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Alzheimer’s Disease (AD) is the most frequent neurodegenerative disorder. Besides the loss of cholinergic neurons, AD is characterized by plaque deposition, the formation of neurofibrillary tangles as well as microglial activation. The formation of toxic amyloid-β (Aβ) oligomers is crucial in the degenerative process and leads to synaptic dysfunction and neuronal apoptosis, but also neuroinflammation. Neuroinflammation is a possible starting point for a therapeutic strategy. Here we investigated the effect of naturally occurring autoantibodies against Aβ (nAbs-Aβ) and the SK-channel activator CyPPA (Cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine) on microglial activation.
First, we investigated the effect of nAbs-Aβ on the viability of Aβ-oligomer treated microglial cells. There was no effect on the oligomer-induced reduction of cell viability, whereas there was an increase in the phosphorylation state of the stress pathway p38 MAP Kinase upon co-treatment with nAbs-Aβ and Aβ-oligomers. This rise in p38 phosphorylation was accompanied by an increased secretion of the pro-inflammatory cytokines IL-6 and TNF-α. In addition, the uptake of Aβ-oligomers by microglial cells was enhanced following nAbs-Aβ-treatment. Interestingly, the impact of nAbs-Aβ on Aβ-treated microglial cells’ viability, cytokine secretion and phagocytosis in vitro exerted beneficial effects on primary neurons. We administered supernatants of treated microglial cells to primary neurons and measured the neurons’ viability. Primary neurons that were treated with supernatants of microglial cells co-treated with nAbs-Aβ and Aβ showed a significantly increased viability compared to neurons treated with supernatants of cells treated with Aβ alone. We also investigated the effect of a single dose of nAbs-Aβ in Tg2576 mice and evaluated the levels of pro-inflammatory cytokines in the brain. The cytokine inhibitory property of nAbs-Aβ in vivo seems to be age-dependent. In very old animals there was a reduction in cytokine-levels, whereas in younger animals there was no. With the application of CyPPA we were also able to modulate the activation state of microglial cells. Morphological changes, measured with real time impedance, as well as NO- and cytokine-production induced with LPS could be prevented with the application of CyPPA. Using specific inhibitory peptides for the SK-channel subtypes, we were able to show a SK3-channel dependency for the secretion of IL-6 and NO, but not for TNF-α. IL-6 secretion was also dependent on extracellular calcium, leading to the hypothesis that CyPPA reduces the activation of microglial cells via modulating calcium homeostasis.
In summary we were able to provide insight in the mechanism of action of nAbs-Aβ on microglial cells in vitro. In addition the effects of nAbs-Aβ on microglial cells could be conveyed to primary neurons. Interestingly our in vivo data indicate that nAbs-Aβ should be considered for therapeutic use as there is no induction of neuroinflammation. The second substance, CyPPA, should be investigated further concerning its immune-modulating ability. Both data are very promising and point to a new way of modulating microglial activation. As both substances are also known to exert beneficial effects on neurons, these two approaches are promising candidates of modulating Alzheimer-associated neuroinflammation.