S-XL6 that increases SOD1G93A's in vivo half-life; and that S-XL6 crosses the blood-brain barrier. S-XL6 demonstrated a degree of selectivity by avoiding off-target binding to plasma proteins. Taken together, our results indicate that cyclic thiosulfinate-mediated SOD1 stabilization should receive further attention as a potential therapeutic approach for fALS.

Polyphenolic flavonoid compounds act as the inhibitory potential of aggregation process: Implications for the prevention and therapeutics against FALS-associated D101G SOD1 mutant

Aggregation of proteins is a biological phenomenon caused by misfolded proteins. Human superoxide dismutase (hSOD1) misfolding and aggregation underlie the neurological illness amyotrophic lateral sclerosis (ALS). The most significant contributing factor to ALS is genetic point mutations in SOD1. particularly, D101G mutant is the most harmful because it significantly reduces the life expectancy of patients. Subsequently, the use of natural polyphenolic flavonoids is strongly recommended to reduce the amyloidogenic behavior of protopathic proteins. In this study, using computational parameters such as protein-ligand interaction and molecular dynamics (MD) simulation analyses, we are trying to identify a pharmacodynamically promising flavonoid compound that can effectively inhibit the pathogenic behavior of the D101G mutant. Epigallocatechin-gallate (EGCG), Hesperidin, Isorhamnetin, and Diosmetin were identified as potential leads in a preliminary screening of flavonoids to anti-amyloid action. The results of MD showed that the binding of flavonoids to D101G mutant caused changes in stability, hydrophobicity of protein, and flexibility, as well as significantly led to the restoration of lost hydrogen bonds. Secondary structure analysis showed that protein destabilization and the increased propensity of β-sheet caused by the mutation were restored to the wild-type state upon binding of flavonoids. Besides, to differentiate aggregation, we elucidated alterations in the free energy landscape (FEL) and dynamic cross-correlation matrix (DCCM) of WT-SOD1 and mutant (unbound /bound) states. Among flavonoids, Epigallocatechin-gallate and Hesperidin had the most therapeutic efficacy against the D101G mutant. Therefore, Epigallocatechin-gallate and Hesperidin promise considerable therapeutic potential to develop highly effective inhibitors in reducing fatal and irreversible ALS.

Fighting against amyotrophic lateral sclerosis (ALS) with flavonoids: a computational approach to inhibit superoxide dismutase (SOD1) mutant aggregation

Protein aggregation is a biological process that occurs when proteins misfold. Misfolding and aggregation of human superoxide dismutase (hSOD1) cause a neurodegenerative disease called amyotrophic lateral sclerosis (ALS). Among the mutations occurring, targeting the E21K mutation could be a good choice to understand the pathological mechanism of SOD1 in ALS, whereof it significantly reduces life hopefulness in patients. Naturally occurring polyphenolic flavonoids have been suggested as a way to alleviate the amyloidogenic behavior of proteins. In this study, computational tools were used to identify promising flavonoid compounds that effectively inhibit the pathogenic behavior of the E21K mutant. Initial screening identified Pelargonidin, Curcumin, and Silybin as promising leads. Molecular dynamics (MD) simulations showed that the binding of flavonoids to the mutated SOD1 caused changes in the protein stability, hydrophobicity, flexibility, and restoration of lost hydrogen bonds. Secondary structure analysis indicated that the protein destabilization and the increased propensity of β-sheet caused by the mutation were restored to the wild-type state upon binding of flavonoids. Free energy landscape (FEL) analysis was also used to differentiate aggregation, and results showed that Silybin followed by Pelargonidin had the most therapeutic efficacy against the E21K mutant SOD1. Therefore, these flavonoids hold great potential as highly effective inhibitors in mitigating ALS's fatal and insuperable effects.Communicated by Ramaswamy H. Sarma.

Small molecules targeting different cellular pathologies for the treatment of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease in which the motor neuron circuitry displays progressive degeneration, affecting mostly the motor neurons in the brain and in the spinal cord. There are no effective cures, albeit three drugs, riluzole, edaravone, and AMX0035 (a combination of sodium phenylbutyrate and taurursodiol), have been approved by the Food and Drug Administration, with limited improvement in patients. There is an urgent need to build better and more effective treatment strategies for ALS. Since the disease is very heterogenous, numerous approaches have been explored, such as targeting genetic mutations, decreasing oxidative stress and excitotoxicity, enhancing mitochondrial function and protein degradation mechanisms, and inhibiting neuroinflammation. In addition, various chemical libraries or previously identified drugs have been screened for potential repurposing in the treatment of ALS. Here, we review previous drug discovery efforts targeting a variety of cellular pathologies that occur from genetic mutations that cause ALS, such as mutations in SOD1, C9orf72, FUS, and TARDP-43 genes. These mutations result in protein aggregation, which causes neuronal degeneration. Compounds used to target cellular pathologies that stem from these mutations are discussed and comparisons among different preclinical models are presented. Because the drug discovery landscape for ALS and other motor neuron diseases is changing rapidly, we also offer recommendations for a novel, more effective, direction in ALS drug discovery that could accelerate translation of effective compounds from animals to patients.

Intrathecal Pseudodelivery of Drugs in the Therapy of Neurodegenerative Diseases: Rationale, Basis and Potential Applications

Intrathecal pseudodelivery of drugs is a novel route to administer medications to treat neurodegenerative diseases based on the CSF-sink therapeutic strategy by means of implantable devices. While the development of this therapy is still in the preclinical stage, it offers promising advantages over traditional routes of drug delivery. In this paper, we describe the rationale of this system and provide a technical report on the mechanism of action, that relies on the use of nanoporous membranes enabling selective molecular permeability. On one side, the membranes do not permit the crossing of certain drugs; whereas, on the other side, they permit the crossing of target molecules present in the CSF. Target molecules, by binding drugs inside the system, are retained or cleaved and subsequently eliminated from the central nervous system. Finally, we provide a list of potential indications, the respective molecular targets, and the proposed therapeutic agents.

Probing the polyphenolic flavonoid, morin as a highly efficacious inhibitor against amyloid(A4V) mutant SOD1 in fatal amyotrophic lateral sclerosis

Deposition of misfolded protein aggregates in key areas of human brain is the quintessential trait of various pertinent neurodegenerative disorders including amyotrophic lateral sclerosis (ALS). Genetic point mutations in Cu/Zn superoxide dismutase (SOD1) are found to be the most important contributing factor behind familial ALS. Especially, single nucleotide polymorphism (SNP) A4V is the most nocuous since it substantially decreases life expectancy of patients. Besides, the use of naturally occurring polyphenolic flavonoids is profoundly being advocated for palliating amyloidogenic behavior of proteopathic proteins. In the present analysis, through proficient computational tools, we have attempted to ascertain a pharmacodynamically promising flavonoid compound that effectively curbs the pathogenic behavior of A4V SOD1 mutant. Initial screening of flavonoids that exhibit potency against amyloids identified morin, myricetin and epigallocatechin gallate as promising leads. Further, with the help of feasible and yet adept protein-ligand interaction studies and stalwart molecular simulation analyses, we were able to observe that aforementioned flavonoids were able to considerably divert mutant A4V SOD1 from its distinct pathogenic behavior. Among which, morin showed the most curative potential against A4V SOD1. Therefore, morin holds a great therapeutic potential in contriving highly efficacious inhibitors in mitigating fatal and insuperable ALS.

First Principles Calculation of Protein-Protein Dimer Affinities of ALS-Associated SOD1 Mutants

Cu,Zn superoxide dismutase (SOD1) is a 32 kDa homodimer that converts toxic oxygen radicals in neurons to less harmful species. The dimerization of SOD1 is essential to the stability of the protein. Monomerization increases the likelihood of SOD1 misfolding into conformations associated with aggregation, cellular toxicity, and neuronal death in familial amyotrophic lateral sclerosis (fALS). The ubiquity of disease-associated mutations throughout the primary sequence of SOD1 suggests an important role of physicochemical processes, including monomerization of SOD1, in the pathology of the disease. Herein, we use a first-principles statistical mechanics method to systematically calculate the free energy of dimer binding for SOD1 using molecular dynamics, which involves sequentially computing conformational, orientational, and separation distance contributions to the binding free energy. We consider the effects of two ALS-associated mutations in SOD1 protein on dimer stability, A4V and D101N, as well as the role of metal binding and disulfide bond formation. We find that the penalty for dimer formation arising from the conformational entropy of disordered loops in SOD1 is significantly larger than that for other protein–protein interactions previously considered. In the case of the disulfide-reduced protein, this leads to a bound complex whose formation is energetically disfavored. Somewhat surprisingly, the loop free energy penalty upon dimerization is still significant for the holoprotein, despite the increased structural order induced by the bound metal cations. This resulted in a surprisingly modest increase in dimer binding free energy of only about 1.5 kcal/mol upon metalation of the protein, suggesting that the most significant stabilizing effects of metalation are on folding stability rather than dimer binding stability. The mutant A4V has an unstable dimer due to weakened monomer-monomer interactions, which are manifested in the calculation by a separation free energy surface with a lower barrier. The mutant D101N has a stable dimer partially due to an unusually rigid β-barrel in the free monomer. D101N also exhibits anticooperativity in loop folding upon dimerization. These computational calculations are, to our knowledge, the most quantitatively accurate calculations of dimer binding stability in SOD1 to date.

Repurposing small-molecule drugs for modulating toxic protein aggregates in neurodegenerative diseases

Neurodegenerative diseases (NDs) are often age-related disorders that can cause dementia in people, usually over 65 years old, are still lacking effective therapies. Some NDs have recently been linked to toxic protein aggregates, for example Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington disease; therefore, mulating toxic protein aggregates would be a promising therapeutic strategy. Moreover, drug repurposing, in other words exploiting drugs that are already in use for another indication, has been attracting mounting attention for potential therapeutic purposes in NDs. Thus, in this review, we focus on summarizing a series of repurposed small-molecule drugs for eliminating or inhibiting toxic protein aggregates and further discuss their intricate molecular mechanisms to improve the current ND treatment. Taken together, these findings will shed new light on exploiting more repurposed small-molecule drugs targeting different types of toxic proteins to fight NDs in the future. Teaser: Drug repurposing has been gaining attention to yield therapeutic potential in neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington disease.

Simulating chalcogen bonding using molecular mechanics: a pseudoatom approach to model ebselen

The organoselenium compound ebselen has recently been investigated as a treatment for COVID-19; however, efforts to model ebselen in silico have been hampered by the lack of an efficient and accurate method to assess its binding to biological macromolecules. We present here a Generalized Amber Force Field modification which incorporates classical parameters for the selenium atom in ebselen, as well as a positively charged pseudoatom to simulate the σ-hole, a quantum mechanical phenomenon that dominates the chemistry of ebselen. Our approach is justified using an energy decomposition analysis of a number of density functional theory–optimized structures, which shows that the σ-hole interaction is primarily electrostatic in origin. Finally, our model is verified by conducting molecular dynamics simulations on a number of simple complexes, as well as the clinically relevant enzyme SOD1 (superoxide dismutase), which is known to bind to ebselen.

Graphical Abstract

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.

Ebselen, a multi-target compound: its effects on biological processes and diseases

Ebselen is a well-known synthetic compound mimicking glutathione peroxidase (GPx), which catalyses some vital reactions that protect against oxidative damage. Based on a large number of in vivo and in vitro studies, various mechanisms have been proposed to explain its actions on multiple targets. It targets thiol-related compounds, including cysteine, glutathione, and thiol proteins (e.g., thioredoxin and thioredoxin reductase). Owing to this, ebselen is a unique multifunctional agent with important effects on inflammation, apoptosis, oxidative stress, cell differentiation, immune regulation and neurodegenerative disease, with anti-microbial, detoxifying and anti-tumour activity. This review summarises the current understanding of the multiple biological processes and molecules targeted by ebselen, and its pharmacological applications.

Ebselen and Analogues: Pharmacological Properties and Synthetic Strategies for Their Preparation

Ebselen is the leader of selenorganic compounds, and starting from its identification as mimetic of the key antioxidant enzyme glutathione peroxidase, several papers have appeared in literature claiming its biological activities. It was the subject of several clinical trials and it is currently in clinical evaluation for the treatment of COVID-19 patients. Given our interest in the synthesis and pharmacological evaluation of selenorganic derivatives with this review, we aimed to collect all the papers focused on the biological evaluation of ebselen and its close analogues, covering the timeline between 2016 and most of 2021. Our analysis evidences that, even if it lacks specificity when tested in vitro, being able to bind to every reactive cysteine, it proved to be always well tolerated in vivo, exerting no sign of toxicity whatever the administered doses. Besides, looking at the literature, we realized that no review article dealing with the synthetic approaches for the construction of the benzo[d][1,2]-selenazol-3(2H)-one scaffold is available; thus, a section of the present review article is completely devoted to this specific topic.

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.

A Novel SOD1 Intermediate Oligomer, Role of Free Thiols and Disulfide Exchange

Wild-type human SOD1 forms a highly conserved intra-molecular disulfide bond between C57-C146, and in its native state is greatly stabilized by binding one copper and one zinc atom per monomer rendering the protein dimeric. Loss of copper extinguishes dismutase activity and destabilizes the protein, increasing accessibility of the disulfide with monomerization accompanying disulfide reduction. A further pair of free thiols exist at C6 and C111 distant from metal binding sites, raising the question of their function. Here we investigate their role in misfolding of SOD1 along a pathway that leads to formation of amyloid fibrils. We present the seeding reaction of a mutant SOD1 lacking free sulfhydryl groups (AS-SOD1) to exclude variables caused by these free cysteines. Completely reduced fibril seeds decreasing the kinetic barrier to cleave the highly conserved intramolecular disulfide bond, and accelerating SOD1 reduction and initiation of fibrillation. Presence or absence of the pair of free thiols affects kinetics of fibrillation. Previously, we showed full maturation with both Cu and Zn prevents this behavior while lack of Cu renders sensitivity to fibrillation, with presence of the native disulfide bond modulating this propensity much more strongly than presence of Zn or dimerization. Here we further investigate the role of reduction of the native C57-C146 disulfide bond in fibrillation of wild-type hSOD1, firstly through removal of free thiols by paired mutations C6A, C111S (AS-SOD1), and secondly in seeded fibrillation reactions modulated by reductant tris (2-carboxyethyl) phosphine (TCEP). Fibrillation of AS-SOD1 was dependent upon disulfide reduction and showed classic lag and exponential growth phases compared with wild-type hSOD1 whose fibrillation trajectories were typically somewhat perturbed. Electron microscopy showed that AS-SOD1 formed classic fibrils while wild-type fibrillation reactions showed the presence of smaller “sausage-like” oligomers in addition to fibrils, highlighting the potential for mixed disulfides involving C6/C111 to disrupt efficient fibrillation. Seeding by addition of sonicated fibrils lowered the TCEP concentration needed for fibrillation in both wild-type and AS-SOD1 providing evidence for template-driven structural disturbance that elevated susceptibility to reduction and thus propensity to fibrillate.

Identification of ebselen and its analogues as potent covalent inhibitors of papain-like protease from SARS-CoV-2

An efficient treatment against a COVID-19 disease, caused by the novel coronavirus SARS-CoV-2 (CoV2), remains a challenge. The papain-like protease (PL^pro) from the human coronavirus is a protease that plays a critical role in virus replication. Moreover, CoV2 uses this enzyme to modulate the host’s immune system to its own benefit. Therefore, it represents a highly promising target for the development of antiviral drugs. We used Approximate Bayesian Computation tools, molecular modelling and enzyme activity studies to identify highly active inhibitors of the PL^pro. We discovered organoselenium compounds, ebselen and its structural analogues, as a novel approach for inhibiting the activity of PL^proCoV2. Furthermore, we identified, for the first time, inhibitors of PL^proCoV2 showing potency in the nanomolar range. Moreover, we found a difference between PL^pro from SARS and CoV2 that can be correlated with the diverse dynamics of their replication, and, putatively to disease progression.

Crystal structure of SARS-CoV-2 papain-like protease

The pandemic of coronavirus disease 2019 (COVID-19) is changing the world like never before. This crisis is unlikely contained in the absence of effective therapeutics or vaccine. The papain-like protease (PLpro) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) plays essential roles in virus replication and immune evasion, presenting a charming drug target. Given the PLpro proteases of SARS-CoV-2 and SARS-CoV share significant homology, inhibitor developed for SARS-CoV PLpro is a promising starting point of therapeutic development. In this study, we sought to provide structural frameworks for PLpro inhibitor design. We determined the unliganded structure of SARS-CoV-2 PLpro mutant C111S, which shares many structural features of SARS-CoV PLpro. This crystal form has unique packing, high solvent content and reasonable resolution 2.5 Å, hence provides a good possibility for fragment-based screening using crystallographic approach. We characterized the protease activity of PLpro in cleaving synthetic peptide harboring nsp2/nsp3 juncture. We demonstrate that a potent SARS-CoV PLpro inhibitor GRL0617 is highly effective in inhibiting protease activity of SARS-CoV-2 with the IC50 of 2.2 ± 0.3 μmol/L. We then determined the structure of SARS-CoV-2 PLpro complexed by GRL0617 to 2.6 Å, showing the inhibitor accommodates the S3-S4 pockets of the substrate binding cleft. The binding of GRL0617 induces closure of the BL2 loop and narrows the substrate binding cleft, whereas the binding of a tetrapeptide substrate enlarges the cleft. Hence, our results suggest a mechanism of GRL0617 inhibition, that GRL0617 not only occupies the substrate pockets, but also seals the entrance to the substrate binding cleft hence prevents the binding of the LXGG motif of the substrate.

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.

Crystallization and structure of ebselen bound to Cys141 of human inositol monophosphatase

Inositol monophosphatase (IMPase) is inhibited by lithium, which is the most efficacious treatment for bipolar disorder. Several therapies have been approved, or are going through clinical trials, aimed at the replacement of lithium in the treatment of bipolar disorder. One candidate small molecule is ebselen, a selenium-containing antioxidant, which has been demonstrated to produce lithium-like effects both in a murine model and in clinical trials. Here, the crystallization and the first structure of human IMPase covalently complexed with ebselen, a 1.47 Å resolution crystal structure (PDB entry 6zk0), are presented. In the complex with human IMPase, ebselen in a ring-opened conformation is covalently attached to Cys141, a residue located away from the active site. IMPase is a dimeric enzyme and in the crystal structure two adjacent dimers share four ebselen molecules, creating a tetramer with approximate 222 symmetry. In the crystal structure presented in this publication, the active site in the tetramer is still accessible, suggesting that ebselen may function as an allosteric inhibitor or may block the binding of partner proteins.

Novel Selenium-based compounds with therapeutic potential for SOD1-linked amyotrophic lateral sclerosis

Background

Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease as well as Lou Gehrig's disease, is a progressive neurological disorder selectively affecting motor neurons with no currently known cure. Around 20% of the familial ALS cases arise from dominant mutations in the sod1 gene encoding superoxide dismutase1 (SOD1) enzyme. Aggregation of mutant SOD1 in familial cases and of wild-type SOD1 in at least some sporadic ALS cases is one of the known causes of the disease. Riluzole, approved in 1995 and edaravone in 2017 remain the only drugs with limited therapeutic benefits.

Methods

We have utilised the ebselen template to develop novel compounds that redeem stability of mutant SOD1 dimer and prevent aggregation. Binding modes of compounds have been visualised by crystallography. In vitro neuroprotection and toxicity of lead compounds have been performed in mouse neuronal cells and disease onset delay of ebselen has been demonstrated in transgenic ALS mice model.

Finding

We have developed a number of ebselen-based compounds with improvements in A4V SOD1 stabilisation and in vitro therapeutic effects with significantly better potency than edaravone. Structure-activity relationship of hits has been guided by high resolution structures of ligand-bound A4V SOD1. We also show clear disease onset delay of ebselen in transgenic ALS mice model holding encouraging promise for potential therapeutic compounds.

Interpretation

Our finding established the new generation of organo-selenium compounds with better in vitro neuroprotective activity than edaravone. The potential of this class of compounds may offer an alternative therapeutic agent for ALS treatment. The ability of these compounds to target cysteine 111 in SOD may have wider therapeutic applications targeting cysteines of enzymes involved in pathogenic and viral diseases including main protease of SARS-Cov-2 (COVID-19).

Funding

Project funding was supported by the 10.13039/100000971ALS Association grant (WA1128) and Fostering Joint International Research (19KK0214) from the 10.13039/100009950Ministry of Education, Culture, Sports, Science and Technology (10.13039/501100001700MEXT), Japan.