Investigation of the role of nitric oxide synthase 2 in pregnancy using mutant mice

Nitric oxide (NO) has been implicated as a signalling molecule in many cellular processes. As nitric oxide synthase 2 (NOS-2) is the main isoform expressed in mouse decidua and metrial gland, mice with a targeted disruption of the gene encoding NOS-2 were used to determine the potential roles of this enzyme during pregnancy. Reproductive success and the morphology of implantation sites throughout pregnancy were compared in NOS-2 deficient (NOS-2(-/-)) and wild-type (WT) mice. Although there were no significant differences in the duration of gestation or birth weight, NOS-2(-/-) mice had significantly fewer viable embryos at mid-gestation and delivered smaller litters than did WT mice. Histological sections of uteroplacental units from WT and NOS-2(-/-) mice were compared to establish the mechanisms underlying the loss of fetuses. No morphological differences were observed on day 6 or day 8 of gestation, indicating that implantation and early development of implantation sites were unaffected by the absence of NOS-2. However, by mid-gestation, decidua of NOS-2(-/-) mice had reduced cellularity and their decidual arteries had abnormally thickened walls. These observations were quantified by morphometric measurements, which showed a significant reduction in decidual cellular area and a significant increase in the blood vessel wall : lumen ratio in NOS-2(-/-) mice. The increase in the thickness of the blood vessel walls was not due to abnormal cellular infiltration or to altered expression of alpha-actin in vascular smooth muscle. These results indicate that NOS-2 has a functional role in the maintenance of decidual cellular integrity and development of appropriate uterine vasculature, and may play a supportive role in promoting embryo survival.