Tryptophan hydroxylase 1 knockout and tryptophan hydroxylase 2 polymorphism: effects on hypoxic pulmonary hypertension in mice

Novel Tryptophan Hydroxylase Inhibitor TPT-001 Reverses PAH, Vascular Remodeling, and Proliferative-Proinflammatory Gene Expression

The serotonin pathway has long been proposed as a promising target for pulmonary arterial hypertension (PAH)-a progressive and uncurable disease. We developed a highly specific inhibitor of the serotonin synthesizing enzyme tryptophan hydroxylase 1 (TPH1), TPT-001 (TPHi). In this study, the authors sought to treat severe PAH in the Sugen/hypoxia (SuHx) rat model with the oral TPHi TPT-001. Male Sprague Dawley rats were divided into 3 groups: 1) ConNx, control animals; 2) SuHx, injected subcutaneously with SU5416 and exposed to chronic hypoxia for 3 weeks, followed by 6 weeks in room air; and 3) SuHx+TPHi, SuHx animals treated orally with TPHi for 5 weeks. Closed-chest right- and left heart catheterization and echocardiography were performed. Lungs were subject to histologic and mRNA sequencing analyses. Compared with SuHx-exposed rats, which developed severe PAH and right ventricular (RV) dysfunction, TPHi-treated SuHx rats had greatly lowered RV systolic (mean ± SEM: 41 ± 2.3 mm Hg vs 86 ± 6.5 mm Hg; P < 0.001) and end-diastolic (mean ± SEM: 4 ± 0.7 mm Hg vs 14 ± 1.7 mm Hg; P < 0.001) pressures, decreased RV hypertrophy and dilation (all not significantly different from control rats), and reversed pulmonary vascular remodeling. We identified perivascular infiltration of CD3+ T cells and proinflammatory F4/80+ and CD68+ macrophages and proliferating cell nuclear antigen-positive alveolar epithelial cells all suppressed by TPHi treatment. Whole-lung mRNA sequencing in SuHx rats showed distinct gene expression patterns related to pulmonary arterial smooth muscle cell proliferation (Rpph1, Lgals3, Gata4), reactive oxygen species, inflammation (Tnfsrf17, iNOS), and vasodilation (Pde1b, Kng1), which reversed expression with TPHi treatment. Inhibition of TPH1 with a new class of drugs (here, TPT-001) has the potential to attenuate or even reverse severe PAH and associated RV dysfunction in vivo by blocking the serotonin pathway.

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.

Women's mood at high altitude. sexual dimorphism in hypoxic stress modulation by the tryptophan-melatonin axis

Sexual (and gender)-dimorphism in tolerance to hypobaric hypoxia increasingly matters for a differential surveillance of human activities at high altitude (HA). At low altitudes, the prevalence of anxiety and depression in women has already been found to double when compared with men; it could be expected to even increase on exposure to HA. In purposefully caring for the health of women at HA, the present work explores the potential involvement of the tryptophan (Trp)-melatonin axis in mood changes on exposure to hypobaric hypoxia. The present work highlights some already known anxiogenic effects of HA exposure. Hypoxia and insomnia reduce serotonin (5-HT) availability; the latter defect being expressed as failure of brown adipose tissue (BAT) activation and mood disorders. Rapid eye movement (REM) sleep organization and synapsis restoration that are additionally affected by hypoxia impair memory consolidation. Affective complaints may thus surge, evolving into anxiety and depression. Sex-related differences in neural network organization and hormonal changes during the menstrual cycle, and certainly also during the life cycle, underscore the possibility of 5-HT-related mood alterations, particularly in women on HA exposure. The mean brain rate of 5-HT synthesis at sea level is already 1.5-fold higher in males than in females. sexual dimorphism also evidences the overexpression effects of SERT, a 5-HT transporter protein. Gonadal and thyroid hormones, as influenced by HA exposure, further modulate 5-HT availability and its effects in women. Besides caring for adequate oxygenation and maintenance of one's body core temperature, special precautions concerning women sojourning at HA should include close observations of hormonal cycles and, perhaps, also trials with targeted antidepressants.

Serotonin-deficient neonatal mice are not protected against the development of experimental bronchopulmonary dysplasia or pulmonary hypertension

Serotonin (5-hydroxytryptamine, 5-HT) is a potent pulmonary vasoconstrictor and contributes to high pulmonary vascular resistance in the developing ovine lung. In experimental pulmonary hypertension (PH), pulmonary expression of tryptophan hydroxylase-1 (TPH1), the rate limiting enzyme in 5-HT synthesis, and plasma 5-HT are increased. 5-HT blockade increases pulmonary blood flow and prevents pulmonary vascular remodeling and PH in neonatal models of PH with bronchopulmonary dysplasia (BPD). We hypothesized that neonatal tph1 knock-out (KO) mice would be protected from hypoxia-induced alveolar simplification, decreased vessel density, and PH. Newborn wild-type (WT) and tph1 KO mice were exposed to normoxia or hypoxia for 2 weeks. Normoxic WT and KO mice exhibited similar alveolar development, pulmonary vascular density, right ventricular systolic pressures (RVSPs), and right heart size. Circulating (plasma and platelet) 5-HT decreased in both hypoxia-exposed WT and KO mice. Tph1 KO mice were not protected from hypoxia-induced alveolar simplification, decreased pulmonary vascular density, or right ventricular hypertrophy, but displayed attenuation to hypoxia-induced RVSP elevation compared with WT mice. Tph1 KO neonatal mice are not protected against hypoxia-induced alveolar simplification, reduction in pulmonary vessel density, or RVH. While genetic and pharmacologic inhibition of tph1 has protective effects in adult models of PH, our results suggest that tph1 inhibition would not be beneficial in neonates with PH associated with BPD.

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.

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.

Direct Delivery of MicroRNA96 to the Lungs Reduces Progression of Sugen/Hypoxia-Induced Pulmonary Hypertension in the Rat

The 5HT1B receptor (5HT1BR) contributes to the pathogenic effects of serotonin in pulmonary arterial hypertension. Here, we determine the effect of a microRNA96 (miR96) mimic delivered directly to the lungs on development of severe pulmonary hypertension in rats. Female rats were dosed with sugen (30 mg/kg) and subjected to 3 weeks of hypobaric hypoxia. In normoxia, rats were dosed with either a 5HT1BR antagonist SB216641 (7.5 mg/kg/day for 3 weeks), miR96, or scramble sequence (50 μg per rat), delivered by intratracheal (i.t) administration, once a week for 3 weeks. Cardiac hemodynamics were determined, pulmonary vascular remodeling was assessed, and gene expression was assessed by qRT-PCR, and in situ hybridization and protein expression were assessed by western blot and ELISA. miR96 expression was increased in pulmonary arteries and associated with a downregulation of the 5HT1BR protein in the lung. miR96 reduced progression of right ventricular systolic pressure, pulmonary arterial remodeling, right ventricular hypertrophy, and the occurrence of occlusive pulmonary lesions. Importantly, miR96 had no off-target effects and did not affect fibrotic markers of liver and kidney function. In conclusion, direct delivery of miR96 to the lungs was effective, reducing progression of sugen/hypoxia-induced pulmonary hypertension with no measured off-target effects. miR96 may be a novel therapy for pulmonary arterial hypertension, acting through downregulation of 5HT1BR.

D-methionine improves cisplatin-induced anorexia and dyspepsia syndrome by attenuating intestinal tryptophan hydroxylase 1 activity and increasing plasma leptin concentration

BACKGROUND

Cisplatin is a widely used antineoplastic drug. However, cisplatin-induced dyspepsia syndromes, including delayed gastric emptying, gastric distension, early satiety, nausea, and vomiting, often force patients to take doses lower than those prescribed or even refuse treatment. D-methionine has an appetite-enhancing effect and alleviates weight loss during cisplatin treatment.

METHODS

This work established a model of anorexia and dyspepsia symptoms with intraperitoneal injection of cisplatin (5 mg/kg) once a week for three cycles. Presupplementation with or without D-methionine (300 mg/kg) was performed. Orexigenic and anorexigenic hormones (ghrelin, leptin, and glucagon-like peptide-1), tryptophan hydroxylase 1 (TPH1), 5-hydroxytryptamine receptors (5-HT_2C and 5-HT₃ ), and hypothalamic feeding-related peptides were measured by immunohistochemistry staining, enzyme-linked immunosorbent assay, and real-time PCR assay.

KEY RESULTS

Cisplatin administration caused marked decrease in appetite and body weight, promoted adipose and fat tissue atrophy, and delayed gastric emptying and gastric distension, and D-methionine preadministration prior to cisplatin administration significantly ameliorated these side effects. Besides, cisplatin induced an evident increase in serum ghrelin level, TPH1 activity, and 5-HT₃ receptor expression in the intestine and decreased plasma leptin levels and gastric ghrelin mRNA gene expression levels. D-methionine supplementation recovered these changes. The expression of orexigenic neuropeptide Y/agouti-related peptide and anorexigenic cocaine- and amphetamine-regulated transcript proopiomelanocortin neurons were altered by D-methionine supplementation in cisplatin-induced anorexia rats.

CONCLUSIONS AND INFERENCES

D-methionine supplementation prevents cisplatin-induced anorexia and dyspepsia syndrome possibly by attenuating intestinal tryptophan hydroxylase 1 activity and increasing plasma leptin concentration. Therefore, D-methionine can be used as an adjuvant therapy for treating cisplatin-induced adverse effects.

Platelets promote epileptic seizures by modulating brain serotonin level, enhancing neuronal electric activity, and contributing to neuroinflammation and oxidative stress

The drugs currently available for treating epilepsy are only partially effective in managing this condition. Therefore, it is crucial to investigate new pathways that induce and promote epilepsy development. Previously, we found that platelets interact with neuronal glycolipids and actively secrete pro-inflammatory mediators during central nervous system (CNS) pathological conditions such as neuroinflammation and traumatic brain injury (TBI). These factors increase the permeability of the blood-brain barrier (BBB), which may create a predisposition to epileptic seizures. In this study, we demonstrated that platelets substantially enhanced epileptic seizures in a mouse model of pentylenetetrazole (PTZ) -induced seizures. We found that platelets actively secreted serotonin, contributed to increased BBB permeability, and were present in the CNS parenchyma during epileptic seizures. Furthermore, platelets directly stimulated neuronal electric activity and induced the expression of specific genes related to early neuronal response, neuroinflammation, and oxidative phosphorylation, leading to oxidative stress in neurons. The intracranial injection of physiological numbers of platelets that mimicked TBI-associated bleeding was sufficient to induce severe seizures, which resembled conventional PTZ-induced epileptic activity. These findings highlight a conceptually new role of platelets in the development of epileptic seizures, and indicate a potential new therapeutic approach targeting platelets to prevent and treat epilepsy.

Inhibition of serotonin synthesis: A novel therapeutic paradigm

The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). There are two independent serotonin systems in the body characterized by two isoforms of TPH, TPH1 and TPH2. While TPH2 synthesizes serotonin in the brain, TPH1 is expressed in the gut and in other peripheral tissues and supplies platelets in the circulation with serotonin. This duality of the serotonin system is enforced by the blood-brain barrier which is impermeable for serotonin. In the brain serotonin acts as neurotransmitter and is a main target for the treatment of psychiatric disorders. In the periphery it is released by platelets at the site of activation and elicits numerous physiological effects. TPH1 deficient mice were shown to be protected from diverse diseases including hemostatic, inflammatory, fibrotic, gastrointestinal, and metabolic disorders and therefore serotonin synthesis inhibition emerged as a reasonable therapeutic paradigm. Recently the first TPH inhibitor, telotristat ethyl, came on the market for the treatment of carcinoid syndrome. This review summarizes the state of development and the therapeutic opportunities of such compounds.

Platelets mediate protective neuroinflammation and promote neuronal plasticity at the site of neuronal injury

It is generally accepted that inflammation within the CNS contributes to neurodegeneration after traumatic brain injury (TBI), but it is not clear how inflammation is initiated in the absence of infection and whether this neuroinflammation is predominantly beneficial or detrimental. We have previously found that brain-enriched glycosphingolipids within neuronal lipid rafts (NLR) induced platelet degranulation and secretion of neurotransmitters and pro-inflammatory factors. In the present study, we compared TBI-induced inflammation and neurodegeneration in wild-type vs. St3gal5 deficient (ST3^-/-) mice that lack major CNS-specific glycosphingolipids. After TBI, microglial activation and CNS macrophage infiltration were substantially reduced in ST3^-/- animals. However, ST3^-/- mice had a larger area of CNS damage with marked neuronal/axonal loss. The interaction of platelets with NLR stimulated neurite growth, increased the number of PSD95-positive dendritic spines, and intensified neuronal activity. Adoptive transfer and blocking experiments provide further that platelet-derived serotonin and platelet activating factor plays a key role in the regulation of sterile neuroinflammation, hemorrhage and neuronal plasticity after TBI.

Peripheral Serotonin Synthesis as a New Drug Target

The first step in serotonin (5-HT) biosynthesis is catalyzed by tryptophan hydroxylase (TPH). There are two independent sources of the monoamine that have distinct functions: first, the TPH1-expressing enterochromaffin cells (ECs) of the gut; second, TPH2-expressing serotonergic neurons. TPH1-deficient mice revealed that peripheral 5-HT plays important roles in platelet function and in inflammatory and fibrotic diseases of gut, pancreas, lung, and liver. Therefore, TPH inhibitors were developed which cannot pass the blood-brain barrier to specifically block peripheral 5-HT synthesis. They showed therapeutic efficacy in several rodent disease models, and telotristat ethyl is the first TPH inhibitor to be approved for the treatment of carcinoid syndrome. We review this development and discuss further therapeutic options for these compounds.

The serotonin hypothesis in pulmonary hypertension revisited: targets for novel therapies (2017 Grover Conference Series)

Increased synthesis of serotonin and/or activity of serotonin in pulmonary arteries has been implicated in the pathobiology of pulmonary arterial hypertension (PAH). The incidence of PAH associated with diet pills such as aminorex, fenfluramine, and chlorphentermine initially led to the “serotonin hypothesis of pulmonary hypertension.” Over the last couple of decades there has been an accumulation of convincing evidence that targeting serotonin synthesis or signaling is a novel and promising approach to the development of novel therapies for PAH. Pulmonary endothelial serotonin synthesis via tryptophan hydroxlase 1 (TPH1) is increased in patients with PAH and serotonin can act in a paracrine fashion on underlying pulmonary arterial smooth muscle cells (PASMCs), In humans, serotonin can enter PASMCs via the serotonin transporter (SERT) or activate the 5-HT1B receptor; 5-HT1B activation and SERT activity cooperate to induce PASMC contraction and proliferation via activation of downstream proliferative and contractile signaling pathways. Here we will review the current status of the serotonin hypothesis and discuss potential and novel therapeutic targets.

Pressure overload leads to an increased accumulation and activity of mast cells in the right ventricle

Right ventricular (RV) remodeling represents a complex set of functional and structural adaptations in response to chronic pressure or volume overload due to various inborn defects or acquired diseases and is an important determinant of patient outcome. However, the underlying molecular mechanisms remain elusive. We investigated the time course of structural and functional changes in the RV in the murine model of pressure overload‐induced RV hypertrophy in C57Bl/6J mice. Using magnetic resonance imaging, we assessed the changes of RV structure and function at different time points for a period of 21 days. Pressure overload led to significant dilatation, cellular and chamber hypertrophy, myocardial fibrosis, and functional impairment of the RV. Progressive remodeling of the RV after pulmonary artery banding (PAB) in mice was associated with upregulation of myocardial gene markers of hypertrophy and fibrosis. Furthermore, remodeling of the RV was associated with accumulation and activation of mast cells in the RV tissue of PAB mice. Our data suggest possible involvement of mast cells in the RV remodeling process in response to pressure overload. Mast cells may thus represent an interesting target for the development of new therapeutic approaches directed specifically at the RV.

PCPA protects against monocrotaline-induced pulmonary arterial remodeling in rats: potential roles of connective tissue growth factor

The purpose of this study was to investigate the mechanism of monocrotaline (MCT)-induced pulmonary artery hypertension (PAH) and determine whether 4-chloro-DL-phenylalanine (PCPA) could inhibit pulmonary arterial remodeling associated with connective tissue growth factor (CTGF) expression and downstream signal pathway. MCT was administered to forty Sprague Dawley rats to establish the PAH model. PCPA was administered at doses of 50 and 100 mg/kg once daily for 3 weeks via intraperitoneal injection. On day 22, the pulmonary arterial pressure (PAP), right ventricle hypertrophy index (RVI) and pulmonary artery morphology were assessed and the serotonin receptor-1B (SR-1B), CTGF, p-ERK/ERK were measured by western blot or immunohistochemistry. The concentration of serotonin in plasma was checked by ELISA. Apoptosis and apoptosis-related indexes were detected by TUNEL and western blot. In the MCT-induced PAH models, the PAP, RVI, pulmonary vascular remodeling, SR-1B index, CTGF index, anti-apoptotic factors bcl-xl and bcl-2, serotonin concentration in plasma were all increased and the pro-apoptotic factor caspase-3 was reduced. PCPA significantly ameliorated pulmonary arterial remodeling induced by MCT, and this action was associated with accelerated apoptosis and down-regulation of CTGF, SR-1B and p-ERK/ERK. The present study suggests that PCPA protects against the pathogenesis of PAH by suppressing remodeling and inducing apoptosis, which are likely associated with CTGF and downstream ERK signaling pathway in rats.

Serotonergic Control of Metabolic Homeostasis

New treatments are urgently needed to address the current epidemic of obesity and diabetes. Recent studies have highlighted multiple pathways whereby serotonin (5-HT) modulates energy homeostasis, leading to a renewed interest in the identification of 5-HT-based therapies for metabolic disease. This review aims to synthesize pharmacological and genetic studies that have found diverse functions of both central and peripheral 5-HT in the control of food intake, thermogenesis, and glucose and lipid metabolism. We also discuss the potential benefits of targeting the 5-HT system to combat metabolic disease.

Peripheral serotonin regulates postoperative intra-abdominal adhesion formation in mice

The aim of the present study is to investigate the role and potential mechanisms of peripheral serotonin in postoperative intra-abdominal adhesion formation in mice. The caecum-rubbing operations were conducted for intra-abdominal adhesion formation modelling in wild-type and Tph1−/− mice. The deficiency of serotonin significantly decreased the adhesion scores, weight loss, and adhesion thickness as well as levels of collagen fibres and hydroxyproline in the adhesive tissues. The Tph1−/− mice exhibited a milder inflammatory response and oxidative stress in the adhesive tissues than did the wild-type mice. Moreover, the deficiency of serotonin reduced the levels of PAI-1 and fibrinogen, and raised the t-PA and t-PA/PAI levels in the peritoneal fluids. Moreover, the expressions of CD34, VEGF, TGF-β and 5-HT_2B receptor in the adhesive tissues were significantly decreased in the Tph1−/− group mice. Furthermore, the Tph1−/− +5-HTP group showed more severe adhesions than did the Tph1−/− group mice, and the p-chlorophenylalanine (PCPA) could markedly alleviated the adhesion formation in the WT mice. In conclusion, the present study showed that peripheral serotonin regulated postoperative intra-abdominal adhesion formation by facilitating inflammation, oxidative stress, disorder of the fibrinolytic system, angiopoiesis and TGF-β1 expression via the 5-HT_2B receptor in the adhesive tissues.

Tryptophan hydroxylase 1 Inhibition Impacts Pulmonary Vascular Remodeling in Two Rat Models of Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is a progressive disease defined by a chronic elevation in pulmonary arterial pressure with extensive pulmonary vascular remodeling and perivascular inflammation characterized by an accumulation of macrophages, lymphocytes, dendritic cells, and mast cells. Although the exact etiology of the disease is unknown, clinical as well as preclinical data strongly implicate a role for serotonin (5-HT) in the process. Here, we investigated the chronic effects of pharmacological inhibition of tryptophan hydroxylase 1 (TPH1), the rate-limiting enzyme in peripheral 5-HT biosynthesis, in two preclinical models of pulmonary hypertension (PH), the monocrotaline (MCT) rat and the semaxanib (SUGEN, Medinoah, Suzhou, China)-hypoxia rat. In both PH models, ethyl (S)-8-(2-amino-6-((R)-1-(5-chloro-[1,1'-biphenyl]-2-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate and ethyl (S)-8-(2-amino-6-((R)-1-(3',4'-dimethyl-3-(3-methyl-1 H-pyrazol-1-yl)-[1,1'-biphenyl]-4-yl)-2,2,2-trifluoroethoxy)pyrimidin-4-yl)-2,8-diazaspiro[4.5]decane-3-carboxylate, novel orally active TPH1 inhibitors with nanomolar in vitro potency, decreased serum, gut, and lung 5-HT levels in a dose-dependent manner and significantly reduced pulmonary arterial pressure, and pulmonary vessel wall thickness and occlusion in male rats. In the MCT rat model, decreases in lung 5-HT significantly correlated with reductions in histamine levels and mast cell number (P < 0.001, r² = 0.88). In contrast, neither ambrisentan nor tadalafil, which are vasodilators approved for the treatment of PAH, reduced mast cell number or 5-HT levels, nor were they as effective in treating the vascular remodeling as were the TPH1 inhibitors. When administered in combination with ambrisentan, the TPH1 inhibitors showed an additive effect on pulmonary vascular remodeling and pressures. These data demonstrate that in addition to reducing vascular remodeling, TPH1 inhibition has the added benefit of reducing the perivascular mast cell accumulation associated with PH.

Regulation of allergic lung inflammation by endothelial cell transglutaminase 2

Tissue transglutaminase 2 (TG2) is an enzyme with multiple functions, including catalysis of serotonin conjugation to proteins (serotonylation). Previous research indicates that TG2 expression is upregulated in human asthma and in the lung endothelium of ovalbumin (OVA)-challenged mice. It is not known whether endothelial cell TG2 is required for allergic inflammation. Therefore, to determine whether endothelial cell TG2 regulates allergic inflammation, mice with an endothelial cell-specific deletion of TG2 were generated, and these mice were sensitized and challenged in the airways with OVA. Deletion of TG2 in endothelial cells blocked OVA-induced serotonylation in lung endothelial cells, but not lung epithelial cells. Interestingly, deletion of endothelial TG2 reduced allergen-induced increases in respiratory system resistance, number of eosinophils in the bronchoalveolar lavage, and number of eosinophils in the lung tissue. Endothelial cell deletion of TG2 did not alter expression of adhesion molecules, cytokines, or chemokines that regulate leukocyte recruitment, consistent with other studies, demonstrating that deletion of endothelial cell signals does not alter lung cytokines and chemokines during allergic inflammation. Taken together, the data indicate that endothelial cell TG2 is required for allergic inflammation by regulating the recruitment of eosinophils into OVA-challenged lungs. In summary, TG2 functions as a critical signal for allergic lung responses. These data identify potential novel targets for intervention in allergy/asthma.

Serotonin deficiency exacerbates acetaminophen-induced liver toxicity in mice

Acetaminophen (APAP) overdose is a major cause of acute liver failure. Peripheral 5-hydroxytryptamine (serotonin, 5-HT) is a cytoprotective neurotransmitter which is also involved in the hepatic physiological and pathological process. This study seeks to investigate the mechanisms involved in APAP-induced hepatotoxicity, as well as the role of 5-HT in the liver's response to APAP toxicity. We induced APAP hepatotoxicity in mice either sufficient of serotonin (wild-type mice and TPH1-/- plus 5- Hydroxytryptophan (5-HTP)) or lacking peripheral serotonin (Tph1-/- and wild-type mice plus p-chlorophenylalanine (PCPA)).Mice with sufficient 5-HT exposed to acetaminophen have a significantly lower mortality rate and a better outcome compared with mice deficient of 5-HT. This difference is at least partially attributable to a decreased level of inflammation, oxidative stress and endoplasmic reticulum (ER) stress, Glutathione (GSH) depletion, peroxynitrite formation, hepatocyte apoptosis, elevated hepatocyte proliferation, activation of 5-HT2B receptor, less activated c-Jun NH₂-terminal kinase (JNK) and hypoxia-inducible factor (HIF)-1α in the mice sufficient of 5-HT versus mice deficient of 5-HT. We thus propose a physiological function of serotonin that serotonin could ameliorate APAP-induced liver injury mainly through inhibiting hepatocyte apoptosis ER stress and promoting liver regeneration.

Inhibiting peripheral serotonin synthesis reduces obesity and metabolic dysfunction by promoting brown adipose tissue thermogenesis

Mitochondrial uncoupling protein 1 (UCP1) is enriched within interscapular brown adipose tissue (iBAT) and beige (also known as brite) adipose tissue, but its thermogenic potential is reduced with obesity and type 2 diabetes for reasons that are not understood. Serotonin (5-hydroxytryptamine, 5-HT) is a highly conserved biogenic amine that resides in non-neuronal and neuronal tissues that are specifically regulated via tryptophan hydroxylase 1 (Tph1) and Tph2, respectively. Recent findings suggest that increased peripheral serotonin and polymorphisms in TPH1 are associated with obesity; however, whether this is directly related to reduced BAT thermogenesis and obesity is not known. We find that Tph1-deficient mice fed a high-fat diet (HFD) are protected from obesity, insulin resistance and nonalcoholic fatty liver disease (NAFLD) while exhibiting greater energy expenditure by BAT. Small-molecule chemical inhibition of Tph1 in HFD-fed mice mimics the benefits ascribed to Tph1 genetic deletion, effects that depend on UCP1-mediated thermogenesis. The inhibitory effects of serotonin on energy expenditure are cell autonomous, as serotonin blunts β-adrenergic induction of the thermogenic program in brown and beige adipocytes in vitro. As obesity increases peripheral serotonin, the inhibition of serotonin signaling or its synthesis in adipose tissue may be an effective treatment for obesity and its comorbidities.

Elevated transglutaminase 2 activity is associated with hypoxia-induced experimental pulmonary hypertension in mice

Previous studies in human patients and animal models have suggested that transglutaminase 2 (TG2) is upregulated in pulmonary hypertension (PH), a phenomenon that appears to be associated with the effects of serotonin (5-hydroxytryptamine; 5-HT) in this disease. Using chemical tools to interrogate and inhibit TG2 activity in vivo, we have shown that pulmonary TG2 undergoes marked post-translational activation in a mouse model of hypoxia-induced PH. We have also identified irreversible fluorinated TG2 inhibitors that may find use as non-invasive positron emission tomography probes for diagnosis and management of this debilitating, lifelong disorder. Pharmacological inhibition of TG2 attenuated the elevated right ventricular pressure but had no effect on hypertrophy of the right ventricle of the heart. A longitudinal study of pulmonary TG2 activity in PH patients is warranted.

Experimental and transgenic models of pulmonary hypertension

Pulmonary hypertension in human patients can result from increased pulmonary vascular tone, pressure transferred from the systemic circulation, dropout of small pulmonary vessels, occlusion of vessels with thrombi or intimal lesions, or some combination of all of these. Different animal models have been designed to reflect these different mechanistic origins of disease. Pulmonary hypertension models may be roughly grouped into tone-related models, inflammation-related models, and genetic models with unusual or mixed mechanism. Models of tone generally use hypoxia as a base, and then modify this with either genetic modifications (SOD, NOS, and caveolin) or with drugs (Sugen), although some genetic modifications of tone-related pathways can result in spontaneous pulmonary hypertension (Hph-1). Inflammation-related models can use either toxic chemicals (monocrotaline, bleomycin), live pathogens (stachybotrys, schistosomiasis), or genetic modifications (IL-6, VIP). Additional genetic models rely on alterations in metabolism (adiponectin), cell migration (S100A4), the serotonin pathway, or the BMP pathway. While each of these shares molecular and pathologic symptoms with different classes of human pulmonary hypertension, in most cases the molecular etiology of human pulmonary hypertension is unknown, and so the relationship between any model and human disease is unclear. There is thus no best animal model of pulmonary hypertension; instead, investigators must select the model most related to the specific pathology they are studying.

Gender, sex hormones and pulmonary hypertension

Most subtypes of pulmonary arterial hypertension (PAH) are characterized by a greater susceptibility to disease among females, although females with PAH appear to live longer after diagnosis. While this "estrogen paradoxȍ of enhanced female survival despite increased female susceptibility remains a mystery, recent progress has begun to shed light upon the interplay of sex hormones, the pathogenesis of pulmonary hypertension, and the right ventricular response to stress. For example, emerging data in humans and experimental models suggest that estrogens or differential sex hormone metabolism may modify disease risk among susceptible subjects, and that estrogens may interact with additional local factors such as serotonin to enhance the potentially damaging chronic effects of estrogens on the pulmonary vasculature. Regardless, it remains unclear why not all estrogenic compounds behave equally, nor why estrogens appear to be protective in certain settings but detrimental in others. The contribution of androgens and other compounds, such as dehydroepiandrosterone, to pathogenesis and possibly treatment must be considered as well. In this review, we will discuss the recent understandings on how estrogens, estrogen metabolism, dehydroepiandrosterone, and additional susceptibility factors may all contribute to the pathogenesis or potentially to the treatment of pulmonary hypertension, by evaluating current human, cell-based, and experimental model data.

Imatinib attenuates hypoxia-induced pulmonary arterial hypertension pathology via reduction in 5-hydroxytryptamine through inhibition of tryptophan hydroxylase 1 expression

RATIONALE

Whether idiopathic, familial, or secondary to another disease, pulmonary arterial hypertension (PAH) is characterized by increased vascular tone, neointimal hyperplasia, medial hypertrophy, and adventitial fibrosis. Imatinib, a potent receptor tyrosine kinase inhibitor, reverses pulmonary remodeling in animal models of PAH and improves hemodynamics and exercise capacity in selected patients with PAH.

OBJECTIVES

Here we use both imatinib and knockout animals to determine the relationship between platelet-derived growth factor receptor (PDGFR) and serotonin signaling and investigate the PAH pathologies each mediates.

METHODS

We investigated the effects of imatinib (100 mg/kg) on hemodynamics, vascular remodeling, and downstream molecular signatures in the chronic hypoxia/SU5416 murine model of PAH.

MEASUREMENTS AND MAIN RESULTS

Treatment with imatinib reduced all measures of PAH pathology observed in hypoxia/SU5416 mice. In addition, 5-hydroxytryptamine (5-HT) and tryptophan hydroxylase 1 (Tph1) expression were reduced compared with the normoxia/SU5416 control group. Imatinib attenuated hypoxia-induced increases in Tph1 expression in pulmonary endothelial cells in vitro via inhibition of the PDGFR-β pathway. To better understand the consequences of this novel mode of action for imatinib, we examined the development of PAH after hypoxic/SU5416 exposure in Tph1-deficient mice (Tph1(-/-)). The extensive changes in pulmonary vascular remodeling and hemodynamics in response to hypoxia/SU5416 were attenuated in Tph1(-/-) mice and further decreased after imatinib treatment. However, imatinib did not significantly further impact collagen deposition and collagen 3a1 expression in hypoxic Tph1(-/-) mice. Post hoc subgroup analysis suggests that patients with PAH with greater hemodynamic impairment showed significantly reduced 5-HT plasma levels after imatinib treatment compared with placebo.

CONCLUSIONS

We report a novel mode of action for imatinib, demonstrating TPH1 down-regulation via inhibition of PDGFR-β signaling. Our data reveal interplay between PDGF and 5-HT pathways within PAH, demonstrating TPH1-dependent imatinib efficacy in collagen-mediated mechanisms of fibrosis.

[Pulmonary hypertension: from molecular pathophysiology to haemodynamic abnormalities]

Pulmonary hypertension (PH) is a complex disorder resulting from many etiologies that cause disturbances of normal pulmonary haemodynamics. Recent breakthroughs have led to a better understanding of the pathophysiology of the disease. In PH, haemodynamic disturbances are closely linked to structural changes and excessive remodeling of pulmonary vessels, leading to progressive narrowing of the pulmonary vascular lumen. Imbalances between pulmonary vasoconstrictors and vasodilators on the one hand, and factors favoring cell proliferation and apoptosis on the other hand, probably account for most cases of PH. This review aims to update readers with the current knowledge on the molecular physiopathology of PH and how this can progress the therapeutic of this disorder.

Inhibition of allergic inflammation by supplementation with 5-hydroxytryptophan

Clinical reports indicate that patients with allergy/asthma commonly have associated symptoms of anxiety/depression. Anxiety/depression can be reduced by 5-hydroxytryptophan (5-HTP) supplementation. However, it is not known whether 5-HTP reduces allergic inflammation. Therefore, we determined whether 5-HTP supplementation reduces allergic inflammation. We also determined whether 5-HTP decreases passage of leukocytes through the endothelial barrier by regulating endothelial cell function. For these studies, C57BL/6 mice were supplemented with 5-HTP, treated with ovalbumin fraction V (OVA), house dust mite (HDM) extract, or IL-4, and examined for allergic lung inflammation and OVA-induced airway responsiveness. To determine whether 5-HTP reduces leukocyte or eosinophil transendothelial migration, endothelial cells were pretreated with 5-HTP, washed and then used in an in vitro transendothelial migration assay under laminar flow. Interestingly, 5-HTP reduced allergic lung inflammation by 70-90% and reduced antigen-induced airway responsiveness without affecting body weight, blood eosinophils, cytokines, or chemokines. 5-HTP reduced allergen-induced transglutaminase 2 (TG2) expression and serotonylation (serotonin conjugation to proteins) in lung endothelial cells. Consistent with the regulation of endothelial serotonylation in vivo, in vitro pretreatment of endothelial cells with 5-HTP reduced TNF-α-induced endothelial cell serotonylation and reduced leukocyte transendothelial migration. Furthermore, eosinophil and leukocyte transendothelial migration was reduced by inhibitors of transglutaminase and by inhibition of endothelial cell serotonin synthesis, suggesting that endothelial cell serotonylation is key for leukocyte transendothelial migration. In summary, 5-HTP supplementation inhibits endothelial serotonylation, leukocyte recruitment, and allergic inflammation. These data identify novel potential targets for intervention in allergy/asthma.

Role of endothelial AADC in cardiac synthesis of serotonin and nitrates accumulation

Serotonin (5-HT) regulates different cardiac functions by acting directly on cardiomyocytes, fibroblasts and endothelial cells. Today, it is widely accepted that activated platelets represent a major source of 5-HT. In contrast, a supposed production of 5-HT in the heart is still controversial. To address this issue, we investigated the expression and localization of 5-HT synthesizing enzyme tryptophan hydroxylase (TPH) and L-aromatic amino acid decarboxylase (AADC) in the heart. We also evaluated their involvement in cardiac production of 5-HT. TPH1 was weakly expressed in mouse and rat heart and appeared restricted to mast cells. Degranulation of mast cells by compound 48/80 did not modify 5-HT cardiac content in mice. Western blots and immunolabelling experiments showed an abundant expression of AADC in the mouse and rat heart and its co-localization with endothelial cells. Incubation of cardiac homogenate with the AADC substrate (5-hydroxy-L-tryptophan) 5-HTP or intraperitoneal injection of 5-HTP in mice significantly increased cardiac 5-HT. These effects were prevented by the AADC inhibitor benserazide. Finally, 5-HTP administration in mice increased phosphorylation of aortic nitric oxide synthase 3 at Ser (1177) as well as accumulation of nitrates in cardiac tissue. This suggests that the increase in 5-HT production by AADC leads to activation of endothelial and cardiac nitric oxide pathway. These data show that endothelial AADC plays an important role in cardiac synthesis of 5-HT and possibly in 5-HT-dependent regulation of nitric oxide generation.

Inhibition of gut- and lung-derived serotonin attenuates pulmonary hypertension in mice

Decreasing the bioavailability of serotonin (5-HT) by inhibiting its biosynthesis may represent a useful adjunctive treatment of pulmonary hypertension (PH). We assessed this hypothesis using LP533401, which inhibits the rate-limiting enzyme tryptophan hydroxylase 1 (Tph1) expressed in the gut and lung, without inhibiting Tph2 expressed in neurons. Mice treated repeatedly with LP533401 (30-250 mg/kg per day) exhibited marked 5-HT content reductions in the gut, lungs, and blood, but not in the brain. After a single LP533401 dose (250 mg/kg), lung and gut 5-HT contents decreased by 50%, whereas blood 5-HT levels remained unchanged, suggesting gut and lung 5-HT synthesis. Treatment with the 5-HT transporter (5-HTT) inhibitor citalopram decreased 5-HT contents in the blood and lungs but not in the gut. In transgenic SM22-5-HTT+ mice, which overexpress 5-HTT in smooth muscle cells and spontaneously develop PH, 250 mg/kg per day LP533401 or 10 mg/kg per day citalopram for 21 days markedly reduced lung and blood 5-HT levels, right ventricular (RV) systolic pressure, RV hypertrophy, distal pulmonary artery muscularization, and vascular Ki67-positive cells (P < 0.001). Combined treatment with both drugs was more effective in improving PH-related hemodynamic parameters than either drug alone. LP533401 or citalopram treatment partially prevented PH development in wild-type mice exposed to chronic hypoxia. Lung and blood 5-HT levels were lower in hypoxic than in normoxic mice and decreased further after LP533401 or citalopram treatment. These results provide proof of concept that inhibiting Tph1 may represent a new therapeutic strategy for human PH.

Gene therapy by targeted adenovirus-mediated knockdown of pulmonary endothelial Tph1 attenuates hypoxia-induced pulmonary hypertension

Serotonin is produced by pulmonary arterial endothelial cells (PAEC) via tryptophan hydroxylase-1 (Tph1). Pathologically, serotonin acts on underlying pulmonary arterial cells, contributing to vascular remodeling associated with pulmonary arterial hypertension (PAH). The effects of hypoxia on PAEC-Tph1 activity are unknown. We investigated the potential of a gene therapy approach to PAH using selective inhibition of PAEC-Tph1 in vivo in a hypoxic model of PAH. We exposed cultured bovine pulmonary arterial smooth muscle cells (bPASMCs) to conditioned media from human PAECs (hPAECs) before and after hypoxic exposure. Serotonin levels were increased in hypoxic PAEC media. Conditioned media evoked bPASMC proliferation, which was greater with hypoxic PAEC media, via a serotonin-dependent mechanism. In vivo, adenoviral vectors targeted to PAECs (utilizing bispecific antibody to angiotensin-converting enzyme (ACE) as the selective targeting system) were used to deliver small hairpin Tph1 RNA sequences in rats. Hypoxic rats developed PAH and increased lung Tph1. PAEC-Tph1 expression and development of PAH were attenuated by our PAEC-Tph1 gene knockdown strategy. These results demonstrate that hypoxia induces Tph1 activity and selective knockdown of PAEC-Tph1 attenuates hypoxia-induced PAH in rats. Further investigation of pulmonary endothelial-specific Tph1 inhibition via gene interventions is warranted.

Advances in tryptophan hydroxylase-2 gene expression regulation: new insights into serotonin-stress interaction and clinical implications

Serotonin (5-HT) modulates the stress response by interacting with the hormonal hypothalamic-pituitary-adrenal (HPA) axis and neuronal sympathetic nervous system (SNS). Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in 5-HT biosynthesis, and the recent identification of a second, neuron-specific TPH isoform (TPH2) opened up a new area of research. While TPH2 genetic variance has been linked to numerous behavioral traits and disorders, findings on TPH2 gene expression have not only reinforced, but also provided new insights into, the long-recognized but not yet fully understood 5-HT-stress interaction. In this review, we summarize advances in TPH2 expression regulation and its relevance to the stress response and clinical implications. Particularly, based on findings on rhesus monkey TPH2 genetics and other relevant literature, we propose that: (i) upon activation of adrenal cortisol secretion, the cortisol surge induces TPH2 expression and de novo 5-HT synthesis; (ii) the induced 5-HT in turn inhibits cortisol secretion by modulating the adrenal sensitivity to ACTH via the suprachiasmatic nuclei (SCN)-SNS-adrenal system, such that it contributes to the feedback inhibition of cortisol production; (iii) basal TPH2 expression or 5-HT synthesis, as well as early-life experience, influence basal cortisol primarily via the hormonal HPA axis; and (iv) 5'- and 3'-regulatory polymorphisms of TPH2 may differentially influence the stress response, presumably due to their differential roles in gene expression regulation. Our increasing knowledge of TPH2 expression regulation not only helps us better understand the 5-HT-stress interaction and the pathophysiology of neuropsychiatric disorders, but also provides new strategies for the treatment of stress-associated diseases.

On the presence of serotonin in mammalian cardiomyocytes

Pleiotropic effects of serotonin (5-HT) in the cardiovascular system are well documented. However, it remains to be elucidated, whether 5-HT is present in adult mammalian cardiomyocytes. To address this issue, we investigated the levels of 5-HT in blood, plasma, platelets, cardiac tissue, and cardiomyocytes from adult mice and for comparison in human right atrial tissue. Immunohistochemically, 5-HT was hardly found in mouse cardiac tissue, but small amounts could be detected in renal preparations, whereas adrenal preparations revealed a strong positive immunoreaction for 5-HT. Using a sensitive HPLC detection system, 5-HT was also detectable in the mouse heart and human atrium. Furthermore, we could identify 5-HT in isolated cardiomyocytes from adult mice. These findings were supported by detection of the activity of 5-HT-forming enzymes-tryptophan hydroxylase and aromatic L-amino acid decarboxylase-in isolated cardiomyocytes from adult mice and by inhibition of these enzymes with p-chlorophenylalanine and 3-hydroxybenzyl hydrazine. Addition of the first intermediate of 5-HT generation, that is 5-hydroxytryptophan, enhanced the 5-HT level and inhibition of monoamine oxidase by tranylcypromine further increased the level of 5-HT. Our findings reveal the presence and synthesis of 5-HT in cardiomyocytes of the mammalian heart implying that 5-HT may play an autocrine and/or paracrine role in the heart.

Paracrine modulation of cholangiocyte serotonin synthesis orchestrates biliary remodeling in adults

Paracrine signaling between cholangiocytes and stromal cells regulates biliary remodeling. Cholangiocytes have neuroepithelial characteristics and serotonin receptor agonists inhibit their growth, but whether they are capable of serotonin biosynthesis is unknown. We hypothesized that cholangiocytes synthesize serotonin and that cross talk between liver myofibroblasts (MF) and cholangiocytes regulates this process to influence biliary remodeling. Transwell cultures of cholangiocytes ± MF, and tryptophan hydroxylase-2 knockin (TPH2KI) mice with an inactivating mutation of the neuronal tryptophan hydroxylase (TPH) isoform, TPH2, were evaluated. Results in the cell culture models confirm that cholangiocytes have serotonin receptors and demonstrate for the first time that these cells express TPH2 and produce serotonin, which autoinhibits their growth but stimulates MF production of TGF-β(1). Increased TGF-β(1), in turn, counteracts autocrine inhibition of cholangiocyte growth by repressing cholangiocyte TPH2 expression. Studies of TPH2KI mice confirm that TPH2-mediated production of serotonin plays an important role in remodeling damaged bile ducts because mice with decreased TPH2 function have reduced biliary serotonin levels and exhibit excessive cholangiocyte proliferation, accumulation of aberrant ductules and liver progenitors, and increased liver fibrosis after bile duct ligation. This new evidence that cholangiocytes express the so-called neuronal isoform of TPH, synthesize serotonin de novo, and deploy serotonin as an autocrine/paracrine signal to regulate regeneration of the biliary tree complements earlier work that revealed that passive release of serotonin from platelets stimulates hepatocyte proliferation. Given the prevalent use of serotonin-modulating drugs, these findings have potentially important implications for recovery from various types of liver damage.

Mice deficient in Mkp-1 develop more severe pulmonary hypertension and greater lung protein levels of arginase in response to chronic hypoxia

The mitogen-activated protein (MAP) kinases are involved in cellular responses to many stimuli, including hypoxia. MAP kinase signaling is regulated by a family of phosphatases that include MAP kinase phosphatase-1 (MKP-1). We hypothesized that mice lacking the Mkp-1 gene would have exaggerated chronic hypoxia-induced pulmonary hypertension. Wild-type (WT) and Mkp-1(-/-) mice were exposed to either 4 wk of normoxia or hypobaric hypoxia. Following chronic hypoxia, both genotypes demonstrated elevated right ventricular pressures, right ventricular hypertrophy as demonstrated by the ratio of the right ventricle to the left ventricle plus septum weights [RV(LV + S)], and greater vascular remodeling. However, the right ventricular systolic pressures, the RV/(LV + S), and the medial wall thickness of 100- to 300-microm vessels was significantly greater in the Mkp-1(-/-) mice than in the WT mice following 4 wk of hypobaric hypoxia. Chronic hypoxic exposure caused no detectable change in eNOS protein levels in the lungs in either genotype; however, Mkp-1(-/-) mice had lower levels of eNOS protein and lower lung NO production than did WT mice. No iNOS protein was detected in the lungs by Western blotting in any condition in either genotype. Both arginase I and arginase II protein levels were greater in the lungs of hypoxic Mkp-1(-/-) mice than those in hypoxic WT mice. Lung levels of proliferating cell nuclear antigen were greater in hypoxic Mkp-1(-/-) than in hypoxic WT mice. These data are consistent with the concept that MKP-1 acts to restrain hypoxia-induced arginase expression and thereby reduces vascular remodeling and the severity of pulmonary hypertension.

Strain-specific genetics, anatomy and function of enteric neural serotonergic pathways in inbred mice

Serotonin (5-HT) powerfully affects small intestinal motility and 5-HT-immunoreactive (IR) neurones are highly conserved between species. 5-HT synthesis in central neurones and gastrointestinal mucosa depends on tissue-specific isoforms of the enzyme tryptophan hydroxylase (TPH). RT-PCR identified strain-specific expression of a polymorphism (1473C/G) of the tph2 gene in longitudinal muscle-myenteric plexus preparations of C57Bl/6 and Balb/c mice. The former expressed the high-activity C allele, the latter the low-activity G allele. Confocal microscopy was used to examine close contacts between 5-HT-IR varicosities and myenteric neurones immunoreactive for neuronal nitric oxide synthase (NOS) or calretinin in these two strains. Significantly more close contacts were identified to NOS- (P < 0.05) and calretinin-IR (P < 0.01) neurones in C57Bl/6 jejunum (NOS 1.6 +/- 0.3, n = 52; calretinin 5.2 +/- 0.4, n = 54), than Balb/c jejunum (NOS 0.9 +/- 0.2, n = 78; calretinin 3.5 +/- 0.3, n = 98). Propagating contractile complexes (PCCs) were identified in the isolated jejunum by constructing spatiotemporal maps from video recordings of cannulated segments in vitro. These clusters of contractions usually arose towards the anal end and propagated orally. Regular PCCs were initiated at intraluminal pressures of 6 cmH(2)O, and abolished by tetrodotoxin (1 microm). Jejunal PCCs from C57Bl/6 mice were suppressed by a combination of granisetron (1 microm, 5-HT(3) antagonist) and SB207266 (10 nm, 5-HT(4) antagonist), but PCCs from Balb/c mice were unaffected. There were, however, no strain-specific differences in sensitivity of longitudinal muscle contractions to exogenous 5-HT or blockade of 5-HT(3) and 5-HT(4) receptors. These data associate a genetic difference with significant structural and functional consequences for enteric neural serotonergic pathways in the jejunum.

Molecular cloning, characterization and expression of two tryptophan hydroxylase (TPH-1 and TPH-2) genes in the hypothalamus of Atlantic croaker: down-regulation after chronic exposure to hypoxia

Recently we discovered that hypoxia causes marked impairment of reproductive neuroendocrine function in Atlantic croaker, a marine teleost, which is due to a decline in hypothalamic serotonergic activity. As a first step in understanding the molecular responses of the hypothalamic serotonergic system to hypoxia, we cloned and characterized the genes for the enzymes regulating the rate-limiting step in serotonin biosynthesis, tryptophan hydroxylase (TPH-1 and TPH-2) in the croaker brain. The full-length croaker TPH-1 and TPH-2 cDNAs contain open reading frames encoding proteins with 479 and 487 amino acids, respectively, which are highly homologous to the TPH-1 (76-93%) and TPH-2 (64-92%) proteins of other vertebrates. Croaker TPH-1 and TPH-2 mRNA expression was detected throughout the brain but was greatest in the hypothalamic region. Both Northern blot analysis and real-time PCR showed that TPH-1 (transcript size approximately 2.1 kb) and TPH-2 ( approximately 1.9 kb) mRNA levels were significantly decreased in the hypothalami of croaker exposed for 2 weeks to hypoxic conditions compared with those in fish exposed to normoxic conditions. Immunohistochemistry of hypothalamic neurons with TPH antibodies showed reduced expression of TPHs in hypoxia-exposed fish compared with normoxic fish. Western blot analysis confirmed that hypoxia caused a marked decline in hypothalamic TPH protein levels, which was associated with decreases in hypothalamic TPH enzyme activity and 5-hydroxytryptophan levels. These results suggest that TPH is a major site of hypoxia-induced down-regulation of serotonergic function in croaker brains. Moreover, they provide the first evidence that hypoxia decreases the expression of TPH transcripts in vertebrate brains.

Pulmonary hypertension: therapeutic targets within the serotonin system

Pulmonary arterial hypertension (PAH) is characterized by a sustained and progressive elevation in pulmonary arterial pressure and pulmonary vascular remodelling leading to right heart failure and death. Prognosis is poor and novel therapeutic approaches are needed. The serotonin hypothesis of PAH originated in the 1960s after an outbreak of the disease was reported among patients taking the anorexigenic drugs aminorex and fenfluramine. These are indirect serotonergic agonists and serotonin transporter substrates. Since then many advances have been made in our understanding of the role of serotonin in the pathobiology of PAH. The rate-limiting enzyme in the synthesis of serotonin is tryptophan hydroxylase (Tph). Serotonin is synthesized, through Tph1, in the endothelial cells of the pulmonary artery and can then act on underlying pulmonary arterial smooth muscle cells and pulmonary arterial fibroblasts in a paracrine fashion causing constriction and remodelling. These effects of serotonin can be mediated through both the serotonin transporter and serotonin receptors. This review will discuss our current understanding of 'the serotonin hypothesis' of PAH and highlight possible therapeutic targets within the serotonin system.