Glucocorticoids - cortisol and corticosterone - are steroid hormones synthesised in the adrenal gland that are important mediators of the stress response. Glucocorticoids are also vital in development to aid in organ maturation. Endogenous glucocorticoid levels rapidly rise before birth in all mammals to promote fetal organ maturation. Because preterm birth occurs before this natural rise in glucocorticoid levels, pregnant women at risk of preterm delivery are administered synthetic glucocorticoids to mature the fetal lung and aid neonatal survival. Mice that globally lack the glucocorticoid receptor (GR) die at birth, attributed to lung immaturity. Effects on tissues other than the lung remain less well characterised. Previous work has shown endogenous glucocorticoid action is also essential to mature the mouse fetal heart. Mice globally lacking GR have small, functionally and structurally immature hearts. Mice with tissue-specific deletion of GR in cardiomyocytes and vascular smooth muscle cells (SMGRKO mice; generated using Sm22α-Cre) also have an increased risk of death around the time of birth, suggesting that glucocorticoid maturation of the cardiovascular system is important for neonatal survival. GR expression within the fetal mouse heart initiates at E10.5 but GR in the myocardium is not activated and localised to the nucleus until E15.5. This suggests that mice can respond to glucocorticoid from E10.5. Here, it was hypothesised that antenatal glucocorticoid exposure, prior to the increase in endogenous glucocorticoid levels, would advance fetal heart maturation and this will depend on cardiovascular GR. To investigate the effects of antenatal glucocorticoid treatment on fetal heart maturation in mid-gestation and identify effects mediated by GR, mice with a conditional deletion of GR in cardiomyocytes and vascular smooth muscle cells were studied (SMGRKO mice). Pregnant mice received dexamethasone (dex) in the drinking water from E12.5-E15.5. Levels of Fkbp5 mRNA (a marker of glucocorticoid action) were unchanged between control and SMGRKO mice at E15.5 or following dex treatment. This suggested a lack of response to dex treatment. However, liquid chromatography mass spectrometry measurement confirmed the presence of dex and its active metabolite 6- hydroxydexamethasone (6OHDex) in livers of E15.5 fetuses from dex treated dams (fetal: Dex 0.46 ± 0.1 ng/g, 6OHDex 13.6 ± 0.35 ng/g; dam: Dex 7.96 ± 3.65 ng/g, 6OHDex 4.75 ± 1.2 ng/g). Livers of fetuses exposed to dex had lower levels of the naturally occurring active glucocorticoid, corticosterone, compared to vehicle treated fetuses. This suggests HPA axis suppression in dex exposed fetuses. Maternal liver showed no significant difference in corticosterone levels between dex and vehicle treated mice, suggesting that whilst dex suppressed the HPA axis in fetuses, it did not in the dams. To determine any persistent effects of early antenatal dex treatment on fetal heart, a later time point in gestation, E17.5, was also assessed. At E17.5, 2-days following cessation of dex treatment, dex and its metabolites were undetectable in the fetal and maternal liver. However, corticosterone levels remained reduced in fetal liver at E17.5 in dex exposed animals (vehicle treated: 4.31 ± 0.47 ng/g, Dex treated: 1.72 ± 0.42 ng/g, p < 0.01), whilst levels in the dam liver did not differ from vehicle treated controls. This suggests prolonged HPA axis suppression following dex treatment, which reduced the natural late-gestation rise in glucocorticoids required for fetal organ maturation. To determine whether early antenatal dex treatment could advance fetal heart function, Doppler imaging with a Vevo 770 high frequency ultrasound imager was used. Isovolumetric contraction time, isovolumetric relaxation time and ejection time of the left ventricle were unaltered by dex treatment. However, at E15.5 the mitral deceleration index (MDI), a measure of diastolic function that takes into account loading conditions, was 1.5 fold lower in vehicle treated SMGRKO mice than control (Cre-) littermates (p < 0.05). This reduction in SMGRKO mice suggests glucocorticoids are required within the fetal cardiomyocytes and/or vascular smooth muscle cells to mature the diastolic function of the fetal heart. Dex exposure had no effect on MDI in SMGRKO fetuses, but reduced the MDI by 1.5 fold in control mice to similar levels as in SMGRKO mice (p < 0.05). RNA analysis revealed a trend (p=0.09) for reduced levels of Nr3c1 mRNA (encoding GR) in hearts of E15.5 control (Cre-) fetuses following dex treatment. Although this requires confirmation at the level of GR protein, this finding together with the lack of induction of the GR target, Fkbp5, suggests dex may cause glucocorticoid resistance through down-regulation of GR. At E17.5, 2-days following cessation of dex there were no changes in systolic parameters and the reduction in MDI found at E15.5, following dex, had normalised. Litter size was reduced (close to a 50% reduction) at E17.5 in dex treated mice. This was similar between SMGRKO and control fetuses. The cause of death was not established, but potentially could be due to the reduction in the natural rise in glucocorticoids at E17.5, previously shown to be important for fetal heart maturation. It is therefore possible that mice with more immature hearts may die before reaching E17.5. RNA analysis was undertaken to determine any mechanistic alterations following dex treatment, which could support fetal heart functional alterations found at E15.5. In contrast to expectation, dex also decreased expression of mRNA encoding the calcium handling proteins SERCA2a, NCX1, and CaV1.2 in E15.5 fetal mouse hearts in both control and SMGRKO mice (p < 0.05), compared with the respective vehicle treated mice. These proteins had previously shown to be induced by glucocorticoid action in cardiomyocytes. However, the similar down-regulation in both genotypes indicates this effect is not dependent on GR in cardiomyocytes. Lowered SERCA2a activity following dex treatment could contribute to the changes in MDI observed in control mice. Similarly, Scnn1a and Kcnj12 mRNA levels, previously found to be induced by glucocorticoids in cardiomyocytes, were down-regulated in the E15.5 fetal heart in vivo following dex. Collectively, these data are consistent with glucocorticoid resistance or down-regulation of glucocorticoid action in E15.5 fetal hearts following dex administration. Mutations in KCNJ12 are associated with long QT syndrome, which is characterised by a delayed repolarisation of the heart following each contraction. An altered relaxation of the fetal heart found in control mice following dex could therefore be due to a prolongation of the cardiac action potential, particularly with a delayed repolarisation, because of lower Kcnj12 expression. At E17.5, there were no significant differences in expression of calcium handling genes or ion channel mRNAs between genotypes or following earlier dex exposure. Thus, effects of dex on mRNA expression level may not persist, which could account for the lack of functional changes observed 2-days following cessation of treatment. Because effects seen in vivo with dex treatment were contrary to those predicted, and to further investigate the effect of dex upon calcium content, an in vitro model of primary fetal E15.5 cardiomyocytes was used. Cardiomyocytes were treated with dex for 24 hours and effects on membrane potential voltage changes and calcium transients measured. Following dex, isolated fetal cardiomyocytes showed an elongated repolarisation phase of the action potential (untreated: 120.45 ± 13.81 ms, Dex: 142.34 ± 12.97 ms, p < 0.01), and duration of calcium transients (untreated: 103.31 ± 13.78 ms, Dex: 120.43 ± 23.36 ms, p < 0.05). This assessment of fetal cardiomyocytes was preliminary work to aid in the understanding of mechanisms of fetal heart functional alterations associated with glucocorticoid regulation. The results suggest glucocorticoids may be important in regulating calcium levels. In summary, dex treatment in mice from E12.5-E15.5 did not advance fetal heart maturation. It reduced litter size at E17.5, irrespective of whether GR was expressed in cardiomyocytes or not. The normal late-gestation increase in endogenous glucocorticoid levels in the fetus was reduced by dex, even after treatment finished. / The suppression of corticosterone levels following antenatal dex may reduce maturation of the heart at E15.5 and could be responsible for the reduction in litter size. Downregulation of GR in the fetal heart, may be a mechanism that results in glucocorticoid resistance following antenatal dex treatment, which could explain the lack of beneficial effects of antenatal dex upon fetal heart maturation in these experiments in mice.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:743677 |
| Date | January 2018 |
| Creators | Agnew, Emma Jane |
| Contributors | Chapman, Karen ; Gray, Gillian |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/29555 |
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