Amyloid beta aggregation kinetics : The role of intrinsic and extrinsic factors
Abstract: As our knowledge of diseases and the development of new technologies increases,many health problems that were lethal in the past can now be cured. Couple thisto better life quality and improved medical care, we are seeing an increase in theaverage life expectancy. The downside to this, however, is that diseases that developwith age are becoming more common. There is currently an estimated 47 millionpeople living with dementia with this number predicted to be doubled every 20years. Alzheimer’s disease (AD) accounts for 60-70% of all the dementia cases inthe whole world. Most of the AD cases are sporadic and patients develop symptomsafter 65 years of age. A small subset of all AD cases are caused by a modified genethat can be passed through generations and lead to the development of dementia ata much younger age. Patient with AD usually have memory and mental problemsand become totally dependent on others in their late stage. Unfortunately, there isstill no therapy available for AD on the market.The hallmarks of AD are plaques and tangles in the brain which are believed tolead to the death of nerve cells in the brain. The plaques mainly contain fibrilsthat are formed by a small protein called amyloid beta (Abeta). Abeta has a tendencyto clump (aggregate) together and form long fibrils. Many studies suggest that Abetaaggregation links to the development of AD, which is supported by cases of peoplewith genetic modifications. Those modifications lead to an increased production ofAbeta or more aggregation favoured products and are linked to an earlier developmentof AD symptoms.How Abeta causes cell death is not known, but it is believed that the toxic species arenot the fibrils but aggregation intermediates, made up of two or more Abeta peptides.To figure out the link between Abeta peptides and AD, aggregation behaviour of Abetais, therefore, an important question. Chemical kinetics is an area of chemistry inwhich the reaction speed of a chemical process is studied. Specifically, in aggregationkinetics, the detailed reaction steps and rates of the formation or disruption of acomplex is studied. The overall aggregation process can be divided into several steps.This process can be thought of as standing in line at the supermarket. If we considerthe people to be Abeta a long line of people would represent a mature Abeta fibril. Toform the line, only a few people are needed and generally stand close to each other atthe check-out, which is referred to as primary nucleation. The nucleus then growsas more people join and the line becomes longer, a process termed elongation. Asmore and more people join the line, there forms a long queue, which in our case isthe mature fibril. As the line grows, people try to cut in by standing off to the side ofthe line (secondary nucleation), should a neighbouring cashier comes to work, thisgroup of people (new nucleus) are in prime position to break off from the line (fibril) and start queuing at the front of a newline, thus the process repeats and a new line(fibril) is formed. This queuing example is used here only to explain the simplifiedbasic steps in the Abeta aggregation. The general supermarket would not have thecapacity to open enough cashiers to cope with massive secondary nucleation thatis involved in the aggregation kinetics. In reality, the fibril surface serves as a veryefficient catalyser that helps to generate a huge amount of Abeta oligomers, which isthen followed by elongation to produce more fibril surface. These processes form acatalytic cycle that boosts the Abeta fibril formation and is potentially very dangerousto the brain.Our study is mainly focused on solving the aggregation mechanism of Abeta. In mywork, we use experimental tools to follow the aggregation of Abeta and its gene modifiedvariants that link to early-onset AD, Abeta with different length, or at differentpH or salt concentration. The experimental data was then analysed using mathematicalequations to find out the detailed steps that are involved in the aggregation.We found that some gene factors and environmental factors lead to a decreased repulsiveforce between molecules. Thus, the clumping between molecules or withbigger complexes is favoured. The saturated secondary nucleation is observed inall these cases, which means the catalytic fibril surface (queue) is fully covered bymonomer (people), and new nuclei formation speed is limited by the release of thenuclei from the catalytic surface. In this case, the secondary nucleation speed isreaching the maximum. This secondary nucleation speed maximization leads tomassive amount of oligomer production, which is potentially a high-risk factor tothe brain. A chaperone protein, Brichos, is found to selectively hinder this secondarynucleation and drastically decreases the toxic oligomer production.Overall, our study reveals the kinetic details of Abeta aggregation, mainly focusingon the effect of intrinsic and extrinsic factors. We have identified that secondarynucleation is the critical pathway that generates toxic oligomers and as such could bean important target in the development of effective therapy for Alzheimer’s disease.
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