Critical role of mTOR, PPARδ and PPARd signaling in regulating early pregnancy decidual function, embryo viability and feto-placental growth

STUDY QUESTION: What are the consequences of inhibiting mTOR, the mechanistic target of rapamycin (mTOR), and the peroxisome proliferator activated receptor gamma (PPAR gamma) and PPARd pathways in the early post-implantation period on decidual function, embryo viability and feto-placental growth in the rat? SUMMARY ANSWER: mTOR inhibition from Days 7 to 9 of pregnancy in rats caused decidual PPAR. and PPARd upregulation on Day 9 of pregnancy and resulted in embryo resorption by Day 14 of pregnancy. PPAR gamma and PPAR delta inhibition differentially affected decidual mTOR signaling and levels of target proteins relevant to lipid histotrophic nutrition and led to reduced feto-placental weights on Day 14 of pregnancy. WHAT IS KNOWN ALREADY: Although mTOR, PPAR gamma and PPAR delta are nutrient sensors important during implantation, the role of these signaling pathways in decidual function and how they interact in the early post-implantation period are unknown. Perilipin 2 (PLIN2) and fatty acid binding protein 4 (FABP4), two adipogenic proteins involved in lipid histotrophic nutrition, are targets of mTOR and PPAR signaling pathways in a variety of tissues. STUDY DESIGN, SIZE, DURATION: Rapamycin (mTOR inhibitor, 0.75 mg/kg, sc), T0070907 (PPAR. inhibitor, 0.001 mg/kg, sc), GSK0660 (PPARd inhibitor, 0.1 mg/kg, sc) or vehicle was injected daily to pregnant rats from Days 7 to 9 of pregnancy and the studies were performed on Day 9 of pregnancy (n = 7 per group) or Day 14 of pregnancy (n = 7 per group). PARTICIPANTS/MATERIALS, SETTING, METHODS: On Day 9 of pregnancy, rat decidua were collected and prepared for western blot and immunohistochemical studies. On Day 14 of pregnancy, the resorption rate, number of viable fetuses, crown-rump length and placental and decidual weights were determined. MAIN RESULTS AND THE ROLE OF CHANCE: Inhibition of mTOR in the early post-implantation period led to a reduction in FABP4 protein levels, an increase in PLIN2 levels and an upregulation of PPAR gamma and PPAR delta in 9-day-pregnant rat decidua. Most embryos were viable on Day 9 of pregnancy but had resorbed by Day 14 of pregnancy. This denotes a key function of mTOR in the post-implantation period and suggests that activation of PPAR signaling was insufficient to compensate for impaired nutritional/survival signaling induced by mTOR inhibition. Inhibition of PPAR gamma signaling resulted in decreased decidual PLIN2 and FABP4 protein expression as well as in inhibition of decidual mTOR signaling in Day 9 of pregnancy. This treatment also reduced feto-placental growth on Day 14 of pregnancy, revealing the relevance of PPAR gamma signaling in sustaining post-implantation growth. Moreover, following inhibition of PPAR delta, PLIN2 levels were decreased and mTOR complex 1 and 2 signaling was altered in decidua on Day 9 of pregnancy. On Day 14 of pregnancy, PPAR delta inhibition caused reduced fetoplacental weight, increased decidual weight and increased resorption rate, suggesting a key role of PPAR delta in sustaining post-implantation development. LARGE SCALE DATA: Not applicable. LIMITATIONS, REASONS FOR CAUTION: This is an in vivo animal study and the relevance of the results for humans remains to be established. WIDER IMPLICATIONS OF THE FINDINGS: The early post-implantation period is a critical window of development and changes in the intrauterine environment may cause embryo resorption and lead to placental and fetal growth restriction. mTOR, PPAR gamma and PPAR delta signaling are decidual nutrient sensors with extensive cross-talk that regulates adipogenic proteins involved in histotrophic nutrition and important for embryo viability and early placental and fetal development and growth.