Exploring a novel mechanism instructing maternal care and influencing offspring outcomes

Pregnancy induces numerous changes in the physiology of the mother as she adapts to the metabolically demanding fetus developing within her womb. It is during this period of intense physiological stress that she begins to develop the maternal behaviours that will ultimately support the nurturing of the offspring after parturition and protect them from harm. In human pregnancies, women are particularly susceptible to mood disorders which may reflect an enhanced vulnerability induced by pregnancy. The placenta is the key organ of pregnancy driving the physiological and potentially the behavioural changes required for a successful pregnancy. Placental dysfunction may contribute to maternal mood disorders by mis-programming maternal behaviour, a relationship that can be explored in a tractable animal model. Imprinted genes are expressed in the placenta and have been implicated in the regulation of key endocrine lineage of the mouse placenta. Phlda2 is a maternally expressed imprinted gene that regulates one such lineage, the Spongiotrophoblast (SpT), which is a source of placental lactogens and a number of other hormones. Placental lactogens belong to the same hormone family as prolactin and some members of this family mediate their action via the Prolactin Receptor (Prlr). Both prolactin and the Prlr have been directly shown to be required for the establishment of maternal care behaviour in rodents. This led to the hypothesis that Phlda2 might modulate maternal care in rodents by regulating the size of the placental endocrine compartment. To test this hypothesis, we used three cohorts of Wildtype (WT) dams carrying fetuses that possessed either a 50% reduction in the SpT, resulting from a double dose of Phlda2 (Mouseline: Phlda2 +/+ BACx1, TG; 2X) or a 200% increase in the SpT, resulting from a null dose of Phlda2 (Mouseline: Phlda2 -/+), KO; 0X) or 100% of the WT size of the SpT, resulting from a single dose of Phlda2 (Mouseline: Phlda2 +/+, WT; 1X) generated using Recipient Embryo Transfer (RET). Maternal behaviour was studied postpartum and a biomolecular characterisation was performed during pregnancy using microarray and Quantitative Polymerase Chain Reaction (qPCR) to look at changes in maternal gene expression and a histological approach used to examine maternal neurogenesis in the Sub-ventricular Zone (SVZ) of the lateral ventricles and Sub-Granular Zone (SGZ) of the hippocampus. Additionally, a study was made of male offspring carrying the Phlda2 transgene (TG), which were previously shown to be low birth weight, their non-transgenic litter mates (NON-TG) and a fully WT cohort of mice to ask whether placental endocrine dysfunction in utero programmed altered behaviour later in life. The results of this first study showed distinct changes in pup retrieval, nest building and the dam’s grooming and licking behaviour with dams exposed to the smaller endocrine compartment spending less time with their pups. Specific maternal brain regions showed altered transcriptional profiles at Embryonic Day (E) 16.5 of pregnancy and there was a significant reduction in neurogenesis. While there were no differences in anxiety or locomotor activity levels in the offspring cohorts, there were significant changes in hedonic response in both the TG and NON-TG offspring. Together, these data provide the first evidence that imprinted genes can influence both maternal care behaviour and offspring behavioural outcomes via the placental endocrine compartment. This work has wider implications since human studies have shown that elevated placental PHLDA2 is a common features of human growth restricted pregnancies. There is a a co-occurrence of low birth weight and maternal mood disorders with mothers experiencing prenatal depression having a three-fold increased risk of a Low Birth Weight (LBW) baby. Aberrant imprinting in the placenta could account for this co-occurrence.