DAPI (4',6-diamidino-2-phenylindole) as a novel fluorochrome for amyloid staining that binds to β-pleated sheet

Amyloidosis is caused by the extracellular deposition of amyloid fibrils with a β-pleated sheet structure. Diagnosis typically relies on Congo red or Thioflavine T staining. Recently, DAPI (4',6-Diamidino-2-Phenylindole), which is a common nuclear fluorochrome, has been reported to stain amyloid. DAPI staining is simpler than Congo Red and Thioflavine T staining, but its staining mechanism remains unclear. Thus, this study aimed to investigate the mechanism and specificity of DAPI staining for amyloid and its utility. The staining properties of DAPI and Thioflavine T for amyloid were compared on the basis of their stereochemical similarity. In addition, formic acid-treated amyloid specimens were used to investigate the mechanism by which structural changes affect DAPI binding to amyloid fibrils. DAPI staining was also evaluated in four specimens with different types of amyloid. DAPI and Thioflavine T had similar stereochemistry and staining behavior. The amyloid present in formic acid-treated specimens was negative for DAPI staining, indicating that DAPI may recognize the conformation of amyloid fibrils. DAPI stained positively in specimens deposited with AA, Aβ, AL, and AIAPP. DAPI staining recognizes the β-pleated sheet structure of the amyloid fibril structure, and is a simple and sensitive method for detecting amyloid deposition.