Stability of Early-Stage Amyloid-β(1-42) Aggregation Species

Accumulation of aggregated  amyloid-β  protein (Aβ) is an important feature of Alzheimer's disease. There is significant interest in understanding the initial steps of Aβ aggregation due to the recent focus on soluble Aβ  oligomers .  In vitro studies  of Aβ aggregation have been aided by the use of conformation-specific antibodies which recognize shape rather than sequence. One of these, OC antiserum, recognizes certain elements of fibrillar Aβ across a broad range of sizes. We have observed the presence of these fibrillar elements at very early stages of Aβ incubation. Using a  dot blot  assay, OC-reactivity was found in size exclusion chromatography (SEC)-purified Aβ(1–42)  monomer  fractions immediately after isolation (early-stage). The OC-reactivity was not initially observed in the same fractions for Aβ(1–40) or the aggregation-restricted Aβ(1–42) L34P but was detected within 1–2 weeks of incubation. Stability studies demonstrated that early-stage OC-positive Aβ(1–42) aggregates were resistant to 4 M  urea  or  guanidine   hydrochloride  but sensitive to 1%  sodium dodecyl sulfate  (SDS). Interestingly, the sensitivity to SDS diminished over time upon incubation of the SEC-purified Aβ(1–42) solution at 4 °C. Within 6–8 days the OC-positive Aβ42 aggregates were resistant to SDS  denaturation . The progression to, and development of, SDS resistance for Aβ(1–42) occurred prior to  thioflavin  T fluorescence. In contrast, Aβ(1–40) aggregates formed after 6 days of incubation were sensitive to both urea and SDS. These findings reveal information on some of the earliest events in Aβ aggregation and suggest that it may be possible to target early-stage aggregates before they develop significant stability.