Involvement of the bone morphogenetic protein/receptor system during follicle development in the bovine ovary: Hormonal regulation of the expression of bone morphogenetic protein 7 (BMP-7) and its receptors (ActRII and ALK-2).

Bone morphogenetic proteins (BMPs) are crucial factors in follicular growth and development. Among the BMP ligands, BMP-7 which use ActRII as their type II receptor, strongly bind to ALK-2 as their type I receptor. However, whether their receptors are expressed and the regulatory mechanisms controlling their expression during the process of bovine follicle development are still unknown. The aim of the present study was to clarify the involvement of the receptor system for BMP-7 in follicular selection by examining the effects of follicle-stimulating hormone (FSH) and estradiol (E2) on the regulation of ActRII and ALK-2 mRNA expression in bovine granulosa cells (GCs). To observe mRNA expression, follicles were obtained from heifers and GCs were classified into two groups: pre-selection follicles (PRF; follicles with an average diameter of 7 mm and low E2) and post-selection follicles (POF; follicles with an average diameter of 15 mm and high E2). The theca cell (TC) layer and GCs were harvested from aspirated follicles. For in vitro studies, GCs were obtained from bovine follicles of 4-7 mm diameter and cultured in Dulbecco's modified Eagle's/F12 (DMEM/F-12) medium with 10% fetal calf serum for 24h. The medium was then replaced with serum-free DMEM/F-12 supplemented with different doses of E2 (1, 10,100 ng/ml), FSH (1, 5, 10 ng/ml) or combinations of 1 ng/ml of E2 with different FSH doses (1, 5, 10 ng/ml). Total RNA was extracted from GCs and the mRNA expression of ActRII and ALK-2 was estimated by the quantitative PCR method using LightCycler. The expression of BMP-7 mRNA in TCs did not differ between the PRF and POF. ActRII and ALK-2 expression was detected in GCs from bovine antral follicles and was higher in the GCs of POF than in those of PRF, while the expression of the ActRII and ALK-2 genes in the TCs was not different between PRF and POF. Treatment of GCs with E2 (10 ng/ml) alone increased the expression of both ActRII and ALK-2 mRNAs, whereas FSH alone had no effect. However, ActRII and ALK-2 mRNA levels were up-regulated by the combination of E2 (1 ng/ml) and FSH (5 ng/ml). The results of the present study provide the first evidence that FSH and E2 regulate the expression of the ActRII and ALK-2 genes in bovine GCs. Thus, our data suggest that the BMP7/ActRII/ALK-2 system may be critically involved in the process of selection of bovine follicles.