Marcus Fändrich

FTIR reveals structural differences between native β‐sheet proteins and amyloid fibrils

The presence of β‐sheets in the core of amyloid fibrils raised questions as to whether or not β‐sheet‐containing proteins, such as transthyretin, are predisposed to form such fibrils. However, we show here that the molecular structure of amyloid fibrils differs more generally from the β‐sheets in native proteins. This difference is evident from the amide I region of the infrared spectrum and relates to the distribution of the ϕ/ψ dihedral angles within the Ramachandran plot, the average number of strands per sheet, and possibly, the β‐sheet twist. These data imply that amyloid fibril formation from native β‐sheet proteins can involve a substantial structural reorganization.

Myoglobin forms amyloid fibrils by association of unfolded polypeptide segments

Observations that β-sheet proteins form amyloid fibrils under at least partially denaturing conditions has raised questions as to whether these fibrils assemble by docking of preformed β-structure or by association of unfolded polypeptide segments. By using α-helical protein apomyoglobin, we show that the ease of fibril assembly correlates with the extent of denaturation. By contrast, monomeric β-sheet intermediates could not be observed under the conditions of fibril formation. These data suggest that amyloid fibril formation from apomyoglobin depends on disordered polypeptide segments and conditions that are selectively unfavorable to folding. However, it is inevitable that such conditions often stabilize protein folding intermediates.

Small-molecule conversion of toxic oligomers to nontoxic β-sheet–rich amyloid fibrils

Several lines of evidence indicate that prefibrillar assemblies of amyloid-β (Aβ) polypeptides, such as soluble oligomers or protofibrils, rather than mature, end-stage amyloid fibrils cause neuronal dysfunction and memory impairment in Alzheimers disease. These findings suggest that reducing the prevalence of transient intermediates by small molecule-mediated stimulation of amyloid polymerization might decrease toxicity. Here we demonstrate the acceleration of Aβ fibrillogenesis through the action of the orcein-related small molecule O4, which directly binds to hydrophobic amino acid residues in Aβ peptides and stabilizes the self-assembly of seeding-competent, β-sheet-rich protofibrils and fibrils. Notably, the O4-mediated acceleration of amyloid fibril formation efficiently decreases the concentration of small, toxic Aβ oligomers in complex, heterogeneous aggregation reactions. In addition, O4 treatment suppresses inhibition of long-term potentiation by Aβ oligomers in hippocampal brain slices. These results support the hypothesis that small, diffusible prefibrillar amyloid species rather than mature fibrillar aggregates are toxic for mammalian cells.

Oligomeric Intermediates in Amyloid Formation: Structure Determination and Mechanisms of Toxicity

Oligomeric intermediates are non-fibrillar polypeptide assemblies that occur during amyloid fibril formation and that are thought to underlie the aetiology of amyloid diseases, such as Alzheimers disease, Parkinsons disease and Huntingtons disease. Focusing primarily on the oligomeric states formed from Alzheimers disease β-amyloid (Aβ) peptide, this review will make references to other polypeptide systems, highlighting common principles or sequence-specific differences. The covered topics include the structural properties and polymorphism of oligomers, the biophysical mechanism of peptide self-assembly and its role for pathogenicity in amyloid disease. Oligomer-dependent toxicity mechanisms will be explained along with recently emerging possibilities of interference.

Aβ(1-40) Fibril Polymorphism Implies Diverse Interaction Patterns in Amyloid Fibrils

Amyloid fibrils characterize a diverse group of human diseases that includes Alzheimers disease, Creutzfeldt-Jakob and type II diabetes. Alzheimers amyloid fibrils consist of amyloid-beta (Abeta) peptide and occur in a range of structurally different fibril morphologies. The structural characteristics of 12 single Abeta(1-40) amyloid fibrils, all formed under the same solution conditions, were determined by electron cryo-microscopy and three-dimensional reconstruction. The majority of analyzed fibrils form a range of morphologies that show almost continuously altering structural properties. The observed fibril polymorphism implies that amyloid formation can lead, for the same polypeptide sequence, to many different patterns of inter- or intra-residue interactions. This property differs significantly from native, monomeric protein folding reactions that produce, for one protein sequence, only one ordered conformation and only one set of inter-residue interactions.

Directed selection of a conformational antibody domain that prevents mature amyloid fibril formation by stabilizing Aβ protofibrils

The formation of amyloid fibrils is a common biochemical characteristic that occurs in Alzheimers disease and several other amyloidoses. The unifying structural feature of amyloid fibrils is their specific type of β-sheet conformation that differentiates these fibrils from the products of normal protein folding reactions. Here we describe the generation of an antibody domain, termed B10, that recognizes an amyloid-specific and conformationally defined epitope. This antibody domain was selected by phage-display from a recombinant library of camelid antibody domains. Surface plasmon resonance, immunoblots, and immunohistochemistry show that this antibody domain distinguishes Aβ amyloid fibrils from disaggregated Aβ peptide as well as from specific Aβ oligomers. The antibody domain possesses functional activity in preventing the formation of mature amyloid fibrils by stabilizing Aβ protofibrils. These data suggest possible applications of B10 in the detection of amyloid fibrils or in the modulation of their formation.

On the structural definition of amyloid fibrils and other polypeptide aggregates

Abstract.Amyloid fibrils occur inside the human body, associated with ageing or a group of diseases that includes, amongst others, Alzheimer’s disease, atherosclerosis and type II diabetes. Many natural polypeptide chains are able to form amyloid fibrils in vivo or in vitro, and this ability has been suggested to represent an inherent consequence of the chemical structure of the polypeptide chain. Recent literature has provided a wealth of information about the structure of aggregates, precipitates, amyloid fibrils and other types of fibrillar polypeptide assemblies. However, the biophysical meaning associated with these terms can differ considerably depending on the context of their usage. This overview presents a structural comparison of amyloid fibrils and other types of polypeptide assemblies and defines amyloid fibrils, based on structural considerations, as fibrillar polypeptide aggregates with a cross-β conformation.

The aggregation kinetics of Alzheimer’s β-amyloid peptide is controlled by stochastic nucleation

We report here a recombinant expression system that allows production of large quantities of Alzheimers Aβ(1–40) peptide. The material is competent to dissolve in water solutions with “random‐coil properties,” although its conformation and factual oligomerization state are determined by the physico‐chemical solution conditions. When dissolved in 50 mM sodium phosphate buffer (pH 7.4) at 37°C, the peptide is able to undergo a nucleated polymerization reaction. The aggregation profile is characteristically bipartite, consisting of lag and growth phase. From these curves we determined the lag time as well as the rate of aggregation. Both values were found to depend on peptide concentration and addition or formation of seeds. Moreover, they can vary considerably between apparently identical samples. These data imply that the nucleation event is under influence of a stochastic factor that can manifest itself in profound macroscopic differences in the aggregation kinetics of otherwise indistinguishable samples.

Mechanism of amyloid plaque formation suggests an intracellular basis of Aβ pathogenicity

The formation of extracellular amyloid plaques is a common patho-biochemical event underlying several debilitating human conditions, including Alzheimer’s disease (AD). Considerable evidence implies that AD damage arises primarily from small oligomeric amyloid forms of Aβ peptide, but the precise mechanism of pathogenicity remains to be established. Using a cell culture system that reproducibly leads to the formation of Alzheimer’s Aβ amyloid plaques, we show here that the formation of a single amyloid plaque represents a template-dependent process that critically involves the presence of endocytosis- or phagocytosis-competent cells. Internalized Aβ peptide becomes sorted to multivesicular bodies where fibrils grow out, thus penetrating the vesicular membrane. Upon plaque formation, cells undergo cell death and intracellular amyloid structures become released into the extracellular space. These data imply a mechanism where the pathogenic activity of Aβ is attributed, at least in part, to intracellular aggregates.

Structural polymorphism of Alzheimer Aβ and other amyloid fibrils

Deposits of amyloid fibrils characterize a diverse group of human diseases that includes Alzheimer’s disease, Creutzfeldt-Jakob disease and type II diabetes. Amyloid fibrils formed from different polypeptides contain a common cross-β spine. Nevertheless, amyloid fibrils formed from the same polypeptide can occur in a range of structurally different morphologies. The heterogeneity of amyloid fibrils reflects different types of polymorphism: (i) variations in the protofilament number, (ii) variations in the protofilament arrangement and (iii) different polypeptide conformations. Amyloid fibril polymorphism implies that fibril formation can lead, for the same polypeptide sequence, to many different patterns of inter- or intra-residue interactions. This property differs significantly from native, monomeric protein folding reactions that produce, for one protein sequence, only one ordered conformation and only one set of inter-residue interactions.