Natural killer (NK) cells are recruited into the uterine stroma during establishment of the hemochorial placenta and are proposed regulators of uterine spiral artery remodeling. Failures in uterine spiral artery remodeling are linked to diseases of pregnancy. This prompted an investigation of the involvement of NK cells in placentation. NK cell depletion decreased the delivery of proangiogenic factors and delayed uterine spiral artery development, leading to decreased oxygen tension at the placentation site, stabilized hypoxia-inducible factor 1A protein, and redirected trophoblast differentiation to an invasive phenotype. Trophoblast cells replaced the endothelium of uterine spiral arteries extending the depth of the placental vascular bed and accelerating vessel remodeling. Hypoxia-regulated trophoblast lineage decisions, including expansion of invasive trophoblast, could be reproduced in vitro by using rat trophoblast stem cells and were dependent on hypoxia-inducible factor signaling. We conclude that NK cells guide hemochorial placentation through controlling a hypoxiasensitive adaptive reflex regulating trophoblast lineage decisions.

The rat possesses hemochorial placentation with deep intrauterine trophoblast cell invasion and trophoblast-directed uterine spiral artery remodeling; features shared with human placentation. Recognition of these similarities spurred the establishment of in vitro and in vivo research methods using the rat as an animal model to address mechanistic questions regarding development of the hemochorial placenta. The purpose of this review is to provide the requisite background to help move the rat to the forefront in placentation research.

The hemochorial placenta develops from the coordinated multilineage differentiation of trophoblast stem (TS) cells. An invasive trophoblast cell lineage remodels uterine spiral arteries, facilitating nutrient flow, failure of which is associated with pathological conditions such as preeclampsia, intrauterine growth restriction, and preterm birth. Hypoxia plays an instructive role in influencing trophoblast cell differentiation and regulating placental organization. Key downstream hypoxia-activated events were delineated using rat TS cells and tested in vivo, using trophoblast-specific lentiviral gene delivery and genome editing. DNA microarray analyses performed on rat TS cells exposed to ambient or low oxygen and pregnant rats exposed to ambient or hypoxic conditions showed up-regulation of genes characteristic of an invasive/ vascular remodeling/inflammatory phenotype. Among the shared up-regulated genes was matrix metallopeptidase 12 (MMP12). To explore the functional importance of MMP12 in trophoblast celldirected spiral artery remodeling, we generated an Mmp12 mutant rat model using transcription activator-like nucleases-mediated genome editing. Homozygous mutant placentation sites showed decreased hypoxia-dependent endovascular trophoblast invasion and impaired trophoblast-directed spiral artery remodeling. A link was established between hypoxia/HIF and MMP12; however, evidence did not support Mmp12 as a direct target of HIF action. Lysine demethylase 3A (KDM3A) was identified as mediator of hypoxia/HIF regulation of Mmp12. Knockdown of KDM3A in rat TS cells inhibited the expression of a subset of the hypoxia-hypoxia inducible factor (HIF)-dependent transcripts, including Mmp12, altered H3K9 methylation status, and decreased hypoxia-induced trophoblast cell invasion in vitro and in vivo. The hypoxia-HIF-KDM3A-MMP12 regulatory circuit is conserved and facilitates placental adaptations to environmental challenges. placenta | hypoxia | trophoblast invasion | epigenetics | plasticity