Abstract
Islet amyloid polypeptide (IAPP) is a 37-amino acid amyloid protein intimately associated with pancreatic islet β-cell dysfunction and death in type II diabetes. In this study, we combine spectroscopic methods and microscopy to investigate α-helical IAPP-membrane interactions. Using light scattering and fluorescence microscopy, we observe that larger vesicles become smaller upon treatment with human or rat IAPP. Electron microscopy shows the formation of various highly curved structures such as tubules or smaller vesicles in a membrane-remodeling process, and spectrofluorometric detection of vesicle leakage shows disruption of membrane integrity. This effect is stronger for human IAPP than for the less toxic rat IAPP. From CD spectra in the presence of different-sized vesicles, we also uncover the membrane curvature-sensing ability of IAPP and find that it transitions from inducing to sensing membrane curvature when lipid negative charge is decreased. Our in vivo EM images of immunogold-labeled rat IAPP and human IAPP show both forms to localize to mitochondrial cristae, which contain not only locally curved membranes but also phosphatidylethanolamine and cardiolipin, lipids with high spontaneous negative curvature. Disruption of membrane integrity by induction of membrane curvature could apply more broadly to other amyloid proteins and be responsible for membrane damage observed in other amyloid diseases as well.
Highlights
The mechanism behind diabetes-associated membrane damage by islet amyloid polypeptide (IAPP) is poorly understood
Using circular dichroism (CD), spectrophotometry, and fluorescence microscopy of giant unilamellar vesicles (GUVs), we find that ␣-helical IAPP converts large, negatively charged vesicles into much smaller structures
We found that the less POPS the vesicle membranes contained, the less clearance was observed (Fig. 1C). human IAPP (hIAPP) was generally more potent than rat IAPP (rIAPP) at reducing the scattering, and this difference was pronounced at 33% POPS
Summary
The mechanism behind diabetes-associated membrane damage by islet amyloid polypeptide (IAPP) is poorly understood. ␣-synuclein binds to negatively charged membranes as an amphipathic ␣-helix that alone is sufficient to induce membrane curvature, or in other words to remodel liposomes into tubular structures, small, highly curved vesicles, or protein-lipid nanoparticles [52,53,54,55]. Such membrane remodeling and induction of membrane curvature can cause disruption of membrane integrity [52]. Cristae are curved membrane structures rich in cardiolipin and phosphatidylethanolamine, lipids with pronounced negative spontaneous curvature
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