GABA(A) receptor composition is determined by distinct assembly signals within alpha and beta subunits

Intricacies of GABAA Receptor Function: The Critical Role of the β3 Subunit in Norm and Pathology

Neuronal intracellular chloride ([Cl⁻]_i) is a key determinant in γ-aminobutyric acid type A (GABA)ergic signaling. γ-Aminobutyric acid type A receptors (GABA_ARs) mediate both inhibitory and excitatory neurotransmission, as the passive fluxes of Cl⁻ and HCO₃⁻ via pores can be reversed by changes in the transmembrane concentration gradient of Cl⁻. The cation–chloride co-transporters (CCCs) are the primary systems for maintaining [Cl⁻]_i homeostasis. However, despite extensive electrophysiological data obtained in vitro that are supported by a wide range of molecular biological studies on the expression patterns and properties of CCCs, the presence of ontogenetic changes in [Cl⁻]_i—along with the consequent shift in GABA reversal potential—remain a subject of debate. Recent studies showed that the β3 subunit possesses properties of the P-type ATPase that participates in the ATP-consuming movement of Cl⁻ via the receptor. Moreover, row studies have demonstrated that the β3 subunit is a key player in GABA_AR performance and in the appearance of serious neurological disorders. In this review, we discuss the properties and driving forces of CCCs and Cl⁻, HCO₃⁻ATPase in the maintenance of [Cl⁻]_i homeostasis after changes in upcoming GABA_AR function. Moreover, we discuss the contribution of the β3 subunit in the manifestation of epilepsy, autism, and other syndromes.

Expression and purification of a functional heteromeric GABAA receptor for structural studies

The GABA-gated chloride channels of the Cys-loop receptor family, known as GABAA receptors, function as the primary gatekeepers of fast inhibitory neurotransmission in the central nervous system. Formed by the pentameric arrangement of five identical or homologous subunits, GABAA receptor subtypes are defined by the subunit composition that shape ion channel properties. An understanding of the structural basis of distinct receptor properties has been hindered by the absence of high resolution structural information for heteromeric assemblies. Robust heterologous expression and purification protocols of high expressing receptor constructs are vital for structural studies. Here, we describe a unique approach to screen for well-behaving and functional GABAA receptor subunit assemblies by using the Xenopus oocyte as an expression host in combination with fluorescence detection size exclusion chromatography (FSEC). To detect receptor expression, GFP fusions were introduced into the α1 subunit isoform. In contrast to expression of α1 alone, co-expression with the β subunit promoted formation of monodisperse assemblies. Mutagenesis experiments suggest that the α and β subunits can tolerate large truncations in the non-conserved M3/M4 cytoplasmic loop without compromising oligomeric assembly or GABA-gated channel activity, although removal of N-linked glycosylation sites is negatively correlated with expression level. Additionally, we report methods to improve GABAA receptor expression in mammalian cell culture that employ recombinant baculovirus transduction. From these methods we have identified a well-behaving minimal functional construct for the α1/β1 GABAA receptor subtype that can be purified in milligram quantities while retaining high affinity agonist binding activity.

Cell surface expression of homomeric GABAA receptors depends on single residues in subunit transmembrane domains

Cell surface expression of type A GABA receptors (GABA_ARs) is a critical determinant of the efficacy of inhibitory neurotransmission. Pentameric GABA_ARs are assembled from a large pool of subunits according to precise co-assembly rules that limit the extent of receptor structural diversity. These rules ensure that particular subunits, such as ρ1 and β3, form functional cell surface ion channels when expressed alone in heterologous systems, whereas other brain-abundant subunits, such as α and γ, are retained within intracellular compartments. Why some of the most abundant GABA_AR subunits fail to form homomeric ion channels is unknown. Normally, surface expression of α and γ subunits requires co-assembly with β subunits via interactions between their N-terminal sequences in the endoplasmic reticulum. Here, using molecular biology, imaging, and electrophysiology with GABA_AR chimeras, we have identified two critical residues in the transmembrane domains of α and γ subunits, which, when substituted for their ρ1 counterparts, permit cell surface expression as homomers. Consistent with this, substitution of the ρ1 transmembrane residues for the α subunit equivalents reduced surface expression and altered channel gating, highlighting their importance for GABA_AR trafficking and signaling. Although not ligand-gated, the formation of α and γ homomeric ion channels at the cell surface was revealed by incorporating a mutation that imparts the functional signature of spontaneous channel activity. Our study identifies two single transmembrane residues that enable homomeric GABA_AR subunit cell surface trafficking and demonstrates that α and γ subunits can form functional ion channels.

Cognate Ligand Chaperoning: a Novel Mechanism for the Post-translational Regulation of Neurotransmitter Receptor Biogenesis

The functional unit for inter-neuronal communication in the central nervous system is the neuronal synapse. The number of postsynaptic neurotransmitter receptors at the cell surface is an important determinant of synaptic efficacy and plasticity. A diverse array of post-translational processes regulate postsynaptic receptor number, including receptor exocytosis, lateral diffusion, surface stabilization, endocytosis, and recycling, thus highlighting the importance of mechanisms that control postsynaptic receptor levels. Another putative post-translational mechanism for regulating receptor surface expression is cognate ligand chaperoning. It has been proposed that neurotransmitters function as cognate ligand chaperones by binding, within the endoplasmic reticulum (ER) lumen, to their nascent neurotransmitter receptors and facilitating receptor biogenesis. Here we discuss proof-of-concept evidence that small molecules can selectively facilitate the biogenesis of their targets and examine the specific evidence in support of cognate ligand chaperoning of neurotransmitter receptor biogenesis.

Binding site opening by loop C shift and chloride ion-pore interaction in the GABAAreceptor model

Molecular dynamics simulations of the shut α₁β₂γ₂GABA_Aheteropentamer receptor homology model reveal significant differences between intersubunit interfaces (ligand binding G1, G2 and non-binding) compared to homomeric receptor assemblies and possible ion interaction sites in the top part of the transmembrane domain (TMD).

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

Prolonged inhibition of 5-HT₃ receptors by palonosetron results from surface receptor inhibition rather than inducing receptor internalization

BACKGROUND AND PURPOSE

The 5-HT₃ receptor antagonist palonosetron is an important treatment for emesis and nausea during cancer therapy. Its clinical efficacy may result from its unique binding and clearance characteristics and receptor down-regulation mechanisms. We investigated the mechanisms by which palonosetron exerts its long-term inhibition of 5-HT₃ receptors for a better understanding of its clinical efficacy.

EXPERIMENTAL APPROACH

Cell surface receptors (recombinantly expressed 5HT₃A or 5HT₃AB in COS-7 cells) were monitored using [³H]granisetron binding and ELISA after exposure to palonosetron. Receptor endocytosis was investigated using immunofluorescence microscopy.

KEY RESULTS

Chronic exposure to palonosetron reduced the number of available cell surface [³H]granisetron binding sites. This down-regulation was not sensitive to either low temperature or pharmacological inhibitors of endocytosis (dynasore or nystatin) suggesting that internalization did not play a role. This was corroborated by our observation that there was no change in cell surface 5-HT₃ receptor levels or increase in endocytic rate. Palonosetron exhibited slow dissociation from the receptor over many hours, with a significant proportion of binding sites being occupied for at least 4 days. Furthermore, our observations suggest that chronic receptor down-regulation involved interactions with an allosteric binding site.

CONCLUSIONS AND IMPLICATIONS

Palonosetron acts as a pseudo-irreversible antagonist causing prolonged inhibition of 5-HT₃ receptors due to its very slow dissociation. In addition, an irreversible binding mode persists for at least 4 days. Allosteric receptor interactions appear to play a role in this phenomenon.

Time- and behavioral state-dependent changes in posterior hypothalamic GABAA receptors contribute to the regulation of sleep

Sleep-wake behavior is regulated by a circadian rhythm, homeostatically and by additional mechanisms that determine the timing of slow-wave sleep and rapid eye movement sleep (REMS) episodes. The posterior hypothalamus coordinates the neural and humoral signals with the rest-activity cycle. It contains wake-active neurons, and is a site where stimulation of inhibitory GABA_A receptors promotes sleep, whereas their antagonism enhances wakefulness. We explored whether GABAergic mechanisms present in the posterior hypothalamus contribute to the homeostatic and other aspects of sleep-wake regulation. Using micropunches of tissue extracted from either the perifornical (PF) or dorsomedial (DM) regions of the posterior hypothalamus of rats, we determined that mRNA levels for selected subunits of GABA_A receptors (β1, β3 and ε) were higher at the end of the active period or following sleep deprivation, when the need for sleep is high, than after several hours of sleep, when sleep need is partially fulfilled. Such a pattern was present in the PF region only, and was consistent with changes in β1 subunit and GABA synthesizing enzyme (GAD) protein levels. In contrast, in the DM region, the levels of GABA_A receptor subunit mRNAs and proteins (α1, α2, β1) and GAD varied with circadian time, but were not responsive to sleep deprivation. Separate experiments with sleep-wake monitoring and local perfusion of the PF region with the GABA_A receptor antagonist bicuculline revealed that the antagonist had a weaker sleep-reducing effect when sleep need was enhanced by sleep deprivation and that the increased amount of REMS characteristic of the late sleep period was dependent on endogenous GABAergic inhibition. These results support the concept that a varying magnitude of GABAergic inhibition exerted within the PF region contributes to the homeostatic regulation of sleep and shapes its temporal pattern, whereas GABAergic mechanisms in the DM region contribute to circadian regulation.

Sergeeva O. Partial agonism of taurine at gamma-containing native and recombinant GABAA receptors

Taurine is a semi-essential sulfonic acid found at high concentrations in plasma and mammalian tissues which regulates osmolarity, ion channel activity and glucose homeostasis. The structural requirements of GABA_A-receptors (GABA_AR) gated by taurine are not yet known. We determined taurine potency and efficacy relative to GABA at different types of recombinant GABA_AR occurring in central histaminergic neurons of the mouse hypothalamic tuberomamillary nucleus (TMN) which controls arousal. At binary α_1/2β_1/3 receptors taurine was as efficient as GABA, whereas incorporation of the γ_1/2 subunit reduced taurine efficacy to 60–90% of GABA. The mutation γ_2F77I, which abolishes zolpidem potentiation, significantly reduced taurine efficacy at recombinant and native receptors compared to the wild type controls. As taurine was a full- or super- agonist at recombinant α_xβ₁δ-GABA_AR, we generated a chimeric γ₂ subunit carrying the δ subunit motif around F77 (MTVFLH). At α_1/2β₁γ_2(MTVFLH) receptors taurine became a super-agonist, similar to δ-containing ternary receptors, but remained a partial agonist at β₃-containing receptors. In conclusion, using site-directed mutagenesis we found structural determinants of taurine’s partial agonism at γ-containing GABA_A receptors. Our study sheds new light on the β₁ subunit conferring the widest range of taurine-efficacies modifying GABA_AR function under (patho)physiological conditions.

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.

A tight coupling between β₂Y97 and β₂F200 of the GABA(A) receptor mediates GABA binding

The GABA(A) receptor is an oligopentameric chloride channel that is activated via conformation changes induced upon the binding of the endogenous ligand, GABA, to the extracellular inter-subunit interfaces. Although dozens of amino acid residues at the α/β interface have been implicated in ligand binding, the structural elements that mediate ligand binding and receptor activation are not yet fully described. In this study, double-mutant cycle analysis was employed to test for possible interactions between several arginines (α₁R67, α₁R120, α₁R132, and β₂R207) and two aromatic residues (β₂Y97 and β₂F200) that are present in the ligand-binding pocket and are known to influence GABA affinity. Our results show that neither α₁R67 nor α₁R120 is functionally coupled to either of the aromatics, whereas a moderate coupling exists between α₁R132 and both aromatic residues. Significant functional coupling between β₂R207 and both β₂Y97 and β₂F200 was found. Furthermore, we identified an even stronger coupling between the two aromatics, β₂Y97 and β₂F200, and for the first time provided direct evidence for the involvement of β₂Y97 and β₂F200 in GABA binding. As these residues are tightly linked, and mutation of either has similar, severe effects on GABA binding and receptor kinetics, we believe they form a single functional unit that may directly coordinate GABA.

Three arginines in the GABAA receptor binding pocket have distinct roles in the formation and stability of agonist- versus antagonist-bound complexes

Binding of the agonist GABA to the GABA(A) receptor causes channel gating, whereas competitive antagonists that bind at the same site do not. The details of ligand binding are not well understood, including which residues interact directly with ligands, maintain the structure of the binding pocket, or transduce the action of binding into opening of the ion channel gate. Recent work suggests that the amine group of the GABA molecule may form a cation-π bond with residues in a highly conserved "aromatic box" within the binding pocket. Although interactions with the carboxyl group of GABA remain unknown, three positively charged arginines (α(1)Arg67, α(1)Arg132, and β(2)Arg207) just outside of the aromatic box are likely candidates. To explore their roles in ligand binding, we individually mutated these arginines to alanine and measured the effects on microscopic ligand binding/unbinding rates and channel gating. The mutations α(1)R67A or β(2)R207A slowed agonist binding and sped unbinding with little effect on gating, demonstrating that these arginines are critical for both formation and stability of the agonist-bound complex. In addition, α(1)R67A sped binding of the antagonist 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide (SR-95531), indicating that this arginine poses a barrier to formation of the antagonist-bound complex. In contrast, β(2)R207A and α(1)R132A sped antagonist unbinding, indicating that these arginines stabilize the antagonist-bound state. α(1)R132A also conferred a new long-lived open state, indicating that this arginine influences the channel gate. Thus, each of these arginines plays a unique role in determining interactions with agonists versus antagonists and with the channel gate.

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.

A state-dependent salt-bridge interaction exists across the β/α intersubunit interface of the GABAA receptor

The GABA(A) receptor is a multisubunit protein that transduces the binding of a neurotransmitter at an intersubunit interface into the opening of a central ion channel. The structural components that mediate the steps involved in this action are poorly defined. A large amount of work has focused on clarifying the specific functions and interactions of residues believed to surround the GABA binding pocket. Here, we explored two charged residues (β(2)Asp163 and α(1)Arg120), which have been suggested by homology models to participate in a salt-bridge interaction. When mutated to alanine, both single mutants, as well as the double mutant, increase EC(50-GABA), decrease the GABA binding rate, and accelerate deactivation and GABA unbinding rates. Double-mutant cycle analysis demonstrates that the effects of each alanine mutation on the GABA binding rate were additive and independent. In contrast, a significant coupling energy was found during an analysis of deactivation time constants. Using kinetic modeling, we further demonstrated that the GABA unbinding rates, in particular, are strongly coupled. These data suggest that β(2)Asp163 and α(1)Arg120 form a state-dependent salt bridge, interacting when GABA is bound to the receptor but not when the receptor is in the unbound state.

GABA(A) receptor alpha1 subunit mutation A322D associated with autosomal dominant juvenile myoclonic epilepsy reduces the expression and alters the composition of wild type GABA(A) receptors

A GABA(A) receptor (GABA(A)R) alpha1 subunit mutation, A322D (AD), causes an autosomal dominant form of juvenile myoclonic epilepsy (ADJME). Previous studies demonstrated that the mutation caused alpha1(AD) subunit misfolding and rapid degradation, reducing its total and surface expression substantially. Here, we determined the effects of the residual alpha1(AD) subunit expression on wild type GABA(A)R expression to determine whether the AD mutation conferred a dominant negative effect. We found that although the alpha1(AD) subunit did not substitute for wild type alpha1 subunits on the cell surface, it reduced the surface expression of alpha1beta2gamma2 and alpha3beta2gamma2 receptors by associating with the wild type subunits within the endoplasmic reticulum and preventing them from trafficking to the cell surface. The alpha1(AD) subunit reduced surface expression of alpha3beta2gamma2 receptors by a greater amount than alpha1beta2gamma2 receptors, thus altering cell surface GABA(A)R composition. When transfected into cultured cortical neurons, the alpha1(AD) subunit altered the time course of miniature inhibitory postsynaptic current kinetics and reduced miniature inhibitory postsynaptic current amplitudes. These findings demonstrated that, in addition to causing a heterozygous loss of function of alpha1(AD) subunits, this epilepsy mutation also elicited a modest dominant negative effect that likely shapes the epilepsy phenotype.

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.

GABA(A) Receptor Dynamics and Constructing GABAergic Synapses

GABA_A receptors are located on the majority of neurons in the central and peripheral nervous system, where they mediate important actions of the neurotransmitter gamma-aminobutyric acid. Early in development the trophic properties of GABA allow a healthy development of the nervous system. Most neurons have a high intracellular Cl-concentration early in life due to the late functional expression of the Cl-pump KCC2, therefore GABA has excitatory effects at this stage. Upon higher expression and activation of KCC2 GABA takes on its inhibitory effects while glutamate functions as the major excitatory neurotransmitter. Like all multisubunit membrane proteins the GABA_A receptor is assembled in the ER and travels through the Golgi and remaining secretory pathway to the cell surface, where it mediates GABA actions either directly at the synapses or at extrasynaptic sites responding to ambient GABA to provide a basal tonic inhibitory state. In order to adapt to changing needs and information states, the GABAergic system is highly dynamic. That includes subtype specific trafficking to different locations in the cell, regulation of mobility by interaction with scaffold molecules, posttranslational modifications, that either directly affect channel function or the interaction with other proteins and finally the dynamic exchange between surface and intracellular receptor pools, that either prepare receptors for recycling to the surface or degradation. Here we give an overview of the current understanding of GABA_A receptor functional and molecular dynamics that play a major part in maintaining the balance between excitation and inhibition and in changes in network activity.

Mechanisms of reversible GABAA receptor plasticity after ethanol intoxication

The time-dependent effects of ethanol (EtOH) intoxication on GABA(A) receptor (GABA(A)R) composition and function were studied in rats. A cross-linking assay and Western blot analysis of microdissected CA1 area of hippocampal slices obtained 1 h after EtOH intoxication (5 g/kg, gavage), revealed decreases in the cell-surface fraction of alpha4 and delta, but not alpha1, alpha5, or gamma2 GABA(A)R subunits, without changes in their total content. This was accompanied (in CA1 neuron recordings) by decreased magnitude of the picrotoxin-sensitive tonic current (I(tonic)), but not miniature IPSCs (mIPSCs), and by reduced enhancement of I(tonic) by EtOH, but not by diazepam. By 48 h after EtOH dosing, cell-surface alpha4 (80%) and gamma2 (82%) subunit content increased, and cell-surface alpha1 (-50%) and delta (-79%) and overall content were decreased. This was paralleled by faster decay of mIPSCs, decreased diazepam enhancement of both mIPSCs and I(tonic), and paradoxically increased mIPSC responsiveness to EtOH (10-100 mm). Sensitivity to isoflurane- or diazepam-induced loss of righting reflex was decreased at 12 and 24 h after EtOH intoxication, respectively, suggesting functional GABA(A)R tolerance. The plastic GABA(A)R changes were gradually and fully reversible by 2 weeks after single EtOH dosing, but unexplainably persisted long after withdrawal from chronic intermittent ethanol treatment, which leads to signs of alcohol dependence. Our data suggest that early tolerance to EtOH may result from excessive activation and subsequent internalization of alpha4betadelta extrasynaptic GABA(A)Rs. This leads to transcriptionally regulated increases in alpha4 and gamma2 and decreases in alpha1 subunits, with preferential insertion of the newly formed alpha4betagamma2 GABA(A)Rs at synapses.

Generation and functional characterization of fluorescent, N-terminally tagged CB1 receptor chimeras for live-cell imaging

N-terminally tagged CB1 receptor fusion proteins, incorporating enhanced green fluorescent protein (GFP) or super-ecliptic pHluorin (SEP), were generated to study CB1 receptor trafficking and cell surface receptor expression in live COS7 and HEK293 cells and hippocampal neurons. An artificial signal sequence (SS) was required for efficient surface expression of CB1 receptor chimeras, which behaved like wild-type CB1 receptors in functional assays. Treatment with cannabinoid ligands led to a rapid down-regulation of SS-GFP-CB1 from the plasma membrane in COS7 and HEK293 cells, associated with trafficking into cytosolic vesicles. Activation of CB1 receptors was also linked with a time-dependent reduction in cell surface SEP-CB1 fluorescence and incorporation of the construct into acidic endosomes, revealed following exposure to NH4Cl. In live hippocampal neurons, SEP-CB1 fluorescence was largely restricted to the axon, consistent with its polarised surface expression. Thus, these new molecular tools are well suited for studying CB1 receptor trafficking and a new generation of GPCR chimeras incorporating SEP at the N-terminus will be especially useful for monitoring dynamic changes in cell surface receptor expression in living cells.

A gamma 2(R43Q) mutation, linked to epilepsy in humans, alters GABAA receptor assembly and modifies subunit composition on the cell surface

Genetic defects leading to epilepsy have been identified in gamma2 GABA(A) receptor subunit. A gamma2(R43Q) substitution is linked to childhood absence epilepsy and febrile seizure, and a gamma2(K289M) mutation is associated with generalized epilepsy with febrile seizures plus. To understand the effect of these mutations, surface targeting of GABA(A) receptors was analyzed by subunit-specific immunofluorescent labeling of living cells. We first transfected hippocampal neurons in culture with recombinant gamma2 constructs and showed that the gamma 2(R43Q) mutation prevented surface expression of the subunit, unlike gamma2(K289M) substitution. Several gamma2-subunit constructs, bearing point mutations within the Arg-43 domain, were expressed in COS-7 cells with alpha3- and beta3-subunits. R43Q and R43A substitutions dramatically reduced surface expression of the gamma2-subunit, whereas R43K, P44A, and D39A substitutions had a lesser, but still significant, impact and K289M substitution had no effect. Whereas the mutant gamma2(R43Q) was retained within intracellular compartments, alphabeta complexes were still targeted at the cell membrane. Coimmunoprecipitation experiments showed that gamma2(R43Q) was able to associate with alpha3- or beta3-subunits, although the stoichiometry of the complex with alpha3 was altered. Our data show that gamma2(R43Q) is not a dominant negative and that the mutation leads to a modification of GABA(A) receptor subunit composition on the cell surface that impairs the synaptic targeting in neurons. This study reveals an involvement of the gamma2-Arg-43 domain in the control of receptor assembly that may be relevant to the effect of the heterozygous gamma2(R43Q) mutation leading to childhood absence epilepsy and febrile seizure.

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.

Transient change in GABA(A) receptor subunit mRNA expression in Lurcher cerebellar nuclei during Purkinje cell degeneration

Background

Lurcher mice suffer from a complete Purkinje cell (PC) loss in the first four postnatal weeks. Parallel to this degeneration, GABAergic synapses in the deep cerebellar nuclei (DCN), the major recipient of the inhibitory PC projection, increase synaptic conductance. Here, we further investigated this phenomenon, using real-time RT-PCR to assess GABA_A receptor subunit gene expression during PC degeneration.

Results

We observed a specific reduction in γ2 subunit gene expression, while α1–5, β1–2, γ1,3 and δ subunits were unaffected. We made two further specific findings. First, the difference in gene expression was shown in tissue from DCN only. Neither the hippocampus nor coronal sections through the forebrain showed such effects. Furthermore, the involvement of different levels of corticosterone, a possible humeral trigger for differences in gene expression, could be excluded. Second, like the known potentiation of GABAergic synapses, the γ2 down-regulation was present only after the onset of degeneration at p14. The difference in γ2 mRNA expression, however, appeared transient, since it was no longer detectable in adult Lurcher mice.

Conclusion

In conclusion, the down-regulation of γ2 subunits may be related to differences in synaptic efficacy and, as such, may reflect the initial phase of adaptive responses of DCN tissue to massive GABAergic deafferentation. Its transient course, however, does not support the idea that modulations in GABAergic transmission are at the basis of the well-known DCN-based functional benefit of Lurcher mice present throughout their life.

An asymmetric contribution to gamma-aminobutyric type A receptor function of a conserved lysine within TM2-3 of alpha1, beta2, and gamma2 subunits

Mutations that impair the expression and/or function of gamma-aminobutyric acid type A (GABAA) receptors can lead to epilepsy. The familial epilepsy gamma2(K289M) mutation affects a basic residue conserved in the TM2-3 linker of most GABAA subunits. We investigated the effect on expression and function of the Lys --> Met mutation in mouse alpha1(K278M), beta2(K274M), and gamma2(K289M) subunits. Compared with cells expressing wild-type and alpha1beta2gamma2(K289M) receptors, cells expressing alpha1(K278M)beta2gamma2 and alpha1beta2(K274M)gamma2 receptors exhibited reduced agonist-evoked current density and reduced GABA potency, with no change in single channel conductance. The low current density of alpha1beta2(K274M)gamma2 receptors coincided with reduced surface expression. By contrast the surface expression of alpha1(K278M)beta2gamma2 receptors was similar to wild-type and alpha1beta2gamma2(K289M) receptors suggesting that the alpha1(K278M) impairs function. In keeping with this interpretation GABA-activated channels mediated by alpha1(K278M)beta2gamma2 receptors had brief open times. To a lesser extent gamma2(K289M) also reduced mean open time, whereas beta2(K274M) had no effect. We used propofol as an alternative GABAA receptor agonist to test whether the functional deficits of mutant subunits were specific to GABA activation. Propofol was less potent as an activator of alpha1(K278M)beta2gamma2 receptors. By contrast, neither beta2(K274M) nor gamma2(K289M) affected the potency of propofol. The beta2(K274M) construct was unique in that it reduced the efficacy of propofol activation relative to GABA. These data suggest that the alpha1 subunit Lys-278 residue plays a pivotal role in channel gating that is not dependent on occupancy of the GABA binding site. Moreover, the conserved TM2-3 loop lysine has an asymmetric function in different GABAA subunits.

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

Comparative models of GABA(A) receptors composed of alpha1 beta3 gamma2 subunits were generated using the acetylcholine-binding protein (AChBP) as a template and were used for predicting putative engineered cross-link sites between the alpha1 and the gamma2 subunit. The respective amino acid residues were substituted by cysteines and disulfide bond formation between subunits was investigated on co-transfection into human embryonic kidney (HEK) cells. Although disulfide bond formation between subunits could not be observed, results indicated that mutations studied influenced assembly of GABA(A) receptors. Whereas residue alpha1A108 was important for the formation of assembly intermediates with beta3 and gamma2 subunits consistent with its proposed location at the alpha1(+) side of GABA(A) receptors, residues gamma2T125 and gamma2P127 were important for assembly with beta3 subunits. Mutation of each of these residues also caused an impaired expression of receptors at the cell surface. In contrast, mutated residues alpha1F99C, alpha1S106C or gamma2T126C only impaired the formation of receptors at the cell surface when co-expressed with subunits in which their predicted interaction partner was also mutated. These data are consistent with the prediction that the mutated residue pairs are located close to each other.

Endoplasmic reticulum retention and associated degradation of a GABAA receptor epilepsy mutation that inserts an aspartate in the M3 transmembrane segment of the alpha1 subunit

A GABA(A) receptor alpha1 subunit epilepsy mutation (alpha1(A322D)) introduces a negatively charged aspartate residue into the hydrophobic M3 transmembrane domain of the alpha1 subunit. We reported previously that heterologous expression of alpha1(A322D)beta2gamma2 receptors in mammalian cells resulted in reduced total and surface alpha1 subunit protein. Here we demonstrate the mechanism of this reduction. Total alpha1(A322D) subunit protein was reduced relative to wild type protein by a similar amount when expressed alone (86 +/- 6%) or when coexpressed with beta2 and gamma2S subunits (78 +/- 6%), indicating an expression reduction prior to subunit oligomerization. In alpha1beta2gamma2S receptors, endoglycosidase H deglycosylated only 26 +/- 5% of alpha1 subunits, consistent with substantial protein maturation, but in alpha1(A322D)beta2gamma2S receptors, endoglycosidase H deglycosylated 91 +/- 4% of alpha1(A322D) subunits, consistent with failure of protein maturation. To determine the cellular localization of wild type and mutant subunits, the alpha1 subunit was tagged with yellow (alpha1-YFP) or cyan (alpha1-CFP) fluorescent protein. Confocal microscopic imaging demonstrated that 36 +/- 4% of alpha1-YFPbeta2gamma2 but only 5 +/- 1% alpha1(A322D)-YFPbeta2gamma2 colocalized with the plasma membrane, whereas the majority of the remaining receptors colocalized with the endoplasmic reticulum (55 +/- 4% alpha1-YFPbeta2gamma2S, 86 +/- 3% alpha1(A322D)-YFP). Heterozygous expression of alpha1-CFPbeta2gamma2S and alpha1(A322D)-YFPbeta2gamma2S or alpha1-YFPbeta2gamma2S and alpha1(A322D)-CFPbeta2gamma2S receptors showed that membrane GABA(A) receptors contained primarily wild type alpha1 subunits. These data demonstrate that the A322D mutation reduces alpha1 subunit expression after translation, but before assembly, resulting in endoplasmic reticulum-associated degradation and membrane alpha1 subunits that are almost exclusively wild type subunits.

The epilepsy mutation, gamma2(R43Q) disrupts a highly conserved inter-subunit contact site, perturbing the biogenesis of GABAA receptors

Given the association of a gamma2 mutation (R43Q) with epilepsy and the reduced cell surface expression of mutant receptors, we investigated a role for this residue in alpha1beta2gamma2 receptor assembly when present in each subunit. Regardless of which subunit contained the mutation, mutant GABA(A) receptors assembled poorly into functional cell surface receptors. The low level of functional expression gives rise to reduced GABA EC50s (alpha1(R43Q)beta2gamma2 and alpha1beta2(R43Q)gamma2) or reduced benzodiazepine potentiation of GABA-evoked currents (alpha1beta2gamma2(R43Q)). We determined that a 15-residue peptide surrounding R43 is capable of subunit binding, with a profile that reflected the orientation of subunits in the pentameric receptor. Subunit binding is perturbed when the R43Q mutation is present suggesting that this residue is critical for the formation of inter-subunit contacts at (+) interfaces of GABAA subunits. Rather than being excluded from receptors, gamma2(R43Q) may form non-productive subunit interactions leading to a dominant negative effect on other receptor subtypes.

Distinct patterns of expression and regulation of GABA receptors containing the delta subunit in cerebellar granule and hippocampal neurons

Neuronal plasticity is achieved by regulation of the expression of genes for neurotransmitter receptors such as the type A receptor (GABA(A)R) for gamma-aminobutyric acid. We now show that two different rat neuronal populations in culture manifest distinct patterns of GABA(A)R plasticity in response to identical stimuli. Whereas prolonged exposure to ethanol had no effect on expression of the delta subunit of GABA(A)Rs at the mRNA or protein level in cerebellar granule neurons, it increased the abundance of delta subunit mRNA and protein in hippocampal neurons. Subsequent ethanol withdrawal transiently down-regulated delta subunit expression in cerebellar granule neurons and gradually normalized that in hippocampal neurons. These effects of ethanol exposure and withdrawal were accompanied by corresponding functional changes in GABA(A)Rs. GABA(A)Rs containing the delta subunit were also distributed differentially in the cerebellar and hippocampal neurons. These findings reveal complex and distinct mechanisms of regulation of the expression of GABA(A)Rs that contain the delta subunit in different neuronal types.

Genetic manipulations of GABAA receptor in mice make inhibition exciting

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in brain development and behavior. GABA(A) receptor subunits knock-out and knock-in mice have proven that GABA(A) receptors are involved in control of motor coordination, learning, and memory and play a role in anxiety, panic, and epileptogenesis. In addition, these receptors are involved in the molecular mechanisms of action of many drugs and participate actively in cortical plasticity. The use of genetically engineered mice has perhaps never been as successful as in understanding the importance of the heterogeneity of GABA(A) receptors. We review these findings and speculate on the new directions that the use of mice with altered expression of GABA(A) receptor subunits may provide.

Variability in the benzodiazepine response of serotonin 5-HT1A receptor null mice displaying anxiety-like phenotype: evidence for genetic modifiers in the 5-HT-mediated regulation of GABA(A) receptors

Benzodiazepines (BZs) acting as modulators of GABA(A) receptors (GABA(A)Rs) are an important group of drugs for the treatment of anxiety disorders. However, a large inter-individual variation in BZ sensitivity occurs in the human population with some anxiety disorder patients exhibiting diminished sensitivity to BZ and reduced density of GABA(A)Rs. The mechanism underlying BZ treatment resistance is not known, and it is not possible to predict whether an anxiety patient will respond to BZ. 5-hydroxytryptamine1A receptor (5-HT1AR) null mice (R-/-) on the Swiss-Webster (SW) background reproduce several features of BZ-resistant anxiety; they exhibit anxiety-related behaviors, do not respond to BZ, have reduced BZ binding, and have decreased expression of the major GABA(A)R subunits alpha1 and alpha2. Here, we show that R-/- mice on the C57Bl6 (B6) background also have anxiety phenotype, but they respond to BZ and have normal GABA(A)R subunit expression. This indicates that the 5-HT1AR-mediated regulation of GABA(A)R alpha subunit expression is subject to genetic modification. Hybrid SW/B6-R-/- mice also exhibit BZ-resistant anxiety, suggesting that SW mice carry a genetic modifier, which mediates the effect of the 5-HT1AR on the expression of GABA(A)Ralpha subunits. In addition, we show that this genetic interaction in SW mice operates early in postnatal life to influence the expression of GABA(A)R alpha subunits at the transcriptional level. These data indicate that BZ-resistant anxiety results from a developmental arrest of GABA(A)R expression in SW-R-/- mice, and a similar mechanism may be responsible for the BZ insensitivity of some anxiety patients.

Compounds exhibiting selective efficacy for different beta subunits of human recombinant gamma-aminobutyric acid A receptors

Inhibitory GABA(A) receptor modulators are widely used therapeutic agents for a variety of central nervous system disorders. Ltk(-) cells stably expressing human recombinant GABA(A) subunits (alpha1beta1-3gamma2s) were seeded into 96-well plates, loaded with chlorocoumarin-2-dimyristoyl phosphatidylethanolamine and bis(1,3-diethyl-2-thiobarbiturate)trimethineoxonol, and rapid fluorescence resonance energy transfer technique (FRET) measurements were made of GABA-evoked depolarizations in low-Cl(-) buffer using a voltage/ion probe reader. The influence of different betasubunits on the ability of agents to modulate and directly activate the ion channel was examined. GABA evoked concentration-dependent decreases in FRET, increasing fluorescence emission ratio (460/580 nm) at alpha1beta1gamma2, alpha1beta2gamma2, and alpha1beta3gamma2 receptors with similar maximal amplitude (P > 0.05, n = 17) and EC(50) values of 2.4 +/- 0.2, 2.5 +/- 0.2, and 1.3 +/- 0.1 microM, respectively. Piperidine-4-sulfonic acid and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol were less potent, with EC(50) values of 8.7 +/- 0.9, 9.2 +/- 0.5, and 11.7 +/- 1.2, and 43.7 +/- 6.4, 24.8 +/- 1.6, and 26.1 +/- 2.4 microM, respectively. Potency and maximal efficacy of propofol, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, pentobarbital, and steroids, 5alpha-pregnan-3alpha-ol-20-one and 5beta-pregnan-3alpha-ol-20-one, were unaffected by the beta isoform present in the receptor complex. However, several compounds displayed beta2/3 subunit selectivity, notably loreclezole, R(-)-etomidate, and a group of anti-inflammatory agents including mefenamic acid, flufenamic acid, meclofenamic acid, tolfenamic acid, niflumic acid, and diflunisal. The anti-inflammatories exhibited varying levels of efficacy at beta2/3 subunits, with micromolar potency, while having antagonist or weak inverse agonist profiles at alpha1beta1gamma2. Diflunisal was the most efficacious compound, eliciting greater potentiation than loreclezole (90 +/- 14% and 109 +/- 14% at beta3 and beta2, respectively, compared with 62 +/- 6% and 56 +/- 3%), whereas niflumic acid exhibited the lowest efficacy. An additional agent, olsalazine, weakly potentiated responses at all three receptors without any selectivity. This study identifies and characterizes a variety of allosteric modulators for which betasubunits are an important determinant of efficacy and potency.

Expression of distinct alpha subunits of GABAA receptor regulates inhibitory synaptic strength

Distinct alpha subunit subtypes in the molecular assembly of GABA(A) receptors are a critical determinant of the functional properties of inhibitory synapses and their modulation by a range of pharmacological agents. We investigated the contribution of these subunits to the developmental changes of inhibitory synapses in cerebellar granule neurons in primary cultures from wild-type and alpha1 subunit -/- mice. The decay time of miniature inhibitory postsynaptic currents (mIPSCs) halved between 6 days in vitro (DIV6) and DIV12. This was paralleled by the decrease of alpha2 and alpha3 subunits, the increase of alpha1 and alpha6 subunits expression at synapses, and changes in the action of selective alpha subunit modulators. A small but significant shortening of mIPSCs was observed with development in cells from -/- mice together with a decrease in the expression of alpha3 subunit. In contrast, the expression of alpha2 subunit at inhibitory synapses in -/- cells was significantly higher than in +/+ cells at DIV11-12. alpha5 subunit was not detected, and increased sensitivity to a selective alpha4/alpha6 subunit agonist suggests increased expression of extrasynaptic receptors in -/- mice. beta2/beta3 subunit expression and loreclezole sensitivity increased with development in +/+ but not in -/- cells, supporting the preferential association of the alpha1 with the beta2 subunit. Synaptic charge transfer strongly decreased with development but was not different between cells in the +/+ and -/- groups until DIV11-12. Our results uncover a pattern of sequential expression of alpha subunits underlying the changes in functional efficacy of GABAergic networks with development.

Expression of functional receptors by the human gamma-aminobutyric acid A gamma 2 subunit

gamma-Aminobutyric acid A (GABA(A)) receptors are heteromeric membrane proteins formed mainly by various combinations of alpha, beta, and gamma subunits; and it is commonly thought that the gamma 2 subunit alone does not form functional receptors. In contrast, we found that cDNA encoding the gamma 2L subunit of the human GABA(A) receptor, injected alone into Xenopus oocytes, expressed functional GABA receptors whose properties were investigated by using the two-microelectrode voltage-clamp technique. GABA elicited desensitizing membrane currents that recovered after a few minutes' wash. Repetitive applications of GABA induced a "run-up" of GABA currents that nearly doubled the amplitude of the first response. The GABA currents inverted direction at about -30 mV, indicating that they are carried mainly by Cl(-) ions. The homomeric gamma 2L receptors were also activated by beta-alanine > taurine > glycine, and, like some types of heteromeric GABA(A) receptors, the gamma 2L receptors were blocked by bicuculline and were potentiated by pentobarbital and flunitrazepam. These results indicate that the human gamma 2L subunit is capable of forming fully functional GABA receptors by itself in Xenopus oocytes and suggest that the roles proposed for the various subunits that make up the heteromeric GABA(A) receptors in situ require further clarification.

Serine 171, a conserved residue in the gamma-aminobutyric acid type A (GABAA) receptor gamma2 subunit, mediates subunit interaction and cell surface localization

Serine 171 in the GABA(A) receptor gamma2 subunit is highly conserved in the ligand-gated ion channel superfamily. In this paper, we report that mutating serine 171 within gamma2 to glycine or cysteine prevents the interaction of gamma2 with alpha2 and beta1 when these subunits are co-expressed in human embryo kidney 293 cells, resulting in intracellular retention of gamma2. Structure analysis based on a three-dimensional homology model of gamma2 (Ernst, M., Brauchart, D., Boresch, S., and Sieghart, W. (2003) Neuroscience 119, 933-943) reveals that serine 171 may play a critical role in the formation and stabilization of an exposed turn structure that is part of the subunit interaction site. Mutation of serine 171 in the gamma2 subunit could therefore result in alteration of the structure of the subunit interaction site, preventing correct subunit assembly.

Cell surface expression of 5-hydroxytryptamine type 3 receptors is controlled by an endoplasmic reticulum retention signal

Two subunits of the 5-hydroxytryptamine type 3 (5-HT3) have been identified (5-HT3A and 5-HT3B) that assemble into homomeric (5-HT3A) and heteromeric (5-HT3A+5-HT3B) complexes. Unassembled 5-HT3B subunits are efficiently retained within the cell. In this study, we address the mechanism controlling the release of 5-HT3B from the endoplasmic reticulum (ER). An analysis of chimeric 5-HT3A receptor(R).5-HT3BR constructs suggests the presence of elements downstream of the first transmembrane domain of 5-HT3B subunits that inhibit cell surface expression. To investigate this possibility, truncated 5-HT3B subunits were constructed and assessed for their ability to access the cell surface in COS-7 and ts201 cells. Using this approach, we have identified the presence of an ER retention signal located within the first cytoplasmic loop between transmembrane domains I and II of 5-HT3B. Transplantation of this signal (CRAR) into the homologous region of 5-HT3A results in the ER retention of this subunit until rescued by co-assembly with wild-type 5-HT3A. The mutation of this ER retention signal in 5-HT3B (5-HT3BSGER) does not lead to cell surface expression, suggesting the presence of other signals or mechanisms to control the surface expression of 5-HT3BRs. The generation of truncated 5-HT3BSGER constructs confirmed that the CRAR signal does play an important role in the ER retention of 5-HT3B.