Ca²⁺ Interacts with Glu-22 of Aβ(1–42) and Phospholipid Bilayers to Accelerate the Aβ(1–42) Aggregation Below the Critical Micelle Concentration

Abstract

The amyloid cascade hypothesis links the amyloid-β (Aβ) peptide aggregation to neuronal cell damage and ultimately the etiology of Alzheimer's disease (AD). Although Aβ aggregation has been known to accelerate at cell membranes, the exact mechanism of Aβ peptide deposition and the involvement of extracellular species are still largely unclear. Using surface plasmon resonance (SPR) and atomic force microscopy (AFM), we demonstrate that Ca(2+) ions, in conjunction with lipid bilayer, lower the threshold concentration for Aβ aggregation (>a few micromolar in vitro) to physiological levels (low nanomolar). Circular dichroism spectroscopy reveals that Ca(2+) ions and the lipid bilayer concertedly accelerate the conformational change or misfolding of Aβ peptides. Molecular dynamics calculation indicates that Ca(2+) is sandwiched between Glu-22 of Aβ and the lipid phosphate group. SPR experiments conducted using an E22G mutant confirmed the strong interaction among Ca(2+), Aβ(1-42), and the phospholipid bilayer. With the C- and N-termini of the Aβ dimer fully exposed for the attachment of additional Aβ molecules, fibrils formed with the Ca(2+)-anchored Aβ nuclei appear to interact with lipid bilayers differently from those preformed in solution. Thus, similar to the role of Ca(2+) in enriching islet amyloid polypeptides in the pancreas of diabetic patients ( Biophys. J. 2013 , 104 , 173 - 184 ) and the "Ca(2+) bridge" in mediating membrane interaction with α-synuclein in the Parkinson's disease ( Biochemistry , 2006 , 45 , 10947 - 10956 ), the influence of Ca(2+) on the Aβ adsorption at cell membranes, which leads to neuronal membrane damage in AD, cannot be overlooked.

Reduced Lipid Bilayer Thickness Regulates the Aggregation and Cytotoxicity of Amyloid-β

“…DLPC SLBs facilitate the rapid formation of globular aggregates on the bilayer surface of, and possibly within, the membrane. In contrast, most reports of membrane-mediated A␤ aggregation using AFM demonstrate the formation of conventional fibrils (41,47,48). This membrane-associated aggregation is probably responsible for the bilayer disruption, despite not occurring through the canonical amyloid formation pathway.…”

Molecular Simulations of Amyloid Structures, Toxicity, and Inhibition

“…To confirm our simulation observation, we designed surface plasmon resonance (SPR) experiments to determine the Ca 2+ effect on the adsorption of Aβ peptides onto lipid bilayers . SPR results clearly showed that in the presence of Ca 2+ , Aβ 1–42 peptides adsorbed on a supported DPPS bilayer are 9‐fold higher than those in the absence of Ca 2+ (Figure ).…”

Protein folding, misfolding and aggregation: The importance of two-electron stabilizing interactions

“…The expected conformational transition is well documented, cf. Fig 22A(a) and 22A(b) [ 176 , 177 , 181 – 184 ], and may well contribute to the catalysis of polymerization by membranes and lipid rafts [ 247 – 254 ].…”

“…The expected conformational transition is well documented, cf. Fig 22A(a) and 22A(b) [ 176 , 177 , 181 – 184 ], and may well contribute to the catalysis of polymerization by membranes and lipid rafts [ 247 – 254 ]. (D) (a) The ( S )-cysteine can bring together two Aβ molecules via three-point binding of Asp-1, in spite of a degree of steric strain in either chelation mode; (b) The ( R )-cysteine can tightly bind one Aβ molecule in a complex free of strain, but bringing in the second Aβ molecule is impossible due to prohibitive steric hindrance; (c) Thus, the ( S )-cysteine would promote formation and release of the Aβ↑Aβ↓ dimer and therefore accelerate polymerization (the blue curve in the insert), while the ( R )-cysteine would immobilize isolated Aβ molecules on the graphene surface and therefore slow down polymerization (the red curve in the insert).…”