The bone morphogenetic protein system in mammalian reproduction

Are BMP-15 and GDF-9 primary determinants of ovulation quota in mammals?

How do mammals control the number of eggs that are ovulated during the estrous and menstrual cycles? Our understanding of this fundamental process has grown in recent years as a result of intense efforts to identify and characterize the genes that control the ovulation quota. An increasing body of evidence shows that two oocyte-specific factors, bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9), play crucial roles in determining folliculogenesis, ovulation rate and litter size in sheep and mice. In this article, we review recent advances on the physiological, cellular and molecular roles of BMP-15 and GDF-9, which, potentially, link these oocyte-secreted factors to the species-specific determination of ovulation quota and litter size in mammals.

The oocyte and its role in regulating ovulation rate: a new paradigm in reproductive biology

Ovulation rate in mammals is determined by a complex exchange of hormonal signals between the pituitary gland and the ovary and by a localised exchange of hormones within ovarian follicles between the oocyte and its adjacent somatic cells. From examination of inherited patterns of ovulation rate in sheep, point mutations have been identified in two oocyte-expressed genes, BMP15 (GDF9B) and GDF9. Animals heterozygous for any of these mutations have higher ovulation rates (that is, + 0.8–3) than wild-type contemporaries, whereas those homozygous for each of these mutations are sterile with ovarian follicular development disrupted during the preantral growth stages. Both GDF9 and BMP15 proteins are present in follicular fluid, indicating that they are secreted products.In vitrostudies show that granulosa and/or cumulus cells are an important target for both growth factors. Multiple immunisations of sheep with BMP15 or GDF9 peptide protein conjugates show that both growth factors are essential for normal follicular growth and the maturation of preovulatory follicles. Short-term (that is, primary and booster) immunisation with a GDF9 or BMP15 peptide-protein conjugate has been shown to enhance ovulation rate and lamb production. In summary, recent studies of genetic mutations in sheep highlight the importance of oocyte-secreted factors in regulating ovulation rate, and these discoveries may help to explain why some mammals have a predisposition to produce two or more offspring rather than one.

Immunoneutralization of Growth Differentiation Factor 9 Reveals It Partially Accounts for Mouse Oocyte Mitogenic Activity1

Paracrine factors secreted by oocytes play a pivotal role in promoting early ovarian follicle growth and in defining a morphogenic gradient in antral follicles, yet the exact identities of these oocyte factors remain unknown. This study was conducted to determine the extent to which the mitogenic activity of mouse oocytes can be attributed to growth differentiation factor 9 (GDF9). To do this, specific anti-human GDF9 monoclonal antibodies were generated. Based on epitope mapping and bioassays, a GDF9 neutralizing antibody, mAb-GDF9-53, was characterized with very low cross-reactivity with related transforming growth factor (TGF)beta superfamily members, including BMP15 (also called GDF9B). Pep-SPOT epitope mapping showed that mAb-GDF9-53 recognizes a short 4-aa sequence, and three-dimensional peptide modeling suggested that this binding motif lies at the C-terminal fingertip of mGDF9. As predicted by sequence alignments and modeling, the antibody detected recombinant GDF9, but not BMP15 in a Western blot and GDF9 protein in oocyte extract and oocyte-conditioned medium. In a mouse mural granulosa cell (MGC) bioassay, mAb-GDF9-53 completely abolished the mitogenic effects of GDF9, but had no effect on TGFbeta1 or activin A-stimulated MGC proliferation. An unrelated IgG at the same dose had no effect on GDF9 activity. This GDF9 neutralizing antibody was then tested in an established oocyte-secreted mitogen bioassay, where denuded oocytes cocultured with granulosa cells promote cell proliferation in a dose-dependent manner. The mAb-GDF9-53 dose dependently (0-160 microg/ml) decreased the mitogenic activity of oocytes but only by approximately 45% at the maximum dose of mAb. Just 5 microg/ml of mAb-GDF9-53 neutralized 90% of recombinant mGDF9 mitogenic activity, but only 15% of oocyte activity. Unlike mAb-GDF9-53, a TGFbeta pan-specific neutralizing antibody did not affect the mitogenic capacity of the oocyte, but completely neutralized TGF beta 1-induced DNA synthesis. This study has characterized a specific GDF9 neutralizing antibody. Our data provide the first direct evidence that the endogenous GDF9 protein is an important oocyte-secreted mitogen, but also show that GDF9 accounts for only part of total oocyte bioactivity.

Characterization of the bone morphogenetic protein (BMP) system in human pulmonary arterial smooth muscle cells isolated from a sporadic case of primary pulmonary hypertension: roles of BMP type IB receptor (activin receptor-like kinase-6) in the mitotic action

The functional involvement of bone morphogenetic protein (BMP) system in primary pulmonary hypertension (PPH) remains unclear. Here we demonstrate a crucial role of the BMP type IB receptor, activin receptor-like kinase (ALK)-6 for pulmonary arterial smooth muscle cell (pphPASMC) mitosis isolated from a sporadic PPH patient bearing no mutations in BMPR2 gene. A striking increase in the levels of ALK-6 mRNA was revealed in pphPASMC compared with control PASMCs, in which ALK-6 transcripts were hardly detectable. BMP-2 and -7 stimulated the mitosis of pphPASMCs, which was opposite to their suppressive effects on the mitosis of the control PASMCs. BMP-4 and -6 and activin inhibited pphPASMC mitosis, whereas these did not affect control PASMCs. The presence of BMP signaling machinery in pphPASMCs was elucidated based on the analysis on Id-1 transcription and Smad-reporter genes. Overexpression of a dominant-negative ALK-6 construct revealed that ALK-6 plays a key role in the mitosis as well as intracellular BMP signaling of pphPASMCs. Gene silencing of ALK-6 using small interfering RNA also reduced DNA synthesis as well as Id-1 transcription in pphPASMCs regardless of BMP-2 stimulation. Although Id-1 response was not stimulated by BMP-2 in control PASMCs, the gene delivery of wild-type ALK-6 caused significant increase in the Id-1 transcripts in response to BMP-2. Additionally, inhibitors of ERK and p38 MAPK pathways suppressed pphPASMC mitosis induced by BMP-2, implying that the mitotic action is in part MAPK dependent. Thus, the BMP system is strongly involved in pphPASMC mitosis through ALK-6, which possibly leads to activation of Smad and MAPK, resulting in the progression of vascular remodeling of pulmonary arteries in PPH.

Growth Differentiation Factor 9 Regulates Expression of the Bone Morphogenetic Protein Antagonist Gremlin

Growth differentiation factor 9 (GDF9) is an oocyte-expressed member of the transforming growth factor beta (TGF-beta) superfamily and is required for normal ovarian follicle development and female fertility. GDF9 acts as a paracrine factor and affects granulosa cell physiology. Only a few genes regulated by GDF9 are known. Our microarray analysis has identified gremlin as one of the genes up-regulated by GDF9 in cultures of granulosa cells. Gremlin is a known member of the DAN family of bone morphogenetic protein (BMP) antagonists, but its expression and function in the ovary are unknown. We have investigated the regulation of gremlin in mouse granulosa cells by GDF9 as well as other members of the TGF-beta superfamily. GDF9 and BMP4 induce gremlin, but TGF-beta does not. In addition, in cultures of granulosa cells, gremlin negatively regulates BMP4 signaling but not GDF9 activity. The expression of gremlin in the ovary was also examined by in situ hybridization. A distinct change in gremlin mRNA compartmentalization occurs during follicle development and ovulation, indicating a highly regulated expression pattern during folliculogenesis. We propose that gremlin modulates the cross-talk between GDF9 and BMP signaling that is necessary during follicle development because both ligands use components of the same signaling pathway.

Hypergonadotropic ovarian failure associated with an inherited mutation of human bone morphogenetic protein-15 (BMP15) gene

Hypergonadotropic ovarian failure is a common cause of female infertility. It is a heterogeneous disorder that, in the most severe forms, is a result of ovarian dysgenesis (OD). Most OD cases are associated with major X-chromosome abnormalities, but the pathogenesis of this disorder is still largely undefined in patients with a normal karyotype. Animal models showed the important role in female reproduction played by the product of a gene located at Xp11.2 in humans (BMP15). BMP15 is an oocyte-specific growth/differentiation factor that stimulates folliculogenesis and granulosa cell (GC) growth. We report two sisters with a normal karyotype who are affected with hypergonadotropic ovarian failure due to OD. The familial presentation suggested a genetic origin, and candidate genes were screened for mutations. A heterozygous nonconservative substitution in the pro region of BMP15 (Y235C) was identified in both sisters but not in 210 control alleles. This mutation was inherited from the father. Mutant BMP15 appears to be processed abnormally, is associated with reduced GC growth, and antagonizes the stimulatory activity of wild-type protein on GC proliferation. In conclusion, the first natural mutation in human BMP15 is associated with familial OD, indicating that the action of BMP15 is required for the progression of human folliculogenesis. This condition represents an exceptional example of X-linked human disease exclusively affecting heterozygous females who inherited the genetic alteration from the unaffected father. BMP15 defects are involved in the pathogenesis of hypergonadotropic ovarian failure in humans.

Functional and Molecular Characterization of Naturally Occurring Mutations in the Oocyte-secreted Factors Bone Morphogenetic Protein-15 and Growth and Differentiation Factor-9

Bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9) are oocyte-secreted factors that are critical local regulators of ovarian physiology. Recent studies have identified a number of mutations in these genes that cause increased fertility and infertility in heterozygous or homozygous ewes carrying the mutations, respectively. Interestingly, heterozygous ewes with a mutation in both BMP-15 and GDF-9 exhibit higher fertility than those having mutation in only one of the genes. Here, we have produced recombinant human BMP-15 and GDF-9 that carry the mutations identified in those sheep, i.e. I31D and S99I in BMP-15 and S77F in GDF-9. We found that when individually expressed, both BMP-15 mutations had no effect on the processing, secretion, and dimerization of the mature proteins or on the biological activity of the molecules. However, when mutant BMP-15 was co-expressed with wild-type GDF-9, the secretion of BMP-15 and GDF-9 was significantly reduced, suggesting that the mechanisms by which the BMP-15 mutations affect sheep fertility occurs at the level of protein secretion rather than dimerization and biological activity. Moreover, when mutant GDF-9 was co-expressed with mutant BMP-15, the secretion levels of both proteins were significantly lower than those of cells co-expressing wildtype GDF-9 and mutant BMP-15, suggesting a possible mechanism for the extreme fertility observed in the compound heterozygous mutant sheep.

Involvement of activin/BMP system in development of human pituitary gonadotropinomas and nonfunctioning adenomas

Roles of activin/bone morphogenetic protein (BMP) system in the pathogenesis of human pituitary adenoma remain unknown although these factors stimulate follicle-stimulating hormone (FSH) secretion in the normal pituitary. Here we demonstrated that type-I and -II subunit mRNAs of activin/BMP receptors are expressed in Pit-1-negative FSH-producing (FSH-oma) and nonfunctioning pituitary adenomas (NF-oma). Basal levels of serum FSH standardized by luteinizing hormone (LH) were markedly high in FSH-omas in contrast to NF-omas. However, gonadotropin-releasing hormone (GnRH)-induced increment of FSH standardized by that of LH was not changed in FSH-omas, suggesting that imbalanced FSH secretion by FSH-oma is not attributable to GnRH regardless of the expression of GnRH receptor. Although activin betaA subunit was detected in neither adenoma, the betaB subunit was expressed highly in FSH-omas and, to lesser extent, in NF-omas. As for BMPs, BMP-6 and -7 were detected in NF-omas while BMP-4 and -15 were not detected in either type of adenoma. In the presence of pituitary activin/BMP system, the levels of co-expressing follistatin mRNA in the tumors were reduced in FSH-oma compared with NF-oma, suggesting that endogenous follistatin is involved in FSH overproduction through inhibition of activin/BMP system independently of GnRH.