Identification of Key Interactions in the Initial Self-Assembly of Amylin in a Membrane Environment

Islet amyloid polypeptide, also known as amylin, forms aggregates that reduce the amount of insulin-producing cells in patients with type II diabetes mellitus. Much remains unknown about the process of aggregation and cytotoxicity, but it is known that certain cell membrane components can alter the rate of aggregation. Using atomistic molecular dynamics simulations combined with the highly mobile membrane mimetic model incorporating enhanced sampling of lipid diffusion, we investigate interaction of amylin peptides with the membrane components as well as the self-assembly of amylin. Consistent with experimental evidence, we find that an initial membrane-bound α-helical state folds into stable β-sheet structures upon self-assembly. Our results suggest the following mechanism for the initial phase of amylin self-assembly. The peptides move around on the membrane with the positively charged N-terminus interacting with the negatively charged lipid headgroups. When the peptides start to interact, they partly unfold and break some of the contacts with the membrane. The initial interactions between the peptides are dominated by aromatic and hydrophobic interactions. Oligomers are formed showing both intra- and interpeptide β-sheets, initially with interactions mainly in the C-terminal domain of the peptides. Decreasing the pH to 5.5 is known to inhibit amyloid formation. At low pH, His18 is protonated, adding a fourth positive charge at the peptide. With His18 protonated, no oligomerization is observed in the simulations. The additional charge gives a strong midpoint anchoring of the peptides to negatively charged membrane components, and the peptides experience additional interpeptide repulsion, thereby preventing interactions

Protofibrillar Assembly Toward the Formation of Amyloid Fibrils

The formation and growth of amyloid fibrils was investigated using coarse-grained molecular dynamics simulations. In particular, we studied the assembly of amylin-(20-29) peptides, preassembled into protofibril fragments. The systems consisted of 27 protofibril fragments initially distributed onto a regular cubic grid with random orientation. Their association was followed on the µs time scale. At 300 K, it was observed that, while the assemblies formed are mainly disordered, there was an apparent preference for the fragments to associate such that an elongation of the structures predominates over their lateral extension. Increasing the temperature in the simulations resulted in an increase of the contact surfaces and allowed for rearrangement within the prefibrillar aggregates over longer time scales. The preferential elongation-like growth mechanism observed at 300 K was not persistent at higher temperatures indicative of a shift in growth pathway, congruent with experimental observations that changing growth conditions alters the morphology of the fibrils.