Islet Amyloid Polypeptide Membrane Interactions: Effects of Membrane Composition

Abstract

Amyloid formation by islet amyloid polypeptide (IAPP) contributes to β-cell dysfunction in type-2 diabetes. Perturbation of the β-cell membrane may contribute to IAPP induced toxicity. We examine the effects of lipid composition, salt and buffer on IAPP amyloid formation and on the ability of IAPP to induce leakage of model membranes. Even low levels of anionic lipids promote amyloid formation and membrane permeabilization. Increasing the percentage of the anionic lipids, POPS or DOPG, enhances the rate of amyloid formation and increases membrane permeabilization. The choice of zwitterionic lipid has no noticeable effect on membrane catalyzed amyloid formation, but in most cases affects leakage, which tends to decrease in the order DOPC>POPC>sphingomyelin. Uncharged lipids that increase membrane order reduce the ability of IAPP to induce leakage. Leakage is due predominately to pore formation rather than complete disruption of the vesicles under the conditions of these studies. Cholesterol at or below physiological levels significantly reduces the rate of vesicle catalyzed IAPP amyloid formation and decreases susceptibility to IAPP induced leakage. The effects of cholesterol on amyloid formation are masked by 25 mole percent POPS. Overall, there is a strong inverse correlation between the time to form amyloid and the extent of vesicle leakage. NaCl reduces the rate of membrane catalyzed amyloid formation by anionic vesicles, but accelerates amyloid formation in solution. The implications for IAPP membrane interactions are discussed, as is the possibility that loss of phosphatidylserine asymmetry enhances IAPP amyloid formation and membrane damage in vivo via a positive feedback loop.

Extracellular vesicles from human pancreatic islets suppress human islet amyloid polypeptide amyloid formation

“…S3). CH and SM are important membrane components affecting membrane fluidity and lipid raft formation (42), and it has been reported that the presence of SM in PC-based liposomes slows (43), and the addition of CH to synthetic lipid membranes inhibits, IAPP amyloid formation (43,44). These results support the idea that SM and/or CH in healthy pancreatic EVs may help mediate the inhibitory effect on IAPP amyloid formation.…”

The β-cell assassin: IAPP cytotoxicity

“…There is also evidence for association of membrane damage with the growth of amyloid fibrils at the membrane surface (Engel et al 2008). Changes in the composition of the model membranes, including incorporation of cholesterol demonstrated that hIAPP dye leakage from liposomes was more sensitive to membrane composition than amyloid fibril formation at the surface (Zhang et al 2017). However, the time course of sustained leakage and fibril formation are comparable which is consistent with fibrillogenesis at the membrane surface (Engel 2009, Brender et al 2011, Cao et al 2013b.…”

“…Some biophysical studies that included cholesterol also employed high concentrations of anionic lipids and these can mask the effect of the sterol. Higher concentrations of cholesterol in model membranes decrease the rate of fibril formation at the surface, whereas increased POPS leads to increased rate of fibrillogenesis (Zhang et al 2017).…”

“…Synthetic membranes that contain a significant fraction of anionic lipids significantly accelerate human IAPP amyloid formation in vitro, with more highly charged membranes having a larger effect (Zhang et al 2017). Most studies of hIAPP-membrane interactions have made use of simple model membranes comprising pure anionic lipids, such as phosphatidylglycerol (PG) or phosphatidylserine (POPS) or mixtures of anionic lipids with zwitterionic lipids, such as 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC).…”

Changes in glucosylceramide structure affect virulence and membrane biophysical properties of Cryptococcus neoformans

“…LUVs containing 5(6)-carboxyfluorescein (Sigma-Aldrich) were prepared according to the method of Zhang and colleagues (29) and were used in vesicle permeability studies. Briefly, MLVs were prepared using the methods described above, except that 5(6)-carboxyfluorescein was dissolved in phosphate-buffered saline (PBS, pH 7.4) at a concentration of 80 mM before hydration of the lipids.…”