The Effect of an Oxytocin Receptor Antagonist (Retosiban, GSK221149A) on the Response of Human Myometrial Explants to Prolonged Mechanical Stretch

Multiple pregnancies, the myometrium and the role of mechanical factors in the timing of labour

Multiple pregnancy remains a relatively common occurrence, but it is associated with increased risks of adverse outcomes for the mother and her babies and presents unique challenges to healthcare providers. This review will briefly discuss multiple pregnancies, their aetiology and their problems, including preterm birth, before reviewing the processes leading to normal labour onset and how they may be different in a multiple pregnancy. The mechanisms by which mechanical factors i.e., uterine distension or 'stretch' contribute to uterine excitability and the timing of labour onset will be the major focus, and how over distention may pre-dispose multiple pregnancies to preterm birth. This includes current thinking around the role of mechano (stretch) sensitive ion channels in the myometrium and changes to other important regulators of excitability and contraction which have been identified from studies using in vitro and in vivo models of uterine stretch. Physiological stimuli arising from the fetus(es) and placenta(s) will also be discussed. In reviewing what we know about the myometrium in multiple pregnancy in humans, the focus will be on twin pregnancy as it is the most common type of multiple pregnancy and has been the most studied.

Is human labor at term an inflammatory condition?†

Parturition at term in normal pregnancy follows a predictable sequence of events. There is some evidence that a state of inflammation prevails in the reproductive tissues during labor at term, but it is uncertain whether this phenomenon is the initiating signal for parturition. The absence of a clear temporal sequence of inflammatory events prior to labor casts doubt on the concept that normal human labor at term is primarily the result of an inflammatory cascade. This review examines evidence linking parturition and inflammation in order to address whether inflammation is a cause of labor, a consequence of labor, or a separate but related phenomenon. Finally, we identify and suggest ways to reconcile inconsistencies regarding definitions of labor onset in published research, which may contribute to the variability in conclusions regarding the genesis and maintenance of parturition. A more thorough understanding of the processes underlying normal parturition at term may lead to novel insights regarding abnormal labor, including spontaneous preterm labor, preterm premature rupture of the fetal membranes, and dysfunctional labor, and the role of inflammation in each.

Preterm Birth Therapies to Target Inflammation

Preterm birth (PTB; defined as delivery before 37 weeks of pregnancy) is the leading cause of morbidity and mortality in infants and children aged <5 years, conferring potentially devastating short‐ and long‐term complications. Despite extensive research in the field, there is currently a paucity of medications available for PTB prevention and treatment. Over the past few decades, inflammation in gestational tissues has emerged at the forefront of PTB pathophysiology. Even in the absence of infection, inflammation alone can prematurely activate the main components of parturition resulting in uterine contractions, cervical ripening and dilatation, membrane rupture, and subsequent PTB. Mechanistic studies have identified critical elements of the complex inflammatory molecular pathways involved in PTB. Here, we discuss therapeutic options that target such key mediators with an aim to prevent, postpone, or treat PTB. We provide an overview of more traditional therapies that are currently used or being tested in humans, and we highlight recent advances in preclinical studies introducing novel approaches with therapeutic potential. We conclude that urgent collaborative action is required to address the unmet need of developing effective strategies to tackle the challenge of PTB and its complications.

Tools, techniques, and future opportunities for characterizing the mechanobiology of uterine myometrium

The myometrium is the smooth muscle layer of the uterus that generates the contractions that drive processes such as menstruation and childbirth. Aberrant contractions of the myometrium can result in preterm birth, insufficient progression of labor, or other difficulties that can lead to maternal or fetal complications or even death. To investigate the underlying mechanisms of these conditions, the most common model systems have conventionally been animal models and human tissue strips, which have limitations mostly related to relevance and scalability, respectively. Myometrial smooth muscle cells have also been isolated from patient biopsies and cultured in vitro as a more controlled experimental system. However, in vitro approaches have focused primarily on measuring the effects of biochemical stimuli and neglected biomechanical stimuli, despite the extensive evidence indicating that remodeling of tissue rigidity or excessive strain is associated with uterine disorders. In this review, we first describe the existing approaches for modeling human myometrium with animal models and human tissue strips and compare their advantages and disadvantages. Next, we introduce existing in vitro techniques and assays for assessing contractility and summarize their applications in elucidating the role of biochemical or biomechanical stimuli on human myometrium. Finally, we conclude by proposing the translation of "organ on chip" approaches to myometrial smooth muscle cells as new paradigms for establishing their fundamental mechanobiology and to serve as next-generation platforms for drug development.

The impact of progesterone and RU-486 on classic pro-labour proteins & contractility in human myometrial tissues during 24-hour exposure to tension & interleukin-1β

Increased expression of pro-labour genes that encode cyclooxygenase-2 (COX-2), oxytocin receptor (OTR) and connexin-43 (Cx43) at parturition is often attributed to P4 functional withdrawal, based on findings from animal models and human primary myometrial cells. However, the cause of reduced myometrial P4 responsiveness that promotes contractions at labour is not fully determined. Uterine stretch occurs with advancing gestation but most in vitro experimental models do not take this into consideration. We aimed to examine whether tissue-level myometrial stretch influences the ability of P4 to regulate pro-labour protein abundance by using myometrial biopsies from term gestation pregnant women to assess the impact of 24 h exposure to combinations of (i) stretch-mediated tension, (ii) P4 (100 nM) and (iii) an anti-progestin, RU-486 (1 μM). Firstly, we observed baseline COX-2 and Cx43 protein levels increased, whereas P4 content along with calponin-1 and progesterone receptor (PR) protein abundance decreased, in vehicle-treated tissues. P4 supplementation subtly reduced COX-2 levels in un-stretched tissues. Spontaneous and oxytocin-augmented contractility were unchanged by tissue culture exposure to P4 and/or RU-486. Interleukin-1β (IL-1β; 1 ng/ml) enhanced COX-2 protein and PGE₂ content in un-stretched tissues. Overall, tissue stretch may, in part, regulate P4-sensitive pro-labour protein levels, but this is likely to be reliant on interaction with other in utero factors that were absent in our tissue cultures. More complex culture conditions should be evaluated in future to aid further development of a physiologically relevant model to improve our understanding of in utero myometrial P4 responsiveness.

The Oxytocin Receptor: From Intracellular Signaling to Behavior

The many facets of the oxytocin (OXT) system of the brain and periphery elicited nearly 25,000 publications since 1930 (see FIGURE 1 , as listed in PubMed), which revealed central roles for OXT and its receptor (OXTR) in reproduction, and social and emotional behaviors in animal and human studies focusing on mental and physical health and disease. In this review, we discuss the mechanisms of OXT expression and release, expression and binding of the OXTR in brain and periphery, OXTR-coupled signaling cascades, and their involvement in behavioral outcomes to assemble a comprehensive picture of the central and peripheral OXT system. Traditionally known for its role in milk let-down and uterine contraction during labor, OXT also has implications in physiological, and also behavioral, aspects of reproduction, such as sexual and maternal behaviors and pair bonding, but also anxiety, trust, sociability, food intake, or even drug abuse. The many facets of OXT are, on a molecular basis, brought about by a single receptor. The OXTR, a 7-transmembrane G protein-coupled receptor capable of binding to either Gαior Gαqproteins, activates a set of signaling cascades, such as the MAPK, PKC, PLC, or CaMK pathways, which converge on transcription factors like CREB or MEF-2. The cellular response to OXT includes regulation of neurite outgrowth, cellular viability, and increased survival. OXTergic projections in the brain represent anxiety and stress-regulating circuits connecting the paraventricular nucleus of the hypothalamus, amygdala, bed nucleus of the stria terminalis, or the medial prefrontal cortex. Which OXT-induced patterns finally alter the behavior of an animal or a human being is still poorly understood, and studying those OXTR-coupled signaling cascades is one initial step toward a better understanding of the molecular background of those behavioral effects.

Retosiban Prevents Stretch-Induced Human Myometrial Contractility and Delays Labor in Cynomolgus Monkeys

Context

Stretch of the myometrium promotes its contractility and is believed to contribute to the control of parturition at term and to the increased risk of preterm birth in multiple pregnancies.

Objective

To determine the effects of the putative oxytocin receptor (OTR) inverse agonist retosiban on (1) the contractility of human myometrial explants and (2) labor in nonhuman primates.

Design

Human myometrial biopsies were obtained at planned term cesarean, and explants were exposed to stretch in the presence and absence of a range of drugs, including retosiban. The in vivo effects of retosiban were determined in cynomolgus monkeys.

Results

Prolonged mechanical stretch promoted myometrial extracellular signal-regulated kinase (ERK)1/2 phosphorylation. Moreover, stretch-induced stimulation of myometrial contractility was prevented by ERK1/2 inhibitors. Retosiban (10 nM) prevented stretch-induced stimulation of myometrial contractility and phosphorylation of ERK1/2. Moreover, the inhibitory effect of retosiban on stretch-induced ERK1/2 phosphorylation was prevented by coincubation with a 100-fold excess of a peptide OTR antagonist, atosiban. Compared with vehicle-treated cynomolgus monkeys, treatment with oral retosiban (100 to 150 days of gestational age) reduced the risk of spontaneous delivery (hazard ratio = 0.07, 95% confidence interval 0.01 to 0.60, P = 0.015).

Conclusions

The OTR acts as a uterine mechanosensor, whereby stretch increases myometrial contractility through agonist-free activation of the OTR. Retosiban prevents this through inverse agonism of the OTR and, in vivo, reduced the likelihood of spontaneous labor in nonhuman primates. We hypothesize that retosiban may be an effective preventative treatment of preterm birth in high-risk multiple pregnancies, an area of unmet clinical need.

Contractility Measurements of Human Uterine Smooth Muscle to Aid Drug Development

Discovery and characterization of novel pharmaceutical compounds or biochemical probes rely on robust and physiologically relevant assay systems. We describe methods to measure ex vivo myometrium contractility. This assay can be used to investigate factors and molecules involved in the modulation of myometrial contraction and to determine their excitatory or inhibitory actions, and hence their therapeutic potential in vivo. Biopsies are obtained from women undergoing cesarean section delivery with informed consent. Fine strips of myometrium are dissected, clipped and attached to a force transducer within 1 mL organ baths superfused with physiological saline solution at 37 °C. Strips develop spontaneous contractions within 2-3 h under set tension and remain stable for many hours (>6 h). Strips can also be stimulated to contract such as by the endogenous hormones, oxytocin and vasopressin, which cause concentration-dependent modulation of contraction frequency, force and duration, to more closely resemble contractions in labor. Hence, the effect of known and novel drug leads can be tested on spontaneous and agonist-induced contractions. This protocol specifically details how this assay can be used to determine the potency of known and novel agents by measuring their effects on various parameters of human myometrial contraction. We use the oxytocin- and V1a receptor antagonists, atosiban and SR49059 as examples of known compounds which inhibit oxytocin- and vasopressin-induced contractions, and demonstrate how this method can be used to complement and validate pharmacological data obtained from cell-based assays to aid drug development. The effects of novel agonists in comparison to oxytocin and vasopressin can also be characterized. Whilst we use the example of the oxytocin/ vasopressin system, this method can also be used to study other receptors and ion channels that play a role in uterine contraction and relaxation to advance the understanding of human uterine physiology and pathophysiology.

Myometrial cytokines and their role in the onset of labour

Human labour is an inflammatory event, physiologically driven by an interaction between hormonal and mechanical factors and pathologically associated with infection, bleeding and excessive uterine stretch. The initiation and communicators of inflammation is still not completely understood; however, a key role for cytokines has been implicated. We summarise the current understanding of the nature and role of cytokines, chemokines and hormones and their involvement in signalling within the myometrium particularly during labour.