83 BONE MORPHOGENETIC PROTEIN SIGNALING DURING INTERACTION OF THE BOVINE EMBRYO WITH OVIDUCTAL EPITHELIAL CELLS IN VITRO

Abstract

In previous studies, we have demonstrated that different signalling components of bone morphogenetic proteins (BMP) are expressed in an anatomically and temporally regulated fashion in the bovine oviduct. However, a local response of this signalling to the embryo presence has not been elucidated yet. The aim of the present study was to evaluate whether the interaction of the embryo with the oviduct can induce changes in the gene expression of BMP signalling components. For this purpose, we used an in vitro co-culture system of a bovine oviducal epithelial cell (BOEC) monolayer with pre-implantation embryos in 2 developmental time points: before and during the main phase of embryonic genome activation (EGA). Isthmus epithelial cells from post-ovulatory stage oviducts (Day 2–4) were cultured in 500 μL of SOF + 10% FCS in 4-well plates at 38.5°C, 5% CO₂, 5% O₂, and 90% N₂. On Day 6 of culture, medium was replaced with SOF + 5% FCS, and 24 h later BOEC monolayer was cultured in the absence or presence of in vitro-produced embryos from 2- to 8-cell stage [G1 BOEC; 33–54 h post-insemination (hpi)] or from 8- to 16-cell stage (G2 BOEC; 54–98 hpi) in the same conditions. In both groups, a polyester mesh was used to define a local co-culture area, and 30 embryos per well were placed in a 6 × 5 grid over the monolayer. In addition, as control groups, embryos in both developmental stages were cultured either in SOF + 5% FCS (G1 FCS and G2 FCS) or in SOF + 3 mg mL⁻¹ BSA (G1 BSA and G2 BSA). At 54 hpi (G1 BOEC/BSA/FCS) or 98 hpi (G2 BOEC/BSA/FCS), embryos that reached 8- or 16-cell stage, respectively, were transferred to SOF + BSA and cultured until Day 9. The mRNA expression levels of 3 BMP receptors (BMPRIA/IB/II), 2 signalling proteins (SMAD1/5), 1 inhibitor (SMAD6), and 1 target gene (ID2) were analysed by qPCR in 5 samples of BOEC cultured with or without embryos before or during EGA, and in 3 pools of 10 embryos at 8 (54 hpi), 16 (98 hpi), and blastocyst stage (Day 7–8) from all groups. Genes H2A.Z and ACTG1 were used as housekeeping genes, and statistical differences were assessed by ANOVA. The presence of the embryo, irrespective the stage, significantly reduced the expression levels of BMPRIB, BMPRII, SMAD1, SMAD6, and ID2 in BOEC. Embryos that interacted with BOEC before EGA (G1 BOEC) showed a significant increase in the relative abundance of SMAD1 at the 8-cell stage compared with controls. Moreover, embryos that interacted with BOEC during EGA (G2 BOEC) showed a significant increase in the relative abundance of BMPRIB, BMPRII, and ID2 at the 16-cell stage when compared with controls. However, no differences were observed in the mRNA expression levels of BMP signalling components in the blastocysts between groups. In conclusion, local embryo-oviduct interaction in vitro induces changes in the transcriptional levels of BMP signalling, causing a bidirectional response that reduces the expression levels of this signalling in the oviducal cells while increases them in the embryo at early stages. This suggests that BMP signalling pathway could be involved in an early cross-talk between the bovine embryo and the oviduct during first stages of development.