Alzheimer’s disease is the leading cause of dementia worldwide and a major cause of disability.
Now, researchers at Osaka University and Hokkaido University have shown that repeated precipitation-dissolution events of salt crystals do occur even at low salt concentrations in nanoscales, and that it can accelerate the aggregation of the neurotoxic amyloid-β peptides implicated in its pathogenesis.
The human brain comprises around 86 billion neurons, roughly as many grains of sand as in a large dump truck. These neurons juggle electrochemical information as signals among the brain, muscles and organs to orchestrate the symphony of life from survival to self-awareness.
Alzheimer’s disease disrupts this complex neuronal networking, causing functional disability and cell death.
As yet uncurable, available treatments are symptomatic, supportive, or palliative; a breakthrough in understanding its pathogenesis may brighten the prospects for medication, diagnosis and prevention.
The role of amyloid in Alzheimer’s disease has long been recognized. Amyloid-β peptides are derived from amyloid precursor protein and they self-assemble into sizes ranging from low-molecular-weight aggregates and larger oligomers to amyloid fibrils.
These last are known to be neurotoxic but recent research suggests that oligomeric disordered aggregates are also toxic, possibly even more than fibrils.
“Fibril aggregation begins with nucleation followed by an elongation stage,” explains Kichitaro Nakajima, lead author of this study.
“Until now, the early stages of oligomer evolution have been difficult to study because of their morphologic variability, the timeframe for nucleation, and the lack of a suitable fluorescent assay.”
Using liquid-state transmission electron microscopy, the researchers analyzed the aggregation of protein molecules, acquiring time-resolved nanoscale images and electron diffraction patterns.
“Remarkably, we discovered that a salt crystal can precipitate even at a concentration well below its solubility due to local density fluctuation, and its rapid dissolution accelerates the aggregation reaction of amyloid-β peptides,” says Professor Hirotsugu Ogi, the corresponding author.
“This formation of temporary salt crystals provides a mechanism whereby proteins adhere to the surface of the crystal; as it dissolves, the interface shrinks, condensing the proteins at the vanishing point.
This phenomenon resembles the aggregation acceleration by ultrasonic cavitation bubble. Proteins are attached on the bubble surface during the expansion phase, and they are highly condensed by the subsequent bubble collapses by the positive pressure of ultrasonic wave at its center.
This is the artificial catalytic effect. Thus, in an autocatalytic-like nanoscopic aggregation mechanism, salt dissolution accelerates the aggregation reaction, and the aggregate itself can promote salt nucleation.”
Ogi explains the implications of their results:
“The aggregation of amyloid-β peptides is slow and this has been a hindrance to pharmaceutical research.
Establishing an effective acceleration method will help clarify their structural evolution from monomer to fibril.
This knowledge is key to understanding the pathogenesis of Alzheimer’s disease.”
Alzheimer’s disease (AD) is the most common neurodegenerative disease, marked by clinical symptoms such as a decline in cognitive skills, behavioral changes, irreversible memory loss, and language impairment (Mattson, 2004).
This illness is estimated to affect approximately 2% of the population in industrialized countries (Rauk, 2009). The pathological markers of AD are the presence of extracellular amyloid plaques and intracellular neurofibrils because they can form tangles (NFTs) (Chiti and Dobson, 2006).
The major risk factor known for AD is age; about 95% of the AD cases have no clear pattern of inheriting the disease, and it appears that interactions of both the genetic and environmental factors contribute to the etiology of AD (Migliore and Coppede, 2009).
The exact mechanism leading to AD is still not established, and there is no preventative protocol and no effective therapies for AD. Thus, research in all aspects of AD is important because, in general, the aims are to obtain a more complete understanding of the AD, which eventually would lead to its cure or at least a rational treatment.
Extracellular amyloid plaques are commonly referred to as the senile plaques (SP); they are mainly made up of β-amyloid (Aβ) peptides, which exist in fibrilar form (Lovell et al., 1998). Such peptides are made from the amyloid precursor protein (APP) by β- and γ-secretase enzymes; also, these peptides usually contain either 40 or 42 amino acids (Aβ40 and Aβ42), respectively. Aβ40 is the major product of the APP processing, while Aβ42 is the predominant component of the senile plaques (Chiti and Dobson, 2006).
Thus, Aβ neurotoxicity could result from an amyloid fibrilar aggregatie, and it has been suggested that the Aβ-soluble (Aβs) oligomers are the principal neurotoxic agents, which, even at very low concentrations, are capable of inducing marked changes in neuronal long-term potentiation (LTP) as well as in cognitive impairment (Lesne et al., 2005).
Aβ peptides undergo spontaneous self-aggregation. However, the origin of this phenomenon is still unclear. Thus, aggregation and the formation of oligomers of the Aβ molecules are ongoing topics of interest contributing to the etiology and pathogenesis of AD.
Postmortem brain biopsies of AD patients have shown that the aggregated Aβs contain high concentrations of the metals, Fe, Cu and Zn (Miller et al., 2006), and in general, the accumulating evidence strongly indicates that metal ions are physiological and pathological hallmarks of AD (Duce and Bush, 2010, Drew and Barnham, 2010, Molina et al., 2007).
Research has suggested that some metal ions play a distinct role in the aggregation of Aβ (Bush, 2003, Ricchelli et al., 2006). These metal ions behave differently from those that form the slowly self-assembled aggregates; rather, they accelerate the dynamic aggregation of Aβ.
Consequently, the acceleration of the dynamic aggregation process is likely to increase the neurotoxic effects on the neuron cells. Aluminum (Al) is the most widely distributed metal in the environment and is extensively used in daily life (Kumar and Gill, 2009).
It is one of the metals that has a demonstrated human toxicity, and is highly neurotoxic. In this context, Al was suspected to be implicated in the progression of AD, when it was found to induce the formation of neuro-fibrilar tangles in the cerebrum of rabbits (Klatzo et al., 1965, Terry and Pena, 1965).
However, over the years the role of Al in the etiology and pathogenesis of AD has been controversial. While Al(III) is known to aggregate in high concentrations in the AD amyloid deposits, studies concerned with the linkage between Al and AD have not reached consensus (Munoz, 1998, Zatta, 1993, Zatta et al., 2003). Consequently, the question of the link between Al (III) and AD remains of considerable interest (Walton, 2006).
The aims of this work were:
1. to investigate the effect of Al(III) in aqueous solution on the conformation and aggregation of Aβ, and to relate the findings to any effects on the AD;
2. to apply a chemometrics method of data analysis, namely multivariate curve resolution-alternating least squares (MCR-ALS) (Tauler, 1995), so as to explore whether this approach can extract qualitative and/or quantitative information, which otherwise is inaccessible to conventional methods.
It has been demonstrated with the use of fluorescence, UV–vis, CD, and AFM techniques that Al(III) can play an important role as a mediator in the formation of fibrillar amyloid plaques in Alzheimer’s disease.
The above analytical methods provided collectively the information, which enabled the establishment of reactants and products as well as the physical parameters; these characterized the reaction mechanisms and kinetic rates as well as the associated equilibrium constants.
Furthermore, some quantitative and qualitative information, such as the binding constant of the interaction between Al(III) and Aβ40, and the spectrum of the binding complex of Al(III)-Aβ40 can be obtained with the use of the MCR-ALS chemometrics method.
An important aspect of this work was the concentration of Al(III) because it was an important variable which affected the Aβ40 aggregation process.