Small Molecule Inhibitors of Islet Amyloid Polypeptide Fibrillogenesis and Cytotoxicity

Abstract

Protein fiber formation is associated with diseases ranging from Alzheimer's to type II diabetes. The fiber formation is a complex reaction, which includes a number of conformational and oligomeric intermediate states. In recent years, it has become clear that it is these states, and not the end product (i.e. fibers) of amyloid formation that are the toxic agents associated with disease. These insights indicate that small molecule screens must to be directed at assembly mechanism in order to maximize the prospects for success. Islet amyloid polypeptide (IAPP) is a 37 residue peptide hormone co-secreted with insulin by the ß-cells of the pancreas. In patients with type II diabetes, this protein aggregates as amyloid in a process that is correlated with ß-cell dysfunction and the loss of ß-cell mass. In in vitro studies, the addition of soluble IAPP has been shown to be toxic to many ß- and non-ß-cell lines. IAPP fibrillogenesis, as is the case for many other amylodiogenic proteins, can be catalyzed by lipid bilayers. Paradoxically, while amyloid fibers are ß-sheet rich, membrane-stabilized states are α-helical. We have identified a small molecule alpha helix mimetic, IS5, which inhibits bilayer catalysis of fibrillogenesis, and rescues IAPP-induced toxicity in cell culture. Importantly, IAPP:IS5 interactions localize to the putative α-helical region of IAPP, revealing that α-helical states are on pathway to fiber formation. Normally, IAPP is not amyloidogenic as its cosecreted partner, insulin, prevents self-assembly. Here, we show that IS5 inhibition is synergistic with insulin. IS5 therefore represents a new approach to amyloid inhibition as the target is an assembly intermediate that may additionally restore functional IAPP expression.