New perspectives on G-protein coupled receptor signaling in the pregnant myometrium: Exploring the role of regulator of G-protein signaling 2
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2024-01
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Abstract
Prematurity at birth results in neonatal death, or lifelong disability. Early deliveries due to preterm labour may be preventable but remain a significant problem as there are no treatments to reliably stop preterm labour. The ineffectiveness of existing treatments for preterm labour are possibly owing to targeting a limited variety of pro-labour signals that are believed to participate in uterine contractions, such as oxytocin and prostaglandins. Interestingly, prostaglandins (e.g. PGE2) also have functions that may counteract labour activation and maintain uterine quiescence during pregnancy, and these mechanisms may be important to consider in the search for new therapies for preterm labour. This thesis aimed to explore whether the regulator of G protein signaling 2 (RGS2) is involved in the mechanism by which certain prostaglandins suppress uterine contractility. RGS2 functions by inhibiting signals from some pro-contractile G protein-coupled receptors, and it is clinically relevant in the function of other smooth muscle organs. We hypothesized that prostaglandins and glucocorticoids (relevant in the treatment of preterm labour) increase RGS2 expression, and consequently reduce the myometrial pro-contractile response to GPCR agonists like oxytocin. Conversely, pro-inflammatory cytokines implicated in labour were expected to reduce RGS2 expression. We anticipated conditions under which RGS2 expression was reduced would have an opposite effect, thereby increasing uterine contractility. First, we investigated whether the expression of RGS2 in human myometrial smooth muscle (MSM) cells is regulated by mediators relevant to labour including prostaglandins, glucocorticoids, and pro-inflammatory cytokines. Prostaglandin E2 enhanced RGS2 mRNA expression, and the addition of glucocorticoids prolonged this effect. Additionally, PGE2 rescued RGS2 expression that was suppressed by pro-inflammatory cytokines. To understand the function of RGS2 in the uterus, sought to understand the diversity of pro-contractile receptors present in the human myometrium towards the end of pregnancy since these may be acted upon by RGS2. In addition to the well-known highly expressed oxytocin receptor, there were high levels of expression of histamine receptor 1 which is known to have a pro-contractile effect dampened by RGS2 in other smooth muscle. Based on this we selected histamine and oxytocin to study the effects of RGS2 on pro-contractile signaling. In MSM cells prostaglandin- and glucocorticoid-mediated increases in RGS2 failed to significantly reduce either histamine or oxytocin-induced calcium signals which were taken as a measure of contractile activity. The histamine, but not the oxytocin calcium response was reduced by adenoviral overexpression of RGS2 in MSM cells. We next investigated whether a reduction in RGS2 expression increased uterine contractility. We utilized a total Rgs2-knockout mouse model and compared uterine contractility of tissues and cells to that of wildtype mice. The absence of RGS2 increased overall contractility in knockout tissues and in freshly dispersed uterine cells the exaggerated response to oxytocin. Furthermore, the relaxant effects of PGE2 on uterine tissue contractility were in part RGS2-dependent, as they were diminished in knockout tissues. Despite the effects of Rgs2-knockout on isolated myometrial tissues and cells, knockout mice did not undergo labour earlier overall. Studying the effect of RGS2 on the timing of labour in humans is difficult since samples are obtained only at the time of delivery and/or labour. Nevertheless, we found that RGS2 was decreased with preterm but not term labour, implicating different mechanisms that give rise to labour in each setting. Whether the reduction of RGS2 seen in preterm labour gives rise to increased uterine contractility, as we observed in the Rgs2-knockout mouse uterus, remains to be determined. In conclusion we demonstrate that RGS2 expression is regulated by PGE2 and that its function appears to involve the suppression of uterine contractility, as demonstrated through both loss and gain of function experiments in complementary models of study. The effects of RGS2 on the receptors that elicit uterine contractility in labour should be more carefully studied, considering a wider variety of receptors acting in concert. As well, the effects of RGS2 on mouse pregnancies can later be examined to determine whether perturbations in RGS2 expression leave the uterus vulnerable to preterm labour.
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Urrego, D. (2024). New perspectives on G-protein coupled receptor signaling in the pregnant myometrium: exploring the role of regulator of G-protein signaling 2 (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.