Efficient inhibition of amyloid fibrillation and cytotoxicity of α-synuclein and human insulin using biosynthesized silver nanoparticles decorated by green tea polyphenols

Green tea polyphenols (GTPs), particularly epigallocatechin-3-gallate, stand out among natural small molecules screened for their ability to target protein aggregates due to their potent anti-amyloidogenic and neuroprotective activities against various disease-related peptides and proteins. However, the clinical applications of GTPs in amyloid-related diseases have been greatly limited by drawbacks such as poor chemical stability and low bioavailability. To address these limitations, this study utilized an Iranian green tea polyphenolic extract as a reducing agent to neutralize silver ions and facilitate the formation of silver nanoparticle capped by GTPs (GTPs-capped AgNPs). The results obtained from this study demonstrate that GTPs-capped AgNPs are more effective than free GTPs at inhibiting amyloid fibrillation and reducing cytotoxicity induced by amyloid fibrils of human insulin and α-synuclein (α-syn). This improved efficacy is attributed to the increased surface/volume ratio of GTPs-capped AgNPs, which can enhance their binding affinity to amyloidogenic species and boosts their antioxidant activity. The mechanism by which GTPs-capped AgNPs inhibit amyloid fibrillation appears to vary depending on the target protein. For structured protein human insulin, GTPs-capped AgNPs hinder fibrillation by constraining the protein in its native-like state. In contrast, GTPs-capped AgNPs modulate fibrillation of intrinsically disordered proteins like α-syn by redirecting the aggregation pathway towards the formation of non-toxic off-pathway oligomers or amorphous aggregates. These findings highlight polyphenol-functionalized nanoparticles as a promising strategy for targeting protein aggregates associated with neurodegenerative diseases.