Desmosterol replaces cholesterol for ligand binding function of the serotonin(1A) receptor in solubilized hippocampal membranes: support for nonannular binding sites for cholesterol?

The serotonin(1A) receptor is an important member of the G-protein coupled receptor family, and is involved in the generation and modulation of a variety of cognitive and behavioral functions. Solubilization of the hippocampal serotonin(1A) receptor by CHAPS is accompanied by loss of cholesterol that results in a reduction in specific agonist binding activity. Replenishment of cholesterol to solubilized membranes restores membrane cholesterol content and significantly recovers specific agonist binding. In order to test the stringency of cholesterol requirement, we solubilized native hippocampal membranes followed by replenishment with desmosterol. Desmosterol is the immediate biosynthetic precursor of cholesterol in the Bloch pathway differing only in a double bond at the 24th position. Our results show that replenishment with desmosterol restores ligand binding of serotonin(1A) receptors. This is consistent with earlier results showing that desmosterol can replace cholesterol in a large number of cases. However, these results appear to be contradictory to our earlier findings, performed by sterol manipulation utilizing methyl-β-cyclodextrin, in which we observed that replacing cholesterol with desmosterol is unable to restore specific ligand binding of the hippocampal serotonin(1A) receptor. We discuss the possible molecular mechanism, in terms of nonannular lipid binding sites around the receptor, giving rise to these differences.