Background of Alzheimer’s disease
Alzheimer’s disease (AD) is one of the most devastating neurodegenerative disorders affecting more than 25 million people worldwide which is the fifth leading cause of death for those aged 65 years and above. The disease typically starts with a difficulty to remember new information and as it advances, more severe symptoms such as disorientation, change the mood and behaviour, confusions with time, events ending up with the difficulty of speaking, swallowing, walking and almost entire memory loss. The disease develops slowly with the average life span after diagnosis 4-6 years, although it can be as long as 20 years.
There is no current cure for AD, but treatments of symptoms can slow the disease progressing thus improving the quality of life of AD patients.
Model for the AD
The disease was named after a German physician Alois Alzheimer, who in 1906 linked the disease symptoms with the appearing of dense deposits (plaques) outside nerve cells. Additional evidence for the link of the AD symptoms with the brain morphological abnormalities were obtained in the 1960s when the correlation between the cognitive decline and the number of plaques and tangles was found. The application of modern molecular biology techniques in the early 1980s led to the discovery that senile plaques are primarily composed of the rather short amyloid beta-protein (Aβ) with lengths of 40 and 42 amino acids. The in vitro experiments with proteins isolated from the brain or chemically synthesized revealed their ability to spontaneously assemble in large aggregates including fibrils found in amyloid plaques. These findings have led to the model of the AD development termed amyloid cascade hypothesis that is considered as the main model for AD. According to this model, assembly of Aβ proteins in aggregates triggers the disease. Recent studies showed that oligomeric forms of Aβ proteins rather than large insoluble aggregates including fibrils are the neurotoxic Aβ species. The current model considers the accumulation of Aβ oligomers at the synapse as the critical stage of the AD neurodegeneration. The intraneuronal fibrils are formed by tau protein and this process is initiated by Aβ oligomers triggering tau phosphorylation.
Aβ oligomers as therapeutics targets
These recent advances suggest that the treatments of AD should be associated with eliminating Aβ oligomers from the brain. However, assignment to Aβ oligomers of exclusively pathological role should be made with caution. Aβ proteins are components of the synapsis and the existence of cellular receptors for Aβ oligomers points to their physiological role. Indeed, Aβ proteins are present in brains of healthy people and at physiological levels, Aβ proteins regulate synaptic function from the very beginning of the organism development. What happens with the age? Why do Aβ proteins become associated with synaptic failure and lead to Alzheimer’s disease pathology? These are fundamental questions and their answers require a thorough characterisation of Aβ proteins in monomeric and oligomeric states.
Nanoimaging and probing for Aβ oligomers properties
Studies during the past decade identified structural dynamics of Aβ protein termed misfolding as a factor leading to its ability to induce AD. In fact, the similar misfolding property was found for other proteins responsible for the development of neurodegenerative disorders such as Parkinson’s disease,
Down’s syndrome, and Huntington’s disease. Despite the crucial importance of protein misfolding, factors that lead to protein misfolding and aggregation in vitro are poorly understood, not to mention the complexities involved in the formation of protein nanoassembles with different morphologies. A better understanding of the molecular mechanisms of misfolding and aggregation will facilitate rational approaches to prevent protein misfolding and aggregation. Also, such knowledge in conjunction with molecular modelling will fundamentally advance our understanding of cellular nanomachines and their function at normal and the pathological conditions.
The problem in the lack of knowledge lies in the need of techniques required for amyloids characterisation. The real mechanisms of physiological systems and diseases are played out on a very transient level through highly dynamic molecular-scale interactions. A process that may appear simple by traditional techniques may actually have many steps and a variety of important intermediate states – each with its own unique dynamics. Intermediate states are stabilised by weak interactions that are typically transient and difficult to measure. However, they are key control mechanisms in several pathways. The nanotechnology has tools capable of probing the intermediate states of protein self-assembly process and studies with the use of such tools will fill this gap in the knowledge.
We have recently pioneered and developed advanced nanoimaging approaches that allow us to probe and characterise transient misfolded protein states. Importantly, we were able to characterise the amyloid dimer, which is the very first amyloid oligomer. A critical factor in this advancement was the application of such a single-molecule probing technique as Atomic Force Microscopy (AFM) force spectroscopy. Dimers were found to be very stable, suggesting that, at appropriate conditions, further aggregation processes can use dimers as transient seeds. The significance of this findings is supported by the discovery of the Harvard University groups that Aβ dimers were the predominate oligomeric species isolated from the brain of humans with AD and they are capable of causing neuritic degeneration. These studies were extended to other types of amyloidogenic proteins, so we hypothesize that assembly into dimers is the mechanism by which disease-prone, transient misfolded states of peptides and proteins are stabilised by several orders of magnitudes. Along with experimental studies, we applied Molecular Dynamics (MD) computational analysis to demonstrate the role of interpeptide interactions in the formation of misfolded states of peptides at the atomic level. Together, these studies led us to the model of early stages of protein aggregation, in which interaction between monomers is the key to the formation of aggregation-prone misfolded states of proteins.
Future prospects for AD treatments
The recent nanoimaging studies not only advanced our knowledge on the amyloids assembly, but established tools that will propel the development of novel diagnostic and therapeutic treatments of AD. Future characterisation of oligomers with these tools open realistic prospects for the development of efficient immunological preventive, diagnostic, and therapeutic strategies for AD, PD, and other neurodegenerative disorders.
Department of Pharmaceutical Sciences
College of Pharmacy COP 1012
University of Nebraska Medical Center