Ladiwala AR, Dordick JS, Tessier PM.
Aromatic small molecules remodel toxic soluble oligomers of amyloid beta through three independent pathways.
J Biol Chem. 2011 Feb 4;286(5):3209-18.
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This paper addresses the question of how inhibitors of Aβ-induced toxicity work, and how they affect Aβ self-assembly. This is an important and timely question. In the past, when Aβ fibrils were thought to be the cause of Alzheimer’s disease (AD), inhibitor design focused on β-sheet breakers. With the realization that oligomers are the most toxic species, and that better correlation is found between levels of oligomers or soluble Aβ and disease progress than between Aβ deposition and development of dementia, the focus has shifted towards inhibition of Aβ oligomerization. As Ladiwala et al. point out, some people assumed that oligomerization inhibitors would prevent self-association and keep Aβ in a monomeric state, but this turned out not to be the case. Other mechanisms have been found, such as colloidal aggregation (Feng et al., 2008), which raised concerns regarding the applicability of such inhibitors in vivo.
One of the most common observations has been that inhibitors stabilize non-toxic oligomers. Examples include scyllo-inositol (McLaurin et al., 2000; McLaurin et al., 2006), EGCG (Ehrnhoefer et al., 2008), pyrone derivatives (Hong et al., 2009), resveratrol (Feng et al., 2009 and an earlier paper this year by Ladiwala et al., 2010). It is quite remarkable that this general mechanism is shared by compounds of very diverse structure. For example, pyridazine/triazine-diamine derivatives considered to be β-sheet breakers (Nakagami et al., 2002; Nakagami et al., 2002), and our short peptides derived from the C-terminus of Aβ42 (Fradinger et al., 2008; Li et al., 2010) seem to stabilize non-toxic oligomers.
Using a similar approach to that taken by Ladiwala et al., Charlie Glabe's lab examined a large number of inhibitors several years ago and classified them into: 1) compounds that inhibit oligomerization but not fibrillization; 2) compounds that inhibit both oligomerization and fibrillization; and 3) compounds that inhibit fibrillization and do not inhibit oligomerization (Necula et al., 2007). In view of all these data, the work of Ladiwala et al. is another step in this important direction, though in our opinion it represents incremental, rather than quantum leap progress.
Ladiwala et al. used a combination of biochemical, biophysical, and cell culture techniques to characterize the effect of the seven compounds found to be active in their initial screen, which was based on a change in a pattern of silver-stained bands in SDS-PAGE fractionation of Aβ42. This pattern is thought to be artifactual and caused by the interaction of Aβ42 with SDS (Bitan et al., 2005; Hepler et al., 2006), yet it allowed Ladiwala et al. to pick seven compounds that had been previously identified to inhibit Aβ toxicity. The subsequent agreement found among the different assays used, including SDS-PAGE, atomic force microscopy, dot blots with oligomer-specific and fibril-specific antibodies, and Aβ-induced toxicity, was remarkable.
It is difficult, however, to predict how these new data will affect current and future inhibitor design or discovery efforts. To get a more complete picture, it would have been interesting to see the complete list of compounds originally tested by Ladiwala et al., including those that tested negative in their original screen, and compare the results to published data. It is also difficult to generalize the findings because the sample size is small—seven compounds divided into three classes, one of which is further divided into two subclasses. Whether or not these classes are representative of general mechanisms remains to be discovered.
We also had some concerns regarding the interpretation of the data. For example, the classification of class III compounds is based mainly on negative results, which could have been strengthened by the inclusion of additional experiments, for example, dynamic light scattering or analytical ultracentrifugation, providing direct evidence for the size of the Aβ42 species formed in the presence of the compounds in this class. The AFM images show blank mica, which may be consistent with the interpretation that the Aβ structures formed in the presence of the class III compounds are too small to be observed, but also may represent simply inhibition of adsorption to the mica. Another concern is that the cell viability assays were done with non-differentiated PC-12 cells, which are not representative of neurons. An interesting finding that could benefit from further exploration is that in the majority of the experiments presented, both in dose-response and in time-course experiments, the data show "all-or-nothing" phenomena, rather than the gradual changes typically observed in such experiments.
Finally, regarding the relevance of the findings of Ladiwala et al. to clinical development of inhibitors for AD, it may be of interest to compare these findings to in vivo studies of the individual compounds they examined. For example, myricetin, which behaved identically to nordihydroguaiaretic acid (NDGA) in the assays used by Ladiwala et al. (class IA), had different effects from NDGA in the Tg2576 mouse model (Hamaguchi et al., 2009). Myricetin and NDGA also have been reported to have other biological functions in vivo, including an effect on Aβ production. The relative importance of this activity compared to facilitating formation of large amorphous Aβ assemblies shown by Ladiwala et al. is not known. It is also interesting that a previous in vitro study (Moss et al., 2004) did not find disaggregation of Aβ40 fibrils by NDGA, in contrast to the findings of Ladiwala et al. with Aβ42 fibrils.
Benzothiazole derivatives, which were assigned as class IB by Ladiwala et al., are the basis for many amyloid imaging reagents. Presumably, they induce formation of large, non-fibrillar aggregates by binding to Aβ and forming assemblies that consist of both Aβ and the benzothiazole compound. Whether this happens in the brain is not known. It may be an artifact of the high concentrations needed for in vitro experiments. Nonetheless, if such mixed assemblies do form in the brain, they may lead to a false-positive signal in amyloid imaging based on such compounds. Methylene blue, the single compound in class II, is being tested clinically as an inhibitor of tau aggregation and has been reported to have several other biological functions (Atamna and Kumar, 2010). Similar to the compounds in class IA, it is difficult to estimate the relative contribution of these activities compared to induction of Aβ fibrillation, to the overall biological activity of methylene blue. Finally, the data for tannic acid and piceid (class III) confirm previous reports on the inhibition of Aβ toxicity by these compounds, but in the absence of direct evidence for the proposed mechanism of action of these compounds, it is hard to make further conclusions.
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