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Neuronal nicotinic acetylcholine receptors (nAChRs) are made of multiple subunits with diversified functions. The nAChR α[sub 7]-subunit has a property of high Ca[sup 2+] 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 nAChRα[sub 7] existed in low-density, cholesterol-enriched plasma membrane microdomains known as lipid rafts where fiotillin also exists. In contrast, nAChR α[sub 5]- and β[sub 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 nAChRα[sub 7]. Cholesterol depletion from plasma membranes with methylβ-cyclodextrin redistributed nAChRα[sub 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 α-bungarotoxin, a specific antagonist of nAChRα[sub 7], or cholesterol depletion from plasma membranes. The effect of cholesterol depletion was negated by the addition of cholesterol. These data suggest that nAChRα[sub 7] has a specific membrane localization relative to other nAChR subunits and that lipid rafts are necessary to localize nAChRα[sub 7] with AC within plasma membranes. In addition, nAChRα[sub 7] may regulate the AC activity via Ca[sup 2+] within lipid rafts. [ABSTRACT FROM AUTHOR]

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 nAChR alpha 7 existed in low-density, cholesterol-enriched plasma membrane microdomains known as lipid rafts where flotillin also exists. In contrast, nAChR alpha 5- and beta2-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 nAChR alpha 7. Cholesterol depletion from plasma membranes with methyl-beta-cyclodextrin redistributed nAChR alpha 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 nAChR alpha 7, or cholesterol depletion from plasma membranes. The effect of cholesterol depletion was negated by the addition of cholesterol. These data suggest that nAChR alpha 7 has a specific membrane localization relative to other nAChR subunits and that lipid rafts are necessary to localize nAChR alpha 7 with AC within plasma membranes. In addition, nAChR alpha 7 may regulate the AC activity via Ca2+ within lipid rafts.