Cryo-EM structure of the human α1ß3γ2 GABAA receptor in a lipid bilayer.

Type A γ-aminobutyric acid (GABAA) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system1,2. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia3,4. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the α1ß2/3γ2 GABAA receptors5. The ß3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in ß1-ß3 triple knockout neurons6. So far, efforts to generate accurate structural models for heteromeric GABAA receptors have been hampered by the use of engineered receptors and the presence of detergents7-9. Notably, some recent cryo-electron microscopy reconstructions have reported 'collapsed' conformations8,9; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor10,11, the large body of structural work on homologous homopentameric receptor variants12 and the logic of an ion-channel architecture. Here we present a high-resolution cryo-electron microscopy structure of the full-length human α1ß3γ2L-a major synaptic GABAA receptor isoform-that is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator 'megabody' and is in a desensitized conformation. Each GABAA receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of α1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABAA receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.

Author Correction: GABAA receptor signalling mechanisms revealed by structural pharmacology.

In Fig. 5b, d, the arrows showing transmembrane domain rotations were inadvertently pointing clockwise instead of anticlockwise. Similarly, 'anticlockwise' should have been 'clockwise' in the sentence 'This conformational change of the ECD triggers a clockwise rotation of the TMD.' In Extended Data Table 1, the units of the column 'Model resolution' should have been Å instead of Å2. These errors have been corrected online.

GABAA receptor signalling mechanisms revealed by structural pharmacology.

Type-A γ-aminobutyric (GABAA) receptors are ligand-gated chloride channels with a very rich pharmacology. Some of their modulators, including benzodiazepines and general anaesthetics, are among the most successful drugs in clinical use and are common substances of abuse. Without reliable structural data, the mechanistic basis for the pharmacological modulation of GABAA receptors remains largely unknown. Here we report several high-resolution cryo-electron microscopy structures in which the full-length human α1ß3γ2L GABAA receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam. We describe the binding modes and mechanistic effects of these ligands, the closed and desensitized states of the GABAA receptor gating cycle, and the basis for allosteric coupling between the extracellular, agonist-binding region and the transmembrane, pore-forming region. This work provides a structural framework in which to integrate previous physiology and pharmacology research and a rational basis for the development of GABAA receptor modulators.

Barbiturates Bind in the GLIC Ion Channel Pore and Cause Inhibition by Stabilizing a Closed State.

Barbiturates induce anesthesia by modulating the activity of anionic and cationic pentameric ligand-gated ion channels (pLGICs). Despite more than a century of use in clinical practice, the prototypic binding site for this class of drugs within pLGICs is yet to be described. In this study, we present the first X-ray structures of barbiturates bound to GLIC, a cationic prokaryotic pLGIC with excellent structural homology to other relevant channels sensitive to general anesthetics and, as shown here, to barbiturates, at clinically relevant concentrations. Several derivatives of barbiturates containing anomalous scatterers were synthesized, and these derivatives helped us unambiguously identify a unique barbiturate binding site within the central ion channel pore in a closed conformation. In addition, docking calculations around the observed binding site for all three states of the receptor, including a model of the desensitized state, showed that barbiturates preferentially stabilize the closed state. The identification of this pore binding site sheds light on the mechanism of barbiturate inhibition of cationic pLGICs and allows the rationalization of several structural and functional features previously observed for barbiturates.

Protein engineering enables a soakable crystal form of human CDK7 primed for high-throughput crystallography and structure-based drug design.

Cyclin dependent kinase 7 (CDK7) is an important therapeutic kinase best known for its dual role in cell cycle regulation and gene transcription. Here, we describe the application of protein engineering to generate constructs leading to high resolution crystal structures of human CDK7 in both active and inactive conformations. The active state of the kinase was crystallized by incorporation of an additional surface residue mutation (W132R) onto the double phosphomimetic mutant background (S164D and T170E) that yielded the inactive kinase structure. A novel back-soaking approach was developed to determine crystal structures of several clinical and pre-clinical inhibitors of this kinase, demonstrating the potential utility of the crystal system for structure-based drug design (SBDD). The crystal structures help to rationalize the mode of inhibition and the ligand selectivity profiles versus key anti-targets. The protein engineering approach described here illustrates a generally applicable strategy for structural enablement of challenging molecular targets.

The subtypes of nicotinic acetylcholine receptors on dopaminergic terminals of mouse striatum.

This review summarizes studies that attempted to determine the subtypes of nicotinic acetylcholine receptors (nAChR) expressed in the dopaminergic nerve terminals in the mouse. A variety of experimental approaches has been necessary to reach current knowledge of these subtypes, including in situ hybridization, agonist and antagonist binding, function measured by neurotransmitter release from synaptosomal preparations, and immunoprecipitation by selective antibodies. Early developments that facilitated this effort include the radioactive labeling of selective binding agents, such as [(125)I]-alpha-bungarotoxin and [(3)H]-nicotine, advances in cloning the subunits, and expression and evaluation of function of combinations of subunits in Xenopus oocytes. The discovery of epibatidine and alpha-conotoxin MII (alpha-CtxMII), and the development of nAChR subunit null mutant mice have been invaluable in determining which nAChR subunits are important for expression and function in mice, as well as allowing validation of the specificity of subunit specific antibodies. These approaches have identified five nAChR subtypes of nAChR that are expressed on dopaminergic nerve terminals. Three of these contain the alpha6 subunit (alpha4alpha6beta2beta3, alpha6beta2beta3, alpha6beta2) and bind alpha-CtxMII with high affinity. One of these three subtypes (alpha4alpha6beta2beta3) also has the highest sensitivity to nicotine of any native nAChR that has been studied, to date. The two subtypes that do not have high affinity for alpha-CtxMII (alpha4beta2, alpha4alpha5beta2) are somewhat more numerous than the alpha6* subtypes, but do bind nicotine with high affinity. Given that our first studies detected readily measured differences in sensitivity to agonists and antagonists among these five nAChR subtypes, it seems likely that subtype selective compounds could be developed that would allow therapeutic manipulation of diverse nAChRs that have been implicated in a number of human conditions.

Crystal structures of a GABAA-receptor chimera reveal new endogenous neurosteroid-binding sites.

γ-Aminobutyric acid receptors (GABAARs) are vital for controlling excitability in the brain. This is emphasized by the numerous neuropsychiatric disorders that result from receptor dysfunction. A critical component of most native GABAARs is the α subunit. Its transmembrane domain is the target for many modulators, including endogenous brain neurosteroids that impact anxiety, stress and depression, and for therapeutic drugs, such as general anesthetics. Understanding the basis for the modulation of GABAAR function requires high-resolution structures. Here we present the first atomic structures of a GABAAR chimera at 2.8-Å resolution, including those bound with potentiating and inhibitory neurosteroids. These structures define new allosteric binding sites for these modulators that are associated with the α-subunit transmembrane domain. Our findings will enable the exploitation of neurosteroids for therapeutic drug design to regulate GABAARs in neurological disorders.

Inhibitory neurosteroids and the GABAA receptor.

γ-Aminobutyric acid type A receptors (GABAARs) are vital proteins that are engaged in regulating neural circuit activity in the central nervous system. Their effectiveness in this task is dependent on the extent of receptor modulation by naturally occurring ligands that are released in the brain. One of the foremost examples of such ligands is the neurosteroids that can either potentiate GABAAR function or cause direct inhibition. To fully understand the underlying mechanisms by which neurosteroids modulate GABAARs, it is necessary to identify their binding sites on the receptors. For potentiating neurosteroids, recent work has made substantive progress in identifying a binding site located in the transmembrane domains of GABAAR α subunits. However, for the inhibitory neurosteroids, several possibilities exist including an ion channel site as well as potential sites in the transmembrane domain. This review systematically analyzes the evidence behind possible binding sites for the inhibitory neurosteroids. We consider the chemical structure-function properties of such inhibitory neurosteroids, their physiological effects on synaptic inhibition, and whether a binding site exists in the GABA ion channel or in other areas of the transmembrane domain. Finally, we discuss how structural homology modeling and Cys-loop receptor homologues may help to locate the inhibitory neurosteroid-binding site on GABAARs.

Mice expressing the ADNFLE valine 287 leucine mutation of the Β2 nicotinic acetylcholine receptor subunit display increased sensitivity to acute nicotine administration and altered presynaptic nicotinic receptor function.

Several mutations in α4 or ß2 nicotinic receptor subunits are linked to autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE). One such missense mutation in the gene encoding the ß2 neuronal nicotinic acetylcholine receptor (nAChR) subunit (CHRNB2) is a valine-to-leucine substitution in the second transmembrane domain at position 287 (ß2VL). Previous studies indicated that the ß2VL mutation in mice alters circadian rhythm consistent with sleep alterations observed in ADNFLE patients (Xu et al., 2011). The current study investigates changes in nicotinic receptor function and expression that may explain the behavioral phenotype of ß2VL mice. No differences in ß2 mRNA expression were found between wild-type (WT) and heterozygous (HT) or homozygous mutant (MT) mice. However, antibody and ligand binding indicated that the mutation resulted in a reduction in receptor protein. Functional consequences of the ß2VL mutation were assessed biochemically using crude synaptosomes. A gene-dose dependent increase in sensitivity to activation by acetylcholine and decrease in maximal nAChR-mediated [(3)H]-dopamine release and (86)Rb efflux were observed. Maximal nAChR-mediated [(3)H]-GABA release in the cortex was also decreased in the MT, but maximal [(3)H]-GABA release was retained in the hippocampus. Behaviorally both HT and MT mice demonstrated increased sensitivity to nicotine-induced hypolocomotion and hypothermia. Furthermore, WT mice display only a tonic-clonic seizure (EEG recordable) 3 min after injection of a high dose of nicotine, while MT mice also display a dystonic arousal complex (non-EEG recordable) event 30s after nicotine injection. Data indicate decreases in maximal response for certain measures are larger than expected given the decrease in receptor expression.

John Daly's compound, epibatidine, facilitates identification of nicotinic receptor subtypes.

The diversity of nicotinic acetylcholine receptor (nAChR) subtypes was explored by measuring the effects of gene deletion and pharmacological diversity of epibatidine binding sites in mouse brain. All epibatidine binding sites require expression of either the alpha7, beta2, or beta4 subunit. In agreement with general belief, the alpha4beta2*-nAChR and alpha7-nAChR subtypes are major components of the epibatidine binding sites. alpha4beta2*-nAChR sites account for approximately 70% of total high- and low-affinity epibatidine binding sites, while alpha7-nAChR accounts for 16% of the total sites all of which have lower affinity for epibatidine. The other subtypes are structurally diverse. Although these minor subtypes account for only 14% of total binding in whole brain, they are expressed at relatively high concentrations in specific brain areas indicating unique functional roles.