There is a role for proIAPP in islet amyloid fibrillogenesis

• Islet amyloid polypeptide (IAPP) can aggregate into amyloid, a common pathological finding present extracellularly in the islets of Langerhans in individuals with type 2 diabetes. IAPP arises from posttranslational processing of the precursor proIAPP. Accumulation of proIAPP in the secretory granules can result in proIAPP-amyloid formation. We raise the following hypothesis; proIAPP can under not yet defined circumstances aggregate into amyloid-like fibrils intracellularly and at this location act as template and cross-seed amyloid formation of IAPP. We have produced recombinant peptides corresponding to proIAPP and IAPP. These peptides aggregate readily into fibrils with typical amyloid characteristics. Sonicated recproIAPP- and recIAPP- preformed fibrillar aggregates were injected intravenously to +/hIAPP/-mIAPP transgenic mice. Male mice from this strain develop islet amyloid in response to high fat diet. Control animals received an injection of preformed amyloid fibrils from the proinsulin processing intermediate (C-peptide/A-chain) or sodium chloride. All animals were fed a diet high in fat over a ten month period. The presence of islet amyloid was studied after Congo red staining. We found amyloid in 20 % of the islets in animals injected with preformed recIAPP fibrils and in 10 % of the islets in animals injected with preformed recproIAPP fibrils. Control animals developed amyloid in 1-2% of the islets. Our results support the hypothesis that proIAPP-fibrils can act as template and induce conformational changes in soluble IAPP that results in propagation of the amyloid fibrils. This is the first report on in vivo seeding of a localized amyloid form and we present data that support transport of amyloid between islets as a putative route for the spreading of islet amyloid. Our finding suggests that therapies, which use capping of fibril endings, might be useless.

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