Protein aggregation is a hallmark of many diseases, including amyotrophic lateral sclerosis (ALS), where aggregation of Cu/Zn superoxide dismutase (SOD1) is implicated in causing neurodegeneration. Recent studies have suggested that destabilization and aggregation of the most immature form of SOD1, the disulfide-reduced, unmetallated (apo) protein is particularly important in causing ALS. We report herein in depth analyses of the effects of chemically and structurally diverse ALS-associated mutations on the stability and aggregation of reduced apo SOD1. In contrast with previous studies, we find that various reduced apo SOD1 mutants undergo highly reversible thermal denaturation with little aggregation, enabling quantitative thermodynamic stability analyses. In the absence of ALS-associated mutations, reduced apo SOD1 is marginally stable but predominantly folded. Mutations generally result in slight decreases to substantial increases in the fraction of unfolded protein. Calorimetry, ultracentrifugation, and light scattering show that all mutations enhance aggregation propensity, with the effects varying widely, from subtle increases in most cases, to pronounced formation of 40–100 nm soluble aggregates by A4V, a mutation that is associated with particularly short disease duration. Interestingly, although there is a correlation between observed aggregation and stability, there is minimal to no correlation between observed aggregation, predicted aggregation propensity, and disease characteristics. These findings suggest that reduced apo SOD1 does not play a dominant role in modulating disease. Rather, additional and/or multiple forms of SOD1 and additional biophysical and biological factors are needed to account for the toxicity of mutant SOD1 in ALS.

Protein aggregation is a hallmark of many diseases, including amyotrophic lateral sclerosis (ALS) where aggregation of copper/zinc superoxide dismutase (SOD1) is implicated in pathogenesis. We report here that fully metallated (holo) SOD1 under physiologically relevant solution conditions can undergo changes in metallation and/or dimerization over time and form aggregates that do not exhibit classical characteristics of amyloid. The relevance of the observed aggregation to disease is demonstrated by structural and tinctorial analyses, including the novel observation of binding of an anti-SOD1 antibody that specifically recognizes aggregates in ALS patients and mice models. ALS-associated SOD1 mutations can promote aggregation but are not essential. The SOD1 aggregation is characterized by a lag phase, which is diminished by self-or cross-seeding and by heterogeneous nucleation. We interpret these findings in terms of an expanded aggregation mechanism consistent with other in vitro and in vivo findings that point to multiple pathways for the formation of toxic aggregates by different forms of SOD1. Amyotrophic lateral sclerosis (ALS)2 is a devastating, rapidly progressive, and invariably fatal neurodegenerative disease, characterized by motor neuron degeneration and paralysis (1). Approximately 10% of ALS cases are familial (fALS), the remaining cases being sporadic (sALS). The familial and sporadic diseases are clinically indistinguishable and so have been proposed to share common disease mechanisms (1). Mutations in copper/zinc superoxide dismutase (SOD1) account for ϳ20% of fALS and represent a major known cause of the disease. It is generally accepted that fALS-linked SOD1 mutations result in a toxic gain of function rather than a loss of function (1). Much attention has focused on toxic protein aggregation as causing ALS, analogous to pathogenic protein aggregation in other neurodegenerative diseases, including Alzheimer, Huntington, Parkinson, and prion diseases (1-5). SOD1 is present in aggregates in motor neurons of SOD1-linked fALS patients (3, 6) and mice models (3, 7-9) and in some sALS patients (4, 5, 10).Mature SOD1 is a homodimeric protein, with each ␤-barrel subunit containing one catalytic copper ion, one structural zinc ion, one intrasubunit disulfide bond, and two free cysteines (11) (see Fig. 1). More than 147 mainly missense mutations throughout the SOD1 structure have been associated with fALS. Aggregation has been reported previously only for immature (metal and/or disulfide deficient) or aberrant forms of SOD1, often under highly destabilizing conditions, which favor aggregation in general (12-19). The relevance to human disease of such aggregation is not known, nor is it known what forms of SOD1 may give rise to or be present in aggregates in patients (20). Mature (holo) SOD1 is a major form of SOD1 in cells for wild type and most mutant SOD1s (11). Although this form has been proposed to be incompatible with aggregation because of its very high stability (15, 21), several studies have rep...

Many proteins are naturally homooligomers, homodimers most frequently. The overall stability of oligomeric proteins may be described in terms of the stability of the constituent monomers and the stability of their association; together, these stabilities determine the populations of different monomer and associated species, which generally have different roles in the function or dysfunction of the protein. Here we show how a new combined calorimetry approach, using isothermal titration calorimetry to define monomer association energetics together with differential scanning calorimetry to measure total energetics of oligomer unfolding, can be used to analyze homodimeric unmetalated (apo) superoxide dismutase (SOD1) and determine the effects on the stability of structurally diverse mutations associated with amyotrophic lateral sclerosis (ALS). Despite being located throughout the protein, all mutations studied weaken the dimer interface, while concomitantly either decreasing or increasing the marginal stability of the monomer. Analysis of the populations of dimer, monomer, and unfolded monomer under physiological conditions of temperature, pH, and protein concentration shows that all mutations promote the formation of folded monomers. These findings may help rationalize the key roles proposed for monomer forms of SOD1 in neurotoxic aggregation in ALS, as well as roles for other forms of SOD1. Thus, the results obtained here provide a valuable approach for the quantitative analysis of homooligomeric protein stabilities, which can be used to elucidate the natural and aberrant roles of different forms of these proteins and to improve methods for predicting protein stabilities.

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The probability distribution of the drag generated by a two-dimensional square/rectangular obstacle is calculated both for quasi-random input flow patterns and for random surface roughness by employing the multicanonical Monte Carlo procedure in conjunction with the lattice Boltzmann method. The results demonstrate that the multicanonical method can estimate the probability distribution function in low-probability regions with far less computational effort than standard techniques.