The role of the Igf2/H19 locus in regulating murine placental endocrine capacity
The placenta is responsible for materno-fetal nutrient transfer and produces an abundance of hormones and signalling factors that adapt maternal metabolism to support fetal growth. In mice, these functions are performed in two distinct placental regions, the transport labyrinth zone (Lz) and the endocrine junctional zone (Jz). Insulin like growth factor 2 (Igf2) is a growth-promoting paternally expressed imprinted gene found in an imprinting locus with H19, a maternally expressed imprinted long noncoding RNA. In mice, manipulation of the expression of these genes or the differentially methylated imprinting control region (ICR1), which regulates their expression, leads to changes in fetoplacental growth in association with altered placental transport function. While there is evidence that placental Jz formation is impacted, the specific role of this locus in regulating placental endocrine function is unknown. The studies presented in this thesis aimed to test the hypothesis that dysregulation of imprinting at the Igf2/H19 locus, in the mouse placental Jz, alters placental endocrine phenotype. This was assessed using two novel imprinting dysregulation models. The first entailed the under-expression of Igf2 in the placental Jz (Jz-Igf2UE), characterised in Chapters 3 and 4. The second involved Jz deletion of ICR1 (Jz-ΔICR1) leading to increased Igf2 and decreased H19 expression in the Jz, characterised in Chapter 5. The Lz, fetus and mother were not genetically manipulated in either model. Jz-Igf2UE altered placental endocrine function in a sex dependent manner. Female placentas had decreased endocrine cell formation with Jz-Igf2UE, while placentas from males were structurally unaltered. Gene expression of IGF signalling components and Jz hormones were significantly altered in a manner largely dependent on fetal sex. RNA sequencing of isolated Jz samples from female placentas identified 110 dysregulated genes with Jz-Igf2UE. These were linked to hormone synthesis, IGF signalling, metabolism and immune function pathways. Jz-ΔICR1 enhanced endocrine cell formation and Jz hormone expression in both sexes. However, the placental phenotype of females was more severely altered with Jz-ΔICR1. These changes were related to sexually dimorphic alterations in the abundance of IGF2 receptors and downstream signalling pathways (PI3K-AKT and MAPK). Additionally, whilst the Lz was not genetically manipulated and maintained control expression levels of Igf2 and H19, the placental Lz volume and the expression of key nutrient transporters within the Lz was reduced with Jz-ΔICR1. This provides evidence that paracrine signalling exists between the Jz and Lz and this form of signalling may help regulate placental size and function. Taken collectively, this thesis highlights that regulation of the Igf2/H19 locus modulates placental endocrine function and may govern the allocation of resources to the fetus in pregnancy.