The Disulfide Bond, but Not Zinc or Dimerization, Controls Initiation and Seeded Growth in Amyotrophic Lateral Sclerosis-linked Cu,Zn Superoxide Dismutase (SOD1) Fibrillation.

Aggregation of copper-zinc superoxide dismutase (SOD1) is a defining feature of familial ALS caused by inherited mutations in the sod1 gene, and misfolded and aggregated forms of wild-type SOD1 are found in both sporadic and familial ALS cases. Mature SOD1 owes its exceptional stability to a number of post-translational modifications as follows: formation of the intramolecular disulfide bond, binding of copper and zinc, and dimerization. Loss of stability due to the failure to acquire one or more of these modifications is proposed to lead to aggregation in vivo. Previously, we showed that the presence of apo-, disulfide-reduced SOD1, the most immature form of SOD1, results in initiation of fibrillation of more mature forms that have an intact Cys-57-Cys-146 disulfide bond and are partially metallated. In this study, we examine the ability of each of the above post-translational modifications to modulate fibril initiation and seeded growth. Cobalt or zinc binding, despite conferring great structural stability, neither inhibits the initiation propensity of disulfide-reduced SOD1 nor consistently protects disulfide-oxidized SOD1 from being recruited into growing fibrils across wild-type and a number of ALS mutants. In contrast, reduction of the disulfide bond, known to be necessary for fibril initiation, also allows for faster recruitment during seeded amyloid growth. These results identify separate factors that differently influence seeded growth and initiation and indicate a lack of correlation between the overall thermodynamic stability of partially mature SOD1 states and their ability to initiate fibrillation or be recruited by a growing fibril.

Initiation and elongation in fibrillation of ALS-linked superoxide dismutase.

Familial amyotrophic lateral sclerosis (fALS) caused by mutations in copper-zinc superoxide dismutase (SOD1) is characterized by the presence of SOD1-rich inclusions in spinal cords. Similar inclusions observed in fALS transgenic mice have a fibrillar appearance suggestive of amyloid structure. Metal-free apo-SOD1 is a relatively stable protein and has been shown to form amyloid fibers in vitro only when it has been subjected to severely destabilizing conditions, such as low pH or reduction of its disulfide bonds. Here, by contrast, we show that a small amount of disulfide-reduced apo-SOD1 can rapidly initiate fibrillation of this exceptionally stable and highly structured protein under mild, physiologically accessible conditions, thus providing an unusual demonstration of a specific, physiologically relevant form of a protein acting as an initiating agent for the fibrillation of another form of the same protein. We also show that, once initiated, elongation can proceed via recruitment of either apo- or partially metallated disulfide-intact SOD1 and that the presence of copper, but not zinc, ions inhibits fibrillation. Our findings provide a rare glimpse into the specific changes in a protein that can lead to nucleation and into the ability of amyloid nuclei to recruit diverse forms of the same protein into fibrils.