Intracellular retention of recombinant GABAB receptors

Positive regulation of GABA(B) receptors dually coupled to cyclic AMP by the allosteric agent CGP7930

The ability of 2,6 Di-tert-butyl-4-(-hydroxy-2,2-dimethyl-propyl)-phenol (CGP7930), a positive allosteric modulator of GABA(B) receptors, to regulate GABA(B) receptor-induced stimulation and inhibition of adenylyl cyclase activity in rat brain was investigated. In olfactory bulb granule cell layer and in frontal cortex, CGP7930 potentiated the stimulatory effects of (-)-baclofen and gamma-aminobutyric acid (GABA) on basal and corticotropin-releasing hormone-stimulated adenylyl cyclase activities, respectively. In these stimulatory responses, CGP7930 enhanced both agonist potencies and maximal effects. When GABA(B) receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity of frontal cortex was examined, CGP7930 increased the agonist potencies but failed to affect the maximal effect of (-)-baclofen and modestly increased that of GABA. Similar results were obtained for the inhibition of Ca(2+)/calmodulin-stimulated adenylyl cyclase in striatum and cerebellum. Western blot analysis of each membrane preparation showed the presence of GABA(B2) receptor subunit, a putative site of action of CGP7930. These data indicate that CGP7930 positively modulates brain GABA(B) receptors coupled to either stimulation or inhibition of cyclic AMP signalling.

Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent one of the largest gene families in the animal genome. These receptors can be classified into several groups based on the sequence similarity of their common heptahelical domain. The family 3 (or C) GPCRs are receptors for the main neurotransmitters glutamate and gamma-aminobutyric acid, for Ca(2+), for sweet and amino acid taste compounds, and for some pheromone molecules, as well as for odorants in fish. Although none of these family 3 receptors have been found in plants, members have been identified in ancient organisms, such as slime molds (Dictyostelium) and sponges. Like any other GPCRs, family 3 receptors possess a transmembrane heptahelical domain responsible for G-protein activation. However, most of these identified receptors also possess a large extracellular domain that is responsible for ligand recognition, is structurally similar to bacterial periplasmic proteins involved in the transport of small molecules, and is called a Venus Flytrap module. The recent resolution of the structure of this binding domain in one of these receptors, the metabotropic glutamate 1 receptor, together with the recent demonstration that these receptors are dimers, revealed a unique mechanism of activation for these GPCRs. Such data open new possibilities in the development of drugs aimed at modulating these receptors, and raise a number of interesting questions on the activation mechanism of the other GPCRs.

Pharmacological characterisation of a cell line expressing GABA B1b and GABA B2 receptor subunits

The gamma-aminobutyric acid (GABA(B)) receptor has been shown to be a heterodimer consisting of two receptor subunits, GABA(B1) and GABA(B2). We have stably co-expressed these two subunits in a CHO cell line, characterised its pharmacology and compared it to the native receptor in rat brain membranes. Radioligand binding using [3H]CGP54626A demonstrated a similar rank order of potency between recombinant and native receptors: CGP62349>CGP54626A>SCH 50911>3-aminopropylphosphinicacid(3-APPA)>GABA>baclofen>saclofen>phaclofen. However, differences were observed in the affinity of agonists, which were higher at the native receptor, suggesting that in the recombinant system a large number of the receptors were in the low agonist affinity state. In contrast, [35S]GTPgammaS binding studies did not show any differences between recombinant and native receptors with the full agonists GABA and 3-APPA. Measurement of cAMP accumulation in the cells revealed a degree of endogenous coupling of the receptors to G-proteins. This is most likely to be due to the high expression levels of receptors (B(max)=22.5+/-2.5pmol/mg protein) in this experimental system. There was no evidence of GABA(B2) receptors, when expressed alone, binding [3H]CGP54626A, [3H]GABA, [3H]3-APPA nor of GABA having any effect on basal [35S]GTPgammaS binding or cAMP levels.

Mechanisms of GABAA receptor assembly and trafficking: implications for the modulation of inhibitory neurotransmission

Fast synaptic inhibition in the brain is largely mediated by ionotropic GABA receptors, which can be subdivided into GABAA and GABAC receptors based on pharmacological and molecular criteria. GABAA receptors are important therapeutic targets for a range of sedative, anxiolytic, and hypnotic agents and are implicated in several diseases including epilepsy, anxiety, depression, and substance abuse. In addition, modulating the efficacy of GABAergic neurotransmission may play a key role in neuronal plasticity. Recent studies have begun to reveal that the accumulation of ionotropic GABAA receptors at synapses is a highly regulated process that is facilitated by receptor-associated proteins and other cell-signaling molecules. This review focuses on recent experimental evidence detailing the mechanisms that control the assembly and transport of functional ionotropic GABAA receptors to cell surface sites, in addition to their stability at synaptic sites. These regulatory processes will be discussed within the context of the dynamic modulation of synaptic inhibition in the central nervous system (CNS).

G-protein-coupled receptor oligomerization and its potential for drug discovery

G-protein-coupled receptors (GPCRs) represent by far the largest class of targets for modern drugs. Virtually all therapeutics that are directed towards GPCRs have been designed using assays that presume that these receptors are monomeric. The recent realization that these receptors form homo-oligomeric and hetero-oligomeric complexes has added a new dimension to rational drug design. However, this important aspect of GPCR biology remains largely unincorporated into schemes to search for new therapeutics. This review provides a synopsis of the current thinking surrounding GPCR homo-oligomerization and hetero-oligomerization and shows how new models point towards unexplored avenues in the development of new therapies.

Heterodimerization of gamma-aminobutyric acid B receptor subunits as revealed by the yeast two-hybrid system

Several lines of evidence suggested that the first gamma-aminobutyric acid B receptor to be cloned required an additional factor for functional expression. GABA(B1) was retained within the endoplasmic reticulum and failed to couple to signal transduction pathways on stimulation with agonists. In radioligand binding experiments it was found that although the affinity of antagonists showed a close agreement between rat brain membranes and membranes expressing the cloned receptor, agonist ligands were significantly weaker at recombinant receptors. Using the C-terminal tail as bait, a yeast two-hybrid screen was run against a human brain cDNA library and identified a second receptor, GABA(B2), as a major interacting protein. This interaction was confirmed by coimmunoprecipitation as well as extensive colocalization studies. Coexpression of the two seven-transmembrane proteins generated a fully functional receptor, which was expressed at the cell surface confirming the importance of receptor heterodimerization for GABA(B) receptor activity.

Gamma-aminobutyric acidB receptors and the development of the ventromedial nucleus of the hypothalamus

Gamma-aminobutyric acid (GABA) is a highly abundant neurotransmitter in the brain and the ligand for GABA(A), GABA(B), and GABA(C) receptors. Unlike GABA(A) and GABA(C) receptors, which are chloride channels, GABA(B) receptors are G-protein linked and alter cell-signaling pathways. Electrophysiological studies have found GABA(B) receptors in cultured embryonic hypothalamus, but the distribution of these receptors remains unknown. In the present study, we examined the expression of GABA(B) receptors in the ventromedial nucleus of the hypothalamus (VMH) during embryonic mouse development. GABA(B) receptors were present in the VMH at all ages examined, from embryonic day 13 to postnatal day 6. Using a brain slice preparation, we examined the effect of GABA(B) receptor activation on cell movement in the embryonic VMH as the nucleus forms in vitro. The GABA(B) receptor agonist baclofen decreased the rate of cell movement in a dose-dependent manner. Baclofen reduced cell movement by up to 56% compared with vehicle-treated controls. The percentage of cells moving per field and the angles of cell movement were not affected. With our previous findings of GABA(A) receptor activation, it is likely that GABA influences VMH development via multiple mechanisms.

Expression and distribution of metabotropic GABA receptor subtypes GABABR1 and GABABR2 during rat neocortical development

To understand the possible contribution of metabotropic gamma-aminobutyric acid receptors (GABABR) in cortical development, we investigated the expression pattern and the cellular and subcellular localization of the GABABR1 and GABABR2 subtypes in the rat neocortex from embryonic day 14 (E14) to adulthood. At the light microscopic level, both GABABR1 and GABABR2 were detected as early as E14. During prenatal development, both subtypes were expressed highly in the cortical plate. Using double immunofluorescence, GABABR1 colocalized with GABABR2 in neurons of the marginal zone and subplate, indicating that these proteins are coexpressed and could be forming functional GABABRs during prenatal development in vivo. In contrast, only GABABR1 but not GABABR2 was detected in the tangentially migratory cells in the lower intermediate zone. During postnatal development, immunoreactivity for GABABR1 and GABABR2 was distributed mainly in pyramidal cells. Discrete GABABR1-immunopositive cell bodies of interneurons were present throughout the neocortex. In addition, GABABR1 but not GABABR2 was found in identified Cajal-Retzius cells in layer I. At the electron microscopic level, immunoreactivity for GABABR1 and GABABR2 was found in dendritic spines and dendritic shafts at extrasynaptic and perisynaptic sites throughout postnatal development. We further demonstrated the presynaptic localization of GABABR1 and GABABR2, as well as the association of the receptors with asymmetrical synaptic junctions. These results indicate potentially important roles for the GABABRs in the regulation of migratory processes during corticogenesis and in the modulation of synaptic transmission during early development of cortical circuitry.

Cyclic AMP-dependent protein kinase phosphorylation facilitates GABA(B) receptor-effector coupling

GABA (gamma-aminobutyric acid)(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. Here we show that the functional coupling of GABA(B)R1/GABA(B)R2 receptors to inwardly rectifying K(+) channels rapidly desensitizes. This effect is alleviated after direct phosphorylation of a single serine residue (Ser892) in the cytoplasmic tail of GABA(B)R2 by cyclic AMP (cAMP)-dependent protein kinase (PKA). Basal phosphorylation of this residue is evident in rat brain membranes and in cultured neurons. Phosphorylation of Ser892 is modulated positively by pathways that elevate cAMP concentration, such as those involving forskolin and beta-adrenergic receptors. GABA(B) receptor agonists reduce receptor phosphorylation, which is consistent with PKA functioning in the control of GABA(B)-activated currents. Mechanistically, phosphorylation of Ser892 specifically enhances the membrane stability of GABA(B) receptors. We conclude that signaling pathways that activate PKA may have profound effects on GABA(B) receptor-mediated synaptic inhibition. These results also challenge the accepted view that phosphorylation is a universal negative modulator of G protein-coupled receptors.

Importance of the gamma-aminobutyric acid(B) receptor C-termini for G-protein coupling

Functional gamma-aminobutyric acid(B) (GABA(B)) receptors assemble from two subunits, GABA(B(1)) and GABA(B(2).) This heteromerization, which involves a C-terminal coiled-coil interaction, ensures efficient surface trafficking and agonist-dependent G-protein activation. In the present study, we took a closer look at the implications of the intracellular C termini of GABA(B(1)) and GABA(B(2)) for G-protein coupling. We generated a series of C-terminal mutants of GABA(B(1)) and GABA(B(2)) and tested them for physical interaction, surface trafficking, coupling to adenylyl cyclase, and G-protein-gated inwardly rectifying potassium channels in human embryonic kidney (HEK) 293 cells as well as on endogenous calcium channels in sympathetic neurons of the superior cervical ganglion (SCG). We found that the C-terminal interaction contributes only partly to the heterodimeric assembly of the subunits, indicating the presence of an additional interaction site. The described endoplasmic reticulum retention signal within the C terminus of GABA(B(1)) functioned only in the context of specific amino acids, which constitute part of the GABA(B(1)) coiled-coil sequence. This finding may provide a link between the retention signal and its shielding by the coiled coil of GABA(B(2).) In HEK293 cells, we observed that the two well-known GABA(B) receptor antagonists [S-(R*,R*)]-[3-[[1-(3,4-dichlorophenyl)ethyl]amino]-2-hydroxypropyl](cyclohexylmethyl) phosphinic acid (CGP54626) and (+)-(2S)-5,5-dimethyl-2-morpholineacetic acid (SCH50911) CGP54626 and SCH50911 function as inverse agonists. The C termini of GABA(B(1)) and GABA(B(2)) strongly influenced agonist-independent G-protein coupling, although they were not necessary for agonist-dependent G-protein coupling. The C-terminal GABA(B) receptor mutants described here demonstrate that the active receptor conformation is stabilized by the coiled-coil interaction. Thus, the C-terminal conformation of the GABA(B) receptor may determine its constitutive activity, which could be a therapeutic target for inverse agonists.

Recombinant GABA(B) receptors formed from GABA(B1) and GABA(B2) subunits selectively inhibit N-type Ca(2+) channels in NG108-15 cells

Efficient transfection of NG108-15 cells with GABA(B) receptor subunits was achieved using polyethylenimine. Baclofen modulated high voltage-activated Ca(2+) current in differentiated cells transfected with GABA(B1) and GABA(B2) receptor subunits or with the GABA(B2) subunit alone, but not with the GABA(B1) subunit alone. Characteristics of the current modulation were very similar for cells transfected with GABA(B1/2) and GABA(B2) subunits. Using antisense oligonucleotides against GABA(B1) subunits and also western immunoblotting, we are able to show that NG108-15 cells contain endogenous GABA(B1) subunits. Therefore, functional receptors can be formed by the combination of native GABA(B1) subunits with transfected GABA(B2) subunits, in agreement with the proposed heteromeric structure of GABA(B) receptors. Finally, we used selective channel blockers to identify the subtypes of Ca(2+) channels that are modulated by GABA(B) receptors. In fact, in differentiated NG108-15 cells, the recombinant GABA(B) receptors couple only to N-type Ca(2+) channels.

G protein-coupled receptor interacting proteins: emerging roles in localization and signal transduction

The mechanism by which G protein-coupled receptors (GPCRs) translate extracellular signals into cellular changes initially was envisioned as a simple linear model: activation of the receptor by agonist binding leads to dissociation of the heterotrimeric GTP-binding G protein into its alpha and betagamma subunits, both of which can activate or inhibit various downstream effector molecules. The plethora of recently described multidomain scaffolding proteins and accessory/chaperone molecules that interact with GPCR, including GPCR themselves as homo- or heterodimers, provides for diverse molecular mechanisms for ligand recognition, signalling specificity, and receptor trafficking. This review will summarize the recently described GPCR-interacting proteins and their individual functional roles, as understood. Implicit in the search for the functional relevance of these interactions is the expectation that enhancement or disruption of target cell-specific events could serve as highly selective therapeutic opportunities.

GABA(B2) is essential for g-protein coupling of the GABA(B) receptor heterodimer

GABA(B) receptors are unique among G-protein-coupled receptors (GPCRs) in their requirement for heterodimerization between two homologous subunits, GABA(B1) and GABA(B2), for functional expression. Whereas GABA(B1) is capable of binding receptor agonists and antagonists, the role of each GABA(B) subunit in receptor signaling is unknown. Here we identified amino acid residues within the second intracellular domain of GABA(B2) that are critical for the coupling of GABA(B) receptor heterodimers to their downstream effector systems. Our results provide strong evidence for a functional role of the GABA(B2) subunit in G-protein coupling of the GABA(B) receptor heterodimer. In addition, they provide evidence for a novel "sequential" GPCR signaling mechanism in which ligand binding to one heterodimer subunit can induce signal transduction through the second partner of a heteromeric complex.

Intracellular retention of the two isoforms of the D2 dopamine receptor promotes endoplasmic reticulum disruption

The dopamine D2 receptor exists as a long (D2a) and a short (D2b) isoform generated by alternative splicing of the corresponding transcript, which modifies the length of the third cytoplasmic loop implicated in heterotrimeric G-protein-coupling. Anatomical data suggested that this segment regulates the intracellular traffic and localization of the receptor. To directly address this question we used a combination of tagging procedures and immunocytochemical techniques to detect each of the two D2 receptor isoforms. Surprisingly, most of the newly synthesized receptors accumulate in large intracellular compartments, the plasma membrane being only weakly labeled, without significant difference between the two receptor isoforms. Double labeling experiments showed that this localization corresponded neither to endosomal compartments nor to the Golgi apparatus. The D2 receptor is mostly retained in the endoplasmic reticulum (ER), the long isoform more efficiently than the short one. It is accompanied by a striking vacuolization of the ER, roughly proportional to the expression levels of the two receptor isoforms. This phenomenon is partly overcome by treatment with pertussis toxin. In addition, an intrinsic activity of the D2 receptor isoforms is revealed by [35S]-GTPγS binding and cAMP assay, which suggested that expression of weakly but constitutively active D2 receptors promotes activation of heterotrimeric G protein inside the secretory pathway. This mechanism may participate in the regulation of the cellular traffic of the D2 receptors isoforms.

The C-terminal domains of the GABA(b) receptor subunits mediate intracellular trafficking but are not required for receptor signaling

GABA(B) receptors are G-protein-coupled receptors that mediate slow synaptic inhibition in the brain and spinal cord. These receptors are heterodimers assembled from GABA(B1) and GABA(B2) subunits, neither of which is capable of producing functional GABA(B) receptors on homomeric expression. GABA(B1,) although able to bind GABA, is retained within the endoplasmic reticulum (ER) when expressed alone. In contrast, GABA(B2) is able to access the cell surface when expressed alone but does not couple efficiently to the appropriate effector systems or produce any detectable GABA-binding sites. In the present study, we have constructed chimeric and truncated GABA(B1) and GABA(B2) subunits to explore further GABA(B) receptor signaling and assembly. Removal of the entire C-terminal intracellular domain of GABA(B1) results in plasma membrane expression without the production of a functional GABA(B) receptor. However, coexpression of this truncated GABA(B1) subunit with either GABA(B2) or a truncated GABA(B2) subunit in which the C terminal has also been removed is capable of functional signaling via G-proteins. In contrast, transferring the entire C-terminal tail of GABA(B1) to GABA(B2) leads to the ER retention of the GABA(B2) subunit when expressed alone. These results indicate that the C terminal of GABA(B1) mediates the ER retention of this protein and that neither of the C-terminal tails of GABA(B1) or GABA(B2) is an absolute requirement for functional coupling of heteromeric receptors. Furthermore although GABA(B1) is capable of producing GABA-binding sites, GABA(B2) is of central importance in the functional coupling of heteromeric GABA(B) receptors to G-proteins and the subsequent activation of effector systems.

C-terminal interaction is essential for surface trafficking but not for heteromeric assembly of GABA(b) receptors

Assembly of fully functional GABA(B) receptors requires heteromerization of the GABA(B(1)) and GABA(B(2)) subunits. It is thought that GABA(B(1)) and GABA(B(2)) undergo coiled-coil dimerization in their cytoplasmic C termini and that assembly is necessary to overcome GABA(B(1)) retention in the endoplasmatic reticulum (ER). We investigated the mechanism underlying GABA(B(1)) trafficking to the cell surface. We identified a signal, RSRR, proximal to the coiled-coil domain of GABA(B(1)) that when deleted or mutagenized allows for surface delivery in the absence of GABA(B(2)). A similar motif, RXR, was recently shown to function as an ER retention/retrieval (ERR/R) signal in K(ATP) channels, demonstrating that G-protein-coupled receptors (GPCRs) and ion channels use common mechanisms to control surface trafficking. A C-terminal fragment of GABA(B(2)) is able to mask the RSRR signal and to direct the GABA(B(1)) monomer to the cell surface, where it is functionally inert. This indicates that in the heteromer, GABA(B(2)) participates in coupling to the G-protein. Mutagenesis of the C-terminal coiled-coil domains in GABA(B(1)) and GABA(B(2)) supports the possibility that their interaction is involved in shielding the ERR/R signal. However, assembly of heteromeric GABA(B) receptors is possible in the absence of the C-terminal domains, indicating that coiled-coil interaction is not necessary for function. Rather than guaranteeing heterodimerization, as previously assumed, the coiled-coil structure appears to be important for export of the receptor complex from the secretory apparatus.

Allosteric interactions between GB1 and GB2 subunits are required for optimal GABA(B) receptor function

Recent studies on G-protein-coupled receptors revealed that they can dimerize. However, the role of each subunit in the activation process remains unclear. The gamma-amino-n-butyric acid type B (GABA(B)) receptor is comprised of two subunits: GB1 and GB2. Both consist of an extracellular domain (ECD) and a heptahelical domain composed of seven transmembrane alpha-helices, loops and the C-terminus (HD). Whereas GB1 ECD plays a critical role in ligand binding, GB2 is required not only to target GB1 subunit to the cell surface but also for receptor activation. Here, by analysing chimeric GB subunits, we show that only GB2 HD contains the determinants required for G-protein signalling. However, the HD of GB1 improves coupling efficacy. Conversely, although GB1 ECD is sufficient to bind GABA(B) ligands, the ECD of GB2 increases the agonist affinity on GB1, and is necessary for agonist activation of the receptor. These data indicate that multiple allosteric interactions between the two subunits are required for wild-type functioning of the GABA(B) receptor and highlight further the importance of the dimerization process in GPCR activation.

Oligomerization of G-protein-coupled transmitter receptors

Examples of G-protein-coupled receptors that can be biochemically detected in homo- or heteromeric complexes are emerging at an accelerated rate. Biophysical approaches have confirmed the existence of several such complexes in living cells and there is strong evidence to support the idea that dimerization is important in different aspects of receptor biogenesis and function. While the existence of G-protein-coupled-receptor homodimers raises fundamental questions about the molecular mechanisms involved in transmitter recognition and signal transduction, the formation of heterodimers raises fascinating combinatorial possibilities that could underlie an unexpected level of pharmacological diversity, and contribute to cross-talk regulation between transmission systems. Because G-protein-coupled receptors are major pharmacological targets, the existence of dimers could have important implications for the development and screening of new drugs. Here, we review the evidence supporting the existence of G-protein-coupled-receptor dimerization and discuss its functional importance.

Subcellular localization of GABA(B) receptor subunits in rat visual cortex

Although studies in the visual cortex have found gamma-aminobutyric acid B (GABA(B)) receptor-mediated pre- and postsynaptic inhibitory effects on neurons, the subcellular localization of GABA(B) receptors in different types of cortical neurons and synapses has not been shown directly. To provide this information, we have used antibodies against the GABA(B) receptor (R)1a/b and GABA(B)R2 subunits and have studied the localization of immunoreactivities in rat visual cortex. Light microscopic analyses have shown that both subunits are expressed in cell bodies and dendrites of 65-92% of corticocortically projecting pyramidal neurons and in 92-100% of parvalbumin (PV)-, calretinin (CR)-, and somatostatin (SOM)-containing GABAergic neurons. Electron microscopic analyses of immunoperoxidase- and immunogold-labeled tissue revealed staining in the nucleus, cytoplasm and cell surface membranes with both antibodies. Colocalization of both subunits was observed in all of these structures. GABA(B)R1a/b and GABA(B)R2 were concentrated in excitatory and inhibitory synapses and in extrasynaptic membranes. In GABAergic synapses, GABA(B)R1a/b and GABA(B)R2 were more strongly expressed postsynaptically on pyramidal and nonpyramidal cells than presynaptically. In type 1 synapses GABA(B)R1a/b and GABA(B)R2 was found in pre- and postsynaptic membranes. The nuclear localization of GABA(B)R1 and GABA(B)R2 subunits suggests a novel role for neurotransmitter receptors in controlling gene expression. The synaptic colocalization of GABA(B)R1 and GABA(B)R2 indicates that subunits form heteromeric assemblies of the functional receptor in inhibitory and excitatory synapses. Subunit coexpression in GABAergic synapses that include PV-containing and PV-deficient terminals suggests that pre- and postsynaptic GABA(B) receptor activation is provided by several different types of interneurons. The coexpression of both subunits in excitatory synapses suggests a role for GABA(B) receptors in the regulation of glutamate release and raises the question how these receptors are activated in the absence of pre-or postsynaptic GABAergic synaptic inputs to excitatory synapses.

Ontogenic expression of anterior pituitary GABA(B) receptor subunits

gamma-Aminobutyric acid (GABA) is involved in the neuroendocrine control of hypophyseal secretion, acting both in the central nervous system and directly at the pituitary. We have characterized the properties of anterior pituitary GABA(B) receptors. In this work the ontogeny of rat anterior pituitary GABA(B) receptors and the pattern of subunit expression in rats of both sexes were determined. Western blot analysis showed a temporal decrease in GABA(B) subunits GABA(B(1a)) and GABA(B(1b)) expression in female anterior pituitary membranes from day 4 to adulthood, with GABA(B(1a)) being significantly more abundant than GABA(B(1b)) at early stages of development; the GABA(B(2)) subunit was barely detectable. In the male, GABA(B(1a)) followed a similar pattern and appeared to be significantly less abundant than in 4- and 12-day-old females; GABA(B(1b)) and GABA(B(2)) expression in the male was barely detectable. Scatchard plot analysis showed a temporal decrease in binding sites in female anterior pituitary membranes, in agreement with the western blot results. The number of binding sites was significantly higher in female than in male 4-day-old membranes. Dissociation constant values were similar for both sexes at all ages studied. This study reports for the first time the ontogeny of anterior pituitary GABA(B) receptors, showing a particular developmental pattern of subunit expression and a clear sexual dimorphism.

The expression of GABA(B1) and GABA(B2) receptor subunits in the cNS differs from that in peripheral tissues

GABA(B) receptors are G-protein-coupled receptors that mediate the slow and prolonged synaptic actions of GABA in the CNS via the modulation of ion channels. Unusually, GABA(B) receptors form functional heterodimers composed of GABA(B1) and GABA(B2) subunits. The GABA(B1) subunit is essential for ligand binding, whereas the GABA(B2) subunit is essential for functional expression of the receptor dimer at the cell surface. We have used real-time reverse transcriptase-polymerase chain reaction to analyse expression levels of these subunits, and their associated splice variants, in the CNS and peripheral tissues of human and rat. GABA(B1) subunit splice variants were expressed throughout the CNS and peripheral tissues, whereas surprisingly GABA(B2) subunit splice variants were neural specific. Using novel antisera specific to individual GABA(B) receptor subunits, we have confirmed these findings at the protein level. Analysis by immunoblotting demonstrated the presence of the GABA(B1) subunit, but not the GABA(B2) subunit, in uterus and spleen. Furthermore, we have shown the first immunocytochemical analysis of the GABA(B2) subunit in the brain and spinal cord using a GABA(B2)-specific antibody. We have, therefore, identified areas of non-overlap between GABA(B1) and GABA(B2) subunit expression in tissues known to contain functional GABA(B) receptors. Such areas are of interest as they may well contain novel GABA(B) receptor subunit isoforms, expression of which would enable the GABA(B1) subunit to reach the cell surface and form functional GABA(B) receptors.

Association of GABA(B) receptors and members of the 14-3-3 family of signaling proteins

Two GABA(B) receptors, GABA(B)R1 and GABA(B)R2, have been cloned recently. Unlike other G protein-coupled receptors, the formation of a heterodimer between GABA(B)R1 and GABA(B)R2 is required for functional expression. We have used the yeast two hybrid system to identify proteins that interact with the C-terminus of GABA(B)R1. We report a direct association between GABA(B) receptors and two members of the 14-3-3 protein family, 14-3-3eta and 14-3-3zeta. We demonstrate that the C-terminus of GABA(B)R1 associates with 14-3-3zeta in rat brain preparations and tissue cultured cells, that they codistribute after rat brain fractionation, colocalize in neurons, and that the binding site overlaps partially with the coiled-coil domain of GABA(B)R1. Furthermore we show a reduced interaction between the C-terminal domains of GABA(B)R1 and GABA(B)R2 in the presence of 14-3-3. The results strongly suggest that GABA(B)R1 and 14-3-3 associate in the nervous system and begin to reveal the signaling complexities of the GABA(B)R1/GABA(B)R2 receptor heterodimer.

GABA(B(1)) mRNA expression in hippocampal sclerosis associated with human temporal lobe epilepsy

GABA(B) receptors act to inhibit neurotransmitter release from presynaptic terminals, and mediate the late inhibitory postsynaptic potential. Studies of GABA(B) receptor function in rodent models of temporal lobe epilepsy (TLE) suggest that GABA(B) receptor expression and/or function may be perturbed. GABA(B(1)) mRNA levels were investigated in 10 hippocampal resection samples obtained at surgery from intractable hippocampal sclerosis (HS) associated TLE patients and five neurologically normal post-mortem (PM) control samples. In situ hybridisation with a 35S-dATP-labelled oligonucleotide was carried out to measure mRNA levels, along with three-dimensional cell counting, for assessment of neuronal density in hippocampal subregions. GABA(B(1)) mRNA was significantly up-regulated in the subiculum of HS samples as compared with PM controls. When adjusted for the characteristic neuronal density changes observed in HS, GABA(B(1)) mRNA was significantly up-regulated in CA1, hilus and dentate gyrus granule cell layer of HS samples as compared with PM controls. The possibility of increased GABA(B(1)) expression suggests that changes in GABA(B) receptor mechanisms may be involved in the pathogenesis of human HS-associated TLE.

Comparative immunohistochemical localisation of GABA(B1a), GABA(B1b) and GABA(B2) subunits in rat brain, spinal cord and dorsal root ganglion

GABA(B) receptors are G-protein-coupled receptors mediating the slow onset and prolonged synaptic actions of GABA in the CNS. The recent cloning of two genes, GABA(B1) and GABA(B2), has revealed a novel requirement for GABA(B) receptor signalling. Studies have demonstrated that the two receptor subunits associate as a GABA(B1)/GABA(B2) heterodimer to form a functional GABA(B) receptor. In this study we have developed polyclonal antisera specific to two splice variants of the GABA(B1) subunit, GABA(B1a) and GABA(B1b), as well as an antiserum to the GABA(B2) subunit. Using affinity-purified antibodies derived from these antisera we have mapped out the distribution profile of each subunit in rat brain, spinal cord and dorsal root ganglion. In brain the highest areas of GABA(B1a), GABA(B1b) and GABA(B2) subunit expression were found in neocortex, hippocampus, thalamus, cerebellum and habenula. In spinal cord, GABA(B1) and GABA(B2) subunits were expressed in the superficial layers of the dorsal horn, as well as in motor neurones in the deeper layers of the ventral horn. GABA(B) receptor subunit immunoreactivity in dorsal root ganglion suggested that expression of GABA(B1b) was restricted to the large diameter neurones, in contrast to GABA(B1a) and GABA(B2) subunits which were expressed in both large and small diameter neurones. Although expression levels of GABA(B1) and GABA(B2) subunits varied we found no areas in which GABA(B1) was expressed in the absence of GABA(B2). This suggests that most, if not all, GABA(B1) immunoreactivity may represent functional GABA(B) receptors. Although our data are in general agreement with functional studies, some discrepancies in GABA(B1) subunit expression occurred with respect to other immunohistochemical studies. Overall our data suggest that GABA(B) receptors are widely expressed throughout the brain and spinal cord, and that GABA(B1a) and GABA(B1b) subunits can associate with GABA(B2) to form both pre- and post-synaptic receptors.

GABA(B) receptor autoradiography in hippocampal sclerosis associated with human temporal lobe epilepsy

1. Metabotropic gamma-aminobutyric acid receptors (GABA(B)) exist both pre- and postsynaptically throughout the brain, mediating the suppression of neurotransmitter release and late inhibitory postsynaptic potentials. Investigation of GABA(B) receptors in rodent models of temporal lobe epilepsy (TLE) suggests that expression or function of these receptors may be altered in the disorder. 2. The aim of the present study was to investigate the expression of GABA(B) receptors in samples of hippocampus surgically resected from patients with hippocampal sclerosis (HS) related intractable TLE, and compare this expression with samples of neurologically normal post-mortem (PM) control hippocampal tissue. Appropriate measures of neuronal loss associated with HS were investigated for comparison with receptor binding data. 3. Receptor autoradiography with [(3)H]-GABA in the presence of isoguvacine, and quantitative densitometric analysis were used to investigate GABA(B) receptor expression (B(max)) and affinity (K(D)) in 11 HS samples and eight controls. A three-dimensional cell counting technique was used to assess neuronal density in both groups. 4. GABA(B) receptor density was significantly reduced in CA1, CA2, CA3, hilus and dentate gyrus, and increased in the subiculum, of HS cases as compared with PM controls. Neuronal loss was significant in all regions measured. When adjusted for neuronal loss, CA1 GABA(B) receptor expression appeared significantly upregulated (P:<0.05). 5. In HS/TLE, GABA(B) receptor expression per remaining neurone appears increased in CA1. This finding, and increased [(3)H]-GABA affinity at CA3 and hilar GABA(B) receptors, suggests altered GABA(B) receptor function may occur in human HS/TLE, possibly as a result of synaptic reorganization.

Mapping the agonist-binding site of GABAB type 1 subunit sheds light on the activation process of GABAB receptors

The gamma-amino-n-butyric acid type B (GABA(B)) receptor is composed of two subunits, GABA(B)1 and GABA(B)2, belonging to the family 3 heptahelix receptors. These proteins possess two domains, a seven transmembrane core and an extracellular domain containing the agonist binding site. This binding domain is likely to fold like bacterial periplasmic binding proteins that are constituted of two lobes that close upon ligand binding. Here, using molecular modeling and site-directed mutagenesis, we have identified residues in the GABA(B)1 subunit that are critical for agonist binding and activation of the heteromeric receptor. Our data suggest that two residues (Ser(246) and Asp(471)) located within lobe I form H bonds and a salt bridge with carboxylic and amino groups of GABA, respectively, demonstrating the pivotal role of lobe I in agonist binding. Interestingly, our data also suggest that a residue within lobe II (Tyr(366)) interacts with the agonists in a closed form model of the binding domain, and its mutation into Ala converts the agonist baclofen into an antagonist. These data demonstrate the pivotal role played by the GABA(B)1 subunit in the activation of the heteromeric GABA(B) receptor and are consistent with the idea that a closed state of the binding domain of family 3 receptors is required for their activation.

The GABAB receptor interacts directly with the related transcription factors CREB2 and ATFx

gamma-Aminobutyric acid type B (GABA(B)) receptors mediate the metabotropic actions of the inhibitory neurotransmitter GABA. These seven-transmembrane receptors are known to signal primarily through activation of G proteins to modulate the action of ion channels or second messengers. The functional GABA(B) receptor is made up of a heterodimer consisting of two subunits, GABA(B)-R1 and GABA(B)-R2, which interact via coiled-coil domains in their C-terminal tails. By using a yeast two-hybrid approach, we have identified direct interactions between the C-terminal tails of GABA(B)-R1 and GABA(B)-R2 with two related transcription factors, CREB2 (ATF4) and ATFx. In primary neuronal cultures as well in recombinant Chinese hamster ovary cells expressing GABA(B) receptors, CREB2 is localized within the cytoplasm as well as the nucleus. Activation of the GABA(B) receptor by the specific agonist baclofen leads to a marked translocation and accumulation of CREB2 from the cytoplasm into the nucleus. We demonstrate that receptor stimulation results in activation of transcription from a CREB2 responsive reporter gene. Such a signaling mechanism is unique among Family C G protein-coupled receptors and, in the case of the GABA(B) receptor and CREB2, may play a role in long-term changes in the nervous system.

Trafficking and folding defects in hereditary spherocytosis mutants of the human red cell anion exchanger

Hereditary spherocytosis (HS) is a common inherited hemolytic anemia caused by mutations in erythrocyte proteins including the anion exchanger, AE1 (band 3). This study examined seven missense mutations (L707P, R760Q, R760W, R808C, H834P, T837M, and R870W) located in the membrane domain of the human AE1 that are associated with this disease. The HS mutants, constructed in full-length AE1 cDNA, could be transiently expressed to similar levels in HEK 293 cells. Immunofluorescence, cell surface biotinylation, and pulse chase labeling showed that the HS mutants all exhibited defective cellular trafficking from the endoplasmic reticulum to the plasma membrane. Impaired binding to an inhibitor affinity matrix indicated that the mutant proteins had non-native structures and may be misfolded. Further characterization of the HS R760Q mutant showed no change in its oligomeric structure or turnover (half-life = 15 h) compared to wild-type AE1, suggesting the mutant was not aggregated or targeted for rapid degradation via the proteasome. Intracellular retention of HS mutant AE1 would lead to destruction of the protein during erythroid development and would account for the lack of HS mutant AE1 in the plasma membrane of the mature red cell.

Aggresomes, inclusion bodies and protein aggregation

Intracellular and extracellular accumulation of aggregated protein are linked to many diseases, including ageing-related neurodegeneration and systemic amyloidosis. Cells avoid accumulating potentially toxic aggregates by mechanisms including the suppression of aggregate formation by molecular chaperones and the degradation of misfolded proteins by proteasomes. Once formed, aggregates tend to be refractory to proteolysis and to accumulate in inclusion bodies. This accumulation has been assumed to be a diffusion-limited process, but recent studies suggest that, in animal cells, aggregated proteins are specifically delivered to inclusion bodies by dynein-dependent retrograde transport on microtubules. This microtubule-dependent inclusion body is called an aggresome.

The metabotropic GABAB receptor directly interacts with the activating transcription factor 4

G protein-coupled receptors regulate gene expression by cellular signaling cascades that target transcription factors and their recognition by specific DNA sequences. In the central nervous system, heteromeric metabotropic gamma-aminobutyric acid type B (GABA(B)) receptors through adenylyl cyclase regulate cAMP levels, which may control transcription factor binding to the cAMP response element. Using yeast-two hybrid screens of rat brain libraries, we now demonstrate that GABA(B) receptors are engaged in a direct and specific interaction with the activating transcription factor 4 (ATF-4), a member of the cAMP response element-binding protein /ATF family. As confirmed by pull-down assays, ATF-4 associates via its conserved basic leucine zipper domain with the C termini of both GABA(B) receptor (GABA(B)R) 1 and GABA(B)R2 at a site which serves to assemble these receptor subunits in heterodimeric complexes. Confocal fluorescence microscopy shows that GABA(B)R and ATF-4 are strongly coclustered in the soma and at the dendritic membrane surface of both cultured hippocampal neurons as well as retinal amacrine cells in vivo. In oocyte coexpression assays short term signaling of GABA(B)Rs via G proteins was only marginally affected by the presence of the transcription factor, but ATF-4 was moderately stimulated in response to receptor activation in in vivo reporter assays. Thus, inhibitory metabotropic GABA(B)Rs may regulate activity-dependent gene expression via a direct interaction with ATF-4.

Cellular and sub-cellular localisation of GABA(B1) and GABA(B2) receptor proteins in the rat cerebellum

Following the recent discovery that GABA(B) receptors expressed in cell lines are only functional when both GABA(B1) and GABA(B2) are expressed, the present study reports on the development of polyclonal antisera specific for carboxyl-terminal portions of the two related GABA(B) receptor components respectively. Western blotting indicated the specificity of affinity-purified antibodies for native or recombinant expressed GABA(BR1) and GABA(BR2), with no cross-reactivity, both antisera detecting the heterodimer in rat cerebellar membranes. Immunohistochemistry revealed a distinct distribution of both receptor proteins in rat cerebellum. GABA(B1) immunoreactivity was primarily located in the granule cell layer and Purkinje cells, with discrete immuno-positive cell bodies being present in the molecular layer. GABA(B2) staining revealed intense immunoreactivity in the molecular layer, with weaker staining in the granule cell layer. Purkinje cell bodies were less intensely immuno-positive for GABA(B2). Co-localisation of both receptor proteins was observed using double immunofluorescence techniques, consistent with the notion that both proteins are required for the formation of functional GABA(B) receptors in vivo. Immunofluorescence also indicated that GABA(B) receptors did not co-localise with glial fibrillary acid protein, confirming a neuronal localisation for GABA(B) receptors. Electron microscopic analysis of the molecular layer revealed that the distribution of immunolabelling for both GABA(B1) and GABA(B2) was mainly located on the membrane of Purkinje cell dendrites and spines and in parallel fibre terminals.

The human GABA(B1b) and GABA(B2) heterodimeric recombinant receptor shows low sensitivity to phaclofen and saclofen

1. The aim of this study was to characterize the pharmacological profile of the GABA(B1)/GABA(B2) heterodimeric receptor expressed in Chinese hamster ovary (CHO) cells. We have compared receptor binding affinity and functional activity for a series of agonists and antagonists. 2. The chimeric G-protein, G(qi5), was used to couple receptor activation to increases in intracellular calcium for functional studies on the Fluorimetric Imaging Plate Reader (FLIPR), using a stable GABA(B1)/GABA(B2)/G(qi5) CHO cell line. [(3)H]-CGP-54626 was used in radioligand binding studies in membranes prepared from the same cell line. 3. The pharmacological profile of the recombinant GABA(B1/B2) receptor was consistent with that of native GABA(B) receptors in that it was activated by GABA and baclofen and inhibited by CGP-54626A and SCH 50911. 4. Unlike native receptors, the GABA(B1)/GABA(B2)/G(qi5) response was not inhibited by high microMolar concentration of phaclofen, saclofen or CGP 35348. 5. This raises the possibility that the GABA(B1)/GABA(B2)/G(qi5) recombinant receptor may represent the previously described GABA(B) receptor subtype which is relatively resistant to inhibition by phaclofen.

Characterization of gamma-aminobutyric acid receptor GABAB(1e), a GABAB(1) splice variant encoding a truncated receptor

We have identified a splice variant encoding only the extracellular ligand-binding domain of the gamma-aminobutyric acid B (GABA(B)) receptor subunit GABA(B(1a)). This isoform, which we have named GABA(B(1e)), is detected in both rats and humans. While GABA(B(1e)) is a minor component of the total pool of GABA(B(1)) transcripts detected in the central nervous system, it is the primary isoform found in all peripheral tissues examined. When expressed in a heterologous system, the truncated receptor is both secreted and membrane associated. However, GABA(B(1e)) lacks the ability to bind the radiolabeled antagonist [(3)H]CGP 54626A, activate G-protein coupled inwardly rectifying potassium channels, or inhibit forskolin-induced cAMP production when it is expressed alone or together with GABA(B(2)). Interestingly, when co-expressed with GABA(B(2)), not only does the truncated receptor heterodimerize with GABA(B(2)), the association is of sufficient avidity to disrupt the normal GABA(B(1a))/GABA(B(2)) association. Despite this strong interaction, GABA(B(1e)) fails to disrupt G-protein coupled inwardly rectifying potassium activation by the full-length heterodimer pair of GABA(B(1a))/GABA(B(2)).

Neurochemical and molecular pharmacological aspects of the GABA(B) receptor

Metabotropic gamma-aminobutyric acid (GABA)B receptors are known to modulate the synaptic release of various neurotransmitters in the nervous system. Activation of GABA(B) receptor induces the inhibition of adenylyl cyclase activity, while it does not stimulate the formation of inositol phosphates. Activation of a potassium conductance and suppression of a calcium conductance are also recognized, similarly to some of G protein-coupled receptors. Recent molecular cloning has revealed that GABA(B) receptor possesses a large extracellular domain including the binding site for GABA and seven transmembrane domains. Their molecular structures in the brain are unique and interesting because of heterodimerization consisting of two distinct genes: GABABR1 and GABABR2. Such assembled receptors can be classified as a novel type of the metabotropic receptor superfamily.

Analysis of GABAA receptor assembly in mammalian cell lines and hippocampal neurons using gamma 2 subunit green fluorescent protein chimeras

Type A gamma-aminobutyric acid receptors (GABAA), the major sites of fast synaptic inhibition in the brain, are believed to be predominantly composed of alpha, beta, and gamma subunits. To examine the membrane trafficking of GABAA receptors we have produced gamma 2L subunit chimeras with green fluorescent protein (GFP). Addition of GFP to the N-terminus of the gamma 2 subunit (gamma 2L-GFPN) was functionally silent for alpha 1 beta 2 gamma 2L-GFPN receptors expressed in A293 cells. Furthermore, this chimera allowed the visualization of receptor membrane targeting and endocytosis in live cells. In contrast, incorporation of GFP at the C-terminus reduced subunit stability, impairing assembly with receptor alpha and beta subunits. Using gamma 2L-GFPN we were able to demonstrate that targeting of the gamma 2 subunit to GABAergic synapses in hippocampal neurons was dependent upon coassembly with receptor alpha and beta subunits. Together our results demonstrate that the assembly and membrane targeting of GABAA receptors composed of alpha 1 beta 2 gamma 2L-GFPN subunits follow similar itineraries in heterologous systems and neurons.

Tonic and synaptically evoked presynaptic inhibition of sensory input to the rat olfactory bulb via GABA(B) heteroreceptors

Olfactory receptor neurons of the nasal epithelium send their axons, via the olfactory nerve (ON), to the glomeruli of the olfactory bulb (OB), where the axon terminals form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the OB, and with juxtaglomerular (JG) interneurons. Many JG cells are GABAergic. Here we show that, despite the absence of conventional synapses, GABA released from JG cells activates GABA(B) receptors on ON terminals and inhibits glutamate release both tonically and in response to ON stimulation. Field potential recordings and current-source density analysis, as well as intracellular and whole cell recording techniques were used in rat OB slices. Baclofen (2-5 microM), a GABA(B) agonist, completely suppressed ON-evoked synaptic responses of both mitral/tufted cells and JG cells, with no evidence for postsynaptic effects. Baclofen (0.5-1 microM) also reversed paired-pulse depression (PPD) of mitral/tufted cell responses to paired-pulse facilitation (PPF), and reduced depression of JG cell excitatory postsynaptic currents (EPSCs) during repetitive ON stimulation. These results suggest that baclofen reduced the probability of glutamate release from ON terminals. The GABA(B) antagonists CGP35348 or CGP55845A increased mitral/tufted cell responses evoked by single-pulse ON stimulation, suggesting that glutamate release from ON terminals is tonically suppressed via GABA(B) receptors. The same antagonists reduced PPD of ON-evoked mitral/tufted cell responses at interstimulus intervals 50-400 ms. This finding suggests that a single ON impulse evokes sufficient GABA release, presumably from JG cells, to activate GABA(B) receptors on ON terminals. Thus GABA(B) heteroreceptors on ON terminals are activated by ambient levels of extrasynaptic GABA, and by ON input to the OB. The time course of ON-evoked, GABA(B) presynaptic inhibition suggests that neurotransmission to M/T cells and JG cells will be significantly suppressed when ON impulses arrive in glomeruli at 2.5-20 Hz. GABA(B) receptor-mediated presynaptic inhibition of sensory input to the OB may play an important role in shaping the activation pattern of the OB glomeruli during olfactory coding.

A trafficking checkpoint controls GABA(B) receptor heterodimerization

Surface expression of GABA(B) receptors requires heterodimerization of GB1 and GB2 subunits, but little is known about mechanisms that ensure efficient heterodimer assembly. We found that expression of the GB1 subunit on the cell surface is prevented through a C-terminal retention motif RXR(R); this sequence is reminiscent of the ER retention/retrieval motif RKR identified in subunits of the ATP-sensitive K+ channel. Interaction of GB1 and GB2 through their C-terminal coiled-coil alpha helices masks the retention signal in GB1, allowing the plasma membrane expression of the assembled complexes. Because individual GABA(B) receptor subunits and improperly assembled receptor complexes are not functional even if expressed on the cell surface, we conclude that a trafficking checkpoint ensures efficient assembly of functional GABA(B) receptors.

Functional significance of oligomerization of G-protein-coupled receptors

In contrast to other families of cell surface receptors for which dimerization is an integral part of the activation process, G-protein-coupled receptors (GPCRs) were thought, until recently, to function as monomeric units. However, a growing body of evidence indicates that GPCRs could exist and be active as oligomeric complexes. Because they are major pharmacological targets, their existence as homo- or heterodimers could have important implications for the development and screening of new drugs.

Heteromeric assembly of GABA(B)R1 and GABA(B)R2 receptor subunits inhibits Ca(2+) current in sympathetic neurons

Neuronal GABA(B) receptors regulate calcium and potassium currents via G-protein-coupled mechanisms and play a critical role in long-term inhibition of synaptic transmission in the CNS. Recent studies have demonstrated that assembly of GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits into functional heterodimers is required for coupling to potassium channels in heterologous systems. However whether heterodimerization is required for the coupling of GABA(B) receptors to effector systems in neurons remains to be established. To address this issue, we have studied the coupling of recombinant GABA(B) receptors to endogenous Ca(2+) channels in superior cervical ganglion (SCG) neurons using nuclear microinjection to introduce both sense and antisense expression constructs. Patch-clamp recording from neurons injected with both GABA(B)R1a/1b and GABA(B)R2 cDNAs or with GABA(B)R2 alone produced marked baclofen-mediated inhibition of Ca(2+) channel currents via a pertussis toxin-sensitive mechanism. The actions of baclofen were blocked by CGP62349, a specific GABA(B) antagonist, and were voltage dependent. Interestingly, SCGs were found to express abundantly GABA(B)R1 but not GABA(B)R2 at the protein level. To determine whether heterodimerization of GABA(B)R1 and GABA(B)R2 subunits was required for Ca(2+) inhibition, the GABA(B)R2 expression construct was microinjected with a GABA(B)R1 antisense construct. This resulted in a dramatic decrease in the levels of the endogenous GABA(B)R1 protein and a marked reduction in the inhibitory effects of baclofen on Ca(2+) currents. Therefore our results suggest that in neurons heteromeric assemblies of GABA(B)R1 and GABA(B)R2 are essential to mediate GABAergic inhibition of Ca(2+) channel currents.

Export from the endoplasmic reticulum represents the limiting step in the maturation and cell surface expression of the human delta opioid receptor

Synthesis and maturation of G protein-coupled receptors are complex events that require an intricate combination of processes that include protein folding, post-translational modifications, and transport through distinct cellular compartments. Relatively little is known about the nature and kinetics of specific steps involved in these processes. Here, the human delta opioid receptor expressed in human embryonic kidney 293S cells is used as a model to delineate these steps and to establish the kinetics of receptor synthesis, glycosylation, and transport. We found that the receptor is synthesized as a core-glycosylated M(r) 45,000 precursor that is converted to the fully mature M(r) 55,000 receptor with a half-time of about 120 min. In addition to trimming and processing of two N-linked oligosaccharides, maturation involves addition of O-glycans containing N-acetylgalactosamine, galactose, and sialic acid. In contrast to N-glycosylation, which is initiated co-translationally and is completed when the protein reaches the trans-Golgi network, O-glycosylation was found to occur only after the receptor exits from the endoplasmic reticulum (ER) and was terminated as early as the trans-Golgi cisternae. Once the carbohydrates are fully processed and the receptor reaches the trans-Golgi network, it is transported to the cell surface in about 10 min. The exit from the ER was found to be the limiting step in overall processing of the receptor. This indicates that early events in the folding of the receptor are probably rate-limiting and that receptor folding intermediates are retained in the ER until they can adopt the correct conformation. The overall low efficiency of receptor maturation, less than 50% of the precursor being processed to the fully glycosylated protein, further suggests that only a fraction of the synthesized receptors attain properly folded conformation that allows exit from the ER. This indicates that folding and ER export are key events in control of receptor cell surface expression. Whether or not the low efficiency of the ER export is a general feature among G protein-coupled receptors remains to be investigated.

GABA(B) receptor heterodimer-component localisation in human brain

In recombinant cell lines, functional GABA(B) receptors are only formed by the heterodimerisation between two related G-protein coupled receptor proteins GABA(B)R1 (GBR1) and GABA(B)R2 (GBR2), whilst the individual GBR1 or GBR2 do not produce fully functional receptors. To determine whether the heterodimerisation occurs in vivo, novel polyclonal antibodies targeting the C termini of GBR1 and GBR2, were raised in different species, characterised, and used to determine the relative localisation of the reported heterodimer components in human brain tissue, using immunohistochemistry. The use of different species for the raising of the antisera allowed double immunofluorescent labelling of the receptors as an indication of GBR1/GBR2 receptor co-localisation in human brain. The presence of both proteins is reported in cerebellum, hippocampus, cortex, thalamus and basal ganglia. Regions of the brainstem including pons and medulla, also express GBR1 and GBR2 protein. The double immunofluorescence demonstrated that GBR1 and GBR2 are co-localised in the human cerebellar cortex. Together these results suggest the widespread distribution of GABA(B) receptors in human brain, and that GABA(B) receptors GBR1 and GBR2 can exist in the same cell, and therefore may function as a heterodimer in the human brain.

Distribution of the GABA(B) receptor subunit gb2 in rat CNS

We have identified and isolated human and rat cDNAs for a novel receptor, gb2, with 38% homology to the GABA(B) receptors gb1a and gb1b. These receptors comprise a new subfamily of seven transmembrane G protein-coupled receptors (GPCRs) that share structure and sequence similarities with the metabotropic glutamate receptors. In situ hybridization histochemistry using an antisense probe to this novel receptor mRNA shows a distribution in rat CNS nearly identical to that for the gb1 receptor, although some regions showed significant differences. Specifically, message levels for gb2 were virtually absent in the caudate/putamen, and significantly lower in the medial basal hypothalamus, septum and brainstem as compared with gb1 message levels. In contrast to gb1, gb2 mRNA was never detected in white matter suggesting that gb2 message is found exclusively in neurons. Finally, in rat brain regions showing significant overlap of message for gb1 and gb2, the transcripts are often found in the same cells. Data from our previous work showing that coexpression of gb2 with gb1 is necessary for expression of a functional receptor together with the detailed anatomical data presented here indicate that native GABA(B) receptors function as heteromeric proteins, the most abundant form being the gb1/gb2 receptor. However, the more limited distribution of gb2 receptor mRNA suggests that there are brain regions where GABA(B) receptors are composed of gb1 and as yet unidentified family members.

Differences in the ways sympathetic neurons and endocrine cells process, store, and secrete exogenous neuropeptides and peptide-processing enzymes

Most neurons store peptides in large dense core vesicles (LDCVs) and release the neuropeptides in a regulated manner. Although LDCVs have been studied in endocrine cells, less is known about these storage organelles in neurons. In this study we use the endogenous peptide NPY (neuropeptide Y) and the endogenous peptide-processing enzyme PAM (peptidylglycine alpha-amidating monooxygenase) as tools to study the peptidergic system in cultured neurons from the superior cervical ganglion (SCG). Once mature, SCG neurons devote as much of their biosynthetic capabilities to neurotransmitter production as endocrine cells devote to hormone production. Unlike pituitary and atrium, SCG neurons cleave almost all of the bifunctional PAM protein they produce into soluble monofunctional enzymes. Very little PAM or NPY is secreted under basal conditions, and the addition of secretagogue dramatically stimulates the secretion of PAM and NPY to a similar extent. Although endocrine cells typically package "foreign" secretory products together with endogenous products, pro-opiomelanocortin- and PAM-derived products encoded by adenovirus in large part were excluded from the LDCVs of SCG neurons. When expressed in corticotrope tumor cells and primary anterior pituitary cultures, the same virally encoded products were metabolized normally. The differences that were observed could reflect differences in the properties of neuronal and endocrine peptidergic systems or differences in the ability of neurons and endocrine cells to express viral transcripts.

Heterodimerization of a functional GABAB receptor is mediated by parallel coiled-coil alpha-helices

A detailed understanding of GABAB receptor assembly is an important issue in view of its role as attractive target for treatment of epilepsy, anxiety, depression, cognitive defects, and nociceptive disorders. Heteromerization of GABAB-R1 and GABAB-R2 subunits is a prerequisite for the formation of a functional GABAB receptor. Each individual subunit contains one stretch of approximately 30 amino acid residues within its intracellular C-terminal domain that mediates heteromer formation. To investigate the mechanism of the GABAB-R1/GABAB-R2 interaction and to assess the subunit stoichiometry of the complex, recombinant polypeptide chain fragments containing the heteromerization site were produced by heterologous gene expression in Escherichia coli. When mixed in equimolar amounts, these peptides preferentially formed parallel coiled-coil heterodimers under physiological buffer conditions. This demonstrates that the short C-terminal regions are sufficient to determine the specificity of interaction between GABAB receptor subunits. In contrast, isolated GABAB-R1 peptides folded into relatively unstable homodimers, whereas GABAB-R2 peptides were largely unstructured. Together with the data reported in the literature, the results presented here indicate that the functional GABAB receptor is a heterodimer assembled by parallel coiled-coil alpha-helices.

gamma-aminobutyric acid type B receptor splice variant proteins GBR1a and GBR1b are both associated with GBR2 in situ and display differential regional and subcellular distribution

The subunit architecture of gamma-aminobutyric acid, type B (GABA(B)), receptors in situ is largely unknown. The GABA(B) receptor variants, characterized by the constituents GBR1a and GBR1b, were therefore analyzed with regard to their subunit composition as well as their regional and subcellular distribution in situ. The analysis was based on the use of antisera recognizing selectively GBR1a, GBR1b, and GBR2. Following their solubilization, GBR1a and GBR1b were both found by immunoprecipitation to occur as heterodimers associated with GBR2. Furthermore, monomers of GBR1a, GBR1b, or GBR2 were not detectable, suggesting that practically all GABA(B) receptors are heterodimers in situ. Finally, there was no evidence for an association of GBR1a with GBR1b indicating that these two constituents represent two different receptor populations. A size determination of solubilized GABA(B) receptors by sucrose density centrifugation revealed two distinct peaks of which one corresponded to dimeric receptors, and the higher molecular weight peak pointed to the presence of yet unknown receptor-associated proteins. The distribution and relative abundance of GBR2 immunoreactivity corresponded in all brain regions to that of the sum of GBR1a and GBR1b, supporting the view that most if not all GBR1 proteins are associated with GBR2. However, GBR1a was present preferentially at postsynaptic densities, whereas GBR1b may be mainly attributed to presynaptic or extrasynaptic sites. Thus, GBR1a and GBR1b are both associated with GBR2 to form heterodimers at mainly different subcellular locations where they are expected to subserve different functions.

Characterisation and partial purification of the GABA(B) receptor from the rat cerebellum using the novel antagonist [3H]CGP 62349

The novel GABA(B) receptor antagonist [3H]CGP 62349 binds rat cerebellar synaptosomal membranes with high affinity at a single population of sites (K(d) = 0.9 nM, B(max) = 760 fmol/mg protein). Solubilisation with 1% Triton X-100/0.5 M NaCl/10% glycerol resulted in a marked increase in [3H]CGP 62349 binding (K(d) = 0.5 nM, B(max) = 1285 fmol/mg protein). Competition of [3HCGP 35348 = CGP 36742. The GABA(A) ligand isoguvacine did not displace [3H]CGP 62349 binding. Partial purification of [3H]CGP 62349 binding sites was obtained by sucrose density centrifugation and a predominant protein in the peak binding fraction was recognised by an anti-GABA(B) receptor antibody and had a molecular weight similar to the recombinant expressed GABA(B)R1a. These results demonstrate that [3H]CGP 62349 provides a useful additional tool for further characterisation of the pharmacology and biochemistry of the native GABA(B) receptor.

Alternative splicing generates a novel isoform of the rat metabotropic GABA(B)R1 receptor

Here we present a novel isoform of the metabotropic G-protein-coupled receptor for gamma-aminobutyric acid (GABA). The isoform, termed GABA(B)R1c (R1c), differs from the recently identified R1a and R1b receptors by an in-frame insertion of 31 amino acids between the second extracellular loop and the fifth transmembrane region. Analysis of the rat GABA(B)R1 gene demonstrates that the insertion is the result of an alternative splicing event within a 567-bp intron between exons 16 and 17. In situ hybridization in the rat brain shows a wide distribution of R1c transcripts and an overlap with the R1a and R1b transcripts. The highest mRNA levels are found in cerebellar Purkinje cells, cerebral cortex, thalamus and hippocampal CA1 and CA3 regions. Western blots and immunodetection of recombinant epitope-tagged receptors as well as [125I]CGP71872 photoaffinity labelling of cell membranes demonstrate that R1c is correctly expressed, although at a lower level than the previously identified isoforms. When coexpressed with the newly characterized GABA(B)R2, R1c functionally couples to G-protein-activated Kir3.1/3.2 channels in Xenopus oocytes and to PLC-activating chimeric G(alpha)qo subunits in HEK-293 cells with a similar EC50 for agonists. These data suggest that the R1c isoform represents a functional GABA(B)R in the rat brain.

The N-terminal domain of gamma-aminobutyric Acid(B) receptors is sufficient to specify agonist and antagonist binding

The recently identified gamma-aminobutyric acid type B receptors (GABA(B)Rs) share low sequence similarity with the metabotropic glutamate (mGlu) receptors. Like the mGlu receptors, the N-terminal extracellular domain (NTED) of GABA(B)Rs is proposed to be related to bacterial periplasmic binding proteins (PBPs). However, in contrast to the mGlu receptors, the GABA(B)Rs lack a cysteine-rich region that links the PBP-like domain to the first transmembrane domain. This cysteine-rich region is necessary for the PBP-like domain of mGlu receptors to bind glutamate. To delimit the ligand-binding domain of GABA(B)Rs, we constructed a series of chimeric GABA(B)R1/mGluR1 and truncated GABA(B)R1 receptor mutants. We provide evidence that despite the lack of a cysteine-rich region, the NTED of GABA(B)Rs contains all of the structural information that is necessary and sufficient for ligand binding. Moreover, a soluble protein corresponding to the NTED of GABA(B)Rs reproduces the binding pharmacology of wild-type receptors. This demonstrates that the ligand-binding domain of the GABA(B)Rs can correctly fold when dissociated from the transmembrane domains.