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.

The Cys-loop superfamily of ligand-gated ion channels: the impact of receptor structure on function

The Cys-loop receptors constitute an important superfamily of LGICs (ligand-gated ion channels) comprising receptors for acetylcholine, 5-HT3 (5-hydroxytryptamine; 5-HT3 receptors), glycine and GABA (gamma-aminobutyric acid; GABAA receptors). A vast knowledge of the structure of the Cys-loop superfamily and its impact on channel function have been accrued over the last few years, leading to exciting new proposals on how ion channels open and close in response to agonist binding. Channel opening is initiated by the extracellular association of agonists to discrete binding pockets, leading to dramatic conformational changes, culminating in the opening of a central ion pore. The importance of channel structure is exemplified in the allosteric modulation of channel function by the binding of other molecules to distinct sites on the channel, which exerts an additional level of control on their function. The subsequent conformational changes (gating) lead to channel opening and ion transport. Following channel pore opening, ion selectivity is determined by receptor structure in, and around, the ion pore. As a final level of control, cytoplasmic determinants control the magnitude (conductance) of ion flow into the cell. Thus the Cys-loop receptors are complex molecular motors, with moving parts, which can transduce extracellular signals across the plasma membrane. Once the full mechanical motions involved are understood, it may be possible to design sophisticated therapeutic agents to modulate their activity, or at least be able to throw a molecular spanner into the works!

Multiple assembly signals in gamma-aminobutyric acid (type A) receptor subunits combine to drive receptor construction and composition

Mammalian gamma-aminobutyric acid type A (GABAA) receptors are constructed from a large repertoire of subunits (alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, theta and pi) into a pentameric ion channel. GABA(A) receptor assembly occurs within the endoplasmic reticulum (ER) and involves interactions with chaperone molecules. Only specific subunit combinations can produce functional surface receptors (with a fixed stoichiometry); other subunit combinations are retained within the ER and degraded. Thus, receptor assembly occurs by defined pathways to limit the diversity of GABAA receptors. The key to understanding how receptor diversity is achieved and controlled is the identification of assembly signals capable of distinguishing between other subunit partners. Analysis of an assembly box in alpha1 (residues 57-68) has revealed an absolute requirement for this region in the assembly of alphabeta receptors. Furthermore, a selective requirement for a single amino acid (R66) is observed for the assembly of alpha1beta2, but not alpha1beta1 or alpha1beta3, receptors. In addition, we have characterized an assembly signal in the beta3 subunit that is capable of driving the assembly of beta3, gamma2beta3 and alpha1beta3 receptors. Interestingly, this signal does not appear to utilize the alpha1 assembly box, suggesting the presence of alternative assembly signals within the alpha1 subunit. Although this beta3 signal is sufficient to permit the formation of betagamma receptors it is not necessary, suggesting that alternative assembly signals also exist within the beta3 subunit. These findings support the belief that GABAA receptor assembly occurs via multiple defined pathways that may be determined by subunit availability.

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.

Identification of residues within GABA(A) receptor alpha subunits that mediate specific assembly with receptor beta subunits

GABA(A) receptors can be constructed from a range of differing subunit isoforms: alpha, beta, gamma, delta, and epsilon. Expression studies have revealed that production of GABA-gated channels is achieved after coexpression of alpha and beta subunits. The expression of a gamma subunit isoform is essential to confer benzodiazepine sensitivity on the expressed receptor. However, how the specificity of subunit interactions is controlled during receptor assembly remains unknown. Here we demonstrate that residues 58-67 within alpha subunit isoforms are important in the assembly of receptors comprised of alphabeta and alphabetagamma subunits. Deletion of these residues from the alpha1 or alpha6 subunits results in retention of either alpha subunit isoform in the endoplasmic reticulum on coexpression with the beta3, or beta3 and gamma2 subunits. Immunoprecipitation revealed that residues 58-67 mediated oligomerization of the alpha1 and beta3 subunits, but were without affect on the production of alpha/gamma complexes. Within this domain, glutamine 67 was of central importance in mediating the production of functional alpha1beta3 receptors. Mutation of this residue resulted in a drastic decrease in the cell surface expression of alpha1beta3 receptors and the resulting expression of beta3 homomers. Sucrose density gradient centrifugation revealed that this residue was important for the production of a 9S alpha1beta3 complex representing functional GABA(A) receptors. Therefore, our studies detail residues that specify GABA(A) receptor alphabeta subunit interactions. This domain, which is conserved in all alpha subunit isoforms, will therefore play a critical role in the assembly of GABA(A) receptors composed of alphabeta and alphabetagamma subunits.

Cell surface stability of gamma-aminobutyric acid type A receptors. Dependence on protein kinase C activity and subunit composition

Type A gamma-aminobutyric acid receptors (GABA(A)), the major sites of fast synaptic inhibition in the brain, are believed to be composed predominantly of alpha, beta, and gamma subunits. Although cell surface expression is essential for GABA(A) receptor function, little is known regarding its regulation. To address this issue, the membrane stability of recombinant alpha(1)beta(2) or alpha(1)beta(2)gamma(2) receptors was analyzed in human embryonic kidney cells. Alpha(1)beta(2)gamma(2) but not alpha(1)beta(2) receptors were found to recycle constitutively between the cell surface and a microtubule-dependent, perinuclear endosomal compartment. Similar GABA(A) receptor endocytosis was also seen in cultured hippocampal and cortical neurons. GABA(A) receptor surface levels were reduced upon protein kinase C (PKC) activation. Like basal endocytosis, this response required the gamma(2) subunit but not receptor phosphorylation. Although inhibiting PKC activity did not block alpha(1)beta(2)gamma(2) receptor endocytosis, it did prevent receptor down-regulation, suggesting that PKC activity may block alpha(1)beta(2)gamma(2) receptor recycling to the cell surface. In agreement with this observation, blocking recycling from endosomes with wortmannin selectively reduced surface levels of gamma(2)-containing receptors. Together, our results demonstrate that the surface stability of GABA(A) receptors can be dynamically and specifically regulated, enabling neurons to modulate cell surface receptor number upon the appropriate cues.

Identification of amino acid residues within GABA(A) receptor beta subunits that mediate both homomeric and heteromeric receptor expression

GABA(A) receptors are believed to be heteropentamers that can be constructed from six subunit classes: alpha(1-6), beta(1-4), gamma(1-3), delta, epsilon, and pi. Given that individual neurons often express multiple receptor subunits, it is important to understand how these receptors assemble. To determine which domains of receptor subunits control assembly, we have exploited the differing capabilities of the beta2 and beta3 subunits to form functional cell surface homomeric receptors. Using a chimeric approach, we have identified four amino acids in the N-terminal domain of the beta3 subunit that mediate functional cell surface expression of this subunit compared with beta2, which is retained within the endoplasmic reticulum. Substitution of these four amino acids-glycine 171, lysine 173, glutamate 179, and arginine 180-into the beta2 subunit was sufficient to enable the beta2 subunit to homo-oligomerize. The effect of this putative "assembly signal" on the production of heteromeric receptors composed of alphabeta and betagamma subunits was also analyzed. This signal was not critical for the formation of receptors composed of either alpha1beta2 or alpha1beta3 subunits, suggesting that mutation of these residues did not disrupt subunit folding. However, this signal was important in the formation of betagamma2 receptors. These residues did not seem to affect the initial association of beta2 and gamma2 subunits but appeared to be important for the subsequent production of functional receptors. Our studies identify, for the first time, key residues within the N-terminal domains of receptor beta subunits that mediate the selective assembly of GABA(A) receptors.

Modulation of neuronal and recombinant GABAA receptors by redox reagents

1. The functional role played by the postulated disulphide bridge in gamma-aminobutyric acid type A (GABAA) receptors and its susceptibility to oxidation and reduction were studied using recombinant (murine receptor subunits expressed in human embryonic kidney cells) and rat neuronal GABAA receptors in conjunction with whole-cell and single channel patch-clamp techniques. 2. The reducing agent dithiothreitol (DTT) reversibly potentiated GABA-activated responses (IGABA) of alpha1beta1 or alpha1beta2 receptors while the oxidizing reagent 5, 5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) caused inhibition. Redox modulation of IGABA was independent of GABA concentration, membrane potential and the receptor agonist and did not affect the GABA EC50 or Hill coefficient. The endogenous antioxidant reduced glutathione (GSH) also potentiated IGABA in alpha1beta2 receptors, while both the oxidized form of DTT and glutathione (GSSG) caused small inhibitory effects. 3. Recombinant receptors composed of alpha1beta1gamma2S or alpha1beta2gamma2S were considerably less sensitive to DTT and DTNB. 4. For neuronal GABAA receptors, IGABA was enhanced by flurazepam and relatively unaffected by redox reagents. However, in cultured sympathetic neurones, nicotinic acetylcholine-activated responses were inhibited by DTT whilst in cerebellar granule neurones, NMDA-activated currents were potentiated by DTT and inhibited by DTNB. 5. Single GABA-activated ion channel currents exhibited a conductance of 16 pS for alpha1beta1 constructs. DTT did not affect the conductance or individual open time constants determined from dwell time histograms, but increased the mean open time by affecting the channel open probability without increasing the number of cell surface receptors. 6. A kinetic model of the effects of DTT and DTNB suggested that the receptor existed in equilibrium between oxidized and reduced forms. DTT increased the rate of entry into reduced receptor forms and also into desensitized states. DTNB reversed these kinetic effects. 7. Our results indicate that GABAA receptors formed by alpha and beta subunits are susceptible to regulation by redox agents. Inclusion of the gamma2 subunit in the receptor, or recording from some neuronal GABAA receptors, resulted in reduced sensitivity to DTT and DTNB. Given the suggested existence of alphabeta subunit complexes in some areas of the central nervous system together with the generation and release of endogenous redox compounds, native GABAA receptors may be subject to regulation by redox mechanisms.

Subcellular localization and endocytosis of homomeric gamma2 subunit splice variants of gamma-aminobutyric acid type A receptors

The expression of alpha and beta gamma-aminobutyric acid type A receptor subunits produces GABA-gated channels which require the incorporation of either the gamma2 or gamma3 subunit for benzodiazepine modulation. Here we examine the role of the gamma2 subunit splice variants, gamma2S and gamma2L which differ by eight amino acids in the major intracellular domain, in mediating cell surface expression. Using immunocytochemistry we have demonstrated that when expressed alone, the gamma2S subunit can access the cell surface and internalize constitutively. In contrast, alpha1, beta2 and gamma2L are retained predominantly in the endoplasmic reticulum (ER) when expressed alone. Replacing the insert which differentiates gamma2L from gamma2S (LLRMFSFK) with eight alanines produces a phenotype identical to gamma2S. Both gamma2 subunits fail to produce high molecular weight oligomers observed for alpha1beta2 and alpha1beta2gamma2 heterooligomers and do not form functional ion channels. Surface expression of gamma2S is repressed upon the coexpression of alpha1 or beta2 subunits, resulting in ER-retained heterooligomers, suggesting that homomeric gamma2S is unlikely to occur in vivo. However, its independent maturation to surface competence and preferential assembly with alpha and beta subunits may ensure the production of functional benzodiazepine-sensitive receptors. Furthermore, the presence of the gamma2 subunit appears to confer an endocytotic capacity to these heterooligomeric receptors.

Intracellular retention of recombinant GABAB receptors

gamma-Aminobutyric acid type B (GABAB) receptors mediate the transmission of slow and prolonged inhibitory signals in the central nervous system. Two splice variants of GABAB receptors, GABABR1a and GABABR1b, were recently cloned from a mouse cortical and cerebellar cDNA library. As predicted, these receptors belong to the G protein-coupled receptor superfamily. We have used epitope-tagged versions of GABABR1a receptors to study the cellular distribution of these proteins in a variety of non-neuronal and neuronal cell types. Here we report that recombinant GABAB receptors fail to reach the cell surface when expressed in heterologous systems and are retained in the endoplasmic reticulum when introduced into COS cells. In addition, we prove that recombinant GABAB receptors are excluded from the cell surface when overexpressed in ganglion neurons and we further demonstrate that they fail to activate in superior cervical ganglion neurons. Together our observations suggest that recombinant GABAB receptors require additional information for functional targeting to the plasma membrane.

Adjacent phosphorylation sites on GABAA receptor beta subunits determine regulation by cAMP-dependent protein kinase

Activation of cAMP-dependent protein kinase (PKA) can enhance or reduce the function of neuronal GABAA receptors, the major sites of fast synaptic inhibition in the brain. This differential regulation depends on PKA-induced phosphorylation of adjacent conserved sites in the receptor beta subunits. Phosphorylation of beta 3 subunit-containing receptors at S408 and S409 enhanced the GABA-activated response, whereas selectively mutating S408 to alanine converted the potentiation into an inhibition, comparable to that of beta 1 subunits, which are phosphorylated solely on S409. These distinct modes of regulation were interconvertible between beta 1 and beta 3 subunits and depended upon the presence of S408 in either subunit. In contrast, beta 2 subunit-containing receptors were not phosphorylated or affected by PKA. Differential regulation by PKA of postsynaptic GABAA receptors containing different beta subunits may have profound effects on neuronal excitability.

Subcellular localization of gamma-aminobutyric acid type A receptors is determined by receptor beta subunits

gamma-aminobutyric acid type A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. They are constructed from four subunit classes with multiple members: alpha (1-6), beta (1-4), gamma (1-4), and delta (1). The contribution of subunit diversity in determining receptor subcellular targeting was examined in polarized Madin-Darby canine kidney (MDCK) cells. Significant detection of cell surface homomeric receptor expression by a combination of both immunological and electrophysiological methodologies was only found for the beta 3 subunit. Expression of alpha/beta binary combinations resulted in a nonpolarized distribution for alpha 1 beta 1 complexes, but specific basolateral targeting of both alpha 1 beta 2 and alpha 1 beta 3 complexes. The polarized distribution of these alpha/beta complexes was unaffected by the presence of the gamma 2S subunit. Interestingly, delivery of receptors containing the beta 3 subunit to the basolateral domain occurs via the apical surface. These results show that beta subunits can selectively target GABAA receptors to distinct cellular locations. Changes in the spatial and temporal expression of beta-subunit isoforms may therefore provide a mechanism for relocating GABAA receptor function between distinct neuronal domains. Given the critical role of these receptors in mediating synaptic inhibition, the contribution of different beta subunits in GABAA receptor function, may have implications in neuronal development and for receptor localization/clustering.

Proton sensitivity of the GABA(A) receptor is associated with the receptor subunit composition

1. Modulation of GABA(A) receptors by external H(+) was examined in cultured rat sympathetic neurones, and in Xenopus laevis oocytes and human embryonic kidney (HEK) cells expressing recombinant GABA(A) receptors composed of combinations of alpha 1, beta 1, beta 2, gamma 2S and delta subunits. 2. Changing the external pH from 7.4 reduced GABA-activated currents in sympathetic neurones. pH titration of the GABA-induced current was fitted with a pH model which predicted that H(+) interact with two sites (PK(a) values of 6.4 and 7.2). 3. For alpha 1 beta 1 GABA(A) receptors, low external pH (< 7.4) enhanced responses to GABA. pH titration predicted the existence of two sites with PK(a) values of 6.6 and 7.5. The GABA concentration-response curve was shifted to the left by low pH and non-competitively inhibited at high pH (> 7.4). 4. alpha 1 beta 1 gamma 2S receptor constructs were not affected by external pH, whereas exchanging the beta 1 subunit for beta 2 conferred a sensitivity to pH, with predicted PK(a) values of 5.16 and 9.44. 5. Low pH enhanced the responses to GABA on alpha 1 beta 1 delta subunits, whilst high pH caused an inhibition (PK(a) values of 6.6 and 9.9). The GABA concentration-response curves were enhanced (pH 5.4) or reduced (pH 9.4) with no changes in the GABA EC(50). 6. Immunoprecipitation with subunit and epitope-specific antisera to alpha 1, beta 1 and delta subunits demonstrated that these subunits could co-assemble in cell membranes. 7. Expression of alpha 1 beta 1 gamma 2S delta constructs resulted in a 'bell-shaped' pH titration relationship. Increasing or decreasing external pH inhibited the responses to GABA. 8. The pH sensitivity of recombinant GABA(A) receptors expressed in HEK cells was generally in accordance with data accrued from Xenopus oocytes. However, rapid application of GABA to alpha 1 beta 1 constructs at high pH (> 7.4) caused an increased peak and reduced steady-state current, with a correspondingly increased rate of desensitization. 9. Modulation of GABA(A) receptor function was apparently unaffected by the internal pH. Moreover, pH values between 5 and 9.5 did not significantly affect the charge distribution on the zwitterionic GABA molecules. 10. In conclusion, this study demonstrates that external pH can either enhance, have little effect, or reduce GABA-activated responses, and this is apparently dependent on the receptor subunit composition. The potential importance of H(+) sensitivity of GABA(A) receptors is discussed.

Assembly and cell surface expression of heteromeric and homomeric gamma-aminobutyric acid type A receptors

The ability of differing subunit combinations of gamma-aminobutyric acid type A (GABAA) receptors produced from murine alpha 1, beta 2, and gamma 2L subunits to form functional cell surface receptors was analyzed in both A293 cells and Xenopus oocytes using a combination of molecular, electrophysiological, biochemical, and morphological approaches. The results revealed that GABAA receptor assembly occurred within the endoplasmic reticulum and involved the interaction with the chaperone molecules immunoglobulin heavy chain binding protein and calnexin. Despite all three subunits possessing the ability to oligomerize with each other, only alpha 1 beta 2 and alpha 1 beta 2 gamma 2L subunit combinations could produce functional surface expression in a process that was not dependent on N-linked glycosylation. Single subunits and the alpha 1 gamma 2L and beta 2 gamma 2L combinations were retained within the endoplasmic reticulum. These results suggest that receptor assembly occurs by defined pathways, which may serve to limit the diversity of GABAA receptors that exist on the surface of neurons.

Newly synthesized transferrin receptors can be detected in the endosome before they appear on the cell surface

It is well established that a proportion of newly synthesized lysosomal enzymes and class II major histocompatibility complex antigens are delivered directly to the endocytic pathway from the Golgi complex. Here we show that a significant proportion of newly synthesized transferrin receptors can be detected in endosomes before reaching the cell surface. These newly synthesized transferrin receptors are delivered to the endosome more efficiently than either constitutively secreted soluble proteins or glycophosphatidylinositol-anchored plasma membrane proteins suggesting that their transfer to the endosome is signal-dependent. Identification of a signal-dependent transfer step for proteins like the transferrin receptor operating on the exocytic pathway has important implications for membrane biogenesis, especially in the establishment of cell surface polarity.

Transport into and out of the Golgi complex studied by transfecting cells with cDNAs encoding horseradish peroxidase

We have developed a novel technique with which to investigate the morphological basis of exocytotic traffic. We have used expression of HRP from cDNA in a variety of cells in combination with peroxidase cytochemistry to outline traffic into and out of the Golgi apparatus at the electron microscopic level with very high sensitivity. A secretory form of the peroxidase (ssHRP) is active from the beginning of the secretory pathway and the activity is efficiently cleared from cells. Investigation of the morphological elements involved in the itinerary of soluble ER proteins using ssHRP tagged with the ER retention motif (ssHRPKDEL) shows that it progresses through the Golgi stack no further than the cis-most element. Traffic between the RER and the Golgi stack as outlined by ssHRPKDEL occurs via vesicular carriers as well as by tubular elements. ssHRP has also been used to investigate the trans side of the Golgi complex, where incubation at reduced temperatures outlines the trans-Golgi network with HRP reaction product. Tracing the endosomal compartment with transferrin receptor in double-labeling experiments with ssHRP fails to show any overlap between these two compartments.