In order to examine the
neuroprotective effects of the alpha7
nicotinic receptor (nAChR) in relationship to the pathogenesis of
Alzheimer's disease (AD),
neuroblastoma (SH-SY5Y) cells were transfected with small interference RNAs (siRNAs) that targets specifically towards alpha7 nAChR or exposed to 20microM 3-[2,4-dimethoxybenzylidene]
anabaseine (DMXB), a selective agonist of this same receptor. The levels of alpha7 nAChR
mRNA and
protein were measured by RT-PCR and Western blotting, respectively. The levels of the alpha-form of secreted
amyloid precursor
protein (alphaAPPs), total APP and the
extracellular signal-regulated kinase 1/2 (ERK1/2) were also determined by Western blotting. SH-SY5Y cells transfected with
siRNA or exposed to DMXB were then treated with 1microM Abeta(25-35), following which the levels of lipid peroxidation and rate of reduction of MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide] were characterized by utilizing spectrophotometric procedures. Compared to controls, SH-SY5Y cells transfected with
siRNA expressed the decreases in the levels of alpha7 nAChR
mRNA and
protein by 81% and 69% lower levels, respectively; exhibited reduced levels of the alphaAPPs and ERK1/2
proteins; and demonstrated enhanced lipid peroxidation and a decreased rate of MTT reduction. In cells exposed to DMXB, the level of alpha7 nAChR
protein was elevated by 23%, with no alteration in the content of the corresponding
mRNA; the levels of the alphaAPPs and ERK1/2
proteins were increased. Inhibition of the expression of the alpha7 nAChR gene enhanced the toxicity exerted by Abeta, whereas stimulation of this receptor attenuated this toxicity exerted. These findings indicate that alpha7 nAChR may play a significant neuroprotective role by enhancing cleavage of APP by
alpha-secretase, regulating signal transduction, improving
antioxidant defenses and inhibiting the toxicity of Abeta, which is connected with the pathogenesis of AD.