Activation of rat caudate tyrosine hydroxylase phosphatase by tetrahydropterins

Complexity of dopamine metabolism

: Parkinson's disease (PD) coincides with a dramatic loss of dopaminergic neurons within the substantia nigra. A key player in the loss of dopaminergic neurons is oxidative stress. Dopamine (DA) metabolism itself is strongly linked to oxidative stress as its degradation generates reactive oxygen species (ROS) and DA oxidation can lead to endogenous neurotoxins whereas some DA derivatives show antioxidative effects. Therefore, DA metabolism is of special importance for neuronal redox-homeostasis and viability.In this review we highlight different aspects of dopamine metabolism in the context of PD and neurodegeneration. Since most reviews focus only on single aspects of the DA system, we will give a broader overview by looking at DA biosynthesis, sequestration, degradation and oxidation chemistry at the metabolic level, as well as at the transcriptional, translational and posttranslational regulation of all enzymes involved. This is followed by a short overview of cellular models currently used in PD research. Finally, we will address the topic from a medical point of view which directly aims to encounter PD.

Neurotoxin-induced impairment of biopterin synthesis and function: Initial stage of a Parkinson-like dopamine deficiency syndrome

Disorders of the function of the tyrosine hydroxylase play an important role in the occurrence of the Parkinson syndrome. The enzyme that catalyses the first, rate-limiting step in the biosynthesis to dopamine requires the cofactor tetrahydrobiopterin. This compound supplies the reduction equivalent for activation of molecular oxygen. Binding of the cofactor to the enzyme is affected by phosphorylation or dephosphorylation of the enzyme protein and, thereby, influences the activity. Nerve and chromaffin cells that synthesize dopamine, noradrenaline and serotonin are able to synthesize the cofactor tetrahydrobiopterin de novo from guanosine-triphosphate as a precursor. In patients suffering from Parkinson's disease a remarkable decrease in biopterin content was found in the brain. The function of the dopaminergic system was studied with an experimental Parkinson model. The antimetabolite 6-aminonicotinamide induces a dopamine deficit in the striatum with a significant slowdown in the utilization of this transmitter. The abolition of the 6-aminonicotinamide-induced muscular rigidity by l-DOPA and dopamine agonists implies that the antimetabolite produces a Parkinson-like syndrome in rats. There are reports on the molecular basis of this effect which are also important for understanding possible disturbances of the synthesis of biopterins. The effector 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP), which blocks the pentose phosphate pathway, is formed by an enzymatic neurotoxic synthesis. The clonal cell line PC-12 was used to study the molecular basis of the disturbances occurring in the dopaminergic system. These cells contain all the enzymes for catecholamine synthesis, including those for the synthesis of the cofactor tetrahydrobiopterin. Addition of 6-aminonicotinamide to the culture medium resulted in the synthesis of the neurotoxic agent, 6-ANADP, by a glycohydrolase localized in the endoplasmic reticulum. The synthesis of biopterin was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin resulted in reduced DOPA production. The decreased content of biopterin cofactor synthesis was compensated for by the addition of the precursor sepiapterin, indicating that the NADPH-dependent reductases in biopterin synthesis were not inhibited by the antimetabolic nucleotide 6-ANADP. DOPA production was not fully normalized by sepiapterin. Addition of NADH to the medium resulted in a further increase of DOPA production, probably by activation of the recycling pathway. The first step in the synthesis of biopterin from GTP to 7,8-neopterin-triphosphate seems to be particularly sensitive to the action of exogenous neurotoxins. A further sensitive site of action in synthesis to the cofactor BH(4) concerns the function of the dihydropteridin-reductase, which recycles qBH(2) to BH(4). Neurotoxin-induced impairment of biopterin synthesis is probably a pathogenetically important disorder at the initial stage of Parkinson's disease.

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.

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.

The effect of tetrahydrobiopterin on the in situ phosphorylation of tyrosine hydroxylase in rat striatal synaptosomes

Tetrahydrobiopterin (BH4), the obligatory cofactor of the aromatic amino acid hydroxylases, decreased the in situ 32P-phosphorylation of tyrosine hydroxylase (TH) in rat striatal synaptosomes. Incubation of pre-32P-labeled synaptosomes with BH4 in the presence of a permeant analogue of cAMP decreased the cAMP-stimulated level of 32P label incorporation into TH by about 50%, as determined by immunoprecipitation and autoradiography of SDS-polyacrylamide gels. The extent of inhibition mirrored changes in intrasynaptosomal BH4 levels and varied both as a function of BH4 concentration and length of incubation. A similar decrease in the amount of TH 32P-labeling was observed with the precursor of BH4, sepiapterin. This effect, in turn, was reversed by the inhibitor of sepiapterin reductase, N-acetyl-serotonin. Finally, exposure of pre-32P-labeled synaptosomes to the inhibitor of protein phosphatase 2A, okadaic acid, blocked the response to BH4. Collectively, the data suggest that BH4 stimulates the dephosphorylation of TH in situ and thus may play a dual role both as a cofactor for catalysis and a regulator of hydroxylase activity.

Dephosphorylation of tyrosine hydroxylase by brain protein phosphatases: a predominant role for type 2A

Extracts from rat corpus striatum, or striatal proteins resolved by chromatography on DE-52, were tested for protein phosphatase activity using tyrosine hydroxylase, phosphorylated by cAMP-dependent protein kinase, as substrate. The predominant dephosphorylating activity was independent of divalent cations and was inhibited by low concentrations (100 nM) of okadaic acid, defining the phosphatase as type 2A. Phosphatase type 2C (Mg2+ and Mn2+ stimulated) was evident in the presence of okadaic acid but at a level of approximately 10% of type 2A activity. Phosphatase 2B (Ca2+ and calmodulin dependent) mediated dephosphorylation of tyrosine hydroxylase was not apparent. The dephosphorylation of [32P]-tyrosine hydroxylase was not modulated by tetrahydrobiopterin, ATP, or GTP. These results indicate that tyrosine hydroxylase which has been phosphorylated by cAMP dependent protein kinase is dephosphorylated predominantly by phosphatase type 2A in brain, and the activity of this phosphatase is not modulated by pteridines or nucleotides.