Mechanistic Inhibition of Alzheimer's-Associated Aβ Aggregation by Gold Nanoparticles

Alzheimer's disease (AD) is currently the most common type of dementia and the 6^th leading cause of death in the United States.  One pathological hallmark of AD is amyloid plaques, which deposit around the neurons in the brain.  These plaques are composed primarily of amyloid-beta (Aβ), which is a 40-42 amino acid protein that forms from the cleavage of the amyloid precursor protein (APP).  Aβ is a naturally occurring protein in the body; however it self-assembles to create aggregated structures and ultimately insoluble fibrils in a process that is hypothesized to be closely tied to disease progression.  During self-assembly, monomeric Aβ forms nuclei, which progress to form soluble aggregates.  These soluble aggregates can then associate or elongate to produce insoluble fibrils.  In association, soluble aggregates laterally bind to one another to form fibrils of an increased diameter. In elongation, monomeric Aβ binds to the ends of the soluble aggregates to form fibrils with an increased length.  Inhibition of Aβ aggregation has emerged as a therapeutic strategy for AD. We have examined gold nanoparticles for their ability to halt specific Aβ aggregation mechanisms.  Nanoparticles are of particular interest because they cross the blood-brain barrier and have been used for drug delivery. By employing TEM images to evaluate aggregate morphology as well as assays that isolate Aβ aggregation mechanisms, we have tested the mechanistic-specific inhibitory capabilities of gold nanospheres with citrate, cetrimonium bromide (CTAB), poly acrylic acid (PAA), and poly allylamine hydrochloride (PAH) overcoatings.

The effect of gold nanoparticles on Aβ_1-40 monomer aggregation was first evaluated.  Subsequently, mechanistic-specific assays were employed to assess the effect of gold nanoparticles on soluble Aβ_1‑40 aggregate elongation and association.  These assays utilized thioflavin T to monitor increases in amyloid material, dynamic light scattering to monitor increase in aggregate size, and TEM to observe changes in aggregate morphology.

Gold nanoparticles overcoated with citrate, CTAB, and PAH were observed to have little effect on the quantity of amyloid material formed during Aβ_1-40 monomer aggregation. TEM images (Figure 1), however, demonstrate that aggregations performed in the presence of CTAB overcoated nanoparticles favored aggregate association over elongation.  Mechanistic-specific assays confirm these trends and directly tie aggregation mechanisms to aggregate morphology.  PAA nanoparticles abrogated the formation of amyloid material in monomer aggregation assays.  Furthermore, this inhibition was observed at a substiochiomeric ratio of nanoparticles to Aβ of 1:200.  Elongation and association assays further define the mechanism of this inhibition.

Together these results demonstrate that gold nanoparticles serve as effective inhibitors of Aβ self-assembly that modify aggregation at substiochiometric concentrations. Furthermore, inhibition of Aβ aggregation by gold nanoparticles is mechanistic-specific in a manner that is dependent upon the nanoparticle coating. Determining the mechanisms by which these various nanoparticles inhibit Aβ self-assembly may be used to develop an inhibitor capable of targeting multiple mechanisms.

 

Figure 1: TEM image (100,000X), 40µM Aβ monomer aggregated in the presence of 0.2nM CTAB nanoparticles.  Presence of the CTAB nanoparticles allows for the development of associated and non-elongated aggregates.