Amyloid polymorphism: structural basis and neurobiological relevance

Abstract

Our understanding of the molecular structures of amyloid fibrils that are associated with neurodegenerative diseases, of mechanisms by which disease-associated peptides and proteins aggregate into fibrils, and of structural properties of aggregation intermediates has advanced considerably in recent years. Detailed molecular structural models for certain fibrils and aggregation intermediates are now available. It is now well established that amyloid fibrils are generally polymorphic at the molecular level, with a given peptide or protein being capable of forming a variety of distinct, self-propagating fibril structures. Recent results from structural studies and from studies involving cell cultures, transgenic animals, and human tissue provide initial evidence that molecular structural variations in amyloid fibrils and related aggregates may correlate with or even produce variations in disease development. This article reviews our current knowledge of the structural and mechanistic aspects of amyloid formation, as well as current evidence for the biological relevance of structural variations.

Figure 1

Polymorphism of amyloid fibrils and aggregation intermediates, as seen in transmission electron microscope images with negative staining. (A) Synthetic Aβ40 fibrils with “striated ribbon” morphologies. (B) Synthetic Aβ40 fibrils with “twisted” morphologies. (C) Recombinant α-synuclein fibrils with striated ribbon (red arrow), twisted (blue arrow), and rod-like (purple arrow) morphologies. (D) Synthetic Aβ40 aggregation intermediates with protofibrillar (orange arrows) and nonfibrillar (green arrow) morphologies, prepared by quiescent incubation of a 100 μM peptide solution at 24° C and pH 7.4 for 36 hours. Scale bars are 200 nm.

Figure 2

Varieties of cross-β structures in amyloid fibrils. (A) An “in-register” parallel cross-β structure, in which β-strand segments from adjacent protein or peptide molecules align in parallel and with no shift of their amino acid sequences (represented by the varying colors of carbon atoms) relative to one another. (B) An antiparallel cross-β structure. Silver bars indicated hydrogen bonds between backbone carbonyl and backbone amide groups. The fibril growth direction is indicated by the blue arrow. (C,D,E) Schematic representations of cross-β structures that could be formed by a peptide that contains two separate β-strand segments, separated by a loop or turn segment. Colors indicate successive copies of the same peptide molecule. From left to right, the structures are a double-layered, in-register parallel cross-β unit, a double-layered antiparallel cross-β unit, and a double-layered antiparallel β-hairpin structure.

Figure 3

Conceptual representation of molecular mechanisms for amyloid formation. Starting with a peptide or protein monomer A, a series of reversible monomer additions (orange arrows) results in a transient oligomer B that can undergo internal structural rearrangements (i.e., nucleation events, blue arrows) to form cross-β-like structures C₁ and C_2. Subsequent monomer additions to C₁ and C₂ (red arrows) result in fibril polymorphs D₁ and D₂. Variations in growth conditions can affect the relative nucleation rates or subsequent growth rates of the two polymorphs. Fibril growth conditions can also affect the rates of fibril fragmentation or secondary nucleation, favoring one polymorph over the other. In real systems, more than two fibril polymorphs can be formed by the same peptide or protein. In addition, fibril nucleation may compete with the formation of “off-pathway” aggregation intermediates (green arrows), such as a metastable oligomer E and a protofibril G, which grows from cross-β-like nucleus F. Once formed, species D₁, D₂, E, and F remain in dynamic equilibrium with monomers (or small oligomers), allowing a net transfer of peptide or protein molecules from the less stable to the more stable species.

Figure 4

Molecular structural models for three Aβ40 fibril polymorphs, based on data from ssNMR and electron microscopy. (A) Fibrils grown in vitro with the striated ribbon morphology, as in Fig. 1A. (B) Fibrils grown in vitro with the twisted morphology, as in Fig. 2B. (C) Fibrils derived from brain tissue of an AD patient. In each case, the fibril structure is viewed in cross-section, with the fibril growth direction approximately perpendicular to the page. Upper parts are cartoon representations, with colors indicating the different cross-β subunits within two-fold symmetric (A) or three-fold symmetric (B,C) structures. Eight Aβ40 molecules are shown in each subunit. Lower parts are atomic representations, with one molecule in each subunit. Residues 9–40 are shown in panels A and B. Residues 1–40 are shown in panel C. Models in panels A, B, and C are based on Protein Data Bank files 2LMN, 2LMP, and 2M4J, respectively.

Figure 5

Molecular structural models for two types of aggregation intermediates. (A) Protofibrils formed by D23N-Aβ40, viewed in cross-section, with the protofibril growth direction approximately perpendicular to the page. The cartoon representation (upper part) shows eight D23N-Aβ40 molecules in a double-layered antiparallel cross-β structure identified by ssNMR, with alternating colors to clarify the antiparallel alignment of adjacent molecules. The atomic representation (lower part) shows two adjacent molecules. (B) Cylindrin oligomer formed by the peptide KVKVLGDVIEV, which derives from the amino acid sequence of αB-crystallin. In the cartoon representation (upper part), the three-fold symmetry axis of the cylindrin lies vertically in the page and colors indicate different peptide molecules within the hexameric structure. The atomic representation (lower part) shows one pair of antiparallel molecules. Models in panels A and B are based on Protein Data Bank files 2LNQ and 3SGO, respectively.