Oocyte-derived BMP15 and FGFs cooperate to promote glycolysis in cumulus cells

Mammalian oocytes are deficient in their ability to carry out glycolysis. Therefore, the products of glycolysis that are necessary for oocyte development are provided to oocytes by companion cumulus cells. Mouse oocytes secrete paracrine factors that promote glycolysis in cumulus cells. The objective of this study was to identify paracrine factors secreted by oocytes that promote glycolysis and expression of mRNA encoding the glycolytic enzymes PFKP and LDHA. Candidates included growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15) and fibroblast growth factors (FGFs). Bmp15-/- and Gdf9+/- Bmp15-/- (double mutant, DM) cumulus cells exhibited reduced levels of both glycolysis and Pfkp and Ldha mRNA, and mutant oocytes were deficient in promoting glycolysis and expression of Pfkp and Ldha mRNA in cumulus cells of wild-type (WT) mice. Alone, neither recombinant BMP15, GDF9 nor FGF8 promoted glycolysis and expression of Pfkp and Ldha mRNA in WT cumulus cells. Co-treatment with BMP15 and FGF8 promoted glycolysis and increased expression of Pfkp and Ldha mRNA in WT cumulus cells to the same levels as WT oocytes; however, the combinations of BMP15/GDF9 or GDF9/FGF8 did not. Furthermore, SU5402, an FGF receptor-dependent protein kinase inhibitor, inhibited Pfkp and Ldha expression in cumulus cells promoted by paracrine oocyte factors. Therefore, oocyte-derived BMP15 and FGFs cooperate to promote glycolysis in cumulus cells.

Structural and Biophysical Coupling of Heparin and Activin Binding to Follistatin Isoform Functions

Follistatin (FS) regulates transforming growth factor-beta superfamily ligands and is necessary for normal embryonic and ovarian follicle development. Follistatin is expressed as two splice variants (FS288 and FS315). Previous studies indicated differences in heparin binding between FS288 and FS315, potentially influencing the physiological functions and locations of these isoforms. We have determined the structure of the FS315-activin A complex and quantitatively compared heparin binding by the two isoforms. The FS315 complex structure shows that both isoforms inhibit activin similarly, but FS315 exhibits movements within follistatin domain 3 (FSD3) apparently linked to binding of the C-terminal extension. Surprisingly, the binding affinities of FS288 and FS315 for heparin are similar at lower ionic strengths with FS315 binding decreasing more sharply as a function of salt concentration. When bound to activin, FS315 binds heparin similarly to the FS288 isoform, consistent with the structure of the complex, in which the acidic residues of the C-terminal extension cannot interact with the heparin-binding site. Activin-induced binding of heparin is unique to the FS315 isoform and may stimulate clearance of FS315 complexes.

BMP-15 regulation of ovulation quota in mammals

Naturally occurring mutations in the oocyte-specific factor, bone morphogenetic protein-15 (BMP-15), cause infertility in women and in ewes. In contrast to these monoovulatory mammals, the targeted deletion of BMP-15 in polyovulatory mice results in subfertility with only minimal defects in the ovulation process. Given the established role of BMP-15 in governing the progression of folliculogenesis, it is hypothesized that species-specific differences in the BMP-15 system are involved in species-specific determination of ovulation quota and litter size. Recent data using in vitro cell transfection methodology indicate that, in contrast to human BMP-15 which is successfully processed and secreted, the mouse BMP-15 proprotein is resistant to proteolytic cleavage. Thus, no functional mature BMP-15 is secreted in vitro. Further studies have shown that the functional mature form of BMP-15 is barely detectable in mouse oocytes in vivo until just before ovulation, when it is markedly increased. The general hypothesis to emerge from these observations is that the species-specific differences in the defects caused by mutations in the bmp15 gene between monoovulatory ewes and women and polyovulatory mice might be attributed to the timing of the production of BMP-15 mature protein. (Reprod Med Biol 2006; 5: 245-248).

A unique preovulatory expression pattern plays a key role in the physiological functions of BMP-15 in the mouse

Mutations in the bone morphogenetic protein 15 (BMP-15) gene cause female infertility in the monoovulatory human and sheep; however, in the polyovulatory mouse, loss-of-function of BMP-15 results only in reduced ovulation rate. To elucidate the cause of these species-specific differences, we investigated the functional role of BMP-15 in the mouse ovary. Here, we found that the functional mature form of BMP-15 is barely detectable in the mouse oocytes until just before ovulation, when it is markedly increased. Further, we found that BMP-15 induces cumulus expansion in mouse cumulus-oocyte complexes. The oocyte culture medium from immature mice primed with pregnant mare serum gonadotropin followed by human chorionic gonadotropin also stimulated cumulus expansion, and this activity was attenuated by BMP-15 antibody. Interestingly, the oocyte culture medium from mice treated with pregnant mare serum gonadotropin alone had no effect. Moreover, BMP-15 stimulated the expression of EGF-like growth factors in cumulus cells as well as a series of molecules downstream of EGF-like growth factor signaling, including cyclooxygenase 2, hyaluronan synthase 2, tumor necrosis factor-stimulated gene 6, and pentraxin 3, all of which are necessary for normal cumulus expansion. An antagonist of the EGF receptor completely abolished the effect of BMP-15 in inducing cumulus expansion. These results are consistent with the phenotype of BMP-15-null mice, which exhibit normal folliculogenesis but have defects in the ovulation process. The species-specific differences in the phenotypes caused by BMP-15 mutations may thus be attributed to the temporal variations in the production of the mature form of BMP-15.

Activation of follistatin promoter by GnRH in LbetaT2 gonadotroph cells

Follistatin (FS) is produced and secreted from gonadotroph cells in pituitary gland as well as granulosa cells in the ovary. In the present study, we found that the FS promoter is activated by GnRH in the gonadotroph cell line, LbetaT2. Therefore, we examined the signal transduction pathways involved in the mechanism. The activation of the FS promoter by GnRH was inhibited by calphostin C, a protein kinase C inhibitor, and U0126, a MAP kinase kinase (MEK) inhibitor. Phosphorylation by protein kinase C of myristoylated alanine-rich C kinase substrate (MARCKS) in LbetaT2 cells was observed after 3-min treatment with GnRH and declined after 30 min. The subsequent activation of MAP kinase was also transient, and down-regulation of protein kinase C completely inhibited the MAP kinase activation by GnRH, suggesting that the transient activation of protein kinase C led to the transient activation of MAP kinase. Although phorbol 12-myristate 13-acetate treatment increased phosphorylation of MARCKS and activated MAP kinase, it did not activate the FS promoter. Genistein, a tyrosine kinase inhibitor, completely inhibited the GnRH-induced activation of the FS promoter, while no inhibition of the MAP kinase pathway was observed. These results suggest that the activations of both the protein kinase C and tyrosine kinase pathways are necessary for the activation of the FS promoter in gonadotroph cells.

Bone morphogenetic protein-15 in the zebrafish ovary: complementary deoxyribonucleic acid cloning, genomic organization, tissue distribution, and role in oocyte maturation

Bone morphogenetic protein-15 (BMP-15) is a member of the TGFbeta family known to regulate ovarian functions in mammals. The structure and function of BMP-15 in lower vertebrates are less known. In this study, we cloned the zebrafish BMP-15 (zfBMP-15) cDNA and depicted its genomic organization. The zfBMP-15 cDNA encodes a protein of 384 amino acids. The mature protein has 46-51% sequence identities to fugu, chicken, and mammalian BMP-15. It also shares 38-46% homology with growth and differentiation factor-9 in fishes, chicken, and mammals. Phylogenetic analysis further confirms that the zfBMP-15 is most closely related to BMP-15 from other species, whereas the growth and differentiation factor-9 peptides from fish to mammals form a distinct branch. Comparison of zfBMP-15 cDNA with zebrafish genome database revealed that zfBMP-15 is encoded by a gene with two exons and one intron, located on chromosome 6. BMP-15 mRNA is expressed in the ovary and testis and, to a lesser extent, brain, liver, gut, heart, and muscle. Real-time PCR revealed that BMP-15 is expressed in follicles at all stages of development with no significant changes over the course of folliculogenesis. Using in situ hybridization and immunocytochemistry, we detected BMP-15 in both oocytes and follicular cells. Incubation of follicles with antiserum against zfBMP15 increased oocyte maturation, whereas incubation with recombinant human BMP-15 suppressed human chorionic gonadotropin-induced oocyte maturation. These findings suggest that BMP-15 plays a role in regulating gonadal functions in fish, in particular oocyte maturation.

Effects of bone morphogenetic protein-7 (BMP-7) on primordial follicular growth in the mouse ovary

Previously, bone morphogenetic protein-7 (BMP-7) was suggested as a factor that may act to facilitate the transition of follicles from primordial stage to the pool of developed primary, preantral, and antral follicles (Lee et al. 2001: Biol Reprod 65:994-999.). Thus, aim of the present study was to evaluate effect(s) of BMP-7 on the primordial-primary follicle transition. Neonatal mouse ovaries were cultured in the presence or absence of 100 mIU/ml FSH with various doses of BMP-7 (0, 10, and 100 ng/ml). After 4-day culture period, number of follicles was counted and the expression of transcripts for FSH receptor (FSHR), kit ligand (KL), and c-kit was measured by RT-PCR. BMP-7 alone at 100 ng/ml concentration stimulated follicle development with concurrent increase of mRNA for FSHR. BMP-7 alone down-regulated KL expression however, the ratio between KL1 and KL2 was increased. There was no change in the c-kit mRNA expression. Results of the present study suggest that the BMP-7 is one of the factors involved in primordial-primary follicle transition in the mouse ovary and it may play a role in expression of FSHR for further follicular development.

Essential role of the oocyte in estrogen amplification of follicle-stimulating hormone signaling in granulosa cells

The establishment of dominant ovarian follicles that are capable of ovulating fertilizable oocytes is a fundamental determinant of female fertility. This process is governed by pituitary gonadotropins as well as local ovarian factors. Within the follicle, estrogen acts in an autocrine/paracrine manner to enhance FSH action in the granulosa cells. These effects include the augmentation of P450aromatase expression and estradiol production. This feed-forward effect of estrogen is believed to play a key role in follicle dominance. Here we found the essential role of the oocyte in this physiological process using primary cultures of rat granulosa cells. In the presence, but not absence, of oocytes, estrogen amplified FSH-stimulated increases in mRNA expression of P450aromatase, FSH receptor, LH receptor, and inhibin alpha-, betaA-, and betaB-subunits as well as cAMP production. Thus, oocytes mediate the estrogen enhancement of FSH action in the granulosa cells. In comparison with FSH, cotreatment with estrogen and oocytes failed to amplify the stimulatory effects of forskolin or 8-bromoadenosine-cAMP on granulosa cell responses including P450aromatase mRNA expression and cAMP production, indicating that estrogen/oocytes amplify FSH action at a site upstream of adenylate cyclase. These findings support the novel conclusion that communication between the oocyte and granulosa cells plays a crucial role in mediating estrogen action during FSH-dependent folliculogenesis.

Molecular biology and physiological role of the oocyte factor, BMP-15

The oocyte factor, bone morphogenetic protein-15 (BMP-15) has proven to be critical for normal fertility in female mammals. The biological functions of recombinant BMP-15 demonstrate its capacity to promote granulosa cell processes involved in early follicle growth, while simultaneously acting to restrict follicle-stimulating hormone (FSH)-induced granulosa cell differentiation. The in vitro biological activities of BMP-15 demonstrate its role in promoting early follicle growth through the stimulation of granulosa cell mitosis while simultaneously restricting FSH-induced follicle development through the suppression of FSH receptor mRNA expression. The in vivo relevance of the role of BMP-15 was established by the identification of naturally occurring BMP-15 mutations in sheep, which cause infertility in homozygous carrier ewes and, in striking contrast, increased fecundity in heterozygous carrier ewes due to an increase in ovulation quota. The necessity of BMP-15 for folliculogenesis in women has been recently established by the discovery of a BMP-15 mutation that is associated with ovarian dysgenesis. In contrast to the pronounced effects that the BMP-15 mutations have on folliculogenesis in sheep and humans, mice, which are homozygous for targeted deletions of BMP-15 exhibit only minimal defects in the ovulation process, leading to the proposal that there may be causal differences in the BMP-15 system of mono- and polyovulatory animals. Collectively, recent research on the oocyte-secreted factor BMP-15 has provided exciting new opportunities for understanding ovarian physiology and female fertility.

Posttranslational processing of mouse and human BMP-15: potential implication in the determination of ovulation quota

There has been significant attention to the growing recognition that oocytes have a critical capacity to organize and govern surrounding somatic cells. Bone morphogenetic protein 15 (BMP-15) is an oocyte-secreted factor that has raised particular interest due to its established role in determining ovulation quota and female fertility in mammals. As a first step in determining whether there are species-specific differences in the BMP-15 system that may play causal roles in the differences in ovulation quota observed in different mammalian species, we here compare the molecular characteristics of BMP-15 of polyovulatory mice with that of monoovulatory humans. We found that, although human BMP-15 mature protein is readily produced, there are defects in the production of mouse BMP-15 mature protein in an in vitro system of transfected cells. The generation of chimeric constructs consisting of different combinations of mouse and human BMP-15 proregions, cleavage sites, and mature regions indicates that the defects in the production of mouse BMP-15 mature protein depend on the presence of the mouse BMP-15 proregion. The mouse proregion also caused a significant reduction in the production of human BMP-15 mature protein. The coexpression with a convertase cleavage enzyme, furin, results in complete processing of all these chimeras; however, no mouse mature protein is detected in either secreted or cell-confined forms except when associated with the human proregion. Based on the biological role of BMP-15, defects in the production of mouse BMP-15 mature protein could correlate with the high ovulation quota and litter size observed in mice.

Follicle-stimulating hormone regulates oocyte growth by modulation of expression of oocyte and granulosa cell factors

Oocyte-granulosa cell communication is essential for oocyte development. The aims of this study were: 1) to determine the effect of FSH on expression of Kit ligand (KL), growth/differentiation factor-9, bone morphogenetic protein (BMP)-15, and Kit during growth of oocyte-granulosa cell complexes (OGCs) in vitro; 2) to investigate the role of BMP-15 in regulation of KL expression; and 3) to correlate mRNA expression with oocyte growth. OGCs from 12-d-old mice were cultured for up to 7 d in the presence of FSH [0.05 ng/ml (low), 5 ng/ml (high)] or BMP-15 (10 or 100 ng/ml). Transcripts were quantified using real-time RT-PCR, and oocyte and OGC diameters were measured. FSH regulated KL expression in a biphasic manner, with low FSH decreasing the KL-1/KL-2 ratio, and high FSH increasing the KL-1/KL-2 ratio, compared with controls (P < 0.05). The decrease in KL-1/KL-2 ratio with low FSH was due to increased KL-2 mRNA expression. Both FSH concentrations increased OGC diameter (P < 0.05), but only low FSH promoted oocyte growth (P < 0.05). High FSH also decreased BMP-15 expression (P < 0.05). FSH-stimulated oocyte growth was inhibited by Gleevec, an inhibitor of Kit activity. BMP-15 increased both KL-1 and KL-2 mRNA levels in a dose-dependent manner (P < 0.05) but did not alter the KL-1/KL-2 ratio or promote oocyte growth. When the KL-1/KL-2 ratio was increased by exogenous KL-1, FSH-stimulated oocyte growth was suppressed (P < 0.05), suggesting that lowered KL-1/KL-2 ratio is important for oocyte growth. In summary, the correct concentration of FSH is crucial for appropriate modulation of KL and BMP-15 to promote oocyte growth.

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.

Analysis of spatial and temporal expression patterns of bone morphogenetic protein family members in the rat uterus over the estrous cycle

Recent studies have demonstrated that bone morphogenetic proteins (BMPs) play fundamental roles in female fertility. This is particularly evident in terms of the ovary. One major question that is just beginning to be addressed is the role of BMPs in the non-pregnant uterus. To help fill this gap, we used in situ hybridization to investigate the expression of BMP family members in the rat uterus over the estrous cycle. We found that the endometrial/uterine cycle is accompanied by the expression of several components of the BMP pathway - including ligands, receptors and antagonists. The mRNAs encoding BMP receptors are expressed in the epithelial (BMP-RIA, -RIB and -RII), periluminal stroma (BMP-RIA and -RII) and smooth muscle cells (BMP-RIA and -RII). The expression of all three receptors showed clear cyclic variations. The mRNAs encoding BMP ligands were highly expressed in the periluminal stroma (BMP-2 and -7) and glandular epithelium (BMP-7). The expression of BMP-2, but not BMP-7, was cyclical. Notably, the periluminal stroma expressed noggin mRNA. In the blood vascular system, BMP-4, -6 and -RII mRNAs were expressed in myometrial endothelial cells. Interestingly, follistatin, noggin, and BMP-4, -6 and -7 mRNAs were expressed in eosinophilic leukocytes, suggesting unexpected roles for eosinophil-derived BMPs in uterine function. We conclude that BMP ligands, receptors and antagonists are expressed in spatially and temporally restricted patterns that are consistent with a physiological role for these regulatory molecules in promoting uterine cellular processes including cell proliferation, differentiation and apoptosis during the cycle.

Pregnancy-associated plasma protein-a production in rat granulosa cells: stimulation by follicle-stimulating hormone and inhibition by the oocyte-derived bone morphogenetic protein-15

Pregnancy-associated plasma protein-A (PAPP-A) is the major IGF binding protein-4 (IGFBP-4) protease in follicular fluid, consistent with its proposed role in folliculogenesis. Despite growing interest, almost nothing is known about how PAPP-A expression is regulated in any tissue. Here we show that FSH and oocytes regulate PAPP-A expression in granulosa cells (GCs). By in situ hybridization, ovary PAPP-A mRNA was markedly increased by pregnant mare serum gonadotropin treatment, and the message was localized to the membrana GCs but not cumulus GCs (CGCs) of dominant follicles. To explore the mechanism, we used primary cultures of rat GCs. Control (untreated) cells produced little or no PAPP-A spontaneously. Conversely, FSH markedly stimulated PAPP-A mRNA and protein in a dose- and time-dependent fashion. Interestingly, PAPP-A expression in isolated CGCs was also strongly induced by FSH, and the induction was inhibited by added oocytes. To investigate the nature of the inhibition, we tested the effect of oocyte-derived bone morphogenetic protein-15 (BMP-15). BMP-15 alone had no effect on basal levels of PAPP-A expression by cultures of membrana GCs or CGCs. However, BMP-15 markedly inhibited the FSH stimulation of PAPP-A production in a dose-dependent manner. The cleavage of IGFBP-4 by conditioned media from FSH-treated GCs was completely inhibited by anti-PAPP-A antibody, indicating the IGFBP-4 protease secreted by GCs is PAPP-A. These results demonstrate stimulatory and inhibitory roles for FSH and BMP-15, respectively, in regulating PAPP-A production by GCs. We propose that FSH and oocyte-derived BMP-15 form a controlling network that ensures the spatiotemporal pattern of GC PAPP-A expression in the dominant follicle.

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.

The bone morphogenetic protein system in mammalian reproduction

Using molecular, cellular, and genetic approaches, recent studies examining the role of the bone morphogenetic protein (BMP) family of growth factors in the reproductive system have led to significant breakthroughs in our understanding of mammalian reproduction and fertility. Gene expression studies have revealed that key components of the BMP system (ligands, receptors, signaling molecules, and binding proteins) exhibit coordinated spatial and temporal expression patterns in fundamental cell types throughout the reproductive system. Availability of recombinant BMPs has enabled functional studies that have demonstrated important biological activities of BMPs in controlling cellular proliferation, differentiation, and apoptosis in reproductive tissues. The physiological importance of the BMP system for mammalian reproduction has been further highlighted by the elucidation of the aberrant reproductive phenotypes of animals with naturally occurring mutations or targeted deletions of certain BMP family genes. Collectively, these studies have established the concept that the BMP system plays a crucial role in fertility in female and male mammals. The purpose of this article is to review the evidence underpinning the importance of the BMP system in mammalian reproduction.

Effect of intracellular interactions on the processing and secretion of bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9. Implication of the aberrant ovarian phenotype of BMP-15 mutant sheep

Bone morphogenetic protein-15 (BMP-15) and growth and differentiation factor-9 (GDF-9) are members of the transforming growth factor-beta superfamily. Both molecules are closely related in their primary structures and share a nearly identical spatiotemporal expression pattern in the oocyte during folliculogenesis in mammals. Here we have established a series of cell lines, which express recombinant BMP-15, GDF-9, or both, and investigated whether they form homodimers and/or heterodimers. We demonstrate the first evidence that both BMP-15 and GDF-9 can form non-covalent homodimers when expressed individually, while when both are co-expressed BMP-15/GDF-9 heterodimers are produced. Interestingly, when GDF-9 and BMP-15 are co-expressed the processing of both proproteins are significantly impaired as compared with that of the singly expressed proproteins, suggesting that the proprotein heterodimer is less susceptible to proteolytic cleavage than the individual homodimers. Since BMP-15 mutant sheep, called Inverdale, exhibit severe defects in ovarian function we have also established stable transformants expressing the mutant BMP-15 (InvBMP-15) alone or together with GDF-9. Although InvBMP-15 was previously predicted to be unable to form homodimers, we show here that it does form non-covalent dimers; however, the processing efficiency of InvBMP-15 proprotein is significantly lower than wild-type BMP-15. Surprisingly, when GDF-9 is co-expressed, the processing and secretion of InvBMP-15 is abolished, and the processing of GDF-9 is also severely impaired, suggesting that the heterodimers of InvBMP-15/GDF-9 proproteins are not susceptible to proteolytic cleavage and thus degrade in the cells. Based on these findings we propose a novel hypothesis that a decrease in GDF-9 secretion may be involved in causing infertility in homozygous Inverdale ewes.

The spatiotemporal expression pattern of the bone morphogenetic protein family in rat ovary cell types during the estrous cycle

In the mammalian ovary, great interest in the expression and function of the bone morphogenetic protein (BMP) family has been recently generated from evidence of their critical role in determining folliculogenesis and female fertility. Despite extensive work, there is a need to understand the cellular sites of expression of these important regulatory molecules, and how their gene expression changes within the basic ovary cell types through the cycle. Here we have performed a detailed in situ hybridization analysis of the spatial and temporal expression patterns of the BMP ligands (BMP-2, -3, -3b, -4, -6, -7, -15), receptors (BMPR-IA, -IB, -II), and BMP antagonist, follistatin, in rat ovaries over the normal estrous cycle. We have found that: i) all of the mRNAs are expressed in a cell-specific manner in the major classes of ovary cell types (oocyte, granulosa, theca interstitial, theca externa, corpora lutea, secondary interstitial, vascular and ovary surface epithelium); and ii) most undergo dynamic changes during follicular and corpora luteal morphogenesis and histogenesis. The general principle to emerge from these studies is that the developmental programs of folliculogenesis (recruitment, selection, atresia), ovulation, and luteogenesis (luteinization, luteolysis) are accompanied by rather dramatic spatial and temporal changes in the expression patterns of these BMP genes. These results lead us to hypothesize previously unanticipated roles for the BMP family in determining fundamental developmental events that ensure the proper timing and developmental events required for the generation of the estrous cycle.

Molecular basis of bone morphogenetic protein-15 signaling in granulosa cells

Bone morphogenetic protein-15 (BMP-15), an oocyte growth factor belonging to the transforming growth factor-beta superfamily, has recently been shown to be necessary for normal female fertility in mammals. We have previously demonstrated that BMP-15 regulates granulosa cell (GC) proliferation and differentiation; namely, BMP-15 promotes GC mitosis, suppresses follicle-stimulating hormone (FSH) receptor expression, and stimulates kit ligand expression. Although the role of BMP-15 in female reproduction has progressively deserved much attention, there is nothing known to date about the signaling pathway and receptors for BMP-15. Using rat primary GCs and a human GC cell line, COV434, we have now found that administration of BMP-15 causes a rapid and transient phosphorylation, thus activation, of the Smad1/5/8 pathway. BMP-15 also stimulated promoter activity of a selective BMP-responsive reporter construct, further demonstrating the stimulation of Smad1/5/8 signaling by BMP-15. In contrast, BMP-15 stimulation of Smad2 phosphorylation was very weak. To identify the receptors for BMP-15, we utilized recombinant extracellular domains of individual transforming growth factor-beta superfamily receptors and found that activin receptor-like kinase-6 extracellular domain most effectively co-immunoprecipitates with BMP-15, whereas BMP receptor type II extracellular domain was most effective in inhibiting BMP-15 bioactivity on FSH-induced progesterone production and GC thymidine incorporation. We also investigated whether activation of the MAPK pathway is necessary for BMP-15 biological activity and found that the addition of U0126, an inhibitor of ERK1/2 phosphorylation, suppresses BMP-15 activity on GC mitotsis but not on FSH-induced progesterone production, suggesting a selective signaling cascade in GC proliferation and differentiation.

The role of bone morphogenetic proteins in ovarian function

Bone morphogenetic proteins (BMPs) represent the largest subclass of growth factors in the transforming growth factor beta (TGF-beta) superfamily. BMPs have proven to be multifunctional regulators of a wide variety of biological processes in numerous types of cell and tissue. The role of inhibins, activins and TGF-betas (which also belong to the TGF-beta superfamily) in reproduction has been studied extensively over the last 15 years. However, there were no reports on the role of BMPs in the mammalian ovary until 1999 when we reported an intrinsic ovarian BMP system replete with BMP ligands, receptors and novel biological functions. Since this report it has become clear that the BMP system plays an important role in the regulation of ovarian function, evidenced by the ability of BMPs to control granulosa cell proliferation and cytodifferentiation, as well as oocyte development. The physiological relevance of the BMP system has recently been highlighted by the discovery that genetic mutations in the BMP-15 ligand or the BMP type IB receptor lead to critical aberrations in folliculogenesis and ovulation. This review provides a current overview of the rapidly emerging field of the BMP system in mammalian ovarian function.

A novel function of bone morphogenetic protein-15 in the pituitary: selective synthesis and secretion of FSH by gonadotropes

Bone morphogenetic protein-15 (BMP-15) has been known to be an oocyte-specific factor and regulate granulosa cell functions. Here, we report that BMP-15 is also expressed in the well established gonadotrope cell line, LbetaT2 and mouse pituitary tissue. Interestingly, BMP-15 stimulates FSHbeta subunit transcription in LbetaT2 cells without affecting LHbeta and GnRH receptor transcription. These findings, therefore, suggest a possible autocrine control mechanism selective for FSHbeta synthesis by BMP-15. Moreover, BMP-15 exerts a potent and selective stimulation of FSH, but not LH, secretion by primary pituitary cells. Taken together, we hypothesize that BMP-15 could play a physiological role in the monotropic rise of FSH secretion by the pituitary during the estrous and menstrual cycle. These findings, together with the previous evidence of a negative regulation of FSH receptor expression by BMP-15 in the ovary, may constitute a major role in female fertility.

A negative feedback system between oocyte bone morphogenetic protein 15 and granulosa cell kit ligand: its role in regulating granulosa cell mitosis

Although the existence of a regulatory paracrine feedback system between oocytes and follicular somatic cells has been postulated for some time, there has not yet been any definitive evidence that such a communication system exists. Herein we present a previously undescribed oocyte-granulosa cell (GC) feedback communication system involving an oocyte-derived factor, bone morphogenetic protein-15 (BMP-15) and a GC-derived factor, kit ligand (KL), both of which have been shown to be crucial regulators of female reproduction. We used a coculture system of rat oocytes and GCs and found that BMP-15 stimulates KL expression in GCs, whereas KL inhibits BMP-15 expression in oocytes, thus forming a negative feedback loop. Moreover, KL, like BMP-15, exhibited mitotic activity on GCs in the presence of oocytes. Because c-kit (KL receptor) is expressed in oocytes but not GCs, the oocytes must be involved in mediating the KL-induced GC mitosis. Furthermore, the blockage of c-kit signaling in oocytes by using a c-kit neutralizing antibody markedly suppressed BMP-15-induced GC mitosis, suggesting that the oocyte must play a role in the GC responses to BMP-15. In contrast, the c-kit antibody had no effect on the mitotic activities of two other known GC mitogens, activin-A and BMP-7. Altogether, this study presents direct evidence of a negative feedback system governed by oocyte-derived BMP-15 and GC-derived KL, and demonstrates that the mitotic activities of BMP-15 and KL for GCs depend on this oocyte-GC communication system. We hypothesize that the negative feedback system most likely plays a pivotal role in early folliculogenesis.

Genomic organization and promoter analysis of mouse follistatin-related gene (FLRG)

Follistatin (FS) is well characterized as an activin-binding protein. Recently, a novel follistatin-like protein called follistatin-related gene (FLRG) that has a similar domain organization to that of follistatin has been identified. Like follistatins, FLRG binds activins and bone morphogenetic proteins (BMPs). To study the regulation of FLRG expression, we have analyzed the genomic organization and promoter of the mouse FLRG gene. The mouse FLRG gene consists of five exons, and each encodes discrete functional regions. The overall genomic structure of FLRG is similar to that of FS except that the FLRG gene is missing one exon that codes a third FS domain found in FS. The promoter that covers 2.5 kbp and is linked to a luciferase reporter construct is active in human cervical carcinoma HeLa cells as well as in human embryonic kidney (HEK293) cells. Deletion analysis of the promoter regions indicates that a proximal 550 base pairs are enough for basal FLRG promoter activity in the cell lines. FLRG promoter activity is significantly augmented by phorbol 12-myristate 13-acetate (PMA) treatment, but not by cAMP stimulation. By contrast, FS promoter is activatable either by cAMP or PMA. Thus, although FS and FLRG are structurally and functionally related, their modes of regulation by external stimuli are different.

Aberrant expression of growth differentiation factor-9 in oocytes of women with polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a major cause of female infertility. Despite substantial effort, the etiology and pathogenesis of PCOS and polycystic ovaries (PCO) in women remain unknown. Recent studies in laboratory animals have documented a link between dysfunction of two oocyte growth factors, growth differentiation factor-9 (GDF-9) and bone morphogenetic factor-15 (BMP-15), and aberrant folliculogenesis. Because aberrant follicle development is a hallmark of PCOS, we wondered whether the expression patterns of these growth factors might be disrupted in PCOS and PCO oocytes. To address this issue, we compared the pattern and level of expression of GDF-9 and BMP-15 mRNA in ovaries from normal cycling (n = 12), PCOS (n = 5), and PCO (n = 7) patients. In situ hybridization studies showed that the expression of GDF-9 and BMP-15 is restricted to the oocytes in all ovaries examined. Interestingly, a decreased level of GDF-9 signal was observed in developing PCOS and PCO oocytes, compared with normal. This difference was evident throughout folliculogenesis, beginning at recruitment initiation and continuing through the small Graafian follicle stage. By contrast, there were no qualitative or quantitative changes in the expression of BMP-15 mRNA in PCOS oocytes during folliculogenesis. There were also no significant differences between normal and PCOS and PCOs in the levels of the mRNA encoding the housekeeping gene, cyclophilin. Together, these results indicate that the expression of GDF-9 mRNA is delayed and reduced in PCOS and PCO oocytes during their growth and differentiation phase. Because oocyte-derived GDF-9 is crucial for normal folliculogenesis and female fertility, we suggest that a dysregulation of oocyte GDF-9 expression may contribute to aberrant folliculogenesis in PCOS and PCO women.