Albumin Alters the Conformational Ensemble of Amyloid-β by Promiscuous Interactions: Implications for Amyloid Inhibition

Abstract

Human serum albumin (HSA) is a key endogenous inhibitor of amyloid-β (Αβ) aggregation. In vitro HSA inhibits Aβ fibrillization and targets multiple species along the aggregation pathway including monomers, oligomers, and protofibrils. Amyloid inhibition by HSA has both pathological implications and therapeutic potential, but the underlying molecular mechanism remains elusive. As a first step towards addressing this complex question, we studied the interactions of an Aβ42 monomer with HSA by molecular dynamics simulations. To adequately sample the conformational space, we adapted the replica exchange with solute tempering (REST2) method to selectively heat the Aβ42 peptide in the absence and presence of HSA. Aβ42 binds to multiple sites on HSA with a preference to domain III and adopts various conformations that all differ from the free state. The β-sheet abundances of H14-E22 and A30-M33 regions are significantly reduced by HSA, so are the β-sheet lengths. HSA shifts the conformational ensemble towards more disordered states and alters the β-sheet association patterns. In particular, the frequent association of Q15-V24 and N27-V36 regions into β-hairpin which is critical for aggregation is impeded. HSA primarily interacts with the latter β-region and the N-terminal charged residues. They form promiscuous interactions characterized by salt bridges at the edge of the peptide-protein interface and hydrophobic cores at the center. Consequently, intrapeptide interactions crucial for β-sheet formation are disrupted. Our work builds the bridge between the modification of Aβ conformational ensemble and amyloid inhibition by HSA. It also illustrates the potential of the REST2 method in studying interactions between intrinsically disordered peptides and globular proteins.

Testing the link between isoaspartate and Alzheimer’s disease etiology

“…Next, we used the structure-function relationship to understand the reduced Aß binding capacity of aHSA (pTau binding to HSA is poorly understood and thus was not considered). Since Aß can bind to each of the HSA three domains [50], with the exact binding locations not experimentally proven, we used as plausible binding sites those identified previously in our MDS studies that used two protocols [51, 52]. While for fHSA each of the three domains fitted well the crystal structure (RMSD 2.5 Å), for aHSA only domain I superposed satisfactorily with the crystal structure (RMSD 3.2 Å).…”

Multipronged Regulatory Functions of Serum Albumin in Early Stages of Amyloid-β Aggregation

“…The same conclusion has been drawn from our previous simulations of single Aβ42 monomer and HSA. 32 It indicates that the electrostatic interactions are pivotal in the binding process of Aβ monomers to HSA.…”

“…(3) HSA reduced the β-sheet structures in monomeric Aβ42 and rendered its conformational states less compatible to fibrillization. 32 These initial results provide some clues for interactions between higher-order Aβ assemblies and HSA, motivating the hypothesis that HSA could simultaneously trap multiple Aβ peptides and prevent them from forming fibrillar oligomers.…”

“…A single Aβ monomer also prefers to bind to domain III in our previous simulations. 31,32 Therefore, HSA could interfere with the formation of fibril nucleus and effectively reduce the concentration of free monomers, which could account for a lengthened lag phase revealed by ThT fluorescences. 21 Second, HSA could prevent the growth of protofibrils to fibrils by disrupting the structural integrity of protofibrils and blocking the protofibril end for the addition of monomers.…”

“…Our studies have addressed some issues on HSA-monomeric Aβ interactions and the interference by fatty acids with the following primary findings: (1) The C-terminal region of monomeric Aβ is the primary target of HSA while Aβ binds to multiple binding sites on HSA with a preference to domain III. 31,32 (2) Intrinsic flexibilities of HSA were essential for optimal Aβ binding but were quenched by fatty acids, resulting in interferences with preferential Aβ-HSA interactions.…”