A new mechanism for regulation of tyrosine 3-monooxygenase by end product and cyclic AMP-dependent protein kinase

Boosting endogenous dopamine production: a novel therapeutic approach for Parkinson's disease

Innovative therapeutic strategies are urgently needed for Parkinson's disease due to limited efficacy of current treatments and a weak therapeutic pipeline. In this forum article, we propose targeting tyrosine hydroxylase phosphorylation as a novel mechanism of action to address this critical need.

Degradation of Tyrosine Hydroxylase by the Ubiquitin-Proteasome System in the Pathogenesis of Parkinson's Disease and Dopa-Responsive Dystonia

Nigrostriatal dopaminergic systems govern physiological functions related to locomotion, and their dysfunction leads to movement disorders, such as Parkinson’s disease and dopa-responsive dystonia (Segawa disease). Previous studies revealed that expression of the gene encoding nigrostriatal tyrosine hydroxylase (TH), a rate-limiting enzyme of dopamine biosynthesis, is reduced in Parkinson’s disease and dopa-responsive dystonia; however, the mechanism of TH depletion in these disorders remains unclear. In this article, we review the molecular mechanism underlying the neurodegeneration process in dopamine-containing neurons and focus on the novel degradation pathway of TH through the ubiquitin-proteasome system to advance our understanding of the etiology of Parkinson’s disease and dopa-responsive dystonia. We also introduce the relation of α-synuclein propagation with the loss of TH protein in Parkinson’s disease as well as anticipate therapeutic targets and early diagnosis of these diseases.

Dopamine or biopterin deficiency potentiates phosphorylation at (40)Ser and ubiquitination of tyrosine hydroxylase to be degraded by the ubiquitin proteasome system

The protein amount of tyrosine hydroxylase (TH), that is the rate-limiting enzyme for the biosynthesis of dopamine (DA), should be tightly regulated, whereas its degradation pathway is largely unknown. In this study, we analyzed how the TH protein is chemically modified and subsequently degraded under deficiencies of DA and tetrahydrobiopterin (BH4), a cofactor for TH, by using pharmacological agents in PC12D cells and cultured mesencephalic neurons. When inhibition of DA- or BH4-synthesizing enzymes greatly reduced the DA contents in PC12D cells, a marked and persistent increase in phosphorylated TH at (40)Ser (p40-TH) was concomitantly observed. This phosphorylation was mediated by D2 dopamine auto-receptor and cAMP-dependent protein kinase (PKA). Our immunoprecipitation experiments showed that the increase in the p40-TH level was accompanied with its poly-ubiquitination. Treatment of PC12D cells with cycloheximide showed that total-TH protein level was reduced by the DA- or BH4-depletion. Notably, this reduction in the total-TH protein level was sensitive not only to a 26S proteasomal inhibitor, MG-132, but also to a PKA inhibitor, H-89. These data demonstrated that DA deficiency should induce compensatory activation of TH via phosphorylation at (40)Ser through D2-autoreceptor and PKA-mediated pathways, which in turn give a rise to its degradation through an ubiquitin-proteasome pathway, resulting in a negative spiral of DA production when DA deficiency persists.

Complex molecular regulation of tyrosine hydroxylase

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, is strictly controlled by several interrelated regulatory mechanisms. Enzyme synthesis is controlled by epigenetic factors, transcription factors, and mRNA levels. Enzyme activity is regulated by end-product feedback inhibition. Phosphorylation of the enzyme is catalyzed by several protein kinases and dephosphorylation is mediated by two protein phosphatases that establish a sensitive process for regulating enzyme activity on a minute-to-minute basis. Interactions between tyrosine hydroxylase and other proteins introduce additional layers to the already tightly controlled production of catecholamines. Tyrosine hydroxylase degradation by the ubiquitin-proteasome coupled pathway represents yet another mechanism of regulation. Here, we revisit the myriad mechanisms that regulate tyrosine hydroxylase expression and activity and highlight their physiological importance in the control of catecholamine biosynthesis.

The role of tetrahydrobiopterin and catecholamines in the developmental regulation of tyrosine hydroxylase level in the brain

Tyrosine hydroxylase (TH) is a rate-limiting enzyme for dopamine synthesis and requires tetrahydrobiopterin (BH4) as an essential cofactor. BH4 deficiency leads to the loss of TH protein in the brain, although the underlying mechanism is poorly understood. To give insight into the role of BH4 in the developmental regulation of TH protein level, in this study, we investigated the effects of acute and subchronic administrations of BH4 or dopa on the TH protein content in BH4-deficient mice lacking sepiapterin reductase. We found that BH4 administration persistently elevated the BH4 and dopamine levels in the brain and fully restored the loss of TH protein caused by the BH4 deficiency in infants. On the other hand, dopa administration less persistently increased the dopamine content and only partially but significantly restored the TH protein level in infant BH4-deficient mice. We also found that the effects of BH4 or dopa administration on the TH protein content were attenuated in young adulthood. Our data demonstrate that BH4 and catecholamines are required for the post-natal augmentation of TH protein in the brain, and suggest that BH4 availability in early post-natal period is critical for the developmental regulation of TH protein level.

Partial biopterin deficiency disturbs postnatal development of the dopaminergic system in the brain

Postnatal development of dopaminergic system is closely related to the development of psychomotor function. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of dopamine and requires tetrahydrobiopterin (BH4) as a cofactor. To clarify the effect of partial BH4 deficiency on postnatal development of the dopaminergic system, we examined two lines of mutant mice lacking a BH4-biosynthesizing enzyme, including sepiapterin reductase knock-out (Spr(-/-)) mice and genetically rescued 6-pyruvoyltetrahydropterin synthase knock-out (DPS-Pts(-/-)) mice. We found that biopterin contents in the brains of these knock-out mice were moderately decreased from postnatal day 0 (P0) and remained constant up to P21. In contrast, the effects of BH4 deficiency on dopamine and TH protein levels were more manifested during the postnatal development. Both of dopamine and TH protein levels were greatly increased from P0 to P21 in wild-type mice but not in those mutant mice. Serotonin levels in those mutant mice were also severely suppressed after P7. Moreover, striatal TH immunoreactivity in Spr(-/-) mice showed a drop in the late developmental stage, when those mice exhibited hind-limb clasping behavior, a type of motor dysfunction. Our results demonstrate a critical role of biopterin in the augmentation of TH protein in the postnatal period. The developmental manifestation of psychomotor symptoms in BH4 deficiency might be attributable at least partially to high dependence of dopaminergic development on BH4 availability.

Annual and periovulatory changes in tyrosine hydroxylase activity in the ovary of the catfish Heteropneustes fossilis

In the present study, tyrosine hydroxylase (TH, the rate-limiting enzyme in catecholamine synthesis) activity was demonstrated in the ovary of the catfish to elucidate the possible physiological role of catecholamines in the gonad. The ovary is innervated by seven pairs of nerves, originating from the paired sympathetic chain lying dorsal to the posterior kidney. Ovarian TH activity showed a significant annual variation (P<0.001, one-way ANOVA), correlating with gonado-somatic index. Activity was low from December to February (resting phase), increased from March to July (recrudescent phase) and then decreased in post-spawning phase (August-November). The annual pattern was similar to that of the brain. An intraperitoneal injection of 100 IU hCG/fish induced significant periovulatory changes in TH activity with the peak rise at 16 h, and the activity decreased after egg-stripping (P<0.001, one-way ANOVA). Brain TH activity showed similar periovulatory changes. The results suggest that catecholamine synthesis is increased during both ovarian recrudescence and spawning of the annual reproductive cycle, implying a functional role in ovarian growth, maturation and ovulation.

Mutant torsinA interacts with tyrosine hydroxylase in cultured cells

A specific mutation (DeltaE302/303) in the torsinA gene underlies most cases of dominantly inherited early-onset torsion dystonia. This mutation causes the protein to aggregate and form intracellular inclusion bodies in cultured cells and animal models. Co-expression of the wildtype and mutant proteins resulted in the redistribution of the wildtype protein from the endoplasmic reticulum to inclusion bodies in cultured HEK293 cells, and this was associated with increased interaction between the two proteins. Expression of DeltaE302/303 but not wildtype torsinA in primary postnatal midbrain neurons resulted in the formation of intracellular inclusion bodies, predominantly in dopaminergic neurons. Tyrosine hydroxylase was sequestered in these inclusions and this process was mediated by increased protein-protein interaction between mutant torsinA and tyrosine hydroxylase. Analysis in an inducible neuroblastoma cell culture model demonstrated altered tyrosine hydroxylase activity in the presence of the mutant but not wildtype torsinA protein. Our results suggest that the interaction of tyrosine hydroxylase and mutant torsinA may contribute to the phenotype and reported dopaminergic dysfunction in torsinA-mediated dystonia.

Accumulation of phosphorylated tyrosine hydroxylase into insoluble protein aggregates by inhibition of an ubiquitin-proteasome system in PC12D cells

Tyrosine hydroxylase (TH) is a rate-limiting enzyme for the biosynthesis of catecholamines including dopamine. The relationship between proteasomal dysfunction and the etiology of Parkinson's disease has been suggested, but it is unknown if TH protein is affected by proteasomal dysfunctions. Here, we examined the effect of inhibition of ubiquitin-proteasomal pathway on biochemical characteristics of TH protein in the neuronal cells. Inhibition of 20S or 26S proteasome by proteasome inhibitor I, or MG-132 in NGF-differentiated PC12D cells induced dot-like immunoreactivities with the anti-(40)Ser-phosphorylated TH (p40-TH) antibody. These dots were tightly co-localized with ubiquitin and positive to Thioflavine-S staining. These dot-like immunoreactivities were not obvious when immunostaining was performed against total-TH or choline acetyltransferase. Western blotting analysis showed time-dependent increase of p40-TH in the Triton-insoluble fractions. We also examined the effect of okadaic acid, an inhibitor of protein phosphatase 2A, which is a phosphatase acting on p40-TH. Okadaic acid increased the amount of insoluble p40-TH. These data suggest that p40-TH is prone to be insolubilized and aggregated by dysfunction of an ubiquitin-proteasome system in PC12D cells.

Tetrahydrobiopterin shows chaperone activity for tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamine neurotransmitters. Primary inherited defects in TH have been associated with l-DOPA responsive and non-responsive dystonia and infantile parkinsonism. In this study, we show that both the cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) and the feedback inhibitor and catecholamine product dopamine increase the kinetic stability of human TH isoform 1 in vitro. Activity measurements and synthesis of the enzyme by in vitro transcription-translation revealed a complex regulation by the cofactor including both enzyme inactivation and conformational stabilization. Oral BH(4) supplementation to mice increased TH activity and protein levels in brain extracts, while the Th-mRNA level was not affected. All together our results indicate that the molecular mechanisms for the stabilization are a primary folding-aid effect of BH(4) and a secondary effect by increased synthesis and binding of catecholamine ligands. Our results also establish that orally administered BH(4) crosses the blood-brain barrier and therapeutic regimes based on BH(4) supplementation should thus consider the effect on TH. Furthermore, BH(4) supplementation arises as a putative therapeutic agent in the treatment of brain disorders associated with TH misfolding, such as for the human TH isoform 1 mutation L205P.

Molecular mechanism for pterin-mediated inactivation of tyrosine hydroxylase: formation of insoluble aggregates of tyrosine hydroxylase

Tyrosine hydroxylase (TH), an iron-containing enzyme, catalyzes the first and rate-limiting step of catecholamine biosynthesis, and requires tetrahydrobiopterin (BH4) as a cofactor. We found that preincubation of recombinant human TH with BH4 results in the irreversible inactivation of the enzyme at a concentration far less than the Km value toward BH4 in spite of its cofactor role, whereas oxidized biopterin, which has no cofactor activity, does not affect the enzyme activity. We show that TH is inactivated by BH4 in competition with the binding of dopamine. The sequential addition of BH4 to TH results in a gradual decrease in the intensity of the fluorescence and CD spectra without changing their overall profiles. Sedimentation velocity analysis demonstrated an association of TH molecules with each other in the presence of BH4, and studies using gel-permeation chromatography, turbidity measurements, and transmission electron microscopy demonstrated the formation of amorphous aggregates with large molecular weights following the association of the TH proteins. These results suggest that BH4 not only acts as a cofactor, but also accelerates the aggregation of TH. We propose a novel mechanism for regulating the amount of TH protein, and discuss its physiological significance.

Biochemical conservation of recombinant Drosophila tyrosine hydroxylase with its mammalian cognates

Dopamine modulates several behavioral and developmental events; in the fruit fly Drosophila melanogaster, dopamine is a neurotransmitter, a neuromodulator, and a developmental signal. Studies in mammals suggest that these diverse roles for dopamine have been evolutionarily conserved. Fundamental regulation of dopamine occurs via tyrosine hydroxylase (TH), the first and rate-limiting enzyme in the catecholamine biosynthetic pathway. Mammalian TH is acutely regulated via phosphorylation-dephosphorylation mechanisms, which occur as a direct consequence of nerve stimulation. We have shown that the Drosophila homolog of TH, DTH, shares over 50% sequence identity with mammalian TH, and the serine residue corresponding to the major site of phosphorylation is conserved. We demonstrate using recombinant DTH protein generated in E. coli that its regulatory biochemical mechanisms closely parallel those from mammals. Drosophila thus provides a highly conserved and tractable model system in which to test the functional consequences of perturbing TH activity by acute regulatory mechanisms.

Regulatory mechanism of tyrosine hydroxylase activity

Activity of tyrosine hydroxylase is regulated by feedback inhibition and inactivation by catecholamines, and activation by protein phosphorylation. In this article, reaction mechanisms for the conversion of tyrosine hydroxylase to an inactive/stable form by catecholamines, and activation of tyrosine hydroxylase by phosphorylation at Ser-40 are discussed. Inactivation may be induced by sub-stoichiometric amounts of catecholamines, and activation by phosphorylation of Ser-40 may require phosphorylation of three or all four subunits of a tyrosine hydroxylase molecule. Cooperative phosphorylation at Ser-40 in the subunits is also discussed.

Genetically rescued tetrahydrobiopterin-depleted mice survive with hyperphenylalaninemia and region-specific monoaminergic abnormalities

One of the possibly mutated genes in DOPA-responsive dystonia (DRD, Segawa's disease) is the gene encoding GTP cyclohydrolase I, which is the rate-limiting enzyme for tetrahydrobiopterin (BH4) biosynthesis. Based on our findings on 6-pyruvoyltetrahydropterin synthase (PTS) gene-disrupted (Pts(-/-)) mice, we suggested that the amount of tyrosine hydroxylase (TH) protein in dopaminergic nerve terminals is regulated by the intracellular concentration of BH4. In this present work, we rescued Pts(-/-) mice by transgenic introduction of human PTS cDNA under the control of the dopamine beta-hydroxylase promoter to examine regional differences in the sensitivity of dopaminergic neurons to BH4-insufficiency. The DPS-rescued (Pts(-/-), DPS) mice showed severe hyperphenylalaninemia. Human PTS was efficiently expressed in noradrenergic regions but only in a small number of dopaminergic neurons. Biopterin and dopamine contents, and TH activity in the striatum were poorly restored compared with those in the midbrain. TH-immunoreactivity in the lateral region of the striatum was far weaker than that in the medial region or in the nucleus accumbens. We concluded that dopaminergic nerve terminals projecting to the lateral region of the striatum are the most sensitive to BH4-insufficiency. Biochemical and pathological changes in DPS-rescued mice were similar to those in human malignant hyperphenylalaninemia and DRD.

Catecholamines and serotonin are differently regulated by tetrahydrobiopterin. A study from 6-pyruvoyltetrahydropterin synthase knockout mice

(6R)-L-erythro-5,6,7,8-Tetrahydrobiopterin (BH4) is an essential cofactor for tyrosine hydroxylase (TH), tryptophan hydroxylase, phenylalanine hydroxylase, and nitric-oxide synthase. These enzymes synthesize neurotransmitters, e.g. catecholamines, serotonin, and nitric oxide (NO). We established mice unable to synthesize BH4 by disruption of the 6-pyruvoyltetrahydropterin synthase gene, the encoded protein of which catalyzes the second step of BH4 biosynthesis. Homozygous mice were born at the almost expected Mendelian ratio, but died within 48 h after birth. In the brain of homozygous mutant neonates, levels of biopterin, catecholamines, and serotonin were extremely low. The number of TH molecules was highly dependent on the intracellular concentration of BH4 at nerve terminals. Alteration of the TH protein level by modulation of the BH4 content is a novel regulatory mechanism. Our data showing that catecholaminergic, serotonergic, and NO systems were differently affected by BH4 starvation suggest the possible involvement of BH4 synthesis in the etiology of monoamine-based neurological and neuropsychiatric disorders.

Newly synthesized dopamine in the nucleus accumbens is regulated by beta-adrenergic, but not alpha-adrenergic, receptors

Previous microdialysis studies have led to the hypothesis that activation of mesolimbic alpha-adrenoceptors inhibits the release of mesolimbic dopamine from alpha-methyl-p-tyrosine-resistant, reserpine-sensitive pools, and that activation of mesolimbic beta-adrenoceptors stimulates the release of mesolimbic dopamine from alpha-methyl-p-tyrosine-sensitive, reserpine-resistant pools. In the present study we analysed the ability of mesolimbic alpha- and beta-adrenoceptors to modulate the release of dopamine from alpha-methyl-p-tyrosine-sensitive pools in the nucleus accumbens of high and low responders to novelty. Under non-challenged conditions, alpha-methyl-p-tyrosine (10(-4)M, 40 min) produced a decrease in dopamine release that did not differ between high and low responders to novelty. The continuous infusion of 10(-6)M isoproterenol (beta-adrenoceptor agonist) diminished the alpha-methyl-p-tyrosine-induced decrease in dopamine, whereas the continuous infusion of 10(-5)M phenylephrine (alpha-adrenoceptor agonist) remained ineffective. It is concluded that the release of mesolimbic dopamine from alpha-methyl-p-tyrosine-sensitive, reserpine-resistant pools is under excitatory control of beta-adrenergic, but not alpha-adrenergic, receptors in both high and low responders to novelty. In general, this study implies that mesolimbic dopamine that is derived from different pools is regulated via different noradrenergic receptors.

Characterization of a stable form of tryptophan hydroxylase from the human parasite Schistosoma mansoni

A cDNA (Schistosoma mansoni tryptophan hydroxylase; SmTPH) encoding a protein homologous to tryptophan hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of serotonin, was cloned from the human parasite Schistosoma mansoni. Bacterial expression of SmTPH as a histidine fusion protein produced soluble active enzyme, which was purified to apparent homogeneity and a final specific activity of 0.17 micromol/min/mg of protein. The purified enzyme was found to be a tetramer of approximately 240 kDa with a subunit size of 58 kDa. Several of the biochemical and kinetic properties of SmTPH were similar to those of mammalian tryptophan hydroxylase. Unlike the mammalian enzyme, however, SmTPH was found to be stable at 37 degrees C, its t((1)/(2)) being nearly 23 times higher than that of a similarly expressed rabbit tryptophan hydroxylase. A semiquantitative reverse transcription polymerase chain reaction showed that the level of SmTPH mRNA in a larval stage of the parasite (cercaria) is 2.5 times higher than in adult S. mansoni, suggesting possible differences in the level of enzyme expression between the two developmental stages. This study demonstrates for the first time the presence of a functional tryptophan hydroxylase in a parasitic helminth and further suggests that the parasites are capable of synthesizing serotonin endogenously.

Dopamine, in the presence of tyrosinase, covalently modifies and inactivates tyrosine hydroxylase

Dopamine has been implicated as a potential mediating factor in a variety of neurodegenerative disorders. Dopamine can be oxidized to form a reactive dopamine quinone that can covalently modify cellular macromolecules including protein and DNA. This oxidation can be enhanced through various enzymes including tyrosinase and/or prostaglandin H synthase. One of the potential targets in brain for dopamine quinone damage is tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis. The present studies demonstrated that dopamine quinone, the formation of which was enhanced through the activity of the melanin biosynthetic enzyme, tyrosinase, covalently modified and inactivated tyrosine hydroxylase. Dihydroxyphenylalanine (DOPA; the catechol-containing precursor of dopamine) also inactivated tyrosine hydroxylase under these conditions. Catecholamine-mediated inactivation occurred with both purified tyrosine hydroxylase as well as enzyme present in crude pheochromocytoma homogenates. Inactivation was associated with covalent incorporation of radiolabelled dopamine into the enzyme as assessed by immunoprecipitation, size exclusion chromatography, and denaturing sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis. Furthermore, the covalent modification and inactivation of tyrosine hydroxylase was blocked by antioxidant compounds (dithiothreitol, reduced glutathione, or NADH). In addition to kinetic feedback inhibition and the formation of an inhibitory dopamine/Fe+3 complex, these findings suggest that a third mechanism exists by which dopamine (or DOPA) can inhibit tyrosine hydroxylase, adding further complexity to the regulation of catecholamine biosynthesis.

Effects of dopamine on N-terminus-deleted human tyrosine hydroxylase type 1 expressed in Escherichia coli

N-Terminus-deleted mutants and wild-type human tyrosine hydroxylase type 1 were expressed in Escherichia coli (E. coli) and utilized to investigate the dopamine-induced decrease in the enzyme catalytic activity and also to identify the specific portion in the N-terminus that affects the efficiency of the inhibitory action of dopamine. Supernatants of bacterial lysates were used as enzyme samples. The pH profiles of the enzyme catalytic activity were affected according to the degree of the deletion. The deletion up to 39 amino acid residues was enough to abolish the inhibitory effect of dopamine in the basic pH range. These results suggest that the inhibition by dopamine of tyrosine hydroxylase activity is closely related to the amino acid sequence in the N-terminus of the enzyme.

Regulation of N-terminus-deleted human tyrosine hydroxylase type 1 by end products of catecholamine biosynthetic pathway

The N-terminal 52-, 70-, and 157-amino acids-deleted mutants and wild-type tyrosine hydroxylases were expressed in Escherichia coli and utilized to investigate the roles of the N-terminus in the catecholamine inhibition on enzyme activity. Their lysate's supernatants were used as enzyme samples. Three catecholamines, namely dopamine, norepinephrine, and epinephrine, affected both wild-type and mutant enzymes after preincubation in the mode of mixed inhibition, and the most marked alteration among the kinetic parameters produced by the deletion was the increase in the inhibition constants. The deletions also abolished the catecholamine-induced shift of the pH profile of the enzyme activity toward a more acidic pH optimum. All three mutants responded to catecholamines almost in the same way. These results suggest that the three catecholamine end products exert their inhibition on tyrosine hydroxylase to the same extent and that the N-terminal 52 amino acid residues contain the key sequence in mediating the inhibitory action.

Intravenous gamma-glutamyl-tyrosine elevates brain tyrosine but not catecholamine concentrations in normal rats

A number of clinical situations may benefit from intravenous supplements of tyrosine (Tyr). In total parenteral nutrition (TPN), the supply of Tyr is limited by its poor solubility. In both rats and infants maintained on pediatric TPN, plasma Tyr levels are approximately 30% of normal, and in rat brains Tyr concentrations are similarly reduced. We reported previously that supplementing a TPN solution with the soluble peptide, gamma-glutamyl-Tyr [Glu(Tyr)], normalizes plasma Tyr and doubles brain Tyr in rats. To assess more fully the behavior of intravenous Glu(Tyr) in vivo, 20 mmol/L Glu(Tyr) was infused into the inferior vena cava of rats at rates increased every 2 hours over an 8-hour period (300 to 450 mumol Glu(Tyr)/kg body weight/h). The surgical procedure for catheterization is described. At the maximum rate of infusion, plasma Tyr and Glu(Tyr) concentrations reached mean plateau values of 326 and 252 mumol/L, respectively. Brain Tyr concentrations were 71 and 264 nmol/g wet weight in control rats infused with heparinized saline (SAL group) and rats infused with Glu(Tyr) (PEP group) respectively. No differences were found in concentrations of norepinephrine (NE), dopamine (DA), or homovanillic acid (HVA) in prefrontal cortex (PFC), striatum (STR), or remaining brain (RB) tissue in PEP and SAL rats. We did not detect undergraded Glu(Tyr) in the brain, and less than 0.5% of infused Glu(Tyr) appeared in the urine.

Evidence for protein kinase C involvement in the short-term activation by prolactin of tyrosine hydroxylase in tuberoinfundibular dopaminergic neurons

The mechanism of the short-term activation by prolactin (PRL) of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by 3,4-dihydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a dose-dependent increase within 2 h of incubation of the hypothalamic slices with PRL. To determine whether a phosphorylation process was involved in this increase in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition. In control median eminences, two kinetically different forms of TH coexisted, one exhibiting a Ki(DA) value of 29.92 +/- 0.49 microM, the other being approximately 15-fold more sensitive to DA inhibition with a Ki(DA) of 1.96 +/- 0.09 microM, likely corresponding to a phosphorylated and active form and to a nonphosphorylated and less active form, respectively. After PRL treatment, the TH form of low Ki(DA) remained unaffected, whereas the Ki(DA) of the purported active form of TH increased to 62.6 +/- 0.8 microM, suggesting an increase in the enzyme phosphorylation. This increase in the Ki(DA) of TH was selectively prevented by GF 109203X, a potent and selective inhibitor of protein kinase C, but not by a specific inhibitor of protein kinase A or calmodulin. Finally, this action of PRL could be mimicked by 12-O-tetradecanoylphorbol 13-acetate (a direct activator of protein kinase C). These results suggest that PRL, at the median eminence level, activates TH by increasing the enzyme phosphorylation and that this action may involve an activation of protein kinase C.

Deletion mutagenesis of rat PC12 tyrosine hydroxylase regulatory and catalytic domains

The functional organization of rat tyrosine hydroxylase was investigated by deletion mutagenesis of the regulatory and catalytic domains. A series of tyrosine hydroxylase cDNA deletion mutants were amplified by PCR, cloned into the pET3C prokaryotic expression vector, and the mutant proteins were partially purified from E. coli. The results show that the deletion of up to 157 N-terminal amino acids activated the enzyme, but further deletion to position 184 completely destroyed catalytic activity. On the carboxyl end, the removal of 43 amino acids decreased but did not eliminate activity, suggesting that this region may play a different role in the regulation of the enzyme. These findings place the amino end of the catalytic domain between residues 158 and 184 and the carboxyl end at or prior to position 455. Deletions within the first 157 amino acids in the N-terminus caused an increase in hydroxylating activity, a decrease in the apparent Km for tyrosine and phenylalanine substrates, and a substantial increase in the Ki for dopamine inhibition. The results define this region of the N-terminus as the regulatory domain of tyrosine hydroxylase, whose primary functions are to restrict the binding of amino acid substrates and to facilitate catecholamine inhibition. The results also suggest that the well-established role of the regulatory domain in restricting cofactor binding may be secondary to an increase in catecholamine binding, which in turn lowers the affinity for the cofactor. These findings provide new insight into the functional organization and mechanisms of regulation of tyrosine hydroxylase.

Conversion of tyrosine hydroxylase to stable and inactive form by the end products

We have reported previously that tyrosine hydroxylase in the crude extract from rat striatum exists in the inactive form showing almost no activity at the physiological pH and that the inactive form is produced by the action of the end products of the enzyme, such as dopamine. The incubation of the enzyme with the end products resulted in not only the inactivation but also a remarkable stabilization of the enzyme. Catechols possessing amino groups but no negatively charged groups on the side chains (catecholamine-type catechols) were effective at a concentration as low as 10(-7) M in both the inactivation and stabilization of the enzyme. In contrast, catechols not possessing positively or negatively charged side chains (3,4-dihydroxyphenylethyleneglycol-type catechols) were ineffective at a concentration of 10(-7) M but effective at a concentration of 10(-6) M for both the inactivation and stabilization. Catechols possessing negatively charged groups (3,4-dihydroxyphenylacetic acid-type catechols) were ineffective even at a concentration of 10(-6) M. Thus, the end products of tyrosine hydroxylase appear to serve to keep the enzyme inactive and stable. The reaction mechanism of the conversion of the enzyme from the active/labile form to the inactive/stable form by dopamine was also investigated.

Activation of striatal tyrosine hydroxylase by in vivo electrical stimulation: comparison with cyclic AMP-mediated activation

These studies were carried out to characterize the activation of rat striatal tyroxine hydroxylase produced by depolarization of the medial forebrain bundle and to evaluate the possible role of cyclic AMP as a mediator of this activation. The enzymatic properties of tyrosine hydroxylase following in vivo depolarization were compared to those produced by treatment of striatal synaptosomes with dibutyryl cyclic AMP (dbcAMP). Similar effects were observed with regard to enzyme distribution, altered sensitivity to dopamine-induced inhibition, and activity as a function of tyrosine concentration. However, differences between the two treatments were also apparent. First, treatment with dbcAMP shifted the pH optimum from 6.2 to 7.0. In contrast, electrical stimulation decreased the rate of decline in activity as the pH was increased above the optimum, but did not shift the pH optimum. Second, plots of tyrosine hydroxylase activity versus cofactor concentration revealed two enzyme forms for both control and electrically stimulated preparations. However, dbcAMP treatment converted the enzyme to a single high affinity form. These results can be explained by one of the following: (1) cyclic AMP is the sole mediator of enzyme activation, but does not produce a maximally activated enzyme following in vivo depolarization, (2) cyclic AMP is only one of several mediators involved or (3) cyclic AMP is not involved in depolarization-induced activation, with activation occurring via the mediation of other intracellular messengers, such as calcium.

Role of adenylate cyclase in the modulation of the release of dopamine: a microdialysis study in the striatum of the rat

In the present study, we have applied the brain microdialysis technique to investigate the effect of the stimulation of adenylate cyclase on the extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum of freely moving rats. Infusion of 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), 3-isobutyl-1-methylxanthine, or forskolin produced a significant increase in the release of DA. The effect of 8-Br-cAMP was tetrodotoxin, Ca2+, and dose dependent and was saturable. 8-Br-cAMP also caused an increase in the output of DOPAC and HVA. No effects were seen on the output of 5-HIAA, except at the highest 8-Br-cAMP concentration studied. Infusion of 8-Br-cAMP (25 microM, 1.0 mM, and 3.3 mM) together with infusion of (-)-sulpiride (1 microM) or systemic administration of (+/-)-sulpiride (55 mumol/kg i.p.) produced an additive effect on the release of DA. Infusion or peripheral administration of (-)-N-0437 (1 microM or 1 mumol/kg) both decreased the 8-Br-cAMP-induced increase in the release of DA. These results demonstrate that cyclic AMP may stimulate the release of DA, but it is unlikely that this second messenger is linked to presynaptic D2 receptors controlling the release of DA.

The tuberoinfundibular dopaminergic neurons of the brain: hormonal regulation

The role of prolactin and of estradiol and progesterone in the control of the biosynthetic and secretory activity of TIDA neurons has been investigated in the following animal models: young female rats, aged female rats, and young male rats. The indices of TIDA neuronal function employed were a) mass of TH in neurites in the ME, b) total in situ activity of TH in the ME, c) in situ molar activity of TH in the ME, and d) secretion of dopamine into hypophysial portal blood. It was found that prolactin in high concentration in the circulation and in the CSF had little, if any, effect on the mass of TH in the ME. However, a high concentration of prolactin in either the circulation or in the CSF stimulated significantly the in situ TH activity in the ME whether expressed in terms of total activity per ME or activity per mole of TH. The stimulation of TH activity with prolactin was prevented by immunoneutralization of circulating prolactin. A high concentration of prolactin in the CSF was as effective in stimulating TH activity in the ME of rats with intact pituitary glands as in hypophysectomized rats. In addition to prolactin, treatment of animals with intact pituitaries with a combination of estradiol and progesterone markedly stimulated the total in situ activity of TH of the ME as well as the in situ molar activity of TH of the ME, but neither estradiol nor progesterone alone had an effect on TH activity. Hypophysectomy abolished the stimulatory action of estradiol and progesterone on TH activity of the ME. In addition to the in situ activity of TH in the ME, estradiol-progesterone treatment stimulated the secretion of dopamine into hypophysial portal blood. Neither estradiol nor progesterone alone affected dopamine secretion by TIDA neurons. We conclude that exposure to high concentrations of prolactin or to both estradiol and progesterone stimulate the biosynthetic and secretory activity of TIDA neurons. These hormones are effective in old rats and well as young rats and in males as well as females.

The human tyrosine hydroxylase gene

1. Tyrosine hydroxylase (TH) is a rate-limiting enzyme for catecholamine biosynthesis, and it is a pterin-requiring monooxygenase. Both cDNAs and genomic DNA of human TH have been cloned and the nucleotide sequence has been determined. 2. Four similar but distinct mRNAs encode human TH. The results of Southern blot analysis and the nucleotide sequence of the human TH genomic DAN indicate that the four types of human TH mRNA are produced through alternative splicing from a single gene. 3. The human TH gene was split into 4 exons and 13 introns. The 12-bp insertion sequence is encoded by the 3'-terminal portion of the first exon. The 81-bp insertion sequence corresponds to the second exon. Two kinds of alternative splicing are involved: the alternative use of two donor sites in the first exon and the inclusion/exclusion of the second exon. 4. The four types (type 1-4) were expressed in COS cells, and all had enzymatic activities. The type 1 enzyme had the highest homospecific activity (activity per enzyme protein), the values for the other enzymes ranging from 30 to 40%. The Km values of the four types for L-tyrosine and 6-methyl-5,6,7,8-tetrahydropterin were similar.

Comparative studies of the effects of RS-8359 and safrazine on monoamine oxidase in-vitro and in-vivo in mouse brain

The effect of RS-8359, pyrimidine on monoamine oxidase (MAO) has been compared with a hydrazinic MAO inhibitor, safrazine (beta-piperonylisopropylhydrazine hydrochloride,) which is a MAO inhibitor used clinically. In-vitro radiochemical determination of MAO activity showed that the IC50 of RS-8359 was 0.52 microM for the deamination of 5-hydroxytryptamine (5-HT) in the mouse brain mitochondrial preparation, while beta-phenylethylamine (PEA) deamination was inhibited by only 20% at 100 microM of the drug. 5-HT deamination in the brain homogenate prepared from mice killed 60 min after administration of RS-8359 was inhibited significantly by 14 and 48%, at 30 and 100 mg kg-1 (p.o.), respectively, while deamination of PEA was little affected at the same doses. On the other hand, safrazine strongly inhibited both 5-HT and PEA deaminations, but showed no selectivity toward the substrate used. The extent of MAO inhibition by RS-8359, measured fluorometrically with kynuramine as a substrate in the brain homogenate, was independent of preincubation up to 80 min. In contrast, the inhibitory potency of safrazine was strengthened by preincubation in a time-dependent manner. Oral administration of RS-8359 (3-30 mg kg-1) caused a dose-dependent increase in endogenous monoamines in mouse brain, which disappeared a few hours after its administration. Increase in monoamine content caused by safrazine lasted for at least 24 h. These results indicate that RS-8359 is a reversible and specific inhibitor of MAO-A, while safrazine is an irreversible and non-specific MAO inhibitor, in-vivo and in-vitro in mouse brain.

Regulation of the activity of tyrosine hydroxylase in the central nervous system

Among a variety of factors known to affect the activity of tyrosine hydroxylase, catecholamines, cyclic AMP-dependent protein kinase, calmodulin-dependent protein kinase II, and polyanions resulted in the reversible modulation of the enzyme activity to a greater extent than the other factors. The in vitro experiments suggested that the inactivation of the enzyme by catecholamines, the end products of the enzyme, and the activation by cyclic AMP-dependent protein kinase might be most important in controlling the activity of the enzyme. Incubation of the enzyme with catecholamines at a concentration as low as 10-100 nM resulted in a rapid inactivation, and the inactivated enzyme was found to be more stable than the original uninactivated enzyme. The inactive/stable form of the enzyme exhibited no activity under the physiological conditions. Several lines of evidence indicated that tyrosine hydroxylase might usually exist as the catecholamine-induced inactive/stable form in the nervous system. This inactive form of the enzyme was markedly activated by cyclic AMP-dependent protein kinase. This conversion of the enzyme induced by cyclic AMP-dependent protein kinase was a dramatic one from the almost completely inactive form to the extremely active form having an activity as high as 2,800 units per mg of protein, which was the highest so far reported when the enzyme activity was measured at the physiological pH (pH 7). The co-operative action of cyclic AMP-dependent protein kinase, calmodulin-dependent protein kinase II, and polyanion were also examined. Most of the studies on the regulatory mechanism for tyrosine hydroxylase so far reported have been done with the enzyme from the rat. Tyrosine hydroxylase from human pheochromocytoma was found to be remarkably activated by cyclic AMP-dependent protein kinase. Calmodulin-dependent protein kinase II and polyanion also modulated the activity of the human enzyme. These results suggest that tyrosine hydroxylase may be regulated in a similar manner in the human and the rat.

Expression of four types of human tyrosine hydroxylase in COS cells

Alternative splicing from a single gene produces four kinds of human tyrosine hydroxylase (types 1-4), which have structural diversity only in the N-terminal region. We attempted expression of the type 1-4 enzymes in COS cells and performed kinetic analyses. All had enzymatic activities. The Km values of the four types for L-tyrosine and 6-methyl-5,6,7,8-tetrahydropteridine were similar, although their relative homospecific activities were clearly different. The type 1 enzyme displayed the highest activity.

Source and physiological significance of plasma 3,4-dihydroxyphenylalanine in the rat

To elucidate the source and physiological significance of plasma 3,4-dihydroxyphenylalanine, the immediate product of the rate-limiting step in catecholamine biosynthesis, plasma 3,4-dihydroxyphenylalanine was quantified in conscious rats after administration of reserpine, desipramine, clorgyline, or forskolin, treatments that affect tyrosine hydroxylase activity. Plasma 3,4-dihydroxyphenylalanine was also examined during infusions of norepinephrine with or without clorgyline, reserpine, or desipramine pretreatment. After reserpine, the plasma 3,4-dihydroxyphenylalanine level decreased by 22% and then increased by 40%, a result consistent with modulation of tyrosine hydroxylase activity first by an increased axoplasmic norepinephrine content and then by depletion of norepinephrine stores. After desipramine, the plasma 3,4-dihydroxyphenylalanine level decreased by 20%, reflecting the depressant effect of neuronal uptake blockade on norepinephrine turnover. Forskolin increased the plasma 3,4-dihydroxyphenylalanine level by 30%, consistent with activation of tyrosine hydroxylase by cyclic AMP-dependent phosphorylation. Acute administration of clorgyline was without effect on the plasma 3,4-dihydroxyphenylalanine level. Norepinephrine infusions decreased the plasma 3,4-dihydroxyphenylalanine concentration, as expected from end-product inhibition of tyrosine hydroxylase. Pretreatment with desipramine prevented the norepinephrine-induced decrease in plasma dihydroxyphenylalanine content, indicating that inhibition of tyrosine hydroxylase required neuronal uptake of norepinephrine. Both reserpine and clorgyline augmented the norepinephrine-induced decrease in plasma 3,4-dihydroxyphenylalanine level, suggesting that retention of norepinephrine in the axoplasm--due to inhibition of norepinephrine sequestration into storage vesicles or catabolism--caused further inhibition of tyrosine hydroxylase. Changes in plasma 3,4-dihydroxyphenylalanine concentration during norepinephrine infusions were negatively correlated with those in plasma 3,4-dihydroxyphenylglycol level, a finding consistent with modulation of tyrosine hydroxylase activity by axoplasmic norepinephrine. In reserpinized animals, clorgyline and norepinephrine infusion together decreased the plasma 3,4-dihydroxyphenylalanine content by 50%, a result demonstrating that hydroxylation of tyrosine was depressed by at least half. The results indicate that quantification of plasma 3,4-dihydroxyphenylalanine can provide a simple and direct approach for examination of the rate-limiting step in catecholamine biosynthesis.

Effects of protonophores on the synthesis of catecholamines and the intracellular pH in cultured bovine adrenal medullary cells

The protonophores carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) stimulated the synthesis of 14C-catecholamines from [14C]tyrosine in cultured bovine adrenal medullary cells. The stimulatory effect of CCCP but not of FCCP was partially dependent on extracellular Ca2+. CCCP but not FCCP increased the influx of 45Ca2+ to the cells. When cells were incubated with either CCCP or FCCP (0.01-0.2 microgram/ml), the intracellular pH fell from 7.2 to 6.3-6.5 and catecholamine synthesis increased. Tyrosine hydroxylase activity in a soluble fraction prepared from cultured adrenal medullary cells was measured after incubation of the cells with FCCP or CCCP. Although FCCP did not affect the activity of the enzyme, CCCP caused a stable activation of it which was dependent on extracellular Ca2+. Since the optimal pH of soluble tyrosine hydroxylase is around 6.0 in adrenal medullary cells, FCCP may increase the synthesis of catecholamines by shifting the intracellular pH toward it. In addition to this mechanism, CCCP may enhance the synthesis of catecholamines by a Ca2+-dependent mechanism.

Soluble tyrosine hydroxylase (tyrosine 3-monooxygenase) from bovine adrenal medulla: large-scale purification and physicochemical properties

A new procedure that permits large-scale purification of tyrosine 3-monooxygenase (tyrosine hydroxylase) (L-tyrosine,tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2) from the cytosolic fraction of bovine adrenal medulla is described. The homogenous enzyme revealed a subunit Mr of 60,000 and a specific activity of 425 nmol.min-1.mg-1. The N-terminal amino-acid sequence (27 residues) revealed 89% homology with the human pheochromocytoma enzyme as deduced from its cDNA sequence. The pure enzyme contained 0.66 +/- 0.09 mol iron, 0.13 mol zinc and 0.62 +/- 0.04 mol phosphate per mol subunit of Mr = 60,000. A broad light absorption band with its maximum around 700 nm (epsilon 700 nm = 1.3 (mM monomer)-1.cm-1) explains its blue-green color. EPR spectra at 3.6 K revealed high-spin Fe(III) (S = 5/2) in an environment of nearly axial symmetry (g values at 7.2-6.7, 4.7-5.3 and 1.9-2.0). A close correlation was observed between the absorbance at 700 nm and the intensity of the axial type of EPR spectrum. The absorption peak at 700 nm is compatible with a ligand-to-iron charge-transfer transition as a result of catecholate coordination to the iron. Physicochemical studies suggest that the enzyme does not undergo such major substrate- or cofactor-induced conformational changes as have been reported for the related enzyme, phenylalanine hydroxylase.

Effect of spermine on tyrosine hydroxylase activity before and after phosphorylation by cyclic AMP-dependent protein kinase

The effect of spermine on tyrosine hydroxylase (TH) activity purified from bovine adrenal medulla was examined before and after phosphorylation by the catalytic subunit of cyclic AMP-dependent protein kinase (A-kinase). Before phosphorylation, spermine (less than 1 mM) inhibited the enzymatic activity, and negative cooperative effect of spermine on TH (Hill coefficient = 0.7) was observed from the kinetic analysis concerning 6-methyl-5,6,7,8-tetrahydropterin (6MPH4). Spermine interacted noncompetitively toward tyrosine and the Ki for spermine was calculated to be 68 microM. Phosphorylation abolished the ability of spermine to inhibit TH activity in a negative cooperative manner against the pterin cofactor, and also increased four-fold the Ki value against the substrate. These results suggest that spermine may inhibit TH activity by interacting with the pterin binding site of the enzyme molecule in a manner of negative cooperativity, and that this inhibition is reversed by the conformational change of regulatory domain of TH after phosphorylation by A-kinase.

Interaction of tyrosine hydroxylase with ribonucleic acid and purification with DNA-cellulose or poly(A)-sepharose affinity chromatography

Tyrosine hydroxylase in bovine adrenal medulla was activated up to fourfold by incubation with low concentrations (15 micrograms/ml) of ribonucleic acids. At higher RNA concentrations, enzyme activity was inhibited. This interaction with RNA was exploited with the use of poly(A)-Sepharose and DNA-cellulose to effect a rapid purification of stable tyrosine hydroxylase from rat brain and bovine adrenal medulla in high yield (up to 58%). With the purified rat brain enzyme, RNA acted as an uncompetitive inhibitor, a concentration of 15 micrograms/ml lowering the Vmax of tyrosine hydroxylase from 1050 to 569 nmol min-1 mg-1 and lowering the Km for tyrosine from 6.1 to 3.6 microM. With the natural cofactor, tetrahydrobiopterin (BH4), two Km values were obtained, indicating the presence of two forms of the enzyme. Both Km values were decreased only slightly by RNA. The purified brain and adrenal enzymes both contained about 0.07 mol of phosphate/63,000-Da subunit; in both cases, cyclic AMP-dependent protein kinase catalyzed the incorporation of an additional 0.8 mol of phosphate/subunit. The purified enzyme also contains ribonucleic acid, which comprises about 10% of the total mass and appears to be important for full activity.

Reversible conversion of tyrosine hydroxylase to an inactive form by antipain

Tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of catecholamines, was reversibly inactivated by incubation with antipain, which is known as a microbial protease inhibitor. The inactivation was a time-dependent reaction and it was prominent when the enzyme was assayed at a neutral pH but not when assayed at an acidic pH. The inactivated enzyme was markedly activated by cyclic AMP-dependent protein kinase. Other microbial protease inhibitors such as leupeptin, chymostatin, and pepstatin did not induce such as inactivation of the enzyme.

In situ activity and phosphorylation of tyrosine hydroxylase in the median eminence

Intracellular activation and phosphorylation of tyrosine hydroxylase (TH; E.C. 1.14.16.2) in the median eminence of the rat brain were investigated. The in situ activity of TH was assayed by the accumulation of L-dihydroxyphenylalanine (DOPA) in the median eminence of hypothalamic fragments incubated in the presence of NSD 1015. When hypothalamic fragments were incubated with veratridine (0-10(-3) M), maximal stimulation of TH activity was observed at 10(-4) M. The mean concentration of DOPA in the median eminence of hypothalamic fragments incubated with 10(-4) M veratridine was 3 times that seen in its absence. Phosphorylation of TH in the median eminence was evaluated by autoradiographic quantification of [32P]TH in 32P-labelled median eminence tissue. The amount of [32P]TH in 32P-labelled median eminence incubated with 10(-4) M veratridine was 2 times that seen in the absence of veratridine. These data are consistent with the view that in the median eminence phosphorylation and activation of TH are linked events and that phosphorylation may be a means of regulating the biosynthesis of dopamine in this region of the brain.