Interaction Of Human Islet Amyloid Poly Peptide With Phospholipid Membrane Vesicles

Abstract

Amylin, also known as Human Islet Amyloid Polypeptide (hIAPP), is a 37-residue peptide, suspected to play a major role in the malfunction of insulin secretion in diabetes mellitus type II. Co-secreted with insulin in the beta cells, hIAPP, in higher rates destroys the barrier function of the beta-cells, leading to a failure in insulin production. Because of its amyloidogenity, aggregates of fibrils can be observed in the islands of Langerhans due to its overexpression. We studied the physico chemical properties of hIAPP by observing changes in its structure depending on time and the surrounding media using MALDI-TOF-MS, ATR FT-IR- and fluorescence spectroscopy. In water, hIAPP fibrils grow slowly, after a 37°C incubation for 24 hours some alpha-helices are twisted, and after two weeks, no random coil is detected anymore. We determined membrane binding of dansyl-labeled hIAPP to phosphatidylserine (PS)/ phosphatidylcholine (PC) membranes. Additionally, using confocal laser scanning microscopy the binding of TAMRA labelled hIAPP to giant unilamellar vesicles could be observed. At physiological pH, hIAPP is positively charged and thus negative charges at the phospholipid membrane surface accelerate the process of peptide folding. Being random coil as initial state, a mixture of anti-parallel beta-sheets and alpha-helices emerges in time. In the presence of negatively charged PS/PC membranes, hIAPP aggregates can be seen within a few minutes after titration. To understand the process of penetration into cells, we performed leakage measurements of carboxyfluoresceine (CF) filled phospholipid large unilamellar vesicles by means of fluorescence spectroscopy. Titration of hIAPP to CF filled PS/PC liposomes showed different results concerning equilibrium time and maximal extent of leakage depending on the age and preparation of the peptide. In particular the composition of the vesicles seems to determine their stability in the presence of hIAPP.