Cholesterol and Lipid Rafts in the Biogenesis of Amyloid-β Protein and Alzheimer's Disease

Abstract

Cholesterol has been conjectured to be a modulator of the amyloid cascade, the mechanism that produces the amyloid-β (Aβ) peptides implicated in the onset of Alzheimer's disease. We propose that cholesterol impacts the genesis of Aβ not through direct interaction with proteins in the bilayer, but indirectly by inducing the liquid-ordered phase and accompanying liquid-liquid phase separations, which partition proteins in the amyloid cascade to different lipid domains and ultimately to different endocytotic pathways. We explore the full process of Aβ genesis in the context of liquid-ordered phases induced by cholesterol, including protein partitioning into lipid domains, mechanisms of endocytosis experienced by lipid domains and secretases, and pH-controlled activation of amyloid precursor protein secretases in specific endocytotic environments. Outstanding questions on the essential role of cholesterol in the amyloid cascade are identified for future studies.

Keywords: amyloid precursor protein; cellular compartment; cholesterol; lipid bilayer; lipid phase; molecular dynamics.

Figure 1

(A) All-atom representation of the amyloid precursor protein (APP) featuring structured domains predicted by Membranome (entry 117) based on AlphaFold2(56). Secreted APP (sAPP) domain produced from cleavage by β-secretase (BACE1), the variable amyloid beta domain (Aβ), and the intracellular domain visualized with bilayer ecto- and endo-plasmic domains indicated in pink and cyan. (B) Cartoon representation of APP highlighting structured ectodomains 1 and 2, the intrinsically disordered inhibitor domain, and the C99 peptide domain. Within the C99 domain is the variable Aβ subdomain, pending γ-secretase cleavage, as well as a transmembrane hinge at G₇₀₈G₇₀₉ evidenced to significantly modulate Aβ production.

Figure 2

(A) Amyloidogenic and non-amyloidogenic APP processing pathways. (B) The first step in the biogenesis of Aβ is implied to occur in liquid ordered lipid raft domains (blue, saturated lipid tails) is the cleavage of APP by β-secretase (BACE1) to produce C99. This amyloidogenic cleavage of APP occurs primarily in early endosomes and accounts for 10% of APP processing. (C) Non-amyloidogenic cleavage of APP by α-secretase (ADAM10) accounts for 90% of all APP processing(240). It primarily occurs in the plasma membrane, implied to occur in liquid disordered domains (red, unsaturated lipid tails) producing C83.

Figure 3

Processive cleavage of C99 by γ-secretase occurs in the trans-Golgi network, principally late endosomes, and results in the formation of Aβ. The cleavage process, which lacks fidelity, leads to the production of a distribution of Aβ isoforms, 33 to 49 residues in length, principally as Aβ₄₀(224).

Figure 4

In bulk-like environments of liquid disordered phase (red) APP transmembrane domain is evidenced to form a relatively higher population of Gly-out homodimer(58) which can be stabilized with metastable extra-membrane β-strands(166). In the neutral pH plasma membrane, the JM domain K₁₆LVFFAED₂₃ α-helix is destabilized(165, 170). In raft-like environments of liquid ordered phase (blue) APP transmembrane domain is evidenced to form a relatively higher population of Gly-in homodimer, stabilizing the dimer with glycine zipper (white circles) Cα···C=O hydrogen bonds. In the acidic endosomal membrane, the JM domain α-helix is stabilized(98, 155).

Figure 5

(A) Lipid phases and phase transitions exhibited for saturated lipid or sphingomyelin with increasing local concentrations of cholesterol. L_o lateral tail packing illustration is shown looking down the membrane normal. (B) Phase diagram representative of many mixtures involving saturated lipids or sphingomyelins, unsaturated lipids, and cholesterol at fixed temperature and concentration in aqueous solution in the lipid bilayer phase displaying the miscibility gap where phase separation is observed.

Figure 6

(A) Without palmitoylation, γ-secretase, BACE1, and APP may partition to L_d domains, along with ADAM10, which is not evidenced to be amenable to palmitoylation. (B) γ-secretase, BACE1, and APP are hypothesized to more likely partition to L_o domains upon palmitoylation (potential sites are labelled), which will cause conformational changes, particularly the association of extramembrane residues near the palmitoylated site with the lipid surface.

Figure 7

Illustration of subcellular compartments involved in the amyloid and complementary cascade pathways. APP, α-secretase (ADAM10), and β-secretase (BACE1) are represented in purple, blue, and red, respectively. Aβ is displayed in pink. The γ-secretase complex nicastrin, PEN-2, APH-1A, and presenilin 1 domains displayed in blue, pink, orange, and green, respectively. L_o domains represented with blue, ordered saturated lipid tails and a higher concentration of cholesterol, L_d domains represented with red, disordered unsaturated lipid tails and a lower concentration of cholesterol.