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 transcriptio nal regulator that heterodimerizes with PerARNT-Sim (PAS) proteins. ARNT al so dimerizes with hypoxia inducible factor1 alpha (HIF1 alpha), inducing ex pression of vascular endothelial cell growth factor (VEGF) to promote angio genesis. 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 let hal due to abnormal placentation, This study examines the VEGF pathway in G D9.5 embryos of wild-type (+/+), heterozygous (+/-), or knockout (-/-) ARNT genotype, All genotypes expressed abundant VEGF in trophoblastic giant cel ls. However, -/- conceptuses had less VEGFR2 in placental labyrinth and tro phoblastic giant cells. Ang1 and Tie2 decreased in trophoblastic giant cell s and Ang2 was decreased in placental endothelial cells. Abnormal developme nt of the labyrinth correlated with decreased binding of VEGF and decreased expression of VEGFR2, In addition, VEGFR2 seemed to be the primary VEGF bi nding receptor in the labyrinth and blood lacunae of the placenta, as bindi ng could be eliminated by masking the VEGFR2 receptor with inactive antibod y complex. VEGFR1 may be primarily responsible for binding of VEGF to yolk sec 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, th e impact of ARNT deficiency on placental expression of VEGFR2 seems to prov ide an explanation for the failure of the placental labyrinth to progress, whereas the vascularization of the yolk sec and embryo appear relatively un affected on GD9.5. Published 2000 Wiley-Liss, Inc.(dagger).