Distribution and function of nicotinic acetylcholine receptors in glia cells and neurons with focus on the neuroprotective mechanisms of cholesterol lowering drugs in Alzheimer's disease

Abstract

Nicotinic acetylcholine receptors (nAChRs) are involved in the regulation of many processes including synapses regeneration, neurotransmission and neuroprotection in the central nervous system (CNS). There is increasing evidence for the disturbances in nAChRs in Alzheimer's disease (AD). Driven by new findings in molecular biology, the nAChRs in CNS represent a new potential therapeutic target in neurodegenerative disease.

The main purpose of this thesis was to study the distribution and function of nAChRs in glial cells and neurons, to investigate the neuroprotective roles of nAChRs related with cholesterol and cholesterol-lowering drugs in the amyloid precursor protein (APP) processing, which might provide a valuable therapeutic strategy in AD treatment.

Paper I investigated the levels of individual nAChR subunits on primary rat astrocytes and the possible direct influence of Abetas on nAChRs in the cells. The alpha4, alpha7, beta2 and beta3 nAChR subunits and related calcium channel responses were found in rat primary astrocytes, whereas neither alpha2 nor alpha3 could be detected. Elevation in the levels of alpha7, alpha4, beta2 mRNAs and proteins was observed in astrocytes exposed to 0.1 - 100 nM Abeta1-42, which is different from the responses of neurons. The enhanced expression of alpha7, alpha4 and beta2 nAChRs by astrocytes stimulated directly by nanomolar concentrations of API-42 might be related to ongoing defensive or compensative mechanisms.

Paper II investigated weather the cholesterol-lowering drug, lovastatin, enhanced a-form of secreted APP (alphaAPPs) was mediated by activation of alpha7 nAChRs in neuronal cell models. SHSY5Y cells and PC12 cells were treated with lovastatin. Certain concentrations of lovastatin significantly induced up-regulation of alpha7 nAChR expression and alphaAPPs secretion, whereas the increased alphaAPPs secretion was inhibited by prior exposure the cells to alpha-bungarotoxin (alpha-BTX), an antagonist of alpha7 nAChRs. The concentrations of lovastatin used in this study did not decrease the levels of ubiquinone or cholesterol. The upregulation of alpha7 nAChRs stimulated by lovastatin might be involved in a mechanism that enhances production of alphaAPPs in APP processing in neurons.

Paper III investigated the influence of cholesterol or lovastatin on alphaAPPs secretion and alpha nAChR expression in human HTB-15 astrocytes. Cholesterol treatment inhibited the alphaAPPs secretion and alpha7 nAChR expression, while lovastatin treatment enhanced both. The increased alphaAPPs secretion resulted from lovastatin was partially inhibited by the alpha7 nAChR antagonist, alphaBTX or methyllycacoitine (MLA). The results indicated that lovastatin stimulated the alpha-secretase cleavage processing might be regulated by alpha7 nAChRs in astrocytes.

Paper IV investigated the alternative mechanisms among lovastatin, alpha7 nAChRs and mitogen-activated protein kinase/extracellular signal regulated protein kinases (MAPK/ERK) pathway by using SH-SY5Y cells over-expressed human APP695, human HTB-15 astrocytes and primary rat astrocytes. Lovastatin treatment induced high expression of alpha nAChRs, activation of MAPK/ERK pathway and an increase of alphaAPPs secretion in these cultured cells. The stimulation of alphaAPPs induced by lovastatin was inhibited by the alpha7 nAChRs antagonist MLA or the MAPK/ERK inhibitor U0126. Furthermore, the high expression of alpha7 nAChRs resulted form lovastatin treatment was blocked by the MAPK/ERK inhibitor U0126. These results suggested that lovastatin can stimulate the up-regulation of alpha7 nAChRs via activate MAPK/ERK signaling cascade, which therefore, enhances the alpha-secretase cleavage in APP processing.