The amyloid state of proteins in human diseases

Abstract

Amyloid fibers and oligomers are associated with a great variety of human diseases including Alzheimer's disease and the prion conditions. Here we attempt to connect recent discoveries on the molecular properties of proteins in the amyloid state with observations about pathological tissues and disease states. We summarize studies of structure and nucleation of amyloid and relate these to observations on amyloid polymorphism, prion strains, coaggregation of pathogenic proteins in tissues, and mechanisms of toxicity and transmissibility. Molecular studies have also led to numerous strategies for biological and chemical interventions against amyloid diseases.

Figure 1. Properties of Amyloid Fibers

(A) The characteristic cross-β diffraction pattern observed when X-rays are directed on amyloid fibers. The diffuse reflection at 4.8 Å spacing along the meridian (vertical) shows extended protein chains running roughly perpendicular to the fibril and spaced 4.8 Å apart. The even more diffuse reflection at ~10 Å spacing along the equator (horizontal) shows that the extended chains are organized into sheets spaced ~10 Å apart. For less well oriented fibrils, both reflections blur into circular rings. (B) The steric zipper structure of the sequence segment GNNQQNY from the yeast prion Sup35. Five layers of β-strands are shown of the tens of thousands in a typical fibril or microcrystal. The front sheet shows the protein backbones of the strands as gray arrows; the back sheet is in purple. Protruding from each sheet are the sidechains. The arrow marks the fibril axis. (C) The two interdigitating β-sheets are viewed down the axis. Water molecules, shown by red + signs are excluded from the tight interface between the sheets. Red carbonyl groups and blue amine groups form hydrogen bonds up and down between the layers of the sheet (Nelson et al., 2005). Panels B and C reprinted from Nelson et al. 2005.

Figure 2. Steric-zipper protofilaments

Twenty-eight atomic structures of steric-zipper protofilaments from amyloid-forming proteins, determined by X-ray diffraction. All are viewed projected down the protofilament axis, revealing the two sheets (one gold and one purple) with their interdigitated sidechains. Selected zippers are also viewed perpendicular to the protofilament axis, with five layers of β-strands shown with backbones as arrows. Water molecules are shown as aqua spheres; notice their absence from the interfaces between the paired sheets.

Figure 3. Structure of a hetero-zipper

The solid-state NMR-derived structure of Het-s shows hetero-zippers (Wasmer et al., 2008). (A) The protein chain of each molecule (in a single color) contains six β-strands, organized in double loops. The double loops of adjacent molecules sit on top of one another, hydrogen bonded up and down. (B) The two layers are shown schematically with sidechains represented as circles. Each layer may be regarded as a hetero-zipper, in which the sidechains of opposing strands interdigitate. Figure reprinted from Wasmer et al. 2008.

Figure 4. Models for amyloid fibrils larger than a single steric-zipper spine

(A) Model for Aβ1-40 based on solid-state NMR data with additional constrains from electron microscopy (Tycko, 2011). The view is down the fibril axis, showing two molecules of Aβ, each with a U-turn or “β-arch”. Where the green segments of the two molecules abut, they appear to form a homo-steric zipper (Class 1 in Figure 2), and a hetero-zipper could exist between the two arms of each U. Both types of steric zipper need to be confirmed by higher resolution structures. (B) A proposed structure for longer amyloid proteins is a “superpleated β-structure” (Kajava et al., 2010), in which the protein chain forms several U-turns/beta-arches. The view of the upper diagram is down the fibril axis; the view of the lower is perpendicular to the fibril axis. In the lower diagram, each protein chain is hydrogen bonded to the ones above and below (not shown). Hetero-zippers may exist between pairs of differently colored β-strands. This type of structure has been proposed for several proteins in the amyloid state including Ure2p, Sup35p, and α-synuclein. (C) A model for a designed amyloid of ribonuclease A with ten glutamine residue inserted between the core and C-terminal domains (Sambashivan et al., 2005) based on X-ray and electron microscopy data and steric constraints. The view is perpendicular to a cut-away of the fibril. The twisting steric zipper can be seen at the center. Globular subunits of ribonuclease A, which are essentially in their native conformation, are at the periphery. The amyloid-like fibrils of this designed amyloid show enzymatic activity, confirming that ribonuclease molecules retain native-like structure.

Figure 5. Rainbow amyloid

Novel amyloid dyes can be used as surrogate probes of the supramolecular structure of protein aggregates. Shown are Aβ plaques (yellow) and Aβ amyloid angiopathy (green) in an AβPP transgenic mouse (carrying the AβPP Swedish and AβPP Dutch mutation). Note the different spectral signatures upon staining with the luminescent conjugated polythiophene tPTAA (bottom left). The image was recorded using a combination of green and red filters. Scale bar = 20um.

Figure 6. Histopathology of cerebral β-amyloidosis

(A) Aβ immunostaining (brown) reveals severe cerebral amyloid angiopathies (CAA) in superficial cortical vessels in a human case. (B) Ultrastructural analysis of Aβ fibrils (af) in the vessel wall of an arteriole with CAA. Note that the amyloid has displaced nearly the entire vascular wall, disrupting normal vessel-neuron communications (b, basal lamina; e, endothelial cells; l, lumen; m, media; (reprinted with permission from Yamada et al., J Neurol 234: 371-6, 1987). (C) Aβ-immunostaining (brown) of an amyloid plaque in a human Alzheimer’s disease case. Note the dense core and glial nuclei (blue) surrounded by a halo of diffuse Aβ immunostaining. (D) Ultrastructure of an Aβ plaque. Note the dense amyloid core with the amyloid fibrils (af) surrounded by numerous dystrophic neuritis (some are labeled with “dn”). The Aβ plaque is from an AβPP transgenic mouse brain due to better tissue preservation compared to postmortem human tissue. Scale bars = 100 um (A), 1 um (B), 50 um (C) 5 um (D).