Islet amyloid polypeptide demonstrates a persistent capacity to disrupt membrane integrity

Abstract

Amyloid fiber formation is correlated with pathology in many diseases, including Alzheimer's, Parkinson's, and type II diabetes. Although β-sheet-rich fibrillar protein deposits define this class of disorder, increasing evidence points toward small oligomeric species as being responsible for cell dysfunction and death. The molecular mechanism by which this occurs is unknown, but likely involves the interaction of these species with biological membranes, with a subsequent loss of integrity. Here, we investigate islet amyloid polypeptide, which is implicated in the loss of insulin-secreting cells in type II diabetics. We report the discovery of oligomeric species that arise through stochastic nucleation on membranes and result in disruption of the lipid bilayer. These species are stable, result in all-or-none leakage, and represent a definable protein/lipid phase that equilibrates over time. We characterize the reaction pathway of assembly through the use of an experimental design that includes both ensemble and single-particle evaluations. Complexity in the reaction pathway could not be satisfied using a two-state description of membrane-bound monomer and oligomeric species. We therefore put forward a threestate kinetic framework, one of which we conjecture represents a non-amyloid, non-β-sheet intermediate previously shown to be a candidate therapeutic target.amylin | membrane pore | cytotoxicity | disordered protein A lzheimer's, Parkinson's, type II diabetes, and other epidemiologically important diseases are characterized, in part, by the deposition of proteinaceous plaques termed amyloid (1, 2). For each disease, a specific protein is involved in amyloid assembly via a process that is cytotoxic, ultimately leading to degeneration. Although a defining characteristic, it is now widely thought that these deposits are not the origin of cytotoxicity. Instead, it has been suggested that cell dysfunction and death are mediated by small oligomeric species that acquire the capacity to disrupt cellular membrane integrity (3, 4). The energetic and structural basis by which these states mediate toxicity, however, is unclear.Islet amyloid polypeptide (IAPP or amylin) is a 37-residue peptide hormone cosecreted with insulin by pancreatic β-cells. Unmodified, wild-type IAPP self-assembles to form amyloid in type II diabetic patients in a process that is associated with β-cell dysfunction (2, 5). Indeed, the capacity of species-based sequence variants to form amyloid is correlated with the observation of metabolic disease (6). Thus, whereas primates and cats readily form amyloid and acquire diabetes, rats and mice do not spontaneously develop diabetes, and rodent IAPP does not form amyloid or other β-sheet aggregates. Diabetic symptoms can be induced in model rodents either by using toxins or by using rodents transgenic for human IAPP (7).Previous in vitro studies have shown that the binding of human and rat IAPP to lipid bilayers is cooperative (8), an observation accounted for by the presence of oligomeric species. The pop...

Aliphatic peptides show similar self-assembly to amyloid core sequences, challenging the importance of aromatic interactions in amyloidosis

“…This observation supports growing evidence that early intermediates in amyloidosis are the toxic and pathogenic species (10,11). The toxicity of amyloidogenic peptides/ proteins is increasingly linked to their membrane permeabilization and disruption capability (11,(29)(30)(31). The α-helical intermediates observed in our study could play a key role, as α-helical structures are known to facilitate membrane-peptide interaction and subsequent membrane disruption (32).…”

Islet amyloid polypeptide toxicity and membrane interactions

“…The mechanisms of model membrane disruption are not fully defined, and the nature of the membrane active species is not fully elucidated. Pore formation has been proposed to be a key for membrane disruption (21)(22)(23), but a carpeting, detergent-like mechanism has also been advocated (24,25). There is experimental evidence that intermediates permeabilize membranes (26)(27)(28), whereas other studies implicate fibril growth at the membrane surface (29,30).…”

The glucagon-like peptide-1 receptor agonist exenatide restores impaired pro-islet amyloid polypeptide processing in cultured human islets: implications in type 2 diabetes and islet transplantation

“…Several mechanisms have been shown to mediate amyloid-induced beta cell death, including membrane disruption by hIAPP aggregates [15,16], activation of the Fasmediated apoptosis and caspase pathways [7,8,17], interaction of hIAPP aggregates with components of beta cell membranes (such as heparan sulphate proteoglycans [18] or touch receptors [19]), endoplasmic reticulum stress [20,21], oxidative stress [22], activation of the cJUN N-terminal kinase (JNK) pathway [23] and disruption of the autophagy/lysosomal pathway [24]. Growing evidence suggests that small hIAPP aggregates (e.g.…”