In Vitro Interactions Between Amyloid Beta and Islet Amyloid Polypeptide

Abstract

Alzheimer's disease and type II diabetes mellitus are two prevalent protein misfolding diseases. Their co-morbidity has raised questions regarding potential interactions between the peptides that are implicated in each disease state. Islet amyloid polypeptide (IAPP) aggregates in type II diabetes mellitus while amyloid beta 1-42 (Aβ42) aggregates in Alzheimer's disease. Interactions between these amyloidogenic peptides may result in coaggregation that could exacerbate the cellular toxicity associated with each disease by increasing the amount of toxic oligomers over less toxic mature fibrils.We tested this hypothesis in vitro by studying aggregation kinetics, aggregate morphology, stability and cytotoxicity of IAPP / Aβ42 mixtures. Tetramethylrhodamine (TMR)-labeled Aβ42 was incubated with excess AlexaFluor 488 (A488)-labeled IAPP which revealed through colocalization analysis that both peptides coaggregate. A complimentary experiment with excess TMR-labeled Aβ42 showed similar results. We then monitored aggregation kinetics of IAPP and Aβ42 through Thioflavin T fluorescence. Equimolar mixtures of Aβ42 and IAPP displayed aggregation kinetics intermediate of Aβ42 and IAPP. While IAPP prolonged the lag-phase of Aβ42, the opposite was not observed. At the same time, the presence of equimolar or excess IAPP diminished the SDS-resistance of fibrillar Aβ42 aggregates. We then added aggregation intermediates corresponding to the lag phase, early and late growth phases, and the plateau phase to human neuroblastoma (SH-EP) cells to test for cytotoxicity. Surprisingly, the cytotoxicity of Aβ42 / IAPP mixtures was intermediate to that of Aβ and IAPP alone. Taken together, these results indicate a molecular interaction exists between these two disease-relevant peptides and may provide a link between type II diabetes mellitus and Alzheimer's disease but does not support a model in which co-aggregation with IAPP directly exacerbates Aβ toxicity by increasing oligomer formation.