Non-native Soluble Oligomers of Cu/Zn Superoxide Dismutase (SOD1) Contain a Conformational Epitope Linked to Cytotoxicity in Amyotrophic Lateral Sclerosis (ALS)

Accumulating evidence supports a prominent contribution of misfolding and aggregation of SOD1 to the dysfunction and progressive death of motor neurons in ALS. Over 140 mutations (mostly missense) in the SOD1 gene have been identified in patients with familial ALS (FALS), most of which destabilize the native SOD1 homodimer and/or increase aggregation propensity.1,2 Current evidence supports the pathogenic capacity of soluble misfolded SOD1, rather than the large insoluble aggregates that appear only near the onset of paralysis in ALS mouse models.3−7 However, little is known about the structural features of soluble non-native SOD1 conformers or the factors in the cellular environment that influence misfolding and aggregation. Soluble misfolded WT SOD1 has been found in the spinal cord from sporadic ALS patients that do not carry mutations in SOD1,8,9 demonstrating the sufficiency of nongenetic factors to induce the formation of potentially toxic oligomers by SOD1. To identify misfolded SOD1 conformers with greatest relevance to ALS pathology, we probed isolated oligomeric species with a conformation-specific antibody (C4F6) to identify those with potential cytotoxicity. In FALS patients and mouse models, C4F6 specifically recognizes soluble SOD1 found only in disease-affected tissue, revealing a connection between FALS pathology and the as-yet unidentified epitope bound by C4F6.7 Here, we show that higher-order non-native oligomers of mutant SOD1, but not dimers or monomers, contain the epitope recognized by the C4F6 antibody. To assess the impact of the cellular redox environment on the formation of potentially toxic soluble oligomers, we determine the effect of a physiologically prevalent oxidative modification (glutathionylation at Cys-111) on oligomerization. Cys-111 glutathionylation increases both the abundance of soluble oligomers and exposure of the disease-specific epitope recognized by C4F6, revealing a novel mechanism by which oxidative stress modulates potentially toxic SOD1 aggregation. Our results suggest that SOD1 acquires pathogenic features upon the formation of soluble non-native oligomeric assemblies, indicating a particular relevance of these species to neuronal dysfunction in ALS.