Evidence for an inhibitory role of bone morphogenetic protein(s) in the follicular–luteal transition in cattle

Abstract

Bone morphogenetic proteins (BMPs) and their receptors are expressed in ovarian theca cells (TC) and granulosa cells (GC) and BMPs have been implicated in the regulation of several aspects of follicle development including thecal androgen production and granulosal oestrogen production. Their potential involvement in luteal function has received less attention. In this study, we first compared relative abundance of mRNA transcripts for BMPs, activin-bA and BMP/activin receptors in bovine corpus luteum (CL) and follicular theca and granulosa layers before undertaking functional in vitro experiments to test the effect of selected ligands (BMP6 and activin A) on luteinizing bovine TC and GC. Relative to b-actin transcript abundance, CL tissue contained more BMP4 and -6 mRNA than granulosa, more BMP2 mRNA than theca but much less activin-bA mRNA than both granulosa and theca. Transcripts for all seven BMP/activin receptors were readily detected in each tissue and two transcripts (BMPRII, ActRIIA) were more abundant in CL than either theca or granulosa, consistent with tissue responsiveness. In vitro luteinization of TC and GC from antral follicles (4-6 mm) was achieved by culturing with 5% serum for 6 days. Treatment with BMP6 (0, 2, 10, and 50 ng/ml) and activin A (0, 2, 10 and 50 ng/ml) under these conditions dose-dependently suppressed forskolin-induced progesterone (P4) secretion from both cell types without affecting cell number. BMP6 reduced forskolin-stimulated upregulation of STAR mRNA and raised 'basal' CYP17A1 mRNA level in theca-lutein cells without affecting expression of CYP11A1 or hydroxy-D-5-steroid dehydrogenase, 3 b-and steroid D-isomerase 1 (HSD3B1). In granulosa-lutein cells, STAR transcript abundance was not affected by BMP6, whereas forskolin-induced expression of CYP11A1, HSD3B1, CYP19A1 and oxytocin transcripts was reduced. In both cell types, follistatin attenuated the suppressive effect of activin A and BMP6 on forskolin-induced P4 secretion but had no effect alone. Granulosa-lutein cells secreted low levels of endogenous activin A (w1 ng/ml); BMP6 reduced this, while raising follistatin secretion thus decreasing activin A:follistatin ratio. Collectively, these findings support inhibitory roles for BMP/activin signalling in luteinization and steroidogenesis in both TC and GC.

Activin A and follistatin during the oestrous cycle and early pregnancy in ewes

“…In the current study, we have shown that treatment of luteinising ovine granulosa cells with activin A reduced progesterone secretion; thus, the increased activin A concentrations observed around the time of oestrous in the reduced ES ewes may have resulted in poor luteal function in these ewes. Similar to what was observed in the current study, activin A has been shown to reduce luteinisation of granulosa cells in vitro in both humans and cattle (Myers et al 2008, Kayani et al 2009). Activin A also suppressed progesterone production in vitro from granulosa cells collected from preovulatory follicles in rats (Miro et al 1991).…”

“…The activin pathway has been postulated to be involved in regulation of multiple reproductive processes including ovarian follicular development (Knight et al 2012), luteinisation of the follicular cells (Myers et al 2008, Kayani et al 2009), early embryo development (Rajput et al 2013) and uterine function during implantation of the conceptus (Singh et al 2011). As a member of the transforming growth factor beta superfamily of growth factors, activin forms from the dimerisation of two b-subunits of inhibin (INHB), with different combinations forming functionally different isoforms (Knight 1996, Mellor et al 2003.…”

BMP2, 4 and 6 and BMPR1B are altered from early stages of bovine cystic ovarian disease development

“…In this sense, Fatehi et al (2005) found no BMP4 protein expression in granulosa cells of growing follicles, despite having shown mRNA expression in granulosa and theca of antral follicles less than 8 mm in diameter. However, Glister et al (2010) and Kayani et al (2009) showed BMP4 mRNA expression and immunoreactive protein in both theca and granulosa cells, although BMP4 transcripts were more abundant in theca than in granulosa cells, in agreement with earlier immunocytochemical evidence in cattle (Glister et al 2004) and other species (Khalaf et al 2013).…”

“…This inhibition suggests a key role of BMPs as luteinization inhibitors (Otsuka et al 2000(Otsuka et al , 2001. Treatment with BMP4, BMP6 and BMP7 increases basal and insulin-like growth factor 1 (IGF1)-induced estradiol production by bovine granulosa cells in vitro (Glister et al 2004) and decreases forskolin-induced progesterone production in luteinized bovine granulosa cells in vitro (Kayani et al 2009). Similarly, BMP2, added to sheep granulosa cell cultures, results in enhanced estradiol production (Souza et al 2002).…”

Granulosal and thecal expression of bone morphogenetic protein- and activin-binding protein mRNA transcripts during bovine follicle development and factors modulating their expression in vitro

“…However, in our study chordin expression was raised by all four BMPs tested. Previously, BMPs were shown to enhance secretion of follistatin by both non-luteinised (Glister et al 2004) and luteinised (Kayani et al 2009) bovine GC. In contrast, this study found no effect of BMPs on TC expression of follistatin mRNA.…”