The placental programming hypothesis: Placental endocrine insufficiency and the co-occurrence of low birth weight and maternal mood disorders.

Polypeptide hormones and steroid hormones, either expressed by the placenta or dependant on the placenta for their synthesis, are key to driving adaptations in the mother during pregnancy that support growth in utero. These adaptations include changes in maternal behaviour that take place in pregnancy and after the birth to ensure that offspring receive appropriate care and nutrition. Placentally-derived hormones implicated in the programming of maternal caregiving in rodents include prolactin-related hormones and steroid hormones. Neuromodulators produced by the placenta may act directly on the fetus to support brain development. A number of imprinted genes function antagonistically in the placenta to regulate the development of key placental endocrine lineages expressing these hormones. Gain-in-expression of the normally maternally expressed gene Phlda2 or loss-of-function of the normally paternally expressed gene Peg3 results in fewer endocrine cells in the placenta, and pups are born low birth weight. Importantly, wild type dams carrying these genetically altered pups display alterations in their behaviour with decreased focus on nurturing (Phlda2) or heightened anxiety (Peg3). These same genes may regulate placental hormones in human pregnancies, with the potential to influence birth weight and maternal mood. Consequently, the aberrant expression of imprinted genes in the placenta may underlie the reported co-occurrence of low birth weight with maternal prenatal depression.

Imprinted genes influencing the quality of maternal care.

In mammals successful rearing imposes a cost on later reproductive fitness specifically on the mother creating the potential for parental conflict. Loss of function of three imprinted genes in the dam results in deficits in maternal care suggesting that, like maternal nutrients, maternal care is a resource over which the parental genomes are in conflict. The induction of maternal care is a complex, highly regulated process and it is unsurprising that many gene disruptions and environmental adversities result in maternal care deficits. However, recent compelling evidence for a more purposeful imprinting phenomenon comes from observing alterations in the mother's behaviour when expression of the imprinted genes Phlda2 and Peg3 has been manipulated solely in the offspring. This explicit demonstration that imprinted genes expressed in the offspring influence maternal behaviour lends significant weight to the hypothesis that maternal care is a resource that has been manipulated by the paternal genome.

Maternal care boosted by paternal imprinting in mammals.

In mammals, mothers are the primary caregiver, programmed, in part, by hormones produced during pregnancy. High-quality maternal care is essential for the survival and lifelong health of offspring. We previously showed that the paternally silenced imprinted gene pleckstrin homology-like domain family A member 2 (Phlda2) functions to negatively regulate a single lineage in the mouse placenta called the spongiotrophoblast, a major source of hormones in pregnancy. Consequently, the offspring's Phlda2 gene dosage may influence the quality of care provided by the mother. Here, we show that wild-type (WT) female mice exposed to offspring with three different doses of the maternally expressed Phlda2 gene-two active alleles, one active allele (the extant state), and loss of function-show changes in the maternal hypothalamus and hippocampus during pregnancy, regions important for maternal-care behaviour. After birth, WT dams exposed in utero to offspring with the highest Phlda2 dose exhibit decreased nursing and grooming of pups and increased focus on nest building. Conversely, 'paternalised' dams, exposed to the lowest Phlda2 dose, showed increased nurturing of their pups, increased self-directed behaviour, and a decreased focus on nest building, behaviour that was robustly maintained in the absence of genetically modified pups. This work raises the intriguing possibility that imprinting of Phlda2 contributed to increased maternal care during the evolution of mammals.

Loss of offspring Peg3 reduces neonatal ultrasonic vocalizations and increases maternal anxiety in wild-type mothers.

Depression and anxiety are the most common mental health conditions during pregnancy and can impair the normal development of mother-infant interactions. These adversities are associated with low birth weight and increased risk of behavioural disorders in children. We recently reported reduced expression of the imprinted gene PATERNALLY EXPRESSED GENE 3 (PEG3) in placenta of human infants born to depressed mothers. Expression of Peg3 in the brain has previously been linked maternal behaviour in rodents, at least in some studies, with mutant dams neglecting their pups. However, in our human study decreased expression was in the placenta derived from the fetus. Here, we examined maternal behaviour in response to reduced expression of Peg3 in the feto-placental unit. Prenatally we found novelty reactivity was altered in wild-type females carrying litters with a null mutation in Peg3. This behavioural alteration was short-lived and there were no significant differences the transcriptomes of either the maternal hypothalamus or hippocampus at E16.5. In contrast, while maternal gross maternal care was intact postnatally, the exposed dams were significantly slower to retrieve their pups and displayed a marked increase in anxiety. We also observed a significant reduction in the isolation-induced ultrasonic vocalizations (USVs) emitted by mutant pups separated from their mothers. USVs are a form of communication known to elicit maternal care suggesting Peg3 mutant pups drive the deficit in maternal behaviour. These data support the hypothesis that reduced placental PEG3 in human pregnancies occurs as a consequence of prenatal depression but leaves scope for feto-placental Peg3 dosage, during gestation, influencing aspects of maternal behaviour.

Increased dosage of the imprinted Ascl2 gene restrains two key endocrine lineages of the mouse Placenta.

Imprinted genes are expressed primarily from one parental allele by virtue of a germ line epigenetic process. Achaete-scute complex homolog 2 (Ascl2 aka Mash2) is a maternally expressed imprinted gene that plays a key role in placental and intestinal development. Loss-of-function of Ascl2 results in an expansion of the parietal trophoblast giant cell (P-TGC) lineage, an almost complete loss of Trophoblast specific protein alpha (Tpbpa) positive cells in the ectoplacental cone and embryonic failure by E10.5. Tpbpa expression marks the progenitors of some P-TGCs, two additional trophoblast giant cell lineages (spiral artery and canal), the spongiotrophoblast and the glycogen cell lineage. Using a transgenic model, here we show that elevated expression of Ascl2 reduced the number of P-TGC cells by 40%. Elevated Ascl2 also resulted in a marked loss of the spongiotrophoblast and a substantial mislocalisation of glycogen cells into the labyrinth. In addition, Ascl2-Tg placenta contained considerably more placental glycogen than wild type. Glycogen cells are normally located within the junctional zone in close contact with spongiotrophoblast cells, before migrating through the P-TGC layer into the maternal decidua late in gestation where their stores of glycogen are released. The failure of glycogen cells to release their stores of glycogen may explain both the inappropriate accumulation of glycogen and fetal growth restriction observed late in gestation in this model. In addition, using in a genetic cross we provide evidence that Ascl2 requires the activity of a second maternally expressed imprinted gene, Pleckstrin homology-like domain, family a, member 2 (Phlda2) to limit the expansion of the spongiotrophoblast. This "belts and braces" approach demonstrates the importance of genomic imprinting in regulating the size of the placental endocrine compartment for optimal placental development and fetal growth.

A Role for the Placenta in Programming Maternal Mood and Childhood Behavioural Disorders.

Substantial data demonstrate that the early-life environment, including in utero, plays a key role in later life disease. In particular, maternal stress during pregnancy has been linked to adverse behavioural and emotional outcomes in children. Data from human cohort studies and experimental animal models suggest that modulation of the developing epigenome in the foetus by maternal stress may contribute to the foetal programming of disease. Here, we summarise insights gained from recent studies that may advance our understanding of the role of the placenta in mediating the association between maternal mood disorders and offspring outcomes. First, the placenta provides a record of exposures during pregnancy, as indicated by changes in the placental trancriptome and epigenome. Second, prenatal maternal mood may alter placental function to adversely impact foetal and child development. Finally, we discuss the less well established but interesting possibility that altered placental function, more specifically changes in placental hormones, may adversely affect maternal mood and later maternal behaviour, which can also have consequence for offspring well-being.

The imprinted Phlda2 gene modulates a major endocrine compartment of the placenta to regulate placental demands for maternal resources.

Imprinted genes, which are expressed from a single parental allele in response to epigenetic marks first established in the germline, function in a myriad of processes to regulate mammalian development. Recent work suggests that imprinted genes may regulate the signalling function of the placenta by modulating the size of the endocrine compartment. Here we provide in vivo evidence that this hypothesis is well founded. Elevated expression of the imprinted Pleckstrin homology-like domain, family a, member 2 (Phlda2) gene drives a reduction of the spongiotrophoblast endocrine compartment, diminished placental glycogen and asymmetric foetal growth restriction. Using both loss-of-function and gain-in-expression mouse models, here we further show that Phlda2 exclusively modulates the spongiotrophoblast compartment of the placenta without significantly altering the composition of the trophoblast giant cell endocrine lineages that share a common progenitor with this lineage. Additionally, we show that Phlda2 loss-of-function placentae contain nearly three times more placental glycogen than non-transgenic placentae. Remarkably, relative to a fully wild type scenario, wild type placentae also accumulate excessive glycogen. While loss-of-function of Phlda2 increased both placental weight and placental glycogen, the weight of both mutant and non-transgenic fetuses was lower than that found in a fully wild type scenario indicating that excessive glycogen accumulation comes at the cost of foetal growth. This work firstly highlights a novel signalling function for the spongiotrophoblast in stimulating the global accumulation of placental glycogen. Furthermore, this work suggests that Phlda2 manipulates the placenta's demands for maternal resources, a process that must be tightly regulated by epigenetic marks to ensure optimal foetal growth.