Molecular Identification of the 62 kd Form of Glutamic Acid Decarboxylase from the Mouse

Stiff-Person Syndrome

The stiff-person syndrome is a disorder of persistent, painful muscle contractions predominately affecting the axial musculature. We describe a patient with this disorder and review its pathophysiology. Molecular biologic and immunologic techniques have recently added to the understanding of the mechanism of this disorder. Association with diseases such as diabetes, vitiligo and hypothyroidism have strengthened the auto-immune nature of this syndrome. Auto-antibodies against glutamic acid decarboxylase (GAD), an intraneuronal enzyme, have been implicated in the etiology of this unique disease. Therapeutic intervention with agents such as benzodiazepines that modify central GABAergic activity have demonstrated significant benefit in patients with stiff-person syndrome.

A Transgenic Mouse Line Expressing the Red Fluorescent Protein tdTomato in GABAergic Neurons

GABAergic inhibitory neurons are a large population of neurons in the central nervous system (CNS) of mammals and crucially contribute to the function of the circuitry of the brain. To identify specific cell types and investigate their functions labelling of cell populations by transgenic expression of fluorescent proteins is a powerful approach. While a number of mouse lines expressing the green fluorescent protein (GFP) in different subpopulations of GABAergic cells are available, GFP expressing mouse lines are not suitable for either crossbreeding to other mouse lines expressing GFP in other cell types or for Ca2+-imaging using the superior green Ca2+-indicator dyes. Therefore, we have generated a novel transgenic mouse line expressing the red fluorescent protein tdTomato in GABAergic neurons using a bacterial artificial chromosome based strategy and inserting the tdTomato open reading frame at the start codon within exon 1 of the GAD2 gene encoding glutamic acid decarboxylase 65 (GAD65). TdTomato expression was observed in all expected brain regions; however, the fluorescence intensity was highest in the olfactory bulb and the striatum. Robust expression was also observed in cortical and hippocampal neurons, Purkinje cells in the cerebellum, amacrine cells in the retina as well as in cells migrating along the rostral migratory stream. In cortex, hippocampus, olfactory bulb and brainstem, 80% to 90% of neurons expressing endogenous GAD65 also expressed the fluorescent protein. Moreover, almost all tdTomato-expressing cells coexpressed GAD65, indicating that indeed only GABAergic neurons are labelled by tdTomato expression. This mouse line with its unique spectral properties for labelling GABAergic neurons will therefore be a valuable new tool for research addressing this fascinating cell type.

The spatiotemporal segregation of GAD forms defines distinct GABA signaling functions in the developing mouse olfactory system and provides novel insights into the origin and migration of GnRH neurons

Gamma-aminobutyric acid (GABA) has a dual role as an inhibitory neurotransmitter in the adult central nervous system (CNS) and as a signaling molecule exerting largely excitatory actions during development. The rate-limiting step of GABA synthesis is catalyzed by two glutamic acid decarboxylase isoforms GAD65 and GAD67 coexpressed in the GABAergic neurons of the CNS. Here we report that the two GADs show virtually nonoverlapping expression patterns consistent with distinct roles in the developing peripheral olfactory system. GAD65 is expressed exclusively in undifferentiated neuronal progenitors confined to the proliferative zones of the sensory vomeronasal and olfactory epithelia In contrast GAD67 is expressed in a subregion of the nonsensory epithelium/vomeronasal organ epithelium containing the putative Gonadotropin-releasing hormone (GnRH) progenitors and GnRH neurons migrating from this region through the frontonasal mesenchyme into the basal forebrain. Only GAD67+, but not GAD65+ cells accumulate detectable GABA. We further demonstrate that GAD67 and its embryonic splice variant embryonic GAD (EGAD) concomitant with GnRH are dynamically regulated during GnRH neuronal migration in vivo and in two immortalized cell lines representing migratory (GN11) and postmigratory (GT1-7) stage GnRH neurons, respectively. Analysis of GAD65/67 single and double knock-out embryos revealed that the two GADs play complementary (inhibitory) roles in GnRH migration ultimately modulating the speed and/or direction of GnRH migration. Our results also suggest that GAD65 and GAD67/EGAD characterized by distinct subcellular localization and kinetics have disparate functions during olfactory system development mediating proliferative and migratory responses putatively through specific subcellular GABA pools.

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.

Temporal migration of gonadotrophin-releasing hormone-1 neurones is modified in GAD67 knockout mice

Gonadotrophin-releasing hormone (GnRH-1) neurones reside in the forebrain and regulate gonadal function via the hypothalamic-pituitary-gonadal axis. Disruption of this axis results in reproductive dysfunction. During embryonic development, GnRH-1 neurones migrate from the nasal pit through the nasal/forebrain junction (NFJ) into the developing brain. Prenatally gamma-aminobutyric acid (GABA) is excitatory and has been shown to play a role in nervous system development. Both in vivo and in vitro experiments suggest that GABA inhibits migration of GnRH-1 neurones. The present study examines the migration of GnRH-1 neurones in GAD67 knockout (KO) mice to further elucidate the role of GABA on GnRH-1 neuronal development. Three stages were examined, embryonic day (E)12.5, E14.5 and E17.5. GnRH-1 cell number and location were analysed by immunocytochemistry and in situ hybridisation histochemistry. The total number of GnRH-1 immunopositive cells was similar between wild-type (WT) and KO mice. However, significant differences were found in the overall distribution of GnRH-1 immunopositive cells in GAD67 KO compared to WT mice at all stages. Subsequent analysis by area revealed differences occurred at the NFJ with an increase in GnRH-1 cells in GAD67 KO at E14.5 and a decrease in GnRH-1 cells in GAD67 KO at E17.5. Comparable counts for cells expressing GnRH-1 transcript and protein were obtained. These data indicate that attenuated levels of GABA accelerate GnRH-1 cell migration in nasal areas as well as movement of GnRH-1 cells into the central nervous system at the NFJ.

Cellular and subcellular distribution of spinophilin, a PP1 regulatory protein that bundles F-actin in dendritic spines

Spinophilin is an actin binding protein that positions protein phosphatase 1 next to its substrates in dendritic spines. It contains a single PDZ domain and has the biochemical characteristics of a cytoskeletal scaffolding protein. Previous studies suggest that spinophilin is present in most spines, but the concentration of spinophilin varies from brain region to region in a manner that does not simply reflect differences in spine density. Here, we show that spinophilin is enriched in the great majority of dendritic spines in cerebral cortex, caudatoputamen, hippocampal formation, and cerebellum, irrespective of regional differences in spinophilin concentration. In addition, spinophilin is present postsynaptic to asymmetrical contacts on interneuronal dendritic shafts. We further show that, in hippocampus and ventral pallidum, spinophilin is occasionally present in dendritic shafts adjacent to gamma-aminobutyric acid-containing contacts. Thus, the functional role of spinophilin may not be exclusively restricted to excitatory synapses and may be significant at a small fraction of inhibitory contacts. These data also suggest that the concentration of spinophilin per spine is variable and is likely regulated by local physiological factors and/or regional influences.

Structural and functional characterization of mouse glutamate decarboxylase 67 gene promoter

Neuronal expression of the mouse glutamate decarboxylase 67 (mGAD67) gene occurs exclusively in neurons that synthesize and release GABA (GABAergic neurons). This gene is also expressed in pancreatic islet cells and testicular spermatocytes. In order to elucidate the molecular mechanisms underlying the regulation of mGAD67 gene expression, we isolated and characterized the 5'-flanking region of this gene. Sequence analysis of a 10.2-kb DNA fragment of this gene containing a promoter region (8.4 kb) and noncoding exons 0A and 0B revealed the presence of numerous potential neuron-specific cis-regulatory elements. Functional analysis of the 5'-flanking region of exons 0A and 0B by transient transfection into cultured cells revealed that the region -98 to -52 close to exon 0A is important for the transcriptional activity of both exons 0A and 0B. In addition, we used transgenic mice to examine the expression pattern conferred by the 10.2 kb DNA fragment of the mGAD67 gene fused to the bacterial lacZ reporter gene. Transgene expression was observed in neurons of particular brain regions containing abundant GABAergic neurons such as the basal ganglia, in pancreatic islet cells and in testicular spermatocytes and spermatogonia. These results suggest that the 10.2 kb DNA fragment of the mGAD67 gene contains regulatory elements essential for its targeted expression in GABAergic neurons, islet cells and spermatocytes.

Mouse vesicular GABA transporter gene: genomic organization, transcriptional regulation and chromosomal localization

The vesicular GABA transporter (VGAT) loads GABA from neuronal cytoplasm into synaptic vesicles and is selectively expressed in inhibitory neurons that contain GABA and/or glycine. To elucidate the molecular mechanisms of mouse VGAT (mVGAT) gene expression, we have isolated and characterized the mVGAT gene. The mVGAT gene was found to be 4.7 kilobases in size and to contain three exons and two introns by comparison of the cloned genomic DNA with the cDNA (termed mVGATa) sequence reported by Sagne et al. [FEBS Lett. 417 (1997) 177]. Analysis of transcripts and genomic DNA revealed an alternatively spliced mVGAT isoform (termed mVGATb) that retains intron 2 of mVGATa as an exon. This alternative transcript specifies 514 amino acid residues identical to VGATa followed by a unique C-terminal sequence of 11 amino acids encoded by intron 2. Fluorescent in situ hybridization studies showed that the mVGAT gene is localized on chromosome 2. One major transcription start site of the mVGAT gene is an A residue 209 bp upstream from the translational initiation site, as shown using the 5'-RACE method. RT-PCR analysis revealed that the mVGAT gene was expressed at a high level in retinoic acid-treated P19 embryonal carcinoma cells, at a very low level in non-treated P19 cells, and not detectably expressed in Neuro-2a neuroblastoma cells. Sequence analysis of the 5'-flanking region revealed a number of putative regulatory elements including Sp1, Egr-1 and Pitx binding sites. In transient transfection assays, 2 kilobases of the mVGAT 5'-flanking region generated similar levels of luciferase reporter activity in three kinds of cultured cells. Deletion analysis and gel mobility shift assays demonstrated that the region -161 to +155 contained the basal promoter activity of the mVGAT gene and that an activating region from -49 to -27 bound an Sp1-like protein. These results suggest a possible mechanism for regulation of the expression of the mVGAT gene.

Acute and chronic caffeine administration differentially alters striatal gene expression in wild-type and adenosine A(2A) receptor-deficient mice

In order to assess for the respective involvement of adenosine A(1) and A(2A) receptors (A(2A)-R) in the consequences of short- and long-term caffeine exposure on gene expression, the effects of acute caffeine administration on striatal, cortical, and hippocampal expression of immediate early genes (IEG), zif-268 and arc, and the effects of long-term caffeine or 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) exposure (once daily for 15 days) on striatal gene expression of substance P, enkephalin, and glutamic acid decarboxylase isoforms, GAD65 and GAD67, were evaluated in wild-type and A(2A)-R-deficient (A(2A)-R(-/-)) mice. In situ hybridization histochemistry was performed using oligonucleotides followed by quantitative image analysis. Our results demonstrated that a biphasic response of IEG expression to acute caffeine observed in the wild-type striatum was resumed in a monophasic response in the mutant striatum. In the cerebral cortex and hippocampus, the effect of caffeine was weak in wild-type, whereas in mutant mice it induced a 2-3-fold increase in the IEG expression to restore a level similar to the wild-type basal expression. Chronic caffeine and DPCPX-mediated regulation in neuropeptide and GADs striatal gene expression typically showed the mimicking of alterations resulting from the A(2A)-R genetic deficiency in 25 mg/kg caffeine-treated wild-type mice as well as the dose-dependent normalization of substance P and enkephalin expression in A(2A)-R(-/-) mice. These results indicate that, depending on the dose, the blockade of A(2A)-R or A(1) receptors by caffeine is preferentially revealed leading to highly differential alterations in striatal gene expression and they also suggested the central role of these two receptors on the control of dopaminergic functions.

Functional striatal hypodopaminergic activity in mice lacking adenosine A(2A) receptors

Adenosine and caffeine modulate locomotor activity and striatal gene expression, partially through the activation and blockade of striatal A(2A) receptors, respectively. The elucidation of the roles of these receptors benefits from the construction of A(2A) receptor-deficient mice (A(2A)-R(-/-)). These mice presented alterations in locomotor behaviour and striatal expression of genes studied so far, which are unexpected regarding the specific expression of A(2A) receptor by striatopallidal neurones. To clarify the functions of A(2A) receptors in the striatum and to identify the mechanisms leading to these unexpected modifications, we studied the basal expression of immediate early and constitutive genes as well as dopamine and glutamate neurotransmission in the striatum. Basal zif268 and arc mRNAs expression was reduced in mutant mice by 60-80%, not only in the striatum but also widespread in the cerebral cortex and hippocampus. Striatal expression of substance P and enkephalin mRNAs was reduced by about 50% and 30%, respectively, whereas the expression of GAD67 and GAD65 mRNAs was slightly increased and unaltered, respectively. In vivo microdialysis in the striatum revealed a 45% decrease in the extracellular dopamine concentration and three-fold increase in extracellular glutamate concentration. This was associated with an up-regulation of D(1) and D(2) dopamine receptors expression but not with changes in ionotropic glutamate receptors. The levels of tyrosine hydroxylase and of striatal and cortical glial glutamate transporters as well as adenosine A(1) receptors expression were indistinguishable between A(2A)-R(-/-) and wild-type mice. Altogether these results pointed out that the lack of A(2A) receptors leads to a functional hypodopaminergic state and demonstrated that A(2A) receptors are necessary to maintain a basal level in immediate early and constitutive genes expression in the striatum and cerebral cortex, possibly via their control of dopamine pathways.

GABAergic nature of hypothalamic leptin target neurones in the ventromedial arcuate nucleus

Leptin is an adipose tissue-derived cytokine hormone, which reduces body weight via interactions with hypothalamic neurones. Leptin receptors capable of activating the JAK-STAT signal transduction pathway are expressed at high levels in the hypothalamus, particularly in the arcuate nucleus. In order to identify the chemical mediators of leptin's action in the hypothalamus, we have examined whether GABA neurones of the hypothalamic arcuate nucleus contain leptin receptors and the leptin-activated transcription factor STAT3. GABAergic neurones, as visualized by antisera to the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) and GABA, were demonstrated in the ventromedial and ventrolateral parts of the arcuate nucleus. GABA neurones in the ventromedial arcuate nucleus were shown to contain leptin receptor immunoreactivity, as revealed using an antiserum generated to a sequence common to all isoforms of the leptin receptor (Ob-R), as well as an antiserum generated to the carboxy-terminal end of the long leptin receptor (Ob-Rb), and immunoreactivity for the leptin-induced signal transduction molecule STAT3. Ventromedial GABA neurones were also shown to contain neuropeptide Y, whereas ventrolateral proopiomelanocortin-containing neurones lacked GAD and GABA immunoreactivity. Levels of mRNA for GAD65, GAD67 and the vesicular GABA transporter (VGAT) were analysed in the arcuate nucleus of leptin-deficient ob/ob mice and lean control mice by in situ hybridization. No significant differences in GAD65, GAD67 or VGAT mRNA were detected in the arcuate nucleus of ob/ob mice as compared to lean control mice. The presence of leptin receptor and STAT3 in GABAergic arcuate neurones, but absence of changes in gene transcription for GAD and VGAT mRNA suggests, that leptin does not transcriptionally regulate the expression of proteins involved in GABAergic transmission in arcuate neurones. However, mechanisms other than transcriptional regulation for leptin to influence arcuate GABA neurones may exist.

Differential regulation of adult and embryonic glutamate decarboxylases in rat dentate granule cells after kainate-induced limbic seizures

In adult brain, the inhibitory GABAergic neurons utilize two distinct molecular forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD), GAD65 and GAD67. During embryonic development, two truncated forms of GAD67 are also expressed (GAD25 and GAD44), which are translated from two embryonic-specific splice variants of GAD67 messenger RNA. It has recently been established that the excitatory dentate granule cells, in addition to the neurotransmitter glutamate, also contain low levels of GABA and GAD67, which are increased after limbic seizures. To study the seizure-induced activation of glutamate decarboxylase, we investigated the expression of both embryonic and adult glutamate decarboxylase messenger RNAs in the adult rat hippocampus after kainic acid administration by semi-quantitative reverse transcription-coupled polymerase chain reaction, in situ hybridization and immunoblotting. We observed a rapid induction of the embryonic glutamate decarboxylase messenger RNA in the granule cells of dentate gyrus. The expression of embryonic glutamate decarboxylase transcripts, identified here as the splice variant that contains exon 7/B, peaked at about 2h after kainic acid injection and gradually returned to nearly basal levels by 24h. Strikingly, this transient induction of embryonic glutamate decarboxylase messenger RNA was not accompanied by concomitant synthesis of its corresponding protein product GAD25. In contrast, the adult GAD67 messenger RNA and protein were both clearly up-regulated in granule cells, albeit with a certain delay, reaching a maximum around 4-6h after kainic acid injection and gradually returned to control levels by 24h. GAD65 remained unchanged at both messenger RNA and protein levels during the studied period. These characteristic and highly reproducible changes in the synthesis of glutamate decarboxylases indicate that GAD67 is the predominant form of glutamate decarboxylases involved in the elevated synthesis of GABA during seizures and suggest that the transient induction of the embryonic GAD67 messenger RNA that contains exon 7/B, but not GAD25 protein, may exert a role solely in the subsequent up-regulation of adult GAD67 transcription. Expression of the messenger RNA encoding for an alternatively spliced, truncated form of the GABA-synthesizing enzyme glutamate decarboxylase was detected in dentate granule cells briefly after kainic acid-induced seizures. Just as during embryonic development, expression of the alternatively spliced messenger RNA was transient and followed by transcription of its adult form, indicating a possible recapitulation of an embryonic program of gene expression in adult granule cells after epileptic seizures.

Sex steroid regulation of glutamate decarboxylase mRNA expression in goldfish brain is sexually dimorphic

Testosterone and oestradiol can modulate GABA synthesis in sexually regressed goldfish. Here we investigated their effects on the mRNA expression of two isoforms of the GABA synthesizing enzyme glutamate decarboxylase (GAD(65) and GAD(67), EC 4.1.1.15). Full-length GAD clones were isolated from a goldfish cDNA library and sequenced. Goldfish GAD(65) encodes a polypeptide of 583 amino acid residues, which is 77% identical to human GAD(65). Goldfish GAD(67) encodes a polypeptide of 587 amino acid residues and is 82% identical to human GAD(67). Goldfish GAD(65) and GAD(67) are 63% identical. Sexually regressed male and female goldfish were implanted with solid silastic pellets containing testosterone, oestradiol or no steroid. Semiquantitative PCR analysis showed that oestradiol significantly increased GAD(65) mRNA expression in female hypothalamus and telencephalon, while testosterone resulted in a significant increase only in telencephalon. GAD(67) mRNA levels were not affected by steroids in females. In contrast, both steroids induced significant decreases of GAD(65) and GAD(67) mRNA levels in male hypothalamus, but had no effect on GAD mRNA expression in male telencephalon. Our results indicate that modulation of GAD mRNA expression is a possible mechanism for steroid action on GABA synthesis, which may have opposite effects in males and females.

Dynamic expression of a glutamate decarboxylase gene in multiple non-neural tissues during mouse development

BACKGROUND

Glutamate decarboxylase (GAD) is the biosynthetic enzyme for the neurotransmitter gamma-aminobutyric acid (GABA). Mouse embryos lacking the 67-kDa isoform of GAD (encoded by the Gad1 gene) develop a complete cleft of the secondary palate. This phenotype suggests that this gene may be involved in the normal development of tissues outside of the CNS. Although Gad1 expression in adult non-CNS tissues has been noted previously, no systematic analysis of its embryonic expression outside of the nervous system has been performed. The objective of this study was to define additional structures outside of the central nervous system that express Gad1, indicating those structures that may require its function for normal development.

RESULTS

Our analysis detected the localized expression of Gad1 transcripts in several developing tissues in the mouse embryo from E9.0-E14.5. Tissues expressing Gad1 included the tail bud mesenchyme, the pharyngeal pouches and arches, the ectodermal placodes of the developing vibrissae, and the apical ectodermal ridge (AER), mesenchyme and ectoderm of the limb buds.

CONCLUSIONS

Some of the sites of Gad1 expression are tissues that emit signals required for patterning and differentiation (AER, vibrissal placodes). Other sites correspond to proliferating stem cell populations that give rise to multiple differentiated tissues (tail bud mesenchyme, pharyngeal endoderm and mesenchyme). The dynamic expression of Gad1 in such tissues suggests a wider role for GABA signaling in development than was previously appreciated.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

Domain-restricted expression of two glutamic acid decarboxylase genes in midgestation mouse embryos

Glutamic acid decarboxylase (GAD) is the biosynthetic enzyme for gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. In addition to the adult CNS, GABA and GAD also have been detected in embryos, although their precise localization and specific functions in embryonic development have not been elucidated. In this paper, the authors studied the cellular distribution of two GAD isoforms, GAD65 and GAD67, in midgestation mouse embryos by in situ hybridization histochemistry. With few exceptions, it was found that GAD65 and GAD67 mRNAs are localized in overlapping cellular domains of the embryonic CNS that later develop into regions with a strong GABAergic contribution. The GAD-expressing cells are situated in the differentiating zone of the embryonic day 10.5 (E10.5) through E11.5 CNS and in the subventricular zone and the mantle zone of the E12.5 CNS, which suggests that they are committed neuronal precursors. By using a specific serum for GABA, a similar pattern of distribution was obtained, indicating that GAD mRNAs are translated efficiently into enzymatically active GAD, which produces embryonic GABA. The expression domains of GAD overlap with those of genes that are known to be involved in the patterning of the embryonic CNS. The two GAD mRNAs also are detected outside of the embryonic CNS in various cell types, mainly those of placodal and neural crest origin. This pattern of expression is consistent with the notion that GAD and its product, GABA, play a signaling role during development.

Fate of mesencephalic AHD2-expressing dopamine progenitor cells in NURR1 mutant mice

The orphan nuclear receptor NURR1 was previously demonstrated to be required for the generation of mesencephalic dopamine (DA) cells. However, even in the absence of NURR1, which is normally expressed as cells become postmitotic, neuronal differentiation is induced and expression of several genes detected in developing dopamine cells appears normal during early stages of development. These include the homeobox transcription factors engrailed and Ptx-3 as well as aldehyde dehydrogenase 2, here defined as the earliest marker identified in developing DA cells, expressed already in mitotic DA progenitors. We have used the expression of these dopaminergic markers, retrograde axonal tracing, and apoptosis analyses to study the fate of the DA progenitor cells in the absence of NURR1. We conclude that NURR1 plays a critical role in the maturation, migration, striatal target area innervation, and survival of differentiating mesencephalic DA cells.

Detection of glutamate decarboxylase isoforms in rat inferior colliculus following acoustic exposure

The inferior colliculus is a central auditory structure which serves as a site for the integration of ascending and descending auditory information. Changes in central auditory structures may occur with acoustic exposure, which cannot be explained by alterations in cochlear function alone. Rats were exposed to a 10-kHz tone at 100 dB SPL for 9 h. Auditory brainstem response measures showed an initial 25-30-dB threshold shift across all tested frequencies. By 30 days post-exposure, thresholds for clicks and most frequencies returned to near control levels; however, thresholds remained elevated at 10 and 20 kHz. Inner hair cell loss was confined to apical and basal ends of the cochlea, and did not exceed 20%. Inferior colliculus levels of the two isoforms of the GABA synthetic enzyme glutamate decarboxylase (65,000 and 67,000 mol. wt forms) were measured immediately post-exposure (0 h) and at two and 30 days post-exposure using quantitative immunocytochemical and western blotting techniques. Zero-hour measures revealed a significant increase in the level of glutamate decarboxylase (mol. wt 67,000) protein (118%), as well as in the optical density (35%) of immunolabeled cells. By 30 days post-exposure, inferior colliculus protein levels of both glutamate decarboxylase isoforms were significantly below unexposed controls (39% and 21% for the 65,000 and 67,000 mol. wt forms, respectively). These studies describe increased markers for GABA immediately following acoustic exposure, followed by a decline to below control levels from two to 30 days post-exposure. It remains to be determined whether noise trauma-induced changes in glutamate decarboxylase levels in the inferior colliculus reflect protective up-regulation in response to intense stimulation, followed by the establishment of new neurotransmitter equilibrium levels.

Two isoforms of glutamate decarboxylase: why?

Adults express two isoforms of glutamate decarboxylase (GAD), GAD67 and GAD65, which are encoded by different independently regulated genes, a situation that differs from that of other neurotransmitters. In this article, J-J. Soghomonian and David Martin review current knowledge on the differences between these two isoforms. Both isoforms are present in most GABA-containing neurones in the CNS, but GAD65 appears to be targeted to membranes and nerve endings, whereas GAD67 is more widely distributed in cells. Both forms can synthesize transmitter GABA, but GAD67 might preferentially synthesize cytoplasmic GABA and GAD65 might preferentially synthesize GABA for vesicular release. Several lines of evidence suggest that the two forms have different roles in the coding of information by GABA-containing neurones.

Regulation of cell-type specific expression of lacZ by the 5'-flanking region of mouse GAD67 gene in the central nervous system of transgenic mice

The transcriptional regulation of the murine gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was studied by beta-galactosidase histochemistry in transgenic mice carrying fusion genes between progressively longer portions of the 5'-upstream regulatory region of GAD67 and E. coli lacZ. No expression was detected in brains of mice carrying 1.3 kb of upstream sequences including a housekeeping and two conventional promoters, and two negative regulatory elements with homology to known silencers. In mice carrying the same portion of the promoter region plus the first intron, lacZ expression in the adult central nervous system was found in few, exclusively neuronal sites. The number of correctly stained GABAergic centres increased dramatically with increasing the length of the 5'-upstream region included in the construct which suggests that multiple putative spatial enhancers are located in this region. Their action is influenced by epigenetic mechanisms that may be due to site-of-integration and transgene copy-number effects. Additional cis-acting elements are needed to obtain fully correct expression in all GABAergic neurons of the adult central nervous system.

Cleft palate in mice with a targeted mutation in the gamma-aminobutyric acid-producing enzyme glutamic acid decarboxylase 67

The functions of neurotransmitters in fetal development are poorly understood. Genetic observations have suggested a role for the inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) in the normal development of the mouse palate. Mice homozygous for mutations in the beta-3 GABAA receptor subunit develop a cleft secondary palate. GABA, the ligand for this receptor, is synthesized by the enzyme glutamic acid decarboxylase. We have disrupted one of the two mouse Gad genes by gene targeting and also find defects in the formation of the palate. The striking similarity in phenotype between the receptor and ligand mutations clearly demonstrates a role for GABA signaling in normal palate development.

Structure and alternative promoters of the mouse glutamic acid decarboxylase 67 gene

gamma-Aminobutyric acid is synthesized by glutamic acid decarboxylase (GAD), which has two forms, GAD65 and GAD67. Genomic clones coding mouse GAD67 (mGAD67) have been isolated. The restriction map of the overlapping clones covers a region of more than 45 kb of genomic DNA. The mGAD67 gene contains 16 translated exons in addition to an exon which is preferentially expressed in foetal brain. The rapid amplification of 5'-cDNA ends showed that mGAD67 gene transcripts have two different 5'-untranslated regions. Analysis of the genomic clones encompassing the 5'-exons revealed that the two transcripts arose from a single gene by alternative splicing using two different donor sites and a common acceptor. The exons were found 1.5 and 0.6 kb upstream of exon 1. The corresponding promoter regions of these exons have a number of putative regulatory elements, including Sp1- and Krox-24-binding sites. Analysis of mGAD67 transcripts demonstrated that each of the 5'-untranslated exons was expressed in mouse brain. In contrast, exon 0A, but not exon 0B, was expressed in mouse testis and pancreas. These results suggest that these transcripts may be regulated under the control of independent promoters.

Transgenic plants expressing autoantigens fed to mice to induce oral immune tolerance

Oral administration of protein can induce antigen-specific immune hyporesponsiveness. However, the utility of oral tolerance to autoantigens in the treatment of autoimmune diseases may be limited when candidate autoantigens cannot be produced by conventional systems in quantities sufficient for clinical studies. Plants may be ideally suited for this purpose, as they can synthesize, glycosylate and assemble mammalian proteins to provide huge quantities of relatively low cost soluble proteins. Furthermore, edible transgenic plants could provide a simple and direct method of autoantigen delivery for oral tolerance. Therefore, the aim of this study was to determine whether a transgenic plant expression system was capable of synthesizing the diabetes-associated autoantigen, glutamic acid decarboxylase (GAD) in an immunogenic form and whether the oral administration of an autoantigen expressed by a plant could directly induce protective immune responses in a mouse model of diabetes. We show that a GAD-expressing transgenic plant, given as a dietary supplement, inhibits the development of diabetes in the non-obese diabetic (NOD) mouse.

Agrin gene expression in mouse somatosensory cortical neurons during development in vivo and in cell culture

Agrin is an extracellular matrix protein involved in the formation of the postsynaptic apparatus of the neuromuscular junction. In addition to spinal motor neurons, agrin is expressed by many other neuronal populations throughout the nervous system. Agrin's role outside of the neuromuscular junction, however, is poorly understood. Here we use the polymerase chain reaction to examine expression and alternative splicing of agrin in mouse somatosensory cortex during early postnatal development in vivo and in dissociated cell culture. Peak levels of agrin gene expression in developing cortex coincide with ingrowth of thalamic afferent fibres and formation of thalamocortical and intracortical synapses. Analysis of alternatively spliced agrin messenger RNA variants shows that greater than 95% of all agrin in developing and adult somatosensory cortex originates in neurons, including isoforms that have little or no activity in acetylcholine receptor aggregation assays. The levels of expression of "active" and "inactive" isoforms, however, are regulated during development. A similar pattern of agrin gene expression is also observed during a period when new synapses are being formed between somatosensory neurons growing in dissociated cell culture. Changes in agrin gene expression, observed both in vivo and in vitro, are consistent with a role for agrin in synapse formation in the central nervous system.

Cleft palate and decreased brain gamma-aminobutyric acid in mice lacking the 67-kDa isoform of glutamic acid decarboxylase

In addition to its role as an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) is presumed to be involved in the development and plasticity of the nervous system. GABA is synthesized by glutamic acid decarboxylase (GAD), but the respective roles of its two isoforms (GAD65 and 67) have not been determined. The selective elimination of each GAD isoform by gene targeting is expected to clarify these issues. Recently we have produced GAD65 -/- mice and demonstrated that lack of GAD65 does not change brain GABA contents or animal behavior, except for a slight increase in susceptibility to seizures. Here we report the production of GAD67 -/- mice. These mice were born at the expected frequency but died of severe cleft palate during the first morning after birth. GAD activities and GABA contents were reduced to 20% and 7%, respectively, in the cerebral cortex of the newborn GAD67 -/- mice. Their brain, however, did not show any discernible defects. Previous pharmacological and genetic investigations have suggested the involvement of GABA in palate formation, but this is the first demonstration of a role for GAD67-derived GABA in the development of nonneural tissue.

Structure and the promoter region of the mouse gene encoding the 67-kD form of glutamic acid decarboxylase

We have cloned and determined the complete structure of the murine gene encoding the 67-kD form of glutamic acid decarboxylase (GAD67), the gamma-aminobutyric acid synthetic enzyme. Its coding region comprises 18 exons spanning 42 kb of genomic DNA. Exon 1 together with 64 bp of exon 2 defines the 5' untranslated region of GAD67 mRNA. Exon 18 specifies the protein's carboxyl terminal and the entire 3' untranslated region. Exons 7/A and 7/B are solely contained in the coding regions of two alternatively spliced bicistronic embryonic mRNAs, which code for the truncated embryonic GAD forms. The promoter region (P1) corresponding to the main group of transcription initiation sites is devoid of TATA and CAAT boxes but has putative binding sites for the transcription factor SP1 and is embedded in a large G + C-rich domain of a CpG island, features shared by the promoters of constitutively expressed housekeeping genes. Primer extension data suggests the existence of additional transcription start sites at 130 bp and 295 bp upstream from the major initiation site that are utilized less frequently in adult brain. The tentative distal promoters (P2 and P3) that correspond to the minor start sites resemble tissue-specific promoters with TATA and CAAT-like boxes. In 1.3 kb of the 5'-upstream region, we identified several putative transcription factor binding sites such as AP2, Hox, E-box, egr-1, and NF-kappaB and putative neuronal-specific regulatory elements, including the neuronal-restrictive silencer element, which may have functional significance in the developmental and tissue-specific expression of the GAD67 gene.

Comparative distribution of GAD65 and GAD67 mRNAs and proteins in the rat spinal cord supports a differential regulation of these two glutamate decarboxylases in vivo

Gamma-aminobutyric acid (GABA) synthesis can result from the action of at least two glutamic acid decarboxylase (GAD) isoforms, GAD65 and GAD67, possibly involved in distinct mechanisms. We have made the hypothesis that GAD65 may respond to short-term changes and is present in neurons with a phasic activity, while GAD67 may rather provide GABA for the metabolic pool and for supporting tonic levels of synaptic transmission (Erlander et al.: Neuron 7:91-100, 1991; Feldblum et al.: J Neurosci Res 34:689-706, 1993). In the present work we have tested this hypothesis in the rat spinal cord where both types of activities have been identified. The correlation of GABA immunodetection with the distribution of GAD65 and GAD67 mRNAs and proteins has evinced in the dorsal horn a differential regulation of the two isoforms. In situ hybridization has revealed, in the dorsal horn, relatively higher levels of GAD67 mRNA than of GAD65, while immunodetection of the proteins demonstrated numerous punctate profiles with both GAD antisera. Reverse transcription-polymerase chain reaction (RT-PCR) data confirmed the abundance of the GAD67 transcripts compared to GAD65 in the rat spinal cord. In contrast, within the ventral horn, there was a greter number of GAD67-immunoreactive (IR) profiles mostly located around motoneurons. The paucity of GAD65 immunoreactivity in the ventral horn cannot be related to a different accessibility of the antigens to the epitopes since on the same section a dense GAD65 staining was detected in the dorsal horn. Hence, a number of biochemical and electrophysiological data support the concept of the involvement of glycine as the major inhibitory system within the ventral horn which may explain the low levels of GAD transcription in this region. The paucity of GAD65 in the ventral horn may also reflect a functional difference, suggesting a predominance of GAD67 in neurons under tonic activity. In the dorsal horn, where neurons with phasic and tonic firing patterns have been disclosed, GAD65 may, in addition, provide GABA for responses to short-term changes.

GABA and glycine in the central auditory system of the mustache bat: structural substrates for inhibitory neuronal organization

The distribution and morphology of neurons and axonal endings (puncta) immunostained with antibodies to gamma-aminobutyric acid (GABA) and glycine (Gly) were analyzed in auditory brainstem, thalamic, and cortical centers in the mustache bat. The goals of the study were (1) to compare and contrast the location of GABAergic and glycinergic neurons and puncta, (2) to determine whether nuclei containing immunoreactive neurons likewise have a similar concentration of puncta, (3) to assess the uniformity of immunostaining within a nucleus and to consider regional differences that were related to or independent of cytoarchitecture, and (4) to compare the patterns recognized in this bat with those in other mammals. There are nine major conclusions. (1) Glycinergic immunostaining is most pronounced in the hindbrain. (2) In the forebrain, GABA alone is present. (3) Some nuclei have GABAergic or glycinergic neurons exclusively; a few have neither. (4) Although there is sometimes a close relationship between the relative number of immunopositive neurons and the density of the puncta, just as often there is no particular correlation between them; this reflects the fact that many GABAergic and glycinergic neurons project beyond their nucleus of origin. (5) Even nuclei devoid of or with few GABAergic or glycinergic neurons contain relatively abundant numbers of puncta; some neurons receive axosomatic terminals of each type. (6) In a few nuclei there are physiological subregions with specific local patterns of immunostaining. (7) The patterns of immunostaining resemble those in other mammals; the principal exceptions are in nuclei that, in the bat, are hypertrophied (such as those of the lateral lemniscus) and in the medial geniculate body. (8) Cellular colocalization of GABA and Gly is specific to only a few nuclei. (9) GABA and glutamic acid decarboxylase (GAD) immunostaining have virtually identical distributions in each nucleus. Several implications follow. First, the arrangements of GABA and Gly in the central auditory system represent all possible patterns, ranging from mutually exclusive to overlapping within a nucleus to convergence of both types of synaptic endings on single neurons. Second, although both transmitters are present in the hindbrain, glycine appears to be dominant, and it is often associated with circuitry in which precise temporal control of aspects of neuronal discharge is critical. Third, the auditory system, especially at or below the level of the midbrain, contains significant numbers of GABAergic or glycinergic projection neurons. The latter feature distinguishes it from the central visual and somatic sensory pathways.

Expression of tyrosine hydroxylase in the aging, rodent olfactory system

Tyrosine hydroxylase (TH) mRNA, immunoreactivity, and activity were examined as a reflection of dopamine expression in juxtaglomerular neurons intrinsic to the olfactory bulbs of young (6-month-old), middle aged (18-month-old), and aged (25- to 29-month-old) rats and mice. TH expression was maintained at levels observed in young animals in the olfactory bulbs of aged animals from two mouse strains, C57Bl/6JNia and C57Bl/6NNia, and one rat strain, an F1 hybrid between F344 and Brown Norway strains. The parental F344 rat strain exhibited reductions in TH expression of about 20% in 26- to 29-month-old animals as compared to 6- and 18 month-old rats. However, there was significant inter-animal variability. Some aged F344 rats had TH levels that were similar and others had activity levels that were 50% of those in young and middle aged animals. Neither the general condition of the animals nor the presence of adrenal tumors predicted the individuals with reduced TH expression. Olfactory bulb size, estimated from protein content, did not differ between rats and mice of different ages. In addition, expression of olfactory marker protein, a protein found primarily in mature olfactory receptor neurons, also was unchanged indicating the maintenance of afferent innervation. These data suggest that, in contrast to other brain dopamine systems, the expression of the dopamine phenotype is maintained in the aging olfactory bulb.

Distinct protein forms are produced from alternatively spliced bicistronic glutamic acid decarboxylase mRNAs during development

It has been shown that the enzyme glutamic acid decarboxylase (GAD; EC 4.1.1.15), which catalyzes the conversion of L-glutamate to gamma-aminobutyric acid in the central nervous system of vertebrates, can be first detected in rodents at late embryonic stages. In contrast, we have found that the gene coding for the 67-kDa form of GAD is already transcriptionally active at embryonic day E10.5 in the mouse. In addition to the 3.5-kb adult-type mRNA, we have detected two 2-kb embryonic messages that contain alternatively spliced exons of 80 (I-80) and 86 (I-86) bp, respectively. The overlapping stop-start codon TGATG, found in the embryonic exons, converts the monocistronic adult-type transcript into a bicistronic one, coding for a 25-kDa leader peptide and a 44-kDa enzymatically active truncated GAD. A second stop codon at the 3' end of the 86-bp exon abolishes the expression of truncated GAD. The products of the two embryonic mRNAs were identified in a rabbit reticulocyte in vitro translation system, COS cells, and mouse embryos. The two GAD embryonic forms represent distinct functional domains and display characteristic developmental patterns, consistent with a possible role in the formation of the gamma-aminobutyric acid-ergic inhibitory synapses.

Distinct protein forms are produced from alternatively spliced bicistronic glutamic acid decarboxylase mRNAs during development

It has been shown that the enzyme glutamic acid decarboxylase (GAD; EC 4.1.1.15), which catalyzes the conversion of L-glutamate to gamma-aminobutyric acid in the central nervous system of vertebrates, can be first detected in rodents at late embryonic stages. In contrast, we have found that the gene coding for the 67-kDa form of GAD is already transcriptionally active at embryonic day E10.5 in the mouse. In addition to the 3.5-kb adult-type mRNA, we have detected two 2-kb embryonic messages that contain alternatively spliced exons of 80 (I-80) and 86 (I-86) bp, respectively. The overlapping stop-start codon TGATG, found in the embryonic exons, converts the monocistronic adult-type transcript into a bicistronic one, coding for a 25-kDa leader peptide and a 44-kDa enzymatically active truncated GAD. A second stop codon at the 3' end of the 86-bp exon abolishes the expression of truncated GAD. The products of the two embryonic mRNAs were identified in a rabbit reticulocyte in vitro translation system, COS cells, and mouse embryos. The two GAD embryonic forms represent distinct functional domains and display characteristic developmental patterns, consistent with a possible role in the formation of the gamma-aminobutyric acid-ergic inhibitory synapses.

Dopamine receptor blockade increases dopamine D2 receptor and glutamic acid decarboxylase mRNAs in mouse substantia nigra

To study the influence of dopaminergic activity on the expression of dopamine D2 receptors and glutamic acid decarboxylase in substantia nigra, mice were treated daily for several days with an irreversibly acting dopamine D1 and dopamine D2 receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) or with a selective irreversible D2 dopamine receptor antagonist fluphenazine-N-mustard. Mice were killed 24 h after the last injection. Dopamine D1 and dopamine D2 receptors were determined by receptor autoradiography, and dopamine D1 and dopamine D2 receptor mRNA and glutamic acid decarboxylase mRNA were determined by in situ hybridization histochemistry. The results showed that treatment with EEDQ, which blocked 80% to 85% of the dopamine D2 and dopamine D1 receptors in substantia nigra, increased the levels of dopamine D2 receptor mRNA in substantia nigra by about 27%. Treatment with fluphenazine-N-mustard, which blocked about 85% of the dopamine D2 receptors in substantia nigra but had no significant effect on dopamine D1 receptors, increased the levels of dopamine D2 receptor mRNA by about 34%. There were no detectable levels of dopamine D1 receptors, increased the levels of dopamine D2 receptor mRNA by about 34%. There were no detectable levels of dopamine D1 receptor mRNA in substantia nigra either in control animals or in animals treated with the dopamine receptor antagonists. Glutamic acid decarboxylase mRNA was expressed in several regions of the mid-brain but only that expressed in substantia nigra was altered by treatment with dopamine receptor antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology of IDDM

The clinical onset of insulin-dependent diabetes is associated with several autoimmune phenomena including islet cell antibodies, glutamic acid decarboxylase (the GAD65 isoform) autoantibodies (GAD65Ab) as well as insulin autoantibodies. The molecular cloning of these autoantigens has permitted the development of precise and reproducible antibody immunoassays to identify marker-positive patients and control subjects. Among patients with new-onset diabetes about 70% were GAD65Ab positive compared to 1.5% among control subjects while 46% of patients had IAA compared to 1% among control subjects. The autoreactive sites or epitopes of GAD65 and insulin remain to be determined. The disease association with HLA on chromosome 6 may help to define the epitope specificity of the autoimmune reaction. Recent data suggest that 95% of new-onset IDDM children (0-15 years of age) are positive for either DQ2, DQ8 or both compared to about 50% of healthy control subjects. HLA-DQ6 is negatively associated with the disease. Both HLA-DQ2 and DQ8 therefore seem to be necessary, but not sufficient for diabetes. Molecular modelling suggests comparable physicochemical properties of DQ2 and DQ8 but are widely different from DQ6. In 1984, the conclusion was that molecular cloning of the genes for the autoantigens, antibodies, T-cell receptors, as well as HLA class I and II molecules associated with diabetes are essential for analysing the components which control the development of pancreatic beta-cell autoimmunity. In 1994, autoantigens and HLA molecules have been cloned and recombinant reagents developed to be used in experiments aimed at testing whether it will be possible to predict IDDM.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiple evolutionary origin of pyridoxal-5'-phosphate-dependent amino acid decarboxylases

Comparison of the amino acid sequences of nine different pyridoxal-5'-phosphate-dependent amino acid decarboxylases indicated that they can be subdivided into four different groups that seem to be evolutionarily unrelated to each other. Group I is represented by glycine decarboxylase, a component of a multienzyme system; group II comprises glutamate, histidine, tyrosine, and aromatic-L-amino-acid decarboxylases; group III, procaryotic ornithine and lysine decarboxylase as well as the procaryotic biodegradative type of arginine decarboxylase; group IV, eucaryotic ornithine and arginine decarboxylase as well as the procaryotic biosynthetic type of arginine decarboxylase and diaminopimelate decarboxylase. (N-1) profile analysis, a more stringent application of profile analysis, established the homology among the enzymes of each group. A search with the profile of group II indicated a distant relationship with aminotransferases and thus with the alpha family of pyridoxal-5'-phosphate-dependent enzymes. No evidence was obtained that groups I, III and IV were related with other pyridoxal-5'-phosphate-dependent enzymes or any other protein in the database. Unlike the aminotransferases, which, with few possible exceptions, constitute a single group of homologous proteins, the amino acid decarboxylases, by the criterion of profile analysis, have evolved along multiple lineages, in some cases even if they have the same substrate specificity.

Transient forebrain ischemia induces delayed injury in the substantia nigra reticulata: degeneration of GABA neurons, compensatory expression of GAD mRNA

In rodents, transient forebrain ischemia causes preferentially neuron death in small and medium size neurons of the striatum and hilar neurons in the hippocampus within 24 h, and CA1 hippocampal neurons within 72 h. The temporal unfolding of pathological processes after longer time intervals between reperfusion and sacrifice now includes delayed degeneration of the substantia nigra reticulata (SNr). Animals were exposed to 20 min of transient forebrain ischemia and sacrificed within 7 days, or at least 3 weeks after reperfusion. Histological examination and quantitative morphometrics revealed that the degree of volume loss and neuron loss in the SNr depended on the initial ischemic injury. Initial ischemic injury confined to the caudate nucleus produced volume loss but not neuron loss in the SNr. However, initial ischemic injury that included the caudate nucleus and the globus pallidus produced not only greater volume loss but also neuron loss in the SNr. SNr neuron loss was restricted to the medial dorsal area, occurred in animals that survived at least 3 weeks after perfusion, and did not occur in animals that survived 7 days after perfusion, and was accompanied by increased staining of antibody to glial fibrillary acidic protein. The topographic specificity and delayed time course suggest that the mechanism for SNr neuron loss depends on transneuronal events initiated by ischemia but evolving over a longer time period. In situ hybridization with a cDNA probe for glutamic acid decarboxylase (GAD) mRNA demonstrated increased GAD signal in the remaining SNr neurons of animals with CN and GP damage compared to animals with CN damage. The significant increase in GAD mRNA may indicate compensation at the level of gene expression for the loss of GABAergic neurons. This rodent model offers new in vivo opportunities to elucidate the requirements for neuronal viability, and phenotypic expression, and suggests that the current notions of windows of opportunity for therapeutic intervention may be expanded from hours to days to weeks.

Absence of autoantibodies against glutamate decarboxylase (GAD) in the non-obese diabetic (NOD) mouse and low expression of the enzyme in mouse islets

GAD is a major islet cell autoantigen in human type 1 diabetes mellitus. Autoantibodies are preferentially directed against the 65-kD isoform of the enzyme which is the only form expressed in human islets of Langerhans. The NOD mouse is a spontaneous model of type 1 diabetes, frequently employed in studies dealing with the immunopathogenesis of the disease. In the present study the reactivity of sera from 34 prediabetic and 15 diabetic NOD mice was tested against GAD protein present in islets of Langerhans and cerebellum, and against recombinant, semi-purified GAD-65 and GAD-67. A rabbit antiserum (K2) raised against GAD-67 could readily recognize the recombinant GAD-67 and the isoform present in rat and mouse islets and mouse brain. A MoAb (GAD-6) specific for the GAD-65 isoform reacted against the recombinant GAD-65 and the isoform present in rat islets and mouse brain, whereas no reactivity was observed when using mouse islets. However, when testing the NOD mice sera by immunohistochemistry, immunoprecipitation and Western blot, no reactivity against any of the isoforms of GAD could be detected. Using reverse transcription polymerase chain reaction (PCR), GAD-67 mRNA could be detected in mouse and rat islets and in mouse brain. GAD-65 mRNA could also be detected in rat islets and mouse brain, but apparently a much lower copy number is present in mouse islets. These findings stress important differences in the immune response occurring in the animal model NOD mouse compared with human type 1 diabetes, and emphasize that human and animal type 1 diabetes possibly represent the final outcome of several different etiological factors.

Dopaminergic and glutamatergic blocking drugs differentially regulate glutamic acid decarboxylase mRNA in mouse brain

Dopaminergic and glutamatergic inputs play an important role in regulating the activity of GABAergic neurons in basal ganglia. To understand more fully the biochemical interactions between these neurotransmitter systems, the effects of blocking dopamine and glutamate (N-methyl-D-aspartate) (NMDA) receptors on the expression of glutamic acid decarboxylase (GAD) mRNA were examined. Persistent blockade of dopamine receptors was achieved by daily injections of EEDQ, a relatively non-selective irreversible D1 and D2 dopamine receptor antagonist, or FNM, a relatively selective irreversible D2 dopamine receptor antagonist. Persistent blockade of NMDA receptors was achieved by continuously infusing dizocilpine (MK-801), a non-competitive NMDA receptor antagonist. The levels of GAD mRNA in mouse brain were measured by in situ hybridization histochemistry following treatment with these agents. Repeated administration of EEDQ increased the levels of GAD mRNA in corpus striatum and frontal and parietal cortex; the first significant effects were seen after 4 days of treatment. Treatment with FNM elicited effects similar to those produced by EEDQ, except FNM also significantly increased GAD mRNA in nucleus accumbens. Neither EEDQ nor FNM produced significant effects on GAD mRNA in olfactory tubercle or septum. Infusion of MK-801 produced a rapid and marked decrease in the levels of GAD mRNA in corpus striatum, nucleus accumbens, olfactory tubercle, septum and frontal and parietal cortex; significant changes were seen as early as 2 days of treatment. No significant effects were seen in globus pallidus. Cellular analysis of emulsion autoradiograms from corpus striatum revealed that MK-801 reduced the amount of GAD mRNA in individual cells as well as the proportion of cells expressing high levels of GAD mRNA. These results suggest that dopamine, though its interaction with D2 dopamine receptors, exerts an inhibitory effect on the expression of GAD mRNA, and that glutamate, though its interaction with NMDA receptors, exerts a stimulatory effect on GAD mRNA expression. They show further that the regulation of gene expression by dopamine receptors or NMDA receptors is different in different regions of the brain.

Analysis of the anatomical distribution of GAD67 mRNA encoding truncated glutamic acid decarboxylase proteins in the embryonic rat brain

During development of the central nervous system (CNS) the gene that encodes the 67 kDa form of glutamic acid decarboxylase (GAD) undergoes alternative splicing. The alternatively spliced variants include an exon (referred to as ES, for embryonic stop) that contains a premature stop codon. The detection of mRNA containing the ES exon in embryonic rat brain has been previously reported (Proc. Natl. Acad. Sci., 87 (1990) 8771-8775). We have used in situ hybridization to identify the anatomical distribution of ES mRNA in the embryonic rat brain during two stages of development, embryonic day 17 (E17) and E20. At E17, GAD67 mRNA was expressed in several CNS regions that were destined to contain GABAergic neurons when mature. ES transcripts were predominantly localized to ventricular zones and other regions associated with populations of proliferative cells at E17 and E20. At both ages, however, the alternatively spliced variants were also detected in regions of brain associated with migratory or post-mitotic neurons. GAD67 transcripts that did not include the ES exon were localized to anatomical areas that contained post-mitotic, and often post-migratory neurons. The temporal and spatial disappearance of mRNA containing the ES exon generally followed a caudal-to-rostral gradient which paralleled neuronal terminal mitosis and differentiation.

The synaptic vesicle-associated protein amphiphysin is the 128-kD autoantigen of Stiff-Man syndrome with breast cancer

Stiff-Man syndrome (SMS) is a rare disease of the central nervous system (CNS) characterized by progressive rigidity of the body musculature with superimposed painful spasms. An autoimmune origin of the disease has been proposed. In a caseload of more than 100 SMS patients, 60% were found positive for autoantibodies directed against the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). Few patients, all women affected by breast cancer, were negative for GAD autoantibodies but positive for autoantibodies directed against a 128-kD synaptic protein. We report here that this antigen is amphiphysin. GAD and amphiphysin are nonintrinsic membrane proteins that are concentrated in nerve terminals, where a pool of both proteins is associated with the cytoplasmic surface of synaptic vesicles. GAD and amphiphysin are the only two known targets of CNS autoimmunity with this distribution. This finding suggests a possible link between autoimmunity directed against cytoplasmic proteins associated with synaptic vesicles and SMS.

Identification of a dominant epitope of glutamic acid decarboxylase (GAD-65) recognized by autoantibodies in stiff-man syndrome

Glutamic acid decarboxylase (GAD) is the enzyme that synthesizes the neurotransmitter gamma-aminobutyric acid (GABA) in neurons and in pancreatic beta cells. It is a major target of autoimmunity in Stiff-Man syndrome (SMS), a rare neurological disease, and in insulin-dependent diabetes mellitus. The two GAD isoforms, GAD-65 and GAD-67, are the products of two different genes. GAD-67 and GAD-65 are very similar to each other in amino acid sequence and differ substantially only at their NH2-terminal region. We have investigated the reactivity of autoantibodies of 30 Stiff-Man syndrome patients to GAD. All patient sera contained antibodies that recognize strongly GAD-65, but also GAD-67, when tested by immunoprecipitation on brain extracts and by immunoprecipitation or immunocytochemistry on cells transfected with either the GAD-65 or the GAD-67 gene. When tested by Western blotting, all patient sera selectively recognized GAD-65. Western blot analysis of deletion mutants of GAD-65 demonstrated that autoantibodies are directed predominantly against two regions of the GAD-65 molecule. All SMS sera strongly recognized a fragment contained between amino acid 475 and the COOH terminus (amino acid 585). Within this region, amino acids 475-484 and 571-585 were required for reactivity. The requirement of these two discontinuous segments implies that the epitope is influenced by conformation. This reactivity is similar to that displayed by the monoclonal antibody GAD 6, suggesting the presence of a single immunodominant epitope (SMS-E1) in this region of GAD-65. In addition, most SMS sera recognized at least one epitope (SMS-E2) in the NH2-terminal domain of GAD-65 (amino acids 1-95). The demonstration in SMS patients of a strikingly homogeneous humoral autoimmune response against GAD and the identification of dominant autoreactive target regions may help to elucidate the molecular mechanisms of GAD processing and presentation involved in GAD autoimmunity. Moreover, the reactivity reported here of GAD autoantibodies in SMS partially differs from the reactivity of GAD autoantibodies in insulin-dependent diabetes mellitus, suggesting a link between the pattern of humoral autoimmunity and the clinical condition.

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.

Cloning and sequence analysis of a murine cDNA encoding glutamate decarboxylase (GAD65)

We report the cloning and cDNA sequence of murine GAD65. Murine GAD65 is comprised of 585 amino acids and shares a high degree of homology with human and rat GAD65, with most divergences occurring near their amino-termini. The murine GAD65 sequence will allow evaluation of the role of this gene in murine neurogenetic and autoimmune diseases.

Comparative localization of mRNAs encoding two forms of glutamic acid decarboxylase with nonradioactive in situ hybridization methods

Nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes were used to localize two glutamic acid decarboxylase (GAD) mRNAs in rat brain. These mRNAs encode two forms of GAD that both synthesize GABA but differ in a number of characteristics including their molecular size (65 and 67 kDa). For each GAD mRNA, discrete neuronal labeling with high cellular resolution and low background staining was obtained in most populations of known GABA neurons. In addition, the current methods revealed differences in the intensity of labeling among neurons for each GAD mRNA, suggesting that the relative concentrations of each GAD mRNA may be higher in some groups of GABA neurons than in others. Most major classes of GABA neurons were labeled for each GAD mRNA. In some groups of GABA neurons, the labeling for the two mRNAs was virtually identical, as in the reticular nucleus of the thalamus. In other groups of neurons, although there was substantial labeling for each GAD mRNA, labeling for one of the mRNAs was noticeably stronger than for the other. In most brain regions, such as the cerebellar cortex, labeling for GAD67 mRNA was stronger than for GAD65 mRNA, but there were a few brain regions in which labeling for GAD65 mRNA was more pronounced, and these included some regions of the hypothalamus. Finally, some groups of GABA neurons were predominantly labeled for one of the GAD mRNAs and showed little or no detectable labeling for the other GAD mRNA, as, for example, in neurons of the tuberomammillary nucleus of the hypothalamus where labeling for GAD67 mRNA was very strong but no labeling for GAD65 mRNA was evident. The findings suggest that most classes of GABA neurons in the central nervous system (CNS) contain mRNAs for at least two forms of GAD, and thus, have dual enzyme systems for the synthesis of GABA. Higher levels of one or the other GAD mRNA in certain groups of GABA neurons may be related to differences in the functional properties of these neurons and their means of regulating GABA synthesis.

Association of GAD-65, but not of GAD-67, with the Golgi complex of transfected Chinese hamster ovary cells mediated by the N-terminal region

Glutamic acid decarboxylase (GAD) is the enzyme responsible for synthesis of the neurotransmitter gamma-aminobutyric acid in neurons and pancreatic beta cells. It is represented by two isoforms, GAD-65 and GAD-67, which are the products of two different genes and differ substantially only at their N-terminal regions. GAD-65 is a dominant autoantigen in stiff-man syndrome and insulin-dependent diabetes mellitus. In neurons and beta cells, GAD is concentrated around synaptic vesicles and synaptic-like microvesicles, respectively, as well as in the area of the Golgi complex. The mechanisms responsible for specific targeting of GAD to these organelles are not yet understood. The elucidation of the mechanism of subcellular targeting of GAD may be relevant to understanding its role as an autoantigen. In this study, the cloned genes for GAD-65 and GAD-67 were expressed separately in Chinese hamster ovary (CHO) cells and COS cells. While GAD-67 had a diffuse cytoplasmic localization, GAD-65 had a punctate distribution, with most of the immunoreactivity being concentrated in the area of the Golgi complex. A chimeric protein in which the 88 N-terminal amino acids of GAD-67 were replaced by the 83 N-terminal amino acids of GAD-65 was targeted to the Golgi complex, indicating that the N-terminal region of GAD-65 contains a targeting signal sufficient for directing the remaining portion of the molecule, highly similar in GAD-65 and GAD-67, to the Golgi complex-associated structures.

Cholinergic lesions of mouse striatum induced by AF64A alter D2 dopaminergic behavior and reduce D2 dopamine receptors and D2 dopamine receptor mRNA

To determine whether dopamine receptors are expressed in acetylcholine-containing neurons intrinsic to the striatum, and to study further the interactions between the dopaminergic and cholinergic systems, the irreversibly acting cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A), was injected unilaterally into the mouse corpus striatum, and rotational behavior induced by dopamine agonists and certain molecular events associated with this lesion were determined 7 days after lesioning. Brains were analyzed for D2 dopamine receptors by autoradiography, using [3H](-)sulpiride as a ligand, and for D2 dopamine receptor mRNA and glutamic acid decarboxylase mRNA by Northern blot analysis, using selective radiolabelled oligonucleotide probes. Choline uptake sites were determined by binding assays using [3H]hemicholinium-3, a selective choline reuptake blocker, as a ligand. Mice with intrastriatal injections of AF64A showed ipsilateral rotational responses to the non-selective dopamine agonist apomorphine and to the D2 dopamine agonists, pergolide and quinpirole, but not to the D1 dopamine agonist SKF 38393. This was associated with a significant reduction in D2 dopamine receptors in the ipsilateral striatum and a significant decrease in the amount of D2 dopamine receptor mRNA. That AF64A produced a relatively selective cholinergic deficit was supported by the evidence showing that AF64A lesions significantly reduced [3H]hemicholinium-3 binding sites but did not alter glutamic acid decarboxylase (GAD) mRNA. Further, hemicholinium-3, prevented the AF64A-induced changes in rotational behavior. These results suggest that striatal cholinergic interneurons contain D2 dopamine receptors and express the D2 dopamine receptor gene, and that these interneurons are involved in dopamine-mediated rotational behavior.

Irreversible blockade of D2 dopamine receptors by fluphenazine-N-mustard increases glutamic acid decarboxylase mRNA in rat striatum

The influence of dopaminergic activity on the function of GABAergic neurons in striatum was examined by administering rats the irreversible D2 dopamine receptor antagonist, fluphenazine-N-mustard (FNM), and determining the level of glutamic acid decarboxylase (GAD) mRNA in striatum. Rats were given either an acute single injection or chronic daily injections of FNM (20 mumol/kg, i.p.) for 6 days. The level of GAD mRNA in striatum was determined by in situ hybridization histochemistry. The results showed that acute treatment with FNM failed to significantly change striatal GAD mRNA. However, chronic FNM treatment significantly increased in the level of striatal GAD mRNA. These results demonstrate that irreversible blockade of D2 dopamine receptors increases the expression of GAD mRNA in rat striatum.

Effects of increased gamma-aminobutyric acid levels on GAD67 protein and mRNA levels in rat cerebral cortex

Rats were injected with saline or the gamma-aminobutyric acid (GABA) transaminase inhibitor gamma-vinyl-GABA for 7 days and the effects on GABA content and glutamic acid decarboxylase (GAD) activity, and the protein and mRNA levels of the two forms of GAD (GAD67 and GAD65) in the cerebral cortex were studied. gamma-Vinyl-GABA induced a 2.3-fold increase in GABA content, whereas total GAD activity decreased by 30%. Quantitative immunoblotting showed that the decline in GAD activity was attributable to a 75-80% decrease in GAD67 levels, whereas the levels of GAD65 remained unchanged. RNA slot-blotting with a 32P-labeled GAD67 cDNA probe demonstrated that the change in GAD67 protein content was not associated with a change in GAD67 mRNA levels. Our results suggest that GABA specifically controls the level of GAD67 protein. This effect may be mediated by a decreased translation of the GAD67 mRNA and/or a change in the stability of the GAD67 protein.

Regulation of gamma-aminobutyric acid synthesis in the brain

gamma-Aminobutyric acid (GABA) is synthesized in brain in at least two compartments, commonly called the transmitter and metabolic compartments, and because regulatory processes must serve the physiologic function of each compartment, the regulation of GABA synthesis presents a complex problem. Brain contains at least two molecular forms of glutamate decarboxylase (GAD), the principal synthetic enzyme for GABA. Two forms, termed GAD65 and GAD67, are the products of two genes and differ in sequence, molecular weight, interaction with the cofactor, pyridoxal 5'-phosphate (pyridoxal-P), and level of expression among brain regions. GAD65 appears to be localized in nerve terminals to a greater degree than GAD67, which appears to be more uniformly distributed throughout the cell. The interaction of GAD with pyridoxal-P is a major factor in the short-term regulation of GAD activity. At least 50% of GAD is present in brain as apoenzyme (GAD without bound cofactor; apoGAD), which serves as a reservoir of inactive GAD that can be drawn on when additional GABA synthesis is needed. A substantial majority of apoGAD in brain is accounted for by GAD65, but GAD67 also contributes to the pool of apoGAD. The apparent localization of GAD65 in nerve terminals and the large reserve of apoGAD65 suggest that GAD65 is specialized to respond to short-term changes in demand for transmitter GABA.(ABSTRACT TRUNCATED AT 250 WORDS)