GABA(A)-receptor-associated protein links GABA(A) receptors and the cytoskeleton

The COOH terminus of GATE-16, an intra-Golgi transport modulator, is cleaved by the human cysteine protease HsApg4A

Docking of a vesicle at the appropriate target membrane involves an interaction between integral membrane proteins located on the vesicle (v-SNAREs) and those located on the target membrane (t-SNAREs). GATE-16 (Golgi-associated ATPase enhancer of 16 kDa) was shown to modulate the activity of SNAREs in the Golgi apparatus and is therefore an essential component of intra-Golgi transport and post-mitotic Golgi re-assembly. GATE-16 contains a ubiquitin fold subdomain, which is terminated at the carboxyl end by an additional amino acid after a conserved glycine residue. In the present study we tested whether the COOH terminus of GATE-16 undergoes post-translational cleavage by a protease which exposes the glycine 116 residue. We describe the isolation and characterization of HsApg4A as a human protease of GATE-16. We show that GATE-16 undergoes COOH-terminal cleavage both in vivo and in vitro, only when the conserved glycine 116 is present. We then utilize an in vitro assay to show that pure HsApg4A is sufficient to cleave GATE-16. The characterization of this protease may give new insights into the mechanism of action of GATE-16 and its other family members.

Dentate granule cell GABA(A) receptors in epileptic hippocampus: enhanced synaptic efficacy and altered pharmacology

The dentate gyrus (DG) normally functions as a filter, preventing propagation of synchronized activity into the seizure-prone hippocampus. This filter or 'gatekeeper' attribute of the DG is compromised in various pathological states, including temporal lobe epilepsy (TLE). This study examines the role that altered inhibition may play in the deterioration of this crucial DG function. Using the pilocarpine animal model of TLE, we demonstrate that inhibitory synaptic function is altered in principal cells of the DG. Spontaneous miniature inhibitory postsynaptic currents (mIPSCs) recorded in dentate granule cells (DGCs) from epileptic animals were larger, more sensitive to blockade by zinc and less sensitive to augmentation by the benzodiazepine type site 1 modulator zolpidem. Furthermore, mIPSCs examined during a quiescent period following injury but preceding onset of epilepsy were significantly smaller than those present either in control or in TLE DGCs, and had already acquired sensitivity to blockade by zinc prior to the onset of spontaneous seizures. Rapid agonist application experiments demonstrated that prolonged (>35 ms) exposure to zinc is required to block GABAA receptors (GABAARs) in patches pulled from epileptic DGCs. Therefore, zinc must be tonically present to block DGC GABAARs and alter DG function. This would occur only during repetitive activation of mossy fibres. Thus, in the pilocarpine animal model of TLE, an early, de novo, expression of zinc-sensitive GABAARs is coupled with delayed, epilepsy-induced development of a zinc delivery system provided by aberrant sprouting of zinc-containing mossy fibre recurrent collaterals. The temporal and spatial juxtaposition of these pathophysiological alterations may compromise normal 'gatekeeper' function of the DG through dynamic zinc-induced failure of inhibition, predisposing the hippocampal circuit to generate seizures.

Identification of a tubulin binding motif on the P2X2 receptor

To isolate proteins interacting with P2X receptors, GST fusion proteins containing the intracellular C terminal tail of P2X(2), P2X(5), or P2X(7) were used as bait to screen detergent extracts of rat brain synaptosomes. By SDS-PAGE combined with mass spectrometry, two interacting proteins were identified: betaIII tubulin and myelin basic protein. While myelin basic protein bound to all three P2X subunits, betaIII tubulin interacted exclusively with the P2X(2) subunit. The tubulin binding domain could be confined to a proline-rich segment (amino acids 371-412) of the P2X(2) subunit. Our results suggest a role for microtubules in the cellular localisation of the P2X(2) receptor.

Intracellular cross talk and physical interaction between two classes of neurotransmitter-gated channels

Fast chemical communications in the nervous system are mediated by several classes of receptor channels believed to be independent functionally and physically. We show here that concurrent activation of P2X2 ATP-gated channels and 5-HT3 serotonin-gated channels leads to functional interaction and nonadditive currents (47-73% of the predicted sum) in mammalian myenteric neurons as well as in Xenopus oocytes or transfected human embryonic kidney (HEK) 293 cell heterologous systems. We also show that these two cation channels coimmunoprecipitate constitutively and are associated in clusters. In heterologous systems, the inhibitory cross talk between P2X2 and 5-HT3 receptors is disrupted when the intracellular C-terminal domain of the P2X2 receptor subunit is deleted and when minigenes coding for P2X2 or 5-HT3A receptor subunit cytoplasmic domains are overexpressed. Injection of fusion proteins containing the C-terminal domain of P2X2 receptors in myenteric neurons also disrupts the functional interaction between native P2X2 and 5-HT3 receptors. Therefore, activity-dependent intracellular coupling of distinct receptor channels underlies ionotropic cross talks that may significantly contribute to the regulation of neuronal excitability and synaptic plasticity.

Human autophagins, a family of cysteine proteinases potentially implicated in cell degradation by autophagy

We have cloned four human cDNAs encoding putative cysteine proteinases that have been tentatively called autophagins. These proteins are similar to Apg4/Aut2, a yeast enzyme involved in the activation of Apg8/Aut7 during the process of autophagy. The identified proteins ranging in length from 393 to 474 amino acids also contain several structural features characteristic of cysteine proteinases including a conserved cysteine residue that is essential for the catalytic properties of these enzymes. Northern blot analysis demonstrated that autophagins are broadly distributed in human tissues, being especially abundant in skeletal muscle. Functional and morphological analysis in autophagy-defective yeast strains lacking Apg4/Aut2 revealed that human autophagins-1 and -3 were able to complement the deficiency in the yeast protease, restoring the phenotypic and biochemical characteristics of autophagic cells. Enzymatic studies performed with autophagin-3, the most widely expressed human autophagin, revealed that the recombinant protein hydrolyzed the synthetic substrate Mca-Thr-Phe-Gly-Met-Dpa-NH(2) whose sequence derives from that present around the Apg4 cleavage site in yeast Apg8/Aut7. This proteolytic activity was diminished by N-ethylmaleimide, an inhibitor of cysteine proteases including yeast Apg4/Aut2. These results provide additional evidence that the autophagic process widely studied in yeast can also be fully reconstituted in human tissues and open the possibility to explore the relevance of the autophagin-based proteolytic system in the induction, regulation, and execution of autophagy.

GATE-16 and GABARAP are authentic modifiers mediated by Apg7 and Apg3

GATE-16, GABARAP, and LC3 are three mammalian counterparts of yeast Apg8p/Aut7p. Here, we show that GATE-16 and GABARAP are authentic modifiers, as is the case of LC3 modification. The C-terminal Phe(117) of proGATE-16 and the C-terminal Leu(117) of proGABARAP are post-translationally cleaved to expose an essential Gly(116) within GATE-16 and GABARAP, with the products designated GATE-16-I and GABARAP-I, respectively. The Gly(116) within GATE-16 and GABARAP are essential for further formation of the intermediates between them and Apg7p(C572S) and Apg3p(C264S). When Apg7p and Apg3p are expressed, GATE-16-I and GABARAP-I are modified to a secondary ubiquitin-like modified form, GATE-16-II and GABARAP-II, respectively. GATE-16-I and GABARAP-I, but not LC3-I, localize to membrane compartments before their modification. These results indicate that GATE-16 and GABARAP are authentic modifiers, but that they have different biochemical characteristics from those of LC3.

Calcium-dependent maintenance of agrin-induced postsynaptic specializations

Although much progress has been made in understanding synapse formation, little is known about the mechanisms underlying synaptic maintenance and loss. The formation of agrin-induced AChR clusters on cultured myotubes requires both activation of the receptor tyrosine kinase MuSK and intracellular calcium fluxes. Here, we provide evidence that such AChR clusters are maintained by agrin/MuSK-induced intracellular calcium fluxes. Clamping intracellular calcium fluxes after AChR clusters have formed leads to rapid MuSK and AChR tyrosine dephosphorylation and cluster dispersal, even in the continued presence of agrin. Both the dephosphorylation and the dispersal are inhibited by the tyrosine phosphatase inhibitor pervanadate. In contrast, clamping intracellular calcium at the time of initial agrin stimulation has no effect on agrin-induced MuSK or AChR phosphorylation, but blocks AChR cluster formation. These findings suggest an avenue by which postsynaptic stability can be regulated by modification of intracellular signaling pathways that are distinct from those used during synapse formation.

Short-term modulation of GABAA receptor function in the adult female rat

Oxytocin neurons in the supraoptic nucleus (SON) exhibit marked neuronal plasticity during each reproductive cycle. We have previously shown that this neuronal plasticity includes GABAA receptor subunit switching around the time of parturition. Here we focus on addition plasticity in short-term regulatory mechanisms of postsynaptic receptor function before and after parturition, i.e. alterations in metabotropic and allosteric modulation of GABAA receptor activity. Both short- and long-term regulation of the GABAA receptor function affects the electrical behaviour of the oxytocin neurons (Brussaard and Herbison, 2000); however, their causal linkage until recently remained unclear. Non-genomic gonadal steroid feedback to oxytocin neurons is mediated via the neurosteroid allopregnanolone (3 alpha-OH-DHP) that is an allosteric modulator of postsynaptic GABAA receptors. We recently found evidence to support the idea that (1) neurosteroids not only potentiate GABAA receptor function but also prevent its suppression by PKC (Brussaard et al., 2000), and (2) that neurosteroid sensitivity of GABAA receptor is not regulated by subunit switching, but instead, is dependent on the balance between endogenous phosphatase and PKC activity (Koksma et al., 2002). Thus, before pregnancy, the GABAA receptors are sensitive to 3 alpha-OH-DHP, due to a constitutively high level of phosphatase activity. At parturition, endogenous release of oxytocin within the SON shifts the intracellular balance towards a higher level of phosphorylation, leading to 3 alpha-OH-DHP insensitivity of the GABAA receptors. Here we discuss the putative mechanisms underlying these changes in receptor physiology, their causal relations and the functional significance for the hormonal output.

Molecules interacting with PRIP-2, a novel Ins(1,4,5)P3 binding protein type 2: Comparison with PRIP-1

A family of phospholipase C-related, catalytically inactive proteins (designated PRIP) have been identified as a group of novel inositol 1,4,5-trisphosphate binding proteins with a domain organization similar to phospholipase C-delta but lacking the enzymatic activity. The PRIP family consists of at least two types of proteins (PRIP-1 and PRIP-2 subfamilies). In the present study, we examined the tissue distribution of PRIP-2, its expression in rat brain at the mRNA level, and the characteristics of its binding to inositol compounds, protein phosphatase 1, and gamma-amino butyric acid receptor associated protein. We also compared these characteristics with those of PRIP-1. Northern blot analysis and reverse-transcription polymerase chain reaction showed that PRIP-1 was present mainly in the brain, whereas PRIP-2 was expressed ubiquitously. In situ hybridization studies using rat brain revealed that the mRNA for both PRIP-1 and PRIP-2 was similarly expressed; it was detected in the granular cell and Purkinje cell layers in the cerebellum, and in the hippocampal pyramidal cells, dentate granule cells, and pyramidal and/or granule cells of the cerebral cortex in the cerebrum. PRIP-2 bound inositol 1,4,5-trisphosphate and its parent lipid, phosphatidylinositol 4,5-bisphosphate, with a similar affinity, while PRIP-1 preferentially bound the former ligand by about 10-fold. PRIP-1 and PRIP-2 interacted with protein phosphatase 1 and gamma-amino butyric acid receptor associated protein in a similar manner. These results indicate that, similar to PRIP-1, PRIP-2 may be involved in both inositol 1,4,5-trisphosphate-mediated and gamma-amino butyric acid-related signaling.

Cellular sequences in pestivirus genomes encoding gamma-aminobutyric acid (A) receptor-associated protein and Golgi-associated ATPase enhancer of 16 kilodaltons

The presence of cellular protein coding sequences within viral RNA genomes is a unique and particularly interesting feature of cytopathogenic (cp) pestiviruses. Here we report the identification and characterization of two novel cellular sequences in the genomes of cp bovine viral diarrhea virus (BVDV) strains. In BVDV strain CP X604, we detected a duplication of the genomic region encoding NS3, NS4A, and part of NS4B, together with an insertion of sequences that code for cellular gamma-aminobutyric acid (A) receptor-associated protein [GABA(A)-RAP]. Transient-expression studies showed that the GABA(A)-RAP sequence leads to additional processing of the viral polyprotein and thereby to the expression of nonstructural protein NS3. Transfection of bovine cells with RNA transcribed from an infectious cDNA clone revealed that the GABA(A)-RAP-encoding insertion together with the duplicated viral sequences constitutes the genetic basis for the cytopathogenicity of strain CP X604. Surprisingly, molecular analysis of another cp BVDV strain (CP 721) resulted in the identification of a cellular Golgi-associated ATPase enhancer of 16 kDa (GATE-16)-encoding insertion together with duplicated viral sequences. To our knowledge, the genomes of CP X604 and CP 721 are the first viral RNAs found with cellular sequences encoding GABA(A)-RAP and GATE-16, respectively. Interestingly, the two cellular proteins belong to a family of eukaryotic proteins involved in various intracellular trafficking processes. Processing after the C-terminal glycine residue of GABA(A)-RAP and GATE-16 by cellular proteases is essential for covalent attachment to target molecules. Accordingly, it can be assumed that these cellular proteases also recognize the cleavage sites in the context of the respective viral polyproteins and thereby lead to the generation of NS3, the marker protein of cp BVDV.

Interaction between GABAA receptor subunit intracellular loops: implications for higher order complex formation

The majority of fast inhibitory neurotransmission in the CNS is mediated by the GABA type-A (GABAA) receptor, a ligand-gated chloride channel. Of the approximately 20 different subunits composing the hetero-pentameric GABAA receptor, the gamma2 subunit in particular seems to be important in several aspects of GABAA receptor function, including clustering of the receptor at synapses. In this study, we report that the intracellular loop of the gamma2 subunit interacts with itself as well as with gamma1, gamma3 and beta1-3 subunits, but not with the alpha subunits. We further show that gamma2 subunits interact with photolabeled pentameric GABAA receptors composed of alpha1, beta2/3 and gamma2 subunits, and calculate the dissociation constant to be in the micromolar range. By using deletion constructs of the gamma2 subunit in a yeast two-hybrid assay, we identified a 23-amino acid motif that mediates self-association, residues 389-411. We confirmed this interaction motif by inhibiting the interaction in a glutathione-S-transferase pull-down assay by adding a corresponding gamma2-derived peptide. Using similar approaches, we identified the interaction motif in the gamma2 subunit mediating interaction with the beta2 subunit as a 47-amino acid motif that includes the gamma2 self-interacting motif. The identified gamma2 self-association motif is identical to the interaction motif reported between GABAA receptor and GABAA receptor-associated protein (GABARAP). We propose a model for GABAA receptor clustering based on GABARAP and GABAA receptor subunit-subunit interaction.

Autophagy in yeast: a review of the molecular machinery

Autophagy is a membrane trafficking mechanism that delivers cytoplasmic cargo to the vacuole/lysosome for degradation and recycling. In addition to non-specific bulk cytosol, selective cargoes, such as peroxisomes, are sorted for autophagic transport under specific physiological conditions. In a nutrient-rich growth environment, many of the autophagic components are recruited for executing a biosynthetic trafficking process, the cytoplasm to vacuole targeting (Cvt) pathway, that transports the resident hydrolases aminopeptidase I and alpha-mannosidase to the vacuole in Saccharomyces cerevisiae. Recent studies have identified pathway-specific components that are necessary to divert a protein kinase and a lipid kinase complex to regulate the conversion between the Cvt pathway and autophagy. Downstream of these proteins, the general machinery for transport vesicle formation involves two novel conjugation systems and a putative membrane protein complex. Completed vesicles are targeted to, and fuse with, the vacuole under the control of machinery shared with other vacuolar trafficking pathways. Inside the vacuole, a potential lipase and several proteases are responsible for the final steps of vesicle breakdown, precursor enzyme processing and substrate turnover. In this review, we discuss the most recent developments in yeast autophagy and point out the challenges we face in the future.

Autophagosome formation in mammalian cells

Macroautophagy is an intracellular degradation system for the majority of proteins and some organelles. The molecular mechanism of autophagy has been extensively studied using the yeast, Saccharomyces cerevisiae, during these past 10 years. These studies suggested that the molecular machinery of autophagosome formation is well conserved from yeast to higher eukaryotes. Identification and characterization of the mammalian counterparts of the yeast autophagy proteins has facilitated our understanding of mammalian autophagy, particularly of autophagosome formation. These findings are now being applied to studies on the physiological roles of autophagy in mammals.

Rapsyn escorts the nicotinic acetylcholine receptor along the exocytic pathway via association with lipid rafts

The 43 kDa receptor-associated protein rapsyn is a myristoylated peripheral protein that plays a central role in nicotinic acetylcholine receptor (AChR) clustering at the neuromuscular junction. In a previous study, we demonstrated that rapsyn is specifically cotransported with AChR via post-Golgi vesicles targeted to the innervated surface of the Torpedo electrocyte (Marchand et al., 2000). In the present study, to further elucidate the mechanisms for sorting and assembly of postsynaptic proteins, we analyzed the dynamics of the intracellular trafficking of fluorescently labeled rapsyn in the transient-expressing COS-7 cell system. Our approach was based on fluorescence, time-lapse imaging, and immunoelectron microscopies, as well as biochemical analyses. We report that newly synthesized rapsyn associates with the trans-Golgi network compartment and traffics via vesiculotubular organelles toward the cell surface of COS-7 cells. The targeting of rapsyn organelles appeared to be mediated by a microtubule-dependent transport. Using cotransfection experiments of rapsyn and AChR, we observed that these two molecules codistribute within distal exocytic routes and at the plasma membrane. Triton X-100 extraction on ice and flotation gradient centrifugation demonstrated that rapsyn and AChR are recovered in low-density fractions enriched in two rafts markers: caveolin-1 and flotillin-1. We propose that sorting and targeting of these two companion molecules are mediated by association with cholesterol-sphingolipid-enriched raft microdomains. Collectively, these data highlight rapsyn as an itinerant vesicular protein that may play a dynamic role in the sorting and targeting of its companion receptor to the postsynaptic membrane. These data also raise the interesting hypothesis of the participation of the raft machinery in the targeting of signaling molecules to synaptic sites.

Benzodiazepine binding studies on living cells: application of small ligands for fluorescence correlation spectroscopy

We demonstrate the applicability of fluorescence correlation spectroscopy (FCS) for receptor binding studies using low molecular weight ligands on the membranes of living nerve cells. The binding of the benzodiazepine Ro 7-1986/602 (N-des-diethyl-fluorazepam), labeled with the fluorophore Alexa 532, to the benzodiazepine receptor was analyzed quantitatively at the membrane of single rat hippocampal neurons. The values obtained for the dissociation constant Kd = (9.9 +/- 1.9) nm and the rate constant for ligand-receptor dissociation kdisS = (1.28 +/- 0.08) x 10(-3) s(-1) show that there is a specific and high affinity interaction between the dye-labeled ligand (Ro-Alexa) and the receptor site. The binding was saturated at approx. 100 nM and displacement of 10 nM Ro-Alexa, with a 1,000-fold excess of midazolam, showed a non-specific binding of 7-10%. Additionally, two populations of the benzodiazepine receptor that differed in their lateral mobility were detected in the membrane of rat neurons. The diffusion coefficients for these two populations [D(bound1) = (1.32 +/- 0.26) microm2/s; D(bound2) = (2.63 +/- 0.63) x 10(-2) microm2/s] are related to binding sites, which shows a mono-exponential decay in a time-dependent dissociation of the ligand-receptor complex.

Coiled-coils: stability, specificity, and drug delivery potential

The coiled-coil is a ubiquitous protein folding and assembly motif made of alpha-helices wrapping around each other forming a supercoil. The sequences of coiled-coils are made of seven-residue repeats, called heptads, and thus are polymer-like. Due to its simplicity and regularity, the coiled-coil is the most extensively studied protein motif. In this review, results on coiled-coil stability and specificity from structural and biophysical studies are summarized. It is pointed out that the primary sequences of coiled-coils over specify the secondary structure but under specify the tertiary/quaternary structure. This leads to two unique features of coiled-coil structure: linkage between stability and specificity and decoupling of secondary and tertiary/quaternary structural specificity. This is followed by a discussion of the potential of coiled-coils as drug delivery vehicles, particularly the prospect in two-staged pretargeted delivery. Such potentials are intimately related to the unique structural features of coiled-coils. The aim of this review is to illustrate how knowledge on protein stability and specificity can be used in the de novo design of peptide-based drug delivery vehicles with well-defined structure and interaction features.

Light Chain 3 associates with a Sos1 guanine nucleotide exchange factor: its significance in the Sos1-mediated Rac1 signaling leading to membrane ruffling

A 19 kDa protein was identified to associate with the Dbl oncogene homology domain of Sos1 (Sos-DH) and was purified from rat brains by GST-Sos-DH affinity chromatography. Peptide sequencing revealed that the protein is identical to light chain 3 (LC3), a microtubule-associated protein. LC3 coimmunoprecipitated with Sos1, and GST-LC3 was capable of forming complexes with Sos1 in in vitro GST-pull down assay. Furthermore, LC3 was colocalized with Sos1 in cells, as determined by immunohistochemistry. While Sos1 stimulated the guanine nucleotide exchange reaction on Rac1, LC3 suppressed the ability of Sos1 to activate Rac1 in in vitro experiments using COS cell lysates. Consistent with this, overexpression of LC3 decreased the level of active GTP-bound Rac1 in COS cells. Sos1 expression induced membrane ruffling, a downstream target for Rac1, but LC3 expression inhibited this biological effect of Sos1. These findings suggest that LC3 interacts with Sos1 and thereby negatively regulates the Sos1-dependent Rac1 activation leading to membrane ruffling.

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

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

The APG8/12-activating enzyme APG7 is required for proper nutrient recycling and senescence in Arabidopsis thaliana

The vacuole/lysosome serves an important recycling function during starvation and senescence in eukaryotes via a process called autophagy. Here bulk cytosolic constituents and organelles become sequestered in specialized autophagic vesicles, which then deliver their cargo to the vacuole for degradation. In yeasts, genetic screens have identified two novel post-translational modification pathways remarkably similar to ubiquitination that are required for autophagy. From searches of the Arabidopsis genome, we have identified gene families encoding proteins related to both the APG8 and -12 polypeptide tags and orthologs for all components required for their attachment. A single APG7 gene encodes the ATP-dependent activating enzyme that initiates both conjugation pathways. Phenotypic analysis of an APG7 disruption indicates that it is not essential for normal growth and development in Arabidopsis. However, the apg7-1 mutant is hypersensitive to nutrient limiting conditions and displays premature leaf senescence. mRNAs for both APG7 and APG8 preferentially accumulate as leaves senesce, suggesting that both conjugation pathways are up-regulated during the senescence syndrome. These findings show that the APG8/12 conjugation pathways have been conserved in plants and may have important roles in autophagic recycling, especially during situations that require substantial nitrogen and carbon mobilization.

Mammalian Apg12p, but not the Apg12p.Apg5p conjugate, facilitates LC3 processing

A dynamic membrane rearrangement occurs in cells during autophagy to form autophagosomes. In this dynamic process, two ubiquitin-like modifications, Apg12p-conjugation and LC3-modification, are essential for the formation of autophagosomes. Apg7p and Apg10p catalyze the conjugation of Apg12p to Apg5p. The same Apg7p and Apg3p catalyze the processing of LC3 to a membrane-bound form, LC3-II. In this paper, we investigated whether Apg12p has an influence on the second LC3-modification system. A cross-linking experiment revealed that Apg3p interacts with the endogenous Apg12p.Apg5p conjugate. However, Apg3p itself interacts with free Apg12p more preferentially than the Apg12p.Apg5p conjugate, when free Apg12p exists. When Apg12p was overexpressed, LC3 processing was significantly enhanced in the presence of Apg7p. In contrast, when the Apg12p.Apg5p conjugate itself was accumulated by the overexpression of Apg12p and Apg5p, LC3 processing was dominantly inhibited, even in the presence of Apg7p. These results indicate that both Apg12p and the Apg12p.Apg5p conjugate are regulatory factors for LC3 processing.

Alternative splicing of the GABAA receptor α4 subunit creates a severely truncated mRNA

GABA(A) receptors, important sites of drug action, are chloride channels composed of 5 subunits chosen from among 19 or more. Alternative splicing for alpha 5, alpha 6, and rho 1 subunits results in truncated proteins which appear to lack function. We report a similar, relatively common (about 20%) form of alternative splicing of the alpha 4 subunit mRNA in mice and humans which, remarkably, creates a severely truncated message containing only the first two and last coding exons, with a frameshift in between. The only apparent translation product includes a short piece (39 amino acids) of the N-terminus right after the signal peptide. The splicing was developmentally and regionally regulated; the highest proportions of truncated alpha 4 mRNA, about 40%, were observed in embryonic day 18 whole brain and adult cerebellum. The truncated mRNA, when coexpresssed in human embryonic kidney (HEK) 293 cells with the complete alpha 4 subunit and beta1 and gamma 2 S subunits, reduced observed GABA currents without kinetic alterations. No such effect of truncated alpha 4 was observed with alpha1 subunit-containing receptors. Thus, the truncated alpha 4 N-terminus may play a post-translational regulatory role in intracellular folding/glycosylation/assembly of the alpha 4 subunit.

Biochemical identification of the binding domain in the GABA(A) receptor-associated protein (GABARAP) mediating dimer formation

The gamma-aminobutyric acid receptor type A (GABA(A)) receptor-associated protein (GABARAP) is a member of a growing family of intracellular membrane trafficking and/or fusion proteins and has been implicated in plasma membrane targeting and clustering of GABA(A) receptors. GABARAP interacts with microtubules and the gamma2 subunit of GABA(A) receptor and modulates channel kinetics. From crystal structures of GABARAP in high salt concentration it has been proposed that oligomerization of GABARAP might take place in a head-to-tail fashion. In this study, we report that GABARAP self-associates and dimerizes in physiological salt concentrations. We find no evidence for higher order complex larger than a dimer. By using deletion constructs of GABARAP we show that interaction takes place between amino acid 36 and 68. We further narrow the interacting domain by inhibiting the self-association, by adding GABARAP-derived synthetic peptides in GST pull-down assays and shows that the interaction specifically takes place in the previously identified GABARAP-GABA(A) receptor interaction domain from amino acid 41-51. The identification of binding domains in GABARAP allows for the study of GABARAP functions, including GABA(A) receptor dynamics.

Identification, molecular cloning, and characterization of a novel GABAA receptor-associated protein, GRIF-1

A novel 913-amino acid protein, gamma-aminobutyric acid type A (GABA(A)) receptor interacting factor-1 (GRIF-1), has been cloned and identified as a GABA(A) receptor-associated protein by virtue of its specific interaction with the GABA(A) receptor beta 2 subunit intracellular loop in a yeast two-hybrid assay. GRIF-1 has no homology with proteins of known function, but it is the rat orthologue of the human ALS2CR3/KIAA0549 gene. GRIF-1 is expressed as two alternative splice forms, GRIF-1a and a C-terminally truncated form, GRIF-1b. GRIF-1 mRNA has a wide distribution with a major transcript size of 6.2 kb. GRIF-1a protein is only expressed in excitable tissues, i.e. brain, heart, and skeletal muscle major immunoreactive bands of M(r) approximately 115 and 106 kDa and, in muscle and heart only, an additional 88-kDa species. When expressed in human embryonic kidney 293 cells, GRIF-1a yielded three immunoreactive bands with M(r) approximately 115, 106, and 98 kDa. Co-expression of GRIF-1a and alpha 1 beta 2 gamma 2 GABA(A) receptors in mammalian cells revealed some co-localization in the cell cytoplasm. Anti-FLAG-agarose specifically precipitated GRIF-1(FLAG) and GABA(A) receptor beta 2 subunits from human embryonic kidney 293 cells co-transfected with GRIF-1a(FLAG) and beta 2 subunit clones. Further, immobilized GRIF-1-(8-633) specifically precipitated in vitro GABA(A) receptor alpha 1 and beta 2 subunit immunoreactivities from detergent extracts of adult rat brain. The respective GABA(A) receptor beta 2 subunit/GRIF-1 binding domains were mapped using the yeast two-hybrid reporter gene assays. A possible role for GRIF-1 as a GABA(A) receptor beta 2 subunit trafficking factor is proposed.

Influence of non-volatile anesthetics on the migration behavior of the human breast cancer cell line MDA-MB-468

BACKGROUND

Anesthetic agents are known to influence functions of the immune system. Anesthetic drugs also support cancer progression by suppressing the activity of immune cells. In breast carcinoma an increase in expression of peripheral-type benzodiazepine receptors (PBR) and the gamma aminobutyl acid (GABA) level has been discovered. Therefore, an investigation of a direct influence of GABA-A agonist propofol, GABA-A and PBR-agonist etomidate, and the local anesthetic drug lidocaine, which can also bind to the GABA-A receptor and PBR, on migration of breast carcinoma cells was performed.

METHODS

MDA-MB-468 cells were incubated with anesthetic agents using clinically relevant concentrations (propofol 3, 6, 9 mg/l, etomidate 2, 3, 4 mg/l, and lidocaine 1.25, 2.5, 5 mg/l). Locomotion was investigated in a three-dimensional collagen matrix using time-lapse video microscopy and computer-assisted cell-tracking.

RESULTS

The percentage of migrating cells (57.4+/-1.9) as well as the velocity (0.22+/-0.09 microm/min) and distance migrated (89.4+/-66.8 microm/10 h) increased in the presence of propofol in a dose-dependent manner (up to 74.4+/-7.5, 0.30+/-0.09, 143.8+/-89.1, respectively) compared with the long chain triglyceride (LCT) control. In contrast, no influence of etomidate on the number of migrating cells could be observed. The velocity and distance migrated at 3 and 4 mg/l were found to be statistically significantly enhanced. Treatment with lidocaine caused an increase in the percentage of migrating cells (up to 75.0+/-5.6) in velocity dose dependently (up to 0.33+/-0.06) and in distance migrated (up to 151.5+/-92.9).

CONCLUSION

These results show that different anesthetic drugs are able to modulate the migratory machinery of human breast carcinoma cells in vitro.

Targeted trafficking of neurotransmitter receptors to synaptic sites

Emerging data are sheding light on the critical task for synapses to locally control the production of neurotransmitter receptors ultimately leading to receptor accumulation and modulation at postsynaptic sites. By analogy with the epithelial-cell paradigm, the postsynaptic compartment may be regarded as a polarized domain favoring the selective recruitment and retention of newly delivered receptors at synaptic sites. Targeted delivery of receptors to synaptic sites is facilitated by a local organization of the exocytic pathway, likely resulting from spatial cues triggered by the nerve. This review focuses on the various mechanisms responsible for regulation of receptor assembly and trafficking. A particular emphasis is given to the role of synaptic anchoring and scaffolding proteins in the sorting and routing of their receptor companion along the exocytic pathway. Other cellular components such as lipidic microdomains, the docking and fusion machinery, and the cytoskeleton also contribute to the dynamics of receptor trafficking at the synapse.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Lessons from nature: stimuli-responsive polymers and their biomedical applications

Response to stimulus is a basic process of living systems. Based on the lessons from nature, scientists have been designing useful materials that respond to external stimuli such as temperature, pH, light, electric field, chemicals and ionic strength. These responses are manifested as dramatic changes in one of the following: shape, surface characteristics, solubility, formation of an intricate molecular self-assembly or a sol-to-gel transition. Applications of stimuli-responsive, or 'smart', polymers in delivery of therapeutics, tissue engineering, bioseparations, sensors or actuators have been studied extensively and numerous papers and patents are evidence of rapid progress in this area. Understanding the structure-property relationship is essential for the further development and rational design of new functional smart materials. For example, kinetic and thermodynamic control of the coil-to-globule transition could be achieved through changes in polymer composition and topology.

Dynamic regulation of GABA(A) receptors at synaptic sites

gamma-Aminobutyric acid type A receptors (GABA(A)Rs) mediate fast synaptic inhibition in brain and spinal cord. They are ligand-gated ion channels composed of numerous distinct subunit combinations. For efficient synaptic transmission, GABA(A)Rs need to be localized to and anchored at postsynaptic sites in precise apposition to presynaptic nerve terminals that release the neurotransmitter GABA. Neurons therefore require distinct mechanisms to regulate intracellular vesicular protein traffic, plasma membrane insertion, synaptic clustering and turnover of GABA(A)Rs. The GABA(A) receptor-associated protein GABARAP interacts with the gamma2 subunit of GABA(A)Rs and displays high homology to proteins involved in membrane fusion underlying Golgi transport and autophagic processes. The binding of GABARAP with NSF, microtubules and gephyrin together with its localization at intracellular membranes suggests a role in GABA(A)R targeting and/or degradation. Growth factor tyrosine kinase receptor activation is involved in the control of GABA(A)R levels at the plasma membrane. In particular insulin recruits GABA(A)Rs to the cell surface. Furthermore, the regulation of GABA(A)R surface half-life can also be the consequence of negative modulation at the proteasome level. Plic-1, a ubiquitin-like protein binds to both the proteasome and GABA(A)Rs and the Plic1-GABA(A)R interaction is important for the maintenance of GABA-activated current amplitudes. At synaptic sites, GABA(A)Rs are clustered via gephyrin-dependent and gephyrin-independent mechanisms and may subsequently become internalized via clathrin-mediated endocytosis underlying receptor recycling or degradation processes. This article discusses these recent data in the field of GABA(A)R dynamics.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Effects of phosphorylation and cytoskeleton-affecting reagents on GABA(A) receptor recruitment into synaptosomes following acute stress

The [3H]-flunitrazepam receptor density, measured ex vivo in synaptosomes at 4 degrees C, increased by about 30% because of acute stress in chicks. This increase was first reported to be a receptor recruitment due to the fact that the increase induced by subsolubilizing concentrations of Triton X-100 was not additive to the receptor increase induced by acute stress [J Neural Transm 87 (1992) 97]. In synaptosomal membranes from stressed chicks, the incorporation of alkaline phosphatase or ATP into the lumen abolished or increased, respectively, the receptor unmasking after incubation at 4 and 37 degrees C, suggesting that phosphorylation plays a role in the recruitment mechanism. Moreover, both colchicine and vinblastine, but not taxol, abolished the recruitment induced by stress at 37 degrees C only in synaptosomes, suggesting that micrutubule depolymerization plays a role in the masking of receptors. Furthermore, both cytochalasins C and D induced an increase of the receptor density, abolished by N-ethylmaleimide, in both the stressed and nonstressed conditions, suggesting that microfilament depolymerization induced the exposure to the radioligand of a cytosolic vesicular receptor pool, which had not fused yet with the postsynaptic membrane.

Association of human transferrin receptor with GABARAP

A yeast two-hybrid screen identified a specific interaction between the cytoplasmic domain of transferrin receptor (TfR) and GABARAP, a 14 kDa protein that binds to the gamma2 subunit of neuronal GABA(A) receptors. The specificity of the TfR-GABARAP interaction was demonstrated by in vitro binding assays with purified proteins and by co-immunoprecipitation of GABARAP with endogenous TfR from HeLa cell lysates. Replacement of the YTRF internalization motif with ATRA within the cytoplasmic domain of TfR reduced interaction with GABARAP in the yeast two-hybrid screen and in vitro binding assays. The intracellular location of GABARAP using chimeric GABARAP-GFP showed that the majority of GABARAP was located in perinuclear vesicles. Our results show that GABARAP plays a more general role outside the confines of neuronal cells and GABA(A) receptors.

Native and cloned 5-HT(3A)(S) receptors are anchored to F-actin in clonal cells and neurons

Using selective antibodies to visualize the short isoform of the 5-HT(3A) receptor, we report here that both native and cloned 5-HT(3A)(S) receptors formed clusters associated with F-actin in all cell types studied. NG 108-15 cells expressing native 5-HT(3A)(S) receptors, COS-7 cells transiently expressing 5-HT(3A)(S) subunits, and CHO cells stably transfected with a plasmid encoding the 5-HT(3A)(S) sequence all exhibited similar surface receptor topology with 5-HT(3A)(S) receptor cluster accumulation in F-actin-rich lamellipodia and microspikes. Colocalization and coclustering of 5-HT(3A)(S) subunits and F-actin were also observed in transfected hippocampal neurons. Treatment of the neurons with latrunculin-A, a compound altering F-actin polymerization, demonstrated that 5-HT(3A)(S) receptor cluster size and topology were dependent on F-actin integrity. These results suggest that the anchoring of 5-HT(3A)(S) receptor clusters to the cytoskeletal network probably plays a key role in the physiological regulation of the receptor topology and dynamics, as is the case for other members of the 4-TMD ion channel receptor family.

Human Apg3p/Aut1p homologue is an authentic E2 enzyme for multiple substrates, GATE-16, GABARAP, and MAP-LC3, and facilitates the conjugation of hApg12p to hApg5p

Autophagy is a process of bulk degradation of cytoplasmic components by the lysosome/vacuole and has a significant relationship to several neurodegenerative disorders and myopathies in mammals. One of APG gene products essential for autophagy in yeast, Apg3p, is a protein-conjugating enzyme for Apg8p lipidation (Ichimura, Y., Kirisako, T., Takao, T., Satomi, Y., Shimonishi, Y., Ishihara, N., Mizushima, N., Tanida, I., Kominami, E., Ohsumi, M., Noda, T., and Ohsumi, Y. (2000) Nature 408, 488-492). In this study, the cloning of a human Apg3p homologue (hApg3p) as an E2 enzyme essential for human Apg8p homologues (i.e. GATE-16, GABARAP, and MAP-LC3) is shown, and its unique characteristics are described. The predicted amino acid sequence of the isolated clone shows 34.1% identity and 48.1% similarity to yeast Apg3p. Site-directed mutagenesis revealed that Cys(264) of hApg3p is an authentic active-site cysteine residue essential for the formation of hApg3p small middle dothApg8p homologue intermediates. Overexpression of hApg7p enhances the formation of a stable E2-substrate complex between hApg3p(C264S) and each of the hApg8p homologues, and MAP-LC3 is preferred as the substrate over the other two Apg8p homologues. These results indicate that hApg3p is an E2-like enzyme essential for three human Apg8p homologues. Co-immunoprecipitation of hApg7p with hApg3p indicates that hApg3p forms an E1.E2 complex with hApg7p as in the case of yeast Apg3p and Apg7p. Furthermore, hApg3p coimmunoprecipitates with hApg12p, and the overexpression of hApg3p facilitates the formation of the GFPhApg12p.thApg5p conjugate, suggesting that hApg3p cross-talks with the hApg12p conjugation system.

Solution structure of human GABA(A) receptor-associated protein GABARAP: implications for biolgoical funcrion and its regulation

Control of neurotransmitter receptor expression and delivery to the postsynaptic membrane is of critical importance for neural signal transduction at synapses. The gamma-aminobutyric acid, type A (GABA(A)) receptor-associated protein GABARAP was reported to have an important role for movement and sorting of GABA(A) receptor molecules to the postsynaptic membrane. GABARAP not only binds to GABA(A) receptor gamma2-subunit but also to tubulin, gephyrin, and ULK1. We present for the first time the high resolution structure of human GABARAP determined by nuclear magnetic resonance in aqueous solution. One part of the molecule, despite being well ordered and rigid on a MHz time scale, exists in at least two different conformations that interchange with each other on a time scale slower than 25 Hz. An important feature of the solution structure is the observation that amino- and carboxyl-terminal ends of the protein directly interact with each other, which is not seen in recently reported crystal structures. The possible biological relevance of these observations for the regulation of GABARAP interactions and functions is discussed.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Multiple roles of protein kinases in the modulation of gamma-aminobutyric acid(A) receptor function and cell surface expression

gamma-Aminobutyric acid(A) (GABA(A)) receptors are ligand-gated ion channels that mediate the majority of fast synaptic inhibition in the brain and that are also important drug targets for benzodiazepines, barbiturates, and neurosteriods. These receptors are pentameric hetero-oligomers that can be assembled from 7 subunit classes with multiple members: alpha(1-6), beta(1-3), gamma(1-3), delta, epsilon, theta, and pi. Most receptor subtypes in the brain, however, are believed to be composed of alpha-, beta-, and gamma-subunits. Modifications of GABA(A) receptor function are continually implicated in a range of pathologies, including epilepsy, anxiety, insomnia, and substance abuse. Moreover, changes in the efficacy of synaptic inhibition mediated by GABA(A) receptors are believed to be play central roles in certain forms of synaptic plasticity, including rebound potentiation in the cerebellum, and hippocampal long-term potentiation. Given the critical role that GABA(A) receptors play as mediators of synaptic transmission, it is of fundamental importance to understand the endogenous mechanisms used by neurones to control the function of these receptors. This review will focus on the dynamic regulation of GABA(A) receptor phosphorylation state and channel function as mechanisms involved in determining the efficacy of synaptic inhibition. In addition, the possible role of GABA(A) receptor phosphorylation in controlling receptor internalization and recycling will also be explored.

Murine Apg12p has a substrate preference for murine Apg7p over three Apg8p homologs

Apg7p is a unique E1 enzyme which is essential for both the Apg12p- and Apg8p-modification systems, and plays indispensable roles in yeast autophagy. A cDNA encoding murine Apg7p homolog (mApg7p) was isolated from a mouse brain cDNA library. The predicted amino acid sequence of the clone shows a significant homology to human Apg7p and yeast Apg7p. Murine Apg12p as well as the three mammalian Apg8p homologs co-immunoprecipitate with mApg7p. Site-directed mutagenesis revealed that an active-site cysteine within mApg7p is Cys(567), indicating that mApg7p is an authentic E1 enzyme for murine Apg12p and mammalian Apg8p homologs. The mutagenesis study also revealed that Apg12p has a substrate preference for mApg7p over the three Apg8p homologs, suggesting that the Apg12p conjugation by Apg7p occurs preferentially in mammalian cells compared with the modification of the three Apg8p homologs. We also report here on the ubiquitous expression of human APG7 mRNA in human adult and fetal tissues and of rat Apg7p in adult tissues.

Role of the PLC-related, catalytically inactive protein p130 in GABA(A) receptor function

The protein p130 was isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-delta1 but lacking PLC activity. We show that p130 plays an important role in signaling by the type A receptor for gamma-aminobutyric acid (GABA). Yeast twohybrid screening identified GABARAP (GABA(A) receptor-associated protein), which is proposed to contribute to the sorting, targeting or clustering of GABA(A) receptors, as a protein that interacts with p130. Furthermore, p130 competitively inhibited the binding of the gamma2 subunit of the GABA(A) receptor to GABARAP in vitro. Electrophysiological analysis revealed that the modulation of GABA-induced Cl- current by Zn2+ or diazepam, both of which act at GABA(A) receptors containing gamma subunits, is impaired in hippocampal neurons of p130 knockout mice. Moreover, behavioral analysis revealed that motor coordination was impaired and the intraperitoneal injection of diazepam induced markedly reduced sedative and antianxiety effects in the mutant mice. These results indicate that p130 is essential for the function of GABA(A) receptors, especially in response to the agents acting on a gamma2 subunit.

GABAergic innervation organizes synaptic and extrasynaptic GABAA receptor clustering in cultured hippocampal neurons

We have studied the effects of GABAergic innervation on the clustering of GABA(A) receptors (GABA(A)Rs) in cultured hippocampal neurons. In the absence of GABAergic innervation, pyramidal cells form small (0.36 +/- 0.01 micrometer diameter) GABA(A)R clusters at their surface in the dendrites and soma. When receiving GABAergic innervation from glutamic acid decarboxylase-containing interneurons, pyramidal cells form large (1.62 +/- 0.08 micrometer breadth) GABA(A)R clusters at GABAergic synapses. This is accompanied by a disappearance of the small GABA(A)R clusters in the local area surrounding each GABAergic synapse. Although the large synaptic GABA(A)R clusters of any neuron contained all GABA(A)R subunits and isoforms expressed by that neuron, the small clusters not localized at GABAergic synapses showed significant heterogeneity in subunit and isoform composition. Another difference between large GABAergic and small non-GABAergic GABA(A)R clusters was that a significant proportion of the latter was juxtaposed to postsynaptic markers of glutamatergic synapses such as PSD-95 and AMPA receptor GluR1 subunit. The densities of both the glutamate receptor-associated and non-associated small GABA(A)R clusters were decreased in areas surrounding GABAergic synapses. However, no effect on the density or distribution of glutamate receptor clusters was observed. The results suggest that there are local signals generated at GABAergic synapses that induce both assembly of large synaptic GABA(A)R clusters at the synapse and disappearance of the small GABA(A)R clusters in the surrounding area. In the absence of GABAergic innervation, weaker GABA(A)R-clustering signals, generated at glutamatergic synapses, induce the formation of small postsynaptic GABA(A)R clusters that remain juxtaposed to glutamate receptors at glutamatergic synapses.

The X-ray crystal structure and putative ligand-derived peptide binding properties of gamma-aminobutyric acid receptor type A receptor-associated protein

The gamma-aminobutyric acid receptor type A (GABA(A)) receptor-associated protein (GABARAP) has been reported to mediate the interaction between the GABA(A) receptor and microtubules. We present the three-dimensional structure of GABARAP obtained by x-ray diffraction at 1.75 A resolution. The structure was determined by molecular replacement using the structure of the homologous protein GATE-16. NMR spectroscopy of isotope-labeled GABARAP showed the structure in solution to be compatible with the overall fold but showed evidence of conformation heterogeneity that is not apparent in the crystal structure. We assessed the binding of GABARAP to peptides derived from reported binding partner proteins, including the M3-M4 loop of the gamma2 subunit of the GABA(A) receptor and the acidic carboxyl-terminal tails of human alpha- and beta-tubulin. There is a small area of concentrated positive charge on one surface of GABARAP, which we found interacts weakly with all peptides tested, but we found no evidence for specific binding to the proposed physiological target peptides. These results are compatible with a more general role in membrane targeting and transportation for the GABARAP family of proteins.

Crystal structure of the GABA(A)-receptor-associated protein, GABARAP

The GABA(A)-receptor-associated protein (GABARAP) is a member of a growing family of intracellular membrane trafficking and/or fusion proteins and has been implicated in plasma membrane targeting and/or recycling of GABA(A) receptors. GABARAP is localized on intracellular membranes such as the trans-Golgi network, binds to the gamma 2 subunit of GABA(A) receptors and interacts with microtubules and the N-ethylmaleimide-sensitive factor. We report the X-ray crystal structure of mammalian GABARAP at 2.0 A resolution. GABARAP consists of an N-terminal basic helical region, which has been implicated in tubulin binding, and a core structure with a conserved ubiquitin-like fold. Consistent with the high extent of sequence conservation among GABARAP homologues from plants to mammals, one face of the core structure is absolutely conserved while the opposite face shows considerable divergence. These features are in agreement with the conserved surface mediating protein-protein interactions shared by all members of the family, whereas the non-conserved surface region may play specific roles, such as docking to particular membrane receptors.

The amino terminus of the glial glutamate transporter GLT-1 interacts with the LIM protein Ajuba

We have identified a cytoplasmic LIM protein, Ajuba, which interacts with the amino terminus of GLT-1, the most abundant plasma membrane glutamate transporter in the brain. Ajuba has a cytoplasmic location when expressed alone in COS cells, but translocates to colocalize with GLT-1 at the plasma membrane when GLT-1 is coexpressed. Ajuba is expressed in cerebellum, cortex, hippocampus, and retina and also in organs outside the CNS. Ajuba is found with GLT-1 in astrocytes, cerebellar Bergmann glia and retinal neurons, and antibodies to Ajuba coimmunoprecipitate GLT-1 from brain. For GLT-1 expressed in COS cells, coexpression of Ajuba did not affect the transporter's K(m) or V(max) for glutamate. Since Ajuba is known to activate MAP kinase enzymes, and its homologue Zyxin binds to cytoskeletal proteins, we propose that Ajuba is a scaffolding protein allowing GLT-1 to regulate intracellular signaling or interact with the cytoskeleton.

Intact sorting, targeting, and clustering of gamma-aminobutyric acid A receptor subtypes in hippocampal neurons in vitro

The cellular and subcellular distribution of four GABA(A) receptor subtypes, identified by the presence of the alpha1, alpha2, alpha3, or alpha5 subunit, was investigated immunocytochemically in dissociated cultures of hippocampal neurons. We addressed the questions whether (1) cell-type specific expression, (2) axonal/somatodendritic targeting, and (3) synaptic/extrasynaptic clustering of GABA(A) receptor subtypes was retained in vitro. For comparison, the in vivo distribution pattern was assessed in sections from adult rat brain. The differential expression of GABA(A) receptor subunits allowed to identify five morphologically distinct cell types in culture: the alpha1 subunit was observed in glutamic acid decarboxylase-positive interneurons, the alpha2 and alpha5 subunits marked pyramidal-like cells, and the alpha3 subunit labeled three additional cell types, including presumptive hilar cells. All subunits were found in the somatodendritic compartment. In addition, appropriate axonal targeting was evidenced by the intense alpha2, and sometimes alpha3 subunit labeling of axon-initial segments (AIS) of pyramidal cells and hilar cells, respectively. Accordingly, both receptor subtypes were targeted to AIS in vivo, as well. Synaptic receptors were identified by colocalization with gephyrin, a postsynaptic clustering protein, and apposition to presynaptic terminals labeled with synapsin I. In vitro and in vivo, alpha1- and alpha2-receptor subtypes formed numerous synaptic clusters, alpha3-GABA(A) receptors were located either synaptically or extrasynaptically depending on the cell type, whereas alpha5-GABA(A) receptors were extrasynaptic. We conclude that receptor targeting to broad subcellular locations does not require specific GABAergic innervation patterns, which are disturbed in vitro, but depends on protein-protein interactions in the postsynaptic cell that are both subunit- and neuron-specific.

Novel crosslinking methods to design hydrogels

Hydrogels are presently under investigation as matrices for the controlled release of bioactive molecules, in particular pharmaceutical proteins, and for the encapsulation of living cells. For these applications, it is often required that the gels degrade under physiological conditions. This means that the originally three-dimensional structure has to disintegrate preferably in harmless products to ensure a good biocompatibility of the hydrogel. In this overview, different chemical and physical crosslinking methods used for the design of biodegradable hydrogels are summarized and discussed. Chemical crosslinking is a highly versatile method to create hydrogels with good mechanical stability. However, the crosslinking agents used are often toxic compounds, which have been extracted from the gels before they can be applied. Moreover, crosslinking agents can give unwanted reactions with the bioactive substances present in the hydrogel matrix. Such adverse effects are avoided with the use of physically crosslinked gels.