Isolation and characterization of the rat gene encoding glutamate dehydrogenase

Amygdaloid zif268 participated in the D-cycloserine facilitation effect on the extinction of conditioned fear

RATIONALE

The involvement of glutamate in fear extinction is perhaps the most promising in terms of facilitating clinical interventions for posttraumatic stress disorder (PTSD). This study was aimed at elucidating the possible role of zif268 on the D-cycloserine (DCS) facilitation effect on extinction.

OBJECTIVE

We investigated the association between zif268 and the extinction of conditioned fear by using antisense oligodeoxynucleotide (ODN) of zif268 and the fear-potentiated startle paradigm.

METHODS

Male adult Wistar rats were injected DCS (15 mg/kg, IP) 15 min prior to the extinction training, administered with antisense or sense ODN (800 pmol) of zif268 and subjected for fear-potentiated startle paradigm (FPS) and Western blot.

RESULTS

Either context exposure or cue exposure elevated the expression of zif268 in the basolateral nucleus of the amygdala (BLA) (p < 0.05 and p < 0.01, respectively) compared to the control group. Additionally, zif268 expression in BLA was further elevated by the glutamate NMDA receptor agonist DCS administration. Intra-amygdaloid injection of the antisense ODN of zif268 blocked the facilitation effect of DCS on the extinction of conditioned fear. Subsequent control experiments indicated that administration of vehicle or zif268 sense ODN did not alter the facilitation of DCS and that the blockage effect of zif268 antisense ODN was not due to lasting damage to the amygdala.

CONCLUSIONS

Our results suggest that zif268 within the amygdala participates in the DCS facilitation effect on the extinction of conditioned fear.

Expression of human GLUD2 glutamate dehydrogenase in human tissues: functional implications

Glutamate dehydrogenase (GDH), a mitochondrial enzyme with a key metabolic role, exists in the human in hGDH1 and hGDH2 isoforms encoded by the GLUD1 and GLUD2 genes, respectively. It seems that GLUD1 was retroposed to the X chromosome where it gave rise to GLUD2 via random mutations and natural selection. Of these, evolutionary Gly456Ala substitution dissociated hGDH2 from GTP control, while replacement of Arg443 by Ser drastically modified basal activity, heat stability, optimal pH, allosteric regulation and migration pattern in SDS-PAGE, thus suggesting an effect on enzyme's conformation. While GLUD2-specific transcripts have been detected in human brain, retina and testis, data on the endogenous hGDH2 protein are lacking. Given the housekeeping nature of hGDH1 and its high homology to hGDH2, the specific detection of hGDH2 in tissues presents a challenge. To develop an antibody specific for hGDH2, we considered that an epitope containing the Arg443Ser change was an attractive target. We accordingly used a peptide that corresponds to residues 436-447, with Ser at position 443, to immunize rabbits and succeeded in raising a polyclonal antibody specific for hGDH2. Western blots showed that human testis contained equal amounts of hGDH2 and hGDH1 and that both isoproteins localized to the mitochondrial fraction. In human brain, however, hGDH2 expression was lower than that of hGDH1. Immuno-histochemical studies on human testis and cerebral cortex, showed punctuate, organelle-like hGDH2 immuno-labeling in sertoli cells and in astrocytes, respectively, consistent with the mitochondrial localization of the enzyme. Similar studies in kidney revealed that hGDH2 is expressed in epithelial cells of the proximal convoluted tubule. As hGDH2 can metabolize glutamate at relatively low pH without the GTP constrain, it may function efficiently under conditions of relative acidification that prevail in astrocytes following glutamate uptake. Similarly, in the kidney, hGDH2 could contribute to enhanced excretion of ammonia under acidosis.

A gene for neuronal plasticity in the mammalian brain: Zif268/Egr-1/NGFI-A/Krox-24/TIS8/ZENK?

Zif268 is a transcription regulatory protein, the product of an immediate early gene. Zif268 was originally described as inducible in cell cultures; however, it was later shown to be activated by a variety of stimuli, including ongoing synaptic activity in the adult brain. Recently, mice with experimentally mutated zif268 gene have been obtained and employed in neurobiological research. In this review we present a critical overview of Zif268 expression patterns in the naive brain and following neuronal stimulation as well as functional data with Zif268 mutants. In conclusion, we suggest that Zif268 expression and function should be considered in a context of neuronal activity that is tightly linked to neuronal plasticity.

Regulation of Egr-1, SRF, and Sp1 mRNA expression in contracting skeletal muscle cells

The early cellular signals associated with contractile activity initiate the activation and induction of transcription factors that regulate changes in skeletal muscle phenotype. The transcription factors Egr-1, Sp1, and serum response factor (SRF) are potentially important mediators of mitochondrial biogenesis based on the prevalence of binding sites for them in the promoter regions of genes encoding mitochondrial proteins, including PGC-1α, the important regulator of mitochondrial biogenesis. Thus, to further define a role for transcription factors at the onset of contractile activity, we examined the time-dependent alterations in Egr-1, Sp1, and SRF mRNA and the levels in electrically stimulated mouse C2C12skeletal muscle cells. Early transient increases in Egr-1 mRNA levels within 30 min ( P < 0.05) of contractile activity led to threefold increases ( P < 0.05) in Egr-1 protein by 60 min. The increase in Egr-1 mRNA was not because of increased stability, as Egr-1 mRNA half-life after 30 min of stimulation showed only a 58% decline. Stimulation of muscle cells had no effect on Sp1 mRNA but led to progressive increases ( P < 0.05) in SRF mRNA by 30 and 60 min. This was not matched by increases in SRF protein but occurred coincident with increases ( P < 0.05) in SRF-serum response element DNA binding at 30 and 60 min as a result of SRF phosphorylation on serine-103. To assess the importance of the recovery period, 12 h of continuous contractile activity was compared with four successive 3-h bouts, with an intervening 21-h recovery period after each bout. Continuous contractile activity led to a twofold increase ( P < 0.05) in Egr-1 mRNA, no change in SRF mRNA, and a 43% decrease in Sp1 mRNA expression. The recovery period prevented the decline in Sp1 mRNA, produced a decrease in Egr-1 mRNA, and had no effect on SRF mRNA. Thus continuous and intermittent contractile activity evoked different specific transcription factor expression patterns, which may ultimately contribute to divergent qualitative, or temporal patterns of, phenotypic adaptation in muscle.

Calcium-regulated changes in mitochondrial phenotype in skeletal muscle cells

Cytochrome c expression and mitochondrial biogenesis can be invoked by elevated intracellular Ca2+in muscle cells. To characterize the potential role of Ca2+as a messenger involved in mitochondrial biogenesis in muscle, we determined the effects of the Ca2+ionophore A-23187 on the expression of nuclear- and mitochondrially encoded genes. Treatment of myotubes with 1 μM A-23187 for 48–96 h increased nuclear-encoded β-subunit F1ATPase and malate dehydrogenase (MDH) mRNA levels by 50–100% ( P < 0.05) but decreased mRNA levels of glutamate dehydrogenase (GDH) by 19% ( P < 0.05). mRNA levels of the cytochrome c oxidase (COX) nuclear-encoded subunits IV, Vb, and VIc were unchanged, whereas the mitochondrially encoded subunits COX II and COX III were decreased by 30 and 70%, respectively ( P < 0.05). This was paralleled by a 20% decrease ( P < 0.05) in COX activity. These data suggest that cytoplasmic Ca2+differentially regulates the mRNA level of nuclear and mitochondrial genes. The decline in COX II and III mRNA may be mediated by Tfam, because A-23187 modestly reduced Tfam levels by 48 h. A-23187 induced time-dependent increases in Egr-1 mRNA, along with the activation of ERK1/2 and AMP-activated protein kinase. MEK inhibition with PD-98059 attenuated the increase in Egr-1 mRNA. A-23187 also increased Egr-1, serum response factor, and Sp1 protein expression, transcription factors implicated in mitochondrial biogenesis. Egr-1 overexpression increased nuclear-encoded cytochrome c transcriptional activation by 1.5-fold ( P < 0.05) and reduced GDH mRNA by 37% ( P < 0.05) but had no effect on MDH or β-subunit F1ATPase mRNA. These results indicate that changes in intracellular Ca2+can modify mitochondrial phenotype, in part via the involvement of Egr-1.

Osteoclast inhibitory lectin, a family of new osteoclast inhibitors

We have identified two novel type II membrane-bound C-lectins, designated mOCILrP1 and mOCILrP2, of 218 and 217 amino acids, respectively, that share substantial identity with the murine osteoclast inhibitory lectin (OCIL). The extracellular domains of mOCILrP1 and mOCILrP2 share 83 and 75% identity, respectively, with the extracellular domain of mOCIL. When the extracellular domains were expressed as recombinant proteins, each inhibited osteoclast formation in murine bone marrow cultures treated with M-CSF and RANKL with similar potencies to mOCIL (IC(50) of 0.2 ng/ml). Distinct but highly related genes encoded the three OCIL family members, with mOCIL and mOCILrP2 controlled by an inverted TATA promoter, and mOCILrP1 by a TTAAAA promoter. However only mOCIL was robustly regulated by calciotropic agents, while mOCILrP1 was not expressed, and mOCILrP2 was constitutively expressed in osteoblasts. Immunohistochemistry using antipeptide antibodies to the intracellular domain of mOCILrP1/mOCILrP2 and to mOCIL demonstrated that mOCIL and mOCILrP1/mOCILrP2 were concordantly expressed in osteoblasts, chondrocytes, and in extraskeletal tissues. Further, their cellular distribution was identical to that of RANKL. The identification of three distinct genes that were functionally related implies redundancy for OCIL, and their concordant expression with that of RANKL suggests that the RANKL:OPG axis may be further influenced by OCIL family members.

Imbalanced expression of glutamate-glutamine cycle enzymes induced by human T-cell lymphotropic virus type 1 Tax protein in cultivated astrocytes

Human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent involved in the disease HTLV-1-associated myelopathy, or tropical spastic paraparesis (HAM/TSP). The pathogenesis of HAM/TSP is poorly understood, but it is probable that viral infection has an indirect, deleterious effect on neural function. In this regard, dysfunction in astrocytes may be severely detrimental, as they supply neurons with metabolic precursors, control the extracellular levels of ion and excitatory neurotransmitters, and are electrically coupled with oligodendrocytes. In a model in vitro, we demonstrate that HTLV-1 induces an imbalance in the expression of two astrocyte enzymes, at both the transcriptional and translational levels. In both human astrocyte precursors and rat glial cells, the levels of expression of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were increased and decreased, respectively, after coculture with HTLV-1 T cells. The enhancement of GS expression may result from the action of the protein Tax, which is demonstrated to transactivate the GS gene promoter, while the decreased expression of GDH seems to reflect some compensatory mechanism in response to GS induction. GS and GDH are involved in the conversion of glutamate into glutamine or alpha-ketoglutarate, which then acts as a precursor for glutamatergic and gamma-aminobutyric acid (GABA)-ergic neurons. Metabolism in astrocytes altered by Tax protein may lead to deleterious effects if it modifies the extracellular levels of glutamine, glutamate, and GABA and thus modulates neuronal excitability and osmotic equilibrium in the central nervous system of HTLV-1-infected patients.

Cytokines are increased in the rat hippocampus after serotonergic neuron degeneration and upregulate the expression of GDH, an enzyme involved in glutamate detoxification

The degeneration of serotonergic neurons increases the expression of glutamate dehydrogenase (GDH) in hippocampal astrocytes. This process was demonstrated to be independent of the serotonin level. At the same time, upregulation of tumor necrosis factor (TNF) alpha and interleukin (IL)-1 alpha mRNA were observed, whereas levels of transforming growth factor (TGF) beta 1 mRNA remained unchanged. The level of GDH mRNA was increased in primary cultures of hippocampal astrocytes treated with TNF alpha and IL-1 alpha suggesting that these cytokines act on the GDH metabolism. TNF alpha and IL-1 alpha induced an increase in GDH promoter activity in C8S (an astrocytic cell line) transfected with constructs containing 5' flanking genomic sequences of GDH driving the expression of a reporter gene. These observations suggest that cytokines may be signals that upregulate the astrocytic GDH expression in response to the degeneration of serotonergic terminals in the hippocampus.

Glucocorticoid upregulation of glutamate dehydrogenase gene expression in vitro in astrocytes

Glutamate, the major excitatory neurotransmitter, is preferentially catabolized in astrocytes by glutamate dehydrogenase (GDH). Treatment of an astrocytic cell line with hydrocortisone (10(-5) M) resulted in increased expression of GDH mRNA. Transfection of the cells with truncated parts of the GDH promoter showed that genomic responsive elements activated by hydrocortisone are localized in the -557/+1 region of the promoter. This control of GDH expression by glucocorticoids may be involved in their protective effect against glutamate excitotoxicity.

Regulation of glutamate dehydrogenase expression in the developing rat liver: control at different levels in the prenatal period

To study the regulation of the expression of glutamate dehydrogenase (Glu-DH) in rat liver during development, the Glu-DH mRNA concentration in the liver of rats ranging in age from 14 days prenatal development to 3 months after birth was determined. This concentration increased up to two days before birth, decreased rapidly between two days before and one day after birth and increased again in the second and third postnatal week. The ratio of Glu-DH mRNA/protein decreased more than 10-fold in the prenatal period, whereas it did not change significantly after birth. Thus, whereas the ratio between the Glu-DH monomer protein molecules and Glu-DH mRNA molecules is found to be approximately 1400 at 14 days of prenatal development, it is approximately 1700 four weeks after birth. We argue than an increase in the translational efficiency after birth is the most likely cause of the observed developmental changes in Glu-DH mRNA/protein ratio. Our results suggest that the expression after birth is predominantly regulated at the pretranslational level, whereas the prenatal Glu-DH expression is regulated both at the translational level and at the pretranslational level.

Correlation of intron-exon organisation with the three-dimensional structure in glutamate dehydrogenase

The positions of the intron-exon boundaries in the genes for glutamate dehydrogenase from Chlorella sorokiniana rat, and human have been located on the three-dimensional structure of the highly homologous enzyme from Clostridium symbiosum and analysed for their position in the protein structure. This analysis shows no correlation between the positions of these boundaries in the mammalian and Chlorella glutamate dehydrogenase genes and no correlation with units of function in the enzyme and suggests that the present day exons do not represent the protein modules of an ancestral glutamate dehydrogenase. There appears to be no clear preference for the residues at the splice junctions to be either buried or exposed to solvent. However, the frequency with which the introns appear in the loops linking elements of secondary structure, rather than in either the alpha-helical or beta-sheet segments, is higher than predicted on the basis of the proportion of residues in the loops. This is consistent with but not proof of a role for exon modification/exchange in protein evolution since changes at these positions are less likely to disturb the structure and hence maintain function.

A genetic linkage map of the laboratory rat, Rattus norvegicus

We report the construction of the first complete genetic linkage map of the laboratory rat. By testing 1171 simple sequence length polymorphisms (SSLPs), we have identified 432 markers that show polymorphisms between the SHR and BN rat strains and mapped them in a single (SHR x BN) F2 intercross. The loci define 21 large linkage groups corresponding to the 21 rat chromosomes, together with a pair of nearby markers on chromosome 9 that are not linked to the rest of the map. Because 99.5% of the markers fall into one of the 21 large linkage groups, the maps appear to cover the vast majority of the rat genome. The availability of the map should facilitate whole genome scans for genes underlying qualitative and quantitative traits relevant to mammalian physiology and pathobiology.

Identification and analysis of a matrix-attachment region 5' of the rat glutamate-dehydrogenase-encoding gene

Eukaryotic chromatin is thought to be organized into independently regulated loop domains by interaction of matrix-attachment regions (MAR) of the DNA to the nuclear matrix. To define the borders of the chromatin loop containing the glutamate dehydrogenase (GDH) gene, we screened the GDH gene and flanking regions for the presence of MAR sequences. We here report identification, mapping and sequencing of an (A + T)-rich MAR located 2010-1397 bp upstream of the transcription initiation site of GDH, that mediates strong binding to the nuclear matrix. Smaller regions can also confer binding capacity, although at a lower affinity. This (A + T)-rich MAR contained 11 bp and 12 bp (A + T)-rich direct repeats, but not any of the sequences previously described to be associated with MAR activity. We here show that the presence of (A + T)-rich domains of DNA is not sufficient to confer binding capacity, since (A + T)-rich sequences located downstream of the identified MAR did not bind to the nuclear matrix. Moreover, a consensus topoisomerase-II-binding site located downstream of the MAR was found to be insufficient to mediate substantial binding. The number of binding sites in the nuclear matrix for MAR-containing fragments was shown to be approximately 15,000/nucleus. Since organization of the entire rat genome in loops with an average loop size of 100 kbp would require 60,000 binding sites, this suggests that only part of the genome is organized in loops. Alternatively, we might have underestimated the number of binding sites. The GDH MAR, and MAR-containing fragments derived from other species, were found to bind to the same binding sites in the nuclear matrix, although the affinity varied.