Abstract
Photodynamic therapy (PDT) has been explored as a therapeutic strategy to clear toxic amyloid aggregates involved in neurodegenerative disorders such as Alzheimer’s disease. A major limitation of PDT is off-target oxidation, which can be lethal for the surrounding cells. We have shown that a novel class of oligo-p-phenylene ethynylene-based compounds (OPEs) exhibit selective binding and fluorescence turn-on in the presence of pre-fibrillar and fibrillar aggregates of disease-relevant proteins such as amyloid-β (Aβ) and α-synuclein. Concomitant with fluorescence turn-on, OPE also photosensitizes singlet oxygen under illumination through the generation of a triplet state, pointing to the potential application of OPEs as photosensitizers in PDT. Herein, we investigated the photosensitizing activity of an anionic OPE for the photo-oxidation of toxic Aβ aggregates and compared its efficacy to the well-known but non-selective photosensitizer methylene blue (MB). Our results show that while MB photo-oxidized both monomeric and fibrillar conformers of Aβ40, OPE oxidized only Aβ40 fibrils, targeting two histidine residues on the fibril surface and a methionine residue located in the fibril core. Oxidized fibrils were shorter and more dispersed, but retained the characteristic β-sheet rich fibrillar structure and the ability to seed further fibril growth. Importantly, the oxidized fibrils displayed low toxicity. We have thus discovered a class of novel theranostics for the simultaneous detection and oxidization of amyloid aggregates. Importantly, the selectivity of OPE’s photosensitizing activity overcomes the limitation of off-target oxidation of currently available photosensitizers, and represents a significant advancement of PDT as a viable strategy to treat neurodegenerative disorders.
Competing Interest Statement
The authors have declared no competing interest.
ABBREVIATIONS
- AAA
- amino acid analysis
- Aβ
- amyloid β
- CD
- circular dichroism
- CTAB
- cetyltrimethylammonium bromide
- DMEM
- Dulbecco’s Modified Eagle’s Medium
- DMSO
- dimethyl sulfoxide
- DNPH
- 2,4-Dinitrophenylhydrazine
- ESI-MS
- electrospray ionization mass spectrometry
- FBS
- fetal bovine serum
- GAFF
- generalized force field
- HCl
- hydrochloride acid
- RP-HPLC
- reverse phase high performance liquid chromatography
- MB
- methylene blue
- MD
- molecular dynamics
- NaCl
- sodium chloride
- OPE
- oligo-p-phenylene ethynylene
- PDB
- protein databank
- PDT
- photodynamic therapy
- PS
- penicillin-streptomycin
- TPBS
- pH 7.4 phosphate buffered saline containing 0.1% Tween 20
- TEM
- transmission electron microscopy
- TFA
- trifluoroacetic acid