Amyloid oligomers: spectroscopic characterization of amyloidogenic protein states

It is assumed that protein fibrils manifested in amyloidosis result from an aggregation reaction involving small misfolded protein sequences being in an ‘oligomeric’ or ‘prefibrillar’ state. This review covers recent optical spectroscopic studies of amyloid protein misfolding, oligomerization and amyloid fibril growth. Although amyloid fibrils have been studied using established protein‐characterization techniques throughout the years, their oligomeric precursor states require sensitive detection in real‐time. Here, fluorescent staining is commonly performed using thioflavin T and other small fluorescent molecules such as 4‐(dicyanovinyl)‐ julolidine and 1‐amino‐8‐naphtalene sulphonate that have high affinity to hydrophobic patches. Thus, populated oligomeric intermediates and related ‘prefibrillar structures’ have been reported for several human amyloidogenic systems, including amyloid‐beta protein, prion protein, transthyretin, α‐synuclein, apolipoprotein C‐II and insulin. To obtain information on the progression of the intermediate states, these were monitored in terms of fluorescence parameters, such as anisotropy, and quantum efficiency changes upon protein binding. Recently, new antibody stains have allowed precise monitoring of the oligomer size and distributions using multicolor labelling and single molecule detection. Moreover, a pentameric thiophene derivative (p‐FTAA) was reported to indicate early precursors during A‐beta 1‐40 fibrillation, and was also demonstrated in real‐time visualization of cerebral protein aggregates in transgenic AD mouse models by multiphoton microscopy. Conclusively, molecular probes and optical spectroscopy are now entering a phase enabling the in vivo interrogation of the role of oligomers in amyloidosis. Such techniques used in parallel with in vitro experiments, of increasing detail, will probably couple structure to pathogenesis in the near future.