Yoshiaki Furukawa

Accumulation of misfolded proteins is a hallmark of many neurodegenerative diseases. To characterize such misfolded species in vivo, conformation‐specific antibodies are widely used; however, limited knowledge of antibody–epitope interactions often hampers mechanistic insight. To address this, we determined the cryo‐electron microscopy structure of the complex between a monoclonal antibody, 19A9, and Cu/Zn‐superoxide dismutase (SOD1), a protein associated with canine degenerative myelopathy (DM), which is related to human amyotrophic lateral sclerosis. Biochemical analyses confirmed that 19A9 specifically recognizes monomeric SOD1, and the structure revealed binding near the interface normally used for homodimerization in native SOD1, with steric hindrance preventing interaction when the protein is in its homodimeric form. Immunofluorescence staining of spinal cord sections revealed that 19A9 stained a subset of motoneurons in DM‐affected dogs, but not in asymptomatic controls. Structural characterization of the 19A9‐monomeric SOD1 complex enabled us to propose that SOD1 monomers can arise in vivo under pathological conditions.

Misfolding of mutant Cu/Zn‐superoxide dismutase (SOD1) is a well‐established pathological feature of familial amyotrophic lateral sclerosis (ALS). While amino acid substitutions in mutant SOD1 destabilize its structure and promote misfolding, oxidation has also been implicated in the pathological alterations of wild‐type SOD1, particularly in neurodegenerative diseases including sporadic ALS. However, the impact of oxidation on SOD1 folding remains to be fully elucidated. Here, we demonstrate that Cys111 is primarily oxidized to sulfonic acid upon exposure of apo‐SOD1 to hydrogen peroxide, as confirmed by the quantitation of thiol groups and mass spectrometry. Molecular dynamics simulations showed that sulfonylation of Cys111 disrupts the dimer interface and promotes monomerization. This monomeric form then facilitates the subsequent oxidation of buried Cys6, leading to structural disruption, as evidenced by circular dichroism spectroscopy and loss of thiol groups. SOD1 denaturation triggered by Cys111 oxidation became evident when zinc binding was impaired due to pathological mutations and/or under zinc‐deficient conditions. Given that increased oxidative stress is frequently associated with many neurodegenerative diseases, modulating Cys111 oxidation may offer a potential strategy for maintaining SOD1 structural stability and preventing its pathological misfolding.

A homozygous E40K mutation in the gene coding canine Cu/Zn‐superoxide dismutase (cSOD1) causes degenerative myelopathy (DM) in dogs. A pathological hallmark of DM with the cSOD1 mutation is the aggregation of mutant cSOD1 proteins in neurons. The amino acid substitution E40K disrupts a salt bridge between Glu40 and Lys91 and is considered to destabilize the native state of cSOD1; however, the mechanism by which mutant cSOD1 aggregates remains unclear. Here, we show that mutant cSOD1 losing a copper and zinc ion forms oligomers crosslinked via disulfide bonds. The E40K substitution was found to result in the increased solvent exposure of the Cys7 side chain, which then attacked the disulfide bond (Cys57–Cys146) in cSOD1 to form disulfide‐linked oligomers. We also successfully prevented the Cys7 exposure and thus the oligomerization of mutant cSOD1 by a fragment antibody that specifically recognizes the region around the mutation site. The fragment antibody covered the β‐plug region, reinforcing the interactions compromised by the E40K substitution and thus contributing to the maintenance of the structural integrity of the β‐barrel core of cSOD1. Taken together, we propose that the Cys7 exposure in cSOD1 upon the salt bridge disruption plays a central role in the aggregation mechanism of DM‐associated mutant cSOD1.

Misfolding of mutant Cu/Zn‐superoxide dismutase (SOD1) has been implicated in familial form of amyotrophic lateral sclerosis (ALS). A natively folded SOD1 forms a tight homodimer, and the dimer dissociation has been proposed to trigger the oligomerization/aggregation of SOD1. Besides increasing demand for probes allowing the detection of monomerized forms of SOD1 in various applications, the development of probes has been limited to conventional antibodies. Here, we have developed Mb(S4) monobody, a small synthetic binding protein based on the fibronectin type III scaffold, that recognizes a monomeric but not dimeric form of SOD1 by performing combinatorial library selections using phage and yeast‐surface display methods. Although Mb(S4) was characterized by its excellent selectivity to the monomeric conformation of SOD1, the monomeric SOD1/Mb(S4) complex was not so stable (apparent K_d ~ μM) as to be detected in conventional pull‐down experiments. Instead, the complex of Mb(S4) with monomeric but not dimeric SOD1 was successfully trapped by proximity‐enabled chemical crosslinking even when reacted in the cell lysates. We thus anticipate that Mb(S4) binding followed by chemical crosslinking would be a useful strategy for in vitro and also ex vivo detection of the monomeric SOD1 proteins.

Mutations in the gene coding Cu/Zn-superoxide dismutase (SOD1) are linked to a familial form of amyotrophic lateral sclerosis (ALS), and its pathological hallmark includes abnormal accumulation of mutant SOD1 proteins in spinal motorneurons. Mutant SOD1 proteins are considered to be susceptible to misfolding, resulting in the accumulation as oligomers/aggregates. While it remains obscure how and why SOD1 becomes misfolded under pathological conditions in vivo, the failure to bind a copper and zinc ion in SOD1 in vitro leads to the significant destabilization of its natively folded structure. Therefore, genetic and pharmacological attempts to promote the metal binding in mutant SOD1 could serve as an effective treatment of ALS. Here, I briefly review the copper and zinc binding process of SOD1 in vivo and discuss a copper chaperone for SOD1 as a potential target for developing ALS therapeutics.