Evaluation of membrane models and their composition for islet amyloid polypeptide-membrane aggregation

Abstract

Human islet amyloid polypeptide (IAPP) forms amyloid fibrils in the pancreatic islets of patients suffering from type 2 diabetes mellitus (T2DM). The formation of IAPP fibrils has been shown to cause membrane damage which most likely is responsible for the death of pancreatic islet β-cells during the pathogenesis of T2DM. Several studies have demonstrated a clear interaction between IAPP and lipid membranes. However the effect of different lipid compositions and of various membrane mimetics (including micelles, bicelles, SUV and LUV) on fibril formation kinetics and fibril morphology has not yet systematically been analysed. Here we report that the interaction of IAPP with various membrane models promoted different processes of fibril formation. Our data reveal that in SDS and DPC micelles, IAPP adopts a stable α-helical structure for several days, suggesting that the micelle models may stabilize monomeric or small oligomeric species of IAPP. In contrast, zwitterionic DMPC/DHPC bicelles and DOPC SUV accelerate the fibril formation compared to zwitterionic DOPC LUV, indicating that the size of the membrane model and its curvature influence the fibrillation process. Negatively charged membranes decrease the lag-time of the fibril formation kinetics while phosphatidylethanolamine and cholesterol have an opposite effect, probably due to the modulation of the physical properties of the membrane and/or due to direct interactions with IAPP within the membrane core. Finally, our results show that the modulation of lipid composition influences not only the growth of fibrils at the membrane surface but also the interactions of β-sheet oligomers with membranes.

Islet Amyloid Polypeptide Membrane Interactions: Effects of Membrane Composition

“…This is an important control since thioflavin-T is an extrinsic probe and sometimes gives a low signal even in the presence of amyloid fibrils 71 . As expected, and in agreement with earlier studies, the rate of hIAPP amyloid formation was notably enhanced in the presence of vesicles containing high percentages of anionic lipids (Figure-2) 42, 43, 49, 52, 57 . However, significant effects are observed for even modest amounts of DOPG or POPS.…”

“…But in this case a modest acceleration was observed. Comparison of the data presented here with this study and with previous work 51, 52, 59 highlights the high sensitivity of IAPP membrane interactions to lipid composition. In the absence of POPS, addition of 20 mole percent cholesterol to zwitterionic LUVs increased the value of T 50 by 1.5-fold relative to the value in the absence of cholesterol.…”

“…We choose a lipid to peptide ratio of 20:1 and a hIAPP concentration of 20 µM for these and all following studies since this is typical of values used for biophysical studies of membrane catalyzed hIAPP amyloid formation 46, 52, 53 . This concentration will result in an excess of peptide relative to the concentration of vesicles.…”

“…The β-cell membrane contains cholesterol, gangliosides, sphingomyelins, and the level of anionic phospholipids is reported to range from 2.5 to 13.2 mole percent of the total phospholipids 57 . The vast majority of model systems typically contain a much higher percentage of anionic lipids and lack cholesterol and gangliosides, although the effects of cholesterol have been examined in a subset of systems 52, 57–59 . Cholesterol is an important component of membranes; it modulates their biophysical properties and is important for the uptake of hIAPP in vivo 58, 60 .…”

Membrane Curvature-sensing and Curvature-inducing Activity of Islet Amyloid Polypeptide and Its Implications for Membrane Disruption

“…In addition, we tested IAPP clearance of POPG and DOPG vesicles, because phosphatidylglycerol is a membrane component that is widely used in the literature (37,65,66). Both hIAPP and, albeit less so, rIAPP moderately cleared light scattering from POPG more than that from DOPG, although not to the extent that they cleared POPS light scattering (as summarized in Fig.…”