Hilah Gal,¹ Marina Lysenko,² Sima Stroganov,¹  Ezra Vadai,¹ Sameh Youssef,^3,6 Keren Tzadikevitch-Geffen,⁴ Ron Rotkopf,⁵ Tal Biron-Shental,⁴ Alain de Bruin,^3,6 Michal Neeman² and Valery Krizhanovsky¹

¹Department of Molecular Cell Biology, The Weizmann Institute of Science, 76100  Rehovot, Israel

²Department of Biological Regulation, The Weizmann Institute of Science, 76100  Rehovot, Israel

³Dutch Molecular Pathology Center, Department of Pathobiology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584CL Utrecht, The Netherlands

⁴Department of Obstetrics and Gynecology, Meir Medical Center, 44299 Kfar Saba, Israel

⁵Bioinformatics and Biological Computing Unit, Department of Biological Services, The Weizmann Institute of Science, 76100 Rehovot, Israel

⁶Department of Pediatrics, Division of Molecular Genetics, University Medical Center Groningen, University of Groningen, Groningen, 9713 AV, The Netherlands 

Introduction
The placenta is an autonomous organ, essential for the maintenance of fetal integrity during pregnancy. The multinucleated syncytiotrophoblast layer of the placenta, providing fetal nourishment during gestation, exhibits characteristics of cellular senescence. However, the functional role of molecular pathways regulating senescence in syncytiotrophoblast is not understood. We aim to gain an understanding of the role of senescence in the placenta in normal pregnancies and in pregnancies complicated with Intrauterine Growth Restriction (IUGR).

Materials and Methods
We used mouse models and human placentas from normal and IUGR complicated pregnancies. To elucidate the functional pathways regulating senescence in syncytiotrophoblast, we studied placentas of mice with attenuated senescence programs, including Cdkn1a^-/-, p53^-/-, Cdkn2a^-/- and Cdkn2a^-/-;p53^-/-. Using dynamic contrast-enhanced MRI (DCE-MRI), we monitored the signal intensity dynamics in their placentas, compared to wild-type mice. To evaluate senescence pathways in the placenta, we performed expression profiling of human primary trophoblast cultures and immuno-histological studies on placentas, derived from senescence-attenuated mice and from human normal and IUGR complicated pregnancies.

Results
DCE-MRI studies revealed an altered dynamics in placentas of p53^-/-, Cdkn2a^-/- and Cdkn2a^-/-;p53^-/- mice, accompanied by histopathological changes in placental labyrinths. Human primary syncytiotrophoblast upregulated senescence markers and molecular pathways associated with cell-cycle inhibition and a senescence-associated secretory phenotype. The pathways and components of the secretory phenotype were compromised in mouse placentas with attenuated senescence and in human placentas from pregnancies with IUGR. Importantly, human placentas derived from pregnancies complicated with IUGR showed a marked reduction in the expression of key regulators of senescence.

Conclusions
We propose that molecular mediators of senescence regulate placental structure and function, through both cell autonomous and non-autonomous mechanisms.