Importance of poly(A)-binding protein 4 (PABP4) in healthy pregnancy

Abstract

Healthy pregnancy requires a tightly regulated materno-fetal dialogue for processes such as embryo implantation, endometrial decidualisation (in the mouse), placentation and maternal adaptation to occur. Disruption of placental development as well as maternal adaptation can lead to fetal intrauterine growth restriction (IUGR) which increases the risk of late miscarriage/stillbirth (e.g. 53% of preterm stillbirth and 26% of term stillbirth are found to be IUGR). Furthermore, IUGR is a risk factor for neurodevelopmental conditions in childhood and for a spectrum of related adult health disorders such as cardiovascular disease and type II diabetes, often termed metabolic syndrome. Despite these pregnancy disorders being common (e.g. 1 in 200 pregnancies results in stillbirth in the UK) the molecular lesion(s) underlying their pathophysiology are poorly understood and in particular those with placental and/or maternal aetiologies most frequently remain unexplained. Here we investigate the hypothesis that poly(A)-binding protein 4 (PABP4) is required for healthy pregnancy in mice. PABP4 is an RNA-binding protein and a member of the PABP family which are central regulators of mRNA translation and stability. Using all four permutations of wild-type and knock-out crosses, we find that maternal PABP4-deficiency results in a reduced litter size and IUGR. The number of implantations at e8.5 were not reduced in Pabp4-/- females, implying that the reduced litter size was not a consequence of decreased ovulation, fertilisation or implantation frequency. Further longitudinal analysis (at e13.5, e15.5 and e18.5) reveals that fetal death primarily occurred between e18.5 and birth, suggesting these mice may provide a unique opportunity to inform on the maternal causes of stillbirth. The onset of IUGR, which was found to be symmetrical in nature, was established by e15.5 preceding the majority of fetal death. During pregnancy, a materno-fetal dialogue directs and responds to changes in gene expression to give rise to the placenta and adapt the maternal physiology. Defects in these processes may result in reduced growth and/or fetal death and were examined in Pabp4-/- mice to shed light on the mechanistic basis of these related phenotypes. Fetal to placental (F:P) weight ratio, whose changes can be indicative of placental insufficiency or placental adaptation in an attempt to aid fetal growth, was found to be increased in Pabp4-/- dams at e15.5 and e18.5 due to the presence of IUGR fetuses with placentas of normal weight. Consistent with this observation, placental volume was unchanged at e18.5. Total placental weight and volume alone fails to discriminate potential differences in the individual placental zones which include the labyrinth zone, where materno-fetal gas and nutrient exchange occur; the junctional zone, which has endocrine functions including those that promote maternal adaptation; and the decidua basalis, derived from the maternal endometrium and is the site of trophoblast invasion and maternal vascular remodelling in early pregnancy. Therefore, volumetric analysis of these zones and the maternal blood spaces, which transcend the decidua basalis and junctional zone, was undertaken. This showed no change in the maternal blood spaces or the labyrinth, the latter being the zone whose size is most frequently altered in IUGR. Critically however, the size of the maternally-derived decidua basalis was increased with a concurrent decrease in the size of the junctional zone. These morphological changes may play a causative role either through directly affecting placental function and/or by the reduced junctional zone failing to promote appropriate maternal adaptation. Alternatively, they may reflect compensatory adaptations to a primary defect elsewhere in the mother. Complementing these morphological studies, functional studies were undertaken: remodelling of maternal vasculature and the resistance index of vessels delivering blood to the fetus were assessed; as was delivery of nutrients to the fetus (measured by fetal glucose); and systemic maternal adaptations (maternal hormonal profile, circulating glucose levels and organ weights). Uterine, umbilical and decidual spiral arteries were examined, but displayed no apparent differences suggestive of normal blood supply to the fetus. However fetal blood glucose was reduced suggesting a reduced delivery of nutrients important for fetal growth. This was not due to lower circulating maternal blood glucose levels, and mRNA levels of the placental glucose transporters Glut-1 and Glut-3 were not reduced but upregulated, suggestive of an attempt to compensate for reduced fetal glucose. Furthermore, upregulation of at least one system A amino acid transporter mRNA, Snat-2, was observed. The maternal physiological state of PABP4-deficient dams showed deviations in some organ weights (e.g. spleen weight is reduced at e13.5 and e15.5) and the levels of some circulating hormones (e.g. estradiol is deceased whereas progesterone is increased at e18.5). However, future work will be required to determine which, if any, of these changes are primary defects rather than downstream consequences and to identify which mis-regulated mRNAs/pathways within in the materno-fetal dialogue underlie the phenotype. Taken together, my results suggest that the regulation of mRNA translation/stability by PABP4 is critical to achieving the correct pattern of gene expression within the materno-fetal dialogue to enable appropriate placentation and maternal adaptation. Furthermore, my results suggest that Pabp4-/- mice provide a unique opportunity to further understand the maternal causes of a spectrum of related pregnancy complications including IUGR, late miscarriage and stillbirth.