Galloyl-terminated dendrimers block the formation of amyloid fibrils and rescue cell viability

Amyloid plaques, deposits of beta-amyloid (Ab42) peptide, is a hallmark of Alzheimer’s disease (AD). These plaques are generated by the self-assembling of Ab42 monomers into supramolecular nanofibrillar structures stabilized by the peptide’s β-sheets. For several years these plaques have been considered the basis of cell toxicity that lead to neuronal cell death in AD. However, recent studies suggest that the neurotoxic effect are derived from the smaller Ab42 protein aggregates, i.e. oligomers [1, 2].

We have previously demonstrated that specific natural polyphenols (e.g. vescalagin and castalagin) can rescue cell viability affected by the cytotoxicity of Ab42 fibrils. In fact, the use of polyphenols (e.g. EGCG) as modulators of Ab42 fibre formation has been studied, and their ability to block the Ab42 self-assembly process has been reported [3]. The activity of EGCG is reported to occur through the interference of the Pi-Pi stacking within the Ab42 supramolecular arrangement [4]. In general, most of the natural polyphenols reportedto modulate Ab42 self-assembly presentgalloyl-type moieties. Based on this observation, we designed dendrimers displaying this type of moiety on their surface and tested them for their ability to modulate Ab42 fibrillization. 

METHODS

Gallic Acid (GA) based dendrimers were synthetized from 3,4,5-trimethoxybenzoic acid and 2,2′-(Etilenodioxi)bis-ethylamine. The capacity of the dendrimers to modulate the Ab42 fibrillization was evaluated by CD, DLS and fluorescence spectroscopy.

Cell studies were conducted with SHSY-5Y neuroblastoma cell line. The evaluation of the capacity to rescue cell viability in the presence of cytotoxic concentrations of Ab42 due to the reduced formation of nanofibers and/or oligomeric structures was executed by Live/dead assay, alamarBlue® assay and immunostaining.

RESULTS AND DISCUSSION

Dendrimer GA-G1 with six galloyl units (figure 1-A) is able to reduce the aggregation of Ab42 (figure 1-B), while decreasing the β-sheet content of the Ab42 supramolecular assemblies (measured by CD), compared with GA-G0 (presenting only two gallate units). These results suggested that the activity is directlty correlated with the number of galloyl units present in the dendrimer. Cells studies, confirmed these assumption, since GA-G1 (and not GA-G0) has the capacity of rescue SH-SY5Y cell viability in the presence of Ab42, reducing the oligomeric Aβ42 assemblies in the cytoplasm of the cells (figure 1-C).

CONCLUSION

Our results suggest that galloyl-terminated dendrimers are a promising versatile platform to design nanodevices able to reduce the toxicity of Ab42 assemblies in the AD context.