141 TRANSCRIPTIONAL PROFILING OF EQUINE ENDOMETRIUM DURING THE TIME OF MATERNAL RECOGNITION OF PREGNANCY

Early embryonic development, implantation, and maintenance of a pregnancy are critically dependent upon a precisely orchestrated embryo-maternal interaction leading to a receptive uterine environment. The embryo has to signal its presence to the dam in order for estrous cyclicity to subside; however, the horse is one of the few domestic species in which the conceptus-derived pregnancy recognition signal has not been identified. To gain new insights into molecular mechanisms underlying this complex process in the horse, transcriptional profiling of Day 13.5 pregnant and cyclic endometrial tissue samples was carried using custom-designed 2-color microarrays. Endometrial tissue samples collected from 4 pregnant and cyclic mares each were labeled with either Cy3 or Cy5, paired, and analyzed on 4 individual arrays. Transcripts showing a difference in signal intensity of at least 1.3 and a P-value of 0.01 or less in a mixed model analysis were considered differentially expressed. Array data were validated performing quantitative RT-PCR and changes in relative gene expression between pregnant and non-pregnant mares were determined using the delta delta cycle threshold method. Proteins of interest were localized using immunohistochemistry. One hundred six transcripts were up-regulated, whereas 47 transcripts showed lower expression levels in pregnant mares. Quantitative RT-PCR for five up- and down-regulated transcripts each confirmed expression data obtained by microarray with a validation rate of 100%. Half of the genes with known or inferred function have previously been described as regulated by estrogens. Given the large quantities of estrogen synthesized by the equine conceptus, this finding confirms validity of the data. Elevated transcript levels were found for genes involved in cell-cell signaling, heat shock response, and secretory proteins among others. Solute carrier 36 member 2, SLC36A2, was the most highly up-regulated gene, reflecting the nutritional needs of the rapidly developing embryo. The beta subunit of luteinizing hormone (LHB) showed higher expression levels in pregnant mares. It is likely that the observed up-regulation of LHB leads to increased release of bioactive LH/CG into the uterine environment during early pregnancy where we hypothesize it to exert paracrine effects preparing the uterus for conceptus implantation. Among the down-regulated genes, estrogen receptor 1 was of particular interest because of its potential involvement in the initiation of luteolysis in cyclic mares. We hypothesize that either embryonic estrogen or uterine-derived choriogonadotropin is involved in the observed down-regulation of estrogen receptor 1, which may be the key to maternal recognition of pregnancy. Several of the genes identified in the current study are known to play a role in early pregnancy in species other than the horse. We thus hypothesize that a subset of genes crucial to endometrial receptivity does not differ between species, representing a common pattern during early pregnancy, whereas each species has a distinct mechanism ensuring ongoing corpus luteum function.