Mechanisms of chronic nicotine treatment-induced enhancement of the sensitivity of cortical neurons to the neuroprotective effect of donepezil in cortical neurons

Long-term Stimulation of α7 Nicotinic Acetylcholine Receptor Rescues Hemorrhagic Neuron Loss via Apoptosis of M1 Microglia

We previously revealed that long-term treatment with nicotine suppresses microglial activation, resulting in a protective effect against thrombin-induced shrinkage of the striatal tissue in organotypic slice cultures. Here, the effect of nicotine on impaired M1 and protective M2 microglial polarization was investigated using the BV-2 microglial cell line in the presence or absence of thrombin. Following nicotine treatment, α7 nicotinic acetylcholine receptor expression transiently increased and then gradually decreased until 14 days. Treatment with nicotine for 14 days slightly polarized M0 microglia to M2b and d subtypes. Co-exposure of thrombin and low concentration of interferon-γ recruited inducible NO synthase (iNOS)- and interleukin-1β-double-positive M1 microglia in a thrombin-concentration-dependent manner. Treatment with nicotine for 14 days significantly decreased the thrombin-induced increase of iNOS mRNA levels and conversely showed a tendency to increase arginase1 mRNA levels. Moreover, treatment with nicotine for 14 days suppressed thrombin-induced phosphorylation of p38 MAPK through the α7 receptor. Repeated intraperitoneal administration of α7 agonist PNU-282987 for 14 days selectively evoked the apoptosis of iNOS-positive M1 microglia at the perihematomal area and showed a neuroprotective effect in an in vivo intracerebral hemorrhage model. These findings revealed that long-term stimulation of α7 receptor causes suppression of thrombin-induced activation of p38 MAPK followed by apoptosis in neuropathic M1 microglia.

Donepezil modulates amyloid precursor protein endocytosis and reduction by up-regulation of SNX33 expression in primary cortical neurons

Donepezil, a therapeutic drug for Alzheimer’s disease, ameliorates cognitive dysfunction through selective inhibition of acetylcholinesterase. However, recent studies have also reported off-target effects of donepezil that likely contribute to its therapeutic effects. In this study, we investigated the (i) role of donepezil in amyloid precursor protein (APP) processing and (ii) involvement of sorting nexin protein 33 (SNX33), a member of the sorting nexin protein family, in this processing. Results showed that donepezil induces an increase in SNX33 expression in primary cortical neurons. The secretion of sAPPα in culture media increased, whereas the expression of full-length APP in the cell lysate remained unchanged. Exposure of cortical cultures to donepezil led to a decrease in amyloid β (Aβ) protein levels in a concentration- and time-dependent manner. This decrease was not affected by concomitant treatment with acetylcholine receptor antagonists. SNX33 knockdown by target-specific morpholino oligos inhibited the effects of donepezil. Donepezil treatment increased cell membrane surface expression of APP in SNX33 expression-dependent manner. These results suggested that donepezil decreases the level of Aβ by increasing SNX33 expression and APP cleavage by α-secretase in cortical neurons.