Perspective: the ovarian follicle--a perspective in 2001

Temporal and spatial expression of liver receptor homologue-1 (LRH-1) during embryogenesis suggests a potential role in gonadal development

Liver receptor homologue-1 (LRH-1), an orphan member of the nuclear receptor family highly expressed in adult mouse ovary, is closely related to steroidogenic factor 1 (SF-1), known to be important in gonadal formation. To analyze the potential role of LRH-1 in gonadal differentiation, we compared LRH-1 and SF-1 expression during mouse embryonic and postnatal development. LRH-1 expression was first detected in the urogenital ridge before sexual determination, in primordial germ cells and surrounding somatic cells; expression persisted after differentiation into testes and ovaries. Of interest, LRH-1 expression declined in the developing ovary and testis at embryonic day 15.5 but increased again just after birth in the ovary in granulosa cells and transiently in oocytes of developing follicles. By comparing and contrasting LRH and SF-1 expression with the two tissue-specific steroidogenic markers, cytochromes P450 aromatase and P450 17alpha-hydroxylase/17,20 lyase, we provide evidence for a potential role for LRH-1 in gonadal development, the initiation of folliculogenesis and regulation of estrogen biosynthesis within the ovary.

Adenoviral gene transfer allows Smad-responsive gene promoter analyses and delineation of type I receptor usage of transforming growth factor-beta family ligands in cultured human granulosa luteal cells

In the human ovary, cell growth and differentiation are regulated by members of the TGF-beta superfamily, including growth differentiation factor-9 (GDF9), TGF-beta, and activin. TGF-beta and activin are known to signal via Smad3 activation, and we have recently shown the involvement of Smad3 in cellular responses to GDF9. Recent studies with Smad3-deficient mice have also indicated a key role for this signaling mediator in ovarian folliculogenesis. We now demonstrate the use of a Smad3 reporter (CAGA-luciferase) adenovirus in primary cultures of human granulosa-luteal (hGL) cells to detect GDF9, TGF-beta, and activin responses. In rodent granulosa cells, TGF-beta and GDF9 signal through the TGF-beta type I receptor or activin receptor-like kinase 5 (Alk5), whereas the effect of activin is mediated though the activin type IB receptor, also known as Alk4. We now show that the GDF9 response in hGL cells is markedly potentiated upon overexpression of Alk5 by adenoviral gene transduction, as measured by the CAGA-luciferase reporter activity. A similar response to Alk5 overexpression was observed for TGF-beta, but not for activin. Adenoviral overexpression of the activin type IB receptor Alk4 in hGL cells specifically potentiated activin signaling, but not GDF9 or TGF-beta signaling. Alk5 overexpression in hGL cells also potentiated the GDF9 response when inhibin B production was used as the read-out. These results indicate that the CAGA-luciferase adenovirus can be used to study Smad3 signaling in primary cultures of human cells, and that adenoviral overexpression of wild-type receptors of the TGF-beta superfamily can be used to amplify the cellular response to ligands such as GDF9, TGF-beta, and activin. Furthermore, these studies indicate the involvement of Alk5 in GDF9 signaling in human cells and therefore, along with other recent studies, highlight how a limited number of type I and II receptors cooperate to generate specificity of action within the TGF-beta superfamily.

Targeted disruption of luteinizing hormone beta-subunit leads to hypogonadism, defects in gonadal steroidogenesis, and infertility

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) act on gonadal cells to promote steroidogenesis and gametogenesis. Clarifying the in vivo roles of LH and FSH permits a feasible approach to contraception involving selective blockade of gonadotropin action. One way to address these physiologically important problems is to generate mice with an isolated LH deficiency and compare them with existing FSH loss-of-function mice. To model human reproductive disorders involving loss of LH function and to define LH-responsive genes, we produced knockout mice lacking the hormone-specific LHbeta-subunit. LHbeta-null mice are viable but demonstrate postnatal defects in gonadal growth and function resulting in infertility. Mutant males have decreased testes size, prominent Leydig cell hypoplasia, defects in expression of genes encoding steroid biosynthesis pathway enzymes, and reduced testosterone levels. Furthermore, spermatogenesis is blocked at the round spermatid stage, causing a total absence of the elongated spermatids. Mutant female mice are hypogonadal and demonstrate decreased levels of serum estradiol and progesterone. Ovarian histology demonstrates normal thecal layer, defects in folliculogenesis including many degenerating antral follicles, and absence of corpora lutea. The defects in both sexes are not secondary to aberrant FSH regulation, because FSH levels were unaffected in null mice. Finally, both male and female null mice can be pharmacologically rescued by exogenous human chorionic gonadotropin, indicating that LH-responsiveness of the target cells is not irreversibly lost. Thus, LHbeta null mice represent a model to study the consequences of an isolated deficiency of LH ligand in reproduction, while retaining normal LH-responsiveness in target cells.

Cloning and characterization of granulosa cell high-mobility group (HMG)-box protein-1, a novel HMG-box transcriptional regulator strongly expressed in rat ovarian granulosa cells

Specific events in the ovary are dependent on gene expression in the tissue. By screening a rat ovarian granulosa cell cDNA library, a cDNA clone encoding a novel transcription factor-like protein containing a high-mobility group-box, referred to as granulosa cell high-mobility group-box protein-1 (GCX-1), was identified. The expression of GCX-1 is restricted to the hypothalamus, pituitary, testis, uterus, and ovary but was not detected in the adrenal gland. An in situ hybridization study revealed that the expression of GCX-1 was restricted to granulosa cell layers in early-stage follicles, and the expression was very low in large antral follicles and the corpus luteum, but localized expression in the testis or pituitary was not clear. Endogenous GCX-1 protein in the granulosa cells was identified by a Western blot analysis, and an analysis using the green fluorescence protein-GCX-1 fusion protein revealed that the GCX-1 protein was localized in the cell nucleus. GAL4 fusion protein-based assays demonstrated that GCX-1 is a potent transcriptional activator, and its putative transactivation domain was mapped to the region between amino acid residues 25 and 63 from the N terminus. These data strongly suggest that GCX-1 is likely a novel transcriptional activator that is exclusively expressed in reproductive tissues involving the hypothalamo-pituitary-gonadal axis, and functions as a specific regulator of follicular development, and may also participate in other specific events related to reproduction, particularly in the female.

Small nuclear RING finger protein expression during gonad development: regulation by gonadotropins and estrogen in the postnatal ovary

Small nuclear RING finger protein (SNURF/RNF4) is a steroid receptor coregulator that is down-regulated in testicular germ cell cancer. In this work, we examined SNURF expression during murine fetal gonad development and postnatal ovarian folliculogenesis by in situ hybridization and immunohistochemical staining. SNURF mRNA was detectable in gonads of both sexes from embryonic 10.5 days post conception onward. SNURF protein localized to gonocytes and somatic Leydig and Sertoli cells of fetal testis and in oogonia and supporting cells of fetal ovary. In murine postnatal ovary, SNURF mRNA and protein were expressed throughout folliculogenesis, peaking in the oocytes of preantral follicles. Lower amounts of SNURF mRNA and protein were also present in granulosa cells of secondary, antral, and preovulatory follicles and in luteal glands. Exposure of immature female mice and rats to gonadotropin from pregnant mare serum and human chorionic gonadotropin did not change dramatically SNURF mRNA levels in ovary. SNURF mRNA expression was increased in ovaries of immature mice treated with diethylstilbestrol, an effect that was blocked by the pure antiestrogen ICI 182,780. SNURF protein was constitutively expressed in oocytes of hypophysectomized rats, and its content was augmented by estradiol in granulosa cells. In granulosa cell culture, SNURF mRNA accumulation was transiently increased by treatment with the LH agonists phorbol myristate and forskolin at 4 h after treatment and at 48 h in differentiated cells expressing markers of the preovulatory phenotype. These results suggest a role for SNURF in fetal germ cell development as well as in oocyte and granulosa cell maturation in an estrogen- and gonadotropin-regulated fashion.

Oocyte-selective expression of MT transposon-like element, clone MTi7 and its role in oocyte maturation and embryo development

Previously, we found MT transposon-like element, clone MTi7 (MTi7) is highly expressed in the mouse ovary. Here, we show that the MTi7 is expressed in the oocyte from the primordial to the preovulatory follicles. For RNA interference (RNAi), double stranded RNAs (dsRNAs) were prepared for MTi7 and c-mos, a control gene with known functions. Each dsRNA was microinjected into germinal vesicle (GV) stage oocytes or zygotes with pronuclei (PN), after which developmental changes, mRNA expression, and nuclear and microtubular organization were analyzed. We found a 43.4-53% GV arrest in the microinjected oocytes with a concomitant decrease in targeted mRNA expression. In MTi7 dsRNA-injected early and late PN zygotes, a 92.9% 1-cell arrest and 76.9% 2-cell arrest were observed, respectively. This is the first report of an oocyte-selective expression of MTi7 mRNA, and our results strongly suggest that MTi7 involved in the nuclear membrane breakdown during oocyte maturation and embryo development.

Egr-1 induction in rat granulosa cells by follicle-stimulating hormone and luteinizing hormone: combinatorial regulation by transcription factors cyclic adenosine 3',5'-monophosphate regulatory element binding protein, serum response factor, sp1, and early growth response factor-1

Early growth response factor (Egr-1) is an inducible zinc finger transcription factor that binds specific GC-rich enhancer elements and impacts female reproduction. These studies document for the first time that FSH rapidly induces Egr-1 expression in granulosa cells of small growing follicles. This response is transient but is reinitiated in preovulatory follicles exposed to the LH analog, human chorionic gonadotropin. Immunohistochemical analysis also showed gonadotropin induced Egr-1 in theca cells. The Egr-1 gene regulatory region responsive to gonadotropin signaling was localized within -164 bp of the transcription initiation site. Binding of Sp1/Sp3 to a proximal GC-box at -64/-46 bp was enhanced by FSH in immature granulosa cells but reduced after human chorionic gonadotropin stimulation of preovulatory follicles despite constant protein expression. This dynamic regulation of Sp1 binding was dependent on gonadotropin-regulated mechanisms that modulate Sp1/3-DNA binding activity. Serum response factor was active in granulosa cells and bound a consensus CArG-box/serum response element site, whereas two putative cAMP response elements within the -164-bp region bound cAMP regulatory element (CRE) binding protein (CREB) and a second cAMP-inducible protein immunologically related to CREB. Transient transfection analyses using Egr-1 promoter-luciferase constructs and site-specific mutations show that the serum response element, GC-box, and CRE-131 are involved in gonadotropin regulation of Egr-1 expression in granulosa cells. Specific kinase inhibitors of Erk or protein kinase A antagonized this induction while exogenously expressed Egr-1 enhanced reporter expression. These observations indicate that the Egr-1 gene is a target of both FSH and LH action that may mediate molecular programs of proliferation and/or differentiation during follicle growth, ovulation, and luteinization.

Water permeability of an ovarian antral follicle is predominantly transcellular and mediated by aquaporins

Ovarian folliculogenesis is characterized, in part, by the formation and expansion of the fluid-filled antrum. Development of this cavity requires water influx, which may occur by transcellular or pericellular transport mechanisms. To assess the contribution of these mechanisms to the water permeability of an antral follicle, the rate of (3)H(2)O and (14)C-inulin (a complex sugar restricted to the extracellular compartment) uptake into isolated follicles was determined. The rate of H(2)O movement was 3.5-fold greater than that of inulin, suggesting that water enters a follicle primarily by transcellular pathways. Preincubation of the follicles with 50 microM HgCl(2) [a nonspecific aquaporin (Aqp) inhibitor] decreased H(2)O movement to levels seen with inulin, indicating that transcellular water movement is mediated through Aqp. To demonstrate the functional presence of Aqp in granulosa cells, we show that swelling in response to a hypotonic insult is attenuated by preincubation with 50 microM HgCl(2). Flow cytometry demonstrated the presence of Aqps-7, -8, and -9, thus identifying candidate Aqp potentially mediating water movement into antral follicles. These results suggest that water permeability of antral follicles occurs primarily through transcellular mechanisms, which may be mediated by Aqps -7, -8, and/or -9 in granulosa cells.

Involvement of inhibitory nuclear factor-kappaB (NFkappaB)-independent NFkappaB activation in the gonadotropic regulation of X-linked inhibitor of apoptosis expression during ovarian follicular development in vitro

Increased X-linked inhibitor of apoptosis (XIAP) expression and suppressed follicular apoptosis are important determinants in the regulation of follicular development by FSH. The objective of the present study was to examine the role and regulation of nuclear factor-kappaB (NFkappaB) in the gonadotropic control of granulosa cell XIAP expression and follicular growth in vitro. FSH (100 ng/ml) increased rat granulosa cell XIAP mRNA abundance and protein content. The gonadotropin also induced granulosa cell p65 subunit-containing NFkappaB translocation from cytoplasm to nucleus and increased NFkappaB-DNA binding activity. Supershift EMSA indicated the FSH-activated NFkappaB contained p65 and p50 subunits. Unlike TNFalpha, FSH failed to elicit a significant change in granulosa cell phospho- and total-inhibitory NFkappaB (IkappaB) IkappaB contents in vitro and dominant-negative IkappaB expression was ineffective in blocking the increase in NFkappaB-DNA-binding activity and XIAP protein content induced by the gonadotropin. In contrast, SN50 (a cell permeable inhibitory peptide of NFkappaB translocation, 50-200 ng/ml) suppressed FSH-stimulated NFkappaB-DNA binding, XIAP expression, and follicular growth. FSH also increased granulosa cell phospho-Akt contents, a response sensitive to the PI-3K inhibitor LY294002 (10 microM). In conclusion, the present studies demonstrate that the FSH-induced XIAP expression is mediated through the NFkappaB pathway through activation of phosphatidylinositol 3-kinase rather than the classical IkappaB kinase.

Rat zinc-fingers and homeoboxes 1 (ZHX1), a nuclear factor-YA-interacting nuclear protein, forms a homodimer

Zinc-fingers and homeoboxes 1 (ZHX1) is a protein which interacts with the activation domain of the A subunit of nuclear factor-Y. To analyze the physiological role(s) of ZHX1, we searched ZHX1-interacting protein(s) using a yeast two-hybrid system. The rat counterpart of ZHX1 cDNAs was cloned from an ovarian granulosa cell complementary DNA (cDNA) library, indicating that ZHX1 is able to form a homodimer. An analysis of the nucleotide sequence and its deduced amino acid sequence show that rat ZHX1 consists of 873 amino acid residues. Northern blot analysis shows that ZHX1 messenger RNA is expressed ubiquitously and that the level in the ovary are not regulated by gonadotropins. Furthermore, transfection experiments with green fluorescence protein (GFP) expression vectors into human embryonic kidney HEK293 cells reveal that full-length ZHX1 fused to the GFP is localized in the nuclei. Thus, we report on the molecular cloning, expression and characterization of full-length rat ZHX1 cDNA.

The preovulatory rise of ovarian ornithine decarboxylase is required for progesterone secretion by the corpus luteum

Ovarian progesterone secretion during the diestrus stage of the estrous cycle is produced by luteal cells derived from granulosa and thecal cells after the differentiation process that follows ovulation. Our results show that blockade of the preovulatory rise of ovarian ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, by treatment with the specific inhibitor alpha-difluoromethylornithine (DFMO) leads to a significant decrease in the ovarian progesterone content and a dramatic fall in the plasma levels of this hormone during the following diestrus. The same inhibition was produced in spite of the fact that both luteinizing and follicle stimulating hormones were given concomitantly with DFMO. On the other hand, the acute rise in the plasma progesterone levels observed after administration of human chorionic gonadotropin to mice at different periods of the estrous cycle was not affected by DFMO administration. Our results indicate that although elevated levels of ODC are not required for acute ovarian steroidogenesis, the preovulatory peak of ovarian ODC activity observed in the evening of proestrus may be critical for the establishment of a constitutive steroidogenic pathway and progesterone secretion by the corpus luteum during the diestrus stage of the murine estrous cycle.

Regulated expression of Wnts and Frizzleds at specific stages of follicular development in the rodent ovary

Wnt ligands and Frizzled (Fz) G protein-coupled receptors impact cell fate, including embryonic development of the ovary. Because the role of these regulatory molecules during follicular development in the adult is not known, an RT-PCR survey was done. Wnt-4, Fz-4, and Fz-1 were among the transcripts detected, and each exhibited a specific pattern of expression. Fz-1 mRNA was low in preovulatory follicles of PMSG-treated mice but was increased within 4-12 h after an ovulatory surge of human CG. By in situ analysis, Fz-1 transcripts increased first in the theca cells and then in the granulosa cells of ovulating follicles but were low in corpora lutea. In contrast, Wnt-4, a critical factor in early ovarian development, was expressed in small preantral follicles. In addition, Wnt-4 was detected in preovulatory follicles and exhibited high levels in corpora lutea. A potential receptor for Wnt-4 in corpora lutea is Fz-4 that was also elevated in this tissue. Although Wnt-4 has been shown to function downstream of the PR in other tissues, Wnt-4 was not altered in follicles of PR-null mice that fail to ovulate. Rather expression of Fz-1 was lower in ovaries of PR knockout mice, compared with normal littermates. Thus, specific Wnt/Fz are expressed at distinct stages of follicular development, suggesting multiple functions for this signaling pathway in the ovary.

Oestrogen receptors, receptor variants and oestrogen actions in the hypothalamic-pituitary axis

Information on oestrogen action has grown exponentially in the past decade, and recent studies have begun to define the mechanism of ligand-dependent activation and cell-specific effects. Oestrogen-mediated gene transcription in a specific tissue depends on several factors, the most important of which is the presence of at least one of the two nuclear oestrogen receptor (ER) isoforms, ER(alpha) and ERbeta. The presence and levels of specific ER isoform variants, along with receptor coactivator, corepressor and integrator proteins, directly modulate overall nuclear ER activity. The structure of the ligand, including both physiological oestrogens and synthetic oestrogen receptor modulators, influences ER interactions with these other proteins and thus determines the biological response. Furthermore, peptide and neurotransmitter-stimulated intracellular signalling pathways activate specific enzyme cascades and may modify the receptors and their interacting proteins, resulting in either independent or ligand-enhanced ER-mediated responses. Finally, several rapid effects of oestrogen probably occur at the membrane through nongenomic pathways that may or may not require the same ER proteins that are found in the nucleus. This review concentrates on the pituitary-hypothalamic axis and the genomic effects of oestrogen, and discusses the current knowledge of each of these factors in determining oestrogen actions in the neuroendocrine system.