Placental defects in ARNT-knockout conceptus correlate with localized decreases in VEGF-R2, Ang-1, and Tie-2

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a transcriptional regulator that heterodimerizes with Per-ARNT-Sim (PAS) proteins. ARNT also dimerizes with hypoxia inducible factor1alpha (HIF1alpha), inducing expression of vascular endothelial cell growth factor (VEGF) to promote angiogenesis. The angiogenesis/vasculogenesis pathway is required for embryonic survival and includes several receptors (VEGFR1, VEGFR2, Tie2) and ligands (VEGF, Ang1, Ang2, neuropillin). Transgenic knockout of ARNT in mice is lethal due to abnormal placentation. This study examines the VEGF pathway in GD9.5 embryos of wild-type (+/+), heterozygous (+/-), or knockout (-/-) ARNT genotype. All genotypes expressed abundant VEGF in trophoblastic giant cells. However, -/- conceptuses had less VEGFR2 in placental labyrinth and trophoblastic giant cells. Ang1 and Tie2 decreased in trophoblastic giant cells and Ang2 was decreased in placental endothelial cells. Abnormal development of the labyrinth correlated with decreased binding of VEGF and decreased expression of VEGFR2. In addition, VEGFR2 seemed to be the primary VEGF binding receptor in the labyrinth and blood lacunae of the placenta, as binding could be eliminated by masking the VEGFR2 receptor with inactive antibody complex. VEGFR1 may be primarily responsible for binding of VEGF to yolk sac and embryonic tissues, as masking VEGFR2 did not reduce VEGF binding in those areas, and it is interesting that major structural defects were also not found in those regions. In summary, in the ARNT knockout conceptus, the impact of ARNT deficiency on placental expression of VEGFR2 seems to provide an explanation for the failure of the placental labyrinth to progress, whereas the vascularization of the yolk sac and embryo appear relatively unaffected on GD9.5. Published 2000 Wiley-Liss, Inc.