Insulin Inhibits Aß42 Aggregation and Prevents Aß42-Induced Membrane Disruption.

Alzheimer's disease (AD) is associated with self-assembly of amyloid ß-protein (Aß) into soluble oligomers. Of the two predominant Aß alloforms, Aß40 and Aß42, the latter is particularly strongly linked to AD. Longitudinal studies revealed a correlation between AD and type 2 diabetes (T2D), characterized by abnormal insulin levels and insulin resistance. Although administration of intranasal insulin is explored as a therapy against AD, the extent to which insulin affects Aß dynamics and activity is unclear. We here investigate the effect of insulin on Aß42 self-assembly and characterize the capacity of insulin, Aß42, and Aß42 co-incubated with insulin to disrupt the integrity of biomimetic lipid vesicles. We demonstrate that quiescently incubated insulin, which does not form amyloid fibrils, over time develops membrane-disrupting capacity, which we propose to originate in misfolded insulin monomers. These hypothetically toxic misfolded monomers might contribute to the development of insulin resistance in early stages of T2D that are associated with abnormally high insulin levels. We show that in contrast to quiescent incubation, insulin incubated under agitated conditions readily forms amyloid fibrils, which protect against membrane permeation. Insulin quiescently incubated with Aß42 attenuates both Aß42 fibril formation and the ability of Aß42 to disrupt membranes in a concentration-dependent manner. Our findings offer insights into interactions between insulin and Aß42 that are relevant to understanding the molecular basis of intranasal insulin as a therapy against Aß-induced AD pathology, thereby elucidating a plausible mechanism underlying the observed correlations between AD and T2D.

Soluble State of Villin Headpiece Protein as a Tool in the Assessment of MD Force Fields.

Protein self-assembly plays an important role in cellular processes. Whereas molecular dynamics (MD) represents a powerful tool in studying assembly mechanisms, its predictions depend on the accuracy of underlying force fields, which are known to overly promote protein assembly. We here examine villin headpiece domain, HP36, which remains soluble at concentrations amenable to MD studies. The experimental characterization of soluble HP36 at concentrations of 0.05 to 1 mM reveals concentration-independent 90% monomeric and 10% dimeric populations. Extensive all-atom MD simulations at two protein concentrations, 0.9 and 8.5 mM, probe the HP36 dimer population, stability, and kinetics of dimer formation within two MD force fields, Amber ff14SB and CHARMM36m. MD results demonstrate that whereas CHARMM36m captures experimental HP36 monomer populations at the lower concentration, both force fields overly promote HP36 association at the higher concentration. Moreover, contacts stabilizing HP36 dimers are force-field-dependent. CHARMM36m produces consistently higher HP36 monomer populations, lower association rates, and weaker dependence of these quantities on the protein concentration than Amber ff14SB. Nonetheless, the highest monomer populations and dissociation constants are observed when the TIP3P water model in Amber ff14SB is replaced by TIP4P/2005, showcasing the critical role of the water model in addressing the protein solubility problem in MD.