β‐Amyloid activates presynaptic α7 nicotinic acetylcholine receptors reconstituted into a model nerve cell system: involvement of lipid rafts

Abstract

Beta amyloid (Abeta) plays a central role in the pathogenesis of Alzheimer's disease. Abeta is the major constituent of senile plaques, but there is a significant presence of Abeta in the brain in soluble forms. The results of functional studies indicate that soluble Abeta interacts with the alpha7 nicotinic acetylcholine receptor (nAChR) complex with apparent high affinity. However, conflicting data exist as to the nature of the Abeta-alpha7 nAChR interaction, and whether it is the result of specific binding. Moreover, both agonist-like and antagonist-like effects have been reported. In particular, agonist-like effects have been observed for presynaptic nAChRs. Here, we demonstrate Abeta(1-42)-evoked stimulatory changes in presynaptic Ca(2+) level via exogenous alpha7 nAChRs expressed in the axonal varicosities of differentiated hybrid neuroblastoma NG108-15 cells as a model, presynaptic system. The Abeta(1-42)-evoked responses were concentration-dependent and were sensitive to the highly selective alpha7 nAChR antagonist alpha-bungarotoxin. Voltage-gated Ca(2+) channels and internal Ca(2+) stores were both involved in Abeta(1-42)-evoked increases in presynaptic Ca(2+) following activation of alpha7 nAChRs. In addition, disruption of lipid rafts by cholesterol depletion led to substantially attenuated responses to Abeta(1-42), whereas responses to nicotine were largely intact. These results directly implicate the nicotinic receptor complex as a target for the agonist-like action of pico- to nanomolar concentrations of soluble Abeta(1-42) on the presynaptic nerve terminal, including the possible involvement of receptor-associated lipid rafts. This interaction probably plays an important neuromodulatory role in synaptic dynamics.