The hydrophobic Aβ peptide is highly aggregation prone; it first forms soluble oligomers, which then convert into the amyloid fibrils found in the cerebral plaques of Alzheimer's disease. It is generally understood that as the peptide concentration of Aβ increases, the fibrillization process is accelerated, but we examine the limits on this phenomenon. We found that once a threshold concentration of Aβ is exceeded, a stable oligomer is formed at the expense of fibril formation. The suppression of fibril formation was observed by amyloid-binding dye Thioflavin T and solution nuclear magnetic resonance (NMR). Small-angle X-ray scattering, size exclusion chromatography, and analytical ultracentrifugation demonstrated that Aβ peptides form a range of compact species, with a dimer being an early highly populated oligomer. Solution NMR allowed us to define the secondary structure of this Aβ dimer, which shows interlocking contacts between C-terminal peptide strands. Thus, we present a novel Aβ oligomer that resists conversion to fibrils and remains stable for more than one year.

The amyloid-β (Aβ) peptide of Alzheimer's disease (AD) forms polymorphic fibrils on the micrometer and molecular scales. Various fibril growth conditions have been identified to cause polymorphism, but the intrinsic amino acid sequence basis for this polymorphism has been unclear. Several single-site mutations in the center of the Aβ sequence cause different disease phenotypes and fibrillization properties. The E22G (Arctic) mutant is found in familial AD and forms protofibrils more rapidly than wild-type Aβ. Here, we use solid-state NMR spectroscopy to investigate the structure, dynamics, hydration and morphology of Arctic E22G Aβ40 fibrils. (13)C, (15)N-labeled synthetic E22G Aβ40 peptides are studied and compared with wild-type and Osaka E22Δ Aβ40 fibrils. Under the same fibrillization conditions, Arctic Aβ40 exhibits a high degree of polymorphism, showing at least four sets of NMR chemical shifts for various residues, while the Osaka and wild-type Aβ40 fibrils show a single or a predominant set of chemical shifts. Thus, structural polymorphism is intrinsic to the Arctic E22G Aβ40 sequence. Chemical shifts and inter-residue contacts obtained from 2D correlation spectra indicate that one of the major Arctic conformers has surprisingly high structural similarity with wild-type Aβ42. (13)C-(1)H dipolar order parameters, (1)H rotating-frame spin-lattice relaxation times and water-to-protein spin diffusion experiments reveal substantial differences in the dynamics and hydration of Arctic, Osaka and wild-type Aβ40 fibrils. Together, these results strongly suggest that electrostatic interactions in the center of the Aβ peptide sequence play a crucial role in the three-dimensional fold of the fibrils, and by inference, fibril-induced neuronal toxicity and AD pathogenesis.

An increasing number of studies continue to show that the amyloid β (Aβ) peptide adopts an alternative conformation and acquires transmissibility; hence, it becomes a prion. Here, we report on the attributes of two strains of Aβ prions formed from synthetic Aβ peptides composed of either 40 or 42 residues. Modifying the conditions for Aβ polymerization increased both the protease resistance and prion infectivity compared with an earlier study. Approximately 150 d after intracerebral inoculation, both synthetic Aβ40 and Aβ42 prions produced a sustained rise in the bioluminescence imaging signal in the brains of bigenic Tg(APP23:Gfap-luc) mice, indicative of astrocytic gliosis. Pathological investigations showed that synthetic Aβ40 prions produced amyloid plaques containing both Aβ40 and Aβ42 in the brains of inoculated bigenic mice, whereas synthetic Aβ42 prions stimulated the formation of smaller, more numerous plaques composed predominantly of Aβ42. Synthetic Aβ40 preparations consisted of long straight fibrils; in contrast, the Aβ42 fibrils were much shorter. Addition of 3.47 mM (0.1%) SDS to the polymerization reaction produced Aβ42 fibrils that were indistinguishable from Aβ40 fibrils produced in the absence or presence of SDS. Moreover, the Aβ amyloid plaques in the brains of bigenic mice inoculated with Aβ42 prions prepared in the presence of SDS were similar to those found in mice that received Aβ40 prions. From these results, we conclude that the composition of Aβ plaques depends on the conformation of the inoculated Aβ polymers, and thus, these inocula represent distinct synthetic Aβ prion strains.

The self-propagation of misfolded conformations of tau underlies neurodegenerative diseases, including Alzheimer's. There is considerable interest in discovering the minimal sequence and active conformational nucleus that defines this self-propagating event. The microtubule-binding region, spanning residues 244-372, reproduces much of the aggregation behaviour of tau in cells and animal models. Further dissection of the amyloid-forming region to a hexapeptide from the third microtubule-binding repeat resulted in a peptide that rapidly forms fibrils in vitro. We show that this peptide lacks the ability to seed aggregation of tau_244-372 in cells. However, as the hexapeptide is gradually extended to 31 residues, the peptides aggregate more slowly and gain potent activity to induce aggregation of tau_244-372 in cells. X-ray fibre diffraction, hydrogen-deuterium exchange and solid-state NMR studies map the beta-forming region to a 25-residue sequence. Thus, the nucleus for self-propagating aggregation of tau_244-372 in cells is packaged in a remarkably small peptide.

Point mutations in the amyloid-β (Aβ) coding region produce a combination of mutant and WT Aβ isoforms that yield unique clinicopathologies in familial Alzheimer's disease (fAD) and cerebral amyloid angiopathy (fCAA) patients. Here, we report a method to investigate the structural variability of amyloid deposits found in fAD, fCAA, and sporadic AD (sAD). Using this approach, we demonstrate that mutant Aβ determines WT Aβ conformation through prion template-directed misfolding. Using principal component analysis of multiple structure-sensitive fluorescent amyloid-binding dyes, we assessed the conformational variability of Aβ deposits in fAD, fCAA, and sAD patients. Comparing many deposits from a given patient with the overall population, we found that intrapatient variability is much lower than interpatient variability for both disease types. In a given brain, we observed one or two structurally distinct forms. When two forms coexist, they segregate between the parenchyma and cerebrovasculature, particularly in fAD patients. Compared with sAD samples, deposits from fAD patients show less intersubject variability, and little overlap exists between fAD and sAD deposits. Finally, we examined whether E22G (Arctic) or E22Q (Dutch) mutants direct the misfolding of WT Aβ, leading to fAD-like plaques in vivo. Intracerebrally injecting mutant Aβ40 fibrils into transgenic mice expressing only WT Aβ induced the deposition of plaques with many biochemical hallmarks of fAD. Thus, mutant Aβ40 prions induce a conformation of WT Aβ similar to that found in fAD deposits. These findings indicate that diverse AD phenotypes likely arise from one or more initial Aβ prion conformations, which kinetically dominate the spread of prions in the brain.