A rapid and sensitive assay for tyrosine-3-monooxygenase based upon the release of 3H2O and adsorption of [3H]-tyrosine by charcoal

The A328 V/E (rs2887147) polymorphisms in human tryptophan hydroxylase 2 compromise enzyme activity

Human tryptophan hydroxylase 2 (hTPH2) is the rate-limiting enzyme for serotonin biosynthesis in the brain. A number of naturally-occurring single nucleotide polymorphisms (SNPs) have been reported for hTPH2. We investigated the activity and kinetic characteristics of the most common missense polymorphism rs2887147 (A328 V/E; 0.92% allelic frequency for the two different reported SNPs at the same site) using bacterially expressed hTPH2. The recombinant full-length enzyme A328E had no measurable enzyme activity, but A328V displayed decreased enzyme activity (Vmax). A328V also displayed substrate inhibition and decreased stability compared to the wild-type enzyme. By contrast, in constructs lacking the N-terminal 150 amino acid regulatory domain, the A328V substitution had no effect; that is, there was no substrate inhibition, enzyme stabilities (for wild-type and A328V) were dramatically increased, and Vmax values were not different (while the A328E variant remained inactive). These findings, in combination with molecular modeling, suggest that substitutions at A328 affect catalytic activity by altering the conformational freedom of the regulatory domain. The reduced activity and substrate inhibition resulting from these polymorphisms may ultimately reduce serotonin synthesis and contribute to behavioral perturbations, emotional stress, and eating disorders.

Role of Brain Endothelin Receptor Type B (ETB) in the Regulation of Tyrosine Hydroxylase in the Olfactory Bulb of DOCA-Salt Hypertensive Rats

BACKGROUND

We previously reported that endothelins (ETs) regulate tyrosine hydroxylase (TH) activity and expression in the olfactory bulb (OB) of normotensive and hypertensive animals. Applying an ET receptor type A (ETA) antagonist to the brain suggested that endogenous ETs bind to ET receptor type B (ETB) to elicit effects.

OBJECTIVE

The aim of the present work was to evaluate the role of central ETB stimulation on the regulation of blood pressure (BP) and the catecholaminergic system in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

METHODS

DOCA-salt hypertensive rats were infused for 7 days with cerebrospinal fluid or IRL-1620 (ETB receptor agonist) through a cannula placed in the lateral brain ventricle. Systolic BP (SBP) and heart rate were recorded by plethysmography. The expression of TH and its phosphorylated forms in the OB were determined by immunoblotting, TH activity by a radioenzymatic assay, and TH mRNA by quantitative real-time polymerase chain reaction.

RESULTS

Chronic administration of IRL-1620 decreased SBP in hypertensive rats but not in normotensive animals. Furthermore, the blockade of ETB receptors also decreased TH-mRNA in DOCA-salt rats, but it did not modify TH activity or protein expression.

CONCLUSION

These findings suggest that brain ETs through the activation of ETB receptors contribute to SBP regulation in DOCA-salt hypertension. However, the catecholaminergic system in the OB does not appear to be conclusively involved although mRNA TH was reduced. Present and previous findings suggest that in this salt-sensitive animal model of hypertension, the OB contributes to chronic BP elevation.

Role of endothelin receptor type A on catecholamine regulation in the olfactory bulb of DOCA-salt hypertensive rats: Hemodynamic implications

Increasing evidence shows that the olfactory bulb is involved in blood pressure regulation in health and disease. Enhanced noradrenergic transmission in the olfactory bulb was reported in hypertension. Given that endothelins modulate catecholamines and are involved in the pathogenesis of hypertension, in the present study we sought to establish the role of the endothelin receptor type A on tyrosine hydroxylase, the rate limiting enzyme in catecholamine biosynthesis, in the olfactory bulb of DOCA-salt hypertensive rats. Sprague-Dawley male rats, randomly divided into Control and DOCA-Salt hypertensive groups, were used to assess endothelin receptors by Western blot and confocal microscopy, and their co-localization with tyrosine hydroxylase in the olfactory bulb. Blood pressure and heart rate as well as tyrosine hydroxylase expression and activity were assessed following BQ610 (ET_A antagonist) applied to the brain. DOCA-Salt hypertensive rats showed enhanced ET_A and decreased ET_B expression. ET_A co-localized with tyrosine hydroxylase positive neurons. Acute ET_A blockade reduced blood pressure and heart rate and decreased the expression of total tyrosine hydroxylase and its phosphorylated forms. Furthermore, it also diminished mRNA tyrosine hydroxylase expression and accelerated the enzyme degradation through the proteasome pathway as shown by pretreatment with MG132, (20s proteasome inhibitor) intracerebroventricularly applied. Present findings support that the brain endothelinergic system plays a major role through ET_A activation in the increase of catecholaminergic activity in the olfactory bulb of DOCA-Salt hypertensive rats. They provide rationale evidence that this telencephalic structure contributes in a direct or indirect way to the hemodynamic regulation in salt dependent hypertension.

Tetrahydrobiopterin treatment reduces brain L-Phe but only partially improves serotonin in hyperphenylalaninemic ENU1/2 mice

Hyperphenylalaninemia (HPA) caused by hepatic phenylalanine hydroxylase (PAH) deficiency has severe consequences on brain monoamine neurotransmitter metabolism. We have studied monoamine neurotransmitter status and the effect of tetrahydrobiopterin (BH4) treatment in Pahenu1/enu2 (ENU1/2) mice, a model of partial PAH deficiency. These mice exhibit elevated blood L-phenylalanine (L-Phe) concentrations similar to that of mild hyperphenylalaninemia (HPA), but brain levels of L-Phe are still ~5-fold elevated compared to wild-type. We found that brain L-tyrosine, L-tryptophan, BH4 cofactor and catecholamine concentrations, and brain tyrosine hydroxylase (TH) activity were normal in these mice but that brain serotonin, 5-hydroxyindolacetic acid (5HIAA) and 3-methoxy-4-hydroxyphenylglycol (MHPG) content, and brain TH protein, as well as tryptophan hydroxylase type 2 (TPH2) protein levels and activity were reduced in comparison to wild-type mice. Parenteral L-Phe loading conditions did not lead to significant changes in brain neurometabolite concentrations. Remarkably, enteral BH4 treatment, which normalized brain L-Phe levels in ENU1/2 mice, lead to only partial recovery of brain serotonin and 5HIAA concentrations. Furthermore, indirect evidence indicated that the GTP cyclohydrolase I (GTPCH) feedback regulatory protein (GFRP) complex may be a sensor for brain L-Phe elevation to ameliorate the toxic effects of HPA. We conclude that BH4 treatment of HPA toward systemic L-Phe lowering reverses elevated brain L-Phe content but the recovery of TPH2 protein and activity as well as serotonin levels is suboptimal, indicating that patients with mild HPA and mood problems (depression or anxiety) treated with the current diet may benefit from supplementation with BH4 and 5-OH-tryptophan.

Chronic Blockade of Brain Endothelin Receptor Type-A (ETA) Reduces Blood Pressure and Prevents Catecholaminergic Overactivity in the Right Olfactory Bulb of DOCA-Salt Hypertensive Rats

Overactivity of the sympathetic nervous system and central endothelins (ETs) are involved in the development of hypertension. Besides the well-known brain structures involved in the regulation of blood pressure like the hypothalamus or locus coeruleus, evidence suggests that the olfactory bulb (OB) also modulates cardiovascular function. In the present study, we evaluated the interaction between the endothelinergic and catecholaminergic systems in the OB of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Following brain ET receptor type A (ET_A) blockade by BQ610 (selective antagonist), transcriptional, traductional, and post-traductional changes in tyrosine hydroxylase (TH) were assessed in the OB of normotensive and DOCA-salt hypertensive rats. Time course variations in systolic blood pressure and heart rate were also registered. Results showed that ET_A blockade dose dependently reduced blood pressure in hypertensive rats, but it did not change heart rate. It also prevented the increase in TH activity and expression (mRNA and protein) in the right OB of hypertensive animals. However, ET_A blockade did not affect hemodynamics or TH in normotensive animals. Present results support that brain ET_A are not involved in blood pressure regulation in normal rats, but they significantly contribute to chronic blood pressure elevation in hypertensive animals. Changes in TH activity and expression were observed in the right but not in the left OB, supporting functional asymmetry, in line with previous studies regarding cardiovascular regulation. Present findings provide further evidence on the role of ETs in the regulation of catecholaminergic activity and the contribution of the right OB to DOCA-salt hypertension.

Stabilization of Human Tyrosine Hydroxylase in Maltodextrin Nanoparticles for Delivery to Neuronal Cells and Tissue

Enzyme replacement therapy (ERT) is a therapeutic approach envisioned decades ago for the correction of genetic disorders, but ERT has been less successful for the correction of disorders with neurological manifestations. In this work, we have tested the functionality of nanoparticles (NP) composed of maltodextrin with a lipid core to bind and stabilize tyrosine hydroxylase (TH). This is a complex and unstable brain enzyme that catalyzes the rate-limiting step in the synthesis of dopamine and other catecholamine neurotransmitters. We have characterized these TH-loaded NPs to evaluate their potential for ERT in diseases associated with TH dysfunction. Our results show that TH can be loaded into the lipid core maltodextrin NPs with high efficiency, and both stability and activity are maintained through loading and are preserved during storage. Binding to NPs also favored the uptake of TH to neuronal cells, both in cell culture and in the brain. The internalized NP-bound TH was active as we measured an increase in intracellular L-Dopa synthesis following NP uptake. Our approach seems promising for the use of catalytically active NPs in ERT to treat neurodegenerative and neuropsychiatric disorders characterized by dopamine deficiency, notably Parkinson's disease.

The Molecular Chaperone Hsc70 Interacts with Tyrosine Hydroxylase to Regulate Enzyme Activity and Synaptic Vesicle Localization

We previously reported that the vesicular monoamine transporter 2 (VMAT2) is physically and functionally coupled with Hsc70 as well as with the dopamine synthesis enzymes tyrosine hydroxylase (TH) and aromatic amino acid decarboxylase, providing a novel mechanism for dopamine homeostasis regulation. Here we expand those findings to demonstrate that Hsc70 physically and functionally interacts with TH to regulate the enzyme activity and synaptic vesicle targeting. Co-immunoprecipitation assays performed in brain tissue and heterologous cells demonstrated that Hsc70 interacts with TH and aromatic amino acid decarboxylase. Furthermore, in vitro binding assays showed that TH directly binds the substrate binding and carboxyl-terminal domains of Hsc70. Immunocytochemical studies indicated that Hsc70 and TH co-localize in midbrain dopaminergic neurons. The functional significance of the Hsc70-TH interaction was then investigated using TH activity assays. In both dopaminergic MN9D cells and mouse brain synaptic vesicles, purified Hsc70 facilitated an increase in TH activity. Neither the closely related protein Hsp70 nor the unrelated Hsp60 altered TH activity, confirming the specificity of the Hsc70 effect. Overexpression of Hsc70 in dopaminergic MN9D cells consistently resulted in increased TH activity whereas knockdown of Hsc70 by short hairpin RNA resulted in decreased TH activity and dopamine levels. Finally, in cells with reduced levels of Hsc70, the amount of TH associated with synaptic vesicles was decreased. This effect was rescued by addition of purified Hsc70. Together, these data demonstrate a novel interaction between Hsc70 and TH that regulates the activity and localization of the enzyme to synaptic vesicles, suggesting an important role for Hsc70 in dopamine homeostasis.

The V81M variant of tyrosine hydroxylase is associated with more severe freezing of gait in Parkinson's disease

INTRODUCTION

Many of the symptoms and signs of Parkinson's disease (PD) arise from the death of midbrain dopamine neurons that utilize tyrosine hydroxylase (TH) as the rate-limiting enzyme in catecholamine biosynthesis.

METHODS

We investigated whether the presence of a common TH polymorphism affects the clinical outcomes in 101 PD subjects. We further examined the effect of this polymorphism on the purified recombinant enzyme.

RESULTS

PD subjects homozygous for the common V81M polymorphism, have higher overall freezing of gait scores after controlling for disease duration, although this polymorphism does not associate with the occurrence of PD or FOG. In vitro functional assays on pure recombinant wild type TH and V81M TH revealed that the Km of the mutant enzyme for tyrosine was twice that of the wild-type. This polymorphism, however, did not change the stability of the enzyme, nor did it affect the Vmax or Km for the co-substrate BH4.

CONCLUSION

The data suggest that presence of a homozygous V81M polymorphism is associated with more severe FOG, possibly due to lower catecholamine synthetic capacity. Further studies are warranted to investigate the role of subtle changes in catecholamine availability in the development of FOG.

Brain catecholamine depletion and motor impairment in a Th knock-in mouse with type B tyrosine hydroxylase deficiency

Tyrosine hydroxylase catalyses the hydroxylation of L-tyrosine to l-DOPA, the rate-limiting step in the synthesis of catecholamines. Mutations in the TH gene encoding tyrosine hydroxylase are associated with the autosomal recessive disorder tyrosine hydroxylase deficiency, which manifests phenotypes varying from infantile parkinsonism and DOPA-responsive dystonia, also termed type A, to complex encephalopathy with perinatal onset, termed type B. We generated homozygous Th knock-in mice with the mutation Th-p.R203H, equivalent to the most recurrent human mutation associated with type B tyrosine hydroxylase deficiency (TH-p.R233H), often unresponsive to l-DOPA treatment. The Th knock-in mice showed normal survival and food intake, but hypotension, hypokinesia, reduced motor coordination, wide-based gate and catalepsy. This phenotype was associated with a gradual loss of central catecholamines and the serious manifestations of motor impairment presented diurnal fluctuation but did not improve with standard l-DOPA treatment. The mutant tyrosine hydroxylase enzyme was unstable and exhibited deficient stabilization by catecholamines, leading to decline of brain tyrosine hydroxylase-immunoreactivity in the Th knock-in mice. In fact the substantia nigra presented an almost normal level of mutant tyrosine hydroxylase protein but distinct absence of the enzyme was observed in the striatum, indicating a mutation-associated mislocalization of tyrosine hydroxylase in the nigrostriatal pathway. This hypomorphic mouse model thus provides understanding on pathomechanisms in type B tyrosine hydroxylase deficiency and a platform for the evaluation of novel therapeutics for movement disorders with loss of dopaminergic input to the striatum.

Functional characterization of the S41Y (C2755A) polymorphism of tryptophan hydroxylase 2

Human TPH2 (hTPH2) catalyzes the rate-limiting step in CNS serotonin biosynthesis. We characterized a single-nucleotide polymorphism (C2755A) in the hTPH2 gene that substitutes tyrosine for serine at position 41 in the regulatory domain of the enzyme. This polymorphism is associated with bipolar disorder and peripartum depression in a Chinese population. Recombinant h TPH2 human proteins were expressed in bacteria and also stably expressed in PC12 cells. Following bacterial expression and purification, the tyrosine for serine substitution at position 41 (S41Y) polymorphic enzyme displayed increased Vmax with unchanged Km values. By contrast, enzyme stability was decreased in vitro from 32 min to 4 min (37 °C) for the S41Y enzyme (as compared to the wild-type enzyme). The S41Y polymorphism decreased cyclic AMP-dependent protein kinase A-mediated phosphorylation ~ 50% relative to wild-type hTPH2, suggesting that the S41Y mutation may disrupt the post-translational regulation of this enzyme. Transfected PC12 cells expressed hTPH2 mRNA, active protein, and synthesized and released serotonin. Paradoxically, while S41Y-transfected PC12 cells expressed higher levels of hTPH2 than wild type, they synthesized less serotonin. These findings suggest a modified regulation of the S41Y gene variant leading to altered regulation and reduced neurotransmitter synthesis that may contribute to association of the polymorphism with bipolar disorder and depression. We report the functional implications of a polymorphic human tryptophan hydroxylase-2 gene associated with depression and bipolar disorder. The polymorphic enzyme (serine-41 converted to tyrosine) has increased activity, but decreased enzyme stability and serotonin production. Moreover, cyclic AMP-dependent protein kinase (PKA)-mediated phosphorylation of the mutant enzyme is decreased suggesting modified regulation of the S41Y variant leading to altered serotonin.

Evidence for a functionally important histidine residue in human tyrosine hydroxylase

Recombinant human tyrosine hydroxylase isozyme 1 (hTH1) shows a time- and concentration-dependent loss of catalytic activity when incubated with diethylpyrocarbonate (DEP) after reconstitution with Fe(II). The inactivation follows pseudo-first order kinetics with a second order rate constant of 300 M(-1) min(-1) at pH 6.8 and 20°C and is partially reversed by hydroxylamine. The difference absorption spectrum of the DEP-modified vs native enzyme shows a peak at 244 nm, characteristic of mono-N-carbethoxy-histidine. Up to five histidine residues are modified per enzyme subunit by a five-fold excess of the reagent, and two of them are protected from inactivation by the active site inhibitor dopamine. However, derivatization of only one residue appears to be responsible for the inactivation. Thus, no inactivation by DEP was found when the apoenzyme was preincubated with this reagent prior to its reconstitution with Fe(II), modifying four histidine residues.

Enhanced assymetrical noradrenergic transmission in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

The ablation of olfactory bulb induces critical changes in dopamine, and monoamine oxidase activity in the brain stem. Growing evidence supports the participation of this telencephalic region in the regulation blood pressure and cardiovascular activity but little is known about its contribution to hypertension. We have previously reported that in the olfactory bulb of normotensive rats endothelins enhance noradrenergic activity by increasing tyrosine hydroxylase activity and norepinephrine release. In the present study we sought to establish the status of noradrenergic activity in the olfactory bulb of deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Different steps in norepinephrine transmission including tyrosine hydroxylase activity, neuronal norepinephrine release and uptake were assessed in the left and right olfactory bulb of DOCA-salt hypertensive rats. Increased tyrosine hydroxylase activity, and decreased neuronal norepinephrine uptake were observed in the olfactory bulb of DOCA-salt hypertensive rats. Furthermore the expression of tyrosine hydroxylase and its phosphorylated forms were also augmented. Intriguingly, asymmetrical responses between the right and left olfactory bulb of normotensive and hypertensive rats were observed. Neuronal norepinephrine release was increased in the right but not in the left olfactory bulb of DOCA-salt hypertensive rats, whereas non asymmetrical differences were observed in normotensive animals. Present findings indicate that the olfactory bulb of hypertensive rats show an asymmetrical increase in norepinephrine activity. The observed changes in noradrenergic transmission may likely contribute to the onset and/or progression of hypertension in this animal model.

Tyrosine hydroxylase phosphorylation in catecholaminergic brain regions: a marker of activation following acute hypotension and glucoprivation

The expression of c-Fos defines brain regions activated by the stressors hypotension and glucoprivation however, whether this identifies all brain sites involved is unknown. Furthermore, the neurochemicals that delineate these regions, or are utilized in them when responding to these stressors remain undefined. Conscious rats were subjected to hypotension, glucoprivation or vehicle for 30, 60 or 120 min and changes in the phosphorylation of serine residues 19, 31 and 40 in the biosynthetic enzyme, tyrosine hydroxylase (TH), the activity of TH and/or, the expression of c-Fos were determined, in up to ten brain regions simultaneously that contain catecholaminergic cell bodies and/or terminals: A1, A2, caudal C1, rostral C1, A6, A8/9, A10, nucleus accumbens, dorsal striatum and medial prefrontal cortex. Glucoprivation evoked phosphorylation changes in A1, caudal C1, rostral C1 and nucleus accumbens whereas hypotension evoked changes A1, caudal C1, rostral C1, A6, A8/9, A10 and medial prefrontal cortex 30 min post stimulus whereas few changes were evident at 60 min. Although increases in pSer19, indicative of depolarization, were seen in sites where c-Fos was evoked, phosphorylation changes were a sensitive measure of activation in A8/9 and A10 regions that did not express c-Fos and in the prefrontal cortex that contains only catecholaminergic terminals. Specific patterns of serine residue phosphorylation were detected, dependent upon the stimulus and brain region, suggesting activation of distinct signaling cascades. Hypotension evoked a reduction in phosphorylation in A1 suggestive of reduced kinase activity. TH activity was increased, indicating synthesis of TH, in regions where pSer31 alone was increased (prefrontal cortex) or in conjunction with pSer40 (caudal C1). Thus, changes in phosphorylation of serine residues in TH provide a highly sensitive measure of activity, cellular signaling and catecholamine utilization in catecholaminergic brain regions, in the short term, in response to hypotension and glucoprivation.

Engineering specialized metabolic pathways--is there a room for enzyme improvements?

Recent advances in enzyme engineering enable dramatic improvements in catalytic efficiency and/or selectivity, as well as de novo engineering of enzymes to catalyze reactions where natural enzymes are not available. Can these capabilities be utilized to transform biosynthesis pathways? Metabolic engineering is traditionally based on combining existing enzymes to give new, or modified, pathways, within a new context and/or organism. How efficient, however, are the individual enzyme components? Is there room to improve pathway performance by enzyme engineering? We discuss the differences between enzymes in central versus specialized, or secondary metabolism and highlight unique features of specialized metabolism enzymes participating in the synthesis of natural products. We argue that, for the purpose of metabolic engineering, the catalytic efficiency and selectivity of many enzymes can be improved with the aim of achieving higher rates, yields and product purities. We also note the relative abundance of spontaneous reactions in specialized metabolism, and the potential advantage of engineering enzymes that will catalyze these steps. Specialized metabolism therefore offers new opportunities to integrate enzyme and pathway engineering, thereby achieving higher metabolic efficiencies, enhanced production rates and improved product purities.

Gender-specific regulation of tyrosine hydroxylase in thymocyte differentiation antigen-1 knockout mice

Thymocyte differentiation antigen-1 (Thy-1) is a cell surface glycoprotein found on T cells and neurons and is involved in cell-to-cell interactions. In addition, Thy-1 knockouts (KO) are a potential mouse model of restless legs syndrome (RLS) based on clinical observations and the role of dopamine in the disease. In this study, we analyzed the activity and quantity of tyrosine hydroxylase (TH; the rate-limiting enzyme in dopamine production) and determined phosphorylation levels for the enzyme phosphoserine-40 (pSer-40). There was no significant difference in the total TH activity and pSer-40 TH levels between Thy-1 KO and control groups; however, TH specific activity was significantly lower (by 26%) in Thy-1 KO mice. This difference is due in part to increased TH protein levels in this group (increased by 29%). When analyzed by gender, Thy-1 KO female mouse striata contained less TH specific activity compared with control females (decreased by 41%) and male control or Thy-1 KO animals (decreased by 30%). TH specific activity and pSer-40 TH levels in male Thy-1 KO and control displayed no differences. However, pSer-40 TH was significantly higher in control females (38%) compared with control or Thy-1 KO males. The Thy-1 KO females exhibited significantly lower (28%) pSer-40 TH (normalized to GAPDH or TH) than control females. Indeed, the Thy-1 KO females had 50% of the pSer-40 TH found in controls. Our results suggest a gender effect on TH specific activity, TH protein levels, and serine-40 phosphorylation of TH in Thy-1 KO female mice.

Modulating effects of selenium in adrenal medulla of rats exposed to 7,12-dimethylbenz[a]anthracene

OBJECTIVE

The aim of this study was to evaluate the chemopreventive potential of organoselenium compounds (Se I and Se II) in the well-established rat model treated with 7,12-dimethylbenz[a]anthracene (DMBA), by monitoring the extent of tyrosine hydroxylase (TH) activity, adrenomedullin (ADM) level and total RNA level in adrenal medulla. Organic pollutants are the most important environmental factor for the biologic systems. DMBA exposure appears to be associated with a number of physiological disease processes.

METHODS

The effects of Se I and Se II compounds were investigated on TH activity, ADM and total RNA levels in adrenal medulla of rats exposed to DMBA.

RESULTS

TH activity, ADM and total RNA levels were found to be increased significantly due to the effect of DMBA (p < 0.05). This increase was restricted in the Se I- and Se II-treated groups (p < 0.05).

CONCLUSION

The present data showed that the organoselenium compounds may have important effects in the maintainance of homeostasis against stress induced by DMBA.

Progressive neurodegenerative and behavioural changes induced by AAV-mediated overexpression of α-synuclein in midbrain dopamine neurons

Parkinson's disease (PD) is characterised by the progressive loss of nigral dopamine neurons and the presence of synucleinopathy. Overexpression of α-synuclein in vivo using viral vectors has opened interesting possibilities to model PD-like pathology in rodents. However, the attempts made so far have failed to show a consistent behavioural phenotype and pronounced dopamine neurodegeneration. Using a more efficient adeno-associated viral (AAV) vector construct, which includes a WPRE enhancer element and uses the neuron-specific synapsin-1 promoter to drive the expression of human wild-type α-synuclein, we have now been able to achieve increased levels of α-synuclein in the transduced midbrain dopamine neurons sufficient to induce profound deficits in motor function, accompanied by reduced expression of proteins involved in dopamine neurotransmission and a time-dependent loss of nigral dopamine neurons, that develop progressively over 2-4 months after vector injection. As in human PD, nigral cell loss was preceded by degenerative changes in striatal axons and terminals, and the appearance of α-synuclein positive inclusions in dystrophic axons and dendrites, supporting the idea that α-synuclein-induced pathology hits the axons and terminals first and later progresses to involve also the cell bodies. The time-course of changes seen in the AAV-α-synuclein treated animals defines distinct stages of disease progression that matches the pre-symptomatic, early symptomatic, and advanced stages seen in PD patients. This model provides new interesting possibilities for studies of stage-specific pathologic mechanisms and identification of targets for disease-modifying therapeutic interventions linked to early or late stages of the disease.

Divergence in enzyme regulation between Caenorhabditis elegans and human tyrosine hydroxylase, the key enzyme in the synthesis of dopamine

TH (tyrosine hydroxylase) is the rate-limiting enzyme in the synthesis of catecholamines. The cat-2 gene of the nematode Caenorhabditis elegans is expressed in mechanosensory dopaminergic neurons and has been proposed to encode a putative TH. In the present paper, we report the cloning of C. elegans full-length cat-2 cDNA and a detailed biochemical characterization of the encoded CAT-2 protein. Similar to other THs, C. elegans CAT-2 is composed of an N-terminal regulatory domain followed by a catalytic domain and a C-terminal oligomerization domain and shows high substrate specificity for L-tyrosine. Like hTH (human TH), CAT-2 is tetrameric and is phosphorylated at Ser35 (equivalent to Ser40 in hTH) by PKA (cAMP-dependent protein kinase). However, CAT-2 is devoid of characteristic regulatory mechanisms present in hTH, such as negative co-operativity for the cofactor, substrate inhibition or feedback inhibition exerted by catecholamines, end-products of the pathway. Thus TH activity in C. elegans displays a weaker regulation in comparison with the human orthologue, resembling a constitutively active enzyme. Overall, our data suggest that the intricate regulation characteristic of mammalian TH might have evolved from more simple models to adjust to the increasing complexity of the higher eukaryotes neuroendocrine systems.

TPH2 in the ventral tegmental area of the male rat brain

This study surveyed the distribution of tryptophan hydroxylase 2 (TPH2) mRNA, protein, and enzymatic activity throughout the male Sprague-Dawley rat brain. TPH2 is the genetic isoform of TPH that catalyzes the rate-limiting step in serotonin biosynthesis within the central nervous system. Although cell bodies of serotonergic neurons are located mainly in the raphe, serotonin-containing axons innervate many regions of the brain. In the present study, we assessed the levels of mRNA, protein expression, and enzyme activity of TPH2 in the rat raphe, ventral tegmental area (VTA), substantia nigra, hippocampus, cerebellum, dorsal striatum, nucleus accumbens, amygdala, and medial prefrontal cortex to more fully understand the distribution of this enzyme throughout the central nervous system. The pineal gland was used as a control tissue that expresses TPH1 (the peripheral enzyme), but not TPH2. As expected, the raphe showed the highest brain TPH2 activity and protein expression. In the contrast to other reports, however, the VTA followed the raphe as the region with the second-highest amount of TPH2 activity, mRNA and protein expression. There were significantly lower TPH activities and levels of TPH2 protein in the other regions. In addition, TPH2 immunocytochemistry demonstrated the presence of TPH-positive cell bodies within the VTA. The results of this study indicate that TPH2 and serotonergic signaling may play an important role in the mesolimbic/mesocortical reward pathway.

Co-species housing in mice and rats: effects on physiological and behavioral stress responsivity

Co-species housing of mice and rats is common practice at most breeding facilities and research laboratories, neglecting the possible effects on the animals. We investigated physiological as well as behavioral stress-reactivity in mice and rats which were either derived from a co-species or species-separated housing condition at the breeding facilities. The animals were kept under the housing condition they were used to or assigned to the opposite one. Co-species housing had a significant impact on acute stress reactivity in mice and rats but only if they were used to this housing condition throughout their lives. Moreover, the stress-effects appeared to be long lasting. Assigning animals, derived from a species-separated housing condition, to co-species housing led to chronic stress in mice and affected experimental behavior of rats. Our findings led to the conclusion that co-species housing in mice and rats should be avoided, supporting the recommendations by the U.S. National Institutes of Health (NIH) and the Dutch Ministry of Health, Welfare and Sport (VWS). In order to support the interpretation, facilitate the reproducibility and comparability and subsequently the generalizability of experimental results, breeding facilities should at least provide detailed information about their housing conditions.

Role of selenium compounds on tyrosine hydroxylase activity, adrenomedullin and total RNA levels in hearts of rats

Synthetic organoselenium compounds can be tailored to achieve greater chemopreventive efficacy with minimal toxic side effects by structural modifications. Two organoselenium compounds (Se I and Se II) were synthesized and evaluated for their antihypertensive and therapeutic properties by adrenomedullin (ADM) levels and tyrosine hydroxylase (TH) activity assays in rat heart tissue. 7,12-Dimethylbenz[a]anthracene (DMBA) is known to generate DNA-reactive species during their metabolism, which may enhance oxidative stress in cells. TH is thought to be a rate-limiting enzyme in the biosynthesis of catecholamines. ADM, a potent endogenous vasodilating and natriuretic peptide, may play an important role in the pathophysiology of chronic heart failure. The effects of Se I and Se II were investigated on TH activity, ADM and total RNA levels in the hearts of albino Wistar rats. TH activity was found to be increased significantly by the effect of DMBA (P<0.05). This increase was restricted in the Se I and Se II treated groups. ADM level was found to be decreased insignificantly by the effect of DMBA (P>0.05). Total RNA level was found to be decreased significantly by the effect of DMBA (P<0.05). This study demonstrates that synthetic organoselenium compounds can regulate DMBA-induced stress related changes in rat heart.

A biochemical and functional protein complex involving dopamine synthesis and transport into synaptic vesicles

Synaptic transmission depends on neurotransmitter pools stored within vesicles that undergo regulated exocytosis. In the brain, the vesicular monoamine transporter-2 (VMAT(2)) is responsible for the loading of dopamine (DA) and other monoamines into synaptic vesicles. Prior to storage within vesicles, DA synthesis occurs at the synaptic terminal in a two-step enzymatic process. First, the rate-limiting enzyme tyrosine hydroxylase (TH) converts tyrosine to di-OH-phenylalanine. Aromatic amino acid decarboxylase (AADC) then converts di-OH-phenylalanine into DA. Here, we provide evidence that VMAT(2) physically and functionally interacts with the enzymes responsible for DA synthesis. In rat striata, TH and AADC co-immunoprecipitate with VMAT(2), whereas in PC 12 cells, TH co-immunoprecipitates with the closely related VMAT(1) and with overexpressed VMAT(2). GST pull-down assays further identified three cytosolic domains of VMAT(2) involved in the interaction with TH and AADC. Furthermore, in vitro binding assays demonstrated that TH directly interacts with VMAT(2). Additionally, using fractionation and immunoisolation approaches, we demonstrate that TH and AADC associate with VMAT(2)-containing synaptic vesicles from rat brain. These vesicles exhibited specific TH activity. Finally, the coupling between synthesis and transport of DA into vesicles was impaired in the presence of fragments involved in the VMAT(2)/TH/AADC interaction. Taken together, our results indicate that DA synthesis can occur at the synaptic vesicle membrane, where it is physically and functionally coupled to VMAT(2)-mediated transport into vesicles.

Differential regulation of human tyrosine hydroxylase isoforms 1 and 2 in situ: Isoform 2 is not phosphorylated at Ser35

The major human tyrosine hydroxylase isoforms (hTH1 and 2) differ in their ability to be phosphorylated in vitro. hTH1 is phosphorylated at Ser31 by extracellular signal-regulated kinase (ERK). This kinase is not capable of phosphorylating hTH2 at Ser35 (the residue that corresponds to Ser31 in hTH1). We have stably transfected SH-SY5Y cells with hTH1 or hTH2 to determine if hTH2 can be phosphorylated at Ser35 in situ. Forskolin increased the phosphorylation of Ser40 in hTH1 and Ser44 in hTH2. Muscarine increased the phosphorylation of both Ser19 and Ser40/44 in both hTH1 and hTH2. EGF increased the phosphorylation of Ser31 in hTH1. Phosphorylation of Ser35 in hTH2 was not detected under any of the conditions tested. Inhibition of ERK by UO126 decreased the phosphorylation of Ser31 and this lead to a 50% decrease in the basal level of phosphorylation of Ser40 in hTH1. The basal level of Ser44 phosphorylation in hTH2 was not altered by treatment with UO126. Therefore, phosphorylation of Ser31 contributes to the phosphorylation of Ser40 in hTH1 in situ; however, this effect is absent in hTH2. This represents a major difference between the two human TH isoforms, and has implications for the regulation of catecholamine synthesis in vivo.

Dose optimization for long-term rAAV-mediated RNA interference in the nigrostriatal projection neurons

Short-hairpin RNA (shRNA)-mediated gene knockdown is a powerful tool for targeted gene silencing and an emerging novel therapeutic strategy. Recent publications, however, reported unexpected toxicity after utilizing viral-mediated shRNA knockdown in vivo. Thus, it is currently unclear whether shRNA-mediated knockdown strategy can be used as a safe and efficient tool for gene silencing. In this study, we have generated rAAV vectors expressing shRNAs targeting the rat tyrosine hydroxylase (TH) mRNA (shTH) for testing the efficacy of in vivo TH knockdown in the nigral dopaminergic neurons. At high titers, not only the shTH vectors but also the scrambled and green fluorescence protein (GFP)-only controls caused cell death. In a dose-response study, we identified a dose window leading to >60% decrease in TH(+) neurons without any change in vesicular monoamine transporter-2 (VMAT2) expression. Moreover, using the safe and efficient dose, we showed that dopamine (DA) synthesis rate was significantly reduced and this lead to emergence of motor deficits in the shTH-expressing rats. Interestingly, these animals showed very robust and long-lasting recovery after a single systemic L-3,4-dihydroxyphenylalanine (L-DOPA) administration beyond what can be achieved in 6-hydroxydopamine (6-OHDA)-lesioned rats. Our results have implications for both mechanistic and therapeutic studies utilizing long-term shRNA-mediated gene silencing in the nigrostriatal projection system.

The low affinity dopamine binding site on tyrosine hydroxylase: the role of the N-terminus and in situ regulation of enzyme activity

Tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis, is inhibited in vitro by catecholamines binding to two distinct sites on the enzyme. The N-terminal regulatory domain of TH contributes to dopamine binding to the high affinity site of the enzyme. We prepared an N-terminal deletion mutant of TH to examine the role of the N-terminal domain in dopamine binding to the low affinity site. Deletion of the N-terminus of TH removes the high affinity dopamine binding site, but does not affect dopamine binding to the low affinity site. The role of the low affinity site in situ was examined by incubating PC12 cells with L-DOPA to increase the cytosolic catecholamine concentration. This resulted in an inhibition of TH activity in situ under both basal conditions and conditions that promoted the phosphorylation of Ser40. Therefore the low affinity site is active in situ regardless of the phosphorylation status of Ser40.

Individual housing of mice--impact on behaviour and stress responses

The replicability of results derived from studies in rodents might be influenced by stress caused by inappropriate housing conditions. Here we compared the experimental behaviour and stress response (circulating corticosterone level and adrenal tyrosine hydroxylase activity) of individually-housed male and female inbred mice with that of animals housed in social groups. All mice were behaviourally tested in the modified hole board test (mHB). Male C57BL/6, BALB/c and A mice housed in groups of 3 were compared with individually-housed mice. In a subsequent experiment female C57BL/6 and BALB/c mice were housed under similar conditions. To exclude the possible effects of within-cage order of testing, only one individual per group was behaviourally tested. Neither male nor female mice housed individually showed stronger signs of stress than their socially-housed counterparts. However, we observed a within-cage order effect on the hormonal stress response (corticosterone) in socially-housed female C57BL/6 mice. No effects of individual housing on behaviour in the mHB were found.

Chlamydia pneumoniae encodes a functional aromatic amino acid hydroxylase

Chlamydia pneumoniae is a community-acquired respiratory pathogen that has been associated with the development of atherosclerosis. Analysis of the C. pneumoniae genome identified a gene (Cpn1046) homologous to eukaryotic aromatic amino acid hydroxylases (AroAA-Hs). AroAA-Hs hydroxylate phenylalanine, tyrosine, and tryptophan into tyrosine, dihydroxyphenylalanine, and 5-hydroxytryptophan, respectively. Sequence analysis of Cpn1046 demonstrated that residues essential for AroAA-H enzymatic function are conserved and that a subset of Chlamydia species contain an AroAA-H homolog. The chlamydial AroAA-Hs are transcriptionally linked to a putative bacterial membrane transport protein. We determined that recombinant Cpn1046 is able to hydroxylate phenylalanine, tyrosine, and tryptophan with roughly equivalent activity for all three substrates. Cpn1046 is expressed within 24 h of infection, allowing C. pneumoniae to hydroxylate host stores of aromatic amino acids during the period of logarithmic bacterial growth. From these results we can conclude that C. pneumoniae, as well as a subset of other Chlamydia species, encode an AroAA-H that is able to use all three aromatic amino acids as substrates. The maintenance of this gene within a number of Chlamydia suggests that the enzyme may have an important role in shaping the metabolism or overall pathogenesis of these bacteria.

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.

Tyrosine hydroxylase activity is regulated by two distinct dopamine-binding sites

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of the catecholamines dopamine, noradrenaline and adrenaline, is regulated acutely by feedback inhibition by the catecholamines and relief of this inhibition by phosphorylation of serine 40 (Ser40). Phosphorylation of serine 40 abolishes the binding of dopamine to a high affinity (K(D) < 4 nM) site on TH, thereby increasing the activity of the enzyme. We have found that TH also contains a second low affinity (K(D) = 90 nM) dopamine-binding site, which is present in both the non-phosphorylated and the Ser40-phosphorylated forms of the enzyme. Binding of dopamine to the high-affinity site decreases V(max) and increases the K(m) for the cofactor tetrahydrobiopterin, while binding of dopamine to the low-affinity site regulates TH activity by increasing the K(m) for tetrahydrobiopterin. Kinetic analysis indicates that both sites are present in each of the four human TH isoforms. Dissociation of dopamine from the low-affinity site increases TH activity 12-fold for the non-phosphorylated enzyme and 9-fold for the Ser40-phosphorylated enzyme. The low-affinity dopamine-binding site has the potential to be the primary mechanism responsible for the regulation of catecholamine synthesis under most conditions.

Anabolic function of phenylalanine hydroxylase in Caenorhabditis elegans

In humans, liver phenylalanine hydroxylase (PAH) has an established catabolic function, and mutations in PAH cause phenylketonuria, a genetic disease characterized by neurological damage, if not treated. To obtain novel evolutionary insights and information on molecular mechanisms operating in phenylketonuria, we investigated PAH in the nematode Caenorhabditis elegans (cePAH), where the enzyme is coded by the pah-1 gene, expressed in the hypodermis. CePAH presents similar molecular and kinetic properties to human PAH [S(0.5)(L-Phe) approximately 150 microM; K(m) for tetrahydrobiopterin (BH(4)) approximately 35 microM and comparable V(max)], but cePAH is devoid of positive cooperativity for L-Phe, an important regulatory mechanism of mammalian PAH that protects the nervous system from excess L-Phe. Pah-1 knockout worms show no obvious neurological defects, but in combination with a second cuticle synthesis mutation, they display serious cuticle abnormalities. We found that pah-1 knockouts lack a yellow-orange pigment in the cuticle, identified as melanin by spectroscopic techniques, and which is detected in C. elegans for the first time. Pah-1 mutants show stimulation of superoxide dismutase activity, suggesting that cuticle melanin functions as oxygen radical scavenger. Our results uncover both an important anabolic function of PAH and the change in regulation of the enzyme along evolution.

Assay of tyrosine hydroxylase based on high-performance liquid chromatography separation and quantification of L-dopa and L-tyrosine

An assay of L-tyrosine (Tyr) hydroxylating activity operating in lincomycin biosynthesis is described. The assay development consisted of HPLC procedure development, assessing the effect of reaction mixture components on non-enzymatic Dopa and Tyr oxidation, and sample stability evaluation. The HPLC procedure with isocratic elution and fluorescence detection was developed and validated. The method showed a wide linear range of Dopa determination of 0.125-25 micromol/L with lower limit of quantification (LLOQ) of 0.125 micromol/L, RSD of 7.2% and accuracy of 101.7%. The studied linear range of Tyr was 15.625 mmol/L to 500 mmol/L with LLOQ of 15.625 mmol/L, RSD of 1.1%, and accuracy of 98.1%. Recoveries for Dopa and Tyr were 100.66 +/- 0.89% and 94.76 +/- 0.94%, respectively. The inter- and intra-day accuracies and precisions were all within 10%. Samples of the reaction mixture were stable for at least 24 h at room temperature (RT) and 28 days at -20 degrees C. The method was tested for the enzyme activity monitoring in purified as well as crude preparations and enabled micro preparation of the enzyme product during confirmation of its identity. The influence of pH and ascorbic acid content in reaction mixture was studied with respect to non-enzymatic Tyr oxidation.

Long-term modulation of tyrosine hydroxylase activity and expression by endothelin-1 and -3 in the rat anterior and posterior hypothalamus

Brain catecholamines are involved in the regulation of biological functions, including cardiovascular activity. The hypothalamus presents areas with high density of catecholaminergic neurons and the endothelin system. Two hypothalamic regions intimately related with the cardiovascular control are distinguished: the anterior (AHR) and posterior (PHR) hypothalamus, considered to be sympathoinhibitory and sympathoexcitatory regions, respectively. We previously reported that endothelins (ETs) are involved in the short-term tyrosine hydroxylase (TH) regulation in both the AHR and PHR. TH is crucial for catecholaminergic transmission and is tightly regulated by well-characterized mechanisms. In the present study, we sought to establish the effects and underlying mechanisms of ET-1 and ET-3 on TH long-term modulation. Results showed that in the AHR, ETs decreased TH activity through ETBreceptor activation coupled to the nitric oxide, phosphoinositide, and CaMK-II pathways. They also reduced total TH level and TH phosphorylated forms (Ser 19 and 40). Conversely, in the PHR, ETs increased TH activity through a G protein-coupled receptor, likely an atypical ET receptor or the ETCreceptor, which stimulated the phosphoinositide and adenylyl cyclase pathways, as well as CaMK-II. ETs also increased total TH level and the Ser 19, 31, and 40 phosphorylated sites of the enzyme. These findings support that ETs are involved in the long-term regulation of TH activity, leading to reduced sympathoinhibition in the AHR and increased sympathoexcitation in the PHR. Present and previous studies may partially explain the cardiovascular effects produced by ETs when applied to the brain.

Retinol activates tyrosine hydroxylase acutely by increasing the phosphorylation of serine40 and then serine31 in bovine adrenal chromaffin cells

Tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of the catecholamines. It has been reported that retinol (vitamin A) modulates tyrosine hydroxylase activity by increasing its expression through the activation of the nuclear retinoid receptors. In this study, we observed that retinol also leads to an acute activation of tyrosine hydroxylase in bovine adrenal chromaffin cells and this was shown to occur via two distinct non-genomic mechanisms. In the first mechanism, retinol induced an influx in extracellular calcium, activation of protein kinase C and serine40 phosphorylation, leading to tyrosine hydroxylase activation within 15 min. This effect then declined over time. The retinol-induced rise in intracellular calcium then led to a second slower mechanism; this involved an increase in reactive oxygen species, activation of extracellular signal-regulated kinase 1/2 and serine31 phosphorylation and the maintenance of tyrosine hydroxylase activation for up to 2 h. No effects were observed with retinoic acid. These results show that retinol activates tyrosine hydroxylase via two sequential non-genomic mechanisms, which have not previously been characterized. These mechanisms are likely to operate in vivo to facilitate the stress response, especially when vitamin supplements are taken or when retinol is used as a therapeutic agent.

Structure of phenylalanine hydroxylase from Colwellia psychrerythraea 34H, a monomeric cold active enzyme with local flexibility around the active site and high overall stability

The characteristic of cold-adapted enzymes, high catalytic efficiency at low temperatures, is often associated with low thermostability and high flexibility. In this context, we analyzed the catalytic properties and solved the crystal structure of phenylalanine hydroxylase from the psychrophilic bacterium Colwellia psychrerythraea 34H (CpPAH). CpPAH displays highest activity with tetrahydrobiopterin (BH(4)) as cofactor and at 25 degrees C (15 degrees C above the optimal growth temperature). Although the enzyme is monomeric with a single L-Phe-binding site, the substrate binds cooperatively. In comparison with PAH from mesophilic bacteria and mammalian organisms, CpPAH shows elevated [S(0.5)](L-Phe) (= 1.1 +/- 0.1 mm) and K(m)(BH(4))(= 0.3 +/- 0.1 mm), as well as high catalytic efficiency at 10 degrees C. However, the half-inactivation and denaturation temperature is only slightly lowered (T(m) approximately 52 degrees C; where T(m) is half-denaturation temperature), in contrast to other cold-adapted enzymes. The crystal structure shows regions of local flexibility close to the highly solvent accessible binding sites for BH(4) (Gly(87)/Phe(88)/Gly(89)) and l-Phe (Tyr(114)-Pro(118)). Normal mode and COREX analysis also detect these and other areas with high flexibility. Greater mobility around the active site and disrupted hydrogen bonding abilities for the cofactor appear to represent cold-adaptive properties that do not markedly affect the thermostability of CpPAH.

Alpha-synuclein inhibits aromatic amino acid decarboxylase activity in dopaminergic cells

Alpha-synuclein is a presynaptic protein strongly implicated in Parkinson's disease (PD). Because dopamine neurons are invariably compromised during pathogenesis in PD, we have been exploring the functions of alpha-synuclein with particular relevance to dopaminergic neuronal cells. We previously discovered reduced tyrosine hydroxylase (TH) activity and minimal dopamine synthesis in stably-transfected MN9D cells overexpressing either wild-type or A53T mutant (alanine to threonine at amino acid 53) alpha-synuclein. TH, the rate-limiting enzyme in dopamine synthesis, converts tyrosine to l-dihydroxyphenylalanine (L-DOPA), which is then converted to dopamine by the enzyme, aromatic amino acid decarboxylase (AADC). We confirmed an interaction between alpha-synuclein and AADC in striatum. We then sought to determine whether wild-type or A53T mutant alpha-synuclein might have affected AADC activity in dopaminergic cells. Using HPLC with electrochemical detection, we measured dopamine and related catechols after L-DOPA treatments to bypass the TH step. We discovered that while alpha-synuclein did not reduce AADC protein levels, it significantly reduced AADC activity and phosphorylation in our cells. These novel findings further support a role for alpha-synuclein in dopamine homeostasis and may explain, at least in part, the selective vulnerability of dopamine neurons that occurs in PD.

Nicotinic Cholinergic Regulation of Tyrosine Hydroxylase Gene Expression and Catecholamine Synthesis in Isolated Bovine Adrenal Chromaffin Cells

Isolated bovine adrenal chromaffin cells were used to study the nicotinic regulation of tyrosine hydroxylase (TH) gene expression. Continuous exposure of the cells to carbachol or the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) produces a time- and concentration-dependent increase in TH enzyme activity, whereas muscarine has no effect. DMPP at 1 microM (EC50 = 0.3 microM) elicits a two- to threefold elevation of both TH activity and TH immunoreactive protein level after 3-5 days in the presence of 2.5 mM calcium; the increase in enzyme levels is significantly less at lower extracellular calcium levels. The rate of hydroxylation of tyrosine to dopamine (DA) in intact cells, an index of endogenous TH activity, increases in parallel with the rise in TH levels. The TH mRNA level is elevated before the increase in protein levels. As determined by nuclear run-on assays, TH gene transcription is stimulated two- to threefold within 30 min of addition of 1 microM DMPP to the cells; transcription returns to basal levels by 2 h. Nitrendipine (20 microM) blocks the stimulation of transcription by DMPP. Pretreatment of the cells with cycloheximide (5 microM) does not prevent the DMPP stimulation of transcription. Forskolin (10 microM) also increases TH transcription (fourfold in 15 min) by a mechanism that is not blocked by cycloheximide. These results show that nicotinic receptor stimulation increases TH mRNA synthesis, TH protein levels, and TH activity in a calcium-dependent manner. Furthermore, the nicotinic influence on TH gene expression does not appear to require the synthesis of a protein factor for its effects. That in situ DA synthesis rates are elevated consequent to the rise in TH levels demonstrates that TH induction serves as a mechanism for enhancing the catecholamine-synthesizing capacity of the chromaffin cell on a long-term basis.

Specific interaction of the diastereomers 7(R)- and 7(S)-tetrahydrobiopterin with phenylalanine hydroxylase: implications for understanding primapterinuria and vitiligo

Pterin-4a-carbinolamine dehydratase (PCD) is an essential component of the phenylalanine hydroxylase (PAH) system, catalyzing the regeneration of the essential cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin [6(R)BH4]. Mutations in PCD or its deactivation by hydrogen peroxide result in the generation of 7(R,S)BH4, which is a potent inhibitor of PAH that has been implicated in primapterinuria, a variant form of phenylketonuria, and in the skin depigmentation disorder vitiligo. We have synthesized and separated the 7(R) and 7(S) diastereomers confirming their structure by NMR. Both 7(R)- and 7(S)BH4 function as poor cofactors for PAH, whereas only 7(S)BH4 acts as a potent competitive inhibitor vs. 6(R)BH4 (Ki=2.3-4.9 microM). Kinetic and binding studies, as well as characterization of the pterin-enzyme complexes by fluorescence spectroscopy, revealed that the inhibitory effects of 7(R,S)BH4 on PAH are in fact specifically based on 7(S)BH4 binding. The molecular dynamics simulated structures of the pterin-PAH complexes indicate that 7(S)BH4 inhibition is due to its interaction with the polar region at the pterin binding site close to Ser-251, whereas its low efficiency as cofactor is related to a suboptimal positioning toward the catalytic iron. 7(S)BH4 is not an inhibitor for tyrosine hydroxylase (TH) in the physiological range, presumably due to the replacement of Ser-251 by the corresponding Ala297. Taken together, our results identified structural determinants for the specific regulation of PAH and TH by 7(S)BH4, which in turn aid in the understanding of primapterinuria and acute vitiligo.

Functional Domains of Human Tryptophan Hydroxylase 2 (hTPH2)

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in serotonin biosynthesis. A novel gene, termed TPH2, has recently been described. This gene is preferentially expressed in the central nervous system, while the original TPH1 is the peripheral gene. We have expressed human tryptophan hydroxylase 2 (hTPH2) and two deletion mutants (NDelta150 and NDelta150/CDelta24) using isopropyl beta-D-thiogalactopyranoside-free autoinduction in Escherichia coli. This expression system produced active wild type TPH2 with relatively low solubility. The solubility was increased for mutants lacking the NH(2)-terminal regulatory domain. The solubility of hTPH2, NDelta150, and NDelta150/CDelta24 are 6.9, 62, and 97.5%, respectively. Removal of the regulatory domain also produced a more than 6-fold increase in enzyme stability (t((1/2)) at 37 degrees C). The wild type hTPH2, like other members of the aromatic amino acid hydroxylase superfamily, exists as a homotetramer (236 kDa on size exclusion chromatography). Similarly, NDelta150 also migrates as a tetramer (168 kDa). In contrast, removal of the NH(2)-terminal domain and the COOH-terminal, putative leucine zipper tetramerization domain produces monomeric enzyme (39 kDa). Interestingly, removal of the NH(2)-terminal regulatory domain did not affect the Michaelis constants for either substrate but did increase V(max) values. These data identify the NH(2)-terminal regulatory domain as the source of hTPH2 instability and reduced solubility.