Identification of the Neurosteroid Binding Site at GABAA Receptors

LAUTARO D. ALVAREZ

Bariloche

Simposio; 3rd South American Symposium in Signal Transduction and Molecular Medicine; 2015

Neurosteroids are the principal endogenous modulators of GABAA receptors (GABAARs), which are pentameric membrane-bound proteins. As consequence of their ability to modify inhibitory functions in the brain, neurosteroids have high physiological and clinical importance and may act as anesthetic, anticonvulsant and anxiolytic drugs. Despite their relevance, essential issues regarding neurosteroid action on GABAARs are still unsettled. In particular, the precise location of the binding site and residues taking part of the steroid recognition are not definitely identified.Taking as starting point the first reported crystal structure of a human GABAA receptor (a β3 homopentamer), the binding mode of two structurally different representative neurosteroids, pregnanolone and allopregnanolone, was explored through a combination of computational modeling methods. Binding sites between the TM3 of one subunit and TM1 and TM4 of the adjacent subunit were identified. These sites were able to properly accommodate both overall torsioned and flat steroidal structures and, notably, they specifically recognize their 3-OH group, explaining the requirement of a 3β-configuration for the activity. Remarkably, these theoretical findings are well supported by the set of experimental data reported for the action of neurosteroids on β3 homopentamers, providing for first time convincing information about the molecular interaction between neurosteroids and a GABAAR. This finding could be taken as an initial step in the analysis of the allosteric mechanism by which these small lipophilic molecules affect the receptor structure and dynamics. A better understanding of the structure-activity relation would contribute in a rational design of novel analogues with selective medical applications.