Serotonin 5-HT1B and 5-HT1D receptors form homodimers when expressed alone and heterodimers when co-expressed

Predicting therapeutic and side effects from drug binding affinities to human proteome structures

Evaluation of the binding affinities of drugs to proteins is a crucial process for identifying drug pharmacological actions, but it requires three dimensional structures of proteins. Herein, we propose novel computational methods to predict the therapeutic indications and side effects of drug candidate compounds from the binding affinities to human protein structures on a proteome-wide scale. Large-scale docking simulations were performed for 7,582 drugs with 19,135 protein structures revealed by AlphaFold (including experimentally unresolved proteins), and machine learning models on the proteome-wide binding affinity score (PBAS) profiles were constructed. We demonstrated the usefulness of the method for predicting the therapeutic indications for 559 diseases and side effects for 285 toxicities. The method enabled to predict drug indications for which the related protein structures had not been experimentally determined and to successfully extract proteins eliciting the side effects. The proposed method will be useful in various applications in drug discovery.

Functional Dimerization of Serotonin Receptors: Role in Health and Depressive Disorders

Understanding the neurobiological underpinnings of depressive disorder constitutes a pressing challenge in the fields of psychiatry and neurobiology. Depression represents one of the most prevalent forms of mental and behavioral disorders globally. Alterations in dimerization capacity can influence the functional characteristics of serotonin receptors and may constitute a contributing factor to the onset of depressive disorders. The objective of this review is to consolidate the current understanding of interactions within the 5-HT receptor family and between 5-HT receptors and members of other receptor families. Furthermore, it aims to elucidate the role of such complexes in depressive disorders and delineate the mechanisms through which antidepressants exert their effects.

Modulation of Neuron and Astrocyte Dopamine Receptors via Receptor-Receptor Interactions

Dopamine neurotransmission plays critical roles in regulating complex cognitive and behavioral processes including reward, motivation, reinforcement learning, and movement. Dopamine receptors are classified into five subtypes, widely distributed across the brain, including regions responsible for motor functions and specific areas related to cognitive and emotional functions. Dopamine also acts on astrocytes, which express dopamine receptors as well. The discovery of direct receptor-receptor interactions, leading to the formation of multimeric receptor complexes at the cell membrane and providing the cell decoding apparatus with flexible dynamics in terms of recognition and signal transduction, has expanded the knowledge of the G-protein-coupled receptor-mediated signaling processes. The purpose of this review article is to provide an overview of currently identified receptor complexes containing dopamine receptors and of their modulatory action on dopamine-mediated signaling between neurons and between neurons and astrocytes. Pharmacological possibilities offered by targeting receptor complexes in terms of addressing neuropsychiatric disorders associated with altered dopamine signaling will also be briefly discussed.

First quantitative dosages: Strong correlations between non-5-HT2Rs serotonin receptors on normal human heart valves

Objectives

Although critical in animal and human development and pathology, a measurement of the quantitative expression of 5-HTR serotonin receptors on animal or human valvular tissues has never been performed.

Methods

Quantification of the most frequent 5-HTRs reported as being present in human peripheral tissue was performed using radiolabeled agonists/antagonists. A membrane protein extract from normal human valves (aortic/mitral/tricuspid and some pulmonary) and associated diseased left myocardium, all unusable in clinics, were obtained from the Homograft bank.

Results

We analyzed 5-HT1AR/5-HT1B/DR/5-HT2AR/5-HT2BR/5-HT 2CR/5-HT4R/5-HT7R from 28 hearts. We confirmed the presence of tissue and measured the quantitative content for respective proteins in femtomol/mg of protein extracts: for 5-HT2AR (35.9+/−0.7), 5-HT2BR (28.8+/−1.3) but also a newly observed and robust expression for 5-HT4R (38+/−4.2). We identified one, 5-HT1ARs (4.9+/−0.3), and the possible expression, but at a very low level, of previously reported 5-HT1B/DRs (1.3+/−0.5) as well as the new 5-HT7Rs (3.5+/0.1) and 5-HT2CRs (1.2+/−0.1). Interestingly, by using univariate analysis, we were able to observe many correlations between the different 5-HTR levels of expression especially between 5-HT1AR/5-HT1B/DR and also between 5-HT4R/5-HT7R, but none were observed between 5-HT2AR and 5-HT2BR. Using multivariate analyses for a specific 5-HTR level of expression, after adjustment for implantation sites and other 5-HTRs, we found that 5-HT1AR was correlated with 5-HT1B/DR;5-HT4R with 5-HT7R and 5-HT1AR;5-HT2BR with 5-HT2AR only. For 5-HT2C, no correlation was observed.

Conclusion

5-HT2AR/5-HT2BR and 5-HT4R were all observed to have a high and equal level of expression on human valves, but that of 5-HT1AR was more limited. Since these non-5-HT2Rs are coupled with different G-proteins, with specific signaling, theoretically they may control the main 5-HT2R signaling (i.e., PLC/DAG-PKC-ERK/Ras/Src signaling) involved in valvular fibrosis and degeneration.

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor-receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

Ste2p Under the Microscope: the Investigation of Oligomeric States of a Yeast G Protein-Coupled Receptor

Oligomerization of G protein-coupled receptors (GPCRs) may play important roles in maturation, internalization, signaling, and pharmacology of these receptors. However, the nature and extent of their oligomerization is still under debate. In our study, Ste2p, a yeast mating pheromone GPCR, was tagged with enhanced green fluorescent protein (EGFP), mCherry, and with split florescent protein fragments at the receptor C-terminus. The Förster resonance energy transfer (FRET) technique was used to detect receptors' oligomerization by calculating the energy transfer from EGFP to mCherry. Stimulation of Ste2p oligomers with the receptor ligand did not result in any significant change on observed FRET values. The bimolecular fluorescence complementation (BiFC) assay was combined with FRET to further investigate the tetrameric complexes of Ste2p. Our results suggest that in its quiescent (nonligand-activated) state, Ste2p is found at least as a tetrameric complex on the plasma membrane. Intriguingly, receptor tetramers in their active form showed a significant increase in FRET. This study provides a direct in vivo visualization of Ste2p tetramers and the pheromone effect on the extent of the receptor oligomerization.

Receptor-Receptor Interactions and Glial Cell Functions with a Special Focus on G Protein-Coupled Receptors

The discovery that receptors from all families can establish allosteric receptor-receptor interactions and variably associate to form receptor complexes operating as integrative input units endowed with a high functional and structural plasticity has expanded our understanding of intercellular communication. Regarding the nervous system, most research in the field has focused on neuronal populations and has led to the identification of many receptor complexes representing an important mechanism to fine-tune synaptic efficiency. Receptor-receptor interactions, however, also modulate glia-neuron and glia-glia intercellular communication, with significant consequences on synaptic activity and brain network plasticity. The research on this topic is probably still at the beginning and, here, available evidence will be reviewed and discussed. It may also be of potential interest from a pharmacological standpoint, opening the possibility to explore, inter alia, glia-based neuroprotective therapeutic strategies.

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.

G-protein signaling is required for increasing germline stem cell division frequency in response to mating in Drosophila males

Adult stem cells divide to renew the stem cell pool and replenish specialized cells that are lost due to death or usage. However, little is known about the mechanisms regulating how stem cells adjust to a demand for specialized cells. A failure of the stem cells to respond to this demand can have serious consequences, such as tissue loss, or prolonged recovery post injury. Here, we challenge the male germline stem cells (GSCs) of Drosophila melanogaster for the production of specialized cells, sperm cells, using mating experiments. We show that repeated mating reduced the sperm pool and increased the percentage of GSCs in M- and S-phase of the cell cycle. The increase in dividing GSCs depended on the activity of the highly conserved G-proteins. Germline expression of RNA-Interference (RNA-i) constructs against G-proteins, or a dominant negative G-protein eliminated the increase in GSC division frequency in mated males. Consistent with a role for the G-proteins in regulating GSC division frequency, RNA-i against seven out of 35 G-protein coupled receptors (GPCRs) within the germline cells also eliminated the capability of males to increase the numbers of dividing GSCs in response to mating.

5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and β-Arrestin Proteins

5-HT_1B receptors modulate synaptic serotonin (5-HT) levels and play a significant role in the regulation of emotional behaviors. These receptors are Gα_i/o-coupled and inhibit adenylyl cyclase but have also been reported to activate MAP kinases; however, the details of signaling cascades downstream of 5-HT_1B receptor activation remain unclear, particularly in neuronal cells. We generated a stable 5-HT_1B receptor-expressing Neuro2A (N2A-1B) neuronal cell line and demonstrate that activation of these receptors by the selective 5-HT_1B agonist CP-94253 results in activation of ERK1/2 but not of other closely related MAP kinases. Phosphoproteomics revealed four novel phosphorylation sites on the third intracellular loop of the 5-HT_1B receptor, and mutations of serine-256 and serine-291 to alanine led to reduced levels of ERK1/2 phosphorylation following receptor activation. Inhibition of Gα_i/o signaling with pertussis toxin, as well as MEK1/2 inhibition with U0126, also reduced 5-HT_1B-mediated ERK1/2 phosphorylation. Finally, we found that knockout of either β-arrestin 1 or β-arrestin 2 prevented 5-HT_1B-mediated phosphorylation of ERK1/2. Taken together, these results show that 5-HT_1B receptor activation selectively induces ERK1/2 activation through both the Gα_i subunit and β-arrestin proteins. This work elucidates the signal transduction pathway of 5-HT_1B receptors, as well as key phosphorylation sites within the receptor that modulate ERK1/2 activation, and further characterizes the intracellular mechanisms that underlie 5-HT_1B receptor function.

Dimers of serotonin receptors: Impact on ligand affinity and signaling

Membrane receptors often form complexes with other membrane proteins that directly interact with different effectors of the signal transduction machinery. G-protein-coupled receptors (GPCRs) were for long time considered as single pharmacological entities. However, evidence for oligomerization appeared for various classes and subtypes of GPCRs. This review focuses on metabotropic serotonin (5-hydroxytryptamine, 5-HT) receptors, which belong to the rhodopsin-like class A of GPCRs, and will summarize the convergent evidence that homo- and hetero-dimers containing 5-HT receptors exist in transfected cells and in-vivo. We will show that complexes involving 5-HT receptors may acquire new signal transduction pathways and new physiological roles. In some cases, these complexes participate in disease-specific deregulations, that can be differentially affected by various drugs. Hence, selecting receptor complex-specific responses of these heterodimers may constitute an emerging strategy likely to improve beneficial therapeutic effects.

G protein-coupled receptor-receptor interactions give integrative dynamics to intercellular communication

The proposal of receptor-receptor interactions (RRIs) in the early 1980s broadened the view on the role of G protein-coupled receptors (GPCR) in the dynamics of the intercellular communication. RRIs, indeed, allow GPCR to operate not only as monomers but also as receptor complexes, in which the integration of the incoming signals depends on the number, spatial arrangement, and order of activation of the protomers forming the complex. The main biochemical mechanisms controlling the functional interplay of GPCR in the receptor complexes are direct allosteric interactions between protomer domains. The formation of these macromolecular assemblies has several physiologic implications in terms of the modulation of the signaling pathways and interaction with other membrane proteins. It also impacts on the emerging field of connectomics, as it contributes to set and tune the synaptic strength. Furthermore, recent evidence suggests that the transfer of GPCR and GPCR complexes between cells via the exosome pathway could enable the target cells to recognize/decode transmitters and/or modulators for which they did not express the pertinent receptors. Thus, this process may also open the possibility of a new type of redeployment of neural circuits. The fundamental aspects of GPCR complex formation and function are the focus of the present review article.

5-HT Receptor Nomenclature: Naming Names, Does It Matter? A Tribute to Maurice Rapport

The naming of 5-HT receptors has been challenging, especially in the early days when the concept of multiple receptors for a single neurotransmitter was considered to be unrealistic at best. Yet pharmacological (rank orders of potency in functional or biochemical settings) and transductional evidence (second messengers, electrophysiology) clearly indicated the existence of receptor families and subfamilies. The genetic revolution, with the cloning and study of recombinantly expressed receptors, and eventually the cloning of the human and other genomes have made such reservations obsolete. Further, the advances in structural biology, with the possibility to study ligand receptor complexes as crystals and/or using solution NMR have largely confirmed the complexity of the 5-HT receptor system: species differences, existence of multiple receptor active and inactive states, splice variants, editing variants, complexes with multiple interacting proteins and transduction bias. This is a short personal history on how advances in biochemistry, molecular biology, biophysics, imaging and medicinal chemistry, some lateral thinking, and a decent amount of collaborative spirit within the 5-HT receptor nomenclature committee and the 5-HT community at large have helped to better define the pharmacology of the 5-HT receptor family.

Sequential Co-immunoprecipitation and Immunoblot Approach to Determine Oligomerisation of G-Protein-Coupled Receptors

G-protein-coupled receptors (GPCRs) play a major role in psychiatric disorders and are the targets of several current therapeutic approaches in this field. A number of studies have now shown that GPCRs can assemble as high molecular weight homo- and hetero-oligomers, which could affect ligand binding, intracellular signalling or trafficking. This information could be critical in design of new drugs to treat neurological and psychiatric disorders. This chapter describes a sequential co-immunoprecipitation and immunoblot protocol for determining oligomerisation of the 5-hydroxytryptamine (HT)_1A receptor with other GPCRs in co-transfected HEK-293 cells.

Heterodimers of serotonin receptor subtypes 2 are driven by 5-HT2C protomers

The serotonin receptor subtypes 2 comprise 5-HT_2A, 5-HT_2B, and 5-HT_2C, which are Gα_q-coupled receptors and display distinct pharmacological properties. Although co-expressed in some brain regions and involved in various neurological disorders, their functional interactions have not yet been studied. We report that 5-HT₂ receptors can form homo- and heterodimers when expressed alone or co-expressed in transfected cells. Co-immunoprecipitation and bioluminescence resonance energy transfer studies confirmed that 5-HT_2C receptors interact with either 5-HT_2A or 5-HT_2B receptors. Although heterodimerization with 5-HT_2C receptors does not alter 5-HT_2C Gα_q-dependent inositol phosphate signaling, 5-HT_2A or 5-HT_2B receptor-mediated signaling was totally blunted. This feature can be explained by a dominance of 5-HT_2C on 5-HT_2A and 5-HT_2B receptor binding; in 5-HT_2C-containing heterodimers, ligands bind and activate the 5-HT_2C protomer exclusively. This dominant effect on the associated protomer was also observed in neurons, supporting the physiological relevance of 5-HT₂ receptor heterodimerization in vivo Accordingly, exogenous expression of an inactive form of the 5-HT_2C receptor in the locus ceruleus is associated with decreased 5-HT_2A-dependent noradrenergic transmission. These data demonstrate that 5-HT₂ receptors can form functionally asymmetric heterodimers in vitro and in vivo that must be considered when analyzing the physiological or pathophysiological roles of serotonin in tissues where 5-HT₂ receptors are co-expressed.

Distribution of 5-HT1F Receptors in Monkey Vestibular and Trigeminal Ganglion Cells

Background

Evidence of serotonergic involvement in vestibular pathway contributions to migraine and balance disorders is compelling. Serotonergic 5-HT_1B and 5-HT_1D receptors are expressed extensively in inner ear ganglia of monkeys and rats. The serotonergic 5-HT_1F receptor is also a target of triptans. This study describes its distribution in vestibular and trigeminal ganglia of monkeys.

Methods

Using primary polyclonal antibodies raised against oligopeptides specific for the human 5-HT_1F receptor, neuronal somatic area and intensity of immunoreactive vestibular and trigeminal ganglia were quantified.

Results and Discussion

Virtually all vestibular and considerable trigeminal ganglia showed positive 5-HT_1F receptor immunoreactivity. Inferior and superior vestibular ganglia staining appeared confined to distinct cell regions, varying considerably among cells of different sizes: more intense in small, punctate in some medium and regionally polarized in some large cells. Analyses of average somatic vestibular neuronal immunoreactive intensity identified mainly medium sized cells with high standard deviation of intensity corresponding to punctately stained cells. Less variability occurred in somatic intensity staining and cellular distribution among 5-HT_1F receptor immunopositive trigeminal ganglia. Most exhibited similar punctate staining patterns, higher mean somatic immunoreactive intensity and larger neuronal somatic size proportions per size distribution subpopulation compared to vestibular ganglia size distribution populations. Centrally directed vestibular ganglion neuronal processes, cochlear inner hair cells, vestibular hair cells and blood vessels in vestibular maculae and cristae were immunoreactive. The 5-HT_1F receptor expression in vestibular ganglia shows complex variable staining intensity patterns associated with cell size of immunopositive neurons, not seen in immunopositive trigeminal ganglia and not previously evident with 5-HT_1B and 5-HT_1D receptor subtype immunoreactivity in vestibular ganglia. These data motivate exploration of 5-HT₁ receptor oligomerization and ligand functional selectivity in differential serotonergic involvement in co-morbidity of migraine and balance disorders. Similar findings in cochlear inner hair cell afferents are applicable to migraine-related tinnitus or hypercusis (phonophobia).

Regional distribution of serotonergic receptors: a systems neuroscience perspective on the downstream effects of the multimodal-acting antidepressant vortioxetine on excitatory and inhibitory neurotransmission

Previous work from this laboratory hypothesized that the multimodal antidepressant vortioxetine enhances cognitive function through a complex mechanism, using serotonergic (5-hydroxytryptamine, 5-HT) receptor actions to modulate gamma-butyric acid (GABA) and glutamate neurotransmission in key brain regions like the prefrontal cortex (PFC) and hippocampus. However, serotonergic receptors have circumscribed expression patterns, and therefore vortioxetine's effects on GABA and glutamate neurotransmission will probably be regionally selective. In this article, we attempt to develop a conceptual framework in which the effects of 5-HT, selective serotonin reuptake inhibitors (SSRIs), and vortioxetine on GABA and glutamate neurotransmission can be understood in the PFC and striatum-2 regions with roles in cognition and substantially different 5-HT receptor expression patterns. Thus, we review the anatomy of the neuronal microcircuitry in the PFC and striatum, anatomical data on 5-HT receptor expression within these microcircuits, and electrophysiological evidence on the effects of 5-HT on the behavior of each cell type. This analysis suggests that 5-HT and SSRIs will have markedly different effects within the PFC, where they will induce mixed effects on GABA and glutamate neurotransmission, compared to the striatum, where they will enhance GABAergic interneuron activity and drive down the activity of medium spiny neurons. Vortioxetine is expected to reduce GABAergic interneuron activity in the PFC and concomitantly increase cortical pyramidal neuron firing. However in the striatum, vortioxetine is expected to increase activity at GABAergic interneurons and have mixed excitatory and inhibitory effects in medium spiny neurons. Thus the conceptual framework developed here suggests that vortioxetine will have regionally selective effects on GABA and glutamate neurotransmission.

Effects of serotonin in the hippocampus: how SSRIs and multimodal antidepressants might regulate pyramidal cell function

The hippocampus plays an important role in emotional and cognitive processing, and both of these domains are affected in patients with major depressive disorder (MDD). Extensive preclinical research and the notion that modulation of serotonin (5-HT) neurotransmission plays a key role in the therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs) support the view that 5-HT is important for hippocampal function in normal and disease-like conditions. The hippocampus is densely innervated by serotonergic fibers, and the majority of 5-HT receptor subtypes are expressed there. Furthermore, hippocampal cells often co-express multiple 5-HT receptor subtypes that can have either complementary or opposing effects on cell function, adding to the complexity of 5-HT neurotransmission. Here we review the current knowledge of how 5-HT, through its various receptor subtypes, modulates hippocampal output and the activity of hippocampal pyramidal cells in rodents. In addition, we discuss the relevance of 5-HT modulation for cognitive processing in rodents and possible clinical implications of these results in patients with MDD. Finally, we review the data on how SSRIs and vortioxetine, an antidepressant with multimodal activity, affect hippocampal function, including cognitive processing, from both a preclinical and clinical perspective.

Immunomodulatory effects mediated by serotonin

Serotonin (5-HT) induces concentration-dependent metabolic effects in diverse cell types, including neurons, entherochromaffin cells, adipocytes, pancreatic beta-cells, fibroblasts, smooth muscle cells, epithelial cells, and leukocytes. Three classes of genes regulating 5-HT function are constitutively expressed or induced in these cells: (a) membrane proteins that regulate the response to 5-HT, such as SERT, 5HTR-GPCR, and the 5HT₃-ion channels; (b) downstream signaling transduction proteins; and (c) enzymes controlling 5-HT metabolism, such as IDO and MAO, which can generate biologically active catabolites, including melatonin, kynurenines, and kynurenamines. This review covers the clinical and experimental mechanisms involved in 5-HT-induced immunomodulation. These mechanisms are cell-specific and depend on the expression of serotonergic components in immune cells. Consequently, 5-HT can modulate several immunological events, such as chemotaxis, leukocyte activation, proliferation, cytokine secretion, anergy, and apoptosis. The effects of 5-HT on immune cells may be relevant in the clinical outcome of pathologies with an inflammatory component. Major depression, fibromyalgia, Alzheimer disease, psoriasis, arthritis, allergies, and asthma are all associated with changes in the serotonergic system associated with leukocytes. Thus, pharmacological regulation of the serotonergic system may modulate immune function and provide therapeutic alternatives for these diseases.

G-protein-coupled receptor type A heteromers as an emerging therapeutic target

INTRODUCTION

The discovery of receptor-receptor interactions (RRIs) in the early 1980s provided evidence that G-protein-coupled receptors (GPCRs) operate not only as monomers but also as heteromers, in which integration of the incoming signals takes place already at the plasma membrane level through allosteric RRIs. These integrative mechanisms give sophisticated dynamics to the structure and function of these receptor assemblies in terms of modulation of recognition, G-protein signaling and selectivity and switching to β-arrestin signaling.

AREAS COVERED

The present review briefly describes the concept of direct RRI and the available data on the mechanisms of oligomer formation. Further, pharmacological data concerning the best characterized heteromers involving type A GPCRs will be analyzed to evaluate their profile as possible targets for the treatment of various diseases, in particular of impacting diseases of the CNS.

EXPERT OPINION

GPCR heteromers have the potential to open a completely new field for pharmacology with likely a major impact in molecular medicine. Novel pharmacological strategies for the treatment of several pathologies have already been proposed. However, several challenges still exist to accurately characterize the role of the identified heteroreceptor complexes in pathology and to develop heteromer-specific ligands capable of efficiently exploiting their pharmacological features.

Down-regulation of 5-HT1B and 5-HT1D receptors inhibits proliferation, clonogenicity and invasion of human pancreatic cancer cells

Pancreatic ductal adenocarcinoma is characterized by extensive local tumor invasion, metastasis and early systemic dissemination. The vast majority of pancreatic cancer (PaCa) patients already have metastatic complications at the time of diagnosis, and the death rate of this lethal type of cancer has increased over the past decades. Thus, efforts at identifying novel molecularly targeted therapies are priorities. Recent studies have suggested that serotonin (5-HT) contributes to the tumor growth in a variety of cancers including prostate, colon, bladder and liver cancer. However, there is lack of evidence about the impact of 5-HT receptors on promoting pancreatic cancer. Having considered the role of 5-HT-1 receptors, especially 5-HT_1B and 5-HT_1D subtypes in different types of malignancies, the aim of this study was to investigate the role of 5-HT_1B and 5-HT_1D receptors in PaCa growth and progression and analyze their potential as cytotoxic targets. We found that knockdown of 5-HT_1B and 5-HT_1D receptors expression, using specific small interfering RNA (siRNA), induced significant inhibition of proliferation and clonogenicity of PaCa cells. Also, it significantly suppressed PaCa cells invasion and reduced the activity of uPAR/MMP-2 signaling and Integrin/Src/Fak-mediated signaling, as integral tumor cell pathways associated with invasion, migration, adhesion, and proliferation. Moreover, targeting 5-HT_1B and 5-HT_1D receptors down-regulates zinc finger ZEB1 and Snail proteins, the hallmarks transcription factors regulating epithelial-mesenchymal transition (EMT), concomitantly with up-regulating of claudin-1 and E-Cadherin. In conclusion, our data suggests that 5-HT_1B– and 5-HT_1D–mediated signaling play an important role in the regulation of the proliferative and invasive phenotype of PaCa. It also highlights the therapeutic potential of targeting of 5-HT_1B/1D receptors in the treatment of PaCa, and opens a new avenue for biomarkers identification, and valuable new therapeutic targets for managing pancreatic cancer.

Functional characterization of G-protein-coupled receptors: a bioinformatics approach

Complex molecular and cellular mechanisms regulate G protein-coupled receptors (GPCRs). It is suggested that proteins intrinsically disordered regions (IDRs) are to play a role in GPCR's intra and extracellular regions plasticity, due to their potential for post-translational modification and interaction with other proteins. These regions are defined as lacking a stable three-dimensional (3D) structure. They are rich in hydrophilic and charged, amino acids and are capable to assume different conformations which allow them to interact with multiple partners. In this study we analyzed 75 GPCR involved in synaptic transmission using computational tools for sequence-based prediction of IDRs within a protein. We also evaluated putative ligand-binding motifs using receptor sequences. The disorder analysis indicated that neurotransmitter GPCRs have a significant amount of disorder in their N-terminus, third intracellular loop (3IL) and C-terminus. About 31%, 39% and 53% of human GPCR involved in synaptic transmission are disordered in these regions. Thirty-three percent of receptors show at least one predicted PEST motif, this being statistically greater than the estimate for the rest of human GPCRs. About 90% of the receptors had at least one putative site for dimerization in their 3IL or C-terminus. ELM instances sampled in these domains were 14-3-3, SH3, SH2 and PDZ motifs. In conclusion, the increased flexibility observed in GPCRs, added to the enrichment of linear motifs, PEST and heteromerization sites, may be critical for the nervous system's functional plasticity.

The G protein-coupled receptor heterodimer network (GPCR-HetNet) and its hub components

G protein-coupled receptors (GPCRs) oligomerization has emerged as a vital characteristic of receptor structure. Substantial experimental evidence supports the existence of GPCR-GPCR interactions in a coordinated and cooperative manner. However, despite the current development of experimental techniques for large-scale detection of GPCR heteromers, in order to understand their connectivity it is necessary to develop novel tools to study the global heteroreceptor networks. To provide insight into the overall topology of the GPCR heteromers and identify key players, a collective interaction network was constructed. Experimental interaction data for each of the individual human GPCR protomers was obtained manually from the STRING and SCOPUS databases. The interaction data were used to build and analyze the network using Cytoscape software. The network was treated as undirected throughout the study. It is comprised of 156 nodes, 260 edges and has a scale-free topology. Connectivity analysis reveals a significant dominance of intrafamily versus interfamily connections. Most of the receptors within the network are linked to each other by a small number of edges. DRD2, OPRM, ADRB2, AA2AR, AA1R, OPRK, OPRD and GHSR are identified as hubs. In a network representation 10 modules/clusters also appear as a highly interconnected group of nodes. Information on this GPCR network can improve our understanding of molecular integration. GPCR-HetNet has been implemented in Java and is freely available at http://www.iiia.csic.es/~ismel/GPCR-Nets/index.html.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Functional significance of serotonin receptor dimerization

The original model of G-protein activation by a single G-protein-coupled receptor (GPCR) is giving way to a new model, wherein two protomers of a GPCR dimer interact with a single G-protein. This article will review the evidence suggesting that 5-HT receptors form dimers/oligomers and will compare the findings with the results obtained from the studies with other biogenic amine receptors. Topics to be covered include the origin or biogenesis of dimer formation, potential dimer interface(s), and oligomer size (dimer vs. tetramer or higher order). The functional significance will be discussed in terms of G-protein activation following ligand binding to one or two protomers in a dimeric structure, the formation of heterodimers, and the development of bivalent ligands.

Molecular and pharmacological characterization of serotonin 5-HT2α and 5-HT7 receptors in the salivary glands of the blowfly Calliphora vicina

Secretion in blowfly (Calliphora vicina) salivary glands is stimulated by the biogenic amine serotonin (5-hydroxytryptamine, 5-HT), which activates both inositol 1,4,5-trisphosphate (InsP(3))/Ca(2+) and cyclic adenosine 3',5'-monophosphate (cAMP) signalling pathways in the secretory cells. In order to characterize the signal-inducing 5-HT receptors, we cloned two cDNAs (Cv5-ht2α, Cv5-ht7) that share high similarity with mammalian 5-HT(2) and 5-HT(7) receptor genes, respectively. RT-PCR demonstrated that both receptors are expressed in the salivary glands and brain. Stimulation of Cv5-ht2α-transfected mammalian cells with 5-HT elevates cytosolic [Ca(2+)] in a dose-dependent manner (EC(50) = 24 nM). In Cv5-ht7-transfected cells, 5-HT produces a dose-dependent increase in [cAMP](i) (EC(50) = 4 nM). We studied the pharmacological profile for both receptors. Substances that appear to act as specific ligands of either Cv5-HT(2α) or Cv5-HT(7) in the heterologous expression system were also tested in intact blowfly salivary gland preparations. We observed that 5-methoxytryptamine (100 nM) activates only the Cv5-HT(2α) receptor, 5-carboxamidotryptamine (300 nM) activates only the Cv5-HT(7) receptor, and clozapine (1 µM) antagonizes the effects of 5-HT via Cv5-HT(7) in blowfly salivary glands, providing means for the selective activation of each of the two 5-HT receptor subtypes. This study represents the first comprehensive molecular and pharmacological characterization of two 5-HT receptors in the blowfly and permits the analysis of the physiological role of these receptors, even when co-expressed in cells, and of the modes of interaction between the Ca(2+)- and cAMP-signalling cascades.

The serotonergic system in Parkinson's disease

Although the cardinal manifestations of Parkinson's disease (PD) are attributed to a decline in dopamine levels in the striatum, a breadth of non-motor features and treatment-related complications in which the serotonergic system plays a pivotal role are increasingly recognised. Serotonin (5-HT)-mediated neurotransmission is altered in PD and the roles of the different 5-HT receptor subtypes in disease manifestations have been investigated. The aims of this article are to summarise and discuss all published preclinical and clinical studies that have investigated the serotonergic system in PD and related animal models, in order to recapitulate the state of the current knowledge and to identify areas that need further research and understanding.

Distribution of 5-HT1B and 5-HT1D receptors in the inner ear

Migraine and anxiety disorders are frequently co-morbid with balance disorders. This study examined the relative distribution of subtypes of serotonin (5-HT) receptor in the inner ear of monkeys and rats. Most vestibular ganglion cells were immunoreactive for 5-HT(1B) and 5-HT(1D) receptors in macaques and rats. In the inner ear, 5-HT(1B) and 5-HT(1D) receptor immunopositivity was associated with endothelial cells of the vestibular ganglion, spiral ganglion, vestibulocochlear nerve, spiral ligament and stria vascularis. It was noteworthy that 5-HT(1B) and 5-HT(1D) receptors are expressed in parallel sites in peripheral vestibular and trigeminal systems, which may be a factor underlying the efficacy of triptans in treating migraine and migrainous vertigo. Because the vestibular ganglion and trigeminal ganglion are both within the subarachnoid space, an interaction between 5-HT(1B) and TRPV1 receptors on blood vessel and ganglion cells may also contribute to the vasospasm and the comorbid headache, dizziness, nausea and vomiting that accompany subarachnoid hemorrhage.

Serotonin modulates glutamatergic transmission in the rat olfactory tubercle

The olfactory tubercle (OT) is found in the brains of mammals that are highly dependent on their sense of smell. Its human analogue is the poorly understood anterior perforated substance. Previous work on rat brain slices identified two types of field potential responses from the OT. The association fibre (AF) pathway was sensitive to muscarinic modulation, whereas the lateral olfactory tract (LOT) fibre pathway was not. Here, we establish that serotonin (5-hydroxytryptamine; 5-HT) also inhibits field potential excitatory postsynaptic potentials (EPSPs) in the AF, but not in the LOT fibre, pathway. Parallel experiments with adenosine (ADO) excluded ADO mediation of the 5-HT effect. Exogenous 5-HT at 30 microm caused a long-lasting approximately 40% reduction in the amplitude of AF postsynaptic responses, without affecting the time-course of EPSP decline, indicating a fairly restricted disposition of the 5-HT receptors responsible. The 5-HT(1)-preferring, 5-HT(5)-preferring and 5-HT(7)-preferring agonist 5-carboxamidotryptamine caused similar inhibition at approximately 100 nm. The 5-HT(1A)-preferring ligand 8-hydroxy-di-n-propylamino-tetralin at 10 microm, and the 5-HT uptake inhibitor citalopram at 3 microm, caused inhibition of AF-stimulated field potential responses in the 5-10% range. Order-of-potency information suggested a receptor of the 5-HT(1B) or 5-HT(1D) subtype. The 5-HT(1D) agonist L-694,247 (1 microm) suppressed the AF response by approximately 10% when used on its own. After washing out of L-694,427, inhibition by 30 microm 5-HT was reduced to negligible levels. Allowing for a partial agonist action of L-694,427 and complex interactions of 5-HT receptors within the OT, these results support the presence of active 5-HT(1D)-type receptors in the principal cell layer of the OT.

Transcriptional expression of serotonergic regulators in laser-captured microdissected dorsal raphe neurons of subjects with major depressive disorder: sex-specific differences

The relationship between serotonin (5-HT) and major depressive disorder (MDD) has been extensively studied but certain aspects are still ambiguous. Given the evidence that 5-HT neurotransmission is reduced in depressed subjects, it is possible that one or more of the 5-HT regulators may be altered in the dorsal raphe nucleus (DR) of depressed subjects. Candidates that regulate 5-HT synthesis and neuronal activity of 5-HT neurons include intrinsic regulators such as tryptophan hydroxylase 2, 5-HT autoreceptors, 5-HT transporter and transcription factors, as well as afferent regulators such as estrogen and brain-derived neurotrophic factor. The present study was designed to quantify mRNA concentrations of the above 5-HT regulators in an isolated population of 5-HT-containing DR neurons of MDD subjects and gender-matched psychiatrically normal control subjects. We found that mRNA concentrations of the 5-HT1D receptor and the transcription factors, NUDR and REST, were significantly increased in DR-captured neurons of female MDD subjects compared to female control subjects. No significant differences were found for the transcripts in male MDD subjects compared to male controls. This study reveals sex-specific alterations in gene expression of the pre-synaptic 5-HT1D autoreceptors and 5-HT-related transcription factors, NUDR and REST, in DR neurons of women with MDD.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

Modified Receptor Internalization upon Coexpression of 5-HT1BReceptor and 5-HT2BReceptors

Serotonin 5-HT(2B) receptors are often coexpressed with 5-HT(1B) receptors, and cross-talk between the two receptors has been reported in various cell types. However, many mechanistic details underlying 5-HT(1B) and 5-HT(2B) receptor cross-talk have not been elucidated. We hypothesized that 5-HT(2B) and 5-HT(1B) receptors each affect the others' signaling by modulating the others' trafficking. We thus examined the agonist stimulated internalization kinetics of fluorescent protein-tagged 5-HT(2B) and 5-HT(1B) receptors when expressed alone and upon coexpression in LMTK(-) murine fibroblasts. Time-lapse confocal microscopy and whole-cell radioligand binding analyses revealed that, when expressed alone, 5-HT(2B) and 5-HT(1B) receptors displayed distinct half-lives. Upon coexpression, serotonin-induced internalization of 5-HT(2B) receptors was accelerated 5-fold and was insensitive to a 5-HT(2B) receptor antagonist. In this context, 5-HT(2B) receptors did internalize in response to a 5-HT(1B) receptor agonist. In contrast, co-expression did not render 5-HT(1B) receptor internalization sensitive to a 5-HT(2B) receptor agonist. The altered internalization kinetics of both receptors upon coexpression was probably not due to direct interaction because only low levels of colocalization were observed. Antibody knockdown experiments revealed that internalization of 5-HT(1B) receptors (expressed alone) was entirely clathrin-independent and Caveolin1-dependent, whereas that of 5-HT(2B) receptors (expressed alone) was Caveolin1-independent and clathrin-dependent. Upon coexpression, serotonin-induced 5-HT(2B) receptor internalization became partially Caveolin1-dependent, and serotonin-induced 5-HT(1B) receptor internalization became entirely Caveolin1-independent in a protein kinase Cepsilon-dependent fashion. In conclusion, these data demonstrate that coexpression of 5-HT(1B) and 5-HT(2B) receptors influences the internalization pathways and kinetics of both receptors.

Activation, internalization, and recycling of the serotonin 2A receptor by dopamine

Serotonergic and dopaminergic systems, and their functional interactions, have been implicated in the pathophysiology of various CNS disorders. Here, we use recombinant serotonin (5-HT) 2A (5-HT2A) receptors to further investigate direct interactions between dopamine and 5-HT receptors. Previous studies in Xenopus oocytes showed that dopamine, although not the cognate ligand for the 5-HT2A receptor, acts as a partial-efficacy agonist. At micromolar concentrations, dopamine also acts as a partial-efficacy agonist on 5-HT2A receptors in HEK293 cells. Like 5-HT, dopamine also induces receptor-internalization in these cells, although at significantly higher concentrations than 5-HT. Interestingly, if the receptors are first sensitized or "primed" by subthreshold concentrations of 5-HT, then dopamine-induced internalization occurs at concentrations approximately 10-fold lower than when dopamine is used alone. Furthermore, unlike 5-HT-mediated internalization, dopamine-mediated receptor internalization, alone, or after sensitization by 5-HT, does not depend on PKC. Dopamine-internalized receptors recycle to the surface at rates similar to those of 5-HT-internalized receptors. Our results suggest a previously uncharacterized role for dopamine in the direct activation and internalization of 5-HT2A receptors that may have clinical relevance to the function of serotonergic systems in anxiety, depression, and schizophrenia and also to the treatment of these disorders.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Computational prediction of homodimerization of the A3 adenosine receptor

Increasing evidence suggests that G protein-coupled receptors form functional dimers or larger oligomeric complexes through homo- or heterodimerization, and that various transmembrane (TM) domains contribute dimerization interfaces. In this study, monomeric receptor structures - either the monomeric crystallographic structure of bovine rhodopsin or an A(3) adenosine receptor (AR) homology model - were dimerized by computational methods assuming various TM contact regions, optimized, and compared. The semi-empirical oligomeric structure of mouse rhodopsin studied in a native disc membrane with atomic force microscopy was used to establish the distance between monomers in the initial dimeric models. Among eight variations of symmetrical homodimers of bovine rhodopsin, the favored dimeric assembly closely resembled the semi-empirical model, in which TM domains 4 and 5 were the contact site, thus validating this approach. We used similar methods to generate eight homodimers of the A(3)AR and found the favored dimeric interface similarly to be TM4-5. By this method, dimeric variations - TM1-2, TM2-3, TM2-4, TM3-4, TM4-5, TM5-6, TM6-7, and TM7-1 - were constructed with the SYBYL 7.0 program by using a novel "fit-centroids-normal" method. Fitting atoms considered one of eight TM-TM centroids or seven-TM centroids, two centroids of each monomer, and a normal atom passing through the plane containing all centroids. Following molecular dynamics, the most energetically favorable contact modes were identified. In addition to TM4-5, which was favored in both rhodopsin and A(3)AR dimeric models, TM1-2 dimers in which helices 8 also contacted each other were judged favorable. The largest contact surface area between the monomers among the various homodimers, determined by van der Waals calculation with the MOLCAD surface program, was for the TM4-5 dimer. This contact surface also showed a high degree of shape complementarity. In addition, the TM4-5 dimers made by this theoretical method were more stable than the semi-empirically determined dimer.

Inhibition of Serotonin 5-Hydroxytryptamine2C Receptor Function through Heterodimerization

Although dimerization appears to be a common property of G-protein-coupled receptors (GPCRs), it remains unclear whether a GPCR dimer binds one or two molecules of ligand and whether ligand binding results in activation of one or two G-proteins when measured using functional assays in intact living cells. Previously, we demonstrated that serotonin 5-hydroxytryptamine2C (5-HT2C) receptors form homodimers (Herrick-Davis, K., Grinde, E., and Mazurkiewicz, J. (2004) Biochemistry 43, 13963-13971). In the present study, an inactive 5-HT(2C) receptor was created and coexpressed with wild-type 5-HT2C receptors to determine whether dimerization regulates receptor function and to determine the ligand/dimer/G-protein stoichiometry in living cells. Mutagenesis of Ser138 to Arg (S138R) produced a 5-HT2C receptor incapable of binding ligand or stimulating inositol phosphate (IP) signaling. Confocal fluorescence imaging revealed plasma membrane expression of yellow fluorescent protein-tagged S138R receptors. Expression of wild-type 5-HT2C receptors in an S138R-expressing stable cell line had no effect on ligand binding to wild-type 5-HT2C receptors, but inhibited basal and 5-HT-stimulated IP signaling as well as constitutive and 5-HT-stimulated endocytosis of wild-type 5-HT2C receptors. M1 muscarinic receptor activation of IP production was normal in the S138R-expressing cells. Heterodimerization of S138R with wild-type 5-HT2C receptors was visualized in living cells using confocal fluorescence resonance energy transfer (FRET). FRET was dependent on the donor/acceptor ratio and independent of the receptor expression level. Therefore, inactive 5-HT2C receptors inhibit wild-type 5-HT2C receptor function by forming nonfunctional heterodimers expressed on the plasma membrane. These results are consistent with a model in which one GPCR dimer binds two molecules of ligand and one G-protein and indicate that dimerization is essential for 5-HT receptor function.

Heterodimerization of G Protein-Coupled Receptors: Specificity and Functional Significance

G protein-coupled receptors (GPCRs) are cell surface receptors that mediate physiological responses to a diverse array of stimuli. GPCRs have traditionally been thought to act as monomers, but recent evidence suggests that GPCRs may form dimers (or higher-order oligomers) as part of their normal trafficking and function. In fact, certain GPCRs seem to have a strict requirement for heterodimerization to attain proper surface expression and functional activity. Even those GPCRs that do not absolutely require heterodimerization may still specifically associate with other GPCR subtypes, sometimes resulting in dramatic effects on receptor pharmacology, signaling, and/or internalization. Understanding the specificity and functional significance of GPCR heterodimerization is of tremendous clinical importance since GPCRs are the molecular targets for numerous therapeutic drugs.

A serotonin receptor antagonist induces oocyte maturation in both frogs and mice: evidence that the same G protein-coupled receptor is responsible for maintaining meiosis arrest in both species

Accumulating evidence has indicated that vertebrate oocytes are arrested at late prophase (G2 arrest) by a G protein coupled receptor (GpCR) that activates adenylyl cyclases. However, the identity of this GpCR or its regulation in G2 oocytes is unknown. We demonstrated that ritanserin (RIT), a potent antagonist of serotonin receptors 5-HT2R and 5-HT7R, released G2 arrest in denuded frog oocytes, as well as in follicle-enclosed mouse oocytes. In contrast to RIT, several other serotonin receptor antagonists (mesulergine, methiothepine, and risperidone) had no effect on oocyte maturation. The unique ability of RIT, among serotonergic antagonists, to induce GVBD did not match the antagonist profile of any known serotonin receptors including Xenopus 5-HT7R, the only known G(s)-coupled serotonin receptor cloned so far in this species. Unexpectedly, injection of x5-HT7R mRNA in frog oocytes resulted in hormone-independent frog oocyte maturation. The addition of exogenous serotonin abolished x5-HT7R-induced oocyte maturation. Furthermore, the combination of x5-HT7R and exogenous serotonin potently inhibited progesterone-induced oocyte maturation. These results provide the first evidence that a G-protein coupled receptor related to 5-HT7R may play a pivotal role in maintaining G2 arrest in vertebrate oocytes.

Heterodimerization of μ- and δ-Opioid Receptors Occurs at the Cell Surface Only and Requires Receptor-G Protein Interactions

Homo- and heterodimerization of the opioid receptors with functional consequences were reported previously. However, the exact nature of these putative dimers has not been identified. In current studies, the nature of the heterodimers was investigated by producing the phenotypes of the 1:1 heterodimers formed between the constitutively expressed mu-opioid receptor (MOR) and the ponasterone A-induced expression of delta-opioid receptor (DOR) in EcR293 cells. By examining the trafficking of the cell surface-located MOR and DOR, we determined that these two receptors endocytosed independently. Using cell surface expression-deficient mutants of MOR and DOR, we observed that the corresponding wild types of these receptors could not rescue the cell surface expression of the mutants, whereas the antagonist naloxone could. Furthermore, studies with constitutive or agonist-induced receptor internalization also indicated that MOR and DOR endocytosed independently and could not "drag in" the corresponding wild types or endocytosis-deficient mutants. Additionally, the heterodimer phenotypes could be eliminated by the pretreatment of the EcR293 cells with pertussis toxin and could be modulated by the deletion of the RRITR sequence in the third intracellular loop that is involved in the receptor-G protein interaction and activation. These data suggest that MOR and DOR heterodimerize only at the cell surface and that the oligomers of opioid receptors and heterotrimeric G protein are the bases for the observed MOR-DOR heterodimer phenotypes.

Evidence for serotonin influencing the thalamic infiltration of mast cells in rat

Serotonin (5-HT) is involved in neuroimmunomodulation. We analyzed the effects of sumatriptan, a 5-HT(1B/1D) receptor agonist, and ondansetron, a 5-HT(3) receptor antagonist, on thalamic mast cell (TMC) population, the only immunocytes known to infiltrate the brain in physiological conditions. Only sumatriptan was effective, significantly increasing TMC numbers versus controls, and especially those containing 5-HT. 5-HT(1B) receptors are concentrated in the median eminence on non-serotonergic axonal endings, probably hypothalamic terminal fibers, involved in hypothalamic-pituitary neuroendocrine modulating processes. TMC variations could reflect serotonergic actions on these fibers. TMCs would thus be cellular interfaces mediating immune action in the nervous system in relation with the hormonal status of the organism.

Functional consequences of 7TM receptor dimerization

7TM receptors work as signaling platforms that activate multiple signalling systems at the intracellular face of the plasma membrane. It is an emerging concept that 7TM receptors form homo- and hetero-dimers or -oligomers in vitro and in vivo. Numerous studies suggest dimerization is important for receptor function including agonist/antagonist affinity, efficacy, trafficking, and specificity of signal transduction, yet it remains unknown whether dimerization is a prerequisite for 7TM receptor signaling. The current review provides an overview of the biochemical support for 7TM homodimerization, followed by a discussion of the characteristics of homodimerization, with focus on dimer organization, and the functional consequences of dimerization. Heterodimerization will not generally be discussed in this review although we have included a few examples to illustrate specific points, and a table that summarises the current literature on this subject.

Expression of serotonin receptors and role of serotonin in human prostate cancer tissue and cell lines

BACKGROUND

Increase in the number of serotonin (5-HT) releasing neuroendocrine (NE) cells has been shown to be correlated with tumor progression, loss of androgen dependence, and poor prognosis. Serotonin is a well-known mitogen which mediates a wide variety of physiological effects via multiple receptors, of which receptor subtype 1 (5-HTR1) has been identified in prostate cancer (PC) cell lines. Recently, 5-HT has been found to show growth-promoting activity and to be functionally related to oncogenes.

MATERIALS AND METHODS

Localization, protein content, and mRNA expression of 5-HTR subtype 1A, 1B, and 1D was studied in prostatic tissue (35 patients), metastases, PC cell lines, a benign prostatic stromal cell line (human prostate cell preparation (hPCP)), and xenografts of PC-3 cells by immunohistochemistry (IHC), Western blotting, and RT-PCR, respectively. The growth-inhibition effect of a 5-HT1A antagonist (NAN-190) on PC cell lines was studied using a bromodeoxyuridine (BrdU) assay.

RESULTS

A strong immunoreaction of 5-HTR1A and 1B was demonstrated in high-grade tumor cells (35/35) and a small number of BPH cells, whereas 5-HTR1D was confined to vascular endothelial cells. 5-HTR1A was also demonstrated in PC cells metastasized to lymph node and bone, PC-3, DU145, LNCaP, and in xenografts of PC-3 cells and hPCP. Western blot analysis gave strong bands from PC tissue extracts compared to BPH tissue. Using RT-PCR, 5-HTR1A mRNA was demonstrated in all PC cell lines. An antagonist of 5-HTR1A (NAN-190) inhibited the growth of PC-3, DU145, and LNCaP cells but not of hPCP cells.

CONCLUSIONS

This is the first study demonstrating an overexpression of 5-HTR subtypes 1A and 1B in PC cells, especially in high-grade tumors. Moreover, 5-HT stimulates proliferation of PC cells and 5-HTR1A antagonists inhibit proliferation. Thus, we propose that 5-HT has an important role in tumor progression, especially in the androgen-independent state of the disease. The design of specific antagonists for this type of receptor might be useful for the growth control of androgen-independent tumors.

Dimerization of G-protein-coupled receptors: roles in signal transduction

Recently, many G-protein-coupled receptors (GPCRs) have been demonstrated to form constitutive dimers consisting of identical or distinct monomeric subunits. The discovery of GPCR dimerization has revealed a new level of molecular cross-talk between signalling molecules and may define a general mechanism that modulates the function of GPCRs under both physiological and pathological conditions. The heterodimerization between distinct GPCRs could be responsible for the generation of pharmacologically defined receptors for which no gene has been identified so far. Elucidating the role of dimerization in the activation processes of GPCRs will lead us to develop novel pharmaceutical agents that allosterically promote activation or inhibition of GPCR signalling.

Cell surface expression of alpha1D-adrenergic receptors is controlled by heterodimerization with alpha1B-adrenergic receptors

alpha(1)-Adrenergic receptors (ARs) belong to the large Class I G protein-coupled receptor superfamily and comprise three subtypes (alpha(1A), alpha(1B), and alpha(1D)). Previous work with heterologously expressed C-terminal green fluorescent protein (GFP)-tagged alpha(1)-ARs showed that alpha(1A)- and alpha(1B)-ARs localize to the plasma membrane, whereas alpha(1D)-ARs accumulate intracellularly. We recently showed that alpha(1D)- and alpha(1B)-ARs form heterodimers, whereas alpha(1D)- and alpha(1A)-ARs do not. Here, we examined the role of heterodimerization in regulating alpha(1D)-AR localization using both confocal imaging of GFP- or CFP-tagged alpha(1)-ARs and a luminometer-based surface expression assay in HEK293 cells. Co-expression with alpha(1B)-ARs caused alpha(1D)-ARs to quantitatively translocate to the cell surface, but co-expression with alpha(1A)-ARs did not. Truncation of the alpha(1B)-AR extracellular N terminus or intracellular C terminus had no effect on surface expression of alpha(1D)-ARs, suggesting primary involvement of the hydrophobic core. Co-transfection with an uncoupled mutant alpha(1B)-AR (Delta12alpha(1B)) increased both alpha(1D)-AR surface expression and coupling to norepinephrine-stimulated Ca(2+) mobilization. Finally, GFP-tagged alpha(1D)-ARs were not detected on the cell surface when expressed in rat aortic smooth muscle cells that express no endogenous ARs, but were almost exclusively localized on the surface when expressed in DDT(1)MF-2 cells, which express endogenous alpha(1B)-ARs. These studies demonstrate that alpha(1B)/alpha(1D)-AR heterodimerization controls surface expression and functional coupling of alpha(1D)-ARs, the N- and C-terminal domains are not involved in this interaction, and that alpha(1B)-AR G protein coupling is not required. These observations may be relevant to many other Class I G protein-coupled receptors, where the functional consequences of heterodimerization are still poorly understood.