Fibril Formation by Glucagon in Solution and in Membrane Environments

Glucagon is a 29-amino acid peptide hormone secreted by pancreatic α-cells and interacts with specific receptors located in various organs. Glucagon tends to form gel-like fibril aggregates that are cytotoxic because they activate apoptotic signaling pathways. First, fibril formation by glucagon in acidic solution is discussed in light of morphological and structural changes during elapsed time. Second, we provide kinetic analyses using a two-step autocatalytic reaction mechanism; the first step is a homogeneous nuclear formation process, and the second step is an autocatalytic heterogeneous fibril elongation process. Third, the processes of fibril formation by glucagon in a membrane environment are discussed based on the structural changes in the fibrils. In the presence of bicelles in acidic solution, glucagon interacts with the bicelles and forms fibril intermediates on the bicelle surface and grows into elongated fibrils. Glucagon-dimyristoylphosphatidylcholine (DMPC) bilayers in neutral solution mimic the environment for fibril formation by glucagon under near-physiological condition. Under these conditions, glucagon forms fibril intermediates that grow into elongated fibrils inside the lipid bilayer. Many days after preparing the glucagon-DMPC bilayer sample, the fibrils form networks inside and outside the bilayer. Furthermore, fibril intermediates strongly interact with lipid bilayers to form small particles