104 EXPRESSION OF ACTIVIN A AS A LOCAL REGULATOR IN THE BOVINE OVIDUCT

Abstract

Activin (ACV) is known as a local regulator of several reproductive functions including follicular development and implantation in mammals. ACVA is a glycopeptide belonging to the transforming growth factor β superfamily, and is a homodimer of inhibin ßA (INHBA) subunits. Follistatin (FST), an ACV-specific binding protein, inhibits ligand-receptor binding. ACV receptor (ACVR) is a hetero-tetramer consisting of 2 kinds of protein, ACVR1 or ACVR1B and ACVR2A or ACVR2B. The oviduct provides an optimal environment for sperm capacitation, fertilization, and early embryonic development. Previous reports have demonstrated that ACVRs were expressed in bovine oocytes and embryos, and that early embryonic development is facilitated by ACVA in vitro. ACVA produced by the bovine oviduct may affect gametes and embryos as well as oviductal cells as a local regulator in cow. Bovine oviductal samples were classified into 6 stages of the oestrous cycle (day of ovulation; Days 2–3 after ovulation; Days 5–6; Days 8–12; Days 15–17; Days 19–21). We examined (1) protein expression of ACVA and FST in oviductal fluid collected from the ampulla and isthmus, (2) mRNA expression of INHBA and FST in the ampullary and isthmic oviductal tissues during the oestrous cycle, (3) the effects of oestradiol-17β (E2: 0.1, 1, 10 nM) and progesterone (P4: 1, 10, 100 nM) on mRNA expressions of INHBA and FST in cultured oviductal epithelial cells isolated from the ampulla and isthmus, and (4) mRNA expression of ACVRs in tissues and in cultured epithelial and stromal cells. The main findings were as follows: (1) Both ACVA and FST were detected throughout the oestrous cycle in the oviductal fluid of the ampulla and isthmus. (2) INHBA expression was higher in the isthmus than in the ampulla. FST expression in the ampulla was lowest at peri-ovulation, INHBA expression in the isthmus was highest on the day of ovulation and FST in the isthmus during Days 2–6 was highest. Because an increase of ACVA production and a decrease of FST production raise ACVA bioactivity, ACVA seems to be most active at peri-ovulation in both the ampulla and isthmus. (3) In the cultured isthmic oviductal epithelial cells, 10 nM E2 significantly stimulated INHBA expression, but tended to suppress FST expression. Therefore, the bioactivity of ACVA seems to be controlled by E2 during the oestrous cycle in the isthmus. (4) The expression of ACVR1B and ACVR2A was clearly detected in the tissues as well as in cultured epithelial and stromal cells. The overall findings suggest that ACVA secreted into oviductal fluid plays an important role in oviductal functions, including fertilization in the ampulla and sperm motility and viability in the isthmus. It is also suggested that ACVA acts on both epithelial and stromal cells as a local regulator of cellular functions, such as cellular proliferation and secretion in the cow.