Aggregation of Amyloid (A)
Amyloid peptides are secreted as soluble monomer in healthy body, whereas it reportedly acquires neurotoxicity when aggregated. Accumulation of the aggregtes of the A peptide in brain injuries the neuron cells thereof, and the A peptide is considered to be a possible cause of Alzheimer's disease.
Inhibition of fibril formation
One of the aggregates is amyloid fibril, a needle-like aggregate of many peptides. We tried to inhibit the fibrillation of the amyloid (1-40) (A1-40) by adding N-methylated peptide fragments having the partial sequence at 30-40 region of A1-40 (N-Me A30-40). The amount of amyloid fibril was evaluated from fluorescence of thioflavin T (ThT) dye. We found that the addition of the N-Me A30-40 peptides decreased the ThT fluorescence, whereas it did not actually inhibit the formation of insoluble aggregate. We concluded that the N-Me peptides disrupted the regular b-sheet structure of A1-40 fibrils and affected the ThT fluorescence count. The monomer-dimer equilibrium of N-Methylated peptides was (partly) responsible for the observed dependence of their inhibitory effect on the concentration of N-Methylated peptide and the number and position of N-Methylated groups. Our study provides a hint to design new N-Methylated inhibitor peptides of fibrillation.
[1] H.Ochiai, T.Komuro, H.Hiramatsu, Chemistry Letters, 2015, 44, 35-37.
[2] H.Hiramatsu, H.Ochiai, T.Komuro, Chemical Biology and Drug Design, 2016, 87, 425-433.
Effect of posttranslational modifications
Following expression, proteins often undergoes posttranslational modification. We investigated effects of two posttranslational modifications, the deamidation (-CONH2 -COOH [Asn27Asp]) and the citrullination (-NHC(=NH)NH2 -NHC(=O)NH2 [Arg5Cit]) on the fibrillation property of the amyloid (1-40) (A1-40) and the amyloid (1-42) (A1-42). These modifications increase the number of negative charges on the peptide. The decrease in the number of charges is considered to largely affect the fibrillation property.
Each modifiction remarkably decreased the amount of fibril and alternatively increased the soluble aggregates (oligomers). This effect was not explained in terms of the increase in the negative charge on the peptide because different effect was seen at basic pH, at the condition the number of negative charge increased. Instead, the observed effect of each modification on the fibrillation property would be attributed to roles of Asn27 or Arg5 in the fibrillation and/or the oligomerization process.
[3] D.Osaki, H.Hiramatsu, Amyloid, 2016, 23, 234-241.