Structural basis for GABAA receptor potentiation by neurosteroids

Abstract

Type A γ-aminobutyric acid receptors (GABARs) are the principal mediators of inhibitory neurotransmission in the human brain. Endogenous neurosteroids interact with GABARs to regulate acute and chronic anxiety and are potent sedative, analgesic, anticonvulsant and anesthetic agents. Their mode of binding and mechanism of receptor potentiation, however, remain unknown. Here we report crystal structures of a chimeric GABAR construct in apo and pregnanolone-bound states. The neurosteroid-binding site is mechanically coupled to the helices lining the ion channel pore and modulates the desensitization-gate conformation. We demonstrate that the equivalent site is responsible for physiological, heteromeric GABAR potentiation and explain the contrasting modulatory properties of 3a versus 3b neurosteroid epimers. These results illustrate how peripheral lipid ligands can regulate the desensitization gate of GABARs, a process of broad relevance to pentameric ligand-gated ion channels.

Balancing tonic and phasic inhibition in hypothalamic corticotropin‐releasing hormone neurons

“…The mechanism by which CORT modulates I Tonic is unknown, but a recent study found a neurosteroid binding site within a conserved transmembrane region of α5 GABA A Rs (Miller et al . ). Furthermore, the 3α and 5α reductases responsible for production of neurosteroids also convert CORT to 3α, 5α tetrahydrocorticosterone (3α, 5α THB) (Traish et al .…”

The dual‐gate model for pentameric ligand‐gated ion channels activation and desensitization

“…This structure shows a wide open activation gate in the M2 9 /13 region, but a hydrophobic constriction at the −2 proline, where the pore radius narrows down to 1.6Å, thereby precluding the flow of chloride ions whose Pauling radius is 1.8Å. More recently, several pLGICs have been solved in similar conformations: the human α3 GlyR in complex with both glycine and a positive allosteric modulator, AM-3607 (Huang et al 2017), as well as a GLIC-(GABA A α1) and a (GABA A β3)-(GABA A α5) chimera, the former carrying a α1 G258V mutation promoting desensitization (Laverty et al 2017;Miller et al 2017). All these structures show a conserved pore conformation and were assigned to a desensitized state, since they correspond to agonist-bound shut states.…”

“…; Miller et al . ). All these structures show a conserved pore conformation and were assigned to a desensitized state, since they correspond to agonist‐bound shut states.…”

“…; Miller et al . ), while inhibitory neurosteroids likely bind in an intra‐subunit site, at the intracellular end of the M3–M4 interface (Laverty et al . ).…”

A tale of ligands big and small: An update on how pentameric ligand-gated ion channels interact with agonists and proteins

“…How are these processes influenced by endogenous modulators, such as neurosteroids (i.e. GABA A R-pregnanolone interaction; [56])?…”