Activin and inhibin receptor gene expression in the ewe pituitary throughout the oestrous cycle

Abstract

In mammals, activin and inhibin are important regulators of FSH secretion. Previous studies have demonstrated that primary ovine pituitary cells express different activin receptor subtypes: activin receptor-like (ALK)2, ALK4, activin type II receptor A (ActRIIA), ActRIIB and Smad proteins in vitro. Here, we have carried out physiological studies to investigate the pattern of mRNA expression of the activin receptor subunits in the ewe pituitary throughout the oestrous cycle. The oestrous cycles of ewes were synchronized with progestagen sponges. The animals were killed 36 h (before the preovulatory surge, n=4), 48 h (during the preovulatory surge, n=4), 72 h (during the second surge of FSH, n=6) and 192 h (during the luteal phase, n=4) after sponge removal. Using Northern blots, we have shown that the levels of ALK2, ALK4 and ActRIIB mRNA were significantly higher before the preovulatory surge and during the secondary surge of FSH as compared with both during the preovulatory surge and the luteal phase, whereas the level of the ActRIIA mRNA was similar throughout the oestrous cycle. Using Western blots we have also demonstrated that the level of phosphoSmad2 did not vary during the reproductive cycle. Inhibin binding protein (InhBP/p120) and the transforming growth factor-type III receptor, betaglycan, have been identified as putative inhibin co-receptors. In this study, we cloned a fragment of both InhBP/p120 and betaglycan cDNAs in the ewe and showed by Northern blot that pituitary betaglycan and InhBP/p120 mRNA levels did not fluctuate across the oestrous cycle nor did they correlate with serum FSH levels.

Dynamic regulation of pituitary mRNAs for bone morphogenetic protein (BMP) 4, BMP receptors, and activin/inhibin subunits in the ewe during the estrous cycle and in cultured pituitary cells

“…Chronic treatment of pituitary cells with GnRH for 6 h, mimicking a situation similar to the preovulatory surge, induced a decrease in the level of FSHb mRNA. This is consistent with the reduction observed in vivo in pituitaries collected during the LH surge (Fafioffe et al 2004). Moreover, this agrees with reports showing that chronic GnRH inhibits the activity of the oFSHb promoter and the rat FSHb expression in vivo and in vitro (Attardi et al 1989, Lerrant et al 1995, Shafiee-Kermani et al 2007).…”

“…However, BMPR2 is present on the gonadotrope cells. Moreover, the level of ActR2A mRNA is similar throughout the estrous cycle (Fafioffe et al 2004). All together, these data led us to study preferentially BMPR2, BMPR1A, and BMPR1B.…”

“…A lot of studies raised the question of the mechanism of action of BMPs at the gonadotrope cell level, but no data are available concerning the possible changes of pituitary BMP system during the estrous cycle. To determine whether the variations in FSHb mRNA and plasma FSH concentrations are associated with changes in the expression of BMP4 and BMP receptor mRNA as well as activin bB mRNA, three stages of the estrous cycle exhibiting variations of plasma FSH as previously shown (Leung et al 1988, Fafioffe et al 2004 were studied: late follicular phase, before the secondary FSH surge, and the luteal phase.…”

“…In addition, the levels of BMPR1A (ALK3) and BMPR2 mRNAs, as well as those of BMPR1B (ALK6) (also known as Acvr1) is able to induce BMP signaling (ten Dijke et al 1994, Macias-Silva et al 1998, Lee et al 2007 and was previously shown to increase at the secondary FSH surge in ewe (Fafioffe et al 2004). Nonetheless, BMP4 and BMP2 are known to bind to BMPR1A (ALK3) and BMPR1B (ALK6), whereas other BMPs bind more strongly to ALK2 (ten Dijke et al 1994, Miyazono et al 2010.…”

“…In the ewe, it is true for activin receptors since the mRNA for ActR1A (ALK2), ActR1B (ALK4), and ActR2B fluctuates across the estrous cycle (Fafioffe et al 2004). Whether activin ligands are changing in the pituitary is not known.…”