Metabolomic Analysis of Uterine Luminal Fluid During the Peri-Implantation Period of Pregnancy in Pigs

Bangmin Liu, Likun Duan, Xiaojing Liu, Xiaoqiu Wang

During the peri-implantation period of pregnancy in pigs, the rapid elongation of conceptuses (embryonic/fetus and its extraembryonic membranes) is highly dependent on the composition of histotroph secreted from uterine luminal (LE) and glandular (GE) epithelial and stroma cells, as well as selective transport of nutrients. However, little is known about the metabolites in the uterine luminal fluid (ULF) associated with the uterine-conceptus communication during early pregnancy in pigs. Thus, this study was conducted to profile the metabolome in porcine ULF between days 10 and 16 of estrus cycle (C) and pregnancy (P). Gilts were observed for estrus and/or bred via artificial insemination at 12 and 24 h after onset of estrus (day 0). On days 10, 12, 14, or 16 of the estrous cycle and pregnancy (n = 6 gilts per day and status), uteri were flushed with 20 mL sterile PBS (pH 7.2) after gilts were subjected to a midventral laparotomy. Pregnancy was confirmed by the presence of one or more morphologically normal conceptuses. Recoverable uterine flushings (i.e., ULF) were then subjected to metabolomic analysis by LC-MS. Overall, 222 metabolites were detected in the ULF of which 102 were altered by status and 63 were altered by days (P < 0.05; fold change>1.5). Comparing to cyclic day 10 (10C), pregnancy stimulated increases in citric acid, lysine, arginine and other 19 metabolites in the ULF at pregnant day 10 (10P). At 12P when porcine conceptus initiates its morphological changes, ornithine, α-D-glucose, phenylalanine and other 13 metabolites were up-regulated as compared with 12C. At 14P when conceptus initiates implantation, 15 metabolites changed in ULF. At 16P, 123 metabolites were altered in ULF as compared with 16C. Interestingly, of 15 altered metabolites in ULF, 12 were downregulated including asparagine, nicotinamide riboside, and citrulline at 14P. Whereas 102 of 123 altered metabolites were upregulated including phosphorylcholine, 6-phosphate fructose and 6-phosphate glucose in ULF at 16P. To determine the absolute amount of amino acids in the ULF, we performed the targeted metabolomic analyses for 19 amino acids. As a result, arginine, asparagine, glutamic acid, glutamine, histidine, leucine/isoleucine, lysine, phenylalanine, proline, and valine were increased between 10P and 16P. They were less expressed in the ULF of estrus cycle as compared with pregnancy, and remained unchanged between 10C and 16C. Pathway analysis based on KEGG database indicating that arginine biosynthesis, alanine, aspartate and glutamate metabolism, and aminoacyl-tRNA biosynthesis have the highest degree of enrichment. These novel findings provide the foundation for future investigation of metabolomic and functional studies on both endometrium and conceptus required for elongation of porcine conceptus during peri-implantation period of pregnancy.