Modulation of nicotinic acetylcholine receptor conformational state by free fatty acids and steroids.

FERNÁNDEZ NIEVAS, G; BARRANTES FJ; ANTOLLINI S

JOURNAL OF BIOLOGICAL CHEMISTRY

AMER SOC BIOCHEMISTRY MOLECULAR BIOLOGY INC

Lugar: Bethesda, Maryland; Año: 2008 p. 1 - 1

0021-9258

Steroids and free fatty acids (FFA) are noncompetitive antagonists of the nicotinic acetylcholine receptor (AChR). Their site of action is purportedly located at the lipid-AChR interface, but their exact mechanism of action is still unknown. Here we studied the effect of structurally different FFA and steroids on the conformational equilibrium of the AChR in Torpedo californica receptor-rich membranes. We took advantage of the higher affinity of the fluorescent AChR open channel blocker, crystal violet, for the desensitized state than for the resting state. Increasing concentrations of steroids and FFA decreased the KD of crystal violet in the absence of agonist; however, only cisunsaturated FFA caused an increase in KD in the presence of agonist. This latter effect was also observed with treatments that caused the opposite effects on membrane polarity, such as phospholipase A2 treatment or temperature increase (decreasing or increasing membrane polarity, respectively). Quenching by spin-labeled fatty acids of pyrene-labeled AChR reconstituted into model membranes, with the label located at the
M4 transmembrane segment, disclosed the occurrence of conformational changes induced by steroids and cis-unsaturated FFA. The present work is a step forward in understanding the mechanism of action of this type of molecules, suggesting that the direct contact between exogenous lipids and the AChR transmembrane segments removes the AChR from its resting state and that membrane polarity modulates the AChR activation equilibrium by an independent mechanism.   RESUMEN Steroids and free fatty acids (FFA) are non-competitive antagonists of the nicotinic acetylcholine receptor (AChR). Their site of action is purportedly located at the lipid-AChR interface, but their exact mechanism of action is still unknown. Here, we studied the effect of structurally different FFA and steroids on the conformational equilibrium of the AChR in T. californica receptor-rich membranes. We took advantage of the higher affinity of the fluorescent AChR open channel blocker, crystal violet (CrV), for the desensitized state than for the resting state. Increasing concentrations of steroids and FFA decreased the KD of CrV in the absence of agonist; however, only cis-FFA caused an increase in KD in the presence of agonist. This latter effect was also observed with treatments that caused opposite effects on membrane polarity, such as phospholipase A2 treatment or temperature increase (decreasing or increasing membrane polarity, respectively). Quenching by spinlabeled fatty acids of pyrene-labeled AChR reconstituted into model membranes, with the label located at the ãM4 transmembrane segment, disclosed the occurrence of conformational changes induced by steroids and cis-FFA. The present work is a step forward in understanding the mechanism of action of this type of molecules, suggesting that the direct contact between exogenous lipids and the AChR transmembrane segments removes the AChR from its resting state and that membrane polarity modulates the AChR activation equilibrium by an independent mechanism.