A unique central tryptophan hydroxylase isoform

Serotonin signalling in cancer: Emerging mechanisms and therapeutic opportunities

BACKGROUND

Serotonin (5-hydroxytryptamine) is a multifunctional bioamine serving as a neurotransmitter, peripheral hormone and mitogen in the vertebrate system. It has pleiotropic activities in central nervous system and gastrointestinal function via an orchestrated action of serotonergic elements, particularly serotonin receptor-mediated signalling cascades. The mitogenic properties of serotonin have garnered recognition for years and have been exploited for repurposing serotonergic-targeted drugs in cancer therapy. However, emerging conflicting findings necessitate a more comprehensive elucidation of serotonin's role in cancer pathogenesis.

MAIN BODY AND CONCLUSION

Here, we provide an overview of the biosynthesis, metabolism and action modes of serotonin. We summarise our current knowledge regarding the effects of the peripheral serotonergic system on tumourigenesis, with a specific emphasis on its immunomodulatory activities in human cancers. We also discuss the dual roles of serotonin in tumour pathogenesis and elucidate the potential of serotonergic drugs, some of which display favourable safety profiles and impressive efficacy in clinical trials, as a promising avenue in cancer treatment.

KEY POINTS

Primary synthesis and metabolic routes of peripheral 5-hydroxytryptamine in the gastrointestinal tract. Advanced research has established a strong association between the serotonergic components and carcinogenic mechanisms. The interplay between serotonergic signalling and the immune system within the tumour microenvironment orchestrates antitumour immune responses. Serotonergic-targeted drugs offer valuable clinical options for cancer therapy.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved are regarded as shy. Brain monoamines (i.e. serotonin, dopamine and noradrenaline) have been found to play a role in a variety of behaviors related to risk taking. Using zebrafish, we investigated whether there was a relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness during the initial exposure to the tank in female animals. The DOPAC:DA ratio correlated with boldness behavior on the third day in male fish. There was no relationship between boldness and noradrenaline. To probe differences in serotonergic function in bold and shy fish, we administered a selective serotonin reuptake inhibitor, escitalopram, and assessed exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in bold and shy female animals: the drug caused bold fish to spend more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings indicate that variation in serotonergic function has sex-specific contributions to individual differences in risk-taking behavior.

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune 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.

Activation of GPR55 Ameliorates Maternal Separation-Induced Learning and Memory Deficits by Augmenting 5-HT Synthesis in the Dorsal Raphe Nucleus of Juvenile Mice

Maternal separation (MS) represents a profound early life stressor with enduring impacts on neuronal development and adult cognitive function in both humans and rodents. MS is associated with persistent dysregulations in neurotransmitter systems, including the serotonin (5-HT) pathway, which is pivotal for mood stabilization and stress-coping mechanisms. Although the novel cannabinoid receptor, GPR55, is recognized for its influence on learning and memory, its implications on the function and synaptic dynamics of 5-HT neurons within the dorsal raphe nucleus (DRN) remain to be elucidated. In this study, we sought to discern the repercussions of GPR55 activation on 5-HT synthesis within the DRN of adult C57BL/6J mice that experienced MS. Concurrently, we analyzed potential alterations in excitatory synaptic transmission, long-term synaptic plasticity, and relevant learning and memory outcomes. Our behavioral assessments indicated a marked amelioration in MS-induced learning and memory deficits following GPR55 activation. In conjunction with this, we noted a substantial decrease in 5-HT levels in the MS model, while GPR55 activation stimulated tryptophan hydroxylase 2 synthesis and fostered the release of 5-HT. Electrophysiological patch-clamp analyses highlighted the ability of GPR55 activation to alleviate MS-induced cognitive deficits by modulating the frequency and magnitude of miniature excitatory postsynaptic currents within the DRN. Notably, this cognitive enhancement was underpinned by the phosphorylation of both NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In summary, our findings underscore the capacity of GPR55 to elevate 5-HT synthesis and modify synaptic transmissions within the DRN of juvenile mice, positing GPR55 as a promising therapeutic avenue for ameliorating MS-induced cognitive impairment.

Dysregulation of platelet serotonin, 14-3-3, and GPIX in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the leading cause of post-neonatal infant mortality, but the underlying cause(s) are unclear. A subset of SIDS infants has abnormalities in the neurotransmitter, serotonin (5-hydroxytryptamine [5-HT]) and the adaptor molecule, 14-3-3 pathways in regions of the brain involved in gasping, response to hypoxia, and arousal. To evaluate our hypothesis that SIDS is, at least in part, a multi-organ dysregulation of 5-HT, we examined whether blood platelets, which have 5-HT and 14-3-3 signaling pathways similar to brain neurons, are abnormal in SIDS. We also studied platelet surface glycoprotein IX (GPIX), a cell adhesion receptor which is physically linked to 14-3-3. In infants dying of SIDS compared to infants dying of known causes, we found significantly higher intra-platelet 5-HT and 14-3-3 and lower platelet surface GPIX. Serum and plasma 5-HT were also elevated in SIDS compared to controls. The presence in SIDS of both platelet and brainstem 5-HT and 14-3-3 abnormalities suggests a global dysregulation of these pathways and the potential for platelets to be used as a model system to study 5-HT and 14-3-3 interactions in SIDS. Platelet and serum biomarkers may aid in the forensic determination of SIDS and have the potential to be predictive of SIDS risk in living infants.

The Serotonin 4 Receptor Subtype: A Target of Particular Interest, Especially for Brain Disorders

In recent years, particular attention has been paid to the serotonin 4 receptor, which is well expressed in the brain, but also peripherally in various organs. The cerebral distribution of this receptor is well conserved across species, with high densities in the basal ganglia, where they are expressed by GABAergic neurons. The 5-HT4 receptor is also present in the cerebral cortex, hippocampus, and amygdala, where they are carried by glutamatergic or cholinergic neurons. Outside the central nervous system, the 5-HT4 receptor is notably expressed in the gastrointestinal tract. The wide distribution of the 5-HT4 receptor undoubtedly contributes to its involvement in a plethora of functions. In addition, the modulation of this receptor influences the release of serotonin, but also the release of other neurotransmitters such as acetylcholine and dopamine. This is a considerable asset, as the modulation of the 5-HT4 receptor can therefore play a direct or indirect beneficial role in various disorders. One of the main advantages of this receptor is that it mediates a much faster antidepressant and anxiolytic action than classical selective serotonin reuptake inhibitors. Another major benefit of the 5-HT4 receptor is that its activation enhances cognitive performance, probably via the release of acetylcholine. The expression of the 5-HT4 receptor is also altered in various eating disorders, and its activation by the 5-HT4 agonist negatively regulates food intake. Additionally, although the cerebral expression of this receptor is modified in certain movement-related disorders, it is still yet to be determined whether this receptor plays a key role in their pathophysiology. Finally, there is no longer any need to demonstrate the value of 5-HT4 receptor agonists in the pharmacological management of gastrointestinal disorders.

Serotonin regulation of mitochondria in kidney diseases

Serotonin, while conventionally recognized as a neurotransmitter in the CNS, has recently gained attention for its role in the kidney. Specifically, serotonin is not only synthesized in the kidney, but it also regulates glomerular function, vascular resistance, and mitochondrial homeostasis. Because of serotonin's importance to mitochondrial health, this review is focused on the role of serotonin and its receptors in mitochondrial function in the context of acute kidney injury, chronic kidney disease, and diabetic kidney disease, all of which are characterized by mitochondrial dysfunction and none of which has approved pharmacological treatments. Evidence indicates that activation of certain serotonin receptors can stimulate mitochondrial biogenesis (MB) and restore mitochondrial homeostasis, resulting in improved renal function. Serotonin receptor agonists that induce MB are therefore of interest as potential therapeutic strategies for renal injury and disease. SIGNIFICANCE STATEMENT: Mitochondrial dysfunction is associated with many human renal diseases such as acute kidney injury, chronic kidney disease, and diabetic kidney disease, which are associated with increased morbidity and mortality. Unfortunately, none of these pathologies has an FDA-approved pharmacological intervention, underscoring the urgency of identifying new therapeutics for such disorders. Studies show that induction of mitochondrial biogenesis via serotonin (5-hydroxytryptamine, 5-HT) receptors reduces kidney injury markers, restores mitochondrial and renal function after kidney injury, and decreases mortality, suggesting that targeting 5-HT receptors may be a promising therapeutic avenue for mitochondrial dysfunction in kidney diseases. While numerous reviews describe the importance of mitochondria and mitochondrial quality control mechanisms in kidney disease, the relevance of 5-HT receptor-mediated mitochondrial metabolic modulation in the kidney has yet to be thoroughly explored.

Serotonin Signaling through Lipid Membranes

Serotonin (5-HT) is a vital modulatory neurotransmitter responsible for regulating most behaviors in the brain. An inefficient 5-HT synaptic function is often linked to various mental disorders. Primarily, membrane proteins controlling the expression and activity of 5-HT synthesis, storage, release, receptor activation, and inactivation are critical to 5-HT signaling in synaptic and extra-synaptic sites. Moreover, these signals represent information transmission across membranes. Although the lipid membrane environment is often viewed as fairly stable, emerging research suggests significant functional lipid-protein interactions with many synaptic 5-HT proteins. These protein-lipid interactions extend to almost all the primary lipid classes that form the plasma membrane. Collectively, these lipid classes and lipid-protein interactions affect 5-HT synaptic efficacy at the synapse. The highly dynamic lipid composition of synaptic membranes suggests that these lipids and their interactions with proteins may contribute to the plasticity of the 5-HT synapse. Therefore, this broader protein-lipid model of the 5-HT synapse necessitates a reconsideration of 5-HT's role in various associated mental disorders.

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).

The impact of estradiol on serotonin, glutamate, and dopamine systems

Estradiol, the most potent and prevalent member of the estrogen class of steroid hormones and is expressed in both sexes. Functioning as a neuroactive steroid, it plays a crucial role in modulating neurotransmitter systems affecting neuronal circuits and brain functions including learning and memory, reward and sexual behaviors. These neurotransmitter systems encompass the serotonergic, dopaminergic, and glutamatergic signaling pathways. Consequently, this review examines the pivotal role of estradiol and its receptors in the regulation of these neurotransmitter systems in the brain. Through a comprehensive analysis of current literature, we investigate the multifaceted effects of estradiol on key neurotransmitter signaling systems, namely serotonin, dopamine, and glutamate. Findings from rodent models illuminate the impact of hormone manipulations, such as gonadectomy, on the regulation of neuronal brain circuits, providing valuable insights into the connection between hormonal fluctuations and neurotransmitter regulation. Estradiol exerts its effects by binding to three estrogen receptors: estrogen receptor alpha (ERα), estrogen receptor beta (ERβ), and G protein-coupled receptor (GPER). Thus, this review explores the promising outcomes observed with estradiol and estrogen receptor agonists administration in both gonadectomized and/or genetically knockout rodents, suggesting potential therapeutic avenues. Despite limited human studies on this topic, the findings underscore the significance of translational research in bridging the gap between preclinical findings and clinical applications. This approach offers valuable insights into the complex relationship between estradiol and neurotransmitter systems. The integration of evidence from neurotransmitter systems and receptor-specific effects not only enhances our understanding of the neurobiological basis of physiological brain functioning but also provides a comprehensive framework for the understanding of possible pathophysiological mechanisms resulting to disease states. By unraveling the complexities of estradiol's impact on neurotransmitter regulation, this review contributes to advancing the field and lays the groundwork for future research aimed at refining understanding of the relationship between estradiol and neuronal circuits as well as their involvement in brain disorders.

Individual differences in the boldness of female zebrafish are associated with alterations in serotonin function

One of the most prevalent axes of behavioral variation in both humans and animals is risk taking, where individuals that are more willing to take risk are characterized as bold while those that are more reserved as shy. Brain monoamines (i.e., serotonin, dopamine, and norepinephrine) have been found to play a role in a variety of behaviors related to risk taking. Genetic variation related to monoamine function have also been linked to personality in both humans and animals. Using zebrafish, we investigated the relationship between monoamine function and boldness behavior during exploration of a novel tank. We found a sex-specific correlation between serotonin metabolism (5-HIAA:5-HT ratio) and boldness that was limited to female animals; there were no relationships between boldness and dopamine or norepinephrine. To probe differences in serotonergic function, we administered a serotonin reuptake inhibitor, escitalopram, to bold and shy fish, and assessed their exploratory behavior. We found that escitalopram had opposing effects on thigmotaxis in female animals with bold fish spending more time near the center of the tank and shy fish spent more time near the periphery. Taken together, our findings suggest that variation in serotonergic function makes sex-specific contributions to individual differences in risk taking behavior.

Emerging Roles of Gut Serotonin in Regulation of Immune Response, Microbiota Composition and Intestinal Inflammation

Although the exact etiology of inflammatory bowel diseases (IBD) is unknown, studies have shown that dysregulated immune responses, genetic factors, gut microbiota, and environmental factors contribute to their pathogenesis. Intriguingly, serotonin (5-hydroxytryptamine or 5-HT) seems to be a molecule with increasingly strong implications in the pathogenesis of intestinal inflammation, affecting host physiology, including autophagy and immune responses, as well as microbial composition and function. 5-HT may also play a role in mediating how environmental effects impact outcomes in IBD. In this review, we aim to explore the production and important functions of 5-HT, including its impact on the gut. In addition, we highlight the bidirectional impacts of 5-HT on the immune system, the gut microbiota, and the process of autophagy and how these effects contribute to the manifestation of intestinal inflammation. We also explore recent findings connecting 5-HT signalling and the influence of environmental factors, particularly diet, in the pathogenesis of IBD. Ultimately, we explore the pleiotropic effects of this ancient molecule on biology and health in the context of intestinal inflammation.

The brain serotonin system in autism

Autism spectrum disorders (ASDs) are among the most common neurodevelopmental diseases. These disorders are characterized by lack of social interaction, by repetitive behavior, and often anxiety and learning disabilities. The brain serotonin (5-HT) system is known to be crucially implicated in a wide range of physiological functions and in the control of different kinds of normal and pathological behavior. A growing number of studies indicate the involvement of the brain 5-HT system in the mechanisms underlying both ASD development and ASD-related behavioral disorders. There are some review papers describing the role of separate key players of the 5-HT system in an ASD and/or autistic-like behavior. In this review, we summarize existing data on the participation of all members of the brain 5-HT system, namely, 5-HT transporter, tryptophan hydroxylase 2, MAOA, and 5-HT receptors, in autism in human and various animal models. Additionally, we describe the most recent studies involving modern techniques for in vivo regulation of gene expression that are aimed at identifying exact roles of 5-HT receptors, MAOA, and 5-HT transporter in the mechanisms underlying autistic-like behavior. Altogether, results of multiple research articles show that the brain 5-HT system intimately partakes in the control of some types of ASD-related behavior, and that specific changes in a function of a certain 5-HT receptor, transporter, and/or enzyme may normalize this aberrant behavior. These data give hope that some of clinically used 5-HT-related drugs have potential for ASD treatment.

Intestinal tryptophan metabolism in disease prevention and swine production

Tryptophan (Trp) is an essential amino acid that cannot be synthesized by animals. It has been characterized into two different isomers, levorotation-Trp (L-Trp) and dextrorotation-Trp (D-Trp), based on their distinct molecule orientation. Intestinal epithelial cells and gut microbiota are involved in metabolizing L-Trp in the gut via the activation of the kynurenine, serotonin, and indole pathways. However, knowledge regarding D-Trp metabolism in the gut remains unclear. In this review, we briefly update the current understanding of intestinal L/D-Trp metabolism and the function of their metabolites in modulating the gut physiology and diseases. Finally, we summarize the effects of Trp nutrition on swine production at different stages, including growth performance in weaned piglets and growing pigs, as well as the reproduction performance in sows.

A Review of Serotonin in the Developing Lung and Neonatal Pulmonary Hypertension

Serotonin (5-HT) is a bioamine that has been implicated in the pathogenesis of pulmonary hypertension (PH). The lung serves as an important site of 5-HT synthesis, uptake, and metabolism with signaling primarily regulated by tryptophan hydroxylase (TPH), the 5-HT transporter (SERT), and numerous unique 5-HT receptors. The 5-HT hypothesis of PH was first proposed in the 1960s and, since that time, preclinical and clinical studies have worked to elucidate the role of 5-HT in adult PH. Over the past several decades, accumulating evidence from both clinical and preclinical studies has suggested that the 5-HT signaling pathway may play an important role in neonatal cardiopulmonary transition and the development of PH in newborns. The expression of TPH, SERT, and the 5-HT receptors is developmentally regulated, with alterations resulting in pulmonary vasoconstriction and pulmonary vascular remodeling. However, much remains unknown about the role of 5-HT in the developing and newborn lung. The purpose of this review is to discuss the implications of 5-HT on fetal and neonatal pulmonary circulation and summarize the existing preclinical and clinical literature on 5-HT in neonatal PH.

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.

Possible Role of 5-Hydroxytryptamine (5-HT) Receptor on Human Sperm Motility Regulation

The purpose of this study is to examine whether 5-hydroxytryptamine (5-HT, also known as serotonin) regulates human sperm motility, focusing on 5-HT receptors. Immunofluorescent staining revealed the existence of seven types of 5-HT receptors with a heterogeneous pattern of reactive sites. In detail, 5-HT1B, 5-HT6, and 5-HT7 were detected in the post-acrosomal and mid-piece regions. The 5-HT2A and 5-HT5A receptors were mainly localized in the equatorial segment. 5-HT3A and 5-HT4 receptors were present in the neck and post-acrosomal regions. When examining the effects of 5-HT receptor antagonists on sperm motility, only the 5-HT2A receptor antagonist significantly reduced sperm motility. This suggests that the 5-HT2A receptor may have a regulatory function in sperm motility. Eventually, progressive motility should be attenuated to penetrate the oocyte for fertilization. The current study indicated heterogenous expression patterns and plausible functions of 5-HT receptors in human sperm.

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.

The A328 V/E (rs2887147) polymorphisms in human tryptophan hydroxylase 2 compromise enzyme activity

Human tryptophan hydroxylase 2 (hTPH2) is the rate-limiting enzyme for serotonin biosynthesis in the brain. A number of naturally-occurring single nucleotide polymorphisms (SNPs) have been reported for hTPH2. We investigated the activity and kinetic characteristics of the most common missense polymorphism rs2887147 (A328 V/E; 0.92% allelic frequency for the two different reported SNPs at the same site) using bacterially expressed hTPH2. The recombinant full-length enzyme A328E had no measurable enzyme activity, but A328V displayed decreased enzyme activity (Vmax). A328V also displayed substrate inhibition and decreased stability compared to the wild-type enzyme. By contrast, in constructs lacking the N-terminal 150 amino acid regulatory domain, the A328V substitution had no effect; that is, there was no substrate inhibition, enzyme stabilities (for wild-type and A328V) were dramatically increased, and Vmax values were not different (while the A328E variant remained inactive). These findings, in combination with molecular modeling, suggest that substitutions at A328 affect catalytic activity by altering the conformational freedom of the regulatory domain. The reduced activity and substrate inhibition resulting from these polymorphisms may ultimately reduce serotonin synthesis and contribute to behavioral perturbations, emotional stress, and eating disorders.

Unraveling the periprandial changes in brain serotonergic activity and its correlation with food intake-related neuropeptides in rainbow trout Oncorhynchus mykiss

This study explored changes in brain serotonin content and activity together with hypothalamic neuropeptide mRNA abundance around feeding time in rainbow trout, as well as the effect of one-day fasting. Groups of trout fed at two (ZT2) and six (ZT6) hours after lights on were sampled from 90 minutes before to 240 minutes after feeding, while additional groups of non-fed trout were also included in the study. Changes in brain amine and metabolite contents were measured in hindbrain, diencephalon and telencephalon, while in the diencephalon the mRNA abundance of tryptophan hydroxylase (tph1, tph2), serotonin receptors (5htr1a, 5htr1b and 5htr2c) and several neuropeptides (npy, agrp1, cartpt, pomca1, crfb) involved in the control of food intake were also assessed. The results showed changes in the hypothalamic neuropeptides that were consistent with the expected role for each in the regulation of food intake in rainbow trout. Serotonergic activity increased rapidly at the time of food intake in the diencephalon and hindbrain and remained high for much of the postprandial period. This increase in serotonin abundance was concomitant with elevated levels of pomca1 mRNA in the diencephalon, suggesting that serotonin might act on brain neuropeptides to promote a satiety profile. Furthermore, serotonin synthesis and neuronal activity appear to increase already before the time of feeding, suggesting additional functions for this amine before and during food intake. Exploration of serotonin receptors in the diencephalon revealed only small changes for gene expression of 5htr1b and 5htr2c receptors during the postprandial phase. Therefore, the results suggest that serotonin may play a relevant role in the regulation of feeding behavior in rainbow trout during periprandial time, but a better understanding of its interaction with brain centers involved in receiving and processing food-related signals is still needed.

Urinary 5-Hydroxyindolacetic Acid Measurements in Patients with Neuroendocrine Tumor-Related Carcinoid Syndrome: State of the Art

Carcinoid syndrome (CS), mostly associated with small intestinal neuroendocrine tumors (SI-NETs) or lung-related NETs, is characterized by symptoms related to hormonal secretion and long-term complications, including carcinoid heart disease (CHD), which is potentially life-threatening. In the early stages of the disease, symptoms are non-specific, which leads to delayed diagnoses. The availability of reliable tumor markers is crucial for a prompt diagnosis and proper management. This review summarizes available evidence on the role of 24 h urinary 5-hydroxyindolacetic acid (24u5HIAA), which is the urinary breakdown metabolite of serotonin, in the diagnosis/follow-up of NET-related CS, with a focus on its potential prognostic role, while eventually attempting to suggest a timeline for its measurement during the follow-up of NET patients. The use of 24u5HIAA is an established biomarker for the diagnosis of NETs with CS since it shows a sensibility and specificity of 100% and 85-90%, respectively. The downside of 24u5-HIAA is represented by the need for 24 h urine collection and the risk of confounding factors (foods and medication), which might lead to false positive/negative results. Moreover, 24u5HIAA is useful in the follow-up of NETs with CS since a shorter double time correlates to a higher risk of disease progression/disease-specific mortality. Furthermore, an elevation in 24u5-HIAA is correlated with a dismal prognosis because it is associated with an increased likelihood of CHD development and disease progression/mortality. Other potentially interesting biochemical markers have been proposed, including plasmatic 5HIAA, although further standardization and prospective studies are required to define their role in the management of NETs. Meanwhile, 24u5HIAA remains the most accurate CS biomarker.

Unraveling the serotonin saga: from discovery to weight regulation and beyond - a comprehensive scientific review

The prevalence of obesity is rapidly increasing worldwide, while the development of effective obesity therapies lags behind. Although new therapeutic targets to alleviate obesity are identified every day, and drug efficacy is improving, adverse side effects and increased health risks remain serious issues facing the weight-loss industry. Serotonin, also known as 5-HT, has been extensively studied in relation to appetite reduction and weight loss. As a result, dozens of upstream and downstream neural targets of 5-HT have been identified, revealing a multitude of neural circuits involved in mediating the anorexigenic effect of 5-HT. Despite the rise and fall of several 5-HT therapeutics in recent decades, the future of 5-HT as a therapeutic target for weight-loss therapy looks promising. This review focuses on the history of serotonin, the state of current central serotonin research, previous serotonergic therapies, and the future of serotonin for treating individuals with obesity.

Molecular biology of serotonergic systems in avian brains

Serotonin (5-hydroxytryptamine, 5-HT) is a phylogenetically conserved neurotransmitter and modulator. Neurons utilizing serotonin have been identified in the central nervous systems of all vertebrates. In the central serotonergic system of vertebrate species examined so far, serotonergic neurons have been confirmed to exist in clusters in the brainstem. Although many serotonin-regulated cognitive, behavioral, and emotional functions have been elucidated in mammals, equivalents remain poorly understood in non-mammalian vertebrates. The purpose of this review is to summarize current knowledge of the anatomical organization and molecular features of the avian central serotonergic system. In addition, selected key functions of serotonin are briefly reviewed. Gene association studies between serotonergic system related genes and behaviors in birds have elucidated that the serotonergic system is involved in the regulation of behavior in birds similar to that observed in mammals. The widespread distribution of serotonergic modulation in the central nervous system and the evolutionary conservation of the serotonergic system provide a strong foundation for understanding and comparing the evolutionary continuity of neural circuits controlling corresponding brain functions within vertebrates. The main focus of this review is the chicken brain, with this type of poultry used as a model bird. The chicken is widely used not only as a model for answering questions in developmental biology and as a model for agriculturally useful breeding, but also in research relating to cognitive, behavioral, and emotional processes. In addition to a wealth of prior research on the projection relationships of avian brain regions, detailed subdivision similarities between avian and mammalian brains have recently been identified. Therefore, identifying the neural circuits modulated by the serotonergic system in avian brains may provide an interesting opportunity for detailed comparative studies of the function of serotonergic systems in mammals.

Impact of genetic variants within serotonin turnover enzymes on human cerebral monoamine oxidase A in vivo

Variants within the monoamine oxidase A (MAO-A, MAOA) and tryptophan hydroxylase 2 (TPH2) genes, the main enzymes in cerebral serotonin (5-HT) turnover, affect risk for depression. Depressed cohorts show increased cerebral MAO-A in positron emission tomography (PET) studies. TPH2 polymorphisms might also influence brain MAO-A because availability of substrates (i.e. monoamine concentrations) were shown to affect MAO-A levels. We assessed the effect of MAOA (rs1137070, rs2064070, rs6323) and TPH2 (rs1386494, rs4570625) variants associated with risk for depression and related clinical phenomena on global MAO-A distribution volume (VT) using [11C]harmine PET in 51 participants (21 individuals with seasonal affective disorder (SAD) and 30 healthy individuals (HI)). Statistical analyses comprised general linear models with global MAO-A VT as dependent variable, genotype as independent variable and age, sex, group (individuals with SAD, HI) and season as covariates. rs1386494 genotype significantly affected global MAO-A VT after correction for age, group and sex (p < 0.05, corr.), with CC homozygotes showing 26% higher MAO-A levels. The role of rs1386494 on TPH2 function or expression is poorly understood. Our results suggest rs1386494 might have an effect on either, assuming that TPH2 and MAO-A levels are linked by their common product/substrate, 5-HT. Alternatively, rs1386494 might influence MAO-A levels via another mechanism, such as co-inheritance of other genetic variants. Our results provide insight into how genetic variants within serotonin turnover translate to the cerebral serotonin system. Clinicaltrials.gov Identifier: NCT02582398. EUDAMED Number: CIV-AT-13-01-009583.

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 (A^Y) 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/A^Ya, B6-1473GG/aa are investigated. B6-1473GG/A^Ya genotypes were studied. The 1473G and A^Y 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 A^Y allele increases body weight gain and depressive-like immobility. The combination of these alleles results in hind limb dystonia in the B6-1473GG/A^Ya mice. This is the first evidence for the interaction between the C1473G and A^Y mutations.

Immunohistochemical analysis of glutamatergic and serotonergic signaling pathways in chemosensory cell clusters in the pharynx and larynx of rats

The present study examined cellular components and the localization of vesicular glutamate transporter (VGLUT) 1 and 2 and serotonin (5-HT) in chemosensory cell clusters in the rat pharynx and larynx. Triple immunolabeling for guanine nucleotide-binding protein G (t), subunit ⍺3 (GNAT3) and nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) with synaptotagmin-1 (Syt1) revealed NTPDase2-immunoreactive type I-like cells in addition to GNAT3-immunoreactive type II-like and Syt1-immunoreactive type III-like cells in pharyngolaryngeal chemosensory cell clusters. Therefore, these clusters appear to comprise similar cell types to those in the lingual taste buds with slight morphological modifications. An immunofluorescence analysis of VGLUT1 or VGLUT2 and GNAT3 with P2X3 purinoceptors revealed that VGLUTs co-localized to P2X3-immunoreactive spherical nerve terminals closely associated with GNAT3-immunoreactive type II-like cells. Moreover, triple immunolabeling for Syt1/synaptosomal-associated protein, 25 kDa (SNAP25) and P2X3 with VGLUT1 or VGLUT2 revealed punctate immunoreactive products for VGLUT1 and VGLUT2 within P2X3-immunoreactive flat axon terminals wrapped around Syt1/SNAP25-immunoreactive type III-like cells. The afferent nerve fibers innervating cell clusters may contain glutamate and release it by exocytosis. On the other hand, immunoreactive products for 5-HT and dopa decarboxylase were detected in Syt1-immunoreactive cells, indicating the release of 5-HT by these cells. The present results suggest that chemosensory cell clusters in the pharynx and larynx may be modulated by intrinsic glutamate and 5-HT.

Tetrahydrobiopterin: Beyond Its Traditional Role as a Cofactor

Tetrahydrobiopterin (BH4) is an endogenous cofactor for some enzymatic conversions of essential biomolecules, including nitric oxide, and monoamine neurotransmitters, and for the metabolism of phenylalanine and lipid esters. Over the last decade, BH4 metabolism has emerged as a promising metabolic target for negatively modulating toxic pathways that may result in cell death. Strong preclinical evidence has shown that BH4 metabolism has multiple biological roles beyond its traditional cofactor activity. We have shown that BH4 supports essential pathways, e.g., to generate energy, to enhance the antioxidant resistance of cells against stressful conditions, and to protect from sustained inflammation, among others. Therefore, BH4 should not be understood solely as an enzyme cofactor, but should instead be depicted as a cytoprotective pathway that is finely regulated by the interaction of three different metabolic pathways, thus assuring specific intracellular concentrations. Here, we bring state-of-the-art information about the dependency of mitochondrial activity upon the availability of BH4, as well as the cytoprotective pathways that are enhanced after BH4 exposure. We also bring evidence about the potential use of BH4 as a new pharmacological option for diseases in which mitochondrial disfunction has been implicated, including chronic metabolic disorders, neurodegenerative diseases, and primary mitochondriopathies.

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.

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.

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.

Age-Related Alterations in the Level and Metabolism of Serotonin in the Brain of Males and Females of Annual Turquoise Killifish (Nothobranchius furzeri)

The annual turquoise killifish (Nothobranchius furzeri) is a laboratory model organism for neuroscience of aging. In the present study, we investigated for the first time the levels of serotonin and its main metabolite, 5-hydroxyindoleacetic acid, as well as the activities of the key enzymes of its synthesis, tryptophan hydroxylases, and degradation, monoamine oxidase, in the brains of 2-, 4- and 7-month-old male and female N. furzeri. The marked effect of age on the body mass and the level of serotonin, as well as the activities of tryptophan hydroxylases and monoamine oxidase in the brain of killifish were revealed. The level of serotonin decreased in the brain of 7-month-old males and females compared with 2-month-old ones. A significant decrease in the tryptophan hydroxylase activity and an increase in the monoamine oxidase activity in the brain of 7-month-old females compared to 2-month-old females was shown. These findings agree with the age-related alterations in expression of the genes encoding tryptophan hydroxylases and monoamine oxidase. N. furzeri is a suitable model with which to study the fundamental problems of age-related changes of the serotonin system in the brain.

Association of Functional Polymorphism in TPH2 Gene with Alcohol Dependence and Personality Traits: Study in Cloninger's Type I and Type II Alcohol-Dependent Inpatients

Alcohol dependence (AD) is a complex disorder with a poorly understood etiology. In this study, we investigated the relationship between genetic variation in the TPH2 gene, which encodes the enzyme responsible for serotonin synthesis in the brain, and both AD and personality traits, with attention to Cloninger's types of AD. The study included 373 healthy control subjects, 206 inpatients with type I AD, and 110 inpatients with type II AD. All subjects were genotyped for the functional polymorphism rs4290270 in the TPH2 gene, and AD patients completed the Tridimensional Personality Questionnaire (TPQ). The AA genotype and the A allele of the rs4290270 polymorphism were more frequent in both patient groups compared with the control group. In addition, a negative association was found between the number of A alleles and TPQ scores for harm avoidance in patients with type II, but not type I, AD. These results support the involvement of genetic variations of the serotonergic system in the pathogenesis of AD, especially type II AD. They also suggest that in a subset of patients, genetic variation of TPH2 could potentially influence the development of AD by affecting the personality trait of harm avoidance.

Development of the Placenta and Brain Are Affected by Selective Serotonin Reuptake Inhibitor Exposure During Critical Periods

Selective serotonin reuptake inhibitors (SSRIs) are usually prescribed to treat major depression and anxiety disorders. Fetal brain development exhibits dependency on serotonin (5-hydroxytryptamine, 5-HT) from maternal, placental, and fetal brain sources. At very early fetal stages, fetal serotonin is provided by maternal and placental sources. However, in later fetal stages, brain sources are indispensable for the appropriate development of neural circuitry and the rise of emergent functions implied in behavior acquisition. Thus, susceptible serotonin-related critical periods are recognized, involving the early maternal and placental 5-HT synthesis and the later endogenous 5-HT synthesis in the fetal brain. Acute and chronic exposure to SSRIs during these critical periods may result in short- and long-term placental and brain dysfunctions affecting intrauterine and postnatal life. Maternal and fetal cells express serotonin receptors which make them susceptible to changes in serotonin levels influenced by SSRIs. SSRIs block the serotonin transporter (SERT), which is required for 5-HT reuptake from the synaptic cleft into the presynaptic neuron. Chronic SSRI administration leads to pre- and postsynaptic 5-HT receptor rearrangement. In this review, we focus on the effects of SSRIs administered during critical periods upon placentation and brain development to be considered in evaluating the risk-safety balance in the clinical use of SSRIs.

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.

Pathogenic Mechanisms of Pulmonary Arterial Hypertension: Homeostasis Imbalance of Endothelium-Derived Relaxing and Contracting Factors

Pulmonary arterial hypertension (PAH) is a progressive and fatal disease. Sustained pulmonary vasoconstriction and concentric pulmonary vascular remodeling contribute to the elevated pulmonary vascular resistance and pulmonary artery pressure in PAH. Endothelial cells regulate vascular tension by producing endothelium-derived relaxing factors (EDRFs) and endothelium-derived contracting factors (EDCFs). Homeostasis of EDRF and EDCF production has been identified as a marker of the endothelium integrity. Impaired synthesis or release of EDRFs induces persistent vascular contraction and pulmonary artery remodeling, which subsequently leads to the development and progression of PAH. In this review, the authors summarize how EDRFs and EDCFs affect pulmonary vascular homeostasis, with special attention to the recently published novel mechanisms related to endothelial dysfunction in PAH and drugs associated with EDRFs and EDCFs.

Influence of photoperiod variations on the mRNA expression pattern of melatonin bio-synthesizing enzyme genes in the pineal organ and retina: A study in relation to the serum melatonin profile in the tropical carp Catla catla

Surface-dwelling C. catla were exposed to different photoperiods (8L:16D, 12L:12D, 12D:12L and 16L:8D) and the mRNA level profile of enzymes involved in melatonin synthesis was evaluated in the pineal gland and retina. Furthermore, a comparative analysis of the serum melatonin profile with the mRNA level was also performed. The results indicated diurnal variations in the transcripts of tph1, aanat and hiomt in the pineal organ and retina, and these variations change with the change in lighting regime. The serum melatonin profile showed rhythmicity in the natural photoperiod, but the serum melatonin level increased proportionally with increasing daylength. In short photoperiods, the peak value (though lower than in long photoperiods) of melatonin maintains a longer duration in serum. Moreover, the comparative analysis revealed a similar profile of mRNA of pineal aanat1 and aanat2 with serum melatonin under the same lighting conditions. This indicates that serum melatonin is produced by the pineal gland. Our results specify the importance of day length and the timing of onset or offset of the dark for maintaining the oscillating levels of serum melatonin and mRNA levels of melatonin biosynthesizing enzyme genes in the pineal organ and retina as well. The findings in this study highlight the distinctive pattern of mRNA levels in the pineal organ and retina under different photoperiods. The pineal melatonin biosynthesizing enzyme genes showed a similar pattern with serum melatonin levels while the retinal genes changed dramatically with photoperiod. We also revealed a light-dependent transcriptional regulation of pineal aanat genes in C. catla. Moreover, our results suggest that ALAN and skyglow can influence the levels of serum melatonin and its biosynthesis, resulting in desynchronization of the entire biological clock as well as the overall physiology of the animal.

Loss of autism-candidate CHD8 perturbs neural crest development and intestinal homeostatic balance

Individuals with mutations in CHD8 present with gastrointestinal complaints, yet the underlying mechanisms are understudied. Here, using a stable constitutive chd8 mutant zebrafish model, we found that the loss of chd8 leads to a reduced number of vagal neural crest cells (NCCs), enteric neural and glial progenitors, emigrating from the neural tube, and that their early migration capability was altered. At later stages, although the intestinal colonization by NCCs was complete, we found the decreased numbers of both serotonin-producing enterochromaffin cells and NCC-derived serotonergic neurons, suggesting an intestinal hyposerotonemia in the absence of chd8 Furthermore, transcriptomic analyses revealed an altered expression of key receptors and enzymes in serotonin and acetylcholine signaling pathways. The tissue examination of chd8 mutants revealed a thinner intestinal epithelium accompanied by an accumulation of neutrophils and the decreased numbers of goblet cells and eosinophils. Last, single-cell sequencing of whole intestines showed a global disruption of the immune balance with a perturbed expression of inflammatory interleukins and changes in immune cell clusters. Our findings propose a causal developmental link between chd8, NCC development, intestinal homeostasis, and autism-associated gastrointestinal complaints.

Blast Exposure Dysregulates Nighttime Melatonin Synthesis and Signaling in the Pineal Gland: A Potential Mechanism of Blast-Induced Sleep Disruptions

Blast-induced traumatic brain injury (bTBI) frequently results in sleep-wake disturbances. However, limited studies have investigated the molecular signaling mechanisms underlying these sleep disturbances, and potentially efficacious therapies are lacking. We investigated the levels of melatonin and genes involved in melatonin synthesis pathway in the pineal glands of Sprague Dawley rats exposed to single and tightly coupled repeated blasts during the night and daytime. Rats were exposed to single and tightly coupled repeated blasts using an advanced blast simulator. The plasma, cerebrospinal fluid (CSF), and pineal gland were collected at 6 h, 24 h, or 1 month postblast at two different time points: one during the day (1000 h) and one at night (2200 h). Differential expressions of genes involved in pineal melatonin synthesis were quantified using quantitative real-time polymerase chain reaction (qRT-PCR). Plasma and CSF melatonin levels were assessed using a commercial melatonin ELISA kit. The plasma and CSF melatonin levels showed statistically significant decreases at 6 h and 24 h in the blast-exposed rats euthanized in the night (in dim light), with no significant alterations noted in rats euthanized in the morning (daylight) at all three-time points. Blast-exposed rats showed statistically significant decreases in Tph1, Aanat, Asmt, and Mtnr1b mRNA levels, along with increased Tph2 mRNA, in the pineal gland samples collected at night at 6 h and 24 h. No significant changes in the mRNA levels of these genes were noted at 1 month. These findings imply that the melatonin circadian rhythm is disrupted following blast exposure, which may be a factor in the sleep disturbances that blast victims frequently experience.

Microbiome-Related Indole and Serotonin Metabolites are Linked to Inflammation and Psychiatric Symptoms in People Living with HIV

Background:

People living with HIV (PLHIV) exhibit dysregulation of tryptophan metabolism. Altered gut microbiome composition in PLHIV might be involved. Mechanistic consequences within the 3 major tryptophan metabolism pathways (serotonin, kynurenine, and indoles), and functional consequences for platelet, immune and behavioral functions are unknown. We investigated plasma tryptophan metabolites, gut microbiome composition, and their association with platelet function, inflammation, and psychiatric symptoms.

Methods:

This study included 211 PLHIV on long-term antiretroviral treatment (ART). Plasma tryptophan pathway metabolites were measured using time-of-flight mass spectrometry. Bacterial composition was profiled using metagenomic sequencing. Platelet reactivity and serotonin levels were quantified by flowcytometry and ELISA, respectively. Circulating inflammatory markers were determined using ELISA. Symptoms of depression and impulsivity were measured by DASS-42 and BIS-11 self-report questionnaires, respectively.

Results:

Plasma serotonin and indole metabolites were associated with gut bacterial composition. Notably, species enriched in PLHIV were associated with 3-methyldioxyindole. Platelet serotonin concentrations were elevated in PLHIV, without effects on platelet reactivity. Plasma serotonin and indole metabolites were positively associated with plasma IL-10 and TNF-α concentrations. Finally, higher tryptophan, serotonin, and indole metabolites were associated with lower depression and anxiety, whereas higher kynurenine metabolites were associated with increased impulsivity.

Conclusion:

Our results suggest that gut bacterial composition and dysbiosis in PLHIV on ART contribute to tryptophan metabolism, which may have clinical consequences for immune function and behavior.

The modified outer membrane protein Amuc_1100 of Akkermansia muciniphila improves chronic stress-induced anxiety and depression-like behavior in mice

Akkermansia muciniphila is a next-generation probiotic. The interaction between outer membrane protein Amuc_1100 of A. muciniphila and toll-like receptor 2 (TLR2) in intestinal epithelial cells influences the level of intestinal 5-hydroxytryptamine (5-HT). Amuc_1100^Δ80 is a truncated form of Amuc_1100 lacking the first 80 N-terminal amino acids and has a higher affinity for TLR2 than the wild-type protein. Here, we report that Amuc_1100^Δ80 could significantly reduce anxiety and depression-like behavior of mice when they were exposed to chronic unpredictable mild stress (CUMS). The experimental results of the rat insulinoma cell line RIN-14B showed that Amuc_1100^Δ80 also induced a significantly higher upregulation of tryptophan hydroxylase 1 (Tph1), a rate-limiting enzyme of intestinal 5-HT synthesis. The imbalance of the gut microflora could be diminished when CUMS mice were fed with Amuc_1100^Δ80. These results reveal that Amuc_1100^Δ80 could affect the 5-HT level and the downstream 5-HTR1A-CREB-BDNF signal pathway via interacting with TLR2 and by altering the gut microbial composition. In parallel, the downregulation exerted by Amuc_1100^Δ80 on the inflammation and hyperactivated HPA axis was closely related to the improvement of depression-like symptoms in CUMS mice. This study not only provides new insights into the antidepressant effect of A. muciniphila and its outer membrane protein Amuc_1100 but also identifies new potential targets and pathways in the gut for future research and the development of antidepressant drugs.

Vibrio cholerae high cell density quorum sensing activates the host intestinal innate immune response

Quorum sensing fundamentally alters the interaction of Vibrio cholerae with aquatic environments, environmental hosts, and the human intestine. At high cell density, the quorum-sensing regulator HapR represses not only expression of cholera toxin and the toxin co-regulated pilus, virulence factors essential in human infection, but also synthesis of the Vibrio polysaccharide (VPS) exopolysaccharide-based matrix required for abiotic and biotic surface attachment. Here, we describe a feature of V. cholerae quorum sensing that shifts the host-pathogen interaction toward commensalism. By repressing pathogen consumptive anabolic metabolism and, in particular, tryptophan uptake, V. cholerae HapR stimulates host intestinal serotonin production. This, in turn, activates host intestinal innate immune signaling to promote host survival.

Serotonin and Pulmonary Hypertension; Sex and Drugs and ROCK and Rho

Serotonin is often referred to as a "happy hormone" as it maintains good mood, well-being, and happiness. It is involved in communication between nerve cells and plays a role in sleeping and digestion. However, too much serotonin can have pathogenic effects and serotonin synthesis is elevated in pulmonary artery endothelial cells from patients with pulmonary arterial hypertension (PAH). PAH is characterized by elevated pulmonary pressures, right ventricular failure, inflammation, and pulmonary vascular remodeling; serotonin has been shown to be associated with these pathologies. The rate-limiting enzyme in the synthesis of serotonin in the periphery of the body is tryptophan hydroxylase 1 (TPH1). TPH1 expression and serotonin synthesis are elevated in pulmonary artery endothelial cells in patients with PAH. The serotonin synthesized in the pulmonary arterial endothelium can act on the adjacent pulmonary arterial smooth muscle cells (PASMCs), adventitial macrophages, and fibroblasts, in a paracrine fashion. In humans, serotonin enters PASMCs cells via the serotonin transporter (SERT) and it can cooperate with the 5-HT1B receptor on the plasma membrane; this activates both contractile and proliferative signaling pathways. The "serotonin hypothesis of pulmonary hypertension" arose when serotonin was associated with PAH induced by diet pills such as fenfluramine, aminorex, and chlorphentermine; these act as indirect serotonergic agonists causing the release of serotonin from platelets and cells through the SERT. Here the role of serotonin in PAH is reviewed. Targeting serotonin synthesis or signaling is a promising novel alternative approach which may lead to novel therapies for PAH. © 2022 American Physiological Society. Compr Physiol 12: 1-16, 2022.

Nutritional importance of tryptophan for improving treatment in depression and diabetes

The importance of nutrients in our diet is becoming increasingly recognized. From the viewpoint of protein synthesis and other physiologic and metabolic functions, all amino acids are important, but some of these amino acids are not synthesized endogenously. This subset, called essential amino acids, comprise dietarily indispensable nutrients. Tryptophan, an essential amino acid, is the sole precursor of neuronal as well as peripheral serotonin (5-hydroxytryptamine). Its systemic or oral administration increases serotonin synthesis because tryptophan hydroxylase, the rate-limiting enzyme of 5-hydroxytryptamine biosynthesis, is physiologically unsaturated with its substrate. Central serotonin is implicated in a number of psychiatric illnesses, including depression, and in responses to stress. Acting peripherally, serotonin affects vasoconstriction, intestinal motility, control of T cell–mediated immunity, and liver and pancreatic functions. Depression and diabetes are 2 highly prevalent diseases that often coexist. There is evidence that occurrence of depression is 2–3 times higher in people with diabetes mellitus. A comorbid condition of diabetes and depression worsens the treatment and increases risk for death. Stress, known for its causal role in depression, can also enhance risk for diabetes. Stress-induced decreases in the circulating levels of tryptophan can impair brain and pancreatic serotonin-dependent functions to precipitate these diseases. The importance of tryptophan supplementation for improving therapeutic intervention in depression and diabetes is the focus of this article. A deficiency of this essential amino acid may enhance risk for depression as well as diabetes, and can also weaken treatment efficacy of medicinal compounds for treating these diseases. Guidelines for optimal levels of circulating tryptophan can help if supplements of this amino acid can improve treatment efficacy.

Cryo-EM Structure and Activator Screening of Human Tryptophan Hydroxylase 2

Human tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in the synthesis of serotonin. Its dysfunction has been implicated in various psychiatric disorders such as depression, autism, and bipolar disorder. TPH2 is typically decreased in stability and catalytic activity in patients; thus, screening of molecules capable of binding and stabilizing the structure of TPH2 in activated conformation is desired for drug development in mental disorder treatment. Here, we solved the 3.0 Å cryo-EM structure of the TPH2 tetramer. Then, based on the structure, we conducted allosteric site prediction and small-molecule activator screening to the obtained cavity. ZINC000068568685 was successfully selected as the best candidate with highest binding affinity. To better understand the driving forces and binding stability of the complex, we performed molecular dynamics simulation, which indicates that ZINC000068568685 has great potential to stabilize the folding of the TPH2 tetramer to facilitate its activity. The research might shed light on the development of novel drugs targeting TPH2 for the treatment of psychological disorders.

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.

TLR4 in Tph2 neurons modulates anxiety-related behaviors in a sex-dependent manner

TLR4 belongs to the TLR receptor family and can induce a proinflammatory response to invading pathogens. Recent studies have identified that TLR4 is associated with major anxiety disorder. Tph2 is a rate-limiting enzyme for 5-HT biosynthesis that is expressed at high levels in the DRN, which includes the main 5-HT projection to the hippocampus and prefrontal cortex and regulates anxiety disorder. Here, we show that TLR4 expressed in Tph2 neurons in the DRN can modulate anxiety-like behaviors in a sex-dependent manner. Deletion of TLR4 in Tph2 neurons decreases anxiety-like behaviors in male but not in female mice. Meanwhile, a similar phenotype was found by selectively ablating TLR4 in the DRN of adult male but not female mice using AAV-Cre-GFP virus. Inhibition of TLR4 in DRN by infusion of LPS-RS via intra-Aq is sufficient to reverse anxiety-like behavior induced by chronic immobilization stress (CIS). The underlying mechanisms seem to involve alterations in the excitability of Tph2 neurons and key components of 5-HT transmission, including synthesis, reuptake, and transmission. Our results suggest that TLR4 in Tph2 neurons is a key modulator in anxiety-like behaviors and the 5-HT system in the brain between different sexes.

Current research in pathophysiology of opioid induced respiratory depression

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In Oprm1-deleted KO mice, both the preBötC and the KF/PBN are major contributors to OIRD but RD is not totally eliminated.

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PreBötC neurons expressing NK-1R are necessary for breathing. Their deletion results in ataxic breathing and/or apnoea.

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GIRK channels are involved in inhibiting respiration by mu opioid receptors. KCNQ channels modulate OIRD independent of mu signaling.

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Morphine depresses normal but not gasping breathing in rats indicating distinct mechanisms for these 2 forms of breathing.

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Methadone, given to alleviate symptoms of neonatal opioid withdrawal syndrome, desensitizes rats to respiratory depression.

Respiratory depression (RD) is the primary cause of death due to opioids. Opioids bind to mu (µ)-opioid receptors (MORs) encoded by the MOR gene Oprm1, widely expressed in the central and peripheral nervous systems including centers that modulate breathing. Respiratory centers are located throughout the brainstem. Experiments with Oprm1-deleted knockout (KO) mice undertaken to determine which sites are necessary for the induction of opioid-induced respiratory depression (OIRD) showed that the pre-Bötzinger complex (preBötC) and the pontine Kölliker-Fuse nucleus (KF) contribute equally to OIRD but RD was not totally eliminated. Morphine showed a differential influence on preBötC and KF neurons – low doses attenuated RD following deletion of MORs from preBötC neurons and an increase in apneas after high doses whereas deletion of MORs from KF neurons but not the preBötC attenuated RD at both high and low doses. In other KO mice studies, morphine administration after deletion of Oprm1 from both the preBötC and the KF/PBN neurons, led to the conclusion that both respiratory centres contribute to OIRD but the preBötC predominates. MOR-mediated post-synaptic activation of GIRK potassium channels has been implicated as a cause of OIRD. A complementary mechanism in the preBötC involving KCNQ potassium channels independent of MOR signaling has been described. Recent experiments in rats showing that morphine depresses normal, but not gasping breathing, cast doubt on the belief that eupnea, sighs, and gasps, are under the control of preBötC neurons. Methadone, administered to alleviate symptoms of neonatal opioid withdrawal syndrome (NOWES), desensitized rats to OIRD. Protection lost between postnatal days 1 and 2 coincides with the preBötC becoming the dominant generator of respiratory rhythm. Neonatal antidepressant exposure syndrome (NADES) and serotonin toxicity (ST) show similarities including RD. Enzyme CYP2D6 involved in opioid detoxification is polymorphic. Individuals of different CYP2D6 genotype may show increased, decreased, or no enzyme activity, contributing to the variability of patient responses to different opioids and OIRD.

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.

Photoperiodic Changes in Both Hypothalamus Neurotransmitters and Circulating Gonadal Steroids Metabolomic Profiles in Relation to Seasonal Reproduction in Male Quail

Both hypothalamic neurotransmitters and serum steroid hormones are impacted by photoperiod and have effects on physiology and seasonal reproductive. However, the relationship between circulating gonadal steroids and hypothalamic neurotransmitters underlying different photoperiod is still unclear. To further understand the crosstalk of neurotransmitters and steroids in seasonal reproduction, metabolic changes of 27 neurotransmitters concentrated in hypothalamus tissues and 42 steroids hormones in serum were assessed during two artificial photoperiodic programs. The results showed that photoperiod induce testicular atrophy and recrudescence. In L-to-S groups, significantly decreased levels of testosterone concentration were found in serum (P < 0.001) and increased 11-Dehydrocorticosterone (P < 0.05); Testosterone were almost undetectable at SD_14d. In addition, the hypothalamus exhibited significantly increased arginine and 4-aminobutyric acid (GABA) concentration and decreased serotonin and epinephrine content (P < 0.01 or P < 0.05). Accordingly, serum testosterone and androstenedione became detectable at LD_3d in the S-to-L group and were markedly increase at LD_7d. Furthermore, Serum androstenedione showed a significant increase with long light expose (P < 0.01). Additionally, the hypothalamus exhibited both significantly increased L.Tryptophan and phenylalanine concentration, as well as decreased L-glutamine and L-glutamine.acid content (P < 0.01 or P < 0.05). Serotonin metabolism showed significant differences between L-to-S group and S-to-L group. Furthermore, in the correlation analysis, serum testosterone had a positive correlation with 5-Hydroxyindole-3-acetic acid (5-HIAA), while Androstenedione was significantly negative with L.Tryptophan in L-to-S (P < 0.05). However, in S-to-L group, serum testosterone showed strong negative correlation with both serotonin and 5-HIAA (P < 0.05), but positive correlation with L.Tryptophan (P < 0.01), while Androstenedione was significantly negative correlation with both serotonin (P < 0.05) and L-Glutamine (P < 0.01). Photoperiod also had significant effects on the mRNA expression. We found significant differences in gene expression patterns of both serotonin signaling and steroid biosynthesis, while MAOB, NR5A1, and 3β-HSD showed an opposite tendency between two groups. Taken together, our results revealed that circulating gonadal steroids and hypothalamic neurotransmitters were significantly impact quail’s seasonal reproduction. Circulating gonadal steroids have different effects on neurotransmitter at different photoperiodism, which may coordinately influence the seasonal reproduction of quails.