Constitutive endocytosis of GABAA receptors by an association with the adaptin AP2 complex modulates inhibitory synaptic currents in hippocampal neurons

Regulation of native GABAA receptors by PKC and protein phosphatase activity

RATIONALE AND OBJECTIVE

Protein kinase C (PKC) modulation of ionotropic receptors is a common mechanism for regulation of channel function. The effects of PKC and phosphatase activation on native gamma-aminobutyric acid (GABA(A)) receptors in adult brain are unknown. Previous studies of recombinant GABA(A) receptors have provided evidence that PKC activation inhibits receptor function, whereas other studies suggest that PKC either increases or does not alter GABA(A) receptor function. The present study explored (a) the effects of PKC and phosphatase activity on GABA-mediated (36)Cl(-) uptake in cerebral cortical synaptoneurosomes and (b) the effect of PKC activity on muscimol-induced loss of righting reflex (LORR) in adult rats.

METHODS

GABA(A) receptor function in vitro was measured by muscimol-induced (36)Cl(-) uptake into cerebral cortical synaptoneurosomes. The in vivo effect of PKC on GABA(A)-mediated function was measured by intracerebroventricular (i.c.v.) injection of 4-beta-phorbol-12,13-dibutyrate (PDBu) or calphostin C followed by determination of muscimol-induced LORR.

RESULTS

Adenosine triphosphate (ATP) and PDBu produced a concentration-dependent and specific reduction in muscimol-stimulated (36)Cl(-) uptake that was blocked by the PKC inhibitor calphostin C. Both adenosine diphosphate and 4alphaPDBu were ineffective. Phosphatase inhibition produced similar inhibition of muscimol responses. Furthermore, i.c.v. administration of PDBu and calphostin C produced opposing effects on both the onset and the duration of muscimol-induced LORR in rats.

CONCLUSIONS

The present study provides evidence that PKC activation reduces GABA(A) receptor function in native receptors both in vitro and in vivo. Phosphatase inhibitors decrease muscimol-mediated Cl(-) uptake in GABA(A) receptors demonstrating coordinated regulation of native receptors by PKC and phosphatases.

Amyloid precursor protein expression in the rat hippocampal dentate gyrus modulates during memory consolidation

Despite advances in our understanding of the basic biology of amyloid precursor protein (APP), the normal physiological function(s) of APP in learning and memory remains unclear. Here we show increased APP degradation in the hippocampus to be associated with the consolidation of a passive avoidance response. Neurone-specific APP695 expression became transiently reduced 2-4 h post-training through association with endosomal adaptin proteins and enhanced internalization. By contrast, internalization of glial-associated APP containing a Kunitz protease inhibitor-like domain (APP-KPI) was dependent on the low-density lipoprotein receptor-related protein (LRP). In addition, LRP expression and association with apolipoprotein E increased in the 2-4 h post-training period. The LRP antagonist receptor-associated protein prevented the APP-KPI internalization and LRP-apolipoprotein E association and this resulted in amnesia. Degradation of APP695 and APP-KPI did not appear to be related to alpha-secretase activity, as no learning-associated increase of secreted APP was observed in the CSF. Moreover, as internalization of APP isoforms was observed only in dentate gyrus, it probably relates to the learning-associated restructuring of the perforant path terminals. Memory-associated APP processing in both neuronal and glial compartments points to a role for glial unsheathing of synaptic connections, an event required for the synaptic restructuring that accompanies memory consolidation. These observations may have a direct relevance to understanding the pathophysiology of Alzheimer's disease as beta/gamma-secretase-derived beta-amyloid is formed following internalization of cell surface APP into the endosomal compartment.

Direct interaction of N-ethylmaleimide-sensitive factor with GABAA receptor β subunits

GABA(A) receptors mediate most of the fast inhibitory neurotransmission in the brain, and are believed to be composed mainly of alpha, beta, and gamma subunits. It has been shown that GABA(A) receptors interact with a number of binding partners that act to regulate both receptor function and cell surface stability. Here, we reveal that GABA(A) receptors interact directly with N-ethylmaleimide-sensitive factor (NSF), a critical regulator of vesicular dependent protein trafficking, as measured by in vitro protein binding and co-immunoprecipitation assays. In addition, we established that NSF interacts with residues 395-415 of the receptor beta subunits and co-localizes with GABA(A) receptors in hippocampal neurons. We also established that NSF can regulate GABA(A) receptor cell surface expression depending upon residues 395-415 in the beta3 subunit. Together, our results suggest an important role for NSF activity in regulating the cell surface stability of GABA(A) receptors.

Phospho-dependent binding of the clathrin AP2 adaptor complex to GABAA receptors regulates the efficacy of inhibitory synaptic transmission

The efficacy of synaptic inhibition depends on the number of gamma-aminobutyric acid type A receptors (GABA(A)Rs) expressed on the cell surface of neurons. The clathrin adaptor protein 2 (AP2) complex is a critical regulator of GABA(A)R endocytosis and, hence, surface receptor number. Here, we identify a previously uncharacterized atypical AP2 binding motif conserved within the intracellular domains of all GABA(A)R beta subunit isoforms. This AP2 binding motif (KTHLRRRSSQLK in the beta3 subunit) incorporates the major sites of serine phosphorylation within receptor beta subunits, and phosphorylation within this site inhibits AP2 binding. Furthermore, by using surface plasmon resonance, we establish that a peptide (pepbeta3) corresponding to the AP2 binding motif in the GABA(A)R beta3 subunit binds to AP2 with high affinity only when dephosphorylated. Moreover, the pepbeta3 peptide, but not its phosphorylated equivalent (pepbeta3-phos), enhanced the amplitude of miniature inhibitory synaptic current and whole cell GABA(A)R current. These effects of pepbeta3 on GABA(A)R current were occluded by inhibitors of dynamin-dependent endocytosis supporting an action of pepbeta3 on GABA(A)R endocytosis. Therefore phospho-dependent regulation of AP2 binding to GABA(A)Rs provides a mechanism to specify receptor cell surface number and the efficacy of inhibitory synaptic transmission.

Multiple motifs regulate the trafficking of GABA(B) receptors at distinct checkpoints within the secretory pathway

gamma-Aminobutyric acid type B receptors (GABA(B)) are G-protein-coupled receptors that mediate GABAergic inhibition in the brain. Their functional expression is dependent upon the formation of heterodimers between GABA(B)R1 and GABA(B)R2 subunits, a process that occurs within the endoplasmic reticulum (ER). However, the mechanisms that regulate receptor surface expression remain largely unknown. Here, we demonstrate that access to the cell surface for GABA(B)R1 is sequentially controlled by an RSR(R) motif and a LL motif within its cytoplasmic domain. In addition, we reveal that msec7-1, a guanine-nucleotide-exchange factor (GEF) for the ADP-ribosylation factor (ARF) family of GTPases, critical regulators of vesicular membrane trafficking, interacts with GABA(B)R1 via the LL motif in this subunit. Finally, we establish that msec7-1 modulates the cell surface expression of GABA(B) receptors, a process that is dependent upon the integrity of the LL motif in GABA(B)R1. Together, our results demonstrate that the cell surface expression of the GABA(B)R1 subunit is regulated by multiple motifs, which act at distinct checkpoints in the secretory pathway, and also suggest a novel role for msec7-1 in regulating the membrane trafficking of GABA(B)R1 subunits.

Early expression of KCC2 in rat hippocampal cultures augments expression of functional GABA synapses

The development of GABAergic synapses is associated with an excitatory to inhibitory shift of the actions of GABA because of a reduction of [Cl-]i. This is due to a delayed postnatal expression of the K+ -Cl- cotransporter KCC2, which has low levels at birth and peaks during the first few postnatal weeks. Whether the expression of the cotransporter and the excitatory to inhibitory shift have other consequences on the operation of GABA(A) receptors and synapses is not yet known. We have now expressed KCC2 in immature neurones at an early developmental stage and determined the consequences on the formation of GABA and glutamate synapses. We report that early expression of the cotransporter selectively enhances GABAergic synapses: there is a significant increase of the density of GABA(A) receptors and synapses and an increase of the frequency of GABAergic miniature postsynaptic currents. The density of glutamate synapses and frequency of AMPA miniature postsynaptic currents are not affected. We conclude that the expression of KCC2 and the reduction of [Cl-]i play a critical role in the construction of GABAergic networks that extends beyond the excitatory to inhibitory shift of the actions of GABA.

Dynamic mobility of functional GABAA receptors at inhibitory synapses

Importing functional GABAA receptors into synapses is fundamental for establishing and maintaining inhibitory transmission and for controlling neuronal excitability. By introducing a binding site for an irreversible inhibitor into the GABAA receptor alpha1 subunit channel lining region that can be accessed only when the receptor is activated, we have determined the dynamics of receptor mobility between synaptic and extrasynaptic locations in hippocampal pyramidal neurons. We demonstrate that the cell surface GABAA receptor population shows no fast recovery after irreversible inhibition. In contrast, after selective inhibition, the synaptic receptor population rapidly recovers by the import of new functional entities within minutes. The trafficking pathways that promote rapid importation of synaptic receptors do not involve insertion from intracellular pools, but reflect receptor diffusion within the plane of the membrane. This process offers the synapse a rapid mechanism to replenish functional GABAA receptors at inhibitory synapses and a means to control synaptic efficacy.

Interactions between ρ and γ2 subunits of the GABA receptor

The gamma-aminobutyric acid type C (GABA(C)) receptor is a ligand-gated chloride channel with distinct physiological and pharmacological properties. Although the exact subunit composition of native GABA(C) receptors has yet to be firmly established, there is general agreement that GABA rho subunits participate in their formation. Recent studies on white perch suggest that certain GABA rho subunits can co-assemble with the GABA(A) receptor gamma2 subunit to form a heteromeric receptor with electrophysiological properties that correspond more closely to the native GABA(C) receptor on retinal neurons than any of the homomeric rho receptors. In the present study we examined the interactions among various perch GABA rho and gamma2 subunits. When co-expressed in Xenopus oocytes, the gamma2 subunit co-immunoprecipitated with Flag-tagged perch rho1A, rho1B, and rho2B subunits, but not with the Flag-tagged perch rho2A subunit. Immunocytochemical studies indicated that the membrane surface expression of the gamma2 subunit was detected only when it was co-expressed with perch rho1A, rho1B, or rho2B subunit, but not with the perch rho2A subunit or when expressed alone. In addition, co-immunoprecipitation of perch rho1B and gamma2 subunits was also detected in protein samples of the teleost retina. Taken together, these findings suggest that a heteromeric rho(gamma2) receptor could represent one form of GABA(C) receptor on retinal neurons.

Internalized GABA-receptor subunits are transferred to an intracellular pool associated with the postsynaptic density

Endocytosis represents an important mechanism regulating cell-surface expression of neurotransmitter receptors, including GABAA receptors, in neurons. Little is known, however, about trafficking of internalized receptors. Here, we used antibody tagging in living rat hippocampal neurons in culture to monitor GABAA receptor internalization. We show that cell-surface receptors have a homogeneous distribution reflecting their mobility in the membrane. Unexpectedly, internalized GABAA receptors were detected mainly in a subsynaptic pool associated with gephyrin at postsynaptic sites, whereas AMPA-type glutamate receptors were accumulated in the soma. This process was time-dependent and could be prevented by blocking clathrin-coated vesicle endocytosis. In control experiments, the existence of an intracellular pool of GABAA receptors associated with gephyrin was confirmed independently of internalization of surface receptors, and constitutive endocytosis, unrelated to antibody-tagging, could be demonstrated for both AMPA and GABAA receptors using a biotinylation assay. These results suggest that cycling of GABAA receptors between the cell surface and the subsynaptic pool provides a mechanism for the short-term regulation of GABAergic neurotransmission. Furthermore, the close association of gephyrin with internalized GABAA receptors suggests a role in intracellular receptor trafficking.

Paraneoplastic stiff-person syndrome: passive transfer to rats by means of IgG antibodies to amphiphysin

BACKGROUND

Stiff-person syndrome (SPS) with antibodies to amphiphysin is a paraneoplastic disorder of the central nervous system with a putative autoimmune pathogenesis. Proof of a causal role of the antibodies is still lacking for this and all other antibody-associated paraneoplastic syndromes of the central nervous system.

METHODS

We obtained the plasma filtrate of a patient with breast cancer and SPS that responded to therapeutic plasmapheresis. The purified IgG fraction included high-titre antibodies to the synaptic protein amphiphysin. In a cotransfer design, this IgG fraction was injected intraperitoneally into female Lewis rats that had received encephalitogenic T-helper (Th) lymphocytes specific for myelin basic protein, to induce an immune-mediated leaky blood-brain barrier. The rats were followed up with behavioural tests, video photography, and electromyography.

FINDINGS

The injection of the IgG fraction including antibodies to amphiphysin resulted in a dose-dependent stiffness with spasms resembling human SPS. Control IgG injected into rats that had received the same encephalitogenic Th cells had no effect. IgG binding was demonstrated in the central nervous system of rats that showed signs of the disorder.

INTERPRETATION

These experiments support the hypothesis of a pathogenetic role of antibodies to amphiphysin, thus adding paraneoplastic SPS to the group of antibody-mediated autoimmune disorders.

RELEVANCE TO PRACTICE

These findings provide a strong argument for a direct pathogenetic role of anti-amphiphysin in this type of SPS and support therapeutic attempts to eliminate these autoantibodies by plasmapheresis. The experimental approach used could help to elucidate the role of autoantibodies in other paraneoplastic syndromes, such as SPS with antibodies to glutamic acid decarboxylase, and others including anti-Hu-associated subacute cerebellar degeneration and limbic encephalitis.

Regulation of N-methyl-D-aspartate receptors by calpain in cortical neurons

The N-methyl-D-aspartate (NMDA) receptor is a cation channel highly permeable to calcium and plays critical roles in governing normal and pathologic functions in neurons. Calcium entry through NMDA receptors (NMDARs) can lead to the activation of the Ca2+-dependent protease, calpain. Here we investigated the involvement of calpain in regulation of NMDAR channel function. After prolonged (5-min) treatment with NMDA or glutamate, the whole-cell NMDAR-mediated current was significantly reduced in both acutely dissociated and cultured cortical pyramidal neurons. The down-regulation of NMDAR current was blocked by bath application of selective calpain inhibitors. Intracellular injection of a specific calpain inhibitory peptide also eliminated the down-regulation of NMDAR current induced by prolonged NMDA treatment. In contrast, dynamin inhibitory peptide had no effect on the depression of NMDAR current, suggesting the lack of involvement of dynamin/clathrin-mediated NMDAR internalization in this process. Immunoblotting analysis showed that the NR2A and NR2B subunits of NMDARs were markedly degraded in cultured cortical neurons treated with glutamate, and the degradation of NR2 subunits was prevented by calpain inhibitors. Taken together, our results suggest that prolonged activation of NMDARs in neurons activates calpain, and activated calpain in turn down-regulates the function of NMDARs, which provides a neuroprotective mechanism against NMDAR overstimulation accompanying ischemia and stroke.

Cultured Hippocampal Pyramidal Neurons Express Two Kinds of GABAA Receptors

We combined a study of the subcellular distribution of the alpha1, alpha2, alpha4, beta1, beta2/3, gamma2, and delta subunits of the GABAA receptor with an electrophysiological analysis of GABAA receptor currents determine the to types of receptors expressed on cultured hippocampal pyramidal neurons. The immunocytochemistry study demonstrated that alpha1, alpha2, beta2/3, and gamma2 subunits formed distinct clusters of various sizes, which were colocalized with clusters of glutamate decarboxylase (GAD) immunoreactivity at rates ranging from 22 to 58%. In contrast, alpha4, beta1, and delta subunits were distributed diffusely over the cell soma and neuronal processes of cultured neurons and did not colocalize with the synaptic marker GAD. Whole-cell GABA receptor currents were moderately sensitive to GABAA and were modulated by diazepam. The whole-cell currents were also enhanced by the neurosteroid allopregnanolone (10 nM). Tonic currents, measured as changes in baseline current and noise, were sensitive to Zn2+, furosemide, and loreclezole; they were insensitive to diazepam. These studies suggest that two kinds of GABAA receptors are expressed on cultured hippocampal neurons. One kind of receptor formed clusters, which were present at GABAergic synapses and in the extrasynaptic membrane. The alpha1, alpha2, beta2/3, and gamma2 subunits were contained in clustered receptors. The second kind was distributed diffusely in the extrasynaptic membrane. The alpha4, beta1, and delta subunits were contained in these diffusely distributed receptors. The properties of tonic currents recorded from these neurons were similar to those from recombinant receptors containing alpha4, beta1, and delta subunits.

Association of GRIP1 with a GABA(A) receptor associated protein suggests a role for GRIP1 at inhibitory synapses

GABA(A) receptors mediate the majority of fast synaptic inhibition in the mammalian central nervous system. GABA(A) receptors associate with a number of cytosolic proteins important for regulating their function including the GABA(A) receptor gamma2 subunit associated protein GABARAP. Here we show GABARAP associates with the synaptic PDZ domain containing protein GRIP1. GRIP1 has been localized to inhibitory synapses however the role of this protein with respect to neuronal inhibition remains unclear. Using in vitro protein interaction assays we show that GABARAP interacts directly with PDZ domains 4-6 of GRIP1. Furthermore, using coimmunoprecipitation assays we show that GABARAP interacts with GRIP1 in vivo. Finally, we show that GRIP1 colocalizes with gamma2 subunit containing GABA(A) receptors in cultured hippocampal neurons. Our findings provide evidence that GRIP1 can associate with proteins important for regulating GABA(A) receptor function and suggest that GRIP1 may play a role at inhibitory synapses.

PKC modulation of GABAA receptor endocytosis and function is inhibited by mutation of a dileucine motif within the receptor β2 subunit

The modulation of GABAA receptors by protein kinase C is complex and involves effects on both ion channel function and receptor trafficking. Although PKC regulates receptor cell surface expression the mechanism is not well understood. Using immunofluorescence studies in HEK 293 cells, we demonstrate that activation of PKC by the phorbol ester PMA promotes receptor endocytosis and is dependent on the presence of a gamma subunit. This endocytosis is blocked by the dominant negative dynamin mutant K44A indicating that PKC-induced receptor endocytosis involves the dynamin endocytic pathway. Mutation of a dileucine motif within the receptor beta2 subunit inhibits the effect of PKC activation on receptor endocytosis. Using patch clamp analysis, we show that PKC activation produces a robust inhibition of GABA-gated chloride currents in cells expressing wildtype GABAA receptors, but it is ineffective in modulating receptors lacking the dileucine motif. Furthermore, the introduction into the patch pipette of a 10-amino acid peptide corresponding to the dileucine motif present in the receptor beta2 subunit prevents PKC modulation of wildtype recombinant receptors. Furthermore, in cerebral cortical neuronal slices inclusion of this peptide in the patch pipette prevents PKC modulation of native GABAA receptors. Using limited chymotrypsin digestion assays, we also show that PKC increases receptor internalization in primary cultures of cerebral cortical neurons. Lastly, PKC inhibitors do not block constitutive receptor endocytosis or affect GABA-gated chloride currents suggesting that PKC-dependent phosphorylation is not required for GABAA receptor endocytosis but plays a modulatory role in the process.

Receptor trafficking and synaptic plasticity

Long-term potentiation and long-term depression are processes that have been widely studied to understand the molecular basis of information storage in the brain. Glutamate receptors are required for the induction and expression of these forms of plasticity, and GABA (gamma-aminobutyric acid) receptors are involved in their modulation. Recent insights into how these receptors are rapidly moved into and out of synaptic membranes has profound implications for our understanding of the mechanisms of long-term potentiation and long-term depression.

The GABAB2 subunit is critical for the trafficking and function of native GABAB receptors

Studies in heterologous systems have demonstrated that heterodimerisation of the two GABA(B) receptor subunits appears to be crucial for the trafficking and signalling of the receptor. Gene targeting of the GABA(B1) gene has demonstrated that the expression of GABA(B1) is essential for GABA(B) receptor function in the central nervous system (CNS). However, the contribution of the GABA(B2) subunit in the formation of native GABA(B) receptors is still unclear, in particular whether other proteins can substitute for this subunit. We have created a transgenic mouse in which the endogenous GABA(B2) gene has been mutated in order to express a C-terminally truncated version of the protein. As a result, the GABA(B1) subunit does not reach the cell surface and concomitantly both pre- and post-synaptic GABA(B) receptor functions are abolished. Taken together with previous gene deletion studies for the GABA(B1) subunit, this suggests that classical GABA(B) function in the brain is exclusively mediated by GABA(B1/2) heteromers.

NMDA receptors are movin' in

Dynamic modulation of the number of postsynaptic glutamate receptors is considered one of the main mechanisms for altering the strength of excitatory synapses in the central nervous system (CNS). However, until recently N-methyl-d-aspartate (NMDA) receptors were considered relatively stable once in the plasma membrane, especially in comparison with alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors that are internalized at a high rate. A series of recent studies has changed this viewpoint by revealing that NMDA receptors are subject to constitutive as well as agonist-induced internalization through clathrin-mediated endocytosis. Surprisingly, agonist-induced internalization is not dependent on current flow through the NMDA channel, and the receptors are primed for this type of internalization by selective stimulation of the glycine site but not of the glutamate site. Endocytosis of NMDA receptors provides a fundamental mechanism for dynamic regulation of the number of NMDA receptors at synapses, which might be important for physiological and pathological functioning of the CNS.

Binge ethanol exposure delays development of GABAergic miniature postsynaptic currents in septal neurons

Whole cell GABA(A)R currents of septal neurons isolated from rat pups increase rapidly during the first weeks of life when inhibitory synapses are forming. Early postnatal binge ethanol intubation on days 4-9 delays this maturational up-regulation in septal neurons isolated several days later suggesting inhibitory synapse formation could be disrupted [S.-H. Hsiao, J.L. Acevedo, D.W. DuBois, K.R. Smith, J.R. West, G.D. Frye, Early postnatal ethanol intubation blunts GABA(A) receptor upregulation and modifies 3alpha-hydroxy-5alpha-pregnan-20-one sensitivity in rat MS/DB neurons, Brain Res. Dev. Brain Res. 130 (2001) 25-40]. Surprisingly, whole cell GABA(A)R function does not increase rapidly when septal neurons are grown for the same period in vitro and is not blunted by comparable ethanol exposure of the cultures [S.-H. Hsiao, D.W. DuBois, R.C. Miranda, G.D. Frye, Critically timed ethanol exposure reduces GABA(A)R function on septal neurons developing in vivo but not in vitro, Brain Res Dev. Brain Res. 1008 (2004) 69-80]. Because GABAergic miniature postsynaptic currents (mPSCs) show parallel patterns of maturation whether cortical neurons are growing in vivo or in vitro [D.D. Dunning, C.L. Hoover, I. Soltesz, M.A. Smith, D.K. ODowd, GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons, J. Neurophysiol. 82 (1999) 3286-3297], we examined the impact of binge ethanol exposure on synaptic receptors activated by these currents in septal cultures. Binge ethanol treatment of embryonic septal neurons over 6-11 days in vitro (DIV) slightly reduced GABA(A)R-mediated mPSC amplitude and frequency, but also substantially slowed decay kinetics when mPSCs were recorded later on DIV 13-18. Decreased frequency and slowed mPSC decay kinetics after ethanol were consistent with parameters measured in immature neurons. Untreated septal neurons exhibited decreased mPSC amplitude and frequency with acute 30-100 mM ethanol, without changing decay kinetics suggesting a direct inhibition of postsynaptic receptors. Sustained inhibition of GABA(A)Rs with 100 microM picrotoxin on DIV 6-11 decreased mPSC amplitude and frequency and slowed decay kinetics similar to binge ethanol exposure. These results suggest that binge ethanol exposure delays mPSC maturation by interfering with trophic postsynaptic GABA(A)R signaling during the early development of septal neurons.

Epileptogenesis causes acute and chronic increases in GABAA receptor endocytosis that contributes to the induction and maintenance of seizures in the hippocampal culture model of acquired epilepsy

Altered GABAergic inhibitory tone has been observed in association with a number of both acute and chronic models of epilepsy and is believed to be the result, in part, of a decrease in function of the postsynaptic GABAA receptor (GABAAR). This study was carried out to investigate if alterations in receptor internalization contribute to the decrease in GABAAR function observed with epilepsy, utilizing the hippocampal neuronal culture model of low-Mg2+-induced spontaneous recurrent epileptiform discharges (SREDs). Analysis of GABAAR function in "epileptic" cultures showed a 62% reduction in [3H]flunitrazepam binding to the GABAA alpha receptor subunit and a 50% decrease in GABA currents when compared with controls. Confocal microscopy analysis of immunohistochemical staining of GABAAR beta2/beta3 subunit expression revealed approximately a 30% decrease of membrane staining in hippocampal cultures displaying SREDs immediately after low-Mg2+ treatment and in the chronic epileptic state. Low-Mg2+-treated cultures internalized antibody labeled GABAA receptor with an increase in rate of 68% from control. Inhibition of GABAAR endocytosis in epileptic cultures resulted in both a recovery to control levels of membrane GABAA beta2/beta3 immunostaining and a total blockade of SREDs. These results indicate that altered GABAAR endocytosis contributes to the decrease in GABAAR expression and function observed in this in vitro model of epilepsy and plays a role in causing and maintaining SREDs. Understanding the mechanisms underlying altered GABAA R recycling may offer new insights into the pathophysiology of epilepsy and provide novel therapeutic strategies to treat this major neurological condition.

Huntingtin-associated protein 1 regulates inhibitory synaptic transmission by modulating gamma-aminobutyric acid type A receptor membrane trafficking

Gamma-aminobutyric acid type A receptors (GABA(A)Rs) are the major sites of fast synaptic inhibition in the brain. An essential determinant for the efficacy of synaptic inhibition is the regulation of GABA(A)R cell surface stability. Here, we have examined the regulation of GABA(A)R endocytic sorting, a critical regulator of cell surface receptor number. In neurons, rapid constitutive endocytosis of GABA(A)Rs was evident. Internalized receptors were then either rapidly recycled back to the cell surface, or on a slower time scale, targeted for lysosomal degradation. This sorting decision was regulated by a direct interaction of GABA(A)Rs with Huntingtin-associated protein 1 (HAP1). HAP1 modulated synaptic GABA(A)R number by inhibiting receptor degradation and facilitating receptor recycling. Together these observations have identified a role for HAP1 in regulating GABA(A)R sorting, suggesting an important role for this protein in the construction and maintenance of inhibitory synapses.

Preferential accumulation of GABAA receptor gamma 2L, not gamma 2S, cytoplasmic loops at rat spinal cord inhibitory synapses

Alternative splicing generates two variants of the GABAAR gamma2-subunit, gamma2S and gamma2L, which differ by insertion of the amino acid sequence LLRMFSFK into the large cytoplasmic loop between transmembrane domains 3 and 4. This additional sequence within the GABAAR gamma2L-subunit contains the potential protein kinase C (PKC) phosphorylation site serine 343 (Ser343). In the present study we intended to determine the capacity of these two splice variants to accumulate at inhibitory synaptic terminals and to colocalize with gephyrin, and to find out whether phosphorylation of Ser343 has any effect on GABAAR distribution. Green fluorescent protein (GFP)-tagged large cytoplasmic loops of GABAAR gamma2S and gamma2L (GFP::gamma2S/L) were used as surrogates for full-length receptors to study the function of the individual gamma2S and gamma2L peptides in transfected spinal cord neurones (SCNs) and COS-7 cells. It was found that GFP::gamma2L displayed a significantly higher capacity to accumulate at inhibitory synapses than GFP::gamma2S. GABAAR GFP::gamma2S accumulation at inhibitory postsynaptic sites was suppressed to the extent that GFP::gamma2S assumed a diffuse cytosolic distribution. PKC activation facilitated the postsynaptic clustering of GFP::gamma2L but not of GFP::gamma2S. This required the Ser343 residue, since substituting Ala343 for Ser343 produced a diffuse cytosolic localization pattern, like that of GFP::gamma2S. Furthermore, upon PKC activation Discosoma Red2-tagged GABAAR gamma2L (DsRed 2::gamma2L) colocalized with gephyrin in transfected COS-7 cells. These results support the idea that alternative splicing regulates the access of GABAARs to inhibitory postsynaptic sites in a Ser343 phosphorylation-regulated way.

Quantitative effects produced by modifications of neuronal activity on the size of GABAA receptor clusters in hippocampal slice cultures

The number and strength of GABAergic synapses needs to be precisely adjusted for adequate control of excitatory activity. We investigated to what extent the size of GABA(A) receptor clusters at inhibitory synapses is under the regulation of neuronal activity. Slices from P7 rat hippocampus were cultured for 13 days in the presence of bicuculline or 4-aminopyridine (4-AP) to increase neuronal activity, or DNQX to decrease activity. The changes provoked by these treatments on clusters immunoreactive for the alpha1 and alpha2 subunits of the GABA(A) receptor or gephyrin were quantitatively evaluated. While an increase in activity augmented the density of these clusters, a decrease in activity provoked, in contrast, a decrease in their density. An inverse regulation was observed for the size of individual clusters. Bicuculline and 4-AP decreased whilst DNQX increased the mean size of the clusters. When the pharmacological treatments were applied for 2 days instead of 2 weeks, no effects on the size of the clusters were observed. The variations in the mean size of individual clusters were mainly due to changes in the number of small clusters. Finally, a regulation of the size of GABA(A) receptor clusters occurred during development in vivo, with a decrease of the mean size of the clusters between P7 and P21. This physiological change was also the result of an increase in the number of small clusters. These results indicate that neuronal activity regulates the mean size of GABA(A) receptor- and gephyrin-immunoreactive clusters by modifying specifically the number of synapses with small clusters of receptors.

A four PDZ domain-containing splice variant form of GRIP1 is localized in GABAergic and glutamatergic synapses in the brain

We have isolated, from a rat brain cDNA library, a clone corresponding to a 2779-bp cDNA encoding a novel splice form of the glutamate receptor interacting protein-1 (GRIP1). We call this 696-amino acid splice form GRIP1c 4-7 to differentiate it from longer splice forms of GRIP1a/b containing seven PDZ domains. The four PDZ domains of GRIP1c 4-7 are identical to PDZ domains 4-7 of GRIP1a/b. GRIP1c 4-7 also contains 35 amino acids at the N terminus and 12 amino acids at the C terminus that are different from GRIP1a/b. In transfected HEK293 cells, a majority of GRIP1c 4-7 was associated with the plasma membrane. GRIP1c 4-7 interacted with GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor. In low density hippocampal cultures, GRIP1c 4-7 clusters colocalized with GABAergic (where GABA is gamma-aminobutyric acid) and glutamatergic synapses, although a higher percentage of GRIP1c 4-7 clusters colocalized with gamma-aminobutyric acid, type A, receptor (GABA(A)R) clusters than with alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor clusters. Transfection of hippocampal neurons with hemagglutinin-tagged GRIP1c 4-7 showed that it could target to the postsynaptic complex of GABAergic synapses colocalizing with GABA(A)R clusters. GRIP1c 4-7-specific antibodies, which did not recognize previously described splice forms of GRIP1, recognized a 75-kDa protein that is enriched in a postsynaptic density fraction isolated from rat brain. EM immunocytochemistry experiments showed that in intact brain GRIP1c 4-7 concentrates at postsynaptic complexes of both type I glutamatergic and type II GABAergic synapses although it is also presynaptically localized. These results indicate that GRIP1c 4-7 plays a role not only in glutamatergic synapses but also in GABAergic synapses.

Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses

Neural inhibition in the brain is mainly mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors. Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at perisynaptic and extrasynaptic locations where they prolong phasic inhibition and mediate tonic inhibition, respectively. Of special interest are mechanisms that modulate the stability and function of postsynaptic GABA(A) receptor subtypes and that are implicated in functional plasticity of inhibitory transmission in the brain. We will summarize recent progress on the classification of synaptic versus extrasynaptic receptors, the molecular composition of the postsynaptic cytoskeleton, the function of receptor-associated proteins in trafficking of GABA(A) receptors to and from synapses, and their role in post-translational signaling mechanisms that modulate the stability, density, and function of GABA(A) receptors in the postsynaptic membrane.

The brefeldin A-inhibited GDP/GTP exchange factor 2, a protein involved in vesicular trafficking, interacts with the β subunits of the GABAA receptors

We have found that the brefeldin A-inhibited GDP/GTP exchange factor 2 (BIG2) interacts with the beta subunits of the gamma-aminobutyric acid type-A receptor (GABA(A)R). BIG2 is a Sec7 domain-containing guanine nucleotide exchange factor known to be involved in vesicular and protein trafficking. The interaction between the 110 amino acid C-terminal fragment of BIG2 and the large intracellular loop of the GABA(A)R beta subunits was revealed with a yeast two-hybrid assay. The native BIG2 and GABA(A)Rs interact in the brain since both coprecipitated from detergent extracts with either anti-GABA(A)R or anti-BIG2 antibodies. In transfected human embryonic kidney cell line 293 cells, BIG2 promotes the exit of GABA(A)Rs from endoplasmic reticulum. Double label immunofluorescence of cultured hippocampal neurons and electron microscopy immunocytochemistry of rat brain tissue show that BIG2 concentrates in the trans-Golgi network. BIG2 is also present in vesicle-like structures in the dendritic cytoplasm, sometimes colocalizing with GABA(A)Rs. BIG2 is present in both inhibitory GABAergic synapses that contain GABA(A)Rs and in asymmetric excitatory synapses. The results are consistent with the hypotheses that the interaction of BIG2 with the GABA(A)R beta subunits plays a role in the exocytosis and trafficking of assembled GABA(A)R to the cell surface.

Hetero-oligomerization between GABAA and GABAB Receptors Regulates GABAB Receptor Trafficking

The neurotransmitter gamma-aminobutyric acid (GABA) mediates inhibitory signaling in the brain via stimulation of both GABA(A) receptors (GABA(A)R), which are chloride-permeant ion channels, and GABA(B) receptors (GABA(B)R), which signal through coupling to G proteins. Here we report physical interactions between these two different classes of GABA receptor. Association of the GABA(B) receptor 1 (GABA(B)R1) with the GABA(A) receptor gamma2S subunit robustly promotes cell surface expression of GABA(B)R1 in the absence of GABA(B)R2, a closely related GABA(B) receptor that is usually required for efficient trafficking of GABA(B)R1 to the cell surface. The GABA(B)R1/gamma2S complex is not detectably functional when expressed alone, as assessed in both ERK activation assays and physiological analyses in oocytes. However, the gamma2S subunit associates not only with GABA(B)R1 alone but also with the functional GABA(B)R1/GABA(B)R2 heterodimer to markedly enhance GABA(B) receptor internalization in response to agonist stimulation. These findings reveal that the GABA(B)R1/gamma2S interaction results in the regulation of multiple aspects of GABA(B) receptor trafficking, allowing for cross-talk between these two distinct classes of GABA receptor.

Ethanol regulation of γ-aminobutyric acidA receptors: genomic and nongenomic mechanisms

gamma-Aminobutyric acid(A) (GABA(A)) receptors are ligand-gated ion channels that, predominantly, mediate inhibitory synaptic transmission in the CNS. These receptors are pentameric complexes that are comprised of subunits from several classes (alpha, beta, gamma, delta, ), with each class consisting of several isoforms. Chronic ethanol consumption alters GABA(A) receptor function producing cellular tolerance to GABA and ethanol, cross-tolerance to benzodiazepines and barbiturates, and sensitization to inverse agonists. Recent studies have clearly demonstrated that GABA(A) receptors play an important role in ethanol dependence and functional properties of GABA(A) receptor are altered following chronic ethanol administration. However, the exact mechanisms that account for alterations in GABA(A) receptor function following chronic ethanol administration have not been resolved. The mechanisms responsible for adaptation of GABA(A) receptors to chronic ethanol exposure may involve ethanol-induced changes in cell surface expression, subcellular localization, synaptic localization, receptor phosphorylation, neurosteroids, and/or changes in GABA(A) receptor subunit composition. In this review, we provide an overview of recent data pertaining to mechanisms that could be responsible for altered properties and expression of GABA(A) receptors following chronic ethanol administration.

[The life cycle of the GABA(A) receptor and its regulating molecules]

Gamma-aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the central nervous system. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their function or cell surface stability will have major consequences for neuronal excitation. This review highlights recent findings on the regulation of GABA(A)-receptor expression and function, focusing on the mechanisms of sorting, targeting, synaptic clustering, and endocytic events of GABA(A) receptors, all which are regulated by their associated proteins. Now these topics are an area of active interest in studies on inhibitory neurotransmission.

Marlin-1, a novel RNA-binding protein associates with GABA receptors

GABA(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. Whereas heterodimerization between GABA(B) receptor GABA(B)R1 and GABA(B)R2 subunits is essential for functional expression, how neurons coordinate the assembly of these critical receptors remains to be established. Here we have identified Marlin-1, a novel GABA(B) receptor-binding protein that associates specifically with the GABA(B)R1 subunit in yeast, tissue culture cells, and neurons. Marlin-1 is expressed in the brain and exhibits a granular distribution in cultured hippocampal neurons. Marlin-1 binds different RNA species including the 3'-untranslated regions of both the GABA(B)R1 and GABA(B)R2 mRNAs in vitro and also associates with RNA in cultured neurons. Inhibition of Marlin-1 expression via small RNA interference technology results in enhanced intracellular levels of the GABA(B)R2 receptor subunit without affecting the level of GABA(B)R1. Together our results suggest that Marlin-1 functions to regulate the cellular levels of GABA(B) R2 subunits, which may have significant effects on the production of functional GABA(B) receptor heterodimers. Therefore, our observations provide an added level of regulation for the control of GABA(B) receptor expression and for the efficacy of inhibitory synaptic transmission.

Phosphorylation and chronic agonist treatment atypically modulate GABAB receptor cell surface stability

GABA(B) receptors are heterodimeric G protein-coupled receptors that mediate slow synaptic inhibition in the central nervous system. The dynamic control of the cell surface stability of GABA(B) receptors is likely to be of fundamental importance in the modulation of receptor signaling. Presently, however, this process is poorly understood. Here we demonstrate that GABA(B) receptors are remarkably stable at the plasma membrane showing little basal endocytosis in cultured cortical and hippocampal neurons. In addition, we show that exposure to baclofen, a well characterized GABA(B) receptor agonist, fails to enhance GABA(B) receptor endocytosis. Lack of receptor internalization in neurons correlates with an absence of agonist-induced phosphorylation and lack of arrestin recruitment in heterologous systems. We also demonstrate that chronic exposure to baclofen selectively promotes endocytosis-independent GABA(B) receptor degradation. The effect of baclofen can be attenuated by activation of cAMP-dependent protein kinase or co-stimulation of beta-adrenergic receptors. Furthermore, we show that increased degradation rates are correlated with reduced receptor phosphorylation at serine 892 in GABA(B)R2. Our results support a model in which GABA(B)R2 phosphorylation specifically stabilizes surface GABA(B) receptors in neurons. We propose that signaling pathways that regulate cAMP levels in neurons may have profound effects on the tonic synaptic inhibition by modulating the availability of GABA(B) receptors.

Cloning and Characterization of a GABAA Receptor γ2 Subunit Variant

We have cloned a novel gamma-aminobutyric acid type A (GABAA) receptor gamma2 subunit variant named gamma2XL. gamma2XL contains an alternatively spliced exon, resulting in the addition of 40 amino acids to the N-terminal extracellular domain between Ser171 and Tyr172. We show that gamma2XL failed to localize to the cell surface when it was coexpressed with the alpha2 and beta1 subunits in human embryonic kidney 293 cells. Expression of gamma2XL in 293 cells suppressed GABAA receptor binding in a dose-dependent manner by preventing GABAA receptor cell-surface localization. We also generated a gamma2 mutant with Ser171 and Tyr172 converted to glycine and threonine, respectively. We demonstrate that this mutant has a significantly lower affinity for the alpha2 and beta1 subunits and failed to reach the cell surface when coexpressed with these subunits. Together, our results indicate that Ser171 and Tyr172 in the gamma2 subunit constitute a critical motif. When this motif is disrupted by insertion of the alternative exon, access of the gamma2 subunit to the cell surface is prevented.

A rapid increase in the total number of cell surface functional GABAA receptors induced by brain-derived neurotrophic factor in rat visual cortex

The number of postsynaptic gamma-aminobutyric acid type A (GABAA) receptors is a fundamental determinant of the variability of inhibitory synaptic responses in the central nervous system. In rat visual cortex, [3H]SR-95531 binding assays revealed that brain-derived neurotrophic factor (BDNF), one of the neurotrophins, induced a rapid increase in the total number of cell surface GABAA receptors, through the activation of Trk B receptor tyrosine kinases. We also demonstrated that BDNF rapidly induced a sustained potentiation of GABAA receptor-mediated currents, using nystatin-perforated patch clamp recordings, in visual cortical layer 5 pyramidal neurons freshly isolated from P14 rats. The potentiation was caused by the activation of Trk B receptor tyrosine kinase and phospholipase C-gamma. In addition, intracellular Ca2+ was important for the potentiation of GABAA responses induced by BDNF. The selective increase in mean miniature inhibitory postsynaptic (mIPSC) current amplitude without effects on mIPSC time courses supports the idea that BDNF rapidly induces an increase in the total number of cell surface functional GABAA receptors in visual cortical pyramidal neurons. These results suggest that BDNF could alter the number of cell surface GABAA receptors in a region-specific manner.

Long-term depression of synaptic inhibition is expressed postsynaptically in the developing auditory system

Inhibitory transmission is critically involved in the functional maturation of neural circuits within the brain. However, the mechanisms involved in its plasticity and development remain poorly understood. At an inhibitory synapse of the developing auditory brain stem, we used whole cell recordings to determine the site of induction and expression of long-term depression (LTD), a robust activity-dependent phenomenon that decreases inhibitory synaptic gain and is postulated to underlie synapse elimination. Recordings were obtained from lateral superior olivary (LSO) neurons, and hyperpolarizing inhibitory potentials were evoked by stimulation of the medial nucleus of the trapezoid body (MNTB). Both postsynaptic glycine and GABAA receptors could independently display LTD when isolated pharmacologically. Focal application of GABA, but not glycine, on the postsynaptic LSO neuron was sufficient to induce depression of the amino acid-evoked response, or MNTB-evoked inhibitory postsynaptic potentials. This GABA-mediated depression, in the absence of MNTB stimulation, was blocked by a GABAB receptor antagonist. To assess whether a change in neurotransmitter release is associated with the LTD, the polyvalent cation, ruthenium red, was used to increase the frequency of miniature inhibitory synaptic events. Consistent with a postsynaptic locus of expression, we found that the mean amplitude of miniature events decreased after LTD with no change in their frequency of occurrence. Furthermore, there was no change in the paired-pulse ratio or release kinetics of evoked inhibitory responses. Together, these results provide direct evidence that activity-dependent LTD of inhibition has a postsynaptic locus of induction and alteration, and that GABA but not glycine plays a pivotal role.

Chronic ethanol consumption enhances internalization of alpha1 subunit-containing GABAA receptors in cerebral cortex

The molecular mechanisms that underlie ethanol dependence involve alterations in the functional properties and subunit expression of GABAA receptors. Chronic ethanol exposure decreases GABAA receptor alpha1 subunits and increases alpha4 subunit levels in cerebral cortical membranes. This study explored the effect of chronic ethanol exposure on internalization of GABAA/benzodiazepine receptors. Chronic ethanol exposure increased alpha1 subunit levels by 46 +/- 12% and [3H]flunitrazepam binding by 35 +/- 9% in the clathrin-coated vesicle (CCV) fraction. There was a corresponding 34 +/- 8% decrease in alpha1 peptide expression and 37 +/- 6% decrease in [3H]flunitrazepam binding in the synaptic fraction. Chronic ethanol consumption also increased the alpha1 subunit immunoprecipitate in the cytosolic fraction (77 +/- 22%), measured by western blot analysis. Moreover, co-immunoprecipitation of both clathrin and adaptin-alpha with alpha1 subunits was increased in the cytosolic fraction, suggesting that alpha1 subunit endocytosis is enhanced by chronic ethanol consumption. In contrast, alpha4 subunit peptide levels were not altered in the CCV fraction despite a 39 +/- 13% increase in peptide levels in the synaptic fraction of cortex. Moreover, acute ethanol exposure did not alter alpha1 subunit peptide expression or [3H]flunitrazepam binding in the synaptic or CCV fractions. These results suggest that chronic ethanol consumption selectively increases internalization of alpha1 subunit-containing GABAA receptors in cerebral cortex.

Constitutive GABAA receptor endocytosis is dynamin-mediated and dependent on a dileucine AP2 adaptin-binding motif within the beta 2 subunit of the receptor

Receptor endocytosis is an important mechanism for regulating the synaptic efficacy of neurotransmitters. There is strong evidence that GABA(A) receptor endocytosis is clathrin-dependent; however, this process is not well understood. Here we demonstrate that in HEK 293 cells, endocytosis of GABA(A) receptors composed of either alpha1beta2gamma2Lor alpha1beta2 subunits is blocked by the dominant negative dynamin construct K44A. Furthermore, we identify a dileucine AP2 adaptin-binding motif within the receptor beta2 subunit that is critical for endocytosis. Internalization of GABAA receptors lacking this motif is dramatically inhibited, and the receptors appear to accumulate on the cell surface. Patch clamp analysis of receptors lacking the dileucine motif show that there is an increase in the peak amplitude of GABA-gated chloride currents compared with wild-type receptors. Additionally, GABA-gated chloride currents in HEK 293 cells expressing wild-type receptors are increased by introduction of a peptide corresponding to the dileucine motif region of the receptor beta2 subunit but not by a control peptide containing alanine substitutions for the dileucine motif. In mouse brain cerebral cortical neurons, the dileucine motif peptide increases GABA-gated chloride currents of native GABA(A) receptors. This is the first report to our knowledge that an AP2 adaptin dileucine recognition motif is critical for the endocytosis of ligand-gated ion channels belonging to this superfamily.

Control of synaptic strength, a novel function of Akt

Akt (also known as PKB), a serine/threonine kinase involved in diverse signal-transduction pathways, is highly expressed in the brain. Akt is known to have a strong antiapoptotic action and thereby to be critically involved in neuronal survival, but its potential role in the dynamic modulation of synaptic transmission is unknown. Here we report that Akt phosphorylates, both in vitro and in vivo, the type A gamma-aminobutyric acid receptor (GABA(A)R), the principal receptor mediating fast inhibitory synaptic transmission in the mammalian brain. Akt-mediated phosphorylation increases the number of GABA(A)Rs on the plasma membrane surface, thereby increasing the receptor-mediated synaptic transmission in neurons. These results identify the GABA(A)R as a novel substrate of Akt, thereby linking Akt to the regulation of synaptic strength. This work also provides evidence for the rapid regulation of neurotransmitter receptor numbers in the postsynaptic domain by direct receptor phosphorylation as an important means of producing synaptic plasticity.

Modulation of GABAA receptor activity by phosphorylation and receptor trafficking: implications for the efficacy of synaptic inhibition

Fast synaptic inhibition in the brain is largely mediated by GABA(A) receptors. These ligand-gated ion channels are crucial in the control of cell and network activity. Therefore, modulating their function or cell surface stability will have major consequences for neuronal excitation. It has become clear that the stability and activity of GABA(A) receptors at synapses can be dynamically modulated by receptor trafficking and phosphorylation. Here, we discuss these regulatory mechanisms, and their consequences for the efficacy of GABA(A) receptor mediated synaptic inhibition.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

ATP-sensitive potassium channel traffic regulation by adenosine and protein kinase C

ATP-sensitive potassium (K(ATP)) channels activate under metabolic stress to protect neurons and cardiac myocytes. However, excessive channel activation may cause arrhythmia in the heart and silence neurons in the brain. Here, we report that PKC-mediated downregulation of K(ATP) channel number, via dynamin-dependent channel internalization, can act as a brake mechanism to control K(ATP) activation. A dileucine motif in the pore-lining Kir6.2 subunit of K(ATP), but not the site of PKC phosphorylation for channel activation, is essential for PKC downregulation. Whereas K(ATP) activation results in a rapid shortening of the action potential duration (APD) in metabolically inhibited ventricular myocytes, adenosine receptor stimulation and consequent PKC-mediated K(ATP) channel internalization can act as a brake to lessen this APD shortening. Likewise, in hippocampal CA1 neurons under metabolic stress, PKC-mediated, dynamin-dependent K(ATP) channel internalization can also act as a brake to dampen the rapid decline of excitability due to K(ATP) activation.

Glycine binding primes NMDA receptor internalization

NMDA (N-methyl-d-aspartate) receptors (NMDARs) are a principal subtype of excitatory ligand-gated ion channel with prominent roles in physiological and disease processes in the central nervous system. Recognition that glycine potentiates NMDAR-mediated currents as well as being a requisite co-agonist of the NMDAR subtype of 'glutamate' receptor profoundly changed our understanding of chemical synaptic communication in the central nervous system. The binding of both glycine and glutamate is necessary to cause opening of the NMDAR conductance pore. Although binding of either agonist alone is insufficient to cause current flow through the channel, we report here that stimulation of the glycine site initiates signalling through the NMDAR complex, priming the receptors for clathrin-dependent endocytosis. Glycine binding alone does not cause the receptor to be endocytosed; this requires both glycine and glutamate site activation of NMDARs. The priming effect of glycine is mimicked by the NMDAR glycine site agonist d-serine, and is blocked by competitive glycine site antagonists. Synaptic as well as extrasynaptic NMDARs are primed for internalization by glycine site stimulation. Our results demonstrate transmembrane signal transduction through activating the glycine site of NMDARs, and elucidate a model for modulating cell-cell communication in the central nervous system.

Lateral mobility and specific binding to GABA(A) receptors on hippocampal neurons monitored by fluorescence correlation spectroscopy

The binding behavior of a fluorescently labeled muscimol derivative to the GABA(A) receptor was analyzed at rat hippocampal neurons by fluorescence correlation spectroscopy. After muscimol had been labeled with the fluorophore Alexa Fluor 532, specific binding constants for binding of the dye-labeled ligand (Mu-Alexa) to the GABA(A) receptor were determined. We found a high specific binding affinity of Mu-Alexa with a K(D) value of 3.4 +/- 0.5 nM and a rate constant of ligand-receptor dissociation (k(diss)) of (5.37 +/- 0.95) x 10(-2) s(-1). A rate constant of ligand-receptor association (k(ass)) of (1.57 +/- 0.28) x 10(7) L mol(-1) s(-1) was calculated. The following diffusion coefficients were observed: D(free) = 233 +/- 20 microm(2)/s (n = 66) for free diffusing Mu-Alexa, D(bound1) = 2.8 +/- 0.9 microm(2)/s (n = 64) for the lateral mobility, and D(bound2) = 0.14 +/- 0.05 microm(2)/s (n = 56) for the hindered mobility of the GABA(A) receptor-ligand complex in the cell membrane. Saturation of Mu-Alexa binding was observed at a concentration of 50 nM. A maximum number of binding sites [B(max) = 18.4 +/- -0.4 nM (n = 5)] was found. Similar K(i) values of 4.5 +/- 1.0 nM for nonlabeled muscimol and 8.8 +/- 1.8 nM for Mu-Alexa were found by RRAs using [(3)H]muscimol as a radioligand. A concentration-dependent increase in the level of specific Mu-Alexa binding was demonstrated by the positive cooperative activity of co-incubated midazolam, which was selectively found in GABA(A) receptor-ligand complexes with hindered mobility.

Interaction of calcineurin and type-A GABA receptor gamma 2 subunits produces long-term depression at CA1 inhibitory synapses

Long-term depression (LTD) is an activity-dependent weakening of synaptic efficacy at individual inhibitory synapses, a possible cellular model of learning and memory. Here, we show that the induction of LTD of inhibitory transmission recruits activated calcineurin (CaN) to dephosphorylate type-A GABA receptor (GABA(A)Rs) via the direct binding of CaN catalytic domain to the second intracellular domain of the GABA(A)R-gamma(2) subunits. Prevention of the CaN-GABA(A) receptor complex formation by expression of an autoinhibitory domain of CaN in the hippocampus of transgenic mice blocks the induction of LTD. Conversely, genetic expression of the CaN catalytic domain in the hippocampus depresses inhibitory synaptic responses, occluding LTD. Thus, an activity-dependent physical and functional interaction between CaN and GABA(A) receptors is both necessary and sufficient for inducing LTD at CA1 individual inhibitory synapses.

Constitutive cycling: a general mechanism to regulate cell surface proteins

Cells can change their function by rapidly modulating the levels of certain proteins at the plasma membrane. This rapid modulation is achieved by using a specialised trafficking process called constitutive cycling. The constitutive cycling of a variety of transmembrane proteins such as receptors, channels and transporters has recently been directly demonstrated in a wide range of cell types. This regulation is thought to underlie important biological phenomena such as learning and memory, gastric acid secretion and water and blood glucose homeostasis. This review discusses the molecular mechanisms of constitutive cycling, its regulation by extracellular agents such as hormones and its misregulation in disease states.

A-kinase anchoring protein 79/150 facilitates the phosphorylation of GABA(A) receptors by cAMP-dependent protein kinase via selective interaction with receptor beta subunits

GABA(A) receptors, the key mediators of fast synaptic inhibition in the brain, are predominantly constructed from alpha(1-6), beta(1-3), gamma(1-3), and delta subunit classes. Phosphorylation by cAMP-dependent protein kinase (PKA) differentially regulates receptor function dependent upon beta subunit identity, but how this kinase is selectively targeted to GABA(A) receptor subtypes remains unresolved. Here we establish that the A-kinase anchoring protein 150 (AKAP150), directly binds to the receptor beta1 and beta3, but not to alpha1, alpha2, alpha3, alpha6, beta2, gamma2, or delta subunits. Furthermore, AKAP79/150 is critical for PKA-mediated phosphorylation of the receptor beta3 subunit. Together, our observations suggest a mechanism for the selective targeting of PKA to GABA(A) receptor subtypes containing the beta1 or beta3 subunits dependent upon AKAP150. Therefore, the selective interaction of beta subunits with AKAP150 may facilitate GABA(A) receptor subtype-specific functional modulation by PKA activity which may have profound local effects on neuronal excitation.

Dynamics and regulation of clathrin coats at specialized endocytic zones of dendrites and spines

Endocytosis is a fundamental mechanism by which neurons control intercellular signaling, nutrient uptake, and synaptic transmission. This process is carried out by the assembly of clathrin coats and the budding of clathrin-coated vesicles from the neuronal plasma membrane. Here, we demonstrate that in young neurons, clathrin assembly and disassembly occur rapidly, locally, and repeatedly at "hot spots" throughout dendrites and at the tips of dendritic filopodia. In contrast, clathrin coats in mature dendrites reside in stable, long-lasting zones at sites of endocytosis, where clathrin undergoes continuous exchange with local cytosolic pools. In dendritic spines, endocytic zones lie lateral to the postsynaptic density (PSD) where they develop and persist independent of synaptic activity, akin to the PSD itself. These results reveal the presence of a novel specialization dedicated to endocytosis near the postsynaptic membrane.

Effects of early postnatal ethanol intubation on GABAergic synaptic proteins

Fetal alcohol syndrome includes brain damage from aberrant synaptogenesis, altered cell-cell signaling and blunted plasticity in surviving neurons. Distortion of neurotrophic GABA signals by ethanol-mediated allosteric modulation of GABA(A) receptor (GABA(A)R) activity during brain maturation may play a role. In this regard, early postnatal binge-like ethanol treatment on postnatal days (PDs) 4-9 acutely inhibits whole cell GABA(A)R Cl(-) current and subsequently blunts GABA(A)R function in medial septum/diagonal band (MS/DB) neurons and cerebellar Purkinje cells [Dev. Brain Res. 130 (2001) 25-40; Brain Res. 810 (1998) 100-113; Brain Res. 832 (1999) 124-135]. In light of these functional changes, we hypothesized that ethanol treatment also would decrease levels of proteins important for assembly of GABAergic synapses in maturing brain. To test this relationship, binge-like ethanol intubation was administered to rat pups on PDs 4-9 producing peak blood ethanol concentrations in the range of 302.5+/-6.3 mg/dl. GABAergic synaptic proteins were measured in brain tissue on PDs 13-14 when GABA(A)R currents in individual MS/DB neurons are reduced, but those of cerebellar Purkinje neurons are not yet altered [Dev. Brain Res. 130 (2001) 25-40; Brain Res. 810 (1998) 100-113; Brain Res. 832 (1999) 124-135]. Surprisingly, ethanol did not decrease protein levels of GABA(A)R alpha1/beta2 subunits, GAD(67) or gephyrin in MS/DB at this time when whole cell recordings indicate GABA(A)R function is impaired in acutely dissociated individual neurons. However, in cerebellum where ethanol treated Purkinje cell GABA(A)R function remains normal on PDs 13-14 [Brain Res. 832 (1999) 124-135], reduced levels of several GABAergic synaptic proteins including: GAD(67), GABA(A)R alpha1 subunit, ClC-2 a voltage-gated Cl(-) channel, synaptotagmin a synaptic vesicle protein, and N-cadherin, a synapse associated cell adhesion molecule, were found. These results indicate that binge-like ethanol exposure differentially decreases GABAergic synaptic proteins in some brain areas in a pattern that does not parallel reductions in GABA(A)R function of individual neurons that survive this ethanol insult.

Identification of a non-canonical tyrosine-based endocytic motif in an ionotropic receptor

Rapid modulation of the surface number of certain ionotropic receptors is achieved by altering the relative rates of insertion and internalization. These receptors are internalized by a clathrin-mediated pathway; however, a motif that is necessary for endocytosis of ionotropic receptors has not yet been identified. Here, we identified a motif that is required for constitutive and agonist-regulated internalization of the ionotropic P2X(4) receptor. Three amino acids in the C terminus of P2X(4) (Tyr(378), Gly(381), and Leu(382)) compose a non-canonical tyrosine-based sorting signal of the form YXXGL. We found that P2X(4) protein was present in clathrin-coated vesicles isolated from rat brain and that a glutathione S-transferase fusion of the P2X(4) C terminus pulled down the adaptor protein-2 complex from brain extract. Mutation of either the tyrosine-binding pocket of the mu2 subunit of adaptor protein-2 or the YXXGL motif in the receptor C terminus caused a decrease in receptor internalization and a dramatic increase in the surface expression of P2X(4) receptors. The YXXGL motif represents a non-canonical tyrosine-based sorting signal that is necessary for efficient endocytosis of the P2X(4) receptor. Similar motifs are present in other receptors and may be important for the control of their functional expression.

P2X receptor trafficking in neurons is subunit specific

P2X receptors within the CNS mediate excitatory synaptic transmission and also act presynaptically to modulate neurotransmitter release. We have studied the targeting and trafficking of P2X4 and P2X2 receptors heterologously expressed in cultured olfactory bulb neurons. Homomeric P2X4 receptors had a punctate distribution, and many of the puncta colocalized with early endosomes. In contrast, P2X2 receptors were primarily localized at the plasma membrane. By antibody-labeling of surface receptors in living neurons, we showed that P2X4 receptors undergo rapid constitutive internalization and subsequent reinsertion into the plasma membrane, whereas P2X2 receptors were not regulated in such a way. The internalization of P2X4 receptors was dynamin-dependent, and the binding of ATP enhanced the basal rate of retrieval in a Ca2+-independent manner. The presence of the P2X4 subunit in a P2X4/6 heteromer governed the trafficking properties of the receptor. P2X receptors acted presynaptically to enhance the release of glutamate, suggesting that the regulated cycling of P2X4-containing receptors might provide a mechanism for modulation of synaptic transmission.