Binding of serum-derived amyloid-associated proteins to amyloid fibrils

Amyloid signature proteins such as serum amyloid P component, apolipoprotein E (ApoE), and ApoA-IV generally co-localise with amyloid, regardless of the types of amyloid precursor protein or the organs. Most of these proteins derive from serum and have reportedly been involved in amyloid fibril formation and stabilisation, as well as in excretion and degradation of amyloid precursor proteins. However, the processes and mechanisms by which these specific proteins deposit together with amyloid fibrils have not been clarified.We analysed the binding of serum proteins to amyloid fibrils derived from amyloid β and insulin in vitro by using liquid chromatography-tandem mass spectrometry (LC-MS/MS).Specific serum proteins including ApoA-I, ApoE, ApoA-IV, ApoC-III and vitronectin adhered to amyloid fibrils at high concentrations in vitro. In addition, the profile of these proteins commonly occurred in both amyloid β and insulin amyloid fibrils and was mostly consistent with the composition of amyloid signature proteins. We also showed that high concentrations of serum proteins can adhere to amyloid fibrils in a short time.Our in vitro results suggest that amyloid signature proteins coexist with amyloid primarily dependent on the binding of each serum protein, in the extracellular fluid, to amyloid fibrils.