The proximal portion of the COOH terminus of the oxytocin receptor is required for coupling to g(q), but not g(i). Independent mechanisms for elevating intracellular calcium concentrations from intracellular stores

[Emergent role of astrocytes in oxytocin-mediated modulatory control of neuronal circuits and brain functions]

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity are critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, and give details of underlying intracellular cascades.

The modulation of emotional and social behaviors by oxytocin signaling in limbic network

Neuropeptides can exert volume modulation in neuronal networks, which account for a well-calibrated and fine-tuned regulation that depends on the sensory and behavioral contexts. For example, oxytocin (OT) and oxytocin receptor (OTR) trigger a signaling pattern encompassing intracellular cascades, synaptic plasticity, gene expression, and network regulation, that together function to increase the signal-to-noise ratio for sensory-dependent stress/threat and social responses. Activation of OTRs in emotional circuits within the limbic forebrain is necessary to acquire stress/threat responses. When emotional memories are retrieved, OTR-expressing cells act as gatekeepers of the threat response choice/discrimination. OT signaling has also been implicated in modulating social-exposure elicited responses in the neural circuits within the limbic forebrain. In this review, we describe the cellular and molecular mechanisms that underlie the neuromodulation by OT, and how OT signaling in specific neural circuits and cell populations mediate stress/threat and social behaviors. OT and downstream signaling cascades are heavily implicated in neuropsychiatric disorders characterized by emotional and social dysregulation. Thus, a mechanistic understanding of downstream cellular effects of OT in relevant cell types and neural circuits can help design effective intervention techniques for a variety of neuropsychiatric disorders.

Emerging role of astrocytes in oxytocin-mediated control of neural circuits and brain functions

The neuropeptide oxytocin has been in the focus of scientists for decades due to its profound and pleiotropic effects on physiology, activity of neuronal circuits and behaviors, among which sociality. Until recently, it was believed that oxytocinergic action exclusively occurs through direct activation of neuronal oxytocin receptors. However, several studies demonstrated the existence and functional relevance of astroglial oxytocin receptors in various brain regions in the mouse and rat brain. Astrocytic signaling and activity is critical for many important physiological processes including metabolism, neurotransmitter clearance from the synaptic cleft and integrated brain functions. While it can be speculated that oxytocinergic action on astrocytes predominantly facilitates neuromodulation via the release of specific gliotransmitters, the precise role of astrocytic oxytocin receptors remains elusive. In this review, we discuss the latest studies on the interaction between the oxytocinergic system and astrocytes, including detailed information about intracellular cascades, and speculate about future research directions on astrocytic oxytocin signaling.

Oxytocin stimulates lipolysis, prostaglandin E2 synthesis, and leptin secretion in 3T3-L1 adipocytes

A model of oxytocin in the regulation of metabolic status has described one of oxytocin synthesis and release from the neurohypophysis in response to leptin, to suppress further leptin release. In addition, a lipogenic role for oxytocin has been suggested, consistent with an insulinergic action. This model, however, may be incorrect. Oxytocin reduces fat mass in the absence of either leptin or leptin receptor signalling, thereby challenging the interdependence between leptin and oxytocin. An oxytocin induced production of the anti-lipolytic prostaglandin E₂ (PGE₂) might account for this. Media from 3T3-L1 differentiated adipocytes treated with oxytocin (0-50 nmol.L^-1) for 24 hrs were assayed for PGE₂, leptin, adiponectin, and glycerol. Harvested cells were analysed for lipid droplet triglyceride and cytosolic free fatty acid (FFA) by flow cytometry, and for altered expression of lipolytic and lipogenic associated gene ontology transcripts by cDNA array. Both PGE₂ and leptin secretion were significantly increased by oxytocin treatment whilst adiponectin secretion was not. A significant increase in cytosolic FFA was detected following oxytocin treatment, similar to that determined following treatment with isoproterenol (positive control). A significant increase in glycerol release to the culture media confirmed a lipolytic effect. No enrichment of lipolytic and lipogenic associated gene ontology transcripts was determined, but significant overrepresentation of chemosensory olfactory transcripts was. In conclusion, oxytocin stimulates lipolysis in 3T3-L1 adipocytes, mediated by autocrine/paracrine actions of PGE₂ and leptin. To confirm that this response is mediated solely by the oxytocin receptor, further experiments would require those effects being blocked by a specific oxytocin antagonist.

Oxytocin in the Male Reproductive Tract; The Therapeutic Potential of Oxytocin-Agonists and-Antagonists

In this review, the role of oxytocin and oxytocin-like agents (acting via the oxytocin receptor and belonging to the oxytocin-family) in the male reproductive tract is considered. Previous research (dating back over 60 years) is revised and connected with recently found aspects of the role oxytocin plays in male reproductive health. The local expression of oxytocin and its receptor in the male reproductive tract of different species is summarized. Colocalization and possible crosstalk to other agents and receptors and their resulting effects are discussed. The role of the newly reported oxytocin focused signaling pathways in the male reproductive tract, other than mediating contractility, is critically examined. The structure and effect of the most promising oxytocin-agonists and -antagonists are reviewed for their potential in treating male disorders with origins in the male reproductive tract such as prostate diseases and ejaculatory disorders.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Arginine 313 of the putative 8th helix mediates Gαq/14 coupling of human CC chemokine receptors CCR2a and CCR2b

In man, two CC chemokine receptor isoforms, CCR2a and CCR2b, are present that belong to the rhodopsin-like G protein-coupled receptor family, and couple to G_i and G_q family members. The CCR2 receptors are known to regulate canonical functions of chemokines such as directed migration of leukocytes, and to potentially control non-canonical functions such as differentiation, proliferation, and gene transcription of immune and non-immune cells. We recently reported on the activation of phospholipase C isoenzymes and RhoA GTPases by coupling of the two CCR2 receptors to members of the G_q family, in particular Gα_q and Gα₁₄. So far little is known about the structural requirements for the CCR2/G_q/14 interaction. Interestingly, the CCR2 receptor isoforms are identical up to arginine 313 (R³¹³) that is part of the putative 8th helix in CCR2 receptors, and the 8th helix has been implicated in the interaction of rhodopsin-like G protein-coupled receptors with Gα_q. In the present work we describe that the 8th helix of both CCR2a and CCR2b is critically involved in selectively activating Gα_q/14-regulated signaling. Refined analysis using various CCR2a and CCR2b mutants and analyzing their cellular signaling, e.g. ligand-dependent (i) activation of phospholipase C isoenzymes, (ii) stimulation of serum response factor-mediated gene transcription, (iii) activation of mitogen-activated protein kinases, (iv) internalization, and (v) changes in intracellular calcium concentrations, identified arginine 313 within the amino terminal portion of helix 8 to play a role for the agonist-mediated conformational changes and the formation of a Gα_q/14 binding surface. We show that R³¹³ determines Gα_q/14 protein-dependent but not G_i protein-dependent cellular signaling, and plays no role in G_q/G_i-independent receptor internalization, indicating a role of R³¹³ in biased signaling of CCR2 receptors.

Molecular Basis of Oxytocin Receptor Signalling in the Brain: What We Know and What We Need to Know

Oxytocin (OT), a hypothalamic neuropeptide involved in regulating the social behaviour of all vertebrates, has been proposed as a treatment for a number of neuropsychiatric disorders characterised by deficits in the social domain. Over the last few decades, advances focused on understanding the social effects of OT and its role in physiological conditions and brain diseases, but much less has been done to clarify the molecular cascade of events involved in mediating such effects and in particular the cellular and molecular pharmacology of OT and its target receptor (OTR) in neuronal and glial cells.The entity and persistence of OT activity in the brain is closely related to the expression and regulation of the OTR expressed on the cell surface, which transmits the signal intracellularly and permits OT to affect cell function. Understanding the various signalling mechanisms mediating OTR-induced cell responses is crucial to determine the different responses in different cells and brain regions, and the success of OT and OT-derived analogues in the treatment of neurodevelopmental and psychiatric diseases depends on how well we can control such responses. In this review, we will consider the most important aspects of OT/OTR signalling by focusing on the molecular events involved in OT binding and coupling, on the main signalling pathways activated by the OTR in neuronal cells and on intracellular and plasma membrane OTR trafficking, all of which contribute to the quantitative and qualitative features of OT responses in the brain.

Expression of Transient Receptor Channel Proteins in Human Fundal Myometrium in Pregnancy

OBJECTIVE

Cation channels comprised of transient receptor potential (TrpC) proteins may play a role in signal-regulated calcium entry and calcium homeostasis in myometrium. The objective of this study was to determine the relative abundance of specific TrpC mRNAs expressed in human myometrium and determine if TrpC mRNA and protein concentrations differ in fundal myometrium before and after the onset of labor.

METHODS

A quantitative real-time polymerase chain reaction (Q-RT-PCR) procedure was developed for determining the concentration of TrpC mRNA expression in immortalized and primary human myometrial cells and myometrial fundus tissues from patients before and after the onset of labor. The corresponding TrpC proteins were detected by Western blot analysis and immunohistochemistry.

RESULTS

hTrpC1, 3, 4, 5, 6, and 7 mRNAs were expressed in two lines of immortalized human myometrial cells and in primary human myocytes. In all of these cells, hTrpC1 and hTrpC4 mRNAs were the most abundant, followed by hTrpC6. A similar distribution was observed in fundal myometrium samples from patients before and after the onset of labor. hTrpC4 mRNA was significantly lower after the onset of labor; there were no significant changes in the concentrations of other TrpC mRNAs. Immunohistochemistry identified hTrpC1, 3, 4, and 6 proteins in myometrial smooth muscle cells. Western blot analysis of myometrial membranes demonstrated no statistically significant changes in hTrpC1, 3, 4, and 6 proteins between samples collected before and after the onset of labor.

CONCLUSIONS

We have demonstrated that hTrpC1 and hTrpC4 are the most abundant TrpC mRNAs in human myometrium, with TrpC6 being the next most abundant. There was no increase in TrpC mRNA or protein in fundal myometrium with the onset of labor. Nonetheless, these isoforms may play significant roles in signal regulated calcium entry in human myometrium.

Neuropeptide diversity and the regulation of social behavior in New World primates

Oxytocin (OT) and vasopressin (AVP) are important hypothalamic neuropeptides that regulate peripheral physiology, and have emerged as important modulators of brain function, particularly in the social realm. OT structure and the genes that ultimately determine structure are highly conserved among diverse eutherian mammals, but recent discoveries have identified surprising variability in OT and peptide structure in New World monkeys (NWM), with five new OT variants identified to date. This review explores these new findings in light of comparative OT/AVP ligand evolution, documents coevolutionary changes in the oxytocin and vasopressin receptors (OTR and V1aR), and highlights the distribution of neuropeptidergic neurons and receptors in the primate brain. Finally, the behavioral consequences of OT and AVP in regulating NWM sociality are summarized, demonstrating important neuromodulatory effects of these compounds and OT ligand-specific influences in certain social domains.

Oxytocin has therapeutic effects on cancer, a hypothesis

Oxytocin (OT) is the first peptide hormone structurally assessed and chemically synthesized in biologically active form. This hormone acts as an important factor in a human reproductive system particularly during pregnancy and lactation in women. So far, different therapeutic roles for OT have been identified as a spectrum from central and peripheral actions on male and female reproductive systems, circulatory system, musculoskeletal system, etc. Some in vitro and in vivo studies also revealed that OT is responsible for bivariate biological functions involved in cancer as following. By activating OT receptor in tumoral cells, OT enacts as a growth regulator, whether activator or inhibitor. Regarding the increase of OT in some conditions such as breastfeeding, exercise, and multiparity, we can relate the effect of these conditions on cancer with OT effects. Based on this hypothesis, we present a review on the effects of this neuropeptide on various types of cancer and also the influence of these conditions on the same cancer.

Tandem duplications in the C-terminal domain of the mesotocin receptor exclusively identified among East Eurasian thrushes

Mesotocin is a neurohypophyseal hormone found in some non-mammalian vertebrates, including birds, reptiles, and amphibians. In this study, we identified and characterized 18-amino acid duplications in the C-terminal domain of the mesotocin receptor (MTR), specifically found in Turdus thrushes (Aves: Passeriforms: Turdidae). These duplicated elements are located in the distal part of the C-terminal tails of MTR and consist of amino acids that are highly conserved among major vertebrates. Intraspecific polymorphisms in a variable number of tandem duplications are commonly found in East Eurasian Turdus, but not in any other genus of Turdidae. Moreover, the genus Turdus can be further classified into 2 groups according to the presence or absence of a 3-amino acid deletion just adjacent to the putative palmitoylation site in the cytoplasmic C-terminal tail. The phylogeny presented here strongly supports the conspecific group of 4 East Eurasian thrushes (Turdus pallidus, T. chrysolaus, T. obscurus, and T. celaenops). Our findings, therefore, provide a new synapomorphy that can be used for phylogenetic assumptions and shed a light on the history of diversification within Eurasian Turdus clades.

Individual differences underlying susceptibility to addiction: Role for the endogenous oxytocin system

Recent research shows that the effects of oxytocin are more diverse than initially thought and that in some cases oxytocin can directly influence the response to drugs and alcohol. Large individual differences in basal oxytocin levels and reactivity of the oxytocin system exist. This paper will review the literature to explore how individual differences in the oxytocin system arise and examine the hypothesis that this may mediate some of the individual differences in susceptibility to addiction and relapse. Differences in the oxytocin system can be based on individual factors, e.g. genetic variation especially in the oxytocin receptor, age or gender, or be the result of early environmental influences such as social experiences, stress or trauma. The paper addresses the factors that cause individual differences in the oxytocin system and the environmental factors that have been identified to induce long-term changes in the developing oxytocin system during different life phases. Individual differences in the oxytocin system can influence effects of drugs and alcohol directly or indirectly. The oxytocin system has bidirectional interactions with the stress-axis, autonomic nervous system, neurotransmitter systems (e.g. dopamine, serotonin and GABA/glutamate) and the immune system. These systems are all important, even vital, in different phases of addiction. It is suggested that early life adversity can change the development of the oxytocin system and the way it modulates other systems. This in turn could minimise the negative feedback loops that would normally exist. Individuals may show only minor differences in behaviour and function unless subsequent stressors or drug use challenges the system. It is postulated that at that time individual differences in oxytocin levels, reactivity of the system or interactions with other systems can influence general resilience, drug effects and the susceptibility to develop problematic drug and alcohol use.

Noncanonical control of vasopressin receptor type 2 signaling by retromer and arrestin

The vasopressin type 2 receptor (V2R) is a critical G protein-coupled receptor (GPCR) for vertebrate physiology, including the balance of water and sodium ions. It is unclear how its two native hormones, vasopressin (VP) and oxytocin (OT), both stimulate the same cAMP/PKA pathway yet produce divergent antinatriuretic and antidiuretic effects that are either strong (VP) or weak (OT). Here, we present a new mechanism that differentiates the action of VP and OT on V2R signaling. We found that vasopressin, as opposed to OT, continued to generate cAMP and promote PKA activation for prolonged periods after ligand washout and receptor internalization in endosomes. Contrary to the classical model of arrestin-mediated GPCR desensitization, arrestins bind the VP-V2R complex yet extend rather than shorten the generation of cAMP. Signaling is instead turned off by the endosomal retromer complex. We propose that this mechanism explains how VP sustains water and Na(+) transport in renal collecting duct cells. Together with recent work on the parathyroid hormone receptor, these data support the existence of a novel "noncanonical" regulatory pathway for GPCR activation and response termination, via the sequential action of β-arrestin and the retromer complex.

Non-synonymous single-nucleotide variations of the human oxytocin receptor gene and autism spectrum disorders: a case-control study in a Japanese population and functional analysis

Background

The human oxytocin receptor (hOXTR) is implicated in the etiology of autism spectrum disorders (ASDs) and is a potential target for therapeutic intervention. Several studies have reported single-nucleotide polymorphisms (SNPs) of the OXTR gene associated with ASDs. These SNPs, however, reside outside the protein-coding region. Not much is known about genetic variations that cause amino acid substitutions that alter receptor functions.

Methods

Variations in the OXTR gene were analyzed in 132 ASD patients at Kanazawa University Hospital in Japan and 248 unrelated healthy Japanese volunteers by re-sequencing and real-time polymerase chain reaction-based genotyping. Functional changes in variant OXTRs were assessed by radioligand binding assay and measurements of intracellular free calcium concentrations ([Ca²⁺]_i) and inositol 1,4,5-trisphosphate (IP₃) levels.

Results

Six subjects (4.5%) in the ASD group and two in the control group (0.8%) were identified as heterozygotes carrying the R376G variation (rs35062132; c.1126C>G); one individual from the ASD group (0.8%) and three members of the control group (1.2%) were found to be carrying R376C (c.1126C>T). The C/G genotype significantly correlated with an increased risk of ASDs (odds ratio (OR) = 5.83; 95% confidence interval (CI) = 1.16 to 29.33; P = 0.024, Fisher’s exact test). Consistently, the G allele showed a correlation with an increased likelihood of ASDs (OR = 5.73; 95% CI = 1.15 to 28.61; P = 0.024, Fisher’s exact test). The frequencies of the C/T genotype and the T allele in the ASD and control groups did not differ significantly. We also examined changes in agonist-induced cellular responses mediated by the variant receptors hOXTR-376G and hOXTR-376C. OXT-induced receptor internalization and recycling were faster in hOXTR-376G-expressing HEK-293 cells than in cells expressing hOXTR-376R or hOXTR-376C. In addition, the elevation in [Ca²⁺]_i and IP₃ formation decreased in the cells expressing hOXTR-376G and hOXTR-376C tagged with enhanced green fluorescent protein (EGFP), in comparison with the cells expressing the common-type hOXTR-376R tagged with EGFP.

Conclusions

These results suggest that the rare genetic variation rs35062132 might contribute to the pathogenesis of ASDs, and could provide a molecular basis of individual differences in OXTR-mediated modulation of social behavior.

DNA methylation of specific CpG sites in the promoter region regulates the transcription of the mouse oxytocin receptor

Oxytocin is a peptide hormone, well known for its role in labor and suckling, and most recently for its involvement in mammalian social behavior. All central and peripheral actions of oxytocin are mediated through the oxytocin receptor, which is the product of a single gene. Transcription of the oxytocin receptor is subject to regulation by gonadal steroid hormones, and is profoundly elevated in the uterus and mammary glands during parturition. DNA methylation is a major epigenetic mechanism that regulates gene transcription, and has been linked to reduced expression of the oxytocin receptor in individuals with autism. Here, we hypothesized that transcription of the mouse oxytocin receptor is regulated by DNA methylation of specific sites in its promoter, in a tissue-specific manner. Hypothalamus-derived GT1-7, and mammary-derived 4T1 murine cell lines displayed negative correlations between oxytocin receptor transcription and methylation of the gene promoter, and demethylation caused a significant enhancement of oxytocin receptor transcription in 4T1 cells. Using a reporter gene assay, we showed that methylation of specific sites in the gene promoter, including an estrogen response element, significantly inhibits transcription. Furthermore, methylation of the oxytocin receptor promoter was found to be differentially correlated with oxytocin receptor expression in mammary glands and the uterus of virgin and post-partum mice, suggesting that it plays a distinct role in oxytocin receptor transcription among tissues and under different physiological conditions. Together, these results support the hypothesis that the expression of the mouse oxytocin receptor gene is epigenetically regulated by DNA methylation of its promoter.

Genetic variation in the C-terminal domain of arginine vasotocin receptor in avian species

Arginine vasotocin (AVT) is a neurohypophysial hormone that plays an essential role in various social behaviours. We investigated the degree of polymorphisms in the C-terminal domain of the AVT V2-type receptor (AVT2R) among avian species to determine the mechanism by which genetic polymorphisms in the neuropeptide receptor may contribute to different levels of signal transduction. In passerine birds, AVT2R was characterised by 2 variable regions, both of which were managed by insertion/deletion (indel); however, indels were rarely found in other avian taxa. The presence or absence of deletions in passerines largely affected the properties of the predicted palmitoylation sites at the proximal part of the C-terminal tail. Moreover, we detected intraspecific polymorphisms in estrildid finches based on the number of tri-amino acid (GHQ/EHQ/EHR) repeats in another variable region. Our results indicate that amino acid substitutions and length variation at the C-terminus may impact subsequent signal transduction and affect behavioural traits in wild birds.

Bridging the gap between GPCR activation and behaviour: oxytocin and prolactin signalling in the hypothalamus

Neuropeptides of the brain are important neuromodulators, controlling behaviour and physiology. They signal through G protein-coupled receptors (GPCR) that couple to complex intracellular signalling pathways. These signalling networks integrate information from multiple sources, resulting in appropriate physiological and behavioural responses to environmental and internal cues. This paper will focus on the neuropeptides oxytocin and prolactin with respect to (1) the regulation of neuroendocrine stress responses and anxiety, and (2) the receptor-mediated molecular mechanisms underlying these actions of the neuropeptides. Besides its significant reproductive functions when released into the bloodstream, brain oxytocin reduces the activity of the hypothalamo-pituitary-adrenal (HPA) axis as well as anxiety-related behaviour in male and female rats. Oxytocin mediates its anxiolytic effect, at least in part, via binding to its GPCR in the hypothalamic paraventricular nucleus, followed by transactivation of the epidermal growth factor receptor, and subsequent activation of a MEK-extracellular signal-regulated kinase (ERK) MAP kinase pathway. Prolactin, by binding to its GPCR receptors, of which there are short and long forms, also activates ERK, and this is necessary for the control of the expression of corticotrophin-releasing hormone-an important regulator of the HPA axis. Liganded oxytocin and prolactin may also recruit other signalling pathways, but how these pathways contribute to the observed behavioural and physiological effects remains to be established. GPCR-mediated oxytocin and prolactin neuronal signalling are illustrative of the complexity of GPCR-activated regulation of appropriate neuroendocrine and behavioural responses to environmental and physiological demands.

Association between neuropeptide oxytocin and male infertility

PURPOSE

To investigate the relationship between oxytocin (OT) and male infertility, serum OT baseline concentration and oxytocin receptor (OTR) gene expression in fertile and infertile men were investigated.

METHODS AND PATIENTS

Twenty obstructive azoospermia patients, twenty five idiopathic asthenozoospermia patients, twenty idiopathic oligozoospermia patients and twenty healthy subjects were taken into consideration. Serum OT baseline concentration was determined by radioimmunoassay. Moreover, serum concentration of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone (T) were determined by chemoluminescence to evaluate the correlation with OT. OTR gene promotor and OTR mRNA expressions were determined by polymerase chain reaction and reverse transcriptase-polymerase chain reaction, respectively. OTR protein expression was also performed by Western Blot.

RESULTS

Serum OT baseline concentrations in infertile groups were significantly higher than in fertile group (F0.05/2(2,82) = 8.29, p < 0.001). Serum baseline concentration of OT was not correlated with that of LH, FSH and T. There was no significant difference in gene sequences of OTR gene promotor and OTR mRNA when comparing infertile patients with fertile. Human OTR was in the form of oligomers and monomers, and the oligomers were in the majority containing tetramers and hexamers. Monomer expression was significantly higher in idiopathic asthenozoospermia and idiopathic oligozoospermia than that in obstructive azoospermia and control group (F0.05/2(2,82) = 115.50, p < 0.001). There was no significant difference in oligomer expression between different groups, but 20% of idiopathic asthenozoospermia cases showed a decrease.

CONCLUSIONS

Significantly different OT baseline concentrations and OTR expressions between fertile and infertile men strongly suggest that OT/OTR system is likely to be linked with male infertility, providing new insights into the pathogenesis and treatment of male infertility.

Species Comparison of the Role of p38 MAP Kinase in the Female Reproductive System

The p38 mitogen-activated protein kinases (MAPKs) are members of discrete signal transduction pathways that have significant regulatory roles in a variety of biological processes, depending on the cell, tissue and organ type. p38 MAPKs are involved in inflammation, cell growth and differentiation and cell cycle. In the female reproductive system, p38 MAPKs are known to regulate various aspects of the reproductive process such as mammalian estrous and menstrual cycles as well as early pregnancy and parturition. p38 MAPKs have also been implicated in alterations and pathologies observed in the female reproductive system. Therefore, pharmacologic modulation of p38 MAPKs, and inter-connected signaling pathways (e.g., estrogen receptor signaling, c-fos, c-jun), may influence reproductive physiology and function. This article provides a critical, comparative review of available data on the roles of p38 MAPKs in the mammalian female reproductive system and in reproductive pathophysiology in humans and preclinical species. We first introduce fundamental differences and similarities of the mammalian female reproductive system that should be considered by toxicologists and toxicologic pathologists when assessing the effects of new pharmacologic agents on the female reproductive system. We then explore in detail the known roles for p38 MAPKs and related molecules in female reproduction. This foundation is then extended to pathological conditions in which p38 MAPKs are thought to play an integral role.

The C-terminal tail of CRTH2 is a key molecular determinant that constrains Galphai and downstream signaling cascade activation

Prostaglandin D(2) activation of the seven-transmembrane receptor CRTH2 regulates numerous cell functions that are important in inflammatory diseases, such as asthma. Despite its disease implication, no studies to date aimed at identifying receptor domains governing signaling and surface expression of human CRTH2. We tested the hypothesis that CRTH2 may take advantage of its C-tail to silence its own signaling and that this mechanism may explain the poor functional responses observed with CRTH2 in heterologous expression systems. Although the C terminus is a critical determinant for retention of CRTH2 at the plasma membrane, the presence of this domain confers a signaling-compromised conformation onto the receptor. Indeed, a mutant receptor lacking the major portion of its C-terminal tail displays paradoxically enhanced Galpha(i) and ERK1/2 activation despite enhanced constitutive and agonist-mediated internalization. Enhanced activation of Galpha(i) proteins and downstream signaling cascades is probably due to the inability of the tail-truncated receptor to recruit beta-arrestin2 and undergo homologous desensitization. Unexpectedly, CRTH2 is not phosphorylated upon agonist-stimulation, a primary mechanism by which GPCR activity is regulated. Dynamic mass redistribution assays, which allow label-free monitoring of all major G protein pathways in real time, confirm that the C terminus inhibits Galpha(i) signaling of CRTH2 but does not encode G protein specificity determinants. We propose that intrinsic CRTH2 inhibition by its C terminus may represent a rather unappreciated strategy employed by a GPCR to specify the extent of G protein activation and that this mechanism may compensate for the absence of the classical phosphorylation-dependent signal attenuation.

Multiple roles for the C-terminal tail of the chemokine scavenger D6

D6 is a heptahelical receptor that suppresses inflammation and tumorigenesis by scavenging extracellular pro-inflammatory CC chemokines. Previous studies suggested this is dependent on constitutive trafficking of stable D6 protein to and from the cell surface via recycling endosomes. By internalizing chemokine each time it transits the cell surface, D6 can, over time, remove large quantities of these inflammatory mediators. We have investigated the role of the conserved 58-amino acid C terminus of human D6, which, unlike the rest of the protein, shows no clear homology to other heptahelical receptors. We show that, in human HEK293 cells, a serine cluster in this region controls the constitutive phosphorylation, high stability, and intracellular trafficking itinerary of the receptor and drives green fluorescent protein-tagged beta-arrestins to membranes at, and near, the cell surface. Unexpectedly, however, these properties, and the last 44 amino acids of the C terminus, are dispensable for D6 internalization and effective scavenging of the chemokine CCL3. Even in the absence of the last 58 amino acids, D6 still initially internalizes CCL3 but, surprisingly, exposure to ligand inhibits subsequent CCL3 uptake by this mutant. Progressive scavenging is therefore abrogated. We conclude that the heptahelical body of D6 on its own can engage the endocytotic machinery of HEK293 cells but that the C terminus is indispensable for scavenging because it prevents initial chemokine engagement of D6 from inhibiting subsequent chemokine uptake.

Interaction of extracellular signal-regulated protein kinase 1/2 with actin cytoskeleton in supraoptic oxytocin neurons and astrocytes: role in burst firing

Neuronal firing patterns determine the manner of neurosecretion, the underlying mechanisms of which are poorly understood. Using supraoptic nuclei in brain slices from lactating rats, we examined the involvement of extracellular signal-regulated protein kinase 1/2 (ERK1/2) and filamentous actin (F-actin) in burst generation by oxytocin (OT) neurons. Blocking phosphorylation of ERK1/2 (pERK1/2) decreased miniature EPSCs and blocked OT-evoked bursts, as did intracellularly loading an antibody against pERK1/2. OT (10 pM) increased cytosolic pERK1/2 close to the cell membrane within the first 5 min, subsiding by 30 min, whereas OT elicited pERK1/2 nuclear translocation in closely associated supraoptic astrocytes. The increased pERK1/2 was tightly correlated with spatiotemporal actin dynamics. In OT neurons, OT initially increased F-actin, particularly at membrane subcortical areas, and then decreased it after 30 min. Both polymerization and depolymerization of actin cytoskeleton were associated with bursts, but only polymerization facilitated OT-evoked bursts. Blocking ERK1/2 activation blocked OT-evoked actin polymerization, whereas depolymerizing F-actin increased pERK1/2 expression. These changes were further identified in vivo. In intact animals, suckling increased ERK1/2 activation in the cytosol and membrane subcortical area F-actin formation in OT neurons, whereas it increased F-actin concentration in astrocytic somata. Coimmunoprecipitation showed that suckling increased molecular interactions between pERK1/2 and actin. Finally, two different blockers of ERK1/2 kinase injected intracerebroventricularly reduced suckling-evoked milk ejections. This is the first demonstration that OT mediation of suckling-evoked bursts/milk ejections is via interactions between pERK1/2 and actin cytoskeleton.

Oxytocin-induced activation of eukaryotic elongation factor 2 in myometrial cells is mediated by protein kinase C

The nonapeptide oxytocin (OT) mediates a wide spectrum of biological action, many of them related to reproduction. Recently, we have shown that OT exerts a trophic effect on uterine smooth muscle cells and induces dephosphorylation, and thus activation, of the translation elongation factor eukaryotic elongation factor 2 (eEF2). The present study was designed to elucidate the mechanisms underlying this novel action of OT in the well-characterized human myometrial cell line hTERT-C3. Pathways known to induce eEF2 dephosphorylation are mammalian target of rapamycin (mTOR), and the MAPKs ERK1/2 and p38. Using a panel of chemical inhibitors of specific signaling pathways, we determined that none of these pathways played a role in OT-mediated eEF2 dephosphorylation. Because the OT receptor is a G protein-coupled receptor linked to Galphaq, we tested the possibility that this OT action was mediated via protein kinase C (PKC). PKC activity was blocked by application of the general PKC chemical inhibitor Go6983 or by incubation with the cell-permeable PKC inhibitor peptide myr-psi PKC. With either approach, the effect of OT on eEF2 dephosphorylation was suppressed, indicating that the PKC pathway is essential for this OT action. Consistent with this idea, we also found that direct stimulation of PKC with the phorbol ester phorbol 12-myristate 13-acetate induced eEF2 dephosphorylation. Moreover, we observed that the stimulatory effect of OT on [(35)S]methionine incorporation into nascent proteins was blocked by PKC inhibition. Overall, these results define a novel hormonal signaling pathway that leads to eEF2 dephosphorylation and activation of protein synthesis.

Modeling the human oxytocin receptor for drug discovery efforts

The oxytocin receptor belongs to class A receptors within the great family of G protein-coupled receptors. The endogenous ligand oxytocin is a nonapeptide hormone that induces the uterine contractions at parturition and is used to induce the labor. The peptide oxytocin and, even more, its non-peptide antagonist, could be valuable tools in tocolysis. The knowledge of the three-dimensional structure of the oxytocin receptor and the determination of the main interaction points between the receptor and the ligands may help to develop selective oxytocin agonists and antagonist. This review summarizes the knowledge about the mapping of the binding domain of the oxytocin receptor and the efforts in the field of molecular modeling studies related to oxytocin receptor-ligand interactions.

Preterm labour. Biochemical and endocrinological preparation for parturition

Preterm delivery is a common obstetric problem occurring in about 1 in 10 of all births. Preterm babies have a high risk of morbidity and mortality. Such births account for 75% of all major neonatal problems. At the other end of the spectrum, prolonged pregnancy is also a subject of concern because it too is associated with increased fetal morbidity and mortality. Despite extensive research, the mechanisms that control the length of human pregnancy and signal the onset of labour have not been fully determined. This chapter will discuss basic principles in the biology of parturition and the regulation of contraction-associated proteins including the oxytocin receptor. The major pathways regulating contractions and the transcriptional regulation of the main genes that are known to be involved in the onset of labour and parturition will be examined. Some new potentially therapeutic strategies for the biochemical management of preterm labour will be discussed.

Hormonal signaling and signal pathway crosstalk in the control of myometrial calcium dynamics

Understanding the basis for the control of myometrial contractant and relaxant signaling pathways is important to understanding how to manage myometrial contractions. Signaling pathways are influenced by the level of expression of the signals and signal pathway components, the location of these components in the appropriate subcellular environment, and covalent modification. Crosstalk between these pathways regulates the effectiveness of signal transduction and represents an important way by which hormones can regulate phenotype. This review deals primarily with signaling pathways that control Ca2+ entry and intracellular release, as well as the interplay between these pathways.

Amino acids in the COOH-terminal region of the oxytocin receptor third intracellular domain are important for receptor function

Previously, residue K6.30 in the COOH-terminal region of the third intracellular domain (3iC) of the oxytocin (OT) receptor (OTR) was identified as important for receptor function leading to phospholipase C activation in both OTR and the vasopressin V(2) receptor (V(2)R) chimera V(2)ROTR3iC. Substitution of either A6.28K or V6.30K in wild-type V(2)R did not recapitulate the increase in phosphatidylinositide (PI) turnover observed in V(2)ROTR3iC. Hence, the role of K6.30 may be context-specific. Deletion of two NH(2)-terminal OTR3iC segments in the V(2)ROTR3iC chimera did not diminish vasopressin-stimulated PI turnover, whereas deletion of RVSSVKL (residues 6.19-6.25) reduced receptor expression. Deletion of this sequence in wild-type OTR reduced expression by 50% without affecting affinity for [(3)H]OT. This OTR mutant was unable to activate PI turnover or extracellular signal-regulated kinase 1/2 phosphorylation. The effects of alanine substitution for individual residues in RVSSVKL indicated differential importance for OTR function. The R6.19A substitution lost high-affinity sites for [(3)H]OT and the ability to stimulate PI turnover. Affinity for [(3)H]OT and membrane expression was not affected by any other substitutions. OTR-V6.20A and OTR-K6.24A mutants functioned as well as wild-type OTR, whereas OTR S6.21A, S6.22A, and V6.23A mutants exhibited impaired abilities to activate PI turnover (20-40% of OTR), and the OTR-L6.25A mutant exhibited constitutive activity. In conclusion, specific amino acids in the RVSSVKL segment in the COOH-terminal region of the third intracellular domain of OTR influence the ability of OTR to activate G protein-mediated actions.

Dominant role of betagamma subunits of G-proteins in oxytocin-evoked burst firing

Pulsatile neuropeptide secretion is associated with burst firing patterns; however, intracellular signaling cascades leading to bursts remain unclear. We explored mechanisms underlying burst firing in oxytocin (OT) neurons in the supraoptic nucleus in brain slices from lactating rats. Application of 10 pm OT for 30 min or progressively rising OT concentrations from 1 to 100 pm induced burst firing in OT neurons in patch-clamp recordings. Burst generation was blocked by OT antagonist and ionotropic glutamate receptor blockers or tetanus toxin. Blocking G-protein activation with suramin or intracellular GDP-beta-S, but not intracellularly administered antibody against the OT-receptor (OTR) C terminus, blocked bursts. Moreover, pretreatment of slices with pertussis toxin, an inhibitor of G(i/o)-proteins, did not block OT-evoked bursts, suggesting that G(i)/G(o) activation is unnecessary for burst generation. Thus, we further examined G alpha(q/11)-associated signaling pathways in OT-evoked bursts. Inhibition of phospholipase C or RhoA/Rho kinase did not block bursts. Activation of G betagamma subunits using myristoylated G betagamma-binding peptide (mSIRK) caused bursts, whereas intracellularly loaded antibody against G beta subunit blocked OT-evoked bursts. Blocking Src family kinase, but not phosphatidylinositol 3-kinase, occluded OT-evoked bursts. Similar to the effects of OT on EPSCs, mSIRK inhibited tonic EPSCs and elicited EPSC clustering. Finally, suckling caused dissociation of OTRs and G beta subunits from G alpha(q/11) subunits shown by coimmunoprecipitation and immunocytochemistry, supporting crucial roles for OTRs and G betagamma subunits in the milk-ejection reflex. We conclude that G betagamma subunits play a dominant role in burst firing evoked by applied OT or by suckling.

Oxytocin and the human prostate in health and disease

Oxytocin is a peptide hormone produced by the neurohypophysis. The discovery that the peptide is produced locally within the male and female reproductive tracts has raised the possibility that oxytocin may have paracrine and autocrine actions outside of the nervous system. Oxytocin and its receptor have been identified in the human prostate. The prostate is an androgen-dependent organ whose function is to secrete components of the seminal fluid. Oxytocin has been shown to modulate contractility of prostate tissue and also to regulate local concentrations of the biologically active androgens. Oxytocin has also been shown to regulate cell growth. Prostate disease is common and results from abnormal growth of the gland. Oxytocin concentrations are altered in both benign and malignant prostate diseases and in vitro studies suggest that the peptide may be involved in the pathophysiology of these diseases.

Helix 8 of the Viral Chemokine Receptor ORF74 Directs Chemokine Binding

The constitutively active G-protein-coupled receptor and viral oncogene ORF74, encoded by Kaposi sarcoma-associated herpesvirus (human herpesvirus 8), binds a broad range of chemokines, including CXCL1 (agonist), CXCL8 (neutral ligand), and CXCL10 (inverse agonist). Although chemokines interact with the extracellular N terminus and loops of the receptor, we demonstrate that helix 8 (Hx8) in the intracellular carboxyl tail (C-tail) of ORF74 directs chemokine binding. Partial deletion of the C-tail resulted in a phenotype with reduced constitutive activity but intact regulation by ligands. Complete deletion of the C-tail, including Hx8, resulted in an inactive phenotype that lacks CXCL8 binding sites and has an increased number of binding sites for CXCL10. Similar effects were obtained with the single R7.61(322)W or Q7.62(323)P mutations in Hx8. We propose that the conserved charged or polar side chain at position 7.61 has a specific role in stabilizing the end of transmembrane domain 7 (TM7). Disruption of Hx8 by deletion or mutation distorts an H-bonding network, involving highly conserved amino acids within TM2, TM7, and Hx8, that is crucial for positioning of the TM domains, coupling to Galphaq, and CXCL8 binding. Thus, Hx8 appears to exert a key role in receptor stabilization through the conserved residue R7.61, directing the ligand binding profile of ORF74 and likely also that of other class A G-protein-coupled receptors.

Residues in the Hydrophilic Face of Putative Helix 8 of Oxytocin Receptor Are Important for Receptor Function†

Oxytocin receptor (OTR) activates the GTP-binding protein Galpha(q). To investigate whether the N-terminal region of the fourth intracellular domain of this receptor, which forms putative helix 8, plays a role in coupling, its hydrophilic residues (H7.59, H7.62, E7.63, Q7.66, and R7.67) were mutated individually to alanine. In COSM6 cells, these mutants were expressed at equivalent concentrations, but at lower concentrations than OTR. Alanine substitution for H7.62 or Q7.66 did not substantially affect the affinity for OT (K(d) = 0.63 and 0.48 nM, respectively, vs 0.52 nM for the wild type), whereas substitution for H7.59, E7.63, or R7.67 reduced the affinity 5-6-fold. When expressed at equal concentrations, OTR-H7.62/A and OTR-Q7.66/A stimulated phosphatidylinositide turnover as well as OTR, whereas OTR-H7.59/A, OTR-E7.63/A, and OTR-R7.67/A exhibited an impaired ability to respond to OT. Therefore, residues H7.59, E7.63, and R7.67 within the putative hydrophilic interface appeared to influence both the OTR conformation and Galpha(q) coupling. To explore this further, five multiple alanine substitution mutants were constructed. Alanine modification at H7.62 and Q7.66 did not substantially affect the affinity for OT (K(d) = 0.75 nM), whereas any combination of alanine substitutions for H7.59, E7.63, and R7.67 produced mutant receptors that lost high-affinity ligand binding. While OTR-(H7.62,Q7.66)/A exhibited PLC activation equivalent to that of OTR, receptors with two or more changes in H7.59, E7.63, and R7.67 lost the ability to respond to OT in a dose-dependent manner. Five residues (L7.60, F7.61, L7.64, V7.65, and F7.68) in the opposite hydrophobic interface were also mutated to alanine. None of these substitutions affected ligand binding; only OTR-(L7.60,F7.61)/A had a somewhat weaker ability to activate PLC. These data are consistent with the prediction that these residues lie within an amphipathic alpha-helix and emphasize the importance of this hydrophilic interface, and particularly of H7.59, E7.63, and R7.67, in OTR function.

Adult cardiac Sca-1-positive cells differentiate into beating cardiomyocytes

Although somatic stem cells have been reported to exist in various adult organs, there have been few reports concerning stem cells in the heart. We here demonstrate that Sca-1-positive (Sca-1+) cells in adult hearts have some of the features of stem cells. Sca-1+ cells were isolated from adult murine hearts by a magnetic cell sorting system and cultured on gelatin-coated dishes. A fraction of Sca-1+ cells stuck to the culture dish and proliferated slowly. When treated with oxytocin, Sca-1+ cells expressed genes of cardiac transcription factors and contractile proteins and showed sarcomeric structure and spontaneous beating. Isoproterenol treatment increased the beating rate, which was accompanied by the intracellular Ca(2+) transients. The cardiac Sca-1+ cells expressed oxytocin receptor mRNA, and the expression was up-regulated after oxytocin treatment. Some of the Sca-1+ cells expressed alkaline phosphatase after osteogenic induction and were stained with Oil-Red O after adipogenic induction. These results suggest that Sca-1+ cells in the adult murine heart have potential as stem cells and may contribute to the regeneration of injured hearts.

Extracellular signal-regulated kinase 1/2 activation by myometrial oxytocin receptor involves Galpha(q)Gbetagamma and epidermal growth factor receptor tyrosine kinase activation

The mechanisms by which oxytocin (OT) stimulates extracellular signal-regulated kinase 1/2 (ERK1/2) are only partially understood. OT receptor (OTR) signals predominantly through Galpha(q), but ERK1/2 phosphorylation (ERK1/2-P) in PHM1 myometrial cells was not eliminated by inhibition of downstream effectors such as phospholipase C or protein kinase C. Inconsistent with a Galpha(i)-coupled response, pertussis toxin inhibition of OT-induced ERK1/2-P was reversed by the protein kinase A inhibitors Rp-cAMPS and KT5720. Consistent with an inhibitory role for protein kinase A, pertussis toxin pretreatment raised cellular cAMP and 8-(4-chlorophenylthio)-cAMP inhibited OT-induced ERK1/2-P. Attenuation of the OT response by the Gbetagamma scavenger carboxyl terminus of the beta-adrenergic receptor kinase implicated a Gbetagamma-mediated pathway. In both COSM6 cells overexpressing OTR (OTR-COSM6) and in PHM1 cells, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor AG1478 markedly reduced OT-induced ERK1/2-P, whereas the platelet-derived growth factor receptor tyrosine kinase inhibitor AG1296 had no effect. Furthermore, OT increased EGFR tyrosine phosphorylation in OTR-COSM6 cells, which was inhibited by AG1478 or EGTA plus thapsigargin pretreatment. AG1478 did not affect inositol 1,4,5-triphosphate production by OT or protein kinase C-stimulated ERK1/2-P but completely blocked ionomycin-induced ERK1/2-P and EGFR tyrosine phosphorylation. In both OTR-COSM6 and PHM1 cells, EGTA reduced OT-stimulated ERK1/2-P; no ERK1/2-P was observed when intracellular calcium increases were blocked by pretreatment with thapsigargin plus EGTA. These data are consistent with activation of a Gbetagamma-mediated pathway as a consequence of Galpha(q) activation in myometrium and OTR-COSM6 cells that results in increased ERK1/2-P. This pathway involves both EGFR activation and an influence of calcium.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Cholesterol and steroid hormones: modulators of oxytocin receptor function

The function and physiological regulation of the oxytocin-receptor system is strongly steroid-dependent. This is, unexpectedly, only partially reflected by the promoter sequences in the oxytocin receptor and favors the idea that posttranscriptional mechanisms may also play a significant role for the physiological regulation of the oxytocin-receptor system. Our data indicate that cholesterol acts as an allosteric modulator of the oxytocin receptor and stabilizes both membrane-associated and solubilized OT receptors in a high-affinity state for agonists and antagonists. Moreover, high-affinity OT receptors are 2-fold enriched in cholesterol-rich plasma membrane domains in HEK293 fibroblasts stably expressing the human OT receptor. Biochemical data suggest a direct and cooperative molecular interaction of cholesterol molecules with OT receptors. To localize the cholesterol interacting domain of the oxytocin receptor the C-terminal part including the last two transmembrane domains have been exchanged by the corresponding sequences of the cholecystokinin type B receptor, which is functionally not dependent on cholesterol. Concerning its ligand-binding behavior this chimeric receptor protein showed the same dependence on cholesterol and its analogues as the wild type oxytocin receptor. From mutagenesis experiments and studies with receptor chimera between the OTR and cholecystokinin type B receptor, we conclude that a major part of the cholesterol interacting domain may be localized in the first part of the oxytocin receptor, possibly in a domain nearby the agonist binding site. Progesterone is considered to be essential to maintain the uterine quiescence. High concentrations of progesterone (> 10 microM) attenuate or block the signaling of several GPCRs, including the OT receptor via a fast, reversible and non-genomic pathway. Progesterone is known to inhibit both cholesterol biosynthesis and the intracellular trafficking of cholesterol. We therefore test the hypothesis that progesterone affects the signal transduction and subdomain localization of receptors via its influence on cholesterol trafficking. Since cholesterol-rich subdomains (rafts) are considered to be organization centers for cellular signal transduction, changes of the level or distribution of cholesterol may have profound effects on receptor-mediated signaling in general. Using fluorescence recovery after photobleaching (FRAP) measurements with GFP-tagged oxytocin receptors the influence of steroids on the mobility and distribution of the oxytocin receptor in the plasma membrane was analyzed. Progesterone had no effect on the lateral mobility of the oxytocin receptor, but it led to marked inhibition of cellular motility such as vesicle trafficking and movements of filopodia. Non-genomic effects of progesterone and estradiol with respect to receptor signaling as well as the influence of cholesterol on signal transduction will be discussed in more detail.

Localization of the human oxytocin receptor in caveolin-1 enriched domains turns the receptor-mediated inhibition of cell growth into a proliferative response

In this study, we investigated the functional role of the localization of human OTR in caveolin-1 enriched membrane domains. Biochemical fractionation of MDCK cells stably expressing the WT OTR-GFP indicated that only minor quantities of receptor are partitioned in caveolin-1 enriched domains. However, when fused to caveolin-2, the OTR protein proved to be exclusively localized in caveolin-1 enriched fractions, where it bound the agonist with increased affinity and efficiently coupled to Galpha(q/11). Interestingly, the chimeric protein was unable to undergo agonist-induced internalization and remained confined to the plasma membrane even after prolonged agonist exposure (120 min). A striking difference in receptor stimulation was observed when the OT-induced effect on cell proliferation was analysed: stimulation of the human WT OTR inhibited cell growth, whereas the chimeric protein had a proliferative effect. These data indicate that the localization of human OTR in caveolin-1 enriched microdomains radically alters its regulatory effects on cell growth; the fraction of OTR residing in caveolar structures may therefore play a crucial role in regulating cell proliferation.

A sexual arousability model involving steroid effects at the plasma membrane

This review will discuss the status of research related to sexual arousability. It will also present a model for sexual arousability based on current knowledge of steroids effects at the membranes of cells. Steroids have multiple rapid actions that are suggested to result from actions at membrane-associated receptors. When stimulated by steroids these receptors alter G-protein coupling in a manner unique to this complex. Initial stimulation of the receptors by steroids alters the coupling pattern of G-proteins and of other binding sites associated with the complex. This change in G-protein coupling is a stable alteration and thus may serve as a long-term change in the system, which is a requirement of sexual arousability. Stimulation of this receptor system by a surge of oxytocin at ejaculation or orgasm then decouples the G-protein and reduces arousability. Sex hormone binding globulin may be an important ligand at this complex. This model suggests completely new relationships among steroids and their receptors that may complement or diverge from actions at known intracellular receptors.

Regulation of MAP kinase activity by peptide receptor signalling pathway: paradigms of multiplicity

G protein-coupled receptors (GPCRs) can stimulate the mitogen-activated protein kinase (MAPK) cascade and thereby induce cellular proliferation like receptor tyrosine kinases (RTKs). Work over the past 5 years has established several models which reduce the links of G(i)-, G(q)-, and G(s)-coupled receptors to MAPK on few principle pathways. They include (i) Ras-dependent activation of MAPK via transactivation of RTKs such as the epidermal growth factor receptor (EGFR), (ii) Ras-independent MAPK activation via protein kinase C (PKC) that converges with the RTK signalling at the level of Raf, and (iii) activation as well as inactivation of MAPK via the cAMP/protein kinase A (PKA) pathway in dependency on the type of Raf. Most of these generalizing hypotheses are founded on experimental data obtained from expression studies and using a limited set of individual receptors. This review will compare these models with pathways to MAPK found for a great variety of peptide hormone and neuropeptide receptor subtypes in various cells. It becomes evident that under endogenous conditions, the transactivation pathway is less dominant as postulated, whereas pathways involving isoforms of PKC and, especially, phosphoinositide 3-kinase (PI-3K) appear to play a more important role as assumed so far. Highly cell-specific and unusual connections of signalling proteins towards MAPK, in particular tumour cells, might provide points of attacks for new therapeutic concepts.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

Phorbol ester differentially regulates oxytocin receptor binding activity in hypothalamic cultured neurons and astrocytes

Hypothalamic cultured neurons and astrocytes were used to investigate the cellular mechanisms underlying the oxytocin receptor-mediated downregulation through a possible involvement of protein kinase C (PKC). For this purpose, the effects of PKC activators, inhibitor and of OT on OT receptor binding activity were compared in both cultures. In neurons, phorbol-myristate-acetate (PMA), a potent PKC activator, increased the binding of an OT receptor antagonist whereas in astrocytes, a decrease was observed. Pre-treatment of the cells with bisindolylmaleimide (10(-4) M), a PKC inhibitor, prevented the PMA-induced up- and downregulation. In contrast, receptor downregulation resulting from treatment of both cells with OT (10(-9) M) was not affected by the PKC inhibitor. On the other hand, when PMA (10(-7) M) was tested along with OT (10(-9) M), a subsequent decrease in ligand binding was observed in astrocytes. In neurons, PMA attenuated the OT-induced downregulation. Structural analysis of neuron and astrocyte OT receptor mRNA by RT-PCR, subcloning and sequencing, demonstrated identical sequence to rat uterine receptor. In conclusion, these data suggest that activation of PKC has opposite effect on OT receptor binding activity in neurons and astrocytes but they do not support the involvement of PKC in the OT-induced downregulation.

Stimulation of intracellular sphingosine-1-phosphate production by G-protein-coupled sphingosine-1-phosphate receptors

Recently, a family of G-protein-coupled receptors named endothelial differentiation gene (Edg) receptor family has been identified, which are specifically activated by the two serum lipids, sphingosine-1-phosphate and lysophosphatidic acid. Sphingosine-1-phosphate can also act intracellularly to release Ca2+ from intracellular stores. Since in several cell types, G-protein-coupled lysophosphatidic acid or sphingosine-1-phosphate receptors mobilize Ca2+ in the absence of a measurable phospholipase C stimulation, it was analysed here whether intracellular sphingosine-1-phosphate production was the signalling mechanism used by extracellular sphingosine-1-phosphate for mobilization of stored Ca2+. Sphingosine-1-phosphate and the low affinity sphingosine-1-phosphate receptor agonist, sphingosylphosphorylcholine, induced a rapid, transient and nearly complete pertussis toxin-sensitive Ca2+ mobilization in human embryonic kidney (HEK-293) cells. The G-protein-coupled sphingosine-1-phosphate receptors, Edg-1, Edg-3 and Edg-5, were found to be endogenously expressed in these cells. Most interestingly, sphingosine-1-phosphate and sphingosylphosphorylcholine did not induce a measurable production of inositol-1,4,5-trisphosphate or accumulation of inositol phosphates. Instead, sphingosine-1-phosphate and sphingosylphosphorylcholine induced a rapid and transient increase in production of intracellular sphingosine-1-phosphate with a maximum of about 1.4-fold at 30 s. Stimulation of sphingosine-1-phosphate formation by sphingosine-1-phosphate and sphingosylphosphorylcholine was fully blocked by pertussis toxin, indicating that extracellular sphingosine-1-phosphate via endogenously expressed G(i)-coupled receptors induces a stimulation of intracellular sphingosine-1-phosphate production. As sphingosine-1-phosphate- and sphingosylphosphorylcholine-induced increases in intracellular Ca2+ were blunted by sphingosine kinase inhibitors, this sphingosine-1-phosphate production appears to mediate Ca2+ signalling by extracellular sphingosine-1-phosphate and sphingosylphosphorylcholine in HEK-293 cells.

Activation of functional oxytocin receptors stimulates cell proliferation in human trophoblast and choriocarcinoma cell lines

Despite oxytocin receptors (OTR) being present in human chorio-decidual tissues, their expression and role in placental trophoblast cells in the context of tumor growth or physiological functions related to cell proliferation have never been examined. In the present study we demonstrate the presence and functionality of OTR in normal human trophoblast cell lines (ED77 and ED27) and a choriocarcinoma cell line (BeWo). RT-PCR and immunofluorescence analysis revealed the presence of OTR messenger RNA and protein in these cells. Binding studies using [(125)I]oxytocin ([(125)I]OT) antagonist confirmed the presence of specific binding sites in ED27, ED77, and BeWo cells. OTR functionality was demonstrated by measuring the OT-induced increase in the intracellular calcium concentrations. This effect was dose dependent and was blocked by the selective OT antagonist d(CH(2))(5)[Tyr(Me)(2),Thr(4), Tyr-NH(2)(9)]OVT (OT antagonist). Furthermore, two proteins with apparent molecular masses of 125 and 60 kDa became tyrosine phosphorylated in all of the cell lines after OT stimulation (and an additional protein of 45 kDa in BeWo choriocarcinoma cells), suggesting that this peptide can stimulate tyrosine kinase activity. Finally, we observed a dose-dependent OT stimulation of cell proliferation associated with OTR activation that was completely abolished by the selective OT antagonist. These findings provide the first evidence of the presence of functional OTR in normal trophoblast cell lines as well as in choriocarcinoma cells and show that a specific effect of OT on normal and neoplastic trophoblast is to promote cellular proliferation.

Characterization and expression of oxytocin and the oxytocin receptor

Oxytocin, a nonapeptide hormone and neurotransmitter, is expressed in a variety of tissues, as are its receptors. In vivo, oxytocin acts as a paracrine and/or autocrine mediator of multiple biological effects. These effects are exerted primarily through interactions with G-protein-coupled oxytocin/vasopressin receptors, which, via G(q) and G(i), stimulate phospholipase C-mediated hydrolysis of phosphoinositides. It is generally recognized that, during pregnancy, oxytocin plays a major role in increasing myometrial contractility at term, and that it acts on its cardiac receptor to decrease the cardiac rate and force of contraction. It is, however, doubtful that increased endocrine oxytocin concentration is involved in the onset and progression of normal human labor.