Spatio-specific functional transcriptomic signature of bovine endometrium 7 days after insemination

Establishment of pregnancy depends on a well-balanced interplay between sex-steroid profiles, the maternal reproductive tract and the developing conceptus. The bovine endometrium is a dynamic tissue that undergoes spatiotemporal functional changes directed by the ovarian hormones, estradiol (E2) and progesterone (P4). Due to local vascular arrangements, sex steroid input to the uterus varies both along its wide and the broad axis. A long standing unresolved question is how early can the embryo signal to the reproductive tract and change its function. The present study tested two hypotheses simultaneously: (1) there is a spatio-specific transcriptomic signature in the endometrium and, (2) presence of an embryo influences such signature 7 days after AI (D7). It was expected that abundance of sex-steroid-responsive, receptivity related genes would decrease from anterior to posterior thirds of the uterine horn and from mesometrial to antimesometrial side. Moreover, due to its limited number of cells, presence of a D7 embryo would not affect endometrial gene expression. Estrous cycles of multiparous, non-lactating Nelore cows were synchronized and animals were allocated to one of two experimental groups at estrus (D0): Control (Con; n = 8), cows were sham-inseminated and received semen diluent; or Pregnant (Preg; n = 16), cows were inseminated with semen from the same batch of a commercial bull, 12h post estrus. Size of the pre-ovulatory follicle, subsequent CL area and plasmatic P4 concentrations on D7 were similar between groups (P > 0.1). Cows were slaughtered on D7 and the uterine horn ipsilateral to the CL was isolated and divided in anterior, middle and posterior thirds, starting from the uterotubaric junction. Each uterine third was washed individually with DMPBS and presence of an embryo in the flushing was confirmed in the Preg group. All embryos found (n = 10) were in the anterior third flushings. Subsequently, intercaruncular endometrial samples were collected from each uterine third in the mesometrial and antimesometrial region. Relative abundance of transcripts for 89 key genes was measured by PCR using Fluidigm platform (Biomark HD). Data were analyzed by split-split-plot ANOVA and included the effects of group (Con vs. Preg.), third (anterior, middle and posterior) and region (mesometrial and antimesometrial) and interactions. Spatial regulation of gene expression across the uterine horn was verified with upregulation for expression of genes associated to secretory activity, transporters, prostaglandin cascade and redox environment pathways on the anterior and decreasing to the posterior third. In contrast, for transcripts associated with extracellular matrix remodeling and growth factors, there was an upregulation of transcript abundance from the anterior towards the posterior third. In conclusion, the expression pattern of specific genes varied among regions of the female reproductive tract.