Labour-associated changes in adrenomedullin content in human placenta and fetal membranes

Molecular Mechanism and Pathways of Normal Human Parturition in Different Gestational Tissues: A Systematic Review of Transcriptome Studies

Objective: Genome-wide transcriptomic studies on gestational tissues in labor provide molecular insights in mechanism of normal parturition. This systematic review aimed to summarize the important genes in various gestational tissues around labor onset, and to dissect the underlying molecular regulations and pathways that trigger the labor in term pregnancies.

Data sources: PubMed and Web of Science were searched from inception to January 2021.

Study Eligibility Criteria: Untargeted genome-wide transcriptomic studies comparing the gene expression of various gestational tissues in normal term pregnant women with and without labor were included.

Methods: Every differentially expressed gene was retrieved. Consistently expressed genes with same direction in different studies were identified, then gene ontology and KEGG analysis were conducted to understand molecular pathways and functions. Gene-gene association analysis was performed to determine the key regulatory gene(s) in labor onset.

Results: A total of 15 studies, including 266 subjects, were included. 136, 26, 15, 7, and 3 genes were significantly changed during labor in the myometrium (seven studies, n = 108), uterine cervix (four studies, n = 64), decidua (two studies, n = 42), amnion (two studies, n = 44) and placenta (two studies, n = 41), respectively. These genes were overrepresented in annotation terms related to inflammatory and immune responses. TNF and NOD-like receptor signaling pathways were overrepresented in all mentioned tissues, except the placenta. IL6 was the only gene included in both pathways, the most common reported gene in all included studies, and also the gene in the central hub of molecular regulatory network.

Conclusions: This systematic review identified that genes involved in immunological and inflammatory regulations are expressed in specific gestational tissues in labor. We put forward the hypothesis that IL6 might be the key gene triggering specific mechanism in different gestational tissues, eventually leading to labor onset through inducing uterine contraction, wakening fetal membranes and stimulating cervical ripening.

Systematic Review Registration: Identifier [CRD42020187975].

Increased adrenomedullin protein content and mRNA expression in human fetal membranes but not placental tissue in pre-eclampsia

The relationship between Pre-eclampsia (PE) and placental production of Adrenomedullin (AdM) is not completely understood. This study measured placental and fetal membrane AdM protein concentrations by specific radioimmunoassay and mRNA expression by Northern blot analysis in samples obtained at either term or preterm gestation from women either in labour or not in labour. Samples were obtained from women with normotensive and pre-eclamptic pregnancies. There were significant increases in immunoreactive AdM protein concentration (pg/mg DNA) in choriodecidua and amnion of women with PE compared to normal pregnancy for the preterm not-in-labour group (choriodecidua: control 124 +/- 16, n = 10, PE 361 +/- 35, n = 10; amnion: control 94 +/- 12, n = 10, PE 153 +/- 19, n = 10) and for the term not-in-labour (choriodecidua: control 128 +/- 17, n = 14, PE 459 +/- 51, n = 8; amnion: control 112 +/- 15, n = 14, PE 253 +/- 57, n = 8) and in-labour (choriodecidua: control 531 +/- 74, n = 14, PE 881 +/- 188, n = 8; amnion: control 545 +/- 84, n = 14, PE 1008 +/- 230, n = 8) groups. AdM mRNA relative abundance was greater in preterm, not-in-labour choriodecidual samples in PE, but not in amnion. In addition, this study observed labour-associated increases in choriodecidual and amniotic irAdM in term pre-eclamptic and control patients. However, there were no significant changes in AdM protein or mRNA expressions between any of the groups for placental tissue. These results suggest that fetal membranes, but not placental, production of AdM is increased at the post-translational level during PE in preterm and term tissues and at the pre-translational level during PE in preterm tissues. Fetal membranes, AdM may play an important role in the regulation of feto-placental hemodynamics and fetal physiology during pre-eclampsia.

Expression of adrenomedullin mRNA is altered with gestation and labour in human placenta and fetal membranes

The aim of the present study was to investigate whether placental and fetal membrane AdM (adrenomedullin) mRNA expression changes with gestation and human labour, as we have previously found labour-associated changes in AdM content in fetal membranes [Al-Ghafra, Gude, Brennecke and King (2003) Clin. Sci. 105, 419-423]. Placentas and fetal membranes were collected either at term or pre-term from women either in-labour or not-in-labour, and AdM mRNA abundance was measured in tissue extracts by Northern blot analysis. Increases were found in the relative abundance of amniotic tissue AdM mRNA in both in-labour and not-in-labour groups at term compared with those at pre-term, and there were positive correlations with gestational age. Relative abundance of choriodecidual tissue AdM mRNA was also significantly elevated in the not-in-labour groups between pre-term and term tissues, although there was no significant correlation with gestational age. However, placental AdM mRNA expression was neither significantly increased at term (compared with pre-term) nor correlated with gestational age. In addition, there were significant increases in AdM mRNA in amnion and choriodecidua in the in-labour group compared with the not-in-labour group for both pre-term and term gestations. There was no difference in AdM mRNA in placental tissues between labour groups. In conclusion, the present study provides evidence that AdM production by fetal membranes is increased in amniotic and choriodecidual tissues at term, compared with pre-term, and in response to labour.

Growth and function of the normal human placenta

The placenta is the highly specialised organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure the exchange of nutrients and waste products between the maternal and fetal circulatory systems operates at maximal efficiency. The main functional units of the placenta are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. After implantation, trophoblast cells proliferate and differentiate along two pathways described as villous and extravillous. Non-migratory, villous cytotrophoblast cells fuse to form the multinucleated syncytiotrophoblast, which forms the outer epithelial layer of the chorionic villi. It is at the terminal branches of the chorionic villi that the majority of fetal/maternal exchange occurs. Extravillous trophoblast cells migrate into the decidua and remodel uterine arteries. This facilitates blood flow to the placenta via dilated, compliant vessels, unresponsive to maternal vasomotor control. The placenta acts to provide oxygen and nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functional changes occur that accommodate the increasing metabolic demands of the developing fetus throughout gestation.

Adrenomedullin: Multiple functions in human pregnancy

Adrenomedullin is a 52 amino acid peptide originally isolated from human phaeochromocytoma in 1993. It was initially demonstrated to have profound effects on the vasculature including vasodilatation and subsequently promotion of angiogenesis. Since then it has become apparent that it has a wide range of other biological actions including regulation of cell growth and differentiation. Successful pregnancy outcome relies on establishing and maintaining throughout gestation an efficient blood supply to the fetus. This allows the exchange of nutrients, oxygenation of fetal blood and removal of cytotoxins from the fetus, such as carbon dioxide. One of the most important local adaptations to pregnancy is the change in maternal blood flow to the implantation site. Evidence now points towards a vital role for adrenomedullin in the regulation of placentation. It appears that adrenomedullin may play important roles in the regulation of fetal perfusion both in normal and in compromised pregnancies. However, most studies have focused on measuring adrenomedullin levels and studying its expression as well as that of its receptors. More functional studies are now required to elucidate the underlying mechanisms involved.