Fertility in luteinizing hormone receptor-knockout mice after wild-type ovary transplantation demonstrates redundancy of extragonadal luteinizing hormone action

Elevated chorionic gonadotropic hormone in transgenic mice induces parthenogenetic activation and ovarian teratomas

Both male and female reproductive functions are impacted by altered gonadotrophin secretion and action, which may also influence the development of endocrine tumours. To ascertain if chronic hypersecretion of human chorionic gonadotropin (hCG) contributes to the development of gonadal tumours, double transgenic (TG) mice that overexpress hCGα- and β-subunits were analysed. By the age of two months, ovarian tumours with characteristics of teratomas developed with 100% penetrance. Teratomas were also seen in wild-type ovaries orthotopically transplanted into TG mice, demonstrating an endocrine/paracrine mechanism for the hCG-induced ovarian tumorigenesis. Both in vitro and in vivo experiments showed oocyte parthenogenetic activation in TG females. In addition, ovaries showed reduced ovulatory gene expression, inhibited ERK1/2 phosphorylation, and impaired cumulus cell expansion. Hence, persistently high endocrine hCG activity causes parthenogenetic activation and development of ovarian teratomas, along with altered follicle development and impaired ERK1/2 signalling, offering a novel mechanism associated with the molecular pathogenesis of ovarian teratomas.

Novel roles of luteinizing hormone (LH) in tissue regeneration-associated functions in endometrial stem cells

Luteinizing hormone (LH) stimulates the synthesis and secretion of the key steroid hormone estrogen, which subsequently promotes ovarian follicular growth and development. Therefore, the administration of exogenous LH to achieve superovulation (multiple ovulations) and an LH surge is commonly used as the most effective therapeutic option in a majority of in vitro fertilization (IVF) clinics. However, a relatively low pregnancy rate (between 20% and 35%) is one of the most challenging aspects of LH-based infertility treatment. Furthermore, the major cause of this low pregnancy rate in LH-based infertility treatment remains unidentified. Recent studies have shown that endometrial stem cell loss or deficiency can significantly decrease tissue regeneration ability during the menstrual cycle and reduce endometrial receptivity. In this context, we postulated that the low pregnancy rates following LH-based ovarian hyperactivation may be the result of the adverse effects of consecutive exogenous LH administration on endometrial stem cells. To the best of our knowledge, this study revealed for the first time that in addition to its previously reported roles in stimulating ovarian functions through the pituitary-gonadal axis, LH brings about the extragonadal suppression of various tissue regeneration-associated functions in endometrial stem cells, such as self-renewal, migration ability, multilineage differentiation potential, and pluripotency/stemness, by inhibiting pro-survival Akt and ERK1/2 signaling pathways in vitro and in vivo, and as a consequence, it decreases the endometrial receptivity.

Unconventional Actions of Glycoprotein Hormone Subunits: A Comprehensive Review

The glycoprotein hormones (GPH) are heterodimers composed of a common α subunit and a specific β subunit. They act by activating specific leucine-rich repeat G protein-coupled receptors. However, individual subunits have been shown to elicit responses in cells devoid of the receptor for the dimeric hormones. The α subunit is involved in prolactin production from different tissues. The human chorionic gonadotropin β subunit (βhCG) plays determinant roles in placentation and in cancer development and metastasis. A truncated form of the thyrotropin (TSH) β subunit is also reported to have biological effects. The GPH α- and β subunits are derived from precursor genes (gpa and gpb, respectively), which are expressed in most invertebrate species and are still represented in vertebrates as GPH subunit paralogs (gpa2 and gpb5, respectively). No specific receptor has been found for the vertebrate GPA2 and GPB5 even if their heterodimeric form is able to activate the TSH receptor in mammals. Interestingly, GPA and GPB are phylogenetically and structurally related to cysteine-knot growth factors (CKGF) and particularly to a group of antagonists that act independently on any receptor. This review article summarizes the observed actions of individual GPH subunits and presents the current hypotheses of how these actions might be induced. New approaches are also proposed in light of the evolutionary relatedness with antagonists of the CKGF family of proteins.

Cutting the ovarian surface improves the responsiveness to exogenous hormonal treatment in aged mice

Purpose

Ovarian vascular abnormality and ovarian fibrosis are observed in the low responder patients and aging mice. Vascularization and fibrosis are regulated by injury‐repair system, such as wound. Thus, in this study, the authors tried to investigate the effect of the surgical treatment to ovarian surface with cutting on the functions of ovary in aging mouse model, gcNrg1KO.

Method

The ovarian surface of gcNrg1KO was surgically cut, and then the ovary was returned inside of bursa ovarica. To assess the effect of cutting on fertility, mating test, smear analysis, and exogenous hormonal treatment were done. Additionally, the histological analysis was used for observing the remodeling of ovarian stroma after the surgical approach.

Result

Ovarian fibrosis disappeared at 7 days after surgery. With the abrogation of fibrosis, the blood vessels were fluently observed around the follicles, and the follicular development was re‐started. The responses against exogenous hormone were recovered at 21 days after the surgery, and estrous cycle and delivery were also recovered by the surgery and the fertility was maintained for 3 months.

Conclusion

This cutting method of ovarian surface becomes a good option against low responder patients.

Cellular Heterogeneity of the Luteinizing Hormone Receptor and Its Significance for Cyclic GMP Signaling in Mouse Preovulatory Follicles

Meiotic arrest and resumption in mammalian oocytes are regulated by 2 opposing signaling proteins in the cells of the surrounding follicle: the guanylyl cyclase natriuretic peptide receptor 2 (NPR2), and the luteinizing hormone receptor (LHR). NPR2 maintains a meiosis-inhibitory level of cyclic guanosine 5'-monophosphate (cGMP) until LHR signaling causes dephosphorylation of NPR2, reducing NPR2 activity, lowering cGMP to a level that releases meiotic arrest. However, the signaling pathway between LHR activation and NPR2 dephosphorylation remains incompletely understood, due in part to imprecise information about the cellular localization of these 2 proteins. To investigate their localization, we generated mouse lines in which hemagglutinin epitope tags were added to the endogenous LHR and NPR2 proteins, and used immunofluorescence and immunogold microscopy to localize these proteins with high resolution. The results showed that the LHR protein is absent from the cumulus cells and inner mural granulosa cells, and is present in only 13% to 48% of the outer mural granulosa cells. In contrast, NPR2 is present throughout the follicle, and is more concentrated in the cumulus cells. Less than 20% of the NPR2 is in the same cells that express the LHR. These results suggest that to account for the LH-induced inactivation of NPR2, LHR-expressing cells send a signal that inactivates NPR2 in neighboring cells that do not express the LHR. An inhibitor of gap junction permeability attenuates the LH-induced cGMP decrease in the outer mural granulosa cells, consistent with this mechanism contributing to how NPR2 is inactivated in cells that do not express the LHR.

Pregnancy and Live Birth In Women With Pathogenic LHCGR Variants Using Their Own Oocytes

CONTEXT

The LH/chorionic gonadotropin receptor (LHCGR) is mainly expressed in gonads and plays important roles in estradiol production, ovulation, and luteal formation. Women with pathogenic LHCGR variants suffer from infertility, and successful fertility treatments for such women have never been reported.

OBJECTIVE

The purpose of this study was to determine whether women with pathogenic LHCGR variants can achieve successful pregnancies through in vitro fertilization.

DESIGN

Three women with LH resistance and infertility and their parents underwent exome sequencing. The biochemical characteristics and functional effects of LHCGR mutation were assessed in transfected human embryonic kidney -293T cells and primary granulosa cells.

RESULTS

All affected women harbored pathogenic LHCGR variants. The LHCGR variants lacked cell surface localization and signal transduction abilities in vitro and in vivo. After dual triggering and prolonging the interval between triggering and oocyte pick-up, all three patients achieved oocytes and high-quality embryos. After frozen embryo transfer, one woman successfully birthed twins, and one woman successfully birthed a live boy. Apart from difficulties in oocyte retrieval, no obvious abnormalities in fertilization or during embryo development and pregnancy were identified in these patients.

CONCLUSIONS

This study is, to our knowledge, the first to report successful assisted reproductive treatment of women with pathogenic LHCGR variants using their own oocytes. Our results supported that defects in LHCGR disrupted ovulation but had no effect on fertilization and embryo development.

Gonadotropins and their receptors: coevolution, genetic variants, receptor imaging, and functional antagonists

Gonadotropins belong to the family of dimeric glycoprotein hormones and regulate gonadal physiology mediated by G protein-coupled, seven-transmembrane receptors. These glycoprotein hormones are widely used in the clinic to promote ovarian follicle development and for treating some cases of male infertility. We traced the coevolution of dimeric gonadotropin hormones and their receptors, together with thyrotropin and its receptor. We updated recent findings on human genetic variants of these genes and their association with dizygotic twining, polycystic ovarian syndrome, primary ovarian insufficiency, male-limited precocious puberty, and infertility. In addition to the known physiological roles of gonadotropin-receptor signaling in gonadal tissues, we also discussed emerging understanding of extragonadal functions of gonadotropins in bones and adipose tissues, together with recent advances in in vivo imaging of gonadotropin receptors in live animals. Recent development of gonadotropin receptor agonists and antagonists were summarized with an emphasis on the development of functional antagonists for FSH receptors to alleviate osteoporosis and obesity associated with menopause.

The gonadotropin system, lessons from animal models and clinical cases

This review article centers upon family of gonadotropin hormones which consists of two pituitary hormones - follicle-stimulating hormone (FSH) and luteinizing hormone (LH) as well as one non-pituitary hormone - human chorionic gonadotropin (hCG) secreted by placenta, and their receptors. Gonadotropins play an essential role in proper sexual development, puberty, gametogenesis, maintenance of pregnancy and male sexual differentiation during the fetal development. They belong to the family of glycoprotein hormones thus they constitute heterodimeric proteins built of common α subunit and hormone-specific β-subunit. Hitherto, several mutations in genes encoding both gonadotropins and their receptors have been identified in humans. Their occurrence resulted in a number of different phenotypes including delayed puberty, primary amenorrhea, hermaphroditism, infertility and hypogonadism. In order to understand the effects of mutations on the phenotype observed in affected patients, detailed molecular studies are required to map the relationship between the structure and function of gonadotropins and their receptors. Nonetheless, in vitro assays are often insufficient to understand physiology. Therefore, several animal models have been developed to unravel the physiological roles of gonadotropins and their receptors.

Confirmation of Luteinizing Hormone (LH) in Living Human Vitreous and the Effect of LH Receptor Reduction on Murine Electroretinogram

Luteinizing hormone (LH), produced in the anterior pituitary, has been detected in cadaver eyes and LH receptors (LHRs) have been identified in the retina, with the highest density in cone photoreceptors. Our aim was to confirm the presence of LH in the living, human eye as well as to examine the potential impact of a reduction in LHR signaling on visual processing. Vitreous samples were collected from 40 patients (23 diabetics, 17 non-diabetics) who were undergoing vitrectomies for various indications. LH concentration was quantified in each sample via an electro-chemiluminescence immunoassay and Meso Scale Discovery platform and normalized to total protein. In addition, full-field electroretinography (ERG) was performed on 11 adult LHR knockout heterozygous mice (B6;129X1-Lhcgr^tm1Zmlei/J) and 11 wild types using the Celeris-Diagnosys system. The median LH values (pg/mg total protein) for non-diabetics, diabetics without proliferative diabetic retinopathy (PDR) and diabetics with PDR were 40.7, 41.9 and 167.8 respectively. LH levels were significantly higher in diabetics with PDR. In our ERG investigation, heterozygous LHRKOs were found to have significantly reduced amplitudes of a-wave and b-waves at high stimulus intensities with no significant change in a-wave or b-wave amplitudes at lower intensities; this is consistent with a selective impairment of cone-mediated responses. Our findings confirm LH is present in the adult human eye. Our findings also suggest that a reduction in LH receptor signaling negatively impacts visual processing of the cone photoreceptors. Overall, our study results support the theory that LH likely plays a physiologic role in the eye.

The acceleration of reproductive aging in Nrg1flox/flox ;Cyp19-Cre female mice

Irregular menstrual cycles, reduced responses to exogenous hormonal treatments, and altered endocrine profiles (high FSH/high LH/low AMH) are observed in women with increasing age before menopause. In this study, because the granulosa cell‐specific Nrg1 knockout mice (gcNrg1KO) presented ovarian and endocrine phenotypes similar to older women, we sought to understand the mechanisms of ovarian aging and to develop a new strategy for improving fertility in older women prior to menopause. In the ovary of 6‐month‐old gcNrg1KO mice, follicular development was blocked in bilayer secondary follicles and heterogeneous cells accumulated in ovarian stroma. The heterogeneous cells in ovarian stroma were distinguished as two different types: (i) the LH receptor‐positive endocrine cells and (ii) actin‐rich fibrotic cells expressing collagen. Both the endocrine and fibrotic cells disappeared following long‐term treatment with a GnRH antagonist, indicating that the high levels of serum LH induced the survival of both cell types and the abnormal endocrine profile to reduce fertility. Moreover, follicular development to the antral stages was observed with reduced LH and the disappearance of the abnormal stromal cells. Mice treated with the GnRH antagonist regained normal, recurrent estrous cycles and continuously delivered pups for at least for 3 months. We conclude that endocrine and matrix alternations occur within the ovarian stroma with increasing age and that abolishing these alternations resets the cyclical release of LH. Thus, GnRH antagonist treatments might provide a new, noninvasive strategy for improving fertility in a subset of aging women before menopause.

A novel compound heterozygous mutation of the luteinizing hormone receptor -implications for fertility

The luteinizing hormone/chorionic gonadotropin receptor (LHCGR) belongs to the family of G-protein coupled receptors and binds both luteinizing hormone (LH) and human chorionic gonadotropin (hCG). Ligand-receptor interaction mediates a downstream cascade of events which is essential for ovulation in women, and expression of the male phenotype in men. The human LHCGR gene consists of 11exons and 10 introns. Homozygous and compound heterozygous mutations may inactivate the receptor by altering its structure and subsequent function. Herein we reported a novel, compound heterozgygous inactivating LHCGR mutation in a woman who presented with secondary infertility, having previously carried to term a donor oocyte pregnancy. A 27 bp deletion was detected in exon I at amino acid number 12. This mutation involved the signal peptide region, which is important for protein targeting, maturation and cellular expression. Another mutation involving a 2 base pair (thymine and cytosine) deletion was detected in exon 11 at amino acid number 586. This deletion produced a frameshift resulting in a premature stop codon and a truncated protein. An XY sibling with the same mutations was phenotypically female and misdiagnosed as complete androgen insensitivity syndrome. Other unaffected family members were genetically tested and carried one of the two mutations.

Gonadal and nongonadal FSHR and LHR dysfunction during lipopolysaccharide induced failure of blastocyst implantation in mouse

PURPOSE

The purpose of the present study was to investigate the impact of lipopolysaccharide (LPS) on follicle-stimulating hormone (FSH), luteinizing hormone (LH) and their receptors during preimplantation days of pregnancy.

METHOD

The PBS or lipopolysaccharide (LPS) was injected intraperitoneally in the pregnant females on day 0.5 of pregnancy and serum, embryos, ovaries and uterine horns were collected on days 1.5, 2.5, 3.5, 4.0, 4.125, 4.33 and 4.42 of pregnancy.

RESULT(S)

In the LPS-treated pregnant females, the secretion of FSH and LH is disturbed with respect to normal pregnancy. Furthermore, the expression of FSHR mRNA in embryos and ovaries, LHR mRNA in embryos and uterus get modulated in response to LPS during preimplantation days of pregnancy.

CONCLUSION(S)

The disturbance in the serum level of FSH and LH in response to LPS leads implantation failure in mouse which suggests that these gonadotropins plays an integral role in the process of the successful implantation. This study also suggests a possible nongonadal role of FSHR and LHR in LPS-induced implantation failure in the mouse.

Transcriptome analysis reveals an unexpected role of a collagen tyrosine kinase receptor gene, Ddr2, as a regulator of ovarian function

Mice homozygous for the smallie (slie) mutation lack a collagen receptor, discoidin domain receptor 2 (DDR2), and are dwarfed and infertile due to peripheral dysregulation of the endocrine system of unknown etiology. We used a systems biology approach to identify biological networks affected by Ddr2(slie/slie) mutation in ovaries using microarray analysis and validate findings using molecular, cellular, and functional biological assays. Transcriptome analysis indicated several altered gene categories in Ddr2(slie/slie) mutants, including gonadal development, ovulation, antiapoptosis, and steroid hormones. Subsequent biological experiments confirmed the transcriptome analysis predictions. For instance, a significant increase of TUNEL-positive follicles was found in Ddr2(slie/slie) mutants vs. wild type, which confirm the transcriptome prediction for decreased chromatin maintenance and antiapoptosis. Decreases in gene expression were confirmed by RT-PCR and/or qPCR; luteinizing hormone receptor and prostaglandin type E and F receptors in Ddr2(slie/slie) mutants, compared with wild type, confirm hormonal signaling pathways involved in ovulation. Furthermore, deficiencies in immunohistochemistry for DDR2 and luteinizing hormone receptor in the somatic cells, but not the oocytes, of Ddr2(slie/slie) mutant ovaries suggest against an intrinsic defect in germ cells. Indeed, Ddr2(slie/slie) mutants ovulated significantly fewer oocytes; their oocytes were competent to complete meiosis and fertilization in vitro. Taken together, our convergent data signify DDR2 as a novel critical player in ovarian function, which acts upon classical endocrine pathways in somatic, rather than germline, cells.

Molecular characterization of two sea bass gonadotropin receptors: cDNA cloning, expression analysis, and functional activity

The follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) play central roles in vertebrate reproduction. They act through their cognate receptors to stimulate testicular and ovarian functions. The present study reports the cloning and characterization of two sea bass (Dicentrarchus labrax) cDNAs encoding a FSH receptor (sbsFSHR) and a LH receptor (sbsLHR). The mature proteins display typical features of the glycoprotein hormone receptor family members, but the sbsFSHR also contains some remarkable differences when compared with other fish or mammalian FSHRs. Among them, a distinct extracellular N-terminal cysteine domain as regards to its length and cysteine number, and the presence of an extra leucine-rich repeat. Expression analysis revealed that the sbsFSHR is exclusively expressed in gonadal tissues, specifically in the follicular wall of previtellogenic and early-vitellogenic follicles. On the contrary, sbsLHR mRNA was found to be widely distributed in sea bass somatic tissues. When stably expressed in mammalian cell lines, sbsFSHR was specifically stimulated by bovine FSH, while sbsLHR was activated by both bovine LH and FSH. Nevertheless, specific stimulation of the sbsLHR was observed when recombinant sea bass gonadotropins were used. The isolation of a FSHR and a LHR in sea bass opens new ways to study gonadotropin action in this species.

The past, present and future of nongonadal LH/hCG actions in reproductive biology and medicine

The past and present published studies reaffirm that nongonadal LH and hCG actions are real and here to stay. These actions have led to a better understanding of the biology of the hormones and more importantly begin to pave the way for novel therapies in reproductive medicine and in other areas.

Extragonadal LH/hCG action--not yet time to rewrite textbooks

Gonadotropins are indispensable in both sexes in the regulation of gonadal sex steroid production and gametogenesis. In addition to their well-established classical actions, numerous recent publications have indicated the presence and function of luteinizing hormone/chorionic gonadotropin receptors (LH/hCG-R) in a variety of extragonadal tissues. However, the physiological significance of such effects has remained unclear. We have generated two genetically modified mouse models, one with excessive production of hCG and the other with targeted disruption of LH/hCG-R gene, and used them to address the functions of LH and hCG. Numerous gonadal and extragonadal phenotypes were found in the models with the two extremes of LH/hCG action. However, when the extragonadal effects were scrutinized in greater detail, they all appeared to arise through modification of gonadal function, either through enhanced or inhibited response to LH/hCG stimulation. Hence, further evidence is needed before the extragonadal LH/hCG-R expression can be considered functionally significant.

Normalization of hormonal imbalances, ovarian follicular dynamics and metabolic effects in follitrophin receptor knockout mice

Genetically modified follitrophin receptor knockout female mice with total FSH-receptor (FSH-R) deletion are sterile and their combined estrogen deficiency-hyperandrogenemic status provides an experimental paradigm to study the effect of hormonal imbalances on ovarian function and metabolic alterations. Elevated LH levels causing hyperandrogenemia perturb normal folliculogenesis. To control diverse pathophysiology associated with hormonal imbalances, we investigated the effects of transplanting a single normal mouse ovary in young mutants. An intact FSH-R signalling system in the graft responded promptly to the up-regulated pituitary gonadotrophins circulating in the host mutant. Resumption of regular estrous cycles validated stimulation of uterine functions. Secretions from the viable functioning grafts partially corrected follicular abnormalities originally present in host ovaries. Stromal hyperplasia responsible for high ovarian LH-receptor and key enzymes in host thecal/interstitial complex and hyperandrogenemia was reduced in host ovaries. Increases in plasma estradiol and reduced LH and free testosterone re-established the negative-feedback system. Reduced android obesity and activation of mammary glands indicated the combined beneficial effects of normalized steroid hormones on target organs. These data provide evidence that ovarian transplantation in mutants corrects estrogen loss and hyperandrogenemia. However, correction of hormonal imbalances is not sufficient to fully restore effects of FSH-R loss in host granulosa cells.

Phenotypic characterisation of mice with exaggerated and missing LH/hCG action

In order to study the physiology and pathophysiology of gonadotrophin action, we have produced transgenic (TG) mice overexpressing human chorionic gonadotrophin (hCG) alpha and beta subunits (hCG+ mice) and knockout (KO) mice for the luteinising hormone receptor (LHR; LuRKO mice). The two extremes in LH function, i.e. strong LH/hCG stimulation and total blockade of this action, confirm numerous earlier concepts about LH function, but they also reveal new aspects about gonadal function during excessive LH production and in the absence of this trophic stimulus. The purpose of this review is to summarise the key findings on these two genetically modified mouse models.

Ovarian wedge resection restores fertility in estrogen receptor beta knockout (ERbeta-/-) mice

Ovulation rarely occurs in mice in which the estrogen receptor beta (ERbeta) gene has been inactivated (ERbeta-/- mice). Here, we investigated whether this subfertility is due to a defect in the ovary itself or to more general endocrine changes in ERbeta-/- mice. We transplanted ERbeta-/- ovaries into WT mice and WT ovaries into ERbeta-/- mice. Upon mating with ERbeta-/- males, fertility increased from 20% in control intact ERbeta-/- group to 40% in the WT recipients with ERbeta-/- ovaries. The transplantation procedure was not efficient, and when WT ovaries were transplanted into WT mice, fertility was only 36%. Surgical ovarian wedge resection, a procedure which induces ovulation in anovulatory women with polycystic ovarian syndrome, resulted in 100% fertility of ERbeta-/- mice. In ERbeta-/- mice, as the follicles enlarged, the thecal layer remained very compact (revealed by H&E and collagen staining), and there was no increase in vascularization (measured as smooth muscle actin). In addition, there was an increase in PDGF receptor alpha (PDGFRalpha) and a decrease in PDGFbeta expression in the granulosa cells, similar to what has been found in follitropin receptor knockout mice. After wedge resection, expression of both smooth muscle actin and PDGFRs was normalized. During normal follicular development, increased vascularization of the thecal layer is a prerequisite for further follicular growth. We suggest that the defect in ERbeta-/- mouse ovaries is a failure of communication between the granulosa and thecal layers. The follicles do not mature because of insufficient blood supply. This problem is overcome by stimulating neovascularization by simple wedge resection of the ovaries.

Gonadotrophin resistance

Gonadotrophin resistance is caused by inactivating mutations in receptors (Rs) of the two gonadotrophins, i.e. luteinizing hormone (LH) and follicle-stimulating hormone (FSH), presenting as hypergonadotrophic hypogonadism and infertility/subfertility in both sexes. These conditions are extremely rare, but must be kept in mind upon differential diagnosis of disorders of sexual maturation, hypogonadism and infertility. In 46,XY individuals inactivation of LHR causes a disturbance in male-type sexual differentiation that ranges from male pseudohermaphroditism (complete lack of genital masculinization) to mild conditions such as cryptorchidism and hypospadias, depending on completeness of the receptor inactivation. In women, the phenotype is milder, presenting mainly as anovulatory amenorrhoea and hypo-oestrogenization. Inactivation of FSHR causes in otherwise normally masculinized men small testis size and variably reduced spermatogenesis, but not azoospermia or absolute infertility. In women the phenotype is more severe, with primary or early secondary amenorrhoea, arrested follicular maturation and anovulatory infertility. Incomplete forms with milder phenotype and partial responsiveness to FSH have also been described. Although gonadotrophin resistance is a very rare condition, its correct diagnosis is important for the selection of adequate treatment.

Mutations along the pituitary-gonadal axis affecting sexual maturation: novel information from transgenic and knockout mice

During the last 10 years, numerous activating and inactivating mutations have been detected in the genes encoding the two gonadotrophins, luteinising hormone (LH) and follicle-stimulating hormone (FSH), as well as their cognate receptors (R), LHR and FSHR. Because activation of the hypothalamic-pituitary-gonadal axis is a crucial event in the onset and progression of puberty, mutations affecting gonadotrophin action have major influence on this developmental process. Many of the phenotypic effects observed have been expected on the basis of the existing information about gonadotrophin action (e.g. delayed puberty), but also many unexpected findings have been made, including the lack of phenotype in women with activating LHR mutations, and the discrepancy in phenotypes of men with inactivating mutations of FSHbeta (azoospermia and infertility) and FSHR (oligozoospermia and subfertility). Some of the possible mutations, such as inactivating LHbeta and activating FSHR mutations in women, have not yet been detected. Genetically modified mice provide relevant phenocopies for the human mutations and serve as good models for studies on molecular pathogenesis of these conditions. They may also predict phenotypes of the mutations that have not yet been detected in humans. We review here briefly the effects of gonadotrophin subunit and receptor mutations on puberty in humans and contrast the information with findings on genetically modified mice with similar mutations.

Genetically modified mouse models in studies of luteinising hormone action

Numerous genetically modified mouse models have recently been developed for the study of the pituitary-gonadal interactions. They include spontaneous or engineered knockouts (KO) of the gonadotrophin-releasing hormone (GnRH) and its receptor, the gonadotrophin common-alpha(Calpha), luteinising hormone (LH) beta and follicle-stimulating hormone (FSH) beta subunits, and the two gonadotrophin receptors (R), LHR and FSHR. In addition, there are also transgenic (TG) mice overexpressing gonadotrophin subunits and producing supraphysiological levels of these hormones. These models have offered relevant phenocopies for similar mutations in humans and to a great extent expanded our knowledge on normal and pathological functions of the hypothalamic-pituitary-gonadal (HPG) axis. The purpose of this article is to review some of our recent findings on two such mouse models, the LHR KO mouse (LuRKO), and the hCG overexpressing TG mouse (hCG+).

Ovarian follicle development and transgenic mouse models

Ovarian follicle development is a complex process that begins with the establishment of what is thought to be a finite pool of primordial follicles and culminates in either the atretic degradation of the follicle or the release of a mature oocyte for fertilization. This review highlights the many advances made in understanding these events using transgenic mouse models. Specifically, this review describes the ovarian phenotypes of mice with genetic mutations that affect ovarian differentiation, primordial follicle formation, follicular growth, atresia, ovulation and corpus luteum (CL) formation. In addition, this review describes the phenotypes of mice with mutations in a variety of genes, which affect the hormones that regulate folliculogenesis. Because studies using transgenic animals have revealed a variety of reproductive abnormalities that resemble many reproductive disorders in women, it is likely that studies using transgenic mouse models will impact our understanding of ovarian function and fertility in women.

What have we learned about gonadotropin function from gonadotropin subunit and receptor knockout mice?

A number of biochemical and physiological studies elucidated the roles of pituitary and placental glycoprotein hormones. Advances in the past two decades in manipulating the mouse genome by random or site-specific mutagenesis have heralded a new dimension to our understanding of the biology of gonadotropins. It is now possible to model many human reproductive disorders involving gonadotropins/gonadotropin-signaling in the mouse. Mutant mice selectively lacking either FSH or LH or their cognate receptors have been generated. The gonadotropin ligand and the corresponding receptor knockout mice mostly phenocopy each other. Analyses with these genetic models confirmed earlier physiological studies; in addition they also revealed novel roles for gonadotropins previously unrecognized. While FSH action seems dispensable for male but not female fertility, absence of LH causes infertility in both the sexes. While Sertoli cell number and germ cell carrying capacity of the Sertoli cells in compromised in FSH mutants, both somatic and germ cell lineages are affected in the LH mutants resulting in complete male infertility. FSH mutant females demonstrate a preantral stage block in folliculogenesis and FSH alone is not sufficient to promote full folliculogenesis in the absence of LH. Pre-ovulatory stage follicles do not form and most of the follicles undergo apoptosis in the absence of LH. Many extra-gonadal phenotypes have been described for the receptor knockout mice and whether these bear any resemblances to those in patients with similar inactivating mutations in the receptors for FSH and LH remains an open question. Thus the in vivo models will continue to have a significant impact in understanding gonadotropin physiology and pathophysiology and serve as novel genetic tools to study signaling mechanisms in the gonads.