Heterotypic interactions in amyloid function and disease

Amyloid aggregation results from the self-assembly of identical aggregation-prone sequences into cross-beta-sheet structures. The process is best known for its association with a wide range of human pathologies but also as a functional mechanism in all kingdoms of life. Less well elucidated is the role of heterotypic interactions between amyloids and other proteins and macromolecules and how this contributes to disease. We here review current data with a focus on neurodegenerative amyloid-associated diseases. Evidence indicates that heterotypic interactions occur in a wide range of amyloid processes and that these interactions modify fundamental aspects of amyloid aggregation including seeding, aggregation rates and toxicity. More work is required to understand the mechanistic origin of these interactions, but current understanding suggests that both supersaturation and sequence-specific binding can contribute to heterotypic amyloid interactions. Further unravelling these mechanisms may help to answer outstanding questions in the field including the selective vulnerability of cells types and tissues and the stereotypical spreading patterns of amyloids in disease.

Cross-Species and Cross-Polymorph Seeding of Lysozyme Amyloid Reveals a Dominant Polymorph

The ability to self-propagate is one of the most intriguing characteristics of amyloid fibrils, and is a feature of great interest both to stopping unwanted pathological amyloid, and for engineering functional amyloid as a useful nanomaterial. The sequence and structural tolerances for amyloid seeding are not well understood, particularly concerning the propagation of distinct fibril morphologies (polymorphs) across species. This study examined the seeding and cross-seeding reactions between two unique fibril polymorphs, one long and flexible (formed at pH 2) and the other short and rigid (formed at pH 6.3), of human lysozyme and hen egg-white lysozyme. Both polymorphs could cross-seed aggregation across species, but this reaction was markedly reduced under physiological conditions. For both species, the pH 6.3 fibril polymorph was dominant, seeding fibril growth with a faster growth rate constant at pH 2 than the pH 2 polymorph. Based on fibrillation kinetics and fibril morphology, we found that the pH 2 polymorph was not able to faithfully replicate itself at pH 6.3. These results show that two distinct amyloid polymorphs are both capable of heterologous seeding across two species (human and hen) of lysozyme, but that the pH 6.3 polymorph is favored, regardless of the species, likely due to a lower energy barrier, or faster configurational diffusion, to accessing this particular misfolded form. These findings contribute to our better understanding of amyloid strain propagation across species barriers.

The Structural Basis of Amyloid Strains in Alzheimer's Disease

Amyloid fibrils represent one of the defining features of Alzheimer's disease (AD). They are made up of protofilaments composed of amyloid β (Aβ) peptides that are held together with extraordinary stability by a network of tight steric zippers and axial hydrogen bonds. This review explores the hypothesis that the peptide conformation within a protofilament represents the physical embodiment of a "strain" of AD. Evidence suggests that within a single strain the fold of individual peptides is invariant. However, the fibrils are capable of structural polymorphism that includes variation in the arrangement of protofilaments into fibrils, the pitch of the resultant fibrils, and the higher-order organization of the plaques into which they aggregate. These intrastrain polymorphisms are separated by low energy barriers, allowing multiple configurations to coexist within a single preparation or tissue. Clinical presentation of different strains may be determined by variation in the way different protofilament structures generate the relevant toxic species, be they monomers, oligomers, or higher-order structures. Evidence reviewed here is consistent with a model in which disease progression is concomitant with a gradual, progressive annealing of amyloid fibrils from benign, loosely packed structures into dense neurotoxic aggregates. This model challenges the commonly held hypothesis that oligomers of Aβ peptides are the only active proximate species in neurodegeneration. However, the data do not implicate fibrils themselves. Rather, they cast suspicion on larger-scale supramolecular aggregates as toxic agents. Electron tomography of amyloid plaques in situ strongly suggests that the formation of amyloid aggregates results in perturbation of the cellular membrane integrity, warranting further investigation of this as a potential mode of neurotoxicity. If dense supramolecular amyloid aggregates prove to be important agents of neurodegeneration in AD, this model may also have relevance to other forms of amyloidoses.

Context dependence of protein misfolding and structural strains in neurodegenerative diseases

Vast arrays of structural forms are accessible to simple amyloid peptides and environmental conditions can direct assembly into single phases. These insights are now being applied to the aggregation of the Aβ peptide of Alzheimer's disease and the identification of causative phases. We extend use of the imaging agent Pittsburgh compound B to discriminate among Aβ phases and begin to define conditions of relevance to the disease state. Also, we specifically highlight the development of methods for defining the structures of these more complex phases.

Cross-seeding and conformational selection between three- and four-repeat human Tau proteins

In Alzheimer's disease and frontotemporal dementias, the microtubule-associated protein Tau forms intracellular paired helical filaments. The filaments can form not only by the full-length human Tau protein, but also by the three repeated (K19) or four repeated (K18) Tau segments. However, of interest, experimentally, K19 can seed K18, but not vice versa. To obtain insight into the cross-seeding between K18 and K19 aggregates, here, K18 and K19 octamers with repeat 3 (R3) in U-shaped, L-shaped, and long straight line-shaped (SL-shape) conformations are assembled into different structures. The simulation results show that K18-8/K19-8 (K18 and K19 assemblies number 8) with R3 in an L shape and K18-9/K19-9 with R3 in an SL shape are highly populated and present the highest structural similarity among all simulated K18 and K19 octamers, suggesting that similar folding of K18/K19 may serve as structural core for the K18-K19 co-assembled heterogeneous filament. We demonstrate that formation of stable R2 and R3 conformations is the critical step for K18 aggregation, and R3 is critical for K19 fibrillization. The different core units in K18 and K19 may create a cross-seeding barrier for the K18 seed to trigger K19 fibril growth because R2 is not available for K19. Our study provides insights into cross-seeding involving heterogeneous structures. The polymorphic nature of protein aggregation could be magnified in the cross-seeding process. If the seeding conformations lead to too much divergence in the energy landscape, it could impede fibril formation. Such an effect could also contribute to the asymmetric barrier between K18 and K19.