Seeding of oligomers: new strategies for Alzheimer´s diagnosis and possible implications for the progression of the disease

Abstract

Alzheimer's disease (AD) is a complex neurodegenerative condition which has become a major public health problem because of its increasing prevalence, long duration and high cost of care. It is estimated than more than 25 million people worldwide are affected to some degree by AD. Unfortunately, there is not effective treatment or accurate pre-clinical diagnosis for AD. The neuropathological hallmarks of AD are neuronal loss in regions related to memory and cognition, neurotransmitter depletion, synaptic alteration and the deposition of two types of abnormal protein aggregates: neuroflbrillary tangles and amyloid beta plaques. Alzheimer's disease form part of a group of disorders called Protein Misfolding disorders (PMD). In general, proteins fold properly into their native conformation and, if they do not, the misfolding is corrected by chaperone proteins. In PMDs however, misfolding of a protein results in its aggregation and accumulation as amyloid deposits in diverse tissues. Compelling evidence suggest that the misfolding and aggregation of amyloid-beta peptide (AP), the major component of plaques, is the triggering factor of AD pathology. J7c w.fro studies have shown that AP misfolding and aggregation follows a seeding-nucleation mechanism. Analogous to a crystallization process, in this model the limiting step is the formation of small oligomeric intermediates that act as seeds to catalyze the polymerization process. I hypothesize that oligomers are present in biological fluids of AD patients long before the clinical symptoms of the disease appear. My experimental approach is to use the functional property of oligomers to catalyze (to seed) the polymerization of monomeric protein as a way to measure their presence in biological fluids. For this puapose I have developed a technology consisting in the cyclic amplification of seeds combined a sensitive detection of AP aggregates. Using this technique I was able to detect an increase of AJ3 oligomers in human biological fluids such as cerebrospinal fluid (CSF) and blood. The results of this research may help to design a biochemical test for Alzheimer's disease and other Protein misfolding disorders. Prion disease is the only member of the PMD group classified as transmissible. Compelling evidence supports the concept that the misfolded prion protein is the only component of the infectious agent and that it can `replicate' in the brain in the absence of nucleic acid by converting the natively folded prion protein. Interestingly, the data available indicate that amyloid formation in all PMD disorders follows a seeding-nucleation mechanism. This suggests that disease propagation by a protein oligomer may perhaps be occuring in other Protein misfolding disorders Qf higher prevalence such as Alzheimer's disease. Indeed, recent developments in the field have demonstrated that misfolded proteins associated with some PMDs can initiate the conversion of the normal form of the protein into the misfolded form and propagate these changes to neighboring cells in experimental models. Taking these antecedents into account, I want to investigate in this thesis if misfolded AP oligomers implicated in Alzheimer's disease are capable to transmit the disease under experimental conditions. Another intersting aspect that misfolded aggregates composed of one protein may interact and promote the aggregation of another protein by a phenomenon known as heterologous-seeding. Evidence for this phenomenon has been found for a number of PMDs. Thus, I would like to determine if exposure to AP misfolded protein may influence the development of another PMD, such as Prion disease, both z.72 1;I.two and z.7€ vz.1;o.