Distinctive interactions in the holoenzyme formation for two isoforms of glutamate decarboxylase

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.

Anxiolytic-like effect of pyridoxine in mice by elevated plus maze and light and dark box: Evidence for the involvement of GABAergic and NO-sGC-cGMP pathway

Present study was carried out to investigate the 'anxiolytic-like' effect of pyridoxine in mice. Pyridoxine (90, 180 and 360 mg/kg) was administered by intraperitoneal (i.p.) route to the experimental mice and anxiety-related behavior was evaluated by light and dark box (LDB) and elevated plus maze (EPM) models. Glutamate, GABA and nitrite levels were also determined in the isolated whole brain of mice. It was observed that pyridoxine (180 mg/kg, i.p.) exerted 'anxiolytic-like' effect in mice in EPM and LDB models. Also, there was a significant increase in the levels of GABA whereas; the levels of glutamate and nitrite were decreased as compared to the control group. Administration of pentamethylene tetrazole (PTZ; 20 mg/kg, i.p.) exerted anxiogenic effects in mice, but the combination of PTZ and pyridoxine (180 mg/kg, i.p.) abolished the 'anxiolytic-like' effect of pyridoxine, thereby, suggesting the possible role of GABA in the 'anxiolytic-like' effect of pyridoxine in mice. Further, the influence of NO-sGC-cGMP pathway was investigated by administering the sub-effective dose of pyridoxine in combination with sub-threshold doses of NO modulators i.e. l‑arginine (50 mg/kg, i.p.; NO donor); methylene blue (1 mg/kg, i.p.; NO and soluble guanylate cyclase inhibitor) and sildenafil (1 mg/kg, i.p.; phosphodiesterase inhibitor and cGMP modulator). It was observed that the 'anxiolytic-like' effect of pyridoxine in mice was counteracted by the NO donor and potentiated by the NO inhibitors. Thus, the present study confirmed the involvement of GABAergic and NO-sGC-cGMP pathway in the 'anxiolytic-like' effect of pyridoxine in mice.

Cofactor-dependent conformational heterogeneity of GAD65 and its role in autoimmunity and neurotransmitter homeostasis

The human neuroendocrine enzyme glutamate decarboxylase (GAD) catalyses the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) using pyridoxal 5'-phosphate as a cofactor. GAD exists as two isoforms named according to their respective molecular weights: GAD65 and GAD67. Although cytosolic GAD67 is typically saturated with the cofactor (holoGAD67) and constitutively active to produce basal levels of GABA, the membrane-associated GAD65 exists mainly as the inactive apo form. GAD65, but not GAD67, is a prevalent autoantigen, with autoantibodies to GAD65 being detected at high frequency in patients with autoimmune (type 1) diabetes and certain other autoimmune disorders. The significance of GAD65 autoinactivation into the apo form for regulation of neurotransmitter levels and autoantibody reactivity is not understood. We have used computational and experimental approaches to decipher the nature of the holo → apo conversion in GAD65 and thus, its mechanism of autoinactivation. Molecular dynamics simulations of GAD65 reveal coupling between the C-terminal domain, catalytic loop, and pyridoxal 5'-phosphate-binding domain that drives structural rearrangement, dimer opening, and autoinactivation, consistent with limited proteolysis fragmentation patterns. Together with small-angle X-ray scattering and fluorescence spectroscopy data, our findings are consistent with apoGAD65 existing as an ensemble of conformations. Antibody-binding kinetics suggest a mechanism of mutually induced conformational changes, implicating the flexibility of apoGAD65 in its autoantigenicity. Although conformational diversity may provide a mechanism for cofactor-controlled regulation of neurotransmitter biosynthesis, it may also come at a cost of insufficient development of immune self-tolerance that favors the production of GAD65 autoantibodies.

Pyridoxine enhances cell proliferation and neuroblast differentiation by upregulating the GABAergic system in the mouse dentate gyrus

We investigated the effects of pyridoxine (vitamin B(6)) on cell death, cell proliferation, neuroblast differentiation, and the GABAergic system in the mouse dentate gyrus. We administered pyridoxine (350 mg/kg intraperitoneally) to 8 week old mice twice a day for 14 days and sacrificed them at 10 weeks of age. Pyridoxine treatment did not induce neuronal death or activate microglia in the dentate gyrus, while glial fibrillary acidic protein (GFAP)-positive cells were significantly increased in the subgranular zone of the dentate gyrus. The increase in GFAP-positive cells was confirmed to be due to proliferating cells based on double immunofluorescence staining. GFAP-positive cells, which were also labeled with Ki67, a marker for cell proliferation, and doublecortin, a marker for neuroblast differentiation, were significantly increased in the pyridoxine-treated group compared to those in the vehicle-treated group. Pyridoxine treatment also increased the protein levels of glutamic acid decarboxylase (GAD) 67, an enzyme for GABA synthesis, and pyridoxal 5'-phosphate (PNP) oxidase, an enzyme for pyridoxal phosphate synthesis, in the dentate gyrus. These results suggest that pyridoxine treatment distinctly increases cell proliferation, neuroblast differentiation, and upregulated the GABAergic system, as revealed by the increases of GAD67 and PNP oxidase in the mouse dentate gyrus.

Transient increases of glutamic acid decarboxylase 67 immunoreactivity and its protein levels in the somatosensory cortex after transient cerebral ischemia in gerbils

In this study, we investigated changes in glutamic acid decarboxylase 67 (GAD67) immunoreactivity and its protein levels in the gerbil somatosensory cortex after ischemia/reperfusion. GAD67 immunoreactivity was significantly increased in layers III and V of the somatosensory cortex 12 hr after ischemia/reperfusion. Thereafter, GAD67 immunoreactivity was decreased with time after ischemia/reperfusion. GAD67 immunoreactivity in the somatosensory cortex 4 days after ischemia/reperfusion was similar to that in the sham-operated group. In addition, GAD67 protein levels were also significantly increased 12 hr after transient forebrain ischemia. These results suggest that the transient increase of GAD67 immunoreactivity in layers III and V may be associated with responses to transient ischemia-induced neuronal damage.

GAD isoforms exhibit distinct spatiotemporal expression patterns in the developing mouse lens: correlation with Dlx2 and Dlx5

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter of the adult nervous system and its biosynthetic enzyme glutamic acid decarboxylase (GAD) are abundantly expressed in the embryonic nervous system and are involved in the modulation of cell proliferation, migration, and differentiation. Here we describe for the first time the expression of GABA and embryonic and adult GAD isoforms in the developing mouse lens. We show that the GAD isoforms are sequentially induced with specific spatiotemporal profiles: GAD65 and embryonic GAD isoforms prevail in primary fibers, while GAD67 is the predominant GAD expressed in the postnatal secondary fibers. This pattern correlates well with the expression of Dlx2 and Dlx5, known as upstream regulators of GAD. GABA and GAD are most abundant at the tips of elongating fibers and are absent from organelle-free cells, suggesting their involvement is primarily in shaping of the cytoskeleton during fiber elongation stages.

Prenatal hypoxia down regulates the GABA pathway in newborn mice cerebral cortex; partial protection by MgSO4

The fetal and newborn brain is particularly susceptible to hypoxia, which increases the risk for neurodevelopmental deficits, seizures, epilepsy and life-span motor, behavioral and cognitive disabilities. Here, we report that prenatal hypoxia at gestation day 17 in mice caused an immediate decrease in fetal cerebral cortex levels of glutamate decarboxylase, a key proteins in the GABA pathway. While maternal MgSO4 treatment prior to hypoxia did not have an early effect, it did accelerate maturation at a later stage based on the observed protein expression profile. In addition, MgSO4 reversed the hypoxia-induced loss of a subpopulation of inhibitory neurons that express calbindin in cortex at postnatal day 14. In the hippocampus, responses to prenatal hypoxia were also evident 4 days after the hypoxia. However, in contrast to the observations in cerebral cortex, hypoxia stimulated key protein expression in the hippocampus. The hippocampal response to hypoxia was also reversed by maternal MgSO4 treatment. The data presented here suggests that decreased levels of key proteins in the GABA pathway in the cerebral cortex may lead to high susceptibility to seizures and epilepsy in newborns after prenatal or perinatal hypoxia and that maternal MgSO4 treatment can reverse the hypoxia-induced deficits in the GABA pathway.

Development of GABA innervation in the cerebral and cerebellar cortices

In many areas of the vertebrate brain, such as the cerebral and cerebellar cortices, neural circuits rely on inhibition mediated by GABA (gamma-aminobutyric acid) to shape the spatiotemporal patterns of electrical signalling. The richness and subtlety of inhibition are achieved by diverse classes of interneurons that are endowed with distinct physiological properties. In addition, the axons of interneurons display highly characteristic and class-specific geometry and innervation patterns, and thereby distribute their output to discrete spatial domains, cell types and subcellular compartments in neural networks. The cellular and molecular mechanisms that specify and modify inhibitory innervation patterns are only just beginning to be understood.

Time course of changes in pyridoxal 5'-phosphate (vitamin B6 active form) and its neuroprotection in experimental ischemic damage

In the present study, we investigated ischemia-induced changes of pyridoxal 5'-phosphate synthesizing enzyme and degrading enzyme and neuroprotective effects and roles of pyridoxal 5'-phosphate against ischemic damage in the gerbil hippocampal CA1 region. Pyridoxal 5'-phosphate oxidase and pyridoxal phosphate phosphatase immunoreactivities were changed in neurons up to 2 days after ischemia, while 4 days after ischemia their immunoreactivities were expressed in astrocytes. Pyridoxal 5'-phosphate oxidase immunoreactivity and its protein level were highest 12 h after ischemia, while those in pyridoxal phosphate phosphatase were highest 2 days after ischemia. Total activities of these enzymes were changed after ischemia, but specific activities of the enzymes were not altered. Treatment with pyridoxal 5'-phosphate into brains (4 microg/5 microl, i.c.v.) at 30 min before transient ischemia protected about 80% of CA1 pyramidal cells 4 days after ischemia and induced elevation of glutamic acid decarboxylase 67 immunoreactivity in the CA1 region. However, pyridoxal 5'-phosphate treatment into ischemic brains decreased GABA transaminase immunoreactivity in the CA1 region after ischemia. These results indicate that pyridoxal 5'-phosphate may be associated with the inhibitory discharge of GABA in the hippocampal CA1 neurons, and the increased level of GABA may protect hippocampal CA1 pyramidal cells from ischemic damage.

Glucagon-like peptide-1 stimulates GABA formation by pancreatic beta-cells at the level of glutamate decarboxylase

Pancreatic beta-cells are the major extraneural site of glutamate decarboxylase expression (GAD). During culture of isolated beta-cells, the GAD product gamma-aminobutyrate (GABA) is rapidly released in the medium, independently of insulin. It is considered as a possible mediator of beta-cell influences on alpha-cells, acinar cells, and/or infiltrating lymphocytes. In this perspective, we investigated the regulation of GABA release by rat beta-cells during a 24-h culture period. Glucose was previously reported to inhibit GABA release by diverting cellular GABA to mitochondrial breakdown through activation of GABA transferase (GABA-T). In the present study, glucagon-like peptide-1 (GLP-1) was shown to stimulate GABA formation at the level of GAD, its effect being suppressed by the GAD inhibitor allylglycine and remaining unaltered by the GABA-T inhibitor gamma-vinyl-GABA. The stimulatory action of GLP-1 is cAMP dependent, being reproduced by the adenylate cyclase activator forskolin and the cAMP analog N(6)-benzoyladenosine-3',5'-cAMP and inhibited by a PKA inhibitor. It is dependent on protein synthesis and associated with an increased expression of GAD67 but not GAD65. The GLP-1-induced stimulation of GAD activity in beta-cells can elevate medium GABA levels in conditions of glucose-driven intracellular GABA breakdown and thus maintain GABA-mediated beta-cell influences on neighboring cells.

Reversal of brain injury-induced prefrontal glutamic acid decarboxylase expression and working memory deficits by D1 receptor antagonism

Working memory (WM), the ability to transiently hold information in mind, is essential for high-level cognitive functions that are often impaired in brain-injured patients. The cellular and molecular mechanisms contributing to WM deficits, which can manifest in the absence of overt damage, in these patients are unknown. The function of the dorsolateral prefrontal cortex in humans and monkeys, and the medial prefrontal cortex (mPFC), in rodents is critical for WM. We demonstrate that controlled cortical impact injury of rats causes a long-lasting WM impairment that is associated with increased levels of the GABA-synthesizing enzyme glutamic acid decarboxylase 67 (GAD67) in the mPFC for up to 1 month after injury. A single administration of dopamine D1 antagonists at 14 d after injury is sufficient to decrease GAD67 levels and restore WM for at least 1 week. These findings indicate that inhibition of prefrontal neuronal activity contributes to WM deficits and that strategies to reduce GAD67 expression can offer prolonged WM improvement in brain-injured patients.

Characterisation of an autoreactive conformational epitope on GAD65 recognised by the human monoclonal antibody b78 using a combination of phage display, in vitro mutagenesis and molecular modelling

Autoantibodies to the diabetes autoantigen, the 65kDa isoform of glutamic acid decarboxylase (GAD65), react with conformational epitopes defined according to linear sequences but not according to structural information, or contact sites with the antibody paratope. To ascertain such information for an exemplary human monoclonal antibody (mAb) to GAD65, b78, we combined antibody screening of phage-displayed peptide libraries, alanine mutagenesis of selected motifs, homology modelling of the PLP and C-terminal regions of GAD65, and molecular dynamics to examine for structural effects of mutagenesis. By phage display, mAb b78 selected phagotopes containing acidic residues (D, E), hydrophobic residues (Y, F or W) and LRS that localised to a possible surface-exposed conformational epitope on the combined homology model. Alanine mutants of GAD65 based on deduced contact residues were examined for binding with b78 and control sera. Mutation of (524)SRL(526), (572)DF(573) and (498)KPQ(500) reduced reactivity of b78 with mutant GAD65 > 50%. Molecular dynamics indicated that mutation of (498)KPQ(500) caused structural changes that could account for effects of this mutation. Thus phage display in combination with molecular modelling identified contact residues within a highly conformational epitope for mAb b78 in the C-terminus of GAD65. These techniques should have broad applicability to definition of epitope structure.

Analysis of GAD65 autoantibodies in Stiff-Person syndrome patients

Autoantibodies to the 65-kDa isoform of glutamate decarboxylase GAD65 (GAD65Ab) are strong candidates for a pathological role in Stiff-Person syndrome (SPS). We have analyzed the binding specificity of the GAD65Ab in serum and cerebrospinal fluid (CSF) of 12 patients with SPS by competitive displacement studies with GAD65-specific rFab-derived from a number of human and mouse mAbs specific for different determinants on the Ag. We demonstrate considerable differences in the epitope specificity when comparing paired serum and CSF samples, suggesting local stimulation of B cells in the CSF compartment of these patients. Moreover, these autoantibodies strongly inhibit the enzymatic activity of GAD65, thus blocking the formation of the neurotransmitter gamma-aminobutyric acid. The capacity of the sera to inhibit the enzymatic activity of GAD65 correlated with their binding to a conformational C-terminal Ab epitope. Investigation of the inhibitory mechanism revealed that the inhibition could not be overcome by high concentrations of glutamate or the cofactor pyridoxal phosphate, suggesting a noncompetitive inhibitory mechanism. Finally, we identified a linear epitope on amino acids residues 4-22 of GAD65 that was recognized solely by autoantibodies from patients with SPS but not by serum from type 1 diabetes patients. A mAb (N-GAD65 mAb) recognizing this N-terminal epitope was successfully humanized to enhance its potential therapeutic value by reducing its overall immunogenicity.

Age-dependent changes of pyridoxal phosphate synthesizing enzymes immunoreactivities and activities in the gerbil hippocampal CA1 region

In the present study, age-related changes of pyridoxal 5'-phosphate (PLP) synthesizing enzymes, pyridoxal kinase (PLK) and pyridoxine 5'-phosphate oxidase (PNPO), their protein contents and activities were examined in the gerbil hippocampus proper. Significant age-dependent changes in PLK and PNPO immunoreactivities were found in the CA1 region, but not in the CA2/3 region. In the postnatal month 1 (PM 1) group, PLK and PNPO immunoreactivities were detected mainly in the stratum pyramidale of the CA1 region. PLK and PNPO immunoreactivities and their protein contents were highest in the PM 6 group, showing that many CA1 pyramidal cells had strong PLK and PNPO immunoreactivities. Thereafter, PLK and PNPO immunoreactivities started to decrease and were very low at PM 24. Alterations in the change patterns in protein contents and total activities of PLK and PNPO corresponded to the immunohistochemical data, but their specific activities were not altered in any experimental group. Based on double immunofluorescence study, PLK and PNPO immunoreactive cells in the strata oriens and radiatum were identified as GABAergic cells. Therefore, decreases of PLK and PNPO in the hippocampal CA1 region of aged brains may be involved in aging processes related with gamma-aminobutyric acid (GABA) function.

Learning-induced plasticity of cortical representations does not affect GAD65 mRNA expression and immunolabeling of cortical neuropil

Two forms of glutamic acid decarboxylase (GAD) are present in inhibitory neurons of the mammalian brain, a 65-kDa isoform (GAD65) and a 67-kDa isoform (GAD67). We have previously found that GAD67 is upregulated during learning-dependent plasticity of cortical vibrissal representations of adult mice. After sensory conditioning involving pairing stimulation of vibrissae with a tail shock, the increase in mRNA expression and density of GAD67-immunoreactive neurons was observed in barrels representing vibrissae activated during the training. In the present study, using the same experimental model, we examined GAD65 mRNA and protein levels in the barrel cortex. For this purpose, we used in situ hybridization and immunohistochemistry. No changes in the level of GAD65 mRNA expression were detected after the training. The pattern of GAD65 mRNA expression was complementary to that observed for GAD67. Immunocytochemical analysis found no changes in immunolabeling of neuropil of the barrels representing the vibrissae activated during the training. The results show that, in contrast to GAD67, cortical plasticity induced by sensory learning does not affect the expression of GAD65.

Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis

Glutamate decarboxylase (GAD) exists as two isoforms, GAD65 and GAD67. GAD activity is regulated by a cycle of activation and inactivation determined by the binding and release of its co-factor, pyridoxal 5'-phosphate. Holoenzyme (GAD with bound co-factor) decarboxylates glutamate to form GABA, but it also catalyzes a slower transamination reaction that produces inactive apoGAD (without bound co-factor). Apoenzyme can reassociate with pyridoxal phosphate to form holoGAD, thus completing the cycle. Within cells, GAD65 is largely apoenzyme (approximately 93%) while GAD67 is mainly holoenzyme (approximately 72%). We found striking kinetic differences between the GAD isoforms that appear to account for this difference in co-factor saturation. The glutamate dependent conversion of holoGAD65 to apoGAD was about 15 times faster than that of holoGAD67 at saturating glutamate. Aspartate and GABA also converted holoGAD65 to apoGAD at higher rates than they did holoGAD67. Nucleoside triphosphates (such as ATP) are known to affect the activation reactions of the cycle. ATP slowed the activation of GAD65 and markedly reduced its steady-state activity, but had little affect on the activation of GAD67 or its steady-state activity. Inorganic phosphate opposed the effect of ATP; it increased the rate of apoGAD65 activation but had little effect on apoGAD67 activation. We conclude that the apo-/holoenzyme cycle of inactivation and reactivation is more important in regulating the activity of GAD65 than of GAD67.