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Abstract
Islet Amyloid Polypeptide (IAPP) is a 37-residue neuropancreatic peptide hormone implicated in the etiology type II diabetes. Secreted from insulin producing β-cells, IAPP plays an adaptive role in glucose homoeostasis. IAPP helps to regulate blood sugar by suppressing the release of glucagon locally in the pancreas, and acts on receptors in the brain to regulate satiety and gastric emptying. Upon the onset of type II diabetes, β-cell stress from the overexpression of insulin results in IAPP aggregation and the formation of amyloid deposits in the islets of Langerhans. The processes that govern amyloid formation are still not entirely understood. Along the pathway of amyloid formation, IAPP forms oligomeric species that are toxic to β-cells. To date, few studies have addressed the nature of IAPP oligomerization in solution. IAPP sequences derived from other species, most notably rats, do not exhibit cytotoxicity or form amyloid. While the sequences of these variants are known, there is a lack of structure-activity relationships to identify residues responsible for their non-aggregating properties and lack of toxicity Identification of key residues will inform the design of next generation analogues of IAPP that can be used in the treatment of type I and II diabetes.
In this work, an in situ photochemical crosslinking technique is used in combination with biophysical assays to study the oligomerization of IAPP and IAPP analogues in solution. The results complement previous mass spectrometry studies done in the gas phase and indicate IAPP and variants rapidly form low-order oligomers in solution. Variants of IAPP with point substitutions are investigated to determine the residue-specific effects responsible for mitigating cell toxicity and amyloid formation. A serine to proline substitution at position 28 is identified as a key residue in the modulation of amyloidogenicity and toxicity, while the same substitution at position 29 has very little effect. The role of the highly conserved disulfide loop region of IAPP was also investigated. The loop is found to play a protective role in amyloid formation and contributes to the efficacy of IAPP towards target receptors.
