GISP binding to TSG101 increases GABA receptor stability by down-regulating ESCRT-mediated lysosomal degradation

The neuron-specific G protein-coupled receptor interacting scaffold protein (GISP) is a multidomain, brain-specific protein derived from the A-kinase anchoring protein-9 gene. We originally isolated GISP as an interacting partner for the GABA(B) receptor subunit GABA(B1). Here, we show that the protein tumour susceptibility gene 101 (TSG101), an integral component of the endosomal sorting machinery that targets membrane proteins for lysosomal degradation, also interacts with GISP. TSG101 co-immunoprecipitates with GISP from adult rat brain, and using GST pull-downs, we identified that the eighth coiled-coiled region of GISP is critical for TSG101 association. Intriguingly, although there is no direct interaction between GISP and the GABA(B2) subunit, their co-expression in HEK293 cells increases levels of GABA(B2). GISP also inhibits TSG101-dependent GABA(B2) down-regulation in human embryonic kidney 293 cells whereas over-expression of a mutant GISP lacking the TSG101 binding domain has no effect on GABA(B2) degradation. These data suggest that GISP can function as a negative regulator of TSG101-dependent lysosomal degradation of transmembrane proteins in neurons to promote receptor stability.     

Influence of positive allosteric modulators on GABA(B) receptor coupling in rat brain: a scintillation proximity assay characterisation of G protein subtypes

Little is known concerning coupling of cerebral GABA_B receptors to G protein subtypes, and the influence of positive allosteric modulators (PAMs) has not been evaluated. These questions were addressed by an antibody-capture/scintillation proximity assay strategy. GABA concentration-dependently enhanced the magnitude of [³⁵S]GTPγS binding to Gαo and, less markedly, Gαi_1/3 in cortex, whereas Gq and Gs/olf were unaffected. ( R )-baclofen and SKF97581 likewise activated Gαo and Gαi_1/3, expressing their actions more potently than GABA. Similar findings were acquired in hippocampus and cerebellum, and the GABA_B antagonist, CGP55845A, abolished agonist-induced activation of Gαo and Gαi_1/3 in all structures. The PAMs, GS39783, CGP7930 and CGP13501, inactive alone, enhanced efficacy and potency of agonist-induced [³⁵S]GTPγS binding to Gαo in all regions, actions abolished by CGP55845A. In contrast, they did not modify efficacies at Gαi_1/3. Similarly, in human embryonic kidney cells expressing GABA_B(1a+2) or GABA_B(1b+2) receptors, allosteric modulators did not detectably enhance efficacy of GABA at Gαi_1/3, though they increased its potency. To summarise, GABA_B receptors coupled both to Gαo and to Gαi, but not Gq and Gs/olf, in rat brain. PAMs more markedly enhanced efficacy of coupling to Go versus Gi_1/3. It will be of interest to confirm these observations employing complementary techniques and to evaluate their potential therapeutic significance.     

Developing oligodendrocytes express functional GABA(B) receptors that stimulate cell proliferation and migration

GABA(B) receptors (GABA(B)Rs) are involved in early events during neuronal development. The presence of GABA(B)Rs in developing oligodendrocytes has not been established. Using immunofluorescent co-localization, we have identified GABA(B)R proteins in O4 marker-positive oligodendrocyte precursor cells (OPCs) in 4-day-old mouse brain periventricular white matter. In culture, OPCs, differentiated oligodendrocytes (DOs) and type 2 astrocytes (ASTs) express both the GABA(B1abcdf) and GABA(B2) subunits of the GABA(B)R. Using semiquantitative PCR analysis with GABA(B)R isoform-selective primers we found that the expression level of GABA(B1abd) was substantially higher in OPCs or ASTs than in DOs. In contrast, the GABA(B2) isoform showed a similar level of expression in OPCs and DOs, and a significantly higher level in ASTs. This indicates that the expression of GABA(B1) and GABA(B2) subunits are under independent control during oligodendroglial development. Activation of GABA(B)Rs using the selective agonist baclofen demonstrated that these receptors are functionally active and negatively coupled to adenylyl cyclase. Manipulation of GABA(B)R activity had no effect on OPC migration in a conventional agarose drop assay, whereas baclofen significantly increased OPC migration in a more sensitive transwell microchamber-based assay. Exposure of cultured OPCs to baclofen increased their proliferation, providing evidence for a functional role of GABA(B)Rs in oligodendrocyte development. The presence of GABA(B)Rs in developing oligodendrocytes provides a new mechanism for neuronal-glial interactions during development and may offer a novel target for promoting remyelination following white matter injury.     

Discovery of a novel potent GABA(B) receptor agonist

Structure-activity studies have led to a discovery of 3-(4-pyridyl)methyl ether derivative 9d that has 25- to 50-fold greater functional potency than R-baclofen at human and rodent GABA(B) receptors in vitro. Mouse hypothermia studies confirm that this compound crosses the blood-brain barrier and is approximately 50-fold more potent after systemic administration.     

GABA(B) receptors in 5-HT transporter- and 5-HT1A receptor-knock-out mice: further evidence of a transduction pathway shared with 5-HT1A receptors

The functional properties of GABA(B) receptors were examined in the dorsal raphe nucleus (DRN) and the hippocampus of knock-out mice devoid of the 5-HT transporter (5-HTT-/-) or the 5-HT(1A) receptor (5-HT(1A)-/-). Electrophysiological recordings in brain slices showed that the GABA(B) receptor agonist baclofen caused a lower hyperpolarization and neuronal firing inhibition of DRN 5-HT cells in 5-HTT-/- versus 5-HTT+/+ mice. In addition, [(35)S]GTP-gamma-S binding induced by GABA(B) receptor stimulation in the DRN was approximately 40% less in these mutants compared with wild-type mice. In contrast, GABA(B) receptors appeared functionally intact in the hippocampus of 5-HTT-/-, and in both this area and the DRN of 5-HT(1A)-knock-out mice. The unique functional changes of DRN GABA(B) receptors closely resembled those of 5-HT(1A) autoreceptors in 5-HTT-/- mice, further supporting the idea that both receptor types are coupled to a common pool of G-proteins in serotoninergic neurons.     

Activation of D1 dopamine receptors increases surface expression of AMPA receptors and facilitates their synaptic incorporation in cultured hippocampal neurons

Considerable evidence indicates that neuroadaptations leading to addiction involve the same cellular processes that enable learning and memory, such as long-term potentiation (LTP), and that psychostimulants influence LTP through dopamine (DA)-dependent mechanisms. In hippocampal CA1 pyramidal neurons, LTP involves insertion of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors into excitatory synapses. We used dissociated cultures to test the hypothesis that D1 family DA receptors influence synaptic plasticity in hippocampal neurons by modulating AMPA receptor trafficking. Brief exposure (5 min) to a D1 agonist increased surface expression of glutamate receptor (GluR)1-containing AMPA receptors by increasing their rate of externalization at extrasynaptic sites. This required the secretory pathway but not protein synthesis, and was mediated mainly by protein kinase A (PKA) with a smaller contribution from Ca2+-calmodulin-dependent protein kinase II (CaMKII). Prior D1 receptor stimulation facilitated synaptic insertion of GluR1 in response to subsequent stimulation of synaptic NMDA receptors with glycine. Our results support a model for synaptic GluR1 incorporation in which PKA is required for initial insertion into the extrasynaptic membrane whereas CaMKII mediates translocation into the synapse. By increasing the size of the extrasynaptic GluR1 pool, D1 receptors may promote LTP. Psychostimulants may usurp this mechanism, leading to inappropriate plasticity that contributes to addiction-related behaviors.     

Adenosine receptors interacting proteins (ARIPs): Behind the biology of adenosine signaling

Adenosine is a well known neuromodulator in the central nervous system. As a consequence, adenosine can be beneficial in certain disorders and adenosine receptors will be potential targets for therapy in a variety of diseases. Adenosine receptors are G protein-coupled receptors, and are also expressed in a large variety of cells and tissues. Using these receptors as a paradigm of G protein-coupled receptors, the present review focus on how protein-protein interactions might contribute to neurotransmitter/neuromodulator regulation, based on the fact that accessory proteins impinge on the receptor/G protein interaction and therefore modulate receptor functioning. Besides affecting receptor signaling, these accessory components also play a key role in receptor trafficking, internalization and desensitization, as it will be reviewed here. In conclusion, the finding of an increasing number of adenosine receptors interacting proteins, and specially the molecular and functional integration of these accessory proteins into receptorsomes, will open new perspectives in the understanding of particular disorders where these receptors have been proved to be involved.     

Formation of a physiological complex between TRPV2 and RGA protein promotes cell surface expression of TRPV2

The transient receptor potential, sub-family Vanilloid (TRPV)(2) cation channel is activated in response to extreme temperature elevations in sensory neurons. However, TRPV2 is widely expressed in tissues with no sensory function, including cells of the immune system. Regulation of GRC, the murine homolog of TRPV2 has been studied in insulinoma cells and myocytes. GRC is activated in response to certain growth factors and neuropeptides, via a mechanism that involves regulated access of the channel to the plasma membrane. This is likely to be an important primary control mechanism for TRPV2 outside the CNS. Here, we report that a regulated trafficking step controls the access of TRPV2 to the cell surface in mast cells. In mast cells, elevations in cytosolic cAMP are sufficient to drive plasma membrane localization of TRPV2. We have previously proposed that the recombinase gene activator protein (RGA), a four-transmembrane domain, intracellular protein, associates with TRPV2 during the biosynthesis and early trafficking of the channel. We use a polyclonal antibody to RGA to confirm the formation of a physiological complex between RGA and TRPV2. Finally, we show that over-expression of the RGA protein potentiates the basal surface localization of TRPV2. We propose that trafficking and activation mechanisms intersect for TRPV2, and that cAMP mobilizing stimuli may regulate TRPV2 localization in non-sensory cells. RGA participates in the control of TRPV2 surface levels, and co-expression of RGA may be a key component of experimental systems that seek to study TRPV2 physiology.     

The cell adhesion molecule neuroplastin-65 inhibits hippocampal long-term potentiation via a mitogen-activated protein kinase p38-dependent reduction in surface expression of GluR1-containing glutamate receptors

Neuroplastin-65 is a brain-specific, synapse-enriched member of the immunoglobulin (Ig) superfamily of cell adhesion molecules. Previous studies highlighted the importance of neuroplastin-65 for long-term potentiation (LTP), but the mechanism was unclear. Here, we show how neuroplastin-65 activation of mitogen-activated protein kinase p38 (p38MAPK) modified synapse strength by altering surface glutamate receptor expression. Organotypic hippocampal slice cultures treated with the complete extracellular fragment of neuroplastin-65 (FcIg1-3) sustained an increase in the phosphorylation of p38MAPK and an inability to induce LTP at hippocampal synapses. The LTP block was reversed by application of the p38MAPK inhibitor SB202190, suggesting that p38MAPK activation occurred downstream of neuroplastin-65 binding and upstream of the loss of LTP. Further investigation revealed that the mechanism underlying neuroplastin-65-dependent prevention of LTP was a p38MAPK-dependent acceleration of the loss of surface-exposed glutamate receptor subunits that was reversed by pretreatment with the p38MAPK inhibitor SB202190. Our results indicate that neuroplastin-65 binding and associated stimulation of p38MAPK activity are upstream of a mechanism to control surface glutamate receptor expression and thereby influence plasticity at excitatory hippocampal synapses.     

Conformational changes at benzodiazepine binding sites of GABA(A) receptors detected with a novel technique

Benzodiazepines are widely used for their anxiolytic, sedative, myorelaxant and anticonvulsant properties. They allosterically modulate GABA(A) receptor function by increasing the apparent affinity of the agonist GABA. We studied conformational changes induced by channel agonists at the benzodiazepine binding site. We used the rate of covalent reaction between a benzodiazepine carrying a cysteine reactive moiety with mutated receptor having a cysteine residue in the benzodiazepine binding pocket, alpha1H101Cbeta2gamma2, as a sensor of its conformation. This reaction rate is sensitive to local conformational changes. Covalent reaction locks the receptor in the conformation stabilized by positive allosteric modulators. By using concatenated subunits we demonstrated that the covalent reaction occurs either exclusively at the alpha/gamma subunit interface, or if it occurs in both alpha1 subunits, exclusively reaction at the alpha/gamma subunit interface can modulate the receptor. We found evidence for an increased rate of reaction of activated receptors, whereas reaction rate with the desensitized state is slowed down. The benzodiazepine antagonist Ro15-1788 efficiently inhibited the covalent reaction in the presence of 100 microm GABA but only partially in its absence or in the presence of 10 microm GABA. It is concluded that Ro15-1788 efficiently protects activated and desensitized states, but not the resting state.     

Hippocampal gene expression profiling reveals the possible involvement of Homer1 and GABA(B) receptors in scopolamine-induced amnesia

Scopolamine-treated rats are commonly used as a psychopharmacological model of memory dysfunction and have been extensively studied to establish the effectiveness of acetylcholinesterase inhibitors in the treatment of Alzheimer's disease. Scopolamine is a muscarinic acetylcholine receptor antagonist that induces memory deficits in young subjects similar to those occurring during aging. The amnesic effect of scopolamine is well established but the molecular and cellular mechanisms that sustain its neuropharmacological action are still unclear. The present genome wide study investigates hippocampal gene expression profiling in scopolamine-treated adult rats following stimulation in a spatial memory task. Using microarray and quantitative real-time RT-PCR approaches, we identified several genes previously known to be associated with memory processes (Homer1, GABA(B) receptor, early growth response 1, prodynorphin, VGF nerve growth factor inducible) and multiple novel candidate genes possibly involved in cognition (including calcium/calmodulin-dependent protein kinase kinase 2, dual specificity phosphatase 5 and 6, glycophorin C) that were altered following scopolamine treatment. Moreover, we found that stable over-expression of glutamatergic components Homer1a and 1c in the hippocampus of adult rats induced by recombinant adeno-associated virus vector abolished memory improvement produced by the GABA(B) receptor antagonist SGS742 in scopolamine-treated rats. Taken together, these results reveal novel genes and mechanisms involved in scopolamine-induced amnesia, and demonstrate the involvement of both GABA and glutamate neurotransmission in this animal model of cognitive dysfunctions.     

Palmitoylation cycles and regulation of protein function (Review)

The efficacy and success of many cellular processes is dependent on a tight orchestration of proteins trafficking to and from their site(s) of action in a time-controlled fashion. Recently, a dynamic cycle of palmitoylation/de-palmitoylation has been shown to regulate shuttling of several proteins, including the small GTPases H-Ras and N-Ras, and the GABA-synthesizing enzyme GAD65, between the Golgi compartment and either the plasma membrane or synaptic vesicle membranes. These proteins are peripheral membrane proteins that in the depalmitoylated state cycle rapidly on and off the cytosolic face of ER/Golgi membranes. Palmitoylation of one or more cysteines, by a Golgi localized palmitoyl transferase (PAT) results in trapping in Golgi membranes, and sorting to a vesicular pathway in route to the plasma membrane or synaptic vesicles. A depalmitoylation step by an acyl protein thioesterase (APT) releases the protein from membranes in the periphery of the cell resulting in retrograde trafficking back to Golgi membranes by a non-vesicular pathway. The proteins can then enter a new cycle of palmitoylation and depalmitoylation. This inter-compartmental trafficking is orders of magnitude faster than vesicular trafficking. Recent advances in identifying a large family of PATs, their protein substrates, and single PAT mutants with severe phenotypes, reveal their critical importance in development, synaptic transmission, and regulation of signaling cascades. The emerging knowledge of enzymes involved in adding and removing palmitate is that they provide an intricate regulatory network involved in timing of protein function and transport that responds to intracellular and extracellular signals.     

斜纹夜蛾酵母双杂交文库的构建及其在蜕皮激素受体互作蛋白筛选中的应用

【目的】蜕皮激素在昆虫变态发育中起着关键的调控作用。蜕皮激素活化为20-羟基蜕皮酮(20-hydroxyecdysone,20E)后与蜕皮激素受体二聚体〔蜕皮激素受体(ecdysone receptor,EcR)和超气门蛋白(ultraspiracle protein,USP)〕结合,启动20E诱导的级联反应。本研究旨在从互作蛋白角度探究EcR/USP自身调控的分子机理。【方法】以重要农业害虫斜纹夜蛾Spodoptera litura为研究对象,通过构建酵母双杂交筛选系统分别筛选了与EcR和USP相互作用的蛋白。【结果】文库转化效率达到3.0×106cfu/μg,文库滴度达到1.3×108cfu/m L,文库插入片段大小在500~2 000 bp之间。4种诱饵质粒(EcRA/p GBKT7,EcRB1/p GBKT7,USP1/p GBKT7和USP2/p GBKT7)对酵母细胞无毒性,并且无自激活性,说明构建的酵母双杂交文库质量可靠。用以上4种诱饵质粒筛选酵母双杂交文库,共得到110个互作蛋白,其中与EcRB1,USP1和USP2互作的蛋白分别有26,52和32个,未筛选到与EcRA互作的蛋白。随后,从中挑选了Dna J-5(Hsp40 homolog 5,一种热激蛋白分子伴侣),MBF2(mediator of Bm FTZ-F1 type 2,一种转录共激活子),polyubiquitin(多聚泛素类蛋白),esr16(ecdysteroid regulated 16k Da protein,一种蜕皮激素调控蛋白)和NEDD8-like(neural precursor cell expressed,developmentally down-regulated protein 8,一种泛素调节相关蛋白)5种蛋白,利用酵母双杂交和Far-Western印迹法进一步验证了蛋白间的互作关系。【结论】分子伴侣和泛素化修饰等在蜕皮激素受体调控中可能起着重要作用。本研究对深入理解昆虫变态发育的分子机理具有重要意义。     

Protein tyrosine phosphatase interacting protein 51 (PTPIP51) mRNA expression and localization and its in vitro interacting partner protein tyrosine phosphatase 1B (PTP1B) in human placenta of the first, second, and third trimester

The cellular localization of protein tyrosine phosphatase 51 (PTPIP51) and its in vitro interacting partner protein tyrosine phosphatase 1B (PTP1B) was studied in human placentae of different gestational stages. The expression of PTPIP51 protein and mRNA was observed in the syncytiotrophoblast and cytotrophoblast layer of placentae from the first, second, and third trimesters. In contrast, PTP1B expression was restricted to the syncytiotrophoblast during all gestational stages. Cells of the cytotrophoblasts and parts of the syncytiotrophoblasts expressing high amounts of PTPIP51 were found to execute apoptosis as shown by TdT-mediated dUTP-biotin nick end labeling assay, cytokeratin 18f, and caspase 3 expression. PTPIP51 could also be traced in the endothelium and smooth muscle cells of placental arterial and venous vessels, identified by double immunostainings with antibodies directed against van Willebrand factor and alpha-smooth muscle actin. Some of these cells showing a high PTPIP51 reactivity were Ki67 positive, indicating proliferation. Additionally, a small population of placental CD14-positive macrophages and mesenchymal cells within the villous stroma were detected as PTPIP51 positive. Our data suggest that both proteins, PTPIP51 and PTP1B, play a role in differentiation and apoptosis of the cytotrophoblast and syncytiotrophoblast, respectively. Moreover, PTPIP51 may also serve as a cellular signaling partner in angiogenesis and vascular remodeling.     

Modulation of cell surface GABA(B) receptors by desensitization, trafficking and regulated degradation

Inhibitory neurotransmission ensures normal brain function by counteracting and integrating excitatory activity.-Aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian central nervous system,and mediates its effects via two classes of receptors:the GABA A and GABA B receptors.GABA A receptors are heteropentameric GABA-gated chloride channels and responsible for fast inhibitory neurotransmission.GABA B receptors are heterodimeric G protein coupled receptors (GPCR) that mediate slow and prolonged inhibitory transmission.The extent of inhibitory neurotransmission is determined by a variety of factors,such as the degree of transmitter release and changes in receptor activity by posttranslational modifications (e.g.,phosphorylation),as well as by the number of receptors present in the plasma membrane available for signal transduction.The level of GABA B receptors at the cell surface critically depends on the residence time at the cell surface and finally the rates of endocytosis and degradation.In this review we focus primarily on recent advances in the understanding of trafficking mechanisms that determine the expression level of GABA B receptors in the plasma membrane,and thereby signaling strength.     

T3JAM, a novel protein that specifically interacts with TRAF3 and promotes the activation of JNK(1)

Previous studies suggest that localization of tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family members is important for regulating their signal transduction. During a screen for TRAF3-associated proteins that potentially alter TRAF3 subcellular localization and enable signal transduction, we identified a novel protein, T3JAM (TRAF3-interacting Jun N-terminal kinase (JNK)-activating modulator). This protein associates specifically with TRAF3 but not other TRAF family members. Coexpression of T3JAM with TRAF3 recruits TRAF3 to the detergent-insoluble fraction. More importantly, T3JAM and TRAF3 synergistically activate JNK but not nuclear factor (NF)-kappaB. Our studies indicate that T3JAM may function as an adapter molecule that specifically regulates TRAF3-mediated JNK activation.