Alzheimer’s disease: Amyloid oligomers, therapeutic agents, their target and mode of action

The broad objective of this study is to unravel the potential effect of naturally occurring autoantibodies (NAbs) and CNI-1493 (immunomodulatory compound) on Amyloid beta peptide (Aβ) oligomers. Extracellular accumulation of Aβ peptide in the brain is suspected to cause and trigger the progression of Alzheimer’s disease (AD). The exact physiological role of Aβ remains unclear. The oligomeric forms Aβ impair synaptic activity. Hence, immunotherapeutic approach targeting Aβ is one of the promising strategies to treat AD. One of the challenges in the immunotherapy approach is to specifically target the toxic misfolded forms of Aβ without disturbing the physiological levels of Aβ monomers. Naturally occurring autoantibodies against Aβ are reduced in AD patients. In the first chapter, I have characterized the Aβ epitope region of Aβ specific-naturally occurring autoantibodies (NAbs-Aβ) using binding studies. I compared the binding ability of NAbs-Aβ towards a variety of mutated and truncated Aβ peptides. NAbs-Aβ had greater binding ability towards the C-terminal sequence of Aβ, which is membrane bound. The C-terminal end of Aβ is crucial for the formation of Aβ oligomerization. Hence, C-terminal specificity of NAbs-Aβ could help in the minimizing of Aβ oligomerization. Additionally, this specificity also rules out the possible interactions of NAbs-Aβ with membrane bound amyloid precursor protein (APP), Aβ monomers and fibrils. Further studies on Aβ epitope characterization of NAbs-Aβ were performed using site-directed mutagenesis. The results indicated that the lysine at 28th and isoleucine at 32nd position of Aβ are crucial for the binding of NAbs-Aβ. Deletion or substitution of any of these two amino acids at their respective positions lead to loss of binding. Additional binding studies using conformationally fixed ‘click’ Aβ peptides indicated that the NAbs-Aβ are specific for low-molecular weight anti-parallel dimers. Interestingly, NAbs-Aβ showed excellent binding towards Aβ with two substitution mutations at C-terminal (Aβ1-40;G29,33Ile) end. In the second chapter, characterization of Aβ1-40;G29,33Ile showed that the peptide undergo a different oligomerization process to form SDS-stable trimers. Cytotoxicity studies showed that Aβ wild type forms toxic oligomers at certain incubation conditions where as Aβ1-40;G29,33Ile remains non-toxic under the same conditions. In-line with the results, according to my current understanding, Aβ1-40;G29,33Ile is the first oligomeric Aβ species that is stable and has high affinity to NAbs-Aβ. Non-steroidal anti-inflammatory drugs are reported to provide beneficial effects in AD. In the third chapter, I have investigated the role of CNI-1493, which is an anti-inflammatory drug. In vitro studies show that CNI-1493 binds to Aβ oligomers and destabilizes the complex to monomers. In this study, I have characterized in detail, the effect of two immune system related AD drug candidates, NAbs-Aβ and CNI-1493, on Aβ oligomers. These findings help our understanding about the NAbs-Aβ and support their potential role as a safe and efficient AD immunotherapy candidates. The immunomodulatory effect combined with its role on Aβ oligomers make CNI-1493 an interesting drug candidate for AD therapeutic studies. In addition, the novel Aβ oligomer could be developed as a potential vaccine and diagnostic tool for NAbs-Aβ after further characterization.