If an easy-to-use method of analyzing fibril structures can be developed, it will become a great help to cover a wide range of samples including pathological tissues as targets of investigations, thereby contributing to progress toward a unified understanding of relationships between fibril structures and pathologies. However, the use of these techniques requires a lot of labor and time and therefore making them unsuitable for high throughput use. The relevance of amyloid polymorphism to in vivo pathology has also been clarified by the structural investigation of Aβ 1-40 amyloid fibrils that were prepared with seeds derived from brain tissues of AD patients with distinct clinical histories 20. The clarified structures have revealed distinct molecular structures of amyloid polymorphs. Recently, detailed structural investigations of amyloid fibrils are progressing by using several powerful methods, such as solid-state NMR (ssNMR) 9, 11, 20, cryo-electron microscopy (cryo-EM) 12, 13, 14, 21, and X-ray crystallography 8. In another case, distinct fibril structures can be also induced in a stochastic manner even under the same conditions 19. Subtle variations in amino acid sequence among different species also result in different fibril structures 18, although it does not correspond to amyloid polymorphism because amino acid sequences are not the same. Although further researches are needed to elucidate the pathological significance, the formation of such conformationally distinct amyloid fibrils now attracts much attention for understanding pathogenic mechanisms of amyloid-related diseases and developing curative treatments in terms of protein structure.Īmyloid polymorphs have been successfully reproduced in vitro depending on growth conditions, e.g., temperature 15, protein concentration 16, and solvent 17 however, the most critical factors have not been fully revealed yet 5. While oligomer species have been proposed as major cytotoxic agents especially in the case of AD, amyloid polymorphs also exert different neurotoxic effects on living cells 9, and it has recently been suggested that tau protein adopted distinct amyloid conformations in the human brain in different diseases 14. In studies of prion diseases, different physicochemical properties among amyloid polymorphs, such as fragility and growth rates, have been suggested to be the molecular basis underlying the presence and propagation of distinct prion strains 15. Variations in the number and the arrangement of protofilaments are an example of polymorphism 6, 7, and more microscopic variations at the level of secondary or tertiary structure of polypeptide chains have been also identified in many proteins 8, 9, 10, 11, 12, 13, 14. While straight unbranched morphology and cross-β structure are known as a common basic structure of amyloid fibrils, it was revealed that amyloid fibrils show structural variations even from the same protein this phenomenon is referred to as amyloid polymorphism 3, 4, 5. It was concluded that the polyiodide species and conformations formed are sensitive to surface structure of amyloid fibrils, and the resultant differences in color will be useful for detecting polymorphism in a wide range of diagnostic samples.Īmyloid fibrils are protein aggregates associated with many amyloidosis and neurodegenerative diseases including Alzheimer’s disease (AD) and Parkinson’s disease 1, 2. The analysis of resonance Raman spectra and polarization microscope suggested that several polyiodide ions composed of I 3 − and/or I 5 − were formed on the grooves or the edges of β-sheets. The colors were fundamentally represented as a sum of three absorption bands in visible region between 400 and 750 nm, and the bands showed different titration curves against iodine, suggesting that there are three specific iodine binding sites. Each of the colors was inherited to daughter fibrils by seeding reactions. When insulin fibrils were prepared and iodine-stained, they exhibited different colors depending on polymorphs. Here, we propose that iodine staining, a historically known reaction that was firstly found by Virchow, can be used as a method for distinguishing amyloid polymorphs. However, its prompt recognition is almost halted due to insufficiency of analytical methods for detecting polymorphism of amyloid fibrils sensitively and quickly. It is recently suggested that amyloid polymorphism, i.e., structural diversity of amyloid fibrils, has a deep relationship with pathology.

Iodine absorption spectrum