Distinctive contribution of two additional residues in protein aggregation of Aβ42 and Aβ40 isoforms

Abstract

Amyloid-β (Aβ) is one of the amyloidogenic intrinsically disordered proteins (IDPs) that self-assemble to protein aggregates, incurring cell malfunction and cytotoxicity. While Aβ has been known to regulate multiple physiological functions, such as enhancing synaptic functions, aiding in the recovery of the blood-brain barrier/brain injury, and exhibiting tumor suppression/antimicrobial activities, the hydrophobicity of the primary structure promotes pathological aggregations that are closely associated with the onset of Alzheimer's disease (AD). Aβ proteins consist of multiple isoforms with 37-43 amino acid residues that are produced by the cleavage of amyloid-β precursor protein (APP). The hydrolytic products of APP are secreted to the extracellular regions of neuronal cells. Aβ 1-42 (Aβ42) and Aβ 1-40 (Aβ40) are dominant isoforms whose significance in AD pathogenesis has been highlighted in numerous studies to understand the molecular mechanism and develop AD diagnosis and therapeutic strategies. In this review, we focus on the differences between Aβ42 and Aβ40 in the molecular mechanism of amyloid aggregations mediated by the two additional residues (Ile41 and Ala42) of Aβ42. The current comprehension of Aβ42 and Aβ40 in AD progression is outlined, together with the structural features of Aβ42/Aβ40 amyloid fibrils, and the aggregation mechanisms of Aβ42/Aβ40. Furthermore, the impact of the heterogeneous distribution of Aβ isoforms during amyloid aggregations is discussed in the system mimicking the coexistence of Aβ42 and Aβ40 in human cerebrospinal fluid (CSF) and plasma. [BMB Reports 2024; 57(6): 263-272].

Fig. 1

Amyloid-β (Aβ) aggregation. (A) The primary structure of Aβ42 and aggregation mechanism of Aβ. Unstructured monomeric Aβ proteins self-assemble to oligomeric states; the oligomeric intermediates then elongate to insoluble fibrils. (B) Trajectories of Aβ biomarker abnormality and AD progression. Aβ as a biomarker can be assessed using cerebrospinal fluid Aβ and PET Aβ imaging. Aβ the panel image was adopted and reformatted from the review by Sperling et al. (26). (C) Enzymatic hydrolysis of amyloid-β precursor protein (APP) following non-pathogenic/pathogenic cleavage pathway. APP is sequentially cleaved by β-secretase and γ-secretase. As a result of the sequential cleavage, Aβ 1-42 (Aβ42) or Aβ 1-40 (Aβ40) is released to the extracellular space of neuronal cells. The inset of panel c shows the cleavage pathways of Aβ42/Aβ40 by γ-secretase.

Fig. 2

Conformations of Aβ42 and Aβ40 fibrils. (A, B) Cryo-electron microscopy (cryo-EM) structures of (A) Type I and (B) Type II Aβ42 fibrils isolated from the brain tissues of sporadic and familial AD patients. The cross-sections of both Type I/II Aβ42 protofibrils are oriented as the S-shaped conformation, while the inter-fibrillar contact areas are not identical. (C) Cryo-EM structures of Aβ40 fibrils generated by seeding fresh Aβ40 with sonicated cortex tissue extract of an AD patient. Unlike Aβ42, the C-shaped conformation of Aβ40 is observed in the protofibril structure. The protofibrils of Aβ40 are in parallel alignment, forming the mature fibril structure. In the left panel, the β-sheet structures are highlighted in orange, while in the right panel, each amino acid is colored differently according to its polarity and charge state.

Fig. 3

Aggregation kinetics and structural dynamics of Aβ42 and Aβ40. (A) Schematic of the aggregation pathways that contribute the overall kinetics of Aβ: Primary nucleation, elongation, and secondary nucleation. (B) Relative aggregation rates of Aβ42 and Aβ40 in the primary nucleation, elongation, and secondary nucleation steps. The normalization of the rate constants was performed for each step. The rate constants of Aβ42 were set as a reference. The primary nucleation and elongation of Aβ42 are faster than those of Aβ40. Relative ratio of the secondary nucleation of Aβ42 to Aβ40 is in the range of the same magnitude. (C) Monomeric conformations of Aβ42 and Aβ40 that are involved in protein aggregation. In the structural pool of Aβ with wide distribution radius of gyration (R_g), representative features of aggregation-inducing conformers (Extended conformer in panel (C)) of Aβ42 show the exposed hydrophobic central regions, due to the intramolecular interaction of the C-terminal region.