Nicotinic acetylcholine receptor α7 regulates cAMP signal within lipid rafts

Neuronal nicotinic acetylcholine receptors (nAChRs) are made of multiple subunits with diversified functions. The nAChR alpha(7)-subunit has a property of high Ca2+ permeability and may have specific functions and localization within the plasma membrane as a signal transduction molecule. In PC-12 cells, fractionation by sucrose gradient centrifugation revealed that nAChRalpha(7) existed in low-density, cholesterol-enriched plasma membrane microdomains known as lipid rafts where flotillin also exists. In contrast, nAChR alpha(5)- and beta(2)-subunits were located in high-density fractions, out of the lipid rafts. Type 6 adenylyl cyclase (AC6), a calcium-inhibitable isoform, was also found in lipid rafts and was coimmunoprecipitated with nAChRalpha(7). Cholesterol depletion from plasma membranes with methyl-beta-cyclodextrin redistributed nAChRalpha(7) and AC6 diffusely within plasma membranes. Nicotine stimulation reduced forskolin-stimulated AC activity by 35%, and this inhibition was negated by either treatment with alpha-bungarotoxin, a specific antagonist of nAChRalpha(7), or cholesterol depletion from plasma membranes. The effect of cholesterol depletion was negated by the addition of cholesterol. These data suggest that nAChRalpha(7) has a specific membrane localization relative to other nAChR subunits and that lipid rafts are necessary to localize nAChRalpha(7) with AC within plasma membranes. In addition, nAChRalpha(7) may regulate the AC activity via Ca2+ within lipid rafts.