118 Deep Sequencing of the Porcine Endometrial Transcriptome on Days 12 and 14 of Pregnancy

Abstract

Establishment of pregnancy in mammals requires prolongation of luteal life span for sustained progesterone (P4) production. P4 stimulates secretory functions of the endometrium required for conceptus growth and development and is essential for endometrial receptivity. The porcine conceptus undergoes rapid differentiation and expansion of its trophoblastic membranes between Day 11 and Day 12 of gestation. With trophoblast elongation production of estrogen, the porcine embryonic pregnancy recognition signal, by the conceptus increases. A complex interplay of estrogen signaling and prostaglandin (PG) metabolism in the endometrium finally results in prevention of luteolysis. Conceptus attachment to the uterine surface epithelium starts around Day 14 of pregnancy, preceded by a pronounced vascularization at the implantation zones, initiating the epitheliochorial placentation. To characterize the complex transcriptome changes in the endometrium in the course of maternal recognition of pregnancy and initial placentation, deep sequencing of endometrial RNA samples of pregnant animals (n = 4) and corresponding non-pregnant controls (n = 4) was performed on Day 12 and Day 14 after ovulation using Illumina RNA-Seq (Illumina Inc., San Diego, CA, USA). Between 21 and 36 million 76-bp single-end reads were produced per sample. Reads were mapped with TopHat to the porcine genome assembly Sscrofa10.2 and relative expression values were calculated based on the National Center for Biotechnology Information’s gene annotation for the analysis of differential gene expression. Statistical analysis revealed more than 3500 differentially expressed genes (DEG) for Day 12 and almost 2500 DEG for Day 14 of pregnancy (DESeq, FDR 1%). Differential expression of selected genes was validated by the use of quantitative real-time PCR (RT-PCR). Comparison of the RNA-Seq data from Day 12 and Day 14 of pregnancy based on a comparison of the DEGs and of overrepresented functional categories revealed distinct differences in gene regulation, reflecting the different functions of the endometrium during these stages; that is, recognition of pregnancy on Day 12 and preparation for conceptus implantation on Day 14. The data from Day 14 was also compared to the published microarray data set from Day 14 of pregnancy (Østrup et al. 2010 Biol. Reprod.) and other sets of DEG by the use of gene set enrichment analysis (GSEA). Overall, a very good agreement with the results of the previous microarray study was found. Furthermore, a significant overlap with genes upregulated in bovine endometrium on Day 18 of pregnancy was found for the genes upregulated in porcine endometrium on Day 14. More than half of these genes were known interferon-regulated genes. In conclusion, RNA-Seq revealed distinct transcriptome changes on Day 12 and Day 14 of pregnancy in porcine endometrium associated with MRP and initiation of implantation.