Delineating the placental mechanisms underpinning poor pregnancy outcomes in advanced maternal age and exploring the therapeutic potential of melatonin

Abstract

In high income countries there is an ongoing increase in the proportion of the obstetric population classed as advanced maternal age (AMA; ≥35 years). AMA is associated with an increased risk of adverse pregnancy outcomes, including fetal growth restriction (FGR) and stillbirth. The aetiology behind this increased risk is unknown but there is evidence for placental dysfunction in AMA. Many pregnancy pathologies are associated with changes in placental oxidative stress and mitochondrial dysfunction. Similar changes are also observed in ageing. Elevated placental oxidative stress and mitochondrial dysfunction may underlie placental dysfunction and adverse pregnancy outcome in AMA. Further to this, melatonin, a neurohormone with antioxidant properties, has been proposed as a therapeutic in pregnancy complications, potentially enhancing fetal growth. This project tested the hypotheses (i) that there is elevated placental oxidative stress and mitochondrial dysfunction in mice of AMA (ii) that melatonin, an antioxidant, would increase fetal growth via alleviation of placental oxidative stress in AMA mice. A systematic review with meta-analysis, initially conducted to assess whether murine AMA models demonstrate reproducible effects on pregnancy outcome, confirmed consistent reports of reduced litter size and fetal weight in AMA mice. These reproducible phenotypes allowed AMA mice to be used in subsequent studies. C57Bl/6J AMA (36-42 weeks) and young (12-16 weeks) mice were used to assess placental oxidative stress, apoptosis (immunostaining) and mitochondrial function (respirometry and citrate synthase activity). To assess melatonin as a potential therapeutic, melatonin (10 μg/ml) or vehicle was administered via drinking water from embryonic day (E)12.5 to E17.5 to AMA and young mice. Fetal and placental weight were recorded and immunostaining for oxidative stress and measurement of citrate synthase activity were performed in placentas from melatonin and vehicle-treated dams. In placentas from AMA versus young dams, oxidative stress (lipid peroxidation) and apoptosis were increased. When normalised to sample mitochondrial content, complex I, II and IV oxygen consumption were significantly reduced in AMA. However, when normalised to protein content, these differences were not observed. Melatonin treatment significantly increased fetal weight and placental efficiency in both male and female fetuses in AMA dams and significantly reduced placental lipid peroxidation in both young and AMA dams in male fetuses only. Placental mitochondrial content (citrate synthase activity) was increased in AMA dams. Melatonin significantly reduced mitochondrial content in placentas from male and female fetuses from both young and AMA dams. This project confirmed the hypothesis that AMA in mice is associated with elevated placental oxidative stress and apoptosis. Mitochondrial dysfunction was observed but due to the concurrent increase in mitochondrial number, there were no overall differences in placental bioenergetic capacity. Collectively, this project indicates that the increase in placental mitochondrial content in AMA could be an adaptation by the placenta to maintain appropriate bioenergetic capacity and that oxidative damage may contribute to placental dysfunction in AMA pregnancies. The ability of melatonin to increase fetal growth in AMA mice highlights it as potential therapeutic in women of AMA but the mechanisms through which melatonin improves placental function to enhance fetal growth need to be explored further.

Date of Award	31 May 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Susan Greenwood (Supervisor), Michelle Desforges (Supervisor), Mark Dilworth (Supervisor) & Alexander Heazell (Supervisor)