Amyloid Aggregation of Amylin: Gain of Function along Aggregation Pathway?

Abstract

Abstract: Aggregation of human amylin, a 37 amino acid residue neuropeptide of pancreatic origin, into amyloid aggregates is implicated in the etiology of diabetes mellitus type II. Despite its clinical significance, details of progression of nontoxic amylin monomers into cytotoxic oligomers are not very clear. Studies based on Amyloid-β (Aβ), a peptide with similar size, have suggested that a conformational transition along the aggregation pathway makes the oligomers of Aβ membrane-binding competent (1). We have explored the possibility of amylin undergoing a similar transition during aggregation as it could shed light on potential commonalities in the conformation and function of different toxic amyloid species. Using fluorescence correlation spectroscopy, we identify two distinct oligomers of amylin along the aggregation pathway having hydrodynamic radius of 0.90 nm and 1.6 nm respectively. The membrane affinity of amylin increases remarkably from ∼15 % in smaller species to ∼ 85% in the larger one as assessed by an in vitro membrane-binding assay developed in our lab (2). We observe similar difference in the cell membrane attachment ability of these two species in RIN5mf cell lines using confocal microscopy. A preliminary conformational study in artificial lipid bilayers using a SERS based methodology (3) suggests a temporal conformational reorganization in the peptide backbone. Our data suggest that amylin might acquire toxic function by a mechanism which depends on similar conformational features as Aβ in presence of membranes. Further studies aimed at obtaining high-resolution structural details of amylin oligomers in solution and in membranes are currently in progress.