Identification of amino acid residues important for assembly of GABA receptor alpha1 and gamma2 subunits

Phosphorylation of GABAA receptors influences receptor trafficking and neurosteroid actions

RATIONALE

Gamma-aminobutyric acid type A receptors (GABAARs) are the principal mediators of inhibitory transmission in the mammalian central nervous system. GABAARs can be localized at post-synaptic inhibitory specializations or at extrasynaptic sites. While synaptic GABAARs are activated transiently following the release of GABA from presynaptic vesicles, extrasynaptic GABAARs are typically activated continuously by ambient GABA concentrations and thus mediate tonic inhibition. The tonic inhibitory currents mediated by extrasynaptic GABAARs control neuronal excitability and the strength of synaptic transmission. However, the mechanisms by which neurons control the functional properties of extrasynaptic GABAARs had not yet been explored.

OBJECTIVES

We review GABAARs, how they are assembled and trafficked, and the role phosphorylation has on receptor insertion and membrane stabilization. Finally, we review the modulation of GABAARs by neurosteroids and how GABAAR phosphorylation can influence the actions of neurosteroids.

CONCLUSIONS

Trafficking and stability of functional channels to the membrane surface are critical for inhibitory efficacy. Phosphorylation of residues within GABAAR subunits plays an essential role in the assembly, trafficking, and cell surface stability of GABAARs. Neurosteroids are produced in the brain and are highly efficacious allosteric modulators of GABAAR-mediated current. This allosteric modulation by neurosteroids is influenced by the phosphorylated state of the GABAAR which is subunit dependent, adding temporal and regional variability to the neurosteroid response. Possible links between neurosteroid actions, phosphorylation, and GABAAR trafficking remain to be explored, but potential novel therapeutic targets may exist for numerous neurological and psychological disorders which are linked to fluctuations in neurosteroid levels and GABAA subunit expression.

Gephyrin, the enigmatic organizer at GABAergic synapses

GABA_A receptors are clustered at synaptic sites to achieve a high density of postsynaptic receptors opposite the input axonal terminals. This allows for an efficient propagation of GABA mediated signals, which mostly result in neuronal inhibition. A key organizer for inhibitory synaptic receptors is the 93 kDa protein gephyrin that forms oligomeric superstructures beneath the synaptic area. Gephyrin has long been known to be directly associated with glycine receptor β subunits that mediate synaptic inhibition in the spinal cord. Recently, synaptic GABA_A receptors have also been shown to directly interact with gephyrin and interaction sites have been identified and mapped within the intracellular loops of the GABA_A receptor α1, α2, and α3 subunits. Gephyrin-binding to GABA_A receptors seems to be at least one order of magnitude weaker than to glycine receptors (GlyRs) and most probably is regulated by phosphorylation. Gephyrin not only has a structural function at synaptic sites, but also plays a crucial role in synaptic dynamics and is a platform for multiple protein-protein interactions, bringing receptors, cytoskeletal proteins and downstream signaling proteins into close spatial proximity.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

Regulation of GABA(A) receptor dynamics by interaction with purinergic P2X(2) receptors

γ-Aminobutyric acid type A receptors (GABA(A)Rs) in the spinal cord are evolving as an important target for drug development against pain. Purinergic P2X(2) receptors (P2X(2)Rs) are also expressed in spinal cord neurons and are known to cross-talk with GABA(A)Rs. Here, we investigated a possible "dynamic" interaction between GABA(A)Rs and P2X(2)Rs using co-immunoprecipitation and fluorescence resonance energy transfer (FRET) studies in human embryonic kidney (HEK) 293 cells along with co-localization and single particle tracking studies in spinal cord neurons. Our results suggest that a significant proportion of P2X(2)Rs forms a transient complex with GABA(A)Rs inside the cell, thus stabilizing these receptors and using them for co-trafficking to the cell surface, where P2X(2)Rs and GABA(A)Rs are primarily located extra-synaptically. Furthermore, agonist-induced activation of P2X(2)Rs results in a Ca(2+)-dependent as well as an apparently Ca(2+)-independent increase in the mobility and an enhanced degradation of GABA(A)Rs, whereas P2X(2)Rs are stabilized and form larger clusters. Antagonist-induced blocking of P2XRs results in co-stabilization of this receptor complex at the cell surface. These results suggest a novel mechanism where association of P2X(2)Rs and GABA(A)Rs could be used for specific targeting to neuronal membranes, thus providing an extrasynaptic receptor reserve that could regulate the excitability of neurons. We further conclude that blocking the excitatory activity of excessively released ATP under diseased state by P2XR antagonists could simultaneously enhance synaptic inhibition mediated by GABA(A)Rs.

Structural model of rho1 GABAC receptor based on evolutionary analysis: Testing of predicted protein-protein interactions involved in receptor assembly and function

The homopentameric rho1 GABA(C) receptor is a ligand-gated ion channel with a binding pocket for gamma-aminobutyric acid (GABA) at the interfaces of N-terminal extracellular domains. We combined evolutionary analysis, structural modeling, and experimental testing to study determinants of GABA(C) receptor assembly and channel gating. We estimated the posterior probability of selection pressure at amino acid residue sites measured as omega-values and built a comparative structural model, which identified several polar residues under strong selection pressure at the subunit interfaces that may form intersubunit hydrogen bonds or salt bridges. At three selected sites (R111, T151, and E55), mutations disrupting intersubunit interactions had strong effects on receptor folding, assembly, and function. We next examined the role of a predicted intersubunit salt bridge for residue pair R158-D204. The mutant R158D, where the positively charged residue is replaced by a negatively charged aspartate, yielded a partially degraded receptor and lacked membrane surface expression. The membrane surface expression was rescued by the double mutant R158D-D204R, where positive and negative charges are switched, although the mutant receptor was inactive. The single mutants R158A, D204R, and D204A exhibited diminished activities and altered kinetic profiles with fast recovery kinetics, suggesting that R158-D204 salt bridge perhaps stabilizes the open state of the GABA(C) receptor. Our results emphasize the functional importance of highly conserved polar residues at the protein-protein interfaces in GABA(C) rho1 receptors and demonstrate how the integration of computational and experimental approaches can aid discovery of functionally important interactions.

The subunit arrangement and assembly of ionotropic receptors

Ionotropic receptors mediate rapid communication between neurons. These receptors are oligomers and are usually assembled from multiple subunit types. Receptors built from different subunit combinations have distinct functional properties, such as single-channel conductances, rates of desensitization and sensitivities to activators and inactivators; they can also have different intracellular locations. Methods are now available for determining not only the subunit stoichiometry but also the subunit arrangement within ionotropic receptors. This information will inform experiments designed to understand the molecular basis of receptor assembly and function. It will also permit the modelling of potential ligand-binding sites at the interfaces between the subunits and should lead to a more rational approach to drug development.

Multiple modes for conferring surface expression of homomeric beta1 GABAA receptors

The gamma-aminobutyric acid type A (GABA(A)) receptor assembles from individual subunits to form ligand-gated ion channels. Human (h) beta3 subunits assemble to form homomeric surface receptors in somatic cells, but hbeta1 subunits do not. We have identified three distinct sets of amino acid residues in the N-terminal extracellular domain of the hbeta1 subunit, which when mutated to the homologous residue in hbeta3 allow expression as a functional homomeric receptor. The three sets likely result in three modes of assembly. Mode 1 expression results from a single amino acid change at residue hbeta1 Asp-37. Mode 2 expression results from mutations of residues between positions 44 and 73 together with residues between positions 169 and 173. Finally, mode 3 results from the mutations A45V and K196R. Examination of homology-based structural models indicates that many of the residues are unlikely to be involved in physical inter-subunit interactions, suggesting that a major alteration is stabilization of an assembly competent form of the subunit. These mutations do not, however, have a major effect on the surface expression of heteromeric receptors which include the alpha1 subunit.

Structure-guided drug design: conferring selectivity among neuronal nicotinic receptor and acetylcholine-binding protein subtypes

Neuronal nicotinic receptors, encoded by nine genes of the alpha and three of the beta type of subunits, and whose gene products assemble in distinct permutations as pentameric molecules, constitute a fertile area for structure-guided drug design. Design strategies are augmented by a wide variety of peptide, alkaloid and terpenoid toxins from various marine and terrestrial species that interact with nicotinic receptors. Also, acetylcholine-binding proteins from mollusks, as structural surrogates of the receptor that mimic its extracellular domain, provide atomic resolution templates for analysis of structure and response. Herein, we describe a structure-guided approach to nicotinic ligand design that employs crystallography of this protein as the basic template, but also takes into consideration the dynamic properties of the receptor molecules in their biological media. We present the crystallographic structures of several complexes of various agonists and antagonists that associate with the agonist site and can competitively block the action of acetylcholine. In so far as the extracellular domain is involved, we identify additional non-competitive sites at those subunit interfaces where agonists do not preferentially bind. Ligand association at these interface sites may modulate receptor function. Ligand binding is also shown by solution-based spectroscopic and spectrometric methods to affect the dynamics of discrete domains of the receptor molecule. The surrogate receptor molecules can then be employed to design ligands selective for receptor subtype through the novel methods of freeze-frame, click chemistry that uses the very structure of the target molecule as a template for synthesis of the inhibitor.

17 β-estradiol modulates GABAergic synaptic transmission and tonic currents during development in vitro

Estrogens exert a variety of modulatory effects on the structure and function of the nervous system. In particular, 17 beta-estradiol was found to affect GABAergic inhibition in adult animals but its action on GABAergic currents during development has not been elucidated. In the present study, we investigated the effect of 17 beta-estradiol on hippocampal neurons developing in vitro. In this model, mIPSC kinetics showed acceleration with age along with increased alpha1 subunit expression, similarly as in vivo. Long-term treatment with 17 beta-estradiol increased mIPSC amplitudes in neurons cultured for 6-8 and 9-11DIV and prolonged the mIPSC decaying phase only in the 9-11DIV group. The time needed for the onset of 17 beta-estradiol effect on mIPSC amplitude was approximately 48 h. In the period of 9-11DIV, treatment with 17 beta-estradiol strongly reduced the tonic conductance activated by low GABA concentrations. The effects of 17 beta-estradiol on mIPSCs and tonic conductance were not correlated with any change in expression of considered GABAAR subunits (alpha1-3, alpha5-6, gamma2) while alpha4 and delta subunits were at the detection limit. In conclusion, we provide evidence that 17 beta-estradiol differentially affects the phasic and tonic components of GABAergic currents in neurons developing in vitro.

Galanthamine and non-competitive inhibitor binding to ACh-binding protein: evidence for a binding site on non-alpha-subunit interfaces of heteromeric neuronal nicotinic receptors

Rapid neurotransmission is mediated through a superfamily of Cys-loop receptors that includes the nicotinic acetylcholine (nAChR), gamma-aminobutyric acid (GABA(A)), serotonin (5-HT(3)) and glycine receptors. A class of ligands, including galanthamine, local anesthetics and certain toxins, interact with nAChRs non-competitively. Suggested modes of action include blockade of the ion channel, modulation from undefined extracellular sites, stabilization of desensitized states, and association with annular or boundary lipid. Alignment of mammalian Cys-loop receptors shows aromatic residues, found in the acetylcholine or ligand-binding pocket of nAChRs, are conserved in all subunit interfaces of neuronal nAChRs, including those that are not formed by alpha subunits on the principal side of the transmitter binding site. The amino-terminal domain containing the ligand recognition site is homologous to the soluble acetylcholine-binding protein (AChBP) from mollusks, an established structural and functional surrogate. We assess ligand specificity and employ X-ray crystallography with AChBP to demonstrate ligand interactions at subunit interfaces lacking vicinal cysteines (i.e. the non-alpha subunit interfaces in nAChRs). Non-competitive nicotinic ligands bind AChBP with high affinity (K(d) 0.015-6 microM). We mutated the vicinal cysteine residues in loop C of AChBP to mimic the non-alpha subunit interfaces of neuronal nAChRs and other Cys loop receptors. Classical nicotinic agonists show a 10-40-fold reduction in binding affinity, whereas binding of ligands known to be non-competitive are not affected. X-ray structures of cocaine and galanthamine bound to AChBP (1.8 A and 2.9 A resolution, respectively) reveal interactions deep within the subunit interface and the absence of a contact surface with the tip of loop C. Hence, in addition to channel blocking, non-competitive interactions with heteromeric neuronal nAChR appear to occur at the non-alpha subunit interface, a site presumed to be similar to that of modulating benzodiazepines on GABA(A) receptors.

Spontaneous cross-link of mutated alpha1 subunits during GABA(A) receptor assembly

gamma-Aminobutyric acid, type A (GABA(A)) receptor alpha1 subunits containing a cysteine mutation at a position in the channel mouth (H109C) surprisingly formed a spontaneous cross-link with each other in receptors composed of alpha1H109C, beta3, and gamma2 subunits. Cross-linking of two alpha1H109C subunits did not significantly change the affinity of [(3)H]muscimol or [(3)H]Ro15-1788 binding in alpha1H109Cbeta3gamma2 receptors, but GABA displayed a reduced potency for activating chloride currents. On reduction of the disulfide bond, however, GABA activation as well as diazepam modulation was similar in mutated and wild-type receptors, suggesting that these receptors exhibited the same subunit stoichiometry and arrangement. Disulfide bonds could not be reoxidized by copper phenanthroline after having been reduced in completely assembled receptors, suggesting that cross-linking can only occur at an early stage of assembly. The cross-link of alpha1H109C subunits and the subsequent transport of the resulting homodimers to the cell surface caused a reduction of the intracellular pool of alpha1H109C subunits and a reduced formation of completely assembled receptors. The formation of alpha1H109C homodimers as well as of correctly assembled GABA(A) receptors containing cross-linked alpha1H109C subunits could indicate that homodimerization of alpha1 subunits via contacts located in the channel mouth might be one starting point of GABA(A) receptor assembly. Alternatively the assembly mechanism might have started with the formation of heterodimers followed by a cross-link of mutated alpha1 subunits at the heterotrimeric stage. The formation of cross-linked alpha1H109C homodimers would then have occurred independently in a separate pathway.

The role of GABAA receptor biogenesis, structure and function in epilepsy

Maintaining the correct balance in neuronal activation is of paramount importance to normal brain function. Imbalances due to changes in excitation or inhibition can lead to a variety of disorders ranging from the clinically extreme (e.g. epilepsy) to the more subtle (e.g. anxiety). In the brain, the most common inhibitory synapses are regulated by GABAA (γ-aminobutyric acid type A) receptors, a role commensurate with their importance as therapeutic targets. Remarkably, we still know relatively little about GABAA receptor biogenesis. Receptors are constructed as pentameric ion channels, with α and β subunits being the minimal requirement, and the incorporation of a γ subunit being necessary for benzodiazepine modulation and synaptic targeting. Insights have been provided by the discovery of several specific assembly signals within different GABAA receptor subunits. Moreover, a number of recent studies on GABAA receptor mutations associated with epilepsy have further enhanced our understanding of GABAA receptor biogenesis, structure and function.