Impact of sustained exposure to β-amyloid on the regulation of synaptic function and cell integrity in a model nerve cell system expressing α4β2 nicotinic acetylcholine receptors

Ph.D. University of Hawaii at Manoa 2013.Includes bibliographical references.Alzheimer's disease is the leading cause of dementia in the elderly. It is characterized by the accumulation of extracellular deposits of β-amyloid (Aβ) in senile plaques, intracellular formation of neurofibrillary tangles (composed of abnormally phosphorylated tau), and substantial loss of synapses and neurons, most notably in the hippocampus. In addition, cholinergic neurons in the basal forebrain degenerate. Recent studies have reported that Aβ can affect nicotinic receptor (nAChR)-mediated synaptic transmission. Although the regulation of nAChRs by Aβ has been widely studied, the impact of prolonged exposure to Aβ on nAChR expression and signaling is not known. The objective of this study was therefore to understand the impact of sustained exposure of Aβ on neuronal and synaptic function. In this study, we employed a neuronal culture model to address the impact of sustained exposure of Aβ on nerve cells expressing exogenous high-affinity type α4β2 nAChRs. Specifically the differentiated rodent neuroblastoma cell line NG108-15 expressing α4β2 nAChRs from transfected plasmids containing the mouse receptor sequences or not (mock-transfected control cells) were exposed to oligomeric Aβ for several days, following which Ca2+ responses, receptor expression levels, axonal mitochondrial dynamics, oxidative stress, cell viability/toxicity and expression levels of different signaling molecules were compared. The results demonstrated that prolonged daily exposure to Aβ potentiated nicotinic receptor function and promoted mitochondrial and cellular toxicity specifically in the nerve cells expressing α4β2 nAChRs. Increased nAChR-mediated Ca2+ responses to acute applications of Aβ or nicotine were first observed after 3 days of Aβ treatment, paralleled by increased expression levels of α4β2 nAChRs, likely the result of enhanced receptor recycling. The rate of mitochondrial movement and average mitochondrial size were also drastically reduced, while the mitochondrial fission protein Drp-1 was increased. In addition, the presence of α4β2 nAChRs dramatically enhanced Aβ-induced increases in the levels of ROS and nuclear fragmentation, leading to apoptosis. Lastly, these changes were paralleled by altered levels of the phosphorylated forms of ERK, JNK, CREB and Fyn. Together, the findings suggest that the presence of nAChRs sensitizes neurons to the toxic actions of soluble oligomeric Aβ, perhaps contributing to the cholinergic deficit in Alzheimer's disease