| Alzheimer's disease (AD) is a progressive neurodegenerative disorder, with asymptomatic and symptomatic phases. Hallmark lesions of AD include extracellular deposits of fibrillar amyloid-beta (A beta) and intracellular Neurofibrillary tangle formations (NFTs). However, recent evidence seems to support soluble oligomeric forms of amyloid proteins as bioactive species in AD. Amyloid-beta oligomers (Abetao), such as Abeta*56, Abeta dimers and trimers have been demonstrated to be synaptotoxic species in AD. In particular, one of these oligomers, Abeta*56, was found to cause cognitive decline in the AD mouse model Tg2576, despite the absence of plaques and neuronal loss. In addition, cross-sectional studies suggest its possible involvement in the asymptomatic or preclinical phase of AD. However, it is currently unclear how this specific oligomer (Abeta*56) influences cellular and molecular processes to lead to cognitive deficits. My thesis focused on how Abeta*56 is able to disrupt cognition at the cellular and molecular level. First, we demonstrate that Abeta*56 forms a complex with NMDA receptors (NMDARs) resulting in an aberrant increase in intracellular calcium driven by synaptic NMDARs and the specific activation of the Ca2+/calmodulin dependent protein kinase CaMKIIalpha. Active CaMKIIalpha induces selective pathological changes in tau in vivo and in vitro, involving hyperphosphorylation and missorting. Importantly, other forms of endogenous Abeta oligomers do not appear to trigger these effects. Furthermore, other kinases such as GSK3, Cdk5 and fyn are not modulated by Abeta*56 in vitro. Interestingly, CaMKII phosphorylation is elevated in brain tissue of aged individuals, correlating with Abeta*56 abundance. These findings indicate that distinct Abeta oligomers activate specific neuronal signaling pathways in a highly selective manner in vitro. By extrapolation, these observations may have important consequences relative to our understanding of the different stages of AD. |
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