Induction of Negative Curvature as a Mechanism of Cell Toxicity by Amyloidogenic Peptides: The Case of Islet Amyloid Polypeptide

The death of insulin-producing β-cells is a key step in the pathogenesis of type 2 diabetes. The amyloidogenic peptide Islet Amyloid Polypeptide (IAPP, also known as amylin) has been shown to disrupt β-cell membranes leading to β-cell death. Despite the strong evidence linking IAPP to the destruction of β-cell membrane integrity and cell death, the mechanism of IAPP toxicity is poorly understood. In particular, the effect of IAPP on the bilayer structure has largely been uncharacterized. In this study, we have determined the effect of the amyloidogenic and toxic hIAPP₁₋₃₇ peptide and the nontoxic and nonamyloidogenic rIAPP₁₋₃₇ peptide on membranes by a combination of DSC and solid-state NMR spectroscopy. We also characterized the toxic but largely nonamyloidogenic rIAPP₁₋₁₉ and hIAPP₁₋₁₉ fragments. DSC shows that both amyloidogenic (hIAPP₁₋₃₇) and largely nonamyloidogenic (hIAPP₁₋₁₉ and rIAPP₁₋₁₉) toxic versions of the peptide strongly favor the formation of negative curvature in lipid bilayers, while the nontoxic full-length rat IAPP₁₋₃₇ peptide does not. This result was confirmed by solid-state NMR spectroscopy which shows that in bicelles composed of regions of high curvature and low curvature, nontoxic rIAPP₁₋₃₇ binds to the regions of low curvature while toxic rIAPP₁₋₁₉ binds to regions of high curvature. Similarly, solid-state NMR spectroscopy shows that the toxic rIAPP₁₋₁₉ peptide significantly disrupts the lipid bilayer structure, whereas the nontoxic rIAPP₁₋₃₇ does not have a significant effect. These results indicate IAPP may induce the formation of pores by the induction of excess membrane curvature and can be used to guide the design of compounds that can prevent the cell-toxicity of IAPP. This mechanism may be important to understand the toxicity of other amyloidogenic proteins. Our solid-state NMR results also demonstrate the possibility of using bicelles to measure the affinity of biomolecules for negatively or positively curved regions of the membrane, which we believe will be useful in a variety of biochemical and biophysical investigations related to the cell membrane.