Control of FSH receptor mRNA expression in rat granulosa cells by 3',5'-cyclic adenosine monophosphate, activin, and follistatin

Comprehensive analysis of differences in N6-methyladenosine RNA methylomes in the rat adenohypophysis after GnRH treatment

N6-methyladenosine is considered to be the most common and abundant internal chemical modification among the more than 150 identified chemical RNA modifications. It is involved in most biological processes and actively participates in the regulation of animal reproduction. However, the potential function of m⁶ A in the pituitaries of mammals is not yet clear. It is also unknown whether m⁶ A is involved in the secretion and regulation of FSH by GnRH, which in turn affects mammalian reproduction. In this study, rats were treated with gonadorelin to simulate physiological GnRH-mediated regulation of FSH synthesis and secretion, and m⁶ A-seq was used to analyze the differential m⁶ A modification of the rat pituitary after gonadorelin treatment. A whole-transcriptome map of m⁶ A in the rat pituitary gland before and after gonadorelin treatment was successfully created. A total of 6413 differential peaks were identified, of which 3764 m⁶ A peaks were upregulated and 2649 m⁶ A peaks were downregulated. Among the 709 differentially expressed genes, 250 genes were discovered with differential methylation modifications. Intriguingly, the altered m⁶ A peaks within mRNAs were enriched in steroid biosynthetic processes and responses to cAMP. The results of the study will lay a foundation for further exploration of the potential role of m⁶ A modification in the regulation of reproductive hormone secretion and provide a theoretical basis for the application of GnRH analogs in mammalian artificial reproduction.

Expression and localization of Smad2 and Smad4 proteins in the porcine ovary

The objective of the present study was to investigate the temporal and spatial expression of Smad2 and Smad4 proteins, the downstream signaling molecules of the transforming growth factor beta (TGF-β) superfamily, in the porcine ovary. Cellular localization of Smad2 and Smad4 proteins was examined using immunohistochemistry. The specificity of the antibodies was examined using Western blot assay. Western blot analyses demonstrated that 52kDa Smad2 and 60kDa Smad4 proteins were expressed in the porcine ovary. Immunohistochemistry revealed that Smad2 and Smad4 were widely expressed in the porcine ovary, mainly localized in the oocyte, granulosa and thecal cells at different stages of folliculogenesis. Within the primordial and primary follicles, Smad2 and Smad4 showed strong staining in oocytes and follicular cells. In the antral follicle, strong staining was observed in oocytes, granulosa and theca cells. These findings suggest that Smad2 and Smad4 may be a key regulator of follicular development and growth of oocytes in the porcine ovary.

Research resource: small RNA-seq of human granulosa cells reveals miRNAs in FSHR and aromatase genes

The granulosa cells in the mammalian ovarian follicle respond to gonadotropin signaling and are involved in the processes of folliculogenesis and oocyte maturation. Studies on gene expression and regulation in human granulosa cells are of interest due to their potential for estimating the oocyte viability and in vitro fertilization success. However, the posttranscriptional gene expression studies on micro-RNA (miRNA) level in the human ovary have been scarce. The current study determined the miRNA profile by deep sequencing of the 2 intrafollicular somatic cell types: mural and cumulus granulosa cells (MGCs and CGCs, respectively) isolated from women undergoing controlled ovarian stimulation and in vitro fertilization. Altogether, 936 annotated and 9 novel miRNAs were identified. Ninety of the annotated miRNAs were differentially expressed between MGCs and CGCs. Bioinformatic prediction revealed that TGFβ, ErbB signaling, and heparan sulfate biosynthesis were targeted by miRNAs in both granulosa cell populations, whereas extracellular matrix remodeling, Wnt, and neurotrophin signaling pathways were enriched among miRNA targets in MGCs. Two of the nine novel miRNAs found were of intronic origin: one from the aromatase and the other from the FSH receptor gene. The latter miRNA was predicted to target the activin signaling pathway. In addition to revealing the genome-wide miRNA signature in human granulosa cells, our results suggest that posttranscriptional regulation of gene expression by miRNAs could play an important role in the modification of gonadotropin signaling. miRNA expression studies could therefore lead to new prognostic markers in assisted reproductive technologies.

Differential regulation of gonadotropin receptors by bone morphogenetic proteins in the zebrafish ovary

Follicle-stimulating hormone receptor (fshr) and luteinizing hormone/choriogonadotropin receptor (lhcgr) exhibit differential temporal expression patterns during zebrafish folliculogenesis with fshr being dominant during vitellogenic growth and lhcgr increasing its expression dramatically before maturation. The dynamic and distinct expression patterns of fshr and lhcgr suggest that they are under tight regulatory control. However, the underlying mechanisms for the differential expression of the two receptors remain unknown. We have recently demonstrated that members of bone morphogenetic protein (BMP) family are largely expressed in the oocyte, while their receptors are exclusively localized on the follicle cells, suggesting a potential paracrine signaling from the oocyte to the follicle cells by BMPs. In this study, we investigated the effects of zebrafish BMP2b (zfBmp2b) and BMP4 (zfBmp4) on the expression of fshr and lhcgr using a novel co-culture approach. The recombinant zfBmp2b or zfBmp4-producing CHO cells were co-cultured with the zebrafish follicle cells followed by real-time qPCR analysis of fshr and lhcgr expression. Our results showed that zfBmp2b and zfBmp4 both down-regulated fshr, while up-regulated lhcgr expression at 24 h of co-culturing. This finding, together with the high expression level of BMP receptors in the follicle cells prior to oocyte maturation, strongly suggests a potential role for BMPs in the differential expression of fshr and lhcgr, especially in the full-grown follicles before maturation. As BMPs are largely expressed in the oocyte, this also implies an important role for the oocyte in orchestrating the differentiation and function of the follicle cells.

A comprehensive curated resource for follicle stimulating hormone signaling

BACKGROUND

Follicle stimulating hormone (FSH) is an important hormone responsible for growth, maturation and function of the human reproductive system. FSH regulates the synthesis of steroid hormones such as estrogen and progesterone, proliferation and maturation of follicles in the ovary and spermatogenesis in the testes. FSH is a glycoprotein heterodimer that binds and acts through the FSH receptor, a G-protein coupled receptor. Although online pathway repositories provide information about G-protein coupled receptor mediated signal transduction, the signaling events initiated specifically by FSH are not cataloged in any public database in a detailed fashion.

FINDINGS

We performed comprehensive curation of the published literature to identify the components of FSH signaling pathway and the molecular interactions that occur upon FSH receptor activation. Our effort yielded 64 reactions comprising 35 enzyme-substrate reactions, 11 molecular association events, 11 activation events and 7 protein translocation events that occur in response to FSH receptor activation. We also cataloged 265 genes, which were differentially expressed upon FSH stimulation in normal human reproductive tissues.

CONCLUSIONS

We anticipate that the information provided in this resource will provide better insights into the physiological role of FSH in reproductive biology, its signaling mediators and aid in further research in this area. The curated FSH pathway data is freely available through NetPath (http://www.netpath.org), a pathway resource developed previously by our group.

Molecular cloning of two gonadotropin receptors and their distinct mRNA expression profiles in daily oogenesis of the wrasse Pseudolabrus sieboldi

In fish, asynchronous development of ovarian follicles, the simultaneous advance of vitellogenesis and oocyte maturation in one ovary, is a rational reproductive strategy to spawn consecutively in one spawning season. In this study, to clarify the mode of action of follicle stimulating hormone (FSH) and luteinizing hormone (LH) in asynchronous ovarian follicle development in daily egg production, we cloned cDNAs of the follicle-stimulating hormone receptor (FSHR) and luteinizing hormone receptor (LHR) in the bambooleaf wrasse (Pseudolabrus sieboldi), which exhibits clear diurnal spawning rhythms over 1 month. In addition, different developmental stages of ovarian follicles were isolated from whole ovaries at various daily time points on 1 day in the spawning season, and mRNA expression levels of FSHR and LHR were analyzed. Sequence analysis showed distinct differences in the number of putative leucine-rich repeats at the extracellular domain between FSHR and LHR, suggesting a difference in ligand-specificity. Real-time PCR analyses revealed that FSHR mRNA was highly expressed in early yolk-stage follicles but decreased at the end of vitellogenesis. In contrast, the expression of LHR mRNA was maintained at low levels in vitellogenic stage follicles but markedly elevated at the end of the vitellogenic and early migratory nucleus stages, thereafter markedly dropping in the late migratory nucleus stage. The present results suggest that co-regulation of vitellogenesis and oocyte maturation in one ovary is controlled by the stage-distinctive expression levels of FSHR and LHR mRNA in ovarian follicles, and daily switching of sensitivity from FSH to LH is required for daily egg production.

Enhancement of vinorelbine-induced cytotoxicity and apoptosis by clomipramine and lithium chloride in human neuroblastoma cancer cell line SH-SY5Y

The aim of this work is to investigate whether clomipramine (CIM) and lithium chloride (LiCl) potentiate the cytotoxicity of vinorelbine (VNR) on SH-SY5Y human neuroblastoma cells in vitro and whether midkine (MK) can be a resistance factor for these treatments. Four groups of experiments were performed for 96 h using both monolayer and spheroid cultures of SH-SY5Y cells: (1) control group, (2) singly applied VNR, CIM, and LiCl, (3) VNR with CIM, and (4) VNR with LiCl. Their effects on monolayer and spheroid cultures were determined by evaluating cell proliferation, bromodeoxyuridine labeling index (BrdU-LI), apoptosis, cyclic adenosine monophosphate (cAMP) and midkine levels, colony-forming efficiency, spheroid size, and ultrastructure. In comparison with the control group, single and combination drug treatments significantly reduced the proliferation index (PI) for 96 h. The most potent reduction of PI was observed with VNR in combination with CIM and LiCl for all time intervals. VNR with CIM and LiCl seemed to be ineffective in reducing BrdU-LI of both monolayer cell and spheroid cultures, spheroid size, and cAMP level. VNR with LiCl increased apoptosis at 24 h, however VNR with CIM increased apoptosis at 96 h. VNR was the most potent drug in inhibiting colony-forming efficiency. The combination of VNR with CIM was the most potent in reducing midkine levels among all groups. Interestingly, the combination of VNR with LiCl led to both nuclear membrane breakdown and disappearance of the cellular membranes inside the spheroids. Both CIM and LiCl seemed to potentiate VNR-induced cytotoxicity, and MK was not a resistance factor for VNR, LiCl, and CIM.

Smad3 is required for normal follicular follicle-stimulating hormone responsiveness in the mouse

Follicle-stimulating hormone (FSH) is the major regulator of folliculogenesis, but other factors modulate its action, including members of the transforming growth factor (TGF) beta family. The intersection of signal transduction pathways that integrate the follicular response to FSH remains to be elucidated. Herein, we investigated the role of Smad3, a critical molecule mediating the intracellular TGFbeta family proteins, in follicle development and the expression of FSH receptors. We found that gonadotropin stimulation could not induce normal ovulation in Smad3-deficient mice. Moreover, FSH could not stimulate early follicle growth in Smad3-deficient mice in in vivo or in vitro systems. Cultured granulosa cells from Smad3-deficient animals had reduced cell division rates following FSH treatment compared with granulosa cells derived from the ovaries of wild-type (WT) mice. Whole ovaries and isolated granulosa cells from Smad3-deficient animals had lower basal expression of FSH receptor (Fshr), aromatase (Cyp19a1), and cyclin D2 (Ccnd2) mRNA compared with WT mice. Follicle-stimulating hormone treatment of granulosa cells from WT ovaries upregulated Fshr, Cyp19a1, and Ccnd2 expression. However, FSH did not increase these mRNAs in Smad3-deficient granulosa cells. When Smad3 was introduced into Smad3-deficient granulosa cells with adenovirus vectors, FSH responsiveness was restored, and FSH was able to upregulate Fshr expression. Furthermore, SMAD3 interacts with a palindromic SMAD binding element in the Fshr promoter, and TGFB can activate promoter constructs containing this element. Collectively, these observations establish an essential role for Smad3 in regulating the response of ovarian follicles to FSH.

Expression of activin pathway genes in granulosa cells of dominant and subordinate bovine follicles

We examined the association between the expression profiles of genes of the activin signalling pathway and ovarian follicular dominance in cattle. In monovular species such as cattle, one ovarian follicle of a cohort is selected to become dominant, whereas all others (i.e. the subordinate follicles) eventually succumb to apoptosis. We showed that Inhibin-betaA, coding for the betaA chain found in the A isoforms of activin, Inh-alpha coding for the inhibin-specific alpha chain, and the activin antagonist follistatin were expressed at higher levels in dominant follicle granulosa cells from Day 3.5 (ovulation=Day 0). Before selection, Inh-betaA but not Inh-alpha was significantly correlated with potential dominant follicles, as defined by high aromatase expression and follicular fluid estrogen concentrations. Follistatin expression marked the largest follicles at Day 1.5, but displayed large variation in levels among cows. The third inhibin gene, Inh-betaB, could only be detected at very low levels from Day 7 and thus was unlikely to play a prominent role in activin/inhibin signalling in cattle during these stages. There was a decrease in activin tone (P=0.07) specifically in the aromatase-high/dominant follicles, as measured by the ratio of Inh-betaA to Inh-alpha plus follistatin transcripts between Days 1.5 and 7. Messenger RNA for both activin type II receptors and the nuclear effector Smad2 were detected in granulosa cells, consistent with an autocrine role for activin signalling. Additionally, expression of the putative activin target genes Smad2 and FSHreceptor were, respectively, either strongly (P<0.001) or weakly (P=0.09) associated with dominant follicles.

Maintenance of in vitro granulosa cell function by adenoviral mediated follicle stimulating hormone receptor gene transduction

PURPOSE

To maintain in vitro granulosa cell function by adenoviral-mediated FSHR gene transduction.

METHODS

Rat granulosa cells were cultured and transduced with adenovirus carrying FSHR gene. The number of receptors and the rate of steroidogenesis were assessed.

RESULTS

The number of FSHR on the granulosa cells was 4,874 per cell immediately after extraction, it was 2,176 by the third day, and had further reduced to 693 by the seventh day. On the third day of culture, the amount of production of estradiol by FSH stimulation also decreased to about one-quarter of the first day's quantity. Compared to the untransduced granulose cells, when the cells contained FSHR gene, the FSHR expression and steroidogenesis were both enhanced (2,176 vs. 7,206 per cell (p<0.001) and 192 vs. 5940 pg/mL (p<0.01), respectively).

CONCLUSION

Granulosa cell functions can be maintained or increased by novel gene therapy. This can be a useful component of assisted reproductive therapy.

Effect of follicle-stimulating hormone and estrogen on the expression of betaglycan messenger ribonucleic acid levels in cultured rat granulosa cells

Betaglycan (TGFbeta type III receptor) was recently identified as a coreceptor to enhance the binding of inhibin A to activin type II receptor. This inhibin/betaglycan/activin type II receptor complex prevents activins from binding to their own receptors. The present study was undertaken to identify the expression and the regulation of the betaglycan gene in cultured rat granulosa cells. Northern blot analysis indicated betaglycan mRNA transcript of approximately 6.4 kbp. The treatment of the cells with FSH increased the betaglycan mRNA level, and a concurrent treatment with estradiol brought a significant increase in betaglycan mRNA. The protein kinase A activator, 8-bromoadenosine-cAMP, also increased the expression of its mRNA. Furthermore, betaglycan mRNA was induced additively by estradiol, which was blocked by estrogen receptor antagonists [ICI 182780, (R, R)-cis-diethyltetrahydro-2,8-chrysenediol]. In the luciferase assay, FSH altered the promoter activity of betaglycan. Moreover, when FSH plus estradiol was added to the granulosa cells, a significant increase in the half-life of betaglycan mRNA transcript was seen. In summary, FSH and estradiol increased betaglycan mRNA expression, most possibly through the protein kinase A pathway and the estrogen receptor-beta. The increase of betaglycan mRNA was due to an increase in transcription and altered mRNA stability. In ovarian regulatory function, the expression of betaglycan may involve the functional antagonism of inhibin A in activin signal transduction.

Developmental Profiles of Activin βA, βB, and Follistatin Expression in the Zebrafish Ovary: Evidence for Their Differential Roles During Sexual Maturation and Ovulatory Cycle1

Our recent experiments showed that gonadotropin(s) stimulated activin betaA and follistatin expression through the cAMP-PKA pathway but suppressed betaB via a cAMP-dependent but PKA-independent pathway in cultured zebrafish follicle cells. Given that pituitary gonadotropins are the major hormones controlling the development and function of the ovary, the differential expression of activin betaA and betaB as well as follistatin in response to gonadotropin(s) raises an interesting question about the temporal expression patterns of these molecules in vivo during sexual maturation and ovulatory cycle. Three experiments were performed in the present study. In the first experiment using sexually immature zebrafish, we followed the expression of activin betaA, betaB, and follistatin at the whole ovary level during a 10-day period in which the ovary developed from the primary growth stage to the one with nearly full-grown follicles. Activin betaA expression was very low at the primary growth stage but significantly increased with the growth of the ovary, and its rise was accompanied by an increase in follistatin expression. In contrast, the expression of activin betaB could be easily detected in the ovary of all stages; however, it did not exhibit an obvious trend of variation during the development. The second experiment examined the stage-dependent expression of activin betaA, betaB, and follistatin at the follicle level in the adult mature zebrafish. The expression of activin betaA was again low in the follicles during the primary growth stage, but exhibited a phenomenal increase after the follicles entered vitellogenesis with the peak level reached at midvitellogenic stage; in contrast, activin betaB mRNA could be easily detected at all stages with a slight increase during follicle growth. The expression of follistatin, on the other hand, also increased significantly during vitellogenesis; however, its level dropped sharply after reaching the peak at the midvitellogenic stage. In the third experiment, we investigated the dynamic changes of the ovarian activin betaA, betaB, and follistatin expression during the daily ovulatory cycle. The expression of activin betaA and follistatin gradually increased from 1800 h onward and reached the peak level around 0400 h when the germinal vesicles had migrated to the periphery in the full-grown oocytes. In contrast, activin betaB expression steadily declined, although not statistically significant, during the same period, but increased sharply at 0700 h when mature oocytes started to appear in most of the ovaries collected. In conclusion, activin betaA and betaB exhibit distinct expression patterns during the development of the ovary and the daily ovarian cycle of the zebrafish. It seems that activin betaA is involved in promoting ovary and follicle growth, whereas activin betaB may have a tonic role throughout follicle development but becomes critical at the late stage of oocyte maturation and/or ovulation.

Spatial expression patterns of activin and its signaling system in the zebrafish ovarian follicle: evidence for paracrine action of activin on the oocytes

We have previously demonstrated that activin is likely an ovarian mediator of pituitary gonadotropin(s) and local epidermal growth factor in their stimulating oocyte maturation and maturational competence in the zebrafish. However, the downstream events controlled by activin remain unknown. One possible mechanism is that activin may directly work on the oocytes to promote the development of oocyte maturational competence. To substantiate this hypothesis, we performed the present study to demonstrate the expression of the activin system in different compartments of zebrafish follicles, namely, the follicle cells and oocytes. The proteins examined include activin subunits (betaA and betaB), activin-binding protein (follistatin), activin type II receptors (type IIA and IIB), the type I activin receptor-like kinases (ALK1-like, ALK2-like, and ALK4-like), and the intracellular activin signaling molecules (Smad2, Smad3, Smad4, and Smad7). The results showed that the entire activin signaling system is expressed by the full-grown immature zebrafish oocytes ( approximately 0.65 mm in diameter), including ALK4-like (ActRIB), ALK2-like (ActRIA), ActRIIA, ActRIIB, Smad2, Smad3, Smad4, and Smad7, therefore supporting our hypothesis that the oocytes are one of the direct targets of activin actions in the zebrafish ovary. In contrast, activin itself (betaA and betaB) and ALK1-like type I receptor are predominantly expressed in the follicle cells surrounding the oocytes. Interestingly, although follistatin is expressed in both the follicle cells and oocytes, its level of expression is significantly higher in the oocytes than the follicle cells, implying that follistatin may serve as a signal from the oocytes to modulate the activity of activin produced by the follicle cells. Taken together, the present study provides convincing evidence that although all members of the activin system are expressed in the whole follicle, they exhibit distinct spatial patterns of expression among different compartments of the follicle. It is likely that activin works directly on the oocytes in a paracrine manner to promote oocyte maturation and maturational competence. On the other hand, instead of being controlled passively by the follicle cells, the oocytes may actively participate in the regulation of follicle development by releasing various modulating molecules such as follistatin.

Lysyl oxidase gene expression and enzyme activity in the rat ovary: regulation by follicle-stimulating hormone, androgen, and transforming growth factor-beta superfamily members in vitro

Lysyl oxidase (LOX) catalyzes the final enzymatic reaction required for cross-linking of collagen and elastin fibers and therefore has a crucial role in regulating the formation and maintenance of extracellular matrix in the ovary. LOX mRNA is abundantly expressed in rat granulosa cells. To examine how regulation of LOX in the ovary might influence follicular development, we studied LOX mRNA expression and enzyme activity in rat granulosa cells from late preantral/early antral follicles in vitro. FSH dose dependently inhibited LOX mRNA and enzyme activity (50% reduction at 10 ng/ml) in vitro, and FSH action was mimicked by 8-bromo-cAMP, suggesting FSH action via elevation of cAMP. Dihydrotestosterone alone enhanced LOX mRNA and enzyme activity, but potentiated the effect of FSH, causing a further reduction. TGFbeta1 alone dose dependently enhanced LOX mRNA (5-fold increase at 10 ng/ml) and activity (1.5-fold increase). FSH dose dependently inhibited the increase in LOX mRNA and activity caused by TGFbeta1 (by up to 84% and 80%, respectively). Growth differentiation factor-9 (GDF-9) and activin A, at the same concentration as TGFbeta1 (10 ng/ml), stimulated LOX mRNA and activity within 6 h, although overall expression was higher at 48 h. All three factors when combined with FSH further reduced both mRNA and enzyme activity (by up to 60%) compared with FSH alone. These findings indicate control of LOX at endocrine, paracrine, and autocrine levels within the ovary and suggest coordinated regulation of ovarian extracellular matrix during follicular development, with FSH determining whether local factors act as stimulators or inhibitors of LOX.

Retinoic acid mediates transcriptional repression of ovine follicle-stimulating hormone receptor gene via a pleiotropic nuclear receptor response element

The FSH receptor (FSHR) and retinoid receptors are critical regulators of gonadal function. Unlike the latter, the FSH receptors are expressed exclusively in ovarian granulosa and testicular Sertoli cells in a developmental fashion. Toward understanding the nature of various transcription factors that direct a tissue- and stage-specific expression of the FSHR gene, we have studied FP4, one of the two footprinting regions (FP3 and FP4) mapped at -241 to -269 and -284 to -303, respectively, upstream of the transcription start site of the ovine FSHR gene. Gel mobility shift assays with FP4 probe revealed two sequence-specific DNA-protein complexes in the presence of nuclear extracts from two immortal gonadal cell lines. Antibody supershift assays demonstrated that retinoic acid receptor (RAR) was involved in the complex 1 whereas steroidogenic factor-1 (SF-1) was present in the complex 2. Mutation studies revealed that DNA binding sites for RAR and SF-1 were overlapping each other within a 19-base pair length of nucleotide sequence of FP4, and a mutation in the half RAR binding site seriously affected SF-1 binding. Reporter assays showed that FP4 conferred SF-1 transactivation as well as RAR-mediated, ligand-dependent repression. Overexpression of SF-1 in a transformed Sertoli cell line partially overcame RAR-mediated suppression. For the first time, our studies reveal a direct retinoid modulation of the gonadotropin receptor promoter and suggest a mechanism by which activators and repressors compete for composite elements providing antagonistic pathways that could modulate the expression of FSHR.

Stage-specific expression of Smad2 and Smad3 during folliculogenesis

Paracrine and autocrine growth factors can affect many different aspects of ovarian follicle development. Many members of the transforming growth factor beta (TGFbeta) family of growth factors and their receptors are expressed in developing follicles. However, the presence and function of the family of the TGFbeta signaling molecules known as Smads have not been evaluated during follicle development. We have demonstrated that two Smad family members that function as mediators for both activin and TGFbeta are expressed in granulosa cells of preantral follicles but not in large antral follicles. Smad2 expression, but not Smad3 expression, returns in luteal cells. Both Smad2 and Smad3 are translocated to the nucleus of granulosa cells in response to treatment with either TGFbeta or activin. However, Smad2 is more responsive to activin stimulation, and Smad3 is more responsive to TGFbeta stimulation. Stage-specific expression and differing ligand sensitivity of signaling molecules may work together to allow different effects of TGFbeta family ligands using the same signaling pathways over the course of follicular development.

Gonadotropin regulation of activin betaA and activin type IIA receptor expression in the ovarian follicle cells of the zebrafish, Danio rerio

We have previously demonstrated that both activin and its receptors are expressed in the zebrafish ovary, suggesting paracrine roles for activin in the ovarian functions. Activin significantly stimulated zebrafish oocyte maturation in vitro, and this effect could be blocked by follistatin, an activin-binding protein. Interestingly, follistatin also blocked the stimulatory effect of gonadotropin (hCG) on the oocyte maturation. Taken together, these results have led to a hypothesis that the ovarian activin system may play a role in mediating the actions of gonadotropin in the ovary. To test this hypothesis, the present study was undertaken to investigate if gonadotropin has any effect on the expression of activin betaA subunit and activin type IIA (ActRIIA) receptor in the zebrafish ovary. A primary culture of zebrafish ovarian follicle cells was established in the present study, and the cultured cells expressed both activin betaA and ActRIIA receptor when assayed with RT-PCR. The primary culture consisted of three major types of cells, presumably the fibroblasts, the thecal cells and the granulosa cells, according to the morphological features, histochemical staining for 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and RT-PCR for aromatase. Using a semi-quantitative RT-PCR with beta-actin as the internal control, we demonstrated that hCG significantly stimulated mRNA expression of both activin betaA and ActRIIA receptor in the cultured follicle cells in a time- and dose-dependent manner. Treatment of the cells with hCG quickly increased the steady-state mRNA levels of activin betaA and ActRIIA receptor, and the effect peaked at 2 h of treatment. The stimulatory effect of gonadotropin diminished with longer treatment and no effect was observed at 8 h of treatment. The effect of hCG also exhibited strong dose dependence when assayed at 2 h of treatment. The levels of activin betaA and ActRIIA receptor mRNA elevated with increasing dose of hCG; however, the effect significantly decreased at dosage higher than 15 IU/ml. Consistent with the stimulatory effect of gonadotropin on the expression of activin betaA and ActRIIA receptor, IBMX, forskolin and 8-Br-cAMP all significantly increased the mRNA levels of activin betaA and ActRIIA receptor. These results suggest that gonadotropin activates the activin system in the zebrafish ovary by increasing the expression of both activin and its receptors.

Localization of an activin/activin receptor system in the porcine ovary

The aim of this study was to locate a possible activin/activin receptor system within porcine ovaries containing functional corpora lutea. In situ hybridization was used to assess the gene expression of beta(A)- and beta(B)-activin subunits, and immunohistochemical studies were done to detect activin-A protein and activin receptor type II. mRNA expression of the beta(A)- and beta(B)-activin subunits was found in the granulosa from the unilaminar follicle stage onward, in the developing thecal layer of multilaminar and small antral follicles, in the theca interna of mid-sized antral follicles, in corpora lutea, and in the ovarian surface epithelium. Immunoreactive activin A protein could be detected at the same ovarian sites, but in thecal tissue of small antral follicles only. This protein was also demonstrated at the peripheral zone of oocytes from multilaminar and antral follicles. A positive immunoreaction for activin receptor was found in granulosa cells from multilaminar and older follicles and in oocytes from the earliest stages of follicular development onward. In late multilaminar follicles and in antral follicles, the oolemma was stained. Except for small antral follicles, a positive activin receptor immunoreaction was absent in the follicular theca. Activin receptor immunoreaction was furthermore present in corpora lutea and in the ovarian surface epithelium. It is concluded that, within porcine ovaries containing functional corpora lutea, an activin/activin receptor system is present in all intact follicles, the corpora lutea and the surface epithelium. Within follicles, granulosa and theca cells are the main sites of activin synthesis, while oocytes and granulosa cells are the main activin binding sites.

Effect of activin A on in vitro development of rat preantral follicles and localization of activin A and activin receptor II

Preantral follicles (140-160 microm) were isolated mechanically from the ovaries of 10-day-old rats and cultured in groups of four to six for 6 days in medium containing 0, 1, 10, or 100 ng/ml of activin A. Activin stimulated (P < 0.05) the growth of preantral follicles in a dose-dependent fashion and enhanced the proliferation of preantral, oocyte-free follicular cells. Furthermore, treatment with activin induced the majority of follicles to form an antrum-like structure and helped to maintain the ultrastructure of these follicles during culture. Activin A also induced further changes characteristic of follicle and oocyte maturation, such as the elongation of granulosa cells contacting the oocyte and migration of the cortical granules to the oocyte cortex. In addition, gene expression for activin and activin receptor type II (ActR II) was demonstrated in both the oocytes and the somatic cells using the reverse transcription-polymerase chain reaction, and immunohistochemical studies demonstrated the presence of activin and ActR II proteins in the somatic tissue and, especially, the oocytes of these follicles. It is concluded that, in vitro, activin A stimulates the growth of rat preantral follicles and promotes antrum formation. Furthermore, because activin A and ActR II are synthesized within preantral follicles, intrafollicular activin likely plays an important role in early follicular development.

Biological function and cellular mechanism of bone morphogenetic protein-6 in the ovary

The process of ovarian folliculogenesis is composed of proliferation and differentiation of the constitutive cells in developing follicles. Growth factors emitted by oocytes integrate and promote this process. Growth differentiation factor-9 (GDF-9), bone morphogenetic protein (BMP)-15, and BMP-6 are oocyte-derived members of the transforming growth factor-beta superfamily. In contrast to the recent studies on GDF-9 and BMP-15, nothing is known about the biological function of BMP-6 in the ovary. Here we show that, unlike BMP-15 and GDF-9, BMP-6 lacks mitogenic activity on rat granulosa cells (GCs) and produces a marked decrease in follicle-stimulating hormone (FSH)-induced progesterone (P(4)) but not estradiol (E(2)) production, demonstrating not only the first identification of GCs as BMP-6 targets in the ovary but also its selective modulation of FSH action in steroidogenesis. This BMP-6 activity resembles BMP-15 but differs from GDF-9 activities. BMP-6 also exhibited similar action to BMP-15 by attenuating the steady state mRNA levels of FSH-induced steroidogenic acute regulatory protein (StAR) and P450 side-chain cleavage enzyme (P450scc), without affecting P450 aromatase mRNA level, supporting its differential function on FSH-regulated P(4) and E(2) production. However, unlike BMP-15, BMP-6 inhibited forskolin- but not 8-bromo-cAMP-induced P(4) production and StAR and P450scc mRNA expression. BMP-6 also decreased FSH- and forskolin-stimulated cAMP production, suggesting that the underlying mechanism by which BMP-6 inhibits FSH action most likely involves the down-regulation of adenylate cyclase activity. This is clearly distinct from the mechanism of BMP-15 action, which causes the suppression of basal FSH receptor (FSH-R) expression, without affecting adenylate cyclase activity. As assumed, BMP-6 did not alter basal FSH-R mRNA levels, whereas it inhibited FSH- and forskolin- but not 8-bromo-cAMP-induced FSH-R mRNA accumulation. These studies provide the first insight into the biological function of BMP-6 in the ovary and demonstrate its unique mechanism of regulating FSH action.

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

Activins and their receptors in female reproduction

Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins consisting of two inhibin β subunits. The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor proteins, both of which are serine/threonine kinases. Subsequently, downstream signals such as Smad proteins are phosphorylated. Activins and their receptors are present in many tissues of mammals and lower vertebrates where they function as autocrine and (or) paracrine regulators of a variety of physiological processes, including reproduction. In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone. In the pituitary, activins increase follicle-stimulating hormone secretion and up-regulate gonadotropin-releasing hormone receptor expression. In the ovaries of vertebrates, activins are expressed predominantly in the follicular layer of the oocyte where they regulate processes such as folliculogenesis, steroid hormone production, and oocyte maturation. During pregnancy, activin-A is also involved in the regulation of placental functions. This review provides a brief overview of activins and their receptors, including their structures, expression, and functions in the female reproductive axis as well as in the placenta. Special effort is made to compare activins and their receptors in different vertebrates.

Key words: activins, activin receptors, reproductive axis, placenta.

The mechanisms of retinoic acid-induced regulation on the follicle-stimulating hormone receptor in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of retinoic acid (RA) on follicle-stimulating hormone receptor (FSH-R) in rat granulosa cells. Treatment with FSH produced a substantial increase in FSH-R mRNA level, as was expected, while concurrent treatment with increasing concentrations of RA brought about dose-dependent decreases in FSH-induced FSH-R mRNA, with a maximal inhibition one-third lower than that induced by FSH alone. RA, either alone or in combination with FSH, did not affect intracellular cAMP levels, while it inhibited the effect of 8-Br-cAMP on FSH-R mRNA production. These results suggested that RA diminished the action of FSH on FSH-R expression at sites distal to cAMP generation in the granulosa cells. Whether the effect of RA and FSH on FSH-R mRNA levels was the result of decreased transcription and/or altered mRNA stability was also investigated. The rate of FSH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, was found to decrease by the addition of RA. On the other hand, the decay curves for the 2.4 kb FSH-R mRNA transcript in primary granulosa cells did not alter the slope of the FSH-R mRNA decay curve in the presence of RA. Our data suggests for the first time that the effect of RA on FSH-R expression is possibly mediated by the reduction of the FSH-R mRNA level due to a negative regulation of the FSH-R gene in the presence of FSH. These findings assist in understanding the molecular mechanism underlying the effect of RA on reproductive function in rat granulosa cells.