Phenylalanine hydroxylase contributes to serotonin synthesis in mice

Elevated sortilin expression discriminates functional from non-functional neuroendocrine tumors and enables therapeutic targeting

A subset of neuroendocrine tumors (NETs) can cause an excessive secretion of hormones, neuropeptides, and biogenic amines into the bloodstream. These so-called functional NETs evoke a hormone-related disease and lead to several different syndromes, depending on the factors released. One of the most common functional syndromes, carcinoid syndrome, is characterized mainly by over-secretion of serotonin. However, what distinguishes functional from non-functional tumors on a molecular level remains unknown. Here, we demonstrate that the expression of sortilin, a widely expressed transmembrane receptor involved in intracellular protein sorting, is significantly increased in functional compared to non-functional NETs and thus can be used as a biomarker for functional NETs. Furthermore, using a cell line model of functional NETs, as well as organoids, we demonstrate that inhibition of sortilin reduces cellular serotonin concentrations and may therefore serve as a novel therapeutic target to treat patients with carcinoid syndrome.

Serotonin Transporter-dependent Histone Serotonylation in Placenta Contributes to the Neurodevelopmental Transcriptome

Brain development requires appropriate regulation of serotonin (5-HT) signaling from distinct tissue sources across embryogenesis. At the maternal-fetal interface, the placenta is thought to be an important contributor of offspring brain 5-HT and is critical to overall fetal health. Yet, how placental 5-HT is acquired, and the mechanisms through which 5-HT influences placental functions, are not well understood. Recently, our group identified a novel epigenetic role for 5-HT, in which 5-HT can be added to histone proteins to regulate transcription, a process called H3 serotonylation. Here, we show that H3 serotonylation undergoes dynamic regulation during placental development, corresponding to gene expression changes that are known to influence key metabolic processes. Using transgenic mice, we demonstrate that placental H3 serotonylation is dependent on 5-HT uptake by the serotonin transporter (SERT/SLC6A4). SERT deletion robustly reduces enrichment of H3 serotonylation across the placental genome, and disrupts neurodevelopmental gene networks in early embryonic brain tissues. Thus, these findings suggest a novel role for H3 serotonylation in coordinating placental transcription at the intersection of maternal physiology and offspring brain development.

Structure-Based Design of Xanthine-Imidazopyridines and -Imidazothiazoles as Highly Potent and In Vivo Efficacious Tryptophan Hydroxylase Inhibitors

Tryptophan hydroxylases catalyze the first and rate-limiting step in the biosynthesis of serotonin, a well-known neurotransmitter that plays an important role in multiple physiological functions. A reduction of serotonin levels, especially in the brain, can cause dysregulation leading to depression or insomnia. In contrast, overproduction of peripheral serotonin is associated with symptoms like carcinoid syndrome and pulmonary arterial hypertension. Recently, we developed a class of TPH inhibitors based on xanthine-benzimidazoles, characterized by a tripartite-binding mode spanning the binding sites of the cosubstrate pterin and the substrate tryptophan and by chelation of the catalytic iron ion. Herein, we describe the structure-based development of a second generation of xanthine-imidiazopyridines and -imidazothiazoles designed to inhibit TPH1 in the periphery while preventing the interaction with TPH2 in the brain. Lead compound 32 (TPT-004) shows superior pharmacokinetic and pharmacodynamic properties as well as efficacy in preclinical models of peripheral serotonin attenuation and colorectal tumor growth.

Bridging of host-microbiota tryptophan partitioning by the serotonin pathway in fungal pneumonia

The aromatic amino acid L-tryptophan (Trp) is essentially metabolized along the host and microbial pathways. While much is known about the role played by downstream metabolites of each pathways in intestinal homeostasis, their role in lung immune homeostasis is underappreciated. Here we have examined the role played by the Trp hydroxylase/5-hydroxytryptamine (5-HT) pathway in calibrating host and microbial Trp metabolism during Aspergillus fumigatus pneumonia. We found that 5-HT produced by mast cells essentially contributed to pathogen clearance and immune homeostasis in infection by promoting the host protective indoleamine-2,3-dioxygenase 1/kynurenine pathway and limiting the microbial activation of the indole/aryl hydrocarbon receptor pathway. This occurred via regulation of lung and intestinal microbiota and signaling pathways. 5-HT was deficient in the sputa of patients with Cystic fibrosis, while 5-HT supplementation restored the dysregulated Trp partitioning in murine disease. These findings suggest that 5-HT, by bridging host-microbiota Trp partitioning, may have clinical effects beyond its mood regulatory function in respiratory pathologies with an inflammatory component.

The monoaminergic system is a bilaterian innovation

Monoamines like serotonin, dopamine, and adrenaline/noradrenaline (epinephrine/norepinephrine) act as neuromodulators in the nervous system. They play a role in complex behaviours, cognitive functions such as learning and memory formation, as well as fundamental homeostatic processes such as sleep and feeding. However, the evolutionary origin of the genes required for monoaminergic modulation is uncertain. Using a phylogenomic approach, in this study, we show that most of the genes involved in monoamine production, modulation, and reception originated in the bilaterian stem group. This suggests that the monoaminergic system is a bilaterian novelty and that its evolution may have contributed to the Cambrian diversification.

Tryptophan Hydroxylase-2-Mediated Serotonin Biosynthesis Suppresses Cell Reprogramming into Pluripotent State

The monoamine neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) has important functions both in the neural system and during embryonic development in mammals. In this study, we set out to investigate whether and how endogenous serotonin affects reprogramming to pluripotency. As serotonin is synthesized from tryptophan by the rate limiting enzymes tryptophan hydroxylase-1 and -2 (TPH1 and TPH2), we have assessed the reprogramming of TPH1- and/or TPH2-deficient mouse embryonic fibroblasts (MEFs) to induced pluripotent stem cells (iPSCs). The reprogramming of the double mutant MEFs showed a dramatic increase in the efficiency of iPSC generation. In contrast, ectopic expression of TPH2 alone or in conjunction with TPH1 reverted the rate of reprogramming of the double mutant MEFs to the wild-type level and besides, TPH2 overexpression significantly suppressed reprogramming of wild-type MEFs. Our data thus suggest a negative role of serotonin biosynthesis in the reprogramming of somatic cells to a pluripotent state.

Parallel pathways for serotonin biosynthesis and metabolism in C. elegans

The neurotransmitter serotonin plays a central role in animal behavior and physiology, and many of its functions are regulated via evolutionarily conserved biosynthesis and degradation pathways. Here we show that in Caenorhabditis elegans, serotonin is abundantly produced in nonneuronal tissues via phenylalanine hydroxylase, in addition to canonical biosynthesis via tryptophan hydroxylase in neurons. Combining CRISPR-Cas9 genome editing, comparative metabolomics and synthesis, we demonstrate that most serotonin in C. elegans is incorporated into N-acetylserotonin-derived glucosides, which are retained in the worm body and further modified via the carboxylesterase CEST-4. Expression patterns of CEST-4 suggest that serotonin or serotonin derivatives are transported between different tissues. Last, we show that bacterial indole production interacts with serotonin metabolism via CEST-4. Our results reveal a parallel pathway for serotonin biosynthesis in nonneuronal cell types and further indicate that serotonin-derived metabolites may serve distinct signaling functions and contribute to previously described serotonin-dependent phenotypes.

Glutamine increases stability of TPH1 mRNA via p38 mitogen-activated kinase in mouse mastocytoma cells

Expression changes for tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in serotonin synthesis, by environmental glutamine (GLN) were examined in mouse mastocytoma-derived P815-HTR cells. GLN-treated cells exhibited a robust increase in TPH1 mRNA after a 6 h exposure to GLN. 6-Diazo-5-oxo-L-norleucine (DON), a glutamine-utilizing glutaminase inhibitor, significantly inhibited the GLN-induction of TPH1 mRNA. Nuclear run-on assays and mRNA decay experiments demonstrated that the primary mechanism leading to increased TPH1 mRNA levels was not due to transcriptional changes, but rather due to increased TPH1 RNA stability induced by GLN. Treatment with GLN also led to activation of p38 MAP kinase, but not p42/44 MAPK. In addition, SB203580, a p38 MAP kinase specific inhibitor, completely abolished the GLN-mediated increase of TPH1 mRNA levels, suggesting the pathway stabilizing TPH1 mRNA might be mediated by the activated p38 MAP kinase pathway. Additionally, SB203580 significantly reduced the stability of TPH1 mRNA, and this reduction of the stability was not affected by GLN in the culture medium, implying a sequential signaling from GLN being mediated by p38 MAP kinase, resulting in alteration of TPH1 mRNA stability. TPH1 mRNA stability loss was also dependent on de novo protein synthesis as shown by treatment of cells with a transcriptional/translational blocker. We provide evidence that TPH1 mRNA levels are increased in response to increased exogenous GLN in mouse mastocytoma cells via a stabilization of TPH1 mRNA due to the activity of the p38 MAP kinase.

Structure-Based Design of Xanthine-Benzimidazole Derivatives as Novel and Potent Tryptophan Hydroxylase Inhibitors

Tryptophan hydroxylases catalyze the first and rate-limiting step in the synthesis of serotonin. Serotonin is a key neurotransmitter in the central nervous system and, in the periphery, functions as a local hormone with multiple physiological functions. Studies in genetically altered mouse models have shown that dysregulation of peripheral serotonin levels leads to metabolic, inflammatory, and fibrotic diseases. Overproduction of serotonin by tumor cells causes severe symptoms typical for the carcinoid syndrome, and tryptophan hydroxylase inhibitors are already in clinical use for patients suffering from this disease. Here, we describe a novel class of potent tryptophan hydroxylase inhibitors, characterized by spanning all active binding sites important for catalysis, specifically those of the cosubstrate pterin, the substrate tryptophan as well as directly chelating the catalytic iron ion. The inhibitors were designed to efficiently reduce serotonin in the periphery while not passing the blood-brain barrier, thus preserving serotonin levels in the brain.

Peripheral Serotonin Deficiency Affects Anxiety-like Behavior and the Molecular Response to an Acute Challenge in Rats

Serotonin is synthetized through the action of tryptophan hydroxylase (TPH) enzymes. While the TPH2 isoform is responsible for the production of serotonin in the brain, TPH1 is expressed in peripheral organs. Interestingly, despite its peripheral localization, alterations of the gene coding for TPH1 have been related to stress sensitivity and an increased susceptibility for psychiatric pathologies. On these bases, we took advantage of newly generated TPH1^-/- rats, and we evaluated the impact of the lack of peripheral serotonin on the behavior and expression of brain plasticity-related genes under basal conditions and in response to stress. At a behavioral level, TPH1^-/- rats displayed reduced anxiety-like behavior. Moreover, we found that neuronal activation, quantified by the expression of Bdnf and the immediate early gene Arc and transcription of glucocorticoid responsive genes after 1 h of acute restraint stress, was blunted in TPH1^-/- rats in comparison to TPH1^+/+ animals. Overall, we provided evidence for the influence of peripheral serotonin levels in modulating brain functions under basal and dynamic situations.

Cytochrome P450 2D (CYP2D) enzyme dysfunction associated with aging and serotonin deficiency in the brain and liver of female Dark Agouti rats

Among the enzymes that support brain metabolism, cytochrome P450 (CYP) enzymes occupy an important place. These enzymes catalyze the biotransformation pathways of neuroactive endogenous substrates (neurosteroids, neurotransmitters) and are necessary for the detoxification processes. The aim of the present study was to assess changes in the CYP2D activity and protein level during the aging process and as a result of serotonin deficiency in the female brain. The CYP2D activity was measured in brain and liver microsomes of Dark Agouti wild type (WT) female rats (mature 15-week-old and senescent 18-month-old rats) and in tryptophan hydroxylase 2 (TPH2)-deficient senescent female rats. The CYP2D activity in mature WT Dark Agouti females was independent of the changing phases of the estrous cycle. In senescent WT females rats, the CYP2D activity and protein level were decreased in the cerebral cortex, hippocampus, cerebellum and liver, but increased in the brain stem. In the other examined structures (frontal cortex, hypothalamus, thalamus, striatum), the enzyme activity did not change. In aging TPH2-deficient females, the CYP2D activity and protein levels were decreased in the frontal cortex, hypothalamus and brain stem (activity only), remaining unchanged in other brain structures and liver, relative to senescent WT females. In summary, the aging process and TPH2 deficit affect the CYP2D activity and protein level in female rats, which may have a negative impact on the compensatory capacity of CYP2D in the synthesis of serotonin and dopamine in cerebral structures involved in cognitive and emotional functions. In the liver, the CYP2D-catalyzed drug metabolism may be diminished in elderly females. The results in female rats are compared with those obtained previously in males. It is concluded that aging and serotonin deficiency exert sex-dependent effects on brain CYP2D, which seem to be less favorable in females concerning CYP2D-mediated neurotransmitter synthesis, but beneficial regarding slower neurosteroid metabolism.

The roles of serotonin in cell adhesion and migration, and cytoskeletal remodeling

Serotonin is well known as a neurotransmitter. Its roles in neuronal processes such as learning, memory or cognition are well established, and also in disorders such as depression, schizophrenia, bipolar disorder, and dementia. However, its effects on adhesion and cytoskeletal remodelling which are strongly affected by 5-HT receptors, are not as well studied with some exceptions for e.g. platelet aggregation. Neuronal function is strongly dependent on cell-cell contacts and adhesion-related processes. Therefore the role played by serotonin in psychiatric illness, as well as in the positive and negative effects of neuropsychiatric drugs through cell-related adhesion can be of great significance. In this review, we explore the role of serotonin in some of these aspects based on recent findings.