Modulation Of Nicotinic Acetylcholine Receptor Conformational States By Free Fatty Acids And Steroids

ANTOLLINI, S.S.; FERNÁNDEZ NIEVAS, G.A.; BARRANTES, F.J.

Boston

Congreso; 53 Annual Meeting of the Biophysical Society; 2009

Modulation Of Nicotinic Acetylcholine Receptor Conformational States By Free Fatty Acids And Steroids Silvia S. Antollini, Gaspar Fernandez Nievas, Francisco J. Barrantes. UNESCO Chair Biophys & Mol Neurobiol, Bahia Blanca, Argentina. 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 free fatty acids 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 certain cis-unsaturated free fatty acids decreased the KD of CrV in the absence of agonist. The position of the double bond at the hydrocarbon chain of cis-monounsaturated fatty acids appears to be critical for their effect on the AChR resting conformation state. All cis-unsaturated fatty acids tested, but not trans-unsaturated or saturated fatty acids, caused an increase of the KD value in the presence of agonist. This latter effect was also observed with membrane treatments that caused opposite effects on membrane polarity (phospholipase A2 treatment or temperature increase, which decreased or increased the membrane polarity, respectively). Quenching by spin-labeled fatty acids of pyrene-labeled AChR reconstituted into model membranes, with the label located at the gM4 transmembrane segment, disclosed the occurrence of conformational changes induced by steroids and cis-FFA. These results suggest that the direct contact between exogenous lipids and the AChR transmembrane segments removes the AChR from its resting state and that membrane polarity modulates theAChRactivation equilibrium by an independent mechanism.