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In order to characterize the transition of the corpora lutea (CL) from acquisition of luteolytic sensitivity to rescue of luteal function: i) the expression of 38 factors associated with steroids, prostanoids, and angiogenic systems and ii) concentrations of the main hormones responsible for maintenance of CL function in cyclic and pregnant pigs were examined. Additionally, the effect of prostaglandin (PG) E₂ and F_{2 α} on luteal function during the estrous cycle and pregnancy was evaluated in vitro. Significantly up-regulated gene expression was revealed in CL collected on day 14 of the estrous cycle (CYP19A1, ESR2, PTGS2, HIF1A, and EDN1) and on days 12–14 of pregnancy (SCARB1, PGRMC1, STAR, HSD3B1, NR5A1, PTGFR, PTGER4, and VEGFA). Elevated concentrations of estradiol-17β and PGE₂ occurred in CL on days 12 and 14 of pregnancy respectively, while an increased intraluteal PGF_{2 α} content was noted on day 14 of the estrous cycle. Both PGs increased the synthesis of progesterone by cultured luteal slices obtained on day 14 of pregnancy, in contrast to the action of PGF_{2 α} on the corresponding day of the estrous cycle. PGE₂ stimulated cAMP production via PTGER2 and PTGER4, while PGF_{2 α} elevated the content of CREB in cultured luteal slices from CL of pregnant pigs. In silico analysis showed that infiltration of lymphocytes and apoptosis of microvascular endothelium were activated in CL on day 12 of the estrous cycle vs pregnancy. Summarizing, an abundance of E₂ and PGE₂ during pregnancy regulates specific pathways responsible for steroidogenesis, the prostanoid signaling system and angiogenesis during rescue from luteolysis in porcine CL.

The new corpora lutea (CLs) in pigs are formed from the preovulatory follicles after the luteinizing hormone (LH) surge. However, total autonomy and independence of CLs from LH up to Day 12 of cycle has recently been questioned. Transformation of estrous cycle CL to CL of pregnancy initiated by embryonic signals requires not only the cessation of prostaglandin F2 (PGF_2α) supply to the luteal tissue but also needs the CL to overcome luteolytic acquisition and/or changing its sensitivity to PGF_2α during Days 12–14 of pregnancy. The luteolytic cascade is prevented by inhibition of lymphocyte infiltration and leucocyte recruitment, limitation of cell apoptosis, upregulation of pregnancy-associated genes and an enhanced antiluteolytic role of PGE₂. Our ‘two-signal switch hypothesis’ highlights the importance of post PGF_2α and PGE₂ receptor signaling pathways activation in CLs during luteolysis and rescue. The ‘luteolytic switch’ involves increased expression of many regression mediators and activation of the post PTGFR signaling pathway. The ‘rescue switch’ initiated by embryonic signals – estradiol 17β and PGE₂ – induces post PTGER2/4 pathway, turning the ‘luteolytic switch’ off and triggering activity of genes responsible for CL maintenance. In mid and late pregnancy, CLs are maintained by LH and the synergistic action of metabolic hormones. This paper provides an outline of recent views on CL regression, rescue and maintenance during pregnancy in pigs that conflict with previous paradigms and highlights new findings regarding the actions of prostaglandins, role of microRNAs (miRNA) and immune system and signaling pathways governing the life cycle of porcine CL.

Circulating miRNAs were proposed to be indicators of normal or complicated pregnancies. Based on this knowledge and our recent transcriptomic approach showing expression of miRNAs in the porcine endometrium, conceptuses and uterine extracellular vesicles during pregnancy, we have hypothesized that signs of ongoing local embryo-maternal crosstalk involving miRNAs can be detected in the circulation of pregnant gilts as early as a few days after maternal recognition of pregnancy. By applying several molecular biology techniques that differ in dynamic range and precision in maternal serum of Day 16 pregnant pigs, we were able to show for the first time increased levels of several miRNAs, previously reported to be expressed in either conceptuses and extracellular vesicles (miR-26a and miR-125b) or pregnant endometrium (miR-23b). Our results clearly showed that real-time RT-PCR and digital PCR are the most reliable methods, being able to detect small-fold changes of low-abundant circulating miRNAs. Further validation in a separate group of gilts confirmed an increase in miR-23b and miR-125b levels. In silico analyses identified pregnancy-related biological processes and pathways affected by these miRNAs. Target prediction analysis revealed hundreds of porcine transcripts with conserved sites for these miRNAs, which were classified into signaling pathways relevant to pregnancy. We conclude that a unique set of miRNAs can already be observed in the circulation of pigs during the first weeks of pregnancy, as a result of the initiation of embryo-maternal communication.

Interferon-tau (IFNT), a maternal recognition of pregnancy (MRP) signals in domestic ruminants, suppresses the release of luteolytic pulses of uterine prostaglandin F2a (PGF2a), thus extending the corpus luteum (CL) life span. We hypothesized that IFNT also exerts anti-luteolytic actions in bovine CL. To examine the direct effects of IFNT on bovine CL, luteal slices and enriched luteal endothelial cells (LECs) were utilized. We found that recombinant ovine IFNT (roIFNT) markedly elevates interferon-associated genes (STAT1, STAT2 and IRF9) and interferon-stimulated genes (ISGs: MX2, ISG15 and OAS1Y) in both models. Furthermore, IFNT time-dependently induced STAT1 phosphorylation in LECs without affecting total STAT1. roIFNT-stimulated viable LECs numbers and the knockdown of protein inhibitor of activated STAT1 (PIAS1) abolished this effect, suggesting that PIAS1 may mediate the proliferative effect of IFNT. IFNT significantly downregulated luteolytic genes such as TGFB1, thrombospondin-1 (THBS1), endothelin-1 (EDN1) and serpin family E member-1 (SERPINE1) in LECs. However, less robust effects were observed in luteal slices. Moreover, PGF2a alone induced THBS1, SERPINE1 and EDN1 mRNA in CL slices whereas in the presence of IFNT, THBS1 and SERPINE1 stimulation was abolished. Collectively, these results indicate that IFNT acts via STAT1- IRF9-dependent and independent pathways and affects diverse luteal functions. Most interestingly, this study suggests the existence of an anti-luteolytic effect of IFNT in bovine CL, namely, inhibiting key PGF2a-induced luteolytic genes. The proliferative effect of IFNT may constitute an additional mechanism that promotes luteal cell survival, thus, extending the luteal life span during early pregnancy in cows.