There are more than fifty protein-misfolding diseases characterised by the pathological self-assembly of peptides and proteins into amyloid fibrils, such as Alzheimer’s disease (AD), type 2 diabetes (T2D), Parkinson’s disease (PD) and systemic amyloidosis. Human islet amyloid polypeptide (hIAPP or amylin) is a 37-residue neuropancreatic hormone that is co-secreted with insulin. Its physiological functions include regulation of blood glucose homeostasis and prevention of gastric emptying through mediating the central nervous system. Under pathophysiological conditions, hIAPP abnormally self-associates into amyloid fibrils and is the major protein constituent found in amyloid deposits in pancreatic islets in patients with T2D—a disease affecting more than 300 million individuals worldwide. Pancreatic islet amyloidosis is believed to contribute to the progression of T2D, β-cell loss and islet transplant failure. The sequence of hIAPP is similar (ca. 50% similarity) to that of β-amyloid polypeptide (Aβ), and both hIAPP and Aβ have been detected in the brain plaques in AD. Indeed, studies in vitro have shown that hIAPP can co-aggregate with Aβ, and others have shown an epidemiological link between diabetes and neurodegenerative diseases including AD, proposing an AD-associated type 3 diabetes (T3D). There is only one natural genetic mutation of hIAPP reported so far (the S20G variant). This variant is associated with early-onset T2D in the Japanese population and several in vitro experiments have demonstrated that S20G is more aggregation-prone compared with wild-type (wt) hIAPP.
Despite being first identified as the amyloidogenic peptide associated with T2D more than 30 years ago, and the recent solution of near-atomic resolution cryo-EM structures of wt hIAPP and S20G fibrils formed in vitro, the aggregation mechanism of hIAPP is still unclear. Numerous studies of hIAPP fibrillation using synthetic peptide variants have illustrated the influence of particular residues or regions of the sequence on the aggregation processes (reviewed in). Others have reported that the kinetics of hIAPP self-assembly follow a characteristic nucleation-dependent polymerisation process involving primary and secondary pathways, and highlighted the importance of secondary nucleation in fibril formation. Recently, chemical kinetic analysis by global fitting of kinetic models has been used to determine the molecular mechanisms of AD-associated Aβ aggregation, including the effects of sequence, pH, agitation and chaperones on the relative contribution of the different processes in amyloid fibril formation (primary nucleation, secondary nucleation, elongation and fragmentation). By contrast, only a single report to date has applied this approach to study IAPP, using a non-natural variant in which the C-terminus lacks its natural amidation: a moiety that forms key interactions in the fibril structures and is known to affect the rate of aggregation (note that a seco
