Evidence for metabolism of creatine by the conceptus, placenta, and uterus for production of ATP during conceptus development in pigs.

In pigs, the majority of embryonic mortality occurs when free-floating conceptuses (embryos/fetuses and associated placental membranes) elongate and the uterine-placental interface undergoes folding and develops areolae. Both periods involve proliferation, migration, and changes in morphology of cells that require ATP. We hypothesize that insufficient ATP in conceptus and uterine tissues contributes to conceptus loss in pigs. Creatine is stored in cells as phosphocreatine (PCr) for ATP regeneration through the creatine (Cr)-creatine kinase (CK)-PCr pathway. However, the expression of components of this pathway in pigs has not been examined throughout gestation. Results of qPCR analyses indicated increases in AGAT, GAMT, CKM, CKB, and SLC6A8 mRNAs in elongating porcine conceptuses and immunofluorescence microscopy localized GAMT, CKM, and CKB proteins to the trophectoderm of elongating conceptuses, to the columnar chorionic epithelial cells at the bottom of chorioallantoic troughs, and to endometrial luminal epithelium (LE) at the tops of the endometrial ridges of uterine-placental folds on Days 40, 60, and 90 of gestation. GAMT protein is expressed in endometrial LE at the uterine-placental interface, but immunostaining is more intense in LE at the bottoms of the endometrial ridges. Results of this study indicate that key elements of the pathway for creatine metabolism are expressed in cells of the conceptus, placenta, and uterus for potential production of ATP during two timepoints in pregnancy with a high demand for energy; elongation of the conceptus for implantation and development of uterine-placental folding during placentation.

Pig conceptuses utilize extracellular vesicles for IFNG-mediated paracrine communication with the endometrium.

Interferon-gamma (IFNG) is a pro-inflammatory cytokine secreted by the porcine conceptus (embryo and extra-embryonic membranes) during the peri-implantation period of pregnancy. IFNG modifies the endometrial inflammatory immune response and is required for the implantation and survival of the conceptus. It is not known how IFNG from the conceptus trophectoderm is transported across the endometrial luminal epithelium (LE). In the present study, immunofluorescence analyses detected immunoreactive IFNG protein in both the trophectoderm and endometrial LE on Day 15 of pregnancy, while our previous research localized IFNG mRNA only to conceptus trophectoderm. Using minced endometrial explants to disrupt the barrier posed by the intact endometrial LE, treatment with recombinant IFNG induced the expression of genes that were not induced when IFNG was infused into the uterine lumen in vivo by McLendon et al. (Biology of Reproduction. 2020;103(5):1018-1029). We hypothesized that during pregnancy extracellular vesicles (EVs) serve as intercellular signaling vehicles to transport conceptus-derived IFNG across the intact endometrial LE and into the stromal compartment of the uterus. Western blotting detected the presence of IFNG in EVs isolated from the uterine fluid of pregnant gilts, but not nonpregnant gilts. Real-time PCR demonstrated increased expression of IFNG-stimulated genes in EV-treated endometrial explants and EV-mediated IFNG transport was confirmed in whole uterine sections cultured with EVs from day 15 of pregnancy. These results suggest that EVs are involved in IFNG transport across the endometrial LE to enable paracrine communication between the conceptus and cells within the endometrial stroma.

Understanding placentation in ruminants: a review focusing on cows and sheep.

Mammals differ regarding their placentae, but in all species placental trophoblasts interact intimately with the uterine endometrium to mediate the transfer of nutrients from the mother to the embryo/fetus through the closely juxtaposed microcirculatory systems of the uterus and placenta. Placentation in ruminants is intermediate between the non-invasive type, as observed in the epitheliochorial placenta of pigs, and the invasive type, as observed in the haemochorial placentae of mice and humans. In ruminants, placental trophoblast cells invade uterine endometrial tissue, but invasion is believed to be limited to the endometrial luminal epithelium (LE). In the LE there are varying degrees of syncytialisation among species, with syncytialisation being more extensive in sheep than cows. The hallmarks of placentation in ruminants include: (1) an extended period in which conceptuses (embryos and associated placental membranes) elongate and must be supported by secretions (histotroph) from the uterus; (2) a cascade involving an array of adhesion molecules that includes integrin-mediated attachment of the conceptus trophoblast to the endometrial LE for implantation; (3) syncytialisation of the developing early placenta, a process for which there is currently limited understanding; and (4) development of placentomes that define the cotyledonary placentae of cows and sheep, and provide haemotrophic support of fetal development.

Integrins and their potential roles in mammalian pregnancy.

Integrins are a highly complex family of receptors that, when expressed on the surface of cells, can mediate reciprocal cell-to-cell and cell-to-extracellular matrix (ECM) interactions leading to assembly of integrin adhesion complexes (IACs) that initiate many signaling functions both at the membrane and deeper within the cytoplasm to coordinate processes including cell adhesion, migration, proliferation, survival, differentiation, and metabolism. All metazoan organisms possess integrins, and it is generally agreed that integrins were associated with the evolution of multicellularity, being essential for the association of cells with their neighbors and surroundings, during embryonic development and many aspects of cellular and molecular biology. Integrins have important roles in many aspects of embryonic development, normal physiology, and disease processes with a multitude of functions discovered and elucidated for integrins that directly influence many areas of biology and medicine, including mammalian pregnancy, in particular implantation of the blastocyst to the uterine wall, subsequent placentation and conceptus (embryo/fetus and associated placental membranes) development. This review provides a succinct overview of integrin structure, ligand binding, and signaling followed with a concise overview of embryonic development, implantation, and early placentation in pigs, sheep, humans, and mice as an example for rodents. A brief timeline of the initial localization of integrin subunits to the uterine luminal epithelium (LE) and conceptus trophoblast is then presented, followed by sequential summaries of integrin expression and function during gestation in pigs, sheep, humans, and rodents. As appropriate for this journal, summaries of integrin expression and function during gestation in pigs and sheep are in depth, whereas summaries for humans and rodents are brief. Because similar models to those illustrated in Fig. 1, 2, 3, 4, 5 and 6 are present throughout the scientific literature, the illustrations in this manuscript are drafted as Viking imagery for entertainment purposes.

Metabolic pathways utilized by the porcine conceptus, uterus, and placenta.

Conceptus elongation and early placentation involve growth and remodeling that requires proliferation and migration of cells. This demands conceptuses expend energy before establishment of a placenta connection and when they are dependent upon components of histotroph secreted or transported into the uterine lumen from the uterus. Glucose and fructose, as well as many amino acids (including arginine, aspartate, glutamine, glutamate, glycine, methionine, and serine), increase in the uterine lumen during the peri-implantation period. Glucose and fructose enter cells via their transporters, SLC2A, SLC2A3, and SLC2A8, and amino acids enter the cells via specific transporters that are expressed by the conceptus trophectoderm. However, porcine conceptuses develop rapidly through extensive cellular proliferation and migration as they elongate and attach to the uterine wall resulting in increased metabolic demands. Therefore, coordination of multiple metabolic biosynthetic pathways is an essential aspect of conceptus development. Oxidative metabolism primarily occurs through the tricarboxylic acid (TCA) cycle and the electron transport chain, but proliferating and migrating cells, like the trophectoderm of pigs, enhance aerobic glycolysis. The glycolytic intermediates from glucose can then be shunted into the pentose phosphate pathway and one-carbon metabolism for the de novo synthesis of nucleotides. A result of aerobic glycolysis is limited availability of pyruvate for maintaining the TCA cycle, and trophectoderm cells likely replenish TCA cycle metabolites primarily through glutaminolysis to convert glutamine into TCA cycle intermediates. The synthesis of ATP, nucleotides, amino acids, and fatty acids through these biosynthetic pathways is essential to support elongation, migration, hormone synthesis, implantation, and early placental development of conceptuses.

Elongating porcine conceptuses can utilize glutaminolysis as an anaplerotic pathway to maintain the TCA cycle†.

During the peri-implantation period of pregnancy, the trophectoderm of pig conceptuses utilize glucose via multiple biosynthetic pathways to support elongation and implantation, resulting in limited availability of pyruvate for metabolism via the TCA cycle. Therefore, we hypothesized that porcine trophectoderm cells replenish tricarboxylic acid (TCA) cycle intermediates via a process known as anaplerosis and that trophectoderm cells convert glutamine to α-ketoglutarate, a TCA cycle intermediate, through glutaminolysis. Results demonstrate: (1) that expression of glutaminase (GLS) increases in trophectoderm and glutamine synthetase (GLUL) increases in extra-embryonic endoderm of conceptuses, suggesting that extra-embryonic endoderm synthesizes glutamine, and trophectoderm converts glutamine into glutamate; and (2) that expression of glutamate dehydrogenase 1 (GLUD1) decreases and expression of aminotransferases including PSAT1 increase in trophectoderm, suggesting that glutaminolysis occurs in the trophectoderm through the GLS-aminotransferase pathway during the peri-implantation period. We then incubated porcine conceptuses with 13C-glutamine in the presence or absence of glucose in the culture media and then monitored the movement of glutamine-derived carbons through metabolic intermediates within glutaminolysis and the TCA cycle. The 13C-labeled carbons were accumulated in glutamate, α-ketoglutarate, succinate, malate, citrate, and aspartate in both the presence and absence of glucose in the media, and the accumulation of 13C-labeled carbons significantly increased in the absence of glucose in the media. Collectively, our results indicate that during the peri-implantation period of pregnancy, the proliferating and migrating trophectoderm cells of elongating porcine conceptuses utilize glutamine via glutaminolysis as an alternate carbon source to maintain TCA cycle flux.