Poor Decision Making and Sociability Impairment Following Central Serotonin Reduction in Inducible TPH2-Knockdown Rats.

Serotonin is an essential neuromodulator for mental health and animals' socio-cognitive abilities. However, we previously found that a constitutive depletion of central serotonin did not impair rat cognitive abilities in stand-alone tests. Here, we investigated how a mild and acute decrease in brain serotonin would affect rats' cognitive abilities. Using a novel rat model of inducible serotonin depletion via the genetic knockdown of tryptophan hydroxylase 2 (TPH2), we achieved a 20% decrease in serotonin levels in the hypothalamus after three weeks of non-invasive oral doxycycline administration. Decision making, cognitive flexibility, and social recognition memory were tested in low-serotonin (Tph2-kd) and control rats. Our results showed that the Tph2-kd rats were more prone to choose disadvantageously in the long term (poor decision making) in the Rat Gambling Task and that only the low-serotonin poor decision makers were more sensitive to probabilistic discounting and had poorer social recognition memory than other low-serotonin and control individuals. Flexibility was unaffected by the acute brain serotonin reduction. Poor social recognition memory was the most central characteristic of the behavioral network of low-serotonin poor decision makers, suggesting a key role of social recognition in the expression of their profile. The acute decrease in brain serotonin appeared to specifically amplify the cognitive impairments of the subgroup of individuals also identified as poor decision makers in the population. This study highlights the great opportunity the Tph2-kd rat model offers to study inter-individual susceptibilities to develop cognitive impairment following mild variations of brain serotonin in otherwise healthy individuals. These transgenic and differential approaches together could be critical for the identification of translational markers and vulnerabilities in the development of mental disorders.

Alterations in the brain serotonin system and serotonin-regulated behavior during aging in zebrafish males and females.

The brain serotonin (5-HT) system performs a neurotrophic function and supports the plasticity of the nervous system, while its age-related changes can increase the risk of senile neurodegeneration. Zebrafish brain is highly resistant to damage and neurodegeneration due to its high regeneration potential and it is a promising model object in searching for molecular factors preventing age-related neurodegeneration. In the present study alterations in 5-HT-related behavior in the home tank and the novel tank diving test, as well as 5-HT, 5-HIAA levels, tryptophan hydroxylase (TPH), monoamine oxidase (MAO) activity and the expression of genes encoding TPH, MAO, 5-HT transporter and 5-HT receptors in the brain of 6, 12, 24 and 36 month old zebrafish males and females are investigated. Marked sexual dimorphism in the locomotor activity in the novel tank test is revealed: females of all ages move slower than males. No sexual dimorphism in 5-HT-related traits is observed. No changes in 5-HT and 5-HIAA levels in zebrafish brain during aging is observed. At the same time, the aging is accompanied by a decrease in the locomotor activity, TPH activity, tph2 and htr1aa genes expression as well as an increase in the MAO activity and slc6a4a gene expression in their brain. These results indicate that the brain 5-HT system in zebrafish is resistant to age-related alterations.

A microbially produced AhR ligand promotes a Tph1-driven tolerogenic program in multiple sclerosis.

Multiple sclerosis is a debilitating autoimmune disease, characterized by chronic inflammation of the central nervous system. While the significance of the gut microbiome on multiple sclerosis pathogenesis is established, the underlining mechanisms are unknown. We found that serum levels of the microbial postbiotic tryptophan metabolite indole-3-carboxaldehyde (3-IAld) inversely correlated with disease duration in multiple sclerosis patients. Much like the host-derived tryptophan derivative L-Kynurenine, 3-IAld would bind and activate the Aryl hydrocarbon Receptor (AhR), which, in turn, controls endogenous tryptophan catabolic pathways. As a result, in peripheral lymph nodes, microbial 3-IAld, affected mast-cell tryptophan metabolism, forcing mast cells to produce serotonin via Tph1. We thus propose a protective role for AhR-mast-cell activation driven by the microbiome, whereby natural metabolites or postbiotics will have a physiological role in immune homeostasis and may act as therapeutic targets in autoimmune diseases.

Enhancing thermostability of tryptophan hydroxylase via protein engineering and its application in 5-hydroxytryptophan production.

5-Hydroxytryptophan (5-HTP), as the precursor of serotonin and melatonin in animals, can regulate mood, sleep, and behavior, which is widely used in pharmaceutical and health products industry. The enzymatic production of 5-hydroxytryptophan (5-HTP) from L-tryptophan (L-Trp) using tryptophan hydroxylase (TPH) show huge potential in application due to its advantages, such as mild reaction conditions, avoidance of protection/deprotection processes, excellent regioselectivity and considerable catalytic efficiency, compared with chemical synthesis and natural extraction. However, the low thermostability of TPH restricted its hydroxylation efficiency toward L-Trp. In this study, we aimed to improve the thermostability of TPH via semi-rational design guided by (folding free energy) ΔΔG fold calculation. After two rounds of evolution, two beneficial mutants M1 (S422V) and M30 (V275L/I412K) were obtained. Thermostability evaluation showed that M1 and M30 possessed 5.66-fold and 6.32-fold half-lives (t1/2) at 37 °C, and 4.2 °C and 6.0 °C higher melting temperature (Tm) than the WT, respectively. The mechanism behind thermostability improvement was elucidated with molecular dynamics simulation. Furthermore, biotransformation of 5-HTP from L-Trp was performed, M1 and M30 displayed 1.80-fold and 2.30-fold than that of WT, respectively. This work provides important insights into the thermostability enhancement of TPH and generate key mutants that could be robust candidates for practical production of 5-HTP.

Engineering of human tryptophan hydroxylase 2 for efficient synthesis of 5-hydroxytryptophan.

L-Tryptophan hydroxylation catalyzed by tryptophan hydroxylase (TPH) presents a promising method for synthesizing 5-hydroxytryptophan (5-HTP), yet the limited activity of wild-type human TPH2 restricts its application. A high-activity mutant, MT10 (H318E/H323E), was developed through semi-rational active site saturation testing (CAST) of wild-type TPH2, exhibiting a 2.85-fold increase in kcat/Km over the wild type, thus enhancing catalytic efficiency. Two biotransformation systems were developed, including an in vitro one-pot system and a Whole-Cell Catalysis System (WCCS). In the WCCS, MT10 achieved a conversion rate of only 31.5 % within 32 h. In the one-pot reaction, MT10 converted 50 mM L-tryptophan to 44.5 mM 5-HTP within 8 h, achieving an 89 % conversion rate, outperforming the M1 (NΔ143/CΔ26) variant. Molecular dynamics simulations indicated enhanced interactions of MT10 with the substrate, suggesting improved binding affinity and system stability. This study offers an effective approach for the efficient production of 5-HTP.

Neonatal treatment with para-chlorophenylalanine (pCPA) induces adolescent hyperactivity associated with changes in the paraventricular nucleus Crh and Trh expressions.

Disruption of the brain serotoninergic (5-HT) system during development induces long-lasting changes in molecular profile, cytoarchitecture, and function of neurons, impacting behavioral regulation throughout life. In male and female rats, we investigate the effect of neonatal tryptophan hydroxylase (TPH) inhibition by using para-chlorophenylalanine (pCPA) on the expression of 5-HTergic system components and neuropeptides related to adolescent social play behavior regulation. We observed sex-dependent 5-HT levels decrease after pCPA-treatment in the dorsal raphe nucleus (DRN) at 17 and 35 days. Neonatal pCPA-treatment increased playing, social and locomotory behaviors assessed in adolescent rats of both sexes. The pCPA-treated rats demonstrated decreased Crh (17 days) and increased Trh (35 days) expression in the hypothalamic paraventricular nucleus (PVN). There was sex dimorphism in Htr2c (17 days) and VGF (35 days) in the prefrontal cortex, with the females expressing higher levels of it than males. Our results indicate that neonatal pCPA-treatment results in a long-lasting and sex-dependent DRN 5-HT synthesis changes, decreased Crh, and increased Trh expression in the PVN, resulting in a hyperactivity-like phenotype during adolescence. The present work demonstrates that the impairment of TPH function leads to neurobehavioral disorders related to hyperactivity and impulsivity, such as attention deficit hyperactivity disorder (ADHD).

Confirmation of the presence of serotonergic cells in the fetal cerebral cortex in cultures and in situ.

BACKGROUND: Early appearance of serotonin in the fetal brain and its effects on brain morphogenesis support its neurotrophic role. OBJECTIVE: To determine the presence of serotonergic cells and the expression of tryptophan-5-hydroxylase (TPH), 5-hydroxytryptamine (5-HT), serotonin transporter (SERT), 5-HT1A receptor and Pet-1 during the development of the cerebral cortex, both in situ and in tissue cultures. MATERIAL AND METHODS: A descriptive, observational study was carried out in pregnant Wistar rats. The presence of the plug was regarded as the beginning of gestation. On days 13, 16 and 17, cesarean sections were performed to obtain the fetuses, and the brains were then immediately dissected to identify the presence of serotonergic cells, TPH, 5-HT, SERT, 5-HT1A and Pet-1 in tissue cultures and in situ by immunostaining detected on a confocal microscope. RESULTS: Serotonergic cells and terminals were observed in the midbrain on day 17 of gestation, and in neopallium cocultures on days 13 and 16. TPH, 5-HT, SERT and Pet-1 immunopositive cells were also observed in the neopallium on day 12 of culture. CONCLUSIONS: The presence of serotonergic cells and other elements of the serotonergic system in the early cerebral cortex was confirmed, which may be transient and participate in cortical maturation processes during brain development.

ANTECEDENTES: La aparición temprana de serotonina en el cerebro fetal y sus efectos en la morfogénesis cerebral apoyan su papel neurotrófico. OBJETIVO: Determinar la presencia de células serotoninérgicas y la expresión de triptófano-5-hidroxilasa (TPH), 5-hidroxitriptamina (5-HT), transportador de serotonina (SERT), receptor 5-HT1A y Pet-1 durante el desarrollo de la corteza cerebral, tanto in situ como en cultivo de tejidos. MATERIAL Y MÉTODOS: Se realizó estudio observacional descriptivo en ratas Wistar preñadas. La presencia del tapón se consideró el inicio de la gestación; en los días 13, 16 y 17 se practicaron cesáreas para obtener los fetos e inmediatamente se disecaron los cerebros para identificar células serotoninérgicas, TPH, 5-HT, SERT, 5-HT1A y Pet-1 en cultivo de tejido e in situ mediante inmunomarcaje detectado en un microscopio confocal. RESULTADOS: Células y terminales serotoninérgicas fueron observadas en el mesencéfalo el día 17 de gestación y en cocultivos de neopalio los días 13 y 16. También se observaron células inmunopositivas a TPH, 5-HT, SERT y Pet-1 en el neopalio en el día 12 del cultivo. CONCLUSIONES: Se confirmó la presencia de células serotoninérgicas y otros elementos del sistema serotoninérgico en la corteza cerebral temprana, la cual puede ser transitoria y participar en los procesos de maduración cortical durante el desarrollo cerebral.

Effect of Prolonged Exposure to Short Daylight and a Tryptophan Hydroxylase Inhibitor on the Behavior and Brain Serotonin System in Danio rerio.

We studied the effect of reduced tryptophan hydroxylase (TPH) activity and short daylight exposure on the behavior and the 5-HT system of the brain in D. rerio. Male and female D. rerio were exposed for 30 days to standard (12:12 h light:dark) and short (4:20 h light:dark) photoperiods in the presence or absence of TPH inhibitor (p-chlorophenylalanine, pCPA, 5 mg/liter). On day 31, the fish behavior in the "novel tank diving" test, their sex and body weight were determined, and the levels of pCPA, 5-HT, and its metabolite 5-HIAA were measured by HPLC; the levels of the key genes encoding metabolism enzymes (Tph1a, Tph1b, Tph2, and Mao) and receptors of 5-HT (Htr1aa, Htr2aa) were assessed by real-time PCR with reverse transcription. The short daylight exposure caused masculinization of females, reduced body weight, and motor activity in the "novel tank diving" test, but did not affect the 5-HT system of the brain. Long-term pCPA treatment had no effect on sex and body weight, significantly reduced the 5-HIAA level, but increased Tph1a and Tph2 gene expression in the brain. No effects of the interaction of short daylight and pCPA exposure on the sex, body weight, behavior, and 5-HT system of the brain were found. Thus, a moderate decrease in TPH activity cannot potentiate the negative effects of short daylight exposure on the sex, body weight, behavior, and 5-HT system of D. rerio.

In Vitro and In Vivo Chaperone Effect of (R)-2-amino-6-(1R, 2S)-1,2-dihydroxypropyl)-5,6,7,8-tetrahydropterin-4(3H)-one on the C1473G Mutant Tryptophan Hydroxylase 2.

Tryptophan hydroxylase 2 (TPH2) is the key and rate-limiting enzyme of serotonin (5-HT) synthesis in the mammalian brain. The 1473G mutation in the Tph2 gene decreases TPH2 activity in the mouse brain by twofold. (R)-2-amino-6-(1R, 2S)-1,2-dihydroxypropyl)-5,6,7,8-tetrahydropterin-4(3H)-one (BH4) is a pharmacological chaperone for aromatic amino acid hydroxylases. In the present study, chaperone effects of BH4 on the mutant C1473G TPH2 were investigated in vitro and in vivo. In vitro BH4 increased the thermal stability (T50 value) of mutant and wild-type TPH2 molecules. At the same time, neither chronic (twice per day for 7 days) intraperitoneal injection of 48.3 mg/kg of BH4 nor a single intraventricular administration of 60 µg of the drug altered the mutant TPH2 activity in the brain of Balb/c mice. This result indicates that although BH4 shows a chaperone effect in vitro, it is unable to increase the activity of mutant TPH2 in vivo.

Molecular signature of excessive female aggression: study of stressed mice with genetic inactivation of neuronal serotonin synthesis.

Aggression is a complex social behavior, critically involving brain serotonin (5-HT) function. The neurobiology of female aggression remains elusive, while the incidence of its manifestations has been increasing. Yet, animal models of female aggression are scarce. We previously proposed a paradigm of female aggression in the context of gene x environment interaction where mice with partial genetic inactivation of tryptophan hydroxylase-2 (Tph2+/- mice), a key enzyme of neuronal 5-HT synthesis, are subjected to predation stress resulting in pathological aggression. Using deep sequencing and the EBSeq method, we studied the transcriptomic signature of excessive aggression in the prefrontal cortex of female Tph2+/- mice subjected to rat exposure stress and food deprivation. Challenged mutants, but not other groups, displayed marked aggressive behaviors. We found 26 genes with altered expression in the opposite direction between stressed groups of both Tph2 genotypes. We identified several molecular markers, including Dgkh, Arfgef3, Kcnh7, Grin2a, Tenm1 and Epha6, implicated in neurodevelopmental deficits and psychiatric conditions featuring impaired cognition and emotional dysregulation. Moreover, while 17 regulons, including several relevant to neural plasticity and function, were significantly altered in stressed mutants, no alteration in regulons was detected in stressed wildtype mice. An interplay of the uncovered pathways likely mediates partial Tph2 inactivation in interaction with severe stress experience, thus resulting in excessive female aggression.

Advances in the microbial synthesis of the neurotransmitter serotonin.

Serotonin, as a monoamine neurotransmitter, modulates the activity of the nervous system. Due to its importance in the coordination of movement and regulation of mood, impairments in the synthesis and homeostasis of serotonin are involved in numerous disorders, including depression, Parkinson's disease, and anxiety. Currently, serotonin is primarily obtained via natural extraction. But this method is time-consuming and low yield, as well as unstable supply of raw materials. With the development of synthetic biology, researchers have established the method of microbial synthesis of serotonin. Compared with natural extraction, microbial synthesis has the advantages of short production cycle, continuous production, not limited by season and source, and environment-friendly; hence, it has garnered considerable research attention. However, the yield of serotonin is still too low to industrialization. Therefore, this review provides the latest progress and examples that illustrate the synthesis pathways of serotonin as well as proposes strategies for increasing the production of serotonin. KEY POINTS: • Two biosynthesis pathways of serotonin are introduced. • L-tryptophan hydroxylation is the rate-limiting step in serotonin biosynthesis. • Effective strategies are proposed to improve serotonin production.

Effects of the Combination of the C1473G Mutation in the Tph2 Gene and Lethal Yellow Mutations in the Raly-Agouti Locus on Behavior, Brain 5-HT and Melanocortin Systems in Mice.

Tryptophan hydroxylase 2 (TPH2) is the key and rate-limited enzyme of serotonin (5-HT) synthesis in the brain. The C1473G mutation in the Tph2 gene results in a two-fold decrease in enzyme activity in the mouse brain. The lethal yellow (AY) mutation in the Raly-Agouti locus results in the overexpression of the Agouti gene in the brain and causes obesity and depressive-like behavior in mice. Herein, the possible influences of these mutations and their combination on body mass, behavior, brain 5-HT and melanocortin systems in mice of the B6-1473CC/aa. B6-1473CC/AYa, B6-1473GG/aa are investigated. B6-1473GG/AYa genotypes were studied. The 1473G and AY alleles increase the activity of TPH2 and the expression of the Agouti gene, respectively, but they do not alter 5-HT and 5-HIAA levels or the expression of the genes Tph2, Maoa, Slc6a4, Htr1a, Htr2a, Mc3r and Mc4r in the brain. The 1473G allele attenuates weight gain and depressive-like immobility in the forced swim test, while the AY allele increases body weight gain and depressive-like immobility. The combination of these alleles results in hind limb dystonia in the B6-1473GG/AYa mice. This is the first evidence for the interaction between the C1473G and AY mutations.

The Role of C1473G Polymorphism in Mouse Triptophan Hydroxylase 2 Gene in the Acute Effects of Ethanol on the c-fos Gene Expression and Metabolism of Biogenic Amines in the Brain.

Tryptophan hydroxylase 2 is a key enzyme in the synthesis of the neurotransmitter serotonin, which plays an important role in the regulation of behavior and various physiological functions. We studied the effect of acute ethanol administration on the expression of the early response c-fos gene and metabolism of serotonin and catecholamines in the brain structures of B6-1473C and B6-1473G congenic mouse strains differing in the single-nucleotide substitution C1473G in the Tph2 gene and activity of the encoded enzyme. Acute alcoholization led to a significant upregulation of the c-fos gene expression in the frontal cortex and striatum of B6-1473G mice and in the hippocampus of B6-1473C mice and caused a decrease in the index of serotonin metabolism in the nucleus accumbens in B6-1473C mice and in the hippocampus and striatum of B6-1473G mice, as well as to the decrease in the norepinephrine level in the hypothalamus of B6-1473C mice. Therefore, the C1473G polymorphism in the Tph2 gene has a significant effect of acute ethanol administration on the c-fos expression pattern and metabolism of biogenic amines in the mouse brain.

Comparison of Fluorometric and Chromatographic Methods of In Vitro Assay of Tryptophan Hydroxylase 2, the Key Enzyme of Serotonin Synthesis in the Brain.

We present rapid and sensitive assay of tryptophan hydroxylase 2 enzyme activity based on the fluorescence of the complex of 5-hydroxytryptophan (5-HTP) with o-phthalic aldehyde. This method was compared with the standard method based on chromatographic isolation of 5-HTP followed by its quantification using an electrochemical detector. High sensitivity of the developed fluorometric method and similarity of the results obtained by fluorometric and chromatographic methods were demonstrated. The use of this rapid, cheap, and effective fluorometric method can simplify and facilitate measurements of tryptophan hydroxylase 2 activity and can make this assay available for a wide range of neurochemical and pharmacological laboratories.

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.

[Alterations in the Level of mRNA of TPH1, TPH2 Genes, Tryptophan Hydroxylase Activity and Serotonin Metabolism in Mouse Brain Five Days after Lipopolysaccharide Administration].

Tryptophan hydroxylases 1 and 2 (TPH1 and TPH2) play a key role in the synthesis of serotonin (5-HT), a hormone and neurotransmitter, in the peripheral organs and brain, respectively. The main aim of this study was to clarify the distribution of mRNA of the Tph1 and Tph2 genes in brain structures under normal conditions and after inflammation. The experiments were carried out on young (4 weeks old) male C57BL/6 mice. The animals were divided into three groups: intact, control, injected ip with saline, and injected ip with 2 mg/kg of bacterial lipopolysaccharide (LPS). Markers of inflammation, spleen mass and thymus mass were assayed 5 days after the saline or LPS administration. In the frontal cortex, hippocampus, striatum, hypothalamus, and midbrain the concentrations of 5-HT and its main metabolite, 5-hydroxyindole acetic acid (5-HIAA), and TPH activity were assayed using HPLC, while Tph1 and Tph2 mRNA were quantified using quantitative real-time RT-PCR. A dramatic increase of spleen mass and decrease of thymus mass 5 days after LPS administration was shown. A significant increase of 5-HT and 5-HIAA levels in the midbrain as well as decrease of 5-HIAA concentration and TPH activity in hypothalamus in mice treated with LPS and saline compared with intact animals was revealed. The highest concentration of Tph2 gene mRNA was observed in the midbrain in 5-HT neuron bodies, while this gene mRNA level was lower in 5-HT endings (cortex, hippocampus, striatum, and hypothalamus). Trace amounts of Tph1 mRNA was found in all studied brain structures in mice of the three groups. Thus, Tph1 gene expression in the mouse brain is too low to significantly affect 5-HT synthesis in normal conditions and during inflammation.

Rational design of tryptophan hydroxylation 1 for improving 5-Hydroxytryptophan production.

5-Hydroxytryptophan (5-HTP) is a chemical precursor of serotonin, which synthesizes melatonin and serotonin in animals and regulates mood, sleep, and behavior. Tryptophan hydroxylase (TPH) uses tetrahydrobiopterin (BH4) as a cofactor to hydroxylate L-tryptophan (L-Trp) to 5-HTP, and the low catalytic activity of TPH limits the rate of hydroxylation of L-Trp. In this study, the catalytic mechanism and structural features of L-Trp-TPH1-BH4 were investigated, and the catalytic activity was improved using a rational design strategy. Then the S337A/F318Y beneficial mutation was obtained. Molecular dynamics simulations showed that the S337A/F318Y mutant formed a salt bridge with TPH1 while forming an additional hydrogen bond with the substrate indole ring, stabilizing the indole ring and enhancing the binding affinity of the variant to L-Trp. As a result, the yield of 5-HTP was increased by 2.06-fold, resulting in the production of 0.91 g/L of 5-HTP. The rational design of the TPH structure to improve the hydroxylation efficiency of L-Trp offers the prospect of green production of 5-HTP.

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.

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.

Normal Pregnancy-Induced Islet Beta Cell Proliferation in Mouse Models That Are Deficient in Serotonin-Signaling.

During mouse pregnancy placental lactogens stimulate prolactin receptors on pancreatic islet beta cells to induce expression of the tryptophan hydroxylase Tph1, resulting in the synthesis and secretion of serotonin. Presently, the functional relevance of this phenomenon is unclear. One hypothesis is that serotonin-induced activation of 5-HT2B receptors on beta cells stimulates beta cell proliferation during pregnancy. We tested this hypothesis via three different mouse models: (i) total Tph1KO mice, (ii) 129P2/OlaHsd mice, which are incompetent to upregulate islet Tph1 during pregnancy, whereas Tph1 is normally expressed in the intestine, mammary glands, and placenta, and (iii) Htr2b-deficient mice. We observed normal pregnancy-induced levels of beta cell proliferation in total Tph1KO mice, 129P2/OlaHsd mice, and in Htr2b-/- mice. The three studied mouse models indicate that islet serotonin production and its signaling via 5-HT2B receptors are not required for the wave of beta cell proliferation that occurs during normal mouse pregnancy.