Lührs T, Ritter C, Adrian M, Riek-Loher D, Bohrmann B, Döbeli H, Schubert D, Riek R.
3D structure of Alzheimer's amyloid-beta(1-42) fibrils.
Proc Natl Acad Sci U S A. 2005 Nov 29;102(48):17342-7.
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The work by Luhrs et al. truly is a tour de force that integrates hydrogen exchange/NMR studies, electron microscopy, thioflavin T-binding, MTT assays, and molecular modeling into studies of the structure of fibrils formed by wild-type Aβ42 and a variety of rationally designed mutants. The result is a provocative model of the organization of Aβ protofilaments, the component structures comprising the mature 10 nm amyloid-type fibrils.
Evaluation of the work requires answers to two questions: Is the model correct, and is the model relevant? Additional experimentation will be required to provide these answers.
The clever single (asymmetric) and multiple (symmetric) amino acid substitution experiments performed by Luhrs et al. yield data consistent with their initial postulation of two β-strand regions connected by a short turn. The molecular modeling studies produce a model with low root-mean-square deviation (RMSD) error, again suggesting consistency and correctness of the derived protofilament structure. However, as with any modeling endeavor, the results depend on the assumptions. For example, the Luhrs model assumes the correctness of a certain feature of an earlier model, that of Robert Tycko's group, and thus becomes a "model based on a model based on experimental data" (with apologies to Professor Dan Kirschner for stealing his line).
The Luhrs model also integrates experimental data suggesting Met35 is not involved in intersheet side-chain packing. However, substantial experimental data exist showing that Met35 oxidation interferes significantly with Aβ aggregation (e.g., see Hou et al., 2002; Hou et al., 2004; Butterfield and Boyd-Kimball, 2005; Palmblad et al., 2002; Bitan et al., 2003).
These studies emphasize an important structural role for Met35 in fibril formation, one that can be rationalized mechanistically by models such as that of the Tycko group and those derived by molecular dynamics simulations (Urbanc et al., 2004). The contribution of Met35 to fibril formation is consistent with the fact that fibril formation of the [35L]Aβ(1-42) peptides used in the study of Luhrs et al. occurred over a relatively long time period (two months).
The use of oxidized Aβ (Met35 sulfoxide) also raises the question of relevance. What is the primary structure of the Aβ peptide(s) in vivo that gives rise to fibrils? Are the fibrils thus formed structurally equivalent to those studied by Luhrs et al? Evidence suggests that the predominant Aβ species is not oxidized. In addition, a large and increasing body of work supports the hypothesis that the most potent neurotoxins in Alzheimer disease and other diseases (e.g., prion diseases, see Silveira et al., 2005) may be oligomers and not fibrils. It would be exciting and informative if the approach of Luhrs et al. were applied to an analysis of the structure and dynamics of oligomer formation.
In conclusion, the fine work of Luhrs et al. provides a wonderful working model of Aβ protofilament structure that is rich in testable hypotheses.