Mechanistic Insights into Amyloid Formation by IAPP and Inhibition Concepts

The self-assembly of endogenous proteins into amyloid fibrils is the hallmark of a group of diverse human disorders including Alzheimer`s disease, Parkinson`s disease, and type II diabetes. Many inhibition strategies have been developed in the attempt to prevent this key process. A leading approach is the search for natural entities that stabilize the native protein conformation and prevent amyloid formation in healthy individuals. Such a stabilizing agent might be insulin, in the case of the highly amyloidogenic islet amyloid polypeptide (IAPP), which forms amyloid fibrils in the pancreas of type II diabetes patients. Insulin was shown to be an exceptionally potent inhibitor of IAPP fibrilization and is thought to form a complex with IAPP within the pancreatic β-cells. Here, we investigated the molecular mechanism underlying the IAPP-insulin interaction. First, we compared the inhibition abilities of the A and B chains of insulin. Using Thioflavin T fluorescence and circular dichroism measurements, we found that the B chain, but not the A chain, is an excellent inhibitor of IAPP amyloid formation. Next, we mapped the entire B chain using consecutive overlapping peptide arrays and identified a region that inhibits IAPP amyloid formation. We propose that this domain mediates recognition between IAPP and insulin. Moreover, by using a reciprocal assay, we located the interaction domain within IAPP as well. Interestingly, we found it to correspond to the IAPP self-recognition site. The molecular mapping of this interaction is of major importance in elucidating the mechanism underlying amyloid formation in type II diabetes. Moreover, it may be utilized for the design of peptidomimetic inhibitors of amyloid formation.