Species and tissue specificity of mammalian GTP cyclohydrolase I messenger RNA

The protein partners of GTP cyclohydrolase I in rat organs

Objective

GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme for tetrahydrobiopterin biosynthesis and has been shown to be a promising therapeutic target in ischemic heart disease, hypertension, atherosclerosis and diabetes. The endogenous GCH1-interacting partners have not been identified. Here, we determined endogenous GCH1-interacting proteins in rat.

Methods and Results

A pulldown and proteomics approach were used to identify GCH1 interacting proteins in rat liver, brain, heart and kidney. We demonstrated that GCH1 interacts with at least 17 proteins including GTP cyclohydrolase I feedback regulatory protein (GFRP) in rat liver by affinity purification followed by proteomics and validated six protein partners in liver, brain, heart and kidney by immunoblotting. GCH1 interacts with GFRP and very long-chain specific acyl-CoA dehydrogenase in the liver, tubulin beta-2A chain in the liver and brain, DnaJ homolog subfamily A member 1 and fatty aldehyde dehydrogenase in the liver, heart and kidney and eukaryotic translation initiation factor 3 subunit I (EIF3I) in all organs tested. Furthermore, GCH1 associates with mitochondrial proteins and GCH1 itself locates in mitochondria.

Conclusion

GCH1 interacts with proteins in an organ dependant manner and EIF3I might be a general regulator of GCH1. Our finding indicates GCH1 might have broader functions beyond tetrahydrobiopterin biosynthesis.

The eNOS cofactor tetrahydrobiopterin improves endothelial dysfunction in livers of rats with CCl4 cirrhosis

In cirrhosis, intrahepatic endothelial dysfunction is one of the mechanisms involved in the increased resistance to portal blood flow and therefore in the development of portal hypertension. Endothelial nitric oxide synthase (eNOS) uncoupling due to deficiency of tetrahydrobiopterin (BH4) results in decreased production of NO and plays a major role in endothelial dysfunction in other conditions. We examined whether eNOS uncoupling is involved in the pathogenesis of endothelial dysfunction of livers with cirrhosis. Basal levels of tetrahydrobiopterin and guanosine triphosphate (GTP)-cyclohydrolase (BH4 rate-limiting enzyme) expression and activity were determined in liver homogenates of control and rats with CCl4 cirrhosis. Thereafter, rats were treated with tetrahydrobiopterin, and eNOS activity, NO bioavailability, assessed with a functional assay, and the vasodilator response to acetylcholine (endothelial function) were evaluated. Livers with cirrhosis showed reduced BH4 levels and decreased GTP-cyclohydrolase activity and expression, which were associated with impaired vasorelaxation to acetylcholine. Tetrahydrobiopterin supplementation increased BH4 hepatic levels and eNOS activity and significantly improved the vasodilator response to acetylcholine in rats with cirrhosis. In conclusion, the impaired response to acetylcholine of livers with cirrhosis is modulated by a reduced availability of the eNOS cofactor, tetrahydrobiopterin. Tetrahydrobiopterin supplementation improved the endothelial dysfunction of cirrhotic livers.

A potential mechanism for the impairment of nitric oxide formation caused by prolonged oral exposure to arsenate in rabbits

We have recently found evidence for impairment of nitric oxide (NO) formation and induction of oxidative stress in residents of an endemic area of chronic arsenic poisoning in Inner Mongolia, China. To investigate the underlying mechanisms responsible for these phenomena, a subchronic animal experiment was conducted using male New Zealand White rabbits. After 18 weeks of continuous exposure of rabbits to 5 mg/l of arsenate in drinking water, a significant decrease in systemic NO production occurred, as shown by significantly reduced plasma NO metabolites levels (76% of control) and a tendency towards decreased serum cGMP levels (81.4% of control). On the other hand, increased oxidative stress, as shown by significantly increased urinary hydrogen peroxide (H(2)O(2)) (120% of control), was observed in arsenate-exposed rabbits. In additional experiments measuring aortic tension, the addition of either the calcium ionophore A23187 or acethylcholine (ACh) induced a transient vasoconstriction of aortic rings prepared from arsenate-exposed rabbits, but not in those prepared from control animals. This calcium-dependent contractility action observed in aorta rings from arsenate-exposed rabbits was markedly attenuated by the superoxide (O2(.-)) scavenging enzyme Cu, Zn-SOD, as well as diphenyleneiodonium (DPI) or N(G)-nitro-L-arginine methyl ester (L-NAME), which are inhibitors for nitric oxide synthase (NOS). However, the cyclooxygenase inhibitor indomethacin or the xanthine oxidase blocker allopurinol had no effect on this vasoconstriction. These results suggest that arsenate-mediated reduction of systemic NO may be associated with the enzymatic uncoupling reaction of NOS with a subsequent enhancement of reactive oxygen species such as O2(.-), an endothelium-derived vasoconstricting factor. Furthermore, hepatic levels of (6R)-5,6,7,8-tetrahydro-L-biopterin (BH(4)), a cofactor for NOS, were markedly reduced in arsenate-exposed rabbits to 62% of control, while no significant change occurred in cardiac L-arginine levels. These results suggest that prolonged exposure of rabbits to oral arsenate may impair the bioavailability of BH(4) in endothelial cells and, as a consequence, disrupt the balance between NO and O2(.-) produced from endothelial NOS, such that enhanced free radicals are produced at the expense of NO.

Hormonal control of GTP cyclohydrolase I gene expression and enzyme activity during color pattern development in wings of Precis coenia

Color patterns of butterfly wings are composed of single color points represented by each scale. In the case of Precis coenia, at the end of pupal development, different types of pigments are synthesized sequentially in the differently colored scales beginning with white (pterins) followed by red (ommatins) and then black (melanin). In order to explain how formation of these different colors is regulated, we examined the expression of an mRNA-encoding guanosine triphosphate-cyclohydrolase I (GTP-CH I; EC 3.5.4.16), the first key enzyme in the biosynthesis of pteridines, during pigment formation in the wings of P. coenia. The strongest positive signal was recognized around pigment formation one day before butterfly emergence. This GTP-CH I gene expression is paralleled by GTP-CH I enzyme activity measured in wing extracts. We also investigated the effect of 20-hydroxyecdysone on the expression of GTP-CH I mRNA and the enzyme activity during color formation. The results strongly suggest that the onset and duration of the expression of a GTP-CH I mRNA is triggered by a declining ecdysteroid hormone titer during late pupal development.

cAMP inhibits cytokine-induced biosynthesis of tetrahydrobiopterin in human umbilical vein endothelial cells

We studied the effects of cAMP on cytokine (interferon-gamma plus tumor necrosis factor-alpha)-induced stimulation of tetrahydrobiopterin (BH4) synthesis in human umbilical vein endothelial cells (HUVEC). The cytokine mixture caused a marked increase in the biosynthesis and release of BH4 by HUVEC. Dibutyryl-cAMP produced a dose-dependent inhibition of this cytokine-induced stimulation of synthesis and release of BH4 by these cells. 8-Bromo-cAMP also caused a significant inhibition, although the effects were less marked than those of dibutyryl-cAMP. Both forskolin and the stable analog of prostacyclin, iloprost, caused cAMP accumulation and a concomitant diminution of the cytokine-induced BH4 synthesis in HUVEC. Dibutyryl-cAMP and iloprost also significantly inhibited the cytokine-induced stimulation of GTP cyclohydrolase I (GCHI) activity and mRNA production. We concluded that the suppression by the cAMP messenger system of cytokine-induced stimulation of synthesis and release of BH4 by HUVEC can be attributed to the inhibition of the activity of GCHI, the rate-limiting enzyme in BH4 biosynthetic pathway, in HUVEC. The data also suggest that the cAMP-mediated reduction in the GCHI mRNA level may at least partially explain the decline in GCHI activity. It is reasoned that under inflammatory conditions, cAMP-elevating agents such as prostacyclin exert regulatory effects on circulation by inhibiting cytokine-induced synthesis and release of BH4 by HUVEC.

GTP cyclohydrolase I mRNA: novel splice variants in the slime mould Physarum polycephalum and in human monocytes (THP-1) indicate conservation of mRNA processing

GTP cyclohydrolase I (EC 3.5.4.16) is the first enzyme in the biosynthesis of tetrahydrobiopterin [(6R)-5,6,7,8-tetrahydro-L-biopterin, H(4)-biopterin] in mammals and of folic acid in bacteria. Here we have characterized the GTP cyclohydrolase I gene structure and two mRNA species from Physarum polycephalum, an acellular slime mould that synthesizes H(4)-biopterin and metabolites of the folic acid biosynthetic pathway. Its GTP cyclohydrolase I gene consists of seven exons, and the two GTP cyclohydrolase I cDNA species isolated from Physarum encode for proteins with 228 (25.7 kDa) and 195 (22.1 kDa) amino acids. Furthermore, we identified two previously undescribed mRNA species in interferon-gamma-treated human myelomonocytoma cells (THP-1) in addition to the cDNA coding for the fully functional 250-residue (27.9 kDa) protein, which is identical with that in human phaeochromocytoma cells. One of the new splice variants codes for a 233-residue (25.7 kDa) protein, whereas the other codes for the full-length protein but is alternatively spliced within the 3'-untranslated region. In heterologous expression, the shorter proteins of Physarum as well as of THP-1 cells identified here are degraded by proteolysis. Accordingly, only the 27.9 kDa protein was detectable in Western blots from THP-1 cell extracts. Quantification of GTP cyclohydrolase I mRNA species in different human cell types with and without cytokine treatment showed that in addition to the correct mRNA the two splice variants isolated here, as well as the two splice variants known from human liver, are strongly induced by cytokines in cell types with inducible GTP cyclohydrolase I (THP-1, dermal fibroblasts), but not in cell types with constitutive GTP cyclohydrolase I expression (SK-N-SH, Hep-G2). As in human liver, splicing of the new mRNA variant found in THP-1 cells occurs at the boundary of exons 5 and 6. Strikingly, the 195-residue protein from Physarum is alternatively spliced at a homologous position, i.e. at the boundary of exons 6 and 7. Thus alternative splicing of GTP cyclohydrolase I at this position occurs in two species highly distant from each other in terms of evolution. It remains to be seen whether variant proteins encoded by alternatively spliced GTP cyclohydrolase I mRNA transcripts do occur in vivo and whether they participate in regulation of enzyme activity.

Pterin-4alpha-carbinolamine dehydratase in rat brain. I. Patterns of co-localization with tyrosine hydroxylase

The bifunctional protein, PCD/DCoH, is both a pterin-4alpha-carbinolamine dehydratase (PCD) and a dimerization cofactor of the hepatic nuclear factor 1alpha (DCoH). In association with brain tyrosine hydroxylase (TH), which is required for dopamine synthesis, PCD catalyses dehydration and thus recycling of the cofactor tetrahydrobiopterin (BH(4)). PCD immunoreactivity in the catecholaminergic system of the rat brain was studied using a rabbit polyclonal antibody. Double immunofluorescence was performed to establish intracellular co-localization with TH. PCD immunoreactivity was found to be high and consistently present in all the neuron groups expressing TH. More than 90% of the TH+ cells were also expressing PCD. The highest co-expression (99-100% of TH+ cells) was observed in pontine catecholaminergic cell groups including locus coeruleus. Lower co-expression was observed in substantia nigra (17% of TH+ cells without PCD) and particularly in arcuate nucleus (41% of TH+ cells without PCD). Our results argue in favor of a generalized recycling of BH(4) in catecholaminergic neurons except when the neuron terminal field is located outside the blood-brain barrier. The respective roles of synthesis and recycling of BH(4) in the control of TH activity are discussed.

Phosphorylation of GTP cyclohydrolase I and modulation of its activity in rodent mast cells. GTP cyclohydrolase I hyperphosphorylation is coupled to high affinity IgE receptor signaling and involves protein kinase C

GTP cyclohydrolase I controls the de novo pathway for the synthesis of tetrahydrobiopterin, which is the essential cofactor for tryptophan 5-monooxygenase and thus, for serotonin production. In mouse bone marrow-derived mast cells, the kit ligand selectively up-regulates GTP cyclohydrolase I activity (Ziegler, I., Hültner, L. , Egger, D., Kempkes, B., Mailhammer, R., Gillis, S., and Rödl, W. (1993) J. Biol. Chem. 268, 12544-12551). Immunoblot analysis now confirms that this long term enhancement is caused by increased expression of the enzyme. Furthermore we show that GTP cyclohydrolase I is subject to modification at the post-translational level. In vivo labeling with [32P]orthophosphate demonstrates that in primary mast cells and in transfected RBL-2H3 cells overexpressing GTP cyclohydrolase I, the enzyme exists in a phosphorylated form. Antigen binding to the high affinity receptor for IgE triggers an additional and transient phosphorylation of GTP cyclohydrolase I with a concomitant rise in its activity, and in consequence, cellular tetrahydrobiopterin levels increase. These events culminate 8 min after stimulation and can be mimicked by phorbol ester. The hyperphosphorylation is greatly reduced by the protein kinase C inhibitor Ro-31-8220. In vitro phosphorylation studies indicate that GTP cyclohydrolase I is a substrate for both casein kinase II and protein kinase C.

GTP-cyclohydrolase-I like immunoreactivity in rat brain

GTPCH-I immunoreactive structures in the rat brain were studied using a polyclonal antibody raised in the chick. General mapping was made using the avidin-biotin-peroxidase technique and compared with the distribution of tyrosine hydroxylase and serotonin immunoreactivities. Double immunofluorescence was performed in order to establish real intracellular colocalization. GTPCH-I immunoreactivity was generally found to be low. Immunostained neurons were present in all the serotonin cell groups. In catecholaminergic neurons, although tyrosine hydroxylase immunoreactivity was always very high, GTPCH-I immunoreactivity was extremely variable, from relatively strong (substantia nigra, ventral tegmental area) to low (locus coeruleus, caudal part of the hypothalamus), extremely low (rostral hypothalamus, ventral brainstem) or almost absent (dorsal brainstem, some hypothalamic nuclei). When feasible, double immunolabeling revealed that all the serotonin cells and most of the tyrosine hydroxylase cells were also expressing GTPCH-I. Our results argue in favor of a regulation of tyrosine hydroxylase activity by the intracellular synthesis of BH4.

GTP cyclohydrolase I mRNA induction and tetrahydrobiopterin synthesis in human endothelial cells

The key role of tetrahydrobiopterin (BH4) in the synthesis of nitric oxide by human umbilical vein endothelial cells (HUVEC) has been demonstrated. We characterized the induction of BH4 synthesis in a cell line (ECV) derived from HUVEC and primary HUVEC. A significant induction of guanosine triphosphate cyclohydrolase I (GTPCH) mRNA was observed in response to TNF, IL-1beta, and IFNgamma in ECV and HUVEC. The induction of GTPCH mRNA was abolished by actinomycin D. The cytokines led to an increased accumulation of BH4 in ECV. This effect was prevented by 2,4-diamino-6-hydroxypyrimidine, a selective inhibitor of GTPCH, as well as by actinomycin D and by cycloheximide. Results provide evidence for an increase in GTPCH activity and in BH4 levels in response to immunostimulants in human endothelial cells.

Immobilization Stress Elevates GTP Cyclohydrolase I mRNA Levels in Rat Adrenals Predominantly by Hormonally Mediated Mechanisms

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, the cofactor for catecholamine, indolamine and nitric oxide biosynthesis. In this study we examined the effect of immobilization stress on GTPCH mRNA levels and the mechanism(s) of stress-induced changes in adrenomedullary GTPCH mRNA levels. We used reverse-polymerase chain reaction to isolate and clone a cDNA corresponding to nucleotides 269 to 570 of rat GTPCH. Northern blot analysis with a cRNA probe revealed two species of GTPCH mRNA (about 3.6 and 1.2 kb) in rat adrenal medulla and cortex, and in PC12 cells. The levels of both forms of GTPCH mRNA were significantly increased 3-5 fold in adrenal medulla by a single 2 hour immobilization and by repeated immobilizations (2 hours a day for 2 days). Hypophysectomy had little effect on their basal levels but prevented the stress elicited rise in both GTPCH mRNAs. In contrast, unilateral transection of the splanchnic nerve did not affect induction of the 3.6 kb GTPCH mRNA by stress. Combined denervation with hypophysectomy completely blocked the induction of both GTPCH mRNA species by immobilization stress. Thus, stress elicits elevation of both forms of GTPCH mRNA by a mechanism requiring an intact pituitary-adrenocortical axis.

Cloning, sequencing and functional studies of the gene encoding human GTP cyclohydrolase I

We have identified a genomic clone containing the 5' regulatory region of the gene GTP-CH encoding human GTP cyclohydrolase I. The transcription start point (tsp) was mapped by 5'-rapid amplification of cDNA ends (5'-RACE). The 2.6-kb region upstream from the tsp showed promoter activity when ligated upstream from a reporter gene. The truncation of approximately 2 kb of the promoter did not change expression activity, while a further removal of 243 bp halved the activity. The promoter contains CCAAT and TATA boxes. The GC-rich region close to the tsp, which contains several putative Sp1-responsive elements, is required for maximum promoter activity. Interferon-gamma treatment of B-cells transfected with reporter constructs had no influence on the expression activity.

Tetrahydrobiopterin biosynthesis in the rat brain: heterogeneity of GTP cyclohydrolase I mRNA expression in monoamine-containing neurons

GTP cyclohydrolase I is the first and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin. A quantitative in situ hybridization technique was used to study the expression of GTP cyclohydrolase I mRNA in the rat brain at the cellular level. Coronal sections between the diencephalon and myelencephalon were exposed to a 35S-labelled antisense GTP cyclohydrolase I cRNA probe. Sections serial to these were hybridized with a 35S-labelled antisense cRNA probe complementary to tyrosine hydroxylase mRNA. Tyrosine hydroxylase and GTP cyclohydrolase I mRNAs were found to colocalize within catecholamine neurons located throughout the brain. The overall distribution of neurons expressing GTP cyclohydrolase I mRNA was observed to correspond exactly to the known distribution of the dopamine, norepinephrine/epinephrine and serotonin-containing cell groups. Overall, a 30-fold range of GTP cyclohydrolase I mRNA expression was observed, with the transcript being significantly more abundant in serotonin than in dopamine or norepinephrine/epinephrine neurons. Comparisons across serotonin cell groups indicated that neurons of the median raphe nucleus, caudal linear nucleus raphe (B8) and the dorsal raphe (B6/B7) expressed the highest levels of GTP cyclohydrolase I mRNA. Comparisons across dopamine cell groups indicated that the transcript was more abundant in neurons of the ventral tegmental area (A10) than in neurons of the substantia nigra pars compacta (A9) and that both A9 and A10 dopamine neurons exhibited higher levels of expression than the DA neurons of the hypothalamus (A11-A14). Norepinephrine neurons of the locus coeruleus (A6) and subcoeruleus (A6v) exhibited significantly higher levels of GTP cyclohydrolase I mRNA than did neurons in other norepinephrine (A1 and A2) or epinephrine (C1 and C2) cell groups. GTP cyclohydrolase I mRNA could not be detected unequivocally in neurons known to contain nitric oxide synthase. Heterogeneity in the level of expression of GTP cyclohydrolase I mRNA by monoamine-containing neurons may play an important role in determining steady state levels of tetrahydrobiopterin and, ultimately, the regulation of monoamine biosynthesis.

Control of 6-(D-threo-1',2'-dihydroxypropyl) pterin (dictyopterin) synthesis during aggregation of Dictyostelium discoideum. Involvement of the G-protein-linked signalling pathway in the regulation of GTP cyclohydrolase I activity

6-(D-threo-1',2'-Dihydroxypropylpterin (dictyopterin) has been identified in extracts of growing Dictyostelium dicoideum cells [Klein, Thiery and Tatischeff (1990) Eur. J. Biochem. 187, 665-669]. We demonstrate that it originates from GTP by de novo biosynthesis and that the first committed step is catalysed by GTP cyclohydrolase I, yielding dihydroneopterin triphosphate [neopterin is 6-(D-erythro-1',2',3'-trihydroxypropyl) pterin]. The GTP cyclohydrolase I activity is found in the cytosolic fraction and in a membrane-associated form. The level of a 0.9 kb mRNA coding for GTP cyclohydrolase I decreases to about 10% of its initial value within 2 h after Dictyostelium cells start development induced by starvation. In the cytosolic fraction, the specific activities of GTP cyclohydrolase I, as well as the concentrations of (6R/S)-5,6,7,8-tetrahydrodictyopterin (H4dictyopterin), follow this decline of the mRNA level. In the particulate fraction, however, the specific activities of GTP cyclohydrolase I and, in consequence, H4dictyopterin synthesis, transiently increase and reach a maximum after 4-5 h of development. The time-course of H4dictyopterin concentrations in the starvation medium closely correlates with its production in the membrane fraction. The activity of membrane-associated GTP cyclohydrolase I can be increased by pre-incubation of the cell lysate with guanosine 5'-[gamma-thio]triphosphate and Mg2+. This GTP analogue does not serve as a substrate and has no direct effect on the enzyme activity, indicating that a G-protein-linked signalling pathway is involved in the regulation of GTP cyclohydrolase I activity and thus in H4dictyopterin production during early development of D. discoideum.

Molecular cloning of the GTP-cyclohydrolase structural gene RIB1 of Pichia guilliermondii involved in riboflavin biosynthesis

The structural gene of GTP-cyclohydrolase, involved in riboflavin biosynthesis, was cloned from a Pichia guilliermondii genomic library. A 1855 bp genomic DNA fragment complementing the riboflavin auxotrophies of an Escherichia coli ribA mutant, defective in GTP-cyclohydrolase II, and a P. guilliermondii rib1 mutant was isolated and sequenced. An open reading frame with the potential to encode a protein of 344 amino acids with a predicted molecular mass of 38,711 Da was detected. The P. guilliermondii enzyme shows a high degree of homology to GTP-cyclohydrolases type II from E. coli and Baccillus subtilis and to GTP-cyclohydrolase from Saccharomyces cerevisiae. Functional GTP-cyclohydrolase from P. guilliermondii may consist of four identical subunits.

Atomic structure of GTP cyclohydrolase I

BACKGROUND

Tetrahydrobiopterin serves as the cofactor for enzymes involved in neurotransmitter biosynthesis and as regulatory factor in immune cell proliferation and the biosynthesis of melanin. The biosynthetic pathway to tetrahydrobiopterin consists of three steps starting from GTP. The initial reaction is catalyzed by GTP cyclohdrolase I (GTP-CH-I) and involves the chemically complex transformation of the purine into the pterin ring system.

RESULTS

The crystal structure of the Escherichia coli GTP-CH-I was solved by single isomorphous replacement and molecular averaging at 3.0 A resolution. The functional enzyme is a homodecameric complex with D5 symmetry, forming a torus with dimensions 65 A x 100 A. The pentameric subunits are constructed via an unprecedented cyclic arrangement of the four-stranded antiparallel beta-sheets of the five monomers to form a 20-stranded antiparallel beta-barrel of 35 A diameter. Two pentamers are tightly associated by intercalation of two antiparallel helix pairs positioned close to the subunit N termini. The C-terminal domain of the GTP-CH-I monomer is topologically identical to a subunit of the homohexameric 6-pyruvoyl tetrahydropterin synthase, the enzyme catalyzing the second step in tetrahydrobiopterin biosynthesis.

CONCLUSIONS

The active site of GTP-CH-I is located at the interface of three subunits. It represents a novel GTP-binding site, distinct from the one found in G proteins, with a catalytic apparatus that suggest involvement of histidines and, possibly, a cystine in the unusual reaction mechanism. Despite the lack of significant sequence homology between GTP-CH-I and 6-pyruvoyl tetrahydropterin synthase, the two proteins, which catalyze consecutive steps in tetrahydrobiopterin biosynthesis, share a common subunit fold and oligomerization mode. In addition, the active centres have an identical acceptor site for the 2-amino-4-oxo pyrimidine moiety of their substrates which suggests an evolutionarily conserved protein fold designed for pterin biosynthesis.

Characterization of mouse and human GTP cyclohydrolase I genes. Mutations in patients with GTP cyclohydrolase I deficiency

GTP cyclohydrolase I is the first and rate-limiting enzyme for the biosynthesis of tetrahydrobiopterin in mammals. Previously, we reported three species of human GTP cyclohydrolase I cDNA in a human liver cDNA library (Togari, A., Ichinose, H., Matsumoto, S., Fujita, K., and Nagatsu, T. (1992) Biochem. Biophys. Res. Commun. 187, 359-365). Furthermore, very recently, we found that the GTP cyclohydrolase I gene is causative for hereditary progressive dystonia with marked diurnal fluctuation, also known as DOPA-responsive dystonia (Ichinose, H., Ohye, T., Takahashi, E., Seki, N., Hori, T., Segawa, M., Nomura, Y., Endo, K., Tanaka, H., Tsuji, S., Fujita, K., and Nagatsu, T. (1994) Nature Genetics 8, 236-242). To clarify the mechanisms that regulate transcription of the GTP cyclohydrolase I gene and to generate multiple species of mRNA, we isolated genomic DNA clones for the human and mouse GTP cyclohydrolase I genes. Structural analysis of the isolated clones revealed that the GTP cyclohydrolase I gene is encoded by a single copy gene and is composed of six exons spanning approximately 30 kilobases. We sequenced all exon/intron boundaries of the human and mouse genes. Structural analysis also demonstrated that the heterogeneity of GTP cyclohydrolase I mRNA is caused by an alternative usage of the splicing acceptor site at the sixth exon. The transcription start site of the mouse GTP cyclohydrolase I gene and the 5'-flanking sequences of the mouse and human genes were determined. We performed regional mapping of the mouse gene by fluorescence in situ hybridization, and the mouse GTP cyclohydrolase I gene was assigned to region C2-3 of mouse chromosome 14. We identified missense mutations in patients with GTP cyclohydrolase I deficiency and expressed mutated enzymes in Escherichia coli to confirm alterations in the enzyme activity.

Isolation of a full-length cDNA clone for human GTP cyclohydrolase I type 1 from pheochromocytoma

Although the existence of three different cDNA forms of human GTP cyclohydrolase I (GCH I) have been reported (Togari et al., 1992), the full-length sequence of any human GCH I cDNA involving poly (A) tail has not yet been documented. In the present study, we first isolated a full-length cDNA clone encoding human GCH I type 1 from human pheochromocytoma cDNA library. The length of the cDNA insert was 2,921 base pairs including poly (A) tail. RNA blot analysis showed a single mRNA species of 4.0 kb in human pheochromocytoma tissue.

Human GTP cyclohydrolase I: only one out of three cDNA isoforms gives rise to the active enzyme

GTP cyclohydrolase I catalyses the first and rate-limiting step of tetrahydrobiopterin biosynthesis. Its expression is regulated by interferon-gamma or kit ligand in a tissue-specific manner. Three different cDNA forms have been reported for human GTP cyclohydrolase I [Togari, Ichinose, Matsumoto, Fujita and Nagatsu (1992) Biochem. Biophys. Res. Commun. 187, 359-365]. We have isolated, from a human liver cDNA library, two clones which contained inserts identical with two of the cDNAs reported by Togari et al. (1992). The three open reading frames corresponding to all reported cDNA sequences were expressed in Escherichia coli. Only the recombinant protein corresponding to the longest reading frame catalysed the conversion of GTP into dihydroneopterin triphosphate. The proteins corresponding to the shorter reading frames failed to catalyse not only the generation of dihydroneopterin triphosphate but also the release of formate from GTP, an intermediate step of the reaction. Recombinant human GTP cyclohydrolase I showed sigmoidal substrate kinetics and maximum activity at 60 degrees C. These findings are well in line with the published properties of the enzyme isolated from rat liver. The data indicate that cytokine-mediated induction of GTP cyclohydrolase I is not due to the expression of enzyme isoforms.

Induction of GTP-cyclohydrolase I mRNA expression by lectin activation and interferon-gamma treatment in human cells associated with the immune response

The development of tetrahydrobiopterin synthesis during lectin stimulation of resting human T lymphocytes (Kerler et al. [1989] FEBS Lett., 250:622-624), the interferon-gamma induced neopterin production by human monocytes/macrophages (Huber et al. [1984] J. Exp. Med., 160:310-316), and the control of tetrahydrobiopterin synthesis in activated T cells by the synergistic action of interferon-gamma and interleukin 2 (Ziegler et al. [1990] J. Biol. Chem. 265:17026-17030) were previously explained by modulation of the apparent GTP-cyclohydrolase I activation. In this study we demonstrate that increases in GTP-cyclohydrolase I activity which occur after lectin induction and after cytokine treatment correlate with increased steady state mRNA levels specific for this enzyme. The enhancement of interferon-gamma induced enzyme activity in primed T cells by interleukin 2 also corresponds to further increases in mRNA expression. The steady state GTP-cyclohydrolase I mRNA levels in primed T cells, however, do not correlate with the steep decline which follows the culmination of enzyme activity 44 hours after treatment. This indicates that the down-regulation of apparent GTP-cyclohydrolase I activity is caused by posttranslational modification of the protein.