Binding of methylene blue to a surface cleft inhibits the oligomerization and fibrillization of prion protein.

Neurodegenerative protein misfolding diseases, including prionopathies, share the common feature of accumulating specific misfolded proteins, with a molecular mechanism closely related. Misfolded prion protein (PrP) generates soluble oligomers that, in turn, aggregate into amyloid fibers. Preventing the formation of these entities, crucially associated with the neurotoxic and/or infectious properties of the resulting abnormal PrP, represents an attractive therapeutic strategy to ameliorate prionopathies. We focused our attention into methylene blue (MB), a well-characterized drug, which is under study against Alzheimer's disease and other neurodegenerative disorders. Here, we have undertaken an in vitro study on the effects of MB on oligomerization and fibrillization of human, ovine and murine PrP. We demonstrated that MB affects the kinetics of PrP oligomerization and reduces the amount of oligomer of about 30%, in a pH-dependent manner, by using SLS and DSC methodologies. Moreover, TEM images showed that MB completely suppresses fiber formation at a PrP:MB molar ratio of 1:2. Finally, NMR revealed a direct interaction between PrP and MB, which was mapped on a surface cleft including a fibrillogenic region of the protein. Our results allowed to surmise a mechanism of action in which the MB binding to PrP surface markedly interferes with the pathway towards oligomers and fibres. Therefore MB could be considered as a general anti-aggregation compound, acting against proteinopathies.