The Familial British Dementia Mutation Promotes Formation of Neurotoxic Cystine Cross-linked Amyloid Bri (ABri) Oligomers

Familial British dementia (FBD) is an inherited neurodegenerative disease believed to result from a mutation in the BRI2 gene. Post-translational processing of wild type BRI2 and FBD-BRI2 result in the production of a 23-residue long Bri peptide and a 34-amino acid long ABri peptide, respectively, and ABri is found deposited in the brains of individuals with FBD. Similarities in the neuropathology and clinical presentation shared by FBD and Alzheimer disease (AD) have led some to suggest that ABri and the AD-associated amyloid beta-protein (A beta) are molecular equivalents that trigger analogous pathogenic cascades. But the sequences and innate properties of ABri and A beta are quite different, notably ABri contains two cysteine residues that can form disulfide bonds. Thus we sought to determine whether ABri was neurotoxic and if this activity was regulated by oxidation and/or aggregation. Crucially, the type of oxidative cross-linking dramatically influenced both ABri aggregation and toxicity. Cyclization of Bri and ABri resulted in production of biologically inert monomers that showed no propensity to assemble, whereas reduced ABri and reduced Bri aggregated forming thioflavin T-positive amyloid fibrils that lacked significant toxic activity. ABri was more prone to form inter-molecular disulfide bonds than Bri and the formation of covalently stabilized ABri oligomers was associated with toxicity. These results suggest that extension of the C-terminal of Bri causes a shift in the type of disulfide bonds formed and that structures built from covalently cross-linked oligomers can interact with neurons and compromise their function and viability.