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

Tyrosine hydroxylase phosphorylation is under the control of serine 40

Tyrosine hydroxylase catalyzes the initial and rate-limiting step in the biosynthesis of the neurotransmitter dopamine. The phosphorylation state of Ser40 and Ser31 is believed to exert a direct effect on the enzymatic activity of tyrosine hydroxylase. Interestingly, some studies report that Ser31 phosphorylation affects Ser40 phosphorylation, while Ser40 phosphorylation has no effect on Ser31 phosphorylation, a process named hierarchical phosphorylation. Here, we provide a detailed investigation into the signal transduction mechanisms regulating Ser40 and Ser31 phosphorylation in dopaminergic mouse MN9D and Neuro2A cells. We find that cyclic nucleotide signaling drives Ser40 phosphorylation, and that Ser31 phosphorylation is strongly regulated by ERK signaling. Inhibition of ERK1/2 with UO126 or PD98059 reduced Ser31 phosphorylation, but surprisingly had no effect on Ser40 phosphorylation, contradicting a role for Ser31 in the regulation of Ser40. Moreover, to elucidate a possible hierarchical mechanism controlling tyrosine hydroxylase phosphorylation, we introduced tyrosine hydroxylase variants in Neuro2A mouse neuroblastoma cells that mimic either phosphorylated or unphosphorylated serine residues. When we introduced a Ser40Ala tyrosine hydroxylase variant, Ser31 phosphorylation was completely absent. Additionally, neither the tyrosine hydroxylase variant Ser31Asp, nor the variant Ser31Ala had any significant effect on basal Ser40 phosphorylation levels. These results suggest that tyrosine hydroxylase is not controlled by hierarchical phosphorylation in the sense that first Ser31 has to be phosphorylated and subsequently Ser40, but, conversely, that Ser40 phosphorylation is essential for Ser31 phosphorylation. Overall our study suggests that Ser40 is the crucial residue to target so as to modulate tyrosine hydroxylase activity.

The Effects of Chronic Unpredictable Mild Stress and Semi-Pure Diets on the Brain, Gut and Adrenal Medulla in C57BL6 Mice

Chronic stress is known to perturb serotonergic regulation in the brain, leading to mood, learning and memory impairments and increasing the risk of developing mood disorders. The influence of the gut microbiota on serotonergic regulation in the brain has received increased attention recently, justifying the investigation of the role of diet on the gut and the brain in mood disorders. Here, using a 4-week chronic unpredictable mild stress (CUMS) model in mice, we aimed to investigate the effects of a high-fat high-glycaemic index (HFD) and high-fibre fruit & vegetable "superfood" (SUP) modifications of a semi-pure AIN93M diet on behaviour, serotonin synthesis and metabolism pathway regulation in the brain and the gut, as well as the gut microbiota and the peripheral adrenal medullary system. CUMS induced anxiety-like behaviour, dysregulated the tryptophan and serotonin metabolic pathways in the hippocampus, prefrontal cortex, and colon, and altered the composition of the gut microbiota. CUMS reduced the catecholamine synthetic capacity of the adrenal glands. Differential effects were found in these parameters in the HFD and SUP diet. Thus, dietary modifications may profoundly affect the multiple dynamic systems involved in mood disorders.

Sex-Dependent Effects of Chronic Restraint Stress on Mood-Related Behaviours and Neurochemistry in Mice

Anxiety and depressive disorders are closely associated; however, the pathophysiology of these disorders remains poorly understood. Further exploration of the mechanisms involved in anxiety and depression such as the stress response may provide new knowledge that will contribute to our understanding of these disorders. Fifty-eight 8-12-week-old C57BL6 mice were separated into experimental groups by sex as follows: male controls (n = 14), male restraint stress (n = 14), female controls (n = 15) and female restraint stress (n = 15). These mice were taken through a 4-week randomised chronic restraint stress protocol, and their behaviour, as well as tryptophan metabolism and synaptic proteins, were measured in the prefrontal cortex and hippocampus. Adrenal catecholamine regulation was also measured. The female mice showed greater anxiety-like behaviour than their male counterparts. Tryptophan metabolism was unaffected by stress, but some basal sex characteristics were noted. Synaptic proteins were reduced in the hippocampus in stressed females but increased in the prefrontal cortex of all female mice. These changes were not found in any males. Finally, the stressed female mice showed increased catecholamine biosynthesis capability, but this effect was not found in males. Future studies in animal models should consider these sex differences when evaluating mechanisms related to chronic stress and depression.

Personalized Medicine to Improve Treatment of Dopa-Responsive Dystonia-A Focus on Tyrosine Hydroxylase Deficiency

Dopa-responsive dystonia (DRD) is a rare movement disorder associated with defective dopamine synthesis. This impairment may be due to the fact of a deficiency in GTP cyclohydrolase I (GTPCHI, GCH1 gene), sepiapterin reductase (SR), tyrosine hydroxylase (TH), or 6-pyruvoyl tetrahydrobiopterin synthase (PTPS) enzyme functions. Mutations in GCH1 are most frequent, whereas fewer cases have been reported for individual SR-, PTP synthase-, and TH deficiencies. Although termed DRD, a subset of patients responds poorly to L-DOPA. As this is regularly observed in severe cases of TH deficiency (THD), there is an urgent demand for more adequate or personalized treatment options. TH is a key enzyme that catalyzes the rate-limiting step in catecholamine biosynthesis, and THD patients often present with complex and variable phenotypes, which results in frequent misdiagnosis and lack of appropriate treatment. In this expert opinion review, we focus on THD pathophysiology and ongoing efforts to develop novel therapeutics for this rare disorder. We also describe how different modeling approaches can be used to improve genotype to phenotype predictions and to develop in silico testing of treatment strategies. We further discuss the current status of mathematical modeling of catecholamine synthesis and how such models can be used together with biochemical data to improve treatment of DRD patients.

Peripheral inflammation induces long-term changes in tyrosine hydroxylase activation in the substantia nigra

Inflammation plays a role in neuropathology. We hypothesised that inflammation, induced by a single intraperitoneal injection of lipopolysaccharide (LPS), would induce long-term changes in the regulation of tyrosine hydroxylase (TH) in the rat midbrain. The level of 12 cytokines was initially analysed from one day to six months after LPS injection to confirm that peripheral inflammation led to neuroinflammatory changes in the midbrain. In the substantia nigra (SN), the levels of 8 of the 12 measured cytokines was significantly increased at one day. Granulocyte-macrophage colony-stimulating factor showed a threefold increased level at 6 months. The ventral tegmental area (VTA) showed a completely different pattern, with no increases in the levels of the 12 cytokines at one day and no changes beyond one week. TH activity was determined using a tritiated water release assay, TH protein and phosphorylation levels (Ser19, Ser31 and Ser40) were determined using western blotting. TH-specific activity in the SN was unchanged at one day but was substantially increased at one week and one month with no concomitant increase in TH phosphorylation. Substantial changes in TH activation without changes in TH phosphorylation have not previously been observed in the brain in response to a range of stressors. TH-specific activity was increased in the SN, and in the caudate putamen, at 6 months and was associated with increased TH phosphorylation at Ser19 and Ser40 at both locations. TH-specific activity in the VTA showed only a transient increase at day one associated with increased phosphorylation at Ser19 and Ser31 but thereafter showed no changes. This study shows that inflammation induced by LPS generated two distinct long-term changes in TH activity in the SN that are caused by different mechanisms, but there were no long-term changes in the adjacent VTA.

Neural-Induced Human Adipose Tissue-Derived Stem Cells Conditioned Medium Ameliorates Rotenone-Induced Toxicity in SH-SY5Y Cells

Parkinson’s disease (PD) is an age-related neurodegenerative disease (NDD) characterized by the degenerative loss of dopaminergic neurons in the substantia nigra along with aggregation of α-synuclein (α-syn). Neurogenic differentiation of human adipose-derived stem cells (NI-hADSCs) by supplementary factors for 14 days activates different biological signaling pathways. In this study, we evaluated the therapeutic role of NI-hADSC-conditioned medium (NI-hADSC-CM) in rotenone (ROT)-induced toxicity in SH-SY5Y cells. Increasing concentrations of ROT led to decreased cell survival at 24 and 48 h in a dose- and time-dependent manner. Treatment of NI-hADSC-CM (50% dilution in DMEM) against ROT (0.5 μM) significantly increased the cell survival. ROT toxicity decreased the expression of tyrosine hydroxylase (TH). Western blot analysis of the Triton X-100-soluble fraction revealed that ROT significantly decreased the oligomeric, dimeric, and monomeric phosphorylated Serine129 (p-S129) α-syn, as well as the total monomeric α-syn expression levels. ROT toxicity increased the oligomeric, but decreased the dimeric and monomeric p-S129 α-syn expression levels. Total α-syn expression (in all forms) was increased in the Triton X-100-insoluble fraction, compared to the control. NI-hADSC-CM treatment enhanced the TH expression, stabilized α-syn monomers, reduced the levels of toxic insoluble p-S129 α-syn, improved the expression of neuronal functional proteins, regulated the Bax/Bcl-2 ratio, and upregulated the expression of pro-caspases, along with PARP-1 inactivation. Moreover, hADSC-CM treatment decreased the cell numbers and have no effect against ROT toxicity on SH-SY5Y cells. The therapeutic effects of NI-hADSC-CM was higher than the beneficial effects of hADSC-CM on cellular signaling. From these results, we conclude that NI-hADSC-CM exerts neuroregenerative effects on ROT-induced PD-like impairments in SH-SY5Y cells.

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.

Neonatal overfeeding increases capacity for catecholamine biosynthesis from the adrenal gland acutely and long-term in the male rat

A poor nutritional environment during early development has long been known to increase disease susceptibility later in life. We have previously shown that rats that are overfed as neonates (i.e. suckled in small litters (4 pups) relative to control conditions (12 pups)) show dysregulated hypothalamic-pituitary-adrenal axis responses to immune stress in adulthood, particularly due to an altered capacity of the adrenal to respond to an immune challenge. Here we hypothesised that neonatal overfeeding similarly affects the sympathomedullary system, testing this by investigating the biochemical function of tyrosine hydroxylase (TH), the first rate-limiting enzyme in the catecholamine synthesis. We also examined changes in adrenal expression of the leptin receptor and in mitogen-activated protein kinase (MAPK) signalling. During the neonatal period, we saw age-dependent changes in TH activity and phosphorylation, with neonatal overfeeding stimulating increased adrenal TH specific activity at postnatal days 7 and 14, along with a compensatory reduction in total TH protein levels. This increased TH activity was maintained into adulthood where neonatally overfed rats exhibited increased adrenal responsiveness 30 min after an immune challenge with lipopolysaccharide, evident in a concomitant increase in TH protein levels and specific activity. Neonatal overfeeding significantly reduced the expression of the leptin receptor in neonatal adrenals at postnatal day 7 and in adult adrenals, but did not affect MAPK signalling. These data suggest neonatal overfeeding alters the capacity of the adrenal to synthesise catecholamines, both acutely and long term, and these effects may be independent of leptin signalling.

Bioactive constituents from cinnamon, hemp seed and polygonum cuspidatum protect against H2O2 but not rotenone toxicity in a cellular model of Parkinson's disease

Mitochondrial dysfunction and oxidative stress are two factors that are thought to contribute to the pathogenesis of Parkinson's disease (PD), a debilitating progressive neurodegenerative disorder that results in the loss of catecholamine producing cells throughout specific regions of the brain. In this study we aimed to compare the effects of hydrogen peroxide (H₂O₂) and rotenone (a pesticide and mitochondrial complex 1 inhibitor) on cell viability and the expression of tyrosine hydroxylase (TH) in a cellular model of PD. We also sought to investigate the potential neuroprotective benefits of bioactive constituents from cinnamon, hemp seed and polygonum cuspidatum. To create a model, SH-SY5Y cells transfected with human TH isoform 1 were treated with varying concentrations of H₂O₂ and rotenone, in the presence or absence of bioactive constituents. The effect of these toxins and constituents on cell viability, apoptosis and protein expression was assessed using MTT viability assays and western blotting. Rotenone treatment caused a significant decrease in cell viability but a significant increase in TH in the remaining cells. H₂O₂ treatment caused a significant decrease in cell viability but had no significant effect on TH expression. Curcumin, cinnamaldehyde, caffeoyltyramide (hemp seed extract) and piceatannol glucoside (polygonum cuspidatum extract) were unable to attenuate rotenone induced cell death, however they were able to provide protection against H₂O₂ induced cell death. This is the first study to demonstrate the neuroprotective properties of cinnamaldehyde, caffeoyltyramide and piceatannol glucoside in a dopaminergic cell line in response to H₂O₂.

Tyrosine hydroxylase regulation in adult rat striatum following short-term neonatal exposure to manganese

Manganese (Mn) is an essential trace element required for a range of physiological processes, but Mn can also be neurotoxic especially during development. Excess levels of Mn accumulate preferentially in the striatum and can induce a syndrome called manganism, characterized by an initial stage of psychiatric disorder followed by motor impairment. In the present study, we investigated the effects of Mn exposure on the developing dopaminergic system, specifically tyrosine hydroxylase (TH) protein and phosphorylation levels in the striatum of rats. Neonatal rats were exposed to Mn intraperitoneally (ip) from post-natal day 8 up to day 12 (PND8-12). Striatal tissue was analysed on PND14 or PND70, to detect either short-term or long-term effects induced by Mn exposure. There was a dose dependent increase in TH protein levels in the striatum at PND14, reaching significance at 20 mg kg(-1) Mn, and this correlated with an increase in TH phosphorylation at serines 40, 31 and 19. However, in the striatum at PND70, a time by which Mn levels were no longer elevated, there was a dose dependent decrease in TH protein levels, reaching significance at 20 mg kg(-1) Mn, and this correlated with TH phosphorylation at Ser40 and Ser19. There was however a significant increase in phosphorylation of TH at serine 31 at 20 mg kg(-1) Mn, which did not correlate with TH protein levels. Taken together our findings suggest that neonatal Mn exposure can have both short-term and long-term effects on the regulation of TH in the striatal dopaminergic system.

Phosphorylation at serine 31 targets tyrosine hydroxylase to vesicles for transport along microtubules

Tyrosine hydroxylase (TH) catalyzes the conversion of l-tyrosine into l-DOPA, which is the rate-limiting step in the synthesis of catecholamines, such as dopamine, in dopaminergergic neurons. Low dopamine levels and death of the dopaminergic neurons are hallmarks of Parkinson's disease (PD), where α-synuclein is also a key player. TH is highly regulated, notably by phosphorylation of several Ser/Thr residues in the N-terminal tail. However, the functional role of TH phosphorylation at the Ser-31 site (THSer(P)-31) remains unclear. Here, we report that THSer(P)-31 co-distributes with the Golgi complex and synaptic-like vesicles in rat and human dopaminergic cells. We also found that the TH microsomal fraction content decreases after inhibition of cyclin-dependent kinase 5 (Cdk5) and ERK1/2. The cellular distribution of an overexpressed phospho-null mutant, TH1-S31A, was restricted to the soma of neuroblastoma cells, with decreased association with the microsomal fraction, whereas a phospho-mimic mutant, TH1-S31E, was distributed throughout the soma and neurites. TH1-S31E associated with vesicular monoamine transporter 2 (VMAT2) and α-synuclein in neuroblastoma cells, and endogenous THSer(P)-31 was detected in VMAT2- and α-synuclein-immunoprecipitated mouse brain samples. Microtubule disruption or co-transfection with α-synuclein A53T, a PD-associated mutation, caused TH1-S31E accumulation in the cell soma. Our results indicate that Ser-31 phosphorylation may regulate TH subcellular localization by enabling its transport along microtubules, notably toward the projection terminals. These findings disclose a new mechanism of TH regulation by phosphorylation and reveal its interaction with key players in PD, opening up new research avenues for better understanding dopamine synthesis in physiological and pathological states.

Reconsidering the role of glial cells in chronic stress-induced dopaminergic neurons loss within the substantia nigra? Friend or foe?

Exposure to psychological stress is known to seriously disrupt the operation of the substantia nigra (SN) and may in fact initiate the loss of dopaminergic neurons within the SN. In this study, we aimed to investigate how chronic stress modified the SN in adult male mice. Using a paradigm of repeated restraint stress (an average of 20h per week for 6weeks), we examined changes within the SN using western blotting and immunohistochemistry. We demonstrated that chronic stress was associated with a clear loss of dopaminergic neurons within the SN. The loss of dopaminergic neurons was accompanied by higher levels of oxidative stress damage, indexed by levels of protein carbonylation and strong suppression of both microglial and astrocytic responses. In addition, we demonstrated for the first time, that chronic stress alone enhanced the aggregation of α-synuclein into the insoluble protein fraction. These results indicate that chronic stress triggered loss of dopaminergic neurons by increasing oxidative stress, suppressing glial neuroprotective functions and enhancing the aggregation of the neurotoxic protein, α-synuclein. Collectively, these results reinforce the negative effects of chronic stress on the viability of dopaminergic cells within the SN.

Early life peripheral lipopolysaccharide challenge reprograms catecholaminergic neurons

Neonatal immune challenge with the bacterial mimetic lipopolysaccharide has the capacity to generate long-term changes in the brain. Neonatal rats were intraperitoneally injected with lipopolysaccharide (0.05 mg/kg) on postnatal day (PND) 3 and again on PND 5. The activation state of tyrosine hydroxylase (TH) was measured in the locus coeruleus, ventral tegmental area and substantia nigra on PND 85. In the locus coeruleus there was an approximately four-fold increase in TH activity. This was accompanied by a significant increase in TH protein together with increased phosphorylation of all three serine residues in the N-terminal region of TH. In the ventral tegmental area, a significant increase in TH activity and increased phosphorylation of the serine 40 residue was seen. Neonatal lipopolysaccharide had no effect on TH activation in the substantia nigra. These results indicate the capacity of a neonatal immune challenge to generate long-term changes in the activation state of TH, in particular in the locus coeruleus. Overall, the current results demonstrate the enduring outcomes of a neonatal immune challenge on specific brain catecholaminergic regions associated with catecholamine synthesis. This highlights a novel mechanism for long-term physiological and behavioural alterations induced by this model.

The Effects of Insulin-Induced Hypoglycaemia on Tyrosine Hydroxylase Phosphorylation in Rat Brain and Adrenal Gland

In this study we investigated the effects of insulin-induced hypoglycaemia on tyrosine hydroxylase (TH) protein and TH phosphorylation in the adrenal gland, C1 cell group, locus coeruleus (LC) and midbrain dopaminergic cell groups that are thought to play a role in response to hypoglycaemia and compared the effects of different concentrations of insulin in rats. Insulin (1 and 10 U/kg) treatment caused similar reductions in blood glucose concentration (from 7.5-9 to 2-3 mmol/L); however, plasma adrenaline concentration was increased 20-30 fold in response to 10 U/kg insulin and only 14 fold following 1 U/kg. Time course studies (at 10 U/kg insulin) revealed that in the adrenal gland, Ser31 phosphorylation was increased between 30 and 90 min (4-5 fold), implying that TH was activated to increase catecholamine synthesis in adrenal medulla to replenish the stores. In the brain, Ser19 phosphorylation was limited to certain dopaminergic groups in the midbrain, while Ser31 phosphorylation was increased in most catecholaminergic regions at 60 min (1.3-2 fold), suggesting that Ser31 phosphorylation may be an important mechanism to maintain catecholamine synthesis in the brain. Comparing the effects of 1 and 10 U/kg insulin revealed that Ser31 phosphorylation was increased to similar extent in the adrenal gland and C1 cell group in response to both doses whereas Ser31 and Ser19 phosphorylation were only increased in response to 1 U/kg insulin in LC and in response to 10 U/kg insulin in most midbrain regions. Thus, the adrenal gland and some catecholaminergic brain regions become activated in response to insulin administration and brain catecholamines may be important for initiation of physiological defences against insulin-induced hypoglycaemia.

Investigation of tyrosine hydroxylase and BDNF in a low-dose rotenone model of Parkinson's disease

Tyrosine hydroxylase (TH, the rate limiting-enzyme in catecholamine synthesis) is regulated acutely via phosphorylation of 3 serine residues--Ser19, 31 and 40, and chronically via changes in TH protein levels. In this study, we aimed to investigate how TH is regulated in the brain, gut and adrenal gland as well as changes in mature brain-derived neurotrophic factor (mBDNF) and proBDNF levels in a low-dose (2 mg/kg, 5 days/week for 4 weeks) rotenone model of Parkinson's disease (PD). Rearing behaviour decreased by week 3 in the rotenone group (p<0.01), with further decreases in rearing by week 4 (p<0.001); however, TH remained unchanged in the substantia nigra (SN) and striatum; TH levels were also unaltered in other catecholaminergic cell groups of the brainstem such as A1C1 neurons or locus coeruleus. In the olfactory bulb, TH protein decreased (2.5-fold, p<0.01) while Ser31 phosphorylation increased (1.4-fold, p<0.05) in the rotenone group. In contrast, TH protein was increased in the adrenal gland (2-fold, p<0.05) and colon (5-fold, p<0.05) of rotenone rats. mBDNF levels were not changed in the SN but were significantly reduced in plasma and significantly increased in the colon (2-fold, p<0.01) of rotenone-treated rats. This is the first study to assess TH and BDNF in the brain and periphery in the rotenone model before SN/striatum degeneration is evident. Together these results suggest that low-dose rotenone may have some potential to model the early stages of PD.

Functional studies of tyrosine hydroxylase missense variants reveal distinct patterns of molecular defects in Dopa-responsive dystonia

Congenital tyrosine hydroxylase deficiency (THD) is found in autosomal-recessive Dopa-responsive dystonia and related neurological syndromes. The clinical manifestations of THD are variable, ranging from early-onset lethal disease to mild Parkinson disease-like symptoms appearing in adolescence. Until 2014, approximately 70 THD patients with a total of 40 different disease-related missense mutations, five nonsense mutations, and three mutations in the promoter region of the tyrosine hydroxylase (TH) gene have been reported. We collected clinical and biochemical data in the literature for all variants, and also generated mutant forms of TH variants previously not studied (N = 23). We compared the in vitro solubility, thermal stability, and kinetic properties of the TH variants to determine the cause(s) of their impaired enzyme activity, and found great heterogeneity in all these properties among the mutated forms. Some TH variants had specific kinetic anomalies and phenylalanine hydroxylase, and Dopa oxidase activities were measured for variants that showed signs of altered substrate binding. p.Arg233His, p.Gly247Ser, and p.Phe375Leu had shifted substrate specificity from tyrosine to phenylalanine and Dopa, whereas p.Cys359Phe had an impaired activity toward these substrates. The new data about pathogenic mechanisms presented are expected to contribute to develop individualized therapy for THD patients.

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.

ser31 Tyrosine hydroxylase phosphorylation parallels differences in dopamine recovery in nigrostriatal pathway following 6-OHDA lesion

Compensatory mechanisms in dopamine (DA) signaling have long been proposed to delay onset of locomotor symptoms during Parkinson's disease progression until ~ 80% loss of striatal DA occurs. Increased striatal dopamine turnover has been proposed to be a part of this compensatory response, but may occur after locomotor symptoms. Increased tyrosine hydroxylase (TH) activity has also been proposed as a mechanism, but the impact of TH protein loss upon site-specific TH phosphorylation in conjunction with the impact on DA tissue content is not known. The tissue content of DA was determined against TH protein loss in the striatum and substantia nigra (SN) following 6-hydroxydopamine lesion in the medial forebrain bundle in young Sprague-Dawley male rats. Although DA predictably decreased in both regions following 6-hydroxydopamine, there was a significant difference in DA loss between the striatum (75%) and SN (40%), despite similar TH protein loss. Paradoxically, there was a significant decrease in DA against remaining TH protein in striatum, but a significant increase in DA against remaining TH in SN. In the SN, increased DA per remaining TH protein was matched by increased ser31, but not ser40, TH phosphorylation. In striatum, both ser31 and ser40 phosphorylation decreased, reflecting decreased DA per TH. However, in control nigral and striatal tissue, only ser31 phosphorylation correlated with DA per TH protein. Combined, these results suggest that the phosphorylation of ser31 in the SN may be a mechanism to increase DA biosynthesis against TH protein loss in an in vivo model of Parkinson's disease. Properties of dopamine biosynthesis were evaluated in the 6-OHDA model of Parkinson's disease by studying the impact of tyrosine hydroxylase (TH) protein loss on its own phosphorylation and dopamine (DA) tissue content in rat nigrostriatal pathway. A dichotomous response was observed between striatum and substantia nigra in that dopamine per remaining TH decreased in striatum, but increased in substantia nigra. Phosphorylation at ser31 reflected these differences, indicating that ser31 phosphorylation may be critical to maintain dopamine with progressive TH protein loss. Drawings are from slides purchased from Motifolio (http://motifolio.com/).

The N-terminal sequence of tyrosine hydroxylase is a conformationally versatile motif that binds 14-3-3 proteins and membranes

Tyrosine hydroxylase (TH) catalyzes the rate-limiting step in the synthesis of catecholamine neurotransmitters, and a reduction in TH activity is associated with several neurological diseases. Human TH is regulated, among other mechanisms, by Ser19-phosphorylation-dependent interaction with 14-3-3 proteins. The N-terminal sequence (residues 1-43), which corresponds to an extension to the TH regulatory domain, also interacts with negatively charged membranes. By using X-ray crystallography together with molecular dynamics simulations and structural bioinformatics analysis, we have probed the conformations of the Ser19-phosphorylated N-terminal peptide [THp-(1-43)] bound to 14-3-3γ, free in solution and bound to a phospholipid bilayer, and of the unphosphorylated peptide TH-(1-43) both free and bilayer bound. As seen in the crystal structure of THp-(1-43) complexed with 14-3-3γ, the region surrounding pSer19 adopts an extended conformation in the bound state, whereas THp-(1-43) adopts a bent conformation when free in solution, with higher content of secondary structure and higher number of internal hydrogen bonds. TH-(1-43) in solution presents the highest mobility and least defined structure of all forms studied, and it shows an energetically more favorable interaction with membranes relative to THp-(1-43). Cationic residues, notably Arg15 and Arg16, which are the recognition sites of the kinases phosphorylating at Ser19, are also contributing to the interaction with the membrane. Our results reveal the structural flexibility of this region of TH, in accordance with the functional versatility and conformational adaptation to different partners. Furthermore, this structural information has potential relevance for the development of therapeutics for neurodegenerative disorders, through modulation of TH-partner interactions.

Does exposure to chronic stress influence blood pressure in rats?

The principal aim of this study was to determine whether prolonged chronic footshock stress can evoke sustained changes in blood pressure in rats and to elucidate possible underlying neurochemical mechanisms as mediated by the sympathoadrenal system. Adult male Wistar rats instrumented for telemetric recording of arterial pressure, heart rate and locomotor activity were subjected to six weeks of inescapable unpredictable electrical footshocks (FS+) or were exposed to shock chambers but were not shocked (FS-). Compared to FS- animals, FS+ animals had significantly reduced body weight gain (by 30%), locomotor activity (by 25%) and social interaction time (by 30%)--symptoms commonly induced by chronic stress and depression in humans. These changes were associated with small, but significant increases in systolic blood pressure (by 7%) and pulse pressure (by 11%) in FS+ rats relative to FS- rats. We have also found neurochemical alterations in sympathoadrenal pathways (that lasted for at least one week post-stress) including about 2-3 fold increases in the levels of tyrosine hydroxylase phosphorylation in the sympathetic ganglia and adrenal gland and a 1.8-fold increase in the expression of the Angiotensin II receptor type 1 protein in the adrenal gland of FS+ rats relative to FS- rats. We conclude that uncontrollable and unpredictable footshock stress can lead to elevation in systolic blood pressure when applied for an extended period of time (six weeks) in Wistar rats, and that these changes could be mediated by stress-induced modifications in sympathoadrenal pathways.

Tyrosine hydroxylase: regulation by feedback inhibition and phosphorylation

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of the catecholamines dopamine, noradrenaline, and adrenaline. In response to short-term stimuli, TH activity is regulated by feedback inhibition by the catecholamines and relief of that inhibition by phosphorylation. This chapter examines the current understanding of these regulatory mechanisms and the roles that they play in different catecholaminergic cells. This chapter also examines hierarchical phosphorylation in TH and how it provides a mechanism for the differential regulation of the major human TH isoforms.

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

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

Neonatal immune challenge alters reproductive development in the female rat

Neonatal lipopolysaccharide (LPS) exposure alters neuroendocrine, immune and behavioural responses in adult rats. Recent findings indicate that neonatal LPS treatment may have a more pronounced effect on the mating behaviours of females compared to males. The current study further explored the impact of neonatal inflammation on reproductive development in the female rat. Wistar rats were administered LPS (0.05 mg/kg, i.p.) or saline (equivolume) on postnatal days (PNDs) 3 and 5. The immediate effect of treatment was assessed on plasma corticosterone and tyrosine hydroxylase (TH) phosphorylation in the adrenal medulla. Weight gain and vaginal opening were recorded, and oestrous cyclicity was monitored post-puberty and in late adulthood. Blood and ovaries were collected throughout development to assess HPA and HPG hormones and to examine ovarian morphology. Reproductive success in the first (F1) generation and reproductive development in the second (F2) generation were also assessed. Neonatal LPS exposure resulted in increased TH phosphorylation in the neonatal adrenals. LPS treatment increased the corticosterone concentrations of females as juveniles, adolescents and adults, and reduced FSH in adolescence. Increased catch-up growth was evident in LPS-treated females, prompting earlier onset of puberty. Diminished follicular reserve was observed in neonatally LPS-treated females along with the advanced reproductive senescence. While fertility rates were not compromised, higher mortality and morbidity were observed in litters born to LPS-treated mothers. Female offspring of LPS-treated mothers displayed increased corticosterone on PND 14, increased catch-up growth and delayed emergence of the first oestrous cycle. No differences in any of the parameters assessed were observed in F2 males. These data suggest that neonatal immunological challenge has a profound impact on the female reproductive development, via the alteration of metabolic and neuroendocrine factors which regulate sexual maturation. Evidence of altered development in the female, but not male offspring of LPS-treated dams suggests increased susceptibility of females to the deleterious effects of neonatal immunological stress and its possible transferability to a subsequent generation.

Increased microglial activation in the rat brain following neonatal exposure to a bacterial mimetic

Neonatal lipopolysaccharide (LPS) exposure increases anxiety-like behaviour in adulthood. Our current aim was to examine whether neonatal LPS exposure is associated with changes in microglial activation, and whether these alterations correspond with alterations in behaviour. In adulthood, LPS-treated animals exhibited significantly increased anxiety-like behaviour and hippocampal microglial activation. The efficacy of the LPS challenge was confirmed by increased neonatal plasma corticosterone and tyrosine hydroxylase (TH) phosphorylation in the adrenal medulla. These findings suggest a neuroimmune pathway which may underpin the long-term behavioural and neuroendocrine changes following neonatal infection.

Epigenetic, transcriptional and posttranscriptional regulation of the tyrosine hydroxylase gene

The activity of tyrosine hydroxylase (TH, EC 1.14.16.2) gene and protein determines the catecholamine level, which, in turn, is crucial for the organism homeostasis. The TH gene expression is regulated by near all possible regulatory mechanisms on epigenetic, transcriptional and posttranscriptional levels. Ongoing molecular characteristic of the TH gene reveals some of the cis and trans elements necessary for its proper expression but most of them especially these responsible for tissue specific expression remain still obscure. This review will focus on some aspects of TH regulation including spatial chromatin organization of the TH locus and TH gene, regulatory elements mediating basal, induced and cell-specific activity, transcriptional elongation, alternative TH RNA processing, and the regulation of TH RNA stability in the cell.

Neuronal activity regulates expression of tyrosine hydroxylase in adult mouse substantia nigra pars compacta neurons

Striatal delivery of dopamine (DA) by midbrain substantia nigra pars compacta (SNc) neurons is vital for motor control and its depletion causes the motor symptoms of Parkinson's disease. While membrane potential changes or neuronal activity regulates tyrosine hydroxylase (TH, the rate limiting enzyme in catecholamine synthesis) expression in other catecholaminergic cells, it is not known whether the same occurs in adult SNc neurons. We administered drugs known to alter neuronal activity to mouse SNc DAergic neurons in various experimental preparations and measured changes in their TH expression. In cultured midbrain neurons, blockade of action potentials with 1 μM tetrodotoxin decreased TH expression beginning around 20 h later (as measured in real time by green fluorescent protein (GFP) expression driven off TH promoter activity). By contrast, partial blockade of small-conductance, Ca(2+) -activated potassium channels with 300 nM apamin increased TH mRNA and protein between 12 and 24 h later in slices of adult midbrain. Two-week infusions of 300 nM apamin directly to the adult mouse midbrain in vivo also increased TH expression in SNc neurons, measured immunohistochemically. Paradoxically, the number of TH immunoreactive (TH+) SNc neurons decreased in these animals. Similar in vivo infusions of drugs affecting other ion-channels and receptors (L-type voltage-activated Ca(2+) channels, GABA(A) receptors, high K(+) , DA receptors) also increased or decreased cellular TH immunoreactivity but decreased or increased, respectively, the number of TH+ cells in SNc. We conclude that in adult SNc neurons: (i) TH expression is activity-dependent and begins to change ∼20 h following sustained changes in neuronal activity; (ii) ion-channels and receptors mediating cell-autonomous activity or synaptic input are equally potent in altering TH expression; and (iii) activity-dependent changes in TH expression are balanced by opposing changes in the number of TH+ SNc cells.

Tyrosine hydroxylase and regulation of dopamine synthesis

Tyrosine hydroxylase is the rate-limiting enzyme of catecholamine biosynthesis; it uses tetrahydrobiopterin and molecular oxygen to convert tyrosine to DOPA. Its amino terminal 150 amino acids comprise a domain whose structure is involved in regulating the enzyme's activity. Modes of regulation include phosphorylation by multiple kinases at four different serine residues, and dephosphorylation by two phosphatases. The enzyme is inhibited in feedback fashion by the catecholamine neurotransmitters. Dopamine binds to TyrH competitively with tetrahydrobiopterin, and interacts with the R domain. TyrH activity is modulated by protein-protein interactions with enzymes in the same pathway or the tetrahydrobiopterin pathway, structural proteins considered to be chaperones that mediate the neuron's oxidative state, and the protein that transfers dopamine into secretory vesicles. TyrH is modified in the presence of NO, resulting in nitration of tyrosine residues and the glutathionylation of cysteine residues.

Human neuroblastoma cells transfected with tyrosine hydroxylase gain increased resistance to methylmercury-induced cell death

In a previous study we demonstrated that human neuroblastoma SH-SY5Y cells transfected with human tyrosine hydroxylase isoform 1 (SH+TH cells) were substantially more resistant to cell death induced by pro-oxidants than wild type SH-SY5Y cells (SH cells). In the present communication we used methylmercury as a model of cell stress in order to test whether SH+TH cells would behave in a similar manner in response to this stressor. Incubation with methylmercury (0.1-3 microM) for 24h caused a significant reduction in cell viability and increased apoptotic markers in both cell types. However, the effects were significantly reduced in the SH+TH cells when compared to the SH cells. Activation of p38(MAPK) was also reduced in the SH+TH compared to the SH cells after methylmercury exposure. Since p38(MAPK) is known to participate in signal transduction pathways during cell stress, our data suggest that SH+TH cells develop an increased resistance to environmental stress caused by neurotoxins such as methylmercury. In conclusion our results show that insertion of the human TH gene in cells that originally do not express this protein leads to alterations in cell homeostasis and triggers defense mechanisms against pro-oxidative insults.