Expression of microRNAs and isomiRs in the porcine endometrium: implications for gene regulation at the maternal-conceptus interface

Embryo implantation is a complex, synchronized process that requires establishment of a reciprocal dialogue between a receptive endometrium and developing blastocysts. Recently, microRNAs (miRNAs), known to modulate gene expression through post-transcriptional mechanisms, were implicated in regulation of early pregnancy events including maternal recognition of pregnancy and implantation. To characterize complex transcriptomic changes, expression of miRNAs in pregnant and cyclic endometria collected on days 12, 16 and 20 was analyzed using Illumina deep sequencing and analyzed with bioinformatic pipeline. Moreover, expression profiles of ten genes related to miRNA synthesis and transport such as DROSHA, DGCR8, XPO5, DICER, TARBP2, TNRC6A, and AGO1-4 were determined. Among genes involved in miRNA transport and synthesis DROSHA, XPO5, DICER1, TARBP, and AGO1 expression was affected by the reproductive status. Moreover, DICER1 and AGO2 proteins were localized in luminal and glandular epithelium with immunofluorescence staining. Several hundred mature, canonical and non-canonical miRNAs were found to be expressed in the endometrial samples. Detailed analysis revealed that miRNA length variants, isomiRs, accounted for the vast majority of defined sequences. Both miRNA and isomiR of miR-140-3p were shown to affect expression of putative targets in endometrial stromal cells in vitro. Computational analysis of putative target genes for miRNAs differentially expressed (DE) between pregnant and cyclic animals resulted in lists of biological processes and regulatory pathways indicating their role in cellular development, cell cycle, immunological response and organismal development. Among predicted target genes for DE miRNAs, vascular endothelial growth factor (VEGF), progesterone and estradiol receptors (PGR, ESR1) and leukemia inhibitory factor (LIF) were found. This research revealed a repertoire of pregnancy-related miRNAs in porcine endometrium during initial stages of conceptus implantation and during the estrous cycle, and sheds light on mechanisms regulating miRNA-mediated gene expression at the maternal-conceptus interface.