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The amyloidogenic homotetrameric plasma protein transthyretin (TTR) has an affinity for bicyclic small molecule ligands in its two thyroxine (T4) binding sites. We have shown that native tetrameric TTR binds to amyloid ligands based on the trans‐stilbene scaffold. The fluorescent Congo‐red analogue, X34, is a symmetric bi‐trans‐stilbene that contains two salicylic acid motifs. We used fluorescence spectroscopy methods to interrogate X34 binding to the TTR tetramer and fibril. We discovered two binding sites in both TTR forms by tryptophan FRET, ligand self‐quenching, Stern‐Volmer plots and binding curves, for the latter including the competitive ligand diflunisal. X34 binds with the similar affinity as diflunisal in the first binding site (Kd1=150 nM), and negative cooperativity renders the binding to the second site with lower affinity very similar compared to diflunisal (Kd2= 1.1 μM). This behavior is coherent with the salicylic acid moiety of diflunisal binding into the binding pocket of TTR (reverse mode). Interestingly X34 binding to TTR fibrils was also well fitted to two binding sites, however with overall lower affinity (Kd1=1.2 μM; Kd2=2.1 μM) compared to binding to the native tetramer. X34 fluorescence when bound to TTR‐fibrils was significantly blue shifted compared to binding to the TTR‐tetramer. [ABSTRACT FROM AUTHOR]

Recently, it has been hypothesized that alpha-synuclein protein strain morphology may be associated with clinical subtypes of alpha-synucleinopathies, like Parkinson's disease and multiple system atrophy. However, direct evidence is lacking due to the caveat of conformation-specific characterization of protein strain morphology. Here we present a new cell model based in vitro method to explore various alpha-synuclein (αsyn) aggregate morphotypes. We performed a spectroscopic investigation of the HEK293 cell model, transfected with human wildtype-αsyn and A53T-αsyn variants, using the amyloid fibril-specific heptameric luminescent oligomeric thiophene h-FTAA. The spectral profile of h-FTAA binding to aggregates displayed a blue-shifted spectrum with a fluorescence decay time longer than in PBS, suggesting a hydrophobic binding site. In vitro spectroscopic binding characterization of h-FTAA with αsyn pre-formed fibrils suggested a binding dissociation constant Kd < 100 nM. The cells expressing the A53T-αsyn and human wildtype-αsyn were exposed to recombinant pre-formed fibrils of human αsyn. The ensuing intracellular aggregates were stained with h-FTAA followed by an evaluation of the spectral features and fluorescence lifetime of intracellular αsyn/h-FTAA, in order to characterize aggregate morphotypes. This study exemplifies the use of cell culture together with conformation-specific ligands to characterize strain morphology by investigating the spectral profiles and fluorescence lifetime of h-FTAA, based upon its binding to a certain αsyn aggregate. This study paves the way for toxicity studies of different αsyn strains in vitro and in vivo. Accurate differentiation of specific alpha-synucleinopathies is still limited to advanced disease stages. However, early subtype-specific diagnosis is of the utmost importance for prognosis and treatment response. The potential association of αsyn aggregates morphotypes detected in biopsies or fluids to disease phenotypes would allow for subtype-specific diagnosis in subclinical disease stage and potentially reveal new subtype-specific treatment targets. Notably, the method may be applied to the entire spectrum of neurodegenerative diseases by using a combination of conformation-specific ligands in a physicochemical environment together with other types of polymorphic amyloid variants and assess the conformation-specific features of various protein pathologies. [ABSTRACT FROM AUTHOR]

We recently reported on 8 different pentameric oligothiophenes, denoted proteophenes, with different amino acid substitution patterns along the conjugated thiophene backbone. The proteophenes were forming chiral both molecular (solvent dissolved) and self‐assembled nano‐architectonic structures, the latter having approximately 5–6 times greater ICD responses. Herein, two proteophenes, HS‐84‐E−E and HS‐84‐K−K, were further investigated in terms of binding to two chiral variants of insulin amyloid fibrils. Binding curves based on fluorescence revealed strong primary binding sites for both proteophenes and more unspecific secondary binding for especially the latter. Interestingly, their induced circular dichroism (ICD) responses were similar to or stronger than for their solvent form and showed a complicated alteration upon binding suggesting that the microstructure at the binding site governs the helical structure of the ligand. Hyperspectral fluorescence microscopy and FLIM revealed more details on the molecular binding in terms of proteophene emission decay‐times. Vibrational circular dichroism (VCD) analysis showed that VCD signal of amyloid fibrils was enhanced upon binding of proteophenes, suggesting changes in the amyloid microstructures. [ABSTRACT FROM AUTHOR]