Effects of the islet amyloid polypeptide on mitochondrial membranes

Abstract

Human islet amyloid polypeptide (hIAPP) is a 37-residue residue peptide hormone reported to be a common factor in protein misfolding disorders such as type 2 diabetes mellitus (T2DM) and Alzheimer’s disease (AD), due to deposition of insoluble hIAPP amyloid in the pancreas and brain, respectively. Multiple studies point to the importance between hIAPP toxicity and the peptide’s interaction with biological membranes. However, research has not adequately addressed such interplay with regards to mitochondrial membranes, which are unique in their composition, particularly due to a structurally unusual phospholipid, cardiolipin (CL), fundamental for mitochondrial membrane function. Using mito-mimetic liposomes (LUVs) with defined lipid compositions that physiologically reflect those of the inner mitochondrial membrane (15% CL content) and mitochondrial contact sites (30% CL), it was determined that, at low lipid:protein ratios, CL accelerated hIAPP monomer aggregation at mitochondrial membranes into on-pathway oligomers. Furthermore, CL-enriched membranes promoted the conversion of fresh hIAPP monomers into a permeabilising oligomeric species at the vesicle surface, possibly by formation of large, 30-40 nm size pores. It is likely that this is due to a combination of biophysico-chemical membrane properties attributed to CL, including an observed increased bilayer fluidity facilitating aggregate penetration. Human IAPP oligomers were found to however increase bilayer rigidity in both LUVs and isolated respiring mitochondria, particularly at the inner fatty acyl region, and this correlated with membrane disruption. Thus, oligomeric, but also fibrillar, hIAPP was able to evoke the efflux of cytochrome c from mitochondria – however, oligomer-induced cytochrome c release was much more dependent upon the presence of CL in the membrane than fibrils, implying a different membrane-disrupting mechanism in which oligomers exploit a direct interaction with CL in membranes. This was indeed confirmed by demonstrating a specific affinity of hIAPP oligomers to CL in protein-lipid overaly assays. The discussed framework, therefore, highlights the uniqueness of CL-enriched membranes and its effects on IAPP interactions, providing ideal conditions for the formation of toxic hIAPP oligomers at such sites, which include the mitochondrial contact sites and the inner mitochondrial membrane. In parallel with the above mechanistic studies, small-molecule compounds such as black tea extract polyphenols (>80% theaflavins) and the di-phenyl-pyrazole compound sery166a were shown to significantly inhibit hIAPP aggregation and membrane permeabilisation in the presence of the pro-aggregative conditions afforded by 30% CL liposomes or isolated mitochondria. These compounds may therefore be considered for future studies on mitochondrial membrane-based therapeutics for diseases involving hIAPP.