gad1b (glutamate decarboxylase 1b; formerly gad1 or GAD67) is one of two zebrafish co-orthologs of vertebrate GAD1, retained from the teleost-specific genome duplication; its paralog is gad1a. It encodes a pyridoxal 5'-phosphate-dependent group II decarboxylase that converts L-glutamate to the inhibitory neurotransmitter GABA in the cytoplasm of GABAergic neurons. The protein is 83% identical to human GAD1 and keeps its PLP-binding lysine and GABA-binding residues. gad1b is the dominant, pan-GABAergic gad1 copy, expressed from pharyngula stages in GABAergic neurons throughout the brain, retina and spinal cord (including Kolmer-Agduhr neurons, Purkinje cells and lateral-line efferent neurons), and it is the classic zebrafish GAD67 marker. gad1b null mutants and morphants show abnormal, seizure-like brain activity and increased locomotion, consistent with reduced GABA synthesis.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004351 glutamate decarboxylase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Glutamate decarboxylase activity propagated from the GAD node PTN002903772, seeded by fly Gad1, rodent Gad1/Gad2 and human GAD1. Reason: gad1b is a GAD1 co-ortholog inside the node's clade, keeps the PLP-binding lysine and GABA-binding residues, and its expression coincides with GABA immunoreactivity in the embryo. The zebrafish enzyme has not been assayed in vitro, but the null mutant phenotype (seizure-like brain activity, like the GABA-A antagonist PTZ) fits loss of GABA synthesis. Supporting Evidence: PMID:9634146 Immunohistochemistry for gamma-aminobutyric acid (GABA) revealed that GABA is produced at all sites of GAD expression, including the novel cells in the caudal hindbrain. PMID:30200754 The electrophysiological traces of the cMO morphants were comparable to those obtained from larvae null for gad1b and wild-type fish exposed to PTZ, which causes seizures in zebrafish. file:DANRE/gad1a/gad1a-bioinformatics/RESULTS.md Both copies keep the PLP lysine and the GABA-binding residues of human GAD1. |
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | ACCEPT | Summary: Cytoplasm propagated from the group II decarboxylase node PTN000242648. Reason: GAD1 is a soluble cytoplasmic enzyme; Gad1b has no signal peptide or transmembrane segment. Supporting Evidence: PMID:27461130 While both GADs synthesize GABA and are co-expressed in most vertebrate GABAergic neurons, GAD1 synthesizes cytoplasmic GABA that is used for extrasynaptic and metabolic purposes and GAD2 regulates the vesicular pool for release |
| GO:0009449 GABA biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: GABA biosynthetic process propagated from PTN002903772; gad1b itself (ZDB-GENE-030909-3) is one of the donors. Reason: gad1b catalyses the step that makes GABA. Its own experimental annotation (the IEP row from Martin et al. 1998) is among the descendant evidence for this node, which is expected and shows that the function is both inherited and grounded on this gene. Supporting Evidence: PMID:9634146 The zebrafish GAD65 and GAD67 are present in identified neurons in the forebrain, midbrain, hindbrain, and spinal cord, and they catalyze the production of GABA in the developing embryo. PMID:17384644 GAD67 is constitutively active and is responsible for basal GABA production. |
| GO:0009449 GABA biosynthetic process | IEP PMID:9634146 Sequence and expression of glutamic acid decarboxylase isofo... | ACCEPT | Summary: GABA biosynthetic process from Martin et al. 1998, where the zebrafish GAD67 cDNA was cloned and its in situ pattern compared with GABA immunostaining. Reason: The GAD67 cDNA of this paper (GenBank AF017266) is gad1b by sequence (99.6% identical to Gad1b over the partial CDS, vs 90.9% to Gad1a), so the row is on the right paralog. GABA was found at all sites of GAD expression. The evidence is correlative (expression pattern), but the process is the correct one for the enzyme and is supported by the gad1b null mutant. Supporting Evidence: PMID:9634146 Immunohistochemistry for gamma-aminobutyric acid (GABA) revealed that GABA is produced at all sites of GAD expression, including the novel cells in the caudal hindbrain. file:DANRE/gad1a/gad1a-bioinformatics/RESULTS.md The classic GAD67 cDNA AF017266 is gad1b by sequence. |
| GO:0016830 carbon-carbon lyase activity | IEA GO_REF:0000002 | ACCEPT | Summary: Carbon-carbon lyase activity from the InterPro PLP-dependent decarboxylase domain (IPR002129). Reason: Correct but very general; glutamate decarboxylase activity (GO:0004351) is the informative term and is also present. Kept as a correct domain-level inference. Supporting Evidence: file:DANRE/gad1a/gad1a-bioinformatics/RESULTS.md Both copies keep the PLP lysine and the GABA-binding residues of human GAD1. |
| GO:0016831 carboxy-lyase activity | IEA GO_REF:0000002 | ACCEPT | Summary: Carboxy-lyase activity from the InterPro pyridoxal-phosphate attachment site signature (IPR021115). Reason: Correct parent of glutamate decarboxylase activity; the PLP attachment lysine is present. Supporting Evidence: file:DANRE/gad1a/gad1a-bioinformatics/RESULTS.md Both copies keep the PLP lysine and the GABA-binding residues of human GAD1. |
| GO:0030170 pyridoxal phosphate binding | IEA GO_REF:0000002 | ACCEPT | Summary: Pyridoxal phosphate binding from the InterPro PLP-dependent decarboxylase domain. Reason: PLP is the covalently bound cofactor of GAD enzymes, attached to the active-site lysine that Gad1b keeps. Supporting Evidence: PMID:17384644 Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5'-phosphate-dependent enzyme glutamic acid decarboxylase (GAD65 and GAD67). file:DANRE/gad1a/gad1a-bioinformatics/RESULTS.md Both copies keep the PLP lysine and the GABA-binding residues of human GAD1. |
| GO:0048786 presynaptic active zone | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Presynaptic active zone propagated from node PTN002903810, seeded by mouse and rat Gad1. Reason: gad1b is inside the GAD1 clade that carries this node, and ZFIN records Gad1b antibody staining at presynaptic active zones in the adult cerebellum (Bae et al. 2009), which I could not read beyond the abstract. The location is plausible for a GABA-synthesizing enzyme in inhibitory terminals, but GAD1 is mainly cytoplasmic and terminal association is more characteristic of GAD2, so the term is kept as non-core. Supporting Evidence: PMID:27461130 While both GADs synthesize GABA and are co-expressed in most vertebrate GABAergic neurons, GAD1 synthesizes cytoplasmic GABA that is used for extrasynaptic and metabolic purposes and GAD2 regulates the vesicular pool for release |
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Download this section (compressed HTML)Q: Does gad1a expression rise in gad1b null mutants (transcriptional adaptation), and how much GABA remains in gad1b mutant brains?
Q: Why does the gad1b translation-blocking morpholino cause craniofacial defects that gad1b null mutants lack - off-target effects, knockdown of gad1a, or compensation in the mutant?
Experiment: Measure brain GABA by HPLC or mass spectrometry in gad1b, gad2, gad1a single and combined mutants, and record gad1a and gad2 mRNA levels in gad1b mutants.
Hypothesis: gad1b supplies most GABA in the larval brain, with gad1a and gad2 providing a residual pool.
Type: genetic interaction
Experiment: Express recombinant Gad1b and measure glutamate decarboxylase kinetics and PLP saturation, alongside Gad1a.
Hypothesis: Zebrafish Gad1b is constitutively active like mammalian GAD67.
Type: biochemical assay
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