Pancreatic islet amyloid polypeptide membrane binding and permeabilization

Type II diabetes is often associated with the presence of islet amyloid. Islet amyloid is aggregated fibrils composed primarily of the hormone peptide, islet amyloid polypeptide (IAPP), which is co-secreted with insulin from the secretory granules of the beta-cells in the islets of Langerhans of the pancreas. Despite the commonality of IAPP amyloid fibrils in the disease state, recent evidence suggests that the small oligomeric forms of IAPP consisting of a few monomers may be the cause of beta-cell toxicity rather than fibrillar amyloid species. In this thesis we explore the relationship of small oligomeric forms of LAPP with their ability to bind and permeabilize the membranes of our model lipid system. We extend previous research by identifying the region of the peptide responsible for membrane binding, which is required for further membrane permeabilization. Insulin inhibits IAPP fibril formation and because of this inhibitory nature it has been suggested that it may also inhibit IAPP membrane binding and permeabilization. We show that insulin, which is co-secreted with IAPP from the secretory granules, does not inhibit the ability of IAPP to permeabilize membranes. Insulin in the act of inhibiting IAPP fibril formation maintains IAPP in a toxic small oligomeric form that more readily permeabilizes membranes than is observed with IAPP fibrils. Type II diabetes is an aging associated disease; one common change that occurs in the aging process is a decrease in membrane-localized cholesterol. Because of the stabilizing effect of cholesterol on the membrane we looked at the effect of IAPP permeabilization of large unilamellar vesicles (LUVs) with incorporated cholesterol and found that cholesterol dramatically reduces the ability of IAPP to permeabilize lipid membranes. In our dye leakage experiments, cholesterol plays a protective role against membrane permeabilization. In light of the complex environment of the secretory granule and the cell we conclude that insulin in its soluble form does not inhibit the ability of IAPP to permeabilize membranes, and cholesterol has a protective effect against permeabilization by IAPP. Other components of the secretory granule may have a cooperative effect in sequestering the toxic IAPP peptides and protecting the hormone secreting beta-cells of the pancreas.