Equine Inhibin/Activin bA-Subunit mRNA Is Expressed in the Endometrial Gland, but Not in the Trophoblast, During Pregnancy KEITARO YAMANOUCHI, 1 KENSUKE HIRASAWA, 2 TELHISA HASEGAWA, 3 AKIHIRO IKEDA, 1 KYU-TAE CHANG, 1 SHIGEMI MATSUYAMA, 1 MASUGI NISHIHARA, 1 KIYOSHI MIYAZAWA, 4 TORU SAWASAKI, 5 HIDEAKI TOJO, 6 CHIKASHI TACHI, 6 AND MICHIO TAKAHASHI 1 * 1 Department of Veterinary Physiology, Veterinary Medical Science, the University of Tokyo, Tokyo, Japan 2 Department of Biomedical Science, Veterinary Medical Science, the University of Tokyo, Tokyo, Japan 3 Laboratory of Molecular and Cellular Biology, Equine Research Institute, Japan Racing Association, Tokyo, Japan 4 Department of Veterinary Obstetrics and Gynecology, Obihiro University of Agriculture and Veterinary Medicine, Hokkaido, Japan 5 Laboratory of Livestock Biotechnology, Institute of Animal Resource Sciences, Graduate School of Agriculture and Life Science, the University of Tokyo, Tokyo, Japan 6 Laboratory of Applied Genetics, Institute of Animal Resource Sciences, Graduate School of Agriculture and Life Science, the University of Tokyo, Tokyo, Japan ABSTRACT The expression of both inhibin a- and inhibin/ activin bA-subunit mRNA was examined in equine uteroplacental tissues collected during preg- nancy (days 90 to 300). Northern blot analysis revealed that 5 transcripts (7.0, 4.1, 3.4, 2.6, 1.5 kb) of bA-subunit were present, and the most abundantly expressed transcript was the 1.5 kb one. Relatively high levels of the 1.5 kb transcript were seen in the second trimester of pregnancy compared to what was found in the third trimester. To identify the tissue localization of bA-subunit mRNA, in situ hybridization was performed, and the positive signal was observed exclusively in the endometrial glands, but not in the fetal placental tissue (trophoblast) at days 150, 210, and 300 of pregnancy. On the other hand, inhibin a-subunit transcript could not be detected at any stage of pregnancy examined either by Northern blot analysis or in situ hybridization. Although the factor(s) regulating the gene expression of bA-subunit in this equine tissue is currently unknown, these results suggest that activin, but not inhibin, is predominantly produced in the endo- metrial glands of the pregnant mare, and thus produced activin may play a paracrine or endocrine role during pregnancy in this species. Mol. Reprod. Dev. 47:363– 369, 1997. r 1997 Wiley-Liss, Inc. Key Words: horse; inhibin; activin; endometrial gland; trophoblast; pregnancy INTRODUCTION The placenta of the mare consists of fetal and mater- nal tissues that are in apposition for purposes of fetal nutrition, respiration, waste removal, endocrine func- tion, and immune protection of the fetus (Silver, 1984; Antczak et al., 1987). The equine placenta is diffuse, noninvasive, and epitheliochorial. The trophoblastic villi are interdigitated in a complex arrangement with the uterine epithelium forming microplacentomes (Samuel et al., 1975; 1977). The dimeric polypeptides inhibin and activin were originally discovered as gonad-derived molecules which were capable of suppressing and enhancing follicle stimulating hormone (FSH) secretion, respectively, from anterior pituitary (Mather et al., 1992). Inhibin is a heterodimer, composed of a- and b-subunits (bA or bB), while activin is a homodimer of the b-subunit. These polypeptides have been found to be members of trans- forming growth factor-b (TGF-b) family (Vale et al., 1991), and to have many biological potencies in various tissues (Vale et al., 1991). Extragonadal expression of mRNA encoding these subunits in various tissues from rats was demonstrated by Meunier et al. (1988), and placenta was shown to express relatively high levels of bA-subunit mRNA, suggesting that activin is produced predominantly in this tissue rather than inhibin. The expression of both mRNA and protein of a-subunit and bA-subunit was also demonstrated in rat, human, and baboon placenta by others (Billiar et al., 1992; Petraglia et al., 1992; Rabinovici et al., 1992; Roberts and Barth, 1994) by means of Northern blot analysis, in situ hybridization, and immunocytochemistry. Contract grant sponsor: Ministry of Education, Science, Sports, and Culture, Japan; Contract grant sponsor: The Equine Research Insti- tute, Japan Racing Association. Keitaro Yamanouchi’s present address is Laboratory of Applied Genet- ics, Institute of Animal Resource Sciences, Graduate School of Agricul- ture and Life Science, the University of Tokyo, Tokyo 113, Japan. *Correspondence to: Michio Takahashi, Department of Veterinary Physiology, Veterinary Medical Science, the University of Tokyo, Tokyo 113, Japan. Received 4 November 1996; Accepted 6 February 1997 MOLECULAR REPRODUCTION AND DEVELOPMENT 47:363–369 (1997) r 1997 WILEY-LISS, INC.