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

Novel related cDNAs (C184L, C184M, and C184S) from developing mouse brain encoding two apparently unrelated proteins

Three related cDNAs (C184L, C184M, and C184S) were isolated from a developing mouse brain cDNA library. C184S is the 5'-end portion and C184M is the 3'-end portion, respectively, of C184L. C184S and C184M have open reading frames of 199 amino acids (ORF1) and 189 amino acids (ORF2), respectively; C184L has both ORF1 and ORF2 (dicistronic structure), but seems to translate only ORF1. Southern blot analysis suggests that all of the three related mRNAs are transcribed from the same single gene. The intervening region of C184L cDNA between ORF1 and ORF2 contained a promoter sequence for C184M mRNA, which is transcribed from the corresponding genomic sequence. Very recently, a cDNA encoding human homologue of ORF1 (human autoantigen p27) and a cDNA encoding a different mouse isoform of ORF2 (mammary tumor virus receptor) were reported. Our results indicate that the mRNAs encoding these apparently unrelated proteins are transcribed within an adjacent or overlapping area on the genome, suggesting the same origin of the two transcription units.

The two subunits of human molybdopterin synthase: evidence for a bicistronic messenger RNA with overlapping reading frames

Molybdoenzymes are ubiquitous and require a prosthetic group called the molybdenum cofactor for activity. We provide evidence here that the two heteromeric subunits (MOCO1-A and MOCO1-B) of human molybdopterin synthase, which is involved in the conversion of precursor Z to molybdopterin in the molybdenum cofactor biosynthetic pathway, are spe-cified by a single bicistronic mRNA with overlapping reading frames. The transcript is in low abundance and shows variable tissue distribution. We propose that leaky scanning of the first translational initiation codon for MOCO1-A by 40S ribosomal subunits occurs, allowing recognition of the AUG for the downstream MOCO1-B reading frame. Such a genetic arrangement may result in a constant ratio and close proximity of lowly expressed enzyme subunits which should, a priori, be especially advantageous for assembly in complex mammalian cells. The MOCO1 locus resides on human chromosome 5.

Taurine, glutamate and GABA modulate the outgrowth from goldfish retinal explants and its concentrations are affected by the crush of the optic nerve

The amino acid taurine plays an important trophic role during development and regeneration of the central nervous system. Other amino acid systems, such as those for glutamate and gamma-aminobutyric acid (GABA), are modified during the same physiological and pathological processes. After crushing the optic nerve, goldfish retinal explants were plated in the absence and in the presence of different amino acids and amino acid receptor agonists. The length and the density of the neurites were measured at 5 days in culture. Taurine increased the length and the density of neurites. Glutamate and glycine increased them at low concentration, but were inhibitors at higher concentration. The combination of N-methyl-D-aspartate (NMDA) and glycine produced a greater inhibitory effect than NMDA alone. NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) added simultaneously with taurine impaired the stimulatory effect of the latter. GABA stimulated the emission of neurites in a concentration dependent manner. Hypotaurine also elevated the length of neurites, but cysteinsesulfinic acid did not produce a significant effect. The concentrations of taurine, glutamate and GABA were determined by HPLC with fluorescent detection in the retina of goldfish at various days post-crushing the optic nerve. The levels of taurine were significantly increased at 48h after the crush, and were elevated up to 20 days. Glutamate level decreased after the lesion of the optic nerve and was still low at 20 days. GABA concentration was not significantly different from the control. The interaction of these amino acids during the regenerative period, especially the balance between taurine and glutamate, may be a determinant in restoring vision after the crush.

Mutations in a polycistronic nuclear gene associated with molybdenum cofactor deficiency

All molybdoenzymes other than nitrogenase require molybdopterin as a metal-binding cofactor. Several genes necessary for the synthesis of the molybdenum cofactor (MoCo) have been characterized in bacteria and plants. The proteins encoded by the Escherichia coli genes moaA and moaC catalyse the first steps in MoCo synthesis. The human homologues of these genes are therefore candidate genes for molybdenum cofactor deficiency, a rare and fatal disease. Using oligonucleotides complementary to a conserved region in the moaA gene, we have isolated a human cDNA derived from liver mRNA. This transcript contains an open reading frame (ORF) encoding the human moaA homologue and a second ORF encoding a human moaC homologue. Mutations can be found in the majority of MoCo-deficient patients that confirm the functional role of both ORFs in the corresponding gene MOCS1 (for 'molybdenum cofactor synthesis-step 1'). Northern-blot analysis detected only full-length transcripts containing both consecutive ORFs in various human tissues. The mRNA structure suggests a translation reinitiation mechanism for the second ORF. These data indicate the existence of a eukaryotic mRNA, which as a single and uniform transcript guides the synthesis of two different enzymatic polypeptides with disease-causing potential.

Gene clusters and polycistronic transcription in eukaryotes

Sometimes genes are arranged nonrandomly on the chromosomes of eukaryotes. This review considers instances of gene clusters in which two genes or more are expressed from a single promoter. This includes cases in which a polycistronic pre-mRNA is processed to make monocistronic mRNAs in nematodes, as well as isolated examples of polycistronic mRNAs found in mammals, flies, and perhaps plants.

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.

Epilepsy with myoclonus and post-natal development of the motor system in humans: a hypothesis

Epilepsy with myoclonus is thought to be linked to the motor system. At birth, development of the central nervous system in humans is far from being achieved. Post-natal changes take place at different levels in this neuronal system. These modifications suggest that the motor cortex is a highly dynamic structure during post-natal development. They may account for the age-dependence of various epileptic syndromes. (1) The number of synapses increases during the early post-natal years and then decreases to reach the adult level around puberty. (2) Neurons differentiate and synthesize various neurotransmitters. (3) Dendrites grow actively and participate in the formation of local cortical circuits. (4) Electrophysiological properties of cortical neurons change during the first months of rodent development. This could reflect modifications of the ion channels present in the cell membrane. (5) The pyramidal tract myelinate and exuberant collaterals are selectively removed. These two processes are dependent on neuronal electrical activity. It has been demonstrated that selective collateral stabilization is promoted by glutamate release and stimulation of the N-methyl-D-aspartate (NMDA) receptor. So, seizures occurring during the neonatal period may interact with these normal developmental features. Furthermore, neuronal electrical activity and seizures stimulate the transcription of specific messenger RNAs coding for neurotrophic factors like nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF). The overproduction of neurotrophic factors leads to maldevelopment of the cortex.

Temporal and spatial expression of glutamic acid decarboxylases in human fetal brain

The expression of two isoforms of glutamic acid decarboxylase, GAD67 and GAD65, was analyzed in central nervous system (CNS) tissues obtained from normal second trimester human fetuses after elective termination of pregnancy. After RT-PCR amplification of sequences contained in total RNA extracts, Southern blotting indicated that GAD67 and GAD65 mRNAs can be detected in frontal pole tissue as early as the 12th week of gestation (12 GW). GAD67 message is strongly expressed during early second trimester and decreases slightly thereafter but remains abundant. In contrast, GAD65 message decreases rapidly and becomes undetectable by the 19 GW. However, GAD67 and GAD65 are similar in their spatial expression in the CNS at 22 GW. GAD67 and GAD65 messages are highly expressed in the cerebellum but expressed in low levels, if at all, in the spinal cord during this gestational period. These results suggest that GAD67 may have a greater role in neuron differentiation than GAD65 during human brain development.

GABAA receptor subunit expression and assembly in cultured rat cerebellar granule neurons

The assembly of multisubunit GABAA receptors in specific neuronal populations is a complex process which is poorly understood. To begin to examine receptor assembly, alpha 1, beta 2/3, and gamma 2 subunit polypeptide expression and association, as well as receptor binding, were examined in cultured rat cerebellar granule neurons. Western blots revealed two alpha 1-immunoreactive proteins. A 39 kDa species was maximal at 2 days in culture and subsequently declined. In contrast, a 51 kDa polypeptide, the anticipated size of the mature alpha 1 subunit, was first detected at 4 days and increased throughout the culture period. Additional studies demonstrated that the beta 2/3 and gamma 2 subunits were detectable at 2 days and attained maximal levels by 6 days. The level of [3H]Ro15-1788 binding, a measure of assembled receptors, rose in parallel with the increases in the 51 kDa alpha 1, beta 2/3 and gamma 2 subunits. Moreover, the 51 kDa alpha 1, beta 2/3, and gamma 2 subunits were associated in receptor complexes. However, immunohistochemical studies demonstrated the presence of substantial intracellular subunit staining. This finding suggest that only some of the subunits expressed in granule neurons contribute to functional GABAA receptors on the cell surface.

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.

Prominent expression of two forms of glutamate decarboxylase in the embryonic and early postnatal rat hippocampal formation

Immunohistochemical methods were used to determine the earliest times of detection for two forms of glutamate decarboxylase (GAD67 and GAD65) in the embryonic and early postnatal rat hippocampal formation and to determine whether their distribution patterns differed from each other and from those of the adult. Both GAD67- and GAD65-containing neurons were observed as early as embryonic day 17 (E17)-E18 in the hippocampus and E19 in the dentate gyrus, and this was substantially earlier than GAD had been detected previously in the hippocampal formation. The two GAD isoforms displayed very similar distribution patterns, but these patterns were distinctly different from those of the adult. From E17 to E20, GAD67 and GAD65 were expressed in neuronal cell bodies throughout the hippocampal and dentate marginal zones (future dendritic layers), and relatively few existed within the principal cell body layers, where GAD-positive neurons are frequently concentrated in the adult. At E21 to postnatal day 1 (P1), there was a sudden shift from a predominance of GAD-containing cell bodies within the developing dendritic regions to a meshwork of GAD-positive processes with terminal-like varicosities in these same regions. This pattern also contrasted with that of the adult, in which GAD-labeled terminals are highly concentrated in the principal cell layers. Electron microscopic observations of the GAD-labeled processes at P1 confirmed their axon-like appearance and demonstrated that the immunoreactivity was consistently localized in vesicle-filled regions that were often closely apposed to and, in some instances, established synaptic contacts with dendritic profiles. The present identification of an early abundance of GAD-containing structures in the hippocampal formation and the marked change in their distribution during development complement recent observations of developmental changes in the functioning of the GABA system and provide additional support for the early involvement of this neurotransmitter system in hippocampal development.

Tissue-specific alternative splicing generates two synaptojanin isoforms with differential membrane binding properties

Synaptojanin is an Src homology 3 domain-binding inositol 5-phosphatase that is thought to function in synaptic vesicle endocytosis. It is encoded by a cDNA with two open reading frames separated by an in-frame stop codon. The first open reading frame encodes a 145-kDa form of the protein, whereas a 170-kDa isoform appears to be composed of both open reading frames and contains additional Src homology 3 domain-binding consensus sequences. Here, we demonstrate that the two synaptojanin isoforms are generated by the alternative use of an exon containing the stop codon. Whereas the 145-kDa isoform is highly enriched in adult brain, the 170-kDa isoform is excluded from this tissue and has a widespread distribution in non-neuronal cells. Unlike the 145-kDa isoform, which can be removed from membranes by a low salt wash, the 170-kDa isoform remains membrane-associated, even in the presence of 1 salt. Further, the 170-kDa form, but not the 145-kDa form, can be isolated from membranes as part of a large molecular weight complex. These properties may allow the 170-kDa isoform of synaptojanin to play a unique and perhaps more general role in endocytosis as compared with the 145-kDa isoform.

Two glutamate decarboxylase forms corresponding to the mammalian GAD65 and GAD67 are expressed during development of the chick telencephalon

The gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamate decarboxylase (GAD) was studied during development of the chick telencephalon. By means of reverse-phase HPLC analysis, we showed that GABA indeed accumulates during embryogenesis, whereas the levels of glutamate, the substrate for GAD, are more or less unchanged up to later developmental stages. The enzyme activity increased approximately 25-fold from embryonic day 3 to embryonic day 17. Immunoblotting data revealed that two GAD proteins, of approximately 65 and 67 kDa, were present during the period investigated. Furthermore, Northern blot analysis with probes obtained from rat cDNA sequences, as well as a chicken-specific probe for GAD65 generated by means of reverse transcriptase-polymerase chain reaction (RT-PCR), strengthened the interpretation that the chick embryo expresses genes corresponding to GAD65 and GAD67. The rat probes recognized transcript sizes of 3.9 kb (GAD65) and 5.6 kb (GAD67), sizes which are different from those of the rat brain (Erlander et al., Neuron, 7, 91-100, 1991). Sequencing of the RT-PCR products revealed a high level of homology (82% at the nucleotide level) between the mammalian and chick GAD65 genes. Taken together, these findings suggest that the chick embryo expresses two GAD genes during embryogenesis. The functional properties of each gene product remain to be investigated.

Basal expression and induction of glutamate decarboxylase and GABA in excitatory granule cells of the rat and monkey hippocampal dentate gyrus

The excitatory, glutamatergic granule cells of the hippocampal dentate gyrus are presumed to play central roles in normal learning and memory, and in the genesis of spontaneous seizure discharges that originate within the temporal lobe. In localizing the two GABA-producing forms of glutamate decarboxylase (GAD65 and GAD67) in the normal hippocampus as a prelude to experimental epilepsy studies, we unexpectedly discovered that, in addition to its presence in hippocampal nonprincipal cells, GAD67-like immunoreactivity (LI) was present in the excitatory axons (the mossy fibers) of normal dentate granule cells of rats, mice, and the monkey Macaca nemestrina. Using improved immunocytochemical methods, we were also able to detect GABA-LI in normal granule cell somata and processes. Conversely, GAD65-LI was undetectable in normal granule cells. Perforant pathway stimulation for 24 hours, which evoked population spikes and epileptiform discharges in both dentate granule cells and hippocampal pyramidal neurons, induced GAD65-, GAD67-, and GABA-LI only in granule cells. Despite prolonged excitation, normally GAD- and GABA-negative dentate hilar neurons and hippocampal pyramidal cells remained immunonegative. Induced granule cell GAD65-, GAD67-, and GABA-LI remained elevated above control immunoreactivity for at least 4 days after the end of stimulation. Pre-embedding immunocytochemical electron microscopy confirmed that GAD67- and GABA-LI were induced selectively within granule cells; granule cell layer glia and endothelial cells were GAD- and GABA-immunonegative. In situ hybridization after stimulation revealed a similarly selective induction of GAD65 and GAD67 mRNA in dentate granule cells. Neurochemical analysis of the microdissected dentate gyrus and area CA1 determined whether changes in GAD- and GABA-LI reflect changes in the concentrations of chemically identified GAD and GABA. Stimulation for 24 hours increased GAD67 and GABA concentrations sixfold in the dentate gyrus, and decreased the concentrations of the GABA precursors glutamate and glutamine. No significant change in GAD65 concentration was detected in the microdissected dentate gyrus despite the induction of GAD65-LI. The concentrations of GAD65, GAD67, GABA, glutamate and glutamine in area CA1 were not significantly different from control concentrations. These results indicate that dentate granule cells normally contain two "fast-acting" amino acid neurotransmitters, one excitatory and one inhibitory, and may therefore produce both excitatory and inhibitory effects. Although the physiological role of granule cell GABA is unknown, the discovery of both basal and activity-dependent GAD and GABA expression in glutamatergic dentate granule cells may have fundamental implications for physiological plasticity presumed to underlie normal learning and memory. Furthermore, the induction of granule cell GAD and GABA by afferent excitation may constitute a mechanism by which epileptic seizures trigger compensatory interictal network inhibition or GABA-mediated neurotrophic effects.

A presynaptic inositol-5-phosphatase

Synaptojanin is a nerve terminal protein of relative molecular mass 145,000 which appears to participate with dynamin in synaptic vesicle recycling. The central region of synaptojanin defines it as a member of the inositol-5-phosphatase family, which includes the product of the gene that is defective in the oculocerebrorenal syndrome of Lowe. Synaptojanin has 5-phosphatase activity and its amino-terminal domain is homologous with the yeast protein Sac1 (Rsd1), which is genetically implicated in phospholipid metabolism and in the function of the actin cytoskeleton. The carboxy terminus, which is of different lengths in adult and developing neurons owing to the alternative use of two termination sites, is proline-rich, consistent with the reported interaction of synaptojanin with the SH3 domains of Grb2 (refs 1, 2). Synaptojanin is the only other major brain protein besides dynamin that binds the SH3 domain of amphiphysin, a presynaptic protein with a putative function in endocytosis. Our results suggest a link between phosphoinositide metabolism and synaptic vesicle recycling.

Transient expression of glutamate decarboxylase and gamma-amino butyric acid in embryonic lens fibers of the rat

We have determined the localization and developmental expression of glutamate decarboxylase (GAD67) in the rat lens. Immunofluorescence experiments showed that GAD67 was transiently expressed in the nuclear fiber cells of the lens between embryonic days (E) 15 and 20, with maximal immunostaining occurring on E17 and E18. gamma-amino butyric acid (GABA) co-localized with GAD67 in the embryonic nuclear fiber cells. Reverse transcription-polymerase chain reaction (RT-PCR) tests showed that at least three alternatively spliced forms of GAD67 mRNA, including mRNAs with and without the I80 and the I86 insert, were transiently co-expressed with GAD67 in the embryonic lens. The major GAD67 protein in the lens was 67 kDa. We conclude that enzymatically active GAD67 is transiently expressed in the lens nuclear fiber cells of the embryonic rat. The transient expression is regulated by transcriptional and/or posttranscriptional processes. We speculate on the basis of possible common gene regulatory elements for glutamate and ornithine decarboxylases and the involvement of these enzymes with polyamine synthesis, that the transient expression of GAD67 may be connected to nuclear and/or DNA breakdown during lens fiber cell differentiation.