1.A.9.5.16
Gamma-aminobutyric acid receptor subunit alpha-1 (GABAAR1) of 455 aas and 4 TMSs. GABA(A)Rs are the principal inhibitory receptors in the brain and the target of a wide range of clinical agents, including anaesthetics, sedatives, hypnotics and antidepressants (Sun et al. 2023). GABA(A)Rs mediate rapid inhibitory transmission. They are ligand-gated chloride ion channel proteins and exist in about a dozen or more heteropentameric subtypes exhibiting variable age and brain regional localization and thus participation in differing brain functions and diseases. GABA(A)Rs are also subject to modulation by several chemotypes of allosteric ligands that help define structure and function, including subtype definition (Olsen 2015). The channel blocker picrotoxin identified a noncompetitive channel blocker site in GABA(A)Rs. This ligand site is located in the transmembrane channel pore, whereas the GABA agonist site is in the extracellular domain at subunit interfaces, a site useful for low energy coupled conformational changes of the functional channel domain. Two classes of pharmacologically important allosteric modulatory ligand binding sites reside in the extracellular domain at modified agonist sites at other subunit interfaces: the benzodiazepine site and the high-affinity, relevant to intoxication, ethanol site. The benzodiazepine site is specific for certain GABA(A)R subtypes, mainly synaptic, while the ethanol site is found at a modified benzodiazepine site on different, extrasynaptic subtypes. In the transmembrane domain are allosteric modulatory ligand sites for diverse chemotypes of general anesthetics: the volatile and intravenous agents, barbiturates, etomidate, propofol, long-chain alcohols, and neurosteroids (Olsen 2015). The last are endogenous positive allosteric modulators. X-ray crystal structures of prokaryotic and invertebrate pentameric ligand-gated ion channels, and the mammalian GABA(A)R protein, allow homology modeling of GABA(A)R subtypes with the various ligand sites located to suggest the structure and function of these proteins and their pharmacological modulation. Our understanding of GABA(A)R pharmacology has been hindered by the vast number of pentameric assemblies that can be derived from 19 different subunits and the lack of structural knowledge of clinically relevant receptors. Sun et al. 2023 isolated native murine GABA(A)R assemblies containing the widely expressed alpha1 subunit and elucidated their structures in complex with drugs used to treat insomnia (zolpidem (ZOL) and flurazepam) and postpartum depression (the neurosteroid allopregnanolone (APG)). Using cryo-EM analysis,  three major structural populations in the brain were revealed: the canonical alpha1beta2gamma2 receptor containing two alpha1 subunits, and two assemblies containing one alpha1 and either an alpha2 or alpha3 subunit, in which the single alpha1-containing receptors feature a more compact arrangement between the transmembrane and extracellular domains. APG is bound at the transmembrane alpha/beta subunit interface, even when not added to the sample, revealing an important role for endogenous neurosteroids in modulating native GABA(A)Rs. Together with structurally engaged lipids, neurosteroids produce global conformational changes throughout the receptor that modify the ion channel pore and the binding sites for GABA and insomnia medications. The data revealed the major alpha1-containing GABA(A)R assemblies, bound with endogenous neurosteroids, thus defining a structural landscape from which subtype-specific drugs can be developed (Sun et al. 2023).  A QSAR model demonstrated robustness and a high degree of predictability. Moreover, specific molecular fragments were identified that exerted both positive and negative effects on binding activity. This discovery paves the way for the swift prediction of binding activity for emerging benzodiazepines (Antović et al. 2023).