Calcium Reshapes Aβ Aggregation at Anionic Lipid Membranes

Dysregulated calcium homeostasis is a hallmark of neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases. In Alzheimer's disease (AD), the aggregation of amyloid beta (Aβ) peptides at neuronal membranes is shown to be modulated by the presence of Ca2+ ions, yet the molecular mechanism by which Ca2+ reshapes Aβ aggregation at anionic membrane surfaces remains poorly understood. To address this knowledge gap, we employed coarse-grained molecular dynamics simulations to investigate the aggregation of the model amyloidogenic K16LVFFAE22 fragment of Aβ (Aβ16-22), on a mixed bilayer composed of 30% anionic phosphatidylserine (PS) and 70% zwitterionic phosphatidylcholine (PC) (30% POPS, 70% POPC) in the presence of Ca2+ ions. We find that Ca2+ ions screen the surface charge and reduce hydrophobic packing defects at the membrane surface. Since peptide binding is primarily driven by electrostatic interactions between positively charged residues and anionic lipids, followed by hydrophobic interactions, these Ca2+-induced changes delay peptide adsorption onto the bilayer, promoting the formation of larger aggregates in solution that subsequently adsorb as preformed aggregates. This is in contrast with the no-Ca2+ condition, where peptides bind earlier as small oligomers and aggregate on the bilayer. Following adsorption, PS-Ca2+-PS ionic bridges rapidly condense PS lipids around peptide aggregates and reduce their lateral mobility. This leads to larger, less ordered aggregates with shallower insertion and hydrophobic residues exposed to the solution, a structural feature associated with seeding-active aggregates. These findings align with Western blot analyses showing enhanced Aβ aggregation with Ca2+ ions and provide a molecular basis for the observed aggregation behavior. Together, our results provide mechanistic insight into how calcium alters the membrane-mediated aggregation pathway of amyloidogenic peptides, potentially informing therapeutic strategies against amyloid pathology.