Human 5-HT(4) and 5-HT(7) receptor splice variants: are they important?

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.

Detection of single nucleotide polymorphism in HTR4 and its relationship with growth traits in sheep

In this study, a single nucleotide polymorphism of HTR4 (hydroxytryptamine receptor 4) was detected using DNA sequencing and KASPar (Kompetitive Allele-Specific PCR) technique with the aim of analyzing its effect on growth traits in 1102 sheep. A synonymous mutation g.101220 C > T located on the fifth intron of the ovis HTR4 gene was detected, and association analysis showed that this mutation was significantly associated with growth traits in sheep (p ＜.05), with TT genotypes having significantly lower body weight, height, length and chest circumference than TC and CC genotypes. It showed that the polymorphism of this locus was significantly associated with growth traits in sheep. In addition, qRT-PCR results showed that HTR4 was expressed in different tissues of sheep. It is highly expressed in the liver, spleen and duodenum. As important metabolic, immune and digestive absorption organs in animals, the above tissues can regulate the excitability of intestinal smooth muscle by participating in the body metabolism and nutrient metabolism of sheep, so that sheep can show better growth characteristics. In conclusion, the polymorphic locus identified in HTR4 gene can be used as candidate molecular marker in sheep breeding.

Serotonin Receptors as a Potential Target in the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia worldwide that has an increasing impact on aging societies. Besides its critical role in the control of various physiological functions and behavior, brain serotonin (5-HT) system is involved in the regulation of migration, proliferation, differentiation, maturation, and programmed death of neurons. At the same time, a growing body of evidence indicates the involvement of 5-HT neurotransmission in the formation of insoluble aggregates of β-amyloid and tau protein, the main histopathological signs of AD. The review describes the role of various 5-HT receptors and intracellular signaling cascades induced by them in the pathological processes leading to the development of AD, first of all, in protein aggregation. Changes in the functioning of certain types of 5-HT receptors or associated intracellular signaling mediators prevent accumulation of β-amyloid plaques and tau protein neurofibrillary tangles. Based on the experimental data, it can be suggested that the use of 5-HT receptors as new drug targets will not only improve cognitive performance in AD, but will be also important in treating the causes of AD-related dementia.

Molecular basis of signal transduction mediated by the human GIPR splice variants

Glucose-dependent insulinotropic polypeptide receptor (GIPR) is a potential drug target for metabolic disorders. It works with glucagon-like peptide-1 receptor and glucagon receptor in humans to maintain glucose homeostasis. Unlike the other two receptors, GIPR has at least 13 reported splice variants (SVs), more than half of which have sequence variations at either C or N terminus. To explore their roles in endogenous peptide-mediated GIPR signaling, we determined the cryoelectron microscopy (cryo-EM) structures of the two N terminus-altered SVs (referred as GIPR-202 and GIPR-209 in the Ensembl database, SV1 and SV2 here, respectively) and investigated the outcome of coexpressing each of them in question with GIPR in HEK293T cells with respect to ligand binding, receptor expression, cAMP (adenosine 3,5-cyclic monophosphate) accumulation, β-arrestin recruitment, and cell surface localization. It was found that while both N terminus-altered SVs of GIPR neither bound to the hormone nor elicited signal transduction per se, they suppressed ligand binding and cAMP accumulation of GIPR. Meanwhile, SV1 reduced GIPR-mediated β-arrestin 2 responses. The cryo-EM structures of SV1 and SV2 showed that they reorganized the extracellular halves of transmembrane helices 1, 6, and 7 and extracellular loops 2 and 3 to adopt a ligand-binding pocket-occupied conformation, thereby losing binding ability to the peptide. The results suggest a form of signal bias that is constitutive and ligand-independent, thus expanding our knowledge of biased signaling beyond pharmacological manipulation (i.e., ligand specific) as well as constitutive and ligand-independent (e.g., SV1 of the growth hormone-releasing hormone receptor).

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Tegaserod: What's Old Is New Again

Irritable bowel syndrome with constipation (IBS-C) and chronic idiopathic constipation (CIC) are common gastrointestinal disorders imposing considerable impact on the quality of life and well-being of affected individuals. A paucity of evidence-based treatment options exist for CIC and IBS-C sufferers. Tegaserod, a 5-HT₄ agonist, has a substantial body of preclinical and clinical study evidence to support its beneficial role in modulating sensorimotor function of the luminal gastrointestinal tract. Tegaserod was first approved for use by the U.S. Food and Drug Administration for the management of IBS-C and CIC in 2002 and 2004, respectively. Tegaserod enjoyed a successful uptake in the management of these disorders during its first several years of availability in the United States, but was later withdrawn from the market in 2007 over concerns related to adverse cardiovascular events. Since then, additional safety data has been generated, and following a resubmission and review by the Food and Drug Administration, in April 2019, tegaserod was once again approved for use in IBS-C under a more restricted labeling, confining use to women under 65 years of age without heart disease or additional cardiovascular risk factors. This review summarizes the regulatory journey of tegaserod and details the existing pharmacokinetic, physiologic, clinical, and safety data of tegaserod generated over the last 2 decades. The discussion also examines the future of tegaserod in the treatment of these constipation disorders, as well as its potential role in other related disorders of brain-gut interaction.

Alternative Pre-mRNA Splicing of the Mu Opioid Receptor Gene, OPRM1: Insight into Complex Mu Opioid Actions

Most opioid analgesics used clinically, including morphine and fentanyl, as well as the recreational drug heroin, act primarily through the mu opioid receptor, a class A Rhodopsin-like G protein-coupled receptor (GPCR). The single-copy mu opioid receptor gene, OPRM1, undergoes extensive alternative splicing, creating multiple splice variants or isoforms via a variety of alternative splicing events. These OPRM1 splice variants can be categorized into three major types based on the receptor structure: (1) full-length 7 transmembrane (TM) C-terminal variants; (2) truncated 6TM variants; and (3) single TM variants. Increasing evidence suggests that these OPRM1 splice variants are pharmacologically important in mediating the distinct actions of various mu opioids. More importantly, the OPRM1 variants can be targeted for development of novel opioid analgesics that are potent against multiple types of pain, but devoid of many side-effects associated with traditional opiates. In this review, we provide an overview of OPRM1 alternative splicing and its functional relevance in opioid pharmacology.

The bidirectional effect of prelimbic 5-hydroxytryptamine type-4 (5-HT4) receptors on ACPA-mediated aversive memory impairment in adult male Sprague-Dawley rats

Objectives

This study aimed at investigating the effect of serotonergic 5-HT4 receptor agonist/antagonist on memory consolidation deficit induced by ACPA (a potent, selective CB₁ cannabinoid receptor agonist) in the pre-limbic (PL) cortex.

Materials and Methods

We used the step-through passive avoidance test to evaluate memory consolidation of male Sprague-Dawley (SD) rats. Bilateral post-training microinjections of the drugs were done in a volume of 0.6 μl/rat into the PL area (0.3 μl per side).

Results

The results showed a significant interaction between RS67333 hydrochloride (5-HT4 receptor agonist) or RS23597-190 hydrochloride (5-HT4 receptor antagonist) and ACPA on consolidation of aversive memory. RS67333 hydrochloride (0.5 μg/rat) enhanced consolidation of memory and its co-administration at the ineffective dose of 0.005 μg/rat with ineffective (0.001 μg/rat) or effective (0.1 μg/rat) doses of ACPA improved and prevented impairment of memory caused by ACPA, respectively. In other words, RS67333 had a bidirectional effect on ACPA-caused amnesia. While RS23597-190 hydrochloride had no effect on memory at the doses used (0.005, 0.01, 0.1, or 0.5 μg/rat); but its concomitant use with an effective dose of ACPA (0.1 μg/rat) potentiated amnesia. None of the drugs had an effect on locomotor activity.

Conclusion

This study revealed that activation or deactivation of the 5-HT4 receptors in the PL may mediate the IA memory impairment induced by ACPA indicating a modulatory role for the 5-HT4 serotonergic receptors.

Nano-analysis reveals high fraction of serotonin release during exocytosis from a gut epithelium model cell

Electrochemical methods were used to explore the exocytotic nature of serotonin (5‐HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut‐derived 5‐HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5‐HT release in BON cells is 80 % compared to previous work in pancreatic beta cells (34 %). The fractional release increased from 80 % in control BON cells to 87 % with 5‐HT preincubation and nearly 100 % with the combination of 5‐HT and the 5‐HT₄ autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5‐HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Comparative analysis of palmitoylation sites of serotonin (5-HT) receptors in vertebrates

Background

In the vertebrate central nervous system as well as in the periphery, serotonin, also known as 5‐hydroxytriptamine (5‐HT), function as a neurotransmitter, a hormone or a mitogen. 5‐HT receptors are composed of 7 family 5‐HT_1‐7 receptors, comprising of 14 structurally and pharmacologically distinct 5‐HT receptor subtypes. Previous experimental studies showed that mouse 5‐HT_1A, 5‐HT₄ and 5‐HT₇ receptors are regulated by post‐translational protein palmitoylation, the reversible attachment of the lipid palmitate to intracellular cysteine residues. Here, we further focused on conservation of these putative palmitoylation sites found in vertebrate 5‐HT receptor orthologs.

Methods and Results

Analysis of sequence databases provides evidence to suggest that palmitoylation sites of these 5‐HT receptors have been extremely conserved in the vertebrate lineages from jawless fishes to human, in spite of the divergence of 5‐HT_1A, 5‐HT₄ or 5‐HT₇ receptors full‐length amino acid sequences during molecular evolution.

Conclusion

Our findings mean that dynamic regulation of 5‐HT receptors made possible by reversible post‐translational protein palmitoylation may be critical for refined functions of the vertebrate serotonergic systems.

Reversible post‐translational protein palmitoylation sites of 5‐HT1A, 5‐HT4, and 5‐HT7 receptors have been extremely conserved in the vertebrate lineages against mutation pressure.

Medium-Throughput Screen of Microbially Produced Serotonin via a G-Protein-Coupled Receptor-Based Sensor

Chemical biosensors, for which chemical detection triggers a fluorescent signal, have the potential to accelerate the screening of noncolorimetric chemicals produced by microbes, enabling the high-throughput engineering of enzymes and metabolic pathways. Here, we engineer a G-protein-coupled receptor (GPCR)-based sensor to detect serotonin produced by a producer microbe in the producer microbe's supernatant. Detecting a chemical in the producer microbe's supernatant is nontrivial because of the number of other metabolites and proteins present that could interfere with sensor performance. We validate the two-cell screening system for medium-throughput applications, opening the door to the rapid engineering of microbes for the increased production of serotonin. We focus on serotonin detection as serotonin levels limit the microbial production of hydroxystrictosidine, a modified alkaloid that could accelerate the semisynthesis of camptothecin-derived anticancer pharmaceuticals. This work shows the ease of generating GPCR-based chemical sensors and their ability to detect specific chemicals in complex aqueous solutions, such as microbial spent medium. In addition, this work sets the stage for the rapid engineering of serotonin-producing microbes.

Liver 5-HT7 receptors: A novel regulator target of fibrosis and inflammation-induced chronic liver injury in vivo and in vitro

BACKGROUND AND AIM

Hepatocellular cancer (HCC) is the sixth most common cancer and liver fibrosis is strongly associated with HCC. Treatment options are limited, and preventive strategies should be developed. An important step in the beginning of liver fibrosis is a strong inflammatory response. 5-HT7 is the last recognized member of the serotonin receptor family and is expressed in both central nerve system and peripheral system and have a lot of functions like learning, memory, smooth muscular relaxation, in the control of circadian rhythms and thermoregulation, pain and migraine, schizophrenia, anxiety, cognitive disturbances, and even inflammation.

METHODS

We therefore examined the biochemical, histopathological and molecular effects of the 5-HT7 receptor agonist and antagonist on inflammatory liver fibrogenesis in animal models of progressive cirrhosis: a mouse model induced by carbon tetrachloride (CCl4) and in Hep3b cells.

RESULTS

5-HT7 expression was observed in the liver in vivo and in vitro in CCl4-induced damage. 5-HT7 receptor agonist but not the antagonist reduced liver markers in mice and in Hep3b cells in carbon tetrachloride (CCl4) induced damage. 5-HT7 agonist, but not antagonist, protected liver tissue from oxidative stress in fibrosis. 5-HT7 agonist but not antagonist induces anti-inflammatory, anti-fibrinotic and anti-cytokine features in liver fibrosis in vivo and in vitro.

CONCLUSIONS

5-HT7 receptors have modulatory function and are an up-and-coming pharmacological target in the inflammatory fibrotic process. 5-HT7 receptor agonist LP-44 showed significant hepatoprotective effects against liver fibrosis, and LP-44 might become a useful therapeutic target for chronic liver inflammation and fibrosis.

Prucalopride exerts neuroprotection in human enteric neurons

Serotonin (5-hydroxytryptamine, 5-HT) and its transporters and receptors are involved in a wide array of digestive functions. In particular, 5-HT4 receptors are known to mediate intestinal peristalsis and recent data in experimental animals have shown their role in neuronal maintenance and neurogenesis. This study has been designed to test whether prucalopride, a well-known full 5-HT4 agonist, exerts protective effects on neurons, including enteric neurons, exposed to oxidative stress challenge. Sulforhodamine B assay was used to determine the survival of SH-SY5Y cells, human enteric neurospheres, and ex vivo submucosal neurons following H2O2 exposure in the presence or absence of prucalopride (1 nM). Specificity of 5-HT4-mediated neuroprotection was established by experiments performed in the presence of GR113808, a 5-HT4 antagonist. Prucalopride exhibited a significant neuroprotective effect. SH-SY5Y cells pretreated with prucalopride were protected from the injury elicited by H2O2 as shown by increased survival (73.5 ± 0.1% of neuronal survival vs. 33.3 ± 0.1%, respectively; P < 0.0001) and a significant reduction of proapoptotic caspase-3 and caspase-9 activation in all neurons tested. The protective effect of prucalopride was reversed by the specific 5-HT4 antagonist GR113808. Prucalopride promotes a significant neuroprotection against oxidative-mediated proapoptotic mechanisms. Our data pave the way for novel therapeutic implications of full 5-HT4 agonists in gut dysmotility characterized by neuronal degeneration, which go beyond the well-known enterokinetic effect.

Alternative Splicing of G Protein-Coupled Receptors: Relevance to Pain Management

Drugs that target G protein-coupled receptors (GPCRs) represent the primary treatment strategy for patients with acute and chronic pain; however, there is substantial individual variability in both the efficacy and adverse effects associated with these drugs. Variability in drug responses is due, in part, to individuals' diversity in alternative splicing of pain-relevant GPCRs. G protein-coupled receptor alternative splice variants often exhibit distinct tissue distribution patterns, drug-binding properties, and signaling characteristics that may impact disease pathology as well as the extent and direction of analgesic effects. We review the importance of GPCRs and their known splice variants to the management of pain.

Distribution of 5-HT3, 5-HT4, and 5-HT7 Receptors Along the Human Colon

Background/Aims

Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT₃ and 5-HT₄ receptors are clinically relevant. The aim was to examine the distribution of 5-HT₃, 5-HT₄, and 5-HT₇ receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine.

Methods

Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction.

Results

The 5-HT₃ receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT₃ receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT₄ and 5-HT₇ receptors were expressed throughout the colon although the 5-HT₄ receptor d, g, and i variants were expressed less often.

Conclusions

The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT₃ receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.

Phosphodiesterase 4 interacts with the 5-HT4(b) receptor to regulate cAMP signaling

Phosphodiesterase (PDE) 3 and PDE4, which degrade cyclic adenosine monophosphate (cAMP), are important regulators of 5-hydroxytryptamine (5-HT) 4 receptor signaling in cardiac tissue. Therefore, we investigated whether they interact with the 5-HT4(b) receptor, and whether A-kinase anchoring proteins (AKAPs), scaffolding proteins that bind to the regulatory subunit of protein kinase A (PKA) and contribute to the spacial-temporal control of cAMP signaling, are involved in the regulation of 5-HT4(b) receptor signaling. By measuring PKA activity in the absence and presence of PDE3 and PDE4 inhibitiors, we found that constitutive signaling of the overexpressed HA-tagged 5-HT4(b) receptor in HEK293 cells is regulated predominantly by PDE4, with a secondary role for PDE3 that is unmasked in the presence of PDE4 inhibition. Overexpressed PDE4D3 and PDE3A1, and to a smaller extent PDE4D5 co-immunoprecipitate constitutively with the 5-HT4(b) receptor. PDE activity measurements in immunoprecipitates of the 5-HT4(b) receptor confirm the association of PDE4D3 with the receptor and provide evidence that the activity of this PDE may be increased upon receptor stimulation with 5-HT. A possible involvement of AKAPs in 5-HT4(b) receptor signaling was uncovered in experiments using the St-Ht31 inhibitor peptide, which disrupts the interaction of AKAPs with PKA. However, St-Ht31 did not influence 5-HT4(b) receptor-stimulated PKA activity, and endogenous AKAP79 and gravin were not found in immunoprecipitates of the 5-HT4(b) receptor. In conclusion, we found that both PDE3A1 and PDE4D3 are integrated into complexes that contain the 5-HT4(b) receptor and may thereby regulate 5-HT4(b) receptor-mediated signaling.

The serotonin 5-HT7 receptors: two decades of research

Like most neurotransmitters, serotonin possesses a simple structure. However, the pharmacological consequences are more complex and diverse. Serotonin is involved in numerous functions in the human body including the control of appetite, sleep, memory and learning, temperature regulation, mood, behavior, cardiovascular function, muscle contraction, endocrine regulation, and depression. Low levels of serotonin may be associated with several disorders, namely increase in aggressive and angry behaviors, clinical depression, Parkinson's disease, obsessive-compulsive disorder, eating disorders, migraine, irritable bowel syndrome, tinnitus, and bipolar disease. These effects are mediated via different serotonin (5-HT) receptors. In this review, we will focus on the last discovered member of this serotonin receptor family, the 5-HT7 receptor. This receptor belongs to the G protein-coupled receptor superfamily and was cloned two decades ago. Later, different splice variants were described but no major functional differences have been described so far. All 5-HT7 receptor variants are coupled to Gαs proteins and stimulate cAMP formation. Recently, several interacting proteins have been reported, which can influence receptor signaling and trafficking.

HTR4 gene structure and altered expression in the developing lung

BACKGROUND

Meta-analyses of genome-wide association studies (GWAS) have identified single nucleotide polymorphisms (SNPs) spanning the 5-hydroxytryptamine receptor 4 (5-HT₄R) gene (HTR4) associated with lung function. The aims of this study were to i) investigate the expression profile of HTR4 in adult and fetal lung tissue and cultured airway cells, ii) further define HTR4 gene structure and iii) explore the potential functional implications of key SNPs using a bioinformatic approach.

METHODS

Following reverse transcription (RT)-PCR in human brain, 5' rapid amplification of cDNA ends (5' RACE) was used to examine the exonic structure of HTR4 at the 5' end. Quantitative (Q)-PCR was used to quantify HTR4 mRNA expression in total RNA from cultured airway cells and whole lung tissue. Publically available gene microarray data on fetal samples of estimated gestational age 7-22 weeks were mined for HTR4 expression. Immunohistochemistry (IHC; in adult and fetal lung tissue) and a radioligand binding assay (in cultured airway cells) were used to analyze 5-HT₄R protein expression.

RESULTS

IHC in adult lung, irrespective of the presence of chronic obstructive pulmonary disease (COPD), suggested low level expression of 5-HT₄R protein, which was most prominent in alveolar pneumocytes. There was evidence of differential 5-HT₄R protein levels during gestation in fetal lung, which was also evident in gene expression microarray data. HTR4 mRNA expression, assessed by Q-PCR, was <0.5% relative to brain in total adult lung tissue and in human airway smooth muscle (HASM) and bronchial epithelial cells (HBEC) derived from adult donors. Radioligand binding experiments also indicated that HBEC and HASM cells did not express a significant 5-HT₄R population. 5' RACE in brain identified a novel N-terminal variant, containing an extended N-terminal sequence. The functional significance of key HTR4 SNPs was investigated using the encyclopedia of DNA elements consortium (ENCODE) dataset. These analyses identified multiple alterations in regulatory motifs for transcription factors implicated in lung development, including Foxp1.

CONCLUSIONS

Taken together, these data suggest a role for HTR4 in lung development, which may at least in part explain the genetic association with lung function.

Linear motifs: lost in (pre)translation

Pretranslational modification by alternative splicing, alternative promoter usage and RNA editing enables the production of multiple protein isoforms from a single gene. A large quantity of data now supports the notion that short linear motifs (SLiMs), which are protein interaction modules enriched within intrinsically disordered regions, are key for the functional diversification of these isoforms. The inclusion or removal of these SLiMs can switch the subcellular localisation of an isoform, promote cooperative associations, refine the affinity of an interaction, coordinate phase transitions within the cell, and even create isoforms of opposing function. This article discusses the novel functionality enabled by the addition or removal of SLiM-containing exons by pretranslational modifications, such as alternative splicing and alternative promoter usage, and how these alterations enable the creation and modulation of complex regulatory and signalling pathways.

Assessment of the pharmacological properties of 5-methoxyindole derivatives at 5-HT4 receptors

Objectives

The aim was to examine the biological activity of 5-methoxytryptamine derivatives at the 5-hydroxytryptamine (5-HT)4 receptor to explore the effect of substitution on the aliphatic amine of the 5-methoxyamine scaffold.

Methods

Three compounds were tested for affinity at the 5-HT4 receptor by radioligand binding and functional activity using guinea-pig ileum and human colon circular muscle preparations and also in the mouse whole gut transit test.

Key findings

The three compounds all had agonist properties at the 5-HT4 receptor but their efficacy differed in the different functional tests. Compound 3 had the highest affinity for the 5-HT4 receptor and was a full agonist at relaxing human colon circular muscle with efficacy closest to 5-HT. Compounds 1 and 2 were partial agonists in this assay with lower efficacies; compound 2 was a full agonist in the guinea-pig ileum assay whereas compound 3 was a partial agonist. Compounds 1 and 2 also showed activity in the mouse gut transit assay while compound 3 had no activity.

Conclusions

Of the compounds tested, compound 3 was the most promising 5-HT4 receptor agonist and the results highlight the value of using human tissue in functional tests when assessing compounds for potential activity.

Constipation, IBs and the 5-HT4 Receptor: What Role for Prucalopride?

After the problems associated with the non-selective 5-HT4 receptor agonists cisapride and tegaserod, the 5-HT4 receptor is now beginning to come in from the cold. Thus, prucalopride is now the first of a new class of drug defined by selectivity and high intrinsic activity at the 5-HT4 receptor. Prucalopride has been developed for treatment of chronic constipation rather than constipation-predominant irritable bowel syndrome (IBS). This follows the trend of first evaluating new gastrointestinal (GI) prokinetic drugs in disorders where disrupted GI motility is known to exist, rather than in a functional bowel disorder where changes in motility are uncertain. If prucalopride is not progressed towards the IBS indication, it has at least shown the way for other selective 5-HT4 receptor agonists. Most notable among these is TD-5108 (velusetrag), also characterized by good selectivity at the 5-HT4 receptor, high intrinsic activity and efficacy in patients with chronic constipation.

Tissue dependent differences in G-protein coupled receptor kinases associated with 5-HT4 receptor desensitization in the rat gastro-intestinal tract

Desensitization of 5-HT(4) receptors is regulated by G-protein coupled receptor kinases (GRKs). However, the specific GRK(s) that regulates the desensitization of 5-HT(4) receptors in the in vivo setting is unknown. We investigated the in situ expression of 5-HT(4) receptors and the GRKs in the rat gastrointestinal tract using immunohistochemistry and their interaction using coimmunoprecipitation. 5-HT(4) receptors were expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, myenteric plexus, circular muscle, submucosal plexus and muscularis mucosae of both the proximal and distal colon. GRK2 was expressed in longitudinal muscle and occasionally in myenteric plexus whilst GRK5 showed limited expression in the nerve endings of the myenteric plexus and submucosal plexus of the colon. GRK3 was expressed in the tunica muscularis mucosae of the oesophagus, circular muscle, submucosal plexus and muscularis mucosae of the colon. GRK6 was expressed in the tunica muscularis mucosae of the oesophagus, longitudinal muscle, circular muscle, and muscularis mucosae of the colon. Stimulation of tunica muscularis mucosae of the oesophagus and distal colon using the 5-HT(4) receptor agonist, tegaserod, followed by analysis of the 5-HT(4) receptor antibody immunoprecipitate, revealed the coimmunoprecipitation of GRK6 with 5-HT(4) receptors in the tunica muscularis mucosae of oesophagus while GRK2 and GRK6 were coimmunoprecipitated with 5-HT(4) receptors in the distal colon. This study indicates that GRK6 may be involved in the regulation of the desensitization of 5-HT(4) receptors in the rat oesophagus whilst GRK2 and GRK6 may be involved in regulation of the desensitization of 5-HT(4) receptors in the distal colon.

Translating 5-HT receptor pharmacology

Since metoclopramide was first described (in 1964) there have been several attempts to develop compounds which retained gastrointestinal prokinetic activity (via 5-HT(4) receptor activation) but without the limiting side effects associated with dopamine D(2) receptor antagonism. Early compounds (mosapride, cisapride, renzapride, tegaserod) were identified before several of the 5-HT receptors were even described (including 5-HT(4) and 5-HT(2B)), whereas prucalopride came later. Several compounds were hampered by non-selectivity, introducing cardiac liability (cisapride: activity at human Ether-a-go-go Related Gene) or potentially, a reduced intestinal prokinetic activity caused by activity at a second 5-HT receptor (renzapride: antagonism at the 5-HT(3) receptor; tegaserod: antagonism at the 5-HT(2B) receptor). Poor intrinsic activity at gastrointestinal 5-HT(4) receptors has also been an issue (mosapride, tegaserod). Perhaps prucalopride has now achieved the profile of good selectivity of action and high intrinsic activity at intestinal 5-HT(4) receptors, without clinically-meaningful actions on 5-HT(4) receptors in the heart. The progress of this compound for treatment of chronic constipation, as well as competitor molecules such as ATI-7505 and TD-5108, will now be followed with interest as each attempts to differentiate themselves from each other. Perhaps at last, 5-HT(4) receptor agonists are being given the chance to show what they can do.

Translating 5-HT receptor pharmacology

Since metoclopramide was first described (in 1964) there have been several attempts to develop compounds which retained gastrointestinal prokinetic activity (via 5-HT(4) receptor activation) but without the limiting side effects associated with dopamine D(2) receptor antagonism. Early compounds (mosapride, cisapride, renzapride, tegaserod) were identified before several of the 5-HT receptors were even described (including 5-HT(4) and 5-HT(2B)), whereas prucalopride came later. Several compounds were hampered by non-selectivity, introducing cardiac liability (cisapride: activity at human Ether-a-go-go Related Gene) or potentially, a reduced intestinal prokinetic activity caused by activity at a second 5-HT receptor (renzapride: antagonism at the 5-HT(3) receptor; tegaserod: antagonism at the 5-HT(2B) receptor). Poor intrinsic activity at gastrointestinal 5-HT(4) receptors has also been an issue (mosapride, tegaserod). Perhaps prucalopride has now achieved the profile of good selectivity of action and high intrinsic activity at intestinal 5-HT(4) receptors, without clinically-meaningful actions on 5-HT(4) receptors in the heart. The progress of this compound for treatment of chronic constipation, as well as competitor molecules such as ATI-7505 and TD-5108, will now be followed with interest as each attempts to differentiate themselves from each other. Perhaps at last, 5-HT(4) receptor agonists are being given the chance to show what they can do.

Distribution of serotonin receptors and interacting proteins in the human sigmoid colon

This study aimed to examine the distribution of 5-HT receptors in the human colon. 5-HT induces desensitization of the circular muscle and as this is facilitated by G-protein coupled receptor kinases (GRKs) and other proteins, we also examined their distribution. Human sigmoid colon samples were dissected into three separate layers (mucosa, taeniae coli and intertaenial strips) and RNA was amplified by RT-PCR. The 5-HT(2B) receptor and all 5-HT(7) receptor splice variants were expressed in all tissues. 5-HT(4) a,b,c and n splice variants were also expressed in all tissues and 5-HT(4d), 5-HT(4g) and 5-HT(4i) were only detected in some samples. The 5-HT(2A) receptor was seen predominantly in the intertaenial strips of the colon. Only one transcript of the serotonin transporter (SERT) was detected in the muscle layers. Variation was seen in GRK expression with GRK2 and 3 predominantly expressed in the mucosa, while GRK5 and 6 were found more commonly in the taeniae coli. PDZ (named after postsynaptic density protein, Drosophila disc large tumour suppressor and tight junction protein ZO-1) domain containing proteins, which may be involved in 5-HT receptor trafficking, were also detected throughout the sigmoid colon. The 5-HT(3A) subunit was expressed in all tissues, whereas the 5-HT(3E) subunit was mainly found in the mucosa layer while the 5-HT(3B) subunit was more common in the muscle layers. Receptor interacting chaperone (RIC-3), which is involved in transporting 5-HT(3) receptor subunits, is expressed less in mucosa compared to muscle layers. In conclusion, these results show that there is variation in distribution of 5-HT receptors and interacting proteins within the sigmoid colon that may contribute to colonic function.

Synthesis of Quinoline Derivatives as 5-HT4 Receptor Ligands

A general and convenient synthesis of 6-methoxyquinoline-3-carboxamides commencing with a cyclization step that involves ρ-anisidine and diethyl (ethoxymethylene)malonate is described. An additional tetrahydroquinoline scaffold 19 is prepared from 6-methoxyquinoline-3-carboxamide and this represents a novel serotinergic lead structure. These compounds show reasonable affinity at 1 × 10–6 M, and docking experiments suggest that they may bind in a similar manner to serotonin.

Cloning, genomic organization and functionality of 5-HT(7) receptor splice variants from mouse brain

The serotonin (5-HT) 5-HT(7) receptors are expressed in both the central nervous system and in peripheral tissues. Receptor distribution studies and pharmacological studies have established that 5-HT(7) receptors play an important role in the control of circadian rhythms and thermoregulation. Selective 5-HT(7) receptor ligands have potential therapeutic applications for the treatment of pain and migraine, schizophrenia, anxiety, cognitive disturbances and inflammation. We have cloned two novel C-terminal splice variants of the 5-HT(7) receptor from mouse brain. These two new splice variants have almost identical sequences as the rat 5-HT(7(b)) and 5-HT(7(c)) splice variants and so were given the same name. Ligand binding assays ([(3)H]5-CT), membrane localization and functional studies in transiently transfected cells indicated that all three splice variants are well expressed on the membrane and no major differences in their respective pharmacology and their ability to activate adenylyl cyclase were observed. This is in analogy with previous reports comparing either the rat or the human variants.