A small molecule probe discriminating between Aß amyloid oligomers and fibrils

One of the conceptual shifts that the study of Alzheimer's disease has seen in the past few years is the realization that the long-studied insoluble Aß (1-42) amyloid fibrils may not be the real culprits in the disease. Instead the dubious distinction may belong to soluble Aß oligomers whose morphology differs from that of mature fibrils. Thus instead of focusing on one kind of Aß species, researchers are focusing on a broad range of oligomers and fully formed fibrils that differ in their architecture. Some differences between these species are clear; for instance the oligomers seem to permeate the plasma membrane in cells much better than the fibrils. The factors that dictate the exact morphology of these species sometimes might be subtle (such as pH shifts), and I myself have worked a little on such subtle changes leading to drastically different morphologies (see here for instance).

However, a study of different amyloid morphologies will greatly benefit from probes that allow us to selectively target and isolate certain amyloid architectures. The principal method of doing this until now has been to raise species-specific antibodies that target either oligomers or fully formed fibrils. In this regard small molecules have not been very promising as they tend to indiscriminately bind to all amyloid species; for instance Congo Red which is the archetypal amyloid labeling dye does not discriminate between different amyloid species.

Now a group at the University of Michigan has discovered some very simple probes that seem to target only spherical oligomers and not fibrils. The probes consist of tryptophan and some rather simple and well known biological molecules containing tryptophan which exhibit fluorescence when bound to proteins, and especially hydrophobic regions of proteins. The team started by screening about 70 simple organic molecules that exhibit fluorescence. Most of these molecules did fluoresce, but when bound to both forms of amyloid, thus precluding discrimination between the two species. Some did not fluoresce at all. But about 10 of them showed differential fluorescent quenching; the fluorescence was quenched much more when bound to oligomers compared to fibrils, thus allowing their selective labeling and visualization. Antibody labeling and radiography confirmed that it was indeed the oligomers that were getting labeled.

Intriguingly these probes consist of tryptophan itself as well as some very simple naturally occurring molecules containing the Trp moiety, such as melatonin, tryptamine and serotonin. In my mind this raises a very interesting possibility; could these molecules also be interacting selectively with amyloid and somehow modulating its behavior inside living systems?

In any case, what is even more valuable is that the probe seems to selectively label the oligomers in the presence of the fibrils and this can be determined from fluorescence studies. This opens up some potentially very interesting applications; for instance if there was any way at all to use such probes in vivo, we could gain extremely valuable knowledge on the ratios of oligomers to fully formed fibrils. Recalling that amyloid is astonishingly a deformed version of a naturally occurring protein, insights gained from such studies could be critical in shedding light on the natural and unnatural roles of amyloid in living systems.

Reinke, A., Seh, H., & Gestwicki, J. (2009). A chemical screening approach reveals that indole fluorescence is quenched by pre-fibrillar but not fibrillar amyloid-β Bioorganic & Medicinal Chemistry Letters, 19 (17), 4952-4957 DOI: 10.1016/j.bmcl.2009.07.082