A regulatory domain in the N terminus of tryptophan hydroxylase 2 controls enzyme expression

Structural characterization of human tryptophan hydroxylase 2 reveals that L-Phe is superior to L-Trp as the regulatory domain ligand

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in serotonin biosynthesis in the brain. Consequently, regulation of TPH2 is relevant for serotonin-related diseases, yet the regulatory mechanism of TPH2 is poorly understood and structural and dynamical insights are missing. We use NMR spectroscopy to determine the structure of a 47 N-terminally truncated variant of the regulatory domain (RD) dimer of human TPH2 in complex with L-Phe, and show that L-Phe is the superior RD ligand compared with the natural substrate, L-Trp. Using cryo-EM, we obtain a low-resolution structure of a similarly truncated variant of the complete tetrameric enzyme with dimerized RDs. The cryo-EM two-dimensional (2D) class averages additionally indicate that the RDs are dynamic in the tetramer and likely exist in a monomer-dimer equilibrium. Our results provide structural information on the RD as an isolated domain and in the TPH2 tetramer, which will facilitate future elucidation of TPH2's regulatory mechanism.

The aromatic amino acid hydroxylases: Structures, catalysis, and regulation of phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase

The aromatic amino acid hydroxylases phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase are non-heme iron enzymes that catalyze key physiological reactions. This review discusses the present understanding of the common catalytic mechanism of these enzymes and recent advances in understanding the relationship between their structures and their regulation.

Differential expression of serotonin2B receptors in GABAergic and serotoninergic neurons of the rat and mouse dorsal raphe nucleus

The central serotonin_2B receptor (5-HT_2BR) modulates 5-HT and dopamine (DA) neuronal function in the mammalian brain and has been suggested as a potential target for the treatment of neuropsychiatric disorders involving derangements of these monoamine systems, such as schizophrenia, cocaine abuse and dependence and major depressive disorder. Studies in rats and mice yielded contrasting results on the control of 5-HT/DA networks by 5-HT_2BRs, thereby leading to opposite views on the therapeutic potential of 5-HT_2BR agents for treating the above disorders. These discrepancies may result from anatomo-functional differences related to a different cellular location of 5-HT_2BRs in rat and mouse brain. Using immunohistochemistry, we assessed this hypothesis by examining the expression of 5-HT_2BRs in 5-HT and GABAergic neurons of rats and mice within different subregions of the dorsal raphe nucleus (DRN), currently considered as the main site of action of 5-HT_2B agents. Likewise, using in vivo microdialysis, we examined their functional relevance in the control of DRN 5-HT outflow, a surrogate index of 5-HT neuronal activity. In the DRN of both species, 5-HT_2BRs are expressed in 5-HT cells expressing tryptophan hydroxylase 2 (TPH₂), in GABAergic cells expressing glutamic acid decarboxylase 67 (GAD67), and in cells expressing both markers (GAD67 & TPH₂; i.e., GABA-expressing 5-HT neurons). The proportion of 5-HT_2BR-positive cells expressing only TPH₂ was significantly larger in mouse than in rat DRN, whereas the opposite holds true for the expression in cells expressing GAD67 & TPH₂. No major species differences were found in the dorsal and ventral subregions. In contrast, the lateral subregion exhibited large differences, with a predominant expression of 5-HT_2BRs in TPH₂-positive cells in mice (67.2 vs 19.9 % in rats), associated with a lower expression in GAD67 & TPH₂ cells (7.9 % in mice vs 41.5 % in rats). Intra-DRN (0.1 μM) administration of the preferential 5-HT_2BR agonist BW 723C86 decreased and increased DRN 5-HT outflow in rats and mice respectively, both effects being prevented by the intra-DRN perfusion of the selective 5-HT_2BR antagonist RS 127445 (0.1 μM). Altogether, these results show the existence of anatomical differences in the cellular expression of 5-HT_2BRs in the rat and mouse DRN, which translate into an opposite control of 5-HT outflow. Also, they highlight the relevance of the subset of GAD67-positive 5-HT neurons as a key factor responsible for the functional differences between rats and mice in terms of 5-HT neuronal activity modulation.

Advances in the Microbial Synthesis of 5-Hydroxytryptophan

5-Hydroxytryptophan (5-HTP) plays an important role in the regulation of emotion, behavior, sleep, pain, body temperature, and other physiological functions. It is used in the treatment of depression, insomnia, migraine, and other diseases. Due to a lack of effective biosynthesis methods, 5-HTP is mainly obtained by natural extraction, which has been unable to meet the needs of the market. Through the directed evolution of enzymes and the introduction of substrate supply pathways, 5-HTP biosynthesis and yield increase have been realized. This review provides examples that illustrate the production mode of 5-HTP and the latest progress in microbial synthesis.

Enhanced production of 5-hydroxytryptophan through the regulation of L-tryptophan biosynthetic pathway

5-Hydroxytryptophan (5-HTP) is the precursor of the neurotransmitter serotonin and has been used for the treatment of various diseases such as depression, insomnia, chronic headaches, and binge eating associated obesity. The production of 5-HTP had been achieved in our previous report, by the development of a recombinant strain containing two plasmids for biosynthesis of L-tryptophan (L-trp) and subsequent hydroxylation. In this study, the L-trp biosynthetic pathway was further integrated into the E. coli genome, and the promoter strength of 3-deoxy-7-phosphoheptulonate synthase, which catalyzes the first step of L-trp biosynthesis, was engineered to increase the production of L-trp. Hence, the 5-HTP production could be manipulated by the regulation of copy number of L-trp hydroxylation plasmid. Finally, the 5-HTP production was increased to 1.61 g/L in the shaking flasks, which was 24% improvement comparing to the original producing strain, while the content of residual L-trp was successfully reduced from 1.66 to 0.2 g/L, which is beneficial for the downstream separation and purification. Our work shall promote feasible progresses for the industrial production of 5-HTP.

Serotonergic Plasticity in the Dorsal Raphe Nucleus Characterizes Susceptibility and Resilience to Anhedonia

Chronic stress induces anhedonia in susceptible but not resilient individuals, a phenomenon observed in humans as well as animal models, but the molecular mechanisms underlying susceptibility and resilience are not well understood. We hypothesized that the serotonergic system, which is implicated in stress, reward, and antidepressant therapy, may play a role. We found that plasticity of the serotonergic system contributes to the differential vulnerability to stress displayed by susceptible and resilient animals. Stress-induced anhedonia was assessed in adult male rats using social defeat and intracranial self-stimulation, while changes in serotonergic phenotype were investigated using immunohistochemistry and in situ hybridization. Susceptible, but not resilient, rats displayed an increased number of neurons expressing the biosynthetic enzyme for serotonin, tryptophan-hydroxylase-2 (TPH2), in the ventral subnucleus of the dorsal raphe nucleus (DRv). Further, a decrease in the number of DRv glutamatergic (VGLUT3+) neurons was observed in all stressed rats. This neurotransmitter plasticity is activity-dependent, as was revealed by chemogenetic manipulation of the central amygdala, a stress-sensitive nucleus that forms a major input to the DR. Activation of amygdalar corticotropin-releasing hormone (CRH)+ neurons abolished the increase in DRv TPH2+ neurons and ameliorated stress-induced anhedonia in susceptible rats. These findings show that activation of amygdalar CRH+ neurons induces resilience, and suppresses the gain of serotonergic phenotype in the DRv that is characteristic of susceptible rats. This molecular signature of vulnerability to stress-induced anhedonia and the active nature of resilience could be targeted to develop new treatments for stress-related disorders like depression.SIGNIFICANCE STATEMENT Depression and other mental disorders can be induced by chronic or traumatic stressors. However, some individuals are resilient and do not develop depression in response to chronic stress. A complete picture of the molecular differences between susceptible and resilient individuals is necessary to understand how plasticity of limbic circuits is associated with the pathophysiology of stress-related disorders. Using a rodent model, our study identifies a novel molecular marker of susceptibility to stress-induced anhedonia, a core symptom of depression, and a means to modulate it. These findings will guide further investigation into cellular and circuit mechanisms of resilience, and the development of new treatments for depression.

Metabolic pathway engineering for high-level production of 5-hydroxytryptophan in Escherichia coli

Cellular metabolic networks should be carefully balanced using metabolic engineering to produce the desired products at the industrial scale. As the precursor for the biosynthesis of the neurotransmitter serotonin, 5-hydroxytryptophan (5-HTP) is effective in treating a variety of diseases, such as depression, fibromyalgia, obesity, and cerebellar ataxia. Due to the lack of an efficient synthetic method, commercial production of 5-HTP is only achieved by extracting from the seeds of Griffonia Smplicifolia. This study reports efficient microbial production of 5-HTP via metabolically engineered Escherichia coli. Firstly, human tryptophan hydroxylase I (TPH1) gene was functionally expressed. For endogenous supply of the cofactor tetrahydrobiopterin (BH4), human BH4 biosynthesis and regeneration pathway was reconstituted. Whole-cell bioconversion resulted in high-level production of 5-HTP (~1.2 g/L) from 2 g/L L-tryptophan in shake flasks. Further metabolic engineering efforts were employed to achieve 5-HTP biosynthesis from simple carbon sources. The whole biosynthetic pathway was divided into three functional modules, L-tryptophan module, the hydroxylation module, and the BH4 module. By reducing the copy number of L-tryptophan module, replacing TPH1 with a more stable mutant form, and promoter regulation of the BH4 module, 5-HTP was produced at a final titer of 1.3 g/L in the shake flask and 5.1 g/L in a fed-batch fermenter with glycerol as the carbon source, both of which were the highest ever reported for microbial production of 5-HTP.

Stabilization of tryptophan hydroxylase 2 by l-phenylalanine-induced dimerization

Tryptophan hydroxylase 2 (TPH2) catalyses the initial and rate‐limiting step in the biosynthesis of serotonin, which is associated with a variety of disorders such as depression, obsessive compulsive disorder, and schizophrenia. Full‐length TPH2 is poorly characterized due to low purification quantities caused by its inherent instability. Three truncated variants of human TPH2 (rchTPH2; regulatory and catalytic domain, NΔ47‐rchTPH2; truncation of 47 residues in the N terminus of rchTPH2, and chTPH2; catalytic domain) were expressed, purified, and examined for changes in transition temperature, inactivation rate, and oligomeric state. chTPH2 displayed 14‐ and 11‐fold higher half‐lives compared to rchTPH2 and NΔ47‐rchTPH2, respectively. Differential scanning calorimetry experiments demonstrated that this is caused by premature unfolding of the less stable regulatory domain. By differential scanning fluorimetry, the unfolding transitions of rchTPH2 and NΔ47‐rchTPH2 are found to shift from polyphasic to apparent two‐state by the addition of l‐Trp or l‐Phe. Analytical gel filtration revealed that rchTPH2 and NΔ47‐rchTPH2 reside in a monomer–dimer equilibrium which is significantly shifted toward dimer in the presence of l‐Phe. The dimerizing effect induced by l‐Phe is accompanied by a stabilizing effect, which resulted in a threefold increase in half‐lives of rchTPH2 and NΔ47‐rchTPH2. Addition of l‐Phe to the purification buffer significantly increases the purification yields, which will facilitate characterization of hTPH2.

Serotonergic systems in the balance: CRHR1 and CRHR2 differentially control stress-induced serotonin synthesis

Anxiety and affective disorders are often associated with hypercortisolism and dysfunctional serotonergic systems, including increased expression of TPH2, the gene encoding the rate-limiting enzyme of neuronal serotonin synthesis. We previously reported that chronic glucocorticoid exposure is anxiogenic and increases rat Tph2 mRNA expression, but it was still unclear if this also translates to increased TPH2 protein levels and in vivo activity of the enzyme. Here, we found that adult male rats treated with corticosterone (CORT, 100 μg/ml) via the drinking water for 21 days indeed show increased TPH2 protein expression in the dorsal and ventral part of the dorsal raphe nucleus (DRD, DRV) during the light phase, abolishing the enzyme's diurnal rhythm. In a second study, we systemically blocked the conversion of 5-hydroxytryptophan (5-HTP) to serotonin immediately before rats treated with CORT or vehicle were either exposed to 30 min acoustic startle stress or home cage control conditions. This allowed us to measure 5-HTP accumulation as a direct readout of basal versus stress-induced in vivo TPH2 activity. As expected, basal TPH2 activity was elevated in the DRD, DRV and MnR of CORT-treated rats. In response to stress, a multitude of serotonergic systems reacted with increased TPH2 activity, but the stress-, anxiety-, and learned helplessness-related dorsal and caudal DR (DRD/DRC) displayed stress-induced increases in TPH2 activity only after chronic CORT-treatment. To address the mechanisms underlying this region-specific CORT-dependent sensitization, we stereotaxically implanted CORT-treated rats with cannulae targeting the DR, and pharmacologically blocked either corticotropin-releasing hormone receptor type 1 (CRHR1) or type 2 (CRHR2) 10 min prior to acoustic startle stress. CRHR2 blockade prevented stress-induced increases of TPH2 activity within the DRD/DRC, while blockade of CRHR1 potentiated stress-induced TPH2 activity in the entire DR. Stress-induced TPH2 activity in the DRD/DRC furthermore predicted TPH2 activity in the amygdala and in the caudal pontine reticular nucleus (PnC), while serotonin synthesis in the PnC was strongly correlated with the maximum startle response. Our data demonstrate that chronically elevated glucocorticoids sensitize stress- and anxiety-related serotonergic systems, and for the first time reveal competing roles of CRHR1 and CRHR2 on stress-induced in vivo serotonin synthesis.

Structural insights into the regulation of aromatic amino acid hydroxylation

The aromatic amino acid hydroxylases phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase are homotetramers, with each subunit containing a homologous catalytic domain and a divergent regulatory domain. The solution structure of the regulatory domain of tyrosine hydroxylase establishes that it contains a core ACT domain similar to that in phenylalanine hydroxylase. The isolated regulatory domain of tyrosine hydroxylase forms a stable dimer, while that of phenylalanine hydroxylase undergoes a monomer-dimer equilibrium, with phenylalanine stabilizing the dimer. These solution properties are consistent with the regulatory mechanisms of the two enzymes, in that phenylalanine hydroxylase is activated by phenylalanine binding to an allosteric site, while tyrosine hydroxylase is regulated by binding of catecholamines in the active site.

The effects of Acorus tatarinowii Schott on 5-HT concentrations, TPH2 and 5-HT1B expression in the dorsal raphe of exercised rats

ETHNOPHARMACOLOGICAL RELEVANCE

Acorus tatarinowii Schott (Shi Chang Pu) belongs to the family of Acoraceae. The plant is used as an important herb for prolonging life many years in traditional Chinese medicine. It is an ancient herbal tonic nutriment and can be used as anti-fatigue medicine. However, the effects of Acorus tatarinowii Schott on the endurance exercise in relation to central nervous system have not yet been clarified. In this study, the effects of Acorus tatarinowii Schott on treadmill running endurance, 5-HT concentrations, TPH2, 5-HT1B expression in the dorsal raphe of exercised rats were investigated.

MATERIALS AND METHODS

Sixty adult male Sprague-Dawley rats were randomly divided into six groups: the normal group, the exercise group, the exercise and the rhizomes of Acorus tatarinowii Schott (ATS)(1mg/kg)-treated group, the exercise and ATS (10mg/kg)-treated group, the exercise and ATS (100mg/kg)-treated group, the exercise and caffeine (10mg/kg)-treated group. The effects of Acorus tatarinowii Schott on endurance exercise were determined by the time to exhaustion during treadmill exercise. The detection of 5-HT concentrations in the dorsal raphe was performed by HPLC analysis. The levels of TPH2, 5-HT1A and 5-HT1B expression were measured by western blot analysis and real-time PCR.

RESULTS

We found Acorus tatarinowii Schott could prolong the time to exhaustion in treadmill exercise and suppress the exercise-induced increase of 5-HT synthesis, TPH2 mRNA and protein expression and prevent the exercise-induced decrease of 5-HT1B mRNA and protein expression in the dorsal raphe. Acorus tatarinowii Schott was as effective as caffeine in prolonging the exhaustion time in treadmill running and in decreasing the exercise-induced increase of 5-HT synthesis and TPH2 mRNA and protein expression and in preventing the exercise-induced decrease of 5-HT1B mRNA and protein expression in the dorsal raphe.

CONCLUSIONS

The results indicated that the effects of Acorus tatarinowii Schott in inhibiting the exercise-induced synthesis of 5-HT and TPH2 expression and in preventing the exercise-induced decrease of 5-HT1B expression in the dorsal raphe might be the anti-fatigue mechanism of Acorus tatarinowii Schott.

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

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

Tryptophan: the key to boosting brain serotonin synthesis in depressive illness

It has been proposed that focusing on brain serotonin synthesis can advance antidepressant drug development. Biochemical aspects of the serotonin deficiency in major depressive disorder (MDD) are discussed here in detail. The deficiency is caused by a decreased availability of the serotonin precursor tryptophan (Trp) to the brain. This decrease is caused by accelerated Trp degradation, most likely induced by enhancement of the hepatic enzyme tryptophan 2,3-dioxygenase (TDO) by glucocorticoids and/or catecholamines. Induction of the extrahepatic Trp-degrading enzyme indolylamine 2,3-dioxygenase (IDO) by the modest immune activation in MDD has not been demonstrated and, if it occurs, is unlikely to make a significant contribution. Liver TDO appears to be a target of many antidepressants, the mood stabilisers Li(+) and carbamazepine and possibly other adjuncts to antidepressant therapy. The poor, variable and modest antidepressant efficacy of Trp is due to accelerated hepatic Trp degradation, and efficacy can be restored or enhanced by combination with antidepressants or other existing or new TDO inhibitors. Enhancing Trp availability to the brain is thus the key to normalisation of serotonin synthesis and could form the basis for future antidepressant drug development.

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.

An electroporation protocol for efficient DNA transfection in PC12 cells

A wide variety of mammalian cell types is used in gene transfection studies. Establishing transfection methods that enable highly efficient DNA uptake has become increasingly important. PC12 is an established rat pheochromocytoma cell line, which responds to exposure to NGF with cessation of growth, expression of cytoplasmic processes, and differentiation into cells resembling sympathetic neurons. Although PC12 cells represent an important model system to study a variety of neuronal functions, they proved relatively difficult to transfect. We have compared the efficiency of three different chemical transfection reagents (Lipofectamine 2000, Lipofectamine LTX and TransIT-LT1) and of two electroporation systems (Neon and Gene Pulser Xcell) in transiently transfecting undifferentiated PC12 cells. By comparing efficiencies from replicate experiments we proved electroporation (in particular Neon) to be the method of choice. By optimizing different parameters (voltage, pulse width and number of pulses) we reached high efficiency of transfection (90 %) and viability (99 %). We also demonstrated that, upon electroporation, cells are not altered by the transfection and maintain their ability to differentiate.

The role of the serotonergic system at the interface of aggression and suicide

Alterations in serotonin (5-HT) neurochemistry have been implicated in the aetiology of all major neuropsychiatric disorders, ranging from schizophrenia to mood and anxiety-spectrum disorders. This review will focus on the multifaceted implications of 5-HT-ergic dysfunctions in the pathophysiology of aggressive and suicidal behaviours. After a brief overview of the anatomical distribution of the 5-HT-ergic system in the key brain areas that govern aggression and suicidal behaviours, the implication of 5-HT markers (5-HT receptors, transporter as well as synthetic and metabolic enzymes) in these conditions is discussed. In this regard, particular emphasis is placed on the integration of pharmacological and genetic evidence from animal studies with the findings of human experimental and genetic association studies. Traditional views postulated an inverse relationship between 5-HT and aggression and suicidal behaviours; however, ample evidence has shown that this perspective may be overly simplistic, and that such pathological manifestations may reflect alterations in 5-HT homoeostasis due to the interaction of genetic, environmental and gender-related factors, particularly during early critical developmental stages. The development of animal models that may capture the complexity of such interactions promises to afford a powerful tool to elucidate the pathophysiology of impulsive aggression and suicidability, and identify new effective therapies for these conditions.

Intracellular stability of tyrosine hydroxylase: phosphorylation and proteasomal digestion of the enzyme

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of catecholamines, is a key protein involved in the pathogenesis of neurodegenerative diseases such as Parkinson's disease. Elucidation of the mechanisms regulating the synthesis, degradation, and activity of TH should be a first target in order to understand the role of this enzyme in pathogenesis. Recently, several reports suggest that the ubiquitin-proteasome pathway is a prerequisite for the degradation of TH and that the N-terminal part of TH plays a critical role in the degradation. In this report, we propose the mechanism by which the N-terminal part of TH regulates the degradation of this enzyme. Moreover, we integrate our findings with recent progress in other areas of TH regulation.

Boosting serotonin in the brain: is it time to revamp the treatment of depression?

Abnormalities in serotonin systems are presumably linked to various psychiatric disorders including schizophrenia and depression. Medications intended for these disorders aim to either block the reuptake or the degradation of this neurotransmitter. In an alternative approach, efforts have been made to enhance serotonin levels through dietary manipulation of precursor levels with modest clinical success. In the last 30 years, there has been little improvement in the pharmaceutical management of depression, and now is the time to revisit therapeutic strategies for the treatment of this disease. Tryptophan hydroxylase (TPH) catalyzes the first and rate-limiting step in the biosynthesis of serotonin. A recently discovered isoform, TPH2, is responsible for serotonin biosynthesis in the brain. Learning how to activate this enzyme (and its polymorphic versions) may lead to a new, more selective generation of antidepressants, able to regulate the levels of serotonin in the brain with fewer side effects.

A functional alternative splicing mutation in human tryptophan hydroxylase-2

The brain serotonergic system has an essential role in the physiological functions of the central nervous system and dysregulation of serotonin (5-HT) homeostasis has been implicated in many neuropsychiatric disorders. The tryptophan hydroxylase-2 (TPH2) gene is the rate-limiting enzyme in brain 5-HT synthesis, and thus is an ideal candidate gene for understanding the role of dysregulation of brain serotonergic homeostasis. Here, we characterized a common, but functional single-nucleotide polymorphism (SNP rs1386493) in the TPH2 gene, which decreases efficiency of normal RNA splicing, resulting in a truncated TPH2 protein (TPH2-TR) by alternative splicing. TPH2-TR, which lacks TPH2 enzyme activity, dominant-negatively affects full-length TPH2 function, causing reduced 5-HT production. The predicted mRNA for TPH2-TR is present in postmortem brain of rs1386493 carriers. The rs13864923 variant does not appear to be overrepresented in either global or multiplex depression cohorts. However, in combination with other gene variants linked to 5-HT homeostasis, this variant may exhibit important epistatic influences.

Brainstem deficiency of the 14-3-3 regulator of serotonin synthesis: a proteomics analysis in the sudden infant death syndrome

Impaired brainstem responses to homeostatic challenges during sleep may result in the sudden infant death syndrome (SIDS). Previously we reported a deficiency of serotonin (5-HT) and its key biosynthetic enzyme, tryptophan hydroxylase (TPH2), in SIDS infants in the medullary 5-HT system that modulates homeostatic responses during sleep. Yet, the underlying basis of the TPH2 and 5-HT deficiency is unknown. In this study, we tested the hypothesis that proteomics would uncover previously unrecognized abnormal levels of proteins related to TPH2 and 5-HT regulation in SIDS cases compared with controls, which could provide novel insight into the basis of their deficiency. We first performed a discovery proteomic analysis of the gigantocellularis of the medullary 5-HT system in the same data set with deficiencies of TPH2 and 5-HT levels. Analysis in 6 SIDS cases and 4 controls revealed a 42-75% reduction in abundance in 5 of the 6 isoforms identified of the 14-3-3 signal transduction family, which is known to influence TPH2 activity (p < 0.07). These findings were corroborated in an additional SIDS and control sample using an orthogonal MS(E)-based quantitative proteomic strategy. To confirm these proteomics results in a larger data set (38 SIDS, 11 controls), we applied Western blot analysis in the gigantocellularis and found that 4/7 14-3-3 isoforms identified were significantly reduced in SIDS cases (p ≤ 0.02), with a 43% reduction in all 14-3-3 isoforms combined (p < 0.001). Abnormalities in 5-HT and TPH2 levels and 5-HT(1A) receptor binding were associated with the 14-3-3 deficits in the same SIDS cases. These data suggest a potential molecular defect in SIDS related to TPH2 regulation, as 14-3-3 is critical in this process.

Phosphorylation of the N-terminal portion of tyrosine hydroxylase triggers proteasomal digestion of the enzyme

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis, and its N-terminus plays a critical role in the intracellular stability of the enzyme. In the present study, we investigated the mechanism by which the N-terminal region of TH affects this stability. TH molecules phosphorylated at their Ser31 and Ser40 were localized predominantly in the cytoplasm of PC12D cells. However, those molecules phosphorylated at Ser19 were found mainly in the nucleus, whereas they seemed to be negligible in the cytoplasm. The inhibition of proteasomes increased the quantity of TH molecules phosphorylated at their Ser19 and Ser40, although it did not increase that of TH molecules or that of TH phosphorylated at its Ser31. The inhibition of autophagy did not affect the amount of the TH molecule or that of its three phosphorylated forms. Deletion mutants of human TH type-1 lacking the N-terminal region containing the three phosphorylation sites possessed high stability of the enzyme in PC12D cells. These results suggest that the phosphorylation of the N-terminal portion of TH regulates the degradation of this enzyme by the ubiquitin-proteasome pathway.

High-fat diet leads to tissue-specific changes reflecting risk factors for diseases in DBA/2J mice

The aim of this study was to characterize the responses of individual tissues to high-fat feeding as a function of mass, fat composition, and transcript abundance. We examined a panel of eight tissues [5 white adipose tissues (WAT), brown adipose tissue (BAT), liver, muscle] obtained from DBA/2J mice on either a standard breeding diet (SBD) or a high-fat diet (HFD). HFD led to weight gain, decreased insulin sensitivity, and tissue-specific responses, including inflammation, in these mice. The dietary fatty acids were partially metabolized and converted in both liver and fat tissues. Saturated fatty acids (SFA) were converted in the liver to monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), and oleic acid (C18:1) was the preferred MUFA for storage of excess energy in all tissues of HFD-fed mice. Transcriptional changes largely reflected the tissue-specific fat deposition. SFA were negatively correlated with genes in the collagen family and processes involving the extracellular matrix. We propose a novel role of the tryptophan hydroxylase 2 (Tph2) gene in adipose tissues of diet-induced obesity. Tissue-specific responses to HFD were identified. Liver steatosis was evident in HFD-fed mice. Gonadal, retroperitoneal and subcutaneous adipose tissue and BAT exhibited severe inflammatory and immune responses. Mesenteric adipose tissue was the most metabolically active adipose tissue. Gluteal adipose tissue had the highest mass gain but was sluggish in its metabolism. In HFD conditions, BAT functioned largely like WAT in its role as a depot for excess energy, whereas WAT played a role in thermogenesis.

Three-way interaction between 14-3-3 proteins, the N-terminal region of tyrosine hydroxylase, and negatively charged membranes

Tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines, is activated by phosphorylation-dependent binding to 14-3-3 proteins. The N-terminal domain of TH is also involved in interaction with lipid membranes. We investigated the binding of the N-terminal domain to its different partners, both in the unphosphorylated (TH-(1-43)) and Ser(19)-phosphorylated (THp-(1-43)) states by surface plasmon resonance. THp-(1-43) showed high affinity for 14-3-3 proteins (K(d) approximately 0.5 microM for 14-3-3gamma and -zeta and 7 microM for 14-3-3eta). The domains also bind to negatively charged membranes with intermediate affinity (concentration at half-maximal binding S(0.5) = 25-58 microM (TH-(1-43)) and S(0.5) = 135-475 microM (THp-(1-43)), depending on phospholipid composition) and concomitant formation of helical structure. 14-3-3gamma showed a preferential binding to membranes, compared with 14-3-3zeta, both in chromaffin granules and with liposomes at neutral pH. The affinity of 14-3-3gamma for negatively charged membranes (S(0.5) = 1-9 microM) is much higher than the affinity of TH for the same membranes, compatible with the formation of a ternary complex between Ser(19)-phosphorylated TH, 14-3-3gamma, and membranes. Our results shed light on interaction mechanisms that might be relevant for the modulation of the distribution of TH in the cytoplasm and membrane fractions and regulation of L-DOPA and dopamine synthesis.

Posttranslational regulation of TPH1 is responsible for the nightly surge of 5-HT output in the rat pineal gland

Serotonin (5-hydroxytryptamine, 5-HT), a precursor for melatonin production, is produced abundantly in the pineal gland of all vertebrate animals. The synthesis of 5-HT in the pineal gland is rate limited by tryptophan hydroxylase 1 (TPH1) whose activity displays a twofold increase at night. Earlier studies from our laboratory demonstrate that pineal 5-HT secretion exhibits dynamic circadian rhythms with elevated levels during the early night, and that the increase is controlled by adrenergic signaling at night. In this study, we report that (a) 5-HT total output from the pineal gland and TPH1 protein levels both display diurnal rhythms with a twofold increase at night; (b) stimulation of cAMP signaling elevates 5-HT output in vivo; (c) 5-HT total output and TPH1 protein content in rat pineal gland are both acutely inhibited by light exposure at night. Consistent with these findings, molecular analysis of TPH1 protein revealed that (a) TPH1 is phosphorylated at the serine 58 in vitro and in the night pineal gland; and (b) phosphorylation of TPH1 at this residue is required for cAMP-enhanced TPH1 protein stability. These data support the model that increased nocturnal 5-HT synthesis in the pineal gland is mediated by the phosphorylation of TPH1 at the serine 58, which elevates the TPH1 protein content and activity at night.