Select nutrients in the ovine uterine lumen. VI. Expression of FK506-binding protein 12-rapamycin complex-associated protein 1 (FRAP1) and regulators and effectors of mTORC1 and mTORC2 complexes in ovine uteri and conceptuses

The transcription factor XBP1 regulates mitochondrial remodel and autophagy in spontaneous abortion

PURPOSE

Spontaneous abortion (SA) remains a clinical challenge in early pregnancy. It has been reported that endoplasmic reticulum stress (ERS) is implicated in pregnancy-related complications. However, the precise mechanistic role of ERS in SA pathogenesis remains elusive. This study aims to explore the therapeutic potential of targeting ERS-related decidual dysfunction in SA.

METHODS

An ERS model was established in both decidualized stromal cells (DSCs) and pregnant mice through tunicamycin (Tu) administration. Chromatin immunoprecipitation (ChIP) and dual-luciferase reporter assays were performed to determine the interaction between XBP1s and the transcription factor binding site (TFBS) of tumor necrosis factor receptor-associated factor 6 (TRAF6). Mitochondrial membrane potential (MMP) and mitochondrial function were assessed using JC-1 and TMRM staining following ERS induction in DSCs. The effects of XBP1s inhibitors on mitochondrial metabolism and autophagy were evaluated through RT-qPCR, Western blotting, RNA-Seq, TUNEL assays, ROS and MitoSOX detection, and histological analyses in Tu-treated DSCs and SA patients. STF-083010 (STF) or shXBP1 was utilized to assess the inhibitory effects of X-box binding protein 1 (XBP1s) on DSC function both in vitro and in vivo.

RESULTS

We observed significant upregulation of XBP1s in decidual tissues from SA patients and Tu-exposed DSCs. Tu exposure significantly increased the proportion of TUNEL-positive cells and upregulated pro-inflammatory cytokines (IL-1β, TNF-α, IL-6, IL-18) in DSCs. XBP1s inhibition via shXBP1 or pharmacological inhibitor STF attenuated Tu-induced apoptosis and inflammatory cytokine expression. Notably, STF or shXBP1 treatment enhanced MMP and upregulated LC3-II expression in Tu-treated DSCs, indicating autophagy activation.Intriguingly, chloroquine (CQ)-mediated autophagy suppression exacerbated apoptosis in STF/Tu-co-treated DSCs, suggesting that XBP1s inhibition confers cytoprotection through autophagy induction. Mechanistically, XBP1s directly bound to the TFBS of TRAF6, a ubiquitin E3 ligase. TRAF6 overexpression exacerbated mitochondrial dysfunction and apoptosis while suppressing autophagy via inhibition of mTORC2/Akt pathway in Tu-treated DSCs.

CONCLUSION

XBP1s inhibition restored mitochondrial homeostasis and promoted autophagy by modulating the TRAF6/mTORC2 axis under ERS conditions, providing novel mechanistic insights into SA pathogenesis and potential therapeutic targets.

Early Embryonic Development in Agriculturally Important Species

The fertilization of oocytes ovulated by pigs, sheep, cows, and horses is not considered a limiting factor in successful establishment of pregnancy. Pig, sheep, and cow embryos undergo cleavage to the blastocyst stage, hatch from the zona pellucida, and undergo central-type implantation. Hatched blastocysts of pigs, sheep, and cows transition from tubular to long filamentous forms to establish surface area for exchange of nutrients and gases with the uterus. The equine blastocyst, surrounded by external membranes, does not elongate but migrates throughout the uterine lumen before attaching to the uterine luminal epithelium (LE) to begin implantation. Pregnancy recognition signaling in pigs requires the trophectoderm to express interleukin 1 beta, estrogens, prostaglandin E2, and interferon gamma. Sheep and cow conceptus trophectoderm expresses interferon tau that induces interferon regulatory factor 2 that inhibits transcription of estrogen and oxytocin receptors by uterine epithelia. This prevents oxytocin-induced luteolytic pulses of prostaglandin F2-alpha from regressing the corpora lutea, as well as ensuring the secretion of progesterone required for maintenance of pregnancy. The pregnancy recognition signal produced by equine blastocysts is not known. Implantation in these species requires interactions between extracellular matrix (ECM) proteins and integrins as the conceptus undergoes apposition and firm attachment to the uterine LE. This review provides details with respect to early embryonic development and the transition from spherical to filamentous conceptuses in pigs, sheep, and cows, as well as pre-implantation development of equine blastocysts and implantation of the conceptuses.

L-Arginine Nutrition and Metabolism in Ruminants

L-Arginine (Arg) plays a central role in the nitrogen metabolism (e.g., syntheses of protein, nitric oxide, polyamines, and creatine), blood flow, nutrient utilization, and health of ruminants. This amino acid is produced by ruminal bacteria and is also synthesized from L-glutamine, L-glutamate, and L-proline via the formation of L-citrulline (Cit) in the enterocytes of young and adult ruminants. In pre-weaning ruminants, most of the Cit formed de novo by the enterocytes is used locally for Arg production. In post-weaning ruminants, the small intestine-derived Cit is converted into Arg primarily in the kidneys and, to a lesser extent, in endothelial cells, macrophages, and other cell types. Under normal feeding conditions, Arg synthesis contributes 65% and 68% of total Arg requirements for nonpregnant and late pregnany ewes fed a diet with ~12% crude protein, respectively, whereas creatine production requires 40% and 36% of Arg utilized by nonpregnant and late pregnant ewes, respectively. Arg has not traditionally been considered a limiting nutrient in diets for post-weaning, gestating, or lactating ruminants because it has been assumed that these animals can synthesize sufficient Arg to meet their nutritional and physiological needs. This lack of a full understanding of Arg nutrition and metabolism has contributed to suboptimal efficiencies for milk production, reproductive performance, and growth in ruminants. There is now considerable evidence that dietary supplementation with rumen-protected Arg (e.g., 0.25-0.5% of dietary dry matter) can improve all these production indices without adverse effects on metabolism or health. Because extracellular Cit is not degraded by microbes in the rumen due to the lack of uptake, Cit can be used without any encapsulation as an effective dietary source for the synthesis of Arg in ruminants, including dairy and beef cows, as well as sheep and goats. Thus, an adequate amount of supplemental rumen-protected Arg or unencapsulated Cit is necessary to support maximum survival, growth, lactation, reproductive performance, and feed efficiency, as well as optimum health and well-being in all ruminants.

Insights into the lipidome and primary metabolome of the uterus from day 14 cyclic and pregnant sheep†

In ruminants, conceptus elongation requires the endometrium and its secretions. The amino acid, carbohydrate, and protein composition of the uterine lumen during early pregnancy has been defined in sheep; however, a comprehensive understanding of metabolomic changes in the uterine lumen is lacking, particularly with respect to lipids. Here, the lipidome and primary metabolome of the uterine lumen, endometrium, and/or conceptus was determined on day 14 of the estrous cycle and pregnancy. Lipid droplets and select triglycerides were depleted in the endometrium of pregnant ewes. In contrast, select ceramides, diglycerides, and non-esterified fatty acids as well as several phospholipid classes (phosphatidylcholine, phosphatidylinositol, phosphatidylglycerols, and diacylglycerols) were elevated in the uterine lumen of pregnant ewes. Lipidomic analysis of the conceptus revealed that triglycerides are particularly abundant within the conceptus. Primary metabolite analyses found elevated amino acids, carbohydrates, and energy substrates, among others, in the uterine lumen of pregnant ewes. Collectively, this study supports the hypothesis that lipids are important components of the uterine lumen that govern conceptus elongation and growth during early pregnancy.

Immunohistochemical determination of mTOR pathway molecules in ovaries and uterus in rat estrous cycle stages

mTOR is a member of the PI3K/Akt/mTOR signaling pathway that participates in cell growth, proliferation, protein synthesis, transcription, angiogenesis, apoptosis and autophagy. mTOR and MAPK pahways are two important key signal pathways which are related to each other. We investigated the role of mTOR and other signaling molecules in rat ovaries and uteruses in stages of the estrous cycle. Young adult female rats were divided into four groups as proestrous, estrous, metestrous and diestrous according to vaginal smears. Immunohistochemical staining of mTORC1, IGF1, PI3K, pAKT1/2/3, ERK1 and pERK1/2 was performed and pAKT1/2/3 and ERK1 were also analyzed using western blotting on ovarian and uterine tissue samples. According to our results, PI3K/Akt/mTOR and ERK/pERK showed an increase in the rat ovulation period. When all the groups were evaluated the immunoreactivities for all of the antibodies were detected in the oocytes, granulosa and theca cells, corpus luteum and stroma of ovary and lamina propria, surface and glandular epithelium of uterus with the strongest observed with anti-ERK1 antibody and then with a decreasing trend with anti-mTORC1, anti-pAkt1/2/3, anti-IGF1, anti-PI3K and anti-pERK1/2 antibodies in the proestrus and estrus stages. Differently from other parts of the ovary, highest antibody expression in the corpus luteum was observed in the metestrous stage. Moreover, the existence of pAKT1/2/3 and ERK1 proteins was confirmed with the Western blotting technique. We suggest that mTOR and mTOR-related ERK signaling molecules may participate in the rat ovulation process.

Amino Acids in Reproductive Nutrition and Health

Amino acids are not only the building blocks of proteins, an indispensable component of cells, but also play versatile roles in regulating cell metabolism, proliferation, differentiation and growth by themselves or through their derivatives. At the whole body level, the bioavailability and metabolism of amino acids, interacting with other macronutrients, is critical for the physiological processes of reproduction including gametogenesis, fertilization, implantation, placentation, fetal growth and development. In fertilization and early pregnancy, histotroph in oviductal and uterine secretions provides nutrients and microenvironment for conceptus (embryo and extraembryonic membranes) development. These nutrients include select amino acids in histotroph (arginine, leucine and glutamine of particular interest) that stimulate conceptus growth and development, as well as interactions between maternal uterus and the conceptus, thus impacting maintenance of pregnancy, placental growth, development and functions, fetal growth and development, and consequential pregnancy outcomes. Gestational protein undernutrition causes fetal growth restriction and predisposes cardiovascular, metabolic diseases and others in offspring via multiple mechanisms, whereas the supplementation of glycine, leucine and taurine during pregnancy partially rescues growth restriction and beneficially modulates fetal programming. Thus, amino acids are essential for the fertility of humans and all animals.

Differential regulation of mTORC1 and mTORC2 is critical for 8-Br-cAMP-induced decidualization

Human endometrium decidualization, a differentiation process involving biochemical and morphological changes, is a prerequisite for embryo implantation and successful pregnancy. Here, we show that the mammalian target of rapamycin (mTOR) is a crucial regulator of 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP)-induced decidualization in human endometrial stromal cells. The level of mSin1 in mTOR complex 2 (mTORC2) and DEPTOR in mTOR complex 1 (mTORC1) decreases during 8-Br-cAMP-induced decidualization, resulting in decreased mTORC2 activity and increased mTORC1 activity. Notably, DEPTOR displacement increases the association between raptor and insulin receptor substrate-1 (IRS-1), facilitating IRS-1 phosphorylation at serine 636/639. Finally, both S473 and T308 phosphorylation of Akt are reduced during decidualization, followed by a decrease in forkhead box O1 (FOXO1) phosphorylation and an increase in the mRNA levels of the decidualization markers prolactin (PRL) and insulin-like growth factor-binding protein-1 (IGFBP-1). Taken together, our findings reveal a critical role for mTOR in decidualization, involving the differential regulation of mTORC1 and mTORC2.

Select nutrients and their effects on conceptus development in mammals

The dialogue between the mammalian conceptus (embryo/fetus and associated membranes) involves signaling for pregnancy recognition and maintenance of pregnancy during the critical peri-implantation period of pregnancy when the stage is set for implantation and placentation that precedes fetal development. Uterine epithelial cells secrete and/or transport a wide range of molecules, including nutrients, collectively referred to as histotroph that are transported into the fetal-placental vascular system to support growth and development of the conceptus. The availability of uterine-derived histotroph has long-term consequences for the health and well-being of the fetus and the prevention of adult onset of metabolic diseases. Histotroph includes numerous amino acids, but arginine plays a particularly important role as a source of nitric oxide and polyamines required for fetal-placental development in rodents, swine and humans through mechanisms that remain to be fully elucidated. Mechanisms whereby arginine regulates expression of genes via the mechanistic target of rapamycin cell signaling pathways critical to conceptus development, implantation and placentation are discussed in detail in this review.

Factors controlling nutrient availability to the developing fetus in ruminants

Inadequate delivery of nutrients results in intrauterine growth restriction (IUGR), which is a leading cause of neonatal morbidity and mortality in livestock. In ruminants, inadequate nutrition during pregnancy is often prevalent due to frequent utilization of exensive forage based grazing systems, making them highly susceptible to changes in nutrient quality and availability. Delivery of nutrients to the fetus is dependent on a number of critical factors including placental growth and development, utero-placental blood flow, nutrient availability, and placental metabolism and transport capacity. Previous findings from our laboratory and others, highlight essential roles for amino acids and their metabolites in supporting normal fetal growth and development, as well as the critical role for amino acid transporters in nutrient delivery to the fetus. The focus of this review will be on the role of maternal nutrition on placental form and function as a regulator of fetal development in ruminants.

Elements of functional genital asymmetry in the cow

Asymmetry in the cow affects ovarian function and pregnancy. In this work we studied ovarian and uterine asymmetry. Synchronised animals, in which in vitro-produced embryos (n=30-60) had been transferred on Day 5 to the uterine horn ipsilateral to the corpus luteum (CL), were flushed on Day 8. Ovulatory follicle diameter, oestrus response and total protein flushed did not differ between sides. However, a corpus luteum in the right ovary led to plasma progesterone concentrations that were higher than when it was present in the left ovary. Fewer embryos were recovered from the left than the right horn. Among 60 uterine proteins identified by difference gel electrophoresis, relative abundance of nine (acyl-CoA dehydrogenase, very long chain; twinfilin, actin-binding protein, homologue 1; enolase 1; pyruvate kinase isozymes M1/M2 (rabbit); complement factor B Bb fragment ; albumin; fibrinogen gamma-B chain; and ezrin differed (P<0.05) between horns. Glucose concentration was higher, and fructose concentration lower, in the left horn. In a subsequent field trial, pregnancy rates after embryo transfer did not differ between horns (51.0±3.6, right vs 53.2±4.7, left). However, Day 7 blood progesterone concentrations differed (P=0.018) between pregnant and open animals in the left (15.9±1.7 vs 8.3±1.2) but not in the right horn (12.4±1.3 vs 12.4±1.2). Progesterone effects were independent of CL quality (P=0.55). Bilateral genital tract asymmetry in the cow affects progesterone, proteins and hexoses without altering pregnancy rates.

Contributions of an animal scientist to understanding the biology of the uterus and pregnancy

I developed a passion for reproductive biology when taking a course in Physiology of Reproduction at Louisiana State University while preparing to apply for Veterinary School at Texas A&M University. My career path changed. I entered graduate school, obtained a Ph.D. and have enjoyed an academic career conducting research in uterine biology and pregnancy in animal science departments at the University of Florida and at Texas A&M University. My contributions to science include: (1) identification of molecules secreted by or transported by uterine epithelia into the uterine lumen that are critical to successful establishment and maintenance of pregnancy, (2) discovery of steroids and proteins required for pregnancy-recognition signalling and their mechanisms of action in pigs and ruminants, (3) patterns of fetal–placental development and placental transport of nutrients, (4) identification of links between nutrients and components of histotroph that affect fetal–placental development, (5) characterising aspects of the endocrinology of pregnancy and (6) contributing to efforts to exploit the therapeutic value of interferon tau, particularly for treatment of autoimmune and inflammatory diseases. Current research focuses on select nutrients in the uterine lumen, specifically amino acids, glucose and fructose, that affect conceptus development, the therapeutic potential for interferon tau, stromal–epithelial cell signalling whereby progesterone and oestrogen act via steroid receptors in uterine stromal cells to stimulate secretion of growth factors (e.g. fibroblast growth factors and hepatocyte growth factor) that regulate uterine epithelial cells and conceptus trophectoderm, and roles of toll-like receptors expressed by uterine epithelia and conceptus trophectoderm in pregnancy.

Select nutrients, progesterone, and interferon tau affect conceptus metabolism and development

Interferon tau (IFNT), a novel multifunctional type I interferon secreted by trophectoderm, is the pregnancy recognition signal in ruminants that also has antiviral, antiproliferative, and immunomodulatory bioactivities. IFNT, with progesterone, affects availability of the metabolic substrate in the uterine lumen by inducing expression of genes for transport of select nutrients into the uterine lumen that activate mammalian target of rapamycin (mTOR) cell signaling responsible for proliferation, migration, and protein synthesis by conceptus trophectoderm. As an immunomodulatory protein, IFNT induces an anti-inflammatory state affecting metabolic events that decrease adiposity and glutamine:fructose-6-phosphate amidotransferase 1 activity, while increasing insulin sensitivity, nitric oxide production by endothelial cells, and brown adipose tissue in rats. This short review focuses on effects of IFNT and progesterone affecting transport of select nutrients into the uterine lumen to stimulate mTOR cell signaling required for conceptus development, as well as effects of IFNT on the immune system and adiposity in rats with respect to its potential therapeutic value in reducing obesity.

Inhibition of mammalian target of rapamycin signaling by CCI-779 (temsirolimus) induces growth inhibition and cell cycle arrest in Cashmere goat fetal fibroblasts (Capra hircus)

The mammalian target of rapamycin (mTOR) is a Ser/Thr kinase. It plays an evolutionarily conserved role in regulating cell growth, proliferation, survival, and metabolism via different cellular processes. The purpose of this study was to explore the inhibitory effects of CCI-779 (temsirolimus), a specific mTOR inhibitor, on mTOR signaling, and examine the mechanism of cell growth suppression by CCI-779 in Cashmere goat fetal fibroblasts (GFb cells). GFb cells were sensitive to CCI-779 and the survival rate of cells treated with >3.0 μM of CCI-779 was significantly reduced compared with the control (p<0.01). CCI-779 inhibited the phosphorylation of mTOR (at Ser2448) and S6 (at Ser240/244), and the expression of mTOR, p70S6K, and S6. Thus, CCI-779 was toxic to GFb cells, and it induced a dose-dependent decrease in cell proliferation and caused G1/S cell cycle arrest. Taken together, these data show that CCI-779 can inhibit mTOR signaling and proliferation in GFb cells in vitro. Therefore, mTOR is an important regulator for GFb cell growth and proliferation.

Mechanistic mammalian target of rapamycin (MTOR) cell signaling: effects of select nutrients and secreted phosphoprotein 1 on development of mammalian conceptuses

Morphological differentiation of uterine glands in mammals is a postnatal event vulnerable to adverse effects of endocrine disruptors. Exposure of ewe lambs to a progestin from birth to postnatal day 56 prevents development of uterine glands and, as adults, the ewes are unable to exhibit estrous cycles or maintain pregnancy. Uterine epithelia secrete proteins and transport nutrients into the uterine lumen necessary for conceptus development, pregnancy recognition signaling and implantation, including arginine and secreted phosphoprotein 1 (SPP1). Arginine can be metabolized to nitric oxide and to polyamines or act directly to activate MTOR cell signaling to stimulate proliferation, migration, and mRNA translation in trophectoderm cells. SPP1 binds αvβ3 and α5β1 integrins and induces focal adhesion assembly, adhesion and migration of conceptus trophectoderm cells during implantation. Thus, arginine and SPP1 mediate growth, migration, cytoskeletal remodeling and adhesion of trophectoderm essential for pregnancy recognition signaling and implantation.

Deletion of tuberous sclerosis 1 in somatic cells of the murine reproductive tract causes female infertility

Tumors develop with dysregulated activation of mammalian target of rapamycin (mTOR), the kinase activity of which is kept in an inactive state by a tumor suppressor dimer containing tuberous sclerosis 1 (TSC1) and TSC2. We examined whether conditional deletion of TSC1 by a knock-in allele of the anti-Müllerian hormone type 2 receptor (Amhr2) driving Cre expression and subsequent activation of mTOR in granulosa cells and in oviductal and uterine stromal cells affects fertility in female mice. Increased phosphorylation of ribosomal protein S6, a downstream target of activated mTOR, was observed in all AMHR2-expressing tissues examined, indicating loss of TSC1 activity. TSC1 deletion in granulosa cells led to the detection of significantly fewer primordial follicles in mutant mice at 12 wk, suggesting premature ovarian insufficiency, which might be related to the significantly increased time mutant mice spent in estrus. Although the number of good-quality ovulated oocytes was not significantly different compared with controls, there was a significantly higher number of degenerated oocytes after normal and superovulation, suggesting compromised oocyte quality, as well. Natural mating also showed severalfold higher numbers of degenerate bodies in the mutants that collected in bilateral swellings resembling hydrosalpinges that formed in all mice examined because of occlusion of the proximal oviduct. Attempts to transfer control embryos into mutant uteri also failed, indicating that implantation was compromised. Endometrial epithelial cells continued to proliferate, and quantitative RT-PCR showed that mucin 1 expression persisted during the window of implantation in mutant uteri, without any changes in progesterone receptor mRNA expression, suggesting a mechanism that does not involve disrupted estradiol-regulated progesterone receptor expression. Homozygous deletion of TSC1 in reproductive tract somatic tissues of mice rendered females completely infertile, which is likely due to these pleiotropic effects on follicle recruitment, oviductal development, and blastocyst implantation.

Uterine histotroph and conceptus development: select nutrients and secreted phosphoprotein 1 affect mechanistic target of rapamycin cell signaling in ewes

Interferon tau (IFNT), the pregnancy recognition signal in ruminants, abrogates the uterine luteolytic mechanism to ensure maintenance of function for the corpora lutea to produce progesterone (P4). IFNT also suppresses expression of classical IFN-stimulated genes by uterine lumenal epithelium (LE) and superficial glandular (sGE) epithelium but, acting in concert with progesterone, affects expression of a multitude of genes critical to growth and development of the conceptus. The LE and sGE secrete proteins and transport nutrients into the uterine lumen necessary for conceptus development, pregnancy recognition signaling, and implantation. Secretions include arginine and secreted phosphoprotein 1 (SPP1). Arginine can be metabolized to nitric oxide and to polyamines or act directly to activate the mechanistic target of rapamycin cell signaling pathway to stimulate proliferation, migration, and mRNA translation in trophectoderm cells. SPP1 binds alphavbeta3 and alpha5beta1 integrins to induce focal adhesion assembly, adhesion, and migration of conceptus trophectoderm cells during implantation. Thus, arginine and SPP1 mediate growth, migration, cytoskeletal remodeling, and adhesion of trophectoderm essential for pregnancy recognition signaling and implantation. This minireview focuses on components of histotroph that affect conceptus development in the ewe.

Increase of essential amino acids in the bovine uterine lumen during preimplantation development

Amino acids (AAs) are crucial for the developing conceptus prior to implantation. To provide insights into the requirements of the bovine embryo, we determined the AA composition of the uterine fluid. At days 12, 15, and 18 post-estrus, the uteri of synchronized pregnant and non-pregnant Simmental heifers were flushed for the analysis of 41 AAs and their derivatives by liquid chromatography-tandem mass spectrometry. The ipsilateral endometrium was sampled for quantitative PCR. In addition to a pregnancy-dependent increase of the essential AAs (P<0.01), we detected elevated concentrations for most non-essential proteinogenic AAs. Histidine (His) and the expression of the His/peptide transporter solute carrier 15A3 (SLC15A3) were significantly increased at day 18 of pregnancy in vivo. In addition, SLC15A3 was predominantly stimulated by trophoblast-derived interferon-τ in stroma cells of an in vitro co-culture model of endometrial cells. Our results show an increased concentration of AAs most likely to optimally provide the elongating pre-attachment conceptus with nutrients.

Impacts of amino acid nutrition on pregnancy outcome in pigs: mechanisms and implications for swine production

Pigs suffer up to 50% embryonic and fetal loss during gestation and exhibit the most severe naturally occurring intrauterine growth retardation among livestock species. Placental insufficiency is a major factor contributing to suboptimal reproductive performance and reduced birth weights of pigs. Enhancement of placental growth and function through nutritional management offers an effective solution to improving embryonic and fetal survival and growth. We discovered an unusual abundance of the arginine family of AA in porcine allantoic fluid (a reservoir of nutrients) during early gestation, when placental growth is most rapid. Arginine is metabolized to ornithine, proline, and nitric oxide, and these compounds possess a plethora of physiological functions. Nitric oxide is a vasodilator and angiogenic factor, whereas both ornithine and proline are substrates for placental synthesis of polyamines, which are key regulators of protein synthesis and angiogenesis. Additionally, arginine, leucine, glutamine, and proline activate the mammalian target of rapamycin cell-signaling pathway to enhance protein synthesis and cell proliferation in placentae. To translate basic research on AA biochemistry and nutrition into application, dietary supplementation with 0.83% l-arginine to gilts on d 14 to 28 or d 30 to 114 of gestation increased the number and litter birth weight of live-born piglets. In addition, supplementing the gestation diet with 0.4% l-arginine plus 0.6% l-glutamine enhanced the efficiency of nutrient utilization, reduced variation in piglet birth weight, and increased litter birth weight. By regulating syntheses of nitric oxide, polyamines, and proteins, functional AA stimulate placental growth and the transfer of nutrients from mother to embryo or fetus to promote conceptus survival, growth, and development.

Comparative aspects of implantation

Uterine receptivity to implantation of blastocysts in mammals includes hatching from zona pellucida, precontact with uterine luminal (LE) and superficial glandular (sGE) epithelia and orientation of blastocyst, apposition between trophectoderm and uterine LE and sGE, adhesion of trophectoderm to uterine LE/sGE, and, in some species, limited or extensive invasion into the endometrial stroma and induction of decidualization of stromal cells. These peri-implantation events are prerequisites for pregnancy recognition signaling, implantation, and placentation required for fetal-placental growth and development through the remainder of pregnancy. Although there is a range of strategies for implantation in mammals, a common feature is the requirement for progesterone (P(4)) to downregulate expression of its receptors in uterine epithelia and P(4) prior to implantation events. P(4) then mediates its effects via growth factors expressed by stromal cells in most species; however, uterine luminal epithelium may express a growth factor in response to P(4) and/or estrogens in species with a true epitheliochorial placenta. There is also compelling evidence that uterine receptivity to implantation involves temporal and cell-specific expression of interferon (IFN)-stimulated genes that may be induced directly by an IFN or induced by P(4) and stimulated by an IFN. These genes have many roles including nutrient transport, cellular remodeling, angiogenesis and relaxation of vascular tissues, cell proliferation and migration, establishment of an antiviral state, and protection of conceptus tissues from challenges by the maternal immune cells.