Role of serine-19 phosphorylation in regulating tyrosine hydroxylase studied with site- and phosphospecific antibodies and site-directed mutagenesis

Role of NT5DC2 in tyrosine hydroxylase phosphorylation based on the analysis of NT5DC2-binding proteins

The gene encoding 5'-nucleotidase domain-containing protein 2 (NT5DC2) has been associated with neuropsychiatric disorders related to the abnormality of dopamine activity in the brain. However, its physiological functions remain unclear. In this study, we analyzed the features of NT5DC2 that influence its binding with tyrosine hydroxylase (TH) and its effects on dihydroxyphenylalanine (DOPA) synthesis, using NT5DC2 overexpressed in PC12D cells by the pCMV vector. Western blot analysis revealed that the purified NT5DC2-DYKDDDDK-tag (NT5DC2-tag) protein can bind with the phosphorylated form of recombinant human TH type 1 (rhTH1), apart from the endogenous TH in PC12D cells. Proteomic analysis by mass spectrometry revealed that the purified NT5DC2-tag protein has the potential to bind to 41 proteins with multiple phosphorylation sites in PC12D cells (NT5DC2 binding proteins: positive, 391 sites/41 proteins; and negative, 85 sites/27 proteins). Overexpression of NT5DC2 in PC12D cells decreased DOPA levels in the medium. When the lysate of PC12D cells overexpressing NT5DC2 was incubated at 37 °C, the phosphorylated form of endogenous TH in PC12D cells decreased. This decrease was also detected when phosphorylated rhTH1 was incubated with purified NT5DC2-tag. Overall, our results suggest that NT5DC2 regulates DOPA synthesis by promoting the dephosphorylation of TH, similar to a phosphatase. Therefore, our study provides useful information for understanding various disorders associated with abnormalities in dopamine levels in the brain.

Changes in Tyrosine Hydroxylase Activity and Dopamine Synthesis in the Nigrostriatal System of Mice in an Acute Model of Parkinson's Disease as a Manifestation of Neurodegeneration and Neuroplasticity

The progressive degradation of the nigrostriatal system leads to the development of Parkinson’s disease (PD). The synthesis of dopamine, the neurotransmitter of the nigrostriatal system, depends on the rate-limiting enzyme, tyrosine hydroxylase (TH). In this study, we evaluated the synthesis of dopamine during periods of neurodegradation and neuroplasticity in the nigrostriatal system on a model of the early clinical stage of PD. It was shown that the concentration of dopamine correlated with activity of TH, while TH activity did not depend on total protein content either in the SN or in the striatum. Both during the period of neurodegeneration and neuroplasticity, TH activity in SN was determined by the content of P19-TH, and in the striatum it was determined by P31-TH and P40-TH (to a lesser extent). The data obtained indicate a difference in the regulation of dopamine synthesis between DA-neuron bodies and their axons, which must be considered for the further development of symptomatic pharmacotherapy aimed at increasing TH activity.

Enhanced tyrosine hydroxylase activity induces oxidative stress, causes accumulation of autotoxic catecholamine metabolites, and augments amphetamine effects in vivo

In Parkinson's disease, dopamine‐containing nigrostriatal neurons undergo profound degeneration. Tyrosine hydroxylase (TH) is the rate‐limiting enzyme in dopamine biosynthesis. TH increases in vitro formation of reactive oxygen species, and previous animal studies have reported links between cytosolic dopamine build‐up and oxidative stress. To examine effects of increased TH activity in catecholaminergic neurons in vivo, we generated TH‐over‐expressing mice (TH‐HI) using a BAC‐transgenic approach that results in over‐expression of TH with endogenous patterns of expression. The transgenic mice were characterized by western blot, qPCR, and immunohistochemistry. Tissue contents of dopamine, its metabolites, and markers of oxidative stress were evaluated. TH‐HI mice had a 3‐fold increase in total and phosphorylated TH levels and an increased rate of dopamine synthesis. Coincident with elevated dopamine turnover, TH‐HI mice showed increased striatal production of H₂O₂ and reduced glutathione levels. In addition, TH‐HI mice had elevated striatal levels of the neurotoxic dopamine metabolites 3,4‐dihydroxyphenylacetaldehyde and 5‐S‐cysteinyl‐dopamine and were more susceptible than wild‐type mice to the effects of amphetamine and methamphetamine. These results demonstrate that increased TH alone is sufficient to produce oxidative stress in vivo, build up autotoxic dopamine metabolites, and augment toxicity.

Subcellular distribution of human tyrosine hydroxylase isoforms 1 and 4 in SH-SY5Y cells

Tyrosine hydroxylase (TH) is the key enzyme that controls the rate of synthesis of the catecholamines. SH-SY5Y cells with stable transfections of either human tyrosine hydroxylase isoform 1 (hTH1) or human tyrosine hydroxylase isoform 4 (hTH4) were used to determined the subcellular distribution of TH protein and phosphorylated TH, under basal conditions and after muscarine stimulation. Muscarine was previously shown to increase the phosphorylation of only serine 19 and serine 40 in hTH1 cells. Under basal conditions, the hTH1 and hTH4 proteins, their serine 19 phosphorylated forms and hTH1 phosphorylated at serine 40 were all similarly distributed; with ~80% in the cytosolic fraction, ~20% in the membrane fraction, and less than 1%, or not detectable, in the nuclear fraction. However, hTH4 phosphorylated at serine 71 had a significantly different distribution with ~65% cytosolic and ~35% membrane associated. Muscarine stimulation led to hTH1 being redistributed from the cytosol and nuclear fractions to the membrane fraction and hTH4 being redistributed from the cytosol to the nuclear fraction. These muscarine stimulated redistributions were not due to TH phosphorylation at serine 19, serine 40, or serine 71 and were most likely due to TH binding to proteins whose phosphorylation was increased by muscarine. This is the first study to show a difference in subcellular distribution between two human TH isoforms under basal and stimulated conditions.

Dopamine neuron-derived IGF-1 controls dopamine neuron firing, skill learning, and exploration

Midbrain dopamine neurons play a role in motivational and cognitive control of behavior. In addition, they regulate motor functions. Dysregulation of dopamine neurons has been linked to depression, schizophrenia, and addiction and their degeneration is causal to Parkinson’s disease. Peripheral hormones have been shown to regulate dopamine neurons functions. Insulin-like growth factor 1 (IGF-1) is a hormone mainly produced in the liver. With this study we discovered that midbrain dopamine neurons synthesize and release IGF-1 in an activity dependent manner. In addition, dopamine neuron-derived IGF-1 modulates dopamine synthesis and dopamine neuron firing and ultimately it controls dopamine-dependent behaviors. This study highlights the neuromodulatory role of neuron-derived IGF-1 and its role in shaping dopamine transmission in the brain.

Midbrain dopamine neurons, which can be regulated by neuropeptides and hormones, play a fundamental role in controlling cognitive processes, reward mechanisms, and motor functions. The hormonal actions of insulin-like growth factor 1 (IGF-1) produced by the liver have been well described, but the role of neuronally derived IGF-1 remains largely unexplored. We discovered that dopamine neurons secrete IGF-1 from the cell bodies following depolarization, and that IGF-1 controls release of dopamine in the ventral midbrain. In addition, conditional deletion of dopamine neuron-derived IGF-1 in adult mice leads to decrease of dopamine content in the striatum and deficits in dopamine neuron firing and causes reduced spontaneous locomotion and impairments in explorative and learning behaviors. These data identify that dopamine neuron-derived IGF-1 acts as a regulator of dopamine neurons and regulates dopamine-mediated behaviors.

Catestatin regulates vesicular quanta through modulation of cholinergic and peptidergic (PACAPergic) stimulation in PC12 cells

We have previously shown that the chromogranin A (CgA)-derived peptide catestatin (CST: hCgA_352-372) inhibits nicotine-induced secretion of catecholamines from the adrenal medulla and chromaffin cells. In the present study, we seek to determine whether CST regulates dense core (DC) vesicle (DCV) quanta (catecholamine and chromogranin/secretogranin proteins) during acute (0.5-h treatment) or chronic (24-h treatment) cholinergic (nicotine) or peptidergic (PACAP, pituitary adenylyl cyclase activating polypeptide) stimulation of PC12 cells. In acute experiments, we found that both nicotine (60 μM) and PACAP (0.1 μM) decreased intracellular norepinephrine (NE) content and increased ³H-NE secretion, with both effects markedly inhibited by co-treatment with CST (2 μM). In chronic experiments, we found that nicotine and PACAP both reduced DCV and DC diameters and that this effect was likewise prevented by CST. Nicotine or CST alone increased expression of CgA protein and together elicited an additional increase in CgA protein, implying that nicotine and CST utilize separate signaling pathways to activate CgA expression. In contrast, PACAP increased expression of CgB and SgII proteins, with a further potentiation by CST. CST augmented the expression of tyrosine hydroxylase (TH) but did not increase intracellular NE levels, presumably due to its inability to cause post-translational activation of TH through serine phosphorylation. Co-treatment of CST with nicotine or PACAP increased quantal size, plausibly due to increased synthesis of CgA, CgB and SgII by CST. We conclude that CST regulates DCV quanta by acutely inhibiting catecholamine secretion and chronically increasing expression of CgA after nicotinic stimulation and CgB and SgII after PACAPergic stimulation.

Interaction of Cdk5 and cAMP/PKA Signaling in the Mediation of Neuropsychiatric and Neurodegenerative Diseases

Both cyclin-dependent kinase 5 (Cdk5) and cyclic AMP (cAMP)/protein kinase A (PKA) regulate fundamental central nervous system (CNS) functions including neuronal survival, neurite and axonal outgrowth, neuron development and cognition. Cdk5, a serine/threonine kinase, is activated by p35 or p39 and phosphorylates multiple signaling components of various pathways, including cAMP/PKA signaling. Here, we review the recent literature on the interaction between Cdk5 and cAMP/PKA signaling and their role in the mediation of CNS functions and neuropsychiatric and neurodegenerative diseases.

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

Regulation of Tyrosine Hydroxylase Expression and Phosphorylation in Dopamine Transporter-Deficient Mice

Tyrosine hydroxylase (TH) and dopamine transporters (DATs) regulate dopamine (DA) neurotransmission at the biosynthesis and reuptake steps, respectively. Dysfunction or loss of these proteins occurs in impaired locomotor or addictive behavior, but little is known about the influence of DAT expression on TH function. Differences in TH phosphorylation, DA tissue content, l-DOPA biosynthesis, and DA turnover exist between the somatodendritic and terminal field compartments of nigrostriatal and mesoaccumbens pathways. We examined whether differential DAT expression affects these compartmental differences in DA regulation by comparing TH expression and phosphorylation at ser31 and ser40. In heterozygous DAT knockout (KO) (+/-) mice, DA tissue content and DA turnover were unchanged relative to wild-type mice, despite a 40% reduction in DAT protein expression. In DAT KO (-/-) mice, DA turnover increased in all DA compartments, but DA tissue content decreased (90-96%) only in terminal fields. TH protein expression and phosphorylation were differentially affected within DA pathway compartments by relative expression of DAT. TH protein decreased (∼74%), though to a significantly lesser extent than DA, in striatum and nucleus accumbens (NAc) in DAT -/- mice, with no decrease in substantia nigra or ventral tegmental area. Striatal ser31 TH phosphorylation and recovery of DA relative to TH protein expression in DAT +/- and DAT -/- mice decreased, whereas ser40 TH phosphorylation increased ∼2- to 3-fold in striatum and NAc of DAT -/- mice. These results suggest that DAT expression affects TH expression and phosphorylation largely in DA terminal field compartments, further corroborating evidence for dichotomous regulation of TH between somatodendritic and terminal field compartments of the nigrostriatal and mesoaccumbens pathways.

Striatal vessels receive phosphorylated tyrosine hydroxylase-rich innervation from midbrain dopaminergic neurons

Nowadays it is assumed that besides its roles in neuronal processing, dopamine (DA) is also involved in the regulation of cerebral blood flow. However, studies on the hemodynamic actions of DA have been mainly focused on the cerebral cortex, but the possibility that vessels in deeper brain structures receive dopaminergic axons and the origin of these axons have not been investigated. Bearing in mind the evidence of changes in the blood flow of basal ganglia in Parkinson's disease (PD), and the pivotal role of the dopaminergic mesostriatal pathway in the pathophysiology of this disease, here we studied whether striatal vessels receive inputs from midbrain dopaminergic neurons. The injection of an anterograde neuronal tracer in combination with immunohistochemistry for dopaminergic, vascular and astroglial markers, and dopaminergic lesions, revealed that midbrain dopaminergic axons are in close apposition to striatal vessels and perivascular astrocytes. These axons form dense perivascular plexuses restricted to striatal regions in rats and monkeys. Interestingly, they are intensely immunoreactive for tyrosine hydroxylase (TH) phosphorylated at Ser19 and Ser40 residues. The presence of phosphorylated TH in vessel terminals indicates they are probably the main source of basal TH activity in the striatum, and that after activation of midbrain dopaminergic neurons, DA release onto vessels precedes that onto neurons. Furthermore, the relative weight of this "vascular component" within the mesostriatal pathway suggests that it plays a relevant role in the pathophysiology of PD.

Phosphorylation dependence and stoichiometry of the complex formed by tyrosine hydroxylase and 14-3-3γ

Phosphorylated tyrosine hydroxylase (TH) can form complexes with 14-3-3 proteins, resulting in enzyme activation and stabilization. Although TH was among the first binding partners identified for these ubiquitous regulatory proteins, the binding stoichiometry and the activation mechanism remain unknown. To address this, we performed native mass spectrometry analyses of human TH (nonphosphorylated or phosphorylated on Ser19 (TH-pS19), Ser40 (TH-pS40), or Ser19 and Ser40 (TH-pS19pS40)) alone and together with 14-3-3γ. Tetrameric TH-pS19 (224 kDa) bound 14-3-3γ (58.3 kDa) with high affinity (K_d = 3.2 nM), generating complexes containing either one (282.4 kDa) or two (340.8 kDa) dimers of 14-3-3. Electron microscopy also revealed one major population of an asymmetric complex, consistent with one TH tetramer and one 14-3-3 dimer, and a minor population of a symmetric complex of one TH tetramer with two 14-3-3 dimers. Lower phosphorylation stoichiometries (0.15–0.54 phosphate/monomer) produced moderate changes in binding kinetics, but native MS detected much less of the symmetric TH:14-3-3γ complex. Interestingly, dephosphorylation of [³²P]-TH-pS19 was mono-exponential for low phosphorylation stoichiometries (0.18–0.52), and addition of phosphatase accelerated the dissociation of the TH-pS19:14-3-3γ complex 3- to 4-fold. All together this is consistent with a model in which the pS19 residues in the TH tetramer contribute differently in the association to 14-3-3γ. Complex formation between TH-pS40 and 14-3-3γ was not detected via native MS, and surface plasmon resonance showed that the interaction was very weak. Furthermore, TH-pS19pS40 behaved similarly to TH-pS19 in terms of binding stoichiometry and affinity (K_d = 2.1 nM). However, we found that 14-3-3γ inhibited the phosphorylation rate of TH-pS19 by PKA (3.5-fold) on Ser40. We therefore conclude that Ser40 does not significantly contribute to the binding of 14-3-3γ, and rather has reduced accessibility in the TH:14-3-3γ complex. This adds to our understanding of the fine-tuned physiological regulation of TH, including hierarchical phosphorylation at multiple sites.

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.

Comprehensive profiling of dopamine regulation in substantia nigra and ventral tegmental area

Dopamine is a vigorously studied neurotransmitter in the CNS. Indeed, its involvement in locomotor activity and reward-related behaviour has fostered five decades of inquiry into the molecular deficiencies associated with dopamine regulation. The majority of these inquiries of dopamine regulation in the brain focus upon the molecular basis for its regulation in the terminal field regions of the nigrostriatal and mesoaccumbens pathways; striatum and nucleus accumbens. Furthermore, such studies have concentrated on analysis of dopamine tissue content with normalization to only wet tissue weight. Investigation of the proteins that regulate dopamine, such as tyrosine hydroxylase (TH) protein, TH phosphorylation, dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) protein often do not include analysis of dopamine tissue content in the same sample. The ability to analyze both dopamine tissue content and its regulating proteins (including post-translational modifications) not only gives inherent power to interpreting the relationship of dopamine with the protein level and function of TH, DAT, or VMAT2, but also extends sample economy. This translates into less cost, and yet produces insights into the molecular regulation of dopamine in virtually any paradigm of the investigators' choice. We focus the analyses in the midbrain. Although the SN and VTA are typically neglected in most studies of dopamine regulation, these nuclei are easily dissected with practice. A comprehensive readout of dopamine tissue content and TH, DAT, or VMAT2 can be conducted. There is burgeoning literature on the impact of dopamine function in the SN and VTA on behavior, and the impingements of exogenous substances or disease processes therein (1-5). Furthermore, compounds such as growth factors have a profound effect on dopamine and dopamine-regulating proteins, to a comparatively greater extent in the SN or VTA (6-8). Therefore, this methodology is presented for reference to laboratories that want to extend their inquiries on how specific treatments modulate behaviour and dopamine regulation. Here, a multi-step method is presented for the analyses of dopamine tissue content, the protein levels of TH, DAT, or VMAT2, and TH phosphorylation from the substantia nigra and VTA from rodent midbrain. The analysis of TH phosphorylation can yield significant insights into not only how TH activity is regulated, but also the signaling cascades affected in the somatodendritic nuclei in a given paradigm. We will illustrate the dissection technique to segregate these two nuclei and the sample processing of dissected tissue that produces a profile revealing molecular mechanisms of dopamine regulation in vivo, specific for each nuclei (Figure 1).

A synopsis on the role of tyrosine hydroxylase in Parkinson's disease

Parkinson's disease (PD) is a common chronic progressive neurodegenerative disorder in elderly people. A consistent neurochemical abnormality in PD is degeneration of dopaminergic neurons in substantia nigra pars compacta, leading to a reduction of striatal dopamine (DA) levels. As tyrosine hydroxylase (TH) catalyses the formation of L-dihydroxyphenylalanine (L-DOPA), the rate-limiting step in the biosynthesis of DA, the disease can be considered as a TH-deficiency syndrome of the striatum. Problems related to PD usually build up when vesicular storage of DA is altered by the presence of either α-synuclein protofibrils or oxidative stress. Phosphorylation of three physiologically-regulated specific sites of N-terminal domain of TH is vital in regulating its kinetic and protein interaction. The concept of physiological significance of TH isoforms is another interesting aspect to be explored further for a comprehensive understanding of its role in PD. Thus, a logical and efficient strategy for PD treatment is based on correcting or bypassing the enzyme deficiency by the treatment with L-DOPA, DA agonists, inhibitors of DA metabolism or brain grafts with cells expressing a high level of TH. Neurotrophic factors are also attracting the attention of neuroscientists because they provide the essential neuroprotective and neurorestorative properties to the nigrostriatal DA system. PPAR-γ, a key regulator of immune responses, is likewise a promising target for the treatment of PD, which can be achieved by the use of agonists with the potential to impact the expression of pro- and anti-inflammatory cytokines at the transcriptional level in immune cells via expression of TH. Herein, we review the primary biochemical and pathological features of PD, and describe both classical and developing approaches aimed to ameliorate disease symptoms and its progression.

Genetic deletion of trace amine 1 receptors reveals their role in auto-inhibiting the actions of ecstasy (MDMA)

"Ecstasy" [3,4-methylenedioxymetamphetamine (MDMA)] is of considerable interest in light of its prosocial properties and risks associated with widespread recreational use. Recently, it was found to bind trace amine-1 receptors (TA(1)Rs), which modulate dopaminergic transmission. Accordingly, using mice genetically deprived of TA(1)R (TA(1)-KO), we explored their significance to the actions of MDMA, which robustly activated human adenylyl cyclase-coupled TA(1)R transfected into HeLa cells. In wild-type (WT) mice, MDMA elicited a time-, dose-, and ambient temperature-dependent hypothermia and hyperthermia, whereas TA(1)-KO mice displayed hyperthermia only. MDMA-induced increases in dialysate levels of dopamine (DA) in dorsal striatum were amplified in TA(1)-KO mice, despite identical levels of MDMA itself. A similar facilitation of the influence of MDMA upon dopaminergic transmission was acquired in frontal cortex and nucleus accumbens, and induction of locomotion by MDMA was haloperidol-reversibly potentiated in TA(1)-KO versus WT mice. Conversely, genetic deletion of TA(1)R did not affect increases in DA levels evoked by para-chloroamphetamine (PCA), which was inactive at hTA(1) sites. The TA(1)R agonist o-phenyl-3-iodotyramine (o-PIT) blunted the DA-releasing actions of PCA both in vivo (dialysis) and in vitro (synaptosomes) in WT but not TA(1)-KO animals. MDMA-elicited increases in dialysis levels of serotonin (5-HT) were likewise greater in TA(1)-KO versus WT mice, and 5-HT-releasing actions of PCA were blunted in vivo and in vitro by o-PIT in WT mice only. In conclusion, TA(1)Rs exert an inhibitory influence on both dopaminergic and serotonergic transmission, and MDMA auto-inhibits its neurochemical and functional actions by recruitment of TA(1)R. These observations have important implications for the effects of MDMA in humans.

Dichotomy of tyrosine hydroxylase and dopamine regulation between somatodendritic and terminal field areas of nigrostriatal and mesoaccumbens pathways

Measures of dopamine-regulating proteins in somatodendritic regions are often used only as static indicators of neuron viability, overlooking the possible impact of somatodendritic dopamine (DA) signaling on behavior and the potential autonomy of DA regulation between somatodendritic and terminal field compartments. DA reuptake capacity is less in somatodendritic regions, possibly placing a greater burden on de novo DA biosynthesis within this compartment to maintain DA signaling. Therefore, regulation of tyrosine hydroxylase (TH) activity may be particularly critical for somatodendritic DA signaling. Phosphorylation of TH at ser31 or ser40 can increase activity, but their impact on L-DOPA biosynthesis in vivo is unknown. Thus, determining their relationship with L-DOPA tissue content could reveal a mechanism by which DA signaling is normally maintained. In Brown-Norway Fischer 344 F₁ hybrid rats, we quantified TH phosphorylation versus L-DOPA accumulation. After inhibition of aromatic acid decarboxylase, L-DOPA tissue content per recovered TH protein was greatest in NAc, matched by differences in ser31, but not ser40, phosphorylation. The L-DOPA per catecholamine and DA turnover ratios were significantly greater in SN and VTA, suggesting greater reliance on de novo DA biosynthesis therein. These compartmental differences reflected an overall autonomy of DA regulation, as seen by decreased DA content in SN and VTA, but not in striatum or NAc, following short-term DA biosynthesis inhibition from local infusion of the TH inhibitor α-methyl-p-tyrosine, as well as in the long-term process of aging. Such data suggest ser31 phosphorylation plays a significant role in regulating TH activity in vivo, particularly in somatodendritic regions, which may have a greater reliance on de novo DA biosynthesis. Thus, to the extent that somatodendritic DA release affects behavior, TH regulation in the midbrain may be critical for DA bioavailability to influence behavior.

Serine 129 phosphorylation reduces the ability of alpha-synuclein to regulate tyrosine hydroxylase and protein phosphatase 2A in vitro and in vivo

Alpha-synuclein (a-Syn), a protein implicated in Parkinson disease, contributes significantly to dopamine metabolism. a-Syn binding inhibits the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis. Phosphorylation of TH stimulates its activity, an effect that is reversed by protein phosphatase 2A (PP2A). In cells, a-Syn overexpression activates PP2A. Here we demonstrate that a-Syn significantly inhibited TH activity in vitro and in vivo and that phosphorylation of a-Syn serine 129 (Ser-129) modulated this effect. In MN9D cells, a-Syn overexpression reduced TH serine 19 phosphorylation (Ser(P)-19). In dopaminergic tissues from mice overexpressing human a-Syn in catecholamine neurons only, TH-Ser-19 and TH-Ser-40 phosphorylation and activity were also reduced, whereas PP2A was more active. Cerebellum, which lacks excess a-Syn, had PP2A activity identical to controls. Conversely, a-Syn knock-out mice had elevated TH-Ser-19 phosphorylation and activity and less active PP2A in dopaminergic tissues. Using an a-Syn Ser-129 dephosphorylation mimic, with serine mutated to alanine, TH was more inhibited, whereas PP2A was more active in vitro and in vivo. Phosphorylation of a-Syn Ser-129 by Polo-like-kinase 2 in vitro reduced the ability of a-Syn to inhibit TH or activate PP2A, identifying a novel regulatory role for Ser-129 on a-Syn. These findings extend our understanding of normal a-Syn biology and have implications for the dopamine dysfunction of Parkinson disease.

Molecular determinants for PP2A substrate specificity: charged residues mediate dephosphorylation of tyrosine hydroxylase by the PP2A/B' regulatory subunit

Together with protein phosphatase 1, protein phosphatase 2A (PP2A) contributes the bulk of Ser/Thr phosphatase activity in most cell types. The predominant form of PP2A is a heterotrimer of catalytic (C), scaffolding (A), and diverse regulatory subunits (B, B', and B''). We have previously shown that N-terminal phosphorylation sites in tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, are specifically dephosphorylated by PP2A holoenzymes containing the B'beta regulatory subunit. Here, we identify a Glu residue conserved in B' regulatory subunits that is critical for dephosphorylation and inactivation of tyrosine hydroxylase in vitro and in PC12 cells. According to the PP2A heterotrimer crystal structure, Glu153 (B'beta numbering) abuts the catalytic site on the C subunit, and we demonstrate that Glu153 substitution inhibits multisite TH dephosphorylation without compromising PP2A/B'beta holoenzyme assembly or in vitro dephosphorylation of model substrates. Apart from its role in modulating TH activity, Glu153 is also necessary for PP2A/B'beta-mediated enhancement of nerve growth factor signaling. Furthermore, global phosphoproteome analysis suggests that Glu153 mediates dephosphorylation of most B'beta substrates in PC12 cells. With regard to selectivity determinants in the substrate, we show that B'beta Glu153 recognizes Arg37 and Arg38 in TH to direct dephosphorylation of both upstream (Ser31) and downstream (Ser40) sites. These results provide evidence of a subunit-spanning substrate docking site on the PP2A/B' holoenzyme, in which negatively charged side chains in the regulatory subunit interact with positive charges proximal to phosphorylated residues to mediate site-specific dephosphorylation.

Progesterone decreases tyrosine hydroxylase phosphorylation state and increases protein phosphatase 2A activity in the stalk-median eminence on proestrous afternoon

The progesterone (P(4)) rise on proestrous afternoon is associated with dephosphorylation of tyrosine hydroxylase (TH) and reduced TH activity in the stalk-median eminence (SME), which contributes to the proestrous prolactin surge in rats. In the present study, we investigated the time course for P(4) effect on TH activity and phosphorylation state, as well as cAMP levels and protein phosphatase 2A (PP2A) activity and quantity, in the SME on proestrous morning and afternoon. P(4) (7.5 mg/kg, s.c.) treatment on proestrous afternoon decreased TH activity and TH phosphorylation state at Ser-31 and Ser-40 within 1 h, whereas morning administration of P(4) had no 1 h effect on TH. PP2A activity in the SME was enhanced after P(4) treatment for 1 h on proestrous afternoon without a change in PP2A catalytic subunit quantity, whereas P(4) treatment had no effect on PP2A activity or quantity on proestrous morning. cAMP levels in the SME were unchanged with 1 h P(4) treatment. At 5 h after P(4) treatment, TH activity and phosphorylation state declined coincident with an increase in plasma prolactin in both P(4)-treated morning and afternoon groups. PP2A activity in the SME was unchanged in 5 h P(4)-treated rat. Our data suggest that P(4) action on tuberoinfundibular dopaminergic (TIDA) neurons involves at least two components. A more rapid (1 h) P(4) effect engaged only on proestrous afternoon likely involves the activation of PP2A. The longer P(4) action on TIDA neurons is evident on both the morning and afternoon of proestrus and may involve a common, as yet unidentified, mechanism.

Aging reveals a role for nigral tyrosine hydroxylase ser31 phosphorylation in locomotor activity generation

BACKGROUND

Tyrosine hydroxylase (TH) regulates dopamine (DA) bioavailability. Its product, L-DOPA, is an established treatment for Parkinson's disease (PD), suggesting that TH regulation influences locomotion. Site-specific phosphorylation of TH at ser31 and ser40 regulates activity. No direct evidence shows that ser40 phosphorylation is the dominating mechanism of regulating TH activity in vivo, and physiologically-relevant stimuli increase L-DOPA biosynthesis independent of ser40 phosphorylation. Significant loss of locomotor activity occurs in aging as in PD, despite less loss of striatal DA or TH in aging compared to the loss associated with symptomatic PD. However, in the substantia nigra (SN), there is equivalent loss of DA or TH in aging and at the onset of PD symptoms. Growth factors increase locomotor activity in both PD and aging models and increase DA bioavailability and ser31 TH phosphorylation in SN, suggesting that ser31 TH phosphorylation status in the SN, not striatum, regulates DA bioavailability necessary for locomotor activity.

METHODOLOGY AND PRINCIPAL FINDINGS

We longitudinally characterized locomotor activity in young and older Brown-Norway Fischer 344 F(1) hybrid rats (18 months apart in age) at two time periods, eight months apart. The aged group served as an intact and pharmacologically-naïve source of deficient locomotor activity. Following locomotor testing, we analyzed DA tissue content, TH protein, and TH phosphorylation in striatum, SN, nucleus accumbens, and VTA. Levels of TH protein combined with ser31 phosphorylation alone reflected inherent differences in DA levels among the four regions. Measures strictly pertaining to locomotor activity initiation significantly correlated to DA content only in the SN. Nigral TH protein and ser31 phosphorylation together significantly correlated to test subject's maximum movement number, horizontal activity, and duration.

CONCLUSIONS/SIGNIFICANCE

Together, these results show ser31 TH phosphorylation regulates DA bioavailability in intact neuropil, its status in the SN may regulate locomotor activity generation, and it may represent an accurate target for treating locomotor deficiency. They also show that neurotransmitter regulation in cell body regions can mediate behavioral outcomes and that ser31 TH phosphorylation plays a role in behaviors dependent upon catecholamines, such as dopamine.

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.

Neurochemical characterization of tyrosine hydroxylase-immunoreactive interneurons in the developing rat cerebral cortex

Understanding the development of cortical interneuron phenotypic diversity is critical because interneuron dysfunction has been implicated in several neurodevelopmental disorders. Here, tyrosine hydroxylase (TH)-immunoreactive neurons in the developing and adult rat cortex were characterized in light of findings regarding interneuron neurochemistry and development. Cortical TH-immunoreactive neurons were first observed 2 weeks postnatally and peaked in number 3 weeks after birth. At subsequent ages, the number of these cell profiles was gradually reduced, and they were seen less frequently in adults. No DNA fragmentation or active caspase 3 was observed in cortical TH cells at any age examined, eliminating cell death as an explanation for the decrease in cell number. Although cortical TH cells reportedly fail to produce subsequent catecholaminergic enzymes, we found that the majority of these cells at all ages contained phosphorylated TH, suggesting that the enzyme may be active and producing L-DOPA as an end-product. Morphological criteria and colocalization of some TH cells with glutamic acid decarboxylase suggest that these cells are interneurons. Previously, parvalbumin, somatostatin, and calretinin were demonstrated in non-overlapping subsets of interneurons. Cortical TH neurons colocalized with calretinin but not with parvalbumin or somatostatin. These findings suggest that the transitory increase in TH cell number is not due to cell death but possibly due to alterations in the amount of detectable TH present in these cells, and that at least some cortical TH-producing interneurons belong to the calretinin-containing subset of interneurons that originate developmentally in the caudal ganglionic eminence.

Phosphorylation state of tyrosine hydroxylase in the stalk-median eminence is decreased by progesterone in cycling female rats

Progesterone has the capacity to suppress hypothalamic dopaminergic neuronal activity on proestrous afternoon and prolong or amplify the preovulatory prolactin surge in rats. In the present study, we examined enzyme activity and phosphorylation state of tyrosine hydroxylase (TH) in the stalk-median eminence of cycling female rats on proestrus and estrus and related these to circulating progesterone levels. Phospho-TH levels were evaluated by Western blot analysis. TH activity was determined from the rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation. Phospho-TH levels at Ser-19, Ser-31, and Ser-40 were similar at 1100, 1300, and 1500 h on proestrus but declined at 1700, 1900, and 2200 h, coincident with rising serum progesterone levels. Similarly, DOPA accumulation was 30-50% lower at 1700, 1900, and 2200 h as compared with 1100-1500 h on proestrus. Ser-31 and Ser-40 phosphorylation states were increased by 1100 h on estrus to a level similar to 1100 h on proestrus, whereas DOPA accumulation was 30% greater on estrous as compared with proestrous morning. There were no significant differences among the several time points on proestrus and estrus with regard to TH protein or beta-tubulin levels. Exogenous progesterone administration (7.5 mg/kg, sc) before the preovulatory progesterone surge decreased TH activity and phospho-TH at Ser-19, Ser-31, and Ser-40, accompanied by premature increased serum prolactin. Our study suggests that decreased TH phosphorylation at Ser-19, Ser-31, and Ser-40 contributes to the decline in TH activity in the stalk-median eminence on proestrous afternoon and that progesterone may cause this initial cytoplasmic response of TH dephosphorylation.

Alpha-synuclein aggregation alters tyrosine hydroxylase phosphorylation and immunoreactivity: lessons from viral transduction of knockout mice

Tyrosine hydroxylase (TH), the rate limiting enzyme in catecholamine synthesis, is frequently used as a marker of dopaminergic neuronal loss in animal models of Parkinson's disease (PD). We have been exploring the normal function of the PD-related protein alpha-synuclein (alpha-Syn) with regard to dopamine synthesis. TH is activated by the phosphorylation of key seryl residues in the TH regulatory domain. Using in vitro models, our laboratory discovered that alpha-Syn inhibits TH by acting to reduce TH phosphorylation, which then reduces dopamine synthesis [X.-M. Peng, R. Tehranian, P. Dietrich, L. Stefanis, R.G. Perez, Alpha-synuclein activation of protein phosphatase 2A reduces tyrosine hydroxylase phosphorylation in dopaminergic cells, J. Cell. Sci. 118 (2005) 3523-3530; R.G. Perez, J.C. Waymire, E. Lin, J.J. Liu, F. Guo, M.J. Zigmond, A role for alpha-synuclein in the regulation of dopamine biosynthesis, J. Neurosci. 22 (2002) 3090-3099]. We recently began exploring the impact of alpha-Syn on TH in vivo, by transducing dopaminergic neurons in alpha-Syn knockout mouse (ASKO) olfactory bulb using wild type human alpha-Syn lentivirus. At 3.5-21 days after viral delivery, alpha-Syn expression was transduced primarily in periglomerular dopaminergic neurons. Cells with modest levels of alpha-Syn consistently co-labeled for Total-TH. However, cells bearing aggregated alpha-Syn, as revealed by proteinase K or Thioflavin-S treatment had significantly reduced Total-TH immunoreactivity, but high phosphoserine-TH labeling. On immunoblots, we noted that Total-TH immunoreactivity was equivalent in all conditions, although tissues with alpha-Syn aggregates again had higher phosphoserine-TH levels. This suggests that aggregated alpha-Syn is no longer able to inhibit TH. Although the reason(s) underlying reduced Total-TH immunoreactivity on tissue sections await(s) confirmation, the dopaminergic phenotype was easily verified using phosphorylation-state-specific TH antibodies. These findings have implications not only for normal alpha-Syn function in TH regulation, but also for measuring cell loss that is associated with synucleinopathy.

Differential expression of the B'beta regulatory subunit of protein phosphatase 2A modulates tyrosine hydroxylase phosphorylation and catecholamine synthesis

Tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, is stimulated by N-terminal phosphorylation by several kinases and inhibited by protein serine/threonine phosphatase 2A (PP2A). PP2A is a family of heterotrimeric holoenzymes containing one of more than a dozen different regulatory subunits. In comparison with rat forebrain extracts, adrenal gland extracts exhibited TH hyperphosphorylation at Ser(19), Ser(31), and Ser(40), as well as reduced phosphatase activity selectively toward phosphorylated TH. Because the B'beta regulatory subunit of PP2A is expressed in brain but not in adrenal glands, we tested the hypothesis that PP2A/B'beta is a specific TH phosphatase. In catecholamine-secreting PC12 cells, inducible expression of B'beta decreased both N-terminal Ser phosphorylation and in situ TH activity, whereas inducible silencing of endogenous B'beta had the opposite effect. Furthermore, PP2A/B'beta directly dephosphorylated TH in vitro. As to specificity, other PP2A regulatory subunits had negligible effects on TH activity and phosphorylation in situ and in vitro. Whereas B'beta was highly expressed in dopaminergic cell bodies in the substantia nigra, the PP2A regulatory subunit was excluded from TH-positive terminal fields in the striatum and failed to colocalize with presynaptic markers in general. Consistent with a model in which B'beta enrichment in neuronal cell bodies helps confine catecholamine synthesis to axon terminals, TH phosphorylation was higher in processes than in somata of dopaminergic neurons. In summary, we show that B'beta recruits PP2A to modulate TH activity in a tissue- and cell compartment specific fashion.

Subcellular Localization and Structural Function of Endogenous Phosphorylated Phosphatidylinositol 4-Kinase (PI4K92)

Anti-phosphopeptide antibodies were raised against phosphatidylinositol 4-kinase (PI4K92) phosphorylation sites (Suer, S., Sickmann, A., Meyer, H. E., Herberg, F. W., and Heilmeyer, L. M. Jr. (2001) Eur. J. Biochem. 268, 2099-2106). Characterization proved three of them (anti-pSer-294, anti-pSer-496, and anti-pThr-504 antibody) to be highly specific, recognizing solely PI4K92 phosphorylated at these sites, respectively. Indirect immunofluorescence reveals that PI4K92 phosphorylated on Ser-294 localizes exclusively at the Golgi. The enzyme phosphorylated on Ser-496 and Thr-504 is detected in nuclear speckles. Phosphorylation of Ser-294 on PI4K92 increases the lipid kinase activity and thus serves better in maintaining Golgi function and morphology (compare Hausser, A., Storz, P., Martens, S., Link, G., Toker, A., and Pfizenmaier, K. (2005) Nat. Cell Biol. 7, 880-886). Microinjection of anti-pSer-496, but not of anti-pSer-294 or anti-pThr-504 antibody, into the cytoplasm or into the nucleus of HS68 cells leads to development of hotspots, probably representing aggregated PI4K92, and in later stages, cells become apoptotic and finally die. The association of phosphorylated PI4K92 with nuclear speckles is dynamic and follows the morphological alteration of speckles upon inhibition of mRNA transcription with alpha-amanitin. Overexpressed PI4K92 phosphorylated on Ser-294 is not transported to the nucleus, and that phosphorylated on Ser-496 is found in the nucleus and mislocalized at the Golgi complex. We conclude that nuclear phosphatidylinositol 4-phosphate, and consequently, synthesis of polyphosphoinositides are required for a correct nuclear function.

GDNF abates serum deprivation-induced tyrosine hydroxylase Ser19 phosphorylation and activity

High dopamine levels can contribute to neuronal dysfunction, impair plasticity and be toxic to neuronal cells in pathological conditions. The synthesis of dopamine is regulated by phosphorylation of the rate-limiting enzyme tyrosine hydroxylase (TH) under physiological conditions, with the phosphorylation of Ser31 and Ser40 directly increasing TH activity. Although a third phosphorylation site, Ser19, does not appear to directly regulate TH activity in physiological conditions, its role in pathological conditions is poorly understood. In this study, we examined the effects of serum deprivation (to mimic loss of retrogradely/anterogradely transported target-derived neurotrophic factors following axonal injury) and glutamate receptor stimulation (to mimic excitotoxicity) on TH phosphorylation and activity in a cell line and in mesencephalic primary culture cells. In addition, we also tested whether glial cell line-derived neurotrophic factor (GDNF) can alter these changes. We demonstrate that serum-deprivation resulted in a sustained increase in Ser19 phosphorylation beginning at 3 h and lasting up to 10 h without any detectable change in Ser31 or Ser40 phosphorylation within this time frame. This increase in Ser19 phosphorylation was associated with enhanced TH activity and was due, in part, to glutamate-receptor-mediated calcium influx and possibly calcium/calmodulin-dependent protein kinase II (CaMKII) activation. Interestingly in this serum-deprivation model, GDNF blocked the increase in Ser19 phosphorylation and TH activity at the 10-h time point following serum deprivation. Furthermore, GDNF also blocked the glutamate-mediated increase in Ser19 phosphorylation in rat primary mesencephalic neuronal cultures. Taken together, these findings suggest that GDNF may reduce dopamine synthesis in pathological conditions.

Kinetics of regulatory serine variants of tyrosine hydroxylase with cyclic AMP-dependent protein kinase and extracellular signal-regulated protein kinase 2

Rat tyrosine hydroxylase is phosphorylated at four serine residues, at positions 8, 19, 31, and 40 in its amino terminal regulatory domain by multiple protein kinases. Cyclic AMP-dependent protein kinase phosphorylates S40, which results in alleviation of inhibition by dopamine. Extracellular signal-regulated protein kinase 2 phosphorylates S8 and S31. Site-directed serine-to-glutamate mutations were introduced into tyrosine hydroxylase to mimic prior phosphorylation of the regulatory serines; these proteins were used as substrates for cAMP-dependent kinase and extracellular signal-regulated kinase 2. The activity of cAMP-dependent kinase was unaffected by the substitution of serines 8, 19 or 31 with glutamate and the activity of extracellular signal-regulated kinase 2 was unaffected by substitution of serines 19 or 40 with glutamate. Cyclic AMP-dependent kinase was less active in phosphorylating S40 if dopamine was bound to tyrosine hydroxylase, but extracellular signal-regulated kinase 2 phosphorylation at S31 was unaffected by the presence of dopamine.

Tyrosine hydroxylase expression and activity in the rat brain: differential regulation after long-term intermittent or sustained hypoxia

Tyrosine hydroxylase, a hypoxia-regulated gene, may be involved in tissue adaptation to hypoxia. Intermittent hypoxia, a characteristic feature of sleep apnea, leads to significant memory deficits, as well as to cortex and hippocampal apoptosis that are absent after sustained hypoxia. To examine the hypothesis that sustained and intermittent hypoxia induce different catecholaminergic responses, changes in tyrosine hydroxylase mRNA, protein expression, and activity were compared in various brain regions of male rats exposed for 6 h, 1 day, 3 days, and 7 days to sustained hypoxia (10% O2), intermittent hypoxia (alternating room air and 10% O2), or normoxia. Tyrosine hydroxylase activity, measured at 7 days, increased in the cortex as follows: sustained > intermittent > normoxia. Furthermore, activity decreased in the brain stem and was unchanged in other brain regions of sustained hypoxia-exposed rats, as well as in all regions from animals exposed to intermittent hypoxia, suggesting stimulus-specific and heterotopic catecholamine regulation. In the cortex, tyrosine hydroxylase mRNA expression was increased, whereas protein expression remained unchanged. In addition, significant differences in the time course of cortical Ser40tyrosine hydroxylase phosphorylation were present in the cortex, suggesting that intermittent and sustained hypoxia-induced enzymatic activity differences are related to different phosphorylation patterns. We conclude that long-term hypoxia induces site-specific changes in tyrosine hydroxylase activity and that intermittent hypoxia elicits reduced tyrosine hydroxylase recruitment and phosphorylation compared with sustained hypoxia. Such changes may not only account for differences in enzyme activity but also suggest that, with differential regional brain susceptibility to hypoxia, recruitment of different mechanisms in response to hypoxia will elicit region-specific modulation of catecholamine response.

Mutation of regulatory serines of rat tyrosine hydroxylase to glutamate: effects on enzyme stability and activity

Tyrosine hydroxylase is phosphorylated at four serine residues in its amino-terminus by multiple kinases. Phosphorylation of serine 40 by cAMP-dependent protein kinase results in alleviation of dopamine inhibition [J. Biol. Chem. 267 (1992) 12639]. The other serines are at positions 8, 19, and 31. The effect of phosphorylation at these serines has been investigated using mutated forms of tyrosine hydroxylase containing glutamates at the positions of the serines. The S8E, S19E, and S31E tyrosine hydroxylase variants have similar steady-state kinetic parameters and similar binding affinity for catecholamines to wild-type enzyme. The S8E, S19E, S31E, and S40E variants differ in stability at elevated temperatures. The S40E variant is the least stable, while the others are all more stable than wild-type enzyme. The increased stability of S8E, S19E, and S31E tyrosine hydroxylases may be one of the physiological effects of phosphorylation. It may also have implications for the interpretation of activities of heterogeneous mixtures of tyrosine hydroxylase which have been phosphorylated.

Two tyrosine hydroxylase genes in teleosts

We report the finding of two non-allelic genes encoding tyrosine hydroxylase (TH) from the diploid teleost barramundi Lates calcarifer. Barramundi TH1 is the homologue of the higher vertebrate TH genes and encodes a protein of 489 amino acids that shares 90% sequence identity to the THs of other teleost species. A second non-allelic tyrosine hydroxylase gene (TH2) encodes a protein of 472 amino acids and shares 62% identity with TH1 and the vertebrate THs. TH1 mRNA is found in the brain and kidney of barramundi while TH2 mRNA is found only in brain. The TH2 gene is also present in the genomes of the pufferfish Takifugu and zebrafish Danio. Estimates of the rates of nucleotide substitution suggest the teleost TH2 genes are selectively constrained although not to the degree seen in the TH1 genes. Differential regulation of the two TH genes is, however, indicated by differences in transcript distribution, the nature of the Ca2+-responsive elements found in the proximal promoter region and the lack of recognised phosphorylation sites in TH2. Preservation of two apparently functional TH genes in phylogenetically distant teleost species is consistent with the notion of partitioning of function between duplicate genes.

Identification of tyrosine hydroxylase as a physiological substrate for Cdk5

Cyclin-dependent kinase 5 (Cdk5) is emerging as a neuronal protein kinase involved in multiple aspects of neurotransmission in both post- and presynaptic compartments. Within the reward/motor circuitry of the basal ganglia, Cdk5 regulates dopamine neurotransmission via phosphorylation of the postsynaptic signal transduction pathway integrator, DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein, M(r) 32,000). Cdk5 has also been implicated in regulating various steps in the presynaptic vesicle cycle. Here we report that Cdk5 phosphorylates tyrosine hydroxylase (TH), the key enzyme for synthesis of dopamine. Using phosphopeptide mapping, site-directed mutagenesis, and phosphorylation state-specific antibodies, the site was identified as Ser31, a previously defined extracellular signal-regulated kinases 1/2 (ERK1/2) site. The phosphorylation of Ser31 by Cdk5 versus ERK1/2 was investigated in intact mouse striatal tissue using a pharmacological approach. The results indicated that Cdk5 phosphorylates TH directly and also regulates ERK1/2-dependent phosphorylation of TH through the phosphorylation of mitogen-activated protein kinase kinase 1 (MEK1). Finally, phospho-Ser31 TH levels were increased in dopaminergic neurons of rats trained to chronically self-administer cocaine. These results demonstrate direct and indirect regulation of the phosphorylation state of a Cdk5/ERK1/2 site on TH and suggest a role for these pathways in the neuroadaptive changes associated with chronic cocaine exposure.

Tyrosine hydroxylase phosphorylation: regulation and consequences

The rate-limiting enzyme in catecholamine synthesis is tyrosine hydroxylase. It is phosphorylated at serine (Ser) residues Ser8, Ser19, Ser31 and Ser40 in vitro, in situ and in vivo. A range of protein kinases and protein phosphatases are able to phosphorylate or dephosphorylate these sites in vitro. Some of these enzymes are able to regulate tyrosine hydroxylase phosphorylation in situ and in vivo but the identity of the kinases and phosphatases is incomplete, especially for physiologically relevant stimuli. The stoichiometry of tyrosine hydroxylase phosphorylation in situ and in vivo is low. The phosphorylation of tyrosine hydroxylase at Ser40 increases the enzyme's activity in vitro, in situ and in vivo. Phosphorylation at Ser31 also increases the activity but to a much lesser extent than for Ser40 phosphorylation. The phosphorylation of tyrosine hydroxylase at Ser19 or Ser8 has no direct effect on tyrosine hydroxylase activity. Hierarchical phosphorylation of tyrosine hydroxylase occurs both in vitro and in situ, whereby the phosphorylation at Ser19 increases the rate of Ser40 phosphorylation leading to an increase in enzyme activity. Hierarchical phosphorylation depends on the state of the substrate providing a novel form of control of tyrosine hydroxylase activation.

Tyrosine hydroxylase and dopamine transporter expression following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neurodegeneration of the mouse nigrostriatal pathway

Administration of the neurotoxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to C57BL/6 mice targets nigrostriatal dopaminergic neurons, leading to cell death and the depletion of striatal dopamine. After MPTP lesioning in young adult mice, surviving nigrostriatal dopaminergic neurons display robust and reproducible return of striatal dopamine weeks to months after injury. Thus, the mouse provides an excellent model with which to investigate the mechanisms underlying neuroplasticity of the nigrostriatal system following neurotoxic injury. The purpose of this study was to analyze proteins and mRNA transcripts of genes involved in dopamine biosynthesis (tyrosine hydroxylase; TH) and uptake (dopamine transporter; DAT) with regard to time course (7-90 days) after MPTP lesioning. Molecular analysis using immunohistochemistry and Western immunoblotting techniques demonstrated an increase in striatal TH by 30-60 days postlesioning that returned to near-control (prelesioned) levels by 60-90 days. In situ hybridization histochemistry indicated that this increase in TH protein might be due in part to increased TH mRNA expression in surviving nigrostriatal dopaminergic neurons. Analysis of TH protein at 7, 30, 60, and 90 days postlesioning with two-dimensional polyacrylamide gel electrophoresis in conjunction with Western immunoblotting revealed altered TH protein isoforms migrating at isoelectric points different from those of the native isoform. In contrast to TH protein, which returned to prelesioned levels by 60 days, DAT protein analysis showed that increased expression of striatal DAT protein did not return to near-prelesion levels until 90 days postlesioning. These results suggest that TH and DAT may differ in their time course of expression in surviving dopaminergic neurons and may play a role in mediating the return of striatal dopamine.

Activation of tyrosine hydroxylase by intermittent hypoxia: involvement of serine phosphorylation

Regulation of tyrosine hydroxylase (TH) by intermittent hypoxia (IH) was investigated in rat pheochromocytoma 12 (PC-12) cells by exposing them to alternating cycles of hypoxia (1% O2, 15 s) and normoxia (21% O2, 3 min) for up to 60 cycles; controls were exposed to normoxia for a similar duration. IH exposure increased dopamine content and TH activity by approximately 42 and approximately 56%, respectively. Immunoblot analysis revealed that comparable levels of TH protein were expressed in normoxic and IH cells. Removal of TH-bound catecholamines and in vitro phosphorylation of TH in cell-free extracts by the catalytic subunit of protein kinase A (PKA) increased TH activity in normoxic but not in IH cells, suggesting possible induction of TH phosphorylation and removal of endogenous inhibition of TH by IH. To assess the role of serine phosphorylation in IH-induced TH activation, TH immunoprecipitates and extracts derived from normoxic and IH cells were probed with anti-phosphoserine and anti-phospho-TH (Ser-40) antibody, respectively. Compared with normoxic cells, total serine and Ser-40-specific phosphorylation of TH were increased in IH cells. IH-induced activation of TH and the increase in total serine and Ser-40-specific phosphorylation of TH were inhibited by Ca2+/calmodulin-dependent protein kinase (CaMK) and PKA-specific inhibitors but not by inhibitors of the extracellular signal-regulated protein kinase pathway, suggesting that IH activates TH in PC-12 cells via phosphorylation of serine residues including Ser-40, in part, by CaMK and PKA. Our results also suggest that IH-induced phosphorylation of TH facilitates the removal of endogenous inhibition of TH, leading to increased synthesis of dopamine.

Role of protein phosphatase 2C from bovine adrenal chromaffin cells in the dephosphorylation of phospho-serine 40 tyrosine hydroxylase

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of catecholamines. It is dephosphorylated by protein phosphatase (PP) 2A and PP2C. In this study we used a fixed amount of bacterially expressed rat TH (5 microM), phosphorylated only at serine 40 (pSer40TH), to determine the PP activities against this site that are present in extracts from the bovine adrenal cortex, adrenal medulla, adrenal chromaffin cells and rat striatum. We found that PP2C was the main TH phosphatase activity in extracts from the adrenal medulla and adrenal chromaffin cells. In adrenal cortex extracts PP2C and PP2A activities toward pSer40TH did not differ significantly. PP2A was the main TH phosphatase activity in extracts from rat striatum. Kinetic studies with extracts from adrenal chromaffin cells showed that when higher concentrations of pSer40TH (> 5 microM) were used the activity of PP2C increased more than the activity of PP2A. PP2C was maximally activated by 1.25 mM Mn2+ and by 5 mM Mg2+ but was inhibited by calcium. Our data suggest a more important role for PP2C than was previously suggested in the dephosphorylation of serine 40 on TH.

Regulation of tyrosine hydroxylase by stress-activated protein kinases

Recombinant human tyrosine hydroxylase (hTH1) was found to be phosphorylated by mitogen and stress-activated protein kinase 1 (MSK1) at Ser40 and by p38 regulated/activated kinase (PRAK) on Ser19. Phosphorylation by MSK1 induced an increase in Vmax and a decrease in Km for 6-(R)-5,6,7,8-tetrahydrobiopterin (BH4), while these kinetic parameters were unaffected as a result of phosphorylation by PRAK. Phosphorylation of both Ser40 and Ser19 induced a high-affinity binding of 14-3-3 proteins, but only the interaction of 14-3-3 with Ser19 increased the hTH1 activity. The 14-3-3 proteins also inhibited the rate of dephosphorylation of Ser19 and Ser40 by 82 and 36%, respectively. The phosphorylation of hTH1 on Ser19 caused a threefold increase in the rate of phosphorylation of Ser40. These studies provide new insights into the possible roles of stress-activated protein kinases in the regulation of catecholamine biosynthesis.

Acute administration of cocaine regulates the phosphorylation of serine-19, -31 and -40 in tyrosine hydroxylase

Acute cocaine can inhibit catecholamine biosynthesis by regulating the enzymatic activity of tyrosine hydroxylase via alterations in the phosphorylation state of the enzyme. The mechanisms underlying acute cocaine-dependent regulation of tyrosine hydroxylase phosphorylation have not been determined. In this study, 0, 15 or 30 mg/kg cocaine was administered intraperitoneally to rats and the phosphorylation state of tyrosine hydroxylase in the brain was examined using antibodies specific for the phosphorylated forms of serine-19, -31 and -40 in tyrosine hydroxylase. In the caudate and nucleus accumbens, cocaine dose-dependently decreased the levels of phosphorylated serine-19, -31 and -40. In the ventral tegmental area, the levels of phosphorylated serine-19, but not serine-31 and -40, were decreased by 15 and 30 mg/kg cocaine. In the amygdala, the levels of phosphorylated serine-19, but not serine-31 or -40, were decreased. The functional effects of these alterations in phosphorylation state were assessed by measuring tyrosine hydroxylase activity in vivo (accumulation of DOPA after administration of the decarboxylase inhibitor NSD-1015). Acute administration of 30 mg/kg cocaine significantly decreased l-DOPA production in caudate and accumbens but not in amygdala. These data suggest that the phosphorylation of serine-31 or -40, but not serine-19, is involved in the regulation of tyrosine hydroxylase activity by acute cocaine.

Tyrosine hydroxylase dephosphorylation by protein phosphatase 2A in bovine adrenal chromaffin cells

This study was undertaken to characterise the protein phosphatases in bovine adrenal chromaffin cells acting on tyrosine hydroxylase. Cells were pre-labelled with 32Pi and permeabilized with digitonin. The extent of dephosphorylation of Ser-8, Ser-19, Ser-31 and Ser-40 on tyrosine hydroxylase was found to be 30%, 38%, 37% and 71% respectively over 5 min. For Ser-19, Ser-31 and Ser-40 the dephosphorylation was entirely due to protein phosphatase 2A, as the dephosphorylation could be completely blocked by microcystin, but not by the protein phosphatase I inhibitory peptide. Permeabilization did not change the distribution of protein phosphatase 2A or tyrosine hydroxylase, or the activity of PP2A, from that occurring in intact cells. The dephosphorylation of Ser-8 was not altered by any inhibitor, suggesting the involvement of other protein phosphatases. The method developed here can be used to determine the protein phosphatases acting on substrates in conditions closely approximating those in situ, including the endogenous state of substrate phosphorylation and phosphatase location.

Activation of extracellular signal-regulated kinases 1 and 2 by depolarization stimulates tyrosine hydroxylase phosphorylation and dopamine synthesis in rat brain

Production of dopamine is regulated via phosphorylation of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines. Here we have used a preparation of rat striatal slices to examine the involvement of two mitogen-activated protein kinases (MAPKs), extracellular signal-regulated protein kinases 1 and 2 (ERK1/2), in the depolarization-dependent regulation of TH phosphorylation and dopamine synthesis. Depolarization with elevated KCl (45 mm) caused an increase in the phosphorylation state and, thereby, activation of ERK1/2. The same stimulus also increased TH phosphorylation at Ser19, Ser31 and Ser40 (measured using site- and phospho-specific antibodies) and TH activity [measured as 3,4-dihydroxyphenylalanine (DOPA) accumulation]. A MAPK/ERK kinase inhibitor, PD098059, decreased the basal levels of phospho-ERK1/2 and prevented the increase in ERK1/2 phosphorylation induced by depolarization. PD098059 also decreased both basal and depolarization-induced phosphorylation of TH at Ser31 and reduced the increase in Ser40 phosphorylation induced by high potassium, but did not affect Ser19 phosphorylation. PD098059 alone inhibited basal TH activity and decreased the accumulation of DOPA induced by depolarization. These data provide evidence for the involvement of ERK1/2 in the regulation of the state of phosphorylation of TH at Ser31 and Ser40 and a correlation between ERK1/2-dependent phosphorylation of TH and stimulation of dopamine synthesis in the brain.

Interaction of phosphorylated tyrosine hydroxylase with 14-3-3 proteins: evidence for a phosphoserine 40-dependent association

Tyrosine hydroxylase (TH) has been reported to require binding of 14-3-3 proteins for optimal activation by phosphorylation. We examined the effects of phosphorylation at Ser19, Ser31 and Ser40 of bovine TH and human TH isoforms on their binding to the 14-3-3 proteins BMH1/BMH2, as well as 14-3-3 zeta and a mixture of sheep brain 14-3-3 proteins. Phosphorylation of Ser31 did not result in 14-3-3 binding, however, phosphorylation of TH on Ser40 increased its affinity towards the yeast 14-3-3 isoforms BMH1/BMH2 and sheep brain 14-3-3, but not for 14-3-3 zeta. On phosphorylation of both Ser19 and Ser40, binding to the 14-3-3 zeta isoform also occurred, and the binding affinity to BMH1 and sheep brain 14-3-3 increased. Both phosphoserine-specific antibodies directed against the 10 amino acids surrounding Ser19 or Ser40 of TH, and the phosphorylated peptides themselves, inhibited the association between phosphorylated TH and 14-3-3 proteins. This was also found when heparin was added, or after proteolytic removal of the N-terminal 37 amino acids of Ser40-phosphorylated TH. Binding of BMH1 to phosphorylated TH decreased the rate of dephosphorylation by protein phosphatase 2A, but no significant change in enzymatic activity was observed in the presence of BMH1. These findings further support a role for 14-3-3 proteins in the regulation of catecholamine biosynthesis and demonstrate isoform specificity for both TH and 14-3-3 proteins.

Phosphorylation of Ser(19) alters the conformation of tyrosine hydroxylase to increase the rate of phosphorylation of Ser(40)

The effect of phosphorylation on the shape of tyrosine hydroxylase (TH) was studied directly using gel filtration and indirectly using electrospray ionization mass spectrometry. Phosphorylation of Ser(19) and Ser(40) produced a TH molecule with a more open conformation than the non-phosphorylated form. The conformational effect of Ser(19) phosphorylation is less pronounced than that of the Ser(40) phosphorylation. The effect of Ser(19) and Ser(40) phosphorylation appears to be additive. Binding of dopamine produced a more compact form when compared with the non-dopamine-bound TH. The interdependence of Ser(19) and Ser(40) phosphorylation was probed using electrospray ionization mass spectrometry. The rate constants for the phosphorylation of Ser(19) and Ser(40) were determined by electrospray ionization mass spectrometry using a consecutive reaction model. The rate constant for the phosphorylation of Ser(40) is approximately 2- to 3-fold higher if Ser(19) is already phosphorylated. These results suggest that phosphorylation of Ser(19) alters the conformation of tyrosine hydroxylase to allow increased accessibility of Ser(40) to kinases.

Depolarization-stimulated catecholamine biosynthesis: involvement of protein kinases and tyrosine hydroxylase phosphorylation sites in situ

Depolarizing stimuli increase catecholamine (CA) biosynthesis, tyrosine hydroxylase (TH) activity, and TH phosphorylation at Ser19, Ser31, and Ser40 in a Ca(2+)-dependent manner. However, the identities of the protein kinases that phosphorylate TH under depolarizing conditions are not known. Furthermore, although increases in Ser31 or Ser40 phosphorylation increase TH activity in vitro, the relative influence of phosphorylation at these sites on CA biosynthesis under depolarizing conditions is not known. We investigated the participation of extracellular signal-regulated protein kinase (ERK) and cAMP-dependent protein kinase (PKA) in elevated K(+)-stimulated TH phosphorylation in PC12 cells using an ERK pathway inhibitor, PD98059, and PKA-deficient PC12 cells (A126-B1). In the same paradigm, we measured CA biosynthesis. TH phosphorylation stoichiometry (PS) was determined by quantitative blot-immunolabeling using site- and phosphorylation state-specific antibodies. Treatment with elevated K(+) (+ 58 mM) for 5 min increased TH PS at each site in a Ca(2+)-dependent manner. Pretreatment with PD98059 prevented elevated K(+)-stimulated increases in ERK phosphorylation and Ser31 PS. In A126-B1 cells, Ser40 PS was not significantly increased by forskolin, and elevated K(+)-stimulated Ser40 PS was three- to five-fold less than that in PC12 cells. In both cell lines, CA biosynthesis was increased 1.5-fold after treatment with elevated K(+) and was prevented by pretreatment with PD98059. These results suggest that ERK phosphorylates TH at Ser31 and that PKA phosphorylates TH at Ser40 under depolarizing conditions. They also suggest that the increases in CA biosynthesis under depolarizing conditions are associated with the ERK-mediated increases in Ser31 PS.

Ca2+ -dependent activation of tyrosine hydroxylase involves MEK1

Tyrosine hydroxylase (TOH) activity is regulated acutely by phosphorylation of serines 8, 19, 31 and 40. The only kinases known to phosphorylate Ser31 are the mitogen-activated protein kinases MAPK-1 and 2. The involvement of these kinases in TOH activation in situ was therefore investigated using intact bovine chromaffin cells. Nicotine, K+ and A23187 increased TOH activity over 10 min in a Ca2+-dependent manner. The response to all three was reduced by PD098059, a selective inhibitor of the upstream activator of MAPK, MEK1. In contrast, TOH activation by forskolin and phorbol dibutyrate were unaffected by PD098059. The results support a key role for MEK1/MAPK in the acute activation of TOH by nicotinic receptors and by other agonists that increase cytosolic Ca2+.

Tyrosine hydroxylase phosphorylation in bovine adrenal chromaffin cells: the role of MAPKs after angiotensin II stimulation

Angiotensin II (AII, 100 nM) stimulation of bovine adrenal chromaffin cells (BACCs) produced angiotensin II receptor subtype 1 (AT1)-mediated increases in extracellular regulated protein kinase 1/2 (ERK1/2) and stress-activated p38MAPK (p38 kinase) phosphorylation over a period of 10 min. ERK1/2 and p38 kinase phosphorylation preceded Ser31 phosphorylation on tyrosine hydroxylase (TOH). The inhibitors of mitogen-activated protein kinase kinase 1/2 (MEK1/2) activation, PD98059 (0.1-50 microM) and UO126 (0.1-10 microM), dose-dependently inhibited both ERK2 and Ser31 phosphorylation on TOH in response to AII, suggesting MEK1/2 involvement. The p38 kinase inhibitor SB203580 (20 microM, 30 min) abolished Ser31 and Ser19 phosphorylation on TOH and partially inhibited ERK2 phosphorylation produced by AII. In contrast, 1 microM SB203580 did not affect AII-stimulated TOH phosphorylation, but fully inhibited heat shock protein 27 (HSP27) phosphorylation produced by AII. Also, 1 microM SB203580 fully inhibited Ser19 phosphorylation on TOH and HSP27 phosphorylation in response to anisomycin (30 min, 10 microg/mL). The results suggest that ERKs mediate Ser31 phosphorylation on TOH in response to AII, but p38 kinase is not involved. Previous studies suggesting a role for p38 kinase in the phosphorylation of Ser31 are explained by the non-specific effects of 20 microM SB203580 in BACCs. The p38 kinase pathway is able to phosphorylate Ser19 on TOH in response to anisomycin, but does not do so in response to AII.

Functional characterization of phosphorylation of 69-kDa human choline acetyltransferase at serine 440 by protein kinase C

Choline acetyltransferase, the enzyme that synthesizes the transmitter acetylcholine in cholinergic neurons, is a substrate for protein kinase C. In the present study, we used mass spectrometry to identify serine 440 in recombinant human 69-kDa choline acetyltransferase as a protein kinase C phosphorylation site, and site-directed mutagenesis to determine that phosphorylation of this residue is involved in regulation of the enzyme's catalytic activity and binding to subcellular membranes. Incubation of HEK293 cells stably expressing wild-type 69-kDa choline acetyltransferase with the protein kinase C activator phorbol 12-myristate 13-acetate showed time- and dose-related increases in specific activity of the enzyme; in control and phorbol ester-treated cells, the enzyme was distributed predominantly in cytoplasm (about 88%) with the remainder (about 12%) bound to cellular membranes. Mutation of serine 440 to alanine resulted in localization of the enzyme entirely in cytoplasm, and this was unchanged by phorbol ester treatment. Furthermore, activation of mutant enzyme in phorbol ester-treated HEK293 cells was about 50% that observed for wild-type enzyme. Incubation of immunoaffinity purified wild-type and mutant choline acetyltransferase with protein kinase C under phosphorylating conditions led to incorporation of [(32)P]phosphate, with radiolabeling of mutant enzyme being about one-half that of wild-type, indicating that another residue is phosphorylated by protein kinase C. Acetylcholine synthesis in HEK293 cells expressing wild-type choline acetyltransferase, but not mutant enzyme, was increased by about 17% by phorbol ester treatment.

Dopamine D(2) receptors regulate tyrosine hydroxylase activity and phosphorylation at Ser40 in rat striatum

In the striatum, dopamine release is inhibited by activation of dopamine D(2) autoreceptors. Changes in dopamine release have been attributed to changes in the synthesis of dopamine, which is regulated via phosphorylation of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines. Here, we have studied the involvement of dopamine D(2) receptors in the regulation of TH phosphorylation at distinct seryl residues, using phosphorylation site-specific antibodies and a preparation of rat striatal slices. The D(2) receptor agonist, quinpirole, reduced basal TH phosphorylation at Ser40 but not at Ser19 or Ser31. Quinpirole was also able to reduce the increase in Ser40 phosphorylation caused by forskolin, an activator of adenylyl cyclase, without affecting the increase in Ser19 phosphorylation produced by the glutamate receptor agonist, N-methyl-D-aspartate (NMDA). In addition, the dopamine D(2) receptor agonist reduced both basal and forskolin-stimulated activity of TH, measured as 3,4-dihydroxyphenylalanine (DOPA) accumulation. Quinpirole decreased phosphorylation of Ser40 induced by okadaic acid, an inhibitor of protein phosphatase 1 and 2A and Ro-20-1724, a phosphodiesterase inhibitor. In contrast, quinpirole did not affect the increase in Ser40 phosphorylation caused by the cAMP analogue, 8-Br-cAMP. These data indicate that, in the striatum, activation of dopamine D(2) receptors results in selective inhibition of TH phosphorylation at Ser40 via reduction of the activity of adenylyl cyclase. They also provide a molecular mechanism accounting for the ability of dopamine D(2) autoreceptors to inhibit dopamine synthesis and release from nigrostriatal nerve terminals.