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

FERN¨¢NDEZ NIEVAS, G. A.; BARRANTES, F. J.; ANTOLLINI, S. S.

Foz do Iguaz¨², Brasil.

Simposio; XIII International Symposium on Cholinergic Mechasnisms.; 2008

MODULATION OF NICOTINIC ACETYLCHOLINE RECEPTOR CONFORMATIONAL STATE BY FREE FATTY ACIDS AND STEROIDS Gaspar A. Fern¨¢ndez Nievas, Francisco J. Barrantes and Silvia S. Antollini Instituto de Investigaciones Bioqu¨ªmicas de Bah¨ªa Blanca and UNESCO Chair of Biophysics & Molecular Neurobiology, C.C. 857, B8000FWB, Argentina Introduction: 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. Methods: 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. Results: 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 spin-labeled 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. Discussion: 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. This work was supported by The Academy of Sciences for the Developing World (TWAS), the Agencia Nacional de Promoci¨®n Cient¨ªfica (FONCYT), the Universidad Nacional del Sur, and the Consejo Nacional de Investigaciones Cient¨ªficas y T¨¦cnicas (CONICET) to S.S.A and F.J.B.