Study On The Interaction Between Amyloid Polypeptide Oligomers And Lipid Membrane

Misfolding of soluble peptides/proteins such as IAPP,A?,and?-synuclein will form insoluble amyloid deposits inside and outside the cell.The amyloid peptide/protein deposition processes are closely related to the occurrence of type?diabetes mellitus?T2DM?,Alzheimer's disease?AD?,Parkinson's disease?PD?and related clinical syndrome.Deposition of amyloid peptides/proteins on the cell surface produces cytotoxicity,leading to cell death.Although amyloid polypeptides have various amino acid sequences,they all produce insoluble amyloid deposits,suggesting that they may have similar pathogenic mechanism.A large number of experimental studies have shown that the main cause of cytotoxicity may be oligomeric intermediates formed during aggregation.The interaction between these oligomeric intermediates and biofilm may damage the integrity of the membrane,or cause some pathological processes such as oxidative stress,endoplasmic reticulum stress,and mitochondrial dysfunction.In this paper,we choose the aggregation core regions of human islet amyloid polypeptide?hIAPP?and?-amyloid polypeptide?A??,i.e.,hIAPP_18-278-27 and A?_25-35,as model peptides,and choose POPC/POPG 4:1 liposome as a model membrane system,and study their oligomeric structures as well as the relation between the structural feature of the oligomers and the ability of the oligomers to disrupt the lipid membrane.We first design two oligomers using hIAPP_18-278-27 and its isomer with D,L-alternating amino acids,examine the disruptive ability of the oligomers to the model membrane by ³¹P NMR spectroscopy and calcein leakage assay.We find that the D,L-alternating peptide oligomers are more disruptive than the all-L peptide oligomers to the lipid membrane.We also study the structural characteristics of oligomers by CD,TEM,¹H NMR and fluorescence quenching experiments.We show that the D,L-alternating peptide adopts an extended PP? conformation,while all-L peptide adopts a random coil conformation in oligomers.Compared with the all-L peptide oligomers,the structure of the D,L-alternating peptide oligomers is looser and the hydrophobic exposure is greater.Both the changes from PP? to?-sheet in the structure of D,L-alternating peptide and from random coil to?-sheet in the structure of all-L peptide reduce the damage effects of the oligomers on the membrane.Our results show that an oligomer with extended peptide chains could be more potent in membrane disruption than an oligomer with folded peptide chains,and the peptide-peptide interaction could decrease the disruptive ability of oligomer.We also construct two oligomers using A?_25-355-35 and C-terminal amidated A?_25-35respectively.The experimental results show that the C-terminal amidated A?_25-35oligomer has a higher degree of hydrophobic exposure,a slower rate of conformational transition to?-sheet,and a stronger ability to damage the model membrane,although the two types of peptides have random coil conformation in their initial oligomeric states.This is attributed to the fact that the peptide amidated at the C-terminus has more net positive charge,which render the oligomer more loosely packed and the hydrophobic residues in oligomer more water exposed.