Gamma-aminobutric acid and glutamate decarboxylas (l-glutamate 1-carboxy-lyase e.c. 4.1.1.15) in the nervous system of the cockroach, periplaneta americana.i.regional distribution and properties of the enzyme

GABA selectively increases mucin-1 expression in isolated pig jejunum

The inhibitory neurotransmitter GABA (γ-aminobutyric acid) is synthesized by glutamic acid decarboxylase, which is expressed in the central nervous system and in various other tissues including the intestine. Moreover, GABA can be ingested in vegetarian diets or produced by bacterial commensals in the gastrointestinal tract. As previous studies in lung have suggested a link between locally increased GABA availability and mucin 5AC production, the present study sought to test whether the presence or lack of GABA (and its precursor glutamine) has an effect on intestinal mucin expression. Porcine jejunum epithelial preparations were incubated with two different amounts of GABA or glutamine on the mucosal side for 4 h, and changes in the relative gene expression of seven different mucins, enzymes involved in mucin shedding, GABA B receptor, enzymes involved in glutamine/GABA metabolism, glutathione peroxidase 2, and interleukin 10 were examined by quantitative PCR (TaqMan(®) assays). Protein expression of mucin-1 (MUC1) was analyzed by Western blot. On the RNA level, only MUC1 was significantly up-regulated by both GABA concentrations compared with the control. Glutamine-treated groups showed the same trend. On the protein level, all treatment groups showed a significantly higher MUC1 expression than the control group. We conclude that GABA selectively increases the expression of MUC1, a cell surface mucin that prevents the adhesion of microorganisms, because of its size and negative charge, and therefore propose that the well-described positive effects of glutamine on enterocytes and intestinal integrity are partly attributable to effects of its metabolite GABA.

Glutamate decarboxylase of the parasitic arthropods Ctenocephalides felis and Rhipicephalus microplus: gene identification, cloning, expression, assay development, identification of inhibitors by high throughput screening and comparison with the orthologs from Drosophila melanogaster and mouse

Glutamate decarboxylase (l-glutamate 1-carboxylyase, E.C. 4.1.1.15, GAD) is the rate-limiting enzyme for the production of γ-aminobutyric acid (GABA), the major inhibitory neurotransmitter in vertebrates and invertebrates. We report the identification, isolation and characterization of cDNAs encoding GAD from the parasitic arthropods Ctenocephalides felis (cat flea) and Rhipicephalus microplus (cattle tick). Expression of the parasite GAD genes and the corresponding Drosophila melanogaster (fruit fly) GAD1 as well as the mouse GAD(65) and GAD(67) genes in Escherichia coli as maltose binding protein fusions resulted in functional enzymes in quantities compatible with the needs of high throughput inhibitor screening (HTS). A novel continuous coupled spectrophotometric assay for GAD activity based on the detection cascade GABA transaminase/succinic semialdehyde dehydrogenase was developed, adapted to HTS, and a corresponding screen was performed with cat flea, cattle tick and fruit fly GAD. Counter-screening of the selected 38 hit substances on mouse GAD(65) and GAD(67) resulted in the identification of non-specific compounds as well as inhibitors with preferences for arthropod GAD, insect GAD, tick GAD and the two mouse GAD forms. Half of the identified hits most likely belong to known classes of GAD inhibitors, but several substances have not been described previously as GAD inhibitors and may represent lead optimization entry points for the design of arthropod-specific parasiticidal compounds.

Molecular cloning, partial genomic structure and functional characterization of succinic semialdehyde dehydrogenase genes from the parasitic insects Lucilia cuprina and Ctenocephalides felis

The enzyme succinic semialdehyde dehydrogenase (SSADH; EC1.2.1.24) is a component of the gamma-aminobutyric acid degradation pathway in mammals and is essential for development and function of the nervous system. Here we report the identification, cDNA cloning and functional expression of SSADH from the parasitic insects Lucilia cuprina and Ctenocephalides felis. The recombinant proteins possess potent NAD+-dependent SSADH activity, while their catalytic efficiency for other aldehyde substrates is lower. A genomic copy of the L. cuprina SSADH gene contains two introns, while a genomic gene version of C. felis is devoid of introns. In contrast to the single copy SSADH genes in Drosophila melanogaster and mammals, in L. cuprina and C. felis, multiple SSADH gene copies are present in the genome.

Molecular cloning and functional characterization of a GABA transporter from the CNS of the cabbage looper, Trichoplusia ni

A cDNA encoding a GABA transporter in the caterpillar Trichoplusia ni has been cloned and expressed in baculovirus-infected insect cells. The cDNA contains an ORF encoding a 608-residue protein, designated TrnGAT. Hydropathy analysis of the deduced amino acid sequence suggests 12 transmembrane domains, a structure similar to that of all other cloned Na+/Cl(-)-dependent GABA transporters. The deduced amino acid sequence shows high identity with a GABA transporter (MasGAT) expressed in the embryo of Manduca sexta. Expression of TrnGAT mRNA was detected only in the brain. Sf21 cells infected with recombinant baculovirus exhibited a 20- to 30-fold increase in [3H]GABA uptake compared to control-infected cells. Several blockers of GABA uptake were used to determine the pharmacological profile of TrnGAT. Although most similar to mammalian neuronal GABA transporter GAT-1 in its kinetic properties, stoichiometry of ionic dependence and pharmacological properties, TrnGAT may be distinguished from mammalian GAT-1 by the inability of cyclic GABA analogues, such as nipecotic acid and its derivatives, to inhibit GABA uptake by the insect protein. The unique pharmacology of TrnGAT suggests that the GABA transport system in the lepidopteran CNS could be a useful target in the future development of rapidly-acting neuroactive agents used to control agriculturally-important insects.

A rapid purification of L-glutamic acid decarboxylase from the brain of the locust Schistocerca gregaria

L-Glutamic acid decarboxylase (GAD; EC 4.1.1.15) was purified to apparent homogeneity from the brain of the locust Schistocerca gregaria using a combination of chromatofocusing (Mono P) and gel filtration (Superose 12) media. The homogeneity of the enzyme preparation was established by native polyacrylamide gel electrophoresis (PAGE) with silver staining. The molecular weight of the purified enzyme was estimated from native gradient gel electrophoresis and gel filtration chromatography to be 97,000 +/- 4,000 and 93,000 +/- 5,000, respectively. When analysed by sodium dodecyl sulphate-PAGE, the enzyme was found to be composed of two distinct subunits of Mr 51,000 +/- 1,000 and 44,000 +/- 1,500. Tryptic peptide maps of iodinated preparations of these two subunits showed considerable homology, suggesting that the native enzyme is a dimer of closely related subunits. The purified enzyme had a pH optimum of 7.0-7.4 in 100 mM potassium phosphate buffer and an apparent Km for glutamate of 5.0 mM. The enzyme was strongly inhibited by the carbonyl-trapping reagent aminooxyacetic acid with an I50 value of 0.2 microM.

Insect optic lobe neurons identifiable with monoclonal antibodies to GABA

Five monoclonal antibodies against GABA were tested on glutaraldehyde fixed sections of optic lobes of three insect species, blowflies, houseflies and worker bees. The specificity of these antibodies was analyzed in several tests and compared with commercially available anti-GABA antiserum. A very large number of GABA-like immunoreactive neurons innervate all the neuropil regions of these optic lobes. Immunoreactive processes are found in different layers of the neuropils. The immunoreactive neurons are amacrines and columnar or noncolumnar neurons connecting the optic lobe neuropils. In addition some large immunoreactive neurons connect the optic lobes with centers of the brain. Some neuron types could be matched with neurons previously identified with other methods. The connections of a few of these neuron types are partly known from electron microscopy or electrophysiology and a possible role of GABA in certain neural circuits can be discussed.

Inhibition of crayfish glutamic acid decarboxylase by structural analogs of the substrate and product

Crayfish glutamic acid decarboxylase (GAD) is inhibited by some aliphatic carboxylic acid analogs of glutamate and gamma-amino-n-butyric acid (GABA). Variations in the length of the carbon skeleton, substitution of a keto for a methylene group, replacement of the carboxyl group or attachment of a bulky basic moiety to the amino terminus of GABA all lead to a drastic reduction in its inhibitory activity. Substitution of a methyl group for the amino group of GABA is a permissible alteration which does not reduce the inhibitory potency. Some structural analogs of glutamate are inhibitory also, particularly if they possess a comparable carbon skeleton and a keto group in the alpha position or a sulfhydryl group. Most of the sulfhydryl analogs are significantly more potent as inhibitors than the corresponding compounds in which the SH group is replaced by an H atom.

Some characteristics of glutamic acid decarboxylase of chick ampullary cristae

In a previous study, it was demonstrated that enzyme-mediated gamma-aminobutyric acid (GABA) synthesis occurs in the vestibule of the chick inner ear. As deeper knowledge of the properties of its synthesizing enzyme might contribute to the understanding of the role of GABA in inner ear function, some characteristics of glutamate decarboxylase (GAD) were studied in chick isolated ampullary cristae under conditions in which 14CO2 release from [1-14C]glutamate and [14C]GABA formation from [U-14C]glutamate for estimating GAD activity were equal. It was found that Km for glutamate is 5 mM and that the enzyme pH optimum is 7.3. These values fall within the range described for the corresponding enzyme in nervous tissue of other species. Pyridoxal phosphate (PLP) activates the enzyme and aminooxyacetic acid inhibits it, the same as these agents activate or inhibit GAD from several nervous tissue sources. 2-Mercaptoethanol shows some protection from inactivation of the PLP-dependent enzyme and Triton X-100 exerts some inhibition of vestibular GAD activity, as previously shown in other nervous tissue preparations. Although its cellular localization is at present uncertain, these results indicate that GAD of chick vestibular tissue possesses properties resembling those of the brain enzyme and might be controlled in a manner similar to that of GAD in brain, thus possibly participating in the regulation of inner ear function.

Inhibitors of crayfish glutamic acid decarboxylase

Crayfish glutamic acid decarboxylase (GAD), like the homologous enzymes from other species, is inhibited by carbonyl-trapping agents (e.g. aminooxyacetic acid; AOAA) and sulfhydryl reagents (e.g. 5,5'-dithiobis-(2-nitrobenzoic acid); DTNB). It also is inhibited by the product GABA, many anions (e.g. SCN- and Cl-), and some cations (e.g. Zn+2). The inhibition by AOAA, but not that by DTNB, was prevented by increasing the concentration of the pyridoxal phosphate (PLP) coenzyme. GABA blocked the effects of PLP on enzyme activity. The inhibition by AOAA, DTNB, GABA, and chloride all were competitive with substrate. The effect of GABA occurs at physiological concentrations and may contribute to the regulation of GAD activity in vivo. The quantitative effect of anions is dependent on the cation with which they are administered. ATP stimulated GAD activity in homogenates prepared with potassium phosphate or Tris-acetate buffer, even when no exogenous PLP was provided.

Assay and properties of glutamic acid decarboxylase in homogenates of crayfish nervous tissue

The activity of glutamic acid decarboxylase (GAD) was measured in homogenates of crayfish nervous tissue. Radioactive GABA and CO2 were formed from radioactive glutamic acid in approximately equimolar amounts. Product formation was linear for 9.5 hr at 11-32 degrees C with about 1-30 micrograms homogenate protein. Enzyme activity remained high at pH 7-10 but declined steeply above pH 10.5 and below pH 7. Enzyme activity was stimulated by pyridoxal phosphate, 2-mercaptoethanol, and potassium phosphate; at higher than optimal concentrations of each the activity was reduced. Sodium phosphate altered the stimulatory effect of potassium phosphate. Crayfish GAD behaves like a typical neural GAD but is distinguishable biochemically from GAD of other species.

Glutamic acid decarboxylase in sea lamprey (Petromyzon marinus): characterization, localization, and developmental changes

We have carried out assays for glutamic acid decarboxylase (GAD) in homogenates of brain and spinal cord from larval and adult sea lamprey (Petromyzon marinus). The enzyme had similar characteristics in both stages. Optimal pH was 6.8; optimal temperature was 27-30 degrees C; Km at 27 degrees C was 5 mM. GAD activity was distributed uniformly along the length of the spinal cord. Specific activities for the larval cord and brain were 26 and 63 nm CO2/mg protein/h, respectively. The specific activities for the adult cord and brain were 29 and 236 nm CO2/mg protein/h, respectively. Thus, the activity of cord homogenates did not change significantly between larval and adult stages, but that of the brain increased about fourfold.