The present consensus on the the development of Alzheimer's disease is that it starts with the accumulation of amyloid-β, though there are many competing theories as to why only some people exhibit this problem to a great enough degree to produce pathology. The biochemistry of oligomers supporting amyloid-β causes sufficient disarray in brain metabolism to set the stage for neuroinflammation, malfunction of immune cells in the brain, and aggregation of altered forms of tau protein into neurofibrillary tangles that cause most of the damage and cell death in the later stages of the condition. The failure to improve outcomes via attempts to remove amyloid-β from the brains of Alzheimer's patients may be a case of too little, too late, but there is still good reason to remove amyloid-β. Doing so early enough and efficiently enough should prevent the later stages of the condition from developing at all.
The most modern approach to drug development, built atop greatly improved capacities in computation and associated modeling of protein structures and interactions, is to find points of intervention through a greater understanding of how proteins interact with one another, in detail, and how those interactions pertain to disease processes. Researchers can then rationally design molecules that (a) interfere at a vulnerable and highly specific point in a desired interaction and (b) due to this specificity are safe enough for clinical use, as they cause only limited disruption elsewhere in the operation of cellular biochemistry. This is the ideal, in any case. The challenge, as ever, is finding a point of intervention that does in fact turn out to be both specific enough and good enough in practice, in patients.
The research noted here today is an example of this approach to development applied to preventing the aggregation of amyloid-β. In principle, sufficient disruption of the process of forming protein aggregates should allow existing systems of clearance to remove excess or damaged protein molecules before they causes issues. In practice, we shall see how it turns out as this work progresses.
Alzheimer's is a disease of aggregation. Neurons in the human brain make a protein called amyloid beta. Such proteins on their own, called monomers of amyloid beta, perform important tasks for neurons. But in the brains of people with Alzheimer's disease, amyloid beta monomers have abandoned their jobs and joined together. First, they form oligomers - small clumps of up to a dozen proteins - then longer strands and finally large deposits called plaques. For years, scientists believed that the plaques triggered the cognitive impairments characteristic of Alzheimer's disease. But newer research implicates the smaller aggregates of amyloid beta as the toxic elements of this disease.
Now, researchers have developed synthetic peptides that target and inhibit those small, toxic aggregates. Their synthetic peptides - which are designed to fold into a structure known as an alpha sheet - can block amyloid beta aggregation at the early and most toxic stage when oligomers form. The team showed that the synthetic alpha sheet's blocking activity reduced amyloid beta-triggered toxicity in human neural cells grown in culture, and inhibited amyloid beta oligomers in two laboratory animal models for Alzheimer's. These findings add evidence to the growing consensus that amyloid beta oligomers - not plaques - are the toxic agents behind Alzheimer's disease. The results also indicate that synthetic alpha sheets could form the basis of therapeutics to clear toxic oligomers in people.
"This is about targeting a specific structure of amyloid beta formed by the toxic oligomers. What we've shown here is that we can design and build synthetic alpha sheets with complementary structures to inhibit aggregation and toxicity of amyloid beta, while leaving the biologically active monomers intact."
Alzheimer's disease (AD) is characterized by the deposition of β-sheet-rich, insoluble amyloid β-peptide (Aβ) plaques; however, plaque burden is not correlated with cognitive impairment in AD patients; instead, it is correlated with the presence of toxic soluble oligomers. Here, we show, by a variety of different techniques, that these Aβ oligomers adopt a nonstandard secondary structure, termed "α-sheet." These oligomers form in the lag phase of aggregation, when Aβ-associated cytotoxicity peaks, en route to forming nontoxic β-sheet fibrils.
De novo-designed α-sheet peptides specifically and tightly bind the toxic oligomers over monomeric and fibrillar forms of Aβ, leading to inhibition of aggregation in vitro and neurotoxicity in neuroblastoma cells. Based on this specific binding, a soluble oligomer-binding assay (SOBA) was developed as an indirect probe of α-sheet content. Combined SOBA and toxicity experiments demonstrate a strong correlation between α-sheet content and toxicity. The designed α-sheet peptides are also active in vivo where they inhibit Aβ-induced paralysis in a transgenic Aβ Caenorhabditis elegans model and specifically target and clear soluble, toxic oligomers in a transgenic APPsw mouse model. The α-sheet hypothesis has profound implications for further understanding the mechanism behind AD pathogenesis.