Non-invasive fluorescent imaging of gliosis in transgenic mice for profiling developmental neurotoxicity

Gliosis is a universal response of the brain to almost all types of neural insults, including neurotoxicity, neurodegeneration, viral infection, and stroke. A hallmark of gliotic reaction is the up-regulation of the astrocytic biomarker GFAP (glial fibrillary acidic protein), which often precedes the anatomically apparent damages in the brain. In this study, neonatal transgenic mice at postnatal day (PD) 4 expressing GFP (green fluorescent protein) under the control of a widely used 2.2-kb human GFAP promoter in the brain are treated with two model neurotoxicants, 1-methyl-4(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'-CH(3)-MPTP), and kainic acid (KA), respectively, to induce gliosis. Here we show that the neurotoxicant-induced acute gliosis can be non-invasively imaged and quantified in the brain of conscious (un-anesthetized) mice in real-time, at 0, 2, 4, 6, and 8 h post-toxicant dosing. Therefore the current methodology could be a useful tool for studying the developmental aspects of neuropathies and neurotoxicity.

Selectivity and Affinity Determinants for Ligand Binding to the Aromatic Amino Acid Hydroxylases

Hydroxylation of the aromatic amino acids phenylalanine, tyrosine and tryptophan is carried out by a family of non-heme iron and tetrahydrobiopterin (BH4) dependent enzymes, i.e. the aromatic amino acid hydroxylases (AAHs). The reactions catalyzed by these enzymes are important for biomedicine and their mutant forms in humans are associated with phenylketonuria (phenylalanine hydroxylase), Parkinson's disease and DOPA-responsive dystonia (tyrosine hydroxylase), and possibly neuropsychiatric and gastrointestinal disorders (tryptophan hydroxylase 1 and 2). We attempt to rationalize current knowledge about substrate and inhibitor specificity based on the three-dimensional structures of the enzymes and their complexes with substrates, cofactors and inhibitors. In addition, further insights on the selectivity and affinity determinants for ligand binding in the AAHs were obtained from molecular interaction field (MIF) analysis. We applied this computational structural approach to a rational analysis of structural differences at the active sites of the enzymes, a strategy that can help in the design of novel selective ligands for each AAH.

Sustained phosphorylation of tyrosine hydroxylase at serine 40: a novel mechanism for maintenance of catecholamine synthesis

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine synthesis. Its activity is known to be controlled acutely (minutes) by phosphorylation and chronically (days) by protein synthesis. Using bovine adrenal chromaffin cells we found that nicotine, acting via nicotinic receptors, sustained the phosphorylation of TH at Ser40 for up to 48 h. Nicotine also induced sustained activation of TH, which for the first 24 h was completely independent of TH protein synthesis, and the phosphorylation of TH at Ser31. Imipramine did not inhibit the acute phosphorylation of TH at Ser40 or TH activation induced by nicotine, but did inhibit the sustained responses to nicotine seen at 24 h. The protein kinase(s) responsible for TH phosphorylation at Ser40 switched from being protein kinase C (PKC) independent in the acute phase to PKC dependent in the sustained phase. Sustained phosphorylation and activation of TH were also observed with histamine and angiotensin II. Sustained phosphorylation of TH at Ser40 provides a novel mechanism for increasing TH activity and this leads to increased catecholamine synthesis. Sustained phosphorylation of TH may be a selective target for drugs or pathology in neurons that contain TH and synthesize dopamine, noradrenaline or adrenaline.

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.

Insights into the catalytic mechanisms of phenylalanine and tryptophan hydroxylase from kinetic isotope effects on aromatic hydroxylation

Phenylalanine hydroxylase (PheH) and tryptophan hydroxylase (TrpH) catalyze the aromatic hydroxylation of phenylalanine and tryptophan, forming tyrosine and 5-hydroxytryptophan, respectively. The reactions of PheH and TrpH have been investigated with [4-(2)H]-, [3,5-(2)H(2)]-, and (2)H(5)-phenylalanine as substrates. All (D)k(cat) values are normal with Delta117PheH, the catalytic core of rat phenylalanine hydroxylase, ranging from 1.12-1.41. In contrast, for Delta117PheH V379D, a mutant protein in which the stoichiometry between tetrahydropterin oxidation and amino acid hydroxylation is altered, the (D)k(cat) value with [4-(2)H]-phenylalanine is 0.92 but is normal with [3,5-(2)H(2)]-phenylalanine. The ratio of tetrahydropterin oxidation to amino acid hydroxylation for Delta117PheH V379D shows a similar inverse isotope effect with [4-(2)H]-phenylalanine. Intramolecular isotope effects, determined from the deuterium contents of the tyrosine formed from [4-(2)H]-and [3,5(2)H(2)]-phenylalanine, are identical for Delta117PheH and Delta117PheH V379D, suggesting that steps subsequent to oxygen addition are unaffected in the mutant protein. The inverse effects are consistent with the reaction of an activated ferryl-oxo species at the para position of the side chain of the amino acid to form a cationic intermediate. The normal effects on the (D)k(cat) value for the wild-type enzyme are attributed to an isotope effect of 5.1 on the tautomerization of a dienone intermediate to tyrosine with a rate constant 6- to7-fold that for hydroxylation. In addition, there is a slight ( approximately 34%) preference for the loss of the hydrogen originally at C4 of phenylalanine. With (2)H(5)-indole-tryptophan as a substrate for Delta117PheH, the (D)k(cat) value is 0.89, consistent with hydroxylation being rate-limiting in this case. When deuterated phenylalanines are used as substrates for TrpH, the (D)k(cat) values are within error of those for Delta117PheH V379D. Overall, these results are consistent with the aromatic amino acid hydroxylases all sharing the same chemical mechanism, but with the isotope effect for hydroxylation by PheH being masked by tautomerization of an enedione intermediate to tyrosine.

Effects of ligands on the mobility of an active-site loop in tyrosine hydroxylase as monitored by fluorescence anisotropy

Fluorescence anisotropy has been used to monitor the effect of ligands on a mobile loop over the active site of tyrosine hydroxylase. Phe184 in the center of the loop was mutated to tryptophan, and the three native tryptophan residues were mutated to phenylalanine to form an enzyme with a single tryptophan residue in the mobile loop. The addition of 6-methyl-5-deazatetrahydropterin to the enzyme resulted in a significant increase in the fluorescence anisotropy. The addition of phenylalanine did not result in a significant change in the anisotropy in the presence or absence of the deazapterin. The K(d) value for the deazapterin was unaffected by the presence of phenylalanine. Qualitatively similar results were obtained with apoenzyme, except that the addition of phenylalanine led to a slight decrease in anisotropy. Frequency-domain lifetime measurements showed that the distribution of lifetimes was unaffected by both the amino acid and deazapterin. Frequency-domain anisotropy analyses were consistent with a decrease in the motion of the sole tryptophan in the presence of the deazapterin. This could be modeled as a decrease in the cone angle for the indole ring of about 12 degrees . The data are consistent with a model in which binding of a tetrahydropterin results in a change in the conformation of the surface loop required for proper formation of the amino acid binding site.

DJ-1 transcriptionally up-regulates the human tyrosine hydroxylase by inhibiting the sumoylation of pyrimidine tract-binding protein-associated splicing factor

Loss-of-function mutations in DJ-1 cause a subset of familial Parkinson disease (PD). However, the mechanism underlying the selective vulnerability in dopaminergic pathway due to the inactivation of DJ-1 is unclear. Previously, we have reported that DJ-1 is a neuroprotective transcriptional co-activator interacting with the transcriptional co-repressor pyrimidine tract-binding protein-associated splicing factor (PSF). Here we show that DJ-1 and PSF bind and regulate the human tyrosine hydroxylase (TH) promoter. Inactivation of DJ-1 by small interference RNA (siRNA) results in decreased TH expression and l-DOPA production in human dopaminergic cell lines. Consistent with its role as a transcriptional regulator, DJ-1 specifically suppresses the global SUMO-1 modification. High molecular weight sumoylated protein species, including PSF, accumulate in the lymphoblast cells from the patients carrying pathogenic DJ-1 mutations. DJ-1 elevates the TH expression by inhibiting the sumoylation of PSF and preventing its sumoylation-dependent recruitment of histone deacetylase 1. Furthermore, siRNA silencing of DJ-1 decreases the acetylation of TH promoter-bound histones, and histone deacetylase inhibitors restore the DJ-1 siRNA-induced repression of TH. Therefore, our results suggest DJ-1 as a regulator of protein sumoylation and directly link the loss of DJ-1 expression and transcriptional dysfunction to impaired dopamine synthesis.

A flexible loop in tyrosine hydroxylase controls coupling of amino acid hydroxylation to tetrahydropterin oxidation

The role of a polypeptide loop in tyrosine hydroxylase (TyrH) whose homolog in phenylalanine hydroxylase (PheH) takes on a different conformation when substrates are bound has been studied using site-directed mutagenesis. The loop spans positions 177 to 191; alanine was introduced into those positions, introducing one alanine substitution per TyrH variant. Mutagenesis of residues in the center of the loop resulted in alterations in the KM values for substrates, the Vmax value for dihydroxyphenylalanine (DOPA) synthesis, and the coupling of tetrahydropterin oxidation to tyrosine hydroxylation. The variant with the most altered KM value for 6-methyltetrahydropterin was TyrH F184A. The variants with the most affected K(tyr) values were those with substitutions in the center of the loop, TyrH K183A, F184A, D185A, P186A and D187A. These five variants also had the most reduced Vmax values for DOPA synthesis. Alanine substitution in positions 182-186 resulted in lowered ratios of tyrosine hydroxylation to tetrahydropterin oxidation. TyrH F184Y and PheH Y138F, variants with the residue at the center of the loop substituted with the residue present at the homologous position in the other hydroxylase, were also studied. The V/K(tyr) to V/K(phe) ratios for these variants were altered significantly, but the results did not suggest that F184 of TyrH or Y138 of PheH plays a dominant role in determining amino acid substrate specificity.

Intrinsic isotope effects on benzylic hydroxylation by the aromatic amino acid hydroxylases: evidence for hydrogen tunneling, coupled motion, and similar reactivities

Deuterium kinetic isotope effects for hydroxylation of the methyl group of 4-methylphenylalanine have been used as a probe of the relative reactivities of the hydroxylating intermediates in the aromatic amino acid hydroxylases phenylalanine, tyrosine, and tryptophan hydroxylase. When there are three deuterium atoms in the methyl group, all three enzymes exhibit an intrinsic isotope effect of about 13. The temperature dependence of the isotope effect is consistent with moderate tunneling, with the extent of tunneling identical for all three enzymes. In the case of phenylalanine hydroxylase, the presence of the regulatory domain has no effect on the values. The intrinsic primary and secondary isotope effects were determined using 4-methylphenylalanine containing one or two deuterium atoms in the methyl group. With one deuterium atom, the intrinsic primary and secondary effects have average values of 10 and 1.1, respectively. With two deuterium atoms, the primary effects decrease to 7.4 and the secondary effect increases to 1.3, consistent with coupled motion of the primary and secondary hydrogens. The results with all three enzymes are consistent with a hydrogen abstraction mechanism. The similarities of the isotope effects and extent of tunneling establish that the reactivities of the hydroxylating intermediates in the three enzymes are essentially identical.

Reduction and oxidation of the active site iron in tyrosine hydroxylase: kinetics and specificity

Tyrosine hydroxylase (TyrH) is a pterin-dependent enzyme that catalyzes the hydroxylation of tyrosine to form dihydroxyphenylalanine. The oxidation state of the active site iron atom plays a central role in the regulation of the enzyme. The kinetics of reduction of ferric TyrH by several reductants were determined by anaerobic stopped-flow spectroscopy. Anaerobic rapid freeze-quench EPR confirmed that the change in the near-UV absorbance of TyrH upon adding reductant corresponded to iron reduction. Tetrahydrobiopterin reduces wild-type TyrH following a simple second-order mechanism with a rate constant of 2.8 +/- 0.1 mM(-)(1) s(-)(1). 6-Methyltetrahydropterin reduces the ferric enzyme with a second-order rate constant of 6.1 +/- 0.1 mM(-)(1) s(-)(1) and exhibits saturation kinetics. No EPR signal for a radical intermediate was detected. Ascorbate, glutathione, and 1,4-benzoquinone all reduce ferric TyrH, but much more slowly than tetrahydrobiopterin, suggesting that the pterin is a physiological reductant. E332A TyrH, which has an elevated K(m) for tetrahydropterin in the catalytic reaction, is reduced by tetrahydropterins with the same kinetic parameters as those of the wild-type enzyme, suggesting that BH(4) does not bind in the catalytic conformation during the reduction. Oxidation of ferrous TyrH by molecular oxygen can be described as a single-step second-order reaction, with a rate constant of 210 mM(-)(1) s(-)(1). S40E TyrH, which mimics the phosphorylated state of the enzyme, has oxidation and reduction kinetics similar to those of the wild-type enzyme, suggesting that phosphorylation does not directly regulate the interconversion of the ferric and ferrous forms.

Effects of mutations in tyrosine hydroxylase associated with progressive dystonia on the activity and stability of the protein

Tyrosine hydroxylase (TyrH) catalyzes the conversion of tyrosine to dihydroxyphenylalanine (DOPA), the rate-limiting step in the biosynthesis of dopamine. Four mutations in the TyrH gene have recently been described in cases of autosomal recessive DOPA-responsive dystonia (Swaans et al., Ann Hum Genet 2000;64:25-31). All four are predicted to result in changes in single amino acid residues in the catalytic domain of the protein: T245P, T283M, R306H, and T463M. To determine the effects of these mutations on the molecular properties of the enzyme, mutant proteins containing the individual single amino acid changes have been expressed in bacteria and purified. Only the T283M mutation results in a decrease in the enzyme k(cat) value, while the T245P enzyme has a slightly higher value than the wild-type enzyme. The only case in which a K(m) value for either tyrosine or tetrahydrobiopterin is perturbed is the T245P enzyme, for which the K(m) value for tyrosine has increased about 50%. In contrast to the minor effects of the mutations on enzyme activity, the stability is decreased significantly by the mutations. The R306H and T283M enzymes are the least stable, losing activity 30- and 50-fold more rapidly than the wild-type enzyme. The apparent T(m) value for unfolding was decreased by 3.9, 8.2, and 7.2 degrees for the T245P, R306H, and T463M enzymes, while the T283M enzyme was too unstable for measurement of a T(m) value. The results establish that the physiological effects of the mutations are primarily due to the decreased stability of the mutant proteins rather than decreases in their intrinsic activities.

Biochemical conservation of recombinant Drosophila tyrosine hydroxylase with its mammalian cognates

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

A tyrosinase with an abnormally high tyrosine hydroxylase/dopa oxidase ratio. Role of the seventh histidine and accessibility to the active site

The sequencing of the genome of Ralstonia solanacearum[Salanoubat M, Genin S, Artiguenave F, et al. (2002) Nature 415, 497-502] revealed several genes that putatively code for polyphenol oxidases (PPOs). This soil-borne pathogenic bacterium withers a wide range of plants. We detected the expression of two PPO genes (accession numbers NP_518458 and NP_519622) with high similarity to tyrosinases, both containing the six conserved histidines required to bind the pair of type-3 copper ions at the active site. Generation of null mutants in those genes by homologous recombination mutagenesis and protein purification allowed us to correlate each gene with its enzymatic activity. In contrast with all tyrosinases so far studied, the enzyme NP_518458 shows higher monophenolase than o-diphenolase activity and its initial activity does not depend on the presence of l-dopa cofactor. On the other hand, protein NP_519622 is an enzyme with a clear preference to oxidize o-diphenols and only residual monophenolase activity, behaving as a catechol oxidase. These catalytic characteristics are discussed in relation to two other characteristics apart from the six conserved histidines. One is the putative presence of a seventh histidine which interacts with the carboxy group on the substrate and controls the preference for carboxylated and decarboxylated substrates. The second is the size of the residue isosteric with the aromatic F261 reported in sweet potato catechol oxidase which acts as a gate to control accessibility to CuA at the active site.

Regulatory mechanism of tyrosine hydroxylase activity

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

Isoosmotic isolation of rat brain synaptic vesicles, some of which contain tyrosine hydroxylase

Rat brain synaptic vesicles were isoosmotically isolated and examined for Mg(2+)-ATPase [EC 3.6.1.3.] and tyrosine hydroxylase [EC 1.14.16.2.] associated with the synaptic vesicles. Synaptosomes in 0.32 M sucrose were disrupted by freezing and thawing treatment, and the cytosol fraction was fractionated on a Sephacryl S-500 column with a mean exclusion size of 200 nm. Peak I at the void volume was a mixture of large vesicular membranes, small amounts of synaptic vesicles and coated vesicles, etc. Peak II consisted of non- and granulated synaptic vesicles of 35-40 nm diameter, and peak III of soluble proteins. The synaptic vesicles in peak II reacted with antibodies against the H(+)-ATPase A-subunit, vesicular acetylcholine transporter, and vesicular monoamine transporter. However, they showed little Mg(2+)-ATPase activity. Tyrosine hydroxylase was observed in either peak II or III on blotting with an anti-tyrosine hydroxylase antibody. These results imply that tyrosine hydroxylase exists in soluble and bound forms to synaptic vesicles in nerve terminals.

Cyclin-dependent Kinase 5 Phosphorylates Serine 31 of Tyrosine Hydroxylase and Regulates Its Stability

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis, and its activity is regulated by phosphorylation in the N-terminal regulatory domain. The proline-directed serine/threonine kinase cyclin-dependent kinase 5 (cdk5) plays an important role in diverse neuronal processes. In the present study, we identify TH as a novel substrate of cdk5. We show that cdk5 phosphorylates TH at serine 31 and that this phosphorylation is associated with an increase in total TH activity. In transgenic mice with increased cdk5 activity, the immunoreactivity for phosphorylated TH at Ser-31 is enhanced in neurons of the substantia nigra, a brain region enriched with TH-positive neurons. In addition, we demonstrate that co-expression of cdk5 and its regulatory activator p35 with TH increases the stability of TH. Consistent with these findings, TH protein levels are reduced in cdk5 knock-out mice. Importantly, the TH activity and protein turnover of the phosphorylation-defective mutant TH S31A was not altered by cdk5 activity. Taken together, these data suggest that cdk5 phosphorylation of TH is an important regulator of TH activity through stabilization of TH protein levels.

Tetrahydrobiopterin binding to aromatic amino acid hydroxylases. Ligand recognition and specificity

The three aromatic amino acid hydroxylases (phenylalanine, tyrosine, and tryptophan hydroxylase) and nitric oxide synthase (NOS) all utilize (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) as cofactor. The pterin binding site in the three hydroxylases is well conserved and different from the binding site in NOS. The structures of phenylalanine hydroxylase (PAH) and of NOS in complex with BH(4) are still the only crystal structures available for the reduced cofactor-enzyme complexes. We have studied the enzyme-bound and free conformations of BH(4) by NMR spectroscopy and molecular docking into the active site of the three hydroxylases, using endothelial NOS as a comparative probe. We have found that the dihydroxypropyl side chain of BH(4) adopts different conformations depending on which hydroxylase it interacts with. All the bound conformations are different from that of BH(4) free in solution at neutral pH. The different bound conformations appear to result from specific interactions with nonconserved amino acids at the BH(4) binding sites of the hydroxylases, notably the stretch 248-251 (numeration in PAH) and the residue corresponding to Ala322 in PAH, i.e., Ser in TH and Ala in TPH. On the basis of analysis of molecular interaction fields, we discuss the selectivity determinants for each hydroxylase and explain the high-affinity inhibitory effect of 7-tetrahydrobiopterin specifically for PAH.

Phosphorylation of Ser19 increases both Ser40 phosphorylation and enzyme activity of tyrosine hydroxylase in intact cells

We have previously shown that the phosphorylation of Ser19 in tyrosine hydroxylase can increase the rate of phosphorylation of Ser40 in tyrosine hydroxylase threefold in vitro. In this report we investigated the role of Ser19 on Ser40 phosphorylation in intact cells. Treatment of bovine chromaffin cells with anisomycin produced a twofold increase in Ser19 phosphorylation with no increase in Ser31 phosphorylation and only a small increase in Ser40 phosphorylation. Treatment of bovine chromaffin cells with forskolin produced a fourfold increase in Ser40 phosphorylation but no significant increase in either Ser19 or Ser31 phosphorylation. When chromaffin cells were first treated with anisomycin, the level of Ser40 phosphorylation after treatment by forskolin was 76% greater than the level of Ser40 phosphorylation in cells treated with forskolin alone. This potentiation of Ser40 phosphorylation by anisomycin could be completely blocked by the p38 MAP (mitogen-activated protein) kinase inhibitor SB 203580. The potentiation of Ser40 phosphorylation by anisomycin was not due to an increase in Ser40 kinase activity. Anisomycin treatment of chromaffin cells potentiated the forskolin-induced increase in tyrosine hydroxylase activity by 50%. This potentiation of activity was also blocked by SB 203580. These data provide the first evidence that the phosphorylation of Ser19 can potentiate the phosphorylation of Ser40 and subsequent activation of tyrosine hydroxylase in intact cells.

Effects of phosphorylation by protein kinase A on binding of catecholamines to the human tyrosine hydroxylase isoforms

Tyrosine hydroxylase (TyrH), the catalyst for the key regulatory step in catecholamine biosynthesis, is phosphorylated by cAMP-dependent protein kinase A (PKA) on a serine residue in a regulatory domain. In the case of the rat enzyme, phosphorylation of Ser40 by PKA is critical in regulating the enzyme activity; the effect of phosphorylation is to relieve the enzyme from inhibition by dopamine and dihydroxyphenylalanine (DOPA). There are four isoforms of human tyrosine hydroxylase (hTyrH), differing in the size of an insertion after Met30. The effects of phosphorylation by PKA on the binding of DOPA and dopamine have now been determined for all four human isoforms. There is an increase of about two-fold in the Kd value for DOPA for isoform 1 upon phosphorylation, from 4.4 to 7.4 microM; this effect decreases with the larger isoforms such that there is no effect of phosphorylation on the Kd value for isoform 4. Dopamine binds more much tightly, with Kd values less than 3 nM for all four unphosphorylated isoforms. Phosphorylation decreases the affinity for dopamine at least two orders of magnitude, resulting in Kd values of about 0.1 microM for the phosphorylated human enzymes, due primarily to increases in the rate constant for dissociation of dopamine. Dopamine binds about two-fold less tightly to the phosphorylated isoform 1 than to the other three isoforms. The results extend the regulatory model developed for the rat enzyme, in which the activity is regulated by the opposing effects of catecholamine binding and phosphorylation by PKA. The small effects on the relatively high Kd values for DOPA suggest that DOPA levels do not regulate the activity of hTyrH.

Regulation of Dyrk1A kinase activity by 14-3-3

Dual-specificity tyrosine(Y) regulated kinase 1A (DYRK1A) is a serine/threonine protein kinase implicated in mental retardation resulting from Down syndrome. In this study, we carried out yeast two-hybrid screening to find proteins regulating DYRK1A kinase activity. We identified 14-3-3 as a Dyrk1A interacting protein, which is consistent with the previous finding of the interaction between the yeast orthologues Yak1p and Bmh1/2p. We showed the interaction between Dyrk1A and 14-3-3 in vitro and in vivo. The binding required the N-terminus of Dyrk1A and was independent of the Dyrk1A phosphorylation status. Functionally, 14-3-3 binding increased Dyrk1A kinase activity in a dose dependent manner in vitro. In vivo, a small peptide inhibiting 14-3-3 binding, sc138, decreased Dyrk1A kinase activity in COS7. In summary, these results suggest that DYRK1A kinase activity could be regulated by the interaction of 14-3-3.

Activity-dependent phosphorylation of tyrosine hydroxylase in dopaminergic neurons of the rat retina

We studied in vivo activity-dependent phosphorylation of tyrosine hydroxylase (TH) in dopaminergic (DA) neurons of the rat retina. TH phosphorylation (TH-P) was evaluated by immunocytochemistry, using antibodies specific for each of three regulated phosphorylation sites. TH synthesis rate was measured by dihydroxyphenylalanine (DOPA) accumulation in the presence of NSD-1015, an inhibitor of aromatic amino acid decarboxylase. TH-P was increased markedly by light or after intraocular injection of GABA(A) and glycine inhibitors. All three phosphospecific antibodies responded similarly to test drugs or light. A 30 min exposure to light increased DOPA accumulation by threefold over that seen after 30 min in darkness. Immunostaining to an anti-panNa channel antibody was found in all parts of the DA neuron. TTX blocked TH-P induced by light or GABA/glycine inhibitors but only in varicosities of the DA axon plexus, not in perikarya or dendrites. Veratridine increased TH-P in all parts of the DA neuron. The distribution of the monoamine vesicular transporter 2 was shown by immunocytochemistry to reside in varicosities of the DA plexus but not in dendrites, indicating that the varicosities are sites of dopamine release. Collectively, these data indicate that, in the retina, dopamine synthesis in varicosities is affected by the spiking activity of retinal neurons, possibly including that of the DA neurons themselves.

Tyrosine phosphorylation inhibits the interaction of 14-3-3 proteins with the plant plasma membrane H+-ATPase

Interaction of 14-3-3 proteins with their targets depends not only on the phosphorylation status of the target but also on that of 14-3-3 (Fu et al., 2000). In this work we demonstrated that the maize 14-3-3 isoform GF14-6 is a substrate of the tyrosine kinase insulin growth factor receptor 1. By means of site-directed mutants of GF14-6, we identified Tyr-137 as the specific tyrosine residue phosphorylated by the insulin growth factor receptor 1. Phosphorylation of GF14-6 on Tyr-137 lowered its affinity for a peptide mimicking the 14-3-3 binding site of the plant plasma membrane H+-ATPase. Moreover, phosphorylation in planta of 14-3-3 tyrosine residues, resulting from incubation with the tyrosine phosphatase inhibitor, phenylarsine oxide, decreased their association to the H+-ATPase.

14-3-3 gene characterization and description of a second 14-3-3 isoform in both Echinococcus granulosus and E. multilocularis

Members of the 14-3-3 protein family have been identified as regulatory molecules in intracellular signaling pathways and cell cycle control. Previously, the first Echinococcus 14-3-3 isoform (E14-3-3.1) was isolated from E. granulosus and E. multilocularis metacestode stages. Hyperexpression of this isoform was claimed to be associated with non-restricted tumor-like growth of the E. multilocularis metacestode. In this report, we describe the characterization of a 14-3-3 cDNA from E. granulosus and E. multilocularis corresponding to a second isoform of this family, E14-3-3.2. The characterized 14-3-3 gene was interrupted by two introns whose sequence and positions were conserved in both Echinococcus species. The deduced amino acid sequence of E14-3-3.2 showed 88% identity to the E14-3-3.1 isoform and 52% identity to a third Echinococcus isoform (E14-3-3.3) described by other authors. These findings, coupled to Southern blot analysis, suggest the presence of more than one 14-3-3 gene in Echinococcus. Phylogenenetically, the Echinococcus 14-3-3.1 and 14-3-3.2 isoforms appeared to cluster with zeta-type ("pro-tumorigenic") 14-3-3 isoforms from closely related organisms, whereas the E14-3-3.3 isoform grouped with 14-3-3 epsilon isoforms. The presence of more than one 14-3-3 isoform might indicate isoform-specific roles in the different parasite stages of Echinococcus.