Arresting amyloid with coulomb's law: acetylation of ALS-linked SOD1 by aspirin impedes aggregation

Abstract

Although the magnitude of a protein's net charge (Z) can control its rate of self-assembly into amyloid, and its interactions with cellular membranes, the net charge of a protein is not viewed as a druggable parameter. This article demonstrates that aspirin (the quintessential acylating pharmacon) can inhibit the amyloidogenesis of superoxide dismutase (SOD1) by increasing the intrinsic net negative charge of the polypeptide, i.e., by acetylation (neutralization) of multiple lysines. The protective effects of acetylation were diminished (but not abolished) in 100 mM NaCl and were statistically significant: a total of 432 thioflavin-T amyloid assays were performed for all studied proteins. The acetylation of as few as three lysines by aspirin in A4V apo-SOD1-a variant that causes familial amyotrophic lateral sclerosis (ALS)-delayed amyloid nucleation by 38% and slowed amyloid propagation by twofold. Lysines in wild-type- and ALS-variant apo-SOD1 could also be peracetylated with aspirin after fibrillization, resulting in supercharged fibrils, with increases in formal net charge of ∼2 million units. Peracetylated SOD1 amyloid defibrillized at temperatures below unacetylated fibrils, and below the melting temperature of native Cu2,Zn2-SOD1 (e.g., fibril Tm = 84.49°C for acetylated D90A apo-SOD1 fibrils). Targeting the net charge of native or misfolded proteins with small molecules-analogous to how an enzyme's Km or Vmax are medicinally targeted-holds promise as a strategy in the design of therapies for diseases linked to protein self-assembly.

Figure 1

Chemically boosting the net charge (Z) of SOD1. The accumulation of acetyl modifications on native or oligomeric SOD1 by repeated doses of an acylating agent is plausible when considering the long lifetime of native SOD1 in motor neurons (27), and the generally long lifetime of amyloid oligomers in vivo (28,29). (A) The net charge of dimeric WT SOD1 has been measured to vary from ∼−3.7 to ∼−12.1, depending on the metalation state and subcellular pH (25). Acetylation of lysine residues in native SOD1 will increase the magnitude of net negative charge by between 15% and 50%, depending on subcellular pH and metalation state. (B) Peracetylating lysine residues in an amyloid fibril of SOD1 (d = 15 nm, l = 2 μm) with aspirin is expected to increase the magnitude of net negative charge by ∼10⁶ formal units. To see this figure in color, go online.

Figure 2

Acetylation of multiple lysines in WT- and ALS-variant apo-SOD1 by aspirin. Electrospray ionization mass spectra of soluble (A) WT, (B) D90A, and (C) A4V apo-SOD1 after reaction with different concentrations of aspirin (in aqueous buffer). The mean number of acetylated lysines are denoted as Lys-Ac(∼N), and are listed per apo-SOD1 monomer. To see this figure in color, go online.

Figure 3

Fibrillization of unacetylated (top) and acetylated (bottom) apo-SOD1 (0 mM NaCl) as measured by thioflavin-T fluorescence in a 96-well microplate. Eighteen plots of thioflavin-T fluorescence from 18 replicate amyloid assays of WT apo-SOD1-Ac(0) (top) and WT apo-SOD1-Ac(∼6) (bottom) carried out simultaneously in the same 96-well microplate. Each of the 18 aliquots were taken from the same stock solution of acetylated or nonacetylated SOD1 and analyzed simultaneously in the same 96-well microplate to eliminate variations in solution conditions and minimize random error. Black arrows identify the outlier sigmoids in the data set. The raw plots of ThT fluorescence for other acetyl derivatives of WT apo-SOD1 (and D90A and A4V) can be found in the Supporting Material (Figs. S6–S11). The average number of acetylated lysines (Ac(∼N)) is listed per apo-SOD1 monomer. To see this figure in color, go online.

Figure 4

Effect of lysine acetylation on the rate of fibrillization of WT- and ALS-variant apo-SOD1 in 0 mM NaCl. Thioflavin-T fluorescence assays of unacetylated and acetylated (A) WT, (B) D90A, and (C) A4V apo-SOD1 proteins (pH 7.4, 37°C). Each curve is an average of normalized fluorescence measurements from 18 separate replicate experiments (of the sort shown in Fig. 3). Transmission electron micrographs of fibrillar forms of acetylated (D) WT, (E) D90A, and (F) A4V apo-SOD1 after the ThT assay. The mean number of acetylated lysines is denoted as Lys-Ac(∼N), per apo-SOD1 monomer. Table 1 and Fig. S12 list p-values for all kinetic analyses extracted from ThT fluorescence assays. To see this figure in color, go online.

Figure 5

Effect of lysine acetylation on the rate of fibrillization of WT- and ALS-variant apo-SOD1 in 100 mM NaCl. Thioflavin-T fluorescence assays of unacetylated and acetylated (A) WT, (B) D90A, and (C) A4V apo-SOD1 proteins (pH 7.4, 37°C). Each curve is an average of data (normalized) from 18 separate replicate experiments. The mean number of acetylated lysines is denoted as Lys-Ac(∼N), per apo-SOD1 monomer. (D–F, upper panels) Transmission electron micrographs of fibrillar forms of acetylated WT, D90A, and A4V apo-SOD1 after the ThT assay in 100 mM NaCl. (D–F; lower panels) SDS-PAGE of samples before (left) and after (right) ThT aggregation assay in 100 mM NaCl. The gels on the right in each panel are from supernatants of samples after aggregation and centrifugation. Table 1 and Fig. S13 list p-values for all kinetic analyses extracted from ThT fluorescence assays. To see this figure in color, go online.

Figure 6

Effect of peracetylation of lysine in fibrillar WT- and ALS-variant apo-SOD1 on fibril thermostability (i.e., acetylation after fibrillization). (A–C) Mass spectra of unacetylated (left panel), and peracetylated (right panel) WT- and ALS-variant apo-SOD1 thermochemically dissociated from amyloid fibrils (acetylation was performed after fibrillization). (D–F) Thermal stability curves of unacetylated (blue solid squares) and peracetylated (red solid squares) WT, D90A, and A4V apo-SOD1 amyloid fibrils. To see this figure in color, go online.